applied sciences Article
Performance Assessment of a Developed Brake Frame Based on the Application of a Proposed Equivalent Model in a Net Protection System for Debris Flow Sungtae Kim, Sanghyun Hong, Jungjoo Kim and Hankyu Yoo * Department of Civil and Environmental Engineering, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-gu, Ansan 15588, Korea;
[email protected] (S.K.);
[email protected] (S.H.);
[email protected] (J.K.) * Correspondence:
[email protected]; Tel.: +82-31-400-5147; Fax: +82-31-409-4104 Received: 13 June 2018; Accepted: 5 July 2018; Published: 8 July 2018
Abstract: The demand for a net protection system for mitigating debris flow has increased recently for which new energy absorption devices have been hereby developed. The purpose of this study is to analyze the behavior and energy absorption rate of a brake frame, to find an efficient approach for its numerical simulation, and to increase its field applicability by analyzing its effects. Three methodologies were used for this purpose. Firstly, quasi-static load tests were conducted for analyzing the behavior of the brake frame. Secondly, three-dimensional FE modeling was performed to analyze the buckling and fractural behavior of steel rings. Lastly, the effects of the brake frame were numerically analyzed based on a proposed equivalent model. The results show that the brake frame can withstand the external force by elastic/local buckling, fracture of a steel ring, and elastic/plastic behavior of wire rope, and has the energy absorption rate of 53 kJ. It is deduced that the proposed equivalent model is capable to accurately simulate the behavior of the brake frame and can replace the three-dimensional model. The tensile force reduction in the system by using brake frames was observed to be about 75%. It is concluded that the proposed equivalent model for brake frame design is practically applicable. Keywords: debris flow; net protection system; brake frame; quasi-static load test; buckling; equivalent model; three-dimensional FE model; anchor capacity; impact energy
1. Introduction The sudden phenomenon of debris flow is quite destructive and occurs due to climate change. It causes damage to the people, property and structures [1]. In order to minimize the damage, research work is currently going on to predict the phenomenon of debris flow and develop its countermeasures, such as protection systems. The protection system for the debris flow can be classified into closed-type (such as concrete gravity dams) and open-type protection (such as buttress dams). The design standard of the system depends on the amount of debris flow and its pattern of the flow [2]. However, these two traditional protection systems have their own limitations, such as the need for a strong foundation for construction on steep slopes and it is also difficult for heavy machinery to access the construction site. Therefore, some net-type protection systems have been developed recently which depend on the flexibility of the net. For this study, the developed net protection system mainly consists of a net, energy absorption devices namely brake frames, and anchors, as shown in Figure 1. All the elements of the net protection system were designed carefully according to the design details so that these element could be saved from possible damage due to debris flow impact. The net is a kind of mesh or web made by wire ropes and it is designed to contain the debris flow by blocking and holding the material within itself.
Appl. Sci. 2018, 8, 1101; doi:10.3390/app8071101
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ropes and it is designed to contain the debris flow by blocking and holding the material within itself. The importance absorption devices in the net protection system is to absorb the kinetic energy importanceofofenergy energy absorption devices in the net protection system is to absorb the kinetic from the debris flow forcing it to make a halt. Recently, various energy dissipating devices have been energy from the debris flow forcing it to make a halt. Recently, various energy dissipating devices introduced in Italy which are capable of absorbing kJ to 5000 [3]. energy The numerical have been introduced in Italy which are capable of 100 absorbing 100kJ kJimpact to 5000energy kJ impact [3]. The approach is widely used to assess the performance and effects of these devices while developing numerical approach is widely used to assess the performance and effects of these devices while the constitutive and models each element the system with developing the laws constitutive lawsfor and models forofeach elementand of subsequent the system comparison and subsequent the experimental [3,4]. An analytical for analytical a flexible debris system flow was comparison with results the experimental results model [3,4]. An modelflow for protection a flexible debris suggested due to the difficulty in the dynamic analysis and the consideration of non-linearity of the protection system was suggested due to the difficulty in the dynamic analysis and the consideration structures [5]. of the structures [5]. of non-linearity
Figure cross-section of of the the developed developed net net protection protection system. system. Figure 1. 1. Transverse Transverse cross-section
Anchor supports play an important role in the system by transferring the kinetic energy from Anchor supports play an important role in the system by transferring the kinetic energy from the the impact to the system, and the anchor capacity can be determined by observing the interaction impact to the system, and the anchor capacity can be determined by observing the interaction between between the impact energy of the debris flow and the energy absorption devices. The performance of the impact energy of the debris flow and the energy absorption devices. The performance of the energy the energy absorption devices has to be assessed before the field application because the anchor absorption devices has to be assessed before the field application because the anchor support capacity support capacity varies with the efficiency of the energy absorption devices. There are many studies varies with the efficiency of the energy absorption devices. There are many studies related to the related to the full-scale impact tests and physical model tests that analyze the behavior of net full-scale impact tests and physical model tests that analyze the behavior of net protection systems protection systems as per their anchor capacity [6–10]. The anchor design primarily depends on the as per their anchor capacity [6–10]. The anchor design primarily depends on the slope condition, slope condition, engineering judgment, and information from previous experiences [11]. The load engineering judgment, and information from previous experiences [11]. The load transfer and failure transfer and failure characteristics of the anchors used in net protection system were investigated characteristics of the anchors used in net protection system were investigated under vertical and under vertical and horizontal loads based on field test [12]. The most important aspect is the analysis horizontal loads based on field test [12]. The most important aspect is the analysis of the systematic of the systematic relationship between each element. relationship between each element. The purpose of this study is to assess the performance of developed brake frames before field The purpose of this study is to assess the performance of developed brake frames before field application and to ensure the stability of the protection system through predicting the impact load application and to ensure the stability of the protection system through predicting the impact load on on the system. Therefore, an investigation scheme was developed, as explained below. Energy the system. Therefore, an investigation scheme was developed, as explained below. Energy dissipation dissipation is the area under the force-displacement curve obtained during quasi-static load tests. is the area under the force-displacement curve obtained during quasi-static load tests. Firstly, a brake Firstly, a brake frame was developed for providing up to 53 kJ of energy dissipation and then quasiframe was developed for providing up to 53 kJ of energy dissipation and then quasi-static load tests static load tests were performed to analyze the behavioral characteristics of the brake frame, and also were performed to analyze the behavioral characteristics of the brake frame, and also to analyze its to analyze its energy absorption capacity, as explained in Section 2. During the quasi-static load tests, energy absorption capacity, as explained in Section 2. During the quasi-static load tests, permanent permanent deformation was observed in one of the steel rings provided in the brake frame due to deformation was observed in one of the steel rings provided in the brake frame due to buckling and buckling and tensile fracture, resulting in energy dissipation in each brake frame. These tests proved tensile fracture, resulting in energy dissipation in each brake frame. These tests proved to be more to be more economical since a single brake frame used in this study was capable to deform up to 1.5 economical since a single brake frame used in this study was capable to deform up to 1.5 m which is m which is comparatively greater than other devices. comparatively greater than other devices. Subsequently, the behavior and deformation of each of the system components was recorded Subsequently, the behavior and deformation of each of the system components was recorded and an equivalent model was, thus, proposed for the validation of numerical simulations in different and an equivalent model was, thus, proposed for the validation of numerical simulations in conditions, as discussed in Section 3. The proposed equivalent model is based on the forcedifferent conditions, as discussed in Section 3. The proposed equivalent model is based on the displacement relationship and it can be used to simulate the complex behaviors of the brake frame, force-displacement relationship and it can be used to simulate the complex behaviors of the brake including buckling and metal fracture, using a one-dimensional model. This model can be used to frame, including buckling and metal fracture, using a one-dimensional model. This model can be used increase the efficiency of numerical modeling in this study. to increase the efficiency of numerical modeling in this study. Furthermore, in Section 4, the effects of using a brake frame in the system were analyzed through Furthermore, in Section 4, the effects of using a brake frame in the system were analyzed through the numerical approach by comparing its results with those of a system without which the brake the numerical approach by comparing its results with those of a system without which the brake frame frame and the tensile force was observed to be significantly decreased in the wire rope due to the use of the brake frame.
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and the tensile force was observed to be significantly decreased in the wire rope due to the use of the Appl. Sci. 2018, 8, x FOR PEER REVIEW 3 of 14 brake frame. 2.2. Development Developmentof ofaaBrake BrakeFrame Framefor forEnergy EnergyAbsorption Absorption 2.1. 2.1. Development DevelopmentofofaaBrake BrakeFrame Frame This plays an animportant importantrole roleininstopping stoppingthe the impact block well in reducing This brake brake frame frame plays impact block as as well as as in reducing the the stresses on the wire rope (net protection system) and thereby decreasing the tensile on stresses on the wire rope (net protection system) and thereby decreasing the tensile load on theload anchor the anchor by the absorbing impact This study the development of a brake frame by absorbing impact the energy. Thisenergy. study includes theincludes development of a brake frame that consists that consists of the components such as wire rope, guide rope, steel ring, and U-clips, as shown in of the components such as wire rope, guide rope, steel ring, and U-clips, as shown in Figure 2. An Figure 2. An independent wire rope core was used as the wire rope which was designed by using independent wire rope core was used as the wire rope which was designed by using 16 mm diameter. 16 mm diameter. steel ring is a representative of the net protection system in which large The steel ring isThe a representative element of theelement net protection system in which large permanent permanent deformation occurs. For this study, four holes of 20 mm diameter were drilled through the deformation occurs. For this study, four holes of 20 mm diameter were drilled through the steel ring. steel ring. The wire rope of 2900 mm length was attached to the periphery of the steel ring to create The wire rope of 2900 mm length was attached to the periphery of the steel ring to create effective effective and tothe share the tensile strength of rope wire rope the steel achieved bondingbonding and to share tensile strength of wire with with the steel ring.ring. ThisThis waswas achieved by by passing the wire rope in and out through the drilled holes and further connecting it to the steel passing the wire rope in and out through the drilled holes and further connecting it to the steel ring ring by byusing usingfour fourU-bolts. U-bolts. A A 50 50 mm mm wide wide and and 44 mm mm thick thick portion portion of of SS400 SS400 (related (related to totensile tensilestrength strengthof of 400 MPa) steel was used to assemble the 300 mm diameter steel ring. The steel ring was designed 400 MPa) steel was used to assemble the 300 mm diameter steel ring. The steel ring was designed to to be befractured fracturedwith withan anincrease increasein inthe thetensile tensilestrength strengthand andaafracture fracturepoint pointwas wasdesignated designatedmanually manuallyto to the ring so that elongation of the wire rope occurs when the steel ring breaks apart, thereby enabling the ring so that elongation of the wire rope occurs when the steel ring breaks apart, thereby enabling the the brake brake frame frame system system to to absorb absorb the the impact impact energy. energy. In In order order to to assist assist the the fracture fracture initiation initiation and and propagation on the steel ring, a triangular groove with 15 mm breadth and 15 mm height propagation on the steel ring, a triangular groove with 15 mm breadth and 15 mm heightwas wascreated created on onthe theperiphery peripheryof ofsteel steelring. ring. The The wire wire rope ropewas was further furtherattached attachedto toaaguide guiderope ropeof of1500 1500mm mmlength length using four or six U-clips. using four or six U-clips.
(a)
(b)
Figure2.2.Depiction Depictionof ofthe theelements elementsof ofbrake brakeframe frameand andinstallation installationin inthe thesystem, system,(a) (a)components componentsof of Figure brake frame; and (b) brake frame installed in the system. brake frame; and (b) brake frame installed in the system.
2.2. Structural Performance of the Developed Brake Frame 2.2. Structural Performance of the Developed Brake Frame In order to observe the structural performance, and also to analyze the absorbing rate of the In order to observe the structural performance, and also to analyze the absorbing rate of the brake brake frame, quasi-static load tests were performed in the hybrid structural testing center located at frame, quasi-static load tests were performed in the hybrid structural testing center located at Myongji Myongji University, South Korea. A 5000 kN static actuator was used to develop 3500 kN tensile force University, South Korea. A 5000 kN static actuator was used to develop 3500 kN tensile force in the in the brake frame and the piston speed was maintained as 250 mm/min. Subsequently, two types of brake frame and the piston speed was maintained as 250 mm/min. Subsequently, two types of tests tests were performed using two different sets of U-clips attached on the brake frame, as shown in were performed using two different sets of U-clips attached on the brake frame, as shown in Figure 3. Figure 3. The relationship between tensile force and displacement developed in the brake frame is shown The relationship between tensile force and displacement developed in the brake frame is shown in Figure 4. The behavioral characteristics of the brake frame during the tests can be divided into six in Figure 4. The behavioral characteristics of the brake frame during the tests can be divided into six phases. The first phase consists of the resistance of U-clips in which the displacement was observed phases. The first phase consists of the resistance of U-clips in which the displacement was observed to be approximately 0–480 mm. In this displacement range, the peak tensile force was dependent to be approximately 0–480 mm. In this displacement range, the peak tensile force was dependent on the number of U-clips used and the tightening of bolts (workability) resulting in different results observed in test 1 and test 2. The second and third phases are named as elastic/local buckling and plastic collapse of the steel ring, respectively. The displacement was observed to be approximately 480–700 mm in the phase of elastic/local buckling which was developed in both shoulders of the steel
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on the number of U-clips used and the tightening of bolts (workability) resulting in different results observed in test 1 and test 2. The second and third phases are named as elastic/local buckling and plastic collapse of the steel ring, respectively. The displacement was observed to be approximately Appl. Sci. 2018, 8, x FOR PEER REVIEW 4 of 14 Appl. Sci. 2018, PEER REVIEW of 14 480–700 mm8,inx FOR the phase of elastic/local buckling which was developed in both shoulders of the4 steel ring. Afterwards, plastic collapse occurred around the displacement range of 700–900 mm. Tensile ring. Afterwards, plastic collapse occurred around the displacement range of 700–900 mm. Tensile ring. Afterwards, collapse occurred around the displacement rangeatof 700–900 Tensile failure the steel steel plastic ring was was observed in phase phase four. Tensile Tensile failure occurred occurred 72–74 kN atmm. the fracture fracture failure of of the ring observed in four. failure at 72–74 kN at the failure of the steel ring was observed in phase four. Tensile failure occurred at 72–74 kN at the fracture point the number of of U-clips used. In the phase five,five, plastic behavior was point of of the thesteel steelring ringregardless regardlessofof the number U-clips used. In the phase plastic behavior point of the steel ring regardless of the number of U-clips used. In the phase five, plastic behavior observed in the frame in the displacement range of of 1100–1400 mm was observed in brake the brake frame in the displacement range 1100–1400 mmininboth bothtests. tests.In In the the final final was observed in the brakebehaviors frame in the displacement range of 1100–1400 mmdisplacement in both tests. crossed In the final phase, elastic and plastic were observed in the wire rope after the the phase, elastic and plastic behaviors were observed in the wire rope after the displacement crossed phase, elastic and plastic behaviors were observed indeveloped the wire rope after the displacement crossed 1400 mm limit. The tensile failure of the wire rope was around 176–192 kN. The behavior of the 1400 mm limit. The tensile failure of the wire rope was developed around 176–192 kN. The the 1400 mm limit. The tensile failure of the wire rope was developed around 176–192 kN. The the brake of frame in all of the six phases illustrated Figure 5. in Figure 5. behavior the brake frame in all of theissix phases isinillustrated behavior of the brake frame in all of the six phases is illustrated in Figure 5.
(a) (a)
(b) (b)
Figure 3. 3. Quasi-static load tests using different sets of U-clips, (a) brake frame using four U-clips; and Figure Quasi-static load load tests tests using using different different sets sets of of U-clips, U-clips, (a) (a) brake brake frame frame using using four four U-clips; U-clips; and and Figure 3. Quasi-static (b) brake frame using six U-clips. (b) brake frame using six U-clips. (b) brake frame using six U-clips.
Figure 4. Classification of quasi-static load tests result in six phases. Figure 4. Classification of quasi-static load tests result in six phases. Figure 4. Classification of quasi-static load tests result in six phases.
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(a) (a)
(b) (b)
(c) (c)
(d) (d)
(e) (e)
(f) (f)
Figure of deformation phases of the brake frame, (a) phase 1; (b) phase 2; (c) phase 3; Figure 5. 5. Illustration Illustration Figure 5. Illustration of of deformation deformation phases phases of of the the brake brake frame, frame, (a) (a) phase phase 1; 1; (b) (b) phase phase 2; 2; (c) (c) phase phase 3; 3; (d) phase 4; (e) phase 5; and (f) phase 6. (d) phase 4; (e) phase 5; and (f) phase 6. (d) phase 4; (e) phase 5; and (f) phase 6.
3. Model for Brake Frame 3. Equivalent Equivalent 3. Equivalent Model Model for for Brake Brake Frame Frame 3.1. Numerical Modeling Modeling for for Three-Dimensional Three-Dimensional Behavior 3.1. Numerical Behavior In used to analyze the behavior of frame for this study, three-dimensional FE model was used toto analyze thethe behavior of brake brake frame for In this thisstudy, study,three-dimensional three-dimensionalFE FEmodel modelwas was used analyze behavior of brake frame proposing an equivalent model for its analysis and a comparison was made with the results of quasiproposing an equivalent model for its analysis and aand comparison was made with the results of quasifor proposing an equivalent model for its analysis a comparison was made with the results of static load tests. As shown in Figure 6, the 3-D FE models include the steel ring being modelled as aa static load tests. As shown in Figure 6, the 3-D FE models include the steel ring being modelled as quasi-static load tests. As shown in Figure 6, the 3-D FE models include the steel ring being modelled shell element and wire rope being modelled as a truss element. The steel ring was modelled as a fourshell element and wire roperope beingbeing modelled as a truss element. The steel as a fouras a shell element and wire modelled as a truss element. The ring steelwas ringmodelled was modelled as a noded, thin (S4R), and divided in total 7312 elements which noded, doubly-curved, doubly-curved, thin shell shell element (S4R),(S4R), and was was divided in to to aain total oftotal 7312of elements which four-noded, doubly-curved, thinelement shell element and was divided to aof 7312 elements had mm spacing between each modelled as linear 3-D had 2.5 2.5had mm2.5 spacing between each other. other. The wire wire rope was modelled as aa two-noded two-noded linearlinear 3-D truss truss which mm spacing between each The other. Therope wirewas rope was modelled as a two-noded 3-D element (T3D2), and was divided in to a total of 716 elements which had 2 mm spacing between each element (T3D2), and was in to aintotal 716ofelements whichwhich had 2had mm2spacing between each truss element (T3D2), anddivided was divided to a of total 716 elements mm spacing between other. The analysis was with Explicit (SIMULIA, Johnston, RL, other.other. The numerical numerical analysis was performed performed with ABAQUS ABAQUS Explicit (SIMULIA, Johnston, RL, USA USA each The numerical analysis was performed with ABAQUS Explicit (SIMULIA, Johnston, RL, 2016) [13]. 2016) [13]. USA 2016) [13].
(a) (a)
(b) (b)
(c) (c)
Figure Three-dimensional numerical modeling of the system components, (a) ring model; (b) Figure 6. 6. ofof thethe system components, (a) steel steel ringring model; (b) Figure 6. Three-dimensional Three-dimensionalnumerical numericalmodeling modeling system components, (a) steel model; wire rope model; and (c) the assembled model. wire rope model; and (c) the assembled model. (b) wire rope model; and (c) the assembled model.
The The contact contact problem problem in in the the 3-D 3-D FE FE model model was was composed composed of of the the tangential tangential behavior behavior with with the the nonnonThe contact problem in the 3-D FE model was composed of the tangential behavior with the friction condition and the normal behavior with the hard contact condition. The contact conditions friction condition and the normal behavior with the hard contact condition. The contact conditions non-friction condition and the normal behavior with the hard contact condition. The contact conditions must must develop develop when when the the 16 16 mm mm diameter diameter of of wire wire rope rope comes comes in in contact contact with with the the hole hole of of the the steel steel ring ring must develop when the 16 mm diameter of wire rope comes in contact with the hole of the steel ring according to extent of increase in area of wire rope. However, local stress concentration occurs when according to extent of increase in area of wire rope. However, local stress concentration occurs when according to extent of increase in area of wire rope. However, local stress concentration occurs when the the shell shell element element gets gets in in contact contact with with the the truss truss element. element. As, As, the the truss truss element element intrinsically intrinsically ignores ignores the the
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the shell element gets in contact with the truss element. As, the truss element intrinsically ignores the geometric conditions in this model; thus, to avoid this phenomenon, the contact area of the shell element through which the truss element passes was modelled as a rigid body in this study (shown as the gray areas on the steel ring in Figure 6). MPC link was applied in the modeling to simulate the behavior of U-clip in brake frame. The properties of 3-D FE model for the components of brake frame are given in Table 1. A SS400 steel ring having a Young’s modulus of 210 GPa and a yielding stress of 775 MPa was numerically modelled by using the elasto-plastic property with strain hardening and the failure of elements was evaluated by using displacement criterion. The wire rope (IWRC) for the elasto-plastic material has the Young’s modulus of 150 GPa, a yielding stress of 1000.2 MPa and an ultimate stress of 1336.7 MPa at 0.0494 of strain. The effective cross-sectional area comprised of 58% of the actual cross-sectional area. Table 1. Properties of 3-D FE model. Property
Steel Ring
Wire Rope
Young’s modulus (GPa) Yielding stress (MPa) Density (kg/m3 ) Thickness (mm) Cross-sectional area (mm2 )
210 775 7850 4 -
150 1000.2 8730 116.62
The loading condition of the 3-D FE model was set to be the same as that in quasi-static load tests, and was set to apply horizontal displacement at both ends of the wire ropes resulting in the development of tensile forces, whereby, the vertical and transverse displacements were fixed. No specific boundary condition was applied to the steel ring except the contact area with wire rope. When the wire rope was subjected to tensile behavior, the buckling and fracture of the steel ring were set to occur according to the contact condition in the hole. 3.2. Behavior of Brake Frame and Verification of Three-Dimensional Model In order to comprehensively analyze the behavior of the steel ring and to verify the numerical results using experimental results, the displacement in the range between phase 2 and 4 was considered while excluding the effect of the resistance of U-clips from the relationship between the tensile force and the displacement of the brake frame. The comparison of the 3-D numerical modeling results, related to the relationship between tensile force and displacement, with the experimental results and the behavior of steel ring during different phases are shown in Figures 7 and 8. The numerical analysis results match satisfactorily with the experimental results. The 3-D numerical simulations show that the steel ring could withstand a tensile force of 13.6 kN due to primary elastic buckling at a recorded displacement of 479.2 mm. Further, the displacement in the brake frame continued to increase during the phase of plastic collapse and a tensile force of 6.6 kN was recorded at the displacement value of 687.2 mm due to local buckling on the left shoulder. Furthermore, a tensile force of 10 kN was recorded at the displacement value of 980 mm due to local buckling on the right shoulder. A difference was observed between the tensile force values obtained from the experimental and numerical results in the displacement range of 600 mm and 700 mm. The observed difference was due to the fact that the elastic buckling on both the shoulders of steel ring was observed to be occurring together during the quasi-static load tests, as compared to the numerical simulations, in which it happened alternatively. Due to the secondary elastic buckling, a tensile force of 30 kN was recorded at the displacement value of 1057.7 mm and the behavior of the steel ring was observed to be similar to phase 3 behavior (deformation shape) shown in Figure 8. A tensile force of 54.3 kN was recorded at the displacement value of 1105.7 mm due to the tensile failure of the steel ring, which was observed to be matching with the experimental results.
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In this study, the brake frame is devised to absorb the impact load on wire rope through buckling and fracture steelPEER ringREVIEW as well as by ensuring the additional plastic deformation. When the 3-D Appl. Sci. 2018, 8,of x FOR 7 ofFE 14 model was used, it was observed that the tensile force could vary (difference in the numerical and Appl. Sci. 2018, 8, x FOR PEER REVIEW 7 of 14 experimental in some sections to order ringthe buckling; however, when the analysis rangeresults) up to the fracture point according was considered in of thesteel model, accurate behavior of steel ring, as well as the energy absorption rate, could be estimated. analysis range up to the fracture point was considered in the model, the accurate behavior of steel ring, analysis range up to the fracture point was considered in the model, the accurate behavior of steel as well energy rate, could be estimated. ring,asasthe well as the absorption energy absorption rate, could be estimated.
FigureComparison 7. Comparison of three-dimensional model numerical results with experimental results. Figure Figure 7. 7. Comparison of of three-dimensional three-dimensional model model numerical numerical results results with with experimental experimental results. results.
(a)
(b)
(a)
(c)
(b)
(d)
(c)
(e)
Figure 8. Three-dimensional behavior of steel rings during different phases, (d) (e)(a) primary elastic buckling; (b) local buckling on left shoulder; (c) local buckling on right shoulder; (d) secondary elastic Figure 8. Three-dimensional behavior of steel rings during different phases, (a) primary elastic buckling; and (e) tensile failure. Figure 8. Three-dimensional behavior of steel rings during different phases, (a) primary elastic buckling;
buckling; (b) local buckling on left shoulder; (c) local buckling on right shoulder; (d) secondary elastic (b) local buckling on left shoulder; (c) local buckling on right shoulder; (d) secondary elastic buckling; buckling; and Model (e) tensile failure. 3.3. Equivalent Based on One-Dimensional Connector for Brake Frame and (e) tensile failure. The brake frame is an important component of the net protection system and it is necessary to 3.3. Equivalent Model Based on One-Dimensional Connector for Brake Frame inspect its performance and behavior using Connector full-scale tests or FE analysis. Full-scale testing has 3.3. Equivalent Model Based on One-Dimensional for Brake Frame various limitations such as the difficulty in conducting repetitive tests and the simulation various to The brake frame is an important component of the net protection system and it isofnecessary The brake frame is an important component ofexplained the net protection system and it is necessary to testing conditions. When the 3-D FE analysis, as in Section 3.1, is performed for the has inspect its performance and behavior using full-scale tests or FE analysis. Full-scale testing inspect its performance and behavior using full-scale tests ordecreased. FE analysis. Full-scale testing hasthese various verification process, the efficiency of analysis also gets In order to overcome various limitations such as the difficulty in conducting repetitive tests and the simulation of various limitations such the difficulty conducting repetitive tests the simulation of in various testing limitations, anas equivalent modelinbased on one-dimensional axialand connector is proposed this study testing conditions. When the 3-D FE analysis, as explained in Section 3.1, is performed for the which is When capablethe of 3-D replacing the 3-D as FE explained model for brake frame.3.1, Theisequivalent mentioned conditions. FE analysis, in Section performedmodel for the verification verification process, the efficiency of analysis also gets decreased. In order to overcome these abovethe is efficiency the axial connection on nonlinear elasticity and has seven-linear force- an process, of analysis type also based gets decreased. In order to overcome these limitations, limitations, an equivalent model based on one-dimensional axial connector is proposed in this study displacement as shown in Table 2. The length of equivalent model is determined as equivalent modelrelationship, based on one-dimensional axial connector is the proposed in this study which is capable which of the replacing the diameter 3-D FE model brake frame. The were equivalent 0.3 ismcapable based on equivalent of steelfor ring. Various points markedmodel on thementioned forceabove is the axial connection typefrom based on nonlinear and has seven-linear forcedisplacement relationship deduced the maximum force ofelasticity different phases. Points 1 and 2 define
displacement relationship, as shown in Table 2. The length of the equivalent model is determined as 0.3 m based on the equivalent diameter of steel ring. Various points were marked on the forcedisplacement relationship deduced from the maximum force of different phases. Points 1 and 2 define
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of replacing the 3-D FE model for brake frame. The equivalent model mentioned above is the axial connection type based on nonlinear elasticity and has seven-linear force-displacement relationship, as shown in Table 2. The length of the equivalent model is determined as 0.3 m based on the equivalent diameter of steel ring. Various Appl. Sci. 2018, 8, x FOR PEER REVIEWpoints were marked on the force-displacement relationship deduced 8 of 14 from the maximum force of different phases. Points 1 and 2 define the gap initially present between the wire rope and steel ring, and represent the incapability of the system to withstand any externalof force. the gap initially present between the wire rope and steel ring, and represent the incapability the Points 2–5 represent the system resistance of external force by progressing through the elastic buckling system to withstand any external force. Points 2–5 represent the system resistance of external force to tensile fracture stage the steel ring. Points represent thestage resistance external by 5–8 the bythe progressing through theofelastic buckling to the5–8 tensile fracture of theofsteel ring.force Points wire rope.the Theresistance results ofof quasi-static loadby tests the behavior at points 3–4 as well points 5–6 represent external force thedepict wire rope. The results of quasi-static loadastests depict in way thatat either there negative slope5–6 or in there is athat decrease forceisata these points. However, theabehavior points 3–4isasawell as points a way eitherinthere negative slope or there in to maintain of theHowever, equivalent force-displacement phases between these is aorder decrease in forcethe at stability these points. inmodel, order to maintain the stability of the equivalent points were replaced with perfectly plastic conditions in the numerical simulation. model, force-displacement phases between these points were replaced with perfectly plastic conditions in the numerical simulation. Table 2. Force-displacement relationship for equivalent model. Point Point 1 1 2 2 3 3 4 4 5 5 6 6 7 7 8 8
Table 2. Force-displacement relationship for equivalent model. Displacement (mm) Force (kN) Remarks Displacement (mm) Force (kN) Remarks 0 0 Initial gap (Points 1–2) 0 250 0 0 Initial gap (Points 1–2) 250 0 650 45 650 45 Steel ring (Points 2–5) 900 45 900 1100 45 70 Steel ring (Points 2–5) 1100 70 1400 70 1400 70 Wire rope (Points 5–8) 1476 160 1476 160 Wire rope (Points 5–8) 1500 180 1500 180
Figure presents the theresults resultsofofthe thequasi-static quasi-staticload loadtests, tests, the force-displacement relationship Figure 99 presents the force-displacement relationship of of the equivalent model and also the analysis results from the application of equivalent model. the equivalent model and also the analysis results from the application of equivalent model. Pull Pull displacement was applied the of ends of connector axial connector to satisfy the equivalent conditions of displacement was applied at theatends axial to satisfy the equivalent conditions of quasiquasi-static load tests for the simulation of application of equivalent model. The equivalent model static load tests for the simulation of application of equivalent model. The equivalent model which which a 1-Dconnector, axial connector, as shown in Figure 9, observed was observed be able to accurately simulate uses a uses 1-D axial as shown in Figure 9, was to betoable to accurately simulate the the performance of brake frame as well as perfectly replace the complex contact conditions and performance of brake frame as well as perfectly replace the complex contact conditions and fractural fractural themodel 3-D FE model resulting in efficiency a higher efficiency of analysis. behavior behavior in the 3-DinFE resulting in a higher of analysis.
Figure 9. 9. Representation Representation of of the the equivalent equivalent model model derived derived from from quasi-static quasi-static load load tests tests and and numerical numerical Figure analysis results using the model. analysis results using the model.
4. Validation by Numerical Simulation and Results 4.1. Modeling of the Net Protection System The effects of the developed brake frame on the net protection system were numerically validated through ABAQUS Explicit using a specific case in which a spherical impact block of 400 kg
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4. Validation by Numerical Simulation and Results 4.1. Modeling of the Net Protection System The effects of the developed brake frame on the net protection system were numerically validated through ABAQUS Explicit using a specific case in which a spherical impact block of 400 kg mass and Appl. Sci. 2018, 8, x FOR PEER REVIEW 9 of 14 1 m diameter was rolled down a slope at an angle of 54.34◦ , as shown in Figure 10. The slope and the impact block bodies. The and widthwas of slope weretomodelled as 20 monand modelled aswere 20 mmodelled and 14.35as m,rigid respectively. Theheight impact energy assessed be 30 kJ based the 14.35 m, respectively. The impact energy was assessed to be 30 kJ based on the potential energy potential energy related to the height of 7.65 m. The distance between the base of the slope related and the tonet theprotection height of 7.65 m. The between the base the slope the net protection was system was distance set as 0.25 m. Factors likeof height and and velocity of the impactsystem block while set as 0.25 m. Factors like height and velocity of the impact block while rolling are shown in Figure 11. rolling are shown in Figure 11. The coefficient of friction of the slope was 0.60 and the speed of impact The coefficient of friction of the slope was 0.60 and the speed of impact block just before the impact block just before the impact was measured as 10 m/s. The net protection system was assembled using was measured 10 m/s.wire Theropes net protection system was assembled horizontal andwere vertical wire horizontal andasvertical crossing over each other. A totalusing of seven wire ropes installed ropes crossing over each other. A total of seven wire ropes were installed horizontally and twelve horizontally and twelve wire ropes were installed vertically, both at a distance of 0.5 m between each wire ropes a distancelinear of 0.5 3-D m between each other. Theand wirethe rope was other. Thewere wireinstalled rope wasvertically, modelledboth as aat two-noded truss element (T3D2), spacing modelled as aindividual two-nodedelements linear 3-D truss (T3D2), and the of spacing between the individual between the was set element as 25 mm. The number elements constituting the wire elements was set as 25 mm. The number of elements constituting the wire rope without and with rope without and with a brake frame were 3176 and 3260, respectively. The physical properties and athe brake frame were 3176 and 3260, respectively. The physical properties and the sectional area for sectional area for the modeling of wire rope were used the same as in Section 3.1. There were no the modeling of wire rope were used the as in Section specific boundary specific boundary conditions applied to same the twelve vertical 3.1. wireThere ropeswere and no their only role was to conditions applied to the twelve vertical wire ropes and their only role was to prevent the horizontal prevent the horizontal wire ropes from breaking away due to the impact. The seven horizontal wire wire ropes breaking away duethe to the impact. seven horizontal wire ropes were ropes werefrom designed to withstand impact dueThe to the hinged boundary conditions at designed both endstoof withstand the impact due to the hinged boundary conditions at both ends of horizontal wire ropes. horizontal wire ropes. Further, the assembly was then analyzed by attaching brake frames to the Further, the assembly was then analyzed by attaching brake frames to the seven horizontal ropes seven horizontal wire ropes and a comparison was made between the results obtained bywire modeling and comparison was made between the results obtained by modeling ropes with the ahorizontal wire ropes with and without installing brake frames. Inthe thehorizontal numericalwire simulation, the and without installing brake frames. In the numerical simulation, the brake frames were applied at the brake frames were applied at the center of the horizontal wire ropes only, as shown in Figure 10b, center of the horizontal wire ropes only,explained as shownin in Section Figure 10b, and the proposed equivalent modelto and the proposed equivalent model 3.3 was applied afterwards. In order explained in Section 3.3 was applied afterwards. In order to facilitate the contact condition of impact facilitate the contact condition of impact block with the equivalent model, the 0.3 m length of brake block the equivalent the 0.3 m length brake framein installed in each ofwere the horizontal framewith installed in each ofmodel, the horizontal wire ropeofwas divided two parts which connected wire rope was divided in two parts which were connected to each other through a series spring to each other through a series of spring connections. The impact force was observed to be of following connections. The impact force was observed to be following the force-displacement relationship in the force-displacement relationship in Table 2, causing half of the displacement because the brake Table 2, causing half of the displacement because the brake frame was divided in two parts. frame was divided in two parts.
(a)
(b)
Figure10. 10.Numerical Numerical models models with frame; and (b) Figure with and and without withoutusing usingbrake brakeframes, frames,(a) (a)without withoutbrake brake frame; and with brake frame. (b) with brake frame.
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Figure 11. Height Height andvelocity velocity of impact block while rolling, (a) height of impact block; and (b) Figure of of impact block while rolling, (a) height of impact block; and (b) velocity Figure 11. 11. Heightand and velocity impact block while rolling, (a) height of impact block; and (b) velocity of impact block. of impactofblock. velocity impact block.
4.2. Effects of Brake Frame on the Net Protection System for Debris Flow Impact 4.2. Effects of of Brake Brake Frame Frame on on the the Net 4.2. Effects Net Protection Protection System System for for Debris Debris Flow Flow Impact Impact The horizontal wire ropes were numbered from 1 to 7 starting from the bottom wire rope. The The horizontal horizontal wire wireropes ropeswere werenumbered numberedfrom from 1 to 7 starting from bottom 1 to 7 starting from the the bottom wirewire rope.rope. The tensile forces obtained at the end of wire rope without brake frames according to time are shown in The tensile forces obtained at the end of wire rope without brake frames according to time are shown tensile forces obtained at the end of wire rope without brake frames according to time are shown in Figure 12. Without installing brake frames in the net protection system, the maximum tensile forces in Figure Without installing brakeframes framesininthe thenet netprotection protectionsystem, system,the themaximum maximum tensile tensile forces Figure 12.12. Without installing brake (reaction force) among all seven wire ropes were obtained to be 117.8 kN and 117.0 kN, respectively, (reaction 117.0 kN, respectively, in (reaction force) force) among amongall allseven sevenwire wireropes ropeswere wereobtained obtainedtotobebe117.8 117.8kN kNand and 117.0 kN, respectively, in wire ropes 3 and 4. The brake time (time required to attain maximum tensile force) for wire ropes wire ropes 3 and 4. The brake time (time required to attain maximum tensile force) for wire ropes in wire ropes 3 and 4. The brake time (time required to attain maximum tensile force) for wire ropes3 3 and 4 was observed as 0.09 s and 0.12 s, respectively, as shown in Figure 12. The stress contours in and 4 was observed asas 0.09 s and 0.12 s, respectively, as shown in Figure 12. 12. TheThe stress contours in the 3 and 4 was observed 0.09 s and 0.12 s, respectively, as shown in Figure stress contours in the net protection system during this process are shown in Figure 13. When the impact block was net system during this process are shown in Figure 13. When the impact block was rolled the protection net protection system during this process are shown in Figure 13. When the impact block was rolled down the slope, it made an impact with the wire rope 2 in the first step and a tensile force of down slope, madeitan impact with the wirethe rope 2 in the2first stepfirst andstep a tensile of force 32.2 kN rolled the down the itslope, made an impact with wire rope in the and aforce tensile of 32.2 kN was observed to be developed in wire rope 2. In the next step, the impact block made impact was observed to be developed in wire in rope 2. rope In the impact block block mademade impact with 32.2 kN was observed to be developed wire 2. next In thestep, nextthe step, the impact impact with wire ropes 3 and 4 developing maximum tensile forces in it among all the seven wire. wire and 4 3developing maximummaximum tensile forces in it among seven wire. Afterwards, the with ropes wire 3ropes and 4 developing tensile forces all in the it among all the seven wire. Afterwards, the impact block was observed to be rebounding towards the slope and making another impact blockthe wasimpact observed towas be rebounding the slope and making another impact with the Afterwards, block observed totowards be rebounding towards the slope and making another impact with the net protection system due to which the tensile force developed in all the wire ropes net protection due to which the tensile forcethe developed in alldeveloped the wire ropes 3 and impact with thesystem net protection system due to which tensile force in all except the wire ropes4 except 3 and 4 were observed to be around 25 kN, which lasted for 1.2 s after the second impact. The were to beobserved around 25tokN, which lasted 1.2 slasted after the impact. The impact exceptobserved 3 and 4 were be around 25 kN,for which for second 1.2 s after the second impact.block The impact block was observed to get rebounded to a relatively greater extent because it produced was observed to getobserved rebounded to a rebounded relatively greater extent because produced relatively smaller impact block was to get to a relatively greaterit extent because it produced relatively smaller displacement in the net protection system. displacement in the net protection system. relatively smaller displacement in the net protection system.
Figure 12. Tensile force acting on seven wire ropes without brake frame. Figure 12. Tensile force acting on seven wire ropes without brake frame. Figure 12. Tensile force acting on seven wire ropes without brake frame.
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(b) (b)
(c) (c)
(d) (d)
Figure inin the netnet protection system after impact, without using brake frame, (a) Figure13. 13.Stress Stressdistribution distribution the protection system after impact, without using brake frame, Figure 13. Stress distribution in theimpact; net protection system after impact, without using brake frame, (a) 0.015 s after impact; (b) 0.09 s after (c) 0.195 s after impact; and (d) 0.27 s after impact. (a) 0.015 s after impact; (b) 0.09 s after impact; (c) 0.195 s after impact; and (d) 0.27 s after impact. 0.015 s after impact; (b) 0.09 s after impact; (c) 0.195 s after impact; and (d) 0.27 s after impact.
When the brake frames were installed and the impact block was rolled down the slope, the When the the brake frames frameswere wereinstalled installed andthethe impact block was rolled down the slope, When impact block waskN rolled slope, the maximum tensilebrake force developed in wire ropeand 4 was observed to be 9.9 in thedown brakethe time of 0.225 the maximum tensile force developed in wire rope 4 was observed to be 9.9 kN in the brake time of force in wire rope 4tensile was observed to bedeveloped 9.9 kN in the brakeropes time 5ofand 0.225 s,maximum as showntensile in Figure 14.developed Further, the maximum forces were in wire 6 0.225 s, as shown in Figure 14. Further, the maximum tensile forces were developed in wire ropes 5 s, as shown in Figure 14. Further, the maximum tensile forces were developed in wire ropes 5 and due to the impact and were recorded as 28.4 kN and 29 kN in brake times of 0.27 s and 0.33 s,6 and the impact and were recorded as 28.4 and brake times 0.27s sand and0.33 0.33 s, s, due 6todue thetoimpact and contours were recorded as 28.4 kN kN andsystem 29 29 kNkN in in brake ofof0.27 respectively. The stress in the net protection during all times this process are shown in respectively. The stress contours in the net protection system during all this process are shown in respectively. The stress contours in the net protection system during all this process are shown in Figure 15. Figure Figure 15. 15.
Figure 14. Tensile force acting on seven wire ropes with a brake frame. Figure 14. Tensile force acting on seven wire ropes with a brake frame. Figure 14. Tensile force acting on seven wire ropes with a brake frame.
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(a)
(b)
(c)
(d)
Figure net protection system after impact, with using a brake frame, (a) Figure 15. 15.Stress Stressdistribution distributionininthe the net protection system after impact, with using a brake frame, 0.12 s after impact; (b) 0.21 s after impact; (c) 0.27 s after impact; andand (d) (d) 0.315 s after impact. (a) 0.12 s after impact; (b) 0.21 s after impact; (c) 0.27 s after impact; 0.315 s after impact.
5. Discussion 5. Discussion The brake frame physically depicts plastic behavior as the displacements cannot be recovered to The brake frame physically depicts plastic behavior as the displacements cannot be recovered to the initial stages when the external forces are removed after the occurrence of buckling and fracture the initial stages when the external forces are removed after the occurrence of buckling and fracture behavior (phases 1–5, as shown in Figure 4). However, in this study, the proposed equivalent model behavior (phases 1–5, as shown in Figure 4). However, in this study, the proposed equivalent model was axial connection type based on nonlinear elasticity in which the displacements can be recovered was axial connection type based on nonlinear elasticity in which the displacements can be recovered to the initial stages after the removal of external forces. Therefore, when the net protection system is to the initial stages after the removal of external forces. Therefore, when the net protection system is analyzed using the proposed equivalent model, the obtained results must be used at the development analyzed using the proposed equivalent model, the obtained results must be used at the development point of maximum tensile and reaction forces in the wire ropes without considering the results related point of maximum tensile and reaction forces in the wire ropes without considering the results related to restoring forces. to restoring forces. As compared to the cases where the brake frames were used, the tensile forces in each wire rope As compared to the cases where the brake frames were used, the tensile forces in each wire rope were observed to be at least 1.4–18 times greater in the system without using a brake frame, except were observed to be at least 1.4–18 times greater in the system without using a brake frame, except in in the case of wire rope 6, as shown in Figure 16. The results show that the maximum tensile forces the case of wire rope 6, as shown in Figure 16. The results show that the maximum tensile forces in in the wire ropes obtained in the system with brake frame were reduced by 75%, as compared to the wire ropes obtained in the system with brake frame were reduced by 75%, as compared to those those obtained in the system without brake frame. The results also show that the sum of tensile forces obtained in the system without brake frame. The results also show that the sum of tensile forces in in all the wire ropes obtained in the system with the brake frame were reduced by 74%, as compared all the wire ropes obtained in the system with the brake frame were reduced by 74%, as compared to to those obtained in the system without the brake frame. those obtained in the system without the brake frame. When the brake frame is used, the load distribution in the upper wire ropes was observed to When the brake frame is used, the load distribution in the upper wire ropes was observed to increase as compared to the high load distribution in the center portion of the wire ropes when the increase as compared to the high load distribution in the center portion of the wire ropes when the brake frame was not used due to the reason that the deformation in the wire ropes increases when brake frame was not used due to the reason that the deformation in the wire ropes increases when brake frames were used. In both the cases, the wire rope having maximum tensile force encompassed brake frames were used. In both the cases, the wire rope having maximum tensile force encompassed 28.8–30.6% of the load distribution. Furthermore, in both cases, two of the wire ropes comprising the 28.8–30.6% of the load distribution. Furthermore, in both cases, two of the wire ropes comprising the most tensile forces acting in them were observed to be encompassing 58–61% of the load distribution. most tensile forces acting in them were observed to be encompassing 58–61% of the load distribution. It is thus deduced from the results that the concentration of the load distribution in some of the wire It is thus deduced from the results that the concentration of the load distribution in some of the wire ropes must be considered for the design of each individual case. ropes must be considered for the design of each individual case.
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Figure 16. Comparison Comparison of Figure 16. of the the development development of of tensile tensile forces forces and and load load distribution distribution in in each each wire wire rope rope with and without using a brake frame. with and without using a brake frame.
6. Conclusions 6. Conclusions To control the damage caused by debris flow, a net protection system was developed in this To control the damage caused by debris flow, a net protection system was developed in this study, which is equipped with a new energy absorption device called a brake frame. Deducing the study, which is equipped with a new energy absorption device called a brake frame. Deducing the important design parameters was necessary while analyzing the systematic relationship of all important design parameters was necessary while analyzing the systematic relationship of all elements elements comprising the net protection system. The quasi-static load tests were performed in order comprising the net protection system. The quasi-static load tests were performed in order to analyze to analyze the performance of brake frames. Using a three-dimensional FE model, the numerical the performance of brake frames. Using a three-dimensional FE model, the numerical analysis was analysis was performed to investigate the behavior of steel rings causing large deformation and the performed to investigate the behavior of steel rings causing large deformation and the verification of verification of the numerical model was also conducted accordingly. Based on the numerical the numerical model was also conducted accordingly. Based on the numerical approach, the effect approach, the effect of the brake frame in reducing the impact load in specific cases was estimated of the brake frame in reducing the impact load in specific cases was estimated using a proposed using a proposed equivalent model. equivalent model. (1) The developed brake frames were based on the absorption mechanism of the impact energy 1. through The developed brake frames were based on the absorption of the impact energy three elements: rings, wire ropes, and U-clips, mechanism with six phases of behavioral through three elements: rings, wire ropes, and U-clips, with six phases of behavioral characteristics. The brake frames could deform up to 1.5 m, thereby absorbing 53 kJ of impact characteristics. brake frames deform up to 1.5ism, thereby absorbing 53arrangement kJ of impact energy. The fieldThe application of thecould net protection system possible only when the energy. The field application of the net protection system is possible only when the arrangement as well as the number of brake frames is designed in specific cases according to the impact as well as the number of brake frames is designed in specific cases according to the impact energy. energy. 2. The steel steel rings rings in in the the brake brake frame frame were were capable capable of of absorbing absorbing the the impact impact energy energy through through the the (2) The mechanisms of of elastic elastic and and local local buckling, buckling, as as well When the the equivalent mechanisms well as as metallic metallic fracture. fracture. When equivalent model based based on on the the one-dimensional one-dimensional axial axial connector connector having having seven seven linear model linear force-displacement force-displacement relationships is is utilized, utilized, an an accurate accurate simulation simulation is is performed performed and and an an increase increase in in efficiency efficiency of of the the relationships analysis can can be be expected, expected, as as compared compared to to the the use use of of three-dimensional three-dimensional FE FE model. model. analysis 3. The anchor capacity was observed to be depending on the performance of the energy absorption absorption (3) The anchor capacity was observed to be depending on the performance of the energy devices. The Theeffectiveness effectiveness of of using using brake brake frame frame in in the the net netprotection protection system system was was clearly clearly observed observed devices. because the the impact impact load load on on the the anchor anchor positions positions (the (the connection connection between between the the wire rope and the because ground) was was recorded recorded to be about about 75% 75% less less than than that that in in the the case case when when the the brake brake frames frames were not ground) installed. The Theanchor anchorcapacity capacityobserved observedfor forthe thespecific specific cases cases can can be be used used as as a reference for the installed. anchor design design while while using using the the brake brake frames frames in in the the net net protection protection system. anchor Extending Extending the the observations observations and and conclusions conclusions of of this this study study in in the the future, future, the the relationship relationship between between the the impact impact energy energy and and anchor anchor capacity capacity will will be be developed, developed, and and the the tensile tensile forces forces and and load load distribution distribution in be analyzed for various various cases cases of of the the impact impact energy energy based based on on the the full-scale full-scale tests. tests. in the the anchors anchors will will be analyzed for Author Contributions: H.Y. supervised the overall process in this study. S.K. designed the net protection system using the brake frame, as well as performed quasi-static load tests. S.H. performed the numerical simulation and
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Author Contributions: H.Y. supervised the overall process in this study. S.K. designed the net protection system using the brake frame, as well as performed quasi-static load tests. S.H. performed the numerical simulation and proposed the equivalent model. J.K. analyzed the performance and effects of using brake frames, as well as prepared the manuscript by combining the observations of the physical experiments and numerical analysis. Acknowledgments: This work was supported by the Technology Development Program (S2401940) funded by the Ministry of SMEs and Startups (MSS, Korea). Conflicts of Interest: The authors declare no conflict of interest.
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