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Journal of Intelligent Material Systems and Structures http://jim.sagepub.com/ Detection of crack formation and stress distribution for carbon fiber−reinforced polymer specimens through triboluminescent-based imaging Nirupam Aich, Eunho Kim, Mohamed ElBatanouny, Jaime Plazas-Tuttle, Jinkyu Yang, Paul Ziehl and Navid B Saleh Journal of Intelligent Material Systems and Structures published online 23 May 2014 DOI: 10.1177/1045389X14535017 The online version of this article can be found at: http://jim.sagepub.com/content/early/2014/05/22/1045389X14535017

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Original Article

Detection of crack formation and stress distribution for carbon fiber–reinforced polymer specimens through triboluminescent-based imaging

Journal of Intelligent Material Systems and Structures 1–8 Ó The Author(s) 2014 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/1045389X14535017 jim.sagepub.com

Nirupam Aich1, Eunho Kim2, Mohamed ElBatanouny3, Jaime Plazas-Tuttle1, Jinkyu Yang2, Paul Ziehl3 and Navid B Saleh1

Abstract This article demonstrates the ability of surface-coated triboluminescent materials to detect damage in carbon fiber–reinforced polymer specimens. An experimental protocol was developed to test the efficiency of the triboluminescent-based diagnostic method using carbon fiber–reinforced polymer coupons under combined bending– compression conditions. Luminescence, emitted from the triboluminescent coatings under quasi-static loading, was detected by capturing digital images. We employed image processing software to quantify change in luminescence as a function of triboluminescent concentration. We observed that 10%, 20%, and 30% triboluminescent coating resulted in 25.3, 27.9, and 40.4 (arbitrary units) total luminescence, respectively, which shows a positive correlation of triboluminescent concentration with luminescence. Finite element simulation was also performed to understand the stress and strain distribution and to aid in understanding and correlating light emission regions on the carbon fiber–reinforced polymer coupons under bending deformation. This work represents a step toward the development of a robust technology that employs triboluminescent materials for early damage detection, consistent with theoretical predictions of damage occurrence. Keywords Polymers, sensor, structural health monitoring

Introduction Fiber-reinforced polymeric (FRP) materials are widely used in many engineering applications, including aerospace, automotive, marine, and civil engineering. These FRP materials are typically composed of high-strength fibers (e.g. glass or carbon) embedded in a relatively soft polymer matrix, such as epoxy, polyester, and vinyl ester. The high strength-to-weight ratio of FRP composites along with their long life expectancy and corrosion resistance makes them a suitable substitute for metals in many applications. However, they also pose unprecedented challenges for inspection and maintenance due to their unique failure modes; such as fiber breakage, matrix cracking, and delaminations (Davies et al., 1998; Hamdi et al., 2013). Any undetected damage can jeopardize the safety and reliability of structural members that employ FRP composites, potentially leading to catastrophic failure. Therefore, reliable and real-time structural health monitoring (SHM) methods are

strongly desired to detect the unique damage modes in FRP composites. The last two decades have seen numerous studies focused on the development of SHM methods for FRP materials (Austin et al., 2013; Fowler et al., 1989; Ziehl and Fowler, 2003). The most widely investigated nondestructive evaluation (NDE) methods are based on ultrasonic waves that propagate and interact with structural damage in composites, hence enabling condition 1

Department of Civil, Architectural, and Environmental Engineering, University of Texas, Austin, TX, USA 2 William E. Boeing Department of Aeronautics & Astronautics, University of Washington, Seattle, WA, USA 3 Department of Civil and Environmental Engineering, University of South Carolina, Columbia, SC, USA Corresponding author: Paul Ziehl, Department of Civil and Environmental Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA. Email: [email protected]


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assessment of the integrity of the tested component (Hoon and Sang Jun, 2010; Keilers and Chang, 1995). Other techniques, for example, thermography, eddy current method, and X-ray-based imaging, have also been pursued (Adams and Cawley, 1988; Giurgiutiu, 2007; Qin and Bao, 1995). While each of these technologies provides diagnostic information with unique advantages and shortcomings, most methods necessitate complicated software for postprocessing and/or bulky and expensive hardware that often limits their applications. Thus, novel SHM methods capable of real-time damage sensing based on simple detection techniques and inexpensive devices are needed to overcome the limitations of large dataset analysis and the use of complex damage detection equipment. Triboluminescent (TL) materials offer an alternative solution, where real-time crack detection can be accomplished at low cost and with rather simple evaluation methods. TL materials release energy via luminescence, when the crystal planes are subject to strain or fracture. A wide range of TL materials, commercially known as phosphors (Phosphor Technology, Hoddesdone, Herts, UK), are being employed for different applications; for example, manganese-doped ZnS (ZnS:Mn) (Aich et al., 2013; Chandra et al., 2010; Olawale et al., 2012), europium tetrakis (Hurt et al., 1966), and terbium complexes (Sage et al., 2001). Different materials have been used to extract luminescence under mechanical stress. However, ZnS:Mn with two to three orders of magnitude higher luminescence when compared to the other phosphors has been the most widely used for SHM applications (Olawale et al., 2011). ZnS:Mn has thus been selected as the material of choice for this study. Previous studies have demonstrated that these TL materials, including ZnS:Mn, can effectively serve for optical or visible damage detection and sensing (Sage and Bourhill, 2001). The advantageous properties of TL materials have intrigued researchers to study them for detection of mechanical or structural damage in TL films under low velocity impact (Dickens et al., 2011), in aluminum plates coated with TL material under low velocity (Sage et al., 1999) and hypervelocity impact (Bergeron et al., 2006; Sage et al., 1999), in cementitious materials under low velocity impact (Olawale et al., 2012) and compressive loading (Aich et al., 2013; Olawale et al., 2012), and in FRP composites (Sage et al., 2001). These were also used for visualization of stress distribution on the surface of structures under external loading (Xu et al., 2000). However, research on TL applications in FRPs is currently limited to preparation of TL-mixed FRPs and their preliminary evaluation for damage detection (Dickens and Okoli, 2011; Sage et al., 2001). The current gap in the technology development is lack of systematic evaluation and correlation between TL intensity for the case of lower rates of loading, such as those that would normally be applicable as part of a

routine maintenance or evaluation procedure. In addition, correlation of luminescence associated with specific damage and failure mechanisms is not currently available. This article demonstrates the ability of surface coated TL materials to detect severe deformation and damage in carbon fiber reinforced polymer (CFRP) specimens. We developed an experimental protocol to test the efficiency of the TL-based diagnostic method using CFRP coupons under combined bending-compression conditions. Luminescence, emitted from the TL coatings under quasi-static loading, was detected by capturing digital images. Finite element simulation was also performed to understand the stress and strain distribution and to aid in understanding and correlating light emission regions on the CFRP coupons under bending deformation. The current work represents a step toward the development of a robust technology that employs TL material for early damage detection, consistent with theoretical predictions of damage occurrence.

Materials and methods Carbon fiber–reinforced polymer and TL materials Carbon/polyphenylene sulfide (PPS) plastic unidirectional blank coupons (Fiberforge, Glenwood Springs, CO, USA) with dimension of 100 mm 3 10 mm 3 2.1 mm were used as test specimens. These specimens were purchased as panels and cut to the desired dimensions using a diamond saw. The stacking sequence of the specimen was [0/90/0/45/245/90/0/90/0/90/245/45/ 0/90/0]. ZnS:Mn crystals with a median size of 8.5 mm obtained from Phosphor Technology were used as TL coating materials. Characterization protocols and results have been discussed in detail in a previously published article (Aich et al., 2013). IVEX-C410 ArmorStarÒ polystyrene composite resin from CCP Composites (Kansas City, MO, USA) and Hi-Point 90 resin additive from Pergan Marshall LLC (Marshall, TX, USA) were obtained for use in ZnS:Mn dispersion and composite preparation.

Specimen preparation and combined compression– bending testing ZnS:Mn material was first suspended in 40 mL IVEXC410 (ArmorStarVE) by magnetic stirring for 5 min and then 0.6 mL Hi-Point 90 resin catalyst was added to the mix followed by further stirring for 2 min. The suspension was bath sonicated for 5 min to ensure homogenized dispersion of TL particles. Three different sets of suspensions were prepared with 4.6, 10.4, and 17.8 g ZnS:Mn to obtain 10%, 20%, and 30% (w/w) TL concentrations, respectively. The CFRP specimens were immersed in each suspension for 1 h and then dried in room temperature in an arrangement, as shown


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Figure 1. (a) Arrangements of painting and drying of the TL-epoxy coating on the CFRP coupon surfaces and (b) prepared CFRP coupons uniformly coated with 10% TL materials. TL: triboluminescent; CFRP: carbon fiber–reinforced polymer.

Figure 2. Specimen condition (a) before loading and (b) after loading under combined compression–bending stress and fracture formation causing buckling. (c) DSLR camera positioned at equal distance on the opposite sides of the TL-coated CFRP specimen for imaging. DSLR: digital single-lens reflex; TL: triboluminescent; CFRP: carbon fiber–reinforced polymer.

in Figure 1(a). During the drying process, each side of the coupons was retouched with the TL suspension using a fine brush to achieve strong binding with the CFRP and uniformity in coating thickness. To ensure evenness of the coatings on both sides of the specimens, the upper-facing sides were altered every 15 min. Uniform coating of ZnS:Mn suspension with 0.25 mm thickness on each side of the specimens was achieved. Coated specimens with 10% TL materials are shown in Figure 1(b). The TL-coated specimens were then loaded in a combined compression–bending test using a Tinius Olsen TI-5000 electromechanical load frame (Tinius Olsen Inc., Horsham, PA, USA). The overall test setup is presented in Figure 2(a) to (c). Both ends of the specimen were fixed in the grips, which were free to rotate (Figure 3(a)). Also, a 0.2-mm offset from the center of the end cross section was given to both ends to induce a bending deformation under compression loading (the red line in Figure 3(a)). Details of the testing setup are described in Guo et al. (2013). Quasi-static compression

loading was applied to the grips with a 2.54-mm/min loading rate until the specimen failed through bending deformation. The applied force and compression displacements were recorded during the test through a National Instruments data acquisition system controlled with LabVIEW software (National Instruments, Austin, TX).

Imaging and analysis Light emission from the TL coating layers on the CFRP surfaces under combined compression–bending stress was captured in a sufficiently darkened room using two Nikon D-7000 digital single-lens reflex (DSLR) cameras (Nikon Inc., Melville, NY, USA). The cameras were placed on opposite sides (compression and tension sides of the specimen) of the load frame at equal distances from the specimens with similar ISO values, focal length, and shutter speed to maintain controlled image collection (Figure 2(c)). The imaging for each specimen was started at the beginning


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Figure 3. BCs for CFRP beam (a) in test setup and (b) FE model. x and z degrees of freedom are fixed in upper boundary condition and x, y, and z degrees of freedom are fixed in lower boundary condition. CFRP: carbon fiber–reinforced polymer; FE: finite element; DOF: degrees of freedom; BC: boundary condition.

of loading and continued for 3 min taking sequential images every 20 s (i.e. the exposure time of each shot was 20 s). Image analysis was performed with ImageJ software (NIH) to analyze captured luminescence. The threedimensional (3D) profiles for luminescence intensity for each of the samples were generated to measure the effect of increasing TL concentrations, and a MATLAB program was used to quantify luminescence intensity.

Finite element simulation A finite element (FE) program (ABAQUS/Standard) was used for the compression–bending simulation of the composite specimen. The material properties of CFRP materials were as follows: E11 = 134 GPa, E22 = 9:0 GPa, G12 = 3:8 GPa, v12 = 0:27, and G23 = 3:2 GPa, where E, G, and n represent Young’s modulus, shear modulus, and Poisson’s ratio, and the subscripts 1, 2, and 3 denote fiber direction, transverse fiber direction, and through-thickness direction, respectively. The strengths of CFRP material were as follows: ST 11 = 1:78 GPa, SC11 = 1:6 GPa, ST 22 = 77 MPa, and SC22 = 30 MPa, where subscript T and C denote tension and compression, respectively. The mechanical properties were given from the supplier of CFRP (Fiberforge) except the shear stiffness values which were from Kim et al. (2013). The TL coating was also accounted for in the FE model. The volume ratio of TL particles in the coated layer is small. Therefore, we used Young’s

modulus E = 4:0 GPa and Poisson’s ratio v = 0:3 of cured IVEX-C410 resin (ArmorStarVE) for the TL layer. Each composite layer was considered as homogeneous and was modeled with solid elements, type C3D8R, supported by ABAQUS/standard. In the through-thickness direction, one solid element was used for each ply. To simulate the compression–bending behavior, boundary conditions (BCs) were applied to a line that had a 0.2 mm offset from the center of the cross section, as shown in Figure 3(b). For the BC in the lower end section, x, y, and z degrees of freedom are fixed, while only x and z degrees of freedom are fixed at the BC in the upper end section. The Y direction displacement of the upper BC line was controlled to invoke compression in the specimen. The two end sections of the specimen are free to rotate under the compression loading due to applied BCs (see Figure 3(b)). Geometric nonlinearity was considered in the FE simulation for large bending behavior. FE results were used to obtain stress distribution on the CFRP coupons, and failure mechanisms were excluded in the numerical studies.

Results and discussions Progressive failure detection The imaging technique using DSLR provides the capability to detect the effect of loading and damage through TL photo-emission from the compression


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5 Table 1. Load–displacement results for the compression– bending test performed on a CFRP specimen coated with 30% TL material. Time (s)

Compression (mm)

Force (N)

Intensity values (a.u.)

20 40 60 80 100 120 140 160 180 200 220 240 260

0.3069 0.9355 1.8766 2.5319 3.4425 4.3823 5.0122 5.9622 6.9033 7.5281 8.4679 9.4052 10.0084

410 449 456 458 458 458 457 81 84 84 82 82 83

0 0 0 18.17 20.42 18.60 18.67 40.40 0 0 0 0 0

CFRP: carbon fiber–reinforced polymer; TL: triboluminescent.

Figure 4. (a-e) Captured digital images and (f) Increasing TL emission from the compression surface due to increasing load until failure indicating possible early detection of overload and crack detection during the compression–bending test performed on a CFRP specimen coated with 30% TL material. TL: triboluminescent; CFRP: carbon fiber–reinforced polymer; DSLR: digital single-lens reflex.

surface of the CFRP specimens. In previous work with concrete specimens, one single image per specimen was taken for the entire duration of the test, that is, from the initiation of loading until the occurrence of specimen failure (Aich et al., 2013). This single exposure did not provide information of damage progression over time. Rather, it described damage over the course of the entire test. In the current investigation, sequential imaging each 20 s during the loading period shows incremental TL emission as a function of compressive loading. Figure 4(a) to (e) shows the captured images of the incremental TL emission from the compression surface of a CFRP specimen coated with 30% TL material under gradually increasing compressive load. The corresponding load–displacement curve is presented in Figure 4(f), and associated time scales, displacements, and applied forces are listed in Table 1. Figure 4(a) shows an image taken at 80 s after loading was applied. This time scale corresponds to a displacement of 2.53 mm and a force of 458 N. From that point forward, the TL emissions were continuously observed due to the CFRP surface stresses as depicted in Figure 4(a) to (e). These time scales are marked with arrows to show the corresponding displacements of 2.53, 3.44, 4.38, 5.01, and 5.96 mm, respectively, for Figure 4(a) to (e); while corresponding time scales are 80, 100, 120, 140, and 160 s, respectively (Table 1). Quantified values of luminescence are also presented in Table 1.

The detection of light depends on the sensitivity of the camera and intensity of the emitted light from TL materials. It has been reported that crystals of ZnS:Mn require approximately 1 MPa for elastic-triboluminescence; the limit of elasticity is approximately 30 MPa (Chandra et al., 2010; Osip’yan et al., 1986). Also, approximately 100 MPa is required for fractotriboluminescence (Olawale et al., 2012). It has been reported that about 6% of triboluminescence occurs during elastic deformation, 14% at fracture, while 80% occurs during plastic deformation (Alzetta et al., 1970; Olawale et al., 2012). Figure 5(b) represents the stress distributions obtained from FE simulation in the TL layer, located on the compression side of the specimen under five different displacements; that is, 0.3, 0.93, 1.88, 2.53, and 5.01 mm, corresponding to the time intervals of 20, 40, 60, 80, and 140 s, respectively. When the compression of the specimen reaches around 1.88 mm, the maximum stress in the TL layer exceeds 40 MPa in the center (red-colored region). This implies that light emission captured in Figure 4(a) to (e) is mainly due to the plastic deformation of TL, while the low-intensity light emitted during elastic deformation of TL is difficult to be sensed with the camera. It should be noted that according to the FE simulation, the first crack in CFRP beam appears around 0.48 mm compression at the first 90° layer from the compression side. This is in agreement with the experimental observation that the first failure appears around 0.5 mm compression. The stress distributions obtained from FE simulation without considering damage (Figure 5(b)) may differ from those in the experiment. Nonetheless, we found that the FE simulation well captured the moment of strong light emission in the TL layer, implying that minor cracks appearing in the weakest layers do not affect the stress distribution in the relatively soft TL layer prior to the sudden fracture of the specimen.


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Figure 5. Comparison of the (a) image analysis results for TL emission distribution obtained by image analysis and (b) FE simulation for stress distribution in TL layer. The stress distributions at five different compressions, 0.30, 0.93, 1.88, 2.53, and 5.01 mm are presented in (b). The TL emission image in (a) was obtained from the compression surface of a CFRP specimen coated with 30% TL material at the time of failure during the compression–bending test performed on a CFRP specimen coated with 30% TL material. TL: triboluminescent; FE: finite element; CFRP: carbon fiber–reinforced polymer.

We found nearly no light emission from the 180-s time point (Figure 4(f) and Table 1), implying that the failure of the coupon occurred in the time window between 140 and 160 s. It should be noted that the first detection of light occurred between 40 and 60 s, which is much earlier than the failure of the coupon. This implies that TL materials may be used to signal the progressive failure of composites in the host structure prior to fracture. This concept may be helpful for detecting first ply failure within composites, given optimal configurations of TL doping on the surface of composite materials. Recently, high durability of TL systems was reported in the literature (Moon Jeong et al., 2013), where light was consistently emitted under cyclic loading up to ~30,000 cycles. This is promising for the structural prognosis related to fatigue loading, through correlation of the number of cycles and the corresponding light emission.

Detection of fracture As shown in Figure 4(f), the applied force does not change significantly after it reaches about 450 N due to bending deformation and suddenly decreases to 81 N with the fracture of the specimen. At the moment of fracture, TL materials emit exceptionally high-intensity light (flashing) as depicted in Figure 4(e) with the lightintensity value of 40.40 arbitrary units (a.u.) (Table 1). We found that the TL coating layers were also fractured and debonded from the CFRP coupons as the CFRP specimen underwent this rupture. The resinmixed TL coating exhibited sufficient adhesive strength with the CFRP surface and failed only when fracture appeared in the CFRP structure. Figure 5(a) shows the 3D profile of light emission obtained from the 30% TL-coated CFRP surface at the time of failure (i.e. at the displacement of 5.96 mm,

corresponding to 160 s). The left-side image in Figure 5(a) indicates the selected areas for analysis of the TL emission from the CFRP surface. On the right side of the figure, the 3D profile is shown where the height in the Z direction indicates the intensity of the luminescence originating from the surface and Z = 0 indicates the TL-coated compression surface of the CFRP specimen. The Y direction represents the vertical direction of the CFRP specimen during loading. A and D are the two ends of the CFRP specimen attached to the jig, and B and C are the areas near the center. It is noted that the TL emission intensity is highest near the center of the CFRP compressive surface, that is, near B and C. This luminescence map indicates the stress distribution in the CFRP during failure. The stress distribution of the specimen in terms of luminescence is compared with the FE simulation, as shown in Figure 5(b). Combined compression–bending with free rotational end conditions induces bending deformation without stress concentration at the ends. Therefore, the maximum stress and strain appear at the center of the specimen in the X–Y domain. We observe qualitative agreement between the experimental (Figure 5(a)) and numerically predicted results from the FE simulation (Figure 5(b)). Comparison of the luminescence profile in Figure 5(a) with the stress distribution profile in Figure 5(b) indicates the capability of TL materials to effectively detect stress distribution. This also demonstrates the compatibility of the TLembedded epoxy coating with the CFRP specimens. Such compatibility is needed for the determination of local stress distribution. This sequential imaging technique and the TL coating provide unique advantages not only to detect the critical deformation but may also be effectively utilized to track the time of failure for CFRP specimens under


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Figure 6. (a–c) DSLR images of various concentration TL-coated CFRPs during failure and (d–f) corresponding 3D luminescence profile obtained by analysis of the presented DSLR images showing TL emission from the compression surface of a CFRP specimen at the time of failure obtained through image analysis. DSLR: digital single-lens reflex; TL: triboluminescent; CFRP: carbon fiber–reinforced polymer; 3D: three-dimensional.

loading. If a calibration curve is generated using displacement and illumination intensity, it may be possible to image CFRP specimens under stress and to project the remaining time or load to failure. In this way, a predictive tool may be developed for monitoring of structural systems in real time, or alternately for routine inspection/evaluation of composite components. This will require advancement in the area of high-speed, highexposure cameras for the capturing of fractions of seconds with high-quality output.

Luminescence intensity–TL concentration relation Figure 6(a) to (c) presents DSLR images of the TL emission from stressed CFRP coated with 10%, 20%, and 30% TL and Figure 6(d) to (f) presents the corresponding 3D profiles. It is evident that with the increasing TL concentration, stress-induced luminescence is increased. When analyzed to quantify total luminescence using MATLAB, total intensities for these specimens with 10%, 20%, and 30% TL concentrations were found to be 25.3, 27.9, and 40.4 a.u., respectively. Similar observations of TL concentration-dependent luminescence variations were made during a previous study related to concrete cubes (Aich et al., 2013). Optimization of TL concentration for actual TL-based sensing systems will require further investigation using different concentrations of TL materials and rates of loading.

Conclusion This study presents a novel imaging technique using a TL material, which can be used to visually detect sequential stress accumulation during loading and failure of a CFRP element. The technique also enables visual mapping of stresses along the CFRP surface, which has been correlated to an FE model. Moreover, the increase in TL concentration in the amount of 10, 20, and 30 wt% resulted in more intense luminescence quantified at 25.3, 27.9, and 40.4 a.u., respectively. Such correlation of luminescence with TL concentration indicates opportunities of optimization for TL material usage in large-scale applications. This approach has potential application for SHM in aerospace, civil, and other structural systems. It is also potentially well suited as a low-cost, rapid evaluation tool as part of a routine maintenance procedure or during fabrication. Optimization of the process regarding applicable TL concentration and controlling TL coating thickness will be the premise of future TL-based monitoring/evaluation research. Acknowledgements The authors thank Dr Michael Sutton, Mr Siming Guo, and Mr David Westbury in the Department of Mechanical Engineering at the University of South Carolina for their guidance and help with testing CFRP coupons in the Tinius


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Olsen TI-5000 electromechanical load frame. The authors also thank Mr Ifat Jahangir, graduate student in the Department of Electrical Engineering, for the help with MATLAB computations. Authors N.A. and E.K. contributed equally to this work.

Declaration of conflicting interests The authors declare that there is no conflicts of interest.

Funding Portions of this work were supported through the University of South Carolina, Vice President for Research, ASPIRE-I program.

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