Fatigue Life for Type 316L Stainless Steel under Cyclic Loading Khairul Azhar Mohammad1,a, Edi Syams Zainudin1,b, Sapuan Salit1,c, Nur Ismarrubie Zahari1,d and Aidy Ali2,e 1
Department of Mechanical and Manufacturing Engineering, Universiti Putra Malaysia, 43400 UPM, Serdang Selangor, Malaysia
2
Department of Mechanical Engineering, National Defence University of Malaysia, Sungai Besi Camp, 57000 Kuala Lumpur, Malaysia a
[email protected],
[email protected],
[email protected], d
[email protected],
[email protected]
Keywords: Fatigue Life, Fatigue Limit, Austenitic 316L Stainless Steel, Scanning Electron Microscope (SEM)
Abstract. The paper presents the determination of fatigue life of 316L stainless steel at room temperature. Plenty of steel in the world has been investigated for a lot of application in the science and technology market. The mechanisms of fatigue of 316L stainless steels were studied and investigated. Fatigue tests of specimens were performed in accordance with ASTM E466-96. The fatigue tests were performed in constant load amplitude, constant frequency of 5 Hz with load ratio R=0.1. Fracture surface of specimens were examined by using Scanning Electron Microscope (SEM). The results showed that the endurance fatigue limit of 316L stainless steel was 146.45 MPa. Introduction Life prediction is one of the complex and essential in designing a component that working in high temperature and suffers alternative loading. In over the years many researchers focused on tolerate design to determine life of heavy duty pipe that operating under high mechanical pressure and temperature due to a lot usage of cylindrical component and extensive usage of this type of geometry. Fatigue caused the structure failed. Due to the complicated nature of fatigue mechanisms and the large number of factors that influence fatigue life, there is so far no unified approach that can treat all fatigue problems [1]. It is key feature for industry is to provide the material in high strength and can be a challenge to them because of industrial material does not known the composition, and presence of defects such as holes, cavities and crack in their substructure is inevitable. As a result, consideration of fracture mechanics in the design of metallic structures is crucial. Tubular or cylindrical components are commonly employed in most of the industries including oil and gas industries such as pipes, borers, pressure vessels and offshore platforms as a part of engineering structure. As result, these components can be very susceptible to fail during use in non conducive environment. Previous attempts have been done for plate type of structures due to its geometry that is simple to be designed. In safety consideration of mode of failure of pressure vessels and pipes are the crack initiation, crack propagation and fracture is often necessary to consider fatigue failure. Meanwhile, additional high temperature subjected to pipe will lead to shorter life compared to sum of creep damage and fatigue damage incurred separately. According to Chen et al. [2], the starting point of failure under low cycle fatigue is mostly related to the geometrical discontinuities on the specimen surface. Besides, furthermore of creep-fatigueenvironment may enhance the cracking problem. Up to now, research works in investigating the mechanism of cylindrical components is limited due to the complexity of geometry comparing with the simple geometry like a plate structure. Geometry of component and load conditions are the two dominant parameters which will affect the mechanism of failure and crack propagation. Surface damage or scratch cracks are also may be a crack initiation point provided it acts as stress riser while the component experiences a fluctuating
loading [3].The mechanism of the fatigue damage tolerance of steel base pipes structure is still not well understood. Therefore, the present work is concentrated on fatigue life of tubular form’s austenitic stainless steel 316L at room temperature which is subjected to variable loading. Specimen Geometry and Material Properties The austenitic type 316L stainless steel was utilized and investigated in order to characterize fatigue life at room temperature. The specimen was designed and fabricated carefully into hourglass shaped specimen in accordance to ASTM 606 for fatigue test and were provided by local supplier, S.N Machinery Services Sdn. Bhd. The study was undertaken on cyclic loading whereas the dimensions of specimen which are shown in Fig. 1(a) and a schematic drawing of the specimen is depicted in Fig. 1(b). The mechanical properties for Type 316L stainless steel is presented in Table 1 meanwhile Table 2 shows the chemical composition of the material.
(a) (b) Fig. 1: (a) Austenitic 316L Type Stainless Steel and (b) its dimensions Table 1: Mechanical Properties of Type 316L Stainless Steel Mechanical Properties Yield Point, MPa Tensile strength, MPa Modulus of Elasticity, GPa Strength at break, MPa Elongation at break, mm
Type 316L stainless steel 332 673 165 586 35.5
Table 2: Chemical compositions of Type 316L stainless steel [8] Element (%) 316L
C 0.020
Ni 11.21
Cr 17.38
Mn 1.86
P 0.027
S 0.0054
Si 0.51
Mo 2.36
N 0.038
Experimental Fatigue test was conducted at Strength of Material Lab at Tenaga Nasional Berhad Research (TNBR) in Bangi. The fatigue specimens were tested in tension-tension fatigue test using Fast Track Hydraulic Universal Testing Machine Instron 8802 of 250 kN load capacity. In order to collect the data for fatigue test, 7 data plots were adequate enough as recommended by ASTM E606-92 (1998) to establish S-N curve. The specimens were subjected to by several different maximum stress level with starting 0.9 UTS (90% of the ultimate tensile stress) of the materials followed by 0.80 UTS, 0.70 UTS, 0.60 UTS, 0.50 UTS, 0.40 UTS and 0.30 UTS. All fatigue tests for austenitic steel were performed in constant load amplitude, constant frequency of 5 Hz and a stress ratio, R equal to 0.1. The specimen is undergoing fatigue test as shown in the Fig. 2. Fatigue test results were plotted in graph in term of stress versus life data. Best fit curve were plotted for data analysis.
The microstructures of the material and fracture surface analysis were carried out on Scanning Electron Microscope (SEM - Hitachi S-3400N) in order to establish the micromechanics of material failure/identify the initiation site and to determine the crack propagation mode. In microstructural analysis, the stainless steel type 316L was examined after polishing and etching using a solution consisting of 30 ml Glycerol, 20 ml HCL and 10 ml HNO3 to reveal and give clear micrograph features of microstructure using SEM. It is well known that the material in this research is austenitic Type 316L stainless steel which has been widely used in pressure vessel fabrication. Jig
Specimen Type 316L Stainless Steel
Fig. 2: Specimen undergoing fatigue testing Results and Discussion In experimental, fatigue data of constant amplitude load fatigue testing with load ratio of 0.1 and frequency of 5 Hz performed on AISI Type 316L stainless steel specimen. The results are plotted in S-N curve is shown in Fig. 3. From the results, stress amplitude versus fatigue life is plotted to view the significant difference of fatigue life at high stress compared to lower stress. From the figure shows that variation of curve of fatigue test is decrease as different loading applied on specimen till fatigue failure. It is mentioned that type 316L stainless steel undergoes harden under cyclic loading [4]. The data collected from the test have been fitted by power law curves in order to describe the specimen fatigue behavior. The equations of power law curves are given by: y = ax b
(1)
This equation shown the relation between fatigue life and applied load is called Basquin relation. The Basquin relation is the general equation represents typical S-N curve and its expression developed from log–log S-N graphs. It is the most widely used equation. This equation is usable in the stress-based approach to fatigue analysis and design. The relation is given by:
S a = S f′ ( 2N f
)
b
(2)
where Sa is stress amplitude, S f′ is the fatigue strength coefficient and b is the fatigue strength exponent. The fatigue experimental data was fitted with equation of Basquin Equation whereby the value of b exponent obtains from the slope of line in log-log scale. It is show that behavior of fatigue strength on type 316L stainless steel was hardening and followed by softening at surrounding condition [5].
400 350
σa (MPa)
300 250 200 y = axb
150 100 50
0 1,00E+03 1,00E+04 1,00E+05 1,00E+06 1,00E+07 1,00E+08 Fatigue Life Nf
Fig. 3: S-N of fatigue life curve for type 316L stainless steel The result from experimental represented that it can be observed the value of data is decrease as applied load at different variable into fatigue endurance zone. A range of stress amplitude’s value between 334 MPa – 290.93 MPa yields cycle ranging 4628 – 17340 considered the hazardous area where the fatigue life is very low between of 103-104 of stress amplitude. At range of 105 and above, it is considered it would have a much longer fatigue life and it is categorized as high cycle fatigue where has stress amplitude’s value between 290.93 and below ranging 55478 – 7893764 cycles until reached the fatigue limit. In the graph, the arrows represent the specimen beyond 1 millions cycles did not break even after applied many specimen throughout the tests. Each life’s component would be replaced by another one since it reaches at this stage more than a 10000000 cycles. In this research, it has successfully characterized the fatigue limit for type of 316L stainless steel is found to be 146.45 kN at 7,893,764 cycles. Failure Morphology of Fracture Surface In terms of examined the morphological-microstructure relationship of fracture specimen, Scanning Electron Microscope (SEM - Hitachi S-3400N) was used in order to investigate the micromechanisms of material failure. The fracture mode initiate at surface irregularities of specimen such as voids and inclusions in according to Lah et al. [6]. Fractured surface was used in order to break separate into two and then captured by Low Cycle Fatigue (LCF) tested at 334 MPa as shown in the Fig. 4(a), 4(b) and 4(c). As a generally, the surface of fracture specimen exhibits the evidence of beachmark due to fatigue failure. From the captured image of fractured surface shows crack initiation and early propagation exhibits the transgranular fracture mode for first stage in Fig. 4(a). Otherwise, it shows the fracture strip and which reveals the shear surface morphology. In Fig. 4(a), more than one void were found at initiation site at the first stage. Dimple surface
void
Transition of fracture surface
(a) (b) (c) Fig. 4: (a) Nucleation Crack Figure (b) Propagation Crack Figure (c) Unstable Rapid Crack Growth before Rupture
Before specimen goes to the stage of rupture, it revealed a propagation zone in fracture surface where mixed fracture path completely intergranular mode in fracture behavior of specimen in second stage as shown in Fig. 4(b). Meanwhile, Fig. 4(c) shows the captured image on final stage before the specimen fail by separate into two part. In this stages (rupture zone), mechanism interaction of unstable mode between intergranular and trangranular fracture was found to be failure specimen in determination of fatigue life at room temperature. Conclusions The fatigue life of stainless steel type 316L is successfully characterized. From the graph, the variation line of fatigue life comparing with experimental results of between the higher and lower loading causes the specimen to fail tremendously fast instead of lower load. Failures were observed from 9,000 cycles and the imposed limit of fatigue life was set to end at 9 x 107 cycles due to time and cost limitations. From the experimental results, it was found that the specimen has the fatigue limit of 146.45 kN at 7,893,764 cycles. The morphology analyses on the specimen fracture surface revealed that the crack nucleated and crack propagation were transgranular and intergranular mode respectively. Meanwhile the final phase in mechanism of fatigue life reveals both was mixed the transgranular and transgranular mode in continuous cyclic loading. Acknowledgement The author would like to appreciate to University Putra Malaysia for the supporting this research under Research University Grant Scheme (RUGS) (Project No.: 9348000) and to thank one of the authors, Khairul Azhar bin Mohammad, which made to pursuit of this research achievable. References [1] Y. Weixing: Int. J. Fat. Vol 15 (1993), p. 243 [2] L. J. Chen, G. Yao, J. F. Tian, Z.G. Wang and H.Y. Zhao: Int. J. Fat. Vol. 20 (1998), p. 543 [3] U. P. Shine and E.M.S Nair: World Academy of Science, Engineering and Technology, Vol. 46 (2008), p. 616 [4] D.W. Kim, J.H. Chang and W.S. Ryu: Int. J. Press. Vess. Pip. Vol. 85 (2008), p. 378 [5] J.-B. Vogt, J. Foct, C. Regnard, G. Robert and J. Dhers: Metal. Trans. A Vol. 22A (1991), p. 2385 [6] N.A.C. Lah, Aidy Ali, N. Ismail, L. P. Chai and A. A. Mohamed: J. Mat. Des. Vol. 31 (2010), p. 312
