Fatigue Crack Growth Behavior Research Articles

Microstructural causes and mechanisms of crack growth rate transition and fluctuation of additively manufactured titanium alloy

Wire and arc additive manufacturing (WAAM) enables rapid near-net-shape fabrications of large-size parts and in-situ remanufacturing in many industry sectors. A comprehensive understanding of the fatigue failure mechanism of WAAM titanium alloys is a prerequisite for their widespread use in critical structural components subject to fatigue load. Here, the fatigue crack growth behavior of WAAM TA15 material is investigated. Fatigue crack growth tests are performed using compact tension specimens sampled from different locations and with different crack orientations of the WAAM TA15 block. The fatigue crack growth rate (FCGR) data exhibit two governing rates separated by a transition stress intensity factor value, ΔKn, and the degrees of fluctuation of the FCGR data in the two regimes are notably different. A piecewise log-linear model is first proposed by incorporating the Heaviside step function and ΔKn into the classical Paris’ model, allowing for the transition ΔKn to be determined by the data. The potential causes of the transition ΔKn are phenomenologically inferred via fractography and surface roughness profiling results. The critical microstructure affecting the value of ΔKn is identified by relating the crack tip cyclic plastic zone size at ΔKn to the sizes of main microstructures. The cause of different degrees of fluctuations in the two regimes separated by ΔKn is inferred by examining the microstructures within the plastic zone. The microstructural mechanisms of the local FCGR reduction and fluctuation are further identified and explained.

Quasi-in-situ observation of fatigue crack growth behavior of friction stir welded 2024-T4 joint

This study presents a quasi-in situ observation of the fatigue crack growth behavior in a friction stir welded 2024-T4 joint. The microstructure and fatigue properties of the joint were investigated using electron backscatter diffraction (EBSD) technique, scanning electron microscopy (SEM), and fatigue crack growth tests. The fatigue crack growth behavior of the joint was examined by conducting fatigue crack growth tests with different notch locations. The results show that the sample with the notch in the stir zone (SZ) exhibited the highest resistance to fatigue crack growth, followed by the notched samples of the Advancing side (AS) and Retreating side (RS) weldments. Microstructural observations showed a homogeneous microstructure with a fine grain size in SZ and it was observed that this fine-grained structure significantly enhanced the material’s resistance to fatigue crack growth. The experimental results were further analyzed using the Paris model to provide a quantitative understanding of the crack growth behavior. The study underlines the impact of microstructural characteristics and notch location on the fatigue performance of the weldment. Overall, the quasi-in situ observations and experimental findings contribute to a comprehensive understanding of the fatigue crack growth behavior in friction stir welded 2024-T4 joints.

Evaluation of fatigue crack growth behavior of bogie frames under operating load

Evaluation of fatigue crack growth behavior of bogie frames under operating load

Effect of different regions on fatigue crack growth behavior of 6005-T6 aluminum alloy metal inert gas (MIG) butt welded joint: experimental and numerical study

Effect of different regions on fatigue crack growth behavior of 6005-T6 aluminum alloy metal inert gas (MIG) butt welded joint: experimental and numerical study

Isothermal and thermomechanical fatigue crack growth behavior of 316LN stainless steel under load‐ and strain‐controlled modes

AbstractCrack growth tests of 316LN stainless steel under load‐controlled and strain‐controlled thermo‐mechanical fatigue (TMF) loadings at the temperature range of 350°C ~ 550°C were conducted. Moreover, the isothermal fatigue (IF) crack growth tests at the maximum temperature of 550°C were performed for comparison. The differences in IF and TMF crack propagation behavior under different loading conditions were thoroughly discussed, and the underlying mechanisms were revealed by a comprehensive characterization of the deformation state of the crack tip region using digital image correlation (DIC) and electron backscatter diffraction (EBSD) methods. Furthermore, it was found that the EBSD characterization was simply based on a single plane made it difficult to evaluate the complex crack growth behavior. In addition, the limitations of the classical parameter of stress intensity factor under TMF loading were presented.

Additively manufactured Ti‐6Al‐4V microstructure tailoring for improved fatigue life performance

AbstractIn this study, laser powder bed fusion (LPBF)‐produced Ti‐6Al‐4V is subjected to three separate post‐build heat treatments (HTs), specifically (1) a one‐step annealing HT exceeding the ‐transus temperature, (2) a two‐step annealing HT whose first step exceeds the ‐transus temperature and the second step is an anneal below the ‐transus temperature, and (3) a sawtooth HT with temperatures oscillating below the ‐transus temperature. Tensile properties and fatigue crack growth behavior were assessed and compared to wrought Ti‐6Al‐4V. LPBF materials nominally exhibited a 17% yield stress reduction, 1.9% strain‐to‐failure increase, and 9.2% modulus of toughness decrease. Fatigue crack growth curves were used to assess the linear crack growth rate region and approximate fracture toughness ( ). Post‐mortem fractography observed striations indicating the crack growth direction frequently changes direction providing an understanding of the slower crack growth rates observed in LPBF materials compared to their wrought counterpart.

Assessment of fatigue crack growth resistance of newly developed LTT alloy composition for the repair of high strength steel structures

Tensile residual stress (TRS) is a well-known factor that deteriorate the integrity of welded joints. Fatigue failure is accelerated by the existence of TRS introduced during the welding process. There have been efforts in the last two decades to develop filler alloys that can reduce TRS by introducing compressive residual stress (CRS) to oppose the TRS in high strength steel welded joints. These works are based on the theory of austenite (γ) to martensite (α’) transformation and the filler is often called a low transformation-temperature (LTT) alloy. Many studies have reported that the fatigue strength (FS) of weld joint made with LTT alloy is many times better than that of the conventional fillers. It is reported to be particularly useful in the repair of high strength steel structures. However, studies on the fatigue crack growth (FCG) behaviour of these LTT alloys is scarce. In this work, we developed Fe-CrNiMo based LTT weld metal composition, assessed its FCG behaviour and compared the results with that of a conventional welding wire (ER70S-6). It is found that ER70S-6 weld metal obtained under relatively fast cooling is extremely tough, but the associated heat affected zone (HAZ) has poor resistance to FCG which obscured the benefit of the tough weld metal. High heat input or condition that results to slow cooling of the ER70S-6 weldment deteriorates its resistance to FCG. Unfortunately, despite its low martensite start temperature of 231±7 and the anticipated beneficial effect of induced CRS, the LTT alloy studied had the lowest FCG resistance. The LTT alloy appears to have an intrinsic microstructural feature or a ‘fault line’ that reduced its resistance to FCG. While the LTT alloy weld metal has poor resistance to FCG, the associated HAZ resisted FCG more than the HAZ associated with ER70S-6 weld metal. It is observed that aligning the ER70S-6 weld metal perpendicular to the crack front produced the highest resistance to fatigue crack initiation and propagation. In the case of ER70S-6, it is believed that the weld metal induced a CRS at the notch tip which resulted to the high fatigue resistance. In the case of the LTT alloy, perpendicular alignment of the weld metal produced slight improvement.

Overload-induced anisotropy of fatigue crack growth in laser deposited Ti–6Al–4V alloy

Considering the anisotropy of microstructure of additive manufactured metal materials, quantitative assessment of the anisotropy of fatigue crack growth (FCG) behavior is necessary for structural damage tolerance design and evaluation. This study conducted an experimental and mechanistic investigation of the FCG behavior of the laser metal deposited (LMD) Ti–6Al–4V alloy, focusing on the anisotropy of FCG and the effect of overload on FCG. Both the FCG experiments with constant amplitude loading (CAL) and with a single overload (OL) were performed in different directions. Under CAL, the FCG rate exhibits a bilinear correlation with the stress intensity factor. Before the transition point (ΔKT = 16.9 MPa√m), since the grain boundaries are sparser along the columnar crystal growth direction, the FCG rate in the XY specimen is higher than that in the other directions. When the FCG rate further increases, the hindrance effect of grain boundaries on FCG weakens, and the anisotropy of FCG rates disappears. An interesting new finding is that when an overload is applied after ΔKT, the FCG rates in the XZ and ZX directions become different, and the overload-induced anisotropy increases with the overload ratio (OLR). The difference in FCG rates between the two directions exceeds 23.2% when OLR is 2.5. Based on microscopic and theoretical analysis, it was demonstrated that the overload induces residual strain in front of the crack-tip and enhances FCG resistance. The FCG resistance is positively related to the size of the plastic zone and negatively related to the yield strength in the loading direction.

Automatic crack tip localization in enormous DIC images to in-situ characterize high-temperature fatigue crack growth behavior

Digital Image Correlation (DIC), with its advantages of full-field, non-contact and applicability to extreme environments, is a powerful deformation measurement technique to in-situ monitor the high-temperature fatigue crack propagation behavior. For the DIC-based fracture mechanics analyses of cracking, crack tip position in DIC images, which is usually determined visually and manually, plays a significant role. However, the long-term fatigue-DIC experiment at both room-and high-temperatures poses a real challenge due to the huge amount of DIC images, lower image quality at high-temperature, and even the invisibility of cracks. In this paper, a fully automated algorithm for localizing the crack tip is proposed based on the crack tip displacement field and the immune algorithm. Compared to traditional algorithms, it does not necessitate iterative initial values of crack tip location, and permits precise and fast determination of critical fracture mechanics parameters, such as the Stress Intensity Factors (SIFs). The algorithm reliability is validated by both numerical simulation and fatigue testing of nickel-based superalloy GH4169 at room temperature and 650℃. It is shown that crack tip position and SIF values can be precisely and automatically determined within 7 s per image using an ordinary low-cost computer, which significantly improves the analyzing efficiency.

In-situ experimental and numerical investigation on fatigue crack growth in perfluorosulfonic-acid membrane with overloading effect

Fatigue crack growth behavior of perfluorosulfonic-acid membrane is investigated under single overload conditions by in-situ fatigue testing. The results show that the effected range of overload on cyclic plasticity positively correlates with the overload ratio (Rol), indicating that overload-induced residual stress is aggravated with increase of Rol. Besides, crack-tip sharpening occurred at higher Rol can promote cyclic plasticity deformation, thus initially accelerating the crack growth. Moreover, the cyclic plasticity can be used to interpret the post-overload crack growth. Based on plastically dissipated energy, a numerical scheme incorporated with developed elastic-viscoplastic constitutive relationship is established to predict the fatigue crack growth.

Stress ratio influence on Paris parameter and fatigue driving force pertinent to material properties

Paris model is widely used since it was proposed in 1963 due to simplicity and universality, but it cannot consider stress ratio (R) effect and curve fitting is always needed to obtain Paris parameters when R is changed. Based on Goodman curve and analysis of 22 groups of fatigue crack growth (FCG) data, an analytical FCG rate model is proposed by linking Paris parameters and fatigue driving force at R ≠ 0 (from 0 to 0.5) to those at R = 0 and material constants (tensile strength and fatigue limit at R = −1). Results show that proposed model can well predict FCG behavior at different R values based on Paris parameters at R = 0.

Quasi-In Situ Observation of the Microstructural Response during Fatigue Crack Growth of Friction Stir Welded AA2024-T4 Joint.

The reliability of friction stir welded joints is a critical concern, particularly given their potential applications in the aerospace manufacturing industry. This study offers a quasi-in situ observation of the microstructural response during fatigue crack growth (FCG) of a friction stir welded AA2024-T4 joint, aiming to correlate fatigue crack growth behavior with mechanical properties investigated using electron backscatter diffraction (EBSD). Notched compact tension (CT) specimens corresponding to the morphology of the stir zone (SZ), advancing side (AS), and retreating side (RS) were meticulously designed. The findings indicate that the welding process enhances the joint's resistance to fatigue crack growth, with the base metal exhibiting a shorter fatigue life (i.e., ~105 cycles) compared to the welding zones (SZ ~ 3.5 × 105 cycles, AS ~ 2.5 × 105 cycles, and RS ~ 3.0 × 105 cycles). Crack propagation occurs within the stir zone, traversing refined grains, which primarily contribute to the highest fatigue life and lowest FCG rate. Additionally, cracks initiate in AS and RS, subsequently expanding into the base metal. Moreover, the study reveals a significant release of residual strain at the joint, particularly notable in the Structural-CT-RS (Str-CT-RS) sample compared to the Str-CT-AS sample during the FCG process. Consequently, the FCG rate of Str-CT-AS is higher than that of Str-CT-RS. These findings have significant implications for improving the reliability and performance of aerospace components.

Experimental and numerical insights into fatigue crack propagation of railway component with gradient residual stress

As a representative structure with an acute radial gradient in terms of residual stress (RS) and grains, the induction-hardened railway S38C axle exhibits excellent mechanical properties and fatigue resistance. To investigate the effect of gradient features on fatigue crack growth (FCG) behaviors, we designed three-point bending (TPB) tests with different gradient features, including the negative-gradient, positive-gradient, and matrix specimens. Finite element modeling was introduced to simulate the FCG of the TPB specimens in terms of the cyclic J-integral (ΔJ) and equivalent plastic strain at the growing crack tip. The results reveal that the FCG rates in both negative- and positive-gradient structures decrease and a more pronounced tendency of decline is achieved in the positive-gradient ones. The substantial compressive residual stress (CRS) within the negative-gradient structure directly influences the crack tip area, while the CRS in the positive-gradient structure indirectly mitigates the crack driving force by impeding the overall structural deformation. Coarser grains aid in enhancing energy absorption along the zigzag crack path, thereby diminishing the crack driving force. ΔJ and the equivalent plastic zone surrounding the crack tip indicate significant reductions in the presence of CRS.

Study on Corrosion Fatigue Behavior of 304L Austenite Stainless Steel in 325 °C High-Temperature Water Environment

The fatigue crack growth behavior of 304L austenitic stainless steel (SS) in a 325 °C high-temperature and high-pressure water environment were investigated by a corrosion fatigue test system, by electron back scatter diffraction (EBSD), and by a transmission electron microscope (TEM). The experimental results indicated that the crack growth rate (CGR) of 304L SS increases with increasing the stress intensity factor, stress level, and fatigue frequency (f). Compared to dissolved hydrogen (DH) in a high-temperature water environment, dissolved oxygen (DO) significantly enhances the CGR by about an order of magnitude higher. The crack tip of 304L SS after the corrosion fatigue test under higher stress levels is sharper, with more secondary cracks on the fracture surface, while the crack tip under lower stress levels is blunter with relatively fewer secondary cracks. The oxidation behavior at the crack tip was analyzed under different loading and water chemistry conditions, and a related effect on the crack tip and CGR was clarified.

Fatigue crack growth of WC–Co cemented carbides: a comparative study using small indentation flaws and long through-thickness cracks

The fatigue crack growth behavior of a submicron-grained WC–Co hardmetal is investigated by artificially introducing small flaws by means of sharp indentation. Similar fatigue testing is also conducted on notched specimens with long through-thickness cracks for comparison purposes. The use of controlled small indentations flaws is shown to be a valid and successful approach for studying and describing crack growth behavior under cyclic loading for the material under consideration. This statement is based on the similitude found in fatigue mechanics and mechanisms between both crack types. Regarding the former, accounting of the indentation-induced residual stresses is key to rationalize the experimental findings. Concerning the latter, inspection of crack-microstructure interaction as well as fracture surfaces permit to discern similar features and scenarios, at both meso- and micrometric length scales. Results from this research yield an immediate practical implication, as indentation techniques may then be proposed as an alternative testing route for investigating fatigue crack growth behavior of hardmetal grades where sharp indentation is capable to induce well-developed radial crack systems.

Effects of build direction and microstructure on fatigue crack growth behavior of Ti6Al4V fabricated by laser powder bed fusion

AbstractThis study compares the fatigue crack growth (FCG) behavior of Ti6Al4V parts fabricated by laser powder bed melting (LPBF) with four build directions (0°, 30°, 60°, and 90°) and two heat treatments and analyzed the effect of build direction and microstructure on FCG. The results show that the build direction has a significant effect on the FCG rate. Particularly, specimens with 90° build direction have the best FCG resistance while the 0° specimens have the worst. Furthermore, the fragile deposition layer boundary is the main cause of the crack deflection. The microstructure shows that the columnar β grains have less effect on FCG. The increase in resistance to FCG after heat treatment is attributed to the lath‐like α grains consuming the energy for FCG. As compared to needle‐like α′ grains, the lath‐like α grains are more likely to initiate intergranular fracture and secondary cracking, leading to rugged crack tip paths.

Fatigue growth behavior of mode II crack in headed stud steel used in steel–concrete composite structures

The fracture mechanics method has been employed for fatigue performance evaluation of stud used in steel–concrete composite structures while the fatigue crack growth behavior in headed stud steel has not been systematically studied. The present work is proposed to investigate the fatigue growth behavior of mode II crack in headed stud steel and its welded joint. Compact tensile shear (CTS) specimens extracted from the base material (BM) and weld metal (WM) of stud are tested under mode II loading to investigate the fatigue crack growth (FCG) trajectories and rates with different stress ratio. The crack closure level and FCG threshold are determined and discussed. Results show that the crack deflection in BM meets both the MTS criterion and S-criterion, while crack deflection in WM does not meet the two criterions. The 2% compliance offset criterion in the origin ASTM method is suitable to determine the crack opening force both for BM and WM of stud. The low crack closure level in WM indicates the tensile residual stress in welded joint, resulting a more quickly fatigue crack growth rate (FCGR) in WM than that in BM. The calculation formulation of equivalent SIF range proposed by Borrego et al. is verified with the best normalization effects and is recommended to fit the FCGR model both for headed stud steel and its welded joint under mode II loading.

Study on fatigue crack growth behavior of the TA15 titanium alloy repaired by laser deposition

Laser deposition repair was employed to repair the body and surface of the forged TA15 titanium alloy. Subsequently, the fatigue crack growth behavior of the repaired components after annealing was investigated. The results showed that the fatigue crack growth rate (FCGR) of the body-repaired specimen decreased from the initial cracks in the heat-affected zone (HAZ), interface, and repaired zone (RZ). The increase in the repair layer thickness drastically reduced the FCGR in the surface-repaired specimen. The main crack growth mode was crossing the primary α and transformed β in the substrate zone (SZ), primarily crossing the lamellae α and along lamellae α in the HAZ, and crossing the lamellae α in the RZ. The RZ developed a tighter bond with the substrate interface, making crack growth more difficult. Crossing the lamellae α in the basket-weave structure required higher amount of energy and exhibited significant increase in tortuosity. Hence, the FCGR of the initial crack within the RZ exhibited the lowest rate. An increase in the thickness of the repair layer increased the proportion of the basket-weave, improving the resistance against crack growth, and increased the tortuosity associated with the crack path, reducing the FCGR. It was confirmed that the proposed body and surface repair methodology improved the FCGR of investigated specimens.

Characterization of fatigue crack growth behavior in welded tubular T-joint

This paper focuses on investigating the fatigue crack growth (FCG) characteristics and residual fatigue life of tubular T-joints which are prone to suffer fatigue damage at the brace and chord intersections under multi-axial stress. Static and fatigue loading tests are performed to investigate the FCG behaviors of tubular T-joints, combining beach marking technique. The evolution of FCG characteristics is studied during the crack growth, convergence and wall penetration through an efficient co-simulation system that established using the multi-scale modeling technique. Fracture morphology analysis is conducted to gain insight into the FCG behaviors combining the scanning electron microscope (SEM) observation. Results indicate that multiple cracks initiate at the crown regions and subsequently evolve circumferentially around the weldments in a doubly-curved shape towards wall-thickness. Interaction effects between adjacent cracks extend the fatigue life along the outer surface, attributed to the premature exposure of converged crack front and the induced variation of stress intensify factor (SIF) distribution along the crack front. The dominant failure mode of tubular T-joints is characterized by an opening mode crack, with the contribution of anti-plane shear mode crack gradually increasing as structural symmetry diminishes. The established co-simulation system shows advantage in capturing the FCG behavior, predicting the fatigue life and characterizing the FCG characteristics with a good balancing of simulation efficiency and calculation accuracy.

Bonded and prestressed fatigue crack retarders based on Fe-SMA

A bonded and prestressed retarder concept is proposed in this work. The shape memory effect of an iron-based shape memory alloy (Fe-SMA) has been exploited to realize a desirable self-prestressing mechanism, resulting in compressive stresses in the parent structure. The synergistic prestressing and added fail-safe features of bonded and prestressed Fe-SMA retarders desirably restrain the fatigue crack growth. The fatigue crack growth behaviour in metallic plates reinforced with bonded and prestressed retarders is experimentally studied in this work. A test campaign has been implemented to investigate the effects of prestressing, as well as the influence of the relative location between the Fe-SMA retarders and the initial crack tip on the crack growth behaviour and fatigue crack life extension. The experimental results show that the prestressing of retarders desirably extends the fatigue crack growth life. More extension in fatigue life can be achieved by positioning the bonded prestressed retarders from 40 to 20 mm before the initial crack tip, the desirable retardation due to beneficial compressive prestressing and added load path on the small crack is significant. The findings in this work imply that it is beneficial to proactively reinforce the fatigue-prone locations in metallic structures with bonded and prestressed Fe-SMA retarders in order to extend the service life.