Crack Growth Behavior Research Articles

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

Fatigue prediction through quantification of critical defects and crack growth behaviour in additively manufactured Ti-6Al-4V alloy

The study presents a methodology for predicting the fatigue life and identifying critical defects in additively manufactured (AM) Ti-6Al-4V alloy processed via laser powder bed fusion (L-PBF). Predictions were made on L-PBF Ti-6Al-4V alloy in two conditions: as-built and heat-treated by using X-ray μ-Computed Tomography (μ-CT) for the quantification of the defects and fatigue crack growth (FCG) data. For validation, fatigue life predictions were made on the same specimens on which μ-CT was conducted prior to fatigue testing. FCG and fatigue tests (S-N) highlighted differences in the performance between the as-built and heat-treated conditions: the as-built condition had a lower threshold stress intensity factor range (ΔKth) of 2.16 MPa m1/2 than that of the heat-treated condition, 4.96 MPa m1/2. Differences in fatigue limit were attributed to differences in ΔKth in as-built and heat-treated specimens. To gain mechanistic understanding of these differences near ΔKth cracks were examined using Electron Backscatter Diffraction - Kernel Average Misorientation (EBSD-KAM). It was found that upon heat treatment, the dislocation density increased around near threshold fatigue cracks. Increases in ΔKth are attributed to the increases in β-phase content and α-lath thickness caused by heat treatment.

Effects of crack-γ/γ' interface relative distributions on the deformation and crack growth behaviors of a nickel-based superalloy.

Using the molecular dynamics (MD) method, we investigated the effects of crack distributions on the deformation and crack growth of a nickel (Ni)-based superalloy. The results indicated that as the distance between two cracks increased, both tensile strength and plasticity decreased, while the crack growth rate significantly increased. In systems with short crack distances, strong interactions occurred between the dislocations that emitted from two cracks and the γ/γ' interface mismatched dislocation network. These interactions led to an overlap in the plastic zones ahead of the crack tips at the γ/γ' interface, which resulted in significant passivation at the front and middle regions of the cracks. Consequently, the two cracks merged in the X-direction to form a wide crack. The cracks coalescence consumed a lot of external deformation work, resulting in the highest tensile strength and plasticity. In this study, we proposed a potential approach to simultaneously enhance the strength and plasticity of multidefect systems, providing a theoretical basis for explaining deformation mechanisms and crack growth in these systems.

Small crack growth behavior and correlated microstructure-sensitivity during cyclic deformation of a high-chromium/carbon wrought tool steel

The macro to micro-mechanical mechanisms of cyclic deformation behavior of a carbide-containing ferritic steel was studied, and the exceptional properties obtained in low-cycle regimes were discussed, relying on the sensitivity of the small crack propagation to the microstructural characteristics. In this respect, the evolution of boundaries, sub-boundaries, carbides, and micro-textures in the vicinity of the crack growth path has been characterized in detail. Regarding this matter, the subtle alteration of grain size gradient, resulting in the decrement of incompatibility between mechanical properties of grains, is the main factor that halts the crack growth temporarily. Furthermore, both pre-existed and cyclically developed substructures could tortuous the crack propagation path and induce the crack closure effect, respectively. The advantages and disadvantages introduced by primary carbides, serving as initiators of small cracks, and dispersed high volume fraction of submicron carbides, inducing the formation of an effective strengthening layer (ESL) and acting as deflectors of small cracks, have been investigated, respectively. The {001} <110> oriented grains have been identified as preferred micro-texture contributing to small crack propagation. In total, the small crack sensitivity of the elaborated microstructure has resulted in a prolonged lifetime.

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.

Water effect on subcritical crack growth and fracture behavior of marble

Understanding the fracture properties and subcritical crack growth of rocks in aqueous environments is significant for the effective assessment of the long-term stability of the masonry buildings, cultural heritage structures, and art sculptures. We use double torsion techniques to systematically investigate the subcritical crack growth behavior of marble under air, fluid, saturated and saturated + fluid test conditions. In the fluid condition and the saturated + fluid condition, the load reductions during the relaxation test are larger than other conditions, reaching 12.81% and 12.26%, respectively. There is a water-weakening effect on both the subcritical crack growth index (SCI) and KIC, but the water weakening sensitivity in KIC is lower than that of SCI. Compared to the air condition, the SCI values of the marble decreased by 43.69% under the fluid condition, 17.64% under the saturated condition, and 38.09 % under the saturated + fluid condition, respectively. The introduction of water during the relaxation test results in an immediate increase in the crack velocity and the major principal strain of the specimen, as evidenced by the appearance of the Ⅳ region on the KI-V curve. Scanning electron microscope images show that the most triangular pits and dissolution pits are present in the marble under fluid and saturated + fluid conditions, leading to a more pronounced deterioration of the mechanical properties. The test results highlight the significant impact of water on subcritical crack growth in marble. In particular, during rainfall, both dry and saturated marble masonry buildings are more prone to cracking due to the introduction of water.

Crack growth behavior of U71MnG rail steel under overload conditions described using a dislocation correction model

Crack growth behavior of U71MnG rail steel under overload conditions described using a dislocation correction model

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.

Investigation on the growth behaviour of creep crack in 2219 aluminium alloy by nanometre computed tomography

Aluminium alloy is widely utilized in aerospace engineering because of its lightweight and high specific strength. Creep and fracture of aluminium alloy serving at high-temperature conditions are always a long-term concern. Especially creep crack nucleation and growth behaviours under high out-of-plane constraint levels are the significant processes to generate failure. High-resolution nanometre computed tomography equipment can be adopted for obtaining the three-dimensional morphology and information on damage to investigate the mechanism of creep crack growth. According to the obtained results, the stress-field strength has an impact on creep crack propagation under high out-of-plane constraint levels.

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.

Fracture toughness and slow crack growth behaviour of metal-proton conducting ceramic composites

The fracture toughness and slow crack growth behaviour of two load-bearing metal ceramic components adopted in negatrode-supported electrochemical devices, namely Ni-BCZY27 and Ni-BCZY442, are investigated via double torsion technique.The investigation showed that the two metal-ceramic composites are considerably less sensitive to slow crack growth when compared to oxide ceramics such as stabilized zirconia and alumina, with KI0/KIC equal to approximately 0.8. On the other hand, they also showed lower fracture toughness than Ni-3YSZ, with Ni-BCZY442 and Ni-BCZY27 having a fracture toughness of respectively 1.74 and 3.04 MPa*m1/2 for porosities equal to 25.8% and 19.1% respectively, compared to Ni-3YSZ reported in previous work with similar Ni content and porosity of approximately 30% having a fracture toughness equal to 3.4 MPa*m1/2. Furthermore, the analysis of samples presenting extensive barium depletion indicated fast fracture propagation upon application of loads significantly lower than the critical fracture load.

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.

A comparative study of creep crack growth behaviour amongst the dissimilar P91-IN182-SS316LN and similar P91 weld joints

ABSTRACT Towards ensuring the structural integrity of the high-temperature components, it is essential to evaluate the creep crack growth (CCG) behaviour, especially for the weaker creep regions of the components. In this study, the CCG behaviour was investigated in the modified 9Cr-1Mo (P91) and Inconel (IN182) buttered layer interface regions of dissimilar metal weld (DMW) joints of P91 and type 316LN SS at 798 K and 898 K. For a quantitative assessment, the CCG rates were compared with the heat-affected zone (HAZ) of similar metal P91 welds (P91HAZ). The CCG rate was characterised using the creep fracture C* parameter as per ASTM E1457–17. The CCG rate in the P91/IN182 interface region was found to be higher than the P91HAZ, viz. 3.5 times and 4 times at 798 K and 898 K, respectively. This could be attributed to the propensity for creep cavitation in the interface region of the DMW joint.

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.

Strain-Assisted Phase Transformation in Two-Dimensional Transition-Metal Dichalcogenides.

Two-dimensional polymorphic transition-metal dichalcogenides have drawn attention for their diverse applications. This work explores the complex interplay between strain-induced phase transformation and crack growth behavior in annealed nanocrystalline MoS2. Employing molecular dynamics (MD) simulations, this research focuses on the effect of grain size, misorientation, and annealing on phase evolution and their effects on the mechanical behavior of MoS2. First, examining phase transformation in monocrystalline MoS2 under various stress states reveals distinct behaviors depending on the initial phase (1T or 2H) and crystallographic orientation with respect to loading directions. Notably, transformation from a layered hexagonal to a body-centered tetragonal structure is more noticeable when strain in a zigzag direction is applied to the 1T sample. As such, single crystalline MoS2 with a 1T phase exhibits a 16% lower fracture stress in the armchair direction compared to that with a 2H phase. On the other hand, the 1T phase shows a 5% higher phonon lifetime compared to the 2H phase with similar phonon group velocities. Next, the influence of thermal energy and mechanical stress on the phase transformation of nanocrystalline MoS2 is investigated through annealing and quenching cycles, uncovering 60 and 44% irreversibility of phase transformation for an average grain size of 3 and 11 nm, respectively. Besides, the evolution of nanocrystalline samples with different initial phases and grain sizes is studied under uniaxial and biaxial stress. This study shows an inverse pseudo-Hall-Petch effect with exponents of 0.11 and 0.09 for 2H and 1T, respectively. The study reveals that phase transformation can occur concurrently with crack initiation and propagation with the 1T phase exhibiting a 19% lower grain size sensitivity of fracture stress compared to the 2H phase.

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.