Fatigue Crack Propagation Research Articles

Influence of post-weld explosive treatment on low cycle fatigue of AA7075-T651 friction stir welded joint

In this investigation the friction stir welded joint of 5 mm thick AA7075-T651 alloy has been subjected to the post-weld explosive treatment. To assess the influence of the proposed treatment on the joint properties, a comprehensive analysis was conducted, including microstructural examination, microhardness testing, static tensile tests, low-cycle fatigue testing, and fracture surface observations. The strain amplitudes of 0.35 %, 0.4 %, 0.5 %, and 0.6 % with asymmetry coefficient R = 0.1, were used for the examined samples. It was concluded that the explosively treated FSW joints are characterized by a lower amplitude of plastic deformation, a reduced number of cycles to failure and show a tendency for cyclic softening. The differences in the fatigue crack propagation primarily concern the initiation area of the crack near the hardened surface. Explosive treatment did not influence the location of material decohesion, which occurs in the low-hardness zone.

Mechanical Properties of Additively Manufactured Polymeric Materials-PLA and PETG-For Biomechanical Applications.

The study presented herein concerns the mechanical properties of two common polymers for potential biomedical applications, PLA and PETG, processed through fused filament fabrication (FFF)-Material Extrusion (ME). For the uniaxial tension tests carried out, two printing orientations-XY (Horizontal, H) and YZ (Vertical, V)-were considered according to the general principles for part positioning, coordinates, and orientation typically used in additive manufacturing (AM). In addition, six specimens were tested for each printing orientation and material, providing insights into mechanical properties such as Tensile Strength, Young's Modulus, and Ultimate Strain, suggesting the materials' potential for biomedical applications. The experimental results were then compared with correspondent mechanical properties obtained from the literature for other polymers like ASA, PC, PP, ULTEM 9085, Copolyester, and Nylon. Thereafter, fatigue resistance curves (S-N curves) for PLA and PETG, printed along 45°, were determined at room temperature for a load ratio, R, of 0.2. Scanning electron microscope observations revealed fibre arrangements, compression/adhesion between layers, and fracture zones, shedding light on the failure mechanisms involved in the fatigue crack propagation of such materials and giving design reference values for future applications. In addition, fractographic analyses of the fatigue fracture surfaces were carried out, as well as X-ray Computed Tomography (XCT) and Thermogravimetric (TGA)/Differential Scanning Calorimetric (DSC) tests.

Reliability Analysis of Fatigue Crack Growth in Plain Concrete

Civil engineers know that fatigue indicates themajority of structural failures. In plain and reinforced concrete structures, fatigue might account for excessivedeformation, excessive of crack widths, de-bonding ofreinforcement and rupture of the reinforcement, all of thesecan lead to structural collapse. Fatigue crack propagation can be a method to predict a remaining life of structures.Herein some works that have been done will be shown inorder to analyse the fatigue crack growth for plain concretespecimens. The goals of this study are to estimate the criticalcrack length and remaining life of a plain concrete specimen, determine the state of the specimen either havingfailed or being safe using first order reliability method(FORM) and assess which parameter has most influence onthe reliability index

Fatigue crack evaluation of butt weld joints in full-scale aluminum alloy joints: Experimental and numerical study of traction structural stress

In this study, the digital image correlation technique is used to determine the inconsistent mechanical properties and microstructure non-uniformity in different regions of aluminum alloy butt welded joints. A state monitoring method is proposed based on phased-array ultrasonic total focus imaging for crack evolution. A series of surface fatigue crack propagation tests on full-scale aluminum alloy butt welds are investigated, which innovatively revealed the mechanism and evolution law of crack propagation at the surface of the weld. The analytical function for dynamic crack propagation in welded structures is proposed, which provides a new idea and data support for clarifying the state deterioration process of structures. The results show that the traction structural stress can effectively identify the critical fatigue position of the weld and effectively account for the effect of angular misalignment. The phased-array ultrasonic total-focus imaging monitoring analysis method for surface fatigue crack propagation is proposed to improve the completeness of the fatigue crack evolution law.

Fatigue crack initiation and propagation in plain and notched PBF-LB/M, WAAM, and wrought 316L stainless steel specimens

Additively manufactured (AM) components—either made by laser-powder bed fusion or wire and arc additive manufacturing—typically contain process-related defects on and near surfaces that can be removed by machining. Various studies have shown that post-treatment, such as machining significantly improves the fatigue strength of AM parts. To this day, however, hardly any studies have investigated the fatigue strength of post-treated additively manufactured components with notches. In this study, fatigue tests were performed on plain and notched specimens to determine and compare the crack initiation and crack propagation behavior due to different manufacturing-related effects. Tests were performed on specimens produced by the two aforementioned AM processes and compared to specimens taken from wrought sheets. The fatigue strength of AM materials is influenced by microstructure, defects, residual stress, and notches. PBF-LB/M specimens exhibit the highest fatigue strength in plain, notch-free conditions, attributed to differences in microstructure and static strength affecting fatigue crack initiation. Notched specimens show larger differences among materials, with PBF-LB/M having shorter fatigue crack propagation life related to line-type defect clusters, while the plain PBF-LB/M specimens are less affected as their fatigue strength is primarily determined by fatigue crack initiation.

Effect of grain structure on fatigue crack propagation behavior of 2024 aluminum alloy under different stress ratios

The fatigue load that a material experiences and its microstructure are important factors influencing fatigue crack propagation behavior. This study employed laser scanning microscopy and electron backscatter diffraction (EBSD) technology, along with fatigue crack propagation experiments, to investigate the fatigue crack propagation behavior of 2024 aluminum alloy under varying stress ratios (R). The results showed that the stress amplitude (σp) was the main factor controlling the fatigue crack propagation life (N). Additionally, detailed characterization of the fatigue crack propagation path was conducted using crystal models and EBSD. A fatigue crack propagation model for 2024 aluminum alloy under different R-values was established based on the grain twist angle and Schmid factor. Finally, the impact of R on the crack tip shielding (Ks) was systematically analyzed, elucidating the intrinsic mapping relationship between R, microstructure, and crack propagation characteristics.

Effects of multiple overloads and high-low sequence loading on the fatigue crack propagation of AZ31B magnesium alloy

An experimental study was conducted to examine the initiation of cracks in the AZ31B magnesium (Mg) alloy for tensile overload effects, exploring the influence of overload cycle number, magnitude, and multiplicity of the applied overloads (OL). The number of cycles to failure increased from 8 % to 15 % for overload ratio (OLR) of 1.5 and 2.0 respectively applied as first cycle when compared with constant amplitude loading (CAL). However, the number of cycles to failure increased from 25 % to 43 % for OLR of 1.5 and 2.0 respectively when overload was applied at 5000th cycle. The onset of cracks was delayed when two overloads were applied as the first and 5000th loading cycles. The longitudinal-transverse (L-T) specimens were tested using a two-step high-low sequence loading procedure by varying maximum and minimum load. The fractography results also depict variations in the initial crack formations under different overload conditions. The effects of different overloads on the fracture surfaces of Compact Tension specimens were examined using the Field Emission Scanning Electron Microscopy, indicating substantial reduction in both the number and size of microcracks. 3D profilometer tests were performed, indicating a surface roughness of 0.20 μm to 0.30 μm.

Investigation on the interaction evolution and growth behavior of multiple cracks under fatigue loading

In this study, the cracks interaction is investigated on plates with different crack geometry parameters through finite element analysis and experiments. The presence of multiple cracks may lead to an enhancement or shielding effect on stress intensity factors (SIFs) depending on crack relative positions and sizes, thereby potentially accelerating or decelerating the crack propagation rate. When the crack tips of double parallel non-collinear cracks begin to overlap, the interaction between cracks transitions from an enhancement effect to a shielding effect, and the KII values reach their peak at this point, which are explained by the stress field distributions surrounding the cracks. For non-collinear double cracks with an inclined angle α, the shielding effect of inclined cracks becomes increasingly pronounced in comparison to horizontal cracks as the angle α increases. To verify the numerical results, fatigue crack propagation experiments were carried out on Al–Li alloy samples with double non-collinear cracks with different crack vertical distance. The experimental results show that growth direction of two non-collinear cracks underwent a transition when they overlapped, leading to load mode I changing into mixed mode of I + II. Based on both numerical simulations and experimental results, the material parameters C and m in Paris equation are modified, and then a new method for predicting the multiple cracks growth life is presented. By comparing with the experimental values, it was found that this method exhibits high accuracy and reliability.

Failure analysis of fatigue crack propagation in specimens of AlSi10Mg aluminum alloy produced by L-PBF: Effect of different heat treatments

Fatigue crack growth behavior of AlSi10Mg aluminum alloy processed by laser powder bed fusion (L-PBF) was studied for three material conditions: as-built, stress relieved and hot isostatic pressed. An improvement in the fatigue crack growth resistance was found for the heat-treated conditions which was attributed to the residual stress relief induced by the heat treatments. The stress ratio did not affect the fatigue crack growth in regime II which was explained by the absence of crack closure. Fatigue crack growth rates were predicted using a model based on the cyclic plasticity of the tested aluminum alloy. The proposed model agreed well with the experimental results.

Investigating the effects of preliminary overloading and fatigue on the mechanical strength and fractographic characteristics of CuSn7Zn4Pb7 bronze alloy fractures

This study presents the outcomes of experimental research designed to explore the effects of plastic deformation on the tensile strength and torsional fatigue life of the CuSn7Zn4Pb7 bronze alloy. The primary objective was to understand how plastic deformation impacts these critical mechanical properties of the alloy. The study aimed to determine the nature of damages and attempt to systematise them. Various combinations of loads and material microstructure play a decisive role in the propagation of fatigue cracks. Therefore, in the study, the microstructure of the analysed material, the determination of its chemical composition, and its compliance with relevant material standards were presented. Additionally, microscopic images of fracture surfaces, along with fatigue crack analysis and determining the presence of the fatigue and residual zones, were provided.

“Notch sensitivity and heat treatment effect on the fatigue behaviour of AlSi10Mg aluminium alloy processed by additive manufacturing”

This paper studies the fatigue behaviour of smooth and notched specimens made from AlSi10Mg aluminium alloy via LPBF additive manufacturing. Three material conditions were addressed: as-built, stress relief, and hot isostatic pressing. Heat treatments were conducted at a temperature to avoid Al-Si network rupture, resulting in an increased fatigue strength of both the smooth and notched specimens. This beneficial effect of the heat treatments was attributed to the homogenization of residual stresses through the specimen depth. In addition, the introduction of notches had a detrimental effect on fatigue life. Two fatigue life prediction models were successfully applied. The first one accounts for both the initiation and propagation periods using the SWT parameter and the FITNET FFS procedure, while the second one is based on a calibrated curve relating the SWT parameter and the number of cycles to failure generated from smooth specimens. Fatigue crack propagation from subsurface defects had a primary role on fatigue life of the tested alloy.

Improvement of the fatigue life of an electron-beam welded Ti2AlNb joint subjected to an electromagnetic coupling treatment

The local stress concentration created by the seam welding process often reduces the fatigue life of the material along and around the seam. Post-welding heat treatment is usually used to improve the weld performance, but the improvement effect on fatigue life may not be obvious. In this study, a new treatment method of electromagnetic coupling was applied to regulate the Ti2AlNb electron beam weld after heat treatment, so as to improve the fatigue properties of the welded joint. The results show that after electromagnetic coupling treatment, the fatigue limit of the welded joint is increased by 10.4 %, the residual compressive stress is increased, and the fatigue crack propagation rate is decreased. The microstructure analysis shows that after electromagnetic coupling treatment, the fatigue crack source starts in the subsurface layer, the substructural crystals in the fusion zone and the heat affected zone decrease, the recrystallization content and the large angle grain boundary increase, and the dislocation density increases and tends to be homogenized. An increase of at least 2.3 % in hardness of the welded joints also indicates the increase in the dislocation density. Local high density dislocation is found in the grain, which causes dislocation entanglement and hinders crack initiation and propagation and this provides an important basis for improving fatigue properties. This study provides a new method for improving the fatigue properties of Ti2AlNb electron beam welds which can also be used to study the properties of other welded joints.

Influence of load ratio on fatigue life assessment of AZ31B magnesium alloy under different temperatures

First ever fatigue crack growth experiments were conducted under various load ratios (R = 0.1 to 0.8) at 300 K, and (R = 0.1, 0.3, 0.5, and 0.7) at 327 K and 473 K, respectively, on AZ31B magnesium alloy. The results reveal that temperature-dependent variations in both fatigue crack growth rates and stress intensity factor ranges (ΔK) across different load ratios. It was found that at relatively higher load ratio (R = 0.8) leads to a lower ΔK = 8.0287 MPa√m and a lower load ratio (R = 0.1) result in a higher ΔK = 15.5981 MPa√m, indicating that the material undergoes fatigue crack propagation at a higher level of stress intensity factor range at room temperature. A slower crack propagation was seen at higher load ratio (R = 0.8), whereas lower load ratio (R = 0.1) exhibits faster crack growth at 300 K. Similarly, crack growth generally increases at higher temperatures (i.e., 373 K and 473 K). The fatigue life of the magnesium alloy AZ31B increased from 12,410 to 87,031 cycles at 300 K, 9,620 to 32,667 cycles at 373 K, and 9,140 to 30,143 cycles at 473 K as the load ratio increases. The average fatigue life decreases to 59 % and 63 % as the temperature is varied from 300 K through 373 K to 473 K. The fractographic and surface roughness carried out by field emission scanning electron microscopy and 3D-Profilometer respectively, illustrate that roughness of fracture surfaces correlates with an increase in the fatigue crack growth rate for a specific value of ΔK.

Fatigue crack propagation anisotropy of an Al–Zn–Mg–Cu super-thick plate

The fatigue crack propagation (FCP) rate is an important performance index in the aerospace industry for the safe lifespan evaluation. In this study, the FCP anisotropy of an Al–Zn–Mg–Cu super-thick rolled plate was analyzed. The results show that as the depth increases, the FCP anisotropy gradually becomes significant especially in the low ΔK region. The specimens tested along the transverse direction exhibit lower FCP rate than those tested along the rolling direction in the low ΔK region, which is related to the difference in the grain shape and grain orientation. More specifically, the crack is easy to deflect along the grain boundaries in the deformed grains when loading along the transverse direction. Meanwhile, more obvious grain orientation deviation on both sides of the crack can cause larger angle deflection, longer FCP path, and lower FCP rate. In the Paris region, the FCP anisotropy is not obvious regardless of thickness. The reason is that the main influencing factor is controlled by strength in this region, while the strength anisotropy is very limited leading to this similarity.

Toward developing remanufactured Ti6Al4V alloys with high fatigue crack growth resistance by in-situ cooling during laser remanufacturing

The fatigue performance of remanufactured titanium alloy blade components has garnered significant attention in recent years. The repaired titanium alloy components using the direct energy deposition (DED) method exhibit structural differences, leading to an inhomogeneous microstructure that directly impacts the service life of the remanufactured component. The work presented here aimed to investigate novel approaches for enhancing the resistance to crack propagation and to gain a deeper insight into the fatigue crack propagation characteristics of the Ti6Al4V remanufactured interface with an inhomogeneous microstructure. The research focused on analyzing the microstructure evolution and fracture properties of remanufactured components using in-situ cooling. The results indicate that in-situ cooling has the potential to decrease the anisotropy of the repaired zone (RZ) and enhance the resistance to crack propagation in the heat-affected zone (HAZ) and base metal (BM). The high cooling rate associated with the in-situ cooling can elevate the presence of high-angle grain boundaries (HAGBs) in the RZ, diminish the phase transformation between the HAZ and BM, and decrease the size of the secondary α phase. The research contributes to a deeper comprehension of fatigue crack propagation in remanufactured components and provides a pathway for the improvement of fatigue performance of remanufactured titanium alloy.

Fatigue behavior and damage mechanism of steel plate-UHPC composite bridge deck subjected to negative bending moment

In order to investigate the fatigue behavior and failure mechanism of steel plate-UHPC composite bridge deck under hogging moment, a total of four full-scale specimens with two class of reinforcement ratio (Three specimens with reinforcement ratio of 1.58 %, 1.69 %, 1.86 %, respectively, which called longitudinal specimen. One specimen with reinforcement ratio of 0.89 %, which called transverse specimen.) were designed and tested under fatigue loading. Test results reveal that when the fatigue loading ratio was 0.25 with maximum load of 0.8 times of the static cracking load, taking the crack width of 0.10 mm as the fatigue failure criterion, the fatigue life of longitudinal specimen over 16 million cycles. However, the fatigue life of transverse specimen was only 9.08 million cycles. The longitudinal reinforcement restricts the propagation of fatigue cracks along the depth of the UHPC slab, and tensile fracture of the steel fibers occurred in the region between the top layer of longitudinal reinforcement and the UHPC surface. The fatigue damage mechanism resulting from fatigue degradation of the UHPC layer in the tension zone is identified. In addition, The fatigue design of the steel plate-UHPC composite bridge deck should be controlled by the durability rather than the strength. The established tensile S-N curve with 0.05 mm critical UHPC crack width could be used evaluate the fatigue cracking resistance of composite bridge deck.

Investigation on the fatigue crack propagation of rock-concrete interface under fatigue loading below the initial cracking load

The fatigue crack propagation of the composite rock-concrete interface under constant-amplitude flexural fatigue loading below the initial cracking load was investigated in the present study. Fatigue pre-crack initiation and propagation tests were first conducted on three types of rock-concrete interfaces under three-point bend loading. It was shown that for the rock-concrete interface, the damage evolution throughout the fatigue loading process can be divided into deceleration and acceleration stages according to the rate curves of flexural stiffness, deformation and crack propagation. The crack propagation rate exhibited two distinct regions when plotted on a log scale, namely the small-crack growth region and the long-crack growth region. The small-crack growth region for the studied rock-concrete interfaces was not particularly pronounced. The long-crack growth region was the Paris’ law region and included both the acceleration and part of the deceleration stages. Based on the experimental results, the fatigue crack propagation rate in the Paris’ law region was then described using the empirical Paris’ law. The exponent m1 in the Paris’ law was found to be approximately constant, with a value of m1 ≈ 21 for the rock-concrete interface. The pre-factor C1 was found to be dependent on both the load ratio R and the type of the rock-concrete interface (represented by an elastic mismatch parameter α). By introducing R and α into the empirical Paris’ law, an adjusted Paris’ law was proposed. The adjusted Paris’ law can describe the fatigue crack propagation rate in a unified way for the same rock-concrete interface of different R and different interfaces in the scope of this research. Based on the adjusted Paris’ law, the fatigue cycles during the crack propagation and fatigue lifetimes of all tested rock-concrete interface specimens were reasonably predicted. For practical applications, an approach was suggested involving a combination of the adjusted Paris’ law and a material coefficient β (approximately β ≈ 0.143 in this study) to estimate the fatigue lifetimes of structures or specimens with existing pre-cracks at the rock-concrete interface.

Effect of Mode II in the mixed-mode on the fatigue crack growth behaviour for SAPH440 material

In this study, the effect of mixed-mode loading on fatigue crack growth under varying applied loads for SAPH440 material was experimentally investigated. Previous research has either examined the effect of mixed-mode loading at a fixed angle or its effect on slanted cracks. This paper uses a CTS specimen and a loading device to apply varying angles of load during Mode I loading conditions. Initially, a numerical analysis was performed to validate existing equations for the stress intensity factor, which quantifies the crack driving force in the experiments. In the crack initiation region, load shedding techniques and load increasing techniques were used to confirm fatigue cracking considering mixed-mode effects. The change in the threshold value according to the loading angle was also evaluated. In the crack propagation region, a multi-level loading technique was employed to assess the mixed-mode effects under varying loading angles. This process verified the mixed-mode effect under conditions where the applied load angle varies. The fatigue crack fracture surfaces were examined using digital camera capture and SEM. It was found that when mixed-mode loading occurs during Mode I loading, the fatigue crack propagation rate varies depending on the mixed-mode loading angle.

High-temperature thermal fatigue of Ni3Al-based single crystal alloy affected by wall thickness and peak temperature

Complex thin-walled structures are commonly employed in turbine components to improve cooling efficiency. However, thin-walled blades are prone to thermal fatigue failure due to the high thermal stress induced by frequent and abrupt temperature fluctuations during duty cycles. The thermal fatigue behavior of Ni3Al-based single crystal alloy with thin-walled structure during 25 °C ↔ 1000 °C/1100 °C was investigated by combining thermal fatigue tests and finite element simulation. The thermal fatigue demonstrated a significant thickness debit effect, with thermal fatigue property dropping as peak temperature and wall thickness increased. Wall thickness had a more pronounced effect than peak temperature. Visible slip traces were found on the wall thickness surface, and their density was positively correlated with wall thickness. Further investigation revealed that thermal fatigue is primarily driven by the octahedral slip system ({111}<110>), with significant contributions from (111)[101‾] and (1‾1‾1)[101]. Simulations of thermal stress distribution and J-integral at the crack tip during crack initiation and propagation stages indicated that wall thickness influences thermal fatigue properties by affecting thermal stress concentration at the notch and crack tip. Higher peak temperatures increased thermal stress and reduced yield strength, thus diminishing thermal fatigue performance. A thermal fatigue crack propagation prediction model was constructed incorporating Paris' law and the J-integral. The model revealed an unfavorable correlation between material Cj/ nj and both peak temperature/wall thickness, indicating that both factors adversely affect thermal fatigue resistance.

Dual-graded lattice with mechanical bionics to enhance fatigue performance

Additive manufacturing (AM) is revolutionizing the design and fabrication of orthopedic implants, enabling the creation of complex graded structures to enhance biomechanical bionics. This study aims to enhance fatigue life and reduce stress shielding for a long-term stability by using functionally graded lattice. Various lattice with minimal surface, including uniform array, Z-axis graded, and dual-graded Gyroid, were designed and fabricated by selective laser melting (SLM), followed by mechanical and fatigue tests for validation of design objectives. With a novel Sigmoid-Boltzmann fusion algorithm, the dual-graded lattice enhances the artificial vertebral body (AVB) with an 11 % increase in strength, >200 % improvement in fatigue life and reduced stress shielding on the contact surface, benefiting long-term implant stability and biomechanical fidelity. It displayed outstanding resistance to fatigue crack propagation due to superior load-bearing capacity and energy absorption capabilities contributed by the graded central region. The findings highlight the significance of biomimetic design principles and offer insights into the optimization of orthopedic implants using dual-graded lattice.