Crack initiation in AISI 316 stainless steel has been investigated. Persistent Slip bands (PSBs) were characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). PSBs on the surface of the material increase the surface roughness and result in crack initiation. EBSD data from near the crack initiation region were used to correlate the global and local misorientations of the grains, plastic deformation, and Schmid Factor with the fatigue life of specimens. The crack initiation region was found to have the highest misorientations. The region near crack initiation was found to have more plastic deformation, which was severe in specimens loaded with higher stresses. The Kernel Average Misorientation (KAM) and Grain Reference Orientation Deviation (GROD) maps from the EBSD data were investigated for specimens that failed at different fatigue cycles. It was found that the interaction of high dislocation density, substructuring and misorientation of low-angle grain boundaries in the region of plastic deformation resulted in the fatigue crack initiation.

not-yet-known not-yet-known not-yet-known unknown In this work, the effect of aging variables of a proposed heat treatment and their combined impact on the fatigue life of an Al6061-T6 were evaluated. Al 6061-T6 specimens were prepared according to the ASTM E466 and then heat-treated, followed by fatigue tests in a rotating beam testing machine. The results showed an increase in the fatigue life of Al6061-T6 up to 4.47 times and up to 4.69 regarding the aging temperature and the aging time, respectively. Three empirical models to estimate the fatigue life in function of the aging time, aging temperature, and both aging time and temperature were obtained by fitting the experimental, showing a correlation of R 2 =90%, R 2 =79.2%, and R 2 =80.1%, respectively. The proposed heat treatment also decreased the energy consumed by the process by 67.8% than that consumed by the T6 heat treatment of the raw material as a consequence of reducing the time of the process.

In contrast, but complementary to previous studies, this study examines the fatigue behaviour in Ti-6Al-4V obtained by electron beam powder bed fusion, focusing on damage initiation sites, fatigue damage progression, and correlating these with fatigue life curves. Three material conditions were considered: as-built specimens with original surfaces after printing (AB), as-built specimens with a machined and polished surface (MP), and hot isostatic pressed specimens with a machined and polished surface (H). Fatigue fracture surface topography was analysed using scanning electron microscopy and surface metrology microscopy. Different fatigue responses were observed, with crack initiation at surface roughness in AB, lack of fusion defects in MP, and phase facet formation in H specimens. Interaction between cracks and manufacturing defects was investigated. Kitagawa-Takahashi diagrams were applied successfully to AB and MP specimens. This study aims to enhance understanding of crack initiation and interaction mechanisms, improving life prediction capabilities through microstructure and defect-sensitive modelling.

not-yet-known not-yet-known not-yet-known unknown Fatigue characterization of asphalt mixtures is imperative for predicting pavement performance. Since cracking begins in asphalt binders, determining their fatigue properties helps establish cause-and-effect relationships with cracking in mixtures and pavements. This study analyzes the fatigue of 12 Brazilian asphalt binders and mixtures using the Linear Amplitude Sweep (LAS) test for binders and the Direct Tension Cyclic Fatigue test for mixtures. Results were interpreted with Simplified Viscoelastic Continuous Damage theory (S-VECD) and a pavement was simulated using FlexPAVE TM software. Findings indicate that binder type alone does not predict fatigue class through the fatigue factor of binder (FFB), necessitating rheological damage analysis. Most mixtures showed good performance in pavement simulations, with fatigue factor of mixtures (FFM) and fatigue classes not being sufficient for predicting fatigue performance. A linear correlation was found between FFB and FFM but not between FFB and simulated damage (N/Nf), though higher FFB typically result in lower N/Nf.

The fatigue damage accumulation behavior of GH4169 superalloy was studied through two-level loading fatigue tests based on an electromagnetic resonance testing machine. A total of 178 experimental data points and 13 S-N curves were obtained. By comparing and analyzing experimental data, the influence of stress and fatigue damage in the first-level loading on the residual fatigue life of the material was studied. The effect of first-level fatigue damage on the residual fatigue life of different second-level stresses was explained. A new fatigue damage accumulation rule was proposed and validated using several nonlinear fatigue cumulative damage models. The results indicate that this rule can serve as a criterion for evaluating the accuracy of fatigue cumulative damage models.

This study investigates the impact of various surface finishes on the low cycle fatigue (LCF) properties of laser powder bed fusion GRCop-42. The evaluated surfaces include as-built, machined, and chemically polished finishes (1 .0% and 2 .0%). LCF life of polished GRCop-42 was assessed at cryogenic (−195 ◦ C ), ambient, and elevated temperatures (200 ◦ C , 400 ◦ C , 600 ◦ C , and 800 ◦ C ) across three strain amplitudes. Results indicate that surface finish has minimal impact on LCF life. Stress across different strain levels showed minimal effect of surface finish on cyclic hardening/softening. Cryogenic temperatures led to cyclic hardening followed by stabilization, while ambient and 200 ◦ C temperatures showed initial hardening followed by softening. At 400 ◦ C and above, specimens displayed continuous cyclic softening. Fractography showed that surface finish impacts plastic deformation: as-printed and polished surfaces had brittle fractures, while machined specimens were ductile. Specimens at cryogenic and ambient temperatures exhibited brittle fractures, whereas those at elevated temperatures showed plastic deformation and microcracks.

This study presents a procedure based on constant amplitude (CA) fatigue data to predict the statistical fatigue lifetime of glass/orthopolyester composites subjected to repeated ordered and random two, three, and six sequences of block loadings. A numerical routine was developed to detect cycle-by-cycle the statistical strength degradation progression until failure, assuming that the strength at the end of a block cycle equals the strength at the start of the successive one and that the individual samples’ static strength, residual strength, and fatigue life share the same rank in their respective cumulative distribution function. Predictions conform to the statistically undetectable loading sequence effects and lightly overestimate the lifetimes of random and ordered high-to-low (1/100 cycles) repeated two-block loadings. The vanishing effect of the loading sequence when the block extents remain fixed, the block extent effects for a given three-block sequence, and the lifetimes of three-block loadings were fully predicted. The six-block sequence’s experimental lifetimes with different block loading orders and block extent fell within the predicted lifetimes’ cumulative distribution function. A reliable damage rule based on residual strength was proposed and compared to the Miner’s rule.

The test data of smooth specimens is expanded, by integrating Bayesian theory and the Monte Carlo method, and a probabilistic fatigue model considering size effect under different loading conditions is established. Furthermore, the influence of non-uniform stress fields and notch characteristics on fatigue crack initiation is defined, considering critical distance, with a correction to the stress field damage parameter. The relationship between fatigue life and damage variables under various loading conditions is explored, the virtual sub-sample augmentation method and the weakest link theory are combined, leading to the establishment of a multiaxial fatigue life prediction model, suitable for notched specimens. Finally, the prediction life using proposed method and other models are compared with the experimental life of three materials, the compared results show that the proposed method exhibits a notably heightened level of accuracy, and the fatigue life of notched specimens can be predicted by small samples of smooth specimens.

X-ray computed tomography is an extremely useful tool for the non-destructive analysis of additively manufactured (AM) components. AM components often show manufacturing defects such as high porosity, rough surfaces, or a lack of fusion (LoF) between production layers. These imperfections can be detrimental for the fatigue life of components. To better understand how cracks initiate and grow from internal defects, we fabricated Ti-6Al-4V samples with an internal cavity using electron beam powder bed fusion (PBF-EB/M). The samples were tested in the high-cycle fatigue (HCF) and very high-cycle fatigue (VHCF) regime using ultrasonic testing equipment and analyzed with X-ray computed tomography (XCT). X-ray imaging was used to locate crack initiation sites around defects and to measure characteristic properties of the crack and the defects with the aid of deep learning segmentation tools. LoF defects exposed to the outer surface of the samples due to machining and post-processing, were found to be as detrimental to the fatigue life as the printed central artificial defects. The work presented here can benefit industries that utilize the AM of high-strength, light-weight alloys, such as aerospace and medicine, in the design and manufacturing of components by improving part reliability and fatigue life.

This study investigates the shear fracture behaviors in UHPC under direct shear conditions using Z-shaped specimens and acoustic emission (AE) monitoring. The effect of steel fiber (SSF) contents (1%, 2%, 2.5%, and 3%) on the failure process and the relative slip of cracks at different loading stages were measured and evaluated. The results indicate that increasing the SSF content significantly enhances the ultimate shear stress and ductility, effectively limits crack propagation and formation, and reduces the extent of damage for UHPC. During the failure process, an increase in the SSF content results in higher cumulative AE energy and a tendency for the peak frequency to shift towards the low-frequency range. Additionally, increasing the SSF content expands the range of wavelet entropy values and delays the occurrence of wavelet entropy. Due to the reinforcement effects of SSF, the primary crack type evolved from shear to tensile during the failure process.

This study aims to investigate the low cycle fatigue (LCF) behavior and fatigue life prediction method of an advanced solid solution strengthened nickel-based superalloy at various temperatures and stress concentrations. Cylindrical specimens as well as plates with three types of holes (including straight holes and inclined holes with different orientations) were designed and machined for fatigue tests. The test results revealed fatigue life decreases with the temperature increasing. Meanwhile, the fatigue lives of the plate specimens have decreased differently and are strongly dependent on the geometry and orientation of the holes. The key damage parameter related to the fatigue life was found on the strain gradient path. A criterion based on strain gradient is defined as the boundary of the damage process zone. The results show that the prediction was in good agreement with the experimental data, for the majority of the data is within a scatter band of ±2.

Three fatigue damage models based on entropy, originally developed for constant amplitude loading, were assessed and compared each other in order to state whether its applicability can be extended towards variable amplitude loading conditions. A variable amplitude loading history, applied on a 2024-T3 aluminum alloy reported in the literature, was processed using both rainflow cycle counting and spectral techniques to transform it into a distribution of simple processes with constant amplitudes, then the damage models were assessed under the new loading conditions. The results showed that the model by Khonsari, combined with the rainflow technique, exhibited the highest accuracy with respect to the referenced experimental results with -0.67 standard deviation from the average data and 16% error from the median. Therefore, it is possible to assess the fatigue damage accumulation in metallic materials under variable amplitude loading through a thermodynamic approach with models developed for constant amplitude loading.

A unified phase-field theory is introduced to describe crack initiation and early crack growth due to pitting corrosion of pipelines in contact with near-neutral pH groundwater. The model incorporates a formulation that accounts for stochastic corrosion-induced crack nucleation events at pit sites. This approach supplies a modeling framework to handle the combined effects of electrochemical transformation of the original metal, hydrogen diffusion and resulting embrittlement of the original metal, and mechanical stresses. This is a robust computational approach to tracking the evolving metalelectrolyte interface and embrittlement regions. It was confirmed that dissolved hydrogen into the material promotes crack initiation over a wide potential range and that pitting corrosion, as a precursor of stress corrosion cracking, has a recognizable influence on strength over time. It is expected that this work provides the ground for modeling the pit-to-crack transition required to effectively control very slow crack-in-colonies in pipelines.

In this article, the low cycle fatigue(LCF) behavior of a low silicon cast aluminum alloy Al-3.5Si-0.5Mg-0.4Cu with T6 treat-state at room temperature is investigated by extracting samples from aluminum steering knuckles. Initial microstructure, cyclic stress response behavior and fatigue fracture are studied. Low cycle fatigue experiment results and behavior performance of 6 different total strain amplitudes are included. OM, SEM, XRD and TEM are carried out to observe pre(post)-fatigue microstructures and fracture. The results demonstrate that cracks initiate from surface or subsurface defects and higher total strain amplitude leads to larger sum area of fatigue crack initiation region and steady crack propagation region, and wider fatigue striation bandwidth. Furthermore, crack propagates along the interface between eutectic silicon and α-Al under low strain amplitude while crack grows through eutectic silicon particle if higher strain amplitude is applied.

Variable creep load is one of the most important failure modes for hot-component of aero-engine. To accurately predict creep damage and remaining life, a novel nonlinear creep damage accumulation model is proposed based on the Wilshire function and isodamage line, which takes the influence of load history into account and uses ultimate tension strength to compensate the temperature effect. Experimental result of tests on four kinds of material were utilized to verify the accuracy of the proposed model and to compare it with existing models. It was determined that the novel model was better at predicting damage accumulation than all others model. Furthermore, the proposed model elucidates the evolutionary process of creep damage, and four cases of damage evolution process are discussed.

A comparison study of fatigue behavior in Al-Al and Al-steel refill friction stir spot welding jointHaokun Yang1a*, Detao Cai2,3a, Jiaxin Li1, Chin Yung Kwok1, Yunqiang Zhao3, Yu Li4, Haisheng Liu4, Waiwah Lai11 Smart Manufacturing Division (SMD), Hong Kong Productivity Council (HKPC), Hong Kong 999077, People’s Republic of China2 State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha, Hunan 410082, People’s Republic of China3 China-Ukraine Belt and Road Joint Laboratory on Materials Joining and Advanced Manufacturing, China-Ukraine Institute of Welding Guangdong Academy of Sciences, Guangzhou, Guangdong 510650, People’s Republic of China4 Center for Industrial Analysis and Testing, Guangdong Academy of Science, Guangzhou, Guangdong 510650, People’s Republic of Chinaa These authors contribute equallyCorresponding author: Haokun Yang, +852 27885679,[email protected]: Refill friction stir spot welding, Fatigue fracture, Aluminum, Steel

In this paper, a theoretical analysis model and two simulation methods are applied to characterize the quasi-static and fatigue delamination of composite laminate with uneven thicknesses. The test data of partially reinforced double-cantilever beam (DCB) were used as benchmark to verify the analysis model and simulation, and cohesive zone models (CZMs) and virtual crack closure technique (VCCT) are used in simulation. It’s shown that the partially reinforced DCB has a unique double-peak load-displacement relationship, and produces instability development during the delamination. By comparing the results of simulation and experiment, it is found that the simulation based on the exponential CZM can simulate the delamination process of partially reinforced DCB under both quasi-static and fatigue loading; while VCCT method will generate a straight delamination front edge in the area of reinforcement, and lost the micro-damage of the previous loading step between load steps, and result in an incorrect delamination behavior.

This work discusses the results from tests conducted to investigate the uniaxial compression and creep behavior of red sandstone. The original untreated sample and the 800 ℃ treated sample have been selected to carry out the experiments. It has been found that high temperature has obvious influence on the mechanical properties of red sandstone. The relationship between creep strain and instantaneous strain, as well as instantaneous deformation modulus and creep viscosity coefficient have been analyzed. It has been found that high temperature reduces the ability of red sandstone to resist instantaneous deformation and creep deformation. Acoustic emission (AE) technology has been also used in the loading process of uniaxial compression and creep tests, providing a powerful means for damage evolution analysis of red sandstone.