Predicting very high cycle fatigue (VHCF) life in composites is challenging due to the lengthy testing times required by traditional methods. Ensuring compatibility between VHCF and high cycle fatigue (HCF) data presents an additional challenge. This investigation examines CF-PEKK fatigue behavior under low and ultrasonic cyclic frequencies using a thermographic approach and the Fracture Fatigue Entropy (FFE) concept. Constant-amplitude fatigue (CAF) and increasing-amplitude fatigue (IAF) tests were conducted, with self-heating behavior analyzed for different loading conditions. Digital Image Correlation (DIC) and finite element analysis (FEA) enabled stress estimation for each test setup. Heat dissipation rates from IAF experiments and fatigue data from three CAF experiments at 40 Hz and three at 20 kHz were used to calculate FFE values. Results showed that the self-heating response of CF-PEKK specimens helps achieve comparable fatigue data across frequencies, with consistent FFE ranges identified between HCF and VHCF regimes. This consistency highlights that FFE can provide reliable fatigue life estimation for composites. Thermographic and microscopic analyses further confirmed this approach as an efficient method for estimating fatigue life under various cyclic loading conditions.

In-situ fatigue crack propagation experiment was conducted on laser cladding with coaxial powder feeding (LCPF) K477 under various stress ratios and temperatures. Multiple crack initiation sites were observed by using in-situ scanning electron microscopy (SEM). The fatigue short crack growth rate was measured, and the impacts of temperature and stress ratio on this growth rate were analyzed. Based on these experiments, the experimental data were expanded, and three ensemble learning algorithms, i.e. random forest (RF), extreme gradient boosting (XGBoost) and light gradient boosting machine (LightGBM) were employed to establish a fatigue short crack growth rate model controlled by multiple parameters. It is indicated that the RF model performs the best, achieving a coefficient of determination ( R 2) of up to 0.88. The fatigue life predicted by the machine learning (ML) method agrees well with the experimental one.

In this paper, the damage mechanisms and residual strength prediction models of unidirectional glass-fiber reinforced polymers are investigated by acoustic emission(AE) technique. The material exhibits three damage modes: matrix cracking, fiber fracture, and interface damage. A novel AE descriptor, amplitude/centroid frequency (ACF), is introduced to differentiate interface damage from other damage modes. Moreover, three signal types exhibit a strong clustering effect when correlated with ACF and average frequency. Microscopic damage mechanisms of the samples are observed using scanning electron microscopy and correlated with AE signals. The AE signals are analyzed using machine learning, and the clustering analysis results are used as a training set to obtain classification models using support vector machine (SVM) and K-nearest neighbor (KNN) methods. Leveraging the traditional mechanical regression analysis prediction model, the study achieves prediction of the material’s residual strength post-fatigue through improvement in AE cumulative counting. Additionally, optimization of prediction results can be achieved by a certain kind of signal after clustering. The combination of supervised learning and residual strength prediction models can realize the real-time classification of AE signals and apply them to the prediction of residual strength, which has a significant application value in real-time monitoring.

The enhanced fatigue resistance of superelastic Cu-Al-Mn shape memory wires were investigated via bamboo-like grain structure obtained by cyclic heat treatment (CHT). It was demonstrated that increasing the number of CHT cycles can promote the grain growth to a straight grain boundary, eliminating the triangular structure that generally caused brittle fracture. In particular, the 5CHT and 7CHT specimens displayed negligible residual strain subsequent to unloading, following exposure to cumulative strains ranging from 2% to 10%, thereby showcasing superior superelasticity. From the functional fatigue test, the fatigue life of the shape memory wire subjected to 5 CHT cycles exceeds 4650 cycles at 5 % strain, which is more than 5 times that of traditional polycrystalline quenched samples.

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.

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.

Aluminum alloys are primary structural materials in aircraft fuel systems, where jet fuel can significantly affect the crack propagation behavior of the materials. This paper presents a model for calculating the crack growth rate of aluminum alloys in jet fuel environment. The model is based on elastoplastic fracture mechanics and revises the interaction terms in the linear superposition model by taking into account corrosion damage and crack closure effects. The derivation process of the model is discussed in detail. To validate the efficacy of the model, fatigue crack propagation tests were conducted on three types of aviation aluminum alloys, namely 2524-T3, 7050-T7451, and 7075-T62, in the jet fuel environment. The experimental results were compared with crack growth rates in the laboratory air environment. Findings indicate that the proposed model effectively captures the primary trends observed in the experimental data. In addition, the failure surfaces of the specimens were observed using a super-depth-of-field optical microscopy system. When subjected to jet fuel, among three materials tested, the 7075-T62 alloy was found to have the roughest failure surface. This increased roughness contributing to crack closure was identified as one of the reasons why its fatigue crack growth rate is significantly lower in the jet fuel than in air.

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.

In this paper, the low-cycle fatigue fracture behavior and life prediction of Cr –Ni –Mo –V gun steel at room temperature and 600 ℃ are studied. The results indicated that at room temperature and 600 ℃, the Cr –Ni –Mo –V gun steel exhibited obvious monotony softening and cyclic softening. This behavior could be attributed to the formation of dislocation networks, dislocation walls, dynamic recovery, and dynamic recrystallization. As the temperature increased, the failure mode gradually shifted from mixed transgranular and intergranular fractures to intergranular fractures possibly owing to the reduced grain boundary strength and easy oxidation of the grain boundary at high temperatures. In addition, the fatigue life prediction model considering the influence of temperature is established by using the energy dissipation quadratic function, offering a practical method to improve the fatigue performance evaluation of Cr –Ni –Mo –V gun steel at various temperature.

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 explores the varying thermal response of additively manufactured samples of AlSi10Mg subjected to cyclic loading. The thermal response is driven primarily by the self-heating effect. The paper explores the viability of employing thermographic methods to establish an S-N curve for fatigue life prediction with fewer samples than traditionally required from constant-amplitude tests. Specific Self-Heating tests are conducted, gradually increasing the loading amplitude while monitoring the specimen's temperature. The validity of evaluated solution is compared with the own experimental data including the impact of four heat treatments on the samples from one AM batch, leading to valuable insights and conclusions.

Wave-induced fatigue damage is inevitable for the deepwater steel catenary riser (SCR). To accurately evaluate the fatigue performance of the full-scale SCR using small-size specimens, five kinds of fatigue testing strategies were explored, considering the effect of welding residual stress. Further, the corresponding fatigue tests were conducted regarding the welded joint specimens with the same weld profile, and the relevant differences in fatigue testing results were comprehensively analyzed. Through comparison with the full-scale resonant bending fatigue testing results, the most equivalent strategy using small-scale specimens was confirmed. The relevant results indicated that compared with the fatigue lives of full-scale pipeline joint specimens, the fatigue testing lives under the constant applied maximum stress of the yield strength for base material or the constant applied mean stress of the peak value of transverse welding residual stress strategy are relatively lower. The fatigue life values obtained from the high-stress ratio testing strategy are higher, especially in the low-stress range region. Comparatively, the fatigue lives obtained under the 80 mm wide specimen without the cutting process strategy were higher in the high-stress range region. The variable applied mean stress strategy using the 25 mm wide welded joint specimen was most suitable for equivalence with full-scale fatigue testing results, and the difference in the fatigue life testing results was only 9.7%. The difference between the applied mean stress and the actual transverse welding residual stress under various fatigue testing strategies is the key to affecting the equivalence of fatigue testing results.

The surface morphology has a significant influence on the fatigue behavior of components. For austenitic stainless steels (ASSs), this issue is even more pronounced due to their metastability. Based on the complex deformation mechanisms of metastable ASSs, which include dislocation slip, deformation twinning, and deformation-induced martensitic phase transformation, the metastable stainless steel AISI 347 was investigated in this study together with the stable AISI 904L as a reference material. 4-point bending fatigue tests with load ratio R = 0.1 and testing frequency f = 10 Hz at ambient temperature were carried out on specimens with 5 technically relevant surfaces morphologies: mechanical polished, milled, micro-shot peened, laser shock-peened, and ultrasonic modified. Systematic material characterizations were carried out to clarify the key influences of these morphologies on the fatigue behavior. Deformation-induced martensite layers were proven to improve the fatigue life in metastable austenitic steels, which open perspectives to extend the lifetime of components.

The blasting excavation process during underground rock mass engineering can induce severe stress disturbance, resulting in spalling and damage to the surrounding rock mass in the tunnels, which can seriously compromise the underground engineering construction. In the present work, an experimental blast loading device was developed to study the dynamic response of rocks under explosive loads, which could vary the utilization of explosive gas energy by changing the constraint conditions. The device employed a high-speed camera to record the stress wave propagation and failure characteristics on the surface of the specimen and verified the reliability of the experimental results using an ultra-dynamic strain gauge. The developed apparatus was used to explore the failure characteristics and stress wave propagation laws in red sandstone under different explosion gas energies. The complete process of stress wave propagation in red sandstone was captured under different explosive gas energies, from an intact form to failure, and the attenuation law of stress waves was obtained. The experimental results showed that when the explosive stress wave traversed through the specimen, it primarily experienced tensile strain, with maximum tensile strain observed at the free surface. The stress wave propagation in the specimen varied under different explosive loads, leading to varying overall failure characteristics of the specimen. The larger the amplitude of the stress wave, the greater the spatial attenuation coefficients of the compression wave and the tensile wave. The thickness of the spalling fracture was determined based on the wave width of the stress wave λ 1, the attenuation coefficient of the stress wave α, and the longitudinal wave velocity C 0. The closer the crack is to the bottom of the specimen, the smaller the thickness. The experimental results provide theoretical guidance to understand the strong dynamic disturbance behavior and progressive instability failure phenomenon in deep underground engineering.

Metal Additive Manufacturing technologies provide new opportunities for manufacturing complex components. However, the limited and scattered data on damage tolerance behavior is hindering adoption in safe-critical applications. A design of experiments (DOE) is used in this study to provide an understanding of the Electron Beam Melted (EBM) Ti6Al4V fracture toughness properties. Three builds comprised of 150 compact tension samples were printed representative of the EBM build chamber, followed by surface machining, microstructural characterization, X-ray microcomputed tomography (µCT), and fracture toughness testing per ASTM E399. Analysis of Variance (ANOVA) statistics on the influence and interaction of intra-build design parameters on the As-Built and Machined samples showed orientation, build location, and geometry to contribute to property variation. EBM fracture toughness reported an average of 65 MPa√m, with an increase in build height and proximity to the center of the build envelope. The location- and size-dependent properties resulted from changes in microstructure and porosity throughout the build space. While intra-build design variation was present, the EBM Ti6Al4V fracture toughness properties reported a 10% overall variation comparable to wrought and cast alloys. The extensive experimental work in this study shows EBM Ti6Al4V to be a repeatable and reliable alloy for use in load-bearing applications.

To provide more suggestions for the anti-fatigue design of cold-formed steel, a series of high-cycle fatigue tests are carried out on cold-rolled steel sheets connected by self-tapping screws and blind rivets in this paper. Subsequently, fatigue failure mode, fatigue strength, and fatigue damage are analyzed in detail. The test results show that all the effective data points are below the S- N curve of screws in the code. Meanwhile, the fatigue limit of the fitted curves with 95% survival probability is between 0.3 and 0.67 times the specification. Besides, the fatigue damage expansion rate of cold-rolled steel sheets is less than the standard value. Through further discussion, it is noticed that base metal grade has little influence on the fatigue performance, and thickness has a significantly positive influence on the shear resistance and fatigue performance. Moreover, the fatigue performance of the two types of connections is similar.