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.

Mean stress effect remains as a critical aspect in multiaxial fatigue analysis. This work presents a new criterion which, based on classical Findley criterion, applies a material-dependent exponent to the mean normal stress term and includes the ultimate tensile stress as a fitting parameter. The criterion has been checked against an extended version of the Papuga database of multiaxial experimental tests with 485 results, and compared with the criteria by Findley, Robert and Papuga. The new criterion provides outstanding results for pure uniaxial cases, with multiaxial performance similar to Robert criterion with a smaller range of error and a conservative trend. These features enhance the applicability and versatility of the criterion for its use in fatigue design of structural 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.

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.

This work deals with the analysis in the frequency domain of the temperature signal and mechanical energy rate of C45 steel under two different fatigue stepwise loading series at stress ratios of 0.1 and -1. It was first investigated the energy distribution among the harmonic components of the signals to understand possible variations caused by a different stress ratio. In addition, the second amplitude harmonic (SAH) of heat dissipated and mechanical energy rates have been considered in the analysis and their relationship was investigated. It has been shown as it depends only on the material, hence it is valid whatever the kind of the test is without any assumption on the energy supplied to the material or material hysteresis loop stabilisation. The adopted approach allows the analysis of intrinsic dissipations by means of rapid, full-field and contactless techniques without any specific requirement on loading condition or temperature signal stabilisation.

Laser powder bed fusion (L-PBF) is an additive manufacturing (AM) process which offers several advantages over conventional manufacturing techniques, including material savings and design possibilities. For these reasons several industrial sectors such as biomedical, aerospace and automotive, are considering L-PBF for customized parts production. Among the processable material palette, Ti6Al4V gained increasing interest due to the high specific strength, and stiffness, and excellent bio-compatibility, and corrosion resistance. However, to include L-PBF Ti6Al4V parts in load bearing applications it is important to properly understand the mechanical properties of AM’ed materials with a specific focus on fatigue behaviour. In this study the fatigue notch sensitivity of Ti6Al4V L-PBF manufactured specimens is investigated in depth. Samples with different notches were subjected to stress relieving heat treatment to generate a fine α+β microstructure. The results were used to define a relation between the perturbed stress field generated by the geometrical discontinuity, the microstructure size and the notch sensitivity. Moreover, the critical distance has been calculated using the theory of critical distance formulated with the line method (LM). The estimated value were adopted to predict fatigue properties of notched components with different notch radii.

In order to measure the interfacial Strain Energy Release Rate (SERR) of adhesively bonded joints with metallic substrates, an alternative to the Tapered Double Cantilever Beam (TDCB) specimen is used here. In particular, the dimensions of one of the two substrates were reduced to create an Asymmetric Tapered Double Cantilever Beam (ATDCB) specimen. The ATDCB specimen has been introduced in the past by other research teams to study the mode I+II cohesive fracture properties of structural adhesives. It is shown the geometric dissymmetry of the ATDCB specimen affects the crack propagation path towards the interface with the smaller (less stiff) substrate. However, this substrate plasticises during the fracture test; hence, a strategy is also presented to evaluate the amount of potential energy spent for this phenomenon and subtract it from the total potential energy induced in the system during the fracture test. After this operation, the SERR of the adhesive under investigation was calculated by means of the Linear Elastic Fracture Mechanics (LEFM) theory before and after water ageing. The results showed a large decrease of the SERR with the increase of the ageing time.

To study the thermal cracking characteristics and mechanism of sandstone after high-temperature treatment, the pore size distribution and micromorphology of sandstone were observed by nuclear magnetic resonance and scanning electron microscopy. Then, based on the Weibull distribution theory, a thermal elastic mechanical model of random heterogeneous rock was established for the rock unit, the thermal stress distribution characteristics of sandstone were analysed, and the thermal fracture mechanism of rock was discussed. The results show that the porosities of the samples increased with increasing temperature, and the proportion of large pores increased significantly when exceeded 400 °C. Particularly when reached 1000 °C, thermal cracking was distributed in a complex network. Additionally, different rock units are in different thermal stress states, which leads to the regional differences in the distribution of rock thermal fracture. When exceeded 400 °C, there were obvious thermal cracks near the outer edge that weakened the mechanical properties of rock.

Ultra-light Mg-Li alloy is a promising alloy in aerospace since it is known to the lightest structural alloy at present, but its fatigue behaviors remain to be explored. This work focuses on very-high cycle fatigue (VHCF) strength and small crack initiation behaviors of an extruded dual-phase Mg-Li alloy (LZ91). The fatigue strength of the LZ91 alloy at 109 cycles is about 78 MPa, and the fatigue ratio is approximately 0.46. Microstructure characterization demonstrates that fatigue crack tends to initiate from the β-Li phase-enriched region. The α-Mg phase, presenting <10−10 >  fiber texture with the basal plane, deforms hardly along the extrusion direction and acts as an enhanced phase compared with the β-Li phase. The deformation discrepancy localizes cyclic plasticity at the Li phase and finally leads to the fatigue crack initiation.

A bio-ceramic coating was prepared on the surface of ZK60 magnesium alloys by micro-arc oxidation (MAO) method. The substrate (BM) and coated (MAO) specimens were pre-corroded in a simulated body fluid (SBF) for 12 h. Strain-controlled and stress-controlled loading modes were used to conduct fatigue tests for the two specimens, respectively. The cyclic deformation behavior of the two specimens with non-corroded and pre-corroded was studied. The mechanism of cyclic deformation under different loading conditions is related to twinning and slip. At the same test conditions, the fatigue life of the non-corroded BM specimen is higher than that of the non-corroded MAO specimen, while the fatigue life of the pre-corroded MAO specimen is higher than that of the pre-corroded BM specimen. A modified total strain energy model is proposed and the precision of life prediction is higher than that of traditional fatigue model.