A mechanistic based cavitation model that considers nucleation, early-stage growth and sintering under creep-fatigue interaction is proposed. The number density of cavities ρ and their evolution during multi-cycle creep-fatigue loading are predicted. Both the cavity nucleation and early-stage growth rates, controlled by grain boundary (GB) sliding mechanism during the tension phase, are formulised as a function of local normal stress σ n. The cavity sintering that occurs during the compression phase is described as a function of σ n, but the mechanism switches to the unconstrained GB diffusion. By examining various load waveform parameters, results provide important insights into experimental design of studying the creep-dominated cavitation process under creep-fatigue interaction. First, creep-fatigue test with initial compression will promote higher ρ value compared to that with initial tension, if the unbalanced stress hold time in favour of tension is satisfied. Second, the ρ value does not have a monotonic dependence on either the compressive hold time or stress level, because of their competing effect on nucleation and sintering. Third, the optimum value of stress variation rate exists in terms of obtaining the highest ρ value due to sintering effect.

To exploit the full material potential of short fibre reinforced PA6T/6I, specific component calculations including aniso- tropic material behaviour is necessary. For this, different failure criteria and fatigue models are used to describe the behaviour during a component service life. This paper deals with the determination and consideration of fibre orientations for failure criteria and fatigue calculations. Therefore, a novel method to determine fibre orientation (FO) distributions across injection moulded plates, is proposed. The developed method allows a forecast of FOs for different specimen extraction positions and angles on injection moulded plates by using only a few measured reference points. As a result, fatigue models can be calibrated with the strength values and the corresponding FO, calculated for fracture position. The performed tests show a non-negligible influence of failure positions, due to fibre orientation distributions along the specimens. So, the FO determination method delivers an improvement in strength values estimation.

This work investigates the influence of temperature on the creep damage behaviors of red sandstone. The samples treated at 25 ℃, 200 ℃, 400 ℃ and 600 ℃ are selected to carry out the uniaxial compression and creep experiments. It is found that temperature has obvious influence on uniaxial compressive strength, Young’s modulus and failure modes of red sandstone. It can be also believed that the temperature can degrade the mechanical behaviors of red sandstone. However, as the temperature increases, the damage value does not always increase, the damage variable has a negative value at 25 ~ 400 ℃. It can be found that the higher the temperature, the larger the effect of loading ratio on the ratio of creep strain to instantaneous strain, that is, temperature reduces the ability of red sandstone to resist creep deformation. Acoustic emission (AE) technology has been also used in the loading process of uniaxial compression and creep tests. It is found that the probability density of AE absolute energy of different samples still satisfies the Gutenberg-Richter law well. In uniaxial compression test, as the temperature increases, the absolute value of the critical index increases exponentially. In the uniaxial creep test, as the loading ratio increases, the absolute value of the critical index first decreases and then increases. When the loading ratio is the same, as the temperature increases, the absolute value of the critical index also increases exponentially.

Fatigue behavior of Al-Al and Al-steel refill friction stir spot welding joints Haokun 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

The state function based on fatigue accumulation model has a great influence on accuracy of fatigue reliability analysis of components. A nonlinear fatigue accumulation model considering the interaction of loads is proposed in this paper. By introducing the square ratio of front and back loads as the new load interaction factor into the conventional toughness exhaustion model, thus it is improved to reflect the load sequence and interaction effect simultaneously. Moreover, the fatigue state function based on proposed model is constructed, which is analyzed by the probability density evolution method. The time-varying fatigue reliability curve is obtained by analyzing the generalized density evolution equation. The proposed model are verified using four experimental data, and results shows that the predication residual life fraction such obtained is more accurate. And accuracy and efficiency of the proposed time-varying fatigue reliability analysis method is validated using multi-level loading experimental data.

Using thin-walled cone-pipe welded joints of stainless steels, fatigue tests under bending loads were carried out. The test data were statistically analyzed with the Benard’s approximation, Gaussian, 2P-Weibull, and 3P-Weibull distributions. Stress–life curves at different failure probabilities by a constant strength scatter band model were obtained. The metallographic structures were investigated, and the stress concentration states were analyzed to elucidate the causes of the strengths and scatters. In the high-cycle fatigue regime, the 3P-Weibull distribution was mostly in agreement with the Benard’s approximation, and the coefficient of determination was 0.9642. The microstructure of the weld metal with a high weld opening angle was mainly ferrite phase with 20% austenite distribution. The crack initiation point was close to the weld interface, but the propagation direction was at a right angle, and initially penetrated the heat affected zone of the cone, leading to the high fatigue strength. The stress concentration factors depended on the weld opening angles, indicating the main factor which affected strengths and scatters.

A new perspective of localized shear strain accumulation was proposed to elucidate the formation mechanism of fine granular area (FGA) generated in a high strength steel under very-high-cycle fatigue (VHCF). On the one hand, experiments of VHCF under the negative stress ratio of -1 was carried out, and the microstructure of FGA was found and characterized by using Scanning Electron Microscope, Transmission Electron Microscopy, and Transmission Kikuchi Diffraction. The results show that the FGA consists of high-density dislocations, sub-grains, and fine grains with high angle grain boundaries. On the other hand, the evolution of shear strain and fatigue damage at the vicinity of an inclusion was modelled by using crystal plasticity finite element method at both positive and negative stress ratios. The results show that although the overall strain in VHCF is negligible, significant shear strain is accumulated at the vicinity of the inclusion. Such a large local strain is the driving force for the formation of FGA. The results also suggest that with the accumulation of shear strain and damage, the positive stress ratio is gradually evolved into negative. This may explain why FGA has also been reported at positive stress ratios in some literatures.

Complex civil structures require the cooperation of many building materials. However, it is difficult to accurately monitor and evaluate the inner damage states of various material systems. Based on a convolutional neural network (CNN) and the acoustic emission (AE) time-frequency diagram, we used the transfer learning method for classifying the AE signals of different materials under external loads. The results show the CNN model can accurately classify cracks that come from different materials based on AE signals. The recognition accuracy can reach 90% just by re-training the full connection layer of the pre-trained model, and its accuracy can reach 97% after re-training the top 2 convolutional layers of this model. A realization of cracking source identification mainly depends on the differences in mineral particles in materials. This work highlights the great potential for real-time and quantitative monitoring of the health status of composite civil structures.

This paper describes a microstructure-based multiaxial non-proportional fatigue life prediction model with maximum shear strain and non-proportionality as damage parameters applied to A319 alloy. The materials made with different casting cooling rates and Sr modification are characterized and quantified in terms of secondary dendrite arm spacing (SDAS), size and aspect ratio of eutectic Si particles. Multiaxial non-proportional fatigue tests have been performed on six groups of A319 alloys to systematically analyze the effect of microstructure and loading path on the fatigue properties of Al-Si cast alloy. The first part of the paper is focused on microstructure quantitative characterization to determine the influence of different casting conditions, followed by stress response behavior and fatigue fracture analysis. Finally, quantitative relationship between six fatigue life parameters and microstructure characteristics is established and a new fatigue life prediction model is proposed to predict fatigue life of Al-Si alloy under multiaxial non-proportional loading condition.

This work is an extension of applying a previously developed fracture mechanics cracking damage model to predict the fatigue lifetime of un-notched round specimens made of a ferrite-pearlite 0.4C-70/30 carbon steel in some cases of variable amplitude loading VAL. The model simulates the collective behavior of growing short fatigue cracks originating from the specimen surface roughness. Material grains of different phases, sizes and strengths are randomly distributed over the minimum circumference. Possible activities of surface cracks are predicted against loading cycles. Relevant published experimental data were utilized for comparison. The present predictions are in agreement with the corresponding experimental results.

In this paper a new interpretation and modification of the SWT function in terms of the total damaging energy density is proposed and discussed. The total damaging energy density is the sum of the damaging part of the strain energy density and complementary energy density corresponding to the first quadrant in damaging σD-εD axes. For cyclic loading with positive mean stress (σm≥0) the proposed function reduces to the original SWT formulation. For cyclic loading with negative mean stress (σm<0) the maximum stress is augmented by 1/3 of absolute value of the mean stress. The proposed approach shows a consistent correlation of the mean stress effects for both positive and negative mean stresses.

Gear contact fatigue is becoming a primary limitation for the growing demand of power density and service life in gear-driven equipment. The unchecked surface fatigue crack could further cause premature failure and put a serious risk to the safety and reliability of mechanical systems. In this work, an attempt is made to investigate the effects of rolling-sliding and mild wear on contact fatigue behavior. A comprehensive contact model is developed to capture the variation instantaneous pressure and stress field is calculated with the transient mixed EHL approach. Rolling-sliding contact is simulated with the time-varying roughness topography updated by Archard wear equation. The stress cycles are extracted and the relative contact fatigue life is obtained by using Zaretsky criterion. Results suggest that in rolling-sliding contact the contact fatigue life is obviously lower compared with pure rolling. The increases in the number and amplitude of stress micro-cycles is found to be the main contributors to the reduction of fatigue life. Mild wear tends to smooth the surface, subsequently mitigates the stress concentration and reduces stress cycles, then decrease the risk of surface contact fatigue.

Fatigue cracks initiated from holes in several zones and structural components of the RRJ-95 aircraft frames were investigated. Using the method of quantitative fractography the crack growth duration in the brackets of the in-service airframe and in the wing panels during full-scale bench tests was estimated the spacing of meso-beach-marks (MBM) and fatigue striations. The applied program of bench test consisted of blocks of variable loads that were equivalent to the wing loading in flight and reproducing schematized flight-cycle. It was shown that the duration of fatigue crack propagation in several structural components of the RRJ-95 aircraft frames was approximately the same as for the crack nucleation duration. The total lifetime is sufficiently long for cracks in the structural components to be detected and reliably monitored with a large operating time interval between adjacent inspections.

The influence of maximum principal stress level on true-triaxial unloading behaviors and the failure mechanism of sandstone samples were comprehensively investigated by laboratory tests and discrete element simulations. The results show that the level of σ1 at unloading point significantly affects the deformation and failure characteristics of sandstone samples under true-triaxial unloading conditions. As the level of σ1 at unloading point increases, the ultimate bearing capacity of sandstone sample is increasingly strengthened, while the sample collapses more easily during the unloading process, and the failure mode of sandstone sample changes from mixed tensile-shear failure to shear failure. With the increase in the level of σ1 at unloading point, the accumulative micro-cracks at the unloading point and micro-crack generation rate during the unloading phase exhibit an increasing trend, while the sum of micro-cracks at the unloading phase and the ratio between the amount of tensile micro-cracks and shear micro-cracks roughly show a downward trend. The formation of macro fracture in sandstone sample is closely related to the stress conditions and material inhomogeneity. The tensile fracture in the upper right part of sample when the level of σ1 is relatively low should be attributed to the superiority in tensile contacts between particles in terms of contact number and corresponding tensile force.

The interface fracture toughness of SnSb11Cu6/20steel was measured by calculating the critical energy release rate and stress phase angle of the interface crack. A three-point bending test was used to introduce cracks into the bonding interface, and the cohesion model of the bonding interface was established through experimental data. Through finite element analysis of load-deflection curves with and without interface crack propagation, the crack initiation point is found. Then the energy calculation model of crack propagation is established, and the critical energy release rate is obtained using the virtual crack growth criterion. The calculation results of the stress phase angle show that the crack propagation is greatly affected by the normal stress after the babbitt alloy layer fractures. If the strength of the substrate material is weaker, the crack will continue to expand in the tangent perpendicular to the crack tip.

Abstract: In order to study the dynamic fracture properties of FRP strengthened concrete, the three-point bending experiment with different initial crack-depth ratios was carried out under dynamic loading. The effects of crack-depth ratios on the fracture parameters were analyzed. Combined with Pearson Correlation Coefficient formula, the correlation between crack-depth ratios and fracture parameters was quantitatively verified. The conclusions could be drawn that, there are three critical points in the damage process: the crack initiation point, first peak point and ultimate bearing capacity point. With the increase of crack-depth ratio, the crack initiation load decreases, but the first peak load and ultimate load increase first and then decrease. The stress intensity factors of three points also increase first and then decrease. The increment of critical crack-depth ratio and flexibility coefficient are related to crack-depth ratio, which could reflect the effect of crack-depth ratio on the deformation capacity of FRP strengthened concrete.

Fatigue behaviour of HIPed and stress relieved Ti6Al4V alloy specimen’s produced by SLM was analysed (Rε = -1). The HIP process caused a microstructural transformation decreasing the hardness and monotonic properties that not allowed fatigue strength increase. A bilinear behaviour in the elastic strain-fatigue life curve was observed, because of Young´s modulus decrease during the cyclic elasto-plastic tests consequence of subgrains formation. The Smith-Watson-Topper and total strain energy density models showed a good concordance between predicted and experimental fatigue lives in notched samples.

The effect of pneumatic shot peening (PSP) on the fatigue properties of K403 turbine blades has been investigated under the high and low cycle combined fatigue (CCF) load with two types of blades: untreated blades and PSP treated blades. It is found that there is a threshold vibration stress which should be 194MPa. The PSP has a positive effect on the CCF life of blades mainly due to the compressive residual stress resulting in the reduction of the number of crack sources and propagation rate when vibration stresses are below the threshold vibration stress. However, the PSP treatment has no or negative effect when vibration stresses are above the threshold value. The compressive residual stress is released along with the microstructure changes of K403. Meanwhile, the microstructure changes, reflected in the precipitation of the lamellar MC carbides and σ phases, can accelerate the process of crack initiation and propagation.