Crack Propagation Life Prediction Research Articles

Very high cycle fatigue behavior of TC4 titanium alloy: Faceting cracking mechanism and life prediction based on dislocation characterization

This research analyzes the very-high-cycle-fatigue behavior of TC4 titanium alloy through fatigue tests at R = −1, −0.3, and 0.1. The results show that the S-N curves are all bilinear and exhibit three failure modes as surface slip failure, surface cleavage failure and interior cleavage failure. Transmission electron microscopy analysis reveals the dislocation structure in interior cleavage failure and suggests that the deformation mechanism of faceting cracking involves both anti-phase boundary shearing and stacking fault shearing mechanisms. It concludes that interior failure results from cleavage fracture of α grains due to dislocation slip. Based on the stress intensity factor of the maximum defect, a slip-cleavage competitive failure model was developed by considering factors such as control volume, defect size, external loading, and grain content, with good predictive results. Additionally, on the basis of the failure mechanism and crack propagation rate model, considering the coupled effects of crack tip blunting, stress ratio, Vickers hardness, and material fracture toughness on crack propagation, the crack propagation life prediction model is constructed. The life prediction model is further modified to be more conservative and accurate in predicting life by consideration the maximum defect size, providing important theoretical support and practical guidance for engineering applications.

Effect of Stress Ratio and Evaluation of Crack Sizes on Very-High-Cycle-Fatigue Crack Propagation Life Prediction of Carburized Cr-Ni Steel

Carburized Cr-Ni steel is widely used in the manufacture of components in many fields due to excellent performance, of which the service life has been a concern. In order to investigate the high-cycle-fatigue and very-high-cycle-fatigue properties of carburized Cr-Ni gear steel, axial loading fatigue tests were performed by QBG-100 with stress ratios of −1, 0 and 0.3. The Generalized Pareto distribution was used to evaluate the inclusion size of carburized Cr-Ni gear steel. Based on the stress ratio and the evaluated crack size, a new fatigue life prediction model for carburized Cr-Ni gear steels was constructed. The results show that the S–N characteristics of carburized Cr-Ni gear steel represent the continuously descending tendency. Based on the long crack propagation threshold and the instability propagation threshold of carburized Cr-Ni gear steel, the sizes of FGA, fisheye and surface smooth area (SSA) can be evaluated, respectively. In addition, the maximum size of surface and interior inclusion of carburized Cr-Ni gear steel are 17.50 μm and 6.46 μm with a cumulative probability of 99.9%. By validating the new established model, the prediction result is acceptable according to the good consistency between the predicted life and the experimental life.

Crack Propagation Life Prediction of a Single Lap Shear Joint: A Linear Elastic Fracture Mechanics Based Machine Learning Approach

Estimation of fatigue life in the aerospace industry plays a key role in terms of safety, certification, and inspection. Individual aircraft tracking programs aim to calculate the fatigue index for each aircraft based on its load history. Therefore, accurate estimation of fatigue life based on unique load spectra for each aircraft plays a crucial role. The purpose of this study is to predict the crack growth (CG) life of a single lap shear joint for an aircraft based on different spectral loads using random forest regression and k-nearest neighbors(k-NN) regression. A finite element model was built using the Huth fastener flexibility method to obtain fastener loads and stress values on the structure under maneuver loads. Then, finite element results were compared to analytical calculations. Based on Fighter Aircraft Loading Standard for Fatigue and Fracture (FALSTAFF) spectra, 90 different spectra were developed to be used for the calculation of CG life. The AFGROW software was used to calculate the CG life based on the load spectra and the findings of the finite element model. Analytical calculations of CG lives and features of individual load spectrums were fed to machine learning models. Finally, the CG life predictions of the machine learning models were compared with analytical calculations. According to the findings, a good correlation was observed between the analytical and predicted CG lives.

The whole fatigue crack propagation life prediction of Ni-based single crystal super-alloy specimen with single hole based on EIFS and ‘Fish-eye’ theory

In this work, based on the ‘Fish-eye’ theory and EIFS method, a prediction model of the whole fatigue crack propagation life of the predictable structure under multi-axial stress fatigue is proposed. Then, the fatigue crack propagation life of single-hole nickel-based single crystal superalloy samples under nine working conditions was predicted and verified by finite element simulation. Finally, the effects of three loads and three initial crack lengths on the fatigue crack propagation life of single-hole nickel-based single crystal superalloy were discussed by finite element simulation. The results show that the analysis results of the theoretical model and the finite element model proposed in this work are basically consistent under certain conditions. It is found that the fatigue crack propagation life of the initial crack stage, short crack stage and long crack stage accounts for 2%, 6%–10% and 90% of the whole crack propagation fatigue crack propagation life, respectively. Under the same loading stress condition, the higher the proportion of short crack stage in crack propagation is, the faster the fracture speed of the sample is. At the same time, the progressive crack propagation process of single hole specimen under fatigue load can be seen through the calculation results of finite element model. The above conclusions provide a good theoretical reference for the design and fatigue crack propagation life prediction of nickel-based single crystal superalloy turbine blades with cooling film holes.

Structural fatigue investigation of thermite welded rail joints considering weld-induced residual stress and stress relaxation by cyclic load

This study developed a structural fatigue life prediction model of thermite welded rail joints taking account of the weld-induced residual stress and stress relaxation by external cyclic load. A three-dimensional finite element model including the sequentially coupled thermal–mechanical simulation and cyclic plasticity constitutive simulation was developed to investigate the welding process induced residual stress and the redistribution of the residual stresses due to cyclic bend fatigue loading, respectively. The numerical results of the finite element model were compared with experimental residual stress data to confirm the accuracy of the model developed. The fatigue crack propagation life prediction taking into account the as-welded residual stress and the stress redistribution was performed. The predicted life of weld-toe cracks at web-to-base location will be overestimated by at least 50% if residual stress and relaxation is not considered.

Tunnel boring machine cutterhead crack propagation life prediction with time integration method

Fatigue damage is one of the most common failure modes of large-scale engineering equipment, especially the full-face tunnel boring machine with characteristics of a thick plate structure bearing strong impact load. It is difficult to predict the location and propagation life of crack of cutterhead under strong impact load. Unseasonal maintenance of equipment caused by inaccurate prediction of life cycle of cutterhead seriously affects the construction efficiency of the equipment and the life safety of the operators. Determining the crack location of tunnel boring machine cutterhead structure under strong impact load and predicting the crack propagation life are difficult scientific problems. To solve them, first, the location of the stress concentration of the cutterhead is determined by using finite element analysis method of statics. Second, prediction model for crack propagation life of tunnel boring machine cutterhead characteristic substructure based on time integration is built. And the test of crack growth of cutterhead characteristic substructure is performed. The feasibility and accuracy of the prediction model are verified by contrasting crack prediction models and the results of the test. Finally, the life prediction of tunnel boring machine cutterhead of water diversion project in Northwest Liaoning Province is carried out by using crack propagation model based on time integration. Results show that the maximum error of theoretical prediction and experimental results of crack propagation is 16%. So the feasibility of crack propagation model based on time integration in predicting the crack growth of cutterhead is verified. It is predicted that the tunnel boring machine cutterhead panel can work normally for 5.9 km under the condition of ultimate load. Building the crack propagation model considering the influence of plate thickness and strong impact load has important research value for improving the working efficiency of engineering equipment, prolonging service time, and improving the working safety.

Prediction of Crack Propagation Life of Cast Steel Brake Disc of EMU

Prediction of Crack Propagation Life of Cast Steel Brake Disc of EMU

Study of TBM cutterhead fatigue damage mechanisms based on a segmented comprehensive failure criterion

The cutterhead endures strong, multi-point distributed impact loads when the full face rock tunnel boring machine (TBM) tunnels, which leads to intense vibration and fatigue fracture. Accordingly the structure should meet the requirement of service life considering the fracture failure at weak locations. Hence, cutterhead damage mechanism is crucial to predict and analyze residual life of a operational cutterhead under impact loads. This paper combines with the characteristics of cutterhead life, Paris model and damage cumulative theory, proposes a new failure criterion for the long thick plate in complex stress states, and establishes a segmented crack propagation life prediction model, based on the existing multi-degree-of-freedom coupling dynamic TBM cutterhead model and fatigue crack propagation prediction method for large and complex structures. Meanwhile, a physical fatigue damage test of cutterhead specimens is carried out to validate the theoretical model. Then a TBM cutterhead in a water tunnel project is taken as an example, to calculate the fatigue crack propagation and analyze how different parameters affect fatigue life. The results of the analyses reveal that with fatigue crack propagation, life of actual cutterhead is 36.5km, which gives a meaningful reference for cutterhead design. In addition, the cutterhead life decreases nearly 45.2% with the initial crack depth increasing from 0.5mm to 2mm, and it is inferred that the value of initial crack depth should be controlled below 1.6mm, to ensure the service life. On the other side, the larger the crack shape ratio (a/c), the longer the fatigue life, and the cutterhead life with a round crack (a/c=1) is about 3 times with a narrow crack (a/c=0.1). Furthermore, the crack growth rate parameter m has greater influence on cutterhead life than parameter C, with C increasing 10%, the life decreases about 9%; however, the life decreases nearly 13% when m increases 1%. Then, the critical threshold values of cutterhead material parameters to meet life requirement are obtained. From the results of fatigue damage test, it can be showed that the specimens fracture quickly when the crack depth exceeds 50% of the panel thickness, and the crack shape ratio gradually increases with the loading cycles, which is consistent with the theoretical assumption, as well as the influence rules of parameters.

Study of TBM cutterhead fatigue crack propagation life based on multi-degree of freedom coupling system dynamics

Cutterhead is the core component of TBM tunneling equipment, which endures strong, multi-point distributed impact loads when the TBM tunnels, owing to the extreme surrounding rock environment of high hardness, high temperature and high quartz content. For this reason, the cutterhead works in an extremely severe vibration environment, which leads to engineering fault by a large area crack damage before the service life. Hence, the study on life prediction of TBM cutterhead under the impact loads is a core part of cutterhead design. This paper combines with the technology of system dynamics, linear elastic fracture mechanics and cumulative theory of fatigue damage, for the first time, proposes a method of fatigue crack propagation life prediction for the large and complex structures. In this paper, the TBM cutterhead of an actual project is taken as an example, to predict the fatigue crack propagation life of cutterhead piece and analyze the influences of plate thicknesses on fatigue life, then a new improved scheme of cutterhead structure is presented. The results show that the fatigue crack propagation life of actual cutterhead is 26.6km, which is able to meet the requirement of 20km service life. Moreover, the upper cover plate thickness has the greatest influence on cutterhead fatigue crack propagation life, with the thickness increasing 10%, the life increases nearly by 1.24 times. Then, the other influencing factors are as follows: thickness of the main support plate, thickness of the annular support plate and thickness of the support plate, whereas the influence of the lower cover plate thickness on fatigue life is minimal. Furthermore, the plate thickness limit sizes meeting the life requirement are obtained, and a new structure modified scheme of cutterhead is proposed. Compared with the original scheme, the new cutterhead scheme meets the requirements of structural strength and service life with 8.08% weight decrease, which achieves life determination design and lightweight design. The proposed method of fatigue crack propagation life prediction is feasible in the design and application stage of TBM cutterhead, besides, it is flexible enough and can also be applied in damage strength assessment, dynamic parameters optimization and establishment of nondestructive inspection cycle for the other large and complex structure, and takes on stronger project value and generality.

SIFs of CCT plate repaired with single‐sided composite patch

ABSTRACTConstant amplitude load fatigue tests are performed to obtain crack propagation data for LF2‐aluminium centre crack tension (CCT) plates un‐repaired and repaired with single‐sided composite patches. Then, the James–Anderson method, an experimental method, is used to obtain the stress intensity factor (SIF) formula for the repaired CCT plates with carbon–fibre composite patches. At last, crack propagation life prediction and finite element (FE) calculation are carried out to validate the experimental SIF formula. It is shown that the present SIF formula can exactly predict the fatigue‐crack propagation life of the patched CCT plates and is close to the FE results, which implies the effectivity of the experimental SIF formula in the present paper.

A method of fatigue crack propagation life prediction of weld joints under combined stresses

A method of fatigue crack propagation life prediction of weld joints under combined stresses

Crack propagation life prediction of a perforated plate under thermal fatigue

Superheater outlet headers of boilers are well known to be subjected to the cycling of high pressure and high thermal stress during plant operations. Thermal stresses during cyclic operation are generally severest on the inside surface of the ligaments between the stub-tube holes, where many examples of ligament cracking due to thermal fatigue have been found recently. A method to predict the crack propagation life of the ligaments of boiler headers under thermal fatigue has been required. Firstly in this paper, to model the crack propagation behavior of the ligament regions of boiler headers, a perforated plate of normalized and tempered 2 1/4Cr–1Mo steel was examined under out-of-phase thermal fatigue at a maximum temperature of 600°C in the air. Inelastic analysis of the perforated plate under thermal fatigue was carried out, and the nonlinear fracture mechanics parameters such as the J and C ∗ integral were obtained by the line integral for observed cracks. A simplified method was proposed for predicting these parameters under displacement-controlled conditions such as thermal fatigue. In this method, the change of the macroscopic stress–strain relation of the perforated plate with propagating cracks was combined with the reference stress concept under displacement-controlled conditions. The predicted fracture mechanics parameters from this method coincided well with those from the inelastic analysis. The prediction of the crack propagation life on the basis of the proposed method provided a good correspondence with the test results of the perforated plate under thermal fatigue.

Crack propagation life prediction for Ti-6Al-4V based on striation spacing measurements

A combined experimental and numerical study was conducted on forged Ti-6Al-4V titanium alloy representative of a turbine engine fan blade material. The experimental information on striation spacing in the given material was converted into d a/d N–Δ K data and further extrapolated towards lower crack growth rates assuming a linear relationship between d a/d N and Δ K when plotted in log–log coordinates. The NASA Crack Analysis Code was used with several different sets of the crack growth rate input to predict the number of load cycles required to propagate a semi-circular surface crack from a certain initial size to final fracture in a round bar of nominal diameter 5 mm. For an initial flaw size of 10 to 50 μm, predictions based on the d a/d N–Δ K test results from both compact tension and surface flaw specimens greatly overestimated the fatigue crack propagation life. On the contrary, very good agreement with available experimental data has been shown to exist for predictions based on extrapolated striation spacing measurements.

Short crack growth and life prediction in low-cycle fatigue of smooth specimens

Growth of short fatigue cracks on the surface of smooth specimen was observed in low-cycle fatigue tests of medium-carbon steels, copper and Al-alloy. When growth rate of surface cracks was correlated to J integral range, the data for cracks longer than a few times grain-size and without coalescence growth coincided with the relation obtained for large through-thickness cracks. The other data shifted toward higher rate region than the relation. According to fractographic observation, the deviation in shorter cracks appeared to be ascribed to a change in crack growth mode. On the basis of the observation of crack growth, especially, the prediction of crack propagation life was discussed by introducing an energy density parameter related to J integral range. The prediction was further modified in consideration of crack coalescence, and then it gave a good estimate for each material. The effect of grain-size on fatigue life in medium-carbon steel was also discussed from the viewpoint of crack growth behavior.