Higher Crack Growth Rates Research Articles

Effect of Hydrogen on High Cycle Fatigue Properties of L360 Pipeline Steel Notched Specimens.

The fatigue characteristics of notched specimens of L360 pipeline steel in hydrogen and nitrogen environments were investigated by high cycle fatigue life tests and fatigue crack growth rate tests. The fracture morphology in the nitrogen environment was dominated by microcracks and fatigue strips. The fatigue fracture had distinctly different regions in the hydrogen environment. The outer region of the fracture in the hydrogen environment was similar to the nitrogen environment, but a large number of hydrogen embrittlement features were found in the inner region. The fatigue crack growth rate tests were analyzed in conjunction with fatigue life tests. It was found that more fatigue cycles were required to achieve the stress intensity factor ΔK for rapid hydrogen-promoted crack propagation at lower stress. The region with hydrogen embrittlement features increases with decreasing stress.

Insights into fatigue crack propagation behaviour of WAAM produced SS316 stainless steel: Molecular dynamics simulation and experimental analysis

The current work focuses on the crack growth mechanism / behavior of wire arc additively manufactured SS316L. Beside the experimental investigation, a molecular dynamics based polycrystalline model is developed to simulate the crack propagation under cyclic loading. The purpose of this work is to understand the crack growth behavior of wire arc additively manufactured SS316L, particularly the directional dependence of crack propagation. The experiments and molecular dynamics analysis are performed for the crack propagation along the build and traverse direction for the SS316L deposit. The experimental procedure involves fatigue crack growth rate tests, while the molecular dynamics simulations model the crack propagation in polycrystalline structures of different grain sizes. The ultimate tensile strength in build and traverse direction were 535±2 MPa and 495 ± 2 MPa, respectively. The experimental results reveal faster crack propagation along build direction in comparison to traverse direction. The values for Paris law material constants i.e. C and m are 7x10-15 and 6.78 along build direction while 8x10-14 and 6.02 along traverse direction, respectively. The crack growth along the build direction is primarily intergranular, which leads to high crack growth rate. The molecular dynamics simulations reveal the initiation and closure of secondary cracks for the traverse direction, which leads to slower crack growth rate along the traverse direction. The fractography of the fractured surfaces from fatigue crack growth rate test also indicate the formation of secondary cracks for the case of traverse direction. The molecular dynamics simulations performed with three grain sizes and thickness variation indicate the insensitivity of predicted crack growth mechanisms with respect to grain sizes used for simulations.

Guided analysis of fracture toughness and hydrogen-induced embrittlement crack growth rate in quenched-and-tempered steels using machine learning

This study focuses on developing a machine learning (ML) model, specifically a Bayesian-optimized deep neural network, leveraging numerical simulation data for the prediction of fracture toughness (KIC) and crack growth rate (CGR) in Quenched-and-Tempered Steel (AISI 4140 alloy) under hydrogen embrittlement conditions. The proposed model demonstrated superior accuracy with Root Mean Squared Error (RMSE) values of 0.052 for KIC and 0.084 for CGR, surpassing conventional ML models including random forest (0.094 for KIC, 0.139 for CGR), gradient boosting (0.184 for KIC, 0.196 for CGR), support vector regression (0.142 for KIC, 0.110 for CGR), and decision tree regression (0.119 for KIC, 0.133 for CGR). The findings also revealed that input parameters such as temperature, hydrogen concentration, and hydrostatic stress carry higher weight factors compared to other parameters in predicting KIC. Similarly, the prediction of high CGR values, ranging from 5.9 × 10−5 to 9.5 × 10−5 m/s, was associated with the importance of hydrostatic stress, strain rate, and initial crack length in the prediction model. This underscores the model's ability to capture the intricate dependencies of output objectives on input features. Furthermore, a comprehensive case study, informed by the ML model, highlights the potential for tuning specific processing input parameters to manage hydrogen embrittlement effectively, contributing to a deeper understanding of its dynamics.

Effect of Mg on Inclusion and High Cycle Fatigue Behavior in Titanium Microalloyed Beam Steel

In this paper, the fatigue behavior of titanium microalloyed beam steels were studied by high cycle fatigue test and fatigue crack growth rate test. The effect of Mg addition on the fatigue behavior in titanium microalloyed beam steel was systematically analyzed. According to the experimental results, the addition of magnesium can effectively modify the inclusions by reducing the size of Al2O3 and TiN and promoting the formation of finer complex inclusions with a MgO·Al2O3 core in titanium microalloyed high-strength beam steel. The number of inclusions in the experimental steels had far less of an impact on the fatigue characteristic than inclusion size. With the heterogeneous nucleation effect of MgO·Al2O3, the inclusions are refined after the Mg addition. The tensile strength of Beam-2 steel decreased by approximately 54 MPa, while its fatigue strength increased by about 33 MPa, showing favorable fatigue resistance. These findings are essential for optimize the fatigue properties of titanium microalloy steel and promoting the development of automobile beam steel with excellent fatigue properties.

Predicting fatigue crack growth through the small and long crack regimes for a military transport aircraft loading spectrum using FASTRAN

Predicting fatigue crack growth across the small and long crack regimes in airframe components is a challenging task. Current fracture mechanics methods require, amongst other things, high fidelity small crack growth rate data and a thorough understanding of the different behaviours of small and long cracks. This paper presents a case study where a fatigue crack growth prediction of Aluminium Alloy 7075-T7351 coupons tested to a military transport aircraft loading spectrum, from nucleation to failure, is attempted using the most recent version of the linear-elastic fracture mechanics-based analysis tool FASTRAN v5.76. The military transport aircraft loading spectrum was applied to different coupon configurations to show the effectiveness of FASTRAN for a range of crack growth scenarios. Crack growth rate data for the prediction in the near-threshold region was experimentally obtained from small cracks using quantitative fractography and this data is shown to be a determining factor in the accuracy of the prediction. This paper highlights the challenges faced by engineers in predicting fatigue crack growth under variable amplitude loading from nucleation to failure and demonstrates the ability of FASTRAN to accurately predict load history effects. Areas for continued improvement are identified with the aim of accurate and reliable fatigue crack growth prediction in a range of structures, conditions, and scenarios.

Experimental studies of fracture and fatigue crack growth in dissimilar metal welded joints of nickel-based superalloys: Cracks oriented along the weld

Nickel-based superalloys, Inconel 718 and Nimonic 80A, were welded without any interlayer for Blisk manufacturing in aerospace industries. Fracture toughness and fatigue crack growth rate of the dissimilar metal welded joints were experimentally studied to confirm their structural integrity. Cracks were oriented in and along the weld. Base metals, Inconel 718, and Nimonic 80A differ in yield strength but have the same modulus of elasticity. Electron beam welding was used to butt weld Inconel 718 to Nimonic 80A. J resistance curve ( J vs. crack extension) was experimentally found to determine the elastic–plastic fracture toughness. The welded specimens showed lesser fracture toughness and higher crack growth rates than base metals. An increase in load ratio resulted in an increase in fatigue crack growth rate in the high Δ K region and a decrease in Δ Kth. Fractography studies confirmed linear elastic and plane strain fracture all along the fatigue crack growth rate path except for the final fracture with plane stress.

Characterization of Fatigue Crack Growth Based on Acoustic Emission Multi-Parameter Analysis.

In engineering structures that are subject to cyclic loading, monitoring and assessing fatigue crack growth (FCG) plays a crucial role in ensuring reliability. In this study, the acoustic emission (AE) technique was used to monitor the FCG behavior of 2.25Cr1Mo0.25V steel in real-time. Specifically, an AE multi-parameter analysis was conducted to qualitatively assess the crack growth condition and quantitatively correlate the crack growth rate with AE. Various AE parameters were extracted from AE signals, and the performances of different AE parameters were analyzed and discussed. The results demonstrated that four stages of FCG, which correspond to macrocrack initiation, stable crack growth with low crack growth rate, stable crack growth with high crack growth rate, and unstable crack growth, are distinctly identified by several AE time domain parameters. The sudden and continuous occurrence of many AE signals with high count (>100) and high energy (>40 mV·ms) can provide early and effective warning signs for accelerated crack growth before final failure occurs. Moreover, linear correlations between crack growth rate and different AE parameters are established for quantifying crack growth. Based on the AE multi-parameter analysis, it was found that the count, energy, and kurtosis are superior AE parameters for both qualitatively and quantitatively characterizing the FCG in 2.25Cr1Mo0.25V steel. Results from this research provide an AE strategy based on multi-parameter analysis for effective monitoring and assessment of FCG in engineering materials.

Effect of Mechanical Properties of Rail and Wheel on Wear and Rolling Contact Fatigue

Rolling contact fatigue (RCF) and wear are important problems for the wheel and rail. RCF and wear is caused by contact stress and the slip ratio between the wheel and the rail. The material properties of the wheel and rail are an important factor to prevent the degradation caused by RCF and wear. In this study, the mechanical properties and fatigue characteristics of the two types of wheel and rail were evaluated, and the effects on wear and contact fatigue were examined. We found that the crack growth rate and the hardness were important factors in the contact fatigue and the wear. The rail steel with a higher crack growth rate and hardness had a low resistance to contact fatigue with large size damage. The hardness ratio and the total hardness are important factors in evaluating the wear resistance. In addition, we found that the residual stress increased proportionally to the maximum shear stress.

Optimization scheme of the precision separation process of the notched bar based on the acoustic emission hit energy angle

Low-cycle fatigue cropping is a new precision separation method of metal bars with prefabricated notch effectively solves the problem of high active load and much material waste that occurs in the traditional separation process. However, there is still a difficult contradiction, that is, the high loading frequency improves the efficiency, but will reduce the quality of cross-section. The question of how to balance the loading frequency and the quality of cross-section has become an urgent problem. In this study, a new acoustic emission (AE) parameter, AE hit energy angle (AEHEA), is proposed to solve this problem. From the AE signal vitiate law in a single loading cycle, it can be seen that the interaction of high loading frequency and high crack growth rate leads to the formation of a dislocation obstacle zone, which causes the cracks originally extending in the radial direction to extend in the circumferential direction in the second half of single loading, resulting in a wider spectrum of acoustic emission sources and generating AEHEA. The results of the statistical analysis of the height variance of the cross-section show that AEHEA can well reflect the influence of the loading frequency on the cross-section quality. When AEHEA occurs, the cross-section quality decreases significantly, and the earlier AEHEA occurs, the greater the poor cross-section area. Therefore, the key to controlling the loading strategy is to suppress the occurrence of AEHEA. Before AEHEA occurs, the loading frequency should be as high as possible to improve the efficiency of cropping, and before AEHEA occurs, the loading frequency should be reduced to suppress the occurrence of AEHEA. Based on this method, three loading control strategies are developed and compared to find a better loading control strategy that balances the efficiency and quality of the cross-section.

Microstructural and load hold effects on small fatigue crack growth in α+β dual phase Ti alloys

Microstructurally small crack growth was monitored in Ti-6Al-4V and Ti-6Al-2Sn-4Zr-2Mo with equiaxed and bi-modal microstructures. The influence of the microstructure was assessed to obtain an improved understanding of the lifetime variability observed in Ti alloys. Primary α grains, basal plane cracking and misalignment across boundaries were identified as key features for high crack growth rates. The origin of life debits encountered under dwell-fatigue loadings was also investigated through crack growth monitoring with the introduction of load holds at peaks stress. Dwell periods were found to induce a substantial small crack acceleration.

Study of the low cycle fatigue of 12Cr18Ni10Ti steel based on fractal analysis and artificial intelligence approaches

The evolution of the structure and assessment of the age limit of steel 12Cr18Ni10Ti upon fatigue loading is considered using neural network modeling and approaches of fractal analysis of the microstructure. An algorithm for processing images of the microstructures has been developed to improve their quality. An indicator of the fractal dimension of the image is used as a quantitative indicator for assessing the evolution of the microstructure of the surface metal layer. A quantitative assessment of the structures at different stress amplitudes is carried out in a wide range of low temperatures using the fractal dimension index. Correlation of the fractal dimension index with the run of the sample material is shown. The appearance of the main crack was observed in the range of 0.7 - 0.8 from the number of cycles to failure, after which the crack growth rate increased. At a lower temperature, the main crack is formed later, but further loading results in a higher crack growth rate. Formation of the secondary phases in austenitic steel at a lower temperature occurred at earlier stages than that at a temperature of t = +20°C, which led to hardening of the material. An artificial neural network (ANN) has been developed and trained for assessing structural changes in metal proceeding from the fractal dimensionality of the microstructure images at different stages of fatigue loading. The developed neural network made it possible to estimate with a sufficiently high accuracy the number of cycles before damage of the sample and the residual life of the material. Thus, the developed ANN can be used to assess the current state of the material in a wide range of low temperatures.

Acoustic Emission Testing and Ib-Value Analysis of Ultraviolet Light-Irradiated Fiber Composites

The failure behavior of composites under ultraviolet (UV) irradiation was investigated by acoustic emission (AE) testing and Ib-value analysis. AE signals were acquired from woven glass fiber/epoxy specimens tested under tensile load. Cracks initiated earlier in UV-irradiated specimens, with a higher crack growth rate in comparison to the pristine specimen. In the UV-degraded specimen, a serrated fracture surface appeared due to surface hardening and damaged interfaces. All specimens displayed a linearly decreasing trend in Ib-values with an increasing irradiation time, reaching the same value at final failure even when the starting values were different.

A Study of Fatigue Crack Growth Rate in Steels in Relation to Crack-Tip Plastic Deformation and Fracture. Part 2. Parametric Relation between Fatigue Crack Growth and Plastic Strains

The first part of this study presented low-cycle fatigue and cyclic fracture toughness test methods for 10GN2MFA steel at 20 and 270°C, experimental results to construct kinetic fatigue fracture diagrams and to determine low-cycle plastic strain and fracture characteristics. Here, those results are estimated as to the application of cyclic plasticity to describe the fatigue crack growth rate. The cyclic strain-hardening coefficient and the Paris–Erdogan equation exponent for ferrite-pearlite steels are shown, to be closely related as well as the intersection point of the diagram in the range of high crack growth rates was defined. The parameters of the above relation and coordinates of the intersection point were determined for the steels of this class. In the latter case, the optimization methods were used to find the minimum sum of squares of residual functions. Based on the correlation between the cyclic strain-hardening coefficient and Paris–Erdogan equation exponent, the conclusion was drawn of the lower boundary value of this characteristic predicted for an ideal elastoplastic material (zero cyclic strain-hardening coefficient), to be equal to 1.0. As also follows, other classes of the materials (and other cycle asymmetries) can exhibit similar relations in the form of rays coming from the point (0; 1.0) at a corresponding experimental slope to the coordinate axes. From the common intersection point of the diagram (Gurney point) and their slopes to the axes for ferrite-pearlite steels, the procedure was advanced to construct the linear section of the diagram based on the results of low-cycle fatigue tests with plotting the stress amplitude vs. plastic strain amplitude curve. The procedure was approved on 10GN2MFA steel at 20 and 270°C and SAE-1020 and API5L X60 steels taken from the literature. In view of data scattering for cyclic fracture toughness test results, the estimation of the linear diagram sections is quite adequate, and the procedure of their construction is appropriate and can be extended to other classes of the materials and stress ratios.

Influence of Waveforms on Fatigue Crack Growth Characteristics of Tire Tread Rubber using Finite Element Analysis

ABSTRACT Rubber products are typically subjected to cyclic fatigue loading in service. During prolonged exposure to cyclic loading, damages initiate at intrinsic defect sites at microscopic levels and subsequently propagate, leading to catastrophic failure. Therefore, the material that offers better resistance to fatigue crack growth (FCG) is suitable for a durable product. FCG characteristics of rubber compounds depend on many factors, such as constituent material (rubber, filler, etc.), environment, and operational conditions (loading amplitude, loading pattern, etc.). To simulate the realistic service condition of a product, the choice of loading pattern is a key factor and has emerged as a very important research topic in recent times. The present work focuses on the effect of loading pattern on FCG characteristics of tire tread rubber compounds. In the present study, FCG characteristics of a 100% natural rubber (NR) compound were measured on a tear and fatigue analyzer (TFA, Coesfeld, Germany) using double edge notched pure shear specimen. Fatigue loading was applied using sinusoidal and pulse waveforms over a wide range of tearing energy levels. Pulse mode recorded a very high crack growth rate (∼2 times) compared to sine mode at equivalent peak energy levels. In order to understand the mechanics of the higher crack growth rate in pulse mode, finite element analysis (FEA) of a pure shear specimen was performed wherein FCG experimental conditions were used as boundary conditions. FE analyses were carried out using both linear and nonlinear viscoelastic material models. Nonlinear viscoelastic FEA results revealed that viscous energy dissipation at the crack tip is much lower in the case of pulse mode, which is in support of higher the FCG rate in pulse mode as observed in the experiments.

Effect of deformation level and orientation on SCC of 316L stainless steel in simulated light water environments

The stress corrosion cracking (SCC) behavior of 316L stainless steel (SS) forged to 0%, 12.6%, 21.4%, and 39.8% reduction in thickness was investigated at constant K in light water reactor environments. The yield strength, specimen orientation, and water chemistry were correlated with crack growth rate, and their dependencies are discussed. The crack growth rate (CGR) of 316L SS increased monotonously with yield strength irrespective of the specimen orientation or water chemistry. Higher CGRs were observed when cracks propagate along the plane parallel to forging plane than normal to forging plane. The effect of local deformation on the anisotropic cracking behavior for different orientations, crack paths and CGRs are also discussed.

Role of Impurity Sulphur in the Ductility Trough of Austenitic Iron-Nickel Alloys.

The role of impurity sulphur in the ductility trough of iron–nickel (Fe–Ni) alloys is investigated using hot tensile tests. A strong detrimental effect of some ppm levels of sulphur is demonstrated. In addition, it is shown that, in the ductility trough, material failure occurs through subcritical grain boundary crack propagation, involving dynamic embrittlement at the crack tip, due to the sulphur. Very high intergranular crack growth rates are observed. This is possible because plastic deformation accelerates the transport of sulphur to the crack tip, by several orders of magnitude, compared to normal bulk diffusion. The ductility is recovered at high strain rates, which correlates with a decrease in the sulphur concentration measured on the fracture surface. It is suggested that the main mechanism of sulphur transport is dragging by moving dislocations.

Stress intensity factors KI, KII, KIII, Keq, induced at the crack tip of CT specimens subjected to torsional loading

The paper herein presented shows the Stress Intensity Factors (SIF), KI, KII, KIII, and Keq, calculated for Compact Tension (CT) specimens with different thicknesses (B), namely 2.5 mm; 3 mm; 5 mm, 7.5 mm, and 10 mm, that were subjected to three torsional loads (T): 6 N.m; 7.5 N.m; 9 N.m. Fatigue pre-crack subjected to torsional loading was found to be predominantly under Mode-II loading, and a crack branch was observed.The CT specimens, which were analysed using the Finite Element Method (FEM), were then modelled with two cracks that had grown experimentally from a fatigue pre-crack along two directions (+70º and -70º), and for several crack lengths (a/L=0; a/L=0.25; a/L=0.50; a/L=0.75; a/L=1.0). Therefore, equivalent SIF, Keq, were calculated from the numerical results of KI, KII, ans KIII, and a polynomial regression function was determined in function of the thickness of the specimen (B), the crack length (a/L) and the applied torque (T). It was observed that even in the case of a Mode-III loading is applied, Mode I was locally dominant at the branched crack tip, followed by Mode III and Mode II, in that order of relevance. It was also observed that the maximum equivalent SIF occurred at the two outer lateral surfaces of the CT specimens under test, resulting in high crack growth rates in those locations, and minimum at the midplane of the specimen.

A coupled diffusion and cohesive zone model for intergranular stress corrosion cracking in 316L stainless steel exposed to cold work in primary water conditions

A multi-physics model was developed to simulate intergranular stress corrosion cracking (IGSCC) in austenitic stainless steel. The model is implicit, coupled with a segregated solution scheme including a diffusion equation based on Fick’s second law and a cohesive zone description for the fracture mechanics part. The degradation is modelled with an anodic slip-dissolution equation that uses the effective cohesive traction and concentration as the main parameters. The diffusivity in Fick’s second law creates a moving boundary. The cohesive zone is modelled using the PPR model with extended degradation properties using the degradation parameter χ. The model was evaluated against experiments on the effects of cold work on IGSCC. The model showed good agreements for both shifting amount of cold work, illustrated by only changing the yield stress in the bulk material and for shifting the stress intensity factor. The model versatility was also shown by simulating IGSCC in Alloy 600, also with good agreements. The change in the bulk material made crack propagation more disadvantageous for the lower yield stress where the crack blunts, creates more plastic strain and lowers the cohesive traction. The model predicts that cold work of the bulk material creates a faster crack growth velocity due to lower amount of plastic deformation in the bulk and higher cohesive traction. The higher crack growth rate is a coupled effect of both fracture and oxidation properties.

Experimental study on crack propagation of stainless steel 304 material under amplitude overloads

Fatigue mechanisms caused by dynamic loadings starts with cracks that occurred in a material. Under conditions such as transient conditions, an earthquake or instantaneous external loads, some parts of nuclear reactor that initially receive operating loads can be overloaded. The purpose of this research is to investigate the change of crack propagation in stainless steel 304 material related with amplitude overloads. The crack propagation test used the axial dynamic testing with the maximum stress (σ max) of 145 MPa, stress ratio R = 0.1 and over loading ratio (OVR) of 1.5, 1.75, 2. The results showed that the amplitude overloads caused the crack to propagate slowly as the crack tip formed a branch. The higher crack growth rate also showed the higher stress intensity factors in the crack tip.

Characterisation of microstructure, defect and high-cycle-fatigue behaviour in a stainless steel joint processed by brazing

We report the characterisation of microstructures and high-cycle-fatigue (HCF) properties of Type 304 stainless steel joints processed by brazing. Pure copper was applied as the filler metal for brazing at 1120 °C. A two-phase microstructure was obtained within the joint region: the star-shaped precipitates and copper matrix. The precipitates with an average size of 0.43 μm were rich in iron and chromium. A fixed orientation relationship was found between the precipitates and copper matrix. The joint exhibited much higher tensile strength and HCF life when compared to pure copper. The strength enhancement can be attributed to the presence of precipitates. Furthermore, the effect of joint interface roughness as well as defects was critically investigated. The joint interface roughness showed little influence on the HCF lives. Post-examinations revealed that fatigue crack initiation and propagation occurred entirely within the joint region, hence being consistent with the similar HCF lives regardless of the pre-defined interface roughness conditions. In addition, it was found that the HCF lives decreased exponentially with the increase of initial defect area. Fractography analysis revealed that fatigue striation spacings near the crack initiation zone increased with the increase of defect area, suggesting that the larger defects result in higher crack growth rate, hence shorten the overall fatigue life.