Crack Propagation Threshold Research Articles

Effects of Pre-Deformation in Corrosion Fatigue Crack Growth of Al-Mg-Zn Alloy

This study investigated the effect of pre-deformation on the corrosion fatigue crack propagation (CFCG) of Al-Mg-Zn alloy in a corrosive environment. Tensile tests at different pre-deformation levels and molecular dynamics simulations analyzed changes in dislocation density. Corrosion fatigue experiments were conducted in a 3.5% NaCl solution at room temperature, and crack propagation morphology was characterized using electron backscatter diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The results showed that tensile strength increased by 2.63% and 10.00% for 5% and 10% pre-deformation, respectively. The crack propagation threshold values were L2 (6.36 MPa·m1/2) > L0 (6.05 MPa·m1/2) > L1 (5.13 MPa·m1/2), attributed to increased dislocation density and material strength. At 5% pre-deformation, dislocation pile-ups created stress concentrations that facilitated crack propagation. In contrast, the non-uniform dislocation distribution at 10% pre-deformation enhanced both material strength and resistance to crack growth.

Crack Arrest Analysis of Components With Compressive Residual Stress

ABSTRACTA finite element analysis and fracture mechanics methodology for determining the autofrettage pressure required to cause crack arrest in components under varying pressure loading are presented. Superposition of the autofrettage residual stress distribution and working load stress distribution is combined with ANSYS Separating Morphing and Adaptive Remeshing Technology (SMART) to determine the effective stress intensity factor as the crack grows. The condition for crack arrest is identified by comparison with a crack arrest model defining the crack propagation threshold stress intensity factor range for microstructurally short, physically short, and long cracks. The crack propagation threshold models of El Haddad and Chapetti are implemented and applied to fatigue analysis of stainless steel and low carbon steel double notch tensile test specimens with preinduced compressive residual stress. Based on comparison with fatigue test results, the Chapetti model is selected for use in the analysis of a 3D aluminum alloy valve body. The calculated minimum autofrettage pressure required to give crack arrest under a given working load cycle is found to be in good agreement with experimental observations from the literature.

Identification and comparisons of crack stress threshold for raw coal and briquette under triaxial compression

ABSTRACT The reasonable determination of the coal rock stress threshold holds significant importance for the objective description of coal rock deformation stages and fine-scale crack evolution during failure. This study aims to accurately identify the stress threshold for raw coal and briquette crack propagation under triaxial compression and to determine suitable identification methods. The stress thresholds were determined via triaxial compression experiments, coupled with the monitoring of stress, strain, and acoustic emission (AE) signals. This was achieved by employing the crack axial strain method, the moving point regression technique, and the AE response method. The results indicate that the crack closing stress (σ cc ), crack initiation stress (σ ci ), and crack failure stress (σ cd ) of raw coal were consistently and accurately identified using all three methods. However, in the context of triaxial compression experiments on raw coal, the AE response method proves inadequate for delineating the stress threshold associated with crack propagation, necessitating precise identification via alternative methods. These findings contribute to a deeper understanding of the progressive failure process and the macroscopic failure mechanisms of coal samples, with significant theoretical implications for evaluating the stability of deep coal mining operations.

Non-Propagating Cracks at the Fatigue Limit of Notches: An Analysis of Notch Sensitivity and Size Effect.

The issues of the high-cycle fatigue resistance of notches and the role of non-propagating short cracks in defining the fatigue notch sensitivity and fatigue limit of the configuration are addressed. A fracture mechanics approach is employed to determine the threshold configuration that defines the associated fatigue limit. The influence of notch sharpness, notch size, intrinsic fatigue limit, microstructural dimensions, and the threshold for crack propagation is examined. A simple expression is proposed to estimate the maximum fatigue notch factor, kfMax, which incorporates the influence of these non-propagating cracks. The fatigue limits for both blunt and sharp elliptical notches are analyzed and predicted based on experimental results reported in the literature. Additionally, shallow notches or small defects are analyzed, where it is found that the same hypothesis may not be applicable.

Effect of low temperature on fatigue crack propagation behavior of QP980 steel and laser-welded joint

This paper aims to investigate the fatigue crack propagation behavior of quenching and partitioning 980 steel (as base metal) and its welded joint at low-temperature environments. The fatigue crack propagation tests are carried out at 25°C, −40°C and −80°C under R=0.1, 0.3 and 0.5. The results show that the fatigue crack propagation threshold value increases and fatigue crack propagation rate decrease of base metal, whereas welded joint behaved opposite results as the temperature decreases. The fatigue ductility to brittle transition temperature of base metal is lower than that of welded joint. Compared with the welded joint, the base metal has higher fatigue resistance.

Precipitate-mediated fatigue crack propagation behavior of Al–Zn–Mg–Cu alloy tailored by non-isothermal aging

The fatigue crack propagation behavior of non-isothermally aged Al–Zn–Mg–Cu alloy was examined at different aging stages. It has been revealed that non-isothermal aging (NIA) improved the fatigue crack resistance by adjusting the thermodynamics, and thus modified the precipitation configuration. Enhanced crack propagation threshold and decreased crack propagation rate were achieved when NIA was applied and carried out thoroughly. Fine and dispersed GP zones existing in underaged alloy significantly intensified the slip localization, which effectively mitigated the stress concentration near the crack tip and showed an approximately 8.2 MPa·m1/2 crack propagation threshold. Smaller width of the precipitate-free zone enabled the slip bands to propagate through the grain boundaries. However, relatively low strength and intense slip localization led the fatigue crack to propagate along the crystallographic slip bands, responsible for the higher crack propagation rate. For minorly over-aged alloys, coarsening and elevated number density of intragranular precipitate effectively hindered the movement of dislocation around, where strain concentration was frequently and preferentially induced, causing the degradation of resistance to fatigue crack propagation.

Advancements in fracture toughness testing of ultra-high-strength steel sheets: Unraveling the crack-closure effect and unanticipated thickness independence

A novel testing method was developed to evaluate the fracture toughness of ultra-high-strength (UHS) steel sheets. The study also examined the influence of specimen thickness, utilizing specimens of varying thicknesses (1.6 ∼ 10 mm). While the toughness of all specimens was determined by the threshold for planar crack propagation originating from the fatigue pre-crack-front near the mid-thickness zone, the apparent fracture toughness was dependent on specimen thickness. This phenomenon was attributed to the crack-closure effects experienced during the pre-cracking process. In-depth analyses revealed that, in the absence of crack-closure effects, the intrinsic toughness remained unaffected by changes in specimen thickness.

Enhanced corrosion fatigue strength of additively manufactured graded porous scaffold-coated Ti-6Al-7Nb alloy

Current modifications of Ti-based materials with porous scaffolds for achieving biological fixation often decrease corrosion fatigue strength (σcf) of the resultant implants, thereby shortening their service lifespan. To resolve this issue, in the present, a step-wise graded porous Ti-6Al-7Nb scaffold was additively manufactured on optimally surface mechanical attrition treated (SMATed) Ti-6Al-7Nb (specifically denoted as S-Ti6Al7Nb) using laser powder bed fusion (PBF) technology. The microstructure, bond strength, residual stress distribution, and corrosion fatigue behavior of porous scaffolds modified S-Ti6Al7Nb were investigated and compared with those of mechanically polished Ti-6Al-7Nb (P-Ti6Al7Nb), S-Ti6Al7Nb, and porous scaffolds modified P-Ti6Al7Nb. Results showed that corrosion fatigue of porous scaffolds modified Ti-6Al-7Nb was propagation controlled. Moreover, the crack propagation behavior in the PBF scaffold's fusion zone (FZ) and heat-affected zone (HAZ), exhibiting insensitivity to the microstructural configurations characterized by columnar prior-β grain (PBG) boundaries and acicular α′ martensites, coupled with the PBF-induced residual tensile stresses in these regions, resulted in a considerable decrease in σcf for porous scaffolds modified P-Ti6Al7Nb compared to P-Ti6Al7Nb. In contrast, step-wise graded porous scaffold-modified S-Ti6Al7Nb demonstrated an improved σcf which was even higher than that of P-Ti6Al7Nb. Such an advancement in corrosion fatigue strength is primarily attributed to the presence of residual compressive stresses within the underlying S-Ti6Al7Nb substrate, extending beyond FZ and HAZ. These stresses increased the crack propagation threshold, leading to crack deflection/branching and increased crack-path tortuosity, thereby synergistically markedly enhancing the crack propagation resistance of porous scaffolds modified S-Ti6Al7Nb.

Crack propagation determined by energy release rate in cortical bone ultrasonic vibration assisted cutting

In orthopaedic surgery, bone cutting may occur serious mechanical damage due to large crack propagation, which impedes patient postoperative recovery. Ultrasonic vibration assisted cutting (UVAC) has emerged as a leading technique in bone cutting owing to its remarkable advantages, including minimal damage and cutting force. However, how to diminish its damage during bone UVAC remains challenging. Hence, this study established a sophisticated model to calculate the energy release rate for cortical bone UVAC and predicted the critical cutting depths of shear and fracture crack propagation. This superiority stems from the synergistic effects of periodic tool separation and the acoustic softening effect inherent in UVAC, which contribute to a lower average energy release rate and a subsequently reduced cutting force. Nevertheless, the high instantaneous energy release rate during UVAC can exceed the material's critical threshold for crack propagation, leading to the initiation of microcracks and thereby inducing material embrittlement behaviour. These insights shed light on the material removal mechanism of bone UVAC in medical orthopaedic surgery, thus enriching our understanding and potentially guiding further advancements in this field.

Influence of Microstructure on Tensile Properties and Fatigue Crack Propagation Behavior for Lath Martensitic Steel

This paper addresses the role of multilevel microstructures on the fatigue crack propagation behavior and the tensile properties of lath martensite with different substructure sizes. Microstructure characterization of the alloy was carried out by transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron back-scattering diffraction (EBSD), and optical microscopy (OM). Based on the classic Hall–Petch relationship, the results of tensile tests showed that martensitic block is the effective control unit of yield strength. Furthermore, the plasticity of lath martensite is not sensitive to grain size. The tensile deformation mechanisms were also discussed. Fatigue crack propagation tests revealed that the coarse grain has a higher crack propagation threshold and lower crack propagation rate than the fine grain in lath martensitic steel. The change in the plasticity zone ahead of the crack tip leads to the transitional behavior of the fatigue crack propagation rate. When plasticity zone sizes are equal to the block size, the fatigue crack propagation reverts to a stable propagation stage. Finally, an empirical model was established to predict the fatigue crack propagation rate of the stable propagation stage based on the tensile properties of the lath martensitic steel.

On 50 years of fatigue crack closure dispute

AbstractIn this article, we examine the 50 years of observations and interpretations into the subject of fatigue crack closure in lab air and aqueous solutions: disputes and implications. Over this period, several closure concepts have been invoked to explain why the crack propagation threshold ΔKth should reduce with load ratio R. Among these, three types are the most popular and these are closure induced by crack (i) plasticity (PICC), (ii) oxides (OICC), and (iii) roughness (RICC). Therefore, these three and their role in shielding the fatigue crack tip are critically examined. This analysis finds that the reduction in ΔKth with increasing R‐ratio in lab air and aqueous solutions is weakly linked to decreasing crack closure behind the crack tip but strongly related to the access of the environment to the crack tip region and its influence on fatigue damage at and ahead of the crack tip. Supporting evidence for this interpretation comes from the lack of ΔKth dependence on R in a good vacuum (with partial pressure of 10−5 Pa or less). In chemical environments, our viewpoint is supported by a critical analysis of corrosion processes that found that there is insufficient time for most metallic species to form ions, hydrolyze, and transform into hard phases at the crack tip before closure. Therefore, when closure occurs at the crack tip, most of the oxidized metallic species will exist as aquo‐complexes, gel, or colloids that have insufficient shear strength to wedge crack faces during unloading.

A cohesive fatigue model for composite delamination based on a new material characterization procedure for the Paris law

A new cohesive fatigue model (CF22) is proposed after an extensive characterization of experimental data. To minimize the number of parameters required, experimental Paris law data is transformed into a plot of amplitude versus mean relative endurance, where the data is fit using an empirical function. This novel characterization procedure helps identify trends and inconsistencies in the data and provides the values of the model input parameters without requiring inverse analyses. The new fatigue model significantly improves the predictions of crack propagation rates and thresholds compared to CF20, especially under conditions of high stress ratios and high values of mode mixity.

A short and long crack growth model with mean stress correction based on cyclic resistance curve

In the study, the Chapetti cyclic resistance curve is modified by a R-dependence model based on the cyclic strain energy density to account for the mean stress effects on the long crack propagation threshold and fatigue limit. Accordingly, the crack growth model of the modified NASGRO with mechanically short crack extension and Kitagawa-Takahashi diagram are derived with mean stress sensitivity. Relatively good accuracy can be achieved in terms of the near-threshold propagation behaviors and SN curves at high cycle regime including engineering aluminum alloys and titanium alloys.

Toward developing Ti alloys with high fatigue crack growth resistance by additive manufacturing

Fatigue crack growth behaviors were investigated by three-point bending tests for TA19 alloy fabricated by laser metal deposition and four kinds of heat-treated samples. The crack growth resistance of the TA19 samples in the near-threshold regime and Paris regime was evaluated through the experimental characterization and theoretical analysis of the interaction between fatigue crack and α/β phase interface, columnar prior-β grain boundary and colony boundary. The results show that in the near-threshold regime, the fatigue crack propagation threshold and resistance increase with the increase of widths of lamellar αp phases and colonies, and the decrease of the number of α laths with an angle (φ) relative to the applied stress direction ranging from 75° to 90°. In the Paris regime, the fatigue cracking path can be deflected at colony boundaries or columnar prior-β grain boundaries. The larger the deflection angle, the more tortuous the cracking path and the lower the fatigue crack growth rate. The angle (γ) of the columnar prior-β grain growth direction relative to the build direction affects not only φ of different α variants, but also the fatigue cracking path deflection angle (θij) at columnar prior-β grain boundaries. An optimal combination of γ = 0°-15°-0°-15° for several adjacent columnar prior-β grains is derived from the theoretical analysis, and that can effectively avoid φ being in the range from 75° to 90° and make θij as large as possible. Such findings provide a guide for the selection of scanning strategies and process parameters to additively manufacture Ti alloys with high fatigue damage tolerance.

Simulation and Mechanical Properties of Fine-Grained Heat-Affected Zone Microstructure in 18CrNiMo7-6 Steel.

Heat-affected zones (HAZs) in real welds are usually quite narrow, and consequently most standard mechanical tests are difficult or even impossible. Therefore, simulated microstructures are often used for mechanical tests. However, the most often used weld thermal cycle simulator produces only a few millimeters wide area of simulated microstructure in the middle of specimens. Consequently, these kind of simulated specimen are not suitable for standard tensile tests, and even for Charpy impact tests, the simulated area can be too narrow. Therefore, to investigate the mechanical properties of a fine-grain heat-affected zone in 18CrNiMo7-6 steel, two methods were used for simulation of as-welded microstructures: (a) a weld thermal cycle simulator, and (b) as an alternative, though not yet verified option, austenitizing in a laboratory furnace + water quenching. The microstructures were compared and mechanical properties investigated. The grain sizes of the simulated specimens were 10.9 μm (water-quenched) and 12.6 μm (simulator), whereby the deviations from the real weld were less than 10%. Both types of simulated specimen were used for hardness measurement, Charpy impact tests, and fatigue tests. Water-quenched specimens were large enough to enable standard tensile testing. A hardness of 425 HV, yield strength Rp02 = 1121 MPa, tensile strength Rm = 1475 MPa, impact energy KV = 73.11 J, and crack propagation threshold ΔKthR = 4.33 MPa m0.5 were obtained with the water quenched specimens, and 419 HV, KV = 101.49 J, and ΔKthR = 3.4 MPa m0.5 with the specimens prepared with the simulator. Comparison of the results confirmed that the annealed and quenched specimens were suitable for mechanical tests of FG HAZs, even for standard tensile tests. Due to the use of simulated test specimens, the mechanical properties determined can be linked to the FG HAZ microstructure in 18CrNiMo7-6 steel.

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 initiation and propagation thresholds of Hwangdeung granite under elevated temperature

Crack initiation and propagation thresholds of Hwangdeung granite under elevated temperature

Recent developments in the determination of fatigue crack propagation thresholds

The impact of crack closure and environmental effects on the experimental determination of the fatigue crack propagation threshold is a major problem for the assessment of cyclically loaded components, especially at low stress ratios R. In this work, the influence of the experimental procedure and air humidity on da/dN−ΔK data at different R is discussed. Unlike the results at R=−1, the threshold values obtained at R≈0.8, i.e. under negligible crack closure levels, show a very small scatter band regardless of the variation of the test parameters and environmental conditions.

A theoretical model for predicting the temperature-dependent fatigue crack propagation threshold of nickel-based superalloys

The fatigue crack propagation threshold is an essential predictor for the safety of crack-containing components during their expected life. In this work, the fatigue crack propagation of a material is considered from an energy perspective and a critical energy density related to fatigue propagation was presented. A temperature-dependent crack fatigue propagation criterion was proposed. The temperature-dependent fatigue crack propagation threshold model for superalloys was developed. The model reveals the quantitative relationship between fatigue crack propagation threshold, Young's modulus, and temperature. The model predictions were in good agreement with all the available experimental data for eleven superalloys. A convenient and simple optional method for predicting the fatigue crack propagation threshold of superalloys at elevated temperatures was achieved. In addition, the limitations and applicability of the model are discussed. The results of the research are of potential application to aero-engine structural integrity studies.

Estimation of the Kitagawa-Takahashi diagram by cyclic R curve analysis

The Kitagawa-Takahashi (KT) diagram is a proven concept for describing the fatigue limit in presence of a defect or crack. It can be determined empirically with great experimental effort. It can also be estimated by means of the El Haddad relationship if the endurance limit and the long fatigue crack propagation threshold are available in reasonable accuracy. A third option is the determination using the cyclic R-curve, which describes the dependency of the fatigue crack propagation threshold on the crack growth at the short crack propagation stage. This can be experimentally determined using a closure-free initial pre-crack. It can then be applied to the determination of crack arrest for a given applied load and a given defect or crack size. Compared to the other two methods mentioned above, this option has considerable advantages: It can be applied to any component and any stress ratio. It allows the treatment of multiple cracks and provides estimations of the S-N curve in the finite life regime as well as at the endurance limit. Compared to the empirical determination of the KT diagram, the experimental effort is significantly lower and compared to the El Haddad approach it avoids problems such as the use of non-conservative long fatigue crack propagation thresholds (when the conventional load reduction method is applied to materials prone to corrosion) and the mathematical predetermination of the curve shape. The work introduces the method and provides a critical discussion as well as quantitative comparison between the different methods.