Transient Fatigue Crack Growth Research Articles

Overloading effect on transient fatigue crack growth of Ti-6Al-4V parts produced by Laser Powder Bed Fusion

The fatigue crack growth (FCGR) behavior of titanium alloy Ti-6Al-4V submitted to Hot Isostatic Pressing (HIP) through CT specimens manufactured by Laser Powder Bed Fusion (LPBF) is analyzed. Both the crack growth under constant amplitude loading and the transient crack growth behavior after the application of overloads were studied. Crack closure induced by plasticity for the stress ratio R=0 was observed at the Paris regime. The applied overloads for OLR=2 at R=0 caused a FCGR retardation due to the increase of crack closure effect induced by plasticity resulting from the overloads. Rapid transient periods were registered due to small zone affected by the overloads.

Plastic strain gradients and transient fatigue crack growth: A computational study

Fatigue crack growth is analyzed for steady state and overload conditions. The model combines an elastic-plastic strain gradient hardening material and an irreversible cohesive zone model. Plastic strain gradients are found not to affect steady-state fatigue crack growth but play a key role in the overload response. Plastic strain gradient hardening lowers plastic strain magnitudes and alters the spatial distribution of plastic strain. This leads to reduced crack closure and higher crack growth rates during and after the overload. Using classical plasticity alone to describe fatigue crack growth following an overload is thus nonconservative.

Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements

Synchrotron-based, high-energy X-ray diffraction measurements are used to study the local strain fields underlying the transient fatigue crack growth rate retardation produced by a single overload cycle known as the overload effect. Specifically, 4140 steel compact tension specimens fatigued for varying levels of crack growth after an overload cycle have been studied with in-situ diffraction under varying external loads. The load responses of the strain at the overload-position, versus at the crack tip, are focused upon in detail. The large compressive residual strain at the overload-point is observed to remain essentially unchanged even after the overload-point is left in the wake of the propagating crack tip. The differential strain-load response at the crack-tip/overload position before and immediately after the overload is seen to be unchanged. Once the overload point is behind the crack tip, a highly nonlinear behavior is observed in which the load response of the strain field transfers from the overload -point to the crack tip when the load exceeds a critical value. The results are discussed in terms of plasticity-induced crack face contact at the overload point as an important local mechanism contributing to the “overload effect” in this specific system.

On the Study of Fatigue Crack Propagation in the Time Domain

In-situ SEM observations have revealed that fatigue crack propagation in aluminium alloys is caused by the shear band decohesion around the crack tip and the formation and cracking of the shear band is mainly caused by the plasticity generated in the loading part of the load cycle. This shear band decohesion process has been observed to occur in a continuous way over the time period during the load cycle. Based on this observation, in this study, the transient fatigue crack growth rate, da/dt, has been used to obtain the relationship between the conventional used parameter da/dN and the applied driving force. It is proven that two parameters are necessary in order to accurately describe fatigue crack propagation rate per stress cycle, da/dN. The well known stress ratio effects on fatigue crack propagation rate can be correlated by this model.

Effect of Transient Loads on Fatigue Crack Growth in Solution Treated and Aged Ti-62222 at −54, 25, and 175°C

Abstract Transient loads consisting of single tensile overloads and single tensile overloads followed by single compressive underloads were applied to Ti-62222 solution treated and aged titanium alloy at −54, 25, and 175°C. Tensile overload ratios were 2.0 and 2.5, and the compressive underload ratio was −0.5. Four reference steady state ΔKss values, using constant δK testing at R = 0.1, were investigated at each temperature. Cycles of delay, fatigue crack growth during delay, and minimum fatigue crack growth rate during a transient load were obtained for all tests. Cycles of delay ranged from zero to crack arrest. Higher tensile overloads caused greater delay cycles, and underloads were often detrimental. Low and high temperatures were primarily beneficial to delay cycles relative to those at room temperature. Crack growth delay distance was always greater than the pertinent reversed plastic zone size. Fatigue crack growth life predictions were made using FASTRAN III and AFGROW computer programs and produced both conservative and non-conservative results with more than half the predictions being within ±2 of the experimental results. Macro- and microfractography revealed surface crack closure, Mode II displacements, crack tip blunting, branching, and tunneling contributed to the transient fatigue crack growth behavior.

A numerical study of transient fatigue crack growth by use of an irreversible cohesive zone model

Transient fatigue crack growth is studied for a material system in which the crack tip is shielded due to crack bridging. The process of material separation during fatigue crack growth is described by the use of an irreversible cohesive zone model. In contrast to past developments of cohesive zone models, the traction–separation behavior does not follow a predefined path, but is dependent on the evolution of the damage dependent cohesive zone properties. The model definition is given and the basic uniaxial response of the model documented. The cohesive zone model is subsequently applied in a numerical study of transient fatigue crack growth. Single overload cases are computed to demonstrate the effects of variations in the cohesive zone properties. Block loading sequences with variations in the amplitude and the load ratio are computed. Model predictions qualitatively compare well to experimentally observed effects of fatigue crack growth transients in materials with crack bridging zones.

Effects of delamination on the transient fatigue crack growth behaviour following overloads in 8090-T8 AlLi alloys : Kumai, S. and Higo, Y. Mater. Sci. Forum (1996) 217–222 (3), 1371–1376 Conference: Aluminium Alloys—Their Physical and Mechanical Properties, ICAA5, Part 3, Grenoble, France (1–5 July 1996)

Effects of delamination on the transient fatigue crack growth behaviour following overloads in 8090-T8 AlLi alloys : Kumai, S. and Higo, Y. Mater. Sci. Forum (1996) 217–222 (3), 1371–1376 Conference: Aluminium Alloys—Their Physical and Mechanical Properties, ICAA5, Part 3, Grenoble, France (1–5 July 1996)

Transient fatigue crack growth behaviour following single overloads at high stress ratios : Tsukuda, H., Ogiyama, H. and Shiraishi, T. Fatigue and Fracture of Engineering Materials and Structures (1996) 19, 879–891

Transient fatigue crack growth behaviour following single overloads at high stress ratios : Tsukuda, H., Ogiyama, H. and Shiraishi, T. Fatigue and Fracture of Engineering Materials and Structures (1996) 19, 879–891

The effects of overloads in fatigue crack growth

Several theories have been proposed to explain the transient fatigue crack growth decelerations and accelerations which follow overloads. The mechanisms that have been proposed to explain retardation after a tensile overload, for example, include residual stress, crack deflection, crack closure, strain hardening, and plastic blunting/resharpening. These mechanisms are reviewed in the light of recent experimental results, and implications with regard to their applicability are examined. It is suggested that no single mechanism can be expected to represent observed effects over the entire range of d a/d N versus ΔK; eg, behaviour ranging from the near threshold region to the Paris region.

Prediction of service loading from the width and height of striation in fatigue of 2017-T4 aluminium alloy: Murakami, Y., Furukawa, K. and Shiraishi, N. J. Soc. Mater. Sci., Japan Aug. 1990 39, 1113–1118 (in Japanese)

Several theories have been proposed to explain the transient fatigue crack growth decelerations and accelerations which follow overloads. The mechanisms that have been proposed to explain retardation after a tensile overload, for example, include residual stress, crack deflection, crack closure, strain hardening, and plastic blunting/resharpening. These mechanisms are reviewed in the light of recent experimental results, and implications with regard to their applicability are examined. It is suggested that no single mechanism can be expected to represent observed effects over the entire range of da/dN versus ΔK; eg, behaviour ranging from the near threshold region to the Paris region.

Mechanisms associated with transient fatigue crack growth under variable-amplitude loading: an experimental and numerical study: Ward-Close, C.M., Blom, A.F. and Ritchie, R.O. Eng. Fract. Mech. 1989 32, (4), 613–638

Mechanisms associated with transient fatigue crack growth under variable-amplitude loading: an experimental and numerical study: Ward-Close, C.M., Blom, A.F. and Ritchie, R.O. Eng. Fract. Mech. 1989 32, (4), 613–638

Mechanisms associated with transient fatigue crack growth under variable-amplitude loading: An experimental and numerical study

An experimental and numerical study has been made of the mechanisms of fatigue crack growth and crack-closure behavior in an α β titanium alloy Ti-4A1-4Mo-2Sn-0.5Si (IMI 550), following both single and block tensile overloads. Closure immediately behind the crack front (near-tip closure) was found to be the main factor controlling load-interaction effects. Single tensile overloads were found to remove near-tip closure, and slightly reduce far-field closure along the crack length, resulting in an initial acceleration in fatigue crack growth rates. Subsequent delayed retardation of crack growth rates was accompanied by an increase in the near-tip closure load, due to the enlarged compressive residual stress in the overload plastic zone. High/low block overloads caused greater retardation than single overloads of the same magnitude, and this was attributed to changes in the degree of closure in the wake of the crack. Numerical predictions of such transient behavior, based on a modified Dugdale model, are found to be in close agreement with experimental results, both in terms of observed crack growth rates and crack opening displacements. Load-interaction effects were found to be most severe when the baseline stress intensity range ( ΔK) was close to the fatigue threshold ( ΔK TH ), or, when the overload maximum stress intensity ( K max ) approached the fracture toughness of the material. At low ΔK levels, the magnitude of the delay was sensitive to microstructure and found to be enhanced in coarse-grained β-heat-treated microstructures compared to standard fine-grained α β microstructures. Based on these results, mechanistic sequences are suggested to explain the transient fatigue crack growth behavior following single and block tensile overload cycles.

Transient fatigue crack growth caused by change in environmental conditions

Etude effectuee sur un acier commercial a haute resistance dans de l'eau de mer synthetique, remplacant l'air dans la cuve d'essai sans interruption de la contrainte cyclique. Observation d'une acceleration de la vitesse de propagation des fissures de fatigue

Fatigue crack growth retardation following load reductions in a plain C-Mn steel

Transient fatigue crack growth effects following perturbations in applied loading conditions have been studied using a low strength plain C-Mn steel. Reductions in both the alternating and maximum components of the fatigue loading spectrum have been systematically investigated and evaluated using a linear elastic fracture mechanics approach. Results are discussed in terms of the residual stress concept, and a model based on an effective stress intensity concept is propossed to rationalise growth rates within the retardation affected zone. Using constant amplitude fatigue threshold and crack growth data obtained under similar conditions, the model is shown to be in good agreement with experimental data.