Influence of Fatigue Crack Wake Length and State of Stress on Crack Closure.(Pamphlet) (original) (raw)
Related papers
Engineering Fracture Mechanics, 1989
experimental and numerical study has been made of the mechanisms of fatigue crack growth and crack-closure behavior in an a//? titanium alloy Ti4A14Mo-2SnO.SSi (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-rip 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 (AK) was close to the fatigue threshold (AKrn), or, when the overload maximum stress intensity (K,,) approached the fracture toughness of the material. At low AK levels, the magnitude of the delay was sensitive to microstructure and found to be enhanced in coarse-grained P-heat-treated microstructures compared to standard fine-grained a //I microstructures. Based on these results, mechanistic sequences are suggested to explain the transient fatigue crack growth behavior following single and block tensile overload cycles.
Contribution of the cyclic loading portion below the opening load to fatigue crack growth
Materials Science and Engineering: A, 1996
A simple test procedure involving stress ratio changes at the fatigue threshold is proposed to reveal the role of the lower portion of the loading cycle below KoP in the fatigue crack growth behaviour. It is observed for both Al 2024-T3 and Al 7475T761 alloys that the initially non-propagating fatigue crack at the fatigue threshold resumes growth upon diminishing Kmin while keeping Kth,max constant. These experimental findings can be interpreted by means of a modified crack closure concept in which the contribution of the lower portion of the loading cycle below Kop (including also a part of compressive loading if R < 0) to the variation in the stress state experienced by the fatigue crack tip is taken into account.
The evolution of crack-tip stresses during a fatigue overload event
Acta Materialia, 2010
The mechanisms responsible for the transient retardation or acceleration of fatigue crack growth subsequent to overloading are a matter of intense debate. Plasticity-induced closure and residual stresses have often been invoked to explain these phenomena, but closure mechanisms are disputed, especially under conditions approximating to generalised plane strain. In this paper we exploit synchrotron radiation to report very high spatial resolution two-dimensional elastic strain and stress maps at maximum and minimum loading measured under plane strain during a normal fatigue cycle, as well as during and after a 100% overload event, in ultra-fine grained AA5091 aluminium alloy. These observations provide direct evidence of the material stress state in the vicinity of the crack-tip in thick samples. Significant compressive residual stresses were found both in front of and behind the crack-tip immediately following the overload event. The effective stress intensity at the crack-tip was determined directly from the local stress field measured deep within the bulk (plane strain) by comparison with linear elastic fracture mechanical theory. This agrees well with that nominally applied at maximum load and 100% overload. After overload, however, the stress fields were not well described by classical K fields due to closure-related residual stresses. Little evidence of overload closure was observed sometime after the overload event, in our case possibly because the overload plastic zone was very small. Crown
Role of plasticity_induced crack closure in fatigue crack growth.PDF
The premature contact of crack surfaces attributable to the near-tip plastic deformations under cyclic loading, which is commonly referred to as plasticity induced crack closure (PICC), has long been focused as supposedly controlling factor of fatigue crack growth (FCG). Nevertheless, when the plane-strain near-tip constraint is approached, PICC lacks of straightforward evidence, so that its significance in FCG, and even the very existence, remain debatable. To add insights into this matter, large-deformation elastoplastic simulations of plane-strain crack under constant amplitude load cycling at different load ranges and ratios, as well as with an overload, have been performed. Modeling visualizes the Laird-Smith conceptual mechanism of FCG by plastic blunting and re-sharpening. Simulation reproduces the experimental trends of FCG concerning the roles of stress intensity factor range and overload, but PICC has never been detected. Near-tip deformation patterns discard the filling-in a crack with material stretched out of the crack plane in the wake behind the tip as supposed PICC origin. Despite the absence of closure, load-deformation curves appear bent, which raises doubts about the trustworthiness of closure assessment from the compliance variation. This demonstrates ambiguities of PICC as a supposedly intrinsic factor of FCG and, by implication, favors the stresses and strains in front of the crack tip as genuine fatigue drivers.
Materials Science and Engineering, 1983
It is now well established that variableamplitude loading in the form of single spike or block overloads can result in pronounced transient retardations and even complete crack arrest during mode I fatigue crack propagation. The origin of such effects has been ascribed to one or more of the following mechanisms: crack tip-yield zone interactions , crack tip blunting , branching [4-6], crack closure due to residual plastic strains , fracture surface microroughness and corrosion deposits .
Fatigue crack growth “overload effect”: mechanistic insights from in-situ synchrotron measurements
The Journal of Strain …, 2012
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 overloadposition, 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.
International Journal of Pressure Vessels and Piping, 2013
It was still short of generalized fatigue crack growth (FCG) models in the near-threshold regime due to its complex influencing factors. The near-threshold FCG behaviour of a rotor steel 25Cr2Ni2MoV at different load ratios was investigated experimentally, and the FCG driving mechanism was theoretically analysed based on equivalent driving force model. It was found that the crack growth process was determined by combined effects of equivalent driving force at constant amplitude loading and crack closure. A new crack closure model was proposed by considering the influences of load ratio and FCG rate, which could successfully interpret the effect of load ratio on FCG. The correlation of the crack closure model with the transition of driving forces in crack advance was beneficial to unify crack closure theory and crack growth driving parameters in the near-threshold regime.
50 Years of Controversy on Fatigue Crack Closure
Procedia Structural Integrity, 2024
In this article the 50 years of observations, implications and debates related to fatigue crack closure are discussed. New insights related to plasticity, oxide, and roughness induced crack closures and their role in shielding effects of the fatigue crack tip are reexamined. Supporting evidence for these insights comes from the lack of Kth dependence on R in a high vacuum (with partial pressure of 10-5 Pa or less). The presented new critical chemical-mechanical analyses are based on experimental results reported in the literature that demonstrate the marginal R-ratio effect on Kth of long cracks in vacuum for both planar/wavy slip alloys but show R-dependence in the lab air and in chemical environment. The latter is due to the formation of viscous nature of the oxide, which forms in humid air at the newly expose fresh fracture surfaces. It is demonstrated that the dominant factor related to the experimentally observed R-ratio effects on fatigue crack growth (FCG) behavior (on several alloys) in not related to crack closure but the access of the environment to the crack tip region that affects fatigue damage. 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 crack flanks contact occurs, 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. Dislocation-based models have indicated that the crack tip shielding effect from a single asperity is small. The roughness induced crack closure has been suggested as a mechanical obstruction in the wake of the crack during cyclic unloading for planar slip alloys at the threshold region, the emphasis on the access of the environment to the crack tip for an environmental damage was not considered.