A study of overload effect on fatigue crack propagation using EBSD, FIB–DIC and FEM methods (original) (raw)
Related papers
Analysis of overload effects and related phenomena
International journal of …, 1999
Overloads and underloads perturb steady state fatigue crack growth conditions and affect the growth rates by retarding or accelerating the growth. Clear understanding of these transient effects is important for the reliable life prediction of a component subjected to random loads. The overload effects have predominately been attributed to either plasticity induced crack closure behind the crack tip, residual stresses ahead of the crack tip, or a combination of both. These effects are critically examined in the context of the Unified Approach proposed by the authors. Recent experimental and analytical evaluation of crack closure has confirmed its negligible contribution to crack growth and has demonstrated that changes in the stresses ahead of the crack tip are more important than closure behind the crack tip. It is shown that the overload effects and other transient effects arise due to perturbation of the stresses ahead of the crack tip, and these can be accounted for by the two parametric approach emphasized in the unified theory. It is shown that related phenomena including the role of K max , the existence of propagation threshold K pr , and effects of overloads on K pr and K max etc, are all accounted for by the Unified Approach.
International Journal of …, 2007
High-energy synchrotron X-ray diffraction experiments are used to perform local crack plane strain profiling of 4140 steel compact tension specimens fatigued at constant amplitude, subjected to a single overload cycle, then fatigued some more at constant amplitude. X-ray strain profiling results on a series of samples employing in-situ load cycling are correlated with the crack growth rate (da/dN) providing insight into the da/dN retardation known as the ''overload effect''. Immediately after the overload, the strain under maximum load is greatly reduced but the range of strain, between zero and maximum load, remains unchanged compared to the pre-overload values. At the point of maximum retardation, it is the strain range that is greatly reduced while the maximum-load strain has begun to recover to the pre-overload value. For a sample that has recovered to approximately half of the original da/dN value following the overload, the strain at maximum load is fully recovered while the strain range, though partially recovered, is still substantially reduced. The dominance of the strain range in the overload effect is clearly indicated. Subject to some assumptions, strong quantitative support for a crack growth rate driving force of the suggested form [(K max ) 1Àp (DK) p ] c is found. A dramatic nonlinear load dependence in the spatial distribution of the strain at maximum retardation is also demonstrated: at low load the response is dominantly at the overload position; whereas at high loads it is dominantly at the crack tip position. This transfer of load response away from the crack tip to the overload position appears fundamental to the overload effect for high R-ratio fatigue as studied here.
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
Effect of Overload on Fatigue Crack Growth Behavior of Airframe Structure through FEM Approach
2013
catastrophic structural failures in many engineering fields like aircraft, automobile and ships are primarily due to fatigue. Where any structure experiences fluctuating loading during service its load carrying capacity decreases due to a process known as fatigue. Fatigue damage accumulates during every cycle of loading the structure experiences during its operation. When this accumulated damage reaches a critical value, a fatigue crack appears on the structure under service loading. A structure will have a finite fatigue life during which fatigue cracks initiate and propagate to critical sizes leading to catastrophic failure of the structure. Therefore fatigue life consists of two parts: the first part is the life to the initiation of fatigue crack and the second part is the fatigue crack propagation to final fracture. On the other hand fatigue crack growth is the dominant phase for more ductile structures or material. Large structure like an aircraft has a large number of componen...
Effect of Single Overload Ratio and Stress Ratio on Fatigue Crack Growth
World Academy of Science, Engineering and Technology, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering, 2013
In this investigation variation of cyclic loading effect on fatigue crack growth is the studied. This study is performed on 2024 T351 and 7050-T74 aluminum alloys, used in aeronautical structures. The propagation model used in this study is NASGRO model. In constant amplitude loading (CA), effect of stress ratio has been investigated. Fatigue life and fatigue crack growth rate were affected by this factor. Results showed an increasing in fatigue crack growth rates (FCGRs) with increasing stress ratio. Variable amplitude loading (VAL) can take many forms i.e. with a single overload, overload band... etc. The shape of these loads affects strongly the fracture life and FCGRs. The application of a single overload (ORL) decrease the FCGR and increase the delay crack length caused by the formation of a larger plastic zone compared to the plastic zone due without VAL. The fatigue behavior of the both material under single overload has been compared. Keywords—Fatigue crack growth, overload ...
DIC Study of Fatigue Crack Growth after Single Overloads and Underloads
Procedia Structural Integrity, 2017
During their operation, modern aircraft engine components are subjected to increasingly demanding operating conditions, especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent degradation, one of which is creep. A model using the finite element method (FEM) was developed, in order to be able to predict the creep behaviour of HPT blades. Flight data records (FDR) for a specific aircraft, provided by a commercial aviation company, were used to obtain thermal and mechanical data for three different flight cycles. In order to create the 3D model needed for the FEM analysis, a HPT blade scrap was scanned, and its chemical composition and material properties were obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplified 3D rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The overall expected behaviour in terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a model can be useful in the goal of predicting turbine blade life, given a set of FDR data.
Effect on fatigue crack growth of interactions between overloads
Fatigue <html_ent glyph="@amp;" ascii="&"/> Fracture of Engineering Materials and Structures, 2002
Various types of interactions between overloads were studied in a 0.38% C low carbon steel. The retarding effect due to consecutive overloads is found to increase with the number of overloads, until it reaches a maximum. Similarly, it is found that a critical distance between overloads ensures the highest retarding effect, while shorter or longer spacing are less efficient for retarding crack growth. These effects are successfully explained using FEM calculations of the effective stress intensity factor. The kinematic hardening of the alloy, which is very efficient in ferritic±pearlitic steels, is shown to be mostly responsible for those effects. Taking into account the amplitude of kinematic hardening allows qualitative explanation of the observed effects. The order of application of the cycles during variable amplitude fatigue is thus important and should be taken into account for predicting fatigue lives.
4337 - Residual Stresses Behavior Under Fatigue Crack Cyclic Loading: A Finite Element Analyses
Icf11 Italy 2005, 2013
It is very well known that the residual stresses affect the fatigue crack behavior. However, very little it is known how the residual stresses are affected by the fatigue crack cyclic loading. This paper focuses this aspect of the fatigue behavior. In order to perform numerical finite element computation, a regular rectangular steel plate is modeled with a mesh following the shape characteristics of the Compact Tension Specimen. Plastic strains are induced in the rectangular steel plate, by applying irregular external loading, resulting a residual stress field similar to the one occurs in the welded butt joints. A Compact Tension Specimen with a residual stress field is obtained by deactivating some of the finite elements of the rectangular steel plate mesh. The Compact Tension Specimen is cycled loaded, simulating a fatigue process, and the residual stress field is evaluated under the influence of the crack tip plastic zone. The Crack Opening Displacement results are compared for the cases with and without residual stresses. The non-linear evaluation of the stresses is performed by the finite element code LUSAS.
Elucidating the Mechanism of Fatigue Crack Acceleration Following the Occurrence of an Underload
Advanced Engineering Materials, 2016
Fatigue Crack Growth Rate (FCGR) is altered by a single anomalous load exceeding cyclic maximum (Overload) or compressive load below cyclic minimum (Underload). The authors study fatigue crack acceleration due to a single compressive Underload using residual stress mapping (by synchrotron XRD) and crack closure analysis (by DIC). The relative influence and duration of these two principal causes of FCGR alteration are revealed. Validated FEA model is used for parametric analysis of the effect of baseline cyclic loading ratio and magnitude of Underload on the cyclic J-integral.