Development Of Crack Closure Research Articles

Propagation threshold and crack closure of small fatigue cracks.

The propagation behavior of fatigue crack from pre-cracks in smooth or notched specimens of a low-carbon steel was studied. The development of crack closure with crack growth was measured through the compliance method. An interferometric displacement gage was used to measure the opening displacement of surface cracks. All cracks started to grow from the pre-cracks at the maximum stress intonsity factor equal to the threshold value of the effective stress intensity range ΔKeffth∞ for long cracks. As a crack grew, the crack closure developed and the effective stress intensity range ΔKeff decreased. The crack stopped growing when ΔKeff dropped below ΔKeffth∞. The relation between the crack propagation rate and the effective stress intensity range was unique, irrespective of the crack length or the atress level, and agreed well with that for leng cracks. Non-propagating cracks made under the fatigue limit in smooth specimen were mostly Stage I cracks, and the fatigue limit of smooth specimen was determined by the propagation threshold of these Stage I cracks.

Mechanisms for the retardation of fatigue cracks following single tensile overloads: behavior in aluminum-lithium alloys

Mechanisms for the transient crack growth retardation of fatigue cracks following single tensile overloads are examined in a high-strength aluminium-lithium alloy 2090-T8E41, with specific emphasis on the role of fatigue crack closure. Mechanically, it is found that following the application of the overload, an immediate acceleration in fatigue crack growth is observed, which results from a reduction in (far-field) crack closure levels due to crack tip blunting. Subsequent crack growth retardation, as the crack penetrates the overload plastic zone, is associated with enhanced crack tip shielding prompted by compressive residual stresses ahead of the crack tip, crack deflection and consequent (near-tip) crack closure induced by asperity wedging in the immediate crack wake. Aluminum-lithium alloys, by virtue of their marked anisotropy and planar slip characteristics which promote meandering crack paths and hence high levels of shielding, are found to show large load interaction effects, and thus rank superior to other high strength aluminum alloys under tension-dominated variable amplitude loading spectra. However, under conditions where specimen surface layers are removed, or where cycling is performed at high (positive) load ratios, or involves compressive overload cycles (all processes which tend to limit the development of crack closure), the superior performance of aluminum-lithium alloys under variable amplitude fatigue loading may be compromised.

PROPAGATION AND CLOSURE OF SHORT FATIGUE CRACKS AT NOTCHES UNDER COMPRESSIVE MEAN STRESSES

Abstract— Single‐edge‐notched specimens of a low‐carbon steel were fatigued under cyclic in‐plane bending with compressive mean stresses. The development of crack closure with crack growth was studied both experimentally and theoretically. The relation between the crack opening stress and the crack length was a function of the minimum (compressive) applied stress, irrespective of the maximum stress. The effective stress intensity range was a unique parameter in correlating the crack growth rate, even if the crack was embedded in the compressive plastic zone. Under a constant minimum stress, the length of nonpropagating cracks became longer with increasing maximum applied stress. A theoretical model was proposed for predicting the crack opening stress on the basis of the compressive stress distribution at the minimum applied stress. The predicted value agreed fairly well with the experimental result. The model gave upper bounds of the crack growth rate and the length of nonpropagating fatigue cracks within the plastic zone.

AN ANALYSIS OF CRACK TIP SHIELDING IN ALUMINUM ALLOY 2124: A COMPARISON OF LARGE, SMALL, THROUGH‐THICKNESS AND SURFACE FATIGUE CRACKS

Abstract— The development of crack closure during the plane strain extension of large and small fatigue cracks has been investigated in a 2124 aluminum alloy using both experimental and numerical procedures. Specifically, the growth rate and crack closure behavior of long (∼17–38 mm) cracks, through‐thickness physically‐short (50–400 μm) cracks, and naturally‐occurring microstructurally‐small (2–400 μm) surface cracks have been examined experimentally from threshold levels to instability (over the range 10–12–10–6m/cycle). Results are compared with those predicted numerically using an elastic‐plastic finite element analysis of fatigue crack advance and closure under both plane stress and plane strain conditions. It is shown that both the short through‐thickness and small surface cracks propagate below the long crack threshold at rates considerably in excess of long cracks, consistent with the reduced levels of closure developed in their limited wake. Numerical analysis, however, is found consistently to underpredict the magnitude of crack closure for both large and small cracks, particularly at near‐threshold levels; an observation attributed to the fact that the numerical procedures can only model contributions from cyclic plasticity, whereas in reality significant additional closure arises from the wedging action of fracture surface asperities and corrosion debris. Although such shielding mechanisms are considered to provide a prominent mechanism for differences in the growth rate behavior of large and small cracks, other factors such as the nature of the stress and strain singularity and the extent of local plasticity are shown to play an important role.

Notch-root-radius effect on propagation of short fatigue cracks in notched specimens of low-carbon steel.

The propagation behavior of short fatigue cracks formed at a notch root was studied by using low-carbon steel plates with center notches of various tip radii. The development of crack closure with crack growth was measured through the compliance method. Although the growth rate of short cracks was higher than that predicted from the rate-ΔK relation for long cracks, it was uniquely related to the effective stress intensity range and the relation was identical for long cracks. The increase in Kop with increasing crack length was independent of the notch root radius. The length of a non-propagating crack was affected by the notch root radius when it was a short, part-through crack. As the length of a non-propagating crack became longer, the threshold stress intensity range increased and the effective opening fraction decreased. The threshold value of the effective stress intensity range took a constant value irrespective of the crack length.

Notch-plasticity effect on closure of short fatigue cracks.

A model is proposed to evaluate the effect of notch plasticity on the development of crack closure for a short fatigue crack propagating from a notch root under the zero mean stress or compressive mean stress. The model assumes that a fatigue crack closes completely at the minimum applied stress and that compressive plastic deformation take place near the notch as if there were no crack. Plastic yielding of the Dugdale type is made from the root of an elliptical notch in an infinite plate. The crack opening stress and the effective stress intensity range are computed as functions of crack length, applied stress, and notch geometry.

On the development of crack closure and the threshold condition for short and long fatigue cracks in 7150 aluminum alloy

In an attempt to analyze the behavior of physically “short” cracks, a study has been made of the development, location, and effect of crack closure on the behavior of fatigue cracks arrested at the “long” crack threshold stress intensity range, ΔKTH, in underaged, peak aged, and overaged microstructures in a 7150 aluminum alloy. By monitoring the change in closure stress intensity,Kcl, during thein situ removal of material left in the wake of arrested threshold cracks, approximately 50 pct of the closure was found to be confined to a region within ∼500 μm of the crack tip. Following wake removal, previously arrested threshold cracks recommenced to propagate at low load ratios even though nominal stress intensity ranges didnot exceed ΔKTH, representing the behavior of physically short cracks emanating from notches. No such crack extension at ΔKTH was seen at high load ratios. With subsequent crack extension, crack closure was observed to redevelop leading to a deceleration in growth rates. The development of such closure was found to occur over crack extensions comparable with microstructural dimensions, rather than those associated with local crack tip plasticity. Such results provide further confirmation that the existence of a fatigue threshold and the growth of physically short cracks are controlled primarily by crack closure, and the data are discussed in terms of the micro-mechanisms of closure in precipitation hardened alloy systems.

On the role of compression overloads in influencing crack closure and the threshold condition for fatigue crack growth in 7150 aluminum alloy

A study has been made of the effect of single compression cycles on near-threshold fatigue crack propagation in an I/ M 7150 aluminum alloy. Based on experiments at a load ratio of R= 0.10 on cracks arrested at the fatigue threshold ( δk th ) in under-, peak and overaged microstructures, large compression overload cycles, of magnitude five times the peak tensile load, were found to cause immediate reinitiation of crack growth, even though the applied stress intensity range did not exceed ΔK th . Following an initial acceleration, subsequent crack advance was observed to take place at progressively decreasing growth rates until rearrest occurred. Such behavior is attributed to measured changes in crack closure which vary the effective near-tip driving force for crack extension ( ΔK eff ). Specifically, roughness-induced closure primarily is reduced by the application of compressive cycles via a mechanism involving crack surface abrasion which causes flattening and cracking of fracture surface asperities. Closure, however, is regenerated on subsequent propagation resulting in the rearrest. Such observations provide further confirmation that the existence of a fatigue threshold is controlled principally by the development of crack closure and are discussed in terms of the mechanisms of closure in precipitation hardened alloys.

On the development of crack closure with crack advance in a ferritic steel

Etude du taux de developpement de l'arret des fissures en fonction de leur progression dans un acier ferritique. Le niveau d'arret se developpe rapidement et atteint celui d'une fissure longue, expliquant la croissance anormale des fissures courtes