Crack closure

About: Crack closure is a research topic. Over the lifetime, 28157 publications have been published within this topic receiving 588158 citations.

Combined mode fracture via the cracked Brazilian disk test

Abstract: An analysis is made of the cracked Brazilian disk test. Explicit results are given for the stress intensity factors of a crack aligned at any direction to the applied load. Various approximations are considered and a simplified ‘short crack’ approximation is shown to be accurate for l/a≤0.3 (l is crack length, a the disk radius). At certain angles it is found that crack closure should occur and as a frist step the effect of friction is taken into account for such cases by using this short crack approximation. Finally, experimental results are presented to confirm and elucidate the use of this specimen geometry for the problem of combined mode fracture.

Factors influencing the mechanism of superlong fatigue failure in steels

Abstract: When the fatigue life N f of a specimen of 10 mm in thickness is longer than 10 8 cycles, the average fatigue crack growth rate is much less than the lattice spacing (∼0.1 A or 0.01 nm) that is 10 -11 to 10 -12 m/cycle. In the early stage of the fatigue process, the crack growth rate should be much less than the average growth rate, and accordingly we cannot assume that crack growth occurs cycle by cycle. In this paper, possible mechanisms for extremely high cycle fatigue are discussed. Of some possible mechanisms, a special focus was put on a newly found particular fatigue fracture morphology in the vicinity of the fracture origin (non-metallic inclusions) of a heat-treated alloy steel, SCM435, which was tested to N ≥ 10 8 . The particular morphology observed by SEM and AFM was presumed to be influenced by the hydrogen around inclusions. The predictions of the fatigue limit by the √area parameter model are ∼ 10% unconservative for a fatigue life of N f = ∼10 8 , though it successfully predicts the conventional fatigue limit defined for N = 10 7 . Thus, the fatigue failure for N ≥ 10 8 is presumed to be caused by a mechanism which induces breaking or releasing of the fatigue crack closure phenomenon in small cracks. In the vicinity of a non-metallic inclusion at the fracture origin, a dark area was always observed inside the fish-eye mark for those specimens with a long fatigue life. Specimens with a short fatigue life of N f = ∼10 5 do not have such a dark area in the fish-eye mark. SEM and AFM observations revealed that the dark area has a rough surface quite different from the usual fatigue fracture surface in a martensite lath structure. Considering the high sensitivity of high-strength steels to a hydrogen environment and the high hydrogen content around inclusions, it may be concluded that the extremely high cycle fatigue failure of high-strength steels from non-metallic inclusions is caused by environmental effects, e.g. hydrogen embrittlement coupled with fatigue.

Cohesive models for damage evolution in laminated composites

Abstract: A trend in the last decade towards models in which nonlinear crack tip processes are represented explicitly, rather than being assigned to a point process at the crack tip (as in linear elastic fracture mechanics), is reviewed by a survey of the literature. A good compromise between computational efficiency and physical reality seems to be the cohesive zone formulation, which collapses the effect of the nonlinear crack process zone onto a surface of displacement discontinuity (generalized crack). Damage mechanisms that can be represented by cohesive models include delamination of plies, large splitting (shear) cracks within plies, multiple matrix cracking within plies, fiber rupture or microbuckling (kink band formation), friction acting between delaminated plies, process zones at crack tips representing crazing or other nonlinearity, and large scale bridging by through-thickness reinforcement or oblique crack-bridging fibers. The power of the technique is illustrated here for delamination and splitting cracks in laminates. A cohesive element is presented for simulating three-dimensional, mode-dependent process zones. An essential feature of the formulation is that the delamination crack shape can follow its natural evolution, according to the evolving mode conditions calculated within the simulation. But in numerical work, care must be taken that element sizes are defined consistently with the characteristic lengths of cohesive zones that are implied by the chosen cohesive laws. Qualitatively successful applications are reported to some practical problems in composite engineering, which cannot be adequately analyzed by conventional tools such as linear elastic fracture mechanics and the virtual crack closure technique. The simulations successfully reproduce experimentally measured crack shapes that have been reported in the literature over a decade ago, but have not been reproduced by prior models.

Measurement of adherence of residually stressed thin films by indentation. I. Mechanics of interface delamination

Abstract: A fracture analysis of indentation‐induced delamination of thin films is presented. The analysis is based on a model system in which the section of film above the delaminating crack is treated as a rigidly clamped disc, and the crack extension force is derived from changes in strain energy of the system as the crack extends. Residual deposition stresses influence the cracking response by inducing buckling of the film above the crack and by providing an additional crack driving force once buckling occurs. A relation for the equilibrium crack length is derived in terms of the indenter load and geometry, the film thickness and mechanical properties, the residual stress level, and the fracture toughness of the interface. The analysis provides a basis for using controlled indentation cracking as a quantitative measure of interface toughness and for evaluating contact‐induced damage in thin films.

Crack propagation in high stress fatigue

Abstract: An examination of the fracture surfaces of ductile metal specimens broken in high stress fatigue has revealed the occurrence of fracture ripples similar in appearance to those resulting from low stresses, but considerably larger. By sectioning specimens strained to various stages of the fatigue stress cycle, it has been shown that crack propagation and fracture ripple formation are the consequences of the successive rounding and sharpening of the crack tip during each stress cycle. The hypothesis that high stress fatigue is different in character from low stress fatigue and results from a ‘delayed static fracture’ is not in accord with the present observations.