Onset Of Crack Extension Research Articles

Dependence of crack growth initiation on the form of the stress-strain diagram of the material in tension

The problem of growth of a crack in a plate with extensive inelastic deformation was studied by the strain energy density theory. Considered was a rectangular plate with a center through the thickness crack subjected to a uniform stress perpendicular to the crack plane. The stress, deformation and strain energy density fields of the plate were determined by a finite element code based on the incremental theory of plasticity and the von Mises yield criterion. A host of true stress-true strain diagrams representing a variety of aluminum alloys with different degrees of ductility and strain-hardening were considered in order to study the influence of material behavior on crack growth. The critical stress and the plastic enclaves at the onset of crack extension were determined for the various materials considered. From the present study the dependence of these quantities on the mechanical behavior of the material of the plate was clearly established.

Thickness Effect on Elastic-Plastic Fracture Toughness

This paper examines the effect of plate thickness on ductile fracture toughness. Both numerical and experimental investigations are discussed. Considerable attention is paid to the stress, strain and deformation in the vicinity of the crack tipin order to analyze fracture behavior. Fracture toughness tests are performed for center-cracked specimens of SUS430 stainless steel and compact tension specimens of SS41 mild steel. A method to evaluate the critical J-integral value at the onset of crack extension is also proposed.

Characterization of the sub-critical crack growth of low alloy steels in a hydrogen environment by means of a acoustic-emission.

Slow strain rate tests (SSRT) have been carried out using precracked compact tension specimens of low alloy steels in purified hydrogen gas at ambient temperature. By the use of elastic-plastic fracture mechanics parameters, the influence of hydrogen gas on sub-critical crack growth has been investigated in terms of the J-value at the onset of crack extension and also from a view point of fracture modes. Acoustic-Emission (AE) during the SSRT tests has been monitored to clarify the cracking behavior in hydrogen gas and by mechanical tearing. In a hydrogen gas environment, it is clearly shown that a crack initiates and propagates in a quasi-cleavage manner until applied the J value reaches J1c and after that the fracture mode changes to mixture of ductile dimple and quasi-cleavage. This result suggests that mechanical tearing occurs at J-values greater than J1c even in a hydrogen environment. In SA 533 B steels, the AE activity showed two energy peaks with respect to loading levels, whose origins are attributable to the onset of quasi-cleavage fracturing caused by the environment and to the onset of mechanical tearing.

The stability of growth of a through-wall circumferential crack in a cylindrical pipe subjected to bending deformation

Tada, Paris and Gamble have used the tearing modulus approach to examine the stability of growth of a through-wall circumferential crack in a 304 stainless steel circular cylindrical pipe subject to bending deformation. They showed that crack growth is stable, in the sense that growth requires the rotation imposed at the pipe-ends to be increased, provided the pipe length is less than a critical length L c , which is given by their analysis. The Tada-Paris-Gamble analysis focuses on the question of the stability, or otherwise, of crack growth at the onset of crack extension. The analysis does not consider the possibilities that (a) instability might occur after some stable crack growth, and (b) arrest might occur after some unstable growth. A study of these aspects of the circumferential crack growth problem using the tearing modulus approach is precluded by the geometry dependence of the J- crack growth resistance curve. Consequently the present paper uses a crack tip opening angle criterion to describe crack growth, and thereby demonstrates that possibilities (a) and (b) should both occur, depending on the initial crack length and pipe length. In terms of relevance to the technologically important problem of cracking in Boiling Water Reactor piping, the important conclusion stemming from the paper's analysis is that stability of crack growth after the onset of crack extension is assured if the pipe length is less than a critical length L′ c . L′ c is less than L c , the critical length relevant to the onset of crack extension, but it is still appreciably greater than the pipe run lengths in actual reactor piping systems, and safety against guillotine failure of a pipe is therefore generally assured.

A study on the Instrumented Charpy test.

The Instrumented Charpy test was conducted on U-, V-, and fine notched specimens and a fatigue precracked specimen of A508 C1.3 steel and the effect of specimen configuration, test temperature and loading speed on the dynamic fracture toughness was investigated. Comparing the load-displacement curves of the U-notched and fatigue cracked specimen, the fracture toughness at the onset of crack extension was determined and proved to coincide with the lowest value of the dynamic fracture toughness. The J-integral and R-curve of the fatigue cracked specimen were corrected for crack extension by a proposed direct calculation procedure in place of ASTM's step-by-step one.

Essential work of fracture (w e) versus energy dissipation rate (J c) in plane stress ductile fracture

Two measures of fracture toughness have been investigated. The first is the Cotterell's essential work of fracture (we) which reflects the energy absorbed in the process of localized necking and decohesion occurring within the crack tip region. The second is the familiar critical energy dissipation rate associated with the onset of crack extension and commonly designated by Jc. Total of 48 fracture tests have been performed on thin aluminum double-edge-notched panels and thin compact tension specimens with varying crack size-to-ligament ratios. In a simple experimental procedure it has been established that both measures are equivalent, at least under the plane stress conditions, and that they both represent the fraction of energy which is transmitted through the plastic deformation field into the crack tip region. The ratio “essential work of fracture/total work of fracture” has been suggested as a quantitative measure of the energy transmission process. Certain predictions are made concerning variations of the energy transmission factor (ETF) during the stable phase of ductile fracture propagation.

Effect of crack size on fracture of high yield stress materials

AbstractThe paper considers the behaviour of (brittle) high strength materials for which the onset of crack extension and fracture instability are coincident in a fracture mechanics type test where the crack is sufficiently long that the plastic deformation, which is associated with the onset of crack extension, is highly localized to the vicinity of the crack tip. A theoretical analysis shows that both the J integral at the onset of crack extension and the effective fracture toughness decrease below their long crack values as the pre-existing crack size decreases. However, the predicted decreases are only small (≲515%) as the pre-existing crack size decreases to a level at which the applied stress approaches the tensile yield stress of the material. The theoretical predictions are consistent with the experimental results obtained by Wilshire and Knott.

The transition between stress corrosion crack growth and plastic fracture—II. The effect of loading pattern

The paper addresses the problem of predicting the onset of plastic fracture at the tip of a growing stress corrosion crack, using data from laboratory fracture mechanics tests. A theoretical analysis for a particular model: namely that of the place strain deformation of a solid with two symmetrically situated deep cracks, and with tension of the small remaining ligament, shows that plastic fracture occurs at a J value that is not constant, but depends on whether the loading is load or displacement control. This result, which is valid for materials for which the onset of crack extension and unstable fracture are coincident in a rising load fracture mechanics test, provides valuable support for the view that great care must be exercised when using fracture mechanics procedures to predict the transition between stress corrosion crack growth and plastic fracture in such materials.

The transition between stress corrosion crack growth and plastic fracture

The paper focuses on the behaviour of materials for which the onset of crack extension and unstable fracture coincide in a rising load fracture mechanics test. A theoretical analysis shows that use of the experimental test J Ic value gives a non-conservative prediction of unstable fracture when a stress corrosion crack grows in a solid that is subject to a sustained high stress. Consequently, when an engineering component is to be used at high stress levels in an environment where stress corrosion cracking might be expected, there are clear advantages in having a material which exhibits stable crack growth in a fracture mechanics test.

A note on Orowan mechanism below the root of an yielding crack like notch in a strain-hardening metal

Based on Orowan's historic model, a local fracture initiation criterion is proposed for high strength strain-hardening metals. A crack like notch geometry is studied in the following Von Mises yielding materials: (a) an ideally plastic-elastic solid, and (b) a power law strain-hardening metal. Orowan mechanism is seen to be dominant below the root region of an yielding crack like notch, just before the onset of fracture. Close to the elastic-plastic interface, Orowan's Kink-band forms inside the plastic region, when a mismatch Kink-stress (in radial component only) reaches a critical value in this particular region. It is extremely important to note that, this narrow Kink-band zone, where the micro-cracks are likely to nucleate, lies at a distance approx. 3 4 s , where s is the size of the plastic zone on the crack extension plane under a plane strain deformation. A reverse slip mechanism operates in this region in addition to the presence of a pure hydrostatic tension, just before the release of this critical Kink-stress. Due to this stress history, a large inhomogeneous strain-localization occurs in a narrow band, which could then interact with the free notch surface before the onset of final instability. Thus, at the onset of crack extension, (satisfying Griffith-Irwin criterion of fracture), the stress intensification at the notch tip root is directly proportional to the strength of this critical strain-localization and inversely proportional to the plastic zone size on the crack extension plane. Hence, it is concluded that: Orowan's mechanism and McClintock's criteria for critical strain-localizations should play the most important roles for predicting the local fracture behaviour of metals.

The net-section stress at the onset of crack extension: Part 2—The effects of geometric configurational parameters

A detailed examination of the effects of configurational parameters underlines the limited range of applicability of the approximate approach by which the onset of crack extension at a crack tip is related to the attainment of a critical net-section stress. The examination shows that the net-section stress is very sensitive to configurational geometry.

The net-section stress at the onset of crack extension: Part 1—The effects of material and dimensional parameters

This paper examines the effects of material and dimensional parameters on the magnitude of the net-section stress at the onset of crack extension from a pre-existing defect in a ductile material. Attention is focused on the behaviour of a centre-cracked panel deforming in a plane stress mode. The conditions for which the net-section stress is essentially independent of dimensional parameters are highlighted.

Effect of specimen type on J-integral at the onset of crack extension

Effect of specimen type on J-integral at the onset of crack extension