Crack Growth Propagation Research Articles

THE WEDGE FRACTURE TEST A NEW METHOD FOR MEASUREMENT OF FOOD TEXTURE

ABSTRACTThe fracture properties of foods are relevant to texture but can be difficult to measure because of limitations of size or shape. Many established engineering tests for the measurement of crack growth and unstable propagation of cracks require specific test geometries, sizes and compliances of the specimen. With food materials this is rarely possible. The wedge penetration technique can usefully be adapted to foods to determine fracture parameters of brittle and semi‐brittle foods such as moderately hard cheeses and raw and cooked fruits and vegetables.

Low-cycle fatigue in Ni3Al-based alloys

The low-cycle fatigue properties of hot-extruded powders of a Ni3Al-based alloy, IC 218, with nominal composition Ni-16.5Al-8.0Cr-0.4Zr-0.1B (at %) have been evaluated at room temperature. Tests were conducted under total strain conditions in a laboratory air environment. Results indicate that the low-cycle fatigue performance of the PM processed IC 218 nickel aluminide is superior to other structural alloys especially at higher strain amplitudes. These results are explained in terms of the high ductility of the fine-grain material and good crack growth propagation resistance in these alloys. Stress response curves for annealed IC 218 alloys indicate considerable cyclic hardening followed by cyclic softening. The onset of cyclic softening is found to occur at a constant cumulative plastic strain. The critical cumulative plastic strain criteria are verified for step-loaded IC 218 nickel aluminide coupons.

Dynamic Crack Propagation in Precracked Cylindrical Vessels Subjected to Shock Loading

Previous work has shown that a speed-independent dynamic fracture toughness property can be used in an elastodynamic analysis to describe crack initiation and unstable propagation under impact loading. In this paper, a further step is taken by extending the analysis from simple laboratory test specimens to treat more realistic crack-structure geometries. A circular cylinder with an initial part-through wall crack subjected to an impulsive loading on its inner surface is considered. The crack is in a radial-axial plane and has its length in the axial direction long enough that a state of plane strain exists at the center of the crack. Crack growth initiation and propagation through the wall is then calculated. It is found that, once initiated, crack propagation will continue until the crack penetrates the wall. Crack arrest within the wall does not appear to be possible under the conditions considered in this paper.

The growth rates of diamond cavities in polycarbonate

An attempt to quantify post-yield fracture in polymers has been made by examining the growth rates of diamond cavities in cold-drawn polycarbonate. The application of the basic principles of fracture mechanics show that the growth rate is extremely stress sensitive. Also, a simple Eyring equation, based on the net section stress, appears to fit the available data. The “activation volumes” derived from the crack growth and neck propagation are of the same order which suggests that the rate of simple shear yielding adjacent to the cavity controls the growth rate.

Dynamic fracture toughness parameters for HY-80 and HY-130 steels and their weldments

Lower bound dynamic fracture toughness parameters for HY-80 and HY-130 steel and their weld metals are identified. Specific values of the parameters K Id and K Im obtained from direct measurements are reported together with estimates inferred from the large body of Charpy energy, nil ductility transition temperature and dynamic tear energy measurements. The emphasis is on reasonable lower bound values at 30°F, the lowest anticipated service temperature, for use in elastodynamic analyses of crack growth initiation, propagation, and arrest in ship structures. For these conditions, it has been found that the ratio K Id / σ Y is approximately equal to 2 in 1 2 for HY-80 steel. For HY-130 steel and the HY-80 and HY-130 weld metals under these same conditions, K Id / σ Y is approximately 1 in 1 2 . Consequently, HY-80 plate appears to be substantially more resistant to fracture under dynamic loading than are the other three grades examined.

Fatigue of viscoelastic polymers: 2. Fractography

The results of a fractographic study of fatigue failure in viscoelastic polymers is presented. Tests were conducted on a spherulitic low-density polyethylene in reversed loading using non-symmetrical deformation conditions. The microfractographic features have been found to depend on the deformation programme, the temperature of the test and the position on the fracture surface. The latter has been related to the different stages of growth revealed by plotting crack-growth propagation data. As well as interspherulitic fracture, other mechanisms that are sometimes prevalent involve cold-drawing, (fibrillation), and/or lamellar reorientation. The appearance of fatigue fracture surfaces of a non-spherulitic low-density polyethylene, a high-density polyethylene and a plasticized poly(vinyl chloride) corroborate the structural interpretations proposed to account for the observations with the spherulitic low-density polyethylene.