Acoustic emission from stress corrosion cracks in aligned GRP

Acoustic emission (AE) produced by the propagation of stress corrosion cracks in an aligned glass fibre/polyester resin composite material has been recorded. Tests have been carried out over a range of crack growth rates and the variation of AE with crack velocity/applied stress intensity has been examined. The main source of AE is fibre fracture and there is a one-to-one relationship between the number of fibre fractures and the number of high-amplitude AE signals. This enables crack growth to be monitored directly from acoustic emission. The amplitude of AE signals produced by fibre failure appears to be proportional to the fracture stress of the fibres, although further analysis requires a greater understanding of the generation, transmission and detection of AE signals. This work demonstrates that stress corrosion cracking is an ideal source for the study of AE produced by fibre fracture without complications caused by interface effects, such as fibre debonding or pullout.On leave from the Technical University of Wroclaw, Wroclaw, Poland.     

Calculation of weight functions for three-dimensional bodies with cracks

A new algorithm is proposed for the numerical calculation of weight functions used in the determination of stress intensity factors by the finite elements method. The algorithm is based on the method of equivalent volume integration. It is shown that weight functions can be obtained for cracks in three-dimensional bodies with the stress intensity factor averaged over a small section of the crack front. A numerical example demonstrating the usefulness of the algorithm is presented.Moscow. Moscow Physico-Engineering Institute. Translated from Problemy Prochnosti, No. 11, pp. 32–36, November, 1989.     

Acoustic emission from surface fatigue cracks in SS41 steel

Crack penetration behaviour by fatigue crack propagation and the measurements of acoustic emission (AE) before and after crack penetration were examined using SS41 steel plate. The experimental crack shape of the SU type which has an out-of-plane degree of freedom was in good agreement with the calculated shape. But this was not so for the S type which has a restriction to out-of-plane freedom. The crack propagation behaviour on the front surface appears not to change markedly after penetration. However, crack growth on the back surface appears to accelerate. As a crack propagates, substantial acoustic emission occurred just prior to penetration. Then it decreased and the crack penetrated. Intense AE emanation has been attributed to a transition from a plane strain condition to a plane stress condition.     

Acoustic emission mechanisms during high-cycle fatigue

Acoustic emission was obtained during fatigue crack propagation in a D6 tool steel and a 1015 mild steel using two different load ratios, R. It was found that the slope of the acoustic emission count rate vs stress intensity factor was higher when R = 0.2 than when R = 0.4. These results are described in terms of several models of acoustic emission mechanisms available in the literature as well as by a proposed new model. It is concluded that the observed R dependence can be explained by the combination of the contribution of two different models. These two models are the new plastic yielding as well as the crack tip fracture processes.     

Acoustic emission applied to crack advance in overlay welds in reactor bodies

Summary Acoustic-emission measurements have been made on crack initiation in growth at weldbase metal boundaries in the materials used for oil hydrocracking reactor vessels. Tests have been made on cylindrical specimens 80 m in diameter and 45 mm high having welded protective layers 7–8 mm thick at one end, which have been kept for 24 h in a medium whose parameters were close to working ones. When the specimens had been cooled to room temperature, cracking occurred, which could be detected for 30 h. The mechanisms of cracking at the weld-base metal boundary are discussed. There are time differences in the acoustic emission activity for specimens with and without welds.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 25, No. 2, pp. 79–81, March–April, 1989.     

Acoustic emission during heating of strained aluminum

The phenomenon of acoustic emission during annealing of preliminarily strained polycrystalline aluminum was observed. Although processes accompanying the metal annealing belong to thermoactivated (microscopic motion, diffusion) phenomena, the acoustic oscillations are generated in the events of spontaneous macroscopic motions of grain boundaries. This is confirmed by the results of analyzing the effective activation energy and the investigation of the grain boundary structure.     

Engineerng method of constructing a weight function for plane normal-separation cracks in three-dimensional bodies

On the basis of an analysis of the known solutions for a circular and for a semiinfinite normal-separation crack with straight front in an infinite body we constructed, in analogy to the method of Burns and Oore, a weight function for elliptical cracks. Its use for finding the stress intensity factors K_I at the point of minimal curvature under conditions of uniform loading leads to a maximal error of 10%; at the point of minimal curvature of the crack front the error increases with decreasing ratio of the semiaxes of the ellipse. With the aid of this solution the weight function is found in bounded bodies with a quarter-elliptical, semielliptical, and elliptical crack. A comparison of the data obtained by this method with the values of K_I calculated by the finite-element method under nonuniform loading showed that the suggested method is very accurate.Translated from Problemy Prochnosti, No. 10, pp. 14–22, October, 1992.     

Domain-independent values of the J-integral for cracks in three-dimensional residual stress bearing bodies

This paper describes an approach for computing domain-independent values of the J-integral in the finite element context for three-dimensional bodies containing residual stress. In the analysis of cracked bodies containing residual stress, the usual domain integral formulation results in domain-dependent values of J, and this paper discusses modifications that yield domain independence. Two correction terms are defined. The first of these relates to the spatial gradients of non-mechanical strains in the crack-driving direction, and the second accounts for plastic dissipation included in the material state, but unrelated to fracture. The paper further presents results for two examples recently discussed in the literature. Application of the corrections in these two cases demonstrates the ability of the approach to obtain path-independent domain integral results in residual stress bearing bodies.