by Budden, Peter J.; Ainsworth, Robert A.

This paper describes a preliminary examination of the effect of in-plane constraint on creep crack growth under widespread creep conditions using the Q stress. Plane strain is assumed. Damage models for fracture of the process zone based on both ductility exhaustion and stress rupture are shown to predict a variation of the crack growth rate with Q. Lower levels of constraint lead to lower crack growth rates for a given C*. The results are used to outline a high temperature failure assessment diagram approach to constraint-dependent creep crack growth.

DOI: 10.1023/A:1007416926604
Print publication date: 9/1/1997
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by Marchal, Yves; Walhin, Jean-François; Delannay, Francis

The precision of the measurement of the essential work of fracture requires an accurate determination of the critical ligament length below which the ligament is in a mixed mode stress state and the failure mechanisms considerably change. A statistical procedure is proposed which allows to determine easily and accurately this critical ligament length. After rejection of the specimens having too small ligament lengths, w_e is obtained by extrapolation of the remaining data for zero ligament length. The number of data points from which w_e is calculated is also shown to strongly influence the precision of the measurement. The procedure is applied to the measurement of the toughness of sheets of Al and Zn alloys and of a low density polyethylene.

DOI: 10.1023/A:1007482121146
Print publication date: 9/1/1997
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by Buchanan, Dennis J.; John, Reji; Johnson, David Alan

This paper discusses the development of an optimization procedure to deduce the bridging stress from the crack opening displacements (COD) measured during fatigue crack growth. Finite element analysis was conducted using the center-cracked geometry to verify the optimization procedure. The proposed procedure successfully predicted the bridging stress distributions with zero stresses at the crack tip and the bridging stress distributions with non-zero stresses at the crack tip. The results also showed that COD measurements spaced at ≈ 0.8-1.0 mm are sufficient for reliable prediction of bridging stresses. Accurate prediction of bridging stresses near the crack tip required COD data within ≈ 0.1 mm from the crack tip. The application of the proposed procedure to a metal matrix composite (SCS-6/TIMETAL®21S) is also discussed.

DOI: 10.1023/A:1007495331890
Print publication date: 9/1/1997
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by Hallbäck, N.

Numerical calculations have been carried out to assess the influence of both finite geometry effects as well as material properties on mixed mode fracture of aluminium. These effects have been studied in close connection to experimental data for two aluminium alloys found in the literature. Interactions between the crack tip and the outer boundary have, for one of these alloys, been quantified in two ways. Firstly, by evaluating a number of non-singular stress on mixed mode fracture have been examined within the framework of a recently suggested effective plastic strain criterion. The other alloy was addressed in order to furnish a limited investigation concerning the sensitivity of this criterion with respect to material properties. The main conclusions arrived at in this paper are: (i) Boundary induced constraints may relocate the transition between different operative fracture modes and hence be responsible for scatter of experimental achieved under different testing conditions. (ii) The two alloys under consideration were predicted to behave very differently due to variations in the flow behaviour. Different behaviour was also confirmed by the experimental results.

DOI: 10.1023/A:1007339103082
Print publication date: 9/1/1997
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by Yue-Sheng, Wang; Zi-Mao, Zhang; Gui-Lan, Yu

The anti-plane problem of the transient debonding of an interface between two orthotropic materials is examined. The material principal axes are allowed to be oblique to the interface. The debonding is modeled as an interface crack propagating self-similarly from zero-length. The extending speed is assumed to be subsonic, transonic or supersonic. We first consider the dynamic debonding under the moving concentrated loading. The moving dislocation model of self-similar propagation of an interface crack is used to formulate the problem in a singular integral equation which is solved analytically. The stress singularity at the crack tips is discussed. The order of singularity is found to be one-half for subsonic debonding and to vary between zero and one-half depending on the crack speeds for transonic debonding. The dynamic stress intensity factors/coefficients for these two situations are presented in closed-form. The paper also concludes that supersonic debonding is impossible unless the loads are directly applied to the crack tips. Finally, the results for dynamic debonding under xⁿ-type loads are presented by using the superposition method.

DOI: 10.1023/A:1007400728660
Print publication date: 9/1/1997
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