Vibro-acoustic nonlinear wave modulations are investigated experimentally in a cracked aluminum plate. The focus is on the effect of low-frequency vibration excitation on modulation intensity and associated nonlinear wave interaction mechanisms. The study reveals that energy dissipation – not opening–closing crack action – is the major mechanism behind nonlinear modulations. The consequence is that relatively weak strain fields can be used for crack detection in metallic structures. A clear link between modulations and thermo-elastic coupling is also demonstrated, providing experimental evidence for the recently proposed non-classical, nonlinear vibro-acoustic wave interaction mechanism.

https://journals.sagepub.com/doi/pdf/10.1177/1475921711414236

Nonlinear acoustics for fatigue crack detection – experimental investigations of vibro-acoustic wave modulations:

Mechanisms influencing the plane-strain fracture toughness behavior of commercial aluminum-lithium alloys at cryogenic temperatures are investigated as a function of microstructure and plate orientation. It is confirmed that certain alloys show a markedincrease in tensile ductility and toughness withdecrease in temperature, although such behavior is not found in the short-transverse orientations, or for all alloys and aging conditions. Specifically at lower temperatures, the majority of Al-Li alloys, namely 2090-T8E41, 8091-T8X, 8090-T8X, and 2091-T351, show a significantincrease in fracture toughness in the in-plane orientations (L-T, T-L), without any apparent change in fracture mode. Such behavior is attributed primarily to loss of through-thickness constraint resulting from enhanced short-transverse delamination (termed crack-divider delamination toughening), consistent with observed reductions in plane-strain ductility and short-transverse (S-L, S-T) toughness. Conversely, in underaged microstructures of 8091, 8090, and peak-aged 2091, a decrease in toughness with decreasing temperature is found for both L-T and S-L orientations, behavior, which is associated conversely with a fracture-mode change from ductile void coalescence to brittle transgranular shear and integranular delamination at lower temperatures.

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Cryogenic toughness of commercial aluminum-lithium alloys: Role of delamination toughening

An analysis of crack growth in thin-sheet metal via a cohesive zone model

Plates are easily susceptible to buckling under compression, in particular when plate's thickness becomes sufficiently small with respect to others plate's sizes; such a mode of failure is often prevalent with respect to strength failure The buckling phenomena under tension loading can also occur, especially in plates containing defects such as cracks or holes; when the buckling load is reached, complex wrinkling deflection patterns in compressed regions develops around such imperfections In the present paper, the buckling analysis of variously cracked rectangular elastic thin-plates under tension and compression is considered A short explanation of the buckling phenomena in plates is recalled and several numerical analyses, carried out by using the Finite Element Method (FEM), are performed in order to determine the critical load multiplier, both in compression and in tension, by varying some plates' parameters In particular, the critical load multiplier is determined for different relative crack length, crack orientation and Poisson's coefficient of the plate's material which is made to range between 01 and 049 Moreover a simple approximate theoretical model to explain and predict the buckling phenomena in cracked plates under tension is proposed and some comparisons are made with FE numerical results in order to assess its reliability in predicting buckling load multipliers Finally, the obtained results are graphically summarised (in dimensionless form) in several graphs and some interesting conclusions are drawn

Buckling of cracked thin-plates under tension or compression

A finite element procedure is presented for the analysis of the buckling and postbuckling behavior of cracks in plates loaded in tension. The procedure proposed is applied to the problem of the centrally cracked plate in tension where the loading direction is perpendicular to the crack faces. The results of the analysis show that the buckling deformations can cause a considerable amplification of the stress intensity around the crack tip. This effect, which is due to a redistribution of the stress field in the plate, increases with the length of the crack.

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Karl Markström

Papers

Buckling of cracked members under tension

On fracture toughness and its size dependence for steels showing thickness delamination

Experimental determination of Jc data using different types of specimen