J. Harding

Deformation of thermosetting resins at impact rates of strain. Part I: Experimental study

Abstract Three thermosetting resins have been tested in compression at strain rates between 10 −3 and 5×10 3 s −1 . A smaller number of tests have been performed on the same resins at similar strain rates in tension. The testing technique is described and the results obtained are presented. Significant differences between the three materials were found over the whole range of strain rates. All three exhibited a pronounced strain rate sensitivity. Temperature measurements, performed in compression tests at the higher strain rates, showed evidence of adiabatic heating. This revealed the presence of energy storage mechanisms in both yield and strain–stiffening regimes.

Determination of interlaminar shear strength for glass/epoxy and carbon/epoxy laminates at impact rates of strain

Abstract A brief review is given of some of the techniques which have been used to determine the interlaminar shear strength of laminated composites under impact loading. A new technique employing a double-lap shear specimen, where failure occurs on predetermined interfaces, is then described and results are presented for tests on this design of specimen at both a quasi-static and an impact rate of strain. The strain distribution along the failure plane is determined by using a finite element analysis. Large variations in both the shear strain and the normal strain were observed, the magnitude of this variation being sensitive both to the elastic properties of the different reinforcing plies and to the chosen stacking sequence. Results are presented for the interlaminar shear strength at the interface between (a) two plain-weave glass/epoxy plies, (b) two plain-weave carbon/epoxy plies, and (c) a plain-weave glass/epoxy and a plain-weave carbon/epoxy ply. In each case the mean value of the interlaminar shear stress at failure was found to increase significantly as the loading rate was raised from quasi-static to impact.

The effect of strain rate on the interlaminar shear strength of a carbon/epoxy cross-ply laminate: comparison between experiment and numerical prediction

A previously developed design of single-lap interlaminar shear specimen suitable for tests under impact rates of loading is re-examined and used to determine the interlaminar shear strength of a cross-ply carbon/epoxy material with important applications in the aircraft industry. The test has been modelled dynamically with an explicit-finite element code. The results of this analysis have been used to examine the state of stress just prior to failure and, in terms of a quadratic stress-interaction failure criterion, to predict the onset of delamination. The numerical predictions are compared with the behaviour observed experimentally.

Effect of strain rate on the interlaminar shear strength of carbon-fiber-reinforced laminates

Abstract A detailed study is made of the double-lap design of specimen for determining the effect of loading rate on composite interlaminar shear strength. Finite-element analyses are used to determine the effects of varying stresses along the failure plane, of resin-rich regions at each end of the failure plane and of cracks propagating from both ends of the failure plane. Experimental results are obtained at a quasi-static and an impact rate of loading for the interlaminar shear strenght parallel to the fibres in a undirectional carbon/epoxy laminate and at interfaces across which the fibre orientation is 0/90 and ±45. In each case a small effect of strain rate is observed.

Localization of plastic deformation during high strain rate torsion testing of rolled homogeneous armour

A recently developed technique which allows a study of the localization of plastic flow during high-speed torsion testing is briefly described. High-speed photographs showing successive stages in the deformation of three rolled homogeneous armour (RHA) steel specimens are presented. Three stages are identified corresponding to uniform shear, localized shear and the propagation of ductile failure. The results are analysed to estimate the conditions under which localization and subsequent ductile fracture are initiated. The results are compared with a previous study on a soft iron.

Failure analysis of woven hybrid composites using a finite element method

Abstract A finite element method is used to determine the stress and strain distribution around a failed link in a carbon-reinforced ply in a woven carbon/glass epoxy laminate under tensile loading. On the assumption that delamination follows the initial tensile failure in the carbon link, the new stress and strain distribution is determined. The results are discussed in terms of the effect of stacking sequence on the tensile failure of woven hybrid laminates at quasi-static and impact rates of strain.

Use of high-speed photography to study localisation during high-strain-rate torsion testing of soft iron

Abstract A technique has been developed for using high-speed photography to study the continuous formation of regions of localised shear in thin-walled tubular specimens of very ductile Remco iron tested in a torsional split Hopkinson pressure bar apparatus at nominal strain rates of ∼3000 s−1. A fibre-optic flash unit provides adequate illumination at framing rates up to 200 000 s−1. Photographic grids having 5 or 10 lines (or squares) per mm allow strains up to the order of 1000% to be estimated and the start of ductile tearing to be observed.