Data for Elastic-Plastic Materials Models : Modelling : Measurement Techniques : Advanced Materials : Science + Technology : National Physical Laboratory

Data for Elastic-Plastic Materials Models

At stress and strain levels above the limit for linear behaviour, relationships between stress and strain are non-linear. With elastic-plastic models, calculations of stress and strain distributions at low strains are based on linear elasticity. The onset of non-linearity is due to plastic deformation and occurs at a stress level regarded as the first yield stress. The subsequent increase in stress with strain is associated with the effects of strain hardening. In order to calculate some of the parameters in elastic-plastic models, it is necessary to obtain stress values from different tests under the same state of yielding. Suitable data can be obtained from uniaxial tension, shear or compression tests.

The most commonly used model is based on the von Mises yield criterion. But, our tests show that this model doesn't work well in adhesives, such as rubber-toughened adhesives, where plasticity is sensitive to the hydrostatic component of stress. In the commercial FEA software package ABAQUS the linear and exponent forms of the Drucker-Prager materials model are available. But even these models cannot describe the non-linear behaviour of rubber-toughened adhesives in certain joint configurations (e.g. butt joint). Experimental data from a butt joint test is shown along with predictions using the von Mises, linear Drucker-Prager and exponent Drucker-Prager models. Out of these three models, the exponent Drucker-Prager is the most accurate for this adhesive. A new elastic-plastic model (the NPL cavitation model) has been developed at NPL, which includes the influence of rubber-particle cavitation on the plastic deformation behaviour of the adhesive.

Both the linear and non-linear properties of adhesives are dependent on the rate at which the adhesive is deformed. A proper description of rate-dependent behaviour is needed for accurate predictions of performance under impact loading. NPL have produced guides and reports on the preparation and testing of bulk adhesives.