Determining a threshold Strain Nonuniformity Index (SNI) to predict failure in sheet metal components

The Strain Nonuniformity Index (SNI) was introduced as a strain distribution based failure criterion about 10 years ago and its application to prediction of failure in industrial sheet metal components was demonstrated in the previous Conferences. Quantitatively, the SNI represents the rate of change of spatial strain gradient. This means that higher the SNI, sharper the neck, and hence greater is the likelihood of failure. It is immaterial as to how the neck / strain gradient came into existence, and failure is linked with the threshold value of this quantity, beyond which failure could be expected. Since phenomenological parameters are not linked with the process of establishing a threshold SNI, it does not seriously matter whether it was aluminium that was formed or whether it was steel, it does not very much matter if it was hot forming or cold forming. The present paper describes the methodology of estimating the threshold SNI for each of the numerous critical planes in the deforming sheet metal component. Interestingly, the relationship between SNI of major strain and SNI of the thickness strain was found to lie in a narrow band for a diverse set of materials. This therefore helps in finding the critical planes on which the SNI at a given punch travel has crossed the threshold value and failure locations so determined have correlated well with the shopfloor predictions as presented earlier.