Finite element modelling of smart TRIP steel sensors and systems.

Abstract

Transformation Induced Plasticity (TRIP) steels undergo a phase transformation

when subjected to high levels of mechanical strain. This transformation from a

paramagnetic austenitic parent phase to a ferromagnetic martensitic phase is irreversible

and the resultant magnetic properties may therefore be used as a measure

of strain history. The transformation behaviour of TRIP steels has been recognised

as a potential smart characteristic and various proposals have appeared aimed at

producing a structure that performs its primary structural function as well a strain

sensing function simultaneously. However the strain induced nature of the transformation

implies that transformation will occur in areas of high stress concentration

and therefore engineered stress concentration features will be required to provide a

consistent measure of the changes in the magnetic properties of the material as a

function of applied load. In order to predict the performance of smart TRIP steel

sensors, an analysis method capable of quantifying the effectiveness of a component

in its dual role as structure and sensor is needed. The thesis addresses the development

of a methodology for correlating the changing magnetic permeability of

TRIP steel sensors and structures with martensitic transformation behaviour. The

prediction of the deformation behaviour including transformation is implemented

by considering a mechanical analysis based on the finite element method and a

constitutive model incorporating strain-induced martensitic transformation kinetics.

.Extensions to the model which allow for a wide range of deformation rates

and temperatures are also discussed. In order to demonstrate the application of the

methodology, an analysis of a simple tensile element used in strain measurement

applications is presented. The analysis also includes the effect of temperature on

the performance of the sensor. An analysis of a design proposal for a smart aircraft

bolt is also included to investigate the effects of geometry, particularly engineered

stress concentrations, and sensor placement.