Jonson, David.
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.
