FRACTURE RESPONSE OF LOW CONSTRAINT METALLIC STRUCTURES AT VARYING STRAIN RATES

Abstract

Failure in thin metallic structures is accompanied by pronounced stable tearing during crack propagation, with cracks that can run for long distances. Material deformation and fracture behaviour of such structures are affected by the applied strain rate hence deemed strain rate sensitive. The crack tip opening angle, CTOA, has been identified as the preferred parameter to characterise fracture behaviour in these structures as it becomes stable after some amount of crack extension, therefore can potentially be used to characterise stable tearing. In the intermediate strain rate regime (10-1000/s), the behaviour of these materials has to date not been fully investigated with the literature revealing significant gaps in available engineering data. Moreover, the CTOA has to date not been widely explored as an analytical tool at the aforementioned strain rate range due to experimental constraints and difficulties. Recent developments in recommendations for strain rate testing of metallic sheets at intermediate strain rates have been useful in identifying testing methods and equipment for the determination of strain rate dependent data. It is therefore timely to investigate material and fracture behaviour since in principle these can now be investigated to high precision. Similarly, recent methods developed for determining the CTOA such as digital image correlation has made it possible for the accurate determination of this particular driving parameter. The CTOA is the focus of this thesis and with the use of a high strain rate test frame and the commercial finite element package, Abaqus, the CTOA was investigated as a driving parameter for thin sheets of AISI Type 304 stainless steel of thickness 0.5, 1, 1.5 and 2mm at intermediate strain rate range of 1-1000/s. Values of CTOA were found to be constant after an initial unstable regime of less than twice the specimen thickness. It was discovered however that the CTOA is affected by loading rate, decreasing with increasing rate of loading, for all the thicknesses investigated. Explanations for the behaviour of CTOA are provided in the thesis and in particular its take up of a constant value is reasoned to be a consequence of similitude with similar behaviour transported with the crack arising from the localisation of internal stresses at the crack tip. Understanding this behaviour is expected to lead to improvements in industrial practice in simulating the response of structures at intermediate loading rates.

Date of Award	31 Dec 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Keith Davey (Supervisor) & Michael Smith (Supervisor)