Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.755538
Title: Experimental and numerical characterisation of the plastic zone of a fatigue crack at various values of strain hardening exponent
Author: Shah, K.
ISNI:       0000 0004 7428 531X
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2017
Availability of Full Text:
Access from EThOS:
Access from Institution:
Abstract:
Fatigue crack analysis is an essential tool for life prediction and maintenance of components subjected to constant and variable amplitude loading. Investigation of crack growth mechanisms such as crack closure and crack growth retardation due to overload can lead to design optimization as they cause retardation of crack growth, which implies there is a potential opportunity to extract more life from a mechanical component. Despite of enormous literature suggesting the effect of various variables on crack closure, the effect of value of strain hardening exponent on fatigue crack closure is poorly understood and not validated experimentally. Strain hardening also known as work hardening is the term used to describe the phenomenon that most metals become stronger when plastically deformed. In the current project, experiments were carried out on CT specimens of three different materials viz. pure Titanium, Al2024-T3 and Al6061-T6 whose value of strain hardening exponent was 6, 8 and 14 respectively to investigate the effect of the value of strain hardening exponent on crack closure and it was observed that the extent of crack closure was different for different value of strain hardening exponent. The experiments were carried out on all three materials to identify the effect of value of strain hardening exponent on post-overload fatigue crack growth and it was found that, the post-overload crack growth retardation was higher for the material with higher value of strain hardening exponent. The experimental plastic zones were obtained using Thermoelastic Stress Analysis (TSA) technique and it was concluded that, at a higher value of strain hardening exponent (n=6), the radius of the plastic zone is smaller than for a lower value of strain hardening exponent (n=14) for the same crack length. It has been established that the value of strain hardening exponent plays a major role on the size of the plastic zone, values of ΔK₁ and crack growth retardation demonstrating that it is an important material parameter The pioneering analysis of Hutchinson [17] and Rice and Rosengren [18] to characterise the crack-tip stress fields in the case of strain hardening materials for small scale yielding crack problems in plane strain for mode I or mode II stress distributions was extended by Shih for mixed mode loadings and he plotted the plastic zone contours of a fatigue crack as a function of value of strain hardening exponent, n. Despite of numerous studies have been carried out, these plastic zone contours have never been demonstrated experimentally. Another purpose of this work was to fill that gap by experimentally characterizing the plastic zone of a fatigue crack at different values of strain hardening exponent. It has been revealed that experimental results do not validate Shih's predictions but they are in accordance with Hutchinson's solution. The research to date has tended to focus on the extent of plastic zone parallel to the direction of crack growth; however, little attention has been paid to model the plastic zone perpendicular to the direction of crack growth. The radius of the plastic zone, r_p is not a reliable measure as the shape of the plastic zone changes with specimens of different thickness and during different modes of loading. Therefore it was decided to consider the area of the plastic zone as a reliable parameter to characterise the plastic work ahead of the crack tip instead of relying on either the size or shape of the plastic zone. It has been demonstrated that the plastic zone size is dependent on the value of strain hardening exponent but the shape is independent of the value of strain hardening exponent. Hence, a novel method to model the plastic zone size using a mathematical function has been proposed. The plastic zone size was determined by applying Irwin and Dugdale's plastic zone radius as variables in the equation of a plane algebraic quartic curve. It was illustrated that the novel method has great potential to model the plastic zone size.
Supervisor: Patterson, E. A. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.755538  DOI:
Share: