Current Research Projects of Prof. David Hills

Prof. David Hills

Crack Growth From Fastener Holes In Aircraft Structures

This project is concerned with fasteners used in aircraft structures, their strength, contact conditions, and the growth of fatigue cracks from them. The work done so far is all two-dimensional in nature, and has three elements:

1. The solution of the contact problem between the fastener and the hole. Clearance fit fasteners have a small radial clearance, whilst interference fit fasteners clearly have a negative clearance. In ether case the contact is highly conformal in nature, with a large arc of contact, so that the classical Hertzian calculation cannot be used. Instead, formulations appropriate to a disk and a plate with a hole are employed, under frictionless conditions. If friction is to be taken into account, a novel approach is used whereby the fastener and plate are initial taken as a monolithic body, and the interface is inserted by deploying strain nuclei in the form of dislocations. This is an extremely efficient way of tackling this problem.
2. The solution of the crack tip stress intensity factor for a growing crack. Again a strain nucleus method is employed, using dislocations. The crack is assumed to initiate from a point of maximum tension, and to develop in a curvilinear fashion following a self-determining trajectory, such that the mode I stress intensity factor is maximised at each point. A numerically efficient procedure is developed by first establishing an integral equation, and then inverting it numerically.
3. Limited amounts of plastic flow may occur in the contact, and a method of handling these is being developed which relies on the deployment of dislocations, again. This is numerically extremely efficient as it means that all of the information present in the underlying elasticity solution can be retained, but, at the same time, the Prandtl-Reuss plastic flow rules are correctly followed.

The work has developed along all of the lines outlined, and more progress will be made on the plastic flow modelling aspects.

People

Prof. D. A. Hills and Jinping Hou

Sponsor

CAA

Fretting Fatigue Avoidance In The Design Of Shaft Spline Connections

Spline joints between split shafts in gas turbines are highly conformal connections, nominally locked together, but experiencing a small amount of relative movement, which causes fretting, and the possible development of fatigue cracks. This project is an experimental and theoretical investigation of the phenomenon.

On the experimental front, a novel piece of apparatus has been designed and built which enables conformal contacts, using carefully prepared pads, to be employed in a way which is kinematically correct, and employ direct positional feedback of the location of the pad relative to the main specimen. This apparatus has been used to conduct a large number of tests on high strength shaft steel, using a matched generalised stress intensity factor approach to the characterisation of the spline corner.

Theoretically, the problem of analysis has been tackled by a range of methods. An asymptotic analysis of the spline corner has been carried out, and a detailed FE calculation of the spline joint as a whole performed. The asymptotics have been embedded in the FE solution, to enable a generalised stress intensity to be defined. This generalised stress intensity is reproduced in the test specimen, and so the conditions in the crack initiation (process) zone are also matched.

People

Prof. D. A. Hills, Dr A. Mugadu, L. Limmer (RR), J. Forfar (RR)

Sponsor

Rolls-Royce plc.

Design Against Creep And Creep-fatigue Failure

This project, joint with the companies cited, and the universities of Swansea and Leicester, is concerned with component lifing under creep and creep fatigue conditions. It is also, partly, about exchanging technology developed for the steam turbine industry with the gas turbine industry, and seeing if each can benefit from the other's approach. The problem clearly has many aspects: Swansea are involved with experimental determination of strength under creep fatigue with differing geoemtries, Leicester is concerned with establishing shakedown solution very efficiently, and we, so far, have examined the quality of approximate solutions to the problem of confined plasticity at a notch root. We have done this by examining a simplified problem, to attempt to separate out the various features of the stress state which affect the quality of an approximate solution.

We have also begun a preliminary investigation of crack nucleation criteria, based on Dang Van principles, and will proceed to compare characteristic relaxation times for the various steam/gas turbine duty cycles.

People

Prof. D. A. Hills, G. Peridas, S. Williams (RR), R. A. Ainsworth (BE)

Sponsor

EPSRC Structural Integrity Initiative with Rolls-Royce plc. and British Energy support