Fatigue & Fracture Mechanics

Multiaxial Fatigue

Many engineering components operate under a multiaxial stress state, yet basic fatigue data for safe-life assessment is frequently only available in a uniaxial form. The reason for this is the clear difficulty of performing multiaxial tests when compared to simple uniaxial experiments. The project aims to investigate a range of biaxial load conditions in a nickel superalloy, either using tension/torsion or a novel three-actuator biaxial rig developed at Oxford. Finite element modelling of the experimental configuration will be carried out to predict the stress field under both elastic and elastic-plastic assumptions of material behaviour. A number of different multiaxial fatigue criteria will be investigated, concentrating on the crack nucleation phase.  From these, a validated life prediction method will be developed.  The project is taking place in collaboration with Rolls-Royce plc through the UTC for Solid Mechanics

People: David Nowell, João Sahadi

Sponsor: Brazilian National Council of Technological and Scientific Development (CNPq)

Dates: January 2016 – December 2019

Measurement and Modelling of Contact Stiffness (Siloet 2 Project 10, WP3)

Work on fretting fatigue has led to a more fundamental interest in the origins and modelling of frictional behaviour and the interaction of contacts with engineering levels of surface roughness.  A recent EPSRC project (PAMFJP) was carried out with Imperial College, aimed at developing a more predictive approach to the incorporation of friction in larger scale models, e.g. for the prediction of vibration or structural integrity.  The initial phase of the work focused on developing a robust method of measuring interface response which is largely independent of macroscopic geometry.  Current work under the Siloet 2 project is examining the pressure and surface geometry dependence of normal and tangential stiffness.  Alongside the experimental work we are carrying out fundamental modelling of asperity interaction with the aim of predicting the stiffness of rough contacts.

People: David Nowell; David Hills; Kurien Parel; Robert Paynter

Sponsors: Innovate UK; Rolls-Royce plc

Dates: January 2014 – September 2016

Characterisation and modelling of handling damage and its effect on fatigue life (Siloet 2 Project 17, WP2)

David Nowell has a long-standing interest in the effects of surface damage on the fatigue life of components.  Life prediction requires a detailed knowledge of the geometrical damage, the residual stress field, and the modification of material properties due to plasticity.  Early work was focussed on the effects of foreign object damage (FOD) in aircraft engines, and routine experimental simulation of FOD is still carried out.  More recently, our attention has turned to low velocity handling damage, such as dents and scratches.  A recent project ‘EROMDAT’ was funded by the European Aviation Safety Authority and carried out in collaboration with Institut PPrime in Poitiers and several engine manufacturers.  The current project will investigate the residual stress field caused by dents and by scratches.  Finite element simulations of each type of damage have already been carried out, and work is underway to characterise the stress field experimentally.

People: David Nowell; Rodolfo Fleury

Sponsors: Innovate UK; Rolls-Royce plc

Dates: October 2014 – September 2016

Frictional Contacts and Shakedown

In practice contact problems are almost always analysed now by FEA.  The solution of the contact problem itself involves few nodes, and a method of ‘static reduction’ has been achieved which permits the overall stiffness matrix to be reduced enormously so that only the contact nodes (or contact nodes plus those where external forces are applied) remain.  The contact problem itself is then solved ‘exactly’ (in the sense that the Signorini conditions are imposed with compromise) outside the FEA programme.  This is numerically efficient and is particularly effective where multiple load cases are to be treated.  At the moment it is confined to plane problems.

Frictional shakedown is the self-development of interfacial slip displacements under cyclic load which tend to promote the migration of contacts from partial slip to full stick.  It is more complex than plastic shakedown because of a lack of orthogonality, and the conditions for certain shakedown, possible shakedown, certain cyclic slip being established.    When the algorithms are fully developed they will be implemented within the reduced stiffness matrix method described above.  This will provide a consistent and efficient procedure for analyzing contacts both in a ‘marching in time’ sense and for the direct prediction of the final state of the interface.

People: David Hills, R. Flicek

Sponsor: Rolls Royce

Date: October 2011-October 2014

Fundamentals of Contact

Work is going on to understand the properties of all kinds of contacts – receding, complete, those defined by common edges – to enable the kind of response given by FE analysis of complex prototypical problems to be anticipated, and convergence accelerated.  These more unusual kinds of contact demand asymptotic analysis, and particular attention is focused on the effects of friction and partial slip behavior under oscillatory loading.  Current efforts are aimed at quantifying the influence of self-generated interfacial residual tractions in promoting stick or ‘shakedown’, and predicting this without having to perform a ‘marching in time’ solution.

People: David Hills

Sponsor: Rolls-Royce plc

Date: Ongoing