Dislocation-Microstructure Interaction at a Crack Tip - In Search of a Driving Force for Short Crack Growth

Lead Research Organisation: University of Manchester
Department Name: Materials

Abstract

Nickel-based superalloys are particularly used in applications involving high temperatures and stresses, such as the critical gas-turbine blades and discs in aerospace and power-generation industries. The behaviour of short cracks in nickel superalloys is of particular importance for component design and life prediction, as a large proportion of service life is spent in the growth of small cracks before final failure. Due to the strong influence of local microstructure and heterogeneous stress/strain fields, short cracks are known to grow anomalously under fatigue and tend to exhibit high, irregular and scattered growth rates. The physical driving force for short crack growth is still not well understood yet despite intensive research effort, mainly due to the limited understanding of crack-tip behaviour.

This proposal aims to investigate the fundamental deformation mechanism at the tip of a short crack for nickel-based superalloys under fatigue at a range of temperatures. The research will focus on the influence of evolving local plasticity, induced by dislocation dynamics at the crack tip, on short crack growth. The interaction between dislocation and material microstructure is the major source for heterogeneous plasticity and internal stress concentration, leading to initiation and growth of short cracks. Short crack growth testing in a controlled environment will be carried out to study the anomalous behaviour of short crack growth in these alloys under fatigue, which is the expertise of UoS. Temperature will be varied in order to observe the critical effect of temperature change on the slip behaviour near the crack tip. Following crack growth tests, post-mortem transmission-electron-microscopy analyses of crack-tip zone will be performed to reveal the detailed mechanisms for nucleation and multiplication of dislocations, pile-up and penetration of dislocations at phase/grain boundaries and the influence of grain misorientations on dislocation behaviour. In particular, match-stick samples will be extracted from the crack-tip fracture process zone of fatigue-tested specimens to allow in-situ measurements of crack tip deformation under fatigue, which are the established techniques at UoM. In this case, high resolution digital image correlation, with the assistance of grain orientation mapping and scanning-electron-microscopy imaging of gold remodelled surfaces, will be used to quantify shear strain in slip traces formed near the crack tip during fatigue loading. In addition, high energy synchrotron X-ray diffraction studies will be carried out to measure the elastic strain response and load transfer between different phases around the crack tip, which will provide insight regarding the penetration of dislocations into the gamma-prime precipitates.

To physically simulate the material plasticity behaviour, a three-dimensional discrete-dislocation-dynamics (DDD) approach will be developed to model the interaction between dislocations and material microstructures, which is the strength of LU, based on experimental results. The DDD model will be interfaced with viscoplasticity and crystal plasticity models, and further applied to investigate the role of dislocation dynamics in depicting short crack growth. A multi-scale finite element method will be established for the crack-tip deformation analyses, which aims to identify a micromechanics-based driving force for short crack growth. Computational simulations will be thoroughly validated against local strain measurements (at both mesoscale and microscale), in-situ and post-mortem measurements as well as X-ray tomography of extracted match-stick samples. The ultimate goal is to deliver an efficient finite element procedure to predict short crack growth, with full validation against the experimental data, for end users.

Publications

10 25 50
 
Description - development of large-area high-resolution 2D strain analysis;
- development of combining high resolution strain mapping with dislocation imaging in the SEM
Exploitation Route Work is used to demonstrate the need for investment in in-situ test stations and other infrastructure investment. All this is now done with the M4DE theme within the Royce Institute.
Sectors Aerospace, Defence and Marine,Energy

 
Description Industry increasingly interested in experimental methodology developed in this project.
Sector Aerospace, Defence and Marine,Energy
Impact Types Societal,Economic

 
Description Crack Tip - academic partners 
Organisation Loughborough University
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaborating partners in EPSRC project.
Collaborator Contribution Responsible for complementary project works packages. Regular meetings/interaction and knowledge exchange.
Impact - Presentations at international conferences. - Collaborative publications planned for later project stages.
Start Year 2015
 
Description Crack Tip - academic partners 
Organisation University of Southampton
Department Optical Research Centre
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaborating partners in EPSRC project.
Collaborator Contribution Responsible for complementary project works packages. Regular meetings/interaction and knowledge exchange.
Impact - Presentations at international conferences. - Collaborative publications planned for later project stages.
Start Year 2015
 
Description DSTL 
Organisation Defence Science & Technology Laboratory (DSTL)
Country United Kingdom 
Sector Public 
PI Contribution attending progress meetings
Collaborator Contribution providing new understanding they can use for their business
Impact new understanding
Start Year 2010
 
Description Rolls-Royce plc 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
Start Year 2007