TWI Industrial Member Report Summary 987/2011
By Martin Gallegillo
Understanding material behaviour at scales below continuum mechanics can increase understanding of the performance of engineering structures and processes. Several complementary tools are available for the investigation of multi-scale material behaviour including: macro-mechanical tests, advanced characterisation methods, theoretical models formulating analytical expressions and computational calculations of representative volume elements (RVE). This report deals with the theoretical models and computational calculations.
Computational calculations involving multi-scale modelling are an active field of research that can provide a performance improvement and therefore increase efficiency in engineering design. The understanding of material behaviour at small scales will lead to improved performance and safety in many applications, particularly in extreme conditions (eg high temperature). Improvement in engineering design can also lead to cost savings.
The report 'A review of the UK's nuclear R&D capability' (Sherry et al, 2010) highlighted the need in the nuclear industry for the use of modelling techniques that enable simulation of materials behaviour across a range of scale lengths and their experimental validation through techniques characterising microstructural changes and their influence on bulk physical, mechanical, and corrosion properties during service. An example where this approach would yield significant benefits is micro-scale modelling methods describing the microstructure in FeCr alloys under thermal ageing and irradiation, to correlate micro structural changes to changes in mechanical properties. Another example of the benefits obtained applying multi-scale modelling in the aerospace industry is the development of thermal barrier coatings (TBC). This example is presented as a case study in the present report.
TBCs have a limited service life compared with the component (eg blade) design life requirement. Prediction of the life service is essential to maximise the life and cost of expensive components such as turbine blades. This requires the accurate prediction of cooling cycles and optimisation of the thermal conductivity during the development of new coatings. The use of experimental techniques (eg laser flash measurements) is time consuming, expensive and requires special expertise. Consequently, these measurements are rarely used by turbine part designers during the material development. Instead, finite element models can provide accurate cooling and life service predictions, reducing the number of experimental trials required. They are also an inexpensive alternative to the optimisation of the thermal conductivity during material development.
The multi-scale approach is based on the investigation of micro-mechanisms at a local (generally small) scale and the transfer of this local information to the macro-scale. The range of application of this modelling methodology is wide, including areas such as mechanics of materials (composite materials, metals, ceramics, etc), damage, fluid dynamics, diffusion (fuel cells), etc. The present literature review will focus on multi-scale modelling of mechanics of materials.
- Identify the benefits and limitations of the use of multi-scale methodologies based on the investigation of the micro-mechanisms at a local scale and the use of the fundamental behaviour to determine information about themacroscale.
- Assess the ease of application of these methods.
- Identify shortcomings in present modelling of engineering procedures and make recommendations.