My PhD

The subject of my PhD was the measurement and prediction of the strain in a rolling process at different length-scales. It involved both experimental work with rolling and plane strain compression tests and numerical work with finite element simulations of rolling and compression tests. I also dealt with Finite Element (FE) simulations of the behaviour of austenite and ferrite under thermo-mechanical loading (micro-modelling). Within the University of Sheffield I gained a reputation as a competent FE modeller and I collaborated with other research groups building a range of different models (bolted beams subject to fire load, contact analysis of bolted plates, three-point bending test, etc). As part of my academic work I often gave presentations to sponsors and academic staff and I was involved in laboratory demonstrating and tutoring in the department of Mechanical Engineering.

A scheme of my PhD work is shown in figure 1.

Fig.1 - Scheme of my research activities

The physical conditions of the rolling process are so extreme (high temperature, large deformation, high speed) that none of the traditional methods can be used to measure the strain. Looking at the left-hand side of figure 1, the strain can be measured both in compression and rolling tests using gridded inserts (fig. 2).

Fig. 2 - Aluminium specimen before and after the compression test.

Compression tests are normally used to simulate the material behaviour in a rolling process.

The strain is measured by comparing the grid before and after deformation. This can also be done in a rolling experiment using the specimen shown in figure 3.

Fig. 3 - Rolling specimen and gridded insert (x rolling direction).

After measuring the strain, both compression and rolling FE models are created using ABAQUS in order to reproduce the experimental conditions (fig. 4) including experimental problems such as tools misalignment (fig. 5).

Fig. 4 - 2D model of a compression test  (50% reduction, 400 °C)

Fig. 5 - Insert after deformation with highly misaligned tools and corresponding FE model. Comparison of FE and experimental temperature during compression

Finally the measured temperature and strain are compared with the numerical results. The results of these comparisons were very good and a series of journal articles have been published.

The right-hand side of the scheme in figure 1 shows another aspect of my research work. It is the measurement of the strain at a much smaller scale. Using a special technique a microgrid can be laid on a specimen before compression. This microgrid can have a pitch of up to 5 micron and it allows the measurement of the deformation within a single grain or phase.

Fig. 6 - Microgrid on a duplex stainless steel (detail) and FE model.

A microgrid can be created on a stainless steel insert (figure 6). The microgrid is visible on the two phases austenite and ferrite before deformation (1). The picture of the model material before deformation (1) is used to build a Finite Element (FE) model of the microstructure (2). This model material (1) was cast, rolled and heat treated by myself. Different material properties are assigned to ferrite and austenite in the FE model. The stress-strain curves used to define the material behaviour in the FE model were obtained through axisymmetric tests of pure ferrite and pure austenite in a range of strain, strain rates and temperatures. The area covered by the microgrid is 0.7×0.7 mm and the pitch of the microgrid shown (1) is 5 micron. The FE model covers the whole area of the microgrid with over 40,000 nodes. After the test, the deformed microstructure (3) is analysed and the components of the strain can be measured by using the deformed microgrid. Using this data, contour plots of the measured strain components can be obtained (5) and can be compared with the FE results (4).

This technique can help the development and the validation of those mathematical models that deal with the relationship between strain at different length scales (scale transition models).

My academic skills

• Finite Element modelling of various non linear problems

• Programming and linking commercial codes with user-developed subroutines

• Experience in casting stainless steel with different volume fractions of austenite and ferrite

• Rolling and heat treatments to obtain suitable grain/phase size

• Surface treatments on aluminium and stainless steel (etching, electro-etching, electro-polishing)

• Sample machining

• Experience in compression and rolling tests

• Use of optical microscope, SEM and EBSD techniques

• Writing of articles, reports and Power Point presentations

• Oral presentations to sponsors and academia

• Assisting final year students in their project: teaching ABAQUS, ANSYS, Hypermesh, experimental set-up

• Teaching and demonstrating: Matlab, Charpy test, tensile test, technical drawing,  wind tunnel

• Marking assignments and reports

• Organization of social events within the research group