TU/e Academic Awards 2009 - Technische Universiteit Eindhoven

Mechanical Engineering
dr.ir. T.A.P. Engels
Tom Engels, born December 5th 1978, Panningen, the Netherlands.
Enrolled as a student in the department of Mechanical Engineering in 1999,
and finished his doctorate thesis in the same department in the Polymer
Technology group in 2008.
Current employment: Scientist Mechanical Properties at DSM Research
Performance Materials, Geleen, the Netherlands.
Promotor: prof.dr.ir. Han E.H. Meijer. Copromotors: dr.ir. Leon E. Govaert
and dr.ir. Gerrit W.M. Peters
Predicting Performance of Glassy Polymers
Evolution of the Thermodynamic State during Processing and Service Life
Figure 1: Short-term mechanical performance of polymer
products with different processing histories; predictions (lines)
and experiments (markers).
Figure 2: Long-term mechanical performance of polymer products
with different processing histories; predictions (lines)
and experiments (markers).
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Modern design environments integrate shaping and making and the processes
involved are supported by numerical tools that aid distinct steps in the total
design process. Two important fields can be distinguished in product design:
(i) processing and (ii) functional use of the product. Up till now, no real interaction
between the two fields exists. In polymers, however, the processing step largely
determines the behavior in the solid state.
Using state-of-the-art constitutive models, the thermodynamic state of a polymer
material, e.g. as reflected in the value of the yield stress, is the only variable
required to accurately predict both short- and long-term performance of glassy
polymer products. Although the results are very useful, a drawback exists in the
fact that the initial state of the product still has to be determined. This requires
mechanical testing of a prototype which strongly restricts true product optimization.
We present a method that predicts yield stress distributions in injection molded
products of glassy polymers directly from processing conditions. The approach is
based on the evolution kinetics of the yield stress as a function of effective time
and yields excellent predictions of short- and long-term properties after
processing, see figures 1 and 2.
Predictions on performance are made under the assumption of ductile failure,
and no explicit criterion for embrittlement was incorporated. Under the influence
of progressive aging a transition from a ductile to a brittle failure mode can be
experienced. To predict the failure mode, a critical hydrostatic stress criterion is
introduced that serves as a threshold value for the onset of cavitation which is the
initiation of craze formation. Based on the evolution of the thermodynamic state
this molecular weight dependent threshold can be surpassed and a predictable
transition in failure mode results.
The modeling approach presented combines the two aspects of design. It enables
the prediction of the performance of products made of polymer glasses, starting
from the processing conditions and ending with the way in which the product will
fail. This opens the way to true product optimization in a complete virtual
environment without the need of performing even a single mechanical test.