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Structural Analysis of Thermoplastic Components
By G. Trantina and R. Nimmer. McGrawHill,
Inc., 1993. 384 pp.
The primary purpose of this book is to describe the application
of modern engineering analysis techniques to the design
of components fabricated from thermoplastic materials.
The book, the first of its kind to address the unique
behavioral characteristics of thermoplastics and their
impact on finite element analysis (FEA), points out the
need for plastics designers to move on to nonlinear analysis
in order to truly simulate the behavior of plastic parts.
According to the authors, the easy availability of high
speed computing and efficient analysis codes means that
it is no longer necessary nor cost-effective to restrict
oneself to simple linear analyses.
The
authors discuss drawbacks to treating plastics like
metals for the purpose of FEA. Thermoplastics exhibit
complex behavior when subjected to constant, increasing,
or cyclical mechanical loads. The typical approximation
of a linear relationship between stress and strain is
often invalid because of the extremely non-linear behavior
of plastics. Failure to account for this phenomenon
can lead to over prediction of the stiffness of a plastic
part which might then fail in actual use. The current
solution to this problem is significant over design
which results in wastage of raw material, and sometimes
leads to other unanticipated problems.
The problem of non-linearity is further
compounded due to the large displacements that tend
to occur in plastic components. The elastic modulii
of plastics are routinely as much as two orders of magnitude
less than those of metals. Plastics can undergo an order
of magnitude more strain than metals before incurring
damage Consequently, these materials will tend to undergo
much larger rotations and displacements so that the
deformations carry further into the non-linear regions
of the stress-strain curve. Standard data sheets and
most computerized databases commonly provide design
engineers with three relevant categories of ‘design
properties’: flexural, heat resistance, and impact.
While acceptable for comparative purposes, these properties
are not useful for predicting the structural performance
of plastics components because the data are not
independent of the test method, specimen geometry, and
conditions of the test. The authors present methodologies
for the generation and use of engineering data.
The book seeks to provide a proper
understanding of thermoplastic material behavior and
its relationship to measured properties, so accurate
predictions of component behavior can be made. Because
of the widely differing behavioral characteristics of
these materials, no general procedure for the design
of plastic parts can be proposed; instead, the book
suggests practical approaches to handle the design of
thermoplastic components. Treatments of stiffness, failure,
impact, time dependent behavior, and fatigue are presented.
Numerous examples in the book highlight areas of concern
for design analysis of thermoplastics and illustrate
the expected level of accuracy from such analyses.
- Dr. Gerald Trantina
is manager and Dr. Ronald Nimmer is a mechanical engineer
at the Mechanics of Materials Program in GE’s
Engineering Physics Research Center. The book was reviewed
by Hubert Lobo, president of DatapointLabs.
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