Abstract
D.Ing.
In mechanical design and analysis the mechanical properties of the material used are crucial to achieve
effective design or analysis. In designing structures that are susceptible to dynamic loading different
mechanical properties of the material may be needed than those used for quasi-static situations.
Usually when one refers to the dynamic properties of a metal one refers to the notch toughness of the
material. That is the resistance of the material to crack propagation under dynamic loading. Another less
well known dynamic property of a metal is strain rate sensitivity. This implies that mechanical properties
like yield strength, tensile strength and rupture strain varies according to strain rate.
Typical applications where these properties are of use are in impact situations such as vehicle collisions
and cold and hot working of metals in the manufacturing industry. The mechanical properties of certain
metallic components or structures may change when the component or structure are subjected to dynamic
loading that causes permanent deformation.
The purpose of this investigation is to investigate the strain rate sensitive behaviour of certain stainless
steels. The steels investigated are AISI Types 304, 316 and 430 stainless steels, 3CR12 corrosion resisting
steel (a proprietary alloy also known as Type 1.4003) and mild steel which acts as a reference.
The strain rate sensitivity of the above mentioned steels are investigated experimentally at room
temperature for strain rates between 10' to approximately 100 s -1 . The steels are all tested in as
delivered sheet form and testing is conducted in both rolling directions. The testing at the medium strain
rates necessitated the design and construction of a dynamic tensile tester, the design of which, is also
presented.
The implementation of strain rate sensitive material properties into structural design and analysis are
investigated and a constitutive model is proposed. The implementation of the proposed constitutive model
into numerical methods analysis tools such as the finite element method is discussed and presented. The
practical implementation of the proposed constitutive model is illustrated by numerically analysing the
problem of a clamped beam struck transversely by a mass and comparing this with available experimental
data.
The validity of a typical constant velocity tensile test that is used to determine strain rate sensitive
material properties is also investigated numerically to place the experimental results obtained into
perspective.
All the steels tested are found to be strain rate sensitive. Their behaviour is satisfactorily described by the
constitutive model presented. No general trend regarding strain rate sensitivity is found when the results
of the two rolling directions are compared.
The importance of including strain rate sensitivity into structural design and analysis is illustrated by
the analysis of the clamped beam struck transversely by a mass. The numerical results compare well with
the available experimental data.
It transpires from the numerical analysis of a typical constant velocity tensile test that it is difficult
to obtain a constant strain rate throughout the gauge length of a typical test specimen. It also shows that
there exists an optimum specimen geometry where the strain rate variation in the gauge length is at a
minimum.