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.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:9531 |
| Date | 16 August 2012 |
| Creators | Laubscher, Rudolph Frans |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
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