Improved small punch testing and parameter identification of ductile to brittle materials

• Improved small punch test suited for the whole range of ductile to brittle materials.
• Minimal invasive determination of true material parameters by means of numerical simulations.
• Temperature dependent yield curve parameters were identified using a response surface approach.
• Estimation of Weibull parameters of cleavage fracture for steels at low temperatures.

Minimal invasive material testing is of special interest, when only small amounts of material are available or the material degradation of structural components in service has to be evaluated. The disc-shaped specimens used in the small punch test are small enough for local material sampling but representative for characterizing the macroscopic material behaviour. A small punch test device was developed which enables the testing of materials in the whole range from ductile to brittle failure and from ambient temperature down to −190 °C in a unique experimental set-up. The specimens are not clamped as usually in the small punch test. This is crucial for brittle fracture behaviour with little or without plastic deformation. The measured load displacement curve of the punch represents the non-linear response of the material due to elastic–plastic deformation. It contains relevant information about true material parameters, which can be made accessible by solving the inverse problem. Thus, plastic yield curves and Weibull parameters were identified by combining finite element simulations with non-linear optimization techniques. Examples for measured load displacement curves and related results of parameter identification are shown for a pressure vessel steel and a laser welded joint. The results obtained from the small punch test are verified by data from standard specimen tests.

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