Materials research and testing

Reliably testing fatigue behavior

Reliable material parameters are essential if simulation models are to make safe predictions – especially when durability of the service life is being calculated. Structures in automotive engineering, medicine and energy technology are exposed to load changes of more than ten million cycles over the course of their expected lifetime –due to either high-frequency stress or a long operating time of up to 30 years. This range is referred to as the very high cycle fatigue (VHCF) range and extends far beyond the range of classic S-N curves. Extensive knowledge of the fatigue behaviour of the materials used is thus of vital importance if these high-performance components are to operate reliably in the VHCF range. To be able to go through the high load changes of 10¹⁰ in a reasonable time period of less than one month, the samples are offset in resonance vibration at frequencies in the kHz range.

With regard to verifying the simulated sample model, Polytec’s 3D scanning vibrometry provides a direct way of measuring the real fatigue sample’s eigenmodes and eigenfrequencies. Another potential application lies in the field of high-frequency stress and strain analysis, since contact measurement methods such as strain gages are not suitable for this type of endurance stress.

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Practical examples:

Operational reliability in high-stress train components (wheel rims), wind turbines and ultrasonic scalpels (medical technology).

Determining material parameters in a non-destructive way

Material properties are determined with zero contact using laser vibrometers. Conclusions can be drawn as to a material sample’s strength by determining the resonance frequency during broadband excitation. This method is used to ensure quality when manufacturing different building materials, since it is completely non-destructive. Often expensive test objects can be saved, and it can also be easily automated.

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Practical examples:

Knowledge of the temporal and local changes to the material properties of cement-bonded systems is vitally important for both materials research and for planning and implementing construction projects. The material properties can be described using ultrasound parameters. Cement-bonded building materials have a high ultrasonic wave absorption capacity straight after mixing, as well as a low sound velocity. Depending on how structure development progresses, the sound velocities and signal amplitudes then increase continuously. The surface’s ultrasound-induced deflection is recorded with zero contact using laser vibrometers. Calculating Young’s modulus and Poisson’s ratio – the elastic variables – also allows for a more extensive assessment of the materials inspected.