The right materials in the right place are essential for the quality, service life, weight and ultimately the success of the end product. Optomet laser Doppler vibrometers make important contributions on a global scale to the systematic search for new findings in material research. This applies both to the determination of material parameters or to non-destructive testing/non-destructive inspection (NDT/NDI). . Especially non-contact measurement methods are of utmost importance, as those, contrary to e.g. acceleration sensors, do not change the material characteristics.
Composites such as carbon fibre reinforced plastics (CFRP) with their superior material properties are widely used in aerospace applications. Over the past decades, these new materials have been increasingly replacing conventional light metal elements. Unfortunately, these modern materials can also introduce new kinds of defects, which makes advanced measurement methods indispensable e.g. to detect structural damage in components that are invisible from the outside.
Optomet scanning vibrometers are used to detect and visualize different kinds of material defects such as delamination, cracking or inclusions.
In thin-walled components, defects can be very well localized by measuring very small ultrasonic surface waves, the so-called Lamb waves, by means of sensitive heterodyne laser interferometers. In this process, wave packets are excited at a frequency reaching up into the three-digit kHz range via an ultrasonic transducer (or using different methods) and interact with the defect. Measured is the propagation of the wave over the surface of the object under test over time, with typical amplitudes in the two- or three-digit nanometer range. In this manner, defects can be detected regardless of whether the structural or material damage is inside the material or on one of the sample’s outer surfaces.
Local Defect Resonance (LDR)
Another, also very sensitive method for the detection of material defects is the so-called Local Defect Resonance (LDR). Contrary to the Lamb wave method described above, the laser vibrometer does not measure the spatial propagation of a wave over time but rather measures standing waves as in conventional modal tests. The difference is the frequency bandwidth in which the measurement is carried out. While the classic modal analysis works with frequencies in the single-digit kHz area, measuring ranges in the three-digit kHz range can also acquire and visualize resonances in geometrically very small local defects.
A Split Hopkinson Bar test is a method for material testing, with which material properties are determined under dynamic conditions. In this process, the object under test (e.g. a concrete cylinder or a composite) is located between two bars, the incident bar and the transmission bar. An accelerated striker impacts the incident bar where it causes a shock pulse. The generated wave passes through the first bar and hits the material sample and propagates through it into the transmission bar.
Due to their high sampling rate of 160 Msamples/s and a dynamic range of more than 220 dB, laser Doppler vibrometers (LDV) from Optomet are the perfect measuring tool in order to measure the temporal progression of these highly dynamic shock pulses.