Institute for Applied Materials – Materials Science and Engineering

Non-destructive Material Characterization

Micro Computed Tomography

Micro computed tomography (µCT) allows, analogous to conventional X-ray computed tomography, the non-destructive testing of materials and components with regard to local density differences. Thus, pores, cavities and cracks as well as different phases or components can be detected and analyzed. For this purpose, a virtual 3D model of the material or component is reconstructed from radiographs at different angles using a 3D reconstruction. In contrast to medical tomography, the resolution here can be in the micrometer range, depending on the sample or component size.

Micro-CT is used in medical imaging as well as in industrial computed tomography and material analysis. Here it offers the advantage of negligible sample preparation and a non-destructive measurement procedure.

The Micro-CT System Precision from YXLON GmbH allows both images of large component groups of different materials and high-resolution images of smaller material samples. A tube voltage of up to 225kV allows images of various materials such as aluminum alloys, polymers and steels with a cumulative thickness of up to 10mm.

An in-situ testing device developed at IAM-WK allows loading samples and observing damage mechanisms during scanning. Using digital image registration methods, strain fields at different loading levels can be calculated and visualized three-dimensionally.

The department "Manufacturing and Component Behaviour" uses both commercial software (VGStudio, Avizo) and self-developed open source projects (https://sourceforge.net/p/composight/wiki/Home/, https://github.com/IAM-WK) for µCT analysis in order to realize tailor-made solutions for questions in microstructure characterization. In this area we are working on the topic:

Ultrasonic Testing

Ultrasonic testing is used for non-destructive determination of elastic parameters up to orthotropic material behaviour. In contrast to quasi-static experiments, such as tensile, bending or torsion tests, ultrasonic testing is a dynamic measuring method based on time or frequency measurements. The comparatively low amplitudes of the elastic waves only lead to small deflections and thus do not cause any non-linear effects in the material, which means that higher accuracies can be achieved. Three different methods of ultrasonic testing are used at the IAM-WK:

Pulse-Echo Method

In the pulse-echo method, ultrasonic pulses are passed through the sample to be tested. From the time span required for a pulse to pass through a sample, the propagation speeds of the sound waves can be determined, from which in turn the elastic parameters can be derived.

Ultrasonic Phase Spectroscopy (UPS)

In ultrasonic phase spectroscopy, continuous sound waves are introduced into the sample in a previously defined frequency spectrum. Finally, the propagation speed of the sound waves can be determined by the phase difference between the input and output signal. In contrast to the pulse-echo method, the UPS is particularly suitable for the investigation of strongly sound-attenuating samples.

Resonant Ultrasonic Spectroscopy (RUS)

Resonant ultrasonic spectroscopy allows the determination of the elastic material behavior even on samples in the size of millimeters. The elastic constants are determined via the eigenfrequencies of samples of simple geometry.

We are currently working on the following topic:

Magnetic Barkhausen Noise

When an external magnetic field is applied on ferromagnetic materials the growth of magnetic domains that have a similar magnetization orientation to the applied magnetic field can be investigated. The growth occurs by shifting the so-called Bloch-walls at the expense of the unevenly oriented domains and is hindered by lattice defects. Above a certain field strength, the defects can be overcome and a sudden irreversible wall movement occurs until a new defect will blockade the movement once again. Defects are for example grain boundaries, dislocations, voids or inclusions. This effect leads to a discontinuous change in magnetization. These jumps during magnetization were discovered by Heinrich Barkhausen and are therefore also called magnetic barkhausen noise (MBN). The MBN makes it possible to perform a non-destructive sample characterization because it is dependent on both mechanical and microstructural properties. The MBN can be used to make statements about grain size, plastic deformation and residual stress state.

We are currently working on the following topic: