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Quality assurance and qualification of structural parts

Computed tomography image of a car brake disk made of C/C-SiC

Objective for quality assurance (QA)

• Detection and characterization of material and structural defects
• Evaluation of defects with respect to applicability of the structural part
• Early detection during genesis of material defects
• Increasing liability and lifetime of structural components

Non-destructive methods of failure detection

So far three of the classical non-destructive evaluation techniques are available and intensively used for failure detection and microstructure interpretation: ultrasound analysis (water- and air-coupled), lockin-thermography as well as radiography. Depending to the material and the structural part’s geometry, these techniques are used at different manufacturing stages or in between different test cycles for in-situ material evaluation without disturbing the microstructure or the component’s structural integrity. Thus, valuable information about any kind of visual degradation effects or microstructural conditions at failure critical joining zones can be obtained. The combination of these methods enables not only to analyse and describe the mechanisms of material failure within large structure parts but furthermore to focus on the influence and correlation of material specific failure mechanisms on a microstructural scale.

Ultrasound technology (UT): As a part of structure development, the Institute’s competence is also covering experience with water- and air-coupled ultrasound technology. The latter one is especially used for porous materials. Ultrasound analysis is primarily used to detect local material defects and inhomogeneous areas of CFRP- and CMC-parts of up to 800 mm in diameter. Ultrasound techniques are mainly focused on accompanying measurement in terms of quality assurance of structural component development.

Lockin-thermography (LT): This technique is based on a CEDIP Jade III LWIR-camera (λ = 7.7-9.3 µm) which is measuring intensity and time displacement during propagation of thermal waves. To introduce the thermal impulses (e.g. with halogen lights), different frequencies are used. Measuring from the material’s surface, the in-deep penetration of thermal waves of higher frequency (0.1-1 Hz) is 0.5 mm or less. Thus, higher frequencies are used to evaluate coatings and surface structures. In contrast thermal waves of lower frequencies (0.001-0.05 Hz) are penetrating up to 6 mm deepness. Usually they are used for qualitative large area inspections on adhesive bonds as well as to identify material inhomogeneity or delamination in layered microstructures. With this imaging technique it is even possible to control dynamic processes such as the propagation of the resin front during infiltration of fibre composite materials.
Radiography: is based on x-ray transmission imaging due to different absorption. Compared to UT and LT analysis, the film-based technique provides images with much higher resolution. The facility with its 60 kV x-ray tube can easily be used to inspect plates and more or less plane parts with measurements up to 600 x 600 mm. The method enables the detection of microcracks, contaminant particles, inclusions as well as 2D-imaging of the microstructure and distribution of fibre fabrics (e.g. carbon fabric) and the surrounding ceramic matrix.

Since 2006 these measurement techniques are complemented by computed x-ray tomography (CT) which enables detailed 3D-analysis of even complex shaped structural units with high resolution and to inspect internal structures of almost any orientation. With CT, non-destructive evaluation becomes much more precise and demonstrative. Furthermore it minimizes the risk of misinterpretation of defects and microstructures because of an unfavourable component orientation.