Aluminium cast components play an important role in technical applications, especially in regard to lightweight design and optimized production technologies. Low density materials with sufficient strength are often used for complex part geometries. Aluminium alloys possess excellent castability and feature designs with strongly varying wall thicknesses. Such geometries cause different local cooling conditions, which affect the microstructure as well as the degree of porosity. Not only the ductility of the casting is limited by the porosity content, the fatigue strength is also strongly affected by microstructure condition. Cy-clic loading initiates and propagate fatigue cracks, which can lead to burst fracture. It is well known from technical history, that such crack initiation of microcracks, coalescence and macroscopic crack propagation leads to unsafe part design and some-times cause catastrophic accidents and severe consequences. Hence, critical designs should be checked against crack growth by regular inspection. Therefore, the reproducible detection of technical cracks by engineering feasible methods is of utmost importance for safe service life applications. This work extends an existing cyclic test bench with an optical camera measurement unit to detect the crack initiation lifetime on semi-curved cast aluminium alloy specimen.
The constant amplitude fatigue testing deals with semi-curved specimen with as-cast and machined surface condition, taken from the water jacket region of aluminium crankcases. The specimen cover aluminium alloy EN AC-42100 and EN AC-46200. These ring-sections samples are loaded under tumescent compression force, thereby exhibiting tumescent bending with ten-sion stress at the cast surface as high-stressed region. During cyclic testing, photos are taken with a commercially available camera system to detect crack initiation and crack growth. For improved crack recognition, a graphite surface layer as well as two-component luminescence coating has been examined. The luminescent measurement technique applies a fluorescent base layer on the cast surface first; after drying, it is covered with a black finish coating. If a cyclic surface crack occurs, the black top coasting gets opened and the fluorescent film features improved image detection. Images are taken by different test rig settings, utilizing a user-defined trigger signal for the camera. The crack initiation and crack growth evaluation is performed after burst failure by image processing tools to define the first foto with technical crack growth.
Crack initiation is calculated on the basis of image processing or by evaluating the change in specimen stiffness. In general, the alloy EN AC-42100 exhibits a somewhat enlarged crack growth fraction than EN AC-46200. The remaining lifetime based on optical measurements evaluates as 10-15% for aluminium alloy EN AC-42100, whereat the alloy EN AC-46200 exhibits only about 10%, which can be reasoned with increased surface porosity for the latter aluminium alloy. Moreover, alloy EN AC-46200 features a subtle increase in crack initiation lifetime of about 4% for machined to as-cast surface condition. These findings were based on the luminescent Riluminati surface coating technique. Graphite surface layer features crack initiation results with slightly enlarged crack length, but the effort in image processing is significantly increased. Thus, the luminescent Riluminati coating is preferable for optical crack detection. Crack initiation based on stiffness change leads to similar tenden-cies, but the obtained scatter index depends on the choice of reference point. Technical crack initiation is evaluated at about 15% as mean value.
Concluding, optical measurement methods using commercial camera systems with luminescent surface coatings are recommendable to detect technical crack initiation and growth for aluminium cast components.