Fatigue under service conditions and fracture mechanics Fatigue of materials is the most frequent cause for the failure of technical construction parts and systems. In particular in traffic systems and in power stations fatigue failure of components can have catastrophic consequences (aircraft crashes, railway accidents, bridge collapses...). Under variable service loads, cracks in construction parts can initiate, grow, and finally, at a critical crack length, cause failure of the part and eventually of the whole construction.
Especially in aircraft, for economical (weight) reasons, crack formation cannot be completely excluded. However, with regular inspections and timely repairs critical failures can be excluded with a high degree of certainty. For the determination of reliable inspection intervals it is necessary to understand and to quantitatively describe the failure behaviour of materials and construction parts. On the basis of crack growth investigations, threshold values for crack growth, crack growth rate curves, crack resistance curves, and critical stress intensity factors the life span of construction parts under service loading conditions can be computed.
In the Mechanics of Materials and Joining Technology Section the corresponding material investigations can be performed under uniaxial and biaxial service loading conditions. Fatigue and crack growth measurements are possible under vacuum, in air and in corrosive environments, and up to very high temperatures. In the complex thermal mechanical testing facility for turbine blade materials high well-defined temperature gradients can be applied to hollow specimens. The pictures show the path of a crack at the surface of a sample, the fracture surface of a crack starting at the surface of a specimen, and the fracture surfaces of single crystal samples, which were used for crack growth investigations at 700°C and 900°C on behalf of Siemens. The fracture surfaces are examined in detail in the scanning electron microscope.
The biaxial testing facility allows economical testing of flat construction element samples up to a size of 1 m x 1.5 m under close-to-reality loading conditions. An example of such investigations is e.g. a crack initiation investigation of riveted structural elements, which were tested under combined tension and compression and sheer loads. The biaxial testing equipment can also be used to determine the flow charts of metal sheets under arbitrary biaxial loading.
The in-house research work is focused at present on the damage tolerance behaviour of high strength aluminium alloys for aircraft applications, the mechanical behaviour of friction stir welded Al-alloys, and, in connection with the "initial flaw concept" for aircraft structures, on the growth of "short cracks". A fundamental aspect of the investigations is always the relation between the microstructure of a material and its failure behaviour.
All kinds of testing of materials and, to a limited extent, of construction parts are available to external clients.