Fatigue behaviour and fracture-energy of foamed Aluminium

Aluminium foam, as produced by LKZ Ranshofen (AlulightTM ), for example, is aimed to be used as construction material for vehicle components, owing to its excellent properties as energy absorber ("light metal construction", "crash energy absorber"). Cyclic loads will be responsible for the fatigue behaviour of vehicle components made from this class of materials in the same way as those consisting of conventional materials during their normal use. Knowledge of the fatigue properties of some material is necessary in order to optimise the use of the underlying raw material economically on one hand and to guarantee the operating reliability (life time, inspection intervals etc.) of the components made of this material, on the other hand. Since no experimental data exist until now, the mechanical properties of Aluminium foam have to be measured, before using it as construction material. Components can be constructed properly only, if failure owing to fatigue crack formation cannot take place. The high number of cycles, that have to be suffered by vehicle components and the fact that cast Aluminium has an endurance limit, make it necessary, that a time- and energy-saving testing procedure is applied in order to study the fatigue behaviour of this material at such high numbers of cycles which are typical for its actual use. The ultrasound-resonance method is most appropriate for measurements of the fatigue properties of materials used for vehicles and has proved to be most useful for measurements of the in-service strength of components, as well as for tests which serve to develop new materials for the automobile- and aeroplane industries. The fracture energy is a significant material characteristic value because of the cellular structure of the material and its possible application. Knowledge of this value together with static strength and fatigue strength values are especially necessary for the industrial use of these materials. The splitting test is a testing procedure, which allows to measure the specific fracture energy with the help of load-displacement curves of different materials. Brittle, as well as ductile materials can be tested successfully. The load-displacement curves yield information on the fracture behaviour until complete separation of the test piece into two parts takes place and, together with acoustic emission measurements, therefore are most appropriate to characterize the fracture properties of foam material. An interpretation of the experimental results will be essentially improved by performing SEM and CT studies. With these, especially correlation between fatigue strength, fracture energy and material structure, as well as size and distribution of the pores will become possible. New and interesting answers referring fracture behaviour and material structure may be expected, as new materials on one side and new testing methods on the other side will be used for the planned investigations.

Aluminum foams have got an increased attention as a potential structural material in transportation industry for use in weight sensitive construction parts. Beams and plates made of foamed materials show good functional properties like sound absorption, fire retardation, and heat dissipation. Beneficial mechanical properties are a low mass density and a high specific stiffness, which exceed that of aluminum bulk material. Construction parts in vehicles are frequently subjected to vibrations, and repeated mechanical straining which may lead to fatigue damage of the material. Automotive components, for example, are subjected to 108 to 109 (100 Billion to 1 Trillion) cycles during lifetime. If foams are used in car components, it is necessary to investigate not only their low cycle- but also their high cycle fatigue properties. A time saving method to investigate high cycle fatigue properties of materials is the ultrasonic fatigue testing method, where the specimens are loaded with cyclic frequencies of approximately 20 kHz. The high cycle fatigue data show the existence of an endurance limit which means that loading with lower cyclic forces will not lead to fatigue damage of the foam. Fatigue damage is governed by the formation of cracks, which preferentially initiate in the interior sections of cell walls at initial defects, like precracks or holes. No strain localization and formation of deformation bands was found. Fatigue crack growth preferentially follows areas of cell walls with a minimum wall thickness, and eventually may stop near cell-nodes. The low cycle fatigue properties were investigated at 1 - 10 Hz with a servo hydraulic fatigue testing machine. Endurance data above approx. 104 cycles to failure obtained with servo hydraulic and ultrasonic equipment coincide within the range of scatter. Similar fatigue properties were found therefore changing the cyclic frequency more than 3 decades. Fracture experiments of the foams show high values of fracture energy. The fracture behavior of the foam is very ductile. Cell structure show large amounts of plastic deformation and absorption of mechanical energy.