The fatigue behavior of an aluminium foam sandwich beam under alternating bending

Aluminium foam-sandwiches which are applied in car bodies, e.g., as side impact protection structures, arc loaded not only by quasistatic, but also by cyclic forces. If these fatigue loadings induce damage of the foam-sandwich structure, the stiffness, strength and impact behaviour may be adversely affected. The present study shows results of bending fatigue tests on an Aluminium foam-sandwich material that was manufactured by a powder metallurgy process with succesive rolling and foaming. The sandwich had a foam core with 10 min and outer sheet layers with thickness I min. Alternating bending tests with normal stresses parallel to the sheet plane were realised using a servoelectric fatigue testing machine. The sinusoidal loading was momentum controlled with a load ratio of R = -1 and frequencies up to 50 Hz. The fatigue limit was calculated from 18 tests using the stair case method with an ultimate number of cycles of 10(7). The cyclic deformation behaviour within the HCF- and the LCF-regime was determined from hysteresis loops of the bending moment versus the bending angle which were measured at selected load cycles during each test. The material shows pronounced cyclic hardening at the beginning of the fatigue loading due to work hardening processes mainly within the sheet layers. Afterwards, a cyclic neutral behaviour occurs until the end of the test. Damage by fatigue crack initiation generally starts within the sheet layers, mostly near large and deep pores within the gauge length. Subsequently, the cracks propagate firstly within the sheet layers and after that through the foamed core of the sandwich perpendicular to the bending axis.