Creep of aluminum syntactic foams

Aluminum syntactic foams with densities of 1.2-1.5 g/cm³ were deformed at 500 °C under constant uniaxial compressive stresses ranging from 5 to 14 MPa. The foam creep behavior is characterized by a short primary stage and a long secondary stage where the strain rate is constant and minimum, followed by a tertiary stage at high stresses. The minimum strain rate varies with stress according to an apparent stress exponent n with a low value (n ≈ 1) for stresses below 8 MPa, and a high value (n ≈ 14) above 8 MPa. Finite-element modeling provides predictions for the foam strain rates that are in qualitative agreement with experimental results. Modeling also shows that the matrix transfers load to the ceramic elastic spheres, explaining the exceptionally high creep resistance of these syntactic foams as compared to aluminum foams without ceramic spheres. Modeling finally reveals that stresses vary with position in the matrix and time during creep, and that the onset of tertiary stage is associated with the appearance of sharp stress concentrations in the matrix.