With structural reliability assessment as the scenario, a structural part made of foam material was treated as a series k-out-of-n system. The strength random variable of a large-size part was characterized by the strength random variables of virtual elements with certain size, as well as the number of the elements determined by part size. Specially, the statistical dependence among element failures was expounded with respect to the virtual elements. A series k-out-of-n system failure probability model was presented, and the failure probability of a structural part of foam material was described in terms of the size parameters of the part in two dimensions: the number of elements in width direction and that in length direction. It was shown that strong dependence among element failures exists, the failure probability of a large-size part will approaches to an upper bound with the increase in part length.