material BilinearJ2 (1) (2) (3) (4) [5] [6] [7]
# (1) int, unique material tag
# (2) double, elastic modulus
# (3) double, poissons ratio
# (4) double, yield stress
# [5] double, hardening ratio, default: 0.0
# [6] double, beta, default: 1.0
# [7] double, density, default: 0.0
