A new mechanism for biaxial tension tests was developed for loading an in-plane reinforced composite laminate or any injection-molded polymeric specimen simultaneously in two principal directions. This mechanism can be adapted to any uniaxial tension test machine and, thereby, it can reduce the cost of conducting tests on expensive dedicated machines. The fixture provides a uniform state of equibiaxial tension, necessary for characterizing the biaxial state of loading on any polymeric material system and it can also be reconfigured to test non-equibiaxial tension over a short range. The mechanism is presently utilized for understanding the failure behavior of injection molded short fiber polyamide and nanoparticle-reinforced PP thermoplastic composites.