How Do Engineers Evaluate Plastic Materials and Material Strength . Flexural Strength is the ability of a material to resist deformation under load. ASTMD790; Young's Modulus or Tensile Modulus is used to characterize the stiffness of an elastic material. It was named after the 19th century British scientist Thomas Young. It is often referred to as the Modulus of Elasticity. It is a ratio of the stress..