A PLASTIC HINGE MODEL TO EVALUATE THE STRUCTURAL PERFORMANCE OF CASTS

Published manuscripts have described the material properties of castin g materials by using simple material testing techniques such as three- point bending and by using flexure equations which calculate material strength and stiffness, These equations assume the cast is exposed to small deformations, bends with a uniform curvature and acts as a beam. These conditions are not met in some applications since the cast/limb structure may be designed for compliance and large deformation. The f lexure equation should be replaced with the ''plastic hinge'' concept which provides a better description of the mechanical behavior of cast and a better means to predict the failure behavior of the cast/limb s tructure. A testing procedure and a mathematical model were developed to analyze casting materials as a ''plastic hinge''. The results show the model can define the loading behavior of a rigid cast and a cast w ith compliant properties. The full plastic moment can be used to defin e the load where ''practical'' structural failure of the cast will occ ur. The results indicate that rigid cast bonding helps to produce a lo ading behavior that behaves in a complete structural collapse manner. This is similar to an idealized plastic hinge while more compliant bon ding causes a loading behavior where the full plastic moment has to be estimated.