Materials and Their Characteristics # 1985 York
A method has been developed to obtain plane-strain fractures in ductile plastics materials. This was done by adhesion of stiff and hard layers to the free surfaces of 3-points bending samples. The plane-strain fracture toughness, K,., was derived from the force-displacement curves in two wad§ : via a finite elements analysis and experimentally via compliance measurements. The method was applied to PVC, to CPE modified PVC, to MDPE and for verification, to a PMMA with known fracture-toughness.
To obtain plane-strain fractures, the test-specimens must exceed a certain minimum size.
The critical size mostly refers to the sheet thickness or to the crack length. For many ductile plastics the critical size may be as large as a decimeter which makes it difficult to meet this condition (3). And if one is succesfull in making a plaque of the required thickness, then it is almost impossible to avoid thermal stresses, orientation and variations in physical structure along the thickness (crystallinity, crystallite-size, and for amorphous glasses : degree of physical ageing). To avoid these problems we developed a method to obtain plane-strain conditions in relatively thin specimens. Stiff and hard layers are adhered to the free surfaces of a 3-point bending sample (see fig. 1). These layers suppress local plastic flow at the crack-tip and so enforce brittle behaviour. This method was developed independently of Truss, Duckett and Ward (4), who applied the same principle to obtain plane-strain G,,-values in impact-tests.