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1995
Conference Paper
Titel
Application of dynamic key curves for the determination of the impact fracture toughness of polymers at high rates of loading
Abstract
The Technical Committee on Polymers and Composites of the European Society of Structural Integrity (ESIS) is currently preparing a specification for the determination of the fracture toughness, K Ic, for polymers at high loading-rates. This procedure is based on instrumented tests with pre-cracked specimens at impact velocities of 1 m/s. The fracture toughness has to be evaluated quasi-statically from the critical load measured at fracture. However, this procedure is not applicable at higher loading rates of several metres per second, which are proposed in different standards for impact tests of polymers. In these cases, especially for brittle materials, very short times-to-fracture occur and inertia effects are important and have to be considered. Pronounced oscillations of the load signal indicate dynamic effects like elastic waves and vibrations. In this more complicated situation the method of impact response curves (IRC) can be used for the determination of the impact fracture tou ghness K Id. For a given impact testing arrangement (loading device, specimen geometry, material properties) one unique crack tip loading history (i.e., the IRC) has to be determined in pre-experiments. During routine testing only the time-to-fracture has to be measured and K Id can be evaluated from the IRC. This procedure also applies for polymers; however, for each new material with another elastic modulus another IRC has to be determined. To reduce this effort, the concept of dynamic key curves (DKC) was developed. The DKCs have been measured once in model experiments, and rules have been developed to transfer these result to arbitrary impact arrangements. By applying these DKCs it is possible to determine K Id values without pre-experiments and just by the determination of the time-to-fracture. The application to polymers is shown and the accuracy and limitations are discussed.