APPLICATION OF INSTRUMENTED FALLING DART IMPACT TO THE MECHANICAL CHARACTERIZATION OF THERMOPLASTIC FOAMS

J. I. VELASCO*, A. B. MARTINEZ

Departament de Ciencies dels Materials i Enginyeria Metal.lurgica, Escola Tecnica Superior d’Enginyers Industrials de Barcelona (ETSEIB), Universitat Politecnica de Catalunya (UPC), Avda. Diagonal 647, 08028 Barcelona, Spain. E-mail : JVELASCO@CMEM.UPC.ES

D. ARENCON

Centre Catala del Plastic (CCP), Vapor Universitari de Terrassa. C/Colom 114, 08222 Terrassa, Spain

M. A. RODRIGUEZ-PEREZ, J. A. DE SAJA

Departamento de Fisica de la Materia Condensada, Cristalografia y Mineralogia, Universidad de Valladolid, Facultad de Ciencias. Prado de la Magdalena s/n, 47011 Valladolid, Spain.

The applicability of instrumented falling weight impact techniques in characterizing mechanichally thermoplastic foams at relatively high strain rates is presented. In order to try simulating impact loading of foams against sharp elements, an instrumented dart having a hemispherical headstock was employed in the test. Failure strength and toughness values were obtained from high-energy impact experiments, and the elastic modulus could be measured from both flexed plate and indentation low-energy impact tests. The result indicate a dependence of the failure strength, toughness, and the elastic modulus on the foam density, the foaming process, and the chemical composition. This influence was found to be similar to that of pure nonfoamed materials and also to that observed from low-rate compression tests. The results also indicate that the indentation low-energy impact tests were more accurate in obtaining right values of the elastic modulus than the flexed plate low-energy impact tests usually used to characterize rigid plastics. The foam indentation observed with this test configuration contributes to obtaining erroneous values of the elastic modulus if only a simple flexural analysis of plates is applied.

© 1999 Kluwer Academic Publishers

Traditionally, impact tests have been employed to measure the ability of a sample or a finished part to absorb a shock or impact. Falling weight impact tests stand out among the different types of impact tests because the simply supported or fixed (clamped) sample receives the collision of a mass falling from determined height. These tests have advantage of multiaxiality and the possibility of working with finished articles, if they are properly fixed.

The noninstrumented (analogic) impact tests give neither qualitative information about the energy required for the fracture initiation nor the information about the mechanical behavior of the material. These tests give only statistical plots as a relationship between the probability of failure or survival of the sample according the strictness of the test.