JOURNAL OF MATERIALS SCIENCE 34 (1 9 9 9 ) 431 – 438 Application of instrumented falling dart impact to the mechanical characterization of thermoplastic foams J. I. VELASCO ∗ , A. B. MART ´ INEZ Departament de Ci ` encies dels Materials i Enginyeria Metal.l ´ urgica, Escola T ` ecnica Superior d’Enginyers Industrials de Barcelona (ETSEIB), Universitat Polit ` ecnica de Catalunya (UPC), Avda. Diagonal 647, 08028 Barcelona, Spain E-mail: JVELASCO@CMEM.UPC.ES D. ARENC ´ ON Centre Catal ` a del Pl ` astic (CCP), Vapor Universitari de Terrassa. C/Colom 114, 08222 Terrassa, Spain M. A. RODR ´ IGUEZ-P ´ EREZ,J. A. DE SAJA Departamento de F´ ısica de la Materia Condensada, Cristalograf´ ıa y Mineralog´ ıa, 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 mechanically thermoplastic foams at relatively high strain rates is presented in this paper. In order to try simulating impact loading of foams against sharp elements, an instrumented dart having a hemispherical headstock was employed in the tests. 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 results 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. C  1999 Kluwer Academic Publishers 1. Introduction Traditionally, impact tests have been employed to mea- sure 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 a determined height. These tests have the 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 re- quired 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 ac- cording to the strictness of the test [1]. ∗ To whom correspondence should be addressed. Nevertheless, in instrumented impact tests, the recorded force that the striker supports during the test allows for the obtaining of information about the en- ergy absorbed by the material until the failure. Carry- ing out low-energy tests (rebound), makes it possible to get information about its elastic properties. Therefore, using instrumented impact techniques, it is even pos- sible to characterize the fracture behavior of plastics and composites by applying the linear-elastic fracture mechanics (LEFM) on SENB geometry [2, 3] to ob- tain fracture criteria such as the fracture energy (G IC ) and the fracture toughness ( K IC ), which are indepen- dent of the geometry of the cracked body. However, this application is restricted to brittle materials. The aim of this paper is to present a mechanical characterization of polyolefinic foams at relatively high strain rates by means of instrumented falling weight 0022–2461 C  1999 Kluwer Academic Publishers 431