NDTnet 1998 Aug, Vol.3 No.8

Abstract

The SMC (sheet molding compound) consists of a mixture of unsaturated polyester resin, material filler CaCO3, thermoplastic additive and random oriented glass reinforcing fibers. Favourable mechanical properties, together with some advantages in processability and high volume production, make SMC competitive with metals for automotive applications. The relatively low fibre volume content (~20%) and the uncontrolled phase distribution during processing lead to a highly heterogeneous structure with several kinds of stress concentrators such as fiber ends and interfaces, filler material and defects. The result is that at quite an early stage loading introduces various forms of localized damage, such as fiber-end cracking, fiber-matrix interface debonding and matrix cracking. The aim of this paper is to characterize damage progression during tensile loading by means of different ultrasonic techniques. Since the load-induced damage essentially consists of matrix microfractures difficult to detect by means of non destructive techniques, various parameters obtained by different analyses of the ultrasonic signal (amplitude attenuation, attenuation vs frequency slope, spectral energy, etc.) were evaluated as possible indicators of damage accumulation. Cyclic tests were conducted to study material degradation under incremental loading and to correlate changes in integral damage parameters to local damage events. The specimens were subjected to blocks of loading and unloading cycles at a constant maximum stress level; at the end of each block the specimens were tested inside an immersion tank and scanned in pulse-echo mode with a 5 MHz broadband tranducer. SMC is so inhomogeneous in its composition that the scatter and the attenuation of ultrasonic waves normally precludes the use of frequencies extending beyond 10 MHz. The results obtained by ultrasonic analysis are discussed and compared to the information drawn by mechanical indicators such as elastic modulus or histeresis cycle area.

Abstract Source:
Preview of Abstracts, International Conference on Advanced Materials - 1998, Hurghada, Egypt, 15 - 18 December 1998

Full-Text Source:
International Conference on Advanced Materials - 1998, Hurghada, Egypt, 15 - 18 December 1998

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