NDTnet 1998 Aug, Vol.3 No.8

Abstract

Due to the inherent brittle nature, structures made of composite materials are susceptible to impact damage of various levels which may be caused during manufacture, maintenance and in service. Typical among these are: inclusions, containments, voids, fibre breakage, delaminations, translaminar cracks, incorrect orientation, resin rich and resin starved areas, moisture absorption, creep etc. Non Destructive Evaluation of composites is of paramount importance and defects must be detected with a specified probability and degree of confidence. Once detected, these must be characterized so that the structural performance can be assessed. But unlike metals, the nature of defects in composites is very different and often leads to complex failure mechanisms.

Metallic materials absorb impact energy by undergoing elasto-plastic deformation at low and intermediate incident energies. These cases result in clear visible dent at the point of impact, but at high incident impact energies target perforation may occur and the passage of the impactor will generally result in petalling, cracking and spelling. Although such damage will degrade the load bearing ability of the structure, its effect can be predicted by fracture mechanics principles. In contrast, the response of the composite materials is quite different. There is no clear boundary in terms of numerical values as to what exactly is low velocity or high velocity impact.

Normally low velocity impact is considered to be that energy value under which entire structure responds and the stress waves travel through to the boundary of the structure, and get reflected back. In case of high velocity impact the response of the structure is confined to the point of impact and affected region is local to this point. In this case, stress waves through the thickness of the material are predominant factors, and can lead to the formation and detachment of a conical volume of material extending back from the point of impact. It is the response of composites to low velocity impact which is of major practical concern.

Delamination and matrix cracking are the major modes of failure in composite materials. Up to certain level of impact energy, although delamination exists inside the material, it will not be visible at the point of impact. This is cause of worry, as such the damage would greatly reduce mechanical properties in general, and the residual compressive strength in particular. If the structure goes in service in the damaged state, as the damage is invisible at the surface, catastrophic failure can result when this grows under service loads.

The present work aims at trying out Acoustic Impact Technique as a possible means of deployment at field level for the purpose of flaw detection in composite helicopter blades. The principle underlying Acoustic Impact Testing is simple. The sound produced when a structure is tapped is mainly at the frequencies of major structural modes of vibration, which are structural properties. Any difference in the sound is necessarily due to modification affected to the force input by the defect in the structure. A study of the force time history of response or its fast fourier transform makes it possible to locate and detect sub surface flaws.

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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