Monitoring of Impact Damage in Composites Using Wave Propagation Methods

Abstract

Composite structures in an aircraft are susceptible to impact damage, which can occur during manufacture, service or maintenance. Recent studies show that impacts with ground support equipment are the major cause of in-service damage to composite structures in an aircraft. Other sources of impact include collision with birds, runway stones or ballistic impacts. These impacts can produce various types of damage, including fiber breakage, matrix cracking, delamination, and interfacial debonding. The results of such damage can have detrimental effects on the overall structural performance and safety. A comprehensive structural health monitoring (SHM) system provides a means to significantly reduce life-cycle costs of aerospace vehicles by providing accurate diagnostics and prognostics of structural damage to reduce unnecessary inspections and support vehicle life extension. The main objective of this paper is to develop a methodology to detect and identify the damage sources and their severity in composite laminates subjected to low velocity impact using wave propagation methods. When damage occurs in a material due to mechanical load or impact, an acoustic wave emits and propagates through the material. The material chosen for this work is a 12″ long and 12″ wide, +/− 60 degree braided composite. Two edges of the plate were fixed by clamping the plate between two steel bars and secured by bolts spaced 1″ apart, while the other two edges were free, as shown in Figure 1. In order to characterize the wave propagation and damage process, two resonant type AE sensors and four accelerometers were mounted on the specimen. The specimen was then tapped lightly with a hand-held acoustic impact hammer at several different chosen locations, and stress wave signals were monitored using a commercial dynamic signal process system which contains software capable of detecting impact source location. The impact force was kept to a minimum initially such that no damage occurred in the specimen. After this initial test, the specimens were subjected to low velocity impact using drop weight impact machine with 0.5 inch spherical indenter. The impact force was increased by a number of times until substantial damage observed while monitoring signals generated from the specimen. After each incremental impact, both acoustic hammer tapping test and nondestructive inspection such as ultrasonic C-scan and/or X-ray radiography were carried out to delineate the damage source and severity. Figure 2 is an example of C-Scan of the composite plate after a series of impacts with various drop heights. Recorded signals were analyzed to determine the origin of the source and its severity. The impact hammer produced both an extensional wave and a flexural wave in these composite plate specimens. Because of dispersive characteristics of the flexural wave, the first arrival time of the extensional wave was used for source location algorithm. Besides the source location, discussion will be given on parameters such as amplitude, energy, frequency, number of events related with impact force, and damage size in detail. As an example, Figure 3 is a plot of the measured damage size as a function of the dead-weight drop height for tests conducted on various panels. As expected, the size of the damage increases with amount of drop height (or impact energy). Thus, based on C-scan measurements, critical threshold impact height of approximately 5″ is identified for “any measurable” damage to occur. The corresponding magnitude of the impact energy is ∼ 108 in-lb. On the other hand, the critical threshold for any visual damage to be detected is approximately 502 in-lb for the laminate material investigated. In summary, a methodology has been developed for estimating the damage severity from the amplitude of the signal received. The approach entails constructing design curves relating the size of the damage to impact energy, and establishing relationships between impact energy and the magnitude of the signal. These relationships can then be used to predict the estimated size of the damage based on the amplitude of the arriving signal. A critical threshold impact energy has been identified below which “no measurable” damage occurs. Three regions of damage growth, namely, a decreasing rate with magnitude of impact energy. A constant damage growth rate characterizes the steady-state region, while damage size increases almost exponentially with impact energy in the tertiary region potentially leading to catastrophic failure.