Tracking Accelerated Aging of Composites with Ul Trasonic Attenuation Measurements

Composite materials are steadily replacing traditional materials in many industries. For many carbon composite materials, particularly in aerospace applications, durability is a critical design parameter which must be accurately characterized. Lawrence Livermore National Laboratory (LLNL) and Boeing Commercial Airplane Group have established a cooperative research and development agreement (CRADA) to assist in the high speed researchprogram at Boeing. LLNL's expertise in fiber composites, computer modeling, mechanical testing, chernical analysis and nondestructive evaluation (NDE) will contribute to the study of advanced composite materials in commercial aerospace applications. Through thermo-mechanical experiments with periodic chernical analysis and nondestructive evaluation, the aging mechanisms in several continuous fiber polymer composites will be studied. With a design lifetime and skin temperature of 120,000 hours (13.7 years) at 17TC, real-time durability studies of candidate materials for high speed aircraft structures are timeconsuming and very expensive. Design of materials for durability can become more feasible by understanding aging mechanisms of the composite and identifying measurement parameters which can track these long-term aging mechanisms. Once the real-time aging process is characterized, an accelerated program of aging can be developed so that long-term aging can be accelerated and design of the composite materials can occur in a reasonable time period. Higher temperatures and chemical variations in test environments can aceeierate aging of the composites. The goal of this project is to develop an accelerated lifetime model of several continuous fiber polymer composites, verify accelerated aging techniques with thermo-mechanical experimental data and identify tracking parameters for aging mechanisms. Several measurement techniques are being studied for their correlation with aging, including Fourier-transformed infrared spectroscopy (FTIR), positron annihilation and uhrasonie attenuation. The two surface techniques, FTIR and positron annihilation, quantify chernical changes and free-space volume in the composite, respectively. These two techniques and their results are not discussed here. Likewise, mechanical aging results are not reported here. This paper describes through-transmission uhrasonie attenuation measurements of isothermally aged composite materials and their use as a tracking parameter for accelerated aging.