Effects of aging on compressive strength of composites after impact

Abstract

Compression after impact (CAI) tests were performed on graphitelbismaleimide quasi-isotropic laminate panels after exposure to elevated temperatures over time in order to investigate the effects of aging on CAI strength. Aging temperatures of 300'F, 350°F, and 400°F with aging times up to 10,000 hours (approximately 14 months) were studied. The impact event was performed both before and after aging, with damage states ranging from barely visible to severe. For each type of impact, a representative damage state was determined by C-scan. Elevated temperature aging was found to cause degradation in CAI strength, becoming more severe with increasing time and temperature. For specimens impacted after aging, the characteristic damage size was found to increase with aging time. Both symmetric and asymmetric damage states exist. Under the same impact energy, aging time and aging temperature, the asyummetric damage states exhibited a larger damage area and higher CAI strength than the symmetric damage state. Thermal aging generally causes a change in the material properties of a composite, brought about by chemical reactions occurring at elevated temperature [I-31. The rate of change in the material is determined by many factors, including the chemical composition of the constituent materials, aging temperature, fiber volume fraction, and ply orientation with respect to the laminate surface [21. For a given composite, the reaction is controlled by the temperature, surface area, and duration of the exposure to air. In addition, thermoset matrix materials such as bismaleimide will experience continued post-cure linking of the polymer chains when held at elevated temperature. This additional linking will cause the matrix to become more stiff and brittle. Increased stiffness will raise the ultimate strength properties, while the increased brittleness will reduce toughness. The resulting effect on fracture pmperties is a combination of these two influences. Ex~er imenta l Program S~ecimen Preparation Composite materials have become widely used in aircraft structure due to their strength, stiffness, and weight advantages over conventional metals. The increased performance characteristics of composites, along with the improved technologies in propulsion, make possible the development of an economically viable passenger aircraft capable of supersonic cruise. At a cruise speed of over Mach 2.0, the structural components of the airplane will be subjected to an elevated temperature environment. Therefore, it is important to determine the effect of long duration exposure to high temperature on the performance of composite structural components. An important design criterion for using composite materials in primary structure is the CAI strength. This study focused on the compressive behavior of impacted thermoset composite laminates after long term exposure to elevated temperatures. t Graduate Research Assistant, tt Professor. * Manager. The material tested was graphitelbismaleimide IM715260 in a quasi-isotropic (45101-45/90)3, laminate layup. The material was manufactured by BASF in 24 inch by 26 inch panels. These panels were then machined using a fluid cooled diamond cutting wheel into 4 inch by 6 inch test coupons, with the long axis in the 0 ply direction. The test coupons had an average thickness of 0.138 inch. The specimens were impacted using a Dynatup drop weight impactor by Integrated Technologies, Inc. (Intec). Drop weight and target speed were varied to produce the desired impact energy. Drop weight was either 7.51 lb or 13.21 lb . Target speed ranged from 9.3 ft/s to 15.7 ft/s. The target impact energy levels were 100 in-lb, 200 in-lb, and 300 in-lb. These energy levels produced damage states described as: barely visible, with a 0.005 inch average indentation; visible, with 0.015 inch average indentation and; and severe, with 0.040 inch average indentation. Load Copyright@ 1994 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. 1683 1 and deflection curves were recorded during the impact event, allowing calculation of energy versus time for each impact. C-scans were performed on a representative sample of the impacted specimens, in order to determine characteristic dimensions of the damage state for each type of impact. Damage area, damage width, and damage height were measured from the C-scan. The specimens were isothermally aged in two Despatch LAC 1-38A ovens. Table 1 gives the time and temperature of aging for all tested specimens. Tests of specimens aged after impact are indicated by type a, while type b tests indicate aging before impact. There were three replicas for each indicated data point. After aging, foil resistance strain gauges were used on all specimens to measure the far field strain during compression. The gage location is shown in Figure 1. The surface was prepared for gauging by sanding lightly with 400 grit sandpaper to remove surface irregularities, cleaning with isopropyl alcohol, and washing using a neutralizer. The strain gages were bonded to the surface of the specimen using M-bond 200 adhesive, and soldered to lead wires using connecting terminals.