Accelerated ageing versus realistic ageing in aerospace composite materials. IV. Hot/wet ageing effects in a low temperature cure epoxy composite

Abstract

Samples of an aerospace grade carbon fiber epoxy composite (Hexcel, M20/IM7) were subject to long term (%1 year) hot/wet ageing and thermal spiking under a variety of humidity levels and temperature conditions related to ''in service'' conditions seen by military aircraft. Changes to the chemical and physicochemical structure of the composite were analyzed by a range of different techniques including gravimetric analysis, FTIR, and DMA to compare the effects of the various ageing conditions. The results indicated that the chemical effects of hot/wet and spiking conditions on this incompletely cured type of composite are very complex because of the variations in moisture levels and cure chemistry from the composite surface inwards as the resin ages under the different external environments. Physicochemical changes (such as T g ) and structural effects (such as microcracking) are similarly complex and dependent on composite thickness.

Long‐term aging in a commercial aerospace composite sample: Chemical and physical changes

“…Our recent experiments on the artificial aging of commercial carbon fiber composites has shown that it is possible to obtain highly reproducible and accurate transmission spectra of such composites by minor modifications of previously used FTIR methods. [4][5][6][7] This previous research has also confirmed that, in the aging of a composite of a reasonable thickness, the central region of the composite remains chemically unchanged even under quite vigorous aging regimens. A further advantage of this FTIR analysis technique is that it also allows the removal of volatile substances (e.g., moisture) from a composite and hence can be used to determine real chemical changes in the material.…”

“…This may be consistent with the extraction of low-molecular-weight polar oligomers by moisture and is similar to what we have observed in accelerated aging under hot/wet conditions. 7 Other changes that can be observed include the broad ANHA peak (amide) showing increased absorption at 3230 cm À1 and a sharp ACAOA absorption peak at 1169 cm À1 consistent with further oxidation.…”

Environmental Degradation of Carbon Nanotube Hybrid Aerospace Composites

“…The three stages include an induction period with a slow weight increase followed by the middle stage with a high absorption rate and finally a plateau with a very low weight increase rate as illustrated in Fig. 9.3 (Dao et al 2007b) The duration and intensity of each stage depends mainly on the aging conditions (temperature, relative humidity) as well as the nature of the resin and the level of reaction reached at the time of exposure. The induction period is linked to the various effects of material extraction at the surface of the composite and the defect group (unreacted monomer and oligomer end groups) reactions with water (Dao et al 2007b).…”

“…9.3 (Dao et al 2007b) The duration and intensity of each stage depends mainly on the aging conditions (temperature, relative humidity) as well as the nature of the resin and the level of reaction reached at the time of exposure. The induction period is linked to the various effects of material extraction at the surface of the composite and the defect group (unreacted monomer and oligomer end groups) reactions with water (Dao et al 2007b). The intermediate, high-absorption rate may relate to the osmotic effects of unreacted monomers (Dao et al 2007b).…”

“…The induction period is linked to the various effects of material extraction at the surface of the composite and the defect group (unreacted monomer and oligomer end groups) reactions with water (Dao et al 2007b). The intermediate, high-absorption rate may relate to the osmotic effects of unreacted monomers (Dao et al 2007b).…”