Damage tolerance and hot/wet compressive performance are two key parameters which are required for composite materials to replace aluminum in primary structural applications on aircraft. Conventional toughening approaches involving the use of second-phase rubber modification of tetrafunctional epoxies have not lead to the required balance of these properties. Emphasizing newly developed curative, catalyst, and matrix modifier chemistry, a major advance in the performance of epoxy composites has been made which will allow the design requirements for commercial and transport aircraft to be achieved. It is shown that the stress-strain response of these resins can be effectively analyzed to distinguish between material systems for their performance in composites. The toughness, controlled by the strain to failure, and the hot/wet resin modulus are used as the key parameters to make these comparisons. The performance of new composites based on these materials is quantified in terms of two key design requirements, that is, residual compressive strength after impact damage and hot/wet compressive performance.

The mechanics, failure analysis, and materials science approaches that lead to the development of novel high strain to failure composite systems containing a discrete toughened resin interleaf between the laminate plies are also discussed. The combination of new toughened epoxy resins with novel interleafing materials is shown to result in composites exceeding the ultimate design strain target of 0.006 cm/cm while providing a hot/wet compressive strength to reduce weight by over 40% compared to 2024-T3 aluminum. In addition to this, these improved interlayered systems have two other attractive features: (1) their residual compressive strengths after impact are comparable to those of thermoplastic matrix resin systems and (2) they process and cure like standard epoxy materials.