New aircraft designs, both commercial and military, are increasingly relying on composite structures to reduce aircraft weight and fuel consumption. In the case of high-performance aircraft, the desired performance is often unobtainable using “traditional” structural designs (such as riveted or welded aluminum alloys). Corrosion is a critical design concern when composites are to be used in corrosive environments, and it needs to be addressed in the initial design phases of these aircraft. Corrosive environments, which are characterized by their pH factor, temperature, and moisture, continuously degrade composites until they cannot sustain the load that they were initially designed to carry. Even though the corrosion of metals has been investigated extensively--particularly in regards to test methods and experimental investigation--only a limited number of laminates can be investigated experimentally and in only a limited number of corrosive environments. This leaves a knowledge gap because composite system designs require lots of flexibility. A convenient computational simulation method was needed to predict the ability of composites to endure corrosive environments. Just such a method was developed at the NASA Glenn Research Center. It correlates the pH factor of the corrosive environment with the damage tolerance of the composite laminate, which is simulated by micro-, macro-, and laminate-level composite theories. The corrosive environment is assumed to cause polymer composite degradation on a ply-by-ply basis. The degradation is correlated with the measured pH factor, a parabolic distribution of voids through the laminate thickness, and a linear distribution of temperature and moisture.
Simulations are performed by the Integrated Composite Analyzer (ICAN), a computational composite mechanics computer code that includes microlevel, macrolevel, combined stress failure, and laminate-level theories. A simulation starts with constitutive material properties and models behavior up to the laminate scale, accounting for exposure of the laminate to the corrosive environment. The simulation procedure follows:
Approximate pH correlation with ply degradation of AS/E [0±45/90]s composite (assuming that voids vary parabolically through the laminate thickness and that temperature and moisture vary linearly). (AS/E is an AS graphite fiber in an epoxy matrix.)
Results obtained for one laminate indicate that the ply-by-ply simulation degrades the laminate to the last ply or last several plies. Results also demonstrate that the simulation is applicable to other polymer composite systems. The final figure illustrates the degradation procedure with the negative combined-stress failure criterion. Details of this simulation method are given in reference 1.
ICAN simulation cycle.
Long description of figure 2.
Ply corrosion degradation [0±45/90]s of AS/HMHS laminate. Corrosion simulated by parabolic distribution of voids through the thickness of the laminate and linear distribution of temperature (first ply stresses and combined-stress failure criterion, FC). (AS/HMHS, AS graphite fiber in a high-modulus, high-strength matrix; σ11T, longitudinal tensile stress; σ12S, intraply shear stress.)
Long description of figure 3.
Last updated: December 28, 2007
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