A multi-stage damage evolution was found for a ceramic matrix composite loaded in uniaxial tension along fibres. At low strains the composite behave truly elastic (stage I), at higher strain matrix cracks initiate and form a pattern of multiple matrix cracks (stage II). Matrix cracking saturates so that interfacial sliding is the dominating mechanism (stage III), before distributed fibre failure occurs (stage IV), leading to localisation and fracture (Sørensen and Talreja, 1993).
Stress-strain curve of a unidirectional SiCf /calcium aluminosilicate composite.
Modelling by finite element unit cell (accounting for slipping interfaces and Poisson's effects) provided a link between the complicated slip/stick behaviour along the fibre matrix interface and the overall stress-strain behaviour (Sørensen et al., 1993). However, the model prediction for the stress-transverse strain did not show satisfying agreement with experimental measurements, showing strain reversal during multiple matrix cracking. A model showed that roughness along the fibre matrix interface could explain the observed behaviour (Sørensen, 1993).
Fatigue of continuous fibre reinforced ceramics was studied by cyclic loading experiments until 100 million cycles. Although specimens developed significant damage during cycling, still an engineering run-out fatigue limit was found (Sørensen et al., 2002). However, damage indicators (hysteresis modulus and temperature rise by frictional heating) kept changing during cycling, indicating that the evolution of fatigue damage was still evolving. The fatigue damage evolution appears to be sensitive to interfacial lubrication; specimens having been immersed in oil survived 100 million cycles under conditions that caused fatigue failure after approximate 1 million cycles for un-treated specimens (Sørensen and Holmes, 1995).
Sørensen, B. F., Talreja, R. and Sørensen, O. T., 1993, "Micromechanical Analysis of Damage Mechanisms in Ceramic Matrix Composites During Mechanical and Thermal Loading", Composites, vol. 24, pp. 124-40.
Sørensen, B. F., and Talreja, R., 1993, "Analysis of Damage in Ceramic Matrix Composites", International Journal of Damage Mechanics, Vol. 2, pp. 246-71.
Sørensen, B. F., 1993, "Effect of Fibre Roughness on the Overall Stress-Transverse Strain Response of Ceramic Composites", Scripta Metallurgica et Materialia, Vol. 28, pp. 435-9.
Sørensen, B. F. and Holmes, J. W., 1995, "Improvement in the Fatigue Life of Fiber-Reinforced Ceramics by Use of Interface Lubrication", Scripta Metallurgica et Materialia, Vol. 32, pp. 1393-8.
Sørensen, B. F., Holmes, J. W. and Vanswijgenhoven, E.L., 2002, "Does a True Fatigue Limit Exist for Continuous Fiber-Reinforced Ceramic Matrix Composites?", Journal of the American Ceramic Society, Vol. 85 , pp. 359-65.