July 2007 Thermal Cycling Damage in Ceramic Matrix Composites Under a Constant Stress 2137 0.129 (2) Strain Response During Thermal Cycles a typical strain versus thermal cycle number N curve of the C/ Sic composite under a constant stress of 50 MPa is shown in 0.043 Fig 4. It is apparent that the measured strain is cycled with the lic temperature on a progressively increasing baseline. It wa believed that the increasing strain baseline(also referred to as resulted from damage of the C/SiC composites in terms of ma trix cracking during thermal cycles. As we know. the main -0129 ethodology to increase fracture toughness and inelastic strain of CMCs is derived from the viewpoint of microcracking mech- anisms because no plastic deformation can be expected in such naterials.28.47 As shown in Figs. 6(a)and(b), the damage pro- cess of the CMCs under thermal cycles is the multiplication of coating/matrix cracks that propagate perpendicular to the tensile direction. It has been widely accepted that in case of Thermal cycling Number, N fiber-reinforced ceramic matrix composites, the inelastic strain versus thermal cycle number N curve of the 3D represents a large part of the total strain and they have their C/SiC composites during thermal cycles at a constant stress of 50 MPa urce in the sum of the crack opening displacement of the transverse crack system. The inelastic damage strain is related to the crack density p as Thermal cycle number N △ L nUc (1) 50×10 where L is the extensometer length, n the number of cracks, and Ucop crack opening displacements. Thus, the inelastic strain is mply the density of transverse matrix microcracks p multiplied Tcc strain y their aspect ratio 8( dimension parameter of a crack). During thermal cycles, as also illustrated in Fig. 6. the dam- 0.086 10-4 age mechanisms of the CMCs favored the generation of micro- 0.129 racks oriented normal to the tensile axis and the microcracks 20x10 tended to propagate rapidly inside the brittle matrix across the entire width of fiber spacing. Therefore, according to Eq .(1), anges in the inelastic strains depend significantly on the den- 3.0x10 10502100315042005250 ty of transverse matrix microcracks B, which is associated with the extent of thermal cycling damage. It is commonly recognized Time(s) that the crack density increases with increasing thermal cycle Fig.5. Change in thermal cycle creep(TCC)strain and the correspond- number N and there exists a critical Ne beyond whic ing strain rate of the 3D C/SiC composites subjected to thermal cycles and a constant stress of 50 MPa ing to a stable inelastic strain of the composite. Figure 5 shows that the TCC strain increases gradually from the initial.215% with an increase in thermal cycle number n higher modulus fibers and bundles split manne and then maintains a constant value of.121% with a strain leading to an apparent""(marked with arrow in rate of zero(right y-axis in Fig. 5)after 15 cycles(3150 s), in- on the stress-strain curve. The corresponding cumula- dicating no initiation of the new cracks and no propagation of tive ae energy increased dramatically and coincided extremely the previous cracks once the crack density becomes saturate well with the"stiffening" point on the pic mechanical bviously, the contribution of the inelastic strain to the total esponse. It is therefore feasible to correlate the observed strain was rather large within the initial 15 cycles, and the ther AE events to the actual damage phenomena occurring in the mal cycling damage mainly took place before the critical number nvestigated materia Nc of 15. Therefore, TCC strain is considered to be damage 200um the grc graphs showing(a) multiplication of the coating cracks and(b ) matrix cracks penetrating across the fiber-free ceramic matrix regions in round and polished C/SiC composites after 25 thermal cycleshigher modulus fibers and bundles split in a sudden manner, leading to an apparent ‘‘stiffening’’ (marked with arrow in Fig. 3) on the stress–strain curve. The corresponding cumula￾tive AE energy increased dramatically and coincided extremely well with the ‘‘stiffening’’ point on the macroscopic mechanical response. It is therefore feasible to correlate the observed AE events to the actual damage phenomena occurring in the investigated material. (2) Strain Response During Thermal Cycles A typical strain versus thermal cycle number N curve of the C/ SiC composite under a constant stress of 50 MPa is shown in Fig. 4. It is apparent that the measured strain is cycled with the cyclic temperature on a progressively increasing baseline. It was believed that the increasing strain baseline (also referred to as thermal cycle creep (TCC) strain46; see the left y-axis in Fig. 5) resulted from damage of the C/SiC composites in terms of ma￾trix cracking during thermal cycles. As we know, the main methodology to increase fracture toughness and inelastic strain of CMCs is derived from the viewpoint of microcracking mech￾anisms because no plastic deformation can be expected in such materials.28,47 As shown in Figs. 6(a) and (b), the damage pro￾cess of the CMCs under thermal cycles is the multiplication of coating/matrix cracks that propagate perpendicular to the tensile direction. It has been widely accepted that in case of fiber-reinforced ceramic matrix composites, the inelastic strain represents a large part of the total strain and they have their source in the sum of the crack opening displacement of the transverse crack system. The inelastic damage strain is related to the crack density b as47 ein ¼ DLin L ¼ nUCOD L ¼ db (1) where L is the extensometer length, n the number of cracks, and UCOD crack opening displacements. Thus, the inelastic strain is simply the density of transverse matrix microcracks b multiplied by their aspect ratio d (dimension parameter of a crack). During thermal cycles, as also illustrated in Fig. 6, the dam￾age mechanisms of the CMCs favored the generation of micro￾cracks oriented normal to the tensile axis and the microcracks tended to propagate rapidly inside the brittle matrix across the entire width of fiber spacing. Therefore, according to Eq. (1), changes in the inelastic strains depend significantly on the den￾sity of transverse matrix microcracks b, which is associated with the extent of thermal cycling damage. It is commonly recognized that the crack density increases with increasing thermal cycle number N and there exists a critical Nc beyond which the crack density can reach saturated with damage saturation,37,48–50 lead￾ing to a stable inelastic strain of the composite. Figure 5 shows that the TCC strain increases gradually from the initial 0.215% with an increase in thermal cycle number N, and then maintains a constant value of 0.121% with a strain rate of zero (right y-axis in Fig. 5) after 15 cycles (3150 s), in￾dicating no initiation of the new cracks and no propagation of the previous cracks once the crack density becomes saturated. Obviously, the contribution of the inelastic strain to the total strain was rather large within the initial 15 cycles, and the ther￾mal cycling damage mainly took place before the critical number Nc of 15. Therefore, TCC strain is considered to be damage Fig. 4. Typical strain versus thermal cycle number N curve of the 3D C/SiC composites during thermal cycles at a constant stress of 50 MPa. Fig. 5. Change in thermal cycle creep (TCC) strain and the correspond￾ing strain rate of the 3D C/SiC composites subjected to thermal cycles and a constant stress of 50 MPa. a b 200 um 200 um Fig. 6. Micrographs showing (a) multiplication of the coating cracks and (b) matrix cracks penetrating across the fiber-free ceramic matrix regions in the ground and polished C/SiC composites after 25 thermal cycles. July 2007 Thermal Cycling Damage in Ceramic Matrix Composites Under a Constant Stress 2137