Matrix cracking strength

Composites are generally designed to operate below the proportional limit or matrix cracking strength. However, matrix cracking will inevitably occur by accidental exposures to... 

The interest in development of SiC/Si3N4 composites stems from the fact that (a) SiC and Si3N4 are thermodynamically compatible and stable at temperatures to 1700°C, (b) silicon nitride processing temperature can be tailored to avoid fiber degradation, (c) silicon nitride matrix microstructure can be controlled to inprove composite properties such as matrix cracking strength and thermal conductivity. 

The variation oftensile strength with temperature in airfor a 1-D SiC/RBSN composite is shown in Fig. 4 [16]. The elastic modulus and matrix cracking strength decreases slowly with increase in temperature, but the ultimate tensile strength remains relatively constant from 25 to 600°C. Beyond this temperature it decreases due to oxidation of the carbon coating on the SiC fibers and creep effects. 

Limited property data for HP SCS-6/Si3N4 composites are reported in the literature. The 1-D HP SiCy Si3N4 composite shows -380 MPa and - 450-475 MPa matrix cracking strength and ultimate tensile strength, respectively. A 2-D composite is essentially brittle and shows -300 MPa matrix cracking/ultimate tensile strength. Physical and mechanical property data from three studies are summarized in Table VII. 

But it does not usually do this. We have already seen that, if the length of the fibres is less than 2x, they will not fracture. And if they do not fracture they must instead pull out as the crack opens (Fig. 25.6). This gives a major new contribution to the toughness. If the matrix shear strength is tr (as before), then the work done in pulling a fibre out of the fracture surface is given approximately by... 

Matrix cracking and the ultimate composite strength can be analyzed using a number of approaches, but we will utilize the fracture mechanics approach so that we can compare the results with those we developed in Sections 5.2.1.2 and 5.2.3.1 for... 

Matrix cracking in the SFC tests in the case of low modulus resins has been observed recently by Netravali et al. [19], who have concluded that the SFC technique is experimentally simple but analytically complex. It can be expected that composite strength properties can show different degrees of improvement on silane treatment than indicated by IFSS measurements by the SFC test. 

The increase in ILSS for the epoxy-sized fibers over the bare fibers is 12.4%, approximately 50% of the increase observed in the interfacial shear strength as measured by ITS testing. Changes in the failure mode at the fiber-matrix interface may account for the differences. The sized fibers produced large matrix cracks that grew quickly to catastrophic size under load. This would tend to limit the increase in composite shear properties if at every fiber break in the tensile surface of the coupon a matrix crack was created. The presence of these matrix cracks... 

In the LAS-matrix system matrix cracks parallel to the fibre axis were observed at ATc = 800°C, accompanied by a reduction in Young s modulus, although flexure strength seemed to remain unaffected by thermal shock treatment. This was attributed to the difference in the direction of matrix crack propagation in the two composites due to the formation of a-spodumene-... 

The thermal shock behaviour of a 3-D carbon fibre-reinforced SiC-matrix CMC manufactured by CVI was assessed using the air-quench method by Yin et al. (2002). Damage consisted of matrix cracks that induced a reduction in Young s modulus, strength, and work of fracture for A7 > 700°C. 

Although the two approaches are very similar, the value of A Tc in Boccaccini s model does not depend on the interfacial shear strength t, as a result of the model chosen for the value of matrix cracking stress. Blissett et al. (1997) suggested that their method was valid for the UD material providing that some key parameters (interfacial shear stress, matrix fracture energy) were determined independently. 

Sc Characteristic fiber strength °rme Matrix cracking stress... 

The strength properties of fibers are statistical in nature. Consequently, it is necessary to apply principles of weakest link statistics, which define the properties of fibers within a composite. The initial decision to be made concerns the potential for interactions between failed fibers and matrix cracks. It has generally been assumed that matrix cracks and fiber failure are noninteracting and that global load sharing (GLS) conditions are obtained .16,63,64 In this case, the stress along a material plane that intersects a failed fiber is equally distributed among all of the intact fibers. Experience has indicated that these assumptions are essentially valid for a variety of CMCs. 

Using this simplified approach, simulations of stress-strain curves have been conducted.89,97 These curves have been compared with experimental measurements for several 2-D CMCs. One result is summarized in Fig. 1.35. It is apparent that the simulations lead to somewhat larger flow strengths than the experiments, especially at small inelastic strains. To address this discrepancy, further modeling is in progress, which attempts to couple the behavior of the tunnel cracks with the matrix cracks in the 0° plies. 

Fig. 1.6) indicates that the matrix has a major influence on the shear flow strength ts and the shear ductility, yc. Moreover, it has been found that the shear flow strengths can be ranked using a parameter, W, derived from the matrix cracking stress in the absence of interface sliding,58 given by (Fig. 1.37),... 

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