Properties Fast Fracture

The careful text-books measure (Let all who build beware ) 

So when the buckled girder Lets down the grinding span. 

Gordon, The New Science of Engineering Materials. 2d ed.. Princeton University Press. Princeton, New Jersey, 1976. 

As should be familiar to most, the application of a stress to any solid will initially result in a reversible elastic strain that is followed by either fracture without much plastic deformation (Fig. 11.1a) or fracture that is preceded by plastic deformation (Fig. 11.1Z ). Ceramics and glasses fall in the former category and are thus considered brittle solids, whereas most metals and polymers above their glass transition temperature fall into the latter category. 

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The left-hand side of our equation says that fast fracture will occur when, in a material subjected to a stress a, a crack reaches some critical size a or, alternatively, when material containing cracks of size a is subjected to some critical stress cr. The right-hand side of our result depends on material properties only E is obviously a material constant, and G, the energy required to generate unit area of crack, again must depend only on the basic properties of our material. Thus, the important point about the equation is that the critical combination of stress and crack length at which fast fracture commences is a material constant. 

TABLE 3. Summary of the Fast Fracture Strength Properties of HiPerComp Composites. 

The properties required for thermal stress analysis Included Young s modulus, Poisson s ratio, thermal conductivity, thermal expansion coefficient and specific heat. The CARES analysis requires strength data (from which the Weibull parameters are calculated) and Poisson s ratio. Fracture toughness was also measured. Although this parameter is not required for the fast fracture prediction made by CARES, it 1s used in life-time prediction and is related to the properties used in the reliability analysis. Strength measurements, which are the basis of reliability predictions, are controlled by both the size of flaws inherent in ceramic materials and the fracture toughness. Toughness represents the ability of a material to tolerate flaws. 

The subscript 1 in Eq. (18) indicates values transformed to equivalent values based on the properties of time step 1. uieq,K,Tmax is the maximized fast-fracture stress for the first KZp,Total time steps (the proof test time steps) expressed in terms of the properties of time step k. The maximization procedure is identical to that described previously (see Eq. 7 and Eq. 8) except that the maximization is performed only over the proof test time steps 1 to k. o-ieq,i,Tmax,p can be computed by... 

Table 1. NT551 fast fracture and SCG material properties.

The valve s stress history and other relevant terms (temperature, volume, material properties, element number, etc.) were subsequently read into CARES/Li/e. Figure 9 shows the predicted failure probability as a function of the number of cycles for various scenarios. None of the tested valves failed, 8 of the 15 of which were tested to 1000 hours (which corresponds to approximately 1.1 x 10 cycles). Thus the failure rate of the tested valves was less than 1 out of 8 at 1000 hours of operation. From Figure 9 it can be seen that in fast-fracture (at one cycle with no SCG) about 5 out of 100,000 valves are predicted to fail from the loading. With SCG and the power law about 20 out of 100,000 valves fail after 1000 hours operation. If we conservatively assume that a worst-case load (time step 6) is applied over the whole engine cycle (static loading), then about 40 out of 100,000 valves fail after 1000 hours. Using the combined Walker power law with the hypothetical parameters from Table 1 this rate increases to 70 out of 100,000 valves at 1000 hours. All of these rates were well... 

The mechanical properties of the heat treated and the as recieved specimens were tested in various experiments (i.e. controlled crack growth, fast fracture and elastic moduli) in order to analyze the effects of the heat... 

A series of panels were made with differing number of PIP cycles with the expectation that this would result in different overall porosity levels. The optical and x-ray porosity analysis did not confirm this trend and some of the properties did not follow the expected trend with increasing number of infiltration cycles. It was found fiom fast fracture studies that the overall diffusivity is controlled by the microstructure and therefore the porosity correlates with tensile properties. No clear correlation between porosity and properties was observed from the durability testing done. Durability appears to be controlled by the location and distribution of the porosity. Additional testing and analysis is needed to shed insight on the material performance. 

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