Sliding stresses

Singh, D., Goretta, K.C., Richardson, J.W. Jr., and de Arellano-Lopez, A. (2002), Interfacial sliding stress in Si3N4/BN fibrous monoliths , Scripta Mater., 46, 747-751. 

Sliding stress, r(MPa) Push-out force Pull-out length, h Saturation crack spacing, /s Hysteresis loop, Se Unloading modulus, EL 1-200... 

Measurements of the sliding stress, t, and the debond energy, T, have been obtained by a variety of approaches (Table 1.1). The most direct involve displacement measurements. These are conducted in two ways (1) fiber push-through7push-in, by using a small-diameter indentor 33 38,39 and (2) tensile loading in the presence of matrix cracks,5,44,45 Indirect methods for obtaining r... 

For SDE, with a constant sliding stress, t0, the sliding distance,/, in the absence of fiber failure, is related to the crack surface tractions, ab, by14,31... 

Analyses of the plastic strains caused by matrix cracks, combined with calculations of the compliance change, provide a constitutive law for the material. The important parameters are the permanent strain, e0 and the unloading modulus, E. These quantities, in turn, depend on several constituent properties the sliding stress, r, the debond energy, T, and the misfit strain, il. The most important results are summarized below. 

In practice, the tangent modulus is usually found to be smaller than predicted by Eqn. (58). Two factors are involved changes in the sliding stress and fiber failure. At high fiber stresses, the Poisson condition of the fiber reduces the radial stress, arr. Consequently, whenever the sliding stress can be represented by Eqn. (1), r decreases as the stress increases. The associated tangent modulus at fixed crack spacing is1... 

As the stress is raised above the first matrix cracking stress, further cracking occurs resulting in a periodic crack array. Cao et al.55 have shown that the magnitude of the saturation crack spacing, d, is governed by the sliding stress, t, such that r and d are related by... 

Cho et al.52 developed an analytical model that relates the temperature rise during fatigue to the interfacial frictional sliding stress, as elaborated later. Holmes and Cho12 used the model to show that in SiCf/CAS-II, the interfacial shear stress, r, decreases from a value of around 15 MPa to 5 MPa within the first 25 000 fatigue cycles (Fig. 6.13). The approach used to determine r from temperature rise data is described in greater detail in the following section. 

Hirvonen and co-workers reported that high-energy neon ion bombardment resulted in amorphization of the crystal structure and increased film hardness. However, the friction remained low, and this suggests that the amorphous coating readily recrystallised with basal plane orientation under a sliding stress. 

If abrasive sliding stress is present, the dependence on hardness known for metals cannot really be depicted in the same way. It has been demonstrated that polymers exhibit a good relationship between wear resistance and crack propagation energy, or even between wear and the product of tensile strength and fractnre strain (Figure 20.6). 

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