Brittle crack microscopic

The problems with the ceramics lie as much in reproducibility as in absolute strength. With brittle materials, microscopic flaws concentrate stress and cause failure at stresses, which vary according to the classical crack theory as <5 1/2, where 8 is the characteristic dimension of the flaw. For materials formed from powders, the flaws represent structural... 

Fig. 5.8a. Scanning electron microscopic image of the fracture surface of a brittle crack from a NCTL test spedmea The view is directed fiom the front top on the top of the lower part of the 3.2 mm wide raptured test specimen (see also Fig. 3.17). The edge of the notch is fully recognisable in the foreground. A semicircular fracture surface emerges from the edge. Only when the remaining cross section reduces to such an extent that the local stress exceeds stress at yield, the material yields and breaks after large plastic deformation...

When cyclic loads are applied to a material, cracking and fracture may occur by the process of fatigue. On a microscopic scale, fatigue occurs by localized plastic deformation, resulting in the initiation and growth of macroscopically brittle cracks. 

Figure 35 shows the optical microscopic images of the first crack point on the sample surface. The scratch scar of monolayer Sample 1 has the feature of brittleness. However, there is an obvious crack along the scratch scar of Sample 2 before the coating delamination. This indicates that mono-layer Sample 2 has the feature of ductility, and the adhesion between the film and the substrate is poor. However, there is no obvious crack before the delamination in the scratch scars of other samples. The feature of multilayer Samples 3 and 4 is different from monolayer Samples 1 and 2. There are no obvious cracks in the scratch scars of Samples 5 and 6, except several small cracks along the edge of the scars. These... 

Long before a brittle hair crack has come to its complete development, it is already present as a nucleus, and the material has already undergone irreversible damage. Microscopic cracks are often partially filled with material in the form of very thin fibres, forming bridges between the fracture surfaces. These cracks are denoted as... 

Abrasion removes protective oxidized metal and polarized coatings to expose unoxidized metal, in addition to removing metal particles. Forms microscopic grooves and dents for concentration cell corrosion. Increases microscopic surface area exposed to corrosion. Removes strain-hardened surface layers. Cracks brittle metal constituents, forming sites for impact hydraulic splitting. Plastic deformation by high-stress metal-mineral contact causes strain hardening and susceptibility to chemical attack. 

A brittle behavior. The force-displacement (F-d) curves are linear elastic. The crack propagates in an unstable way. The damage mechanisms have not been initiated before failure. The fracture surfaces are very smooth and mirror-like. At the microscopic scale, they are believed to be associated with the development of a single crack. 

When some critical crack length (between 200 pm and 400 pm, i.e. about 15% of the contact diameter) is reached, a brittle propagation stage is observed which is associated with a sudden and drastic drop in the lateral stiffness, K. The measured crack width in the plane of the contact is then of the order of magnitude of the contact diameter. Post-mortem microscope observation of specimen cross sections in the contact zone (Fig. 8) indicates that the depth of the cracks is of the order of magnitude of the contact radius (i.e. about 900 pm). The two deep cracks induced at the edge of the contact may thus be viewed as some kind of half-penny cracks whose radii are approximately equal to the radius of the contact. In the subsequent part of this paper, the two deep cracks will be referred to as primary cracks . 

Some agglomerates of different materials have been observed to fail because of internal flaws driven by a number of stresses (e.g., internal tensile stress cracks in the surface plastic flow at the surface between the agglomerate and platen and shear stress within the sphere). For brittle particle agglomerates with significant internal flaws, the tensile strength is small compared to the compressive and shear strength, and failure is likely initiated by the internal tensile stress. In any case, a careful microscopic examination of failed pieces can provide much information on the dominant failure mode (Bika et al., 2001). 

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