Brittle crack cross-section

Fourth, molded-in metal should be avoided whenever alternate methods will accomplish the desired objective. If it is essential to incorporate such inserts, they should be shaped so that they will present no sharp inside comers to the plastic. The effect of the sharp edges of a metal insert would be the same as explained in the first point above, namely, brittleness and stress concentration can occur. The cross-section that surrounds a metal insert should be heavy enough that it will not crack upon cooling. A method of minimizing cracking around the insert is to heat the... 

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 . 

Denser foams behave differently in compression tests. The brittle failure of samples involves cracking along the inclined and longitudinal planes (Fig. 29 d). When compressive load is applied to a material of high apparent density, such as polyurethane foam (Fig. 29 e), the load will go on increasing continuously even after has been attained. At the same time, the cross-section of the sample will increase and the sample assume a barrel-like shape. The foam cells will begin to crumple at the same time. 

On the basis of the energy representations of the process of brittle fracture, Griffith stated that the work of external forces is spent for the volume deformation of a material and the formation of new surfaces—microcracks. Stresses in the top of microcracks of the loaded material are many times greater than the average stresses in a cross section of the sample. If the stress value at the top of the main crack is equal to theoretical strength, the crack quickly grows and the sample collapses. 

Fig. 4 Growth of a coherent y-brass product layer by the diffusion limited dezincification of e-brass. 42 h anodic polarization in acidified 1 N Na2S04 at room temperature and Eh = —0.75 V. Dezincification of e-brass is locally enhanced by the formation of cracks in the brittle product layer (see arrows). Light optical micrograph of metallographic cross section. (Reprinted from Ref [24], with permission from Elsevier Science.)...

Fig. 5 Formation of cracks in the surface of brittle 5-Zn—Fe (ZnsiEey) due to the selective anodic dissolution ofZn (dezincification) 236 h anodic polarization in 1 N Na2SO4/0.01 N H2SO4 at room temperature and h = —0.7 V. Large pores at the bottom of the image result from the metallurgical preparation of the alloy. Light optical micrograph of metallographic cross section. (From Ref [27].)...

Williams (49), Ward (79), and Jancar et al. (89) proposed an approximate model of mixed mode of fracture to account for the effect of finite specimen dimensions for Kc and G, respectively. The basic idea in both theories is a substitution of the actual distribution of fracture toughness across the cross-section by a simple bimodal distribution, assuming plane strain value in the center and plane stress value at the surface area of the specimen. Size of the plastic zone IR relative to the specimen width B gives the contribution of plane stress regions and is a measure of the displacement of the state of stress at the crack tip from the plane strain conditions. Note that this approach can be used only if the mode of failure does not change with the test conditions or material composition (i.e., it attains its brittle character). 

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...

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