Tham khảo tài liệu 'introduction to contact mechanics part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Determining Stress Intensity Factors 43 When K1 K1C then Gc becomes the critical value of the rate of release in strain energy for the material which leads to crack extension and possibly fracture of the specimen. The relationship between K1 and G is significant because it means that the K1C condition is a necessary and sufficient criterion for crack growth since it embodies both the stress and energy balance criteria. The value of K1C describes the stresses indirectly at the crack tip as well as the strain energy release rate at the onset of crack extension. It should be remembered that various corrections to K and hence G are required for cracks in bodies of finite dimensions. Whatever the correction the correspondence between G and K is given in Eq. . A factor of n sometimes appears in Eq. depending on the particular definition of K1 used. Consistent use of n in all these formulae is essential especially when comparing equations from different sources. Again we should recognize that Eq. applies to plane stress conditions. In practice a condition of plane strain is more usual in which case one must include the factor 1-v2 in the numerator. Determining Stress Intensity Factors Measuring stress intensity factors experimentally Direct application of Griffith s energy balance criterion is seldom practical because of difficulties in determining work of fracture Y. Furthermore the Griffith criterion is a necessary but not sufficient condition for crack growth. However stress intensity factors are more easily determined and represent a necessary and sufficient condition for crack growth but in determining the stress intensity factor Eq. cannot be used directly because the shape factor Y is not generally known. As mentioned previously Y 2 n applies for an embedded penny shaped circular crack of radius c in an infinite plate. Expressions such as this for other types of cracks and loading geometries are available in standard texts.
