Tham khảo tài liệu 'introduction to contact mechanics part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Pointed Indenters 143 unloading the median cracks extend outward and upward and may join with the radial cracks to form a system of half-penny cracks which are then referred to as median radial cracks. In glass the observed cracks at the corners of the residual impression on the specimen surface are usually fully formed me-dian radial cracks. However in other brittle materials with higher values of E Y radial cracks are usually quite distinct from the median cracks and form upon loading. Experimental observations suggest that there is no one general sequence of indentation cracking. Chiang Marshall and Evans4 5 and also Yoffe6 have proposed analytical models that attempt to describe the cracking sequence in terms of a superposition of the Boussinesq elastic stress field and that associated with the expanding cavity model of Johnson. The Yoffe6 model describes the evolution of stresses for both loading and unloading in terms of material and E Y and appears to account for the appearance of radial and lateral cracking for loading and unloading sequences for a wide range of test materials. However the model as with that of Chiang Marshall and Evans4 5 deals with axis-symmetric pointed indenters. Clearly the corners of a pyramidal indenter play an important role since both median and radial cracks align themselves with the corners of the indentation. However development of a three-dimensional analytical model that deals with this type of geometry would be an extremely difficult undertaking and would be best left to numerical finite-element analysis. The value of the axis-symmetric models is to provide physical insight and that they do quite well. It is the radial and lateral cracks that are of particular importance since their proximity to the surface has a significant influence on the fracture strength of the specimen. Fracture mechanics treatments of these types of cracks seek to provide a measure of fracture toughness based on the length of the radial surface .
