Tham khảo tài liệu 'introduction to contact mechanics part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 204 Indentation Test Methods where E is the combined modulus of the indenter and the specimen given by 1 ùđ 1 - E E E where V and V are Poisson s ratio and E and E are Young s modulus of the specimen and the indenter respectively. Equation assumes linear elasticity and makes no prediction about the onset of nonlinear behavior followed by plastic yielding within the specimen when the indenter load is sufficiently high. In conventional compression tests plastic deformation generally does not occur in brittle materials at normal ambient temperatures and pressures due to tensile induced fractures which inevitably occur before the yield stress of the material is reached. However in tests where there is a significant confining pressure brittle fracture is suppressed in favor of shear faulting and plastic flow. This phenomenon is familiar to workers in the rock mechanics field4. In an indentation stress field the stresses in the compressive zone beneath the indenter can be made sufficiently high to induce plastic deformation even in brittle materials. For the fully elastic case the principal shear stress distribution beneath a spherical indenter can be readily determined see Chapter 5 and the maximum shear stress has a value of about and occurs at a depth in the specimen of about beneath the indenter. Tabor5 uses both the von Mises and Tresca stress criteria to show that plastic deformation beneath a spherical indenter with increasing load can be expected to occur first upon increasing the indenter load when Pm 1 Pm UE As the load on the indenter is increased further the amount of plastic deformation also increases. The mean contact pressure pm also increases with increasing load. At high values of indentation strain the response of the material may be predicted using the various hardness theories described in Chapter 9. Experi-ments5 show that for metals where the indenter load is such that pm is about three times the yield
