Tham khảo tài liệu 'friction and lubrication in mechanical design episode 2 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 330 Chapter 8 where K constant for the material lubricant f coefficient of friction w normal load per unit length b width of the contact band Vị v2 surface velocities C c2 constants of materials which are the square root of the product of the thermal conductivity specific heat and density A modification of Blok s formula was proposed by Kelly 15 for similar materials with consideration of surface roughness. The formula is given as Tt Tb K where Tt total surface temperature Tb material bulk temperature s rms surface roughness pin. K constant for the material lubricant combination Mechanism for Surface Crack Initiation It is generally accepted that the penetration of asperities causes plastic deformation in the surface layers where the yield point is exceeded at the real area of contact. Below the plastically compressed layer are layers under elastic compression. As soon as the asperity moves the elastically compressed layers will exert upon the plastic layer a force which will create in it a state of tension. Consequently tensile stresses will appear on the surface in such conditions. The sliding motion also generates a temperature field which penetrates the surface layers. The maximum temperature occurs at the contact surface and decreases with increasing distance from the surface as discussed in Chapter 5. Accordingly the surface layer is thermally elongated more than the subsurface layers and will experience compressive stresses Wear 331 imposed by the bulk material. If this compressive stress exceeds the yield stress then a tensile residual stress will be induced in the surface after cooling. It should also be noted that the temperature at the real area of contact can be very high at high sliding speeds which results in reducing the yield strength significantly and thus increasing the stressed zone. The tensile thermal stress on the surface can be calculated from 16 a aE gAa r l - v fipc where q heat flux caused by friction Vap n coefficient of .
