applied fracture mechanics

Fracture Mechanics Based Models of Structural and Contact FatigueFracture Mechanics Based Models of Structural and Contact Fatigue 21165642± 2k ⋅10 1n21x no00 2.5 5 7.5Figure 6. The dependence of the normal stress intensity factor k + 1n on the coordinate x0 n for the case of theboundary of half-plane loaded with normal p(x 0 )=π/4 and frictional τ(x 0 )=−λp(x 0 ) stresses,y 0 n = −0.2, α n = π/2, q 0 = 0: λ = 0.1 - curve marked with 1, λ = 0.2 - curve marked with 2 (after Kudishand Covitch [1]). Reprinted with permission from CRC Press.is very beneficial as it reduces the contact friction and wear and facilitates better heat transferfrom the contact. However, in some cases the lubricant presence may play a detrimentalrole. In particular, in cases when the elastic solid (half-plane) has a surface crack inclinedtoward the incoming high contact pressure transmitted through the lubricant. Such a crackmay open up and experience high lubricant pressure applied to its faces. This pressure createsthe normal stress intensity factor k − 1n far exceeding the value of the stress intensity factorsk ± 1n for comparable subsurface cracks while the shear stress intensity factors for surface k− 2nand and subsurface k ± 2n cracks remain comparable in value. That becomes obvious from thecomparison of the graphs from Fig. 4-8 with the graphs from Fig. 10 and 9.In cases when a surface crack is inclined away from the incoming high lubricant pressure thecrack does not open up toward the incoming lubricant with high pressure and it behavessimilar to a corresponding subsurface crack, i.e. its normal stress intensity factor k − 1n inits value is similar to the one for a corresponding subsurface crack. It is customary to seethe normal stress intensity factor for surface cracks which open up toward the incominghigh lubricant pressure to exceed the one for comparable subsurface cracks by two ordersof magnitude. In such cases the compressive residual stress has very little influence on thecrack behavior due to domination of the lubricant pressure.The possibility of high normal stress intensity factors for surface cracks leads to seriousconsequences. In particular, it explains why fatigue life of drivers is usually significantlyhigher than the one for followers [1]. Also, it explains why under normal circumstancesfatigue failure is of subsurface origin [1].