- Email: [email protected]
Reply to comments on “initiation and propagation of fretting fatigue cracks”
269
Reply to comments on “Initiation and propagation of fretting fatigue cracks” We should like to thank Dr. Hoeppner for his interesting and constructive comments on the initiation and propagation of fretting fatigue cracks [l] . A frequency effect was previously obtained in the fretting fatigue of a carbon steel, the fretting fatigue strength decreasing at lower frequency [ 21. The damaged layer due to fretting fatigue is formed in the early period of the total life and from it fine fatigue cracks are initiated. Fretting fatigue damage is caused by a combination of tangential stress due to fretting and normal stress due to repeated bending. The tangential stress, however, depends on the frequency because fretting friction is significantly affected by oxidation of the contact surfaces. This is a main cause of the frequency effect in fretting fatigue. The initial increase of tangential force is more rapid, its maximum value is higher and the number of fretting cycles to the maximum friction is smaller when the frequency is lower. Fretting damage is, therefore, saturated earlier and the damage is more severe at a lower frequency. The initiation and propagation of fretting fatigue cracks are similar in appearance to those of corrosion fatigue but they are caused by the combination of tangential and repeated stresses. They may also be affected by the environment. We are at present studying the effects of oxygen and water vapor in the environment on fretting fatigue of a carbon steel and an aluminum alloy. With carbon steel the effect of water vapor is almost negligible but oxygen plays an important role. Although the tangential force in argon is higher than that in air, the initiation periods of the fretting fatigue cracks in both environments are similar, because the oxygen in air accelerates crack initiation. The subsequent crack propagation rate in air is greater owing to the effect of absorption of oxygen on the crack tip. However, with an aluminum alloy which is very sensitive to environment, the environmental effects predominate over the stress conditions. The effect of oxygen is small but water vapor accelerates the initiation and propagation of fretting fatigue cracks. These results will be published shortly. In Figs. 17 and 18 of the subject paper the crack propagation curves are divided into two straight lines. The authors consider that the tangential force effects the initial crack propagation and that crack propagation after the knee point is that of a cracked specimen under repeated bending stress. As indicated by Hoeppner, it is clear that there is a transition region near the knee point of the crack propagation curve. By considering this circumstance the relation between dZ/dn and AK may be expressed by Fig. l(a) shown by Hoeppner. The vertical chain lines in Fig. 21 of the subject paper are not the lines of crack propagation, but show changes from the upper line segment to the lower line segment. The authors consider that fretting fatigue cracks are initiated under maximum repeated shearing stress and a criterion for the initiation of fretting fatigue cracks has been obtained [ 31. However, the propagation depth of the shear-type crack of a carbon steel is small and thereafter the
crack is considered to propagate perpendicularly to the direction of the repeated principal tensile stress, which consists of the tangential stress of fretting and the repeated stress. As it is difficult to calculate AK of mode I under the combined stress, we have attempted to show the relation between dl/dn and AK under repeated bending stress. The figure shows the effect of the tangential force of fretting on the initial crack propagation. K. END0 and H. GOT0 Department
of Mechanical
Engineering,
Kyoto
Uniuersity,
Kyoto
(Japan)
1 K. Endo and H. Goto, Initiation and propagation of fretting fatigue cracks, Wear, 38 (1976) 311- 324. 2 K. Endo, H. Goto and T. Nakamura, Effects of cycle frequency on fretting fatigue life of carbon steel, Bull. JSME, 12 (54) (1969) 1300 - 1308. 3 K. Endo, H. Goto and T. Nakamura, Fretting fatigue strength of several material combinations, Bull. JSME, 16 (92) (1973) 143 - 150. (Received October 8, 1976)
Reply to comments on “Initiation and propagation of fretting fatigue cracks” We should like to thank Dr. Hoeppner for his interesting and constructive comments on the initiation and propagation of fretting fatigue cracks [l] . A frequency effect was previously obtained in the fretting fatigue of a carbon steel, the fretting fatigue strength decreasing at lower frequency [ 21. The damaged layer due to fretting fatigue is formed in the early period of the total life and from it fine fatigue cracks are initiated. Fretting fatigue damage is caused by a combination of tangential stress due to fretting and normal stress due to repeated bending. The tangential stress, however, depends on the frequency because fretting friction is significantly affected by oxidation of the contact surfaces. This is a main cause of the frequency effect in fretting fatigue. The initial increase of tangential force is more rapid, its maximum value is higher and the number of fretting cycles to the maximum friction is smaller when the frequency is lower. Fretting damage is, therefore, saturated earlier and the damage is more severe at a lower frequency. The initiation and propagation of fretting fatigue cracks are similar in appearance to those of corrosion fatigue but they are caused by the combination of tangential and repeated stresses. They may also be affected by the environment. We are at present studying the effects of oxygen and water vapor in the environment on fretting fatigue of a carbon steel and an aluminum alloy. With carbon steel the effect of water vapor is almost negligible but oxygen plays an important role. Although the tangential force in argon is higher than that in air, the initiation periods of the fretting fatigue cracks in both environments are similar, because the oxygen in air accelerates crack initiation. The subsequent crack propagation rate in air is greater owing to the effect of absorption of oxygen on the crack tip. However, with an aluminum alloy which is very sensitive to environment, the environmental effects predominate over the stress conditions. The effect of oxygen is small but water vapor accelerates the initiation and propagation of fretting fatigue cracks. These results will be published shortly. In Figs. 17 and 18 of the subject paper the crack propagation curves are divided into two straight lines. The authors consider that the tangential force effects the initial crack propagation and that crack propagation after the knee point is that of a cracked specimen under repeated bending stress. As indicated by Hoeppner, it is clear that there is a transition region near the knee point of the crack propagation curve. By considering this circumstance the relation between dZ/dn and AK may be expressed by Fig. l(a) shown by Hoeppner. The vertical chain lines in Fig. 21 of the subject paper are not the lines of crack propagation, but show changes from the upper line segment to the lower line segment. The authors consider that fretting fatigue cracks are initiated under maximum repeated shearing stress and a criterion for the initiation of fretting fatigue cracks has been obtained [ 31. However, the propagation depth of the shear-type crack of a carbon steel is small and thereafter the
crack is considered to propagate perpendicularly to the direction of the repeated principal tensile stress, which consists of the tangential stress of fretting and the repeated stress. As it is difficult to calculate AK of mode I under the combined stress, we have attempted to show the relation between dl/dn and AK under repeated bending stress. The figure shows the effect of the tangential force of fretting on the initial crack propagation. K. END0 and H. GOT0 Department
of Mechanical
Engineering,
Kyoto
Uniuersity,
Kyoto
(Japan)
1 K. Endo and H. Goto, Initiation and propagation of fretting fatigue cracks, Wear, 38 (1976) 311- 324. 2 K. Endo, H. Goto and T. Nakamura, Effects of cycle frequency on fretting fatigue life of carbon steel, Bull. JSME, 12 (54) (1969) 1300 - 1308. 3 K. Endo, H. Goto and T. Nakamura, Fretting fatigue strength of several material combinations, Bull. JSME, 16 (92) (1973) 143 - 150. (Received October 8, 1976)