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Fatigue properties of holes with residual stresses
Engineering FraciureMechmifs Vol. 45, No. 5, pp. 551-557, Printed in Great Britain.
1993 0
0013-7944p3 s6.00 + 0.00 1993 Pergamon Press Ltd.
FATIGUE PROPERTIES OF HOLES WITH RESIDUAL STRESSES M. 0. LAI, J. T. OH and A. Y. C. NEE Mechanical and Production Engineering ~rtment, National University of Singa~m, IO Kent Ridge Crescent, Singapore 0% 1, Republic of Singapore Ahstraet-Residual stresses were induced onto pm-machined holes using the ballising process in which a slightly over-sized tungsten carbide ball was pushed through them without rotation. Residual stresses at the vicinity of a hole, evaluated using a fracture mechanics approach, were found to be compressive at the surface and reached maximum tensile stress some 3.5 mm away from the edge of the hole. The compressive stress rendered a decrease in the stress intensity factor at the tip of a crack emanating from the hole surface. Fatigue testing of the ballised and unballised holes showed that the compressive residual stress, together with the improvement in surface roughness of the bahised hole, resulted in a significant increase in the fatigue lives of the bail&d holes, With a 3.5% interference between the hole and the ball, a minimum two-fold increase was found. It is also shown that fatigue performance is dependent upon whether a baIfised hole is complete and unbroken or is slit. When the hole is slit, the compressive stress is redistributed, giving rise to an entirely tensile stress state at the hole surface. Thii stress state unambi~ousiy manifests itself in the decrease in fatigue lives of ball&d holes that have been slit into halves,
INTRODUCTION IN ENGINEERINGstructures, many components are fastened together using bolts and nuts and rivets through drilled holes. Not only do these fastened joints enable easy assembly and dismantling, they are also able to transfer as well as to distribute loads applied onto the structures. The major drawback of such connections, however, is that the fatigue life of the component is reduced by the presence of the hole since the stress concentration around the hole is increased, In addition, the hole-making process itself may introduce defects or roughness at the surface of the hole that may cause further decrease in fatigue performance of the components. For applications where fatigue loadings are important, one way to compensate the decrease in fatigue life of the component is by the introduction of beneficial compressive residual stress at the surface of the hole using cold working. Processes like coid-expansion of holes [l-5], interference-fit [5& and a combination of interference-fit and cold-expansion [6] have been employed to improve the fatigue life of holes in structures. The purpose of the present study is to evaluate the residual stress distribution at the surfaces of holes that have been ballised by forcing a precision ground ball of a prescribed diameter through slightly smaller pre-machined holes, as shown in Fig. 1, as well as to investigate the fatigue performance of the resulting holes under tensile loading. The ballising process is basically a metal working ~nishing process in which no material is removed, It has a bussing action (except practi~lly no rotation of the ball is involved) that refines the surface st~ctu~ of the hole leaving a plastically deformed hole surface. Protrusions generated by drilling or boring the hole prior to ballising are displaced plastically to fill up depressions resulting in improvements in surface finish, roundness and dimensional tolerance of the holes [7,8]. The improvement in surface finish, together with the compressive residual stresses on the surface due to the cold-working the hole surface has experienced, are believed to enhance the fatigue behaviour of the ballised material.
EXPER~~~AL
PROCEDURES
Assab 750 steel with a typical chemical composition of 0.50% C, 0.3% Si, 0.5% Mn, and 0.04% § (nearest equivalent: AISI 1050 or En 43) was employed in the present study. The material, with a tensile strength of 640 MPa and 0.2% proof stress of 340 MPa, was used in the as-received unannealed condition with an approximate hardness of 200 HP.
552
M. 0. LA1 et al. PRESSURE
t BALL PUSHED THROUGH UNDERSIZED HOLE.
2. DURING BALLISING. SURFACE OF HOLE IS CWRESSED
AND WOW<-HARDENED
3. ON EXIT. PLASTICALLY M=ORMED REGION IS CoMpRES!XO BY OUTER ELASTK REGKM. GMNG RISE TO CCNPRESSIVE RESIDUAL STRESS.
Fig. 1. The ballising process.
Tensile fatigue test specimens of 10 x 60 x 210 mm, as shown in Fig. 2, were cut from a flat stock of 10 mm thickness plate. The 60 mm gauge length section of the specimen was machined down to a thickness of 3 mm to reduce the applied load needed for the test. After centre marking, a pilot hole of 4mm diameter was drilled conventionally. The pilot hole was then carefully enlarged on a lathe with a carbide insert boring bar to the desired diameter so that a range of interferences between the hole and the ball could be obtained. Care was taken to ensure that the surface of the hole was well finished and surface roughness maintained uniformly between test specimens. The hole was then ballised on an automatic ballising machine known as the AUTO-multi-function press set with single stroke one-way operation. A 19.046 mm diameter tungsten carbide ball of grade AFMA 25 was used in the ballising process without the application of lubricant. Fatigue tests were conducted on a 100 kN capacity Instron electromagnetic resonance machine with a mean load of 20 kN and a peak load of 7.5 kN. In order to compare the fatigue lives of specimens with different degrees of interferences, all specimens were subjected to the same load levels. ASTM standard E466-82 was adhered to as far as possible.
Fatigue properties of holes
553
Fig. 2. Tensile fatigue test specimen.
EVALUATION OF RESIDUAL STRESSES A fracture mechanics approach proposed by Kang et al. [9] was employed in the present study to evaluate the residual stress distribution at the surface of a ballised hole. The basis of this approach, more fully documented in ref. [IO], is that as the virtual forces approach zero, the displacement between two points across a crack in a solid body subjected to external forces, as shown in Fig. 3, is a function of the stress intensity factor due to the external forces and the virtual forces. By replacing the external forces with the residual stress, the stress intensity factor due to a crack in a body with residual stress can be determined using the principle of superposition and the Buckner weight function. In order to apply the method to a plate with a centre ballised hole surrounded by a region of residual stress field, the Buckner weight function for a plate with a crack emanating from a centre hole was determined. In the case of a ballised specimen where the hole had been slit, the appropriate weight function was likewise evaluated. Finally, the relationship between the residual stress, stress intensity factor and displacement associated with the residual stress released by the introduction of a crack was developed. The displacements, monitored
/’ / SAW CUT
Fig. 3. Displacement in a body with a crack subjected to a force.
Fig. 4. Geometry of plate in residual stress measurement.
M. 0. LA1 et al.
554
0
tIllSTANCE FROM EDGE OF HOLE, mm.
Fig. 5 Residual stress distribution around a ballised hole.
5
2
INTERFERENCE,
3
(%).
Fig. 5. Fatigue lives of unballised and ball&d holes with varying degrees of interference.
experimentally as the cracks were extended incrementaliy, were used to calculate the magnitude of the residual stress field around a ballised hole. The experimental determination of the residual stress at the hole surface made use of rectangular specimens of 40 x 104 mm machined from the same stock of material as the tensile fatigue test samples. Specimen preparation procedures were identical to those for the fatigue test described earlier. After ballising, a crack in the form of a saw cut of length 2 mm in the direction transverse to the long axis of the plate specimen was then introduced at two diagonally opposite edges of the hole as shown in Fig. 4. The displacement between points C and c’, a separation of 60 mm as selected in the residual stress formulation, was measured to an accuracy of 0.5 pm using a travel&g microscope. The two cracks were extended simultaneously in a stepwise manner with increments of 2 mm until the crack length reached 14 mm. The residual stress dist~bu~on around the ball&d hole was then computed from the displacement measured. Residual stress beyond 14 mm was not studied since the magnitude of the stress was believed to be small and the accuracy of the method may also be affected by the boundary conditions of the test sample. RESULTS
AND DISCUSSION
The residual stress distribution around a ballised hole at different degrees of interference is shown in Fig. 5. It can be observed that the region immediately adjacent to the surface of the hole exhibits a residual stress that is compressive, The compressive nature of the residual stress manifested itself through closure of the crack when the cracks were introduced with a saw. A grip on the saw blade was distinctly felt. The magnitude of this compressive stress decreases until it reaches a maximum tensile stress at a distance about 3.5 mm from the surface of the hole. The tensile residual stress gradually decreases thereafter to approximately zero towards the edge of the plate. Increasing the interferences effectively increases the magnitude of the residual stress, as can be seen from Fig. 5. The residual stress that tapers off away from the edge of the hole tends to reach about the same values, implying that it is independent of the degree of interference. The features depicted in Fig. 5 are similar to those observed in cold-expanded fastener holes [I 1) although the magnitude of the stresses may be different due to the severity of the deformation induced. Figure 6 shows the eff.&t of degree of interference on the fatigue lives of the ball&d holes. The lives of the unballised specimens can be found on the vertical axis where the interference is zero. Despite some scattering of the results which may be due to the slight variation in surface roughness of the holes, it can be clearly discerned that fatigue lives of the ball&d holes are longer than those for the unballised boles and that, generally, the fatigue life increases with increasing interference. At an interference of 3.5%, the increase in fatigue life is at least two times that of the unballised counterparts, indicating the benefits of the presence of the compressive residual stress. The finding of the present study is consistent with the unpublished results of Panchal[123; using stainless steel tension specimens in his work, he found an approximately three-fold increase in fatigue life for the ballised holes.
Fatigue properties of holes
555
Residual stress that is compressive in nature is generally recognised to be favourable to fatigue resistance since fatigue cracks find it much more difficult to propagate through a compressive stress field [13]. To ensure crack initiation and fatigue crack propagation in a situation like that of a ballised hole, a tensile stress of sufficiently large magnitude must be applied to offset the compressive residual stress that surrounds the hole. The result is that either a longer fatigue life is observed for specimens that are moderately loaded, or a higher load is required to fail a specimen within a moderate fatigue life. The beneficial effects of compressive residual stress on the fatigue life of structures containing residual stresses have been widely reported [l 1, 14-161 and extensively applied to shot peening and autofrettage in the ordnance industry. The beneficial effect on fatigue life is generally attributed to the compressive residual stress state on the stress intensity factors at the tip of the fatigue crack. In the present study, the stress intensity factors under an arbitrarily chosen tensile stress of 85 MPa at the crack tip of a crack emanating from ballised as well as unballised holes have been computed. The computations were carried out by superimposing the stress intensity factors at the crack tip due to the applied stress and the stress intensity factor due to the residual stress for a finite plate. No residual stress was assumed to be present in an unballised specimen. The effect of residual stress on the stress intensity factor is depicted in Fig. 7, where it can be observed that the stress intensity factors at the crack tips of the specimens with ballised holes exhibit lower values than those for the unballised specimens. This is expected as specimens with higher degrees of interference possess higher compressive residual stresses (Fig. 5). The stress intensity factors of the specimens decrease with increasing interference, especially at the edge of the hole surface. As the crack extends, the stress intensity factors between specimens with different degrees of interference become less significant. To simulate cases where a fastener or rivet hole that has been ballised or cold-expanded has been opened by cracks joining from a free surface or another hole to a ballised hole, as illustrated in Fig. 8, the ballised specimen was slit in the longitudinal direction across the hole to investigate the change in residual stress distribution. Residual stress measurement was conducted in a manner similar to the unslit case by employing the appropriate Buckner weight function. A fatigue test was, however, conducted with the half specimen loaded under three-point bending as shown in Fig. 9 such that the ballised surface was stressed. To avoid buckling of the test samples, the specimens were made thicker than the tensile fatigue test specimens; 19 mm thick plates were instead used for both the fatigue tests and residual stress measurement. The residual stress distribution of the slit case, as shown in Fig. 10 for a specimen with 5% interference, can be seen to be entirely tensile in nature. The stress distribution was dramatically altered from that of compressive stress in the vicinity immediately adjacent to the hole (Fig. 5) to one in which no trace of compressive residual stress could be observed. The magnitude of the tensile stress, although not high, was observed to increase with the increase in interference. The slitting of the specimen has therefore not only released all the compressive residual stress due to the
50
2
C
6
8
10
12
14
16
CRACK LENGTH, (mm). Fig. 7. stressintensity factor at tip of crack emanating from ballised hole.
M. 0. LA1 et al.
556
BALLISED HOLE
BALLISED HOLE
+ FATIGUE CRACK
FATIGUE CRACK
Fig. 8. Ballised hole opened by crack from hole to hole/surface.
ballising process, but it has also caused the residual stress to redistribute such that a tensile stress results. As the applied stress in the three-point bend fatigue testing of the slit specimens was itself tensile at the surface of the hole, the tensile residual stress is expected to reduce the fatigue lives of the slit holes. The fatigue test results of the present investigation as shown in Fig. 11 can be clearly observed to confirm such a trend. In spite of some scatter in the results, the fatigue life decreases from that for the unballised hole with increase in interference. At an interference of as low as l%, a three-fold decrease in fatigue life can be seen. The slitting of the ball&d hole has drastically reduced its fatigue performance due to the tensile residual stress induced. It appears, therefore, that any intentional introduction of beneficial compressive residual stress at the hole to improve its fatigue performance may turn out to be detrimental once the hole is slit or opened. It is well accepted that fatigue life depends significantly on the condition and properties of the surface layer of a machine component. Modifications to the surface affect significantly the length of the nucleation stage of the fatigue process. Surface unevenness acts as a stress concentrator and shortens the length of the nucleation stage. Likewise, macroscopic tensile stresses in the surface layer also shorten the nucleation stage [17]. When the ballised hole is complete (that is, unbroken), both the improvement in the surface finish and the compressive residual stress induced work together to render an increase in fatigue life. When the ballised hole is broken, however, not only is the beneficial compressive residual stress absent, the stress has now in effect become tensile such that any micro-notches that appeared during the working or processing would tend to open up and therefore lower the fatigue life [ 11. This may be the case depicted in Fig. 11. The benefit derived from the improvement in surface finish appeared to be less than the adverse effect due to the tensile residual stress, giving an overall shortening in fatigue life for the slit specimen.
Fig. 9. Fatigue testing of the slit ballised hole specimen.
Fatigue properties of holes
DISTANCE FROM EDGE OF HOLE, mm. Fig. 10. Residual stress distribution around a ball&d hole slit into two halves.
557
INTERFERENCF.
(?+A:)
Fig. I 1. Fatigue lives of holes slit into halves.
CONCLUSIONS (1) The residual stress at the surface of a ball&d hole was found to be compressive at the immediate hole surface. This compressive residual stress decreased in magnitude until it reached maximum tensile stress at a distance some 3.5 mm from the surface of the hole. Thereafter, the tensile stress gradually decreased to zero towards the edge of the specimen. (2) The compressive residual stress gave rise to a distinct decrease in stress intensity factor at the tip of a crack emanating from a ballised hole. (3) Fatigue lives of the ball&d holes were found to increase with the increase in interference. Compared to those for the unballised specimens, a two-fold increase was observed at an interference of 3.5%. (4) The residual stress at the surface of a ballised hole that had been slit (i.e. broken) was observed to be entirely tensile in nature. (5) Fatigue lives of the slit ballised holes were found to have decreased in comparison to those of the unballised specimens. A three-fold decrease at an interference of 1% was observed. REFERENCES [l] [2] [3] [4]
A. F. Grandt and J. M. Potter, Technical Report, AFML-TR-794048 (1979). W. N. Sharpe, Jr., Technical Report, AFGSR-TR-77-0020 (1976). H. Lowak, LBF Report number FB-157, Darmstadt (1981). D. F. Cannon, J. Sinclair and K. A. Sharpe, Iater~atio~l Conferreee and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Anaiysb (pp. 353-369), Utah (December 1985). [5] M. W. Ozelton and T. G. Coyle, ASTM STP 927, 53-71 (1986). [6] B. N. Leis and S. C. Ford, SAE Technical Paper Series No. 780103 (1978). [7] A. Y. C. Nee and V. C. Venkatesh, J. Mech. Working Technol. 6, 215-226 (1982). [8] A. Y. C. Nee and V. C. Venlcatesh, Ann. CIRP 30(l), 505-508 (1981). [9] K. J. Kang, J. H. Song and Y. Y. Earmme, J. Strain Anal. &gag Des. M(I), 23-30 (1989). [IO] M. 0. Lai, J. T. Oh and A. Y. C. Nee, Intestinal Conference on Fracture of ~g~r~g Materials and Strwtures (pp. 859-865), Singapore (August 1991). (111 J. Y. Mann and 0. S. Jost, Metals Forum. Aust. Inst. Metals 6(l), 44-53 (1983). [12] A. Pan&al, Strain distribution in ballising. Bachelor of Science degree project report, Hatfield Polytechnic, U.K. (1983). [13] J. A. Collins, Failure of Metals in Mechanical Design, Analysts, Prediction, Prevention. John Wiley, New York (1981). [14] S. Poolsuk and W. N. Sharpe, Jr., 1. ~pp/. Mech. 45, 515-520 (1978). [IS] W. F. Alder and D. M. Dupree, AFML-TR-7444, Wright Patterson AFB, OH (1974). [I4 D. L. Rich and L. F. Impefliaaeri, ASTM STP 637, 153475 (1977). [17] A. Puskar and S. A. Golovin, Fatigue in Materials: Cumulative Damage Processes. Blsevier, Barking (1985). (Received 7 December 1992)
1993 0
0013-7944p3 s6.00 + 0.00 1993 Pergamon Press Ltd.
FATIGUE PROPERTIES OF HOLES WITH RESIDUAL STRESSES M. 0. LAI, J. T. OH and A. Y. C. NEE Mechanical and Production Engineering ~rtment, National University of Singa~m, IO Kent Ridge Crescent, Singapore 0% 1, Republic of Singapore Ahstraet-Residual stresses were induced onto pm-machined holes using the ballising process in which a slightly over-sized tungsten carbide ball was pushed through them without rotation. Residual stresses at the vicinity of a hole, evaluated using a fracture mechanics approach, were found to be compressive at the surface and reached maximum tensile stress some 3.5 mm away from the edge of the hole. The compressive stress rendered a decrease in the stress intensity factor at the tip of a crack emanating from the hole surface. Fatigue testing of the ballised and unballised holes showed that the compressive residual stress, together with the improvement in surface roughness of the bahised hole, resulted in a significant increase in the fatigue lives of the bail&d holes, With a 3.5% interference between the hole and the ball, a minimum two-fold increase was found. It is also shown that fatigue performance is dependent upon whether a baIfised hole is complete and unbroken or is slit. When the hole is slit, the compressive stress is redistributed, giving rise to an entirely tensile stress state at the hole surface. Thii stress state unambi~ousiy manifests itself in the decrease in fatigue lives of ball&d holes that have been slit into halves,
INTRODUCTION IN ENGINEERINGstructures, many components are fastened together using bolts and nuts and rivets through drilled holes. Not only do these fastened joints enable easy assembly and dismantling, they are also able to transfer as well as to distribute loads applied onto the structures. The major drawback of such connections, however, is that the fatigue life of the component is reduced by the presence of the hole since the stress concentration around the hole is increased, In addition, the hole-making process itself may introduce defects or roughness at the surface of the hole that may cause further decrease in fatigue performance of the components. For applications where fatigue loadings are important, one way to compensate the decrease in fatigue life of the component is by the introduction of beneficial compressive residual stress at the surface of the hole using cold working. Processes like coid-expansion of holes [l-5], interference-fit [5& and a combination of interference-fit and cold-expansion [6] have been employed to improve the fatigue life of holes in structures. The purpose of the present study is to evaluate the residual stress distribution at the surfaces of holes that have been ballised by forcing a precision ground ball of a prescribed diameter through slightly smaller pre-machined holes, as shown in Fig. 1, as well as to investigate the fatigue performance of the resulting holes under tensile loading. The ballising process is basically a metal working ~nishing process in which no material is removed, It has a bussing action (except practi~lly no rotation of the ball is involved) that refines the surface st~ctu~ of the hole leaving a plastically deformed hole surface. Protrusions generated by drilling or boring the hole prior to ballising are displaced plastically to fill up depressions resulting in improvements in surface finish, roundness and dimensional tolerance of the holes [7,8]. The improvement in surface finish, together with the compressive residual stresses on the surface due to the cold-working the hole surface has experienced, are believed to enhance the fatigue behaviour of the ballised material.
EXPER~~~AL
PROCEDURES
Assab 750 steel with a typical chemical composition of 0.50% C, 0.3% Si, 0.5% Mn, and 0.04% § (nearest equivalent: AISI 1050 or En 43) was employed in the present study. The material, with a tensile strength of 640 MPa and 0.2% proof stress of 340 MPa, was used in the as-received unannealed condition with an approximate hardness of 200 HP.
552
M. 0. LA1 et al. PRESSURE
t BALL PUSHED THROUGH UNDERSIZED HOLE.
2. DURING BALLISING. SURFACE OF HOLE IS CWRESSED
AND WOW<-HARDENED
3. ON EXIT. PLASTICALLY M=ORMED REGION IS CoMpRES!XO BY OUTER ELASTK REGKM. GMNG RISE TO CCNPRESSIVE RESIDUAL STRESS.
Fig. 1. The ballising process.
Tensile fatigue test specimens of 10 x 60 x 210 mm, as shown in Fig. 2, were cut from a flat stock of 10 mm thickness plate. The 60 mm gauge length section of the specimen was machined down to a thickness of 3 mm to reduce the applied load needed for the test. After centre marking, a pilot hole of 4mm diameter was drilled conventionally. The pilot hole was then carefully enlarged on a lathe with a carbide insert boring bar to the desired diameter so that a range of interferences between the hole and the ball could be obtained. Care was taken to ensure that the surface of the hole was well finished and surface roughness maintained uniformly between test specimens. The hole was then ballised on an automatic ballising machine known as the AUTO-multi-function press set with single stroke one-way operation. A 19.046 mm diameter tungsten carbide ball of grade AFMA 25 was used in the ballising process without the application of lubricant. Fatigue tests were conducted on a 100 kN capacity Instron electromagnetic resonance machine with a mean load of 20 kN and a peak load of 7.5 kN. In order to compare the fatigue lives of specimens with different degrees of interferences, all specimens were subjected to the same load levels. ASTM standard E466-82 was adhered to as far as possible.
Fatigue properties of holes
553
Fig. 2. Tensile fatigue test specimen.
EVALUATION OF RESIDUAL STRESSES A fracture mechanics approach proposed by Kang et al. [9] was employed in the present study to evaluate the residual stress distribution at the surface of a ballised hole. The basis of this approach, more fully documented in ref. [IO], is that as the virtual forces approach zero, the displacement between two points across a crack in a solid body subjected to external forces, as shown in Fig. 3, is a function of the stress intensity factor due to the external forces and the virtual forces. By replacing the external forces with the residual stress, the stress intensity factor due to a crack in a body with residual stress can be determined using the principle of superposition and the Buckner weight function. In order to apply the method to a plate with a centre ballised hole surrounded by a region of residual stress field, the Buckner weight function for a plate with a crack emanating from a centre hole was determined. In the case of a ballised specimen where the hole had been slit, the appropriate weight function was likewise evaluated. Finally, the relationship between the residual stress, stress intensity factor and displacement associated with the residual stress released by the introduction of a crack was developed. The displacements, monitored
/’ / SAW CUT
Fig. 3. Displacement in a body with a crack subjected to a force.
Fig. 4. Geometry of plate in residual stress measurement.
M. 0. LA1 et al.
554
0
tIllSTANCE FROM EDGE OF HOLE, mm.
Fig. 5 Residual stress distribution around a ballised hole.
5
2
INTERFERENCE,
3
(%).
Fig. 5. Fatigue lives of unballised and ball&d holes with varying degrees of interference.
experimentally as the cracks were extended incrementaliy, were used to calculate the magnitude of the residual stress field around a ballised hole. The experimental determination of the residual stress at the hole surface made use of rectangular specimens of 40 x 104 mm machined from the same stock of material as the tensile fatigue test samples. Specimen preparation procedures were identical to those for the fatigue test described earlier. After ballising, a crack in the form of a saw cut of length 2 mm in the direction transverse to the long axis of the plate specimen was then introduced at two diagonally opposite edges of the hole as shown in Fig. 4. The displacement between points C and c’, a separation of 60 mm as selected in the residual stress formulation, was measured to an accuracy of 0.5 pm using a travel&g microscope. The two cracks were extended simultaneously in a stepwise manner with increments of 2 mm until the crack length reached 14 mm. The residual stress dist~bu~on around the ball&d hole was then computed from the displacement measured. Residual stress beyond 14 mm was not studied since the magnitude of the stress was believed to be small and the accuracy of the method may also be affected by the boundary conditions of the test sample. RESULTS
AND DISCUSSION
The residual stress distribution around a ballised hole at different degrees of interference is shown in Fig. 5. It can be observed that the region immediately adjacent to the surface of the hole exhibits a residual stress that is compressive, The compressive nature of the residual stress manifested itself through closure of the crack when the cracks were introduced with a saw. A grip on the saw blade was distinctly felt. The magnitude of this compressive stress decreases until it reaches a maximum tensile stress at a distance about 3.5 mm from the surface of the hole. The tensile residual stress gradually decreases thereafter to approximately zero towards the edge of the plate. Increasing the interferences effectively increases the magnitude of the residual stress, as can be seen from Fig. 5. The residual stress that tapers off away from the edge of the hole tends to reach about the same values, implying that it is independent of the degree of interference. The features depicted in Fig. 5 are similar to those observed in cold-expanded fastener holes [I 1) although the magnitude of the stresses may be different due to the severity of the deformation induced. Figure 6 shows the eff.&t of degree of interference on the fatigue lives of the ball&d holes. The lives of the unballised specimens can be found on the vertical axis where the interference is zero. Despite some scattering of the results which may be due to the slight variation in surface roughness of the holes, it can be clearly discerned that fatigue lives of the ball&d holes are longer than those for the unballised boles and that, generally, the fatigue life increases with increasing interference. At an interference of 3.5%, the increase in fatigue life is at least two times that of the unballised counterparts, indicating the benefits of the presence of the compressive residual stress. The finding of the present study is consistent with the unpublished results of Panchal[123; using stainless steel tension specimens in his work, he found an approximately three-fold increase in fatigue life for the ballised holes.
Fatigue properties of holes
555
Residual stress that is compressive in nature is generally recognised to be favourable to fatigue resistance since fatigue cracks find it much more difficult to propagate through a compressive stress field [13]. To ensure crack initiation and fatigue crack propagation in a situation like that of a ballised hole, a tensile stress of sufficiently large magnitude must be applied to offset the compressive residual stress that surrounds the hole. The result is that either a longer fatigue life is observed for specimens that are moderately loaded, or a higher load is required to fail a specimen within a moderate fatigue life. The beneficial effects of compressive residual stress on the fatigue life of structures containing residual stresses have been widely reported [l 1, 14-161 and extensively applied to shot peening and autofrettage in the ordnance industry. The beneficial effect on fatigue life is generally attributed to the compressive residual stress state on the stress intensity factors at the tip of the fatigue crack. In the present study, the stress intensity factors under an arbitrarily chosen tensile stress of 85 MPa at the crack tip of a crack emanating from ballised as well as unballised holes have been computed. The computations were carried out by superimposing the stress intensity factors at the crack tip due to the applied stress and the stress intensity factor due to the residual stress for a finite plate. No residual stress was assumed to be present in an unballised specimen. The effect of residual stress on the stress intensity factor is depicted in Fig. 7, where it can be observed that the stress intensity factors at the crack tips of the specimens with ballised holes exhibit lower values than those for the unballised specimens. This is expected as specimens with higher degrees of interference possess higher compressive residual stresses (Fig. 5). The stress intensity factors of the specimens decrease with increasing interference, especially at the edge of the hole surface. As the crack extends, the stress intensity factors between specimens with different degrees of interference become less significant. To simulate cases where a fastener or rivet hole that has been ballised or cold-expanded has been opened by cracks joining from a free surface or another hole to a ballised hole, as illustrated in Fig. 8, the ballised specimen was slit in the longitudinal direction across the hole to investigate the change in residual stress distribution. Residual stress measurement was conducted in a manner similar to the unslit case by employing the appropriate Buckner weight function. A fatigue test was, however, conducted with the half specimen loaded under three-point bending as shown in Fig. 9 such that the ballised surface was stressed. To avoid buckling of the test samples, the specimens were made thicker than the tensile fatigue test specimens; 19 mm thick plates were instead used for both the fatigue tests and residual stress measurement. The residual stress distribution of the slit case, as shown in Fig. 10 for a specimen with 5% interference, can be seen to be entirely tensile in nature. The stress distribution was dramatically altered from that of compressive stress in the vicinity immediately adjacent to the hole (Fig. 5) to one in which no trace of compressive residual stress could be observed. The magnitude of the tensile stress, although not high, was observed to increase with the increase in interference. The slitting of the specimen has therefore not only released all the compressive residual stress due to the
50
2
C
6
8
10
12
14
16
CRACK LENGTH, (mm). Fig. 7. stressintensity factor at tip of crack emanating from ballised hole.
M. 0. LA1 et al.
556
BALLISED HOLE
BALLISED HOLE
+ FATIGUE CRACK
FATIGUE CRACK
Fig. 8. Ballised hole opened by crack from hole to hole/surface.
ballising process, but it has also caused the residual stress to redistribute such that a tensile stress results. As the applied stress in the three-point bend fatigue testing of the slit specimens was itself tensile at the surface of the hole, the tensile residual stress is expected to reduce the fatigue lives of the slit holes. The fatigue test results of the present investigation as shown in Fig. 11 can be clearly observed to confirm such a trend. In spite of some scatter in the results, the fatigue life decreases from that for the unballised hole with increase in interference. At an interference of as low as l%, a three-fold decrease in fatigue life can be seen. The slitting of the ball&d hole has drastically reduced its fatigue performance due to the tensile residual stress induced. It appears, therefore, that any intentional introduction of beneficial compressive residual stress at the hole to improve its fatigue performance may turn out to be detrimental once the hole is slit or opened. It is well accepted that fatigue life depends significantly on the condition and properties of the surface layer of a machine component. Modifications to the surface affect significantly the length of the nucleation stage of the fatigue process. Surface unevenness acts as a stress concentrator and shortens the length of the nucleation stage. Likewise, macroscopic tensile stresses in the surface layer also shorten the nucleation stage [17]. When the ballised hole is complete (that is, unbroken), both the improvement in the surface finish and the compressive residual stress induced work together to render an increase in fatigue life. When the ballised hole is broken, however, not only is the beneficial compressive residual stress absent, the stress has now in effect become tensile such that any micro-notches that appeared during the working or processing would tend to open up and therefore lower the fatigue life [ 11. This may be the case depicted in Fig. 11. The benefit derived from the improvement in surface finish appeared to be less than the adverse effect due to the tensile residual stress, giving an overall shortening in fatigue life for the slit specimen.
Fig. 9. Fatigue testing of the slit ballised hole specimen.
Fatigue properties of holes
DISTANCE FROM EDGE OF HOLE, mm. Fig. 10. Residual stress distribution around a ball&d hole slit into two halves.
557
INTERFERENCF.
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Fig. I 1. Fatigue lives of holes slit into halves.
CONCLUSIONS (1) The residual stress at the surface of a ball&d hole was found to be compressive at the immediate hole surface. This compressive residual stress decreased in magnitude until it reached maximum tensile stress at a distance some 3.5 mm from the surface of the hole. Thereafter, the tensile stress gradually decreased to zero towards the edge of the specimen. (2) The compressive residual stress gave rise to a distinct decrease in stress intensity factor at the tip of a crack emanating from a ballised hole. (3) Fatigue lives of the ball&d holes were found to increase with the increase in interference. Compared to those for the unballised specimens, a two-fold increase was observed at an interference of 3.5%. (4) The residual stress at the surface of a ballised hole that had been slit (i.e. broken) was observed to be entirely tensile in nature. (5) Fatigue lives of the slit ballised holes were found to have decreased in comparison to those of the unballised specimens. A three-fold decrease at an interference of 1% was observed. REFERENCES [l] [2] [3] [4]
A. F. Grandt and J. M. Potter, Technical Report, AFML-TR-794048 (1979). W. N. Sharpe, Jr., Technical Report, AFGSR-TR-77-0020 (1976). H. Lowak, LBF Report number FB-157, Darmstadt (1981). D. F. Cannon, J. Sinclair and K. A. Sharpe, Iater~atio~l Conferreee and Exposition on Fatigue, Corrosion Cracking, Fracture Mechanics and Failure Anaiysb (pp. 353-369), Utah (December 1985). [5] M. W. Ozelton and T. G. Coyle, ASTM STP 927, 53-71 (1986). [6] B. N. Leis and S. C. Ford, SAE Technical Paper Series No. 780103 (1978). [7] A. Y. C. Nee and V. C. Venkatesh, J. Mech. Working Technol. 6, 215-226 (1982). [8] A. Y. C. Nee and V. C. Venlcatesh, Ann. CIRP 30(l), 505-508 (1981). [9] K. J. Kang, J. H. Song and Y. Y. Earmme, J. Strain Anal. &gag Des. M(I), 23-30 (1989). [IO] M. 0. Lai, J. T. Oh and A. Y. C. Nee, Intestinal Conference on Fracture of ~g~r~g Materials and Strwtures (pp. 859-865), Singapore (August 1991). (111 J. Y. Mann and 0. S. Jost, Metals Forum. Aust. Inst. Metals 6(l), 44-53 (1983). [12] A. Pan&al, Strain distribution in ballising. Bachelor of Science degree project report, Hatfield Polytechnic, U.K. (1983). [13] J. A. Collins, Failure of Metals in Mechanical Design, Analysts, Prediction, Prevention. John Wiley, New York (1981). [14] S. Poolsuk and W. N. Sharpe, Jr., 1. ~pp/. Mech. 45, 515-520 (1978). [IS] W. F. Alder and D. M. Dupree, AFML-TR-7444, Wright Patterson AFB, OH (1974). [I4 D. L. Rich and L. F. Impefliaaeri, ASTM STP 637, 153475 (1977). [17] A. Puskar and S. A. Golovin, Fatigue in Materials: Cumulative Damage Processes. Blsevier, Barking (1985). (Received 7 December 1992)