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Corrosion and biaxial fatigue of welded structures
Journal of Materials Processing Technology 143–144 (2003) 410–415
Corrosion and biaxial fatigue of welded structures M.A. Wahab a,∗ , M. Sakano b a
Mechanical Engineering, Louisiana State University, Baton Rouge, LA 708803, USA b Civil Engineering, Kansai University, Suita-shi, Osaka, Japan
Abstract The aim of this research is to develop an experimental approach for corrosion fatigue behaviour of welded structures subjected to simultaneous biaxial, rotating-bending and torsional loading, results of which could be used in railway bridges, ship, offshore structures, ground vehicles etc. The experimental program developed here considers stress field information due to the variations of applied stress conditions, surface treatments and corrosive environment using simulated heat-affected-zone (HAZ) and base materials. This is aimed at providing a procedure for improved design of new welded structures subjected to biaxial loading. The experimental data have been collected and compared with standard theoretical “stress–life” curves generated by earlier researchers (Juvinall, Shigley and Collin’s method). Experimental data in dry and corrosive environment (3.5% NaCl solutions) for rotating-bending–torsional thrust have been collected. This initial investigation suggests that the effects of secondary thrust is more severe than the effects due to corrosive environment, as it accelerates the initiation of the cracks; and consequently, the fatigue life reduces significantly. Design-engineers must consider reducing the effects of secondary thrust and minimising the environmental effect to improve on crack initiation properties of the structures. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Biaxial and corrosion fatigue; Rotating-bending and torsional fatigue; Fatigue strength reduction; Air and corrosive environment
1. Introduction General fatigue assessment only considers axial fatigue loading but real structures are always subjected to multiaxial loads. The procedure developed in this study was aimed at providing an increased understanding of the failure mechanisms in biaxial loading. The problem of multiaxial fatigue loading becomes more complex for welded structures under dynamic cyclic load. It has been found by several researchers that generally fatigue cracks originate either in the heat-affected-zone (HAZ) or in the weld materials due to fatigue loading and could be the potential source of catastrophic failures in some unfortunate situations. Even though the applied load could be completely rotating-bending or purely cyclic axial, there is always induced shear and principal stresses present on the structures and therefore, biaxial loading invariably is being present in most real life loading situations. However, most reported experimental work only considers uniaxial loading, even though the effect of secondary loading could reduce the fatigue life of any structure significantly. In addition to biaxial fatigue loading, it is well documented that corrosion environment and welding of structural ∗ Corresponding author. E-mail address: [email protected] (M.A. Wahab).
grade steels further reduces the fatigue life of the structures. Only limited studies are available at present which account for combined biaxial rotating-bending–torsional and corrosion effects on the welded structures [1–3]. This study is being attempted to find some basic explanation on the effect of secondary thrust on several structures, such as, steel railway bridge structures, ship and offshore structures subjected to environmental fatigue loading conditions. In this study, a biaxial rotating—bending and corrosion fatigue machine has been designed and built to study the effects of corrosion and in addition, a secondary loading has been introduced by torsional loading mechanism. As the HAZ in a welded structure is small and therefore, to generate essential data on the HAZ, experimental weld thermal simulation has been carried out. The fatigue machine thus accommodates specimen heat-treated in weld thermal simulator so that the fatigue properties of the HAZ of a weld can be evaluated under biaxial corrosion fatigue conditions.
2. Biaxial and corrosion fatigue 2.1. Biaxial fatigue and corrosion fatigue All real structures experience some form of service loading. Axial loading coincides in the centre of the specimen,
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00412-6
M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
411
bending loads are applied transversely to the centre line and torsional loads involve the application of a couple in the plane perpendicular to this line. Axial and bending loads give direct stresses while torsional loads give rise to shear stresses. The bending loading induce two significant stresses, they are tensile stress at the surface and shear stress at the point of greatest stress difference. In most case however, the shear stress can be ignored. The torsional loading induce two significant stresses, which are shear and normal stresses. Corrosion fatigue may be defined as the combined action of an aggressive environment and cyclic stresses leading to premature failure of metals by cracking. When the stresses induced in the specimen cause damage to surface film, fresh metal is exposed, the element is thus capable of being corroded. Corrosion fatigue thus can only occur when a component is subjected to cyclic stressing in a medium, which is able to attack the material continuously.
lags. McDiarmid [8] found that the behaviour of ductile materials under reversals of combined bending and twisting fatigue stress can be predicted by a non-linear equation which takes into account of fatigue limits under reversal bending and torsional and applied normal and shear stresses.
2.2. Fatigue strength of welded components
According to Collins, to estimate the completely reversed S–N curve the ultimate tensile strength (σ u ) of the testing material be used. To construct the curve, simply draw a straight line on log-linear co-ordinates between the ultimate tensile strength at one cycle and half of the ultimate tensile strength values at one million cycles and which is shown in Fig. 1. Shigley and Mishke, and Juvinall and Marshek suggested that for the estimating of S–N curve, the ultimate tensile strength of the material be used which establishes a point at one cycle, and another point at 1000 cycles and then draw a straight line to a fatigue limit at a specified number of the cycles N. The number of cycles experienced, N varies with material in Juvinall’s method, and N = 1 × 106 is used for the Shigley’s methods. A new factor m (ranging from 0.75 to 0.9) is being used for estimating S–N curve. Also, a combined reduction factor m, is taken into account and multiplied by the ultimate tensile strength of the material for a point at one million cycles which is shown in Fig. 1. The range of reduction factors account for the effect of type of loading, size, surface finish and any other effect that may be involved, such as elevated temperature, corrosion effect etc. This may be expressed as m = mt md ms mo , where mt
All methods of joining steel components welding remains the most important one. In arc welding, local heat input melts the base and filler metals, which resulting the formation of the weld pool shape, weld imperfections and defects, the initiation of hot and cold cracks in the molten zone, the microstructure changes in the HAZ of the base metal and produces post-weld residual stresses and distortion in the whole structure; and these are connected mainly with negative effects on strength [4]. Previous researchers have found that the maximum stress had a larger influence in bending than in torsion. It is reported that the fatigue strength of ductile metals in torsion was almost independent of the mean stress, with the fatigue strength in torsion decreased slightly with increasing mean stress. Findley [5] found that the addition of mean stress (the stresses have to be in the same state) caused less than 10% decrease in fatigue strength for bending and torsion at 5 × 105 cycles. Hashin [6] and Sines and Ohgi [7] found that the failure surfaces are generally curved and such a failure surface depends in general, on the mean and alternating parts of all cyclic stress components and their mutual phase
3. Estimation of stress–life (S–N ) curves Three widely used design methods of S–N curves are due to Collins [9], Juvinall and Marshek [10], and Shigley and Mischke [11]. These methods are involved using an estimated fatigue limit and one or two additional points at shorter lives are illustrated in Fig. 1. 3.1. Theoretical estimates of S–N curves by Collins, and Juvinall and Shigley
Fig. 1. Estimating completely reversed S–N curves for a smooth and notched members suggested by (a) Collins and (b) Juvinall and Shigley.
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M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
is the load type factor, md the size factor, ms the surface finish factor, mo the any other factor. The parameters of the reduction factors are summarised by Juvinall and Shigley (1991).
4. Experimental program 4.1. Rotating-bending fatigue testing machine In rotating-bending machine the region of the rotating beam between the inboard bearings is subjected to a constant bending moment all along its length. The segment of the machine with a corrosion chamber is shown in Fig. 2. While under the influence of constant moment, the specimen rotates with the drive spindles about its longitudinal axis. Any point on the surface is subjected to a completely reversed stress time pattern. The machine is designed to shut down automatically when the specimen fractured and the shaft connected to the drive motor drops and activate the micro-switch. 4.2. Experimental corrosion system A 4 A, 12 V DC “Shurflo” Diaphragm Pump capable of circulating 10.6 l of corrosive fluid per minute at 310 kPa is attached on an accumulator and produces a mist through the top of the chamber and onto the rotating specimen. The corrosion chamber was designed as a removable attachment for the rotating-bending machine (Fig. 2). The 3.5% of sodium chloride (NaCl) solution are stored in a tank for continuous supply during the experiment. 4.3. Weld thermal simulation
Fig. 3. Weld thermal simulator.
written program code operates the weld thermal simulator corresponds to the HAZ profile that requires achieving. A component of the weld thermal simulator machine is shown in Fig. 3. The simulator uses resistance heating to add heat to the test specimen. The temperature of the specimen is controlled by a micro-controller, which acquires information from two infrared sensors. The welding transformer in the simulator is capable of delivering up to 20,000 A at 3 V. The current input is controlled by a controller, which receives information from the two heat sensors and accordingly, controls the temperature to the specimen. A compressed system that consists of jets and flowing water are used to provide the cooling as required. 4.4. Design of secondary loading (brake) mechanism
The weld thermal simulator is used to generate a HAZ over the entire test area of a specimen since actual HAZ of welds are very small and would be difficult to produce standard test specimens. In the weld thermal simulator the test specimen undergoes similar welding conditions. The
The braking mechanism was designed to apply secondary (torsional) loading (Fig. 4.) The brake was used as a removable attachment for the rotating-bending machine to provide the torsional loading. Frictional force was created when the shoes of the brake came in contact with the rotating shaft. The frictional force (braking force) thus provides a small counter force for the motor, creating the torsional force.
Fig. 2. Corrosion chamber.
Fig. 4. The braking mechanism.
M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
Fig. 5. Weld thermal profile.
4.5. Test specimen and material properties The general geometry of the test specimen was compliant with the ASTM: E466-82, which describes the standardised procedures in carrying out the fatigue tests. The material
413
used in this investigation is mild steel (C1020) and has the following elemental composition (percentages by weight): C (0.23%), Mn (0.6%), P (0.04%), S (0.05%) and others (0.08%). The mechanical properties of the C1020 mild steel are the ultimate tensile strength 520 MPa, yield strength 275 MPa, modulus of elasticity 198 GPa. To get the required thermal profile, the original thermal profile was plotted and tested on specimens. From Fig. 5, it can be seen that the obtained thermal profile has the similar outline as the required original profile. However, the profile obtained did not match the original profile. This may be due to the relatively long time taken by the thermal simulator to heat up the specimens to the desired temperature. The result is the lagging of the profile achieved, which is 3 s effectively. The maximum temperature achieved by the thermal simulator is 984 ◦ C compared to the desired one of 1000 ◦ C. The difference is only 16 ◦ C. The temperature difference could be due to the cooler surface temperature of the specimens. Microstructures of the tested specimens were then obtained (Fig. 6a) and compared with the microstructure required (Fig. 6b) and the microstructures obtained are found to be very similar to the required microstructures.
Fig. 6. (a) Obtained microstructures 200× and (b) required microstructures 500×.
Fig. 7. S–N curves for four experiments (semi-log scale).
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M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
At a stress level of just 40 MPa lower, the biaxial fatigue life drops 70.11% from bending fatigue life. An approximately 16.48% more drop can occur in fatigue life for biaxial fatigue from rotating-bending fatigue.
5. Experimental results 5.1. Discussions of results Four different types of experiments have been conducted and these are summarised in the S–N curves which is shown in Figs. 7–9. Under medium stresses (360 MPa), the fatigue life for a specimen subjected to bending load alone is marginally higher than the fatigue life for a specimen subjected to biaxial and corrosion fatigue. At a lower stress level (320 MPa), the bending fatigue life is much higher than the biaxial and biaxial corrosion fatigue lives. Specimens that did not fail at 10 million revolutions were stopped and it was assumed that the specimens were run-out. It was noted that the biaxial fatigue life drops 53.63% from the rotating-bending fatigue life at a stress level of 360 MPa.
5.2. Comparisons of experimental and theoretical results To confirm the validity of experimental data, a comparison between the theoretical (Collins’s method, and Juvinall and Shigley’s method) and experimental results using simulated and non-simulated specimens were analysed. Fig. 8 shows the comparison of experimental results with Juvinall and Shigley’s curve for biaxial fatigue and biaxial corrosion fatigue lines of best fit. Fig. 9 shows the comparison of results with Collin’s method for non-weld simulated test specimen, which underwent the biaxial fatigue and weld
Fig. 8. Estimating S–N curves for C1020 mild steel using procedure by Juvinall and Shigley.
Fig. 9. Stress–life (S–N) curves for C1020 mild steel using procedure by Collins.
M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
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Fig. 10. Estimating S–N curves for heat-treated C1020 mild steel using procedure by Collins, and Juvinall and Shigley.
simulated-biaxial corrosion fatigue test. For each lines of best fit, the linear equations are obtained. It can be seen that the percentage error of the experimental results with the Collin’s results is quite far off when compared with the Juvinall and Shigley results. The percentage error for Junivall and Shigley’s results is small and that is because the combined reduction factors have been included in Juvinall and Shigley’s method. The percentage error for the biaxial corrosion fatigue results is large for both theoretical results. There is about 19% change from biaxial fatigue to biaxial and corrosion fatigue which could be attributed to corrosion factor which does not included for both theoretical methods. For weld simulated specimen, which underwent the biaxial and corrosion fatigue test. Fig. 10 compares the Collins, and Juvinall and Shigley’s curves with the experimental best fit curve. Experimental results show that even a small-applied secondary torsional loading with an effective applied period of 1 s for every 45–55 s reduces the life of the structure quite drastically.
6. Conclusions From experimental results, it can be concluded that a secondary thrust reduces the fatigue life of a structure drastically. From the results shown, the fatigue life for a specimen under biaxial loading failed at much lower life compared to a specimen that subjected to bending loading. Moreover, corrosive environment further reduces the life but not as drastically as compared to biaxial loading does to bending fatigue. It is thus can be concluded that biaxial is an important and serious reduction factor for steel structures, more severe than corrosion.
Acknowledgements Authors acknowledge with great appreciation the contribution made in the experimental program by their research students Ben Abraham, Marc Gardner, Lee K. Onn and Soo Y. Haw. Contributions made by Mr. I. Brown during the design stage of the experimental research program are acknowledged with gratitude. References [1] P.L. Andersen, Corrosion fatigue testing, fatigue and fracture, in: ASM International, The Materials Information Society of USA, vol. 2, 1997. [2] P.S. Pao, Mechanisms of corrosion fatigue, fatigue and fracture, in: ASM International, The Materials Information Society of USA, vol. 2, 1997. [3] M.A. Wahab, M. Sakano, Experimental study of corrosion fatigue behaviour of welded steel structures, J. Mater. Process. Technol. 118 (1–3) (2001) 116–121. [4] D. Radaj, Heat Effects of Welding—Temperature Field, Residual Stress, Distortion, Springer, Berlin, September 1992. [5] W.N. Findley, Effects of extremes of hardness and mean stress on fatigue of AISI 4340 steel in bending and torsion, J. Eng. Mater. Technol. 111 (2) (1989), ASME. [6] Z. Hashin, Fatigue failure criteria for combined cyclic stress, Int. J. Fract. 17 (2), Sijthoff & Noordhoff, Alphen a/d Rijn, April 1981. [7] G. Sines, G. Ohgi, Fatigue criteria under combined stresses or strains, J. Eng. Mater. Technol. 103 (2) (1991), ASME. [8] D.L. McDiarmid, A new analysis of fatigue under combined bending and twisting, Aeronaut. J. 78 (752), Royal Aeronautical Society, London, 1974. [9] J.A. Collins, Failure of Materials in Mechanical Design, Wiley, New York, 1993. [10] R.C. Juvinall, K.M. Marshek, Fundamentals of Machine Component Design, Wiley, New York, 1983. [11] J.E. Shigley, C.R. Mischke, Mechanical Engineering Design, McGraw-Hill, New York, 1989.
Corrosion and biaxial fatigue of welded structures M.A. Wahab a,∗ , M. Sakano b a
Mechanical Engineering, Louisiana State University, Baton Rouge, LA 708803, USA b Civil Engineering, Kansai University, Suita-shi, Osaka, Japan
Abstract The aim of this research is to develop an experimental approach for corrosion fatigue behaviour of welded structures subjected to simultaneous biaxial, rotating-bending and torsional loading, results of which could be used in railway bridges, ship, offshore structures, ground vehicles etc. The experimental program developed here considers stress field information due to the variations of applied stress conditions, surface treatments and corrosive environment using simulated heat-affected-zone (HAZ) and base materials. This is aimed at providing a procedure for improved design of new welded structures subjected to biaxial loading. The experimental data have been collected and compared with standard theoretical “stress–life” curves generated by earlier researchers (Juvinall, Shigley and Collin’s method). Experimental data in dry and corrosive environment (3.5% NaCl solutions) for rotating-bending–torsional thrust have been collected. This initial investigation suggests that the effects of secondary thrust is more severe than the effects due to corrosive environment, as it accelerates the initiation of the cracks; and consequently, the fatigue life reduces significantly. Design-engineers must consider reducing the effects of secondary thrust and minimising the environmental effect to improve on crack initiation properties of the structures. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Biaxial and corrosion fatigue; Rotating-bending and torsional fatigue; Fatigue strength reduction; Air and corrosive environment
1. Introduction General fatigue assessment only considers axial fatigue loading but real structures are always subjected to multiaxial loads. The procedure developed in this study was aimed at providing an increased understanding of the failure mechanisms in biaxial loading. The problem of multiaxial fatigue loading becomes more complex for welded structures under dynamic cyclic load. It has been found by several researchers that generally fatigue cracks originate either in the heat-affected-zone (HAZ) or in the weld materials due to fatigue loading and could be the potential source of catastrophic failures in some unfortunate situations. Even though the applied load could be completely rotating-bending or purely cyclic axial, there is always induced shear and principal stresses present on the structures and therefore, biaxial loading invariably is being present in most real life loading situations. However, most reported experimental work only considers uniaxial loading, even though the effect of secondary loading could reduce the fatigue life of any structure significantly. In addition to biaxial fatigue loading, it is well documented that corrosion environment and welding of structural ∗ Corresponding author. E-mail address: [email protected] (M.A. Wahab).
grade steels further reduces the fatigue life of the structures. Only limited studies are available at present which account for combined biaxial rotating-bending–torsional and corrosion effects on the welded structures [1–3]. This study is being attempted to find some basic explanation on the effect of secondary thrust on several structures, such as, steel railway bridge structures, ship and offshore structures subjected to environmental fatigue loading conditions. In this study, a biaxial rotating—bending and corrosion fatigue machine has been designed and built to study the effects of corrosion and in addition, a secondary loading has been introduced by torsional loading mechanism. As the HAZ in a welded structure is small and therefore, to generate essential data on the HAZ, experimental weld thermal simulation has been carried out. The fatigue machine thus accommodates specimen heat-treated in weld thermal simulator so that the fatigue properties of the HAZ of a weld can be evaluated under biaxial corrosion fatigue conditions.
2. Biaxial and corrosion fatigue 2.1. Biaxial fatigue and corrosion fatigue All real structures experience some form of service loading. Axial loading coincides in the centre of the specimen,
0924-0136/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0924-0136(03)00412-6
M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
411
bending loads are applied transversely to the centre line and torsional loads involve the application of a couple in the plane perpendicular to this line. Axial and bending loads give direct stresses while torsional loads give rise to shear stresses. The bending loading induce two significant stresses, they are tensile stress at the surface and shear stress at the point of greatest stress difference. In most case however, the shear stress can be ignored. The torsional loading induce two significant stresses, which are shear and normal stresses. Corrosion fatigue may be defined as the combined action of an aggressive environment and cyclic stresses leading to premature failure of metals by cracking. When the stresses induced in the specimen cause damage to surface film, fresh metal is exposed, the element is thus capable of being corroded. Corrosion fatigue thus can only occur when a component is subjected to cyclic stressing in a medium, which is able to attack the material continuously.
lags. McDiarmid [8] found that the behaviour of ductile materials under reversals of combined bending and twisting fatigue stress can be predicted by a non-linear equation which takes into account of fatigue limits under reversal bending and torsional and applied normal and shear stresses.
2.2. Fatigue strength of welded components
According to Collins, to estimate the completely reversed S–N curve the ultimate tensile strength (σ u ) of the testing material be used. To construct the curve, simply draw a straight line on log-linear co-ordinates between the ultimate tensile strength at one cycle and half of the ultimate tensile strength values at one million cycles and which is shown in Fig. 1. Shigley and Mishke, and Juvinall and Marshek suggested that for the estimating of S–N curve, the ultimate tensile strength of the material be used which establishes a point at one cycle, and another point at 1000 cycles and then draw a straight line to a fatigue limit at a specified number of the cycles N. The number of cycles experienced, N varies with material in Juvinall’s method, and N = 1 × 106 is used for the Shigley’s methods. A new factor m (ranging from 0.75 to 0.9) is being used for estimating S–N curve. Also, a combined reduction factor m, is taken into account and multiplied by the ultimate tensile strength of the material for a point at one million cycles which is shown in Fig. 1. The range of reduction factors account for the effect of type of loading, size, surface finish and any other effect that may be involved, such as elevated temperature, corrosion effect etc. This may be expressed as m = mt md ms mo , where mt
All methods of joining steel components welding remains the most important one. In arc welding, local heat input melts the base and filler metals, which resulting the formation of the weld pool shape, weld imperfections and defects, the initiation of hot and cold cracks in the molten zone, the microstructure changes in the HAZ of the base metal and produces post-weld residual stresses and distortion in the whole structure; and these are connected mainly with negative effects on strength [4]. Previous researchers have found that the maximum stress had a larger influence in bending than in torsion. It is reported that the fatigue strength of ductile metals in torsion was almost independent of the mean stress, with the fatigue strength in torsion decreased slightly with increasing mean stress. Findley [5] found that the addition of mean stress (the stresses have to be in the same state) caused less than 10% decrease in fatigue strength for bending and torsion at 5 × 105 cycles. Hashin [6] and Sines and Ohgi [7] found that the failure surfaces are generally curved and such a failure surface depends in general, on the mean and alternating parts of all cyclic stress components and their mutual phase
3. Estimation of stress–life (S–N ) curves Three widely used design methods of S–N curves are due to Collins [9], Juvinall and Marshek [10], and Shigley and Mischke [11]. These methods are involved using an estimated fatigue limit and one or two additional points at shorter lives are illustrated in Fig. 1. 3.1. Theoretical estimates of S–N curves by Collins, and Juvinall and Shigley
Fig. 1. Estimating completely reversed S–N curves for a smooth and notched members suggested by (a) Collins and (b) Juvinall and Shigley.
412
M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
is the load type factor, md the size factor, ms the surface finish factor, mo the any other factor. The parameters of the reduction factors are summarised by Juvinall and Shigley (1991).
4. Experimental program 4.1. Rotating-bending fatigue testing machine In rotating-bending machine the region of the rotating beam between the inboard bearings is subjected to a constant bending moment all along its length. The segment of the machine with a corrosion chamber is shown in Fig. 2. While under the influence of constant moment, the specimen rotates with the drive spindles about its longitudinal axis. Any point on the surface is subjected to a completely reversed stress time pattern. The machine is designed to shut down automatically when the specimen fractured and the shaft connected to the drive motor drops and activate the micro-switch. 4.2. Experimental corrosion system A 4 A, 12 V DC “Shurflo” Diaphragm Pump capable of circulating 10.6 l of corrosive fluid per minute at 310 kPa is attached on an accumulator and produces a mist through the top of the chamber and onto the rotating specimen. The corrosion chamber was designed as a removable attachment for the rotating-bending machine (Fig. 2). The 3.5% of sodium chloride (NaCl) solution are stored in a tank for continuous supply during the experiment. 4.3. Weld thermal simulation
Fig. 3. Weld thermal simulator.
written program code operates the weld thermal simulator corresponds to the HAZ profile that requires achieving. A component of the weld thermal simulator machine is shown in Fig. 3. The simulator uses resistance heating to add heat to the test specimen. The temperature of the specimen is controlled by a micro-controller, which acquires information from two infrared sensors. The welding transformer in the simulator is capable of delivering up to 20,000 A at 3 V. The current input is controlled by a controller, which receives information from the two heat sensors and accordingly, controls the temperature to the specimen. A compressed system that consists of jets and flowing water are used to provide the cooling as required. 4.4. Design of secondary loading (brake) mechanism
The weld thermal simulator is used to generate a HAZ over the entire test area of a specimen since actual HAZ of welds are very small and would be difficult to produce standard test specimens. In the weld thermal simulator the test specimen undergoes similar welding conditions. The
The braking mechanism was designed to apply secondary (torsional) loading (Fig. 4.) The brake was used as a removable attachment for the rotating-bending machine to provide the torsional loading. Frictional force was created when the shoes of the brake came in contact with the rotating shaft. The frictional force (braking force) thus provides a small counter force for the motor, creating the torsional force.
Fig. 2. Corrosion chamber.
Fig. 4. The braking mechanism.
M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
Fig. 5. Weld thermal profile.
4.5. Test specimen and material properties The general geometry of the test specimen was compliant with the ASTM: E466-82, which describes the standardised procedures in carrying out the fatigue tests. The material
413
used in this investigation is mild steel (C1020) and has the following elemental composition (percentages by weight): C (0.23%), Mn (0.6%), P (0.04%), S (0.05%) and others (0.08%). The mechanical properties of the C1020 mild steel are the ultimate tensile strength 520 MPa, yield strength 275 MPa, modulus of elasticity 198 GPa. To get the required thermal profile, the original thermal profile was plotted and tested on specimens. From Fig. 5, it can be seen that the obtained thermal profile has the similar outline as the required original profile. However, the profile obtained did not match the original profile. This may be due to the relatively long time taken by the thermal simulator to heat up the specimens to the desired temperature. The result is the lagging of the profile achieved, which is 3 s effectively. The maximum temperature achieved by the thermal simulator is 984 ◦ C compared to the desired one of 1000 ◦ C. The difference is only 16 ◦ C. The temperature difference could be due to the cooler surface temperature of the specimens. Microstructures of the tested specimens were then obtained (Fig. 6a) and compared with the microstructure required (Fig. 6b) and the microstructures obtained are found to be very similar to the required microstructures.
Fig. 6. (a) Obtained microstructures 200× and (b) required microstructures 500×.
Fig. 7. S–N curves for four experiments (semi-log scale).
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M.A. Wahab, M. Sakano / Journal of Materials Processing Technology 143–144 (2003) 410–415
At a stress level of just 40 MPa lower, the biaxial fatigue life drops 70.11% from bending fatigue life. An approximately 16.48% more drop can occur in fatigue life for biaxial fatigue from rotating-bending fatigue.
5. Experimental results 5.1. Discussions of results Four different types of experiments have been conducted and these are summarised in the S–N curves which is shown in Figs. 7–9. Under medium stresses (360 MPa), the fatigue life for a specimen subjected to bending load alone is marginally higher than the fatigue life for a specimen subjected to biaxial and corrosion fatigue. At a lower stress level (320 MPa), the bending fatigue life is much higher than the biaxial and biaxial corrosion fatigue lives. Specimens that did not fail at 10 million revolutions were stopped and it was assumed that the specimens were run-out. It was noted that the biaxial fatigue life drops 53.63% from the rotating-bending fatigue life at a stress level of 360 MPa.
5.2. Comparisons of experimental and theoretical results To confirm the validity of experimental data, a comparison between the theoretical (Collins’s method, and Juvinall and Shigley’s method) and experimental results using simulated and non-simulated specimens were analysed. Fig. 8 shows the comparison of experimental results with Juvinall and Shigley’s curve for biaxial fatigue and biaxial corrosion fatigue lines of best fit. Fig. 9 shows the comparison of results with Collin’s method for non-weld simulated test specimen, which underwent the biaxial fatigue and weld
Fig. 8. Estimating S–N curves for C1020 mild steel using procedure by Juvinall and Shigley.
Fig. 9. Stress–life (S–N) curves for C1020 mild steel using procedure by Collins.
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Fig. 10. Estimating S–N curves for heat-treated C1020 mild steel using procedure by Collins, and Juvinall and Shigley.
simulated-biaxial corrosion fatigue test. For each lines of best fit, the linear equations are obtained. It can be seen that the percentage error of the experimental results with the Collin’s results is quite far off when compared with the Juvinall and Shigley results. The percentage error for Junivall and Shigley’s results is small and that is because the combined reduction factors have been included in Juvinall and Shigley’s method. The percentage error for the biaxial corrosion fatigue results is large for both theoretical results. There is about 19% change from biaxial fatigue to biaxial and corrosion fatigue which could be attributed to corrosion factor which does not included for both theoretical methods. For weld simulated specimen, which underwent the biaxial and corrosion fatigue test. Fig. 10 compares the Collins, and Juvinall and Shigley’s curves with the experimental best fit curve. Experimental results show that even a small-applied secondary torsional loading with an effective applied period of 1 s for every 45–55 s reduces the life of the structure quite drastically.
6. Conclusions From experimental results, it can be concluded that a secondary thrust reduces the fatigue life of a structure drastically. From the results shown, the fatigue life for a specimen under biaxial loading failed at much lower life compared to a specimen that subjected to bending loading. Moreover, corrosive environment further reduces the life but not as drastically as compared to biaxial loading does to bending fatigue. It is thus can be concluded that biaxial is an important and serious reduction factor for steel structures, more severe than corrosion.
Acknowledgements Authors acknowledge with great appreciation the contribution made in the experimental program by their research students Ben Abraham, Marc Gardner, Lee K. Onn and Soo Y. Haw. Contributions made by Mr. I. Brown during the design stage of the experimental research program are acknowledged with gratitude. References [1] P.L. Andersen, Corrosion fatigue testing, fatigue and fracture, in: ASM International, The Materials Information Society of USA, vol. 2, 1997. [2] P.S. Pao, Mechanisms of corrosion fatigue, fatigue and fracture, in: ASM International, The Materials Information Society of USA, vol. 2, 1997. [3] M.A. Wahab, M. Sakano, Experimental study of corrosion fatigue behaviour of welded steel structures, J. Mater. Process. Technol. 118 (1–3) (2001) 116–121. [4] D. Radaj, Heat Effects of Welding—Temperature Field, Residual Stress, Distortion, Springer, Berlin, September 1992. [5] W.N. Findley, Effects of extremes of hardness and mean stress on fatigue of AISI 4340 steel in bending and torsion, J. Eng. Mater. Technol. 111 (2) (1989), ASME. [6] Z. Hashin, Fatigue failure criteria for combined cyclic stress, Int. J. Fract. 17 (2), Sijthoff & Noordhoff, Alphen a/d Rijn, April 1981. [7] G. Sines, G. Ohgi, Fatigue criteria under combined stresses or strains, J. Eng. Mater. Technol. 103 (2) (1991), ASME. [8] D.L. McDiarmid, A new analysis of fatigue under combined bending and twisting, Aeronaut. J. 78 (752), Royal Aeronautical Society, London, 1974. [9] J.A. Collins, Failure of Materials in Mechanical Design, Wiley, New York, 1993. [10] R.C. Juvinall, K.M. Marshek, Fundamentals of Machine Component Design, Wiley, New York, 1983. [11] J.E. Shigley, C.R. Mischke, Mechanical Engineering Design, McGraw-Hill, New York, 1989.