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Effect of welding nugget diameter on the fatigue strength of the resistance spot welded joints of different steel sheets
Journal of Materials Processing Technology 176 (2006) 127–132
Effect of welding nugget diameter on the fatigue strength of the resistance spot welded joints of different steel sheets M. Vural ∗ , A. Akkus¸, B. Ery¨urek ˙ Istanbul Technical University, Mechanical Engineering Department, Istanbul, Turkey Received 27 April 2005; received in revised form 23 February 2006; accepted 27 February 2006
Abstract This paper presents an experimental study on the fatigue strength of resistance spot welded galvanized steel sheets and austenitic stainless steel (AISI 304) sheets. The sheet materials were joined by using resistance spot welding as a lap joint. Material combination and nugget diameter were selected as experimental parameters. The high cycle fatigue tests were performed and S–N curves were obtained for each specimen. The results show that galvanized steel sheet combination has the highest fatigue limit. The sheet combination which has the minimum fatigue limit is galvanized–AISI 304 sheet combination. For austenitic stainless steel–galvanized steel sheet joint, the measurements of the nugget diameter and crack length were performed after fatigue tests. Crack growth rate of the spot welded galvanized–AISI 304 joining type is slower than that of base metals given in literature. C and m coefficients of Paris–Erdogan equation for spot welded AISI 304–galvanized steel sheet joints were obtained. © 2006 Elsevier B.V. All rights reserved. Keywords: Resistance spot welding; Fatigue resistance; Galvanized steel; Austenitic stainless steel
1. Introduction Resistance spot welds are crucial to the automotive industry since the typical car body contains about a thousands spot welds joining a mixture of metal material types and sheet thickness [1,2]. The corrosion resistance of the steel sheets is very important in car odies and thus galvanized steel and austenitic stainless steel sheets take the place of the uncoated steel sheets in automotive industries. One of the major concerns in the spotweld industry is fatigue, because these joints are exposed to variable loads in the automobile structures [3,4]. The fatigue crack begins at the interior surface of welded sheets in the heat affected zone (HAZ). It is very important to know crack growth rate in the spot welded member and hence determine the serviceability of the spot weld before rupture [5,6]. Rathbun and Matlock have obtained the fatigue S–N curves of spot welded automotive steel sheets recently [7]. They applied the tensile-shear and cross-tension loading modes to the specimens and reported that the fatigue performance of the welded sheets is determined by geometric factors rather than material strength and microstructure. Linder and Malender have used
austenitic and duplex stainless steel sheets to determine the fatigue behaviour of the spot welded joints [8]. Anastassiou and co-workers [9] have studied the stress distribution and fatigue behaviour of the spot welded thin steel sheets. They reported the effect of the weld parameters (welding time, welding current and electrode force) and post-heating treatments on the weld size, the notch bottom geometry, residual stresses and hence on the fatigue life. In the present work, galvanized steel and austenitic stainless steel (AISI 304) sheets were resistance spot welded as lap joints. The experimental parameters are steel sheet combination in joints and nugget diamater of the spot weld. The high cycle fatigue tests were performed to the spot welded specimens, and S–N curves for the joints were obtained. For fatigue design, material coefficients of Paris–Erdogan equation were determined for galvanized–austenitic stainless steel sheet joints by measuring crack lengths with respect to the number of cycles. The crack growth rate curve was plotted for joints and compared with data given in the literature for base metals. 2. Experimental 2.1. Welding process
∗
Corresponding author. Tel.: +90 212 29313 00/2500; fax: +90 212 2450795. E-mail address: [email protected] (M. Vural).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.02.026
In this work, commercial AISI 304 type stainless steel sheet and galvanized steel sheet were used. The sheet materials were cut into 100 mm × 30 mm
128
M. Vural et al. / Journal of Materials Processing Technology 176 (2006) 127–132
Fig. 1. The initial microstructures of base metals. Table 1 Applied welding currents in pre-tests Sheet combination
Nugget diameter (±0.2 mm)
Welding current (kA)
AISI 304–AISI 304
4 5 6
6.25 7.25 8.75
AISI 304–galvanized steel
4 5 6
7.25 8.75 9.75
Galvanized steel–galvanized steel
4 5 6
8 9.25 10.5
dimensions and the specimens were joined as lap joints for three material combinations. The initial microstructures of base metals are given in Fig. 1. The thicknesses of the galvanized and stainless steel sheets are 0.95 and 1.03 mm, respectively. The nugget diameter of the spot welded joints are varied between 4 ± 0.1 and 6 ± 0.1 mm in industrial applications. In order to obtain the relationship between welding current and nugget diameter, pre-tests were carried out. In these pre-tests, welding current was changed from 5.5 to 13.5 kA. With 0.5 kA current step increment. Table 1 gives applied currents for nugget diameters investigated and Fig. 2 gives the welding current versus nugget diameter. During the spot weld operations, the weld time (15 period), squeezing time (25 period) and cooling time (25 period) were kept constant (1 period = 1/50 sn). Fig. 3 shows a series of the spot welded specimens.
Fig. 2. The effect of the weld current on the nugget diameter.
Fig. 3. A series of the spot welded specimens.
The microVickres measurements were applied to the base metals and the weld area of the spot welded specimens. The microhardness values of the AISI 304 and galvanized steel are 190 HV1 and 82 HV1, respectively. There are no a very major changes in hardness of the weld metal and heat affected zone for AISI 304–AISI 304 and galvanized–galvanized spot welded steel sheet combinations. For AISI 304–galvanized spot welded steel sheet combination, there is a major increment in the hardness values of the weld metal and heat affected zone and it was measured as 350 HV1 approximately.
2.2. Fatigue test The fatigue testing was performed in laboratory conditions using a 60 kN servo-hydrolic Dartec testing machine with a software package specifically designed for running fatigue tests. The specimens were mounted in the grips 1 from both ends. In order to provide symmetry and to prevent a moment being applied at the weld, 25 mm long shims, with the same thickness as the sheet metal, were glued at both ends of the specimen as shown in Fig. 4. All tests were performed using a sinusoidal waveform operating at 10 Hz. During the fatigue experiments, load and specimen displacements were recorded and monitored by the test control system. The stiffness of the spot welded specimen is defined as P/l and it was evaluated during each fatigue test, where, P is the cyclic load range and l is the specimen cyclic elongation range. The failure criterias during fatigue tests were rupture or stiffness drop value of the specimens. Since the specimen stiffness
Fig. 4. Sketch of typical lap joint specimen with shims attached.
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sheet combinations, the endurance limits of specimens decreases while the nugget diameter increases (Fig. 6a). The endurance limits of austenitic steel sheet combinations are approximately 3 kN, 2.4 kN and 1.7 kN for 4, 5 and 6 mm nugget diameter, respectively. The minimum endurance limit observed belongs to AISI 304–galvanized steel sheet combinations and is almost 1.3 kN (Fig. 5b) while the maximum endurance limit observed
Fig. 5. Determination of the crack length. decreases while fatigue crack propogates, the 25% stiffness drop was choosen as stifness drop failure criteria [8]. In all tests, applied force range and number of cycles to failure were recorded. Specimens were exposed to a constant load amplitude until fracture occurred or to a maximum 106 cycles, at a R-value of 0.09. Specimens that survived 106 cycles are called run outs.
2.3. Determination of crack length In order to investigate the variation of the crack length with respect to number of cycles and determine the crack growth rate in the spot welded joints, macroscopic examinations were performed. Four identical galvanized–AISI 304 type sheet specimens having 5 mm diameter of nugget were fatigue tested. The specimens were subjected to 30,000, 60,000, 90,000 and 104,000 cycles, respectively. Rupture was not seen in the welding area for all these specimens. After the tests, the welded sheets were cut from the centre of the weld nugget mechanically. Photographs of the joint area between the sheets were taken. In order to measure the uncracked area between the welded sheets after fatigue tests, a surface measurement software programme called Scion-Pro was used. An equivalent diameter for measuring uncracked area was calculated assuming the uncracked area as a circular shape. Spot welded joints behave like a surface crack due to it is geometry. Thus, half of the differences between initial and remaining diameter of the nugget were taken as crack length, as seen in Fig. 5: crack length a =
D i − Dr 2
(1)
As seen in Fig. 5, Di and Dr initial diameter of weld nugget and measured diameter of weld nugget after fatigue, respectively.
3. Results As shown in Fig. 2 given above, when the weld current is increased, the nugget diameter increases until a current value. After that value, the nugget diameter decreases because of the excessive melting and splashing (Fig. 2). This is valid for three material type combinations. The S–N curves of the spot welded specimens having choosen nugget diameter are shown in Fig. 6. All data points belong to a mean value of three tests. As shown in Fig. 6, while the load range decreases, the fatigue life of the specimens increases as expected. There is an inclination going to a specific endurance limit for AISI 304–galvanized and galvanized–galvanized steel sheet combinations for all nugget diameters, although nugget diameter varies (Fig. 7b and c). Nevertheless for AISI 304–AISI 304 steel
Fig. 6. S–N curves of the spot welded specimens: (a) AISI 304–AISI 304 sheet combination; (b) AISI 304–galvanized sheet combination; (c) galvanized–galvanized sheet combination.
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Fig. 7. SEM photographs of the crack surfaces after fatigue failure for resistance spot welded steel sheets.
belongs to galvanized steel sheet combinations at almost 2.8 kN (Fig. 6c). The low cycle fatigue response of all nugget diameters for each material combinations generally have similar fatigue trend in high load range. It is seen from Fig. 5 that, the trend of load range decrement with respect to number of cycles is maximum for AISI 304–galvanized steel sheet combination, and minimum for galvanized–galvanized steel sheet combination for low cycle fatigue period. In Fig. 7, macroscopic and SEM views of the last crack surfaces after fatigue tests are shown. The specimens for SEM investigation are spot welded sheets having three types of material combinations, in 5 mm nugget diameter. Given SEM views in Fig. 6 are the last rupture surfaces of the specimens which were subjected to high load amplitude and thus, those spot welded specimens were failured as a rupture after fatigue tests. Because the photographs do not show the crack growth surfaces, crack initiation site and fatigue striations cannot be seen from Fig. 7. It can be seen that, there is any plastic deformation and the specimens show a brittle fracture. The photographs of the joining areas between galvanized– AISI 304 type sheet combination having 5 mm nugget diam-
eter, are shown in Fig. 8 together with schematic views. The variation of the crack length as a function of number of cycles is plotted in Fig. 8 using sketchs given in Fig. 8. As expected, while the number of cycles to failure increases, the crack length also increases exponentially as seen from Fig. 9. 4. Discussion The endurance limit of the similar steel sheet combination is higher than that of different steel sheet combinations as mentioned above. The reason for this result is heat unbalance between the steel sheets which occurs during spot welding operations of steel sheets having different material properties, especially electrical resistance. Due to the heat unbalance, the nugget between the sheets cannot occur symmetrically. Antisymmetric nugget formation decreases fatigue strength of the welded sheet combination. In order to clarify fatigue performance difference of the base metals and dissimilar spot welded joints, C and m Paris–Erdogan material coefficients (Eq. (2)) which determine the crack growth characteristic of the spot welded joints are calculated using the obtained data from Figs. 8 and 9 and compared
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131
Fig. 8. Sketch and macroscopy of the joining area after fatigue (A1 , A2 , A3 , A4 : the remaining welded areas after fatigue).
with data given for base metals in the literature: Paris–Erdogan crack growth rate equation;
According to the selected fatigue loading type (Fig. 4), Mode II (Sliding Mode) is valid during the fatigue tests. Mode II stress intensity factor for spot welds is [10]:
da = CKm dN (2)
K = where a is the crack length (mm), N the number of cycles, C and m the material coefficients, and K is the stress intensity factor range: crack growth rate;
da dN
i
=
ai+1 − ai Ni+1 − Ni
(i = 1, 2, 3) (3)
2F √ πDr t
(4)
where F is the fatigue load range (Fmax − Fmin ), t the sheet thickness and Dr is the remaining nugget diameter after fatigue. Using the trend line plotted in Fig. 10, Paris–Erdogan material coefficients of the spot welded AISI 304–galvanized steel sheet
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Fig. 9. Variation of the crack length.
Fig. 11. Comparison of crack growth rate of spot welded galvanized–AISI 304 steel sheets combination with the crack growth rate data for base metals in the room temperature condition [6].
References
Fig. 10. Crack growth rate and stress intensity factor in logarithmic scale.
joint are obtained as m = 2.4
and C = 10−11.3
The following equation gives crack growth rate and fatigue life of this type spot welded joints: da = 10−11.3 K2.4 dN
(5)
The crack growth rate of dissimilar sheet combination is compared with the crack growth rate of galvanized steel and austenitic stainless steel given in the literature [11]. Fig. 10 shows the crack growth rate comparison. It is seen from Fig. 11 that, the crack growth rate of the spot welded AISI 304–galvanized steel sheet joint is slower than that of base metals and the crack growth rate of the spot welded is close to that of galvanized steel.
[1] M. Vural, A. Akkus, On the resistance spot weldability of galvanized interstitial free steel sheets with austenitic stainless steel sheets, J. Mater. Process. Technol. 153/154 (2004) 1–6. [2] T. Satoh, H. Abe, T. Nakaoka, Y. Hayata, The fatigue life of the spot welded joint under a repeated load of R = −1, Welding in the World, 1996, p. 12. [3] J. Neves, A. Loureiro, Fracture toughness of welds—effect of brittle zones and strength mismatch, J. Mater. Process. Technol. 153/154 (2004) 537–543. [4] B. Chang, Y. Shi, L. Lu, Studies on the stress distribution and fatigue behavior of weld-bonded lap shear joints, J. Mater. Process. Technol. 108 (3) (2001) 307–313. [5] T. Ninh Nguyen, M.A. Wahab, The effect of weld geometry and residual stresses on the fatigue of welded joints under combined loading, J. Mater. Process. Technol. 77 (1–3) (1998) 201–208. [6] N.T. Ninh, M.A. Wahab, The effect of residual stresses and weld geometry on the improvement of fatigue life, J. Mater. Process. Technol. 48 (1–4) (1995) 581–588. [7] R.W. Rathbun, D.K. Matlock, J.G. Speer, Fatigue behaviour of spot welded high strength steel sheets, Weld. J. Res. Suppl. (2003) 207. [8] J. Linder, A. Melander, M. Larsson, Y. Bergengren, Fatigue design of spot welded austenitic and duplex stainless sheet steels, Fract. Eng. Mater. Struct. (1998) 673. [9] M. Anastassiou, M. Babit, J.L. Lebrun, Residual stress and microstructure distribution in spot welded steel sheets. Relation with fatigue behaviour, Mater. Sci. Eng. A (1990) 141. [10] S. Zhang, Approximate stress intensity factors and notch stresses for common spot-welded specimens, Weld. J. Res. Suppl. (1999) 173. [11] ASM Handbook, Fatigue and Fracture, vol. 19, November 1997, p. 637.
Effect of welding nugget diameter on the fatigue strength of the resistance spot welded joints of different steel sheets M. Vural ∗ , A. Akkus¸, B. Ery¨urek ˙ Istanbul Technical University, Mechanical Engineering Department, Istanbul, Turkey Received 27 April 2005; received in revised form 23 February 2006; accepted 27 February 2006
Abstract This paper presents an experimental study on the fatigue strength of resistance spot welded galvanized steel sheets and austenitic stainless steel (AISI 304) sheets. The sheet materials were joined by using resistance spot welding as a lap joint. Material combination and nugget diameter were selected as experimental parameters. The high cycle fatigue tests were performed and S–N curves were obtained for each specimen. The results show that galvanized steel sheet combination has the highest fatigue limit. The sheet combination which has the minimum fatigue limit is galvanized–AISI 304 sheet combination. For austenitic stainless steel–galvanized steel sheet joint, the measurements of the nugget diameter and crack length were performed after fatigue tests. Crack growth rate of the spot welded galvanized–AISI 304 joining type is slower than that of base metals given in literature. C and m coefficients of Paris–Erdogan equation for spot welded AISI 304–galvanized steel sheet joints were obtained. © 2006 Elsevier B.V. All rights reserved. Keywords: Resistance spot welding; Fatigue resistance; Galvanized steel; Austenitic stainless steel
1. Introduction Resistance spot welds are crucial to the automotive industry since the typical car body contains about a thousands spot welds joining a mixture of metal material types and sheet thickness [1,2]. The corrosion resistance of the steel sheets is very important in car odies and thus galvanized steel and austenitic stainless steel sheets take the place of the uncoated steel sheets in automotive industries. One of the major concerns in the spotweld industry is fatigue, because these joints are exposed to variable loads in the automobile structures [3,4]. The fatigue crack begins at the interior surface of welded sheets in the heat affected zone (HAZ). It is very important to know crack growth rate in the spot welded member and hence determine the serviceability of the spot weld before rupture [5,6]. Rathbun and Matlock have obtained the fatigue S–N curves of spot welded automotive steel sheets recently [7]. They applied the tensile-shear and cross-tension loading modes to the specimens and reported that the fatigue performance of the welded sheets is determined by geometric factors rather than material strength and microstructure. Linder and Malender have used
austenitic and duplex stainless steel sheets to determine the fatigue behaviour of the spot welded joints [8]. Anastassiou and co-workers [9] have studied the stress distribution and fatigue behaviour of the spot welded thin steel sheets. They reported the effect of the weld parameters (welding time, welding current and electrode force) and post-heating treatments on the weld size, the notch bottom geometry, residual stresses and hence on the fatigue life. In the present work, galvanized steel and austenitic stainless steel (AISI 304) sheets were resistance spot welded as lap joints. The experimental parameters are steel sheet combination in joints and nugget diamater of the spot weld. The high cycle fatigue tests were performed to the spot welded specimens, and S–N curves for the joints were obtained. For fatigue design, material coefficients of Paris–Erdogan equation were determined for galvanized–austenitic stainless steel sheet joints by measuring crack lengths with respect to the number of cycles. The crack growth rate curve was plotted for joints and compared with data given in the literature for base metals. 2. Experimental 2.1. Welding process
∗
Corresponding author. Tel.: +90 212 29313 00/2500; fax: +90 212 2450795. E-mail address: [email protected] (M. Vural).
0924-0136/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2006.02.026
In this work, commercial AISI 304 type stainless steel sheet and galvanized steel sheet were used. The sheet materials were cut into 100 mm × 30 mm
128
M. Vural et al. / Journal of Materials Processing Technology 176 (2006) 127–132
Fig. 1. The initial microstructures of base metals. Table 1 Applied welding currents in pre-tests Sheet combination
Nugget diameter (±0.2 mm)
Welding current (kA)
AISI 304–AISI 304
4 5 6
6.25 7.25 8.75
AISI 304–galvanized steel
4 5 6
7.25 8.75 9.75
Galvanized steel–galvanized steel
4 5 6
8 9.25 10.5
dimensions and the specimens were joined as lap joints for three material combinations. The initial microstructures of base metals are given in Fig. 1. The thicknesses of the galvanized and stainless steel sheets are 0.95 and 1.03 mm, respectively. The nugget diameter of the spot welded joints are varied between 4 ± 0.1 and 6 ± 0.1 mm in industrial applications. In order to obtain the relationship between welding current and nugget diameter, pre-tests were carried out. In these pre-tests, welding current was changed from 5.5 to 13.5 kA. With 0.5 kA current step increment. Table 1 gives applied currents for nugget diameters investigated and Fig. 2 gives the welding current versus nugget diameter. During the spot weld operations, the weld time (15 period), squeezing time (25 period) and cooling time (25 period) were kept constant (1 period = 1/50 sn). Fig. 3 shows a series of the spot welded specimens.
Fig. 2. The effect of the weld current on the nugget diameter.
Fig. 3. A series of the spot welded specimens.
The microVickres measurements were applied to the base metals and the weld area of the spot welded specimens. The microhardness values of the AISI 304 and galvanized steel are 190 HV1 and 82 HV1, respectively. There are no a very major changes in hardness of the weld metal and heat affected zone for AISI 304–AISI 304 and galvanized–galvanized spot welded steel sheet combinations. For AISI 304–galvanized spot welded steel sheet combination, there is a major increment in the hardness values of the weld metal and heat affected zone and it was measured as 350 HV1 approximately.
2.2. Fatigue test The fatigue testing was performed in laboratory conditions using a 60 kN servo-hydrolic Dartec testing machine with a software package specifically designed for running fatigue tests. The specimens were mounted in the grips 1 from both ends. In order to provide symmetry and to prevent a moment being applied at the weld, 25 mm long shims, with the same thickness as the sheet metal, were glued at both ends of the specimen as shown in Fig. 4. All tests were performed using a sinusoidal waveform operating at 10 Hz. During the fatigue experiments, load and specimen displacements were recorded and monitored by the test control system. The stiffness of the spot welded specimen is defined as P/l and it was evaluated during each fatigue test, where, P is the cyclic load range and l is the specimen cyclic elongation range. The failure criterias during fatigue tests were rupture or stiffness drop value of the specimens. Since the specimen stiffness
Fig. 4. Sketch of typical lap joint specimen with shims attached.
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sheet combinations, the endurance limits of specimens decreases while the nugget diameter increases (Fig. 6a). The endurance limits of austenitic steel sheet combinations are approximately 3 kN, 2.4 kN and 1.7 kN for 4, 5 and 6 mm nugget diameter, respectively. The minimum endurance limit observed belongs to AISI 304–galvanized steel sheet combinations and is almost 1.3 kN (Fig. 5b) while the maximum endurance limit observed
Fig. 5. Determination of the crack length. decreases while fatigue crack propogates, the 25% stiffness drop was choosen as stifness drop failure criteria [8]. In all tests, applied force range and number of cycles to failure were recorded. Specimens were exposed to a constant load amplitude until fracture occurred or to a maximum 106 cycles, at a R-value of 0.09. Specimens that survived 106 cycles are called run outs.
2.3. Determination of crack length In order to investigate the variation of the crack length with respect to number of cycles and determine the crack growth rate in the spot welded joints, macroscopic examinations were performed. Four identical galvanized–AISI 304 type sheet specimens having 5 mm diameter of nugget were fatigue tested. The specimens were subjected to 30,000, 60,000, 90,000 and 104,000 cycles, respectively. Rupture was not seen in the welding area for all these specimens. After the tests, the welded sheets were cut from the centre of the weld nugget mechanically. Photographs of the joint area between the sheets were taken. In order to measure the uncracked area between the welded sheets after fatigue tests, a surface measurement software programme called Scion-Pro was used. An equivalent diameter for measuring uncracked area was calculated assuming the uncracked area as a circular shape. Spot welded joints behave like a surface crack due to it is geometry. Thus, half of the differences between initial and remaining diameter of the nugget were taken as crack length, as seen in Fig. 5: crack length a =
D i − Dr 2
(1)
As seen in Fig. 5, Di and Dr initial diameter of weld nugget and measured diameter of weld nugget after fatigue, respectively.
3. Results As shown in Fig. 2 given above, when the weld current is increased, the nugget diameter increases until a current value. After that value, the nugget diameter decreases because of the excessive melting and splashing (Fig. 2). This is valid for three material type combinations. The S–N curves of the spot welded specimens having choosen nugget diameter are shown in Fig. 6. All data points belong to a mean value of three tests. As shown in Fig. 6, while the load range decreases, the fatigue life of the specimens increases as expected. There is an inclination going to a specific endurance limit for AISI 304–galvanized and galvanized–galvanized steel sheet combinations for all nugget diameters, although nugget diameter varies (Fig. 7b and c). Nevertheless for AISI 304–AISI 304 steel
Fig. 6. S–N curves of the spot welded specimens: (a) AISI 304–AISI 304 sheet combination; (b) AISI 304–galvanized sheet combination; (c) galvanized–galvanized sheet combination.
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Fig. 7. SEM photographs of the crack surfaces after fatigue failure for resistance spot welded steel sheets.
belongs to galvanized steel sheet combinations at almost 2.8 kN (Fig. 6c). The low cycle fatigue response of all nugget diameters for each material combinations generally have similar fatigue trend in high load range. It is seen from Fig. 5 that, the trend of load range decrement with respect to number of cycles is maximum for AISI 304–galvanized steel sheet combination, and minimum for galvanized–galvanized steel sheet combination for low cycle fatigue period. In Fig. 7, macroscopic and SEM views of the last crack surfaces after fatigue tests are shown. The specimens for SEM investigation are spot welded sheets having three types of material combinations, in 5 mm nugget diameter. Given SEM views in Fig. 6 are the last rupture surfaces of the specimens which were subjected to high load amplitude and thus, those spot welded specimens were failured as a rupture after fatigue tests. Because the photographs do not show the crack growth surfaces, crack initiation site and fatigue striations cannot be seen from Fig. 7. It can be seen that, there is any plastic deformation and the specimens show a brittle fracture. The photographs of the joining areas between galvanized– AISI 304 type sheet combination having 5 mm nugget diam-
eter, are shown in Fig. 8 together with schematic views. The variation of the crack length as a function of number of cycles is plotted in Fig. 8 using sketchs given in Fig. 8. As expected, while the number of cycles to failure increases, the crack length also increases exponentially as seen from Fig. 9. 4. Discussion The endurance limit of the similar steel sheet combination is higher than that of different steel sheet combinations as mentioned above. The reason for this result is heat unbalance between the steel sheets which occurs during spot welding operations of steel sheets having different material properties, especially electrical resistance. Due to the heat unbalance, the nugget between the sheets cannot occur symmetrically. Antisymmetric nugget formation decreases fatigue strength of the welded sheet combination. In order to clarify fatigue performance difference of the base metals and dissimilar spot welded joints, C and m Paris–Erdogan material coefficients (Eq. (2)) which determine the crack growth characteristic of the spot welded joints are calculated using the obtained data from Figs. 8 and 9 and compared
M. Vural et al. / Journal of Materials Processing Technology 176 (2006) 127–132
131
Fig. 8. Sketch and macroscopy of the joining area after fatigue (A1 , A2 , A3 , A4 : the remaining welded areas after fatigue).
with data given for base metals in the literature: Paris–Erdogan crack growth rate equation;
According to the selected fatigue loading type (Fig. 4), Mode II (Sliding Mode) is valid during the fatigue tests. Mode II stress intensity factor for spot welds is [10]:
da = CKm dN (2)
K = where a is the crack length (mm), N the number of cycles, C and m the material coefficients, and K is the stress intensity factor range: crack growth rate;
da dN
i
=
ai+1 − ai Ni+1 − Ni
(i = 1, 2, 3) (3)
2F √ πDr t
(4)
where F is the fatigue load range (Fmax − Fmin ), t the sheet thickness and Dr is the remaining nugget diameter after fatigue. Using the trend line plotted in Fig. 10, Paris–Erdogan material coefficients of the spot welded AISI 304–galvanized steel sheet
132
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Fig. 9. Variation of the crack length.
Fig. 11. Comparison of crack growth rate of spot welded galvanized–AISI 304 steel sheets combination with the crack growth rate data for base metals in the room temperature condition [6].
References
Fig. 10. Crack growth rate and stress intensity factor in logarithmic scale.
joint are obtained as m = 2.4
and C = 10−11.3
The following equation gives crack growth rate and fatigue life of this type spot welded joints: da = 10−11.3 K2.4 dN
(5)
The crack growth rate of dissimilar sheet combination is compared with the crack growth rate of galvanized steel and austenitic stainless steel given in the literature [11]. Fig. 10 shows the crack growth rate comparison. It is seen from Fig. 11 that, the crack growth rate of the spot welded AISI 304–galvanized steel sheet joint is slower than that of base metals and the crack growth rate of the spot welded is close to that of galvanized steel.
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