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Z. Azari - One of the best experts on this subject based on the ideXlab platform.
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The effect of galvanizing on the mechanical resistance and fatigue toughness of a spot welded assembly made of AISI410 martensite
Materials Science and Engineering: A, 2014Co-Authors: N. Becker, J. Gilgert, Etienne Petit, Z. AzariAbstract:Welding of high strength steels containing martensite is difficult due to the appearance of brittle grains after fast cooling. Galvanizing spot-welded specimens made of AISI410 solves this problem, and strongly improves the performances of spot welds. The mechanisms responsible for these achievements are presently analyzed. The changes of the mechanical strength and fatigue toughness of spot-welded samples made of AISI410 after galvanizing have been studied. AISI410 is a chromium-rich martensite. Tensile tests on normalized specimens indicate that the yield stress is close to 1010 MPa and the UTS at similar to 1270 MPa. The fatigue limit of raw AISI410 is at 690 MPa (Delta sigma(L) at R=0.1). Specimens were cut following the rolling direction and spot-welded. Metallography reveals that spot welding produces: - hard martensite at the center of the nugget; - a heat affected zone, which has been forged by the welding electrodes and recrystallized at high temperature; - and finally, a ring of grains tempered in a subcritical temperature range. Spot welds were tested in tensile-shear mode. Both Overload Failure and fatigue toughness were evaluated. Annealing and galvanizing post-treatment have been used to improve the mechanical properties of the spot-welded specimens. The improvement of the mechanical performances of spot-welded specimens after post-treatments are reported as follows. First, we observed significant changes in the modes of rupture both in Overload Failure and fatigue tests. The mechanical strength of untreated spot welds is controlled by the brittleness of the nugget and the forged heat affected zone. After post-treatment, the ultimate tensile strength is increased, and rupture results from fissures propagating exclusively into the zone recrystallized in the subcritical range of temperature. This mode is genuine to spot welds in martensite because of the high strength of the base metal. Second, three modes of rupture depending on the force range control the fatigue toughness. Annealing and galvanizing modify the ranges. Galvanizing increases the endurance, and the fatigue limit by a factor similar to 3.5. This effect is attributed to zinc brazing that strongly changes the distribution of stresses under tensile-shear solicitation. Hardness measurements and tensile test on specific samples clarify the mechanical behavior of the layers around the spot, and shed some light on the mechanisms of rupture. Lifetimes are controlled by the initiation of fatigue cracks.
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The effect of galvanizing on the mechanical resistance and fatigue toughness of a spot welded assembly made of AISI410 martensite
Materials Science and Engineering: A, 2014Co-Authors: N. Becker, J. Gilgert, Etienne Petit, Z. AzariAbstract:Abstract Welding of high strength steels containing martensite is difficult due to the appearance of brittle grains after fast cooling. Galvanizing spot-welded specimens made of AISI410 solves this problem, and strongly improves the performances of spot welds. The mechanisms responsible for these achievements are presently analyzed. The changes of the mechanical strength and fatigue toughness of spot-welded samples made of AISI410 after galvanizing have been studied. AISI410 is a chromium-rich martensite. Tensile tests on normalized specimens indicate that the yield stress is close to 1010 MPa and the UTS at ~1270 MPa. The fatigue limit of raw AISI410 is at 690 MPa (Δ σ L at R =0.1). Specimens were cut following the rolling direction and spot-welded. Metallography reveals that spot welding produces: – hard martensite at the center of the nugget; – a heat affected zone, which has been forged by the welding electrodes and recrystallized at high temperature; – and finally, a ring of grains tempered in a subcritical temperature range. Spot welds were tested in tensile–shear mode. Both Overload Failure and fatigue toughness were evaluated. Annealing and galvanizing post-treatment have been used to improve the mechanical properties of the spot-welded specimens. The improvement of the mechanical performances of spot-welded specimens after post-treatments are reported as follows. First, we observed significant changes in the modes of rupture both in Overload Failure and fatigue tests. The mechanical strength of untreated spot welds is controlled by the brittleness of the nugget and the forged heat affected zone. After post-treatment, the ultimate tensile strength is increased, and rupture results from fissures propagating exclusively into the zone recrystallized in the subcritical range of temperature. This mode is genuine to spot welds in martensite because of the high strength of the base metal. Second, three modes of rupture depending on the force range control the fatigue toughness. Annealing and galvanizing modify the ranges. Galvanizing increases the endurance, and the fatigue limit by a factor ~3.5. This effect is attributed to zinc brazing that strongly changes the distribution of stresses under tensile–shear solicitation. Hardness measurements and tensile test on specific samples clarify the mechanical behavior of the layers around the spot, and shed some light on the mechanisms of rupture. Lifetimes are controlled by the initiation of fatigue cracks.
Sk Bhaumik - One of the best experts on this subject based on the ideXlab platform.
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Failure analysis of propeller reduction gear shaft of indigenous rotary engine
2011Co-Authors: M.l. Madan, M Sujata, Sk BhaumikAbstract:A failed reduction gear shaft of indigenous rotary engine was sent to this laboratory for investigation. Examination revealed fracture in the collar of the shaft adjacent to the semi-circular groove meant for locking purpose. Fractographic study showed that the fracture was intergranular in nature. Further examination revealed that the shaft was case carburized and hence, the intergranular mode of fracture was a consequence of Overload Failure. It was discovered that at the location of fracture, because of localized grinding, the collar thickness was reduced to less than one third of that of the original thickness. It was learnt that grinding was carried out because of the problem in fixing the locking bolt. Analysis suggests that the thickness of the collar after the grinding operation was not adequate enough to support the design load and hence, the occurrence of Overload Failure.
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Failure analysis of fractured HPTR blade of an aeroengine
2011Co-Authors: M.l. Madan, Ma Venkataswamy, M Sujata, K Raghavendra, Sk BhaumikAbstract:A fractured HPTR blade belonging to an aeroengine, which failed in service was sent to the laboratory for analysis. Examination revealed that the blade had failed by fatigue. Fatigue crack was found to have initiated on the leading edge at about 70 mm height from the blade root platform and propagated progressively over about 50% of the blade cross section before giving way to final Overload Failure. Although micro-fractographic features were largely obliterated due to heat effects, gross fracture features indicated that the fatigue crack had originated on the convex surface, close to the leading edge. At the crack origin region, the blade-wall thickness was found to be very minimal (-200 um). This low wall thickness had resulted due to improper liquid metal flow in the mould during blade casting process. Hence, the primary reason for the fatigue Failure of the HPTR blade can be attributed to casting defect.
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Failure analysis of main rotor blade fork bolt (big) of a helicopter
2009Co-Authors: M.l. Madan, Ma Venkataswamy, M Sujata, R Bhuvana, Sk BhaumikAbstract:A failed main rotor blade fork (big) of a helicopter was forwarded to this laboratory for analyzing the cause of Failure. Examination revealed that the bolt has failed by fatigue. There was multiple fatigue crack initiation on the threads of the bolt. After initiation, a few of the cracks joined together to form major fatigue crack fronts which then propagated progressively over about 95% of the cross section before culminating in static Overload Failure. Evidences suggest that there was relative movement (slip) between the nut and the bolt which resulted in thinning of the threads on the bolt by wear process. Because of reduction in cross section, the load carrying capacity of the threads reduced drastically leading to initiation of a number of fatigue cracks on the threads. The material, microstructure and hardness of the bolt were found to conform to design specification. No metallurgical abnormalities were found responsible for the fatigue crack initiation.
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Analysis of fractured eyering shaft of ramp side attachment fitting of cargo door assembly belonging to an aircraft
2009Co-Authors: M.l. Madan, Ma Venkataswamy, M Sujata, R Bhuvana, Sk BhaumikAbstract:A fractured eyering shaft (port) of the cargo door assembly of an aircraft was sent to the laboratory for analysis. Examination revealed that the shaft had fractured at the locking position resulting in dislodgement of the roller sub-assembly meant for moving the ramp on the track. The fracture surface was found to be extensively damaged and as a result, fractographic features on majority of the fracture surface were obliterated. However, in isolated places, the fracture surface was relatively preserved and examination of these regions showed presence of ductile dimple rupture, typical of Overload Failure. Because of post fracture damage, fractographic study alone was not adequate for establishing the Failure mode. However, the fractographic study along with evidences available on the side track (port) suggests that the fracturing of the eyering shaft occurred most probably due to excessive load. A detailed analysis of the Failure is presented in this report.
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Failure investigation of multiplicator of an aeroengine
2008Co-Authors: M Sujata, M.l. Madan, Sk BhaumikAbstract:Fractographic study confirmed that the driving gear of the multiplicator had failed by fatigue. There was multiple fatigue crack initiation and in all the cases, the fatigue crack had initiated at the tooth root. In two cases, the teeth got fractured and lost by tooth bending fatigue. In another case, after initiation, the fatigue crack had propagated into the hub of the gear and continued propagation further along the hub fillet. After propagation to a critical length, this fatigue crack had resulted in fracturing of a segment of the gear. Following this fracture, the impact with the input gear of the gear cluster had resulted in second fracture and chewing off of remaining teeth in the driving gear. Subsequently, the gear assembly had seized leading to torsional Overload Failure of the quill shaft transmitting power from the AGB to the multiplicator. The fatigue Failure in the driving gear appears to be stress related. Evidences suggest that probably, the load on the gear assembly was more than the usual. The power transmission system down the multiplicator needs to be examined as a whole for establishing the cause(s) for excessive load on the gear assembly.
S. P. H. Marashi - One of the best experts on this subject based on the ideXlab platform.
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influence of fusion zone size and Failure mode on mechanical performance of dissimilar resistance spot welds of aisi 1008 low carbon steel and dp600 advanced high strength steel
Materials & Design, 2011Co-Authors: M Pouranvari, M Goodarzi, S. P. H. Marashi, S M Mousavizadeh, M GhorbaniAbstract:Abstract The work here addresses the investigation of the effect of the welding parameters (welding time, welding current and electrode force) on the Overload Failure mode and mechanical performance of dissimilar resistance spot welds between drawing quality special killed AISI 1008 low carbon steel and DP600 dual phase steel. Mechanical properties of spot welds are described in terms of Failure mode, peak load and energy absorption during the quasi-static tensile-shear test. Three distinct Failure modes were observed during the tensile-shear test: interfacial, pullout and partial thickness–partial pullout Failure modes. Correlations among Failure mode, welding parameters, weld physical attributes and weld mechanical performance are analyzed. Effect of expulsion on mechanical performance of welds is also investigated.
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Microstructure and Failure behaviour of resistance spot welded DP980 dual phase steel
Materials Science and Technology, 2010Co-Authors: F. Nikoosohbat, M Pouranvari, M Goodarzi, Shahram Kheirandish, S. P. H. MarashiAbstract:AbstractIn this research, microstructure and Overload Failure behaviour of resistance spot welded DP980 were investigated. Microstructural characterisation, microhardness test and static tensile shear test were conducted. Fusion zone size proved to be the most important controlling factor of spot weld peak load and energy absorption. The results of this study demonstrated that the conventional weld size recommendation of d=4t1/2 is not sufficient to ensure the pullout Failure mode for DP980 steel resistance spot welds during the tensile shear test. In pullout mode, generally, Failure was initiated at heat affected zone/base metal interface, where softening occurs due to the tempering of martensite. However, when heavy expulsion occurs, pullout Failure tends to be initiated at fusion zone/heat affected zone interface. It was shown that heavy expulsion and associated large electrode indentation can reduce load carrying capacity and energy absorption capability of DP980 spot welds.
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Overload Failure behaviour of dissimilar thickness resistance spot welds during tensile shear test
Materials Science and Technology, 2010Co-Authors: S. P. H. Marashi, M Pouranvari, M. Salehi, A. Abedi, Saeid KavianiAbstract:AbstractResistance spot welding is the dominant process for joining sheet metals in automotive industry. Even thickness combinations are rarely used in practice; therefore, there is clearly a practical need for Failure behaviour investigation of uneven thickness resistance spot welds. The aim of the present paper is to investigate the Failure mode and Failure mechanism of dissimilar thickness low carbon steel resistance spot welds during tensile shear Overload test. Microstructural investigations, microhardness tests and tensile shear tests were conducted. Mechanical properties of the joints were described in terms of peak load, energy absorption and Failure mode. In order to understand the Failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile shear are examined by optical microscopy. It was found that for well established weld nuggets, the final solidification line is located in the geometrical centre of the joint. In pull-out Failure mode, Failure is ini...
M Pouranvari - One of the best experts on this subject based on the ideXlab platform.
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influence of fusion zone size and Failure mode on mechanical performance of dissimilar resistance spot welds of aisi 1008 low carbon steel and dp600 advanced high strength steel
Materials & Design, 2011Co-Authors: M Pouranvari, M Goodarzi, S. P. H. Marashi, S M Mousavizadeh, M GhorbaniAbstract:Abstract The work here addresses the investigation of the effect of the welding parameters (welding time, welding current and electrode force) on the Overload Failure mode and mechanical performance of dissimilar resistance spot welds between drawing quality special killed AISI 1008 low carbon steel and DP600 dual phase steel. Mechanical properties of spot welds are described in terms of Failure mode, peak load and energy absorption during the quasi-static tensile-shear test. Three distinct Failure modes were observed during the tensile-shear test: interfacial, pullout and partial thickness–partial pullout Failure modes. Correlations among Failure mode, welding parameters, weld physical attributes and weld mechanical performance are analyzed. Effect of expulsion on mechanical performance of welds is also investigated.
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Overload Failure behaviour of dissimilar thickness resistance spot welds during tensile shear test
Materials Science and Technology, 2010Co-Authors: S. P. H. Marashi, M Pouranvari, M. Salehi, A. Abedi, Saeid KavianiAbstract:AbstractResistance spot welding is the dominant process for joining sheet metals in automotive industry. Even thickness combinations are rarely used in practice; therefore, there is clearly a practical need for Failure behaviour investigation of uneven thickness resistance spot welds. The aim of the present paper is to investigate the Failure mode and Failure mechanism of dissimilar thickness low carbon steel resistance spot welds during tensile shear Overload test. Microstructural investigations, microhardness tests and tensile shear tests were conducted. Mechanical properties of the joints were described in terms of peak load, energy absorption and Failure mode. In order to understand the Failure mechanism, micrographs of the cross-sections of the spot welded joints during and after tensile shear are examined by optical microscopy. It was found that for well established weld nuggets, the final solidification line is located in the geometrical centre of the joint. In pull-out Failure mode, Failure is ini...
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Microstructure and Failure behaviour of resistance spot welded DP980 dual phase steel
Materials Science and Technology, 2010Co-Authors: F. Nikoosohbat, M Pouranvari, M Goodarzi, Shahram Kheirandish, S. P. H. MarashiAbstract:AbstractIn this research, microstructure and Overload Failure behaviour of resistance spot welded DP980 were investigated. Microstructural characterisation, microhardness test and static tensile shear test were conducted. Fusion zone size proved to be the most important controlling factor of spot weld peak load and energy absorption. The results of this study demonstrated that the conventional weld size recommendation of d=4t1/2 is not sufficient to ensure the pullout Failure mode for DP980 steel resistance spot welds during the tensile shear test. In pullout mode, generally, Failure was initiated at heat affected zone/base metal interface, where softening occurs due to the tempering of martensite. However, when heavy expulsion occurs, pullout Failure tends to be initiated at fusion zone/heat affected zone interface. It was shown that heavy expulsion and associated large electrode indentation can reduce load carrying capacity and energy absorption capability of DP980 spot welds.
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effect of weld nugget size on Overload Failure mode of resistance spot welds
Science and Technology of Welding and Joining, 2007Co-Authors: M Pouranvari, H R Asgari, S M Mosavizadch, Pirooz Marashi, M GoodarziAbstract:AbstractIn the present paper, effects of welding current, welding time, electrode pressure and holding time on the weld nugget size were studied. A Failure mechanism was proposed to describe both interfacial and pullout Failure modes. This mechanism was confirmed by SEM investigations. In the light of this mechanism, the effect of welding parameters on static weld strength and Failure mode was studied. Then, an analytical model was proposed to predict Failure mode and to estimate minimum nugget diameter (critical diameter) to ensure pullout Failure mode in shear tensile test. On the contrary to existing industrial standards, in this model, critical nugget diameter is attributed to metallurgical characterisation of material (weld nugget hardness to Failure location hardness ratio), in addition to sheet thickness. For a given sheet thickness, decreasing HWN/HFL increases interfacial Failure mode tendency. The results of this model were compared with experimental data and also with the literature.
N. Becker - One of the best experts on this subject based on the ideXlab platform.
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The effect of galvanizing on the mechanical resistance and fatigue toughness of a spot welded assembly made of AISI410 martensite
Materials Science and Engineering: A, 2014Co-Authors: N. Becker, J. Gilgert, Etienne Petit, Z. AzariAbstract:Welding of high strength steels containing martensite is difficult due to the appearance of brittle grains after fast cooling. Galvanizing spot-welded specimens made of AISI410 solves this problem, and strongly improves the performances of spot welds. The mechanisms responsible for these achievements are presently analyzed. The changes of the mechanical strength and fatigue toughness of spot-welded samples made of AISI410 after galvanizing have been studied. AISI410 is a chromium-rich martensite. Tensile tests on normalized specimens indicate that the yield stress is close to 1010 MPa and the UTS at similar to 1270 MPa. The fatigue limit of raw AISI410 is at 690 MPa (Delta sigma(L) at R=0.1). Specimens were cut following the rolling direction and spot-welded. Metallography reveals that spot welding produces: - hard martensite at the center of the nugget; - a heat affected zone, which has been forged by the welding electrodes and recrystallized at high temperature; - and finally, a ring of grains tempered in a subcritical temperature range. Spot welds were tested in tensile-shear mode. Both Overload Failure and fatigue toughness were evaluated. Annealing and galvanizing post-treatment have been used to improve the mechanical properties of the spot-welded specimens. The improvement of the mechanical performances of spot-welded specimens after post-treatments are reported as follows. First, we observed significant changes in the modes of rupture both in Overload Failure and fatigue tests. The mechanical strength of untreated spot welds is controlled by the brittleness of the nugget and the forged heat affected zone. After post-treatment, the ultimate tensile strength is increased, and rupture results from fissures propagating exclusively into the zone recrystallized in the subcritical range of temperature. This mode is genuine to spot welds in martensite because of the high strength of the base metal. Second, three modes of rupture depending on the force range control the fatigue toughness. Annealing and galvanizing modify the ranges. Galvanizing increases the endurance, and the fatigue limit by a factor similar to 3.5. This effect is attributed to zinc brazing that strongly changes the distribution of stresses under tensile-shear solicitation. Hardness measurements and tensile test on specific samples clarify the mechanical behavior of the layers around the spot, and shed some light on the mechanisms of rupture. Lifetimes are controlled by the initiation of fatigue cracks.
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The effect of galvanizing on the mechanical resistance and fatigue toughness of a spot welded assembly made of AISI410 martensite
Materials Science and Engineering: A, 2014Co-Authors: N. Becker, J. Gilgert, Etienne Petit, Z. AzariAbstract:Abstract Welding of high strength steels containing martensite is difficult due to the appearance of brittle grains after fast cooling. Galvanizing spot-welded specimens made of AISI410 solves this problem, and strongly improves the performances of spot welds. The mechanisms responsible for these achievements are presently analyzed. The changes of the mechanical strength and fatigue toughness of spot-welded samples made of AISI410 after galvanizing have been studied. AISI410 is a chromium-rich martensite. Tensile tests on normalized specimens indicate that the yield stress is close to 1010 MPa and the UTS at ~1270 MPa. The fatigue limit of raw AISI410 is at 690 MPa (Δ σ L at R =0.1). Specimens were cut following the rolling direction and spot-welded. Metallography reveals that spot welding produces: – hard martensite at the center of the nugget; – a heat affected zone, which has been forged by the welding electrodes and recrystallized at high temperature; – and finally, a ring of grains tempered in a subcritical temperature range. Spot welds were tested in tensile–shear mode. Both Overload Failure and fatigue toughness were evaluated. Annealing and galvanizing post-treatment have been used to improve the mechanical properties of the spot-welded specimens. The improvement of the mechanical performances of spot-welded specimens after post-treatments are reported as follows. First, we observed significant changes in the modes of rupture both in Overload Failure and fatigue tests. The mechanical strength of untreated spot welds is controlled by the brittleness of the nugget and the forged heat affected zone. After post-treatment, the ultimate tensile strength is increased, and rupture results from fissures propagating exclusively into the zone recrystallized in the subcritical range of temperature. This mode is genuine to spot welds in martensite because of the high strength of the base metal. Second, three modes of rupture depending on the force range control the fatigue toughness. Annealing and galvanizing modify the ranges. Galvanizing increases the endurance, and the fatigue limit by a factor ~3.5. This effect is attributed to zinc brazing that strongly changes the distribution of stresses under tensile–shear solicitation. Hardness measurements and tensile test on specific samples clarify the mechanical behavior of the layers around the spot, and shed some light on the mechanisms of rupture. Lifetimes are controlled by the initiation of fatigue cracks.
