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Effect Analysis of Arrangement, Sequence and Plate Thickness of Spot Welding on Residual Stress and Fatigue Life of The Joint

Minaei, Mohammad | 2019

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 51948 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Farrahi, Gholamhosein
  7. Abstract:
  8. There are about 5,000 spot welds in the car body. Simulation of this number of spot welds to optimize the body of the car is practically impossible. In researches done in order to model and optimize the spot weld, standard spot weld sample with a nugget have been always used. Electrical current in spot welding process turns to heat according to the Ohm ratio, and causes the melting of the pieces and bonding. So amount of the electrical resistance, especially at the contact surface of the two pieces, is very important.In order to investigate the residual stresses induced in the connection of a three-sheet spot weld sample and its effect on fatigue life, first, a spot weld sample with specified welding parameters was simulated and the distribution diagram of its radial residual stress was compared and validated with the articles. Furthermore, by using an experimental sample, the nugget diameter between the upper and lower plates for a three-sheet connection was compared with nuggets diameter of simulation with accuracy of 93 percent. Also, the fatigue life of the connection determined by fatigue test.In order to investigate the effects of each parameter, the taguchy test was used and by using the simulation results of taguchy test, equations with 90 percent accuracy according to simulation results was derived for estimating the maximum radial residual stress, nugget diameter of upper and lower plates and electrode penetration depth. Also, the fatigue life of one taguchy sample was compared with its experimental sample and found that with increase in tensile radial residual stress, the high cycle fatigue life decreased while in samples with low difference in their residual stress, low difference in their low cycle fatigue life was seen.Resistance spot welding on plates with different thicknesses was simulated. The results shown that the effect of thickness on nugget diameter and radial residual stress in range of used parametters is negligible. In order to investigate the effect of sequence and arrangement of spot welds on residual stress, spot welding process on three-sheet connections with two, and three nuggets with different sequences was simulated and the best sequence for welding of three-sheet connection with three nuggets was specified. Furthermore, in connections with two and three nuggets, the shunt effect was investigated with two methods and the advantages of electrode displacement curve method than electrical current density measurement method was shown. Also, determined that the effect of deformation of plates on decreasing of shunt effect is more than effect of increasing of shunt effect from multiple nuggets, in long distance nugget situations
  9. Keywords:
  10. Resistance Spot Welding ; Nugget Simulation ; Shunt ; Nugget Multiplicity ; Residual Stress ; Fatigue Life

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