1. Spot Weld Mechanical Properties 4

2. Mechanical Properties 4 Lesson Objectives When you finish this lesson you will understand: Fatigue Mechanical testing techniques Factors which effect performance in fatigue tests The importance of these test for part design Learning Activities 1.View Slides; 2.Read Notes, 3.Listen to lecture 4.Do on-line workbook Keywords Fatigue Test, Multiple Spot Fatigue Test

3. Mechanical Properties of Spot Welds Peel Tests Chisel Test Tensile Test Tensile Shear Ratios Torsion Test Impact Tests Fatigue Test

5. Effect of Specimen Geometry Stress State at Edge of Notch (Weld) Must have Large Effect Sawhill “Spot Weldability of High-Strength Sheet Steels” Welding Journal, Jan 1980 Sample bending During testing Is important

6. Overbeeke “Fatigue Characteristics..” Metal Construction & British Welding Journal, July, 1974 The fatigue strength of spot approaches that of the sheet when bending reduced (see the two sheet failures on this graph) Sample To Reduce Bending

7. A More Complex Fatigue Test Specimen Without Sample Bending Overbeeke “Fatigue Characteristics..” Metal Construction & British Welding Journal, July, 1974

8. When Sample Bending is not present: Fracture initiates in HAZ Fracture Propagates around the nugget in HAZ Fracture then goes through BM Until full nugget pulls out

9. Effect of Sheet Thickness Tensile Shear Double Spot Transverse Allows Bending 2 & 2.5 mm 1.5 mm Capelli “Fatigue Strength of Spot-Welded…” Welding of HSLA (Microalloyed) Structural Steels ASM, 1978

10. Fatigue cracks have been observed as early as 1% of Fatigue Life Life determined by crack growth rate dl/dn In Thicker Materials Crack has to Propagate Further Therefore, longer fatigue life

11. Effect of Sheet Thickness Box Sample No Bending Higher Strength Thickness Wilson “Fatigue Behavior of Spot Welded..” SAE Paper 810354, Feb, 1981

12. Effect of Specimen Width Single Spot Bending Narrower Material More Bending Lower Fatigue Strength Orts “Fatigue Strength of Spot Weld…” SAE Paper 810355, Feb 1981

13. Effect of Nugget Size on Fatigue Strength When spot size exceeds some critical size there is no effect Nugget Pull out Overbeeke “Fatigue Characteristics of Heavy-Duty…” Metal Construction and British Welding Journal, July 1974 Interface Failure

14. Effect of Expulsion on Fatigue Life None at low cycle Expulsion may reduce fatigue limit at high cycles No Expulsion With Expulsion Johnson “Automatic Spot Weld Correction” Metal Construction Feb, 1977

15. Effect of Double Spot (Longitudinal) Overbeeke “Fatigue Characteristics of Heavy-Duty…” Metal Construction and British Welding Journal, July 1974 Double Spot not twice the strength, Maybe only the first and last weld in a row share the bulk of the load

16. Effect of Multiple Spots Orts “Fatigue Strength of Spot …” SAE Paper 810355 Feb, 1981 Strength never multiplied By number of welds. Direct Tension had ~ 1/3 Strength of tensile shear

17. Effect of Base Metal Strength Increase BM Strength

18. Effect of BM Structure Chandel “Mechanical Aspects of Spot-Welded..” Metals Technology Jan 1977

19. Effect of Coating - Galvanized Probably related to the cozating reducing the notch effect at the faying surface Freytag “A Comprehensive study of Spot Welding Galvanized” Welding Journal April 1965

20. Effect of Coating - Aluminized Orts “Aluminized Steel, Resistance Spot Welding” Armco Research Reprot Aug 24, 1978

21. Effect of Post Weld Heat Treatment Balasubramanian “Improvement of Resistance Spot Weld..” Welding Research Abroad Nov 1978 Removal of Residual Stresses

22. Effect of Post Weld Compression Treatment Choquet “Improvement of the Fatigue Life of Spotwelds” Welding Research Council Bulletin 112 Feb 1966 Replacement of Compressive Stresses

23. Effect of Oil Coating Slight Improvement attributed to prevention of atmospheric corrosion Overbeeke “Fatigue Characteristics of Heavy-Duty…” Metal Construction and British Welding Journal, July 1974

24. Effect of Load Ratio – Testing Procedure Overbeeke “Fatigue of Spot Welded Lap Joints” Metal Construction May 1976

25. Small Plastic Zone – Initiates in this region and propgates out into BM – Sample Width May Help Low Applied Stress Slightly Higher Stress Plastic Zone Spreads – Promotes Nugget pullout - Kan “Fatigue Resistance of Spot Welds” Metals Engineering Quarterly Nov 1976

26. Full Nugget Pull-out as Plastic Zone surrounds nugget at Higher Tensile Loads Kan “Fatigue Resistance of Spot Welds” Metals Engineering Quarterly Nov 1976