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Spot Weld Mechanical Properties 4

Spot Weld Mechanical Properties 4. Mechanical Properties 4. Learning Activities View Slides; Read Notes, Listen to lecture Do on-line workbook. Lesson Objectives When you finish this lesson you will understand: Fatigue Mechanical testing techniques

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Spot Weld Mechanical Properties 4

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  1. Spot Weld Mechanical Properties 4

  2. Mechanical Properties 4 • Learning Activities • View Slides; • Read Notes, • Listen to lecture • Do on-line workbook • 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 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

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

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

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

  7. 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

  8. 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

  9. 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

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

  11. 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

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

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

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

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

  16. Effect of Base Metal Strength Increase BM Strength

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

  18. 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

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

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

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

  22. 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

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

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

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

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