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Spot Weld Mechanical Properties 4
Mechanical Properties 4
Lesson ObjectivesWhen 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 Activities1. View Slides; 2. Read Notes, 3. Listen to lecture4. Do on-line
workbook
KeywordsFatigue Test, Multiple Spot Fatigue Test
Mechanical Properties of Spot Welds
• Peel Tests• Chisel Test• Tensile Test• Tensile Shear Ratios• Torsion Test• Impact Tests• Fatigue Test
Effect of Specimen Geometry
Stress State atEdge of Notch (Weld)Must have LargeEffect
Sawhill “Spot Weldability of High-Strength Sheet Steels”Welding Journal, Jan 1980
Sample bending During testingIs important
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 ToReduce Bending
A More Complex Fatigue Test SpecimenWithout Sample Bending
Overbeeke “Fatigue Characteristics..”Metal Construction & British Welding Journal, July, 1974
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
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 SteelsASM, 1978
• 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
Effect of Sheet Thickness Box Sample No Bending
Higher Strength
Thickness
Wilson “Fatigue Behavior of Spot Welded..”SAE Paper 810354, Feb, 1981
Effect of Specimen Width Single Spot Bending
Narrower MaterialMore BendingLower Fatigue Strength
Orts “Fatigue Strength of Spot Weld…”SAE Paper 810355, Feb 1981
Effect of Nugget Size on Fatigue Strength
When spot size exceeds some critical size there is no effectNugget Pull out
Overbeeke “Fatigue Characteristics of Heavy-Duty…”Metal Construction and British Welding Journal, July 1974
InterfaceFailure
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
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
Effect of Multiple Spots
Orts “Fatigue Strength of Spot …”SAE Paper 810355 Feb, 1981
Strength never multiplied By number of welds.Direct Tension had ~ 1/3Strength of tensile shear
Effect of Base Metal Strength
Increase BM Strength
Effect of BM Structure
Chandel “Mechanical Aspects of Spot-Welded..”Metals Technology Jan 1977
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
Effect of Coating - Aluminized
Orts “Aluminized Steel, Resistance Spot Welding”Armco Research Reprot Aug 24, 1978
Effect of Post Weld Heat Treatment
Balasubramanian “Improvement of Resistance Spot Weld..”Welding Research Abroad Nov 1978
Removal of Residual Stresses
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
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
Effect of Load Ratio – Testing Procedure
Overbeeke “Fatigue of Spot Welded Lap Joints”Metal Construction May 1976
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
Full Nugget Pull-out as Plastic Zone surrounds nugget at Higher Tensile Loads
Kan “Fatigue Resistance of Spot Welds”Metals Engineering Quarterly Nov 1976