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