ME-8136ME-8136 Advanced Fatigue Fracture Advanced Fatigue Fracture

AnalysisAnalysisTitle

““Improvement of fatigue Improvement of fatigue resistance of spot welds used in resistance of spot welds used in

automotive applications automotive applications””

--- Gajendra Tawade--- Gajendra Tawade

APPLICATIONS OF SPOT APPLICATIONS OF SPOT WELDING.WELDING.

Automobile outer body complete. (spot welded).

Framing station. Car wheel.

IMPORTANCE OF SPOT WELD IMPORTANCE OF SPOT WELD CONTROL.CONTROL.

Over 7,000 resistance welds of all types are used somewhere during manufacturing and assembly of one vehicle.

Close monitoring over these 7000 critically controlled spot welds Enhance overall life and stability of automobile structure.

Spot Welding Principle.Spot Welding Principle.

WELDING CYCLE: Squeeze. Weld. Forge/Hold. Off.

PROCESS VARIABLES : Effective current [KA] Electrode force [lbf] Weld time [cycles] Hold time [Cycles]

Quality control of spot Quality control of spot welds.welds.

Peel test Tensile shear test Chisel test Fatigue test

Tensile-shear test specimen

spot weld-Fatigue test.spot weld-Fatigue test.

Cross-Tension fatigue Test

Tensile Shear fatigue Test

Fatigue life tested at A) Constant amplitudeB) Variable amplitude

(Load-histories)

Constant amplitude Vs Constant amplitude Vs variable amplitude fatigue testvariable amplitude fatigue test

-30

-20

-10

0

10

20

30

Time

Str

ain

1

Variable load history from simulated service conditions in an automobile donated by FORD MOTOR CO.

Constant amplitude

Testing. (Hardly find in any practical case).

Current study covers both aspects. (CA and VA)

OBJECTIVEOBJECTIVE

Improve fatigue resistance of spot welds in Low ‘C ‘and HSLA steel with,

1. Control of residual stresses. Induce compressive residual stresses. Reduce tensile residual stresses (due to welding).

2. Increase base metal strength. 3. Improve surface condition of

steel sheets used. Check Stability of results

under variable amplitude loading.

Control of residual Control of residual stresses.stresses.

• Tensile preloading Tensile preloading (75% of T.S.) (75% of T.S.)

• Coining.*Coining.*• Stress relief Stress relief

annealing annealing (Furnace)(Furnace)

• Extra Pulse of Extra Pulse of forging force by forging force by electrodeselectrodes.* Loading weld before fatigue test

Base metal strength Base metal strength && fatigue fatigue life.life.

• Base metal strength of sheet steel has negligible influence on the fatigue life of spot welded joints.

AISI CODE Y.S. (MPa)

AISI 1006 (G) 30.7

AISI 060 XK(G) 62.5

AISI 050 XF (G) 49.8

AISI 050XF 51.9

Coating (Galvanizing) Coating (Galvanizing) && fatigue Life.fatigue Life.

• CA Fatigue test result for AISI 050XF-(G)• CA Fatigue test result for ASIS 050XF-Bare• NO deviation from straight line.Straight line indicates fatigue life calculated by analytical methods

(B) (G)

Pre-stressing Pre-stressing & & Fatigue Fatigue life.life.

The effect of tensile

pre-stressing was pronounced in HSLA

At shorter lives (<104 cycles), pre-stressing does not cause in great improvement

Pre-stressing is most potential to dominate residual stresses.

Pre-stressedPre-stressed

AISI 1006 (G)

AISI 050XF (G)

AISI 060XK (G)

HSLA

Forging force Forging force && fatigue life. fatigue life.• Fatigue strength

improves to a lesser extent when weld forge is applied at some time after welding. (cool forge)

• Fatigue strength of HSLA improves by 1.4 times when forge is applied immediately after weld cycle.

(Weld forge)

Cool-forge

Weld-forge

HSLA

HSLA

Post weld annealing Post weld annealing && fatigue life.fatigue life.

• Treatment involves furnace annealing test pieces after welding.

• Annealing Temp. 4000C and 6000C.

• Substantial increase in fatigue resistance only for annealing temperature above 6000C.

Stability of improvements Stability of improvements under variable loading.under variable loading.

• Improvements in fatigue life resulting from Pre-loading and coining proved to be nearly stable under full FORD histories

Preloaded-VA

As-welded-CA

Coined-VA

As-welded-CA

Full ford History.

Conclusion.

• Proved that fatigue resistance of tensile shear spot welds particularly HSLA could be greatly improved by treatments which either reduce tensile residual stresses resulting form welding or induced compressive residual stresses at the crack. Such treatments are:

A) Pre-stressing & B) Coining.• Increasing base metal strength does not

greatly alter the fatigue resistance of as welded tensile shear spot welds presumably because such increase are off set by higher residual stresses in the weld.

• The state of sheet steel surface had little if any influence on the test results.