Knowledge on Fatigue Strength of Welded Structures

Knowledge on Fatigue Strength of Welded Structures

Knowledge on Fatigue Strength of Welded Structures

1. Causes of Fatigue Failure in Welded Structures

The main causes of fatigue failure in welded structures include:

1. Inherent Mechanical Properties:

   • Welded joints generally have static load-bearing capacity comparable to the base metal. However, under cyclic dynamic loads, their capacity is significantly lower and highly dependent on joint type and structural configuration. This is a primary factor leading to premature failure due to fatigue in welded joints.

2. Design Limitations:

   • Early welded structures were primarily designed for static loads, often neglecting fatigue considerations. Incomplete fatigue design standards also led to poorly designed joints by modern standards.

3. Lack of Awareness:

   • Engineers often apply fatigue design criteria from other metal structures to welded joints due to insufficient understanding of their unique fatigue characteristics.

4. Cost and Weight Optimization:

   • The widespread use of welded structures has driven designs toward lower costs and lighter weights, increasing operational loads and exacerbating fatigue risks.

5. Technological Lag:

   • The trend toward high-speed, heavy-load applications demands higher dynamic load capacity, yet research on welded structure fatigue strength lags behind.

2. Main Factors Affecting Fatigue Strength of Welded Structures

2.1 Influence of Static Strength on Fatigue Strength

In steel materials, higher specific strength (strength-to-weight ratio) is desirable. High-strength steels exhibit increased fatigue strength with higher static strength.

However, for welded joints, fatigue strength is largely independent of base metal strength, weld metal strength, or heat-affected zone (HAZ) properties. Identical joint details in high-strength and mild steels show similar fatigue strength (S-N curves). This applies to butt joints, fillet joints, and welded beams.

Key Findings:

• Maddox’s study on C-Mn steels (yield strength: 386–636 MPa) showed minimal influence of mechanical properties on crack growth rates.

• Using high-strength steels for cyclic load designs is ineffective unless stress ratios exceed +0.5, where static strength dominates.

Root Cause:
Sharp slag inclusions (0.075–0.5 mm thick, tip radius <0.015 mm) at weld toes act as fatigue crack initiators. These defects render fatigue strength independent of base/weld metal strength.

2.2 Influence of Stress Concentration on Fatigue Strength

2.2.1 Joint Type Effects

Welded joints include:

• Butt Joints: Minimal stress concentration, highest fatigue strength. Performance varies with specimen size, groove type, welding method, etc.

• Cruciform/T-Joints: Higher stress concentration due to cross-sectional changes. Fatigue strength is lower than butt joints.

• Lap Joints: Severe stress concentration due to load path distortion. "Reinforced" cover plates are detrimental, reducing fatigue strength.

2.2.2 Weld Geometry Effects

• Transition Angle: Fatigue strength decreases with smaller angles (e.g., increased weld height at fixed width).

• Transition Radius: Larger radii improve fatigue strength.

• Fillet Weld Size: Fatigue failure shifts from weld to base metal at a/B>0.7a/B>0.7 (where aa = weld throat, BB = plate thickness).

2.2.3 Welding Defects

Defects reduce fatigue strength by up to 80%. Key types:

1. Cracks: Reduce fatigue strength by 55–65% (e.g., 25 mm × 5.2 mm crack in a 60 mm × 12.7 mm specimen).

2. Lack of Penetration: 25% reduction in fatigue strength at 10% cross-sectional loss.

3. Undercut: Depth (hh) is the critical parameter.

4. Porosity: Surface/subsurface pores are more detrimental than internal ones.

5. Slag Inclusions: More harmful than porosity.

Defect Severity Depends On:

• Surface vs. internal location.

• Orientation relative to loading.

• Residual stress field (tensile > compressive).

2.3 Influence of Welding Residual Stresses

Residual stresses (unique to welding) significantly impact fatigue strength. Studies show:

• Under alternating loads (r=−1r=−1), stress-relieved specimens exhibit higher fatigue strength (130 MPa vs. 75 MPa).

• Under pulsating loads (r=0r=0), residual stresses relax, minimizing their effect.

• At high rr (e.g., r=+0.3r=+0.3), residual stresses become negligible, and heat treatment may soften the material, reducing fatigue strength.

Key Insight:
Residual stresses cause actual stress cycles to oscillate from the yield point downward, regardless of nominal stress ratios. This underpins design codes using stress range instead of rr.

3. Methods to Improve Fatigue Strength of Welded Structures

Fatigue cracks typically initiate at weld toes or roots. Mitigation strategies include:

1. Reducing Defects: Especially surface-breaking defects.

2. Improving Weld Toe Geometry: Lower stress concentration.

3. Inducing Compressive Residual Stresses.

3.1 Weld Toe Geometry Improvement

1. TIG Dressing:

   • Smooths weld toe transitions, removes slag inclusions.

   • Increases fatigue strength by 58% (211 MPa at 2×1062×106 cycles).

2. Machining:

   • Eliminates stress risers; costly but effective for defect-free welds.

3. Grinding:

   • Less effective than machining but practical. Use high-speed (15, ⁣000–40, ⁣00015,000–40,000 rpm) tungsten-carbide grinders.

3.2 Compressive Residual Stress Methods

1. Overloading:

   • Yields tensile zones, creating compensatory compressive stresses.

2. Local Heating:

   • Generates compressive stresses at critical locations (e.g., longitudinal welds).

3. Peening/Hammering:

   • Cold-working induces surface compression. Ultrasonic peening improves fatigue strength by 50–170%.

3.3 Hybrid Methods

• Low-Temperature Transformation (LTT) Electrodes:

  • Leverage phase transformation (martensite) to create compressive stresses.

  • Fatigue strength improvements: 11–59% (depending on joint type).

  • LTT Dressing: Applies LTT electrodes only to weld toes, reducing cost.

4. Emerging Technologies

1. Ultrasonic Impact Treatment (UIT):

   • Portable, low-noise, and highly effective (50–170% fatigue strength improvement).

2. LTT Electrodes:

   • Ideal for hidden/critical welds but costly due to alloy content.

5. Conclusion

With welded structures trending toward high-speed, heavy-load applications, advanced fatigue improvement techniques (e.g., UIT, LTT electrodes) are critical. These methods address stress concentration and residual stresses, offering significant lifecycle extensions.

Contact:

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