研究目的
To evaluate the high cycle fatigue performance of circular shaped laser spot welds in dual phase DP780 steels and understand the failure mechanism under fatigue loading.
研究成果
1. Weld size optimization is crucial for fatigue life, with lower laser power small welds performing marginally better than higher laser power large size welds.
2. The stress intensity factor at the overlapping sheet notch tip is determined by the shear stress and bending stress, causing crack initiation. The effective axial load directs the crack direction towards the sheet thickness.
3. The crack speed observed at the initial stage is slow but increases manifolds with the number of fatigue cycles.
4. An enhanced crack path due to the countering effect of bending stress and shear stress against axial stress during lap-shear loading consumes more cycles in small welds, resulting in a marginally longer life.
5. The stress condition has a major influence on the mode of failure, particularly in small welds, with a transition from interfacial to partial to pull-out failure as fatigue load is reduced. Microstructure has a limited role in fatigue crack propagation.
6. Large welds, irrespective of loading magnitude, fail in pull-out mode.
研究不足
The study focuses on high cycle fatigue performance and does not extensively cover low cycle fatigue behavior. The influence of weld microstructure on fatigue crack propagation is noted to be less significant compared to size effects.
1：Experimental Design and Method Selection:
The study involved evaluating the fatigue performance of laser spot welds in DP780 steel using two different laser parameter sets. Analytical stress models and experimental data were used to explain the failure mechanism under fatigue loading.
2：Sample Selection and Data Sources:
Commercially produced DP780 steel grade in the form of cold rolled and continuous annealed strips of 1 mm thickness was used. The chemical composition was determined using a LECO apparatus and IPS-OES techniques.
3：List of Experimental Equipment and Materials:
Equipment included an IPG make fibre laser system integrated with a 6 axis KUKA robot, Instron 5582 system for uniaxial shear tensile tests, and an Instron 8801 servo-hydraulic system for cyclic loading conditions. Materials included DP780 steel and pure (99.99%) Argon gas for shielding.
4：99%) Argon gas for shielding.
Experimental Procedures and Operational Workflow:
4. Experimental Procedures and Operational Workflow: The test coupons were cut and welded in lap joint configuration. Fatigue tests were conducted under load control mode with a stress ratio of 0.1, sinusoidal waveform, and a frequency of 15 Hz. Interrupted fatigue tests were performed to investigate crack initiation and propagation.
5：1, sinusoidal waveform, and a frequency of 15 Hz. Interrupted fatigue tests were performed to investigate crack initiation and propagation.
Data Analysis Methods:
5. Data Analysis Methods: The crack path was analyzed using a Leica optical microscope and a Quanta FEG 650 Scanning Electron Microscope. Stress intensity factors were evaluated to understand crack initiation and propagation.
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