Miniature CoCr laser welds under cyclic shear: Fatigue evolution and crack growth

Area of Science:

• Materials Science
• Biomedical Engineering
• Mechanical Engineering

Background:

• Miniature laser welds (50-300 μm root depth) are critical for air-tight joints in medical devices like implants and stents.
• Assessing the fatigue life of these small welds, particularly under shear loading, is essential for device reliability.
• Cobalt-chromium (CoCr) alloy welds are widely used in medical applications, necessitating robust fatigue testing methods.

Purpose of the Study:

• To develop and validate a fatigue test specimen and procedure for shear-loaded miniature laser welds.
• To compare the performance of novel tensile-loaded specimens against traditional torque-loaded designs.
• To characterize the fatigue behavior, damage mechanisms, and determine the fatigue limit of CoCr alloy laser welds.

Main Methods:

• Development of two novel tensile-loaded fatigue test specimen designs.
• Application of X-ray analysis, atomic force microscopy (AFM), and scanning electron microscopy (SEM) for damage and fracture surface analysis.
• Finite element simulations using a non-linear material model for CoCr alloy to analyze damage and deformation.
• Fatigue testing to generate S-N curves and determine fatigue lives.

Main Results:

• The new tensile-loaded specimens demonstrated reduced variation in fatigue life determination compared to torque-loaded specimens.
• A well-defined crack onset point in the new specimens led to precise weld seam loading.
• Fatigue damage was observed to concentrate within the weld material, affecting the entire weld before final fracture.
• A regression fatigue limit of 10.8-11.8 MPa was determined for hardened CoCr alloy welds under shear loading.

Conclusions:

• Novel tensile-loaded specimens offer improved reliability and precision for fatigue testing of miniature laser welds.
• The developed methodology accurately captures the fatigue behavior and damage mechanisms in CoCr alloy welds.
• The findings provide crucial data for the design and safety assessment of medical devices utilizing these critical components.