Location, location &size: defects close to surfaces dominate fatigue crack initiation

Area of Science:

• Materials Science
• Mechanical Engineering
• Metallurgy

Background:

• Metallic cast components often contain internal defects, such as shrinkage cavities, impacting fatigue life.
• Quantitative data on the relative danger of internal versus surface casting defects is limited.
• Current industry standards may be overly conservative regarding internal defect tolerance.

Purpose of the Study:

• To quantitatively assess the danger posed by internal casting defects compared to surface defects.
• To investigate the initiation and propagation of fatigue cracks from internal shrinkage pores.
• To challenge existing standards for defect tolerance in metallic castings.

Main Methods:

• Fatigue testing of specimens with controlled internal shrinkage pores.
• In situ monitoring using X-ray synchrotron tomography to observe crack nucleation and propagation.
• Comparative analysis of crack initiation from internal versus surface defects of varying sizes.

Main Results:

• Fatigue crack initiation and propagation from internal defects were consistently outpaced by surface cracks.
• Surface cracks initiated at defects up to ten times smaller than internal defects.
• Internal defects demonstrated lower criticality than anticipated under fatigue loading.

Conclusions:

• Internal casting defects, specifically shrinkage pores, are less critical to fatigue life than surface defects.
• The findings suggest that current standards for internal defect tolerance in cast components may be excessively stringent.
• This research provides a quantitative basis for re-evaluating defect acceptance criteria in metallic castings.