Allowance for the crack-tip constraint in the design against ductile failure

Project Managers

Dr. Geralf Hütter, Prof. Björn Kiefer, Ph.D.

Project Scientist

Dr.-Ing. Omar El’Khatib

Project Partner

Fraunhofer Institute for Mechanics of Materials (IWM), Freiburg

Motivation

Damage-tolerant component design ensures that any potentially present crack-like defect does not lead to the failure of a component. In many cases, such defects cannot be ruled out for manufacturing reasons or arise during operation due to material fatigue, creep or corrosion. In the case of safety-relevant components, a fracture-mechanics assessment assuming postulated cracks is therefore prescribed for approval or continued operation. In other areas, a damage-tolerant design in combination with appropriately coordinated maintenance intervals allows a more efficient use of materials and lighter constructions. In such a fracture mechanics assessment, the loading near the tip of a crack in the component is quantified. Crack initiation or component failure are excluded if the near-tip loading in the component is below a material-specific fracture toughness.

The fracture toughness is determined according to applicable codes on standard specimens, which exhibit a high strain constraint in the region of the crack tip and thus a high local stress multiaxiality. Since the fracture toughness increases with decreasing constraint, the fracture toughness determined on standard specimens with high constraint forms a lower limit, so that a design based on it may be too conservative. As a result, too small permissible defect sizes are determined in a component, its remaining service life is underestimated, maintenance intervals are set too short or the component is unnecessarily overdimensioned.

Aim

By developing a validated assessment method to take account of the constraint in the damage-tolerant design of components made of ductile materials and implementing it in relevant codes, it is possible to reduce the amount of material used and optimize service life and maintenance intervals without risk to the safety of components.

Funding

German Federal Ministry for Economic Affairs and Energy