Vessels and components can be designed using elastic-plastic analysis methods as per ASME BPVC code. To protect vessels and components against local fracture, determination of fracture initiation strain or triaxial strain limits to prevent through-wall fracture formation is crucial for the elastic-plastic analyses. Paragraph 5.3.3 of ASME BPVC Section VIII-2 and Paragraphs FF-1141 and FF-1142 of ASME BPVC Section III Appendices provide very different strain limits including the effect of stress triaxiality.

To evaluate the accuracy of the strain limits, a literature survey was performed. Prediction models for ductile fracture initiation strain are discussed and summarized based on their capabilities and fundamentals to incorporate effects of the stress traixiality and the Lode angle. The Lode angle has recently been recognized as an important parameter to reflect effect of complex stress states. Due to the wide applicability and flexibility, the ductile fracture strain initiation model developed based on the Unified Strength Theory is selected as a reference model for further development in this paper. By simplification of the reference model, a simple and practical fracture initiation strain model is proposed, which is specifically applicable to different regions of vessels and components. This model can be applied to locations at a gross or local structural discontinuity and locations away from a gross or local structural discontinuity.

The proposed fracture strain initiation model contains four material constants: 1) the uniaxial tensile fracture initiation strain; 2) the ratio of the uniaxial tensile strength to the uniaxial compressive strength prior to the onset of fracture; 3) the influence parameter of the intermediate principal stress; and 4) the strain hardening exponent prior to the onset of fracture. These four material constants have clear physical meaning and can be determined by experiments of the uniaxial tension and compression as well as pure shear. Parametric studies are performed to demonstrate the behaviors of the material constants.

Based on available experimental results of three types of steel alloys: low carbon steel, medium carbon steel, and stainless steel, three out of four material constants are calibrated with the only uniaxial tensile fracture initiation strain being remained for determination depending on the materials used. The calibrations are performed for different stress states including uniaxial tension, uniaxial compression, pure shear, shear-tension, pure torsion, torsion-tension, and biaxial compression. The proposed strain limits are compared with that specified in ASME BPVC code. The ASME BPVC Section VIII-2 strain limit may not be conservative overall. The ASME BPVC Section III strain limit may not be conservative in some loading cases, but may be drastically conservative for other loading cases.

The proposed strain limit against local fracture is in simple and practical form. They can be easily implemented in typical elastic-plastic analyses of vessels and components.


Strain Limits Against Local Fracture of Vessels and Components

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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