Prolonged degradation phenomena, unusual loading scenarios, and environmental conditions may raise the concern for the brittle failure of otherwise ductile material in piping and pressure vessels. Years of unattended corrosion processes may eventually lead to through-wall pinhole formation and the consequent development of crack-like flaws; similar flaws can also result from accidental impacts experienced during the installation process and maintenance operations, flaw-induced vibrations pushing the material beyond its fatigue limit, lack of fusion and lack of penetration in welds, strong groove-like localized corrosion, and freezing temperatures experienced during repair and replacement operations facilitated by the deliberate formation of ice-plugs.
Crack-like flaws are especially insidious compared to other planar flaws in that they are predominantly characterized by length and depth and a sharp root radius. Typically, the radius at a crack’s root is so tiny to be unquantifiable and virtually nil, leading to theoretically infinite stress concentration. To deal with these challenges, the tools of both linear and nonlinear fracture mechanics are expertly deployed by our NSI analysts when dealing with surface breaking, embedded, through-wall, and branch-type cracks. When micro-cracks are suspected at the root of volumetric flaws––such as aligned porosity, inclusions, and overlaps––they may be analyzed via the same techniques adopted in assessing planar crack-like flaws.
Our team of experts is well versed in dealing with cracks engendered by a wide range of deficiencies occurring during design and manufacturing. Specific processes that may be associated with crack initiation include
- Material production
- Fabrication
- Welding (e.g., lack of penetration, lack of fusion, delayed hydrogen cracking, weld cracking)
- Heat treatment (leading to embrittlement)
- Construction material selection: Cracks may result from either poor design practices or poor choice of a substitute alloy and heat treatment, which may cause lower-than-expected resistance and inadequate material properties in the installed component
The typology of flaws originating from design and manufacturing deficiencies is varied and unique. Each typology is affected by unique interactions between material properties, industrial processes, and environmental/design conditions. Recurrent typologies induced by service/operating conditions include
- Surface connected cracking, ascribed to mechanical and thermal fatigue and several forms of stress corrosion cracking (SSC)
- Subsurface cracking and microfissuring/microvoid formation, generally ascribed to low-temperature hydrogen-related phenomena or high-temperature mechanisms such as creep and hydrogen attack
- Metallurgic changes leading to embrittlement
Multi-level assessment strategies are pursued in accordance with API 579-1 guidelines, which provides stakeholders with a greater understanding of the degradation mechanism and its driving factors. Three levels of assessment are considered, based on the limitations of each and the complexity of the problem at hand:
- Level 1 analysis is limited to crack-like flaws in pressurized cylinders, spheres, or flat plates away from structural discontinuities
- Level 2 analysis may be used for general shell structures suffering from crack-like flaws located at structural discontinuities, provided that the component being analyzed is not operating within its creep range, dynamic load effects are not significant, and service-induced crack growth is not expected
- Level 3 analysis is the most comprehensive and applicable to any geometry and design condition.
In the case of cylindrical pressure vessels, tanks, and piping, a Level 2 assessment of critical crack length is typically conducted. It requires the determination of input primary and secondary stresses, both in the circumferential and longitudinal directions. This task is routinely accomplished via software either specialized for piping stress analysis or attuned to general-purpose finite element analysis (FEA):
- Piping: Caesar II
- Piping: PIPESTRESS (PepS)
- Piping: AutoPIPE
- Piping: STANPIPES
- FEA: ANSYS Workbench
- FEA: LS-DYNA
- FEA: ABAQUS
- FEA: NASTRAN
Input primary and secondary stresses from dedicated software and material properties such as yield strength, tensile strength, and fracture toughness are used to determine stress intensity factors attributed to postulated cracks developing in the circumferential and longitudinal directions at the inner and outer surfaces. A plasticity interaction factor accounts for the plastic behavior of the material. Fracture toughness is either based on direct testing or estimated from the Rolfe-Novak-Barson correlation with the impact energy measured via the Charpy V-notch test.
Critical crack length values are established, via subsequent iterations, as the greatest values that would cause incipient failure, as predicted by the failure assessment diagram provided in API 579-1. Crack growth by fatigue is estimated based on the critical crack length and a suitable model relating crack growth rate and several cycles to failure. Fatigue models of frequent use include
- Paris model
- Walker model
- Bilinear and trilinear models
- NASGRO model
- Collipriest model
Numerical integration of the equation associated with the selected fatigue model provides the minimum number of cycles to failure. The failure is defined as unbounded crack growth leading to catastrophic collapse.
Analysis tools developed within NSI to facilitate and expedite fracture propagation and fatigue analysis include a comprehensive suite of specialized worksheets and sub-routines, implementing API 579-1 equation and assessment criteria. Our toolkit includes
- Mathcad worksheets
- MAPLE worksheets
- Matlab scripts
- Python scripts
- APDL scripts
- VBA scripts