Agile Manufacturing
Engineering Analysis
IPVD Coatings
Manufacturing Process
Design
Materials Processing
Mechanical Systems
Development
Micro Fabrication
and Testing
NDE Technologies
Robotics
Structural Integrity

D&ME Home

Fracture Analysis

Researchers at PNNL evaluate the effects of crack-like flaws on the integrity of engineering structures and components. Examples include pressure vessels, piping systems, storage tanks, heat exchangers, and aerospace structures. These evaluations have applied the science of engineering fracture mechanics to predict critical combinations of stresses, material fracture toughness, and flaw sizes which can lead to structural failures. Such evaluations provide a basis for determining

• sizes of flaws which must be detected during nondestructive examinations of components
• needs to replace or repair structures and components which are found to have flaws present.
• remaining years of operating life of degraded components.

Calculations are based on PNNL's extensive knowledge and experience in fields of structural mechanics, materials science, and the reliability of nondestructive examination technology. The following describes capabilities and experience which exist at PNNL.

Numerical Solutions - Finite element computer codes have been applied to calculate crack-tip stresss intensity factors for flaws in geometrically complex structures such as storage tanks, pressure vessels, and armor piercing projectiles. These calculations have included the effects of plastic deformation and dynamic loads. In other cases closed form and tabulated handbook solutions have been applied in an innovative manner to predict the effects of defects on structural integrity.

Elastic-Plastic Fracture Mechanics - The effects of extreme loads and large cracks have been predicted by including the nonlinear effects of plastic deformation. Numerical solutions have been generated by application of finite element codes and by making use of published data from the literature. The fracture assessment diagram approach (R6-methodology) has been used to advantage to evaluate the effect of cracks in cast stainless steel piping and in high-level waste storage tanks.

Reactor Pressure Vessels - PNNL has developed probabilistic fracture mechanics codes which predict the likelihood of catastrophic brittle fracture of reactor pressure vessels due to severe pressurized thermal shock transients. Fracture mechanics models have been enhanced to simulate the effects of underclad cracks, residual stresses, random variations in material properties, and random variations of flaw lengths. Staff at the U.S. Nuclear Regulatory Commission have adopted the VISA-II code as a tool to resolve safety issues associated with embrittlement of reactor vessels at operating nuclear power plants.

Inservice Inspection of Piping - Results from PNNL probabilistic fracture mechanics calculations have been used to estimate failure probabilities for high energy piping at nuclear power plants. These results have identified the risk-significant piping which should be given priority for inservice inspections. Other results have identified improved strategies for selecting the most cost-effective inspection methods and inspection frequencies.

Inservice Inspection of Piping Flaw Evaluations by ASME Section XI Procedures - PNNL experts have evaluated the acceptability of flaws in accordance with the flaw acceptance standards of Section XI of the ASME Boiler and Pressure Vessel Code. Staff are active on ASME committees responsible for developing and maintaining the Section XI code and have frequently supported NRC staff in reviewing industry evaluations of fabrication and service related flaws at operating nuclear power plants.

Flaw Size Distributions - PNNL has developed data and models for estimating the number and sizes of flaws in multi-pass welds in thick-wall pressure vessels and in piping girth welds. These estimates have provided essential inputs to fracture mechanics calculations. An expert system methodology permits the effects of the welding and inspection processes to be simulated in order to develop weld specific estimates for the number and sizes of flaws. The models have been validated using published data and using data from PNNL research programs. The PNNL data have resulted in detailed examinations of welds using both nondestructive ultrasonic methods and destructive metallographic methods.

For more information about PNNL Fracture Mechanics capability, please contact Fred Simonen (509) 375-2087 or Moe Khaleel (509) 375-2438

Structural Reliability And Risk Assessment

Researchers at PNNL evaluate the reliability of engineering structures and components by using computational models and by applying databases on operating experience. These evaluations have applied the science of engineering fracture mechanics to predict critical combinations of stresses, material fracture toughness, and flaw sizes, which can lead to structural failures. Risk-based studies have combined results from such evaluations with insights gained by considering the significance of the components to the safety of the structure or industrial facility. The following describes capabilities and experience which exist at PNNL.

Probabilistic Fracture Mechanics - PNNL has developed and applied a number of computer codes which predict the reliability of structures using the methodology of probabilistic fracture mechanics. The pc-PRAISE code has been applied to predict piping failures caused by the mechanisms of fatigue, stress corrosion cracking, and fracture by ductile tearing. These evaluations have included failures due to preexisting defects in welds and failures due to the initiation of cracks by fatigue and stress corrosion cracking caused by service related stresses. Many of the model development efforts and calculations at PNNL have addressed the ability of inservice inspections to reduce structural failure probabilities.

Risk-Informed Inservice Inspection of Piping - In work funded by the U.S. Nuclear Regulatory Commission, PNNL has developed the technical basis for improving inservice inspection programs for piping systems at operating nuclear power plants. The methodology focuses inspections on those components, which are most likely to fail, and whose failures are most likely to have severe safety and/or economic consequences. Failure probabilities are estimated by application of probabilistic fracture mechanics codes and by reference to statistics from databases on failures experienced at operating plants. Failure consequences are estimated from the probabilistic risk assessments (PRAs). PNNL staff have been instrumental in the drafting of changes to regulatory guides and of the ASME Section XI code which are needed to implement the new risk-informed approaches.

Failure Probabilities of Reactor Pressure Vessels - Research performed since 1982 at PNNL for the U.S. Nuclear Regulatory Commission has made major contributions to improved methods for calculating failure probabilities for reactor pressure vessels. The probabilistic fracture mechanics code VISA-II, developed and maintained by PNNL staff, is regularly used by NRC staff to evaluate the structural integrity of embrittled vessels at aging nuclear power plants. Calculations have addressed the effects of welding flaws and inservice inspections on vessel reliability.

Flaw Size Distributions - PNNL has developed data and models for estimating the number and sizes of flaws in multi-pass welds in thick-wall pressure vessels and in piping girth welds. These estimates have provided essential inputs to fracture mechanics calculations. An expert system methodology (RR-PRODIGAL) permits the effects of the welding and inspection processes to be simulated in order to develop weld specific estimates for the number and sizes of flaws. The models have been validated using published data and using data from PNNL research programs. The PNNL data have resulted in detailed examinations of welds using both nondestructive ultrasonic methods and destructive metallographic methods.

Probabilistic Finite Element Analysis - PNNL uses in-house computer codes to address uncertainties in loads and variability of material properties. The stochastic methods provide an alternative to the traditional engineering approach of using arbitrary safety factors to ensure desired levels of reliability and safety. Available software makes use of state-of-the-art techniques in stochastic mechanics including FORM/SORM, Monte-Carlo simulations, and adaptive importance sampling. For aging structures, stochastic mechanics provides the means to quantify the safety of degraded structures. For new designs, stochastic mechanics provides the means to explicitly treat all uncertainties in a consistent manner in order to achieve optimum designs. The safety factors of traditional deterministic approaches do not provide a means to quantify expected structural reliability and can sometimes lead to unbalanced designs wherein some components are over-designed and some may be under-designed.

For more information about PNNL Structural Reliability And Risk Assessment capability, please contact Fred Simonen (509) 375-2087 or Moe Khaleel (509) 375-2438

Present and Past Structural Integrity Activities

Selected Points of Contact