Accounting for Constraint in Probabilistic Fracture Mechanics Analysis of Welds in Floating Production, Storage and Off-Loading (FPSO) Vessels

2003 ◽  
Author(s):  
Jens P. Tronskar
Keyword(s):  
Residual Stress ◽  
Stress Distribution ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
North Sea ◽  
Failure Probability ◽  
Crack Tip ◽  
Probabilistic Fracture Mechanics ◽  
Crack Tip Constraint

Revision 4 of the British Energy R6 document: “Assessment of the integrity of structures containing defects” provides methods to allow for loss of crack tip constraint for shallow weld flaws. The document also provides methods to estimate upper-bound values of the through thickness residual stress distribution for a range of common weld joint configurations. The present paper presents results of analyses where approaches to modify the R6 Option 1 or 2 failure assessment diagrams (FADs) for loss of crack tip constraint pertaining to primary and non-uniform residual stress have been applied. The modified FAD were formulated for probabilistic fracture mechanics analyses of semi-elliptical surface cracks located at transverse deck welds of Floating Production, Storage and Off-loading (FPSO) vessels designed to operate in the North Sea. The objective was to study the influence on the failure probability of modifying the FAD for constraint and allowing for non-uniform residual stress. Another objective was to study the influence of constraint correction on the combined fatigue and fracture failure probability for the vessels subjected to wave loading. Material and weld tensile properties and fracture toughness distributions for input to the probabilistic fracture mechanics analyses were obtained from testing of welded panels prepared using welding procedures for actual FPSO fabrication. The loading conditions were derived based on North Sea wave data pertaining to the offshore field where the FPSO is operating. The stresses were obtained from global FE analysis and fitted Weibull long-term and extreme value distributions. The results of the analyses demonstrate clearly the importance of correcting for crack tip constraint pertaining to both primary and secondary stress and to allow for non-uniform residual stress for shallow surface flaws of known crack heights. However, in combination with fatigue crack growth the effects become less prominent as the failure probability is governed by the uncertainty in the parameters of the crack growth relationship and the long-term stress distribution.

A Probabilistic Evaluation Model for Welding Residual Stress Distribution at Piping Joint in Probabilistic Fracture Mechanics Analysis

2008 ◽  
Author(s):  
Hiroto Itoh ◽  
Jinya Katsuyama ◽  
Kunio Onizawa
Keyword(s):  
Residual Stress ◽  
Stress Distribution ◽  
Fracture Mechanics ◽  
Failure Probability ◽  
Evaluation Model ◽  
Residual Stress Distribution ◽  
Welding Residual Stress ◽  
Probabilistic Fracture Mechanics ◽  
Probabilistic Evaluation ◽  
Aging Knowledge

Stress corrosion cracking (SCC) has been observed at some piping joints made by Austenitic stainless steel in BWR plants. In JAEA, we have been developing probabilistic fracture mechanics (PFM) analysis methods for aged piping based on latest aging knowledge and an analytical code, PASCAL-SP. PASCAL-SP evaluates the failure probability of piping at aged welded joints under SCC by a Monte Carlo method. We proposes a simplified probabilistic model which can be applied to the failure probability analysis based on PFM for welded joint of piping considering the uncertainty of welding residual stress. And the probabilistic evaluation model is introduced to PASCAL-SP. A parametric PFM analysis concerning uncertainties of residual stress distribution using PASCAL-SP was performed. The PFM analysis showed that the uncertainties of residual stress distribution largely influenced break probability. The break probability increased with increasing the uncertainties of residual stress.

Deterministic and Probabilistic Fracture Mechanics Analysis of Pipeline Weld Flaws Accounting for Crack Tip Constraint Effects

2004 ◽  
Author(s):  
Jens P. Tronskar ◽  
Zhang Li
Keyword(s):  
Fracture Mechanics ◽  
Crack Tip ◽  
Line Pipe ◽  
Pipeline Construction ◽  
Probabilistic Fracture Mechanics ◽  
Weld Defects ◽  
Specific Loading ◽  
Wide Range ◽  
Constraint Effects ◽  
Crack Tip Constraint

The acceptability of weld defects during line pipe manufacture and pipeline construction is governed by international codes and standards such as the DNV OS-F101 or API1104. These are universal standards applicable for a wide range of pipeline usage conditions, which include typical workmanship criteria for flaw acceptance. It is, however, possible to establish more precise and often less conservative acceptance criteria using a Fitness-For-Service (FFS) approach through the application of procedures such as those of BS 7910. These are based on applying deterministic or probabilistic fracture mechanics principles on specific loading, materials and toughness properties and service conditions of a pipeline. This paper describes the conventional assessment methodology and more advanced approaches to account for crack tip constraint, dynamic loading due to VIV associated with free-spans. The paper highlights two cases as examples where the approaches have been applied for assessing the criticality of weld defects detected during pipeline construction and their impact on the reliability during service.

Failure Probability Analysis Based on FRI Model for Stress Corrosion Cracking Growth Introducing Residual Stress Distribution by Weld

2012 ◽  
Author(s):  
Noriyoshi Maeda ◽  
Tetsuo Shoji
Keyword(s):  
Residual Stress ◽  
Stainless Steel ◽  
Austenitic Stainless Steel ◽  
Stress Distribution ◽  
Stress Corrosion ◽  
Crack Growth ◽  
Stress Corrosion Cracking ◽  
Failure Probability ◽  
Corrosion Cracking ◽  
Protective Film

Failure probability of welds by stress corrosion cracking (SCC) in austenitic stainless steel piping is analyzed by a probabilistic fracture mechanics (PFM) approach based on an electro-chemical crack growth model (FRI model, where FRI stands for “Fracture and Reliability Research Institute” of Tohoku University in Japan). In this model, crack growth rate da/dt, where a is crack depth, is anticipated as the rate of chemical corrosion process defined by electro-chemical Coulomb’s law. The process is also related to the strain rate at the crack tip, taking the small scale yielding into consideration. Compared to the mechanical crack growth equation like the power law for SCC, FRI model can introduce many parameters affecting the generation and break of protective film on the crack surface such as electric current associated with corrosion, the frequency of protective film break and mechanical parameters such as the stress intensity factor K and its change with time dK/dt. Derived transcendental equation is transformed into non-dimensional form, and then solved numerically by iterative method. The extension of surface crack by SCC under residual stress field is simulated by developing the stress distribution in polynomial form following ASME section XI appendix A. This simulation scheme is introduced into PFM framework to derive the failure probability of austenitic stainless steel piping in nuclear power plants to be used in developing a risk-informed inservice inspection (RI-ISI) program.

Failure Probability Analysis by Probabilistic Fracture Mechanics Based on FRI SCC Model

2010 ◽  
Author(s):  
Noriyoshi Maeda ◽  
Tetsuo Shoji
Keyword(s):  
Fracture Mechanics ◽  
Electric Current ◽  
Crack Growth ◽  
Failure Probability ◽  
Crack Depth ◽  
Threshold Value ◽  
Small Scale ◽  
Protective Film ◽  
Probabilistic Fracture Mechanics ◽  
Crack Growth Model

Failure Probability of a weld by stress corrosion cracking (SCC) in austenitic stainless steel piping was analyzed by probabilistic fracture mechanics (PFM) approach based on electro-chemical crack growth model (FRI model). In this model, crack growth rate da/dt where a is crack depth is anticipated as the rate of chemical corrosion process defined by electro-chemical Coulomb’s law. The process is also related to the strain rate at the crack tip, taking small scale yielding condition into consideration. Derived transcendental equation is solved numerically by iterative method. Compared to the mechanical crack growth equation like Paris’ law for SCC, FRI model can introduce many electro-chemical parameters such as electric current associated with corrosion of newly born SCC crack surface, the frequency of protective film break and mechanical parameters such as stress intensity factor change with time dK/dt. Stratified Monte-Carlo method was introduced which define the cell of sampling space by the ranges of a/c (c is crack length at surface) and the width of K of sampling space, Kw which has to be defined referring to KSCC below which no SCC is caused. Log-normal distributions were anticipated for a/c distribution and K distribution. Parameter survey performed shows that failure probability which is defined as the ratio of crack number whose depth reached 80% of wall thickness to the total crack number depends on many parameters introduced, especially on yielding stress, electric current decay parameter m, strain hardening index n in Ramberg-Osgood equation and dK/dt. From the requirements of FRI model, two types of threshold value of initial crack depth, cracks having smaller depth than this value can not grow, are proposed. Calculated failure probability does not reach 1 when cracks having smaller initial depth than the threshold value are included in the distribution of analyzing cracks.

Numerical and experimental investigations of effects of residual stresses on crack behavior in Aluminum 6082-T6

2012 ◽  
Vol 226 (9) ◽  
pp. 2178-2191 ◽  
Author(s):  
Mohammadreza Farahani ◽  
Iradj Sattari-Far ◽  
Davood Akbari ◽  
Rene Alderliesten
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Finite Element ◽  
Stress Field ◽  
Residual Stresses ◽  
Crack Growth ◽  
Crack Tip ◽  
Residual Stress Field ◽  
Integrity Assessment ◽  
Crack Tip Constraint

In the structural integrity assessment, residual stresses play an important role. The residual stresses affect both the crack driving forces and the crack-tip constraint. To investigate the interaction of residual stresses with mechanical loading during the onset of crack growth in Aluminum 6082-T6, modified single edge-notched bending specimens were used. Aluminum 6082 has the highest strength of the 6000 series alloys with excellent corrosion resistance. A residual stress field was created in the specimens by pre-loading. To accurately quantify the residual stress field created during this test procedure, the strains were measured during loading and unloading and compared with finite element results. After the introduction of the residual stress field, the specimens were tested under three-point bending to determine the load versus displacement behavior and fracture toughness. Also, a post-processor for finite element calculation was developed to enable determination of the J-integral values for the specimens having residual stresses. The constraint parameters Q and R were calculated at the crack-tip to describe the stress field in this region. The parameter Q is used to characterize the loading and geometry constraint, and the parameter R is used for characterizing the crack-tip constraint due to residual stresses. It is observed that tensile residual stresses around the crack-tip increase the crack-tip constraint and decrease the fracture toughness of the bodies. By increasing the external load, the constraint parameter R goes toward zero and the effects of residual stresses on the crack growth resistance become negligible.

Using the Multi-Parameter Fracture Mechanics for more Accurate Description of Stress/Displacement Crack Tip Fields

2013 ◽  
Vol 586 ◽  
pp. 237-240 ◽  
Author(s):  
Lucie Šestáková
Keyword(s):  
Stress Distribution ◽  
Fracture Mechanics ◽  
Boundary Conditions ◽  
Displacement Field ◽  
Singular Term ◽  
Crack Tip ◽  
Higher Order ◽  
Precise Description ◽  
Accurate Knowledge ◽  
Higher Order Terms

Most of fracture analyses often require an accurate knowledge of the stress/displacement field over the investigated body. However, this can be sometimes problematic when only one (singular) term of the Williams expansion is considered. Therefore, also other terms should be taken into account. Such an approach, referred to as multi-parameter fracture mechanics is used and investigated in this paper. Its importance for short/long cracks and the influence of different boundary conditions are studied. It has been found out that higher-order terms of the Williams expansion can contribute to more precise description of the stress distribution near the crack tip especially for long cracks. Unfortunately, the dependences obtained from the analyses presented are not unambiguous and it cannot be strictly derived how many of the higher-order terms are sufficient.

Effect of inhomogeneous distribution of non-metallic inclusions on crack path deflection in G42CrMo4 steel at different loading rates

2015 ◽  
Author(s):  
S. Henschel ◽  
L. Krüger
Keyword(s):  
Fracture Mechanics ◽  
Crack Growth ◽  
Crack Path ◽  
Testing Machine ◽  
Crack Tip ◽  
Crack Growth Resistance ◽  
Impact Testing ◽  
Inhomogeneous Distribution ◽  
Metallic Inclusions ◽  
Crack Tip Blunting

An inhomogeneous distribution of non-metallic inclusions can result from the steel casting process. The aim of the present study was to investigate the damaging effect of an inhomogeneous distribution of nonmetallic inclusions on the crack extension behavior. To this end, the fracture toughness behavior in terms of quasi-static J-?a curves was determined at room temperature. Additionally, dynamic fracture mechanics tests in an instrumented Charpy impact-testing machine were performed. The fracture surface of fracture mechanics specimens was analyzed by means of scanning electron microscopy. It was shown that an inhomogeneous distribution significantly affected the path and, therefore, the plane of crack growth. Especially clusters of non-metallic inclusions with a size of up to 200 ?m exhibited a very low crack growth resistance. Due to the damaging effect of the clusters, the growing crack was strongly deflected towards the cluster. Furthermore, crack tip blunting was completely inhibited when inclusions were located at the fatigue precrack tip. Due to the large size of the non-metallic inclusion clusters, the height difference introduced by crack path deflection was significantly larger than the stretch zone height due to the crack tip blunting. However, the crack path deflection introduced by a cluster was not associated with a toughness increasing mechanism. The dynamic loading ( 1 0.5 5 s MPam 10 ? ? K? ) did not result in a transition from ductile fracture to brittle fracture. However, the crack growth resistance decreased with increased loading rate. This was attributed to the higher portion of relatively flat regions where the dimples were less distinct.

Sign in / Sign up

Export Citation Format

Share Document