SINTAP defect assessment procedure for strength mis-matched structures

This paper describes the defect assessment method for strength mismatched welded structures, resulting from the Brite Euram structural integrity assessment procedures for European industry (SINTAP) project. Finite element and experimental validation of the proposed method is provided. This shows that the proposed method is conservative, and that the degree of conservatism is similar to that embedded in the methods for homogeneous structures. It provides confidence in the use of the proposed SINTAP method for assessing defective weld strength mismatched structures.     

Methods for including constraint effects within the SINTAP procedures

To enable constraint-based approaches to be applied to the practical assessment of defective components, development work within SINTAP has addressed a number of issues: constraint parameters for surface defects, constraint parameters for secondary stresses, simplified weight function methods for calculation of T-stress and relevant fracture toughness data. This paper describes these developments and indicates how the methods have been included in the overall SINTAP procedures.     

Fracture assessment of a clad steel using various SINTAP defect assessment procedure levels

In many critical industries such as nuclear, chemical and petrochemical industries, bi‐material components in the form of clad plates are used due to their superior environmental and mechanical properties. During the service life of these bi‐material components, cracks or crack‐like defects may develop in the clad layer and penetrate into the substrate. The assessment of the integrity of such a cracked as well as complex structure would be an important task. Hence, this study is aimed at applying the recently developed European Structural Integrity Assessment Procedure (SINTAP) to a through thickness centre cracked clad steel wide plate to validate the procedure for this type of bi‐material structure. Based on the available input data, various assessment levels as well as two different assessment routes, namely crack driving force (CDF) and failure assessment diagram (FAD), can be used for predicting the failure loads. In principle both routes should provide similar results and this has been verified by having very similar results for both SINTAP routes. Furthermore, the degree of conservatism has been reduced by using higher analysis levels of SINTAP.     

Incorporation of residual stresses into the SINTAP defect assessment procedure

Residual stresses are an important consideration in the structural integrity assessment of welded joints and reliable predictions of structural integrity therefore require that the residual stress distribution is adequately accounted for. However, distributions are dependent on the weld geometry and, in the absence of comprehensive information for welded joints, it is usually necessary to make conservative assumptions. This can result in unrealistic predictions, and consequently, further information on the nature and behaviour of residual stress distributions is required to enable the provision of improved guidance on this subject.The further development of the BS 7910 and R6 procedures for the assessment of residual stress effects was a principal task in the EC funded project named structural integrity assessment procedures for European industry, SINTAP, and an extensive investigation of this subject was performed. It entailed an extensive literature review of distributions in the principal weld geometries (including plate butt, pipe butt, pipe to plate, T-butt and tubular welded joints), experimental and numerical investigations and the development and validation of procedures.     

Practical use of defect assessment procedures for industrial component integrity assessment

Abstract

The practical use of defect assessment procedures for industrial component integrity assessment is described through two practical examples in this paper. In the first example, the procedure is used to perform low temperature fitness-for-service (FFS) analysis of a longitudinal seam welded vessel manufactured from a duplex stainless steel. The impact energy obtained from the Charpy impact test performed on the weld of the vessel was found lower than the value of the minimum impact energy criteria given by the British Standard BS 5500. Charpy impact energy and fracture toughness correlation and failure assessment diagram (FAD) methodology were used in this FFS analysis to determine the fracture resistance of the vessel. For the assumed defect size used in the assessment, the weld was found to meet the fracture resistance criteria and therefore would still be fit-for-purpose. The FAD analysis was, however, repeated using the J-values obtained from the CTOD test to gain better confidence as the Charpy impact test does not provide direct measurement of the fracture toughness. The FAD was later used to determine the critical surface crack length which was then represented as a function of crack depth. The Charpy impact energy correlation method was found to be more conservative than the method of evaluation using the CTOD J-values. The lack of side wall fusion (LOF) which is a typical defect in this type of vessel would usually be influenced by the size/diameter of the electrode wire used in the weld and the number of runs. In this case, the fracture resistance of the 15mm thick vessel with the longitudinal seam weld should be adequate if less than 5mm diameter of electrode wire is used.

The second example illustrates defect assessment of a high temperature plant component. A defect was found in the high pressure final superheater header. A defect assessment incorporating FAD on the header showed that the defect was non-critical. This led to the need to perform creep and fatigue crack growth calculations and remaining life assessment in order to determine the mitigation plan for the engineers. The deterministic approach, which mainly considers the worst case scenario, suggested that the remaining life of the header was approximately 4.5 years. Probabilistic analysis showed that the component could still be fit for service up to 6 years. This will allow the engineers to mitigate a more efficient plan with a decision to either repair or replace the header and when. The use of probabilistic lifing methodologies and algorithms could consequently bring considerable financial benefits to the plant owners/operators e.g. by avoiding premature component repair or replacement. Nevertheless, it would be in the management’s interest to avoid a forced outage. The recommendation would be that the component could still be fit to use until their next minor outage (in 3 years). From then, the remedy option would be to grind down to a depth of suspected crack, excavate a small area/surrounding which might be affected, followed by correct and regular monitoring, or alternatively replace the component.     

High temperature crack initiation and defect assessment of P22 steel weldments using time dependent failure assessment method

High temperature deformation and crack resistance of low alloy ferritic grade P22 steel weldments applied in power plants are reported. The creep crack initiation (CCI) and creep crack growth (CCG) data were determined using compact type (C(T)) and C-Shape (CS(T)) fracture mechanics specimens at 550 °C. The deformation and crack growth behaviour of similar weldment zones and significance of CCI and CCG in defect assessment of components were addressed. The weldments with industrially relevant properties were produced in butt welded pipe joint from which test specimens are sampled. The studied material covers a spectrum of microstructures and ductility over the weldment zones to give representative for a welded component. The emphasis is placed on the measurement and particularly analysis of crack initiation for failure assessment in P22 steel weldments. The particular importance of construction of isochronous curves for time dependent failure assessment diagram (TDFAD) method is reported. It is aimed to contribute to establishing guidelines for acceptable methodologies for testing, analysis and assessment of welded components using TDFAD for high temperature service.     

Suitability of mixed-mode I/II assessment methods for implementation into the SINTAP procedure

Several engineering methods are available for the assessment of structures containing defects. They belong essentially to two categories: FAD methods and CDF methods. The recent SINTAP procedure unifies both FAD and CDF routes for the assessment of mode I loaded defects. In this paper, fracture assessment methods containing guidelines for mixed-mode I/II loading conditions are examined and compared with the aim to discuss their suitability for implementation into the SINTAP procedure.     

R5 and British Standards defect assessment procedures

In this paper, a basic high temperature defect assessment procedure is first described. This is common to the Nuclear Electric R5 procedure and the British Standards Document PD6539. Then the paper provides some details of the PD6539 method and the R5 procedure. The latter procedure covers a number of situations beyond the scope of the basic procedure, such as crack growth in dissimilar metal welds and creep-fatigue crack growth, and these are discussed.     

Structural integrity assessment of components subjected to low constraint conditions

Fracture assessment of a structural component by different procedures utilises fracture toughness data derived from deeply cracked fracture mechanics test pieces. These represent a high level of constraint that may result in excessive conservatism and possible unnecessary replacement or rejection of structural components which are actually fit for service. A source of the conservatism is that the triaxiality of the stress field in the vicinity of a shallow crack in a structural component can be lower than that in a deeply cracked test specimen. Consequently, the apparent fracture toughness of the material and the load bearing capacity of the structural component are increased. Such a situation is usually referred to as “loss of constraint”. In addition, stress fields with low triaxiality occur near non-sharp defects. The ability to allow for the loss of constraint is potentially of great practical significance. This paper provides a fracture assessment methodology which take into account in-plane loss of constraint. Traditionally, the different sources of constraint (type of loading, crack shallowness and notch effect) have been treated separately and no assessment tools had been proposed for the global treatment of the loss of constraint. It also gathers and analyses the results of the experimental programme performed in order to validate this methodology.     

Assessment of notched structural steel components using failure assessment diagrams and the theory of critical distances

When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. The proposal of this paper consists, on the one hand, in the application of the theory of critical distances for the estimation of the notch fracture toughness and, therefore, for the conversion of the notched situation into an equivalent cracked situation in which the material develops a higher fracture resistance. On the other hand, once the notch fracture toughness has been defined, the assessment is performed using the failure assessment diagram methodology, and assuming that the notch effect on the limit load is negligible. The methodology has been applied to 336 CT notched fracture specimens made of two different structural steels, covering temperatures from the corresponding lower shelf up to the upper shelf, providing satisfactory results and a noticeable reduction in the overconservatism derived from the analyses in which the notch effect is not considered.     

Application of the European flaw assessment procedure SINTAP to thin wall structures: Analytical assessment levels

The recently developed European flaw assessment procedure SINTAP has been applied to 29 thin sheets made of five different materials and loaded in tension, bending and biaxial tension. Based on these data the analytical SINTAP assessment levels are evaluated with respect of their ability to predict the failure loads as well as stable crack extension behaviour.     

Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress. Part I: Methodology using the equivalent CTOD ratio

This part I of a two-part paper presents a method of assessing the effects of welding residual stress and constraint loss on the cleavage fracture of a wide plate subjected to membrane stress based on the Weibull stress criterion. It has been found that the Weibull stress criterion is efficient for evaluating the fracture instability of wide plates with and without a welding residual stress field. The concept of an equivalent crack-tip opening displacement (CTOD) ratio β_r under a welding residual stress field is introduced for assessment of constraint loss effects on CTOD fracture toughness of wide plates. The equivalent CTOD ratio β_r is defined as the ratio of the CTOD in the standard fracture toughness specimen to the CTOD in a wide plate with a welding residual stress at the same level of the Weibull stress. Fracture assessment procedures using β_r for wide plates are shown within the framework of the failure assessment diagram. It has been found that the excessive conservatism observed in the conventional procedure can be reduced reasonably by applying the proposed method. The companion part II of the paper presents applications of the CTOD toughness correction method using β_r to the fracture test data of welded joints and verifies the proposed method in the ductile-brittle transition temperature region.     

Fracture assessment of damaged square hollow section (SHS) K-joint using BS7910:2005

A pre-cracked square hollow section K-joint was tested under static loads up to failure. It is found that the load-displacement curves are in good agreement with the finite element results. Ductile tearing was observed to initiate from the crack front parallel to the chord side wall where fracture toughness is smaller. Using plastic collapse load obtained via twice elastic compliance technique and fracture toughness obtained from crack tip opening displacement, the two fracture parameters K_r and L_r are plotted on the standard failure assessment diagram. It shows a conservative assessment for the cracked K-joint subjected to brace end axial loads.     

The SINTAP fracture toughness estimation procedure

The maximum likelihood (MML) procedure for the analysis of fracture toughness data generated at a single temperature that was developed for the SINTAP flaw assessment procedure is described. The procedure involves a series of steps to ensure that a Weibull toughness distribution fitted to experimental data is conservative. Validation is provided by experimental data from weld metal, heat affected zone and parent plate. Results are compared with those from assessments carried out to existing methods described in BSI PD6493:1991 and BS 7910:1999. It is concluded that for small data sets, the MML procedure provides a greater level of consistency and reduces selection of potentially non-conservative fracture toughness values.     

Experimental analysis of differences in mechanical behaviour of cracked and notched specimens in a ferritic–pearlitic steel: Considerations about the notch effect on structural integrity

This paper analyses the experimental differences found between the mechanical behaviour of cracked and notched specimens. These differences are defined in terms of local strains, global strains, stresses and CTOD values during testing, and also through the correlations established between these variables. The experimental programme has been performed at a wide range of temperatures (from −100 °C to −20 °C), providing an outline of the notch effect throughout the transition region. The corresponding effects on structural integrity assessments (through failure assessment diagrams) are also analysed. From the results obtained, a clear notch effect has been defined in all the mentioned parameters and in the assessment itself for temperatures below the transition temperature (T < T₀) of the material. The effect is less pronounced or even negligible at higher temperatures (T > T₀).     

Constraint loss correction for assessment of CTOD fracture toughness under welding residual stress - Part II: Application of toughness correction methodology

This part II paper presents the verified results of the toughness correction methodology for welded joints of wide plates. The equivalent CTOD ratio, β_r, is applied to the fracture data of the welded joints from lower to upper ductile-brittle transition temperature region. In the part I paper, β_r is defined as the ratio of CTOD in the standard fracture toughness specimen to CTOD in the wide plate with welding residual stress at the same level of the Weibull stress. In this part II paper, the equivalent CTOD ratio, β_r, under the welding residual stress field has been verified for assessment of constraint loss effects on CTOD fracture toughness of wide plate. Fracture assessments have been conducted by applying the methodology for “After Weld Notch” and “Before Weld Notch” type welded joints. It has been found that an excessive conservatism observed in the conventional procedure is reasonably reduced by applying the proposed methodology.     

Assessment of the role of micro defect nucleation in probabilistic modeling of cleavage fracture

The present study is concerned with an experimental and numerical investigation of the local conditions for initiation of cleavage failure in ferritic steels. In the experimental analysis, a variety of SE(B), C(T) and CC(T) specimens has been tested. The cleavage triggering sites and the fracture mechanisms were determined in a subsequent fractographic investigation. In a finite element analysis, the local mechanical field quantities at the cleavage initiation spots at the instant of fracture were investigated. Based on the results, an enhanced local concept for cleavage assessment is proposed, accounting for both, the nucleation of critical micro defects and their possible instability due to local overloading. Together with a comparison to the prediction of previous probabilistic models for cleavage failure assessment, the results reveal the importance of considering the nucleation of possibly critical micro defects due to plastic straining as a second necessary criterion for cleavage fracture.     

A comparison of high temperature defect assessment methods

Cracked high temperature components which are subjected to creep or creep-fatigue loading may fail by crack growth, net section rupture or a combination of both processes. In this paper, models are presented for describing these modes of failure in terms of fracture mechanics concepts, limit analysis methods and cumulative damage laws. It is shown that these models form the basis of a number of high temperature defect assessment procedures that are available for plant. These procedures are then applied to a semi-elliptical defect in a plate which is subjected to creep-fatigue loading. It is found that the predictions are sensitive to the crack initiation criteria assumed and the limit analysis solutions adopted.     

Engineering assessment of cracked structures subjected to dynamic loads using fracture mechanics assessment

This paper presents practical guidance on the assessment of cracked structures subjected to dynamic loading. General reviews of fracture behaviour of structures subjected to dynamic loading are presented. A series of finite element (FE) analyses have been carried out to study the effects of dynamic loading on both fracture toughness specimens under rapid loads and cracked connections in steel framed structures under earthquake loads. FE results of submodel analyses of cracked connections are compared with the results from a simplified method. The simplified method can reduce the analysis time enormously and allows design engineers to assess the possibility of connection fractures, or determine approximate values of toughness and defect size requirements for given peak stress and strain level.