John Sharples

Effects of residual stresses on fracture behaviour—experimental results and assessment methods

The influence of residual stresses on fracture behaviour depends on the level of plasticity in a component. Under predominantly elastic conditions, residual stresses can significantly reduce the load-carrying capacity of a defective structure. Conversely, the effects of residual stresses can be minimal when plasticity is widespread. In this paper, a set of experiments with well-characterized residual stresses is used to demonstrate these effects. A new method for assessing defects in practical structures with residual stresses is then described and related to existing methods. The method is applied to the experimental data and shown to be generally accurate and yet conservative.

Continuing Development of a Simplified Method to Account for the Interaction of Primary and Secondary Stresses

The R6 defect assessment procedure, used commonly in the UK nuclear industry to assess the significance of defects in structures, uses the Failure Assessment Diagram to evaluate limiting parameters whilst accounting for the effects of plasticity. The interaction of primary and secondary stress is accounted for within R6 through the use of the ρ, or an equivalent V, term. ‘Look-up’ tables are provided in order to evaluate parameters required to derive the ρ or V terms. In some circumstances, the current methodology has been shown to be excessively conservative and the use of the ‘look-up’ tables is somewhat complex and cumbersome anyway. Previous work has shown that an Alternative Method derived from the Time-Dependent Failure Assessment Diagram approach of the R5 high temperature procedure could potentially be considered. This has since been further modified and termed the Simplified Method. This Simplified Method has the benefit of being less conservative than the current R6 method and of not requiring ρ (or V) factors, and hence not requiring the use of the ‘Look-up’ tables. This paper presents the Simplified Method through comparisons within an extended range of Finite Element Analyses upon both an axial and circumferentially cracked pipe and a centre cracked plate. In addition to the use of the Simplified Method, further work is presented in order to include both out of plane primary and secondary stresses when determining the combined reference stress. A full range of crack opening and out of plane forces, as well as a full range of thermally induced secondary stresses, have been included to provide a broad basis upon which to compare the different methods investigated. Through comparing the full range of cases, some of the assumptions made within the R6 procedure have been reassessed. This has led to an average characteristic length, a, being defined to account for differences in loading type and in and out of plane ratios. However, within this paper further positive evidence for the use of the Simplified Method has been demonstrated.Copyright

Revised Guidance on Residual Stresses in BS7910

A major revision of the British Standard BS7910 on “Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures” is being planned for issue in 2012. This paper provides an overview of the proposed revised guidance in relation to recommended weld residual stress profiles. As such, the paper is focussed on the proposed revised Annex Q of BS7910 which deals with residual stress distributions in as-welded joints.Copyright

STYLE: Comparison of Leak-Before-Break Methodologies Applied in Europe

The paper presents results of a sub-task of the STYLE project currently running within the 7th Framework EU programme by way of an overview of leak-before-break (LBB) methodologies used in the nuclear industry of several European countries (Germany, France, UK, Netherlands, Czech Republic, Spain and Hungary).In relation to the various participating countries, the overview specifically focuses on: practices and procedures; regulatory position; evolution; application; past, present and planned future LBB research activities; and, future plans.A qualitative comparison of the different methods employed in the various countries is included with the commonalities and differences among the approaches being highlighted.Copyright

Extension of the Simplified Method for the Interaction of Primary and Secondary Stresses

The R6 defect assessment procedure, used commonly in the UK nuclear industry to assess the significance of defects in structures, uses the Failure Assessment Diagram to evaluate limiting parameters whilst accounting for the effects of plasticity. The interaction of primary and secondary stress is accounted for within R6 through the use of the ρ, or an equivalent V, term. Previous work has developed an alternate method to that in R6 and has been termed the “Simplified Method”. This Simplified Method has the benefit of being less conservative than the current R6 method and of not requiring ρ (or V factors, and hence not requiring the use of the ‘Look-up’ tables. This paper builds upon the work presented previously and considers the reduction in displacement controlled secondary stress through primary stress induced plasticity by proposing a new function, g(), which is based upon an Option 2 failure assessment curve. Comparisons are also made upon a range of cracked geometries using both the current R6 Method and the derived Simplified Method including g(). The comparisons are made upon both a thermally induced bending stress and a weld residual stresses obtained from a detailed weld simulation. The range of cases provided is to further validate the method and provide strong evidence not only for different geometries but also for realistic residual stress fields. The application of the Simplified Method to the thermally induced bending stress field show improved estimated of KJ over the R6 Method and provide added value to both the Simplified Method and g(). Application to the weld residual stress field shows potential non-conservatism (of both the R6 and Simplified Method) and potential reasons are discussed.© 2009 ASME

An Analysis of the Roles of J and Q in the Assessment of Fracture for Quasi-Statically Extending Cracks in Residual Stress Fields

It is well known that the crack tip stress and strain fields for a crack in an elastic-plastic body depend on the crack tip contour integral J, the Q-stress, and the elastic-plastic properties of the material. This dependence is the fundamental basis of conventional two-parameter J-Q fracture mechanics assessments. It is normally assumed that the crack is created in an unstressed body, or else is inserted concurrently into an existing non-zero stress and strain field such that the crack tip fields build up monotonically and dominate at the crack tip. In such cases, the crack may be regarded as stationary and the J-Q procedure is valid provided that care is taken to calculate J and Q properly when initial stress and/or strains exist. When a crack is introduced progressively and quasi-statically into a component, the location of the crack tip will move along a distinct path. If the component contains residual stress and this is of a significant size along the crack tip path, a re-distribution of the residual stress will occur as the crack tip moves. Specifically, the stress field ahead of the crack tip will unload as the crack tip advances so that non-proportional loading will occur behind the advancing crack tip. In elastic-plastic materials, a wake of plasticity will usually be deposited in the material behind the moving or growing crack tip. Similar effects will also occur when a stationary crack extends due to critical or sub-critical processes. The presence of a plastic wake alters the stress and strain fields at the crack tip so that they do not generally match the fields of a stationary crack. Moreover, J and Q may not describe the stress and strain fields, invalidating the use of the fracture mechanics procedure for such cases. In this paper, a Finite Element analysis of J and Q is carried out for a quasi-statically extending crack inserted in a strip of elastic-plastic material containing an initial residual stress field. Care is taken to model the crack tip conditions appropriately as the crack extends and J is determined using the JEDI post-processing program which can allow for the effects of initial plastic strains and non-proportional loading. An assessment is made of the crack tip field and the likelihood of further extension or fracture is made using local approach models. The analysis considers both cleavage and ductile fracture. The extent of the relationship between J and Q and the crack tip fields is established and the validity of the J-Q procedure to such cases is discussed. The paper considers whether the procedure is conservative when J and Q are determined from an analysis of a stationary crack of the same size inserted into the same initial field.Copyright

Further Studies of Multiple Co-Planar Surface Breaking Flaws for Cleavage Fracture

In assessing the integrity of structures, complex multiple flaws located in close proximity to each other are generally characterised as one, larger, single flaw. Guidance for the characterisation of multiple flaws is provided in procedures such as R6 and BS 7910, which are routinely used in the UK and elsewhere in the structural integrity assessment of structures and components. For this approach to be valid, the characterisation process must be conservative. That is to say, the probability of failure must be higher for the characterised flaw than for the system of multiple flaws. However, previous studies showed that the current characterisation rules may be non-conservative under some circumstances, in particular under cleavage fracture conditions. A combined experimental and analytical programme of work has been undertaken within the UK in order to further investigate this potential non-conservatism for situations where the possibility of cleavage failure may have to be taken into account when assessing structures or components containing multiple flaws. Details of early stages of the analytical programme were reported at the 2006 and 2007 ASME PVP Conferences and comprised a number of finite element analyses to evaluate cleavage failure probability, via a Master Curve-based approach, for interacting twin flaws and the corresponding characterised single flaw, under applied tensile and bending loads, at low temperatures. These analyses considered surface-breaking semi-elliptical flaws all having the same depth, but with four different aspect ratios. For each aspect ratio the separation of the twin flaws was varied. It was found that non-conservatism of the characterisation rules was indicated for flaws of high depth to length aspect ratio (a/c) in contact. This paper describes further work that has been undertaken to extend the results previously reported. The further work described has been centred on: • for the most onerous aspect ratio (a/c = 1.0) extending the experimental results to higher temperatures in the cleavage transition regime. • performing finite element analyses to complement these experiments. • revisiting the methods for calculation of cleavage failure probability to obtain improved agreement with the experimental results. • examining the rules governing the characterisation process, to determine if modification is necessary.Copyright

Crack Opening Areas During High Temperature Operation

This paper presents results from two-dimensional finite element analyses of a centre cracked plate under both plane stress and plane strain conditions. The plate has been loaded in tension and secondary creep conditions have been assumed. The variation of the crack opening area with time has been calculated. It has been shown that the rate of change of the crack opening areas reduces with time up to the redistribution time which approximates the time to achieve steady creep conditions. Thereafter, the rate of change of crack opening area is constant. From curve fits to finite element results, a simplified expression for the rate of change of crack opening area of a stationary crack has been derived in terms of the elastic crack opening area, the creep strain rate, the elastic strain and two characteristic crack lengths (one for a strain field dominated by elastic strains and one for a strain field dominated by creep strains). This expression predicts the rate of change of the crack opening area both during the transient period up to the redistribution time and at all times thereafter.Copyright

Magnitude and distribution of retained residual stresses in laboratory fracture mechanics specimens extracted from welded components

Quantifying material fracture toughness properties is an important step in ensuring structural integrity of industrial components. Welding of structural components can cause large magnitudes of residual stress to be generated, which can be defined as a stress that exists in a material when it is under no primary loading. These stresses can be retained in laboratory fracture mechanics testing specimens removed from non-stress relieved welds, making the quantification of valid material fracture toughness difficult.The aim of this paper is to investigate, analytically, the levels and distributions of residual stresses retained in fracture mechanics specimens taken from welded components. This was achieved using parametric finite element analyses. Furthermore, in order to ensure the validity of fracture toughness measurements derived from components that contain residual stress, a robust method for the design of stress-free fracture mechanics specimens is proposed.Significant weld residual stresses have been shown to be retained in certain laboratory specimens post extraction from non stress-relieved welds. The magnitude and distribution of retained residual stress has been shown to be dependant on material properties, specimen size, specimen type and removal location. In addition, the stress partitioning method has been shown to provide a useful approach for estimating the levels and distributions of residual stresses retained in fracture mechanics specimens extracted in certain orientations.© 2012 ASME

Advanced assessment of the integrity of ductile components

Nuclear Reactor Pressure Vessels (RPV) are manufactured from medium strength low alloy ferritic steel, specifically selected for its high toughness and good weldability. The ability of the pressure vessel to resist crack growth is crucial given that it is one of the fundamental containment safety systems of the reactor. For most of their lifetime, the pressure vessel operates at sufficiently elevated temperatures to ensure the material is ductile. However, the development of ductile damage, in the form of voids, and the ability to predict the ductile crack growth in RPV materials requires further work.The Gurson-Tvergaard-Needleman (GTN) model of void nucleation, growth and coalescence provides one tool for predicting ductile damage development. The model is normally calibrated against fracture toughness test data. However, recent work [1] has demonstrated the benefit of refining calibrations against measured void volume fractions generated from notched and pre-cracked specimen tests.This paper described the measurement of void distributions below the fracture surface of a range of notched and pre-cracked specimens. The void distribution below the fracture surface is shown to be dependent upon the local stress triaxiality and plastic strain distribution. As a result, pre-cracked specimens show a greater concentration of voids close to the fracture surface, whilst notched tensile specimens show a lower volume fraction of voids close to the crack surface. In both specimen types, voids are observed to extend between 2.5 and 3.5 mm below the fracture surface.Copyright