Non-sharp Defects Research Articles

A constraint-based approach for notch cleavage fracture toughness estimations

A stress-based approach is used to evaluate the role of notch sharpness on effective toughness of S355J2 + N steel samples. A notch corrected constraint method based on a J-Q approach is proposed for the first time. The approach is calibrated using experimental fracture data of pre-cracked and notched SEN(B) samples and validated using notched C(T) data from open literature for the same material. A two-parameter Weibull model is used to describe the probability of failure to incorporate confidence levels into the analysis. The methodology shows to be a promising method for assessment of non-sharp defects where benefit may be taken from resolving non-sharp defects acuity.

Sizing limitations of ultrasonic array images for non-sharp defects and their impact on structural integrity assessments

Existing structural integrity assessment procedures typically assume flaws to be infinitely sharp when they cannot be considered as local thinned areas. This assumption is often over-conservative, resulting in a pessimistic assessment of structural components and a significant underestimation of their margins of safety against fracture. One of the main challenges while adopting non-destructive evaluation techniques is distinguishing between sharp cracks (e.g., fatigue) and non-sharp defects and identifying the more severe ones. Towards this broader challenge, the present work aims to examine the sizing limitation and accuracy of ultrasonic array image-based techniques for non-sharp defects (surface breaking u-notches) and investigate how these measurements would affect the structural integrity assessment of components. Parametric numerical simulations and experimental measurements are performed to generate full-matrix capture datasets, which are then processed using the total focusing method to form an image. The image-based sizing approach is shown to perform efficiently for notch depths higher than the inspection wavelength (λL), i.e. as small as 0.2 mm, and semi-notch widths as small as 0.1 mm. The influence of ultrasonic measurements on structural integrity assessments is highlighted using different case studies in the context of non-sharp defects of fatigue and fracture strength estimates. For the cases under analysis, resolving non-sharp defects led up to 5× and 3× values of effective fracture toughness and fatigue strength, respectively. We have also seen that a 30 % uncertainty in semi-notch width sizing would result in a 30 % and 20 % error in fatigue strength and fracture toughness estimations, respectively.

A mechanism-based gradient damage model for metallic fracture

A new gradient-based formulation for predicting fracture in elastic–plastic solids is presented. Damage is captured by means of a phase field model that considers both the elastic and plastic works as driving forces for fracture. Material deformation is characterised by a mechanism-based strain gradient constitutive model. This non-local plastic-damage formulation is numerically implemented and used to simulate fracture in several paradigmatic boundary value problems. The case studies aim at shedding light into the role of the plastic and fracture length scales. It is found that the role of plastic strain gradients is twofold. When dealing with sharp defects like cracks, plastic strain gradients elevate local stresses and facilitate fracture. However, in the presence of non-sharp defects failure is driven by the localisation of plastic flow, which is delayed due to the additional work hardening introduced by plastic strain gradients.

A transferability approach for reducing excessive conservatism in fracture assessments

A source of uncertainty and conservatism in structural integrity assessments is the value of fracture toughness (Kmat) that is used. For conservative results, the value of Kmat is commonly derived from deeply cracked specimens, such as standard compact tension specimens, C(T). High constraint conditions near the crack tip are ensured and this corresponds to lower-bound toughness values independent of specimen size and geometry. However, the local stress fields in single edge notched tension, SE(T), specimens and pipes, for example, are known to be less severe than those at the tip of a deep sharp crack, resulting in an increased capacity to sustain load and higher toughness. Similar behaviour is expected when assessing non-sharp defects (e.g., pits, gouges, dents). The constraint loss or the notch effect produce a relaxation in the triaxial stress field in comparison to the severe stress fields present at deeply sharp cracked specimens. A methodology providing a simple procedure to evaluate the suitability of the use of a higher fracture toughness to reduce excessive conservatism is then required. This study uses a two-parameter fracture mechanics approach (J-Q) to quantify the level of constraint in a component (e.g. a pipe with a surface crack) and in fracture test specimens, i.e. single edge tension [SE(T]), standard compact tension [C(T)] and notched compact tension [C(T)ρ] specimens. The ability of the structure to resist fracture is given by the fracture toughness of the test specimen with a similar J-Q response. Fracture toughness values for different specimens have been obtained from tearing resistance curves (J-R curves) constructed by means of a virtual testing framework. The proposed engineering approach is used as a platform to perform more accurate fracture assessments by the use of a ductile fracture model that informs a classical fracture mechanics approach (J-Q) by incorporating more fundamental understanding of the driving forces and the role of the geometry and loading conditions.

Failure Prediction of Curved Wide Plates Using the Strain-Based Failure Assessment Diagram With Correction for Constraint and Notch Radius

The strain-based failure assessment diagram (SB-FAD) has been developed for predicting failure from flaws in components subjected to high plastic strains. In this paper, a combined numerical and experimental approach is used to apply the SB-FAD to predict failure from a series of API 5L grades X80 and X100 curved wide plate (CWP) specimens with shallow notches machined into the pipe girth weld. For the CWP specimens tested in this work, the SB-FAD in its unmodified form resulted in over-conservative predictions of failure. This is attributed to the SB-FAD assuming high constraint conditions and the presence of a sharp fatigue crack, whereas the CWP specimens tested in this work were low constraint and contained a shallow machined notch without fatigue cracks. A modification of the SB-FAD is then proposed to account for nonsharp defects loaded to high plastic strains under conditions of low constraint. The resulting predictions of the modified SB-FAD show significantly reduced conservatism compared to the unmodified SB-FAD.

An engineering assessment methodology for non-sharp defects in steel structures – Part II: Procedure validation and constraint analysis

This Part II paper validates a new engineering methodology for assessing non-sharp defects in ferritic steel structures. The new approach, presented in the companion Part I paper, is based on a modification of the Failure Assessment Diagram for notches. A series of worked examples is presented based on a large dataset of U-notched SE(B) specimens machined from a heat-treated structural steel grade S690. Failure predictions of notched specimens using the modified approach result in significantly reduced conservatism compared to the standard Failure Assessment Diagram assessment method. The new methodology is also combined with the existing constraint-modified approach in the R6 defect assessment method to predict cleavage fracture in shallow, blunt-notched SE(B) specimens with a small degree of conservatism.

An engineering assessment methodology for non-sharp defects in steel structures – Part I: Procedure development

This Part I paper describes a new engineering assessment methodology for ferritic steel structures containing non-sharp defects within the context of a Failure Assessment Diagram (FAD) approach. Although the modification of the FAD for non-sharp defects can be applied whether the initiating failure mechanism is cleavage or ductile tearing, this paper focuses on cleavage fracture. The parameters describing the sensitivity of the material toughness to the notch effect can either be measured by testing notched specimens of the same thickness as the structure, or for cleavage fracture they can be obtained using look-up tables generated using the Weibull stress toughness scaling model. The other parameters in the procedure can either be conservatively estimated using simple equations or they can be determined more accurately using finite element analysis. Validation of the new method is presented in the companion Part II paper: this shows that assessments of U-notched SE(B) specimens have significantly reduced conservatism when using the new assessment methodology compared to the standard FAD approach for sharp cracks.

Prediction of cleavage fracture from non-sharp defects using the Weibull stress based toughness scaling model

The structural integrity assessment of engineering plant is based on the principles of fracture mechanics that assume defects to be sharp cracks. Whilst conservative, this assumption may be overly conservative in some situations, e.g. for defects that are non-sharp. This paper describes the prediction of cleavage fracture initiation from blunt notches of varying root radii using the Weibull stress based toughness scaling model. Failure predictions are compared with the results of experiments performed on single edge notch bend SE(B) specimens containing both cracks and notches of varying notch root radii. Cleavage initiation sites were located close to the peak tensile stress ahead of the notch, implying that a tensile stress criterion is the main controlling factor for cleavage fracture. The cleavage fracture predictions from the toughness scaling model correlate well with the experimental data; but care needs to be taken to ensure that calibration of the Weibull parameters references fracture toughness data from constraint levels that span that of the defect of interest. This ensures the model interpolates between the constraint states used for calibration, rather than extrapolating outside the range of applicability.

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.