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.     

Fatigue strength assessment of Invar alloy weld joints using the notch stress approach

The aim of this study investigated the fatigue strength of Invar alloy weld joints. Invar steel (Fe-Ni 36%) is widely used in the primary and secondary barriers of membrane-type liquified natural gas (LNG) containment vessels. The fatigue test was carried out for two different types of welded joints with raised edge specimens and with overlap joint specimens based on the nominal and notch stress approaches. The thickness of the Invar plate is less than 1.5 mm, so the notch stress approach with r_ref = 0.05 mm was applied. Our evaluation of the results in terms of the FAT value and the slopes of the design curves are compared with steel, aluminum, and magnesium weld joints in accordance with International Institute of Welding (IIW) recommendations.     

Non-local modelling of cyclic thermal shock damage including parameter estimation

In this paper, rate dependent evolution laws are identified and characterized to model the mechanical (elasticity-based) and thermal damage occurring in coarse grain refractory material subject to cyclic thermal shock. The interacting mechanisms for elastic deformation driven damage induced by temperature gradients and thermal damage induced by isotropic thermal expansion are combined and represented by a single variable for the total damage. The constitutive model includes the shielding of micro-structural thermal damage by the non-local elasticity-based damage developed at the macroscopic and microscopic scale. Quasi-stationary thermal experiments are used to identify the parameters used in the evolution law for thermal damage. The remaining model parameters, including a micro-structural length scale, are quantified by inverse modelling of cyclic thermal shock experiments. Longitudinal wave propagation measurements through damaged material are simulated, enabling the identification on the basis of the first and second thermal shock cycle. A third thermal shock cycle enabled the evaluation of the quality of the obtained parameter set. The set-up of the thermal shock experiments has been optimized through a parameter identifiability analysis. The damage evolution in three consecutive thermal shock cycles is investigated numerically with the optimized model.     

Numerical simulation of strength test on graphite moderator bricks using a continuum damage mechanics model

Experimental tests on graphite moderator bricks with keyways have been simulated using a failure model recently developed by the authors based on continuum damage mechanics. Failure of the specimens, induced by the application of pure bending moment, is characterised by unstable crack propagation, with very small cracks and damage zones formed at the keyway corners immediately before final rupture. Sensitivity of the predictions to the keyway corner radius, variations in material properties and the presence of methane holes have also been investigated. Good agreement has been achieved between the predicted failure strengths and the available experimental data.     

Microstructural and X-ray tomographic analysis of damage in extruded aluminium weld seams

In the present study, X-ray computer tomography is used to determine damage evolution in extrusion weld seams loaded close to failure for two comparable AlSiMg alloys with distinctively different grain structures. In the dispersoid free alloy, recrystallisation occurs upon extrusion and the weld seam has no effect on mechanical properties. No preferential void formation in the weld seam region was detected. In the non-recrystallising dispersoid rich alloy, the weld seam leads to a reduced ductility. There are no signs of internal void formation, and damage initiation was found to be close to the surface. It is argued that for the non-recrystallising alloy, the effect of the weld seam on the ductility is related to a sharp texture gradient across the weld seam.     

Non-local modeling of thermal shock damage in refractory materials

A non-local damage framework has been coupled with heat transport to model transient thermo-mechanical damage (in particular thermal shock) in refractory materials. The non-locality, to be dealt with to obtain an adequate problem formulation, is introduced by terms accounting for micro-structural strain gradients induced by transient temperature gradients. The parameters figuring in the evolution law for elasticity-based damage are temperature dependent. Damage due to isotropic thermal expansion has been accounted for by proposing a new evolution law. A single variable for the total damage is obtained by combining both damage mechanisms. The influence of non-locality and transient temperature gradients within non-locality is investigated in numerical examples. The phenomenological relevance of the framework is verified by modeling of experiments, which simulate thermal shock under process conditions.     

Evaluation of directional mesh bias in concrete fracture simulations using continuum damage models

In the present comparative study, we investigate the influence of directional mesh bias on the results of failure simulations performed with isotropic and anisotropic damage models. Several fracture tests leading to curved crack trajectories are simulated on different meshes. The isotropic damage model with a realistic biaxial strength envelope for concrete is highly sensitive to the mesh orientation, even for fine meshes. The sensitivity is reduced if the definition of the damage-driving variable (equivalent strain) is based on the modified von Mises criterion, but the corresponding biaxial strength envelope is not realistic for concrete. The anisotropic damage models used in this study capture reasonably well arbitrary crack trajectories even if the biaxial strength envelope remains close to typical experimental data. Their superior performance can be at least partially attributed to their ability to capture dilatancy under shear, which is revealed by a comparative analysis of the behavior of individual models under shear with restricted or free volume expansion.     

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.     

Testing and modelling of creep crack growth in compact tension specimens from a P91 weld at 650 °C

Creep crack growth tests were performed, at 650 °C, on compact tension (CT) specimens machined from the parent material and from the weld region of a P91 weldment. Parent material tests were performed on a number of different CT specimen designs in order to investigate the effects of side grooves on the shape of the crack front. Tests of CT specimens machined from the weld region were performed with the initial cracks located within the heat-affected zone (HAZ) along the interface with the parent material (i.e. the type IV position). All of the specimens were prepared with initial cracks created by wire spark erosion. Good correlations between creep crack growth rates and C^∗ were obtained for both the parent and type IV test results. The results indicate that the crack growth rates in the weld specimens are about four times higher than those of the parent material specimens, at the same C^∗. Microstructural investigations of the fracture surfaces using SEM and hardness measurements have shown that the exact location of the initial crack within the weldment has a large effect on the crack growth rate, at various loading levels. The results of Finite Element (FE) analyses of the parent material specimen tests, using a creep continuum damage material model, compared favourably with those obtained from the experiments.     

Influence of the notch shape of pre-notched small punch specimens on the creep failure time

Nowadays, there are standards for determining the creep-fracture properties of a material. However, a sufficient amount of material to be tested is usually required, something that is not always possible or convenient. In certain cases where not enough material is available for carrying out conventional tests to determine these properties of the material to be analysed, there are now several non-standard tests that can achieve this purpose. One of them, the Small Punch Creep Test (SPCT), basically consists of punching, under a constant load, a miniature specimen in which the sides of the specimen are clamped between two dies. One of the greatest challenges at present is to obtain the creep-fracture properties of a material from this type of test using pre-notched specimens. To achieve this initial notch in the SPCT specimen prior to creep-fracture testing, there are several techniques which are being used at present. The main objective of this paper is to analyse the differences between these techniques, taking into account the shape of the pre-notch obtained and the stress distribution at the pre-notch tip during the test. In this way, it is possible to determine which of them is the most appropriate for estimating the creep-fracture properties of the material used.     

Defect tolerance assessment of ARIANE 5 structures on the basis of damage mechanics material modelling

Damage mechanics based material models have been applied to establish fracture control procedures for the failure prediction of ARIANE 5 main structural components as the booster cases including welds, the main stage, and the upper stage cryogenic tank. The main goals of the damage mechanics based investigations were the accurate failure prediction, the clarification of dependency of fracture toughness on geometry, the calibration of analytical methods and the interpretation and optimisation of small scale fracture tests for material characterization and quality assurance.The results of these investigations allowed a failure prediction accuracy with analytical tools which is close to 3D numerical simulations. This could be demonstrated both with ductile (Gurson) and brittle (RKR) damage mechanics models.     

Cohesive zone with continuum damage properties for simulation of delamination development in fibre composites and failure of adhesive joints

A new approach is developed to implement the cohesive zone concept for the simulation of delamination in fibre composites or crack growth in adhesive joints in tension or shear mode of fracture. The model adopts a bilinear damage evolution law, and uses critical energy release rate as the energy required for generating fully damaged unit area. Multi-axial-stress criterion is used to govern the damage initiation so that the model is able to show the hydrostatic stress effect on the damage development. The damage material model is implemented in a finite element model consisting of continuum solid elements to mimic the damage development. The validity of the model was firstly examined by simulating delamination growth in pre-cracked coupon specimens of fibre composites: the double-cantilever beam test, the end-notched flexure test and the end-loaded split test, with either stable or unstable crack growth. The model was then used to simulate damage initiation in a composite specimen for delamination without a starting defect (or a pre-crack). The results were compared with those from the same finite element model (FEM) but based on a traditional damage initiation criterion and those from the experimental studies, for the physical locations of the delamination initiation and the final crack size developed. The paper also presents a parametric study that investigates the influence of material strength on the damage initiation for delamination.     

A synergistic creep fatigue failure model damage (case of the alloy Z5NCTA at 550 °C)

Creep-fatigue tests have been performed on nickel base alloy Z5NCTA 33-21 grade 1. Hold time varied from 0, 10, 30, 90, 300, 1440 and 10,080 min at two values of strain range. It was found that the number of cycles to failure N_R decreases with holding time t_m, according to a power law:

N_R=1.88×10³·(t_m)^-0.39.     

超大跨斜拉桥横向地震破坏模式与损伤控制分析

地震作用下控制结构合理的破坏模式是保证其震后安全性的基础,而目前对超大跨度缆索桥梁破坏模式及其控制的研究仍显不足。以一座试设计的主跨1400m斜拉桥为例,采用弹塑性分析方法并引入地震损伤指标研究了地震作用下横向约束体系的地震损伤与破坏模式;为提高结构的整体抗倒塌能力,研究了极端地震作用下损伤控制策略对桥梁地震损伤和破坏模式的影响;在此基础上,进一步研究了主塔上横系梁刚度和耗能能力以及附加耗能构件对主塔地震损伤分布的影响。结果表明：在横向极端地震作用下,主塔上、下塔柱区段几乎同时遭受损伤,发生双塑性铰的破坏模式;附加耗能阻尼器的损伤控制策略可显著控制桥墩的地震损伤,但不能完全有效控制主塔损伤;在优化参数的基础上,若在上塔柱区段附加一定数量的耗能构件,则可合理改善主塔的破坏模式使其满足地震损伤控制目标。     

On the study of the creep damage development in circumferential notch specimens

Numerical calculations with K–R damage law have been performed to study the creep damage development in circular notch specimens under constant loading. The emphasis was placed on the roles of notch radius, material constant- and applied stress. The results show that the distributions of stresses under creep conditions are different from those of previous studies. Creep damage development and life are different for different notch specimens, and the distributions of the maximum creep damage in the minimum cross-section vary with the notch radius. The creep damage is remarkably affected by the applied stress, material parameter- and notch radius. Higher stress and tri-axial stress state parameter- can cause the creep damage to develop faster.     

Effect of temperature gradients and stress levels on damage of C/SiC composites in oxidizing atmosphere

Strain response of a C/SiC composite, which is cycled with ΔT₁ of 500 °C at 50 MPa, ΔT₂ of 400 °C at 100 MPa and ΔT₃ of 300 °C at 150 MPa, was investigated. Measured thermo-elastic strain ranges are found to retain 0.209% for ΔT₁, 0.168% for ΔT₂, and 0.122% for ΔT₃, independent upon the applied stress level. Non-linear variations of thermal cycling creep strain can reflect damage evolutions of the composites by changing its rate, which depends on temperature gradient and applied stress. After 104 thermal cycles, strength, modulus, and failure strain of the composites retain 60.29%, 84.2%, and 59% of the initial properties, respectively. The coating cracks of the cycled specimens are observed to be perpendicular to the applied stresses and arranged at relatively regular spacing, through which the fibers are oxidized superficially.     

Probabilistic failure assessment of ferritic steels using the master curve approach including constraint effects

The present study is concerned with an enhancement of the master curve approach for the probabilistic failure assessment of ferritic steel structures considering constraint effects. In an experimental program based on a variety of different specimens, distinct effects of the specimen geometry on the reference temperature T₀ are observed. The experimental data base is examined in terms of the K-T_stress-, J-A₂-, J-Q- and J-h-concepts. Based on the results, a constraint enhancement for the master curve concept is suggested consisting of a constraint dependent temperature shift or an alternative constraint dependent scaling of the stress intensity factor.     

The fracture behaviour of a welded tubular joint--an ESIS TC1.3 round robin on failure assessment methods: Part I: experimental data base and brief summary of the results

Experimental work was carried out to determine the fracture behaviour of welded tubular T-joints made of high strength TMCP-steel. Besides the load parameters, the crack profiles along the crack front were determined for the T-joints as well as for small SE(B)-specimens at different applied loads. This gave a direct information on the crack driving force. The aim of this work--which was part of an ESIS TC1 (subcommittee 3) activity on failure assessment--was an evaluation of various analytical failure assessment methods by carefully controlled experiments on a typical component. The main results of the various flaw assessment methods applied to the T-joints will be briefly summarised.     

The use of generic failure frequencies in QRA: the quality and use of failure frequencies and how to bring them up-to-date

Quantitative Risk Assessment (QRA) is a method which is often used in the chemical industry and, in some countries, also in land-use planning. In QRA calculations the frequency of an accident scenario is most often assessed by a generic failure frequency approach. The credibility and validity of the failure frequencies used in the Netherlands for land-use planning is evaluated by means of an historical review. Furthermore, the possibility is presented how these generic data can be revised and updated.     

Simulations of dynamic crack propagation in brittle materials using nodal cohesive forces and continuum damage mechanics in the distinct element code LDEC

Experimental data indicates that the limiting crack speed in brittle materials is less than the Rayleigh wave speed. One reason for this is that dynamic instabilities produce surface roughness and microcracks that branch from the main crack. These processes increase dissipation near the crack tip over a range of crack speeds. When the scale of observation (or mesh resolution) becomes much larger than the typical sizes of these features, effective-medium theories are required to predict the coarse-grained fracture dynamics. Two approaches to modeling these phenomena are described and used in numerical simulations. The first approach is based on cohesive elements that utilize a rate-dependent weakening law for the nodal cohesive forces. The second approach uses a continuum damage model which has a weakening effect that lowers the effective Rayleigh wave speed in the material surrounding the crack tip. Simulations in this paper show that while both models are capable of increasing the energy dissipated during fracture when the mesh size is larger than the process zone size, only the continuum damage model is able to limit the crack speed over a range of applied loads. Numerical simulations of straight-running cracks demonstrate good agreement between the theoretical predictions of the combined models and experimental data on dynamic crack propagation in brittle materials. Simulations that model crack branching are also presented.