Mechanism of effects of warm prestressing(WPS) on apparent toughness of notched steel specimens: Part I: Experimental

In this investigation, a series of experiments are carried out in blunt notched specimens to explore the various factors controlling the warm prestressing (WPS) effect on apparent toughness of a HSLA steel. A great number of specimens were tested using Cool-Fracture (CF) without WPS, Load-Cool-Fracture (LCF) and Load-Unload-Cool-Fracture (LUCF) cycles. More complex cycles have also been used to produce residual stress distributions and notch deformations different in quantities and signs. All fracture surfaces of the specimens were observed. Some specimens were unloaded after WPS and details of microscopic features in front of notch roots were investigated. Experimental results show that warm prestress cycles raising the residual compressive stress and opening the notch root improve notch toughness at low temperatures. Oppositely, WPS cycles raising the residual tensile stress and closing the notch root deteriorate notch toughness. One distinct effect of WPS involves deactivating inclusions and second phases particles. With increasing the preload of WPS, more and more particles being potential cleavage nuclei are decohered and blunted to cavities. This effect is proposed to be involved in improvement of notch toughness.     

Dynamic ductile crack growth and transition to cleavage – a cell model approach

The fracture behavior of ferritic steel in the transition regime is controlled by the competition between ductile tearing and cleavage. Many test specimens that failed by catastrophic cleavage showed significant amounts of ductile tearing prior to cleavage fracture. The transition from ductile tearing to cleavage has been attributed to the increase in constraint and sampling volume associated with ductile crack growth. This work examines the role of dynamic ductile crack growth on the fracture mode transition by way of a cell model of the material. The cell model incorporates the effects of stress triaxiality and strain rate on material failure characteristics of hole growth and coalescence. Loading rate and microstructure effects on the stress fields that evolve with rapid (ductile) crack growth are systematically studied. The stress fields are employed to compute the Weibull stress which provides probability estimates for the susceptibility to cleavage fracture. A center-cracked panel subjected to remote tension is the model problem under study. The computational model uses an elastic-viscoplastic constitutive relation which incorporates enhanced strain rate hardening at high strain rates. Adiabatic heating due to plastic dissipation and the resulting thermal softening are also accounted for. Under dynamically high loading rate, our model shows the crack speed achieves its peak value soon after crack initiation and quickly falls off to slower speeds with further crack growth. Remarkably, the Weibull stress follows a similar pattern which suggests that the transition to the cleavage fracture is most likely to occur, if at all, at the peak speed of ductile crack growth. Key words: Dynamic fracture, ductile tearing, crack growth, transition regime, cleavage fracture, cell model, finite element.     

Probabilistic treatment of fracture using two failure models

A probabilistic methodology for brittle fracture based on two local failure models is presented. Probabilistic fracture parameters are obtained using a weakest link and a chain-of-bundles formulation. Both models define limiting distributions for the fracture stress described by a two-parameter Weibull distribution. Numerical procedures employing measured toughness data and finite element solutions are also described to calibrate the Weibull parameters. An application of the methodology then follows to predict geometry and stable crack growth effects on the distribution of macroscopic fracture toughness (J_c) for a high-strength steel. Measured fracture toughness values for a high-constraint geometry that exhibit no prior ductile tearing are effectively ‘transferred' to a different geometry having much lower constraint and in which tearing precedes cleavage. The inherent difficulty in predicting the scatter of experimental fracture toughness, as well as constraint and ductile tearing effects, within the scope of conventional procedures appears greatly reduced in the framework presented in this work.     

Analysis of ductile to cleavage transition in part-through cracks using a cell model incorporating statistics

This paper describes an approach to study ductile/cleavage transition in ferritic steels using the methodology of a cell model for ductile tearing incorporating weakest link statistics. The model takes into account the constraint effects and puts no restriction on the extent of plastic deformation or amount of ductile tearing preceding cleavage failure. The parameters associated with the statistical model are calibrated using experimental cleavage fracture toughness data, and the effect of threshold stress on predicted cleavage fracture probability is investigated. The issue of two approaches to compute Weibull stress, the 'history approach' and the 'current approach', is also addressed. The numerical approach is finally applied to surface-cracked thick plates subject to different histories of bending and tension, and a new parameter, ψ, is introduced to predict the location of cleavage initiation.     

Statistical assessment of notch toughness against cleavage fracture of ferritic steels

The work is an initial effort on adopting a statistical approach to correlate the fracture behavior between a notched and a fracture mechanics specimen. The random nature of cleavage fracture process determines that both the microscopic fracture stress and the macroscopic properties including fracture load, fracture toughness, and the ductile to brittle transition temperature are all stochastic parameters. This understanding leads to the proposal of statistical assessment of cleavage induced notch brittleness of ferritic steels according to a recently proposed local approach model of cleavage fracture. The temperature independence of the 2 Weibull parameters in the new model induces a master curve to correlate the fracture load at different temperatures. A normalized stress combining the 2 Weibull parameters and the yield stress is proposed as the deterministic index to measure notch toughness. This proposed index is applied to compare the notch toughness of a ferritic steel with 2 different microstructures.     

Numerical prediction of notch bluntness effect on fracture resistance of SM490A carbon steel

The present work investigates the notch radius effect on fracture resistance using the finite element (FE) damage analysis based on the multiaxial fracture strain model. The damage model was determined from experimental data of notched bar tensile and fracture toughness test data using a sharp‐cracked compact tension specimen. Then, the FE damage analysis was applied to simulate fracture resistance tests of SM490A carbon steel specimens with different notch radii. Comparison of simulated results with experimental data showed good agreement. Further simulation was then performed to see effects of the specimen size, thickness, and side groove on J‐R curves for different notch radii. It was found that effects of the specimen size and thickness became more pronounced for the larger notch radius. Furthermore, it was found that without side groove, tearing modulus for notched specimens was similar to that for cracked specimens, regardless of the notch radius.     

STRUCTURAL COLLAPSE, THE FAILURE ASSESSMENT DIAGRAM AND "OFF-THE-SHELF" OPERATION

Abstract— The criteria for determining whether ferritic material exhibits fully ductile behaviour are generally based on the fracture toughness vs temperature relationship determined from standard laboratory test pieces (e.g. Charpy V Impact tests or fracture toughness specimens). This relationship defines a ductile-brittle transition region. When fully ductile, microvoid coalescence behaviour is experienced, and fracture toughness is described as being on the “upper shelf”. At “off the shelf” temperatures brittle, cleavage fracture is experienced. On the lower shelf the material is entirely brittle, exhibiting 100% crystallinity on the fracture face. As the temperature increases, initiation of tearing by microvoid coalescence occurs and some stable tearing can occur prior to the cleavage event. Material toughness increases with temperature until the upper shelf condition is achieved. The characteristics of fracture toughness tests in terms of the toughness level exhibited and the extent of ductile tearing experienced have been used as a guide to whether the structural application (e.g. a pressure vessel) will behave in a brittle or a ductile manner. This paper reports on a feasibility study where various worked examples have been performed to examine the concept of using a “cut off” on the failure assessment diagram, determined from the conditions required to cause plastic collapse of a pressure vessel, as a criterion for defining effective “ductile” operation. Fracture assessment procedures (R6 revisions 2 and 3 and PD6493 levels 2 and 3) have been utilised to determine the influence on pressure vessel performance of the behaviour of fracture toughness test specimens. The procedure of plotting a structural collapse “cut off” on a failure assessment diagram enables the assessment of whether a particular flaw geometry would result in gross deformation of the structure at failure. The use of this procedure provides an unambiguous demarcation between “fracture dominated” and “collapse controlled” conditions. This procedure facilitates judgements on the level of toughness necessary to ensure ductile operation and whether a “tearing plus toughness” requirement is necessary. It is recommended that consideration be given to including structural collapse into fracture assessment procedures carried out using R6 revision 3 or PD 6493:1991 procedures in order to determine the conditions when enhanced toughness no longer influences structural performance (i.e. when effective “upper shelf” conditions are attained).     

Three dimensional finite element investigations into the effects of thickness and notch radius on the fracture toughness of polycarbonate

Fracture toughness of polycarbonate (PC), a commercially important glassy amorphous polymer, is known to be sensitively dependent on a number of factors including molecular weight, ageing time, loading rate and specimen geometry. In this work, we analyze the effect of notch radius and specimen thickness on the near tip fields and the consequence of these on the mode I fracture initiation. To this end, we have performed extensive three dimensional Finite Element simulations within the framework of large deformation elasto-plasticity based on a realistic constitutive model that has been carefully calibrated for PC. Using a simple set of criteria for fracture initiation by void nucleation or ductile tearing, we are able to reproduce experimentally observed brittle to ductile transitions that occur in PC with decrease in thickness and increase in notch radius.     

A probabilistic model for cleavage fracture with a length scale--parameter estimation and predictions of stationary crack experiments

This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different crack depth over specimen width ratios to capture the strong constraint effect on fracture toughness. Three test temperatures were considered, covering a range of 85 °C. All specimens failed by cleavage fracture prior to ductile tearing. A recently proposed probabilistic model for the cumulative failure by cleavage was applied to the comprehensive sets of experimental data. This modified weakest link model incorporates a length scale, which together with a threshold stress reduce the scatter in predicted toughness distributions as well as introduces a fracture toughness threshold value. Model parameters were estimated by a robust procedure, which is crucial in applications of probabilistic models to real structures. The conformity between predicted and experimental toughness distributions, respectively, were notable at all the test temperatures.     

The fracture behaviour of intermetallic TiAl alloys with and without warm pre-stressing

The fracture behaviour of near- (NG) and fully lamellar (DFL) intermetallic TiAl alloys with and without warm pre-stressing (WPS) is investigated by testing three point bending specimens with fatigue pre-cracks at room temperature and at 700 °C. Detailed fractographic observations and FEM calculations are carried out to find the critical step of cleavage fracture. The results show that the cleavage fracture is induced by direct propagation of the pre-crack and that the stress at the crack tip is the decisive controlling factor. The WPS process improves the fracture initiation toughness of both the NG and the DFL microstructure, but deteriorates slightly the increase of the K-resistance curves, especially for the DFL microstructure. The compressive residual stress induced by WPS plays the main role in improving the fracture initiation toughness.     

基于微观机制的复杂应力状态下钢材韧性断裂行为研究

韧性断裂是钢材最常见的破坏形式,研究钢材韧性断裂机理并准确预测钢材韧性断裂行为具有重要的理论意义和工程实用价值。基于微观机制的断裂预测方法对研究钢材韧性断裂行为有较好的适用性。该文基于体胞模型空穴演化机理改进了现有的韧性断裂模型,校核了Q345钢材断裂模型参数。此外,在韧性断裂模型中引入损伤因子,以考虑应力状态在加载过程中的变化,使断裂模型能准确描述每一加载时刻的累积损伤值。文末采用Fortran语言将断裂模型编写USDFLD子程序,并将其植入有限元程序ABAQUS,对一组十字型刚节点试件单轴拉伸试验进行数值模拟。结果表明,该断裂模型在拉-剪复合应力状态下具有良好的预测精度,且能够准确捕捉钢材断裂起始位置及裂缝扩展路径。该文改进的韧性断裂模型也可用于其它韧性金属材料断裂预测分析。     

Development of the Euro fracture toughness dataset

Ten European laboratories have generated the Euro fracture toughness dataset in order to provide an experimental data base sufficiently large to study specimen size and temperature effects on cleavage fracture toughness in the ductile-to-brittle fracture transition regime. The Euro fracture toughness dataset quantifies the fracture behaviour of the quenched and tempered pressure vessel steel DIN 22NiMoCr37. This material is frequently used in nuclear power plants. About 800 fracture toughness tests were performed using compact tension specimens with a size range from 1/2T to 4T.In the lower shelf temperature regime, no significant specimen size effects on cleavage fracture toughness scatter was observed. At higher temperatures, the lower tails of the toughness scatter bands are not significantly effected by the specimen size but with decreasing specimen size the toughness scatter increases due to the fact that the upper part of the scatter band is extended. The presence of a specimen size effect on fracture toughness scatter coincides with the appearance of single cleavage initiation sites at the fracture surface. At the lower shelf temperature both, cleavage initiation sites and size effects are not observed whereas at higher test temperatures both phenomena are present. The specimen size effect trends and the corresponding fracture surface morphology support a weakest-link type cleavage fracture mechanism in the ductile-to-brittle transition regime. A unique correlation between the amount of ductile tearing and cleavage fracture toughness was observed for the steel investigated. This result offers the possibility to determine cleavage fracture toughness from post-test fracture surface examinations.Due to the large number of tests and the wide range of testing conditions, the Euro fracture toughness dataset gives a comprehensive insight into specimen size effects and temperature effects on ductile-to-brittle transition fracture. The Euro fracture toughness dataset includes a large set of raw test data such as load versus load line displacement curves and raw tensile test data for deriving stress-strain curves. The Dataset can be downloaded from the internet via the address ftp://ftp.gkss.de/pub/eurodataset.     

Assessment of effect of ductile tearing on cleavage failure probability in ductile to brittle transition region

The fracture behaviour of ferritic and ferritic martensitic steels in ductile to brittle transition (DBT) region has been extensively studied in recent years and a probabilistic approach of master curve method is generally used to describe the fracture toughness of BCC steels in DBT region as a function of temperature. The assessment of cleavage failure probability however is still untouched in the upper region of ductile to brittle transition, although various extensions of master curve approach and various local approaches has been explored. Additionally the geometry and loading in tension and bending also adds up to the difficulties when cleavage failure is assisted with prior ductile tearing. In this work the cleavage fracture is investigated in upper region of DBT and a modified master curve approach is presented which can satisfactorily describe the fracture toughness as a function of temperature as well as amount of ductile tearing preceded by cleavage.     

Effect of pre-strain on fracture toughness of ductile structural steels under static and dynamic loading

The ductile fracture process consists of void nucleation, growth and coalescence. The whole ductile process can be divided into two successive steps: (I) the initial state to void nucleation, followed by (II) void growth up to void coalescence. Based on this suggestion, resistance to ductile fracture could be divided into the resistance to stage I and stage II, and accordingly the whole fracture toughness could be regarded to be due to contributions from stages I and II. The fracture toughness contributed from the two steps is, respectively, denoted as void nucleation-contributed fracture toughness and void growth-contributed fracture toughness. The effect of plastic pre-strain on the fracture toughness of ductile structural steels under static and dynamic loading (4.9 m/s) within the ductile fracture range was evaluated by summing contributions due to void nucleation-contributed and void growth-contributed fracture toughness. The effect of strain rate on fracture toughness was also investigated by the same means. The results show that both plastic pre-strain and high-speed loading decrease the void nucleation-contributed fracture toughness while their effects on the void growth-contributed fracture toughness depend on the variations in strength and ductility. Moreover, fracture toughness of structural steels generally decreases with increasing strain rate.     

Simulation of Stable Crack Growth for Welded Joints Including Strength Mismatching

1.IotroductionStrengthmismatchingeffectsonfracturetough-nessofferritesteelsremainakeyissueforthesafetyassessmentofstructures.Structuralandpressureves-selsteelsgenerallyexhibitincreaseinfracturetough-nessoverthefirstfewdistanceofstablecrackgrowth.Laboratorytestingoffracturespecimenstomeasureresistancecurves(R-curves)consistentlyrevealsamarkedeffectofstrengthmismatchingonR-curves.Fortheweldedjoint,whoseweldmetalstrengthishigherthanthatofthebasemetal(overmatching),yieldahigherRcurvecomparingwit…     

Warm pre-stress based pre-cracking criteria for fracture toughness testing

There are presently a magnitude of different fracture toughness testing standards that have different criteria for fatigue pre-cracking specimens prior to testing. The reason for the criteria is that too high pre-fatigue load may influence the subsequently measured fracture toughness value. The criteria have to a large extent been developed specifically for each standard in question and this has lead to the considerable variability in the criteria. The basic reason for the pre-fatigue having an effect on the fracture toughness is the warm pre-stress (WPS) effect. Here, existing data relating to pre-fatigue load levels are examined with the help of a newly developed simple WPS correction and a criteria and correction procedure for too high pre-fatigue loads are proposed. The new criteria focuses on brittle fracture, but is equally applicable for ductile fracture, thus enabling a unification of pre-fatigue criteria in different fracture toughness testing standards.     

Numerical modelling of fracture initiation in large steel specimens at impact

Dynamic mode I fracture initiation in impact loaded single edge bend specimens with a quarter notch is investigated by numerical modelling and the results are compared with sets of experimental data from two different steel qualities. The finite element analysis include 2D (two-dimensional) plane strain, 2D plane stress and 3D models. No crack growth is included in the calculations. The impact velocities are approximately 15, 30 and 45 m/s and the specimen size is 320×75 mm² with a thickness of 20 or 40 mm. Some specimens have side grooves. Details of the deflection of the specimens are accurately reproduced prior to crack initiation both by the 2D plane strain model and by the 3D model.The experiments were performed in the ductile to brittle transition region. It is assumed that cleavage fracture initiation can be predicted by the Ritchie-Knott-Rice (RKR) model, i.e. cleavage fracture initiates when the opening stress exceeds the macroscopic cleavage stress over a fixed, critical distance. At an impact velocity of 15 m/s, fracture initiation by void nucleation and growth is observed, though the RKR-conditions is seemingly fulfilled according to the computational results. Possible limitations in the use of the RKR model are discussed.     

Micromechanism of fracture in Al/SiC composites

An SEM study was made of the micromechanism of fracture in SiC particle-reinforced 6061 aluminium composites. The fracture toughness tests on the composites with SiC volume fractions of 0%, 10% and 20% were conducted on single-edge notched sheet specimens. Both qualitative observations of void nucleation at the notch root of the composite samples and quantitative measurements of crack profiles are made to assess the special role of the particle effects in these composites. The results are discussed with respect to the micromechanism of particle breakage and interface debonding and their effect on the nucleation and propagation of microcracks. Two kinds of void are defined to explain the facts that Al/SiC is brittle macroscopically and ductile microscopically. The direction of crack propagation in Al/SiC and the microstructure in the tip region of the crack are also studied with these results.