Weibull master curves and fracture toughness testing Part III Master curves for the evaluation of dynamic Charpy impact tests

The existence of specimen-size-independent quasi-static Weibull master curves for macroscopically homogeneous solids characterizing strength and failure of both purely brittle materials and rather tough materials, which undergo an amount of stable crack growth prior to failure, has already been proved in earlier publications. In this paper, the concept of Weibull master curves is extended to the case of dynamic testing conditions, being typical for Charpy impact tests performed in the ductile-to-brittle transition temperature-range of ferritic-martensitic steels. Dynamic Weibull master curves can be constructed, if the stress-distributions, which are built up in the process zone of the specimens during the Charpy impact tests, can be described with a dynamic quasi-equilibrium approach. In this case, the dynamic Weibull master curves can be related to the quasi-static Weibull master curves with the help of the toughening exponent , characterizing the rate of toughness increase with increasing crack length. Characteristic magnitudes, being most convenient to estimate the capacity of the tested materials to undergo stable crack growth, microcracking and crack-tip shielding prior to rupture, can be derived as well from dynamic Weibull master curves as from quasi-static Weibull master curves.     

Effects of microstructure, specimen and loading geometries on KIC of brittle honeycombs

Fracture toughness of brittle honeycombs depends on cell microstructure, specimen and geometries. A microstructure coefficient in the K_IC expression of brittle honeycombs can not be found analytically. In this paper, a finite element program is utilized to numerically determine the coefficient. In practice, fracture toughness can be measured from conducting a three-point bend or uniaxial tension test. Specimen geometry restrictions of three-point bend test for honeycombs are examined and proposed here. Meanwhile, fracture toughness of honeycombs under the two loading geometries is compared; results show that K_IC measured from uniaxial tension test is smaller than that from three-point bend test if the K_IC formulations for solid materials are employed. As a result of that, the K_IC formulation of three-point bend test is modified for honeycomb-like materials.     

Quantifying Tstress controlled constraint by the master curve transition temperature T0

Specimen size, crack depth and loading conditions may effect the materials fracture toughness. In order to safeguard against these geometry effects, fracture toughness testing standards prescribe the use of highly constrained deep cracked bend specimens having a sufficient size to guarantee conservative fracture toughness values. One of the more advanced testing standards, for brittle fracture, is the master curve standard ASTM E1921-97, which is based on technology developed at VTT Manufacturing Technology. When applied to a structure with low constraint geometry, the standard fracture toughness estimates may lead to strongly over-conservative estimate of structural performance. In some cases, this may lead to unnecessary repairs or even to an early “retirement” of the structure. In the case of brittle fracture, essentially three different methods to quantify constraint have been proposed, J small scale yielding correction, Q-parameter and the T_stress. Here, a relation between the T_stress and the master curve transition temperature T₀ is experimentally developed and verified. As a result, a new engineering tool to assess low constraint geometries with respect to brittle fracture has been obtained.     

Prediction of prestraining effect on fracture toughness of high strength structural steel by local approach

In the present study, the tension and fracture toughness tests on high strength structural steel of Q420 were carried out in different conditions of non-prestraining and prestraining. The results indicated that the prestrain increased the yield stress and tensile strength, but decreased the fracture toughness. Meanwhile, the local parameters m and σ_u controlling the brittle fracture were obtained using finite element method (FEM) analysis. Based on the Weibull stress fracture criterion, the prestraining effect on the fracture toughness was predicted from fracture toughness results of the virgin material by the local approach. The prediction was in good agreement with the experimental results. It certified that the critical Weibull stress obeys the two-parameter Weibull distribution in the local approach, and the fracture behaviour of the material with prestrain can be characterised well by the local approach.     

Weibull master curves and fracture toughness testing Part IV Dynamic fracture toughness of ferritic-martensitic steels in the DBTT-range

Ferritic-martensitic steels are considered to be promising candidates for structural materials in fusion technology. However, they are sensitive to irradiation embrittlement, being characterized by a shift of the ductile-to-brittle transition temperature (DBTT) to higher values. It has already been shown in an earlier publication, that dynamic Weibull master curves are useful in estimating the capacity of materials to undergo stable (micro)cracking prior to brittle failure, if Charpy impact tests are performed in the DBTT-range. Thus, experimental dynamic Weibull master curves of three ferritic-martensitic steels, having been obtained by performing series of instrumented Charpy impact tests at a defined temperature in the DBTT-range, have been evaluated for subsize and, in the case of the reference ferritic-martensitic steel of the European Fusion Technology Program MANET II, for normal-size specimens. It has been observed, that the capacities of stable (micro)cracking of MANET II are nearly optimal and clearly superior to those of the other two considered steels. On the other side, the capacity of stable (micro)cracking prior to failure has been found to be highly specimen-size-dependent, nevertheless it is thought to be an important factor in predicting DBTT-shifts due to irradiation embrittlement.     

Geometry dependent ductile to brittle transition of 10CrMoNbV ferritic–martensitic steel Manet II in terms of critical crack sizes

Abstract

The 10CrMoNbV Manet II cast has been selected as the reference ferritic–martensitic steel in the framework of the European Fusion Technology Programme. Charpy impact tests have been carried out in the ductile to brittle transition temperature range of this steel, such that a dynamic quasi-equilibrium has been achieved in the process zones of investigated specimens before brittle failure. This type of testing enables the evaluation of dynamic Weibull moduli and, consequently, dynamic Weibull master curves. Thus, Weibull parameters have been calculated for normal size and subsize Charpy impact specimens. The evaluated, geometry dependent dynamic Weibull master curves facilitate computation of the failure probability densities of the investigated steels as functions of scaled critical crack sizes or scaled initial defect sizes.     

INVESTIGATION OF THE ROLE OF RESIDUAL STRESSES IN THE WARM PRESTRESS (WPS) EFFECT. PART I—EXPERIMENTAL

Abstract— The role of residual stresses in the warm prestress (WPS) effect has been investigated. Three types of specimen have been tested in this investigation: smooth uniaxial tensile specimens, blunt notched single edge notched bend (SENB) specimens and sharply precracked SENB specimens. Room temperature prestraining of uniaxial tensile specimens leads to a dramatic decrease in the measured nominal fracture stress at — 196°C. Such an embrittling effect may be expected to reduce the beneficial increase in subsequent fracture toughness commonly observed in WPS sequences. The blunt-notched specimens were prestressed in tension and compression. Compressive prestressing was found to lead to a decrease in subsequent fracture load whereas tensile prestressing leads to an increase. The load decrease following a compressive WPS was greater than the load increase following a tensile WPS. Various sequences of loading, unloading and cooling have been investigated and the differences in the subsequent fracture behaviour of specimens have been explained qualitatively by superposition arguments. The theories of Chell and Curry have been supported by the general trend of results.     

Experimental study on mechanical properties and fracture toughness of structural thick plate and its butt weld along thickness and at low temperatures

The thick plate induces the variation of mechanical properties and fracture toughness, especially in cold regions. At the low temperature, the brittle behaviour of steel becomes worse. A series of tests (such as uniaxial tensile test and three‐point bending test) were carried out at low temperature to investigate the mechanical properties and fracture toughness of structural steel plates of Q345B with thickness of 60 to 150 mm, as well as the fracture toughness of 150 mm thick butt welded plate. The test specimens are all manufactured from plates along thickness with small size, and the tensile test specimens included through‐thickness specimens additionally. The ductility index (percentage reduction of area) and the fracture toughness index (critical CTOD values) all decrease with the temperature decreases and the distance from plate surface increases. The results obtained in this paper provide technical basis for preventing brittle fracture of thick plate steel structures in cold regions.     

Correlation between quasi-static and dynamic crack resistance curves

The correlation between the crack resistance measured from dynamic and quasi-static J-R tests has been investigated based on test data from instrumented Charpy impact tests and quasi-static single edge notched bend (SENB) J-R tests.The method originally proposed by Aurich et al. [Analyse und Weiterentwicklung Bruchmechanischer versagenskonzepte--Lokales Risswachstum, Ermittlung des Risswiderstandsverhaltens aus der Kerbslagarbeit. BAM Forschungsbericht 192, Berlin 1993, ISBN 3-89429-329-2], to develop a correlation coefficient between the net fracture resistance of Charpy V-notch (ISO) and quasi-static compact tension tests, was extended to establish correlations between so-called ductile-brittle transition fracture resistance curves obtained from instrumented Charpy V-notched (ISO) and fatigue precracked impact tests and quasi-static SENB J-R fracture resistance tests.The correlation between the dynamic and quasi-static crack resistance with regard to practical application to fracture assessments appears to be reasonably consistent, bearing in mind the inherent scatter in crack resistance data in general. The findings of the present research project from testing of ship grade NVE 36 and pressure vessel steel to ASTM A516 Gr. 70 as well as two weldments in the NVE 36 steel, are in excellent agreement with the results of the research conducted by Aurich et al. [Analyse und Weiterentwicklung Bruchmechanischer versagenskonzepte--Lokales Risswachstum, Ermittlung des Risswiderstandsverhaltens aus der Kerbslagarbeit. BAM Forschungsbericht 192, Berlin 1993, ISBN 3-89429-329-2] for structural and pressure vessel steels to the German DIN standard steels including St 52-3, StE 355 and StE 460.     

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.     

An experimental study of stress wave transmission at a metallic-rock interface and dynamic tensile failure of sandstone, limestone, and granite

An experimental technique is described for determination of dynamic tensile fracture strength of brittle solids. This technique has been used to determine the dynamic tensile fracture strength of several types of rock. The rocks studied were granite, limestone, and sandstone; the specimens were cored perpendicular to the bedding plane for these rocks. The quasi-static fracture strengths of the same solids were also determined for comparison with the dynamic strengths. The dynamic strengths have been found to be several times the quasi-static strengths, thus showing a strong dependence of fracture strength on strain rate.     

Modelling the quasi-static behaviour of bituminous material using a cohesive zone model

This paper investigates the applicability of a cohesive zone model for simulating the performance of bituminous material subjected to quasi-static loading. The Dugdale traction law was implemented within a finite volume code in order to simulate the binder course mortar material response when subjected to indirect tensile loading. A uniaxial tensile test and a three-point bend test were employed to determine initial stress-strain curves at different test rates and the cohesive zone parameters (specifically, fracture energy and cohesive strength). Numerical results agree well with the experimental data up to the peak load and onset of fracture, demonstrating the value of the cohesive zone modelling technique in successfully predicting fracture initiation and maximum material strength.     

A survey of test methods for scale adhesion measurement

Abstract

The spallation of oxide scales from the steam side of tubes in power generation boilers can severely limit component life time. The process involves through scale cracking and crack propagation at or parallel to the scale/alloy interface. Many techniques have been developed in the past two decades to assess the interfacial fracture toughness of brittle coatings on ductile substrates. Analyses from fracture mechanics approaches also allow for quantitative determination of the fracture toughness, which can be used for life time prediction modelling. This paper gives an overview on techniques that have been used on thermally grown oxides, such as Ni, Fe, Ti, Al, and Cr oxides and spinels, and compare the results obtained on each type of oxide system from different techniques. In doing so, the limitations of each technique are illustrated. The methods discussed are flexure, scratch, indentation, tensile and compression tests and those that perform direct analysis of the spallation process. The data on steam grown oxide is sparse, but the test methods discussed could all be applied to these scale types.     

Tenacite de WC-Co 15 %

Fracture mechanics have been applied to the brittle material, WC-Co 15 wt. % by a bend test of notched specimens in order to determine, by several methods, the fracture energy γ and the fracture toughness. The experimental results indicate that the K_IC value so determined represents a lower limiting value of fracture toughness, which is 19 ± 2 MP√m. The critical notch radius is about 70 μm.     

Reinvestigation of the tensile strength and fracture property of Ni(1 1 1)/α-Al2O3(0 0 0 1) interfaces by first-principle calculations

First-principle calculations are performed to reinvestigate the mechanical tensile property and failure characteristic of Ni/Al₂O₃ interfaces, in order to clear the inconsistence existed in the literatures. Four types of interface models without initial lateral stresses are used, i.e., Al-terminated O-site, O-terminated Al-site, Al-terminated Al-site and Al-terminated H-site models. Two kinds of tensile methods, viz., uniaxial extension and uniaxial tension, are adopted to check the mechanical responses of these interface models. It is found that the results under uniaxial extension are generally consistent with those under uniaxial tension, including the overall shapes of stress–strain curves and the values of tensile strengths. Moreover, the initial lateral stresses have an apparent influence on the mechanical properties of the interfaces during the loading process, such as tensile strength, fracture strain and the work of separation. Our simulation results also clarified that, under tensile loading, the most stable O-terminated Al-site interface model tends to fracture in a brittle way along the sublayer between in-plane Ni–Ni atomic bonds, while all of the Al-terminated interface models will fail in a ductile fracture manner with relatively lower stress levels, breaking along the interlayer between the Ni(1) and Al(1) layers.     

STATISTICAL MODELLING OF INTERGRANULAR BRITTLE FRACTURE IN A LOW ALLOY STEEL

The intergranular brittle fracture (IBF) behaviour of a low alloy steel 16MND5 (A508 Cl. 3) was investigated. A temper embrittlement heat treatment was applied to the material to simulate the effect of local brittle zones (ghost lines) which can be found in the as-received material condition. An increase in the Charpy V toughness transition temperature and a significant decrease in the fracture toughness measured on CT-type specimens were observed in the embrittled material, as compared to the reference material which was submitted to the same austenitizing and tempering heat treatment, but which was not subjected to the temper embrittlement treatment. Tensile tests on notched specimens were carried out to measure the Weibull stress and scatter in the results. A statistical model, the Beremin model, originally proposed for brittle cleavage fracture was applied to IBF. It is shown that this model is not able to fully account for the results, in particular for the existence of two slopes in a Weibull plot. Systematic fractographic observations showed that the low slope regime in this representation was associated with the existence of MnS inclusions initiating brittle fracture, while the larger slope was related to microstructural defects. Initiation of IBF from MnS inclusions can occur when the material is still elastically deformed while the second population of microstructural defects is active in the plastic regime. A modified statistical model based on the Beremin model and taking into account these specific aspects is proposed in the framework of the weakest link theory. The parameters of this model are identified from test results on notched specimens. It is shown that this model is able to predict the temperature dependence of fracture toughness and the scatter in the experimental results.     

Fracture toughness prediction using Weibull statistical method for asphalt mixtures containing different air void contents

Brittle fracture of asphalt mixtures at low temperatures is one of the main deterioration modes of pavements. Hence as an important design parameter, it is required that a reliable value for fracture toughness of asphalt mixtures is known. However, because of natural inhomogeneity of asphalt mixtures and inherent sources of scatters such as random distribution of ingredients and preparation process, the use of statistical analyses might provide better estimations for the crack growth resistance of asphalt mixtures. In this paper by conducting several low temperature fracture toughness experiments on three types of asphalt mixtures with different air void contents, the effects of air void percentage on mode I fracture toughness are studied statistically. Fifty six edge cracked semi-circular bend specimens containing 4, 5 and 7% air voids were tested, and the corresponding two and three-Weibull distribution parameters were determined for each set of data. It was shown that the Weibull model can be used successfully for predicting the statistical nature of tensile cracking phenomenon in asphalt mixtures. The mean fracture toughness values and the Weibull parameters were reduced by increasing the air void content. Furthermore, the distribution parameters obtained experimentally for the mixtures containing 4% and 5% voids were also predicted quite well in terms of the Weibull parameters of a reference mixture containing 7% air void.     

Influence of aggregate deformation and contact behaviour on discrete particle modelling of fracture of concrete

The discrete element method, DEM, has been used in fracture studies of non-homogeneous continuous media adopting circular or spherical particles. A 2D circular rigid DEM formulation developed with the purpose of modelling concrete is described and evaluated in uniaxial tensile and compression tests. According to this model, the aggregate can be modelled either as a rigid macro-particle or as a deformable group of particles. The inter-particle contacts can either be assumed as brittle or follow a given bilinear softening curve. It is shown that aggregate deformability, together with the consideration of pure friction contacts working under compression, increases the fracture energy in compression, leading to a better agreement with concrete tests. The softening contact model, by adding a higher capability of load redistribution, is shown to give a better agreement than the brittle model under tensile loading. The recognized crack mechanisms of the brittle model (tensile splitting, branching, bridging) are also present with softening.     

Fracture toughness testing of hard metals using compression-compression precracking

Compression-compression precracking of brittle materials has recently been applied to fracture toughness testing. This paper reports the results of an experimental programme of fracture toughness testing of a WC-Co alloy containing 10% cobalt by weight. Tests were performed on specimens precracked by cyclic compression and on specimens in which this compression-compression precrack was subsequently extended by tensile fatigue. Toughness data obtained in this way were compared with the results of short-rod toughness tests. The causes of differences etween these various data are discussed.