Temperature Dependence Of Fracture Toughness Research Articles

Indentation strength method to determine the fracture toughness of La0.58Sr0.4Co0.2Fe0.8O3-δ and Ba0.5Sr0.5Co0.8Fe0.2O3-δ

The temperature-dependent fracture toughness of brittle ceramics can be conveniently assessed from bending tests of specimens with defined cracks introduced by indentation. However, the validity of this indentation strength in bending method (ISM) depends critically on the correct consideration of the residual stress induced by the indentation process. The ISM has been applied to La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) and, for comparison, on Ba0.5Sr0.5Co0.2Fe0.8O3-δ (BSCF) perovskite. LSCF with rhombohedral phase exhibits ferro-elastic behavior at ambient temperature, whereas BSCF deforms linear-elastically. Pre-indented specimens of both perovskites were fractured at room temperature in biaxial bending, some of them after an additional annealing step. The fracture toughness values of BSCF match reasonably well when determined with equations which consider the presence or absence of residual indentation stress. Interestingly, annealing has little influence on the apparent toughness results obtained for rhombohedral LSCF, which appears to be related with stress relaxation by ferro-elastic deformation.

Temperature and Geometry Dependence of Fracture Toughness in "Euro Fracture Dataset"

Reliable prediction of fracture conditions is a major concern in the integrity assessment of structural components. This is specifically critical within the transition regime where there is a significant scatter in fracture test data. In recent years local stress based approaches that use a "Weibull distribution function" have been examined to predict probability of cleavage fracture at lower shelf temperature. Furthermore the role of constraint in toughness prediction has been noted. An extensive experimental programme known as "Euro fracture dataset" aimed at characterisation of the "Ductile-to-Brittle" transition (DBT) behaviour of ferritic steels. Recently this data set was used by authors to propose a set of "Global" equations for determination of temperature and thickness dependence of Weibull distribution parameters. In this paper finite element simulations of fracture tests are carried out firstly to verify the experimental findings and secondly to examine and validate the proposed "Global" equations. This objective has been achieved through the comparison between the experimental data, predictions of "Global" curves and the results of performed finite element simulations.

The Effect of Oxygen on the Brittle-to-Ductile Transition in Silicon Single Crystals

The brittle-to-ductile transition (BDT) in Czochralski (CZ) grown silicon single crystals and floating-zone (FZ) grown silicon single crystals was investigated by three-point bending. The temperature dependence of the apparent fracture toughness was measured in three different cross-head speeds. It was found that the BDT temperature in the CZ silicon crystal was higher than that in FZ silicon crystal, suggesting that the solute oxygen decreases dislocation mobility. However, the activation energies obtained from the strain rate dependence of the BDT temperatures were nearly the same in both the CZ and FZ silicon crystals, indicating that the dislocation mobility is not influenced by the solute oxygen. In this paper, the origin of the difference in the BDT temperature is discussed, focusing the role of the solute oxygen on the dislocation glide.

Temperature dependent fracture toughness of glass frit bonding layers

Glass frit bonding is an important technology for the hermetical encapsulation of microsensors. During the manufacturing process or in application the bonding layer is repeatedly exposed to temperature changes. Therefore a meaningful stability assessment must include temperature dependent fracture toughness parameters. This work shows that the influence of temperature changes on the fracture toughness depends on whether the bonding layer is subjected to externally pure tensile or combined tensile and shear loading. The reasons for this effect are discussed by use of finite element simulations. Two different kinds of cracks are compared with regard to the residual stresses that result from the wafer bonding process. These residual stresses have a high influence on the loading conditions at a crack tip, when the crack is kinking into the glass frit material. However the influence of residual stresses on an interfacial crack that propagates parallel to the plane of the bonding layer is almost negligible.

The Effect of Boron/Antimony on the Brittle-to-Ductile Transition in Silicon Single Crystals

The brittle-to-ductile transition (BDT) in boron or antimony doped Czochralski (CZ) silicon single crystals was investigated by three-point bending. The temperature dependence of the apparent fracture toughness was measured in three different crosshead speeds, indicating that the BDT temperature in boron doped silicon is the same as that in non-doped one while the BDT temperature in antimony doped silicon is lower than that in non-doped one. The activation energy was obtained from the deformation rate dependence of the BDT temperature, suggesting that the dislocation velocity in boron doped silicon is the same as that in non-doped while the dislocation velocity in antimony doped is larger than that in non-doped one. [doi:10.2320/matertrans.M2010048]

Prediction of the temperature dependence of fracture toughness of 12Cr–2Ni–Mo refractory steel

We study the influence of temperature and the size of the specimens on the characteristics of static crack resistance of 12Cr-2Ni-Mo refractory steel. It is shown that, in the temperature range 20-450°C, the increase in the thickness of specimens leads to an insignificant increase in fracture toughness obtained along a 5% secant line according to the standards of evaluation of the characteristics of crack resistance. The evaluation of the characteristics of crack resistance of 12Cr-2Ni-Mo steel with regard for the scale effect according to an earlier developed numerical-experimental model reveals the existence of satisfactory agreement with the experimental data in the entire investigated temperature range.

Analysis of applicability of small-sized specimens to prediction of temperature dependence of fracture toughness

The paper addresses the use of small-sized specimens of various types, including those with deep (50%) side grooves, for the purpose of fracture toughness prediction. The experimental data for numerous (more than 500) small-sized specimens prepared from materials of various degrees of embrittlement are compared to the test results for full-sized specimens of C(T) type. The concepts of Master Curve and Unified Curve are applied for the processing of experimental data. To handle the test results for small-sized deep-grooved specimens a calculation procedure has been elaborated, which adjusts the calculation method specified in the ASTM Standard E 1921. We provide recommendations of how to use precracked Charpy type deep-grooved (50%) specimens for prediction of a representative temperature dependence of fracture toughness.

Structural integrity assessment aspects of the Master Curve methodology

The Master Curve (MC) methodology, introduced more than two decades ago, has evolved, from only being a brittle fracture testing and analysis procedure, to a technological tool capable of addressing many more structural integrity issues like constraint and parameter transferability. The MC enables a complete characterization of a material’s brittle fracture toughness based on only a few small size specimens. The MC method has been shown to be applicable for practically all steels with a body-centered cubic lattice structure, generally identified as ferritic steels. The method combines a theoretical description of the scatter, a statistical size effect and an empirically found temperature dependence of fracture toughness. The fracture toughness in the brittle fracture regime is thus described with only one parameter, the transition temperature T 0. The basic MC method has been standardized in the ASTM standard E1921, the first standard that accounts for the statistical specimen size effect and variability in brittle fracture toughness. In this presentation some of the more resent advances of the MC technology are highlighted, with special emphasis on problems related to the use of the Master Curve in structural integrity assessment.

The Effect of Arsenic on the Brittle-to-Ductile Transition in Si Single Crystals

The brittle-to-ductile transition (BDT) in arsenic doped (001) CZ silicon single crystals has been experimentally studied. The temperature dependence of apparent fracture toughness was measured by three-point bending tests at various strain rates. The BDT temperature in arsenic doped silicon was found to be lower than that in non-doped. The activation energy was obtained from the strain rate dependence of the BDT temperature. It was found that the value of the activation energy in the arsenic doped silicon is lower than that in non-doped, suggesting that the dislocation velocity in the silicon single crystal was increased by arsenic doping. The effect of increasing in dislocation velocity on the BDT temperature was also investigated by two-dimensional discrete dislocation dynamics simulations, indicating that the BDT temperature is decreased by increasing in dislocation velocity.

Relationships between Microstructural Parameters and Properties of 15NiCuMoNb5 Grade Steel Used for Special Industry Applications

This article is focused on optimization of fundamental utility properties and technological process of advanced low alloy steel 15NiCuMoNb5 used for power engineering mainly for production of large capacity steam boilers. Consequently problem critical locality in weld after post weld heat treatment is studied. Optimization of welding technology is proposed. Fracture behaviour of welded joint made from boiler plates 80 mm in thickness of 15NiCuMoNb5 type steel after N + T PWHT regime after welding procedure were studied. Stable crack growth measurement and evaluation of fracture toughness of base material as well as weld metal in temperature region from 0 up 350°C were carried out. Unstable cleavage fracture and fracture toughness KJc of both parts of welded joint at test temperatures cover the transition region were evaluated. The reference temperature T0 approach was used for evaluation of temperature dependence of fracture toughness in transition range.

Temperature dependence of fracture toughness of silica/epoxy composites: Related to microstructure of nano- and micro-particles packing

Temperature dependence of the fracture toughness of epoxy composites reinforced with nano- and micro-silica particles was evaluated. Epoxy composites containing varied composition ratios Φ SP of spherical nano- and micro-silica particles, 240 nm and 1.56 μm, were prepared at a fixed volume fraction ( V P = 0.30). The thermo-viscoelasticity and fracture toughness of the composites and neat epoxy were measured at 143 K, 185 K, 228 K, 296 K, 363 K, and 399 K. Experimental results revealed that fracture toughness strongly depended on the microstructure of nano- and micro-particles bidispersion as well as its interactions with the matrix at all temperature, but depended on toughened matrix due to increase in mobility of matrix at the relaxation temperatures.

Fracture toughness of cast ferritic steel applying local approach

Fracture characteristics prediction with temperature dependence has been made and experimentally proved for cast manganese steel. The approach is based on computer simulation of the crack nucleus formation and unstable microcrack equilibrium in polycrystalline metal. Results of low-temperature tests of smooth tensile specimens, tensile specimens with circumferential U and V notches, pre-cracked bend specimens and tensile specimens with circumferential shallow crack have been applied. A good agreement of predicted and experimentally determined fracture toughness temperature dependence for the tested steel has been obtained.

Use of the Master Curve methodology for real three dimensional cracks

At VTT, development work has been in progress for 15 years to develop and validate testing and analysis methods applicable for fracture resistance determination from small material samples. The VTT approach is a holistic approach by which to determine static, dynamic and crack arrest fracture toughness properties either directly or by correlations from small material samples. The development work has evolved a testing standard for fracture toughness testing in the transition region. The standard, known as the Master Curve standard is in a way “first of a kind”, since it includes guidelines on how to properly treat the test data for use in structural integrity assessment. No standard, so far, has done this. The standard is based on the VTT approach, but presently, the VTT approach goes beyond the standard. Key components in the standard are statistical expressions for describing the data scatter, and for predicting a specimens size (crack front length) effect and an expression (Master Curve) for the fracture toughness temperature dependence. The standard and the approach, it is based upon, can be considered to represent the state of the art of small specimen fracture toughness characterization. Normally, the Master Curve parameters are determined using test specimens with “straight” crack fronts and comparatively uniform stress state along the crack front. This enables the use of a single K I value and single constraint value to describe the whole specimen. For a real crack in a structure, this is usually not the case. Normally, both K I and constraint vary along the crack front and in the case of a thermal shock, even the temperature will vary along the crack front. A proper means of applying the Master Curve methodology for such cases is presented here.

The effect of temperature and loading rate on the crack initiation and propagation energy in carbon steel charpy specimens

The Charpy impact tests were carried out at different temperatures and loading rates. The temperature dependences of crack initiation and propagation in carbon steels 45 and St. 3 under impact testing were determined from the obtained force variation plots. The effect of the impact velocity in the range from 1 to 4.4 m/s on the fracture toughness temperature dependence is estimated.

An upper-shelf fracture toughness master curve for ferritic steels

When assessing the resistance of a steel structure to failure under load, fracture toughness is a key input variable. Since it is usually not possible to establish a priori if the anticipated service temperature (or temperatures) is in the fracture mode transition or on the upper shelf, it is useful to know the temperature dependence of fracture toughness in both the transition and on the upper shelf. In the transition, the master curve proposed by Wallin defines both the variation of the median value of fracture toughness with temperature and the scatter of fracture toughness about this median value. However, Wallin's master curve does not quantify fracture toughness on the upper shelf. In this paper we assemble a database of upper shelf fracture toughness data ( J Ic) for ferritic steels. These data demonstrate that the temperature dependence of upper J Ic is consistent for all ferritic steels contained in the database and has the same form as the temperature dependence of the flow strength anticipated from dislocation mechanics considerations. This similarity between the temperature dependence of flow strength and the temperature dependence of J Ic is physically expected because both directly depend upon the energy required for dislocation mobility in the ferrite matrix. Both the empirically derived model and the physical basis for the model are described.

The relationship between the transition and upper‐shelf fracture toughness of ferritic steels

ABSTRACTConventionally, the transition fracture toughness and upper‐shelf fracture toughness of ferritic steels are viewed as separate properties: they are measured by tests conducted according to different standards, and neither toughness property can be estimated (except in very qualitative terms) based on knowledge of the other. Information presented in this paper demonstrates that quite the opposite is true: transition fracture toughness and upper‐shelf fracture toughness are directly related because the microstructural features responsible for both the temperature dependence of fracture toughness and for the magnitude of fracture toughness at any given temperature are the same in both transition and on the upper shelf (the lattice structure controls the temperature dependence, while the size and distribution of second‐phase particles control crack initiation and thus the magnitude of fracture toughness). These features bind the transition fracture toughness and upper‐shelf fracture toughness together in a way that is extremely consistent for all ferritic steels. We present empirical evidence of this relationship based on fracture toughness data from a variety of ferritic steels: various heats of nuclear grade pressure vessel steels and welds (both before and after irradiation), copper precipitation hardened steels used in naval ship construction and mild steels used in ship construction. In total, these data span a broad range of T0: from −180 °C to +140 °C. The combination of this theoretically motivated, empirically calibrated relationship between transition and upper‐shelf toughness with Wallin's Master Curve (for toughness in fracture mode transition), and EricksonKirk's proposed Master Curve for toughness on the upper shelf, produces a model that predicts the temperature dependence of, and scatter in, fracture toughness of ferritic steels throughout the transition and upper‐shelf temperature regimes. As input information, this model needs only the value of T0, which can be estimated by performing fracture toughness tests according to the protocols of ASTM test standard E1921‐05.

Temperature Dependence and Variability of Fracture Toughness in the Transition Regime for A508 Grade 4N Pressure Vessel Steel

Abstract A508 Grade 4N is a high nickel (3.4%) pressure vessel steel with a relatively high yield strength (585 MPa minimum for Class 1) and a relatively low transition temperature (47 J transition temperature of −29°C or below). Published fracture toughness data on this steel are nonexistent, and so they have not been included in prior assessments of the applicability of the Master Curve approach to pressure vessel steels. The purpose of this study is to compare the temperature dependence and data scatter for this steel with the behavior predicted by the Master Curve. The available database consists of approximately 800 fracture toughness tests on 51 material heats, some of which were given a temper embrittling heat treatment. As an additional basis of comparison, test data on A508 Grades 2 and 3, 1% nickel weld metal, and several high nickel (1.3–3.8%) steels (NiCrMo and NiCrMoV) were included in the assessment. Maximum likelihood techniques were used to fit the toughness data, and Monte Carlo methods were used to assess the observed distribution of Weibull slopes (related to the extent of variability). The results of this assessment show that the Master Curve provides a good description of the shape (temperature dependence) of the toughness curve for moderate and high transition temperatures. However, for heats with low transition temperatures below the range observed in other pressure vessel steels, the shape of the toughness curve deviates from the Master Curve. The lower the transition temperature, the larger the deviation from the Master Curve. Based on the test data, a model describing the behavior of A508 Grade 4N has been developed. This model also accurately describes the toughness behavior of the other pressure vessel steels included in this study. With respect to variability, the Weibull slope of A508 Grade 4N is consistent with the value of four prescribed by the Master Curve. However, the Weibull slope for the other pressure vessel steels included in this assessment is close to 3.4. This lower value of the Weibull slope, which indicates larger variability, is likely the result of material inhomogeneity.

Temperature dependence of crack initiation fracture toughness of various nanoparticles filled polyamide 66

In the present study, the crack initiation fracture toughness of various nanoparticles filled polyamide 66 was investigated in a broad temperature range (23–120 °C) by using an essential work of fracture (EWF) approach. Four types of spherical nanoparticles, i.e. two types of TiO 2 (21 nm, with/without surface modification), SiO 2 (13 nm) and Al 2O 3 (13 nm), were selected with a constant volume content of 1% in nanocomposites, which were compounded using a twin-screw-extruder. The addition of nanoparticles led to an enhanced specific EWF item at most test temperatures at the cost of the reduction of the non-EWF item. The value of the specific EWF was also estimated by a crack opening displacement method. Associated with SEM fractograph analysis, it was clear that two basic factors, i.e. crack tip blunting and net section stress, finally determined the EWF value. With the addition of nanoparticles, the item of crack tip blunting was increased at most temperature range, which may be incidental with the formation of numerous dimples and sub-dimples induced by nanoparticles; while the item of net section stress was correlated with the particle distribution, especially at room temperature, which was notably decreased in case of poor nanoparticle distribution.

Fracture toughness of REBaCuO bulk superconductor at liquid nitrogen temperature

The fracture toughness of Y123 and Gd123(Ag) single-grain bulk samples with 25 mol% RE 211 phase (and 10 wt.% Ag for the latter) prepared by the modified quench and melt-growth technique, was evaluated by the 3-point bending test at liquid nitrogen temperature (LNT) and room temperature (RT). The sharp V-notch was introduced into the specimens such that both of the loading and the crack extension directions are in the c-axis. The data for Y123 scattered from 1.2 to 2.0 MPam 1/2 at LNT and 1.4 to 1.7 MPam 1/2 at RT. The average value at LNT, 1.6 MPam 1/2, was slightly higher than that for RT, 1.5 MPam 1/2. For Gd123(Ag) the average fracture toughness at LNT, 1.8 MPam 1/2 was higher than that at RT. The temperature dependence of fracture toughness for two kinds of bulks was consistent with that of the bending strength. Further, the order of the magnitude of the fracture toughness almost corresponded with that of bending strength for both bulks.

Prediction of Temperature Dependence of Fracture Toughness as a Function of Neutron Fluence for Pressure-Vessel Steels by Using the Unified Curve Method

Using the Unified Curve method as proposed by the authors earlier and a probabilistic model based on the local approach and physical background of the influence of impurities and carbide size on the local strength of material, a method has been elaborated for predicting a variation of the temperature dependence of fracture toughness with neutron fluence. The results calculated by this method are compared to the available experimental data.