Variation Of Fracture Toughness Research Articles

Correlations Between Microstructure and Mechanical Properties of Air Plasma‐Sprayed Thermal Barrier Coatings Exposed to a High Temperature

An indentation method is used to study the variations in Young's modulus, hardness and fracture toughness of air plasma‐sprayed thermal barrier coatings at a high temperature. The coatings were exposed to 1100°C during 1700 h. A sudden increase in Young's modulus for the first 600 h was observed, while the hardness increased after 800 h as a consequence of sintering. Conversely, there was a reduction of 25% in fracture toughness after 1700 h, evidencing the thermal barrier coating degradation. The evolution of these mechanical properties was correlated with microstructural changes. After 1700 h, the thermally grown oxide thickness reached 6.8 μm, the volumetric percentage of porosity was reduced from 6.8% to 4.7% and the amount of monoclinic phase increased to 23.4 wt%. These characteristics are closely related to the stress distribution in the top coat, which promotes cracks nucleation and propagation, compromising the coating durability.

Effects of electric field on the fracture toughness (KIc) of ceramic PZT

This work was motivated by the observation that a small percentage of the ceramic lead zirconate titanate (PZT) parts in a device application, one that requires an electrode pattern on the PZT surface, developed fatigue cracks at the edges of the electrodes; yet all of the parts were subjected to similar loading. To obtain additional information on the fracture behavior of this material, similar specimens were run at higher voltage in the laboratory under a microscope to observe the initiation and growth of the fatigue cracks. A sequence of experiments was next performed to determine whether there were fracture toughness variations that depended on material processing. Plates were cut from a single bar in different locations and the Vickers indentation technique was used to measure the relative fracture toughness as a function of position along the bar. Small variations in toughness were found, that may account for some of the devices developing fatigue cracks and not others. Fracture toughness was measured next as a function of electric field. The surface crack in flexure technique was modified to apply an electric field perpendicular to a crack. The results indicate that the fracture toughness drops under a positive electric field and increases under a negative electric field that is less than the coercive field, but as the negative coercive field is approached the fracture toughness drops. Examination of the fracture surfaces using an optical microscope and a surface profilometer reveal the initial indentation crack shape and (although less accurately) the crack shape and size at the transition from stable to unstable growth. These results are discussed in terms of a ferroelastic toughening mechanism that is dependent on electric field.

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

ABSTRACTThe 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.

Prediction of Temperature Dependence and Scatter in Fracture Toughness of Pressure Vessel Steel using Nonlocal Damage Models

Abstract The pressure vessel and piping components may be subjected to operation in the ductile-to-brittle transition regime of the material due to increase in the transition temperature. This increase can be due to irradiation embrittlement, vario s other material aging and degradation mechanisms taking place in a nuclear reactor environment. From fracture mechanics point of view, fracture toughness of the material must be adequate to prevent the failure. However, there is considerable scatter observed in the fracture toughness data and the analyst must account for this in the safety analysis. Master curve approach according to ASTM E1921 standard is popularly employed for this purpose. However, it requires the data for transition temperature T0, which is dependent upon the specimen geometry and loading configurations employed in the laboratory tests. Its transferability to safety analysis of components is questionable. Towards this objective, the authors have recently developed a nonlocal formulation for the Rousselier's damage model and combined this with the Beremin's model to predict the variation of fracture toughness and its scatter in the DBTT regime for the standard compact tension specimens. In this work, application of this new procedure to different types of geometry and loading conditions has been explored.

Aspects of fracture toughness modelling of particle filled polymer composites

Polymer composites filled with hard inorganic particles of nano- to micro-scaled sizes show a very complex variation of fracture toughness with increasing particle fraction. Various failure mechanisms such as particle debonding, matrix yielding and subsequent brittle or ductile matrix fracture develop during crack propagation. In this work, these failure mechanisms within a dissipation zone are considered for the calculation of composite fracture toughness. It is shown that their influencing extent depends on critical values of composite strain or stress, respectively. The variation of composite fracture toughness with material and structural properties, such as the relation between the debonding strain and matrix yield strain, matrix yielding energy and particle volume fraction, is discussed.

On the structural dependence of the stress–strain diagram parameters and fracture toughness of metastable austenitic steels

The results of the investigation into the effect of structure on the variation of yield stresses and fracture toughness of a number of austenitic steels at the temperatures of suppression and evolution of the martensitic transformation are generalized taking into account their chemical composition and mode of loading. The correlations are established between the structural characteristics of steels of the class under consideration and the parameters of the analytic expressions for their stress–strain diagrams that determine the conditions for the competitive influence of the austenite strength, the intensity of the formation process of deformation-induced martensite, its amount and the strength of the strain hardening of the material. The specific character of creating its fracture toughness indices is justified structurally.

Mixed alkali effect on Vickers hardness and cracking

Vickers indentation measurements were carried out on borate, silicate and aluminophosphate glasses, each series comprised of samples of different relative alkali ratios (Na/Na+Li). All the glass series exhibited nonlinear variations of hardness with relative alkali ratio, which was attributed to the reduced plastic flow of mixed alkali glasses. The mixed alkali effect was also present in the length of radial cracks although less strongly than in hardness. Using a semi-empirical model, the variations of residual stresses and fracture toughness were estimated.

Functionally graded hydroxyapatite-alumina-zirconia biocomposite: Synergy of toughness and biocompatibility

Functionally Gradient Materials (FGM) are considered as a novel concept to implement graded functionality that otherwise cannot be achieved by conventional homogeneous materials. For biomedical applications, an ideal combination of bioactivity on the material surface as well as good physical property (strength/toughness/hardness) of the bulk is required in a designed FGM structure. In this perspective, the present work aims at providing a smooth gradation of functionality (enhanced toughening of the bulk, and retained biocompatibility of the surface) in a spark plasma processed hydroxyapatite-alumina-zirconia (HAp-Al2O3-YSZ) FGM bio-composite. In the current work HAp (fracture toughness ~1.5MPa.m1/2) and YSZ (fracture toughness ~6.2MPa.m1/2) are coupled with a transition layer of Al2O3 allowing minimum gradient of mechanical properties (especially the fracture toughness ~3.5MPa.m1/2). The in vitro cyto-compatibilty of HAp-Al2O3-YSZ FGM was evaluated using L929 fibroblast cells and Saos-2 Osteoblast cells for their adhesion and growth. From analysis of the cell viability data, it is evident that FGM supports good cell proliferation after 2, 3, 4days culture. The measured variation in hardness, fracture toughness and cellular adhesion across the cross section confirmed the smooth transition achieved for the FGM (HAp-Al2O3-YSZ) nanocomposite, i.e. enhanced bulk toughness combined with unrestricted surface bioactivity. Therefore, such designed biomaterials can serve as potential bone implants.

Effect of Al2O3 and Fe3Al on the phase formation and mechanical properties of β-sialon

The paper evaluated the mechanical properties of β-sialon composites prepared by hot-pressing sintering at 1600°C in N2 atmosphere using α-Si3N4, Al2O3, Y2O3 and Fe3Al as raw materials. The influence of Al2O3 and Fe3Al content on flexure strength, fracture toughness, hardness, and relative density was investigated. And phase formation and morphology of the composites were characterized by X-ray diffraction and electron microscopy. The experimental results indicate that the raw material Fe3Al reacts with α-Si3N4 to form silicides at elevated temperature, and supplies more liquid phase to assist densification. Besides, the variation of flexure strength, fracture toughness and hardness is mainly consistent, and also in good agreement with the relative density measurements. The values all increase firstly, and then decrease when the Al2O3 content increases. Scanning electron microscopy illustrates that the metal particles act to inhibit the crack propagation.

Residual Fracture Toughness of Concrete Subject to Elevated Temperatures

This paper presents an experimental investigation on the variation of residual fracture toughness of concrete after being exposed to elevated temperatures. A total of 60 specimens, with a uniform size of 200x200x230mm and precast notches of 80mm in height, were heated to constant temperatures of 65°C, 120°C, 200°C, 300°C, 350°C, 400°C, 450°C, 500°C and 600°C respectively. After cooling, standard wedge splitting tests, according to the corresponding Chinese Specification, were employed. The results indicate that the elevated temperature has significant influence on the residual fracture toughness of concrete. The magnitude of fracture toughness decreases drastically with increasing temperature. Additionally the relationship between residual fracture toughness and weight loss of specimens with respect to temperatures is also investigated.

Ferroelastic domain switching in lead titanate zirconate ceramics: Temperature dependence and fracture toughness variations

Ferroelastic domain switching in electrically-poled lead titanate zirconate (PZT) ceramics were investigated during uni-axial compression up to 400MPa, at a series of temperatures from 25°C to 180°C. It is found that both the stress-induced switchable polarization and switchable strain in the electrically-poled PZT samples decrease steadily with the increasing temperature. By measuring the polarization variations and strain variations of unpoled, prepoled and depoled PZT samples during a full cycle of heating-cooling, it is found that the reduced switchable polarization by stress is caused by the pyroelectric effect and the reduced switchable strain is due to the lattice distortion and thermal shrinkage effect. The fracture toughness of electrically poled and mechanically depoled PZT ceramics were both measured by Vickers indentation and it is found that the fracture toughness anisotropy can even reach 3.63 in the later, significantly larger than that of 2.62 in the former.

Advances in cleavage fracture modelling in steels: Micromechanical, numerical and multiscale aspects

Brittle cleavage fracture remains one of the major concerns for structural integrity assessment. The main characteristics of this mode of failure in relation to the stress field ahead of a crack, tip are described in the introduction. The emphasis is laid on the physical origins of scatter and the size effect observed in ferritic steels. It is shown that cleavage fracture is controlled by physical events occurring at different scales: initiation at (sub)micrometric particles, propagation across grain boundaries (10–50 microns) and final fracture at centimetric scale. The two first scales are detailed in this paper. The statistical origin of cleavage is described quantitatively from both microstructural defects and stress–strain heterogeneities due to crystalline plasticity at the grain scale. Existing models are applied to the prediction of the variation of Charpy fracture toughness with temperature. La ruine par rupture fragile reste une des préoccupations majeures pour l'évaluation de l'intégrité mécanique des structures. Les principaux traits de l'amorçage de ce mode de rupture à la pointe d'une fissure macroscopique sont tout d'abord rappelés dans l'introduction. On met l'accent sur la dispersion inhérente à ce mode de rupture en relation avec l'origine diverse des sites d'amorçage ainsi que sur l'effet de taille. On montre que la rupture par clivage est contrôlée par des mécanismes physiques agissant à différentes échelles, celle des particules de seconde phase micrométriques et celle des grains. L'origine statistique du clivage est modélisée en prenant en compte à la fois la distribution spatiale des défauts microstructuraux et la distribution intragranulaire des contraintes et déformations. Les modèles développés sont utilisés pour prévoir la variation de la résilience avec la température.

Investigation of Transition Fracture Toughness Variation within the Thickness of Reactor Pressure Vessel Forgings

Abstract An investigation has been conducted on the influence of the through-thickness sampling position on the tensile and fracture toughness properties of reactor pressure vessel forgings, using material from two actual pressurized water reactor vessels. The aim was to quantify the safety margins entailed by extracting surveillance samples from the 1/4T and 3/4T positions, as recommended by the current legislation. For each forging, seven layers have been considered: Inner surface, 1/8T, 1/4T, 1/2T, 3/4T, 7/8T, and outer surface; for each position, two tensile tests at room temperature and 16 fracture toughness tests in the ductile-to-brittle transition region have been performed. In terms of tensile properties, for both forgings the strength is higher at the surfaces than in the centre, while ductility (elongations and reduction of area) is substantially unaffected. For both materials, fracture toughness is better at the surfaces than in the central portion, although differences in terms of Master Curve reference temperatures are statistically not relevant at the 95 % confidence level. This effect is more pronounced for one of the materials, due to the larger amount of material removed with respect to the original heat-treated forging, and is qualitatively confirmed by metallographic observations. The results obtained from this study are in substantial agreement with similar studies found in the literature, although most authors have reported larger differences between surface and central layers.

Study of fracture behaviour of bond coats on nickel superalloy by three-point bending of microbeams

A microbeam testing geometry is designed to study the variation in fracture toughness across a compositionally graded NiAl coating on a superalloy substrate. A bi-material analytical model of fracture is used to evaluate toughness by deconvoluting load–displacement data generated in a three-point bending test. It is shown that the surface layers of a diffusion bond coat can be much more brittle than the interior despite the fact that elastic modulus and hardness do not display significant variations. Such a gradient in toughness allows stable crack propagation in a test that would normally lead to unstable fracture in a homogeneous, brittle material. As the crack approaches the interface, plasticity due to the presence of Ni 3Al leads to gross bending and crack bifurcation.

Mixed mode (I and II) crack tip fields in bulk metallic glasses

Stationary crack tip fields in bulk metallic glasses under mixed mode (I and II) loading are studied through detailed finite element simulations assuming plane strain, small scale yielding conditions. The influence of internal friction or pressure sensitivity on the plastic zones, notch deformation, stress and plastic strain fields is examined for different mode mixities. Under mixed mode loading, the notch deforms into a shape such that one part of its surface sharpens while the other part blunts. Increase in mode II component of loading dramatically enhances the normalized plastic zone size, lowers the stresses but significantly elevates the plastic strain levels near the notch tip. Higher internal friction reduces the peak tangential stress but increases the plastic strain and stretching near the blunted part of the notch. The simulated shear bands are straight and extend over a long distance ahead of the notch tip under mode II dominant loading. The possible variations of fracture toughness with mode mixity corresponding to failure by brittle micro-cracking and ductile shear banding are predicted employing two simple fracture criteria. The salient results from finite element simulations are validated by comparison with those from mixed mode (I and II) fracture experiments on a Zr-based bulk metallic glass.

Influence of Annealing on Microstructure and Mechanical Properties of Isotactic Polypropylene with β-Phase Nucleating Agent

The microstructure and mechanical properties of β-nucleated iPP before and after being annealed at different temperatures (90−160 °C) have been analyzed. Annealing induced different degrees of variation in fracture toughness of β-nucleated iPP samples, namely, slight enhancement at relatively low annealing temperatures ( 140 °C) has been observed. The variation of fracture toughness of β-nucleated iPP is observed to be dependent on the content of β-NA. Experiments, including scanning electronic microscope (SEM), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA), are performed to study the variations of microstructures as well as the toughening mechanism of the β-nucleated iPP after being annealed. The results indicate that the decreased number of chain segments in the amor...

Using high-purity MgO nanopowder as a stabilizer in two different particle size monoclinic ZrO 2: Its influence on the fracture toughness

The fracture toughness of micron (Aldrich) and submicron (Tosoh 0) ZrO 2 ceramics with additions of 3.11 wt% MgO (9.25 mol% Mg-PSZ) nanopowder “as-received” (AR) and 1200 °C heat treated (HT) sintered at 1720 °C during 1 h in air was investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The change in the starting (fill) density with additions of “as-received” (AR) and 1200 °C (HT) MgO powder, may be attributed to the bimodal particle size distribution and the presence of numerous dense agglomerates indicating that the agglomeration state was not completely destroyed during compaction process. The difference observed in the grain size indicated that the ceramics produced with submicron ZrO 2 powders contained a porosity whose distribution limits the grain growth. An effective tetragonal-to-monoclinic phase transformation occurred and was reflected by the presence of intergranular and intragranular cracking in the specimens. Composites fabricated with Aldrich ZrO 2 powders showed a major retention of transformable submicron tetragonal precipitates than Tosoh 0 ZrO 2 samples. The variation in hardness and fracture toughness seems to be wholly dependent on the amount of the t-phase retained. The use of micron ZrO 2 powder as matrix favors the increase both hardness and fracture toughness for additions of “as-received” and heat treatment MgO powder as stabilizer.

Reactive Compatibilized Polymer-Polymer Interface : Amorphous and Semi-crystalline Polymer Pair

We present an investigation of the reinforcement of the interface between a flexible amorphous polymer (polystyrene, PS) and a semi-crystalline polymer (a polyamide, Ny6). Different processes ( a reactive ion-beam process(RIB), a plasma surface activation process(PSA), and a combined ion-beam process(CIB)) were used for the surface functionalization. Poly(styrene-co-maleic anhydride) was also used as the compatibilizer. Fracture toughness was measured using the asymmetric double cantilever beam test (ADCB). For bonding temperatures above 190°C, the adhesion strength was found to increase with bonding time, pass through a peak value, and then reach a plateau. The fracture toughness increased with increasing bonding temperature, passed through a peak near 200°C, and then decreased with further increase of the bonding temperature. This behavior was more obvious for an amorphous polymer/semi-crystalline polymer pair than for a pair of semi-crystalline polymers. The variation of the fracture toughness with bonding time and temperature can be plausibly explained in terms of two different failure mechanisms : adhesive failure at the interface for short bonding times and when the bonding temperature is low, and for longer bonding times and at high temperatures, cohesive failure between chains at the interface and the bulk PS due to decreased chain entanglement. CIB process showed better efficiency than the others because of surface area expanding effect by etching process and more effective surface functionalization.

A dynamic fracture-toughness-based reference temperature characterization for the weld metals of modified 9Cr–1Mo steel in two different weld positions

The ductile–brittle transition temperatures (DBTT) of modified 9Cr–1Mo steel welds from two different weld positions, namely down hand (1G) and overhead (4G), have been evaluated and compared using the ASTM E 1921-05 based reference temperature ( T 0) approach, but under dynamic-loading conditions. The reference temperatures thus obtained, termed as T 0 dy to signify the dynamic condition, have been found to be higher for the 4G position than the 1G position. A scanning electron microscopic study of the fracture surfaces close to the fatigue crack front reveals that while lath boundary fracture is the dominant mechanism for brittle crack initiation in both the welds, the higher T 0 dy value is linked to the higher concentration of probable crack initiation sites in the 4G position. The experimentally obtained Weibull slope in both the welds has been found to be different (7.526 and 7.205 for the 1G and 4G positions, respectively) from the ‘fixed slope of 4’ assumption, used in ASTM E 1921-05. However, in the present instance, the ‘fixed Weibull slope of 4’ concept yields more conservative T 0 dy values compared to those obtained using the experimentally determined Weibull slope. For these welds, the RT NDT-based ASME K IR curve proved to be ultra-conservative compared to the realistic dynamic fracture toughness variation described by the Master Curve indexed with T 0 dy.

Fracture toughness variation induced by stress corrosion cracking of prestressing steels

AbstractThe stress corrosion cracking process developing in metals is at present an unknown mechanism of deterioration. It is a surface process that implies a corrosion and stress synergy, but the most practical consequence is that stress corrosion cracking can modify the mechanical characteristics of the metal. Due to that it leads to brittle failures; it generally involves high level of uncertainty in the prediction.This research deals with steels for prestressed concrete and aims to show that the fracture toughness changes when the steel is susceptible to stress corrosion cracking, questioning the idea that the toughness is an intrinsic characteristic of the material. The reduction in the fracture toughness of prestressing steels when they are in contact with aggressive media, involves that the material, for the same stress level, may reach a fracture having a lower crack size. That means the material becomes less damage tolerant, which implies that it is necessary to develop techniques able to detect defects of smaller size, as for example, small notches, pits or superficial cracks.