Showing papers on "Fracture toughness published in 1998"

Measurement of mechanical properties by ultra-low load indentation

Abstract: Ultra-low load indentation, also known as nanoindentation, is a widely used tool for measuring the mechanical properties of thin films and small volumes of material. One of the great advantages of the technique is its ability to probe a surface and map its properties on a spatially resolved basis, sometimes with a resolution of better than 1 μm. In this paper, techniques for measuring mechanical properties by ultra-low load indentation techniques are reviewed and discussed. Emphasis is given to the use of sharp indenters and how they can be used to measure elastic modulus, hardness, and fracture toughness. These fundamental mechanical properties characterize the three primary modes of deformation in solids—elasticity, plasticity, and fracture.

Engineered interfaces in fiber reinforced composites

Abstract: Characterisation of Interface Properties . Introduction. Theories of adhesion and types of bonding. Physio-chemical characterization of interfaces. Measurements of Interface/Interlaminar Properties . Introduction. The mechanical properties of fiber-matrix interfaces. Interlaminar/intralaminar properties. Interlaminar fracture toughness. Micromechanics of Stress Transfer Across the Interface . Introduction. Fiber fragmentation test. Fiber pull-out. Fiber push-out. Cyclic loading in fiber pull-out and fiber push-out. Surface Treatments of Fibers and Effects on Composite Properties . Introduction. Glass fibers and silane coupling agents. Carbon fibers. Polymeric fibers. Inorganic fibers. Interface Mechanics and Fracture Toughness Theories . Interface-related fracture toughness theories. Toughness theories for short and randomly oriented fiber composites. Fracture toughness maps. Crack-interface interaction. Improvement of Transverse Fracture Toughness with Interface Control . Introduction. Fiber coating and intermittent bonding concept-experimental studies. Improvement of Interlaminar Fracture Toughness with Interface Control . Introduction. Effects of matrix materials on interlaminar fracture resistance. Delamination resisters. Three-dimensional textile composites concept. References. Appendices. List of Symbols an Abbreviations.

Hardness to toughness relationship of fine-grained WC-Co hardmetals

Abstract: The hardness to toughness relationship of fine-grained WC-Co hardmetals was studied based on Palmqvist indentation toughness measurements. Sixty-five commercial and lab-sintered hardmetals of different composition, microstructure and manufacturing history were investigated to build up a representative hardness/toughness measurement band. This band is then used to discuss the influence of the various alloy- and process-related parameters on the hardness to toughness relationship of WC-Co composites. Beyond that, optimal hardness/toughness combinations can be assessed for the hardness range of 1400–2200 HV30. In general, the higher the hardness of the alloys, the longer were the indentation cracks, indicating a decrease in fracture toughness with increasing hardness. However, at a certain hardness, the toughness of individual alloys varied significantly. For example, at HV30:1670, the sum of crack lengths varied between 287 μm (high toughness) and 449 μm (low toughness), which corresponds to fracture toughness values of 11.5 and 9.2 MNm−32, respectively. Very fine-grained hardmetals (ultrafine grades) were shown to be not necessarily tougher than coarser grained alloys (submicron grades), in particular in the hardness range of 1450–2000 HV30, although they exhibit significantly more binder at a given hardness. Only in the high hardness range of > 2000 HV30 might they be of advantage. Samples, exclusively doped with Cr3C2 as growth inhibitor exhibit more favorable hardness/toughness combinations than comparable VC-doped alloys. However, other parameters, such as sintering temperature, sintering time, or the gross carbon content of the respective alloys must be taken into consideration for obtaining optimal hardness/toughness combinations.

Controlling Factors for the Brittle-to-Ductile Transition in Tungsten Single Crystals

Abstract: Materials performance in structural applications is often restricted by a transition from ductile response to brittle fracture with decreasing temperature. This transition is currently viewed as being controlled either by dislocation mobility or by the nucleation of dislocations. Fracture experiments on tungsten single crystals reported here provide evidence for the importance of dislocation nucleation for the fracture toughness in the semibrittle regime. However, it is shown that the transition itself, in general, is controlled by dislocation mobility rather than by nucleation.

Processing and properties of particulate reinforced steel matrix composites

Abstract: Metal matrix composites are an attractive choice for aerospace and automotive applications because of their high stiffness-to-weight ratio. Composites with aluminum and magnesium matrices have been investigated extensively, while less work has been carried out on steel matrix composites. In the present study the hot isostatic pressing (HIP) process of steel matrix composites is described, and the factors influencing the reinforcement distribution, interface processes, as well as the mechanical and corrosion properties, are revealed. Both stainless steels and tool steels were used as the matrix material, and the particulate reinforcements were Al 2 O 3 , TiC, Cr 3 C 2 , or TiN. The results are compared with those of the corresponding unreinforced alloys and also with those of aluminum and magnesium matrix composites. It was found that the incorporation of a relatively low volume fraction of ceramic particulate reinforcements significantly increases the wear resistance of the steel matrices, without deteriorating the corrosion properties. On the other hand, reductions in the tensile strength, ductility and toughness were observed. The superaustenitic stainless steel–TiN and hot work tool steel–Cr 3 C 2 composites may offer the best combination of properties.

Calibration of Weibull stress parameters using fracture toughness data

Abstract: The Weibull stress model for cleavage fracture of ferritic steels requires calibration of two micromechanics parameters $$(m,\sigma _u ) $$ Notched tensile bars, often used for such calibrations at lower-shelf temperatures, do not fracture in the transition region without extensive plasticity and prior ductile tearing However, deep-notch bend and compact tension specimens tested in the transition region can provide toughness values under essentially small-scale yielding (SSY) conditions to support Weibull stress calibrations We show analytically, and demonstrate numerically, that a nonuniqueness arises in the calibrated values, ie, many pairs of $$(m,\sigma _u ) $$ provide equally good correlation of critical Weibull stress values with the distribution of measured (SSY) fracture toughness values This work proposes a new calibration scheme to find $$(m,\sigma _u ) $$ which uses toughness values measured under both low and high constraint conditions at the crack front The new procedure reveals a strong sensitivity to m and provides the necessary micromechanical values to conduct defect assessments of flawed structural components operating at or near the calibration temperature in the transition region Results of a parameter study illustrate the expected values of m for a typical range of material flow properties and toughness levels A specific calibration is carried out for a mild structural steel (ASTM A36)

Nonlinear Fracture Mechanics for Engineers

Abstract: Overview Introduction Classification of Fracture Mechanics Regimes History of Developments in Fracture Mechanics Review of Solid Mechanics Stress Strain Elasticity Plasticity Consideration of Creep Component Analysis in the Plastic Regime Fully Plastic/Limit Loads Review of Linear Elastic Fracture Mechanics Basic Concepts Crack Tip Plasticity Compliance Relationships Fracture Toughness and Predictive Fracture in Components Subcritical Crack Growth Limitations of LEFM Analysis of Cracks under Elastic-Plastic Conditions Introduction Rice's J-Integral J-Integral, Crack Tip Stress Fields, and Crack Tip Opening Displacement J-Integral as a Fracture Parameter and Its Limitations Methods of Estimating J-Integral Analytical Solutions J-Integral for Test Specimens J for Growing Cracks Numerically Obtained Solutions Tables of J-Solutions Crack Growth Resistance Curves Fracture Parameters under Elastic-Plastic Loading Experimental Methods for Determining Stable Crack Growth and Fracture Special Considerations for Weldments Instability, Dynamic Fracture, and Crack Arrest Fracture Instability Fracture under Dynamic Conditions Crack Arrest Test Methods for Dynamic Fracture and Crack Arrest Constraint Effects and Microscopic Aspects of Fracture Higher Order Terms of Asymptotic Series Cleavage Fracture Ductile Fracture Ductile-Brittle Transition Fatigue Crack Growth under Large-Scale Plasticity Crack Tip Cyclic Plasticity, Damage, and Crack Closure ?J-Integral Test Methods for Characterizing FCGR under Large Plasticity Conditions Behavior of Small Cracks Analysis of Cracks in Creeping Materials Stress Analysis of Cracks Under Steady-State Creep Analysis of Cracks under Small-Scale and Transition Creep Consideration of Primary Creep Effects of Crack Growth on the Crack Tip Stress Fields Crack Growth in Creep-Brittle Materials Creep Crack Growth Test Methods for Characterizing Creep Crack Growth Microscopic Aspects of Creep Crack Growth Creep Crack Growth in Weldments Creep-Fatigue Crack Growth Early Approaches for Characterizing Creep-Fatigue Crack Growth Behavior Time-Dependent Fracture Mechanics Parameters for Creep-Fatigue Crack Growth Methods of Determining (Ct)avg Experimental Methods for Characterizing Creep Crack Growth Creep-Fatigue Crack Growth Correlations Case Studies Applications of Fracture Mechanics Fracture Mechanics Analysis Methodology Case Studies Appendices Index

Load-adaptive crystalline–amorphous nanocomposites

Abstract: Advances in laser-assisted deposition have enabled the production of hard composites consisting of nanocrystalline and amorphous materials Deposition conditions were selected to produce super-tough coatings, where controlled formation of dislocations, nanocracks and microcracks was permitted as stresses exceeded the elastic limit This produced a self-adjustment in the composite deformation from hard elastic to quasiplastic, depending on the applied stress, which provided coating compliance and eliminated catastrophic failure typical of hard and brittle materials The load-adaptive concept was used to design super-tough coatings consisting of nanocrystalline (10–50 nm) TiC grains embedded in an amorphous carbon matrix (about 30 vol%) They were deposited at near room temperature on steel surfaces and studied using X-ray photoelectron spectroscopy, X-ray diffraction, scanning electron microscopy, Raman spectroscopy, nanoindentation and scratch tests Design concepts were verified using composition–structure–property investigations in the TiC–amorphous carbon (a-C) system A fourfold increase in the toughness of hard (32 GPa) TiC–a-C composites was achieved in comparison with nanocrystalline single-phase TiC

Fracture analysis of cellular materials: A strain gradient model

Abstract: A generalized continuum model is developed for cellular materials based on the equivalence of strain energy at the macro- and microscale. It is rather similar to the strain gradient theory, but has a well-defined characteristic length, namely, the cell size. The continuum model enables one to use powerful analytical methods to investigate fracture of cellular materials. The near-tip asymptotic fields and full-field solutions are obtained for cellular materials with hexagonal, triangular, or square lattice. Using the same strain-energy equivalence at the macro- and microscale, displacements and rotation of discrete cell walls are estimated from the continuum near-tip asymptotic fields. By postulating a maximum-tensile-stress failure criterion of cell walls, the fracture toughness of cellular materials is estimated to be proportional to the thickness h of cell walls and inversely proportional to √L, where L is the cell size. Moreover, the mixed-mode fracture toughness can be simply obtained from the fracture toughness in pure mode 1 and mode II, once the mode mixity is known. It is established that, with the same mass density, the hexagonal or triangular lattice in a cellular material can provide much higher fracture toughness than the square lattice.

Switch-toughening of ferroelectrics subjected to electric fields

Abstract: Electric fields can influence the fracture toughness of ferroelectrics. For example, poled ferroelectrics exhibit fracture toughness anisotropy: the material is tougher for a crack parallel to the poling direction but less tough for a crack perpendicular to it. When an electric field is applied to a poled sample, a positive field reduces its fracture toughness but a negative field enhances it. Previous investigations attribute these phenomena to polarization switching. This paper proposes a model of stress-assisted 90 polarization switching to quantify the toughening process. Small scale switching and uniform electric fields are assumed. An analytical solution is presented for a mono-domain ferroelectric crystal undergoing a confined polarization switch. This solution and the domain orientation pattern enable us to estimate the fracture resistance against the steady state crack growth in ferroelectrics by a Reuss-type multiple-domain assembly. A dimensionless group of material parameters and an electric field function emerge, and form the key ingredients of switch-toughening. The model is used to delineate several observations, including: poling-induced anisotropy of the fracture toughness, asymmetric variation of the fracture toughness under positive and negative electric fields of a poled specimen; upside-down butterfly loop for the fracture toughness response under cyclic electric loading.

Deformation of a polydomain, liquid crystalline epoxy-based thermoset

Abstract: Liquid crystalline thermosets (LCT's) were prepared by curing a difunctional LC epoxy monomer, diglycidyl ether of 4,4‘-dihydroxy-α-methylstilbene, with the tetrafunctional cross-linker 4,4‘-methylenedianiline (MDA). A commercial, non-LC epoxy monomer of similar starting molecular weight was also cured with MDA to produce an isotropic thermoset for comparison. Dynamic mechanical analysis revealed reduced glassy moduli, increased stiffness in the rubbery state, and broadened and lowered glass transitions for the LCT's compared to the isotropic thermoset based on the non-LC monomer. At room temperature, the true stress versus true strain curves of the LCT's under uniaxial compression showed no strain softening region, substantial plastic deformation (ef ≈ 50%), and increased strain hardening compared to the isotropic thermoset. LCT's with a smectic type of local order exhibited bulk, homogeneous plastic yielding, which led to slow, stable crack propagation and an increased fracture toughness (GIc = 1.62 kJ/...

Effect of Initial α‐Phase Content on Microstructure and Mechanical Properties of Sintered Silicon Carbide

Abstract: By using α- and β-SiC starting powders with similar particle sizes, the effects of initial α-phase content on the microstructure and the mechanical properties of the liquid-phase-sintered and subsequently annealed materials were investigated. The microstructures developed were analyzed by image analysis. When β-SiC powder was used, the grains became elongated. The average diameter decreased with increasing α-SiC content and the aspect ratio showed a maximum at 10%α-SiC and decreased with increasing α-SiC content in the starting powder. Such results suggest that microstructure can be controlled by changing α-phase content in starting powders. The strength increased with increasing α-SiC content in the starting powder while the fracture toughness decreased with increasing α-SiC content. There may be a trade-off in improving both the strength and toughness in SiC ceramics sintered with oxide additives.

In-situ hot pressing/solid-liquid reaction synthesis of dense titanium silicon carbide bulk ceramics

Abstract: An in-situ hot pressing/solid-liquid reaction process was developed for the synthesis of dense polycrystalline Ti3SiC2 ceramics using Ti, Si, and graphite powders as starting materials. The present work demonstrated that this process was one of the most effective and simple methods for the preparation of dense bulk Ti3SiC2 materials. Lattice constants of a=3.068 and c=17.645 are calculated for Ti3SiC2 made through this process. The synthesis temperature influenced the phase composition, microstructure and mechanical properties of Ti3SiC2 prepared at different temperatures. And bulk materials with flexural strength of 480 MPa and fracture toughness of 7.88 MPa.m1/2 were obtained at 1600°C. The high fracture toughness and strength are discussed based on microstructure analysis.

Processing and mechanical properties of boron carbide sintered with TiC

Abstract: The suitability of TiC as a sintering aid for boron carbide is investigated. The in situ reaction of TiC with boron carbide generates elemental carbon and TiB 2 which both aid the sintering process and permit pressureless sintering at temperatures between 2150 and 2200 °C. Relative densities of as-sintered materials exceed 93% of theoretical, but can be increased to nearly full density by subsequent hot isostatic pressing. The grain size of both B 4 C and TiB 2 increases with sintering temperature and decreases with the amount of sintering aid. The presence of TiB 2 causes a slight increase in Young's modulus and a small decrease of hardness as compared to single phase B 4 C materials. The flexural strength of B 4 C-TiC is found to decline with increasing fracture toughness. This behavior is related to crack length dependent toughness, particularly to the slope and steady state value of the R -curve. The dependence of R -curve properties on the grain size of the B 4 C matrix is discussed.

Direct Correlation between Interfacial Width and Adhesion in Glassy Polymers

Abstract: The correlation between interfacial width and adhesion, as expressed by the fracture toughness of the interface, in glassy polymers is investigated. The interfacial width is measured accurately by neutron reflectivity, and the fracture toughness of the interfaces is determined from a double cantilever beam test. Two experimental situations are considered: the case of an interface between two immiscible polymers, polystyrene (PS) and poly(p-methylstyrene) (PpMS), and the case of a PS−PS interface, where the interfacial width and the fracture toughness are measured for different interdiffusion times. In both cases, a direct correlation between the fracture toughness and the interfacial width is found. After an initial rapid increase in toughness due presumably to chain-end diffusion, most of the subsequent increase in toughness occurs over a relatively narrow range of interfacial widths between 9 and 12 nm. The fracture toughness stays constant with further interdiffusion. The results are consistent with r...

Mechanical performance of quartz‐tempered ceramics: part i, strength and toughness*

Abstract: The effect of quartz temper on the physical and mechanical properties of clay ceramics and the elucidation of the underlying mechanisms that are responsible for these effects are presented here. Characteristics studied included bulk density, open and closed porosity, density of impervious portion and fracture morphology. Mechanical behaviour was studied by measuring energy dissipation during fracture, Young's modulus, initial fracture toughness and strength in flexure. The significant increase in toughness with quartz volume fraction is explained by the development of a model that accounts for the crack distribution around the grains. The archaeological implications of the work are discussed on the basis of all the parameters that might affect the potter's choices of raw materials.

The effect of weave pattern and crack propagation direction on mode I delamination resistance of woven glass and carbon composites

Abstract: The effect of weave structure on the interlaminar fracture behavior of orthogonal woven fabric composite laminates has been examined. Crack propagation along the fill and weft yarns, respectively, was considered for plain, twill and 8H-satin glass/epoxy composites, and a 5H-satin carbon/epoxy composite. Fracture testing employed the mode I fracture DCB test specimen. Microscopic details of crack growth in the interply region were considered after fracture testing. The delamination resistance and the difference in fracture toughness between the fill and weft directions increased with increased weave index. Partial debonding of transversely oriented yarns contributed to the delamination resistance. Fracture of debonded fibers referred to as `fiber bridging' was observed in the twill and satin weave glass/epoxy composites, but not in the plain weave glass/epoxy and the 5H-satin weave carbon/epoxy composites. The interlacing of the yarns limited the extent of fiber bridging.