Showing papers on "Fracture toughness published in 2006"

From brittle to ductile fracture of bone

Abstract: Toughness is crucial to the structural function of bone. Usually, the toughness of a material is not just determined by its composition, but by the ability of its microstructure to dissipate deformation energy without propagation of the crack. Polymers are often able to dissipate energy by viscoplastic flow or the formation of non-connected microcracks. In ceramics, well-known toughening mechanisms are based on crack ligament bridging and crack deflection. Interestingly, all these phenomena were identified in bone, which is a composite of a fibrous polymer (collagen) and ceramic nanoparticles (carbonated hydroxyapatite). Here, we use controlled crack-extension experiments to explain the influence of fibre orientation on steering the various toughening mechanisms. We find that the fracture energy changes by two orders of magnitude depending on the collagen orientation, and the angle between collagen and crack propagation direction is decisive in switching between different toughening mechanisms.

Mixed mode brittle fracture in PMMA—An experimental study using SCB specimens

Abstract: A series of mixed mode I/II fracture tests is conducted on polymethylmethacrylate (PMMA) in the full range from pure mode I to pure mode II using a semi-circular bend (SCB) specimen containing an edge crack. The fracture load and the path of crack growth are obtained from experiments for various crack angles. It is shown that the conventional mixed mode I/II fracture criteria such as the maximum tangential stress (MTS) criterion overestimate the fracture strength of PMMA when the SCB specimen is used for fracture tests, particularly for mode II dominant loading conditions. However, improved predictions of fracture load are achieved when a generalized MTS criterion is employed. While the path of crack growth is straight for pure mode I, it deviates significantly from the angle of fracture initiation for pure mode II and mode II dominant loading conditions. It is shown that the path of crack growth predicted by the generalized MTS criterion is also in a good agreement with the observed fracture path in the fractured SCB samples.

Mechanical behaviour and oxidation resistance of structural silicides

Abstract: The need for high performance materials in aerospace, automotive and industrial components operating at temperatures in the range of 1100–1500°C has led to a surge in the research and development of the refractory metal silicide based intermetallics, multiphase alloys and composites. The silicides of Mo, W, Ti, Nb and Cr are attractive for high melting points, strength retention and ductile behaviour, combined with reasonable to excellent oxidation resistance at elevated temperatures. The major limitation to the widespread use of structural silicides is their inherent brittleness and poor fracture toughness at room temperature, which may be improved further by suitable alloying or tailoring composite microstructures or use of innovative processing routes. The high temperature deformation behaviour of the structural silicides is complex, and depends on the composition and alloy content, crystal structure, character of bonding and orientation, microstructural constitution (nature of phases present ...

Preparation, morphology and thermal/mechanical properties of epoxy/nanoclay composite

Abstract: A novel approach assisted with solvent was developed to disperse clay into epoxy matrix. The dispersion of clay was examined by means of optical microscopy (OM), wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM). Batches of cured samples containing 1–3 wt.% silane-modified clay (SMC) were prepared and their thermal/mechanical properties were studied by dynamic mechanical analysis (DMA), tensile and fracture tests. Improvements on storage modulus, Young’s modulus and fracture toughness were achieved with incorporation of SMC clay. The fracture surfaces of the nanocomposites were imaged by scanning electron microscopy (SEM) to investigate the toughening mechanisms.

Effect of mechanical properties measured at room and elevated temperatures on the wear resistance of cutting tools with TiAlN and AlCrN coatings

Abstract: A comprehensive study of the mechanical properties of TiAlN and AlCrN coated cutting tools has been performed at room and elevated temperatures (up to 500 °C) using Micro Material's NanoTest Platform System. Micro-mechanical properties have been measured such as microhardness, elastic modulus, H/E ratio, microhardness dissipation parameter (MDP), critical load values ( L c1 —first crack event; L c2 —load of dramatic coating failure) during scratch testing; a scratch crack propagation resistance parameter, CPR s = L c1 ( L c2 − L c1 ) as well as nano-impact fracture resistance. Cutting tool life was studied under end milling conditions of the structural AISI 1040 steel. A correlation between CPR s was found with H/E ratio and MDP values. These parameters could be used to characterize the fracture toughness of the coatings. It was shown that mechanical characteristics such as H/E ratio, MDP and CPR s as well as nano-impact fracture resistance can be used to assess the resistance to adhesive-fatigue wear that is typical for end milling conditions. It was found that the microhardness of the coating and the H/E ratio reduces with rising temperature while the MDP value grows. The data obtained during quick laboratory nanohardness, nanoscratch as well as nano-impact fatigue testing can be used to rank the coatings studied and in some cases predict the relative life of a coated tool.

Beneficial effects of an ultra-fine α-SiC incorporation on the sinterability and mechanical properties of ZrB2

Abstract: A fully dense ZrB2 ceramic containing 10 vol. % ultra-fine α-SiC particulate was successfully hot pressed at 1900 °C for 20 min and 40–50 MPa of applied pressure. Faceted ZrB2 grains (average size ≈3 μm) and SiC particles dispersed regularly characterized the base material. No extra secondary phases were found. The introduction of the ultra-fine α-SiC particulate was recognized as the key factor that enabled both the control of the diboride grain growth and the achievement of full density. The mechanical properties offered an interesting combination of data: 4.8±0.2 MPa $\surd{m}$ fracture toughness, 507±4 GPa Young’s modulus, 0.12 Poisson’sratio, and 835±35 MPa flexural strength at room temperature. The flexural strength measured at 1500 °C (in air) provided values of 300±35 MPa. The incorporated ultra-fine α-SiC particulate was fundamental, sinterability apart, to enhancing the strength and oxidation resistance of ZrB2. The latter property was tested at 1450 °C for 20 h in flowing dry air. In such oxidizing conditions, the formation of a thin external borosilicate glassy coating supplied partial protection for the faces of the material exposed to the hot environment. The oxidation attack penetrated into the material’s bulk and created a 200-μm-thick zirconia scale. The SiC particulate included in the oxide scale, lost by active oxidation, left carbon-based inclusions in the formerly occupied sites.

Development of the local approach to fracture over the past 25 years: Theory and applications

Abstract: This review paper is devoted to the local approach to fracture (LAF) for the prediction of the fracture toughness of structural steels. The LAF has been considerably developed over the past two decades, not only to provide a better understanding of the fracture behaviour of materials, in particular the failure micromechanisms, but also to deal with loading conditions which cannot easily be handled with the conventional linear elastic fracture mechanics and elastic-plastic fracture mechanics global approaches. The bases of this relatively newly developed methodology are first presented. Both ductile rupture and brittle cleavage fracture micromechanisms are considered. The ductile-to-brittle transition observed in ferritic steels is also briefly reviewed. Two types of LAF methods are presented: (i) those assuming that the material behaviour is not affected by damage (e.g. cleavage fracture), (ii) those using a coupling effect between damage and constitutive equations (e.g. ductile fracture). The micromechanisms of brittle and ductile fracture investigated in elementary volume elements are briefly presented. The emphasis is laid on cleavage fracture in ferritic steels. The role of second phase particles (carbides or inclusions) and grain boundaries is more thoroughly discussed. The distinction between nucleation and growth controlled fracture is made. Recent developments in the theory of cleavage fracture incorporating both the effect of stress state and that of plastic strain are presented. These theoretical results are applied to the crack tip situation to predict the fracture toughness. It is shown that the ductile-to-brittle transition curve can reasonably be well predicted using the LAF approach. Additional applications of the LAF approach methods are also shown, including: (i) the effect of loading rate and prestressing; (ii) the influence of residual stresses in welds; (iii) the mismatch effects in welds; (iv) the warm-prestressing effect. An attempt is also made to delineate research areas where large improvements should be made for a better understanding of the failure behaviour of structural materials.

Microhardness, toughness, and modulus of Mohs scale minerals

Abstract: We report new results of microhardness and depth-sensing indentation (DSI) experiments for the first nine minerals in the Mohs scale: talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, and corundum. The Mohs scale is based on a relative measure of scratch resistance, but because scratching involves both loading and shearing, scratch resistance is not equivalent to hardness as measured by modern loading (indentation) methods; scratch resistance is also related to other material properties (fracture toughness, elastic modulus). To better understand the relationship of hardness to scratch resistance, we systematically determined hardness, fracture toughness, and elastic modulus for Mohs minerals. We measured hardness and toughness using microindentation, and modulus and hardness with DSI (“nanoindentation”) experiments. None of the measured properties increases consistently or linearly with Mohs number for the entire scale.

The mechanical properties and fracture behaviour of epoxy-inorganic micro- and nano-composites

Abstract: Hybrid materials have been formed using an epoxy polymeric matrix and a range of inorganic particles, including mica and organically-modified montmorillonites (‘organoclays’), with various concentrations of the silicate modifier up to about 30 wt.% depending upon the viscosity increase induced by the presence of the silicate. Wide-angle and small-angle X-ray scattering plus transmission electron microscopy were used to identify the morphologies produced, which included particulate, intercalated and ordered exfoliated. The modulus of these composites increased with the weight fraction of silicate. The morphology had a small effect on the measured modulus; the nano-composites with the ordered exfoliated microstructure showing the highest values of the modulus for a given volume fraction of silicate. The fracture toughness, Kc, and the fracture energy, Gc, initially increased as the weight fraction of the silicate was increased, but then decreased at relatively high concentrations. The measured moduli and toughnesses were compared to theoretical predictions. The measured moduli values showed very good agreement with the predicted values, whilst the agreement for values of the measured fracture energy, Gc, with the predicted values, based upon a crack deflection toughening mechanism, were less convincing. Indeed, analysis of the fracture surfaces using scanning electron microscopy showed that the main toughening effect of the silicate particles is due to plastic deformation of the epoxy matrix around the particles.

Tensile and fracture behavior of polymer foams

Abstract: Tensile and mode-I fracture behavior of cross-linked polyvinyl chloride (PVC) and rigid polyurethane (PUR) foams are examined. Tension tests are performed using prismatic bar specimens and mode-I fracture tests are performed using single edge notched bend (SENB) specimens under three-point bending. Test specimens are prepared from PVC foams with three densities and two different levels of cross-linking, and PUR foam with one density. Tension and quasi-static fracture tests are performed using a Zwick/Rowell test machine. Dynamic fracture tests are performed using a DYNATUP model 8210 instrumented drop-tower test set up at three different impact energy levels. Various parameters such as specimen size, loading rate, foam density, cross-linking, crack length, cell orientation (flow and rise-direction) and solid polymer material are studied. It is found that foam density and solid polymer material have a significant effect on tensile strength, modulus, and fracture toughness of polymer foams. Level of polymer cross-linking is also found to have a significant effect on fracture toughness. The presence of cracks in the rise- and flow direction as well as loading rate has minimal effect. Dynamic fracture behavior is found to be different as compared to quasi-static fracture behavior. Dynamic fracture toughness (Kd) increases with impact energy. Examination of fracture surfaces reveals that the fracture occurs in fairly brittle manner for all foam materials.

The roles of toughness and cohesive strength on crack deflection at interfaces

Abstract: In order to design composites and laminated materials, it is necessary to understand the issues that govern crack deflection and crack penetration at interfaces. Historically, models of crack deflection have been developed using either a strength-based or an energy-based fracture criterion. However, in general, crack propagation depends on both strength and toughness. Therefore, in this paper, crack deflection has been studied using a cohesive-zone model which incorporates both strength and toughness parameters simultaneously. Under appropriate limiting conditions, this model reproduces earlier results that were based on either strength or energy considerations alone. However, the general model reveals a number of interesting results. Of particular note is the apparent absence of any lower bound for the ratio of the substrate to interface toughness to guarantee crack penetration. It appears that, no matter how tough an interface is, crack deflection can always be induced if the strength of the interface is low enough compared to the strength of the substrate. This may be of significance for biological applications where brittle organic matrices can be bonded by relatively tough organic layers. Conversely, it appears that there is a lower bound for the ratio of the substrate strength to interfacial strength, below which penetration is guaranteed no matter how brittle the interface. Finally, it is noted that the effect of modulus mismatch on crack deflection is very sensitive to the mixed-mode failure criterion for the interface, particularly if the cracked layer is much stiffer than the substrate.

Two-dimensional scaling properties of experimental fracture surfaces

Abstract: The self-affine properties of postmortem fracture surfaces in silica glass and aluminum alloy were investigated through the 2D height-height correlation function. They are observed to exhibit anisotropy. The roughness, dynamic, and growth exponents are determined and shown to be the same for the two materials, irrespective of the crack velocity. These exponents are conjectured to be universal.

Challenge of mechanical properties of an acicular ferrite pipeline steel

Abstract: In modern industry, the developing tendency and prospect for productions of the oil and gas pipeline steels is to further improve the strength and toughness by advanced manufacture process of the thermo-mechanical control process (TMCP) to refine microstructure. In this work, the hot deformation behavior as well as its effect on the phase transformation of the clean acicular ferrite pipeline steel with simple chemical composition has been investigated. According to the result, the optimum TMCP parameters were designed. Furthermore, the rolling test was carried out on the experimental rolling mill. The results show that, the high strength and excellent toughness of the clean acicular ferrite pipeline steel can be obtained by controlling the TMCP parameters of the production process appropriately. (c) 2006 Elsevier B.V. All rights reserved.

Ultra High Temperature Ceramics for Hypersonic Vehicle Applications

Abstract: HfB{sub 2} and ZrB{sub 2} are of interest for thermal protection materials because of favorable thermal stability, mechanical properties, and oxidation resistance. We have made dense diboride ceramics with 2 to 20 % SiC by hot pressing at 2000 C and 5000 psi. High-resolution transmission electron microscopy (TEM) shows very thin grain boundary phases that suggest liquid phase sintering. Fracture toughness measurements give RT values of 4 to 6 MPam{sup 1/2}. Four-pt flexure strengths measured in air up to 1450 C were as high as 450-500 MPa. Thermal diffusivities were measured to 2000 C for ZrB{sub 2} and HfB{sub 2} ceramics with SiC contents from 2 to 20%. Thermal conductivities were calculated from thermal diffusivities and measured heat capacities. Thermal diffusivities were modeled using different two-phase composite models. These materials exhibit excellent high temperature properties and are attractive for further development for thermal protection systems.