Showing papers on "Fracture toughness published in 2003"

Supra-Ductile and High-Strength Manganese-TRIP/TWIP Steels for High Energy Absorption Purposes

Abstract: The microstructural properties of advanced high strength and supra-ductile TRIP and TWIP steels with high-manganese concentrations (15 to 25 mass%) and additions of aluminum and silicon (2 to 4mass%) were investigated as a function of temperature (−196 to 400°C) and strain rate (10−4≤e≤103 s−1). Multiple martensitic γfcc (austenie)→ehcpMs (hcp-martensite)→αbccMs (bcc-martensite)-transformations occurred in the TRIP steel when deformed at higher strain rates and ambient temperatures. This mechanism leads to a pronounced strain hardening and high tensile strength (>1 000 MPa) with improved elongations to failure of >50%. The austenitic TWIP steel reveals extensive twin formation when deformed below 150°C at low and high strain rates. Under these conditions extremely high tensile ductility (>80%) and energy absorption is achieved and no brittle fracture transition temperature occurs. The governing microstructural parameter is the stacking fault energy Γfcc of the fcc austenite and the phase stability determined by the Gibbs free energy ΔGγ→e. These factors are strongly influenced by the manganese content and additions of aluminum and silicon.The stacking fault energy Γfcc and the Gibbs free energy G were calculated using the regular solution model. The results show that aluminum increases Γfcc and suppresses the γfcc→ehcpMs transformation, whereas silicon sustains the γfcc→ehcpMs transformation and decreases the stacking fault energy. At the critical value of Γfcc≈25 mJ/mol and for ΔGγ→e>0, the twinning mechanism is favored. At lower stacking fault energy of (Γfcc 0, martensitic phase transformation will be the governing deformation mechanism.The excellent ductility and the enhanced impact properties enable complex deep drawing or stretch forming operations of sheets and the fabrication of crash absorbing frame structures.

Self-healing structural composite materials

Abstract: A self-healing fiber-reinforced structural polymer matrix composite material is demonstrated. In the composite, a microencapsulated healing agent and a solid chemical catalyst are dispersed within the polymer matrix phase. Healing is triggered by crack propagation through the microcapsules, which then release the healing agent into the crack plane. Subsequent exposure of the healing agent to the chemical catalyst initiates polymerization and bonding of the crack faces. Self-healing (autonomic healing) is demonstrated on width-tapered double cantilever beam fracture specimens in which a mid-plane delamination is introduced and then allowed to heal. Autonomic healing at room temperature yields as much as 45% recovery of virgin interlaminar fracture toughness, while healing at 80 °C increases the recovery to over 80%. The in situ kinetics of healing in structural composites is investigated in comparison to that of neat epoxy resin.

Nanomechanical characterisation of solid surfaces and thin films

Abstract: To measure nanomechanical properties of surface layers of bulk materials and thin films, depth-sensing nanoindentation measurement techniques are used commonly. The nanoindentation apparatus continuously monitors the load and the position of the indenter relative to the surface of the specimen (depth of an indent or displacement) during the indentation process. Indentation experiments can be performed at a penetration depth of as low as about 5 nm. This chapter presents an overview of various nanoindentation techniques, various measurement options, and data analysis. Data on elastic-plastic deformation behavior, hardness, elastic modulus, scratch resistance, film-substrate adhesion, residual stresses, time-dependent creep and relaxation properties, fracture toughness, and fatigue are presented.

Fracture mechanisms in bulk metallic glassy materials.

Abstract: We find that the failure of bulk metallic glassy (BMG) materials follows three modes, i.e., shear fracture with a fracture plane significantly deviating from 45degrees to the loading direction, normal tensile fracture with a fracture plane perpendicular to the loading direction, or distensile fracture in a break or splitting mode with a fracture plane parallel to the loading direction. The actually occurring type of failure strongly depends on the applied loading mode and the microstructure of the material. Extensive evidence indicates that the Tresca fracture criterion is invalid, and for the first time, three fracture criteria are developed for isotropic materials with high strength, such as advanced BMGs or the newly developed bulk nanostructural materials.

Creep-strengthening of steel at high temperatures using nano-sized carbonitride dispersions

Abstract: Creep is a time-dependent mechanism of plastic deformation, which takes place in a range of materials under low stress-that is, under stresses lower than the yield stress. Metals and alloys can be designed to withstand creep at high temperatures, usually by a process called dispersion strengthening, in which fine particles are evenly distributed throughout the matrix. For example, high-temperature creep-resistant ferritic steels achieve optimal creep strength (at 923 K) through the dispersion of yttrium oxide nanoparticles. However, the oxide particles are introduced by complicated mechanical alloying techniques and, as a result, the production of large-scale industrial components is economically unfeasible. Here we report the production of a 9 per cent Cr martensitic steel dispersed with nanometre-scale carbonitride particles using conventional processing techniques. At 923 K, our dispersion-strengthened material exhibits a time-to-rupture that is increased by two orders of magnitude relative to the current strongest creep-resistant steels. This improvement in creep resistance is attributed to a mechanism of boundary pinning by the thermally stable carbonitride precipitates. The material also demonstrates enough fracture toughness. Our results should lead to improved grades of creep-resistant steels and to the economical manufacture of large-scale steel components for high-temperature applications.

Analysis of different acicular ferrite microstructures in low-carbon steels by electron backscattered diffraction. Study of their toughness behavior

Abstract: Acicular ferrite formation, promoted by the intragranular nucleation of ferrite plates, is well known to be beneficial for achieving a good combination of mechanical properties. However, the set of microstructures that can be obtained during the subsequent development of the transformation from the primary plates generated at particles can be quite complex and depends on a certain number of variables: steel composition, temperature range, prior austenite grain size, and particle density. In the present work, acicular ferrite microstructures have been produced by isothermal treatments in three different steels with different active particle types and densities. The morphology of the obtained intragranular microstructures has been found to depend on the steel composition, the prior austenite grain size, and the density of particles able to promote intragranular nucleation. Electron backscattered diffraction (EBSD) techniques have been used to define the microstructural unit controlling toughness in these types of microstructures.

On the relationship between microstructure, strength and toughness in AA7050 aluminum alloy

Abstract: The effect of process parameters such as quench rate and precipitation heat treatment on the compromise between the toughness and the yield strength of AA7050 aluminum alloy (AlZnMgCu) are investigated, as well as the anisotropy of this compromise in the rolling plane. Fracture toughness is experimentally approached by the Kahn tear test. The microstructure is studied quantitatively in detail by a combination of scanning electron microscopy, transmission electron microscopy and small-angle X-ray scattering, and the relative fractions of the various fracture modes as a function of microstructural state are quantitatively determined on scanning electron microscopy images. Toughness is confirmed to be minimum at peak strength, and lower for an overaged material than for an underaged material of the same yield strength. A lower quench rate is shown to result in an overall reduction of toughness, and in a reduced evolution of this toughness during the aging heat treatment. The overall toughness is also lowered when the main crack propagation direction is parallel to the preferential elongation direction of the coarse constituent particles (rolling direction). The competition between intergranular and transgranular fracture is explained in terms of the modifications of the work hardening rate, and of grain boundary precipitation. The evolution of fracture toughness is qualitatively explained in terms of evolution of yield stress, strain hardening rate, grain boundary precipitation and intragranular quench-induced precipitates.

Glass breaks like metal, but at the nanometer scale.

Abstract: We report in situ atomic force microscopy experiments which reveal the presence of nanoscale damage cavities ahead of a stress-corrosion crack tip in glass. Their presence might explain the departure from linear elasticity observed in the vicinity of a crack tip in glass. Such a ductile fracture mechanism, widely observed in the case of metallic materials at the micrometer scale, might be also at the origin of the striking similarity of the morphologies of fracture surfaces of glass and metallic alloys at different length scales.

Role of collagen and other organics in the mechanical properties of bone

Abstract: Relevant mechanical properties of bone The mechanical properties of bone material are determined by the relative amounts of its 3 major constituents: mineral, water, and organics (mainly type I collagen); by the quality of these components; and by how the resulting material is arranged in space. For our purposes, the mechanical properties of bone can be summed up as follows: modulus of elasticity, yield stress and yield strain, post-yield stress and post-yield strain, and the total area under the stress-strain curve. Also important are some fracture mechanics properties, but these are not discussed here. A typical tensile stress-strain curve for a bone specimen is shown in Fig. 1. The modulus of elasticity shows how stiff the bone material is. Indeed, stiffness is the prime property of bone, distinguishing it from tendon, which has much less tensile stiffness, almost no shear stiffness, but which is nearly as strong and is much tougher. Yield stress and strain determine how much energy can be absorbed before irreversible changes take place. Post-yield stress and strain determine mainly how much energy can be absorbed after yield but before fracture. Irreversible changes take place at yield, caused by microdamage. The total area under the stress-strain curve is equivalent to the work that must be done per unit volume on the specimen before it breaks. Fracture mechanics properties show the extent to which bone is resistant to crack initiation and to crack travel (which are different things and governed by somewhat different features). In fact, crack travel resistance is given rather well by post-yield stress and strain.

Effect of clustering on the mechanical properties of SiC particulate-reinforced aluminum alloy 2024 metal matrix composites

Abstract: Al 2024–SiC metal matrix composite (MMC) powders produced by centrifugal atomization were hot extruded to investigate the effect of clustering on their mechanical properties. Fracture toughness and tension tests were conducted on specimens reinforced with different volume fractions of SiC. A model was proposed to suggest that the strength of the MMCs could be estimated from the load transfer model approach that takes into consideration the extent of clustering. This model has been successful in predicting the experimentally observed strength and fracture toughness values of the Al 2024–SiC MMCs. On the basis of experimental observations, it is suggested that the strength of particulate-reinforced MMCs may be calculated from the relation: σ y = σ m V m + σ r ( V r − V c )− σ r V c , where σ and V represent the yield strength and volume fraction, respectively, and the subscripts m, r, and c represent the matrix, reinforcement, and clusters, respectively.

Graded coatings for thermal, wear and corrosion barriers

Abstract: The present paper summarizes the main results generated in a German National Science Foundation (DFG) program on projects concerned with functionally graded materials applied to optimize the thermal, wear and corrosion properties of metallic and ceramic materials. Thermal barrier coatings deposited onto Cu substrates by pulsed laser deposition showed improved spallation behavior by a graded lamella microstructure with improved interface fracture toughness. A particle-hardened graded surface structure improved the wear resistance of plasma sprayed thermal barriers. By means of evaporation techniques a graded bonding area was manufactured with a high potential of lifetime improvement. For non-oxide ceramics graded coatings based on Si 3 N 4 and mullite led to improved oxidation resistance of the substrate material. Graded TiC–TiN thin films allowed to improve the wear resistance of cutting tool alloys with good adhesion to the substrate material. On light metal alloys, the limits of grading with respect to corrosion protection as well as wear were determined. Graded layers of arc-sprayed titanium with in situ produced particles or welded alloy gradients led to improved wear characteristics. Stress profiles in graded layers were analyzed with the help of a modified X-ray diffraction analysis.

Fabrication, characterization, and dynamic behavior of polyester/TiO2 nanocomposites

Abstract: Unsaturated polyester resin specimens embedded with small loadings of 36 nm average diameter TiO 2 particles were fabricated using a direct ultrasonification method to study the effects of nanosized particles on nanocomposite bulk mechanical properties. The ultrasonification method employed produced nanocomposites with excellent particle dispersion as verified by transmission electron microscopy (TEM). Quasi-static fracture toughness, tension, and compression testing was carried out. The presence of the particles had the greatest effect on fracture toughness; negligible influence was observed in the remaining quasi-static properties. Scanning electron microscopy (SEM) of fracture surfaces was carried out to identify toughening mechanisms. The inadequacy of the bond between the filler and the matrix and the presence of minor particle agglomerations in specimens containing higher volume fractions of particles were believed to be responsible for a consistent decrease in property values beyond a volume fraction of 1 vol.%. Dynamic fracture toughness testing was carried out, and an increase in dynamic fracture toughness relative to quasi-static fracture toughness was observed. High strain rate testing conducted using a split Hopkinson pressure bar (SHPB) apparatus revealed a moderate stiffening effect with increasing particle volume fraction, although no marked effect was observed on the ultimate strength.

Fabrication and mechanical properties of SiO2 matrix composites reinforced by carbon nanotube

Abstract: Carbon nanotubes (CNTs) and SiO 2 glass powder were mixed by ultrasonication in ethanol solution. CNT–SiO 2 composites with improved mechanical properties were fabricated by hot pressure sintering in 1300 °C 25 MPa, N 2 atmosphere). Effects of CNTs content on the relative density and mechanical properties were investigated. The bending strength and fracture toughness of the 5vol.%CNT–SiO 2 composite, compared with the monolithic SiO 2 , were enhanced 65 and 100%, respectively. The main reinforcing mechanisms include CNTs pulling-out, crack deflxion, bridging and CNTs rupture. The defects and aligning direction of CNTs in the matrix are also the affecting factors on the mechanical properties of composites as prepared.

Hydrogen Assisted Cracking of High Strength Alloys

Abstract: : Two important advances over the past 40 years enable the optimization and management of the structural integrity of components in high performance applications. First, the solid mechanics conununity established linear elastic fracture mechanics as the premier framework for modeling the damage tolerance of fracture critical components (Irwin and Wells, 1997; Paris, 1998). Second, materials scientists developed metals with outstanding balances of high tensile strength and high fracture toughness (Garrison, 1990; Wells, 1993; Boyer, 1993; Starke and Staley, 1995; Olson, 1997; Kolts, 1996). An example of achievable strength-toughness properties is provided in Fig. 1, a plot of plane strain fracture toughness vs. tensile yield strength (sigma-YS) for ultra-high strength steels (UHSS) and beta-Ti alloys precipitation hardened with a phase (Gangloff 2001). New nano-scale characterization and high performance computational methods provide for additional advances in the mechanical performance properties of structural metals. These modem alloys and analysis tools satisfy technological needs for optimization and management of component performance in demanding fatigue and fracture critical applications in the aerospace, marine, energy, transportation, and defense sectors.