Fracture toughness of concretes at high temperature

The fracture toughness of ordinary and refractory concretes in the range of 20–1300C was investigated, and the stress intensity factor, K _Ic, on three-point bent specimens (according to ASTM E-399 recommendation) determined. With an increase in testing temperature, the stress intensity factor decreases for both concretes. The values of K _Ic at 20C for both concretes are comparable, being equal to 0.64 MNm^–3/2 for ordinary concrete, and 0.72 MNm^–3/2 for refractory concrete, respectively. At 1100C, K _Ic has a value of 0.043 MNm^–3/2 for ordinary concrete, and for the refractory concrete at 1300C, K _Ic=0.34 MNm^–3/2. The method presented for predicting the behaviour of concrete at high temperature may be used in engineering practice.     

Shrinkage stresses in glass/resin composites

A simple model has been used to calculate the residual stresses in polyester-resin/glass fibre composites that arise when the material is cooled from the post-curing temperature. An elementary elasticity solution for shrink-fit stresses gives a value of the order of 24 MNm^–2 for interfacial pressures in a single fibre model, and it appears that this stress is between 10 and 20% lower if the matrix is a hypothetical material having the properties of a composite. The pull-out stress for a glass fibre in polyester resin is estimated to be 7.6 MNm^–2, in good agreement with earlier experimental results.     

Anelastic deformation and the friction stress of bone

The elastic and non-elastic tensile deformation characteristics of longitudinal bovine and rabbit compact bone specimens have been measured with a microstrain technique. For both types of specimen, limited elastic deformation was observed, with a microscopic yield stress (MYS) (the stress to produce a non-elastic strain of 2×10^–6) of 12±8MNm^–2. Measurements of stress—strain behaviour during loading, unloading and recovery of residual non-elastic strain with time at zero stress, which were made for stress increments from the MYS to fracture, revealed a series of closed hysteresis loops. From these results, the non-elastic strain is attributed entirely to anelastic deformation and the concept of a friction stress (_F), which defines the onset of anelastic deformation, is introduced.     

The creep of sapphire filament with orientations close to the c-axis

Sapphire filament oriented within 2 1/2° of the crystallographic c-axis underwent creep by a mechanism other than slip on the basal planes at temperatures above 1600° C. There was a stress below which creep could not be detected; this decreased from 180 MNm^–2 at 1600° C to 65 MNm^–2 at 1800° C. The total tensile strain obtained never exceeded 5%. Fracture occurred during a linear stage of creep in which the stress exponent of the strain-rate was approximately 6. The creep mechanism appeared to be slip on {20¯2¯1} 01 T2 (morphological unit cell). A filament in which the c-axis lay at 6° to the filament axis deformed by localized basal slip. The accompanying local latice rotations produced fracture at a small overall strain, usually less than 0.5%. The results demonstrate extreme anisotropy of creep in sapphire crystals.     

Hertzian fracture of Pyrex glass under quasi-static loading conditions

Hertzian fracture tests were conducted using an Instron on Pyrex glass specimens with various surface conditions, including lubricants, employing steel, Al₂O₃, WC and Pyrex glass indentors of 0.79 to 12.7 mm radius under ambient air and high vacuum environments at cross-head speeds of 8.5×10^–6 to 2.1×10^–4m sec^–1. The results were not in strict accord with Auerbach's law, nor any of the existing energy-balance Hertzian fracture theories. Rather, they indicated that surface roughness and friction modified the Hertz stress field so that the maximum tensile stress at the surface occurred outside the contact circle. Further, they indicated that Hertzian fracture occurred by the direct, unstable growth into a cone crack of a pre-existing flaw at the displaced site of the maximum tensile stress, the flaw size responsible for the fracture decreasing with decrease in ball size (contact radius). Once a cone crack occurred, its length and growth were described reasonably well by Roesler's theory; however, his constant appears to be too high by a factor of about 5. A surface energy of @ 4 J m^–2 was derived from bend tests on specimens similar to those used in the Hertzian fracture tests. Using this value, the crack sizes which lead to fracture were estimated to range between 0.6 and 3.5 m for the conditions investigated here. The increase in the critical load for Hertzian fracture with indentation velocity was concluded to be due to kinetic effects of water vapour acting at the tip of the crack.     

A double torsion fracture mechanics and Auger electron spectroscopy approach to the study of stress corrosion cracking in low alloy steels

The applicability of the double torsion fracture mechanics test to the study of stress corrosion cracking (SCC) in steels was assessed by evaluating the behaviour of the low alloy steels AISI 4140 and En3OA, exhibiting both high (1400 MNm^–2) and low (765 MNm^–2) yield strengths respectively. An optical method for measuring crack growth rate and a load relaxation method for computing it were compared and found to give similar results. The test was shown to be eminently suitable for the study of SCC in high yield strength steels and those in a temper embrittled condition. The influence of trace impurities on the SCC susceptibility was examined using Auger electron spectroscopy to determine the type and amount of grain boundary segregants. The degree of segregation of trace impurities was shown to have a profound effect on the stress intensity-crack velocity (K-V) diagram by reducingK _IC and increasing the reaction rate as shown by the increased slope in stage III of theK-V diagram. An anomalously low threshold stress intensity was observed in as-quenched AISI 4140 and this was attributed to residual stresses produced by the phase transformations occurring during quenching.     

The effect of stress triaxiality and strain-rate on the fracture characteristics of ductile metals

Notched tensile tests have been carried out on three common metals (pure iron, mild steel and aluminium alloy BS1474) over a wide range of strain-rates (10^–3 to 10⁴ s^–1) and the strain-to-failure measured. The ductility of all three materials was found to be strongly dependent on the level of stress triaxiality in the specimen, this dependency being greatest for the ferrous materials and least for the aluminium alloy. No significant effect of strain-rate could be ascertained from the experimental results provided fracture remained fully ductile. However, for mild steel, a transition to a brittle fracture mode was observed for a given level of stress triaxiality as the strain-rate was increased. Numerical simulations of the experiments have been used to derive constants of a semi-empirical fracture model from the measured results. This model was found to give reasonable predictions of fracture over the range of conditions investigated.     

Experimental studies on the dynamic tensile behavior of Ti–6Al–2Sn–2Zr–3Mo–1Cr–2Nb–Si alloy with Widmanstatten microstructure at elevated temperatures

The tensile behavior of a newly developed Ti–6Al–2Sn–2Zr–3Mo–1Cr–2Nb–Si alloy, referred as TC21, is investigated at temperatures ranging from 298 to 1023 K and under constant strain rate loadings ranging from 0.001 to 1270 s⁻¹. The results show that temperature and strain rate have significant effects on the tensile behavior of the material. At low strain rates of 0.001 and 0.05 s⁻¹, a discontinuity is found in the yield stress–temperature curve. And the discontinuity temperature increases with increasing strain rate. The analysis of temperature and strain rate dependence of unstable strain indicates a high-velocity-ductility phenomenon at elevated temperatures. Scanning electron microscope (SEM) analysis shows that the material is broken in a mixture manner of ductile fracture and intergranular fracture under low strain rates at room temperature, while the fracture manner changes to totally ductile fracture under other testing conditions. The width and depth of ductile dimples increase with increasing temperature. No adiabatic shear band is found in the tensile deformation of the material.     

Microstructural aspects of the fracture process in human cortical bone

Fracture toughness tests were conducted in the transverse and longitudinal directions to the osteonal orientation of human femoral cortical bone tissue to investigate the resulting damage patterns and their interaction with the microstructure. The time history of damage accumulation was monitored with acoustic emission (AE) during testing and was spatially observed histologically following testing. The fracture toughness of the transverse specimens was almost two times greater than the fracture toughness of the longitudinal specimens (3.47 MNm^–3/2 vs. 1.71 MNm^–3/2, respectively). The energy content of the AE waveforms of transverse specimens were greater than those of the longitudinal specimens implying higher fracture resistance in the transverse crack growth direction. The results showed that the propagation of the main crack involved weakening of the tissue by ultrastructural (diffuse) damage at the fracture plane and formation of linear microcracks away from the fracture plane for the transverse specimens. For the longitudinal specimens, the growth of the main crack occurred in the form of separations at lamellar interfaces. The lamellar separations generally arrested at the cement lines. Linear microcracks occurred primarily in the interstitial tissue for both crack growth directions.     

The effect of matrix stresses on fibre pull-out forces

A pull-out test was developed to measure the bond strengths and frictional forces between steel wires, and polycarbonate and epoxy matrices when the matrix was under tensile stress. Some debonding occurred due to the matrix stress. Despite this, the nominal bond strength, in the polycarbonate case, increased with increasing matrix applied stress. When the pull-out force had caused complete debonding, sliding under approximately constant friction coefficient,, occurred. The value of for steel sliding in polycarbonate was 0.6, and for epoxy it was 0.19. The values were reduced to 0.12 and 0.10 respectively when the steel was coated with a fluorocarbon release agent. The normal stresses at the interface, in the absence of any applied stresses, were found to be about 7 MN m^–2 in the polycarbonate, and 3.0 MN m^–2 in the epoxy case. It was observed that the frictional forces due to these residual stresses could be less than one third of those generated by the applied stresses on the matrix. Thus residual stresses are not as important for fibre reinforcement as are matrix Poisson's shrinkage stresses.     

The effect of hydrostatic pressure on the tensile properties of pultruded CFRP

The failure process in waisted tensile specimens of pultruded 60% volume fraction carbon fibre-epoxide was investigated at atmospheric and superposed hydrostatic pressures up to 300 MN m^–2. The maximum principal stress at fracture decreased from ~ 2.0 GN m^–2 at atmospheric pressure to ~ 1.5 GN m^–2 by 200 MN m^–2 superposed pressure and then remained approximately constant. These latter failures were fairly flat and no damage preceding the catastrophic fracture was detected, which indicates that composite strength is solely controlled by fibre strength. Fracture of fibres at lower pressures appeared to commence also in the range 1.5 to 1.6 GN m^–2, but, as it did not result in catastrophic failure, account has to be taken of the resin and the fibre bundles. Debonding was initiated at ~ 1.2 GN m^–2 at atmospheric pressure and this stress increased to ~ 1.5 GN m^–2 when 150 MN m^–2 superposed pressure was applied; the pressure dependence was related to that of the resin tensile strength. This process is described as the first stage, straightening and debond initiation of curved surface bundles, on our model of tensile failure. The second stage, delamination, i.e. the growth of transverse cracks leading to the detachment of these bundles, was impeded by the transverse pressure, being suppressed beyond 150 MN m^–2. Only below this pressure was load redistribution between bundles possible, but, as the pressure was increased from atmospheric, it become more difficult, resulting in a decrease in the composite tensile strength and reduced fibre pull-out.     

Mechanical properties of vanadium beryllide,VBe12

The mechanical properties of VBe₁₂, both at room and elevated temperatures (up to 1200°C), have been measured. Room-temperature properties, including Young's modulus, flexural strength, and fracture toughness are reported. The material behaved elastically at room temperature but became plastic at temperatures above 1000°C. Creep properties of VBe₁₂ were also studied in temperature ranges from 1000–1200°C and applied stress ranges from 33–58 MPa. At low strain rates (approximately < 10^–5s^–1), the stress exponent was about 4, suggesting deformation was controlled by dislocation climb. Microstructural examination indicated that fracture was initiated from grain boundaries subjected to tensile stresses. The creep behaviour of VBe₁₂ is briefly compared with that of other intermetallics.     

Influence of heat treatment on the strength and fracture behaviour of Fe-12Cr-6Al ferritic stainless steel

The changes in the tensile properties and fracture mode brought about by heat treatment of Fe-12Cr-6Al ferritic stainless steel have been studied. A favourable combination of high strength and good ductility is obtained by heating the material at 1370 K for 2 h followed by a water quench. The high-temperature treatment results in carbide dissolution as well as an increase in the grain size. The mechanism of strengthening has been evaluated from the apparent activation energy (28 kJ mol^–1) and is identified to be the unpinning of dislocations from the atmosphere of carbon atoms. As the heat-treatment temperature is increased, the fracture behaviour changes from ductile to brittle mode and this is related to the changes in grain size and friction stress.     

Deformation and fracture behavior of notched and unnotched unidirectional C/C-Mg composite with Young's modulus 520 GPa and tensile strength 1 GPa

Experimental study on tensile fracture behavior of the newly developed C/C-Mg composite, prepared by infiltration of Mg into the pores in the C/C composite heat-treated at 3000°C, was carried out. The volume fraction of the filled Mg was 9–10%. The composite had a specific density 2.1, Young's modulus 520 GPa and Poisson's ratio 0.26. The average tensile strength measured for the specimen with a nominal width 8 mm, gage length 40 mm and thickness 1 mm was 1 GPa. The Young's modulus was improved from 450 to 520 GPa and the strength from 0.9 to 1.0 GPa by Mg-infiltration. The specific Young's modulus and specific strength based on the average measured values were 2.5 × 10⁷ m and 5 × 10⁴ m, respectively, showing high potential as light-weight, stiff and strong structural material. The strength distribution of the composite was described by the two-parameter Weibull distribution function with a shape parameter 7.6 and scale parameter 1060 MPa. Prior to the overall fracture of the composite, the longitudinal cracking arose at the notch tip, due to which the notch tip was blunted and the ligament portion behaved like an unnotched specimen. As a result, the notched strength could be described by the net stress criterion. The apparent critical energy release rate at formation of the longitudinal crack was around 70–90 J/m².     

The yield and deformation behaviour of some polycarbonate blends

Polycarbonate and its blends with PE and MBS have been tested to investigate the impact modification mechanism. These materials have been tested in tension over the speed range 10^–2 to 10² in. sec^–1 (2.5×10^–2 to 2.5×10² cm sec^–1). The tensile deformation behaviour of these materials is similar except for the magnitude of the yield stresses. The yield stress versus log curves have identical slopes. Based on Eyring's equation for the flow of viscous materials, these materials have identical activation volumes, implying that the mechanical behaviour modification is not due to changes in molecular mechanisms. The modifier particles probably change only the stress state of the matrix material. Three-point bending tests on notched bars of these materials have also been performed over the speed range 10^–2 to 10² in. sec^–1 (2.5×10^–2 to 2.5×10² cm sec^–1). The areas under the load-deflection curves have been measured as the total energy absorbed during the deformation. It was found that both geometric constraint and rate of deformation can bring about ductile-brittle transitions. However, the thickness sensitivity of the blends is less than that of the pure material. Scanning electron micrographs show that the matrix material voids and flows extensively around the modifier particles before the ductilebrittle transition speed is reached. This voiding probably relieves plane strain. However, at higher speeds, the modifier particles cannot relax sufficiently rapidly, and they lose this plane strain relieving capability.     

Processing and properties of Al–Li–SiCp composites

Al–Li–SiC_p composites were fabricated by a modified version of the conventional stir casting technique. Composites containing 8, 12 and 18 vol% SiC particles (40 μm) were fabricated. Hardness, tensile and compressive strengths of the unreinforced alloy and composites were determined. Ageing kinetics and effect of ageing on properties were also investigated. Additions of SiC particles increase the hardness, 0.2% proof stress, ultimate tensile strength and elastic modulus of Al–Li–8%SiC and Al–Li–12%SiC composites. In case of the composite reinforced with 18% SiC particles, although the elastic modulus increases the 0.2% proof stress and compressive strength were only marginally higher than the unreinforced alloy and lower than those of Al–Li–8%SiC and Al–Li–12%SiC composites. Clustering of SiC particles appears to be responsible for reduced the strength of Al–Li–18%SiC composite. The fracture surface of unreinforced 8090 Al-Li alloy (8090Al) shows a dimpled structure, indicating ductile mode of failure. Fracture in composites occurs by a mixed mode, giving rise to a bimodal distribution of dimples in the fracture surface. Cleavage of SiC particles was also observed in the fracture surface of composites. Composites show higher peak hardness and lower peak ageing time compared with unreinforced 8090Al alloy. Macro- and microhardness increase significantly after peak ageing. Ageing also results in considerable improvement in strength of the unreinforced 8090Al alloy and its composites. This is attributed to formation of δ^′ (Al₃Li) and S^′ (Al₂CuMg) precipitates during ageing. Per cent elongation, however, decreases due to age hardening. Al–Li–12%SiC, which shows marginally lower UTS and compressive strength than the Al–Li–8%SiC composite in extruded condition, exhibits higher strength than Al–Li–8%SiC in peak-aged condition.     

The dependence of the strength of zinc sulphide on temperature and environment

The dependence of the strength of zinc sulphide on temperature, environment, surface finish and specimen size has been assessed. Room-temperature fracture stresses were determined using a bursting disc geometry for a number of different surface finishes and for two different sample sizes. High and low-temperature fracture stresses in a dry nitrogen atmosphere were obtained from experiments using the Brazilian test geometry and showed that the average strength of the material remained above or equal to the room-temperature value within the range –70 to +600 °C. The Brazilian test is an indirect tensile technique which is attractive for its experimental simplicity but gives fracture stress values which are consistently below those obtained by direct tensile techniques. The data from this test were therefore compared at room temperature to results obtained from the bursting disc test on samples which had been prepared using the same techniques. The possibility of delayed failure through environmentally enhanced slow crack growth was evaluated using the double-torsion technique which revealed slow crack growth below the critical stress intensity factor.     

The tensile properties of pultruded GRP tested under superposed hydrostatic pressure

The failure mechanisms in waisted tensile specimens of pultruded 60% volume fraction glass fibre-epoxide were investigated at atmospheric and superposed hydrostatic pressures extending to 350 MN m^–2. The maximum principal stress at fracture decreased from 1.7 GN m^–2 at atmospheric pressure to 1.3 GN m^–2 at 250 MN m^–2 superposed pressure and remained approximately constant at higher pressures, as had been observed with carbon fibre reinforced plastic (CFRP) and a nickel-matrix carbon fibre composite. In the high-pressure region the failure surfaces were fairly flat, consistent with the fracture process being solely controlled by fibre strength. Pre-failure damage, in particular debonding, was initiated at 0.95 GN m^–2 at atmospheric pressure and this stress rose to 1.2 GN m^–2 at 300 MN m^–2 superposed pressure, i.e. by about 9% per 100 MN m^–2. Unlike the pressure dependence in CFRP, this contrasts with the pressure dependence of the resin tensile strength, about 25% per 100 MN m^–2, but can be associated with that of the fibre bundle/resin debonding stress, about 12% per 100 MN m^–2 superposed pressure. Consistent with this interpretation, glass fibres of the failure surfaces were resin-free, again in contrast to CFRP.     

Delayed failure in silica glass

Stable crack-propagation behaviour in silica glass as a raw material for optical fibres is studied under static tensile stress in various environments such as distilled water, NaCl aqueous solution, air and dry nitrogen gas, and the influence of these environments is discussed. The crack-growth rate in distilled water is obtained qualitatively as a function of the stress intensity factor and temperature, and the activation energy of the cracking process is determined as 97.6 kcal mol^–1. The growth rate seems to be unaffected by Na⁺ and Cl^– ions in an NaCl acqueous solution, but is influenced significantly by the humidity in the atmosphere. In a dry atmosphere, the growth rate in Region II cannot be expressed as a single function of the stress intensity factor. A plot of the log of time to failure against the initial stress intensity factor reveals a linear relationship in the environments tested. The critical fracture stress of an optical fibre is evaluated taking account of the crack size on the basis of fracture mechanics concept.     

Crack velocity dependence of longitudinal fracture in bone

An evaluation of the fracture characteristics of bovine tibia compact tension specimens associated with controlled crack propagation in the longitudinal direction has been made. The fracture mechanics parameters of critical strain energy release rate (G _c) and critical stress intensity factor (K _c) were determined for a range of crack velocities. A comparative fracture energy (W) was also evaluated from the area under the load-deflection curve. It was found that an increase in the average crack velocity from 1.75 to 23.6×10^–5 m sec^–1 produced increases in G _c (from 1736 to 2796 J m^–2), K _c (from 4.46 to 5.38 MN m^–3/2) and W. At crack velocities >23.6×10^–5 m sec^–1, W decreased appreciably. Microstructural observations indicated that, for crack velocities <23.6 m sec^–1, relatively rough fracture surfaces were produced by the passage of the crack around intersecting osteons (or lamellae), together with some osteon pull-out. In contrast, at a higher crack velocity, fracture was characterized by relatively smooth surfaces, as the crack moved indiscriminately through the microstructural constituents.