Rate-dependent fracture toughness of pure polycrystalline ice

A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (–20°C, –30°C and –40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10^–3 to 10^–1 s^–1. The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (K _IC) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of K _IC, the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10^–3 s^–1, the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen.     

Fracture toughness of microsphere Al2O3–Al particulate metal matrix composites

The fracture toughness and behaviour of COMRAL-85^TM, a 6061 aluminium–magnesium–silicon alloy reinforced with 20 vol% Al₂O₃-based polycrystalline ceramic microspheres, and manufactured by a liquid metallurgy route, have been investigated. Fracture toughness tests were performed using short rod and short bar (chevron-notch) specimens machined from extruded 19 mm diameter rod, heat treated to the T6 condition. The fracture toughness in the R–L orientation was found to be lower than in the C–R or L–R orientations owing to the presence of particle-free bands in the extrusion direction. Short rod tests were also conducted for the R–L orientation on six powder metallurgy composites with particle volume fractions ranging between 5% and 30%. It was found that the fracture toughness decreased progressively with particle volume fraction, but at a decreasing rate. A detailed examination of the fracture behaviour was made for both the liquid metallurgy and powder metallurgy processed composites.     

Thermodynamical approach to the brittle fracture of dry plasters

The evolution of the fracture toughness, K _lc, and fracture energy, G _lc, of set plasters was determined on notched beams as a function of sample porosity, P, and characteristic size, W. Toughness was found to decrease with decreasing crack width. For set plasters of 57.7% porosity, the lowest toughness measured was K _lc=0.13 MPa m^1/2 for a crack width of 0.2 mm. For this crack width, fracture toughness and fracture energy linearly changed with porosity: K _lc=0.5 1–1.3 P) MPa m^1/2 and G _lc= 13.47 (1–1.12 P) Jm^–2. Dense plasters were more difficult to break than porous ones. The fracture energies were affected by the velocity of the fracture propagation, which induces damaging and multicracking of the material, so that the roughly calculated chemical surface energy of set plaster was too high. After correction it was estimated to be 0.4 J m ^–2. Finally, because toughness increased with increasing sample size, it was concluded that fracture toughness and energy were not intrinsic parameters of the material. On the other hand, for our sample porosities and sizes, the reduced rupture force, F _rupt W ^–0.65 is a constant and seems to be a characteristic parameter of the mechanical resistance of set plaster beams.     

Single crystal cleavage of brittle materials

Cleavage of brittle single crystals is reviewed and the historical criteria for the phenomenon are critically examined. Previously proposed criteria, including those based on crystal structure (crystal growth planes, the planes bounding the unit cell, and planar atomic packing) and crystal properties (ionic charge of possible cleavage planes, bond density, elastic modulus, and surface free energy), are found to be applicable only to particular crystals or to isostructural groups, but each lacks universal application. It is concluded that the fracture toughness (K _Ic) of the crystallographic planes is the most appropriate criterion. Measurements reveal that the cleavage toughnesses of brittle single crystals are usually about 1 MPa m^1/2 or less.Measurements of the fracture toughnesses of brittle polycrystalline aggregates are then compared to the single crystal cleavage values in those instances where reliable results are available for the same crystal structures. Polycrystalline toughnesses are consistently higher, in part because of the lack of continuity of cleavage cracks through the polycrystalline aggregates. However, the increment of toughness increase is only 1–2 MPa m^1/2. The role of grain texture or preferred crystal orientation is also addressed. It is concluded that polycrystalline aggregate toughnesses are often highly anisotropic and that the values for intensely oriented microstructures may approach those for single crystal cleavage.     

Positron lifetime spectroscopy characterization of thermal history effects on polycarbonate

The effect of a short-term anneal aboveT _g on the free volume cavity size and concentration and on the fracture toughness of polycarbonate is examined. The positron annihilation lifetime (PAL) technique is used to measure the change in free volume concentration and cavity size during isothermal relaxation experiments at 10, 20 and 30 ° C. An activation energy of 16.5 kJ mol^–1 is calculated for the relaxation of the annealed polycarbonate, compared to 12.3 kJ mol^–1 for the unannealed material. The fracture toughness and brittle fracture morphology of compact tension specimens are unchanged by the anneal. The similarity in the PAL parameters and physical properties between the unannealed polycarbonate and the material annealed aboveT _g suggests that the short-term anneal does not appreciably alter the structural state of glassy polycarbonate.     

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m^–2 for unreinforced Si₃N₄, to 4770 J m^–2; its fracture toughness,K _lc , increased from 3.7 MN m^–3/2 for unreinforced material, to 15.6 MN m^–3/2. The strength remains about the same as unreinforced Si₃N₄ and the thermal expansion coefficient is only 2.51×10^–6 ° C^–1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.     

The effect of the loading rate on the fracture toughness of Poly(methyl methacrylate), Polyacetal,Polyetheretherketone and modified PVC

The mode-l-plane-strain fracture toughness at initiation of various engineering plastics was determined for test speeds between 10^–4ms^–1 and 1.0 ms^–1, using a high-speed, servohydraulic testing apparatus. At high rates of testing, the transient acceleration of the specimen was reduced by the use of a damping technique aimed at overcoming dynamic effects. The results obtained are correlated with fractographic analysis performed by scanning electron microscopy (SEM) and by digital image analysis of macroscopic fracture surfaces. The results show that for some materials the high values of K _lc measured at low testing rates are associated with deformation detectable by either method. The extent of such deformation tends to be reduced as the testing velocity is increased and the fracture toughness drops.     

Thermomechanically induced embrittlement in hot isostatically pressed Si3N4/SiC composites

The macroscopic fracture properties of an Si₃N₄/SiC-platelet composite fabricated by hot isostatic pressing (HIP) without sintering aids were measured by the chevron-notch technique in bending and related to micromechanisms of fracture by means of a quantitative profilometric analysis of the fracture surfaces. Compositional and processing parameters were varied systematically in order to maximize both the fracture toughness and the work of fracture of the composite. Data were compared with those of monolithic Si₃N₄ fabricated by the same process. Cooling-rate from the HI Ping temperature was indicated as a critical parameter especially when cooling was performed under high pressure. A marked embrittlement of the composite body was found by cooling at around 650 °C h^–1 and it could not be completely recovered by successive annealing even up to temperatures above 1700 °C. The highest fracture toughness and work of fracture in the composite (obtained at a cooling rate of about 100 °C h^–1), were measured as 4.6 MPa m^1/2 and 58.6 J m^–2, respectively. In agreement with fractal analysis results, they were estimated to be about 60%–70% of the maximum values, respectively, obtainable in the present composite system, provided that a complete debonding at the platelet/matrix interface can occur.     

Rain erosion behaviour of polymethylmethacrylate

Mass loss, optical transmittance and the microscopic erosion sequence have been monitored for polymethylmethacrylate exposed to a 2.54 cm h^–1 rainfall of 1.8 mm diameter water drops at an impact velocity of 222 m sec^–1. Initial drop impacts produced well-defined fracture patterns consisting of a circular area free of damage surrounded by an annulus containing a dense array of fine, short cracks together with a sparse distribution of deeper fractures initiated along surface scratches. Continued exposure to the rainfield produced crack grown and crack intersections at sites of fracture annuli overlap followed by crevice growth as these fracture systems were enlarged by a hydraulic penetration mechanism. An extensive network of subsurface fractures continued to be produced within the expanding cavities with longer exposures. The transient stress distributions generated during a water drop impact on polymethylmethacrylate are considered in terms of their potential for producing circumferential fractures.     

Silicon carbide fibre reinforced glass-ceramic matrix composites exhibiting high strength and toughness

Silicon carbide fibre reinforced glass-ceramic matrix composites have been investigated as a structural material for use in oxidizing environments to temperatures of 1000° C or greater. In particular, the composite system consisting of SiC yarn reinforced lithium aluminosilicate (LAS) glass-ceramic, containing ZrO₂ as the nucleation catalyst, has been found to be reproducibly fabricated into composites that exhibit exceptional mechanical and thermal properties to temperatures of approximately 1000° C. Bend strengths of over 700 MPa and fracture toughness values of greater than 17 MN m^–3/2 from room temperature to 1000° C have been achieved for unidirectionally reinforced composites of 50 vol% SiC fibre loading. High temperature creep rates of 10^–5 h^–1 at a temperature of 1000° C and stress of 350 MPa have been measured. The exceptional toughness of this ceramic composite material is evident in its impact strength, which, as measured by the notched Charpy method, has been found to be over 50 times greater than hot-pressed Si₃N₄.     

Measurement of the fracture toughness of polycrystalline diamond using the double-torsion test

The double-torsion test was employed to study the processes of crack propagation and to measure the fracture toughness of polycrystalline diamond. The value of fracture toughness of about 13 MPa m^1/2 is surprisingly high. Inhomogeneity in microstructure may cause discontinuous crack propagation which makes it difficult to study the subcritical crack growth behaviour of this polycrystalline material. Subcritical crack growth is shown to be negligible and crack deflection is shown to be an important toughening mechanism in polycrystalline diamond.     

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.     

Fracture mechanics of short glass fibre-reinforced nylon composite

The fracture behaviour of injection-moulded short glass fibre-reinforced, thermoplastic nylon 6.6 plaques has been studied under static loading using compact tension specimens and under impact loading using single-edge notched charpy specimens. The influences of specimen position as taken from the plaque mouldings, notch direction, notch sharpness and the rate of testing on the fracture toughness of this composite system were investigated. Results indicated that the fracture toughness is highest for the cracks perpendicular to the mould fill direction and is lowest for cracks parallel to the mould fill direction. A single fracture parameter, K _c, seems to be inadequate for fracture toughness characterization. Evaluation of the fracture toughness as a function of notch sharpness indicated that for notches perpendicular to the mould fill direction the fracture toughness is not affected by the sharpness of the initial notch. However, for cracks in the mould fill direction, sharpness of the initial notch had a significant effect upon the measured value of the fracture toughness. Results also indicated, that the fracture toughness is rate insensitive over the crosshead speed ranging from 0.5–50 mm min^–1. Finally, the specimen position, as taken from plaque mouldings, had no significant effect on the measured value of the fracture toughness.     

Rate and temperature effects on crack blunting mechanisms in pure and modified epoxies

The failure mechanisms of several epoxy polymers (including pure, rubber- and particulatemodified, as well as rubber/particulate hybrid epoxies) were investigated over a wide range of strain rates (10^–6 to 10² sec^–1) and temperatures (–80 to 60° C). A substantial variation in fracture toughness, G_Ic, with rate was observed at both very high and very low strain rates. Under impact testing conditions, G_Ic for both pure and rubber-modified epoxies displayed peaks at about 23 and –80° C which appeared to correlate with the corresponding size of the crack tip plastic zone. In order to explain these rate and temperature-dependent G_Ic results, two separate crack blunting mechanisms were proposed: thermal blunting due to crack tip adiabatic heating and plastic blunting associated with shear yield/flow processes. Thermal blunting was found to occur in the pure- and rubber-modified epoxies under all impact testing conditions and temperatures above 0° C. For temperatures below –20° C under impact conditions, the fracture toughness is dependent on viscoelastic loss processes and not thermal blunting. Plastic blunting was predominant at very slow strain rates less than 10^–2 sec^–1 for the pure- and rubber-modified epoxies and at impact strain rates for the fibre and hybrid epoxies. Microstructural studies of fracture surfaces provided some essential support for the two proposed crack blunting mechanisms.     

A small-specimen investigation of the fracture toughness of Ti5Si3

The fracture toughness of the refractory hardmetal Ti₅Si₃, with a grain size between 5 and 6 m, was measured using the controlled-flaw method in conjunction with the miniaturized disc-bend test. The specimens used in these experiments were 3 mm diameter and varied in thickness from 150–450 m. They were indented using a Vickers pyramid indentor to indention loads varying from 2.9–79.2 N. Indentation cracking was experienced at all indentation loads, and R-curve behaviour was exhibited. The fracture toughness was determined to be 2.69 ± 0.21 MPam^1/2 using a straightforward graphical procedure involving an empirical R-curve equation. This value is almost 30% higher than that of similar material (2.1 MPam^1/2) with a larger grain size, suggesting that the fracture toughness of this material, which fractures intergranularly, might be grain-size dependent.     

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.     

Fracture toughness of polycrystalline tungsten under mode I and mixed mode I/II loading

The fracture toughness of swaged polycrystalline tungsten was tested parallel and perpendicular to the swaging direction and under mixed mode I/mode II loading. The fracture mode is dominated by the microstructure and changed from all-transgranular cleavage in mode I to almost all-intergranular fracture in mode II. The mixed mode results can be related to two common failure criteria, the maximum tensile stress criterion (Maximum σ) and the maximum energy release rate criterion (Maximum G), but the large scatter in the data prohibits a clear distinction between the two criteria. Tests at 77 K show that the polycrystal is significantly tougher than the single crystal at this temperature. This is a consequence of the deflection of the crack into the grain boundaries and the imperfect texture (as compared to a single crystal) of the polycrystalline material.     

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.     

Creep rupture studies of two alumina-based ceramic fibres

Creep rupture tests were performed in air on two polycrystalline oxide fibres (Al₂O₃, Al₂O₃-ZrO₂) using both filament bundles and single filaments. Tests were performed at applied stresses ranging from 50–150 MPa over the temperature range 1150–1250 °C. Under these conditions, creep rates for the alumina-zirconia fibre ranged from 4.12 × 10^–8–7.70 × 10^–6s^–1. At a given applied stress, at 1200°C, creep rates for the alumina fibre were 2–10 times greater than those of the alumina-zirconia fibre. Stress exponents for both fibres ranged from 1.2–2.8, while the apparent activation energy for creep of bundles of the alumina-zirconia fibre was determined to be 648 ± 100kJmol^–1. For the alumina-zirconia fibre, the two test methods yielded similar steady-state creep rates, but the rupture times were generally found to be longer for bundles than for single filaments. The steady-state creep behaviour of these alumina-based fibres is consistent with an interface-reaction-controlled diffusion-controlling mechanism.     

Viscous creep deformation of polycrystalline CaTiO3 at elevated temperatures

Creep deformation of polycrystalline CaTiO₃ has been investigated in air at temperatures between 1100 and 1200° C and stresses between 4 and 13 MPa. The results indicate that the creep deformation of this material can be described by a threshold process with associated activation energy of 200 kcal mol^–1 (836.8 kJ mol^–1). It is tentatively concluded that creep deformation of polycrystalline CaTiO₃ within the ranges of experimental parameters investigated is rate-controlled by interfacial defect creation and/or annihiliation at grain boundaries.