Showing papers on "Fracture toughness published in 1968"
2,452 citations
01 Jan 1968
TL;DR: In this article, the authors studied the strain energy for two and three dimensional crack systems subjected to varying loads, detailing loading and crack geometry effects on fracture criterion, and showed that crack geometry and crack loading effects affect fracture performance.
Abstract: Strain energy for two and three dimensional crack systems subjected to varying loads, detailing loading and crack geometry effects on fracture criterion
531 citations
TL;DR: In this article, the leading edge of a crack is treated as a line disturbance zone in a manner similar to the treatment of dislocation lines in dislocation mechanics, and the authors discuss interpretation of the brittle-ductile transition in terms of relative plastic zone size, and allowable load estimates which include fracture strength in a rational way.
276 citations
TL;DR: In this article, the effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending.
Abstract: The effective surface energy of four brittle materials, alumina, poly(methylmethacrylate), glass, and graphite, is calculated from load/deflection curves of notched bars deformed in three-point bending. Two of the methods, which are commonly used in fracture mechanics studies,viz the modified Griffith treatment and the compliance analysis method, are concerned with the effective surface energy at the initiation of fracture,γ
I
. The third method, the work of fracture test, is concerned with the mean effective surface energy over the whole fracture process,γ
F
. The two estimates ofγ
I
give consistent values, and there is no systematic variation ofγ
I
with notch depth. Values ofγ
F
decrease with increasing notch depth as the fracture process becomes more controlled. For aluminaγ
I
∼γ
F
. For PMMA and glassγ
I
> γγ
F
because of a multiplicity of crack sources during fracture initiation. For graphiteγ
I
<γ
F
because of subsidiary cracking as fracture proceeds.
224 citations
TL;DR: In this paper, the authors present results of acoustic emission studies on flawed and unflawed specimens of aluminum and beryllium and show that acoustic emission from the flawed specimens is found to begin at stress levels far below the general yield stress.
189 citations
TL;DR: In this paper, a theoretical model for predicting elastic-plastic, plane strain stress distributions in notched bars under bend loading has been developed by combining the elastic solutions of Neuber with the slipline field solutions of Hill.
182 citations
01 Jan 1968
102 citations
TL;DR: A comprehensive review of the development and the current state-of-the-art for the WOL (wedge opening loading) specimen for plane strain fracture toughness testing is described in this paper.
101 citations
TL;DR: The functional forms of local fracture criteria are discussed for different metallurgical mechanisms as discussed by the authors, and it is concluded that Mode I (normal) fracture is unlikely to exist except in cleavage, and there is both theoretical and experimental evidence for the development of both sharp and flat-bottomed cracks.
Abstract: Strain distributions in specimens suitable for studying the initiation of fracture are reviewed, and distributions are developed for the steady-state propagation of cracks in plane strain lension of fully plastic materials. The functional forms of local fracture criteria are discussed for different metallurgical mechanisms. It is concluded that:
a)
pure Mode I (normal) fracture is unlikely to exist except in cleavage.
b)
there is both theoretical and experimental evidence for the development of both: sharp and flat-bottomed cracks.
c)
simultaneous diffuse and concentrated (Dugdale-Muskhelishvili) flow fields can occur in torsion of longitudinally grooved bars if the stress-strain curve has a maximum which causes band formation, so that a displacement criterion becomes appropriate for final fracture.
95 citations
TL;DR: In this paper, the authors discussed the fracture of thermally tempered glass in terms of both the stored elastic strain energy in the glass due to tempering and the elastic energy release rate Of crack extension, 9.
Abstract: The fracture of thermally tempered glass is discussed in terms Of both the stored elastic strain energy in the glass due to tempering and the elastic energy release rate Of crack extension, 9. The latter is used to obtain an analytical correlation between the maximum tensile stress and the average particle size at time of fracture. The theoretical predictions are supported by experimental data obtained for various glass thicknesses and temper levels.
91 citations
TL;DR: In this article, the interrelationships among fine-scale microstructure, strength, and toughness have been investigated, including embrittlement phenomena, the effects of residual or impurity elements, and the micromechanisms of crack propagation.
31 Dec 1968
TL;DR: In this article, the effect of mean stress on fatigue crack propagation in 1 2 in. (12.7 mm) thick specimens of six aluminium alloys of high and low fracture toughness was investigated in terms of the stress intensity factor range, ΔK and the R value.
Abstract: Abstract Experiments have been made to investigate the effect of mean stress on fatigue crack propagation in 1 2 in. (12.7 mm) thick specimens of six aluminium alloys of high and low fracture toughness. The results are presented in terms of the stress intensity factor range, ΔK and the R value where R is the ratio of minimum to maximum stress. It was found that increasing R had little effect on the fatigue crack propagation rate for the alloy of very high fracture toughness ( L 64) but that for the other alloys the rate increased rapidly with increasing R at high values of ΔK . A simple law is proposed relating the fatigue crack propagation rate to ΔK , the R value and the fracture toughness of the material.
01 Apr 1968
TL;DR: In this article, the authors present calculations of the temperature elevations accompanying rapid plastic deformation near a crack tip, and show that the localized tip temperature elevation alone alone governs fracture toughness at very fast rates.
Abstract: : This paper presents calculations of the temperature elevations accompanying rapid plastic deformation near a crack tip. Solutions for the stress and strain distribution in non-hardening materials are employed as a basis for the heating rate distribution. Results are approximate in that temperature independent mechanical and thermal properties are assumed and thermal stressing is neglected. Two cases are considered: a stationary crack under increasing load, and a running crack with locally constant speed and plastic zone size. Numerical results are presented as based on properties of 2024 aluminum alloy, 6Al-4V titanium alloy, and mild steel. Temperature rises predicted for test conditions on these metals seldom exceed 100C. This may, nevertheless, be large enough to influence fracture and to account for the observed rise in roughness at very fast rates. Consequences are examined for the assumption that the localized tip temperature elevation alone governs fracture toughness at very fast rates. (Author)
TL;DR: In this article, the phenomological fracture properties of glass bead filled polyphenylene oxide composites were investigated and yield strength and Young's modulus were obtained as a function of volume fraction of filler using standard microtensile testing techniques.
Abstract: The phenomological fracture properties of glass bead filled polyphenylene oxide composites were investigated.Yield strength and Young's modulus were obtained as a function of volume fraction of filler using standard microtensile testing techniques. Fracture toughness was measured using double edge notched tensile bars. The effects of adhesion were studied by using untreated and A-1100 silane-treated glass beads. The nature of the fracture surfaces was observed by use of an optical microscope, a scanning electron microscope and a transmission electron microscope.It was found that the fracture toughness of these materials de creased by increasing the filler content and by improving the adhesion. In general, an increase in strength and stiffness was accompanied by a decrease in toughness.The fractographic studies gave a detailed mapping of the frac ture front as it propagated through the material. Fracture occurred in two stages. The initial stage was a region of stable crack growth accompanied by, crazing, ...
TL;DR: Elastic displacements for edge cracked plate specimens used as crack extension indicator in plane strain fracture toughness measurements were used as a crack extension measure in this paper, where they measured the elastic displacements of the cracks.
Abstract: Elastic displacements for edge cracked plate specimens used as crack extension indicator in plane strain fracture toughness measurements
TL;DR: A quantitative assessment of the density of hyperbolic markings in the fracture surfaces of polymethyl methacrylate has been made showing that the density is proportional to the fracture toughness as discussed by the authors.
Abstract: A quantitative assessment of the density of hyperbolic markings in the fracture surfaces of polymethyl methacrylate has been made showing that the density is proportional to the fracture toughness. In general the fracture toughness at any instance is a random function, but there is a definite tendency for it to increase with velocity of fracture propagation. It is suggested that the craze material ahead of the true crack tip may be assumed to have constant strength and Dugdale's model for elasto—plastic materials applied to organic glasses as well.
TL;DR: In this article, the fracture toughness of a variety of sharply notched tension, bending and rotating disc specimens of PMMA was examined using linear fracture mechanics, and it was observed that rapid fracture with a brittle glassy appearance usually follows a period of slow crack growth, denoted by fan shaped markings of local ductility.
Abstract: The fracture toughness of a variety of sharply notched tension, bending and rotating disc specimens of PMMA is examined using linear fracture mechanics. It is observed that rapid fracture with a brittle glassy appearance usually follows a period of slow crack growth, denoted by fan shaped markings of local ductility, though still brittle overall. In this near brittle regime the fracture toughness is sensitive to strain rate so that high values of effective surface energy are easily induced by rapid testing or notch bluntness. At impact rates the toughness increases again. For design purposes, in the absence of environmental effects, the onset of slow cracking and rapid (glassy) fracture, can be associated with fracture toughness K1c of about 800 Ibf/in3/2 (90 kg/cm3/2) and 1600 Ibf/in3/2 (180 kg/cm3/2) respectively. Detailed studies have not been made on other materials but a guide to the levels of notch toughness and notch brittle temperatures are given for several plastics.
TL;DR: In this paper, the fracture energy of float and Pyrex glass plates has been determined in various media using a simple double-cantilever cleavage technique, and the results are discussed in terms of Marsh's hypothesis that localized plastic flow occurs at the crack tip.
Abstract: The fracture energy of Float and Pyrex glass plates has been determined in various media using a simple double-cantilever cleavage technique. Although slow growth of the crack at constant displacement of the cantilever arms was observed in all environments, consistent values for the energy expended when the crack propagates rapidly were obtained. This ‘instantaneous fracture energy’ranged from ∼5000 erg/cm2 (Float glass) or 6000 erg/cm2 (Pyrex) in vacuum to 2500 erg/cm2 (for both glasses) in water. The results are discussed in terms of Marsh's hypothesis that localized plastic flow occurs at the crack tip.
TL;DR: In this article, it is shown that for macro-cracks when the crack radius l is not too small, the following formula holds for quasi-brittle solids, which agrees with the Orowan-Irwin modification of Griffith's theory; (dWp/dA)crit denotes the plastic work per unit area of new surface, dissipated in the course of loading before fracture.
Abstract: A penny-shaped crack in a material which is ideally elastic-plastic has been envisaged with the assumption that the plastic zone forms a very thin layer surrounding the crack. The Dugdale hypothesis has been adapted and thus the problem has been reduced to that of an elastic semi-space with properly modified boundary conditions. The entire energy absorbed in the process of creation of a new surface is associated with the work expanded in irreversible plastic deformation, the work of cohesive forces being neglected. The displacements of the crack surfaces are calculated as well as the plastic energy dissipation and the fracture criterion is discussed. The shape of the crack, obtained here, differs considerably from that predicted by the theory of elasticity, particularly at the crack tip. The differences in the values of the critical pressure calculated from the Griffith-Sack-Sneddon formula and those obtained by use of the equations derived here are also significant. It is shown that for macro-cracks when crack radius l is not too small the following formula holds
$$P_{crit} = [\pi E\left( {dW_p /dA} \right)_{crit} /2\left( {1 - v^2 } \right)]^{{\raise0.5ex\hbox{$\scriptstyle 1$}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{$\scriptstyle 2$}}} $$
which agrees with the Orowan-Irwin modification of Griffith's theory; (dWp/dA)crit denotes the plastic work per unit area of new surface, dissipated in the course of loading before fracture. The results of this paper hold for the so called ‘quasi-brittle’ solids. Two schemes of loading are considered: 1. pressure applied on the crack surfaces and 2. applied at infinity. Attention is paid to a slightly different mechanism of fracture in both the cases.
01 Jan 1968
TL;DR: In this paper, the authors extended the Dugdale model to include the influence of the stress-strain curve on the plastic-zone size and subsequently on the fracture strength of the plate.
TL;DR: In this paper, a brief and cursory discussion attempts to point out the more important microstructural factors, which may be expected to affect the toughness of high strength alloys, in which fracture occurs by void nucleation and coalescence.
TL;DR: In this article, the effects of thickness and testing direction upon the plane strain and plane stress fracture toughness of 7075-T6 and -T651 aluminum alloy were examined, both by static center-notched tests and precracked Charpy impact tests.
Book•
01 Jan 1968
TL;DR: In this paper, a model for the effects of mild anisotropy on the relationship between the stress components and the plastic strain increments suggests that the Levy-Mises equations will give serious errors in estimates of the surface stress components in compression tests.
TL;DR: In this article, the phenomenon of crack branching in metals is discussed in terms of the elastic stress intensity at the crack tip when crack branching occurs, and it is shown that KB measurements may be useful in determining the fracture toughness of brittle metals.
Abstract: The phenomenon of crack branching in metals is discussed in terms of KB, the elastic stress intensity at the crack tip when crack branching occurs. It is shown that KB measurements may be useful in determining the fracture toughness of brittle metals.
TL;DR: In this paper, a study of the brittle fracture of iridium was made using the scanning electron microscope, and it was shown that cracks can only be nucleated after extensive work-hardening.
Abstract: A study of the brittle fracture of iridium is made using the scanning electron microscope. Except where macroscopic imperfections are present, cracks can only be nucleated after extensive work‐hardening. Plastic relaxation of the crack is inhibited because impurities substantially increase the critical resolved shear stress. Brittle fracture in iridium is thus basically an impurity effect.
TL;DR: In this paper, the authors review progress in the elastic plastic analysis of crack extension and discuss the elastic-plastic fracture instability as well as transitional behavior and combined rate and thermal effects.
Abstract: This paper briefly reviews progress in the elastic plastic analysis of crack extension. Analytical results for plane strain and plane stress deformation fields are noted, and elastic-plastic fracture instability as well as transitional behavior and combined rate and thermal effects are discussed.
TL;DR: In this article, a theoretical estimate of the effect of pin friction on values of the stress intensity factor for the ASTM type single edge notch fracture toughness test specimen was made, and it was concluded that precautions must be taken to keep friction to a minimum when measuring fracture toughness using this type of specimen.
Abstract: A theoretical estimate is made of the effect of pin friction on values of the stress intensity factor for the ASTM type single edge notch fracture toughness test specimen. It is shown that for a coefficient of friction of 0.2 between pins and specimen the stress intensity factor is reduced by approximately 10 per cent. It is therefore concluded that precautions must be taken to keep friction to a minimum when measuring fracture toughness using this type of specimen.