Showing papers on "Fracture toughness published in 1986"

On the Analysis and Design of the End Notched Flexure (ENF) Specimen for Mode II Testing

Abstract: The end notched flexure (ENF) specimen is examined as a candidate for measuring interlaminar fracture toughness in skew symmetric loading. A simple design study for sizing the ENF specimen to minimize geometric nonlinear response and to avoid nonlinear material behavior or flexural failure is presented. Results indicate that interlaminar shear effects may be significant for tough resin systems requiring large thickness-to-length geometries. The influences of interlaminar shear deformation and friction between the crack surfaces on the strain energy release rate are examined.

On the essential work of ductile fracture in polymers

Abstract: The essential work of fracture concept has been extended to cover ductile tearing of polymeric materials that neck before fracture. It is shown that the plane stress specific essential fracture work (we) can be obtained from deeply edge-notched tension specimens, containing either single or double notches, by extrapolating the straight line relationship between the total specific fracture work (wf) and ligament length (l) to zero ligament. In this way, specific essential fracture works have been obtained for nylon 66 and two polyethylenes. It seems that weis a material property for a given sheet thickness being independent of specimen geometry. The straight line relationship between wfand l breaks down when the ligament length to sheet thickness ratio is less than about three, since the fracture data fall in the plane stress-plane strain transition region. However, a plane strain specific essential fracture work can still be obtained by extrapolating the least squares curve of the data to zero ligament provided the thickness satisfies plane strain condition. If this condition is not satisfied a near plane strain value is obtained which is dependent upon thickness. This method is also appropriate for ductile polymers like the rubber modified polystyrenes that craze rather than neck.

Shear fracture tests of concrete

Abstract: Symmetrically notched beam specimens of concrete and mortar, loaded near the notches by concentrated forces that produce a concentrated shear force zone, are tested to failure. The cracks do not propagate from the notches in the direction normal to the maximum principal stress but in a direction in which shear stresses dominate. Thus, the failure is due essentially to shear fracture (Mode II). The crack propagation direction seems to be governed by maximum energy release rate. Tests of geometrically similar specimens yield maximum loads which agree with the recently established size effect law for blunt fracture, previously verified for tensile fracture (Mode I). This further implies that the energy required for crack growth increases with the crack extension from the notch. The R-curve that describes this increase is determined from the size effect. The size effect also yields the shear fracture energy, which is found to be about 25-times larger than that for Mode I and to agree with the value predicted by the crack band model. The fracture specimen is simple to use but not perfect for shear fracture because the deformation has a symmetric component with a non zero normal stress across the crack plane. Nevertheless, these disturbing effects appear to be unimportant. The results are of interest for certain types of structures subjected to blast, impact, earthquake, and concentrated loads.

A statistical model of brittle fracture by transgranular cleavage

Abstract: A MODEL for brittle fracture by transgranular cleavage cracking is presented based on the application of weakest link statistics to the critical microstructural fracture mechanisms. The model permits prediction of the macroscopic fracture toughness, K,,, in single phase microstructures containing a known distribution of particles, and defines the critical distance from the crack tip at which the initial cracking event is most probable. The model is developed for unstable fracture ahead of a sharp crack considering both linear elastic and nonlinear elastic ~‘elastjc~plastic”) crack tip stress fields. Predictions are evaluated by comparison with experimental results on the low temperature flow and fracture behavior of a low carbon mild steel with a simple ferrite/grain boundary carbide microstructure.

Mechanical properties of toughened ZrO2-Y2O3 Ceramics

Abstract: Tetragonal zirconia polycrystals were produced from high purity powders containing 1.5 to 5.0 mol% Y/sub 2/O/sub 3/ by cold isostatic pressing and sintering, hot-pressing, and hot isostatic pressing. The mechanical properties and microstructures of these resulting materials were examined, with emphasis on the relation between strength and fracture toughness.

Strain‐energy effects on dynamic fragmentation

Abstract: Grady’s model of the dynamic fragmentation process, in which the average fragment size is determined by balancing the local kinetic energy and the surface energy, is modified to include the stored elastic (strain) energy. The revised model predicts that the strain energy should dominate for brittle materials, with low fracture toughness and high fracture‐initiation stress. This conclusion is not borne out, however, by limited experimental data on brittle steels, even when the kinetic‐energy density is small compared with the strain‐energy density.

Determination of specimen-size independent fracture toughness of plain concrete

Abstract: Synopsis Simple analytical expressions are proposed for determining the critical stress intensity factor Kc, and the critical energy release rate Gc, of plain concretes in three-point bending. These expressions take into account the slow crack growth preceding fracture and the complex state of stress existing at a propagating crack front. The fracture toughness so determined is shown to be essentially independent of the test specimen dimensions and to depend only on the mix variables. It is also argued that unlike Kc, or Gc, the fracture energy GF of plain concretes is strongly dependent on the test specimen dimensions. It is therefore advisable to express the fracture toughness of plain concretes through the specimen-size independent parameters Kc, or Gc.

Fracture Toughness of Human Dentin

Abstract: Plane strain fracture toughness (KIC) was determined for coronal dentin using compact tension test-pieces obtained from recently extracted permanent lower molar teeth. Specimens were prepared and tested such that the moving crack front was aligned parallel to the tubule orientation. The fracture toughness (K IC) was temperature-invariant in the range 0-60°C, and the mean value obtained was 3.08 MN.(m)-1.5 (SD: 0.33). The critical strain energy release rate (GIC) of dentin was also calculated utilizing modulus data and was found to increase slightly with temperature, possibly as a consequence of significant temperature-dependence of the modulus:The fracture toughness of dentin is midway in the range (0.23- 6.56) observed for cortical bone and is a factor of two greater than that exhibited by most current restorative materials.

Fracture toughness and fracture energy of concrete

Abstract: Keywords: Beton ; Fracture ; Fissure Reference Record created on 2004-09-07, modified on 2016-08-08

Mechanical properties and thermal stability of CeO/sub 2/ containing tetragonal zirconia polycrystals

Abstract: The strength and fracture toughness of cerium oxide containing tetragonal zirconia polycrystals (Ce-TZP) were studied. The thermal stability of Ce-TZP was also studied by low temperature thermal cycling and aging tests in air and in hot aqueous solution. Ce-TZP (12 mol% cerium oxide) showed very high fracture toughness and inelastic fracture behavior and was resistant to phase transformation during low temperature aging.

Silicon carbide whisker reinforced ceramic composites and method for making same

Abstract: The present invention is directed to the fabrication of ceramic composites which possess improved mechanical properties especially increased fracture toughness. In the formation of these ceramic composites, the single crystal SiC whiskers are mixed with fine ceramic powders of a ceramic material such as Al2 O3, mullite, or B4 C. The mixtures which contain a homogeneous dispersion of the SiC whiskers are hot pressed at pressures in a range of about 28 to 70 MPa and temperatures in the range of about 1600° to 1950° C. with pressing times varying from about 0.75 to 2.5 hours. The resulting ceramic composites show an increase in fracture toughness of up to about 9 MPa.m1/2 which represents as much as a two-fold increase over that of the matrix material.

Fatigue crack growth and fracture toughness behavior of an Al-Li-Cu alloy

Abstract: Slip behavior, fracture toughness, and fatigue thresholds of a high purity Al-Li-Cu alloy with Zr as a dispersoid forming element have been studied as a function of aging time. The fracture toughness variation with aging time has been related to the changes in slip planarity,i.e., slip band spacing and width. Although the current alloy exhibits planar slip for all aging conditions examined, the crack initiation toughness,Klc, compares favorably with those of 2XXX and 7XXX aluminum alloys. Near threshold fatigue crack growth results in air and vacuum suggest that irregularities in the crack profile and the fracture surfaces and slip reversibility are some of the major contributing factors to the crack growth resistance of this alloy.

Mode II interlaminar fracture of graphite/epoxy and graphite/PEEK

Abstract: The end notched flexure (ENF) specimen is employed in an investigation of the interlaminar fracture toughness in Mode II (skew symmetric shear) loading of unidirectional graphite/epoxy and graphite/PEEK composites. Important experimental parameters such as the influence of precracking and the data reduction scheme for the Mode II toughness are discussed. Nonlinear load-deflection response is significant for the tough thermoplastic resin composite but is also present for the brittle thermoset composite. The observed nonlinearities, which are highly rate dependent, are attributed to a combination of slow stable crack growth preceding unstable crack growth and material inelastic behavior in the process zone around the crack tip.

Shear Stress Intensity Factors for a Planar Crack With Slightly Curved Front

Abstract: Recent work (Rice, 1985a) has presented the calculations of the first order variation in an elastic displacement field associated with arbitrary incremental planar advance of the location of the front of a half-plane crack in a loaded elastic full space. That work also indicated the relation of such calculations to a three-dimensional weight function theory for crack analysis and derived an expression for the distribution of the tensile mode stress intensity factor along a slightly curved crack front, to first order accuracy in the deviation of the crack front location from a reference straight line. Here we extend the results on stress intensity factors to the shear modes, solving to similar first order accuracy for the in-plane [Mode 2) and antiplane (Mode 3) shear stress intensity factors along a slightly curved crack front. Implications of results for the configurational stability of a straight crack front are discussed. It is also shown that the concept of line tension, while qualitatively useful in characteriz­ ing the crack extension force (energy release rate) distribution exerted on a tough heterogeneity along a fracture path as the crack front begins to curve around it, does not agree with the exact first order effect that is derived here.

Crack Stability and Toughness Characteristics in Brittle Materials

Abstract: Modern-day theories of the strength of brittle materials stem from the Griffith energy balance description of fracture processes (1). Griffith regarded a cracked body as a thermodynamic system: a crack exists in a state of equilibrium if, for a virtual incremental extension, the release of mechanical energy in the system balances the work to create the new surface area. In current "fracture mechanics" notation this critical state may be expressed as Ga = Ge or Ka = Ke, where the crack extension force Ga and stress intensity factor Ka identify with the mechanical energy term, and Gc and Kc similarly identify with the surface work term (2). From a practical standpoint this simple "crack law" has a broad appeal. By mea­ suring the applied loads needed to cause the extension of well-defined cracks we can determine intrinsic material "resistance" or "toughness" parameters, which can then be used in design against brittle failure. However, simple as it seems, the crack law embodied in the relations G a = G c and Ka = Kc is the source of much misunderstanding and improper application in the fracture testing community, particularly by those who

Mechanical property changes in sapphire by nickel ion implantation and their dependence on implantation temperature

Abstract: Sapphire plates, cut parallel to an {0001} plane, have been implanted with 300 keV nickel ions to doses ranging from 5×1012 to 1×1017 Ni cm−2 at specimen temperatures of 100, 300 and 523 K, in order to investigate the effect of implantation temperature on the mechanical property changes in sapphire caused by ion implantation. The measured changes in surface hardness, surface fracture toughness and bulk flexural strength were found to depend strongly on the implantation temperature, and were largely correlated with the residual surface compressive stress measured by using a cantilever beam technique. The surface amorphization that occurred only by the implantation at 100 K and at doses larger than ∼2×10s15 Ni cm−2 reduced the hardness to ∼0.6 relative to the value of the unimplanted sapphire, and considerably increased the surface plasticity. Furthermore, the amorphization was found to involve a large volume expansion of ∼30% and to change drastically the apparent shape and size of a Knoop indentation flaw made prior to implantation. This effect was suggested to reduce stress concentrations at surface flaws and hence to increase the flexural strength.

Influence of toughness on Weibull modulus of ceramic bending strength

Abstract: It is demonstrated both theoretically and experimentally that fracture toughness does not directly influence the Weibull modulus of ceramic bending strength for materials that obey the Griffith criterion for crack propagation. Weibull modulus remains unchanged as toughness is increased. However, toughness variations with crack length do affect the Weibull modulus. Thus materials that display R-curve behavior or Dugdale character give an increased Weibull modulus and appear more reliable.

Effect of material rate sensitivity on failure modes in the Charpy V-notch test

Abstract: For the C harpy V-notch test the influence of strain rate on competing failure mechanisms is analyzed numerically. The nucleation and growth of micro-voids is represented in terms of an elastic-viscoplastic constitutive model, which describes the mechanism of ductile fracture by void coalescence. Failure by cleavage is assumed to occur if the maximum principal tensile stress exceeds a certain critical value. Attention is focused on the temperature regime where the transition in fracture mode between cleavage and ductile rupture takes place. In the analyses the temperature is taken as constant and the effect of inertia is neglected, so that time dependence enters only through the material strain rate sensitivity. The material model is found to reproduce the experimentally observed change in failure mode from predominantly ductile fracture at low strain rates, to cleavage fracture at high strain rates. The numerical results show that in the transition regime, the porosity in the notch tip region plays a role in the fracture process even when failure occurs by cleavage. Once the transition of failure mode from cleavage to ductile rupture has occurred, the energy absorbed at low rates is greater than that absorbed at high rates.

Fracture toughness design in horse hoof keratin.

Abstract: An engineering fracture mechanics approach was applied to the analysis of the fracture resistance of equine hoof-wall. The relationship between fracture toughness and the morphological organization of the keratin hoof tissue was investigated. Fracture toughness was evaluated using the J-integral analysis method which employs the compact tension test geometry. Tensile tests were also conducted to evaluate the effect of the morphological organization on the stress-strain behaviour. Hoof-wall has greatest fracture resistance for cracks running proximally, parallel to the tubular component of the wall keratin. For fully hydrated material tested in this direction the mean critical J-integral value at failure was 1.19 X 10(4)J m-2. This was nearly three times greater than the value determined for the weakest orientation, in which the crack ran parallel to the material between the tubules. The lower fracture toughness of the intertubular material dominates the fracture behaviour of this tissue. The tubular components of the wall appear to reinforce against fracture along the weak plane and the entire wall organization provides the mechanical capability for limiting and controlling fracture in this tissue.

The effect of centrifugation on the fracture properties of acrylic bone cements.

Abstract: In this study, centrifugation did not alter the static or cyclic fracture properties of bone cement. Tests of fracture toughness and fatigue-crack propagation of centrifuged specimens of commercial cements (with and without antibiotic additions) demonstrated no significant difference from control values. Among the cements tested, Palacos (with and without antibiotic) was found to have a significantly higher fracture toughness than either Simplex or Zimmer. We attributed this difference in fracture toughness to the higher molecular weight measured for the Palacos cements. For the tested cements, only Simplex had a significantly greater volume contraction on setting due to centrifugation. The results of our study demonstrate that centrifugation of bone cement does not improve the cement's resistance to fracture in the presence of surface imperfections, such as those found at the bone-cement interface.