Showing papers on "Fracture toughness published in 1978"

Combined Mode Fracture Toughness Measurement by the Disk Test

Abstract: The disk test in which a circular specimen with an internal crack is subjected to diametral compression is used to investigate combined mode I and mode II fracture. The stress intensity factors in the disk test are calculated numerically by means of the boundary collocation procedure and the dislocation method. Special care was taken to analyze the effect of the compression anvils. This method has the advantage, of allowing successive measurement of mode I, mode II and the combined mode fracture toughness under the same conditions. Some kinds of graphite, plaster and marble are examined to obtain the fracture toughness values, KIC , KIIC and the combined mode fracture criterion.

Effects of microstructure on cleavage fracture stress in steel

Abstract: It is shown, by compiling data from the literature, that there is a general relationship between the ferrite grain size and the size of the largest carbide particle in mild steels which are simply cooled after austenitization. By using this relationship, a cleavage fracture criterion derived by Smith is shown to predict a grain size dependence for the cleavage fracture stress of mild steel that is in good agreement with the results of many workers. These results indicate a value of 14 J m−2 for the effective surface energy of ferrite.Experimental results are presented showing the variation of the cleavage fracture stress of spheroidized steels with carbide particle radius. These support the suggestion that cleavage in such steels is due to the propagation of penny-shaped crack nuclei prod uced when spheroidal carbide particles crack. If the 95th percentile carbide radius is taken to represent the crack nucleus radius, an effective surface energy value of 14 Jm−2 is found to satisfy the fracture st...

The influence of compositional and microstructural variations on the mechanism of static fracture in aluminum alloys

Abstract: The object of the paper is to examine the effects of alloy purity and state of aging on the fracture mechanism and resultant toughness of pure Al-Cu alloys, and commercial duralumin. In pure alloys, the transition from a shear to an intergranular mode of fracture with overaging is associated with changes in the nature and size of the matrix precipitate, which affect the slip character. In the corresponding commercial purity alloys, no such fracture mode transition occurs. The presence of second-phase dispersoids inhibits planar slip, and in the overaged state inclusion-matrix interfaces present a suitable alternative site to the grain boundaries for strain accumulation, resulting in debonding leading to the initiation of voids, which subsequently grow and coalesce. The fracture toughness, as conventionally measured, indicates the material’s resistance to crack initiation rather than propagation and is effectively independent of fracture mode. The work hardening capacity has a marked effect on void size, and is shown to be a sensitive indicator of fracture toughness in both pure and commercial alloys. Based on a simple model, good agreement is obtained between experimental results of toughness and those predicted from a knowledge of the tensile properties.

Further considerations on the inconsistency in toughness evaluation of AISI 4340 steel austenitized at increasing temperatures

Abstract: A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI 4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested in previous work, a systematic trend ofincreasing plane strain fracture toughness(K)Ic anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected. This effect is seen under both static and dynamic (impact) loading conditions, and is rationalized in terms of a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence) mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture, based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures, consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness, on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance of this work on commonly-used Charpy/KIc empirical correlations is briefly discussed.

Brittle fracture in a ductile material with application to hydrogen embrittlement

Abstract: A physical model of fracture in materials is developed which features a brittle crack imbedded in a plastically deformed medium. This model is presented as an alternative to fully ductile failure by hole growth, and general criteria for the two alternatives are discussed. One of these criteria for the existence of an atomically sharp crack is that the dislocation content near the crack tip be limited by the inhomogeneous character of dislocation slip in the crystal. With the dislocation distribution characteristic of Mode III fracture, we derive expressions for the fracture toughness as a function of material parameters. We have extended the theory to the case of hydrogen embrittlement in steels and compare our theoretical predictions with experimental work by others.

A Plastic-Strain, Mean-Stress Criterion for Ductile Fracture

Abstract: We describe a computer model for predicting ductile-fracture initiation and propagation. The model is based on plastic strain. Fracture starts or a crack extends when the integrated product of the equivalent plastic-strain increment and a function of the mean stress exceeds a critical value over a critical length. This critical length is characteristic of the microstructure of the material. The computer fracture model is calibrated by computer simulation of simple and notched round-bar tension tests and a precracked compact tension test. The model is then used to predict fracture initiation and propagation in the standard Charpy V-notch specimen. The computed results are compared with experiments. The model predicts fracture toughness from tests of standard surveillance specimens from nuclear-reactor pressure vessels and can be applied to fracture calculations for these vessels.

An analysis of the crack tip stress field in DCB adhesive fracture specimens

Abstract: The problem of a cracked adhesive bonded DCB-type fracture specimen has been analyzed using a hybrid stress model finite element analysis which incorporates an advanced crack tip element. Stresses in the near and far fields have been studied as a function of adherend/adhesive modulus ratio and adhesive thickness. The results are compared to monolithic systems with regard to the stress intensity factor and the localization of the singular stress domain associated with the crack tip.

Characterization of self-curing acrylic bone cements.

Abstract: Four self-curing acrylic bone cements were surveyed by infrared, solubility, viscometry, quantitative metallography, microscopy, and physical testing techniques: CMW, Palacos R, Sulfix-6, and Surgical Simplex P. Results show that these bone cements were primarily composed of poly(methyl methacrylate) and that no cross-linking was evident. Solubility analysis confirmed this latter observation, as the bone cements dissolved completely except for a small insoluble fraction, which was identified as the radiopaque filler. For each bone cement, the viscosity-average molecular weights of both the powdered phase and the cured two-phase product remained unchanged, varying overall from 1 to 5 × 105. Using standard quantitative metallography, porosity ranged from 1 to 8% and the dispersed powder phase decreased 11–46%. Microscopy revealed the nature of the porosity, radiopaque fillers, the powder size and shape, and the fracture morphology. From tensile and fracture toughness tests, five physical properties were determined at ambient conditions and at 37°C after conditioning in distilled water at 37°C for 10 months: the modulus of elasticity, the ultimate tensile strength, the elongation at break, the fracture energy, and the mean inherent flaw size. At ambient conditions, the ultimate tensile strength decreased 33–55% when compared with commercial unmodified poly(methyl methacrylate), Plexiglas G. While the fracture energy remained rather invariant, the mean inherent flaw size increased fivefold over the commercial acrylic tested. This marked increase in the mean inherent flaw size could lower the fatigue resistance of a material, since more and/or larger fracture initiation sites are available. When tested at 37°C after protracted conditioning, the deleterious trends observed at ambient temperature continued. To some degree, porosity, particle-matrix interfaces, residual stresses, low molecular weight products, inorganic and/or other organic additions, and water contributed to the inherent flaw size at the expense of the working stress. Several modifications are suggested by which the importance of these factors might be minimized.

Computer-Controlled Decreasing Stress Intensity Technique for Low Rate Fatigue Crack Growth Testing

Abstract: An automated test system utilizing computer control was developed to obtain crack growth rate data down to the fatigue crack growth threshold with a decreasing stress intensity technique and compact type specimens. The starting stress intensity range ΔK0 was chosen to yield crack growth rates in the range of 2.54 × 10−8 m/cycle (10−6 in./cycle) and subsequent values of ΔK are controlled to the equation ΔK = ΔK0 exp [C(a − a0)] (a0 and a are the initial and instantaneous crack lengths and C is a test variable). Crack length is continuously monitored by using the elastic compliance technique, thereby enabling ΔK to be decreased continuously. Comparison crack growth data were also obtained by the more conventional constant load amplitude or K-increasing method. Excellent agreement was observed between data obtained from the two procedures for a Society of Automotive Engineers 1045 steel at load ratios R of 0.1 and 0.5, an A356-T6 sand-cast aluminum alloy at load ratios of 0.1 and 0.8, and a 2219-T851 aluminum alloy at a load ratio of 0.1.

Dynamic fracture toughness of Homalite-100: Dynamic fracture toughness of Homalite-100 determined by T. Kobayashi and Dally, as well as those of Araldite B by Kalthoff, Beinert and Winkler, are compared with those of the authors' previous results. Errors generated through the use of static near-field crack-tip stresses for evaluating dynamic stress-intensity factor are also discussed

Abstract: Dynamic fracture toughness of Homalite-100 determined by T. Kobayashi and Dally are compared with those previously obtained by the authors where similarities in the two results for single-edged-notch specimens of various configurations are noted. Dynamic fracture toughness of Araldite B obtained by Kalthoff, Beinert and Winkler and those of Homalite-100 obtained by the authors are then compared and, again, similarities in the two results and, in particular, the scatters in experimental data for wedge-loaded DCB specimens of different sizes are found. All three teams of investigators used static near-field solution to compute the dynamic stress-intensity factors from recored dynamic isochromatics or dynamic caustics. Errors generated through this use of static near-field solutions, as well as through the use of larger isochromatic lobes, are thus discussed.

Tensile, impact and fatigue behavior of an amine‐cured epoxy resin

Abstract: Although crosslinked networks are commonly used as adhesives and composite matrixes, structure-property relationships are not as well established as with thermoplastics. For this reason, an extensive study was begun to systematically examine effects of stoichiometry, morphology, and distribution of crosslink density on viscoelastic behavior and ultimate properties. The system selected was based on a bisphenol-A-type epoxy cured with methylene dianiline. This paper describes and discusses results obtained for resins in which the amine/epoxy ratio ranged between 0.7 and 2.2. In agreement with reports by others, the tensile strength, modulus, and ultimate elongation were relatively insensitive to stoichiometry but did not show slight maxima or minima when the amine was somewhat in excess. Impact strengths, tensile energies-to-break and fracture toughness were, in contrast, quite sensitive, though the patterns of each differed significantly. Both fracture toughness and the stress intensity factor required to drive the crack at a given rate varied directly with the amine/epoxy ratio, as did estimates of the characteristic flaw size. Fatigue striations were observed on the fracture surfaces and corresponded to the incremental advance of the crack in one loading cycle.

Effect of the Phase Transformation on the Fracture Behavior of BaTiO3

Abstract: The fracture energy of BaTiO3 increased with increasing grain size at 25°C in the ferroelectric 4 mm symmetry state and remained constant at 150°C in the paraelectric m 3m state. In general, observed flaw sizes agreed with those predicted from fracture mechanics equations. The fracture energy measurements combined with previously reported strength measurements were analyzed in terms of fracture mechanics principles to determine the effect of internal stresses on fracture. The analysis showed that the effect of internal stress on fracture for a given flaw size can be determined from a combination of fracture energy and strength measurements at two temperatures. The increase in fracture energy with grain size in the ferroelectric state is attributed to ferroelastic twin formation and wall motion.

Fracture energy of epoxy resin under plane strain conditions

Abstract: The fracture behaviour of an epoxy resin has been studied by a method which involves the pressurization of an internal circular crack. The method can be used to study both cohesive fracture and the adhesive failure of an interface. Plane strain conditions are assured because the crack does not intersect a free surface and (for adhesive failure) shrinkage stresses are eliminated as a crack driving force. Using high speed photography, the dependence of crack speed on critical pressure and specimen geometry was determined. An elastic analysis permits the derivation of fracture energy as a function of crack velocity. Fracture energy values lay between 100 and 200 Jm−3 at 35° C with a peak at a crack velocity of 37 m sec−1.

Creep behaviour of components containing cracks—a critical review:

Abstract: The prediction of crack propagation rates at elevated temperatures is important and this paper provides a critical review of available information and models for behaviour. For simplicity, behaviour is divided into three situations. At one extreme, a brittle situation may exist in which the material is brittle and the degree of constraint high, so that substantially plane strain conditions exist and stress redistribution at the crack tip is small; in this situation, the fracture is a local crack tip event and the stress intensity may be of use in correlaiting creep crack propagation data. At the other extreme, ductile behaviour may result if the material is ductile and the constraint is low with plane stress conditions prevailing; in this case, stresses at and near the crack tip will redistribute quickly down to more even values and conventional creep analysis techniques using, say, the reference stress will be most useful, particularly for estimating times to rupture. It is postulated that there ...

Fatigue crack closure over the range of stress ratios from −1 to 0.8 down to stress intensity threshold level in HT80 steel and SUS304 stainless steel

Abstract: The closure phenomena of fatigue cracks were investigated with a 1 mm gage length extensometer over the range of stress ratio, R, from −1 to 0.8. Plate specimens with a center slot of HT80 steel and SUS304 stainless steel were fatigued under push-pull loading, and the crack propagation rate, da/dn, was measured. The stress ratio, R, was found to influence da/dn in both materials. The crack opening stress intensity factor, K op, was determined from the relationship between the crack tip extensometer displacement and the load. The effective stress intensity range ratio, U(=ΔKeff/ΔK), decreases with the decrease of the stress intensity amplitude, ΔK/2. As for the data which show the crack closure phenomena (R≦0.4), the relationship between log(da/dn) and log(ΔK eff/2) falls on a straight line near the stress intensity threshold level. For R=0.8 where the crack tip is fully open over the whole range of loading, the data show a discrepancy from the same line. The strain at the crack tip was also measured with the Moire fringe multiplication method. A large amount of plastic strain at the crack tip was observed even below the crack opening load for R=−1 in HT80 steel. These phenomena show that fatigue damage still exists when the crack is closed. These also show that the crack closure cannot fully account for the effect of R on da/dn.

The fracture toughness of ceramics

Abstract: Recently proposed approaches for enhancing the toughness of ceramics have been examined. Several approaches have been demonstrated to produce appreciable increases in toughness, including (a) controlled microfracture, (b) coherent precipitation, (c) ductile second phase networks, and (d) stress induced phase transformations. The relative merits and limitations of each approach, and the important prerequisites for optimum toughening, have been discussed.

Analysis of crack propagation in isotactic polypropylene with different morphology

Abstract: In bulk isotactic polypropylene it is relatively easy to change the morphology of the material in a wide range, mainly by different isothermal crystallization conditions. Size, distribution and inner structure of the spherulites have a great influence on the crack propagation in this material. Due to our microscopic investigations of crack paths it seems to be significant attributing specific resistances to crack propagation to regions with different molecular structure. From the volume portions of those regions and their orientation with respect to the external load follows their share of the fracture surface; these fractions and their specific resistance values thus determine the critical crack extension force of the bulk material.

Strength improvement of cemented carbides by hot isostatic pressing (HIP)

Abstract: HIP treatment after sintering increases the strength of the investigated cemented carbide alloy by a factor of two whereas hardness, fracture toughness, and work of fracture remain unchanged. HIP does not affect the microstructural parameters of the carbide skeleton and the binder phase, but the residual pores are eliminated entirely. Failure of both the as-sintered and post-densified material occurs by a pure Griffith mechanism. The strength-flaw size relationship is established experimentally and is shown to obey exactly Griffith's basic strength equation. The strength is controlled by the largest microstructural defects, i.e. pores in the as-sintered material, and coarse WC grains and inclusions in the HIP-treated specimens.