Showing papers on "Fracture toughness published in 1975"

Metallurgical factors affecting fracture toughness of aluminum alloys

Abstract: Crack extension in commercial aluminum alloys proceeds by the “ductile” or fibrous mode. The process involves the large, ~1 μm to ~10μm, Fe-, Si-, and Cu-bearing inclusions which break easily, and the growth of voids at the cracked particles. The linking-up of the voids is accomplished by the rupture of the intervening ligaments, and this is affected by the fine, ~0.01μm precipitate particles that strengthen the matrix. The ~0.1μm Cr-, Mn-, and Zr-rich intermediate particles are more resistant to cracking and may enter the process in the linking-up stage. The fracture toughness of aluminum alloys therefore depends on a) the extent of the heavily strained region ahead of the crack tip, which is a function of the yield strength arad modulus, b) the size of the ligaments which is related tof c, the volume fraction of cracked particles, and c) the work of rupturing the ligaments. An approximate analysis predicts KIc varies asf c-1/6, and this is in agreement with measurements on alloys with comparable yield strength levels. Studies in which the aging conditions are altered for the samef cshow that the toughness decreases with increasing yield strength level. This degradation in toughness is related to the localization of plastic deformation. The tendency for localization is illustrated with the help of “plane strain” tension and bend specimens whose behavior is related to the toughness. Measurements of the strain distribution on the microscale show that slip is relatively uniformly distributed in a 7000-type alloy with low inclusion and particle content when the material is in the as-quenched and overaged conditions. In contrast the distribution is highly nonuniform in the peak aged condition where slip is concentrated in widely spaced superbands involving coarse slip bands with large offsets that crack prematurely. The connection between the tendency for slip localization and the fine precipitate particles which strengthen the matrix remains to be established. In overaged alloys grain boundary ruptures occur within the superbands. The amount of intergranular failure increases with grain size and is accompanied by a loss of fracture toughness.

An energy release rate criterion for mixed mode fracture

Abstract: The initial energy release rate for a branch crack propagating at an arbitrary angle from an existing crack tip is obtained in a simple fashion and in closed form by using a continuity assumption. It is then postulated that the branch crack propagates in the direction which causes the energy release rate to be a maximum and that initiation occurs when the value of this release rate reaches a critical value. It is shown that these postulates yield results identical to the maximum stress theory, since the direction in which the maximum circumferential stress occurs is also the direction causing the maximum energy release rate.

The effect of temperature on the fracture of polycarbonate

Abstract: The fracture toughness of polycarbonate was obtained over the temperature range 20 to − 120° C. There is a strong thickness dependence which is described in terms of plane stress and plane strain values which are insensitive to temperatures above −40° C but the plane stress value increases below this temperature. This change is associated with theβ transition and stable crack growth was observed in this region with accompanying instabilities arising from adiabatic heating at the crack tip.

Crack propagation and fracture in silicon carbide

Abstract: Fracture mechanics and strength studies performed on two silicon carbides — a hot-pressed material (with alumina) and a sintered material (with boron) — have shown that both materials exhibit slow crack growth at room temperature in water, but only the hotpressed material exhibits significant high temperature slow crack growth (1000 to 1400‡ C). A good correlation of the observed fracture behaviour with the crack growth predicted from the fracture mechanics parameters shows that effective failure predictions for this material can be achieved using macro-fracture mechanics data.

The creep fracture of wrought nickel-base alloys by a fracture mechanics approach

Abstract: Creep fracture in the 500 to 750°C temperature range was by an intergranular crack growth process involving the formation of microcracks in grain boundaries slightly ahead of the main crack. The crack growth was proportional to an exponential power of the stress intensity. Wide differences in cracking behavior were seen between different alloys, but their differences were due primarily to material processing history and not to compositionper se. Transverse sample orientation and coarser grain sizes significantly improved the resistance to cracking. Both slow crack growth and the final fast fracture toughness changed appreciably with test history. A good correlation was found between the notch properties and the creep behavior of an unnotched sample loaded to the yield strength.

Intermittent bonding for high toughness/ high strength composites

Abstract: High strength and high toughness are usually mutually exclusive in brittle filament/brittle matrix composites. The high tensile strength characteristic of strong interfacial filament/matrix bonding can, however, be combined with the high fracture toughness of weak interfacial bonding, when the filaments are arranged to have alternate sections of high and low shear stress (and low and high toughness). Such weak and strong areas can be achieved by appropriate intermittent coating of the fibres. The strong regions ensure that the filament strength is picked up; weak areas randomly in the path of running cracks serve to blunt them by the Cook/Gordon mechanism which, in turn, produces long pull-out lengths with an associated large contribution to toughness. Boron-epoxy composites of volume fraction 0.20 to 0.25 have been made in this way which have fracture toughnesses of over 200 kd m -2, whilst retaining rule of mixtures tensile strengths (~ 650 MN m-2). At the volume fractions used, this apparently represents Kic values greater than 100 MN m -3/2, An analysis is presented for toughness and strength which demonstrates, in broad terms, the effects of varying the coating parameters of concern. Results show that the "toughness" of interfaces is an important parameter, differences in which may not be shown up in terms of interfacial "strength". The choice of coating material is crucial in getting the desired effect. Some observations are made upon methods of measuring the components of toughness in composites.

Carbon fibre composites with rubber toughened matrices

Abstract: In carbon fibre reinforced plastics (CFRP), the initial resistance to crack propagation parallel to fibres is determined largely by the matrix toughness. The fracture toughness (G IC) of an epoxide resin can be increased considerably by the addition of butadieneacrylonitrile co-polymers (CTBN). These cause the precipitation of small spheres of a second phase and, for example, increaseG IC from ∼ 300 to ∼ 3000 J m−2 on the addition of 9 wt% CTBN. The large increases obtained in bulk resins are not obtained in CFRP, instead significant but modest increases are achieved. The suppression of toughness is related to the thickness of resin film through which the crack propagates.

The fracture stress and its dependence on slow crack growth

Abstract: An analysis is presented which enables the fracture strength, (under constant stress-rate conditions) to be predicted from fracture mechanics data obtained during slow crack growth — by identifying and evaluating several key parameters. The predicted strength characteristics are illustrated using fracture mechanics data obtained for a soda lime silicate glass. Finally, the predicted strengths are compared with strengths measured in flexure on abraded soda lime silicate glass specimens. A good correlation is obtained, indicating an equivalence between micro- and macro-crack propagation conditions for this material.

Control of Cracks by Interfaces in Composites

Abstract: A novel theory is proposed to show how a crack may he accelerated or retarded when it meets an interface between two equally brittle materials of different elastic properties. Measurements of a model crack travelling through a brittle adhesive joint have substantially verified the theory. The results demonstrate that the toughness of a composite material, having a periodic stiffness change along the crack path, may be very much greater than the toughness of the individual components of the composite. The relevance of these ideas to the design of tough composite structures is discussed.

Fracture toughness and mechanical behaviour of an epoxy resin

Abstract: The variation of fracture and mechanical properties of epoxy resin Epikote 828, cured with diaminodiphenyl-methane by variation of the resin/amine ratio has been determined. Observations of the crack tip have shown that fracture toughness variations can be attributed to the different blunting characteristics of the various resin/amine compositions. A systematic variation in the thermal transitions of the resins also occurs as a function of composition. Investigations by chemical etching and small angle X-ray scattering have failed to reveal a structural entity which can be invoked to explain the trends in fracture and mechanical properties. It has been found that care must be taken when choosing the method of evaluation of fracture parameters in order that meaningful comparisons can be made between different resin compositions.

Metallurgical factors affecting high strength aluminum alloy production

Abstract: With the advent of linear elastic fracture mechanics, the detailed effects of processing and microstructure on toughness can be evaluated. The effect of microstructure on plane stress and plane strain fracture toughness is considered in detail together with strength, fatigue behavior and corrosion resistance. It is concluded that second phase particles in all size ranges can influence toughness. Increasing the size and amount of particles or decreasing precipitate coherency all lead to decreases in toughness. Grain structure is also shown to play a prominent role in determining plane stress fracture toughness ; at a given strength level, a fibrous grain structure and the prevention of recrystallization are desirable. The ability to influence fatigue crack propagation by control of processing is more remote though relatively little systematic work has been carried out in this field. Thermomechanical processing is considered to offer another possible route to achieving a desirable balance of toughness, strength and corrosion resistance.

The validity of various fracture mechanics methods at creep temperatures

Abstract: The application of stress intensity factors derived from linear elastic fracture mechanics (LEFM) to fracture at creep temperatures has been considered. From tensile creep rupture tests on single edge notched and notched centre hole specimens of solution treated A.I.S.I. type 316 stainless steel, it is shown that a LEFM approach is inapplicable to predicting creep crack growth rates, whilst the net section stress is found to correlate well with the crack growth rates. These observations have been explained by considering the creep relaxation that takes place at the notch root, smoothing out the local stresses and thus making the LEFM stress distribution inapplicable. The resulting stress distribution supports the observation that the net section stress is a successful criterion on which to predict creep rupture in stainless steel.

The Extension of Crack Tip Damage Zones in Fiber Reinforced Plastic Laminates

Abstract: The size and character of the damage zone at the tip of sharp notches in fiber reinforced plastic laminates have been investigated The variables studied were the stress intensity factor, specimen size, laminate thickness, ply thickness, ply orientation, and fiber properties The damage zone consists of subcracks parallel to the fibers of each ply, in some cases accompanied by delamination between plies The damage zone is found to increase in extent approximately in proportion to K2I up to fracture for notch-sensitive laminates For notch-insensitive laminates, a point is reached where the zone spreads rapidly across the entire specimen prior to fracture A strong dependence of damage zone size and fracture toughness on ply thickness, fiber orientation, and fiber properties is demonstrated and discussed

Ductile-brittle fracture transitions in polycarbonate

Abstract: Test data is described for polycarbonate single edge notch specimens at a range of loading rates and two thicknesses at 20°C. The transition from ductile to brittle failure is shown to be both rate and thickness dependent. The effects may be described in terms of a plane stress and a plane strain fracture toughness coupled with a rate dependent yield stress and a plastic zone size to thickness ratio criterion. An increasing percentage of the incidence of brittle failure with rate rather than a sharp transition is noted and explained in terms of a distribution of toughness values introduced by variations in the notching method.

The effect of mean stress on fatigue crack propagation a literature review

Abstract: Most investigations of fatigue crack propagation have been carried out using pulsating tension loading and rate of crack propagation (da/dN) expressed in terms of range of stress intensity factor (ΔK). However, mean stress can have a significant effect on da/dN, so that generally it should be expressed in terms of mean and range of stress intensity factor. Literature relating to the effect of mean stress on da/dN, mainly in steels and aluminium alloys, is reviewed in the present report. Empitical relationships which allow the correlation of crack propagation data obtained at different mean stresses, crack propagation theories which predict the effect of mean stress, and fundamental explanations of the effects of mean stress are discussed. Suggestions for future research are made.

Some Considerations of the Influence of Sub-Critical Cleavage Growth during Fatigue-Crack Propagation in Steels

Abstract: The results are interpreted in terms of a 3-stage relationship between log growth-rate and log δK, and show that, for intermediate and high growth-rates, effects of mean stress are caused directly by the occurrence of ‘static’ modes. The final acceleration in rate near instability is considered to be associated with the critical linkage of such modes. During this stage it is inappropriate to relate the crack growth-rate to the alternating stress intensity, δK, because the maximum value of stress intensity, Kmax, is critical in determining how much of the total crack front is composed of transgranular cleavage facets and hence how fast the crack is growing. Discussion is also included on the difference between the terminal stress intensity during fatigue and the K Ic value (fracture toughness) under monotonic loading. A possible explanation for such differences is proposed in terms of plasticity-induced heating at the crack tip.

Crack Growth Resistance of Random Fiber Composites

Abstract: Crack growth resistance of a random glass fiber composite has been studied by applying the concepts of Linear Elastic Fracture Mechanics. The crack growth resistance curves (R-curves) for epoxy and polyester composites have been developed to study the total fracture behavior of these composites. It is shown that the R-curve approach is suitable for these type of materials. An attempt has been made to use the information from the R-curves to predict the fracture strength of plate specimens with a hole at the center and subjected to tensile loading.

Apparent interfacial failure in mixed-mode adhesive fracture

Abstract: Aluminium-epoxy adhesive specimens constructed with the bond at 45‡ to the direction of loading appear to fail very close to the interface. The actual locus of failure was investigated by14C labelling of the epoxy polymer and also by Auger spectroscopy profile analysis. Both techniques indicated a residual film of polymer a few hundred angstroms thick on the aluminium surface. The fracture energy of these specimens was determined and found to be affected by the surface roughness of the aluminium. The mixed-mode fracture energy (GI,II) C45° of these specimens in the absence of any surface roughness effect (polished surfaces) was 140 J m−2 compared to 136 J m−2 for the same polymer in simple opening-modeGI C adhesive fracture. The “interfacial” failure and the effect of surface finish on fracture are discussed in terms of the applied stress directing the failure toward the interface but the approach of the crack to the boundary being limited by the size of the crack tip deformation zone.

Material Characterization on the Fracture of Filament-Reinforced Composites

Abstract: The mixed mode fracture of unidirectional composites is investigated by application of the strain energy density theory. Two analytical models are used. The first assumes that the composite is an ideal homogeneous anisotropic body, and the second assumes that failure takes place in a layer of matrix material whose edges are bonded to two orthotropic plates. The expressions for the strain energy density factor, S, are derived for both models, and brittle fracture is assumed to occur when the S-factor in an element of material ahead of the crack reaches a critical value, Sc. Results are presented for E-glass fiber reinforced plastics, stainless steel fiber reinforced aluminum and graphite fiber epoxy materials and compared with available experimental data. The latter model is preferred as it accounts for the effect of nonhomogeneity that is inherent in the filament-reinforced composites.

Fracture of Ni-Fe base metallic glasses

Abstract: The fracture toughnesses of specimens of three transition metal base metallic glasses, Ni48Fe29P14B6Al3, Ni39Fe38P14B6Al3 and Ni49Fe29P14B6Si2 are reported. Each alloy was tested in a characteristic thickness, i.e., 25μm (Ni48), ∼ 43μm (Ni39) and ∼ 72 μm (Ni49) andK C values of ∼ 120, 62 and 30 kg mm−3/2, respectively, were observed. It is suggested that this variation is associated primarily with a transition from plane strain (K IC ≃ 30 kg mm−3/2) toward plane stress conditions as sample thickness is decreased. The fatigue crack propagation rate in the Ni39 alloy is also reported;da/dn (mm/cycle) ≃ 2×10−8 ΔK 2.25, whereΔK has units of kg mm−3/2. When the respective data are plotted in terms of (ΔK/E), whereE is Young's modulus, the crack growth behaviour for the Ni-Fe glasses approximates that for crystalline ferrous alloys. A classical chevron pattern, macroscopically at 90° to the tensile axis, is observed when amorphous metallic alloy strips fracture under plane strain conditions. On a finer scale, the chevrons exhibit a sawtooth structure, and the sawtooth surfaces show a fine scale, equi-axed vein pattern. This indicates that local failure occurs by shear rupture.

Fracture of Composite Compact Tension Specimens

Abstract: : Fracture experiments were carried out on compact tension specimens of unidirectional and cross-ply S-glass/epoxy and graphite/epoxy. Fracture toughness values were determined by the compliance calibration technique and by measuring the area under the load-displacement curve. In unidirectional specimens, crack extension was always parallel to the fibers and was dependent on crack length. Toughness did not vary significantly with fiber orientation relative to the load direction in unidirectional S-glass/epoxy. Tests on cross- ply S-glass specimens were not valid because crack propagation did not occur; instead, a zone containing a system of superficial parallel cracks and other damage developed, which extended with increasing load. Cross-ply graphite specimens, on the other hand, did appear to give valid test results although the cracks propagated were not always straight and other damage mechanisms were also present. Toughness values for cross-ply graphite were approximately two orders of magnitude higher than for unidirectional specimens due chiefly to the fracture resistance of fibers transverse to the crack. Toughness values determined by the compliance calibration method were consistent with reported values obtained by other methods. (Author)