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1.
In part 1 the microstructure and fracture characteristics of a rubber-modified epoxy, and for comparison that of the unmodified epoxy, were examined in detail. Based on this analysis a qualitative mechanism involving cavitation, shear yielding and plastic flow was proposed. As an extension of this work, the present paper considers the yield behaviour of the epoxy material and uses the data determined, together with the previously reported fracture results, to calculate values of the crack opening displacement. The rate/temperature dependence of the crack opening displacement and the correlations established between stress intensity factor, KIc, yield stress and type of crack growth suggest that the extent of crack tip blunting largely governs the relative toughness of the epoxy materials and induces transitions in the types of crack growth observed. A quantitative expression is then presented which successfully describes the fracture toughness values over a wide range of temperatures and rates. The two parameters in this expression are shown to be material constants and therefore provide a unique failure criterion. They can be viewed simply as curve-fitting parameters but they may also have some significance in terms of a critical stress that must act over a critical distance ahead of the crack tip to produce crack growth. 相似文献
2.
《Journal of the European Ceramic Society》2007,27(6):2347-2354
Because of the industrial need for an assessment of fracture resistance, KR from small ceramic parts, KR of Si3N4 ceramics has been measured by the indentation fracture (IF) method using representative formulae to evaluate the compatibility with the fracture toughness, KIc determined from the single-edge-precracked beam (SEPB) technique. KR of the fine Si3N4 showed little dependence on the crack length, whereas the samples with coarse microstructures exhibited a rising R-curve behavior. The IF equation which gave the nearest value to KIc from SEPB was different depending on the microstructures. The assessment of fracture resistance with Miyoshi's equation was considered to be preferable for the flat R-curve behavior. By contrast, in the case of the rising R-curve behavior, it was revealed that the relationship between the IF and SEPB values was difficult to explain unless the effective crack extension against KIc for SEPB was clarified. 相似文献
3.
Hiroyuki Miyazaki Hideki HyugaYu-ichi Yoshizawa Kiyoshi HiraoTatsuki Ohji 《Ceramics International》2009
The reliability of the Vickers indentation fracture (IF) method for various types of silicon nitride (Si3N4) ceramics was assessed by comparing the fracture resistance, KR obtained from the IF test with the fracture toughness, KIc from the surface crack in flexure (SCF) technique in the same crack depth region. The KR of a fine-grained and equiaxed Si3N4 matched with the KIc from the SCF test when Miyoshi's equation was used, while the KIc of a bearing-grade Si3N4 was found to lie between KR values calculated with Niihara's equation (higher side) and Miyoshi's equations (lower side). In the case of coarse Si3N4 with elongated grains, the KR determined using Niihara's equation gave the best fit with KIc. The inconsistent outcomes were explained by the probable mechanisms, indicating that the KR from the IF test cannot be correlated directly with the KIc unless the effective crack length for the IF test was clarified. 相似文献
4.
《Journal of the European Ceramic Society》2017,37(14):4243-4257
The fracture toughness, KIc, of six glasses was measured by the surface crack in flexure (SCF) and single-edged precracked beam (SEPB) methods. Results depended upon the loading rate as well as the test environment. Environmentally-assisted slow crack growth affects the results for tests done in air. Dry nitrogen testing is preferred. Crack healing may be a severe complicating factor with precracked flexure bar type specimens if the specimens are unloaded between the precracking and final fracture test. Success in KIc testing depends to a large degree on upon the ability to make good precracks. 相似文献
5.
Summary. The effect of amine/epoxy ratio on the fracture toughness (KIc) of tetrafunctional epoxy resin was investigated. KIc value was measured by single-edge notch-bend test. The KIc value of the tetrafunctional epoxy resin increased with increasing the amount of amine curing agent. This result was explained
with the structural viewpoint of the epoxy network. The network structure of the tetrafunctional epoxy was analyzed with dynamic
thermomechanical measurement and in-situ near IR technique.
Received: 19 June 1997/Accepted: 17 Juli 1997 相似文献
6.
Measuring the fracture toughness (KIc) of glasses still remains a difficult task, raising experimental and theoretical problems as well. The available methods to estimate KIc are reviewed, with emphasis on their respective advantages and drawbacks. In view of our current understanding, this analysis gives precedence to the SEPB method. The ultimate glass strength, the critical flaw size, and the indentation load for the onset of crack initiation are discussed, in the light of the fundamentals of fracture mechanics and classical background regarding the mechanics of brittle materials. Analytical expressions were further proposed to predict the fracture energy and fracture toughness of glasses from different chemical systems from their nominal compositions. The theoretical values were compared with the experimental ones, as obtained by self‐consistent methods when available. The agreement observed in most cases suggests that measured KIc values correspond to the crack propagation regime (as opposed to the crack initiation threshold), and supports previous investigations in glasses and ceramics, which showed that a crack tip is nearly atomically sharp in these materials (but for metallic glasses). Some ideas to design tougher glasses are finally presented. 相似文献
7.
The plane-strain fracture toughness of two common epoxy systems of different ductility were determined at different loading rates, according to ASTM E 399 for metallic materials. The ASTM standard was applicable, but underestimated slightly the specimen thickness required for KIc. KIc decreased with an increasing loading rate and with an increasing yield stress. The fracture surfaces were all very smooth as long as plane-strain conditions prevailed. 相似文献
8.
Single edge notch fracture tests have been carried out on rigid polyurethane foam in the density range 32 to 360 kg/m3. Fracture properties were characterized in terms of the fracture toughness parameter (KIc), the critical strain energy release rate (GIc) and crack opening displacement (c.o.d.). The values of crack opening displacement lead to a proposed mechanism of crack propagation in foams of density greater than about 140 kg/m3. 相似文献
9.
Relatively tough epoxy-blend polymers are now commercially available for use as adhesives and as the matrices for fibre composites. Nevertheless, another failure property which may be of equal, or even of greater, importance in some applications is the resistance of the epoxy polymer to cyclic-fatigue loading. However, the cyclic-fatigue behaviour of epoxy polymers has not been studied in great detail, especially for epoxy polymers where the material has been modified by forming a polymer blend in order to increase its toughness under quasi-static test rates or impact test rates. Therefore, a major aim of the present work has been to undertake a novel investigation of a range of rubber and thermoplastic materials to modify an epoxy polymer to study whether both a relatively high toughness and a significantly improved cyclic-fatigue behaviour can be simultaneously achieved in a given formulation. The unmodified epoxy-polymer possessed a value of the fracture energy, GIc, of 495 J/m2 and a value for the threshold value of the maximum strain-energy release rate in a fatigue cycle, Gth, (below which no significant crack growth occurs) of 155 J/m2. Several epoxy-polymer blends have been identified which do show major increases in these values and probably the best combination of such properties were for the epoxy-polymers modified with a poly(polypropylene-glycol)-based polyurethane (PU) modifier: either when used by itself or as a ‘hybrid’ polymer-blend in combination with core–shell rubber (CSii) particles, based upon a styrene-butadiene rubber core. For these PU-based epoxy polymers the values of GIc and Gth were found to increase to values of about 2475 J/m2 and 445 J/m2, respectively. The mechanisms of toughening that were induced by the addition of the polymer-blend modifier revealed that the presence of a multiphase in the epoxy-blend polymer was a critical requirement in achieving relatively high values of GIc and Gth. This was due to the second-phase particles initiating plastic deformation of the epoxy-matrix phase, which was the major source of energy dissipation and toughening. In turn, the extent of energy dissipated by the plastic deformation of the epoxy-matrix phase is clearly greatly influenced by the degree of ductility exhibited by this phase of the epoxy-blend polymer. Thus, another important feature of the degree of toughening observed is the effect that the modifier has upon the yield stress and plastic failure strain of the epoxy-matrix phase. 相似文献
10.
Theany To Christina Stabler Emanuel Ionescu Ralf Riedel Fabrice Célarié Tanguy Rouxel 《Journal of the American Ceramic Society》2020,103(1):491-499
Four different SiOC glass ceramics were synthesized and their fracture toughness (KIc) and fracture surface energy (γ) were assessed by means of the single-edge precracked beam (SEPB) method. In addition, the elastic moduli were measured and the Vickers indentation behavior (hardness and microcracking) was characterized. In particular, the dependence of KIc on the free carbon content and on the fraction of crystallized nanoparticles (SiC, ZrO2, HfO2) was investigated. An increase in KIc, from about 0.73 to 0.99 MPa √m is observed as the free carbon content is increased from less than 1 to 12 vol%. The addition of Hf and Zr (resulting in 4.5 to 7.8 vol% HfO2 and ZrO2 nanoparticles) was found to increase KIc to an extent similar to the free carbon content. Moreover, predicted KIc values, assuming that the crack travels through all phases accounting for their respective volume fractions, disrupting the weakest links within the structural units, are in agreement with the experimental values. 相似文献
11.
The fracture energies, GIc, of an unmodified and a rubber-modified have been ascertained as a function of test temperature and rate. A time-temperature superpositioning technique has been adopted in order to produce a master curve of the fracture energy, GIc versus the reduced rate of test, tf/aT where tf is the time-to-failure and aT is the time-temperature shift factor. The value of shift factor, aT, as a function of test temperature has been determined from time-temperature shifting the yield stress and the modulus data for the materials. The master curve of GIc versus has been tf/aT and the values of the constants employed in the model have been deduced The value of the constants so determined are discussed with respect to the microstructures of the epoxy polymers. 相似文献
12.
W. Dll 《Polymer Engineering and Science》1984,24(10):798-808
Irreversible deformations form the main energy consumption routes for absorbing supplied energy and thus determine the resistance of materials to fracture. In many polymeric materials under mode-I loading conditions three regions of irreversible deformation behavior can be distinguished as a function of the stress intensity factor KI or strain energy release rate GI, respectively:
- no crack growth for Ki < KIo
- slow crack growth for KIo < KI < KIc
- rapid crack propagation for KI < KIc
13.
D. Verchere J. P. Pascault H. Sautereau S. M. Moschiar C. C. Riccardi R. J. J. Williams 《应用聚合物科学杂志》1991,43(2):293-304
The mechanical properties of a system consisting of a bisphenol A diglycidylether (DGEBA) expoxy, cured with a cycloaliphatic diamine (4,4′-diamino-3,3 dimethyldicyclohexyl-methane, 3DCM), in the presence of an epoxy-terminated butadiene-acrylonitrile random copolymer (ETBN), was studied as a function of the cure schedule and the initial rubber concentration. Fracture toughness (KIc) and fracture energy (GIc) were increased, while Young's modulus and yield strength decreased slightly with increasing volume fraction of the dispersed phase. We show that there is no significant influence of the precure schedule and of the various observed particle diameters on the mechanical properties for a constant rubber volume fraction. In our case, the main deformation process in the rubber-modified epoxy networks is shear yielding while cavitation is negligible. 相似文献
14.
The concepts of linear elastic fracture mechanics (LEFM) are applied to three grades of high density polyethylene in an attempt to determine their fracture behavior in terms of a linear elastic fracture toughness, Kc. The effect of specimen size (thickness and width), crack length and the mode of loading on Kc has been investigated in order to determine the plane strain fracture toughness, KIc, of these materials. The effect of temperature (between +23 and ?180°C) on their fracture behavior has also been investigated and compared in terms of their plane strain fracture toughness values. 相似文献
15.
Renan Belli Michael Wendler Anselm Petschelt Tanja Lube Ulrich Lohbauer 《Journal of the European Ceramic Society》2018,38(16):5533-5544
The testing of fracture toughness becomes problematic when only limited amount of material is available that hinders the production of typical beam specimens to be tested in bending. Here we explore fracture toughness testing methodologies that allow for small discs and plates having surface cracks to be tested in biaxial flexure using the Ball-on-3-balls (B3B) set-up, or sawed notches as in the Compact Tension geometry. The B3B-KIc test has shown to be versatile and account for a very small overestimation of the KIc-value in the order of 0.8–1.25% due to in-plane crack mispositioning, and a maximum of 4% if a worst-case scenario of additional out-of-plane mispositioning is assumed. The geometrical factor in the standard SCF method, derived by Newman and Raju, resulted in an overestimation of ~8% of the KIc-value compared to the new calculation by Strobl et al. for materials with Poisson’s ratio <0.3. 相似文献
16.
Toughness of syndiotactic polystyrene/epoxy polymer blends: microstructure and toughening mechanisms 总被引:1,自引:0,他引:1
Thermoplastic/epoxy blends were formed using an amine-cured epoxy polymer and a semi-crystalline thermoplastic: syndiotactic polystyrene (sPS). Complete phase-separation of the initially soluble sPS from the epoxy occurred via ‘reaction-induced phase-separation’ (RIPS) or via ‘crystallisation-induced phase-separation’ (CIPS), depending upon the thermal processing history employed. Dynamic mechanical thermal analysis showed that no sPS was retained dissolved in the epoxy polymer. For RIPS, at concentrations of sPS of up to 8 wt%, the sPS is present solely as spherical particles. However, macro phase-separation, giving a co-continuous microstructure, accompanied by local phase-inversion, dominates the RIPS blends containing more than 8 wt% sPS. In the CIPS blends, the sPS is present as spherulitic particles, and this microstructure does not change over the range of sPS concentrations employed, i.e. from 1 to 12 wt% sPS. The pure epoxy polymer was very brittle with a value of fracture energy, GIc, of about 175 J/m2. However, the addition of the sPS significantly increases the value of GIc, though the toughness of the RIPS and CIPS blends differs markedly. For the RIPS blends, there is a steady increase in the toughness with increasing content of sPS and an apparent maximum value of GIc of about 810 J/m2 is obtained for 8-10 wt% sPS. On the other hand, the measured toughness of the CIPS blends increases relatively slowly with the concentration of sPS, and a maximum plateau value of only about 350 J/m2 was measured in the range of 8-12 wt% sPS. The relationships between the microstructure of the RIPS and CIPS sPS/epoxy blends and the measured fracture energies are discussed. Further, from scanning electron microscopy studies of the fracture surfaces and optical microscopy of the damage zone around the crack tip, the nature of the micromechanisms responsible for the increases in toughness of the blends are identified. For the RIPS blends, (i) debonding of the sPS particles, followed by (ii) plastic void growth of the epoxy matrix are the major toughening micromechanisms. The increase in toughness due to such micromechanisms is successfully predicted theoretically using an analytical model. In the case of the CIPS blends, the increase in the value of GIc results from (i) crack deflection and (ii) microcracking and crack bifurcation. 相似文献
17.
《Journal of the European Ceramic Society》2014,34(7):1881-1892
The “Surface Crack in Flexure” method is widely used for fracture toughness (KIc) determination of ceramics. In part I of the paper we developed the theoretical fundamentals to apply this procedure to ceramic balls by using the stress application as developed for the so-called “Notched ball test”. The new test (SCF-NB) can be used to test spherical components without the need to cut out special specimens such as bending bars. In this work the practical part is presented including suggestions for crack introduction and specimen preparation and possible measurement errors are discussed. It is concluded that a measurement error less than ±5% is possible.Experiments on balls and bars made from the same silicon nitride ceramic indicate that SCF-NB delivers the same KIc-values as standardised measurements on bars. Additionally, KIc-values obtained for silicon carbide, alumina and zirconia ceramics are presented. They coincide with KIc-data from the literature. 相似文献
18.
The fracture toughness (KIc) and slow crack growth behavior of four fine-grained polycrystalline graphites, oxidized to 5, 10 and 20% weight losses, were measured in air at room temperature. Exponential decreases in the elastic moduli as well as decreases in the fracture surface energy contributed to lowering KIc. Oxidation generally shifted the stress intensity-crack velocity (KI-VI) diagram to lower stress intensity values, and decreased the slope, or N-value. Scanning electron microscope fractography revealed that a combination of filler particle and binder phase degradation with increasing oxidation was responsible for the decreased toughness and changes in the (KI-VI) characteristics. Oxidation conditions were shown to significantly affect the magnitude of decreases in the physical, elastic and mechanical properties of these graphites. 相似文献
19.
Reproducible starter-cracks for subsequent linear elastic fracture mechanics analysis have been grown in PVC by fatigue cycling at 80 Hz. The crack growth rate has been related to the fracture surface markings and to the opening mode stress intensity factor (KI) of the fatigue cycle. Termination of the fatigue crack growth when crack growth rate is constant ensures a smooth mirror fracture surface and a sharp crack tip. 相似文献
20.
The acoustic emission behavior during tensile loading of two common epoxy systems of different ductility was investigated at different loading rates. At low threshold voltage, it was possible to register acoustic emissions before the final failure. Only very few emissions were recorded compared with the amount commonly recorded for metals and composite materials. The acoustic emissions detected were of burst-type, revealing a brittle damage accumulation process. They originated from the initiation and incremental growth of microcracks of stochastic nature. The events occurred before gross yielding and during the final “brittle” failure process. Basically no events were detected between gross yielding and the final failure during which large scale yielding, necking, and stable crack growth took place. The occurrence of events at the different loading rates was strongly influenced by the yielding behavior and fracture toughness, characterized by the yield stress σy and the plane-strain fracture toughness KIc respectively. KIc was inversely dependent on the total number of events up to gross yielding. The event distribution normalized with respect to the conditions at gross yielding was hardly affected by the loading rate. 相似文献