Fracture toughness of bisphenol A‐type epoxy resin

The relationship between the postcuring conditions and the fracture toughness of a bisphenol A‐type epoxy resin cured with acid anhydride was investigated. The glass transition temperature and fragility parameter, derived from the thermo‐viscoelasticity, were used to characterize the epoxy resin postcured under various conditions. Relationship between these two parameters and the fracture toughness was then investigated, based on the fractography results of a microscopic roughness examination of a fractured surface. The values of the glass transition temperature and fragility greatly depended on the postcuring conditions. The glass transition temperature was approximately 400 K when the crosslinking reaction was saturated. The fragility was independent of the saturation of the reaction and varied between 50 and 180. The results of the fracture test and fractography examination showed that there was no direct correlation between the glass transition temperature, the fracture toughness, and the roughness. On the other hand, there was a correlation between the fragility, fracture toughness, and roughness when the glass transition temperature saturated (at 400 K). As the fragility decreased from 180 to 50, the fracture toughness increased from 0.6 to 1.1 MPa · m^1/2 at the same glass transition temperature. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 10: 2266–2271, 2002     

Fracture toughness of silica particulate‐filled epoxy composite

The relationship between the postcuring conditions and fracture toughness on three silica particulate‐filled epoxy composites was investigated. The glass transition temperature, T_g, and the fragility parameter, m, derived from the thermo‐viscoelasticity, were used to characterize the composites, which were postcured under various conditions. The glass transition temperature and fragility both depended on both of the curing conditions and the volume fraction of silica particles. The glass transition temperature increased with the postcuring time and temperature, while the fragility generally decreased as the volume fraction increased. There was no direct correlation between the glass transition temperature and fragility. The fracture toughness depended on both the glass transition temperature and fragility. The composites with a high glass transition temperature and low fragility had high fracture toughness. These results indicate that the glass transition temperature and fragility are useful parameters for estimating the fracture toughness of the silica particulate‐filled epoxy composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 2261–2265, 2002     

Fracture toughness improvement of epoxy resins with short carbon fibers

This study examined the thermal stability and fracture toughness of diglycidylether of bisphenol-A (DGEBA)/short carbon fiber (SCF) composites using several techniques. The thermal stability of the DGEBA/SCF composites was similar to that of neat epoxy resin. The fracture toughness of the composites was significantly improved relative to the neat resin. The SEM micrographs indicated that a relatively rough surface with shear deformation and tortuous cracks was formed, thereby preventing deformation and crack propagation and inducing higher fracture toughness in the DGEBA/SCF composites.     

Fracture toughness of polysulfone/epoxy semi-IPN with morphology spectrum

The fracture toughness of the semi-IPN's of crosslinked epoxy and linear polysulfone(PSf) having morphology spectrum was investigated. The epoxy resin was based on diglycidyl ether of bisphenol A(DGEBA) and diaminodiphenylsulfone(DDS). The morphology spectrum, which has the gradual change of the morphological feature resulting from the concentration gradient of PSf in the epoxy resin can be obtained by inserting a PSf film in DGEBA/DDS mixture before cure. The relative rate of the dissolution of the PSf in the epoxy oligomer and the rate of curing reaction determine the concentration gradient of the PSf. In the region where the PSf concentration is less then 5%, sea(epoxy)-island(PSf) morphology is observed. As the concentration of PSf increases, the morphology changes to nodular structure, inverted sea-island, and PSf/epoxy homogeneous phase. Up to overall 10wt% of PSf, the fracture toughness of the PSf modified epoxy with morphology spectrum was higher than that of the counterpart with uniform concentration of PSf. These results were ascribed to the plastic deformation of the continuous PSf rich phase which was present in the morphology spectrum. Received: 13 October 1998/Revised version: 15 December 1998/Accepted: 16 December 1998     

Fracture toughness of a hybrid-rubber-modified epoxy. I. Synergistic toughening

The fracture behavior of a hybrid-rubber-modified epoxy system was investigated. The modified epoxy included amine-terminated butadiene acrylonitrile (ATBN) rubber and recycled tire particles as fine and coarse modifiers, respectively. The results of the fracture toughness (K_IC) measurement of the blends revealed synergistic toughening in the hybrid system when 7.5-phr small particles (ATBN) and 2.5-phr large particles (recycled tire) were incorporated. Transmission optical micrographs showed different toughening mechanisms for the blends; fine ATBN particles increased the toughness by increasing the size of the damage zone and respective plastic deformation in the vicinity of the crack tip. However, in the case of hybrid resin, coarse recycled rubber particles acted as large stress concentrators and resulted in the branching of the original crack tip. Mode mixity at the branch tips led to synergistic K_IC in the hybrid system. It seemed that the ductility of the matrix played an effective role in the nature of the crack-tip damage zone in the hybrid epoxies. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Brittle to ductile: Fracture toughness mapping on controlled epoxy networks

A series of epoxy networks with controlled molecular weight between crosslinks (M_c) was constructed with a difunctional epoxy resin and a mixture of aliphatic amines. The glass transition and yield strengths decreased as M_c increased, while the elastic properties were independent of M_c. In determining the effect of M_c on fracture toughness, the fracture behavior changed from a brittle fracture to a ductile fracture as M_c increased. The tests and analyses used to evaluate the fracture energy changed from a linear elastic fracture mechanics approach for brittle failures to an elastic-plastic fracture mechanics approach for ductile failures. The ductile responses also showed an increasing fracture resistance with crack extension. Two popular models were used to describe the fracture energy as a function of yield strength. The analysis showed that a change in fracture energy by a thermally induced change in yield strength was equivalent to a change in fracture energy by a chemically induced change in yield strength. In addition, comparisons between the two models allowed insightful relationships to be drawn.     

Fracture toughness and impact strength of anhydride‐cured biobased epoxy

Biobased neat epoxy materials containing functionalized vegetable oils (FVO), such as epoxidized linseed oil (ELO) and epoxidized soybean oil (ESO), were processed with an anhydride curing agent. A percentage of diglycidyl ether of bisphenol F (DGEBF) was replaced by ELO or ESO. The selection of the DGEBF, FVO, and an anhydride‐curing agent resulted in an excellent combination to produce a new biobased epoxy material having a high elastic modulus and high glass transition temperature. Izod impact strength and fracture toughness were significantly improved dependent on FVO content, which produced a phase‐separated morphology. POLYM. ENG. SCI., 45:487–495, 2005. © 2005 Society of Plastics Engineers     

Studies of epoxy resin systems: Part D: Fracture toughness of an epoxy resin: A study of the effect of crosslinking and sub-Tg aging

The fracture toughness of an epoxy resin system, diglycidyl ether of butanediol, DGEB, cured with 4-4′ diaminodiphenyl sulphone, DDS, has been studied by varying the crosslinking density and state of aging. A stable, but rough, crack propagation was observed with specimens that were 99 percent cured and quenched. When the extent of curing was less than 99 percent or the material was aged for more than 20 min at 62°C, crack propagation was of the unstable stick-slip nature. Aging was found to decrease the initiation fracture toughness dramatically, but the arrest fracture toughness was almost unchanged. This result was associated with a change of relaxation strength of the primary, a, transition with aging. An increase of crosslinking density was found initially to reduce the fracture toughness of this epoxy resin, but the fracture toughness increased after 87 percent of curing. The initial decrease of the fracture toughness was attributed to a decrease of relaxation strength of the primary transition (i.e., the area under the α-relaxation peak), while the increase of the fracture toughness after 87 percent curing was explained by the onset of the stablerough crack propagation, Micrographs taken by scanning electron microscopy-showed possible existence of blunting during crack propagation and a decrease of blunting with the extent of aging.     

Fracture toughness of a hybrid rubber modified epoxy. II. Effect of loading rate

Effect of loading rate on toughness characteristics of hybrid rubber-modified epoxy was investigated. Epoxy was modified by amine-terminated butadiene acrylonitrile (ATBN) and recycled tire. Samples were tested at various loading rates of 1–1000 mm/min. Fracture toughness measurements revealed synergistic toughening in hybrid system at low loading rates (1–10 mm/min); hybrid system exhibited higher fracture toughness value in comparison with the ATBN-modified resin with same modifier content. However, synergistic toughening was eliminated by increasing the loading rate. At higher loading rates (10–1000), the fracture toughness of hybrid system decreased gradually to the level lower than that of ATBN-modified epoxy. Fractography of the damage zones showed the toughening mechanisms of ATBN-modified system was less affected by increasing the loading rate compared to that of hybrid system. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Fracture toughness of highly ordered liquid crystalline epoxy thermosets achieved by optimized composition of curing agents

Liquid crystalline (LC) epoxy resin was prepared by using different compositions of aromatic amine as curing agents, in order to control curing rates and chemical compositions. The progress of the curing reaction was investigated based on the gel fraction and epoxy groups of conversion determined by Fourier-transform infrared spectroscopy. The ordered networked polymer structure was analyzed by polarized optical microscopy and X-ray diffraction. Highly oriented network chains in the obtained epoxy thermosets were promoted by the incorporation of flexible chains in the network and the provision of sufficient time for vitrification. Furthermore, it was clarified that a curing temperature higher than T_g is required to promote the transition to the smectic LC phase in order to prepare highly ordered epoxy thermosets. The increase in the formed smectic LC phase in the network chains resulted in significant higher fracture toughness and achieved up to 2.7 times higher value.     

Fracture of an epoxy polymer containing recycled elastomeric particles

The fracture behavior of elastomer-modified epoxy was investigated using compact-tension geometry. The elastomeric modifiers included a liquid carboxyl-terminated butadiene acrylonitrile and solid rubber particles of different sizes which were obtained from recycled automobile tires. When used with solid rubber alone, no significant improvement in the fracture toughness was observed. However, when used in combination with the liquid rubber modifier, it was observed that the fracture toughness of these hybrid epoxies was higher than that of those toughened with liquid rubber alone. This synergistic effect is explained in terms of crack deflection and localized shear yielding. Furthermore, we observed a slight improvement in the fracture toughness as the size of the solid rubber particles increased. Although using a combination of both reactive rubber liquids and solid rubber particles as toughening agents had been investigated previously, in this study, the solid rubber particles used were from recycled rubber tires. Therefore, we have clearly demonstrated an application of producing high-quality engineering epoxy systems using toughening modifiers that are relatively low in cost and created higher-value products for recycled solid rubber. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 271–277, 1997     

Fracture toughness evaluation of polytetrafluoroethylene

The objective of this preliminary work was to explore the fracture resistance of polytetrafluoroethylene (PTFE) (DuPont tradename Teflon) as part of materials characterization work related to the development of “reactive” material projectiles. Little mechanical property data is available on this material since it is commonly used only as a coating material with the dominant properties being its low friction coefficient and high application temperature. Additional end products of the “7C” derivative, however, includes sheet, gaskets, bearing pads, piston rings, and diaphragms. In this work, standard ASTM E1820 fracture toughness specimens were machined from a 14‐mm‐thick sheet of this material obtained from NSWC Dahlgren Laboratory. These specimens were tested at three test temperatures and four test rates to determine if fracture would occur in this material, and if so, how the fracture toughness depends on the test temperature and specimen loading rate. Standard axial tensile specimens were also tested at quasi‐static and elevated loading rates at temperatures from ambient to ?73°C. The major results are that while crack extension is difficult at ambient (20°C) temperature, for temperatures slightly below ambient, a rapid degradation of fracture resistance occurs. This reduction in fracture resistance is enhanced by rapid loading, and the material loses approximately 75% of its toughness (fracture energy absorption ability) at ?18°C if the crack opening loading rate of the C(T) specimen approaches 0.25 m/s. Further reductions in temperature or increases in the loading rate appear to result in a reduced rate of degradation of fracture toughness.     

Fracture toughness of epoxy resins modified with polyethersulfone: Influence of stoichiometry on the morphology of the mixtures

Toughened mixtures containing 15 wt % polyethersulfone were made with diglycicdyl ether of bisphenol-A resin and 4,4′-diaminodiphenylmethane curing agent, with amine/epoxy group stoichiometric ratios varying from 0.6 to 1.5. Fracture behavior of the modified mixtures has been investigated as a function of the stoichiometry in the matrix. Morphology has been analyzed by transmission and scanning electron microscopy. The increase of amine content in the matrix results in a further increased fracture toughness. This behavior has been related to the changes on the ductility of the matrix upon stoichiometric ratio, but also to the changes on microstructural features of the modified mixtures as stoichiometric amine/epoxy group ratio increased. These morphological changes have been interpreted in terms of spinodal decomposition during curing of the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 183–191, 1998     

Fracture toughness testing of dental biomaterials

The adaptation of a cylindrical geometry fracture toughness test specimen to testing of polymeric dental biomaterials is demonstrated. The specimen configuration facilitates the fabrication of small specimens and simplifies the experimental study of environmental effects on properties of dental biomaterials. The test method is used to measure critical stress intensity factor, K_IC for poly (methyl methacrylate) (PMMA), particulate composite restorative materials, and dental polymers. Values obtained are in agreement with data reported for other specimen configurations.     

Fracture toughness of autoclaved aerated concrete

So far, few experiments to determine fracture toughness of autoclaved aerated concrete have been reported in the literature. Tests on three different types of autoclaved aerated concrete using six different specimen geometries have been carried out. Four of the six specimen geometries chosen lead to nearly identical results for K_IC. The two other types of specimen were used to determine fracture energy G_f. K_IC increases with compressive strength of the material. Fracture energy is about one tenth of the corresponding value of normal concrete. The influence of rate of loading on K_IC can be expressed by means of a power law. These results suggest that linear elastic fracture mechanics is a suitable approximation for the calculation of crack propagation in autoclaved aerated concrete.     

Fracture toughness of inorganic-organic hybrid coatings

The concepts of fracture toughness and the energy release rate at fracture for thin polymeric films are introduced. Fracture toughness and energy release rate data for ceramer films based on a linseed oil alkyd, a sunflower oil alkyd, and a commercial alkyd with titanium diisopropoxide bis(acetylacetonate), titanium(IV) isopropoxide, and zirconium(IV) propoxide are presented and compared to previously reported tensile data. Differences between the fracture data and the tensile data demonstrate the usefulness of fracture toughness testing. The energy release rate at fracture may be the one property to maximize in order to optimize all of the other coating properties. It may therefore be a great aid in the optimization of coating formulations. Data from dynamic mechanical thermal analysis indicate that there may be a correlation between fracture properties and secondary relaxation processes in the films. Department of Polymers & Coatings, Fargo, ND 58105.     

Fracture toughness of urethane-modified methacrylate resins

This paper reports the synthesis of a series of urethanetoughened methacrylate resins and evaluation of the fracture toughness (K_IC) of these materials. The incorporation of high-molecular-weight polyfunctional urethanes produced resins with the best mechanical properties, important to applications as dental biomaterials.     

Fracture toughness of phenolphthalein polyether ketone

The static and impact fracture toughness of phenolphthalein polyether ketone (PEK-C) were studied at different temperatures. The static fracture toughness of PEK-C was evaluated via the linear elastic fracture mechanics (LEFM) and the J-integral analysis. Impact fracture toughness was also analyzed using the LEFM approach. Temperature and strain rate effects on the fracture toughness were also studied. The enhancement in static fracture toughness at 70°C was thought to be caused by plastic crack tip blunting. The increase in impact fracture toughness with temperature was attributed two different mechanisms, namely, the relaxation process in a relatively low temperature and thermal blunting of the crack tip at higher temperature. The temperature-dependent fracture toughness data obtained in static tests could be horizontally shifted to match roughly the data for impact tests, indicating the existence of a time–temperature equivalence relationship. © 1995 John Wiley & Sons, Inc.     

The fracture toughness and stress corrosion cracking characteristics of an anhydride-hardened epoxy adhesive

The toughness and stress corrosion cracking characteristics of an epoxy resin (DER 332) hardened with hexahydrophthalic anhydride (HHPA) were investigated. The epoxy was studied in both the bulk and bond form, and its properties were compared with an amine-hardened (tetraethylene pentamine, TEPA) system. The toughness, ??_Ic, of the anhydride system varied less as a function of ratio of hardener-to-resin content and postcure temperature than it did in the TEPA-hardened system. Like the latter, however, its toughness in the bulk and bond forms could not be correlated, but ??_Ic of the joints was dependent on tensile modulus and/or yield strength of the bulk epoxy. Both systems were also toughened in the vicinity of the crack tip by water for short-time loading, but their long-time load carrying capability was reduced by a water environment. The anhydride hardened system was more sensitive to strength loss in water than the amine system. The fracture morphology for the two systems was the same, i.e., fast cracking occurred cohesively near the center of the bond, and slow cracking occurred at the interface.     

Study of rubber-modified brittle epoxy systems. Part I: Fracture toughness measurements using the double-notch four-point-bend method

The double-notch four-point-bend (DN-4PB) technique is known to produce fruitful information revealing the toughening mechanisms around the sub-critically propagated crack tip. In this study, correlations among the single-edge-notch three-point-bend (SEN-3PB), the single-edge-notch four-point-bend (SEN-4PB), and the DN-4PB toughness measurement techniques have been conducted using various modified-epoxy systems. The toughnesses of both the neat and rubber-toughened epoxy systems are found to be independent of the testing techniques used. Specifically, when the peak load is plotted against B. W^1/2/Y (B: thickness, W : width, and Y: correction factor), the data obtained from SEN-3PB, SEN-4PB, and DN-4PB all fall on the same line. The slope of this line is defined as the stress intensity factor. These results imply that with a single DN-4PB test, it is possible to gain the information needed to describe both the toughening mechanisms and the fracture toughness value of relatively brittle (K_IC ≤ 1.2 MPa. M^1/2) polymeric materials.