Donald L. Hunston

Deformation and fracture behaviour of a rubber-toughened epoxy: 1. Microstructure and fracture studies

The microstructure and fracture behaviour of an unmodified and a rubber-modified epoxy have been studied. Values of the stress intensity factor, KIc, at the onset of crack growth, the type of crack growth, and the detailed nature of the associated fracture surfaces have been ascertained. Both materials exhibit essentially the same types of crack growth but the values of KIc for the rubber-modified material were usually significantly higher than those for the nmodified epoxy. The mechanisms for this increased toughness have been considered and a mechanism that accounts for all the observed characteristics has been proposed.

Deformation and fracture behaviour of a rubber-toughened epoxy: 2. Failure criteria

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.

Effects of Environmental Aging on the Properties of Pultruded GFRP

Abstract Pultruded glass–fiber-reinforced vinyl ester matrix composite coupons were subjected to environmental aging in order to study their durability since such composites are of interest for infrastructure applications. Specimens were tested as-received and after aging in water or salt solutions at room temperature (25°C) or in water at 75°C for various times. The flexural properties (strength and modulus) were determined for bending perpendicular to the 0° orientations (0° being the pull direction) for all aging conditions. In addition, flexural properties in the 90° orientation and tensile properties in the 0° orientation were also measured for the as-received specimens and the specimens exposed to selected aging conditions. Both strengths and moduli were generally found to decrease with environmental aging. Comparing the size of the fracture mirrors on the broken ends of the fibers in aged and un-aged samples suggested that environmental aging decreased the in situ fiber strength. In addition, examination of the failure surfaces and comparisons between the strength of the 90° specimens suggested that degradation of the fiber/matrix interphase region also occurred during the aging process.

Matrix resin effects in composite delamination - Mode I fracture aspects

A number of thermoset, toughened thermoset, and thermoplastic resin matrix systems were characterized for Mode I critical strain energy release rates, and their composites were tested for interlaminar critical strain energy release rates using the double cantilever beam method. A clear correlation is found between the two sets of data. With brittle resins, the interlaminar critical strain energy release rates are somewhat larger than the neat resin values due to a full transfer of the neat resin toughness to the composite and toughening mechanisms associated with crack growth. With tougher matrices, the higher critical strain energy release rates are only partially transferred to the composites, presumably because the fibers restrict the crack-tip deformation zones.

Micromechanics of Fracture in Structural Adhesive Bonds

Abstract The high mode-I fracture surface energies, GIC , of structural adhesives can be attributed to their ability to form large crack-tip deformation zones prior to failure. It has been suggested that this feature also controls the dependence of the adhesive bond GIC on bond thickness. The proposed explanation asserted that the physical constraint of the adherents and the nature of the crack-tip stress field in an adhesive joint alter the size and shape of the deformation zone, and this in turn changes the fracture behaviour. To examine this hypothesis, motion pictures were taken of fracture specimens during loading, and the stress whitening that occurred at the crack tip was used to judge the relative dimensions of the deformation zone. The results generally support the hypothesis. Moreover, the pictures furnish a clear image of the deformation zones growth patterns during loading, and this provides a critical test for future modelling efforts.

The Application of Optical Coherence Tomography to Problems in Polymer Matrix Composites

Abstract The Composites Group at the National Institute of Standards and Technology has found optical coherence tomography (OCT) to be a powerful tool for non-destructive characterization of polymer matrix composites. Composites often exhibit superior properties to traditional materials such as wood and metal. However, the barrier to their widespread infiltration into consumer markets is cost. Composites can be made more cost competitive by improved composite design, process optimization, and quality control. OCT provides a means of evaluating the three aforementioned areas. OCT is a very versatile technique that can be applied to a variety of problems in polymer composites such as: microstructure determination for permeability and mechanical property prediction, void, dry spot, and defect detection, and damage evaluation. Briefly, OCT uses a low coherence source such as a superluminescent diode laser with a fiber optic based Michelson interferometer. In this configuration, the composite is the fixed arm of the interferometer. Reflections from heterogeneities within the sample are mapped as a function of thickness for any one position. Volume information is generated by translating the sample on a motorized stage. Information about the location and size of a feature within the composite is obtained. In this work, the power of OCT for imaging composite microstructure and damage is presented. An example of permeability prediction using the composite microstructure imaged from OCT is demonstrated. The effect of image processing on the value of permeability is discussed. Using the same sample, OCT imaging of composite impact damage is compared to more traditional techniques, X-ray computed tomography and confocal microscopy.

Viscoelastic Fracture of Structural Adhesives

Abstract Experiments conducted on several different elastomer-modified epoxy systems indicated that the high fracture energy of most structural adhesives is achieved through crack-tip deformation processes that are viscoelastic. It is essential therefore that the fracture behavior of such materials be determined as a function of temperature and loading history. The linear viscoelastic properties of the model systems were functions of formulation and thermal history but when these parameters were controlled the behavior was thermo-rheologically simple over a wide range of conditions. The fracture behavior was also dependent on formulation and thermal history although the effects of history were quite small in the range of conditions studied here. The fracture behavior at various temperatures and loading rates could be characterized to a first approximation by a master curve of fracture energy vs. reduced time-to-failure. This characterization makes it possible to compare the properties of different formula...

The influence of elastomer concentration on toughness in dispersions containing preformed acrylic elastomeric particles in an epoxy matrix

The influence of toughener concentration on the fracture behavior of two-phase, rubber-toughened epoxy is studied. To vary the concentration without altering other morphological features, samples generated from dispersions of preformed rubber (acrylic) particles in liquid epoxy monomer are used. By diluting with different amounts of epoxy prior to cure, the toughener concentration can be varied over a wide range. Thermal and microscope studies support the assertion of a constant morphology. The fracture results show that the toughness increases to a maximum and then decreases as the concentration is increased. This suggests an optimum concentration of toughening. Micrographs of the initiation zone on the fracture surface at high concentrations of rubber show less deformation than the equivalent surfaces at lower concentrations. This is consistent with a toughening mechanism based on particles initiating yielding and plastic flow in the matrix.

Long-term environmental fatigue of pultruded glass-fiber-reinforced composites under flexural loading

Abstract Pultruded glass-fiber-reinforced vinyl ester composite coupons were subjected to four-point-bend fatigue in various environments to study long-term durability for infrastructure applications. Several groups of specimens were aged in water or in salt solutions containing mass fractions of either 5% NaCl or 10% NaCl for up to 6570 h. Both as-received and aged specimens were then cyclically tested in air or while immersed in water or in salt solution. For specimens cyclically loaded at or above 45% of the average flexural strength of the dry coupons, no substantial difference in fatigue life was observed among all the specimen groups. For samples cyclically loaded at 30% of the dry flexural strength, however, all specimens tested in air survived beyond 107 cycles while all those tested in water environments did not. It is found that long-term environmental fatigue behavior is not controlled by the quantity of water absorbed; rather, it is governed by a combination of both load and fluid environment. No difference in fatigue life was found for specimens aged in different fluid environments at room temperature prior to fatigue testing. Relative to these samples, however, a significant difference was seen for specimens aged in water at 75°C for 2400 h prior to cyclic test at load levels above 30% of the dry flexural strength. When tested at 30% of the dry flexural strength the differences were within the experimental uncertainty. Microscopic examination of the fatigue specimens revealed evidence of a degraded fiber/matrix interphase region in those specimens where environmental exposure caused premature failure so this is believed to be a controlling factor in the environmental performance of the glass composite.

Comparison of optical coherence tomography, x-ray computed tomography, and confocal microscopy results from an impact damaged epoxy/e-glass composite

Optical coherence tomography (OCT) is an emerging technique for imaging of synthetic materials. OCT is attractive because it combines high sensitivity (>90 dB), high resolution (5 µm to 20 µm), and low cost, approximately US