Deformation-induced volume damage in particulate-filled epoxy resins

In this investigation the microstructural deformation and damage mechanisms in an epoxy resin filled with silica flour are investigated. The unfilled resin exhibits bulk shear yielding in both compression and tension which is localized into fine bands by the addition of the particulate filler. In addition, the filled material stress-whitens in compression and at lower rates and higher temperatures in tension. The shear yielding is recoverable upon heating for a time above the glass transition temperature; however, the stress whitening is irreversible. The addition of filler particles introduces several damage mechanisms including particle-matrix debonding, particle fracture and matrix microcracking. In the following report, these damage mechanisms are characterized microstructurally in terms of the rate, temperature and type of loading and their relative contributions to stress whitening.     

Fractography of unfilled and particulate-filled epoxy resins

The objective of this work was to analyse and understand the types of fracture surface morphology found in unfilled and particulate-filled epoxy resins in the light of the thermomechanical history of the specimen (loading rate or duration of loading, temperature, strain at break). Short-term tensile tests and long-term creep tests were conducted at four different temperatures. The fracture surface features were analysed using the scanning electron and optical microscopes and, where suitable, an image analyser. In order to correlate these morphologies with certain regimes of crack velocity, fracture mechanics tests were also conducted, varying the crack speed between 10^–7 and 10² m sec^–1. In the case of the filled resin, the lifetime under static loading is governed by a phase of slow, sub-critical crack growth which is manifested by resin-particle debonding. Thereafter, the crack accelerates and finally may reach terminal velocities depending on the amount of stored elastic energy available at the moment of fracture.     

Creep failure mechanisms of a Ti-6Al-4V thick plate

The creep failure operating mechanisms of a 17-mm thick plate of a Ti–6Al–4V alloy in various heat treating conditions have been investigated. Specimens in the as-received, mill-annealed, condition (50  min maintenance at 720 °C and air cooled as the final step of the thermomechanical process) showed the lowest creep resistance and their metallographic analysis revealed that the temperature-activated dislocation climb was the mechanism responsible for the failure and that observed holes were generated by plastic deformation, rather than by creep cavitation. Conversely, maximum times to failure were recorded in beta-annealed specimens (1030 °C for 30  min, air cooled and aged for 2  h at 730 °C). The fracture surfaces of these broken specimens exhibited an intergranular morphology that was attributed to grain boundary sliding along the former beta grains. Finally, alpha–beta field-annealed samples (940 °C 4  h, and furnace cooled to 700 °C) possessed intermediate lives between those of mill-annealed and beta-annealed specimens and the failure operating mechanism was diffusional creep by the nucleation and coalescence of the creep cavities generated at the alpha–beta interfaces and the triple points.     

Dynamic observation of fracture in particulate-filled epoxy resins reinforced with rubber

Crack propagation in epoxy resins filled with alumina trihydrate has been observed by dynamic in situ scanning electron microscopy. Double torsion specimens were fractured inside a scanning electron microscope (SEM) connected to a video recorder. Characteristic features of the crack propagation process were observed. The fracture mode was mainly intergranular at low crack velocities, (ca. 10 m/s) with evidence of filler particle cracking (transgranular fracture) at higher velocities or after acid-washing the particles. Extensive shear yielding of the epoxy matrix occurred between closely spaced filler particles within the crack tip damage zone. Post-mortem static observations of the fracture surfaces were also carried out. The addition of rubber toughening agents modified the crack propagation process. In some cases the rubber was present as fine, evenly distributed particles while in others there were coarser precipitates and/or what appeared to be rubber-rich epoxy phases. Ligamentary bridges across crack faces in the crack wake necked to fracture at low crack velocities but failed by cavitation under rapid loading. Energy dissipation by local shear yielding of the matrix was still prominent. Vinyl terminated rubber addition induced especially widespread yielding. Out-of-SEM determinations of tensile and fracture parameters were consistent with dynamic SEM observations.     

Crack propagation in a glass particle-filled epoxy resin

Crack propagation in an epoxy resin reinforced with spherical glass particles has been followed using a double-torsion test. In particular the effect of strain rate, volume fraction and particle size upon the stability of propagation, the Young's modulus, the critical stress intensity factor,K _Ic and the fracture energy,G _Ic has been studied. It has been shown that the crack propagation behaviour can be explained principally in terms of crack pinning, although it has been found that propagation is also affected by blunting the breakdown of the particle—matrix interface. It has been demonstrated that crack-front pinning is consistent with a critical crack opening displacement criterion.     

Creep failure model of a tempered martensitic stainless steel integrating multiple deformation and damage mechanisms

A new model considering both deformation and damage evolution under multiple viscoplastic mechanisms is used to represent high temperature creep deformation and damage of a martensitic stainless steel in a wide range of load levels. First, an experimental database is built to characterise both creep flow and damage behaviour using tests on various kinds of specimens. The parameters of the model are fitted to the results and to literature data for long term creep exposure. An attempt is made to use the model to predict creep time to failure up to 10⁵ h.     

Chemorheology of epoxy resin Part I epoxy resin cured with tertiary amine

Epoxy resin was cured with a tertiary amine. The viscosity and dynamic mechanical properties during the curing reaction were measured by a cone-and-plate rheometer. A dual Arrhenius viscosity model was modified to predict the viscosity profile before gelation during the non-isothermal curing. The viscosity profile coincided with the experimental data. The activation energy of this system calculated using the modified model was 19.8 kcal mol^–1 for the first region, and 17.3 kcal mol^–1 for the second region. After gelation, the dynamic complex modulus was related to the reaction kinetics according to the rubber elasticity theory, and the activation energy was 15.3 kcal mol^–1. Furthermore, the gelling point can be estimated from the rheological measurements.     

Chemorheology of epoxy resin

A secondary hydroxyl group containing epoxy resin was reacted and cross-linked with polyurethane (PU). This PU-cross-linked epoxy resin was cured with a tertiary amine and the viscosity and dynamic mechanical properties were determined by a cone-and-plate rheometer. A dual Arrhenius viscosity model was modified to predict the viscosity profile with time before gelation during non-isothermal curing, and the calculated values coincided with the experimental data. The activation energy of this system (NCO/OH ratio=30 mol%) calculated by the modified model was 13kcal mol^–1 for the initial region, and 16.8 kcal mol^–1 for the final region. After gelation, the dynamic complex modulus was correlated to the reaction kinetics according to the rubber elasticity theory, and the activation energy calculated was 5.2 kcal mol^–1. These activation energies are all lower than those of the unmodified epoxy resin system. Consequently, the reaction rate of the PU-cross-linked epoxide system was less affected by temperature than that of the unmodified epoxide system. It was also found that the rate of increase of viscosity and dynamic moduli decreased with increasing PU content. The gelling point was estimated by rheological measurements.     

Multiple fracture in a steel reinforced epoxy resin composite

Specimens of an epoxy resin reinforced by steel wires have been made to fail by a process which involves extensive cracking of the matrix before the UTS of the composite is reached. Such behaviour can result in energy being absorbed by the composite under constant or rising load conditions even when the composite is composed of two brittle phases. The influence of fibre size and volume fraction on the cracking process have been examined, and it has been shown that for low fibre volume fractions and large fibre sizes, the cracking process is governed by a simple relationship. When the fibre size becomes small, or the volume fraction becomes large, the cracking process is hindered, and this relationship breaks down. Under extreme conditions, cracking of the matrix can be completely suppressed and the matrix can be forced to exhibit properties markedly better than it would have shown when tested by itself. The design of materials which behave in this way may provide an important means for producing ceramic-matrix compositions with very good mechanical properties.     

对位芳香族聚酰胺纤维/环氧树脂复合材料防弹性能及其破坏机制

为研究对位芳香族聚酰胺纤维/环氧树脂（Epoxy resin,EP）复合材料的防弹性能及其破坏机制,采用铅芯弹侵彻复合材料靶片。以对位芳香族聚酰胺纤维作增强纤维,EP作基体树脂,纳米SiO₂和聚乙烯醇缩丁醛（Polyvinyl butyral,PVB）作增韧剂,通过热压工艺制备单向（Unidirectional,UD）结构的对位芳香族聚酰胺纤维/EP复合材料靶片。研究单片纤维面密度、UD片材结构、射击角度和树脂改性对靶片防弹性能的影响;观察弹击实验后靶片的破坏形貌,分析靶片的破坏机制。研究结果表明:对位芳香族聚酰胺纤维/EP复合材料具有优异的防弹性能,随着单层纤维面密度的增大,靶片的防弹性能呈现整体上升、局部上下波动的变化趋势;铺层方式为0°/90°/0°/90°的四层单UD片材（4UD）结构的防弹性能优于铺层方式为0°/90°的两层单UD片材（2UD）结构;角度射击时,靶片的穿透比率更大,背衬凹陷深度（Back face signature,BFS）比率更小;PVB增韧改性EP提升了靶片的防弹性能;纤维拉伸变形破坏、片材分层和基体树脂碎裂是复合材料靶片主要的吸能方式。      

Strain softening in an epoxy resin

Post-yield behaviour, particularly strain softening, has been studied here for an epoxy resin to examine the effects of postcuring and plasticization by water and a non-volatile diluent dibutylphtalate. The significance of necking was examined by conducting compression tests (at room temperature) as well as tensile tests. In the case of tensile tests, elevated temperatures were used in order to achieve yielding. The amount of strain softening was found to be reduced by postcuring or by the addition of a plasticizer. In the case of postcuring, little change in yield stress occurred, whereas, with plasticizers, a reduction in yield stress does occur.     

Mechanical fatigue of epoxy resin

In static bending fatigue tests, epoxy resins show practically no fatigue if the stress given to specimen is lower than a critical value, which is close to the bending strength of the specimen. In cyclic bending fatigue tests, on the other hand, the resins are easily fractured even though the stresses are far below the critical values. Some strain may be accumulated on the surface of specimen through cyclic deformations. However, the strain accumulated is reversible. If the specimen is allowed to rest, the strain disappears. If the strain reaches a critical value, an irreversible transition may be induced, probably in the arrangement of segments on the surface. A crack nucleus thus created may propagate and cause the final fracture of the specimen, following the fracture mechanics of elastic materials. The lifetime of epoxy resins under cyclic bending load is determined by the time required for creating a crack nucleus on surface.     

Deformation characteristics of reinforced epoxy resin

Specimens, cut from a commercially produced epoxide resin reinforced by layers of glass cloth, were tested in tension at various strain-rates and temperatures, and at various orientations of the fibres to the tensile axis. Results are presented for elastic moduli, yield and fracture stresses, and elongations. These results give ample evidence of a decrease in the effectiveness of the reinforcement as deformation proceeds. Strain-ratechange tests show that the rate sensitivity of the composite rises rapidly as the strain increases beyond a critical value, which also indicates that the behaviour is increasingly governed by the properties of the matrix.     

环氧改性酚醛树脂的耐腐蚀性能研究

利用环氧树脂(PF)与酚醛树脂(EP)共混,再冷压-烧结成型得到改性酚醛树脂.通过在70%、50%、30%硫酸、50%盐酸、20%氢氧化钠溶液中改性酚醛树脂的腐蚀实验,证明环氧改性酚醛树脂的耐腐蚀性能优于纯酚醛,其中耐腐蚀性能最好配方是PF:EP=100:50.改性酚醛树脂耐酸性能优于耐碱性能,且在非氧化酸性(盐酸)环境中的耐腐蚀性能优于氧化性酸性(硫酸)环境下的耐腐蚀性能.热重分析(TGA)表明,环氧改性酚醛树脂的耐热性能较纯酚醛有所提高.     

真空辅助成型(VARI)用低黏度环氧树脂研究进展

综述了以环氧树脂为主的国内外真空辅助成型技术(VARI)用低黏度树脂基体的研发现状,以及双酚F及改性双酚F环氧树脂研究现状,分析了双酚F及改性双酚F环氧树脂应用于真空辅助成型技术(VARI)的潜力.