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.     

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.     

The fracture of particulate-filled epoxide resins

The double torsion test technique was used to investigate the fracture properties of two commercial epoxide resins filled with glass beads. The influence of varying the volume fraction of glass, the mean particle size and the pre-treatment of the glass surface on the stress intensity factor have been determined. A correlation has been found between the compressive yield strength and the stability of fracture in these composites, similar to that found for unfilled epoxide resins.     

Creep failure mechanisms in a particulate-filled epoxy resin

The deformation-induced volume damage in a series of creep specimens is examined in this investigation in order to improve the basic understanding of creep failure in particulate-reinforced epoxy resins. The results are correlated with the fracture surface morphology reported elsewhere. Volume damage was found to consist of matrix shear yielding, silica-particle debonding and matrix cracking. Fracture is shown to be initiated by shear yielding and debonding which is followed by sub-critical crack growth, demonstrating the importance of volume damage in fracture. Sub-critical crack growth occurs by debonding or by void coalescence depending on the temperature and loading conditions. The temperature and loading dependence of volume damage and the above crack propagation mechanisms are examined and presented graphically in a damage mechanism map.     

Microscopy of impact damage in particulate-filled glass-epoxy laminates

Low-energy impact damage was induced in particulate-filled glass-epoxy laminates by means of a falling weight device, and the damage features were studied by sectioning through the impacted area. The polished sections were studied by optical and scanning electron microscopy to obtain comparative qualitative information about the effects of several different mineral fillers: solid glass beads, hollow glass microspheres, quartz, alumina trihydrate and mica. Approximately 8.5 mm thick laminates, with and without fillers, were impacted at various energies up to 43 J. At the lower impact-energy levels, the damage in the unfilled laminates in most samples was typically transverse matrix cracking, at some distance below the point of impact. Transverse and interlaminar cracking was more extensive in filled than in unfilled laminates. The type and distribution of the damage features are discussed in terms of the properties of the filler particles, and of the stress distributions involved.     

Fracture in epoxy matrix resins

The occurrence of fracture-energy-enhancing steps and welts on fracture surfaces of crosslinked matrix resins has been studied in an epoxy obtained from a trifunctional epoxy resin cured with an anhydride. It is suggested that the steps and welts arise from an underlying basic longitudinal texture, which was revealed by strongly tilting fracture specimens toward the collector in a scanning electron microscope. A model for the development of the basic longitudinal texture is proposed involving a meniscus instability of the propagating crack front, which gives rise to a series of fingers protruding into the bulk resin ahead of the nominal crack front. The periodicity of the basic longitudinal texture seen in the epoxy specimens studied was roughly 350 nm, which was independent of the epoxy resin: hardener ratio within at least 10% of stoichiometry. Because the periodicity of the basic longitudinal texture is roughly equal to the separation of the fracture surfaces immediately behind the crack, a considerable blunting of the crack by plastic deformation or yielding is suggested, a property that should depend on the matrix resin.     

Failure prediction in toughened epoxy resins

In order to improve the damage tolerance of composites and the performance of adhesives, one of the methods being considered is toughened or modified epoxy resins. The modifiers which are commonly used are CTBN rubber and inorganic fillers. A major toughening mechanism causing the increased toughness is the shear deformation process occurring near the crack tip. The effect of such a deformation process is to blunt the crack tip and increase the size of the plastic zone. Several models are available to predict the toughness on the basis of plastic zone size, crack tip opening displacement or crack tip radius, but these are only applicable to Mode I crack extension. Also, most of these approaches use only one stress component which is normal to the crack plane to predict the fracture toughness. The present paper reviews the existing models and suggests a criterion based on the phenomenological approach to failure in order to study the yielding and fracture toughness behavior of both unmodified and modified epoxies. The proposed yield and fracture criteria give predictions in good agreement with experimental results.     

Improved crack resistant liquid epoxy resins

Recent trends in the electrical industry indicate a growing demand for liquid epoxy castings that combine a high usage temperature (i.e. high glass transition temperature of the cured resin) with good crack resistance.It is well-known that the use of liquid instead of solid epoxy resin results in an increase in the glass transition temperature, but this is usually accompanied by a reduction in crack resistance. Frequently, such liquid epoxy resin based castings have to be flexibilized to improve crack resistance, but then the glass transition temperature is inherently lowered.The present paper describes some approaches to a combination of a high glass transition temperature with excellent crack resistance. A study of a number of criteria showed that this can be achieved by careful control of the crosslink density.The proper ratio of epoxy resin to curing agent, in combination with a suitable catalyst ensures castings with a glass transition temperature ranging from 95 to 125°C, and which pass standard crack tests even at temperatures below -80°C and temperature cycling from +150°C to -75°C.     

Adhesion of epoxy resins to metals

The adhesion of an epoxy resin above its glass transition temperature to aluminium, steel and gold surfaces has been studied using the methods of fracture mechanics. The results are compared with those of a previous study of elastomeric adhesives by Andrews and Kinloch, and the “intrinsic failure energies”,θ ₀, for the epoxy-metal bonds are deduced by similar methods. Correspondence, within a factor of two, is found betweenθ ₀ and the thermodynamic work of adhesion,W _A, for most cases of interfacial failure, indicating both the absence of specific or chemical interactions at the interface and a purging of surface contaminants by the epoxy. An exception occurs when an excess of epoxy groups exist in the uncured resin. Here, for steel and aluminium but not for gold, the interfacial bonding is stronger than the cohesive strength of the resin due probably to the formation of strong bonds with the metal oxide surface layer.     

含侧环氧基硅油复合改性环氧树脂的研究

采用侧基环氧化硅油(ES)及其改性物(PSA)来复合改性双酚A型环氧树脂(EP).通过测定复合改性固化物的冲击强度、拉伸强度、断裂伸长率和玻璃化转变温度(Tg),扫描电子显微镜对改性固化物的断裂面形态分析等,系统探讨了复合改性方法、有机硅组成及其含量等对复合改性材料性能的影响.结果表明,采用ES和PSA复合改性EP后,其韧性和耐热性均有不同程度的提高,且以环氧值高的ES和PSA的改性效果更好.其中环氧树脂经10份ES-16或10份PSA-16改性后,Tg由未改性的156.73℃提高到177.35℃,比改性前提高了近20.62℃,均达到了很好的增韧和提高耐热性的效果,符合电子封装等高性能材料的改性要求.     

The mechanical properties of epoxy resins

Crack propagation in a series of epoxy resins described in Part 1 has been studied as a function of testing rate and temperature. It has been found that crack propagation is continuous at low temperatures but that as the temperature is raised the mode of propagation becomes unstable (stick/slip). Features on the fracture surfaces at the crack arrest lines have been shown to be of the same dimensions as those expected for a Dugdale plastic zone. It has been suggested that the slip process takes place by slow growth of a crack through the plastic zone followed by rapid propagation through virgin material. It has been shown that the stick/slip behaviour is due to blunting of the crack which is controlled by the yield behaviour of the resin. A unique fracture criterion has been shown to be applicable to epoxy resins which is that a critical stress of the order of three times the yield stress must be achieved at a critical distance ahead of the crack. Electron microscope replicas of the fracture surfaces have been obtained and an underlying nodular structure can be resolved. However, no direct correlation between the nodule size and fracture properties has been found.     

Plastic deformation in highly cross-linked rubber-modified epoxy resins

Plastic yielding behaviour of three different cross-link density rubber-modified epoxy resins, at different rubber levels and temperature, were investigated. All the systems studied show decrease in Young's modulus,E, and yield stress, _Y, with increasing temperature and rubber content. The deformation process was analysed using both Bowden and Argon theories. Molecular parameters from each. theory were then compared with chemical structures of the epoxy systems.     

Crack propagation in and fractography of epoxy resins

The relationship between the stability of crack propagation in, and the fracture surface appearance of, DGEBA epoxy resins cured with TETA has been investigated using a linear elastic fracture mechanics approach. In particular, the effect of varying the amount of curing agent and curing conditions and altering external variables such as testing rate, temperature and environment have been studied. Under certain conditions, propagation is found to be stable and fracture surfaces have a smooth appearance. Under other conditions the cracks propagate in an unstable stick-slip manner. In this case, arrest lines can be seen on the fracture surfaces and their intensity and roughness increases with the magnitude of the crack jumps. The roughness of the fracture surfaces has been measured using a Talysurf and this has been shown to be due principally to deviation of the cracks from the original fracture plane rather than any gross plastic deformation.     

Micromechanical modeling of particulate-filled composites using micro-CT to create representative volume elements

International Journal of Mechanics and Materials in Design - A method based on X-ray micro-CT was introduced to create realistic representative volume elements (RVE) for particulate-filled...