Epoxy resins brittle fracture

The reinforcing effects of nanofibers in an epoxy matrix and in a rubber matrix using electrospun nanofibers of polybenzimidazole (PBl) was observed. The average diameter of the electrospun fibers was around 300 nm. The nanofibers toughened the brittle epoxy resin. The fracture toughness and modulus of the nanofiber (15 wt.%)-reinforced epoxy composite were both higher than for an epoxy composite made with PBl fibrils (17 wt.%], which were whisker-like particles. 

High temperature epoxy resins are brittle materials, and one method of improving their fracture properties is to incorporate reactive liquid rubbers in the formulations In situ phase separation occurs during cure the cured rubber-modified epoxy resins consist of finely dispersed rubber-rich domains ( 0.1-S pm) bonded to the epoxy matrix. TTT diagrams can be used to compare different rubber-modified systems. 

The distinct fracture lines characteristic of brittle resins was observed on the fracture surface of control (59). In the blend, the bulk of the sample exhibited features similar to those observed for the control and no dispersed particles were found in the bulk of the sample as the polyolefin phase migrated to the top of the sample. The fracture surface of (20 and 30 phr) AC 5120-RVP blends was different from that of unmodified AC 5120 blends with epoxy resin. Although some of the polyethylene migrated to the surface, dispersed polyethylene was seen on the fracmre surfaces (Fig. 21.10). 

Static and impact fracture data (room temperature measurements) mainly for epoxy resin systems is presented in Table 2. These relatively brittle materials require modification by, for instance, blending with a suitable rubber or thermoplastic to improve fracture toughness. Such improvements, however, depend not only on the types of materials but also on the composition and therefore the type of blend structure (e.g.. continuous-discrete phases. 

Acura SI 40 epoxy resin, (a) Machined notch, with brittle fractures, (b) Stereolithography notch (Hague, R. et o/., J. Mater. Set., 39, 2457, 2004) with kind permission of Springer Science and Business Media. 

When the neat epoxy resin is modified by the incorporation of submicron rubber particles or glass spheres, the fracture energy can be increased by factors of 4-5 at all levels of temperature, as is shown in Fig. 13.41. Moreover, the increase of toughness with temperature is less abrupt. As we demonstrate, these observations represent an almost classical behavior of transformation toughening by crack-tip shielding of a brittle solid (Evans et al. 1986). 

Epoxies provide strong joints and their excellent creep properties make them particularly suitable for structural applications, but the unmodified epoxies have only moderate peel and low impact strength. These properties can be improved by modifying the resin, to produce more flexible materials which have an improved resistance to brittle fracture. These adhesives include combinations such as epoxy-nylon, epoxy polyamide. 

Polymer-based composites can be divided into thermoset and thermoplastic composites, which due to their different properties show diverse fracture mechanism. Due to tight three-dimensional molecular network structure of the most of thermoset matrixes such as epoxy resins, they exhibit inherent brittle fracture behavior and poor... 

A comparison of the yield stress as a function of temperature for different flexibilized epoxy resin systems is given in Fig. 6. The crystalline EP exhibit lower yield stresses than the conventional flexibilized EP. The crystalline system, based on sebacic acid even shows ductile behaviour at -80 C, whereas the adipic acid system reaches its critical yield stress limit at -70°C, and brittle failure at lower temperatures is found. Measurements of Hartwig also showed brittle fracture at cryogenic temperatures. But there also seems to be no evidence of ductile behaviour in the case of the sebacic acid polyester segments. The lowest yield stress at -100°C is exhibited by the rubberlike polymer based on sebacic acid polyester. Here too, the adipic acid polyester shows higher yield stresses, but achieves ductile behaviour at -100°C. The more rapid increase in yield stress is due to the absence of crystallinity. These results show that good low temperature flexibility may be... 

For this reason, HMTA cured novolac networks are generally brittle in nature and have low fracture energies and impact strengths, probably due to the voids combined with the high crossliiik densities. Curing novolac resins with epoxy resins can produce networks without volatile by products. These can be reinforced with glass or carbon fibers to yield tough, void-fiee composites, which also retain much of the flame-retardant properties of phenolics [39],... 

Brittle polymers such as epoxy resin are mostly toughened with CFs, thus their fracture toughness can be well characterized by Kjq. The fracture toughness of CFRP composites can be determined experimentally using compact tension specimens under a plane strain condition. In this case, the specimen thickness should obey the following equation ... 

Fig. 5.51 An SEM image of a fractured rubber toughened epoxy resin exhibits brittle fracture. Holes from the dispersed phase particles show the rubber is incompatible with the matrix resin and there is poor adhesion resulting in rubber particles being pulled out during fracture.

Fig. 5.52 SEM of a rubber toughened epoxy resin shows that brittle fracture occurs through both the matrix and the dispersed phases. Voids (arrows) are observed within the dispersed phase and also within the matrix. Small subinclusions are seen within the dispersed phases.

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