Epoxy crazing

Water molecules combine the tendency to cluster, craze and plasticize the epoxy matrices with the characteristic of easily diffusion in the polymer1 10). The morphology of the thermoset may be adversaly influenced by the presence of the sorbed moisture. The diffusion of the water in glassy polymers able to link the penetrant molecules is, therefore, characterized by various mechanisms of sorption which may be isolated giving useful information on the polymer fine structure. 

In Fig. 26, we schematically illustrate four stages of failure in epoxies under an increasing tensile load. In each stage we document the craze/crack structure, the stress at the craze/crack surface and the resultant fracture topography. 

Transmission electron microscopy (TEM) and birefringence studies of strained and/ or fractured epoxies have revealed more direct experimental evidence that molecular flow can occur in these glasses. Films of DGEBA-DETA ( 11 wt.- % DETA) epoxies, 1 pm thick, were strained directly in the electron microscope and the deformation processes were observed in bright-field TEM 73 110). Coarse craze fibrils yielded in-homogeneously by a process that involved the movement of indeformable 6-9 tan diameter, highly crosslinked molecular domains past one another. The material between such domains yielded and became thinner as plastic flow occurred. 

Fig. 5 TEM micrograph of a craze in bulk HDPE deformed in tension at room temperature (embedded in epoxy and stained in Ru04)...

Polymer Morphology and Failure Mechanisms. A failed tensile bar of unmodified piperidine-cured epoxy resin shows shear deformation before tensile failure when strained slowly (0.127 cm/sec). We could not produce stable crazes in specimens of unmodified epoxy resins. At all stress levels, temperatures, and conditions of annealing only fracture occurred after shear band formation. The failure to observe crazes in unmodified epoxy resins may be explained by a fast equilibrium condition which exists between crazing on loading and recovery on unloading. 

Figure 7. Electron micrograph of a CTBN-toughened epoxy resin with large particles—fracture through a craze volume (X5200)...

Next we looked at the microvoid situation in a bisphenol A modified CTBN-epoxy system. This sample had the highest toughening properties that we developed in the epoxy system because of a two-particle size rubber population that uniquely gives a combination of shear deformation and tensile crazing. Only some of the large particles had microvoid development. Consequently the whitening was much less than when only crazing occurs. The multiple failure sites were still evident. 

Crack tip blunting is attributed to localized yielding at the crack tip. Localized yielding may result from shear deformation, or normal stress deformation. Unlike shear deformation, which occurs at constant colume, normal stress deformation involves a volume dilatation and is considered to be responsible for the formation of crazes in thermoplastics. Since crazes are not observed in highly crosslinked epoxies, it is generally assumed that plastic deformation at the crack tip takes place via a shear yielding process. 

Glassy polymers with highercohesiveness, like polycarbonate and cross-linked epoxies, preferentially exhibit shear yielding [7], and some materials, such as rubber-modified polypropylene, can either craze or shear yield, depending on the deformation conditions [8]. Application of a stress imparts energy to a body which... 

The load-displacement curves for C(T) tests of the neat EpoxyH were almost linear until the final unstable fracture. The fracture toughness value in 77K-LNj was 210 J/m and that in RT-air was 120 J/m. Thus the toughness increased by 1.8 times by changing the test environment from RT-air to 77K-LN. Brown and co-workers have found that amorphous polymers crazed in 77K-LNj, but not in a helium or vacuum at about 78K [20-22]. They have also reported that the stress-strain behavior of all polymers, amorphous and crystalline, is affected by at low temperatures [22]. Kneifel has reported that the fracture toughness of epoxy in 77K-LNj is higher than that in RT-air and 5K, and that the reason for this is the reduced notch effect by plastic deformation [23]. Then, the increase of the fracture toughness of the neat EpoxyH in this study is probably caused by the similar effect. 

Polymeric overlays are used on concrete pavements to protect them from dust, excessive wear, cracking, crazing, spalling and to provide an attractive appearance. These materials are applied on concrete pavements in the form of screeds, self-levelling material or floor coatings. Epoxies, polyurethanes and acrylics are commonly used, epoxies being the most commonly chosen. Table 1.4 gives the suitability of these materials for different environments. 

The contribution of crazing to ten e deformation increases with stress, and therefore with strain rate. This point is illustrated in Fig. 3, which ows changes in mechanism with stress and strain in ABS and in tcai ened epoxy resin. The data are taken from creep tests in which the strain rate increased with time under load, so that the values quoted for low strains are also for low strain rate. The changes in... 

For both the PS-PVP and the dPS-COOH epoxy systems the fact that a substantial fraction of dPS is found on the non-PS side of the interface after fracture at the maximum Qc shows that at least some chains in the craze fibrils fail by disentanglement, i.e.f < fb. 

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