Epoxy fracture

Fig. 8.10 The porous structure and high specific surface of CNT fibers enhances adhesion to polymer matrices (a) shows the cross-section of fiber/epoxy fractured specimen, evidencing good wetting by the polymer [9] (b) shows fragmentation tests on CNT fibers in epoxy, for fibers infiltrated with PVA (a) and PI (b) [78]. With kind permission from Elsevier (2009, 2011).

Calculation of Stress Distribution in Macroporous Epoxies Fracture Toughness of Solvent-Modified and Macroporous Epoxies Prepared via CIPS. 

One of the most curious aspects of crack growth in most epoxies is the apparently unstable manner by which propagation occurs, even over wide ranges of temperature and test rate. This behavior is commonly referred to as stick-slip , and is characterized by the crack growing in a series of discrete, unstable jumps. Even some of the earliest works on epoxy fracture report this mode of crack growth. The suspected origins of stick-slip fracture behavior in epoxies is discussed in a subsequent section. Unhke epoxies, thermoplastic polymers, such as poly(methyl methacrylate) and polystyrene, are characterized by stable, continuous crack growth. This mode of fracture sometimes can be observed in epoxies, in particular, when they are tested at fast rates and/or low temperatures. 

The effects of moisture on epoxy fracture are not conclusive. Scott et al. reported that an amine cured epoxy, normally displaying stick-slip fracture at room temperature and low rates, exhibited stable behavior when immersed in distilled water. Also, they found that the rate necessary to promote the unstable to stable crack growth transition at room temperature was increased by two orders of magnitude in the presence of the water. Yamini and Young , on the other hand, found that testing in water tended to suppress stable behavior and promote stick-slip fracture in an amine cured epoxy over a wider range... 

Epoxy fracture is studied in several ways. Fracture properties are often measured by tensile tests on either notched or unnotched specimens, but may also be measured by a compression. 

Hydroxy terminated polysulfones have been found to increase epoxy fracture toughness (46). Amine terminated polysulfones have been found preferable to hydroxy terminated polysulfones as modifiers for DGEBA because they react directly with the epoxy and do not require a catalyst (32). Fracture toughness of TGAP has been tripled by addition of polysulfones and the system has been studied with frequency dependent electromagnetic sensing technique which can measure vitrification time and provide information about size and shape of occluded particles in the cured resin. 

An island area vs. perimeter plot taken from photographs of a roughly 1 mm area of the epoxy sample at one point in the polishing process is shown in Figure 7. The observed slope of 1.51 implies thatD = 2.32. TheD measurements made from different cuts of this same surface were within 6% of this value. We can therefore conclude that the epoxy fracture surface is indeed fractal. 

PDMS epoxy Amine terminated PDMS improved epoxy fracture toughness 148... 

With prepreg epoxy systems, the first two values are lower by at least 0.5%. As seen from Table 2, these values are much lower than the fracture strain for most composites. Thus, the epoxy fractures much earlier than the fibers. This situation is stressed under dynamic load on carbon- or Kevlar fiber composites, since polymers possess a much lower fatigue-endurance limit than carbon- and Kevlar fiber composites this is not the case for fiber-glass composites. 

FESEM images of SWNT/epoxy fracture surface (a) Pulled out CNT bundles as long as 30-40 can be seen. Plastic deformation of the matrix is obvious. Scale bar is 2fj,m. (b) Fracture surface in lower magnification. The failure modes of the composite portion include SWNT pullout, matrix cracks bridged by SWNT. Scale bar is 10 / m. 

A proposed mechanism for toughening of mbber-modifted epoxies based on the microstmcture and fracture characteristics (310—312) involves mbber cavitation and matrix shear-yielding. A quantitative expression describes the fracture toughness values over a wide range of temperatures and rates. 

Fig. 5. Interlaminar fracture toughness, for a number of thermosetting and thermoplastic composites (36,37). Open white bars represent glass-fiber composites shaded bars are for carbon fibers. The materials are A, polyester (unidirectional) B, vinyl ester (CSM = chopped strand mat) C, epoxy (R/BR1424) D, epoxy (T300/914) E, PPS F, PES and G, PEEK. To convert J/m to fdbf/in. multiply by 2100.

A number of amorphous thermoplastics are presently employed as matrices in long fiber composites, including polyethersulfone (PES), polysulfone (PSU), and polyetherimide (PEI). AH offer superior resistance to impact loading and higher interlaminar fracture toughnesses than do most epoxies. However, the amorphous nature of such polymers results in a lower solvent resistance, clearly a limitation if composites based on such polymers are to be used in aggressive environments. 

Two wooden beams are butt-jointed using an epoxy adhesive (Fig. A1.3). The adhesive was stirred before application, entraining air bubbles which, under pressure in forming the joint, deform to flat, penny-shaped discs of diameter 2fl = 2 mm. If the beam has the dimensions shown, and epoxy has a fracture toughness of 0.5 MN mT , calculate the maximum load F that the beam can support. Assume K = cT Tra for the disc-shaped bubbles. 

Rider and Amott were able to produce notable improvements in bond durability in comparison with simple abrasion pre-treatments. In some cases, the pretreatment improved joint durability to the level observed with the phosphoric acid anodizing process. The development of aluminum platelet structure in the outer film region combined with the hydrolytic stability of adhesive bonds made to the epoxy silane appear to be critical in developing the bond durability observed. XPS was particularly useful in determining the composition of fracture surfaces after failure as a function of boiling-water treatment time. A key feature of the treatment is that the adherend surface prepared in the boiling water be treated by the silane solution directly afterwards. Given the adherend is still wet before immersion in silane solution, the potential for atmospheric contamination is avoided. Rider and Amott have previously shown that such exposure is detrimental to bond durability. 

Several experiments will now be described from which the foregoing basic stiffness and strength information can be obtained. For many, but not all, composite materials, the stress-strain behavior is linear from zero load to the ultimate or fracture load. Such linear behavior is typical for glass-epoxy composite materials and is quite reasonable for boron-epoxy and graphite-epoxy composite materials except for the shear behavior that is very nonlinear to fracture. 

Kinloch, A. J. Mechanics and Mechanics of Fracture of Thermosetting Epoxy Polymers. Vol. 72, pp. 45-68. 

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