Time-temperature dependent fracture toughness of PMMA

A toughness-biased Ree-Eyring relationship gives a good description of fracture toughness data of PMMA over a range of temperatures (283 to 353 K) and crack velocities (10 −5 to 1 m sec −1 ). Fracture toughness was measured by Gurney's sector method. The activation energy associated with the equation supports earlier work which suggests that, in the same temperature and velocity range, cracking in PMMA is controlled by craze growth, which is governed by secondary ( β ) molecular processes. Unstable cracking at moderate velocities (10 −2 to 1 m sec −1 ) seems to be produced by an isothermal/adiabatic transformation; an analysis for the onset of instability is given. At temperatures below 283 K, changes in toughness behaviour are seen, and below 243 K no stable cracking at all was obtained. A discussion is given of various methods of characterizing resistance to cracking, and methods of transforming R (à, T ) and K (à, T ) data are compared.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44668/1/10853_2004_Article_BF00540829.pd

Fractographic study of transverse cracks in a fibre composite

Transverse fracture of unidirectional fibre composites was studied in a model glass/epoxy composite in which 1 mm-diameter rods had been used in place of fibres. The fracture surface resulting from transverse cracking in this model system was studied by scanning electron microscopy (SEM). The interaction of the crack with the epoxy matrix resin and the glass rods was the following: Cracks in the resin appeared to have effected a debonding at the glassmatrix interface before reaching the glass. The debonding then propagated along the interface and induced secondary cracks ahead of the primary debonding crack. The confluence of the secondary and primary cracks resulted in sharp ridges being formed on the matrix resin surface, produced by plastic deformation of the rigid epoxy resin. These appeared as a field of parabolic marks. Considering the brittleness of the resin, the amount of plastic deformation indicated by the ridges was astonishing. As the debonding continued around the glass rod, a transverse corrugated texture developed on the resin surface, again produced by plastic deformation. Finally, the cracks reentered the matrix from small patches of polymer adhering especially strongly to the glass surface. The overall fracture energy of transverse cracking of unidirectional fibre composites is suggested to consist, therefore, of the following elements in addition to crack propagation in the matrix resin: (a) the glass-resin debonding before the incoming cracks reach the glass, (b) the initiation of secondary cracks or debonds at the interface, (c) the plastic deformation in generating the ridges on the rigid resin surface, appearing both as the paraboloids and the transverse corrugation, and (d) cracking of the matrix reinitiated at the opposite side of the glass. The use of an enlarged glass reinforcement in this study provided a more direct observation of the properties of transverse crack propagation in composite materials than would have been possible with the small, roughly 10 μ m fibres.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44691/1/10853_2005_Article_BF01111915.pd

Diffusion mechanisms and degradation of environmentally sensitive composite materials. Progress report, July 1, 1979-June 30, 1980

Progress not previously discussed in reports COO-4440-4 through COO-4440-6 is summarized. Moisture diffusion results in unstressed epoxy samples obtained last year (below 70/sup 0/C) suggested the presence of a 2-phase structure in the cured epoxy resin. Chemical etching studies initiated this year further confirm the existence of this structure. Also, diffusion results obtained at 95/sup 0/C demonstrate a reversal in the trend of diffusivity versus crosslink density as observed on the same samples last year. A damage mechanism at this temperature is proposed to explain the results. Initial results on stressed epoxy samples show the inadequacy of the current test fixtures; as a result, new fixtures have been designed to eliminate stress relaxation problems due to swelling and creep of the resin samples during the experiment

The effects of alkali treatment on the interfacial adhesion of bamboo fibres

In the present work, the potential of using bamboo fibres as reinforcement for polyester composites was evaluated. Two types of bamboo fibres were used: untreated and treated with different NaOH concentrations (1, 3, and 5 wt%). Mechanical properties of both treated and untreated fibres were investigated. In addition, single-fibre pull-out tests were performed to study the interfacial shear strength of the fibres with the polyester matrix at different embedment length of fibres. Scanning electron microscopy was used to study the surface morphology of the fibres before and after the tests. Results revealed that an untreated fibre has the best strength and stiffness but lowest strain at break. An increase in alkali concentration reduces the strain at failure and ductility of bamboo fibres.However, the strength and stiffness of the fibres were increased. In addition, the interfacial shear strength was improved with longer embedment length and higher NaOH concentration

Strengthening and toughening of brittle materials. Progress report, January 31, 1972--January 31, 1973

Studies were conducted on spherical particle and fibrous composites. The six reprints deal with: three-dimensional finiteelement analysis, effects of interface on mechanical properties of low aspect ratio fiber composites, reinforcement of epoxy resins with potassium titanate whiskers, mechanical properties of glass bead-filled epoxy composites, and interfacial layer effects on properties of composites. (DLC