Brittleness measurement

Using brittleness measurement to determine corrected hardness of glazes, Niewiedzielski and A. Szymanski used hardness measurements at... 

Photoreceptors may be characterized by optical microscopy for layer structure and architecture. Surface defects may be characterized by atomic force microscopy. Scratch resistance can be characterized by scratching the photoreceptor surface with a stylus of known dimensions under specified loads. Brittleness measurements are... 

This test attempts to characterize the brittleness of bitumen at low temperatures. It consists of measuring the temperature at which fissures appear on a bitumen film spread on a blade as it is repeatedly flexed. This test is delicate and of questionable reliability, but it is currently the only one that allows the elastic behavior of bitumen on decreasing temperature to be characterized. It is standardized in France (T 66-026). 

Examination of oven-aged samples has demonstrated that substantial degradation is limited to the outer surface (34), ie, the oxidation process is diffusion limited. Consistent with this conclusion is the observation that oxidation rates are dependent on sample thickness (32). Impact property measurements by high speed puncture tests have shown that the critical thickness of the degraded layer at which surface fracture changes from ductile to brittle is about 0.2 mm. Removal of the degraded layer restores ductiHty (34). Effects of embrittled surface thickness on impact have been studied using ABS coated with styrene—acrylonitrile copolymer (35). 

The Rheo-Tex rheometer is an inexpensive, automated instmment using load cell technology to measure indentation and creep. Available software calculates hardness/softness, brittleness, plasticity, and tensile strength. This instmment is particularly valuable for measurements on foods and personal care products. 

The process zone is a measure of the yield stress or plasticity of the material in comparison to its brittleness. Yielding within the process zone may take place either plastically or by dimise microcracking, depending on the brittleness of the material. For plastic yielding, / is also referred to as the plastic zone size. 

The third test shown in Fig. 17.2 is the compression test. For metals (or any plastic solid) the strength measured in compression is the same as that measured in tension. But for brittle solids this is not so for these, the compressive strength is roughly 15 times larger, with... 

Figure 9.3. Stress-strain curves for (a) rigid amorphous plastics material showing brittle fracture and (b) rubbery polymer. The area under the curve gives a measure of the energy required to break the...

As with other rigid amorphous thermoplastic polymers such as PVC and polystyrene (see the next chapter) poly(methyl methacrylate) is somewhat brittle and, as with PVC and polystrene, efforts have been made to improve the toughness by molecular modification. Two main approaches have been used, both of which have achieved a measure of success. They are copolymerisation of methyl methacrylate with a second monomer and the blending of poly(methyl methacrylate) with a rubber. The latter approach may also involve some graft copolymerisation. 

The resins are cross-linked and the molecular segments between the cross-links are rigid and inflexible. As a consequence the resins have an excellent heat resistance, as measured in terms of maintenance of rigidity on heating, but are rather brittle. 

On the commercial scale silicone resins are prepared batchwise by hydrolysis of a blend of chlorosilanes. In order that the final product shall be cross-linked, a quantity of trichlorosilanes must be incorporated into the blend. A measure of the functionality of the blend is given by the R/Si ratio (see Section 29.3). Whereas a linear polymer will have an R/Si ratio of just over 2 1, the ratio when using trichlorosilane alone will be 1 1. Since these latter materials are brittle, ratios in the range 1.2 to 1.6 1 are used in commercial practice. Since chlorophenylsilanes are also often used, the CH3/CgH5 ratio is a further convenient parameter of use in classifying the resins. 

Perhaps the most dramatic exception to the perfectly elastic, perfectly plastic materials response is encountered in several brittle, refractory materials that show behaviors indicative of an isotropic compression state above their Hugoniot elastic limits. Upon yielding, these materials exhibit a loss of shear strength. Such behavior was first observed from piezoelectric response measurements of quartz by Neilson and Benedick [62N01]. The electrical response observations were later confirmed in mechanical response measurements of Waekerle [62W01] and Fowles [61F01]. 

Agitation was stopped and the contents of the flask were removed to a Separatory funnel. The aqueous and resinous layers were separated and the solvent was removed from the resinous layer by vacuum distillation. After the removal of the solvent, heating at a reduced pressure of 1.5 to 2.5 mm and at a temperature of 245° to 250°C was continued for 4 /2 hours. The condensate then had a viscosity of 4.0 poises when measured as a 60% solution in toluene and, on cooling, solidified to a brittle mass. 

The single filament pull out test, sometimes called the microdebond test, has received attention for some years as a way to assess the adhesion between fibers and matrices in fiber composite [90,91]. It provides a direct measure of interfacial adhesion and can be used with both brittle and ductile matrix resins. 

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