Razor notches

There is a transition in the impact behaviour of both HIPS and ABS at low temperatures. At 20 °C, the entire fracture surface is stress-whitened, indicating a tou fracture which cannot be analysed by LEFM methods. At lower temperatures, however, whitening is confined to a small area near the base of the notch, and experiments have shown that this type of fracture can be described by LEFM. The point is illustrated in Fig. 14, which presents data obtained by Bucknall and Reid in Charpy tests on razor-notched HIPS specimens. At -25 °C, impact strength / is proportional to BWZ, as predicted by Eq. (15). At 40 °C, on the other hand, there... 

Small pieces from compression molded plaques were cut into thin strips of approximately 50mm x 8mm. A razor notch was placed on both edges along the center of the strips to help induce a straight fracture. The strip was held between the jaws of a pair of pliers and submerged into liquid nitrogen for approximately five minutes. A second set of pliers were cooled at the same time so that when the sample was removed, it could be quickly freeze fractured. A razor blade was used to... 

The 12 mm plaques were used to manufacture Reversed Charpy specimens, 12 mm wide and 70 mm long. These were razor-notched to leave a ligament. y = 1.5 mm and tested at 0°C, with an impact speed of 1 m/s on a span of 60 mm, and using the procedure developed by Hillmansen [5]. The absorbed energy U is integrated used to determine Wp using the equation ... 

We prepared a strip-type specimen (100 X 50 X 2 mm) from the rubber sheet of SBR filled with HAH carbon black (50 phr), vulcanized for 30 min at 155°C. On the specimen, a slit of different lengths (si = 30 mm, S2 = 20 mm, S3= 10 mm) parallel to the extension direction and a notch of different lengths (2 or 5 mm) at the center of the side surface of the specimen perpendicular to the extension direction were made by razor-cutting (see the inserted figure in Figure 18.14). The distance 8 between slits and between the slit Si and the tip of notch was 1 and 3 mm, respectively. The no-slit specimen means that it only includes a notch, without slits. 

For most of the reported results, a machined notch was made, then a sharp pre-crack formed with a fresh razor blade, using a falling weight apparatus specially designed to operate the blades in reproducible conditions. Quite a satisfactory sharp pre-crack is obtained, as shown in Fig. 10. 

Turn off the power supply and remove the plates containing the gel. Separate the plates to expose the gel and put a notch in one corner of the gel with a razor blade to allow you to orient the gel. Place the gel on a sheet of Whatman 3 MM filter paper. Wrap the gel in plastic wrap and place it in the film cassette. 

In each test, a precrack is first introduced ahead of the machined notch using a razor blade in order to minimize a possible effect of the plastic deformation ahead of the precrack. This initial precrack is then extended by approximately 2 mm under computer control by increasing the load ampli-... 

Fig. 3. Device for machining automatically a sharp notch with a razor blade.

A first notch of 250 micrometers radius at the tip was mill cut with a rotary cutter. In order to prevent heating while machining, specimens were cooled with fresh compressed air during cutting. A sharp notch was further introduced at the tip of the first notch with a razor blade. The displacement of the razor blade was controlled by a micrometric thrust so that slow and careful control of the blade advance could be monitored. Figure 3 shows the device used to machine the sharp notches automatically in order to improve reproducibility. 

Fig. 4. Observations of the notch tip obtained by (a) milling a slit in the material, (b) razor blade for PMMA and (c) razor blade for PC...

Elastic modulus and yield stress were measured by compression tests on 10x5x4 mm prisms cut out from hot pressed plates. The J-integral measurements (J-Aa curve) were performed from tensile tests on CT specimens of 48 x 40 x 4 mm size, with a closed loop controlled machine (Schenk-Trebel). The starting crack was made using a razor blade (notch radius < 50 pm) up to a depth ao of 10 mm. 

A sharp starting notch was machined by sliding a razor bade into the material to a relative depth, a/W, varying from 0.45 to 0.6. For testing, specimens were supported on two parallel rollers spaced 55 mm apart, and the load was applied by slowly releasing, via a pneumatic device a dead load ranging from 4 to 18.5 kg. The latter was connected to two upper rollers, which were symmetrically positioned over the notch 18.3 mm apart. 

The middle of the specimens is notched at both sides by inserting a razor blade (American safety single edged blades, thickness 1.0 mm). The razor blade is inserted by means of a mechanical testing machine at low speed (0.20 mm/min) to minimize the introduction of internal stresses in the specimen. For every notch a new razor blade is used to make sure that every notch has the same sharpness. This notching procedure is carried out according to the notch method used for polyethylene specimens, ASTM F1473 [15]. The notches (2.5 mm) reduce the cross-section of the specimen by 50% (from 40 mm to 20 mm ). 

An ISI model 60A scanning electron microscope was used to examine the morphology of the fracture surfaces. Both the neat resins and the composite laminates were notched at room temperature with a razor blade. The samples were then Immersed in liquid nitrogen and fractured in air Immediately after removal from liquid nitrogen. The neat resins were fractured by bending the samples with pliers and the laminates were fractured along the fiber by opening up the notched cracks with pliers. 

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