Pre-notched samples

In the following sections the fatigue lifetime results for fom polymers will be described. In aU cases, the samples were pre-notched at knit lines by using doublegated tensile bar injection molded samples. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

Before use, all the polymer pellets were dried at Ta +20 K for at least 48 h. Then, sheets of controlled thickness B were firstly compression molded under vacuum at Ta + 50 K and then cooled down slowly through the glass transition region. Next, samples of dimensions suitable for the SENB tests were cut from the sheets with a diamond saw, machined to produce the notch, and annealed once more at Ta +20 K for 72 h. The purpose of this thermal treatment is double first, it allows the elimination of the residual stresses induced by sample pressing and machining and secondly, it permits removal of eventual moisture which is known to affect dramatically the SAPA mechanical properties [12]. The pre-crack was produced in the samples just before testing. 

Notched Izod specimens were cut and notched from the above materials using a diamond-head cutter, following the ASTM D 256 standard (0.254 mm radius, 2.0 mm notch depth). The specimens for Charpy instrumented testing were also diamond cut from the same materials, using the same type of notch. Every sample was run at an ambient temperature of 23 C with no pre-conditioning, except for letting the samples equilibrate with the humidity of the room (50% relative humidity) for at least one day prior to testing. 

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