Unnotched samples

Tensile and flexural properties were studied with an Instron 4204 testing machine. Tensile tests were performed on the drawn strands at a test speed of 3 mm/ min, while three-point-bending tests (ISO 178) at a speed of 5 mm/min were applied to the injection molded specimens. Charpy impact strength was measured of the unnotched samples with a Zwick 5102 pendulum-type testing machine using a span of 70 mm. The specimens (4 X 10 X 112 mm) used for three-point-bending tests were also used for the impact tests. It should be noted that neither the tensile tests for the strands nor the impact tests were standard tests. The samples were conditioned for 88 h at 23°C (50% r.h,) before testing. 

The test method may specify either a notched sample or an unnotched sample. If a notched specimen is specified, the dimensions of the notch are also given. Figure 15.31 is an example of these dimensions for a metal sample. In the Charpy test the notched sample is struck from behind the notch. In the Izod test the notch is facing the hammer. The notch, called a stress raiser, concentrates the stresses applied by the impact load. It makes the material brittle and increases the elastic limit in the notch area. Without the notch, many materials will bend without fracture, and the capacity to absorb energy will not be accurately determined. 

The growth behavior of crazes in unnotched samples has been widely investigated. Figure 21 shows examples of the observed growth of surface crazes in terms of the... 

Under some circumstances, as when unnotched samples of PS are stressed from a maximum to a minimum value in the tensile mode or when notched samples are tested in a similar mode the smooth region near the fracture source show... 

Instrumented Charpy impact tests on unnotched samples are conducted to determine the energy absorbing capability and dynamic fracture behaviour of the C/SiC composites. The dynamic fracture toughness (ak) is calculated using the following equation ... 

A temperature increase of 140 °C was measured using a pyrometer (infrared detector) on an unnotched sample at high deformation rates (but which were much lower than the rates on the fracture surface) and with a large spot size (1 mm2). All these results clearly indicate that the layer next to the fracture plane, without cavities and with no matrix orientation, must have been warm and that it subsequently relaxed. Relaxation of a deformed blend takes place in the melt. If a melt is present in the fracture plane ahead of a crack, the crack will blunt. So melt blunting takes place in ductile blend samples tested at high loading rates. 

Especially at three-point bending impact loading the elastic modulus (dynamic modulus) is proportional to the slope of the linear part of the load - deflection diagram of the unnotched samples assuming that the material behaviour can be approximated to be linear-elastic-plastic in that case (for more information see [13Gre] and [OlGre]). 

The impact strength of the pure ENR and its hybrid composites was measured with a standard Izod impact testing machine (model IT 1.4, Fuel Research Instruments, Maharashtra, India) according to ASTM D 256. The measurements were done on unnotched samples (70x10x4 mm) with an impact speed of 1 m/s and energy of 1.4 joules. For each specimen, six measurements were recorded. 

Fatigue tests on unnotched samples have demonstrated that the fatigue strength (the stress corresponding to failure at a given number of cycles) is reduced by as much as 30% when nylon-6,6 is equilibrated at 50% rh (152) (see Fig. 34). In marked contrast, it has been found that fatigue growth rates in nylon-6,6 exhibit a pronoimced minimum at an absorbed moisture content of 2.6 wt% water, and... 

The fracture toughness of PEEK has been studied via fatigue tests of notched and unnotched samples and three-point bend tests using single-notched specimens. Typical fatigue behaviors of... 

Figure 3.13 Fatigue behavior of unreinforced injection molded PEEK specimens. Tests were done in transverse direction. (A) unnotched samples (B) notched samples.

Early studies of fatigue in polymers concentrate on stress cycling of unnotched samples, to produce S versus N plots similar to those that have proved so useful for characterising fatigue in metals (S being the maximum loading stress and N the number of cycles to failure). An example of this type of plot for PVC [160] is shown in Figure 13.46. A major... 

Stress cycling tests on unnotched samples do not readily distinguish between crack initiation and crack propagation. Further progress requires a similar approach to that adopted in fracture studies, namely the introduction of very sharp initial cracks in order to examine crack propagation utilising fracture mechanics concepts. 

Surface Finish and Modification. Previous examples involve essentially homogeneous polymer-diluent systems. Interesting effects with practical consequences also occur when unnotched samples are exposed to various environmental media (4,159). Because fatigue failure in many cases usually involves initiation at the surface, such effects are to be expected. 

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