Three-point fixture tests

The flexural properties characterize the strength, deformation and stiffness behaviour of plastics under quasi-static bend loading. For these tests commercial universal test systems equipped with three-point and four-point fixtures are used. The most important standard for the bend test of plastics is the ISO 178 Plastics - Determination of flexural properties, which favors the three point bend test (Fig. 4.39). The data collected include also values determined according DIN 53452 and DIN 53457 as well as ASTM D 790. The specimen of preference exhibits dimensions of 80 x 10 x 4 mm and can be produced directly by injection moulding or cutting the shoulders from multipurpose specimens. 

Figure 3-20. A three-point flexural test specimen bending fixture designed per ASTM D 790.

We investigated the data consistency further by conducting SEPB tests on the fragments of some broken SCF and SEPB specimens from billets D and C. The halves were tested on shorter 10 mm x 20 mm four-point or 16 mm span three-point fixtures. Table 5 shows the results. Most of the miniature specimen outcomes... 

We use a variant of flexural testing to measure a sample s heat distortion temperature. In this test, we place the sample in a three point bending fixture, as shown in Fig. 8.6 b), and apply a load sufficient to generate a standard stress within it. We then ramp the temperature of the sample at a fixed rate and note the temperature at which the beam deflects by a specified amount. This test is very useful when selecting polymers for engineering applications that are used under severe conditions, such as under the hoods of automobiles or as gears in many small appliances or inside power tools where heat tends to accumulate. 

Small samples were stressed in three-point flexure while being viewed with scanning electron or optical microscopes. The fracture behavior was recorded by video and correlated with acoustic emission measurements. The samples were oriented in the flexure-test fixture with the plane of the cloth plies perpendicular to the force direction therefore, the tensile and compressive stresses are primarily in the plane of the cloth plies where the fibers resist deformation. 

Experimentally, TMA consists of an analytical train that allows precise measurement of position and can be calibrated against known standards. A temperature control system of a furnace, heat sink, and temperature-measuring device (most commonly a thermocouple) surrounds the samples. Fixtures to hold the sample during the run are normally made out of quartz because of its low CTE, although ceramics and invar steels may also be used. Fixtures are commercially available for expansion, three-point bending or flexure, parallel plate, and penetration tests (Fig. 4). 

The dynstat impact test is performed on notched and unnotched small specimens and serves for the evaluation of toughness behaviour if no specimen according to ISO 179-1 or ISO 180 can be prepared. The total length of these specimens is only 15 mm according to DIN 53435. For the tests a force-fitted three-point bend fixture is used. 

The tests were conducted for each material at room temperature and under displacement control at a cross-head speed of 1 mm/ min using a three-point bend fixture of 72 mm loading span. An electromechanical testing machine (MTS Alliance RF/100) with a load cell of 5 kN was utilized. 

For tensile tests the Instron 1125 tester is fitted with pneumatic-hydraulic grips which can be opened and closed by remote control. We have installed microswitches to indicate when a specimen is in place, and pneumatic actuators to eject the specimens from the grips after each test. For flexure tests either a three or a four-point compression-cell flexure fixture is substituted for the grips. We have attached air jets to the lower supports to eject the specimens. 

Fracture testing was in laboratory conditions using semi to fully-articulating flexure fixtures. Four-point flexure fixtures with 20 mm and 40 mm inner and outer spans, respectively were used to test the full length SCF, SEPB and CNB test specimens. In some instances, broken halves of test specimens were retested in three- and four-point flexure per the SEPB method. 

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