Bent strip test

Figure 35 Stress cracking—bent strip test.

In the bent strip test according to ISO 22088, Part 3 [272] the entire test region of the specimen is under a defined, constant peripheral fiber strain. The strip-shaped specimen is clamped on a forming plate with constant radius of curvature and exposed to flexural load. The peripheral fiber strain in the specimen depends on its thickness and the radius of curvature of the forming plate ... 

The bent strip test is especially appropriate for testing minimally relaxing thermoplastics as well as for testing finished parts, if strip-shaped, flat specimens can be... 

A modified version of the bent strip test is the Dow-Sdbel test (parabola method) for quick testing of brittle thermoplastics (typically polystyrene). Flat test specimens are clamped over a parabola-shaped stainless steel form. This causes a continuous change in peripheral liber strain in the clamped specimens. As a measure for stress cracking sensitivity, the peripheral fiber strain is recorded up to which stress cracking has proceeded after a defined time [268]. 

Terpolymers of chlorotrifluoroethylene and ethylene were developed ] that were stress crack-resistant. The inventors in this case incorporated minor amounts (0.1-10 mole percent) of 3,3,3-trifluoro-2-trifluo-romethyl propene [(CF3)2C=CH2] into copolymers of 40 to 60 mole percent of ethylene with 40-60 mole percent of chlorotrifluoroethylene. The resulting terpolymers had improved stress crack-resistance relative to the copolymers of CTFE and Et. Stress crack-resistance was measured by ASTM D2951 (mandrel wrap test) at 180 or 200°C. Another method is called the bent strip test in which a 6.25 cm x 6.25 cm x 1.5 mm strip of the copolymer film is bent over a 6.25 mm diameter bar while the free ends are clamped together. A load of 1,150 g is attached to the clamp (hung from the strip ends) and the entire assembly is placed in an oven at 180 or 200°C. The time required for the film strip to break is a measure of resistance to stress cracking. 

Polymer Composition, mole % Melt Indexer DSC Melting Point, °C (20°C/min heating rate) Bent Strip Test Time, minntes to break at ... 

Vestoran 1900 Specimen Bent strip test, 3.5% outer fiber strain... 

Vestoran 1900 GF20 Filler 20% Glass fiber Specimen Bent strip test, 2.7% outer... 

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