Tensile Strength (Open Hole) of Polymer Matrix Composite Laminates Testing

Open hole tensile testing provides valuable data for allowable strengths of composites assembled with fasteners.

Open Hole Tensile ASTM D5766 measures the force required to break a polymer composite laminate specimen with a centrally located hole. The hole allows for stress concentration and reduced net section while the test method calculates ultimate strength based on gross cross-sectional area, disregarding the hole. ASTM D5766 is commonly used in the aerospace industry as a practice to develop notched design allowable strengths. It is used to generate data where the final application of the product may require fastener holes or to simulate a flaw in a material component. The test is used for composite material forms (including tape or fabric) and limited to continuous fiber or discontinuous fiber reinforced composites with balances and symmetrical test direction. Since the physical properties of many materials can vary depending on ambient temperature, it is sometimes appropriate to test materials at temperatures that simulate the intended end use environment. 

ASTM D5766 test procedure:
Test specimens are placed in the grips of a universal tester at a specified grip separation and pulled until failure. The test specimens and test protocol are as called out in ASTM D3039 for Tensile Properties of Composite Materials. A typical test speed for standard test specimens is 2 mm/min. Depending upon the reinforcement and type, testing in more than one orientation may be necessary.

Elevated or reduced temperature test procedure:
A thermal chamber is installed on the universal test machine. The chamber is designed to allow the test mounts from the base and crosshead of the Universal tester to pass through the top and bottom of the chamber. Standard test fixtures are installed inside the chamber, and testing is conducted inside the controlled thermal environment the same as it would be at ambient temperature. The chamber has internal electric heaters for elevated temperatures and uses external carbon dioxide gas as a coolant for reduced temperatures.

Specimen size:
The most common specimen for ASTM D5766 has a constant rectangular cross section, 1 1/2 in wide and 10 in long. ASTM D5766 outlines hole preparation. Optional tabs may be bonded to the ends of the specimen to prevent gripping damage, if specified.

Data:
Ultimate Open Hole (notched) Tensile Strength (OHT) is reported in Mpa (psi) and is equal to the maximum force carried by the test specimen prior to failure N (lbf) divided by the gross cross sectional area mm2 (in 2). The area is calculated disregarding the hole and as outlined in ASTM D3039. Laminate failure mode (failure type, failure area or failure location) is reported. Failure must occur at the hole to be an acceptable failure.