Tensile strength ASTM D5766 test method measures the open-hole tensile strength of composite laminates. These multidirectional polymer matrix composite laminates are reinforced by high-modulus fibres with a centrally located hole. The hole maintains stable stress concentration. This test is useful for calculating the ultimate strength of the gross cross-sectional area excluding the hole. ASTM D5766 test is mainly used in the aerospace industry for developing notched design allowable strengths.
This test is helpful in generating data for those final application products that require fastener holes or products simulating a defect in the material part. Test composite materials may be continuous fibre or discontinuous fibre-reinforced. Sometimes the physical properties of materials might vary at ambient temperature so simulation is done for the end-use environment.
The ASTM D5766 test samples are put at a given grip separation in a universal tester holder and pulled until failure. The test samples and test protocol for composite materials are as defined by ASTM D3039. For standard test samples, a normal test speed is kept at 2 mm/min. Testing in more than one direction may be required, depending on the reinforcement and type of the sample.
Elevated or reduced temperature test procedure:
The Universal Test machine is equipped with a thermal chamber. This chamber allows traveling across the top and bottom of the chamber on the foundation and the crosshead of the Universal Tester. Inside the chamber, standard measurement devices are mounted and the thermal monitored atmosphere is tested in the same way as it is at ambient temperature. The chamber has internal electric heaters to maintain high temperatures and external carbon dioxide is used as the coolant to lower temperatures.
Video 01: ASTM D5766 Open hole composite tensile test fixture
The most typical sample for ASTM D5766 is rectangular, 1.5 inches wide and 10 inches long, with a consistent cross-section. The hole preparation is specified by ASTM D5766. The end of the test sample may be bonded with alternative tabs for avoiding gripping damage.
Ultimate Open Hole Tensile Strength (OHT) is expressed in MPa (psi). It is equal to the maximum force carried by the test sample before failure N (lbf) divided by the gross cross-sectional area mm2 (in 2). The area is measured excluding the hole as stated by ASTM D3039 standard. Different types of laminate hole failure mode (failure type, failure area, or failure location) is reported.
Hence, ASTM D5766 is a standard test method for open-hole tensile strength of polymer matrix composite laminates.