13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- reaching a maximum at a different time to the stress. • The cruise condition has a relatively uniform stress and temperature. • Additional local fluctuations in stress and temperature may occur depending upon manoeuvres carried out during the flight 2. Development of the Test Facility In the work presented here a new fatigue crack growth test facility has been developed and this is briefly described. There are three essential elements in the provision of thermo-mechanical fatigue crack growth testing: 1. an adaptable and stable mechanical load application system 2. a responsive heating system 3. a suitable crack monitoring system A standard servo-electric test frame was used for the load application system, because of the long term load stability and previous experience with this equipment. Appropriate calibration of the load cell [1] was undertaken, along with checking the load-train alignment [2]. Figure 2. The general test piece arrangement within the radiant lamp furnace A radiant lamp furnace was decided to be the most appropriate heating system for this application (rather than induction/RF heating) because of the need to obtain a uniform temperature profile around the cracked test piece. Thus a 12kW furnace, with 12 lamps (~250 mm long) vertically aligned around the test piece (Figure 2), was installed on the test frame. A reasonably conventional crack length monitoring approach was used - applying electrical DC potential difference measurements and then cross-calibrated against the physical crack length. The relatively simple thermal cycles required initially (Figure 3) were consistent with this method. A constant current (DC – 20A) source was used. 3. Test Technique Development There is currently no standard methodology for the measurement of fatigue crack growth rates under TMF conditions. However, a proprietary test method [3], based upon an ASTM standard [4], but using a square section corner crack test piece; has formed the basis of the new test technique. The second part of the test technique that is required here is the application of the desired thermal cycle (and mechanical load). A related code-of-practice [5] for strain controlled TMF testing has been used to standardise the method of calibrating the thermal cycle applied to the test piece.
RkJQdWJsaXNoZXIy MjM0NDE=