Cost-Effective Precracking of Charpy V-Notch Specimens for Fracture-Toughness Testing Using a Piezoelectric Actuator

What is it about?

History proved that the presence of cracks in structural components can lead to catastrophic failure. In 1954 for example, the Havilland Comet, the world's first jet airliner, broke apart in mid-flight. The riveting of the square windows created cracks in the plane's fuselage, which eventually led to fatigue failure under the enormous take-off and landing stresses. Because cracks in structural components can lead to disaster with severe environmental and economic impact, injuries and deadly accidents, the prevention, detection and monitoring of cracks is taken very seriously and integrated in many safety regulations and inspections. Boeing for example recently reported to have found 'cracks' in Dreamliner planes still in production. Two Belgian nuclear power plants (Doel 3 and Tihange 2) were shut down after the discovery of cracks in the reactor pressure vessel by a new ultrasonic measurement technique. The property which describes the ability of a material containing a crack to resist further fracture is called 'fracture toughness'. Test methods, procedures and guidelines for the determination of fracture toughness of metallic materials are covered by several international standards. Experience has shown that fatigue cracking is the only method to reproducibly obtain a sharp, narrow notch that will simulate a natural crack well enough to provide satisfactory fracture toughness results. Hence, all fracture toughness specimens have to be precracked in fatigue. To avoid biasing of the test results, it is essential to keep the maximum stress intensity factor during fatigue cracking well below the material fracture toughness measured during the subsequent test. Therefore, precracking requirements are included in the standards (BS 7448, ASTM E1820, ASTM E1921, ISO 12135, etc.). Fatigue cracking is most often performed on a servo-hydraulic tensile test bench. Despite the fairly low operating frequency of about 30 Hz and the high power consumption of the hydraulic power unit - and hence high operating cost - it is the instrument of choice in many mechanical testing laboratories. For the opposite reasons, the precracking cost is more favourable on a fatigue resonant machine but such equipment is not always available in the laboratory. The evolution to more stringent precracking requirements, lower stress intensities, shorter finish sharpening lengths and the differences in precracking procedures between standards demand a flexible and accurate process control. This and the large amount of fracture toughness tests annually performed at SCK•CEN was the motivation to search for novel, cost-effective precracking techniques. The high operating frequency, compact size, fairly low cost, reliable operation, low power consumption and low maintenance made the piezo-electric actuator an excellent candidate for this purpose. The use of piezo-electric actuators for material testing is not new. The first ultrasonic fatigue testing machine was constructed by Mason in 1950. Piezo-electric transducers are presently still the only way to obtain fatigue data in the gigacycle range in an acceptable time (hours instead of months) and at a lower cost compared to traditional fatigue procedures. Despite its long tradition in the field of material research, an in-depth search on the Internet (Google, Scholar Google, Web of Science, Espacenet) revealed that the use of a piezo-electric actuator for fatigue cracking fracture toughness specimens is not a common approach. A dedicated, fully automated precracking device for Charpy V-notch (CVN) specimens based on a piezo-electric actuator was developed at SCK•CEN. In the first section of this paper, the basic operation of the piezo-precracking device is explained. Subsequently, the validation of the device is addressed. The equipment was validated by fatigue cracking an extensive amount of specimens under different conditions. The obtained fatigue crack is verified against the requirements set by the standard. The level of control over the precracking process is assessed by statistical analysis of a complete batch. Then the cost of precracking a specimen on a servo-hydraulic tensile test bench, a resonant fatigue machine and the SCK•CEN Piezomatic is discussed. The installation of a second piezo-precracking device in a hot cell and its validation, the precracking of a significant number of neutron activated specimens, is treated in the last sections.

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