FASTENERS AT LOW TEMPERATURES - CHARACTERIZATION AND METHODS FOR DESIGN

Bolted joints are a major part of wind energy plants. Due to climatic conditions, they are often exposed to temperatures far below the freezing point. Together with the multiaxial state of stress, which results from the notch effect of the thread, and possible dynamic overloads during operation, sufficient ductility of the material is needed. The state of the art method to investigate the ductile behavior of fasteners is the Charpy pendulum impact test with a V-notched specimen. According to international standard DIN EN ISO 898-1 [1] respectively ASTM F568M-07 [2], fasteners made of carbon steel and alloy steel with a body centered cubic lattice structure can be used for temperatures down to 223 K (-50 degrees C, -58 degrees F) as long as a minimum impact energy of 27 J at 253 K (-20 degrees C, -4 degrees F) is met. As there are several disadvantages in using this test method for fasteners, a detailed examination of existing test methods and design concepts is necessary to find alternatives to the Charpy pendulum impact test. Extensive quasi-static and dynamic material tests were conducted on fasteners with property classes 5.6, 10.9 and 12.9 in a temperature range between 203 K (-70 degrees C, -94 degrees F) and room temperature 293 K (20 degrees C, 68 degrees F). Both mechanical properties and the influence of different specimen geometries were evaluated. Analytical concepts for the description of the low temperature applicability of different steels were analyzed.