TY - JOUR A1 - Schempp, Philipp A1 - Cross, C.E. A1 - Häcker, Ralf A1 - Pittner, Andreas A1 - Rethmeier, Michael T1 - Influence of grain size on mechanical properties of aluminium GTA weld metal JF - Welding in the world N2 - Grain refinement is an important possibility to enhance the mechanical properties such as strength, ductility and toughness of aluminium weld metal. In this study, grain refinement was achieved through the addition of commercial grain refiner Al Ti5B1 to gas tungsten arc weld metal of the aluminium alloys 1050A (Al 99.5) and 5083 (Al Mg4.5Mn0.7). The grain refiner additions led to a significant reduction of the weld metal mean grain size (Alloy 1050A, 86 %; Alloy 5083, 44 %) with a change in grain shape from columnar to equiaxed. Tensile tests showed for Alloy 5083 that the weld metal's ductility can be increased through grain refinement. No improvement in weld metal strength (i.e. yield strength and ultimate tensile strength) was observed. Furthermore, tear tests with notched specimens revealed that the resistance against initiation and propagation of cracks in the weld metal can be enhanced through grain refinement. The toughness was observed to increase clearly by grain refinement in weld metal of commercial pure Al (Alloy 1050A). In Alloy 5083 weld metal, the toughness was not improved through grain refinement, likely because of a semi-continuous network of brittle intermetallic phases that facilitate crack propagation. KW - GTA welding KW - Aluminium KW - Grain size KW - Tensile tests KW - Dynamic fracture tests PY - 2013 DO - https://doi.org/10.1007/s40194-013-0026-6 SN - 0043-2288 SN - 1878-6669 VL - 57 IS - 3 SP - 293 EP - 304 PB - Springer CY - Oxford AN - OPUS4-28065 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Klinger, Christian A1 - Mehdianpour, Milad A1 - Klingbeil, Dietmar A1 - Bettge, Dirk A1 - Häcker, Ralf A1 - Baer, Wolfram T1 - Failure analysis on collapsed towers of overhead electrical lines in the region Münsterland (Germany) 2005 JF - Engineering failure analysis N2 - End of November 2005 strong south-west wind and heavy snowfall were predominant in the region Münsterland, north-western part of Germany. This led to accretion of a considerable quantity of wet snow to overhead electrical lines in form of snow rolls on the conductors. Eighty-two transmission towers failed catastrophically, most of them by buckling, however some by brittle fracture. As a consequence nearly 250,000 people have been cut off from electrical power supply for several days with major media attention. This paper describes the forensic analysis in order to investigate the failure cause. Therefore extensive materials investigations, mechanical testing of original components and specimens thereof, estimations for the real wind and snow loads and their combinations, structural analyses as well as detailed evaluations on the basis of previous investigations, literature and regulations were conducted. It was revealed that some of the examined components were manufactured from Thomas steel which was partially in embrittled condition. The investigated towers fulfilled the design codes valid at the time of erection. However the present line loads of the wet snow rolls on the conductors exceeded by far the ones given in the design codes valid at that time. The load case leading to failure was reconstructed by the derived positions of loads mainly caused by unequal and asymmetric distribution of snow rolls on left and right electrical system. The loads and corresponding stresses acting on the structure before failure were estimated. By comparison with the fracture forces from mechanical testing of original members of the collapsed tower the component that primarily failed was localised. The primary fracture occurred on a diagonal member under tension made of Thomas steel which was weakened by embrittlement. The failure cause was a combination of heavy weather conditions (storm, approx. 0 °C and wet snowfall leading to heavy snow rolls on conductors), asymmetric loading conditions and the usage of Thomas steel which was partially embrittled. Finally, recommendations for avoiding future failures are given. KW - Transmission towers KW - Thomas steel KW - Embrittlement KW - Snow loads KW - Ice loads PY - 2011 DO - https://doi.org/10.1016/j.engfailanal.2011.07.004 SN - 1350-6307 SN - 1873-1961 VL - 18 IS - 7 SP - 1873 EP - 1883 PB - Elsevier Science Publ. CY - Oxford AN - OPUS4-24517 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Häcker, Ralf T1 - Failure of a pressure vessel for rail transport of fluid carbon dioxide JF - Engineering failure analysis N2 - In 1976, a pressure vessel of a tank wagon burst during gravity shunting. The pressure vessel was filled with cold fluid carbon dioxide and failed by cleavage fracture explosively. Big parts of the vessel were flung up to 300 m, a building was destroyed, and rails as well as the superstructure of the gravity shunting yard were damaged. The initiation sites of the fracture and the orientations of crack propagation were determined by means of the patterns on the fracture surfaces of the broken parts. The initiation sites were located in the heat-affected zone (HAZ) of welds, close to transitions of cross-sections and the starting and end points, respectively, of the welds. The primary initiation of crack propagation was almost certainly on the left side, close to the weld of the fixed bearing connecting the vessel with the carriage. The results of metallographic investigations indicated the development of cold weld cracks during welding. Mechanical behavior assessments on specimens extracted from the failed vessel indicated sufficient strength and ductility in tensile loading. The values of the notched-bar impact energy or toughness were within the limits according to industry rules and standards, except for one metal sheet of the vessel that was welded to the left fixed bearing. During shunting, two tank wagons were inappropriately braked, and began to roll too quickly. One rail brake used for deceleration of the wagons caused impulsive noises and vibrations, resulting in high impulsive loading of the first wagon. The cold weld cracks in the heat-affected zones near the fixed bearing of the pressure vessel were initiation points for unstable crack growth. As the result of very low notched-bar impact toughness, crack propagation was not arrested and unstable cleavage fracture occurred. Due to a second similar accident, happened one year before, a program was started to assess characteristic values of pressure vessel materials and the influence of weld parameters by the manufacturer, the inspecting authority, and BAM. The scientific findings led to modifications of industry rules and standards with regard to using ISO-V specimens and to higher required values for notched-bar impact energy for fine grain structural steels. KW - Transport failure KW - Pressure vessel KW - Notched-bar impact energy KW - Toughness PY - 2014 UR - http://www.sciencedirect.com/science/article/pii/S1350630713003889 DO - https://doi.org/10.1016/j.engfailanal.2013.12.006 SN - 1350-6307 SN - 1873-1961 VL - 43 IS - Special Issue 'A Tribute to Prof. A. Martens' SP - 92 EP - 106 PB - Elsevier Science Publ. CY - Oxford AN - OPUS4-30101 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER - TY - JOUR A1 - Baer, Wolfram A1 - Wossidlo, Peter A1 - Abbasi, Behboud A1 - Cassau, Matthias A1 - Häcker, Ralf A1 - Kossert, Reimund T1 - Large scale testing and statistical analysis of dynamic fracture toughness of ductile cast iron JF - Engineering fracture mechanics KW - Dynamic fracture toughness KW - Ductile cast iron KW - Large scale test KW - Statistical analysis PY - 2009 DO - https://doi.org/10.1016/j.engfracmech.2009.01.005 SN - 0013-7944 SN - 1873-7315 VL - 76 IS - 8 SP - 1024 EP - 1036 PB - Elsevier Science CY - Kidlington AN - OPUS4-19351 LA - eng AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany ER -