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Experimental and analytical investigation of the TMF-HCF lifetime behavior of two cast iron alloys
(2017)
The combined loading Thermomechanical Fatigue (TMF) with High Cycle Fatigue (HCF) has been experimentally investigated for two cast iron alloys. Both alloys contain globular graphite nodules but the first one has a ferritic structure while the second one has an austenitic crystal structure. In particular, the influences of the HCF frequency, of the HCF loading amplitude and of the location of the superposed HCF cycles have been investigated. It was observed that the HCF frequency has a limited impact on the fatigue life. On the other side, the HCF-strain amplitude has a highly non-linear influence on the fatigue life. The experimental results can be understood in terms of a fracture mechanics based damage mechanism: Cracks quickly initiate due to the TMF loading and the growth of the cracks up to a few mm controls the fatigue life. If HCF-loading cycles are superposed, cyclic crack propagation dramatically accelerates after a threshold has been reached. This threshold is regarded as controlling the fatigue life reduction. The previous ideas have been expressed in a model that can be very simply applied to estimate the fatigue life reduction ratio due to the superposed HCF cycles. It only contains two adjustable parameters and can be combined with any TMF life assessment model.
TMF tests were carried out on EN-GJSA-XNiSiCr35-5-2 at constant minimum temperature (400 °C) and varying maximum temperatures (Tmax = 700 °C, 800 °C, 900 °C) with hold times of 180 s at Tmax and two phase angles (in-phase (IP), 180° out-of-phase (OP)). The results showed a comparable strength under OP- and IP-TMF loading. At Tmax = 700 °C and 900 °C, the lifetime in IP-tests was slightly longer than that of OP-tests, while it is vice versa at Tmax = 800 °C. The IP-tests at Tmax = 900 °C showed a similar lifetime as OP-tests at Tmax = 700 °C and 800 °C, which was unexpected for such a high testing temperature. All IP-tests at Tmax = 900 °C showed a continuous cyclic softening from the beginning on, which was different from all other testing conditions. Complementary metallographic investigations indicated that under this test condition, intergranular creep damage is present in the volume of the test pieces.
The fatigue behaviour of cast iron is usually investigated on machined specimens. Components of cast iron, however, have a casting skin; therefore the investigation of the influence of the casting skin on the lifetime is of interest. To study this influence isothermal fatigue tests were carried out on heat-resisting spheroidal graphite cast iron EN GJS SiMo 4.05 in 4–point-bending set-up at 400 °C. Specimens with and without casting skin were investigated comparatively. The number of cycles to failure was significantly lower for specimens with casting skin. Metallographic investigations underline the reduction of lifetime caused by casting skin.
Cast iron arch bridges were built in the late 18th and early 19th century across Europe. After the famous Iron Bridge in the Ironbridge Gorge in Coalbrookdale in England, being now part of the UNESCO world heritage, many cast iron bridges followed and substituted timber bridges. A series of cast iron bridges has been built in central Europe, in Laasan, Breslau, (now Wrocław), in Berlin, Potsdam, Bad Muskau and Schwerin. The paper gives an overview about the current state of some remaining cast iron arch bridges, typical damage cases, material properties and rehabilitation. Most of the old cast iron arch bridges serve as pedestrian bridges. Some of these bridges have been strengthened to increase the traffic load. Different rehabilitation measures as substitution of parts of the bridges and strengthening using Carbon Reinforced Polymers (CFRP) are discussed.
Experimental and analytical investigation of the TMF-HCF lifetime behavior of two cast iron alloys
(2016)
The superposition of small amplitude, high frequent loading cycles (HCF) to the slow, large amplitude, TMF loading cycles can significantly reduce the TMF life, i.e. the number of TMF blocks until failure. In this work, the combined TMF-HCF loading has been experimentally investigated for two cast iron alloys. Both alloys contain globular graphite nodules but the first one has a ferritic while the second one has an austenitic crystal structure. In particular, the influence of the HCF frequency, of the HCF loading amplitude and of the location of the superposed HCF cycles has been investigated. It was observed that the HCF frequency has a limited impact on the TMF fatigue life. In other words, the number of superposed HCF-cycles has only a slight influence on the TMF fatigue life, which contradicts the linear damage accumulation rule concept. On the other side, the HCF-strain amplitude has a highly non-linear influence on the TMF fatigue life.
The experimental results can be understood in terms of a fracture mechanics based damage mechanism [1]: Cracks readily initiate due to the TMF loading and the duration of the growth of the cracks up to a few mm controls the fatigue life. If HCF-loading cycles are superposed, cyclic crack propagation dramatically accelerates at some stage. This stage is related to the existence of a threshold for crack growth under pure HCF-conditions and largely controls the fatigue life of the combined loading.
The previous ideas have been expressed in a model that can be very simply applied to provide the fatigue life reduction factor due to the superposed HCF cycles. It only contains two additional adjustable parameters and can be combined with any TMF model.