FATIGUE STUDIES ON CARBON STEEL TUBES UNDER MULTIAXIAL LOADING

National Metallurgical Laboratory, Jamshedpur 831 001, India ABSTRACT The tests investigations have been carried out in the area of “Multiaxial Fatigue” with an objective to improve the fatigue damage assessment procedure and design rules. Fatigue tests were conducted on tubular specimens of SA333Gr.6 material under combined axial-torsion in-phase/ out-of-phase load combinations with triangular, sine and trapezoidal load waveforms. A software tool has been developed for the evaluation of multiaxial fatigue damage parameter for analyzing the tests data using invariant based fatigue model of ASME Sec.III code. The fatigue crack initiation life was predicted using the best fit axial fatigue life curve (without use of safety factors). The analyses for fatigue crack initiation life assessment have also been carried out using critical plane based models. A critical plane based model has been proposed for the prediction of fatigue life for tests carried out on tube specimens. INTRODUCTION The loaded mechanical components such as piping, vessels etc. are subjected to periodic cyclic loading during its normal operation, as well as during the design basis accident events and fail due to the fatigue cycling. Generally the fatigue damage is evaluated according to the design codes, which use fatigue design curves. The fatigue design curves are derived from material’s uniaxial fatigue tests. However, due to complex geometry and loadings on a component, the state of induced stresses / strains can be multiaxial and non-proportional. In view of above, systematic experimental and analytical studies have been carried on specimens made of low carbon manganese steel conforming to ASME specification of SA-333 Gr.6. The material specifications of this steel are same as used in Indian Pressurized Heavy Water Reactor’s (PHWR) Primary Heat Transport (PHT) piping. Arora P. et. al. (2011) have conducted various types of tests such as monotonic tensile, tension-compression that is pure axial fatigue tests on solid specimens, pure torsion fatigue tests on tube specimen, in-phase axial-torsion fatigue tests on tube specimens and out of phase axial-torsion fatigue tests on tube specimens. All these tests were conducted at room temperature and in air environment with triangular load waveform. The results of these tests were investigated in details, to understand and model the material’s fatigue damage under multiaxial and non-proportional loading condition. Many models based on critical plane theory have been studied in detail. These models have used different definitions of the critical plane and the Fatigue Damage Parameter (FDP) on the critical plane. The FDP in these models is based on the parameters such as stresses, strains, stress/ strain invariants or the strain energy (that is the multiple of stress and strain components). Recently, Jiang Y. (2007) has made an attempt to combine strain energy and critical plane approaches to develop a parameter which is representative of strain energy on the critical plane. In the past few years, many investigations on multiaxial fatigue have been reported in literature. The energy criterion has been studied by Ellyin (1984, 1986) in detail for life predictions. McDiarmid (1994)

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