Key Engineering Materials Vols. 378-379

Abstract: Fatigue damage accumulation in steel structures under random loading has been studied in a number of investigations at the Technical University of Denmark. The fatigue life of welded joints has been determined both experimentally and from a fracture mechanics analysis. In the experimental part of the investigation, fatigue test series with a total of 540 fatigue tests have been carried through on various types of welded plate test specimens and full-scale offshore tubular joints. The materials that have been used are either conventional structural steel or high-strength steel. The fatigue tests and the fracture mechanics analyses have been carried out using load histories, which are realistic in relation to the types of structures studied, i.e. primarily bridges, offshore structures and chimneys. In general, the test series carried through show a significant difference between constant amplitude and variable amplitude fatigue test results. Both the fracture mechanics analysis and the fatigue test results indicate that Miner’s rule, which is normally used in the design against fatigue in steel structures, may give results, which are unconservative, and that the validity of the results obtained from Miner’s rule will depend on the distribution of the load history in tension and compression.

Abstract: The cyclic hardening–softening response, the cyclic stress–strain curve and the substructure evolution of a high nitrogen duplex stainless steel S32750 have been evaluated and the results compared with reference to low and medium nitrogen duplex stainless steels, S32205 and S32900 grades, respectively. The beneficial effects of nitrogen on the cyclic properties of most modern alloys have been analyzed in terms of the flow stress components, i.e. the internal and the effective stress.

Abstract: Fatigue properties of mild steel are investigated under cyclic tension-compression and cyclic torsion loading using ultrasonic fatigue testing equipment and cycling frequency of approx. 20 kHz. Both S-N curves show a distinct change of slope at about 107 cycles, and endurance limits determined at 107 and 109 cycles differ by less than their respective standard deviations. Endurance limit shear stress determined for cyclic torsion loading is about 60% of the tension-compression endurance limit stress, and the slopes of the S-N curves are comparable. Non-propagating cracks could be found in specimens, which did not fail within 109 cycles in torsion loading endurance tests. The endurance limit can be understood as maximum stress amplitude, where possibly formed small cracks do not propagate to failure.

Abstract: This paper reports on the fatigue performance of an ultrafine-grained (UFG) interstitialfree (IF) steel deformed at various mean stress levels. The UFG microstructure was achieved using equal channel angular extrusion processing at room temperature (RT) and along an “efficient” route, giving way to the formation of high angle grain boundaries (HAGBs) with a high volume fraction. The current study not only confirms the previous finding that a high volume fraction of HAGBs promotes cyclic stability, but also inquires into the role of mean stress level on the cyclic stability. It is shown that the UFG IF steel exhibits a stable cyclic deformation response in the lowcycle fatigue regime within the medium applied mean stress range of -75 to 75 MPa. The corresponding fatigue lives can still be predicted with the Smith-Watson-Topper approach within this range. Furthermore, the present study demonstrates that the evolution of mean strains with cyclic deformation can be linked to the evolution of mean stresses in strain-controlled loading.

Abstract: The strength and fatigue crack growth resistance (FCGR) of ferritic, pearlitic and austenitic nodular cast irons (NCIs), recommended to be used in structural components working at low (up to –60 °C), cryogenic (below –60 °C) and elevated (450 °C) temperatures, are analysed. The influence of the manganese content and the strain-induced martensite transformation on FCGR of the austenitic NCIs is shown. An admissible phosphorus content for the optimum wear resistance and FCGR is determined. The strength and FCGR of the NCIs and corresponding rolled steels are compared.

Abstract: Gigacycle fatigue behavior in high-strength steels tested under rotary bending fatigue was summarized in this paper. Characteristic of the very high cycle fatigue is to be caused the transition of fracture mode from surface-induced fracture to subsurface inclusion-induced one. In the vicinity of an inclusion at the origin of internal crack, granular-bright-facet (GBF) area was formed during extremely long fatigue cycles. It was pointed out that the formation of GBF area was an important factor for the control of the internal fatigue fracture in gigacycle regime. The GBF area revealed a very rough granular morphology compared with the area outside the GBF inside the fish-eye zone, and was related to the carbide distribution in the microstructure of the matrix. From the detailed observation of fracture surface and computer simulation by FRASTA method, the GBF area formation mechanism in a gigacycle fatigue regime was proposed as the ‘dispersive decohesion of spherical carbide model’.

Abstract: Cyclic behavior and life prediction of two tempered martensitic steels (AISI H11 and L6) are investigated under thermo-mechanical loading conditions. Two non isothermal constitutive models developed in the same framework of the thermodynamics of irreversible processes are introduced. The first one, in relation with the tempering state, considers the fatigue-ageing phenomena whereas the second one is intended to take into account more complex loading paths. This last non unified approach allows to define different strain mechanisms which can be related to microstructural considerations. The strain-stress parameters provided by both approaches can be introduced into a lifetime model which is based on continuum damage mechanics.

Abstract: The paper analyzes the role of microstructure on the low cycle fatigue behaviour of duplex stainless steels. The alloys are investigated in their as received condition and after ageing at 475°C. The fatigue resistance and the cyclic accommodation of these DSS are strongly controlled by the volume fraction of α and γ "phases which is related to the chemical composition in particular nitrogen. It is shown that DSS with a high fraction of austenite present a good combination of fatigue resistance and cyclic softening especially in the aged condition. The mutual interaction between ferrite and austenite referred to load transfer is beneficial for increasing the fatigue resistance. Alloying with nitrogen appears to be a promising way to master an optimised microstructure leading to high mechanical resistant DSS.

Abstract: The present paper tries to summarize the relationship between microstructure, extrinsic mechanisms and fatigue crack propagation resistance of α+β and β titanium alloys. Emphasis is placed on microstructural parameters, which can be varied by processing, and their effects on the material inherent fracture properties, governing the resistance against microcrack propagation. Moreover, the resistance against macrocracks as well as small cracks in the presence of notch plasticity has been discussed on the basis of secondary extrinsic mechanics such as crack front geometry, crack bridging and crack closure.