Comment on the paper entitled 'A new cumulative fatigue damage rule based on dynamic residual S-N curve and material memory concept' by Peng Z., Huang H., Zhou J. and Li Y. Published in Metals (2018; 8 (6): 456)

A

Design of a four-point bend test for ultra-low cycle fatigue of pipelines under inelastic bending

This master thesis is situated in the research domain dealing with the ductile failure of pipelines under extreme loading conditions. It is part of an umbrella research aiming to develop innovative experimental and computational methodologies to simulate fracture of steel structural elements under ultralow cycle fatigue. The focus of this study is on steel pipeline applications. The objective of this thesis is to design a large-scale four-point bend test setup to cyclically bend pipes. The feasibility of instrumentation will be evaluated using small scale test specimens. In this paper some ideas, constraints and opportunities for the design are considered, based on a literature review of several test setups for other applications. The design parameters have been calculated to compose the design windows and an initial overview of the possible instrumentation is given

Development of a continuum plasticity model for the commercial finite element code ABAQUS

The present work relates to the development of computational material models for sheet metal forming simulations. In this specific study, an implicit scheme with consistent Jacobian is used for integration of large deformation formulation and plane stress elements. As a privilege to the explicit scheme, the implicit integration scheme is unconditionally stable. The backward Euler method is used to update trial stress values lying outside the yield surface by correcting them back to the yield surface at every time increment. In this study, the implicit integration of isotropic hardening with the von Mises yield criterion is discussed in detail. In future work it will be implemented into the commercial finite element code ABAQUS by means of a user material subroutine

Towards better finite element modelling of elastic recovery in sheet metal forming of advanced high strength steel

The first part of this study discusses the influence of element type on parameters such as accuracy of the FE simulation, simulation time and convergence. Guidelines on optimal implementation of element types are proposed. It is shown that an inappropriate choice of element type results in difficulties in convergence of the simulation or gives rise to problems such as shear locking in elements. In the second part of this study a series of finite element simulations using the Hill’48 planar anisotropic yield criterion and a standard U-shape forming test based on the NUMISHEET’93 benchmark was performed. The effectiveness of different isotropic hardening laws and different contact models is investigated. The most appropriate hardening and contact definitions are defined from the viewpoint of optimal springback prediction. Finally, the influence of the orientation of sheet strips relative to the rolling direction on springback angles is evaluated

Testing methodologies for corrosion fatigue

Offshore constructions are subjected to cyclic loading conditions. This situation is combined with the corrosive nature of the surrounding environment. It is of actual concern whether the combined effect is more damaging or not than the superposition of each effect independently. This literature review first introduces the reader to corrosion fatigue. Thereafter a critical comparison of some typical lab-scale fatigue corrosion test setups is given. Special emphasis is devoted to the instrumentation of the setup. This is followed by a design criteria summary which will be used to design a new corrosion fatigue test set-up for evaluating the fatigue properties of steel components in sea water environment

Online fatigue crack growth monitoring with clip gauge and direct current potential drop

Fatigue is a well-known failure phenomenon which has been and still is extensively studied. Often structures are designed according to the safe-life principle so no crack initiation occurs. Nowadays there is a high emphasis on cost-efficiency, and one might rather opt for a fail-safe design. Therefore a certain amount of crack growth can be allowed in structures, but then a good knowledge of stresses and related crack growth rates is needed. To this end, extensive studies are done to obtain a material’s Paris law curve. Within the framework of research for offshore wind turbine constructions, tests were done to determine the crack growth rate of a high strength low alloy (HSLA) steel. A dedicated LabVIEW program was developed to be able to determine an entire Paris law curve with a single specimen, by controlling the stress intensity factor range (ΔK). The program is controlled by the readings of a clip gauge, which make it possible to plan the amount of crack growth per ΔK block and thus plan an entire test in advance. The potential drop technique was also applied in order to obtain the Paris law curve. Clip gauge results were compared with direct current potential drop monitoring. This comparison was done by means of an a/W-N diagram and the resulting Paris law curves. The results show a very good correlation between both methods and with the visual confirmation

Towards the understanding of variable amplitude fatigue

Fatigue life estimation is of high importance during the design stage of a machine or component. Basic fatigue calculations are made based on the use of an S-N curve. As far as constant amplitude loads are applied, this approach works well. However, most part of components in service are subjected to a variable amplitude load spectrum. In this case, linear approaches for fatigue life estimation can lead to over conservative results, which in other words means a heavier and more expensive machine. To further investigate the effect of (complex) service spectra (measured or statistically calculated), simpler load variations must be studied. This paper aims to show the general trend of these events and suggest the underlying physical phenomena behind load and interaction effects. As it will be highlighted, overloads are frequent in a spectrum and they are believed to be responsible for retardation effects. The plasticity induced crack closure mechanism is the most profound explanation for them to occur