Jonas Ringsberg

Life prediction of rolling contact fatigue crack initiation

Abstract A strategy developed for fatigue life prediction of rolling contact fatigue (RCF) crack initiation is presented. It combines elastic–plastic finite element (FE) analyses, multiaxial fatigue crack initiation models used together with the critical plane concept, fatigue damage summation calculations, and comparison of results from numerical analyses and experiments. The strategy presented is utilised and evaluated for two RCF examples: (i) a twin disc test, and (ii) a railway wheel–rail rolling contact. The results from both of the examples verify that the strategy and evaluation methodology presented can be used for fatigue life predictions of RCF crack initiation caused by low-cycle fatigue and ratchetting failure.

Prediction of fatigue crack initiation for rolling contact fatigue

In finite element (FE) simulations of a twin disc test of a wheel/rail contact, fatigue crack initiation criteria for elastic shakedown, plastic shakedown and ratchetting material responses were evaluated for a pearlitic rail steel BS11 normal grade. The Chaboche material model for nonlinear isotropic and kinematic hardening was used in the FE simulations. The ratchetting material response results were compared with a constitutive ratchetting model, and there was good agreement with respect to the number of cycles to crack initiation and shear strain distribution below the contact surface. In addition, angles for critical planes for crack initiation were calculated for both plastic shakedown and ratchetting material responses. Results from simulations with the ratchetting model at constant contact pressures and varying friction coefficient showed asymptotic values of the friction coefficient at which crack initiation due to ratchetting will not occur.

Rolling contact fatigue analysis of rails inculding numerical simulations of the rail manufacturing process and repeated wheel-rail contact loads

The present work is an investigation on how an initially introduced residual stress-state affects the service life of a rail, i.e. the time to fatigue crack initiation. The finite element (FE) method was used to make two-dimensional thermo-mechanical analyses of the rail cooling and roller straightening processes. The results became the initial conditions in a three-dimensional elastic-plastic rail model; the model is part of an FE tool developed for rolling contact fatigue (RCF) analysis of rails. The results from this tool were analysed for fatigue, for eight wheel passages, according to a method which incorporates a critical plane approach that evaluates fatigue damage on a cycle-by-cycle basis. A heavy-haul (30 tonne) train traffic situation on the Iron-ore Line in Sweden was studied with respect to subsurface fatigue crack initiation in straight track. Three examples using the rail model in the FE tool were assessed: (a) an initially stress-free rail, (b) a measured residual stress field in a newly manufactured rail, and (c) a calculated residual stress field from the cooling and roller straightening analyses. The results from the thermo-mechanical FE analyses of the rail manufacturing process showed tensile residual stresses in the longitudinal direction of the rail; this was validated with experimental measurements on newly manufactured rails. The FE tool and fatigue calculations revealed only small differences in results for the three examples. It was concluded that, because of the very high axle load in the present traffic situation, the local wheel-rail contact loads governed the fatigue life to crack initiation. Additional FE tool calculations were made to show the axle load at which rail manufacturing stresses reduce the fatigue life to crack initiation.

Rolling contact fatigue of rails—finite element modelling of residual stresses, strains and crack initiation

Abstract Numerical models that can be used to evaluate crack initiation in rails owing to rolling contact loads are of great value. Very few models on rail fatigue assessments exist in the literature that consider the non-linear stress and strain response in the rail caused by both global dynamic track response and local wheel-rail contact loads. In the present investigation, such a tool has been developed using finite element (FE) models. The tool can be used to calculate governing fatigue conditions such as residual stresses, plastic strains and orientation of crack planes for crack initiation. The ability to perform parameter studies of, for example, track properties, different materials and load cases is inherent in the tool. The tool was used in a case study of commuter train traffic at a Swedish test site. In the FE simulations, a material model with non-linear isotropic and kinematic hardening, which was able to consider ratchetting behaviour, was used. Ratchetting material response near the railhead surface was found. A criterion for fatigue crack initiation caused by ratchetting and a criterion for fatigue crack initiation caused by low-cycle fatigue (LCF) were compared. The LCF criterion showed the lowest number of cycles to crack initiation, which, given the models used, indicates that cracks will initiate owing to LCF rather than ratchetting. Good agreement for the direction of surface cracks between the rail track at the test site and the results from the simulations was achieved.

Fatigue life and crack closure in specimens subjected to variable amplitude loads under plane strain conditions

Abstract The propagation of fatigue cracks in specimens subjected to variable amplitude loading under plane strain conditions was investigated experimentally and numerically, to find the influence of the variable amplitude loading on the stabilised crack closure level. Experiments on four-point-bend specimens with a Gurney block load scheme, showed that the crack closure level depends on the crack length but not on the stress range of the fluctuations. Numerical simulations performed in the fatigue crack growth program FASTRAN-II showed good agreement with the experimental results. In addition, statistical uncertainty analyses performed on the fatigue life show that, for technical applications, the uncertainties in initial crack length and load levels have a greater influence on the uncertainty in fatigue life, than the fluctuation level of the load.

Characteristics of material, ship side structure response and ship survivability in ship collisions

Research on ship collision and grounding has taken giant steps during the last decade. One reason is that computer capacity has increased and, therefore, also the possibility to simulate various collision scenarios in a realistic way using more advanced and larger models. As a result, it has been possible to investigate in more detail the understanding of structural integrity, characteristics and failure phenomena that interact during, for example, a collision. This article summarises research experiences from a research group that has been working with ship collision safety, using both experiments and numerical simulations by finite element (FE) analysis. Results are presented from tensile and forming limit tests, followed by FE analyses of these with the objective of predicting material rupture using appropriate constitutive material models and damage criteria. An example of an innovative design of a side-shell structure that is considered to be more intrusion-tolerant than most side-shell structures used today is demonstrated. Finally, results from a research project which has a holistic approach on the assessment of survivability of a struck ship are presented. In the project, a methodology has been developed which combines structural analysis and damage stability analysis followed by risk analysis.

Fatigue routing of container ships–assessment of contributions to fatigue damage from wave-induced torsion and horizontal and vertical bending

The traditional method for assessing fatigue damage of ship structures assumes moderate wave amplitudes and linear responses. This method can be questioned when applied to container ships which are characterised by large deck openings that cause low torsion rigidity of the structure. Depending on the heading of the vessel in relation to the wave encounter direction, container ships can therefore be sensitive to, for example, oblique waves. In the current investigation, SESAM software with the three-dimensional hydrodynamic code WASIM is used to simulate a 4400 TEU container ship operating in the North Atlantic. Non-linear wave loads are utilised for the direct calculation of stress histories under severe sea states using the finite element software in SESAM. A method for the separation of normal stress components in an arbitrary cross section and location of the ship is proposed: warping stress from wave-induced torsion and stress components from horizontal and vertical bending. The strain (stress) response calculated using the models in one location is verified against full-scale measurements made on a similar type of container vessel in the same location. A case study is presented for fatigue damage assessment in two cross sections of the container ship. The contributions to fatigue damage from wave-induced torsion and horizontal and vertical bending are calculated and presented in a fatigue damage routing tool by means of polar diagrams, which includes the effects of heading and ship speed on fatigue damage.

Survivability analysis of a struck ship with damage opening - influence from model and material properties uncertainties

The conditions for damage stability and survivability of a ship struck in a collision in arbitrary sea-state are, from a structural point of view, determined by the size and shape of the damage opening in its side-shell. In the current investigation, explicit finite element analyses (FEA) are presented of a ship-to-ship collision scenario in which the damage opening of a struck ship is calculated for a selection of damage degradation models and realistic material properties, here referred to as model and material properties uncertainties. The model uncertainty is considered as a possible (user-related) insecurity in the selection of the most appropriate damage criterion for the analysis; the shear failure and the forming limit diagram (FLD) criteria were compared in the current investigation. The uncertainty in material properties is accounted for in the constitutive material model description and the material parameters used in the two damage criteria. The size and shape of the damage openings predicted by the FEA are used in damage stability analyses in which the struck ship is subjected to wave motions in an arbitrary sea-state and flooding into the damage opening. The survivability of the struck ship is estimated for all of the damage opening cases. One of the main conclusions is that the high degree of accuracy that a researcher on structure analysis strives for has to be considered together with the natural variation of the sea-state that defines the characteristics in the following damage stability analysis. Consequently, by adoption of a holistic approach in which structural integrity and damage stability research are combined using a systematic parameter (sensitivity) and collision-scenario-based analysis, simplified models and criteria can be developed more efficiently and with higher precision. It will also be clearer which variables are the most important to focus on when analysing the survivability or risk for capsizing.

On mechanical interaction between steel and coating in stressed and strained exposed locations

Inspection of coating in ships shows that local coating failures occur before predicted service life of the coating. These failures often arise in spots where high stress-concentrations appear. Failure of ballast tank coatings in strained and stress-concentrated locations was investigated, and was believed to occur due to a weakness in the mechanical interaction system between steel and coating. In non-linear finite element analyses, the mechanical interaction between steel and one polymeric coating system was studied. The geometry, a notched specimen, was assumed located in ballast tanks with seawater present as corrosive media. The model of the notched specimen was protected with one coating system commonly used in ballast tanks. In the analyses, properties for unaged and artificially aged coating materials were studied. The model was subjected to simulated fatigue loading which resulted in local material failure due to reached limit of accumulated plastic strain. The results from the FE-analyses with unaged coating showed failure of the steel before the coating. In the analyses with aged coating material, coating failure occurred before the steel. From the FE-analyses it can be concluded that the mechanical interaction system between steel and coating, and thereby the service life of a coating, is strongly weakened dependent on the degree of ageing of the coating material.

A generic energy systems model for efficient ship design and operation

There is an environmentally and economically motivated need to reduce the fuel consumption and air emissions of ships. To achieve a reduction in energy consumption, the energy flow in the entire energy system of a ship must be analysed in both the component, or subsystem, level as well as in a holistic way to capture the interactions between the components. Of the currently available energy consumption monitoring and prediction methods or models, no single model or method can be used to assess the energy efficiency of an arbitrary vessel in both the early design phase and during operation. This study presents a new generic ship energy systems model that can be used for this purpose. This new model has two parts: one for the assessment of a ship’s energy consumption based on an ordinary static power prediction and one for advanced operational analysis, considering hydrodynamic and machinery systems effects. A Panamax tanker vessel was used as the case study vessel to prove the versatility of the model for five example simulations for the design and operation of ships. The examples include variations of the main dimensions, propeller design, engine layout and the operational profile on a North Atlantic route. From the results, different areas with a potential for energy savings were identified.