Jani Romanoff

Numerical and Experimental Investigation on the Collision Resistance of the X-core Structure

This paper analyses the collision resistance of the X-core structure. The analysis includes a detailed investigation of the non-linear plate and laser weld material behaviour using optical, full-field strain measurements. The resulting material relationships are implemented into the finite element model. Furthermore, the finite element model includes the influence of the ship motions to accurately predict the collision resistance. The verification of the numerical results is done by a comparison of the experimental and numerical force versus penetration curves and by a comparison of the deformed geometries. The latter is achieved through a digitised three-dimensional model of the post-experimental X-core structure. As a result, the accuracy of the collision simulations is presented and discussed.

Analysis and design of marine structures

License restrictions may limit access. ; Description based on online resource; title from PDF title page (ebrary, viewed April 14, 2015). ; Print version: ; International Conference on Marine Structures (5th : 2015 : Southampton, UK) ; Analysis and design

Characterisation of local grain size variation of welded structural steel

Previously, it has been shown that the grain size distribution plays an important role in the mechanical properties of welded steel. In the previous investigation, the volume-weighted average grain size has been shown to capture the influence of grain size distribution, resulting in a better fitting Hall–Petch relationship between grain size and hardness. However, the previous studies exclude the effects arising from local variation in grain size. In this paper, the grain size measurement methods are extended for the characterisation of the local grain size variation, which is significant for welded joints and can have an adverse effect on mechanical properties. The local gradient of grain size variation and its dependency on measurement direction are considered. In addition, examples of grain size and hardness variation are shown for S355 base metal and two weld metals, and characteristic differences are highlighted and discussed. The coarse-grained areas of a heterogeneous microstructure are found to have lower hardness than fine-grained areas. However, the surrounding microstructure, i.e. local grain size gradient, has an influence on the measured hardness values.

Round robin study on local stress and fatigue assessment of lap joints and doubler plates

The fatigue assessment of welded ship structures includes high uncertainties. One source of uncertainty is the determination of relevant local stresses. To quantify the uncertainties and to improve the analysis guidelines, round robin studies were performed within the Network of Excellence on Marine Structures (MARSTRUCT) project. One recent study concerned load-carrying fillet welds, which are treated in different ways in structural stress approaches. In total, five partners participated in the work. Two structural configurations of 12 mm thick plates were analysed, that is, double-sided lap joints and doubler plates, each with two different weld throat thicknesses (2.5 mm and 7 mm). Differences in the results were identified and conclusions were drawn with respect to modelling guidelines and typical scatter of computed fatigue lives.

Microstructure and Strain-Based Fatigue Life Approach for High-Performance Welds

Microstructure and pre-existing surface flaws in smooth notch geometries significantly affect the fatigue life of welded joints. Traditionally, a welded joint is assumed to incorporate crack-like defects and the crack propagation dominates the total fatigue life. For a smooth weld notch geometry, the macro crack initiation period becomes more significant, and this difference cannot be modelled with the existing stress or fracture mechanics ‑based approaches. In this paper, a microstructure and strain ‑based fatigue life approach is presented. In the approach, the fatigue damage process is modelled as a repeated crack initiation process within a material volume related to the microstructure. The novelty of the developed approach is that the size of the damage zone is defined from the grain size statistics without using fracture mechanics. The approach is able to consider the changes in the stress gradient, stress triaxiality and plasticity during the fatigue crack initiation and growth. The developed approach has been validated with experiments on submerged-arc and laser-hybrid welded joints. The predicted fatigue life, crack growth path and rate showed good agreement with the experiments. For a welded joint with smooth and favourable notch shape, the short crack growth, i.e. macro crack initiation period is dominant and it has a significant influence on the fatigue life.

Hull-superstructure interaction in optimised passenger ships

The paper investigates the interaction between the hull and the superstructure in optimised passenger ships when exposed to bending loads. The coupled beam theory was applied to extend the basic beam theory to take into account the vertical and shear stiffness between various decks. Optimisation of passenger ships with respect to weight and vertical centre of gravity (VCG) is carried out to create a set of Pareto-optimal solutions. The responses of these designs are compared in detail. The investigation shows that vertical and shear coupling between different decks significantly affect the response of the passenger ships and changes load-carrying mechanism of the hull girder. In the weight optimal design, the vertical bending moment is shared equally by the hull and the superstructure, while in the VCG optimal design, the neutral axis approaches bottom plating of the ship, considerably increasing the share of load carried by the superstructure. This means that the global response evaluation needs to include vertical and shear coupling along the length of the ship, and thus, the simplified two-dimensional section models are not adequate for the conceptual design of passenger ship structures.

Equivalent shell element for ship structural design

This paper presents an equivalent shell element for assessing the ship global and local static and vibration response in early design phases. The element provides a computationally economic tool for global analysis and the same mesh can be used in primary, secondary and tertiary level. The stiffened panel is considered as a three layer laminate element, where the first layer represents the plate, the second layer represents the stiffener web and the third layer represents the stiffener flange. The layers are described as 2D iso- and orthotropic materials, where elasticity matrices are found by applying the rule of mixtures. The element includes the in-plane, membrane-bending coupling, bending and additionally also shear stiffness, which follows the Reissner-Mindlin plate theory for anisotropic homogenous shells. The local plate bending response between the stiffeners is considered as well. The developed shell formulation has been implemented in commercial FE software FEMAP with NX Nastran and demonstrated through two case studies. Results are validated against 3D fine mesh quasi-static and vibration analyses and very good agreement is observed.

The Effect of the Secondary Bending Moment on the Fracture Strength Evaluation of the Laser Welded Joints from a Web Core Sandwich Structure

During the last period the interest on the sandwich structures has became more favorable due to the strength to weight ratio. In the same manner, in ship building field the lightweight structures became more and more attractive. With increasing the usage fields has increased the need to study the behavior of these structures. In general all the sandwich structures loaded in bending shows an effect of the secondary bending moment. In the case of web core sandwich panels used in ship structures has been observed a pronounced effect of the secondary bending moment on laser welded joints. Considering this, the paper presents an analysis of the fracture strength of laser welded joints of a web core sandwich structure, due to the effect of secondary bending moment. In the first part, the paper analytical formulation of the secondary bending moments and their effect on welded joints. This effect is explained on the basis of angle α defined in the paper and which depends on the thickness of the face plate, the thickness of the web plate and respectively the height. The paper continues with a numerical analysis of the stress and strain state from a web core sandwich beam and where also it is analyzed the effect of the secondary bending moment on the fracture strength of laser welded joints. Based on the carried out study it was observed that for high thickness of the web plate the effect of secondary bending moments is the overloading of the welded joints, instead for small thickness of the web plate the effect of the secondary bending moments can be of the unloading the welded joints. However, a small thickness of the web plate can affect the rigidity of the structure. Therefore, based on this study was proposed a solution to reduce the secondary bending moment without reducing the stiffness of the sandwich panel. The analysis conducted in this paper can be a design criterion for the web core sandwich structures.

Two-scale constitutive modeling of a lattice core sandwich beam

Abstract Constitutive equations are derived for a 1-D micropolar Timoshenko beam made of a web-core lattice material. First, a web-core unit cell is modeled by discrete classical constituents, i.e., the Euler–Bernoulli beam finite elements (FE). A discrete-to-continuum transformation is applied to the microscale unit cell and its strain energy density is expressed in terms of the macroscale 1-D beam kinematics. Then the constitutive equations for the micropolar web-core beam are derived assuming strain energy equivalence between the microscale unit cell and the macroscale beam. A micropolar beam FE model for static and dynamic problems is developed using a general solution of the beam equilibrium equations. A localization method for the calculation of periodic classical beam responses from micropolar results is given. The 1-D beam model is used in linear bending and vibration problems of 2-D web-core sandwich panels that have flexible joints. Localized 1-D results are shown to be in good agreement with experimental and 2-D FE beam frame results.

Influence of three-dimensional weld undercut geometry on fatigue-effective stress

With modern automated welding processes, such as laser-hybrid welding, it is possible to produce high-quality welds with good resistance to fatigue. However, although the overall quality of the weld may be good, the joints can still contain significant geometrical variation and geometrical imperfections at the weld notches. In previous research for thin laser-hybrid welded butt joints, small undercut-type imperfections were observed in both toe and root side. These imperfections are usually short along the weld direction and may not be visible when the geometry of the weld is examined. However, such imperfections work as local stress risers and may influence the fatigue strength significantly based on the weakest link principle. Severity of the local notch geometry is commonly analysed by the two-dimensional analyses, which assumes constant geometry in weld direction. This approach is conservative for short undercuts due to macro-support of the surrounding material. In this study, the difference between two-dimensional and three-dimensional stress analyses for short semi-elliptic undercuts is examined using stress averaging approach. The study utilises parametric notch models, where geometric parameters, such as notch length, depth, radius and opening angle, are varied to examine the difference for different undercut shapes.