Global Loads Research Articles

Reanalysis of Ice Pressure-Area Relationships

In the polar regions of the world, ships and offshore structures may be subject to impacts with large ice features. Pressure-area models are commonly used to determine both local and global ice loads. A general description of the two types of ice pressure-area models is given. Reanalysis of data from the trials on the USCGC Polar Sea suggests rising average pressure with rising contact area, quite the opposite of the expected result. A quick review of other ice pressure data lends support to the hypothesis. The paper concludes with a discussion of the implications and needed research to improve our understanding of ice loads.

Interaction of Ships and Ocean Structures With Ice Loads and Stochastic Ocean Waves

The influence of floating ice on the dynamic behavior of ships and offshore structures depends on many factors such as ice thickness and its relative speed with respect to the floating structure. The ice resistance to ship motion forms an essential problem in ship design and navigation. Furthermore, local or global ice loads acting on ocean systems are random and nonsmooth when impact interaction takes place. Impact loads on the bow of a ship navigating in solid ice may be modeled by a Poisson law. The measured stress amplitudes on the ship frame at the bow follow an exponential distribution. The nonhomogeneity and difference in ice microstructure, as well as the influence of salt and temperature, result in a great uncertainty in the ice strength. Therefore, the current review article aims at assessing the ice related problems encountered by offshore structures as well as by ships during their navigation. It also discusses the impacts of local and global ice loads on floating structures and reviews their existing probabilistic models. Moreover, this article covers the dynamic interaction of ice with flexible and rigid structures, and ships. In view of ice loads on marine systems, new design regulations have been introduced by international organizations that are involved in the design and building of ships as well as offshore structures. The ship stochastic stability and the first-passage roll stabilization problem associated with random ocean waves will also be described in an attempt to stimulate future research work dealing with ice impact loads. Moreover, due to the lack of research activities addressing the control problem of ships operating in icy waters, the current article will briefly discuss passive and active control schemes developed for controlling the ship roll motion. There are 529 references cited in this review article.

Use of local and global limit load solutions for plates with surface cracks under tension

Some available experimental results for the ductile failure of plates with surface cracks under tension are reviewed. The response of crack driving force, J, and the ligament strain near the local and global limit loads are investigated by performing elastic-perfectly plastic finite element (FE) analysis of a plate with a semi-elliptical crack under tension. The results show that a ligament may survive until the global collapse load is reached when the average ligament strain at the global collapse load, which depends on the uniaxial strain corresponding to the flow stress of the material and the crack geometry, is less than the true fracture strain of the material obtained from uniaxial tension tests. The FE analysis shows that ligament yielding corresponding to the local limit load has little effect on J and the average ligament strain, whereas approach to global collapse corresponds to a sharp increase in both J and the average ligament strain. The prediction of the FE value of J using the reference stress method shows that the global limit load is more relevant to J-estimation than the local one.

Numerical Simulation of Intercalation-Induced Stress in Li-Ion Battery Electrode Particles

Severe, particle-level strains induced during both production and cycling have been putatively linked to lifetime limiting damage in lithium-ion cells. Because of the presently unknown contributions of manufacturing and intercalation induced stresses in Li cells, this correlation is critical in determining optimal materials and manufacturing methods for these cells. Both global and localized loads must be estimated, in order to select materials able to resist fracture. Here, we select the system for study. We present results of a set of simulation techniques, ranging from one-dimensional finite difference simulations of spherical particles, to fully three-dimensional (3D) simulations of ellipsoidal particles, to systematically study the intercalation-induced stresses developed in particles of various shapes and sizes, with the latter 3D calculations performed using a commercial finite element code. Simulations of spherical particles show that larger particle sizes and larger discharge current densities give larger intercalation-induced stresses. Simulations of ellipsoidal particles show that large aspect ratios are preferred to reduce the intercalation-induced stresses. In total, these results suggest that it is desirable to synthesize electrode particles with smaller sizes and larger aspect ratios to reduce intercalation-induced stress during cycling of lithium-ion batteries.

Program for the Analysis of Plane Trusses

The ‘TRAP - TRuss Analysis Program’ can assist engineers in the analysis of plane truss-type structures, as well asprove an excellent basis for understanding the “finite – element” method. The Program is completely menu-driven, withall nodes, elements, and load data entered through an input file. TRAP includes plotting for displaying the truss geometryin its original shape showing all nodes and support points. In the analysis process, firstly the structure data; e.g numberof nodes, elements, materials and supports are to be defined. Secondly, element properties and support constraints haveto be specified. Finally, nodal global loads are then defined. The program output include; displacements for each node,axial forces, length of each element and reactions at the supports.The program aims at reducing the time of analysis and increase the accuracy of calculations as well as storage of theanalysed data. The program caters for manual methods of truss analysis such as method of sections or joint method. Ithas been found that the program is efficient and has no limitations on the number of nodes and elements. The program is suitable for use by design engineers in analysing Plane – Trusses of any size.

Computation of wet deck bow slam loads for catamaran arched cross sections

A computational model for catamaran wet deck slamming is developed on the basis of the variation of added mass as the hulls enter the water. In the case of a wave-piercing catamaran the bow cross section has a double arch cross section and slamming occurs when the arches fill. Residual air is entrained at the top of the arch due to bubble formation by turbulent mixing and this modifies the effect of the water added mass on the hull. The computational model therefore introduces a soft connection between the water added mass associated with the slam and the hull. The method has been evaluated by comparison with two-dimensional model drop tests in terms of the maximum forces and acceleration imposed on the hull, the variation of velocity during the slam event and the depth of penetration into the water. It is concluded that the added mass computation is adequate for slam modelling in global motions and loads calculations since it gives a good representation of the maximum total forces on the section and their duration.

Inertial and hydrodynamic inertial loads on floating units

The analysis of the dynamic behavior of floating units usually employs a coordinate system with origin in the unit's center of gravity, which significantly simplifies the global mass matrix. Hydrodynamic coefficients are then computed considering the same coordinate system. However, to analyze other conditions of mass distribution and maintain the simplicity of a global mass matrix, it is necessary to determine again the hydrodynamic coefficients, thereby reducing the efficiency of the entire process. Another important point is that the geometries frequently used in floating units are such that the cross-terms of an added mass are relatively unimportant when compared with the main terms, and it is, therefore, common to use only some of them to analyze the unit's dynamic behavior. Recently, however, in the search for production systems suitable for water depths greater than 3000 m, other geometries have been considered in technical and economic feasibility studies. It is possible that for these new geometries all terms of the added mass matrix must be included in the analysis. This paper presents the full development used to determine the complete global mass matrix, the inertial and hydrodynamic inertial loads that make use of the added mass matrices considering any coordinate system and the six degrees of freedom, including all cross-terms.

Numerical Simulation of the Flow around an Array of Free-Surface Piercing Cylinders in Waves

The flow around four free-surface piercing cylinders in waves is simulated by an Euler solver. Three different angles of the incoming wave front are considered in order to investigate the variation of the resulting global and local loads on the four bodies. The discretization of the fluid domain has been made easy and efficient by the use of a chimera method for the generation of the computational mesh. The free-surface has been simulated by a single-phase level set approach.

A comparison of the influence of global functional loads vs. local contact anatomy on articular cartilage thickness at the knee

Cartilage contact geometry, along with joint loading, can play an important role in determining local articular cartilage tissue stress. Thus individual variations in cartilage thickness can be associated with both individual variations in joint loading associated with activities of daily living as well as individual differences in the anatomy of the contacting surfaces of the joint. The purpose of this study was to isolate the relationship between cartilage thickness predicted by individual variations in contact surface geometry based on the radii of the femur and tibia vs. cartilage thickness predicted by individual variations in joint loading. Knee magnetic resonance (MR) images and the peak knee adduction moments during walking were obtained from 11 young healthy male subjects (age 30.5 ± 5.1 years). The cartilage thicknesses and surface radii of the femoral and tibial cartilage were measured in the weight-bearing regions of the medial and lateral compartments of three-dimensional models from the MR images. The ratio of contact pressure between the medial and lateral compartments was calculated from the radii of tibiofemoral contact surface geometries. The results showed that the medial to lateral pressure ratios were not correlated with the medial to lateral cartilage thickness ratios. However, in general, pressure was higher in the lateral than medial compartments and cartilage was thicker in the lateral than medial compartments. The peak knee adduction moment showed a significant positive linear correlation with medial to lateral thickness ratio in both femur ( R 2 = 0.43 , P < 0.01 ) and tibia ( R 2 = 0.32 , P < 0.01 ) . The results of this study suggest that the dynamics of walking is an important factor to describe individual differences in cartilage thickness for normal subjects.

Troubles cognitifs et sclérose en plaques rémittente : intérêt de leur détection précoce: Corrélations anatomo-fonctionnelles et suivi longitudinal

Cognitive impairment is frequent in relapsing remitting Multiple Sclerosis and is often diagnosed after disruption of occupational and social relations.We studied at baseline a homogeneous population of 32 RRMS patients, diagnosed for less than 5 years, with spontaneous memory complaints, and 20 controls. Sixteen patients were followed for 2 years, combining physical examination, neuropsychological tests, and brain MRI. Neuropsychological tests used evaluated memory capacities, attentional capacities, executive functions, language, and visuo-constructive praxis. Lesion load on brain MRI was measured with semi-automatic segmentation procedures and manual control.Eighty percent of patients presented cognitive impairment, and this proportion was higher than that found in the literature. These disorders were more marked for verbal episodic memory, attention, and executive functions. Patients with brain MRI that initially fulfilled the Barkhof criteria and those with callous lesions had more memory disorders. No link between global T1 and T2 lesion loads and neuropsychological scores was found. A statistical link between posterior fossa lesions and attentional disorders was shown. In the longitudinal follow-up, patients had better performances in memory and attentional domains, and a lower number of cognitive domains with dysfunction for each patient. This improvement on neuropsychological tests, whereas EDSS levels were stable, underlined a possible test-retest effect.During the initial phase of the disease, most of the relapsing remitting patients present a mild cognitive impairment. Early detection, therapeutic propositions, and recognition of disorders are necessary.

Maximum ice force on the Molikpaq during the April 12, 1986 event

Abstract On April 12, 1986, the Molikpaq, a caisson-type offshore drilling structure, experienced a series of loading events when second-year and multi-year ice moved against the structure. The highest loads that the Molikpaq experienced during the 1985–86 season were during this day. Extensive ice thickness measurements had been taken in the ice around the Molikpaq prior to April 12. Thickness of up to 6 m were measured and 8 to 12 m estimated for a multi-year hummock. MEDOF panels on the outer face of the caisson and strain gauges on inner bulkheads of the structure were the primary instrumentation used to calculate forces on the faces. Detailed analysis of strain gauge and extensometer data indicated that the maximum global force on the Molikpaq was no more than 420 MN and most likely was 380 MN. This is in contrast to higher and lower values quoted in the literature for the April 12th event. Other loading events during the day when multiple strain gauges and MEDOF panels were being recorded produced global loads of the order 250 MN. Global ice pressures for the 8 to 12 m thick multi-year hummock crushing on the 59 m long east face were not greater than 0.8 MPa.

Micromechanical modelling of shear deformation of a 90°-ply in Glare® at elevated temperatures

This work focuses on modelling the interlaminar shear deformation of a 90°-composite glass fibre epoxy ply, using finite-element analysis (FEA). The investigated transverse layer is part of a typical Glare® lay-up. This approach uses a unit cell of a single, resin embedded fibre, assuming hexagonal fibre alignment. This representative volume element (RVE) is provided with periodic boundary conditions (PBCs). Matrix yielding and micro mechanical damage due to debonding at the fibre–matrix interface are the main consequences after interlaminar shear load is applied on the RVE. Temperature dependent material properties, thermal residual stresses of the 90°-layer and the Glare® composite, as well as the adhesive strength at the fibre–matrix interface are taken into account. Comparing the computational and experimental results shows that plastic matrix deformation and further occurring damage can be assigned to defined global shear loads at given temperatures. It is shown that the parameters concerning the interfacial strength have a large influence on the global shear behaviour of the RVE. Thus, the basis for further investigations is explored to give design criteria for given thermal and mechanical loads in structural applications.

Limit analysis of flaws in pressurized pipes and cylindrical vessels. Part I: Axial defects

Within the theory of plasticity recently developed primal-dual limit analyses with the finite element method (FEM) give upper and lower bounds of the limit load. The method can be iterated until both bounds have converged to the same value which is then considered the exact plastic collapse load. This new numerical method is used in part I of the paper to derive improved lower bound limit load formulae for axial defects in pressurized cylinders for any defect geometry and loading. Based on the observation that even long slits in thick pipes have a residual strength a simple formula for the stress magnification factor is justified also for thick pipes. Global collapse loads are compared with primal-dual FEM limit analyses and with a large number of burst tests. For axial defects it is possible to find corresponding local collapse loads. Part II of the paper will discuss the FEM discretization of the limit load theorems and will consider circumferential defects.

Effects of the Load History on the Residual Stress Distribution in Welded Components

The lifetime of a welded component is finally influenced by the various service conditions. The stress redistribution effect of subsequent load histories is fundamental to the load-bearing capacity of a structure later in service. Since every welded structure additionally experiences external shrinkage restraint, the resulting structural loads are very important for mechanical assessment. Forward-looking development of materials furthermore requires particular consideration of in-service load histories and of their effects on the global structural loads of a construction under shrinkage restraint. Using a large-scale test facility, stress and residual stress investigations have been performed relating to the subject area of residual stress redistribution resulting from a simulated load history. Based on the presented results, which were all carried out with matching filler materials, further research work concerning the influence of the variation of the mismatch factor on the residual stress distribution has to be done.

Glacial isostatic adjustment and relative sea-level changes: the role of lithospheric and upper mantle heterogeneities in a 3-D spherical Earth

SUMMARY The response of the Earth to the melting of the Late Pleistocene ice sheets is commonly studied by spherically layered models, based on well-established analytical methods. In parallel, a few models have been recently proposed to circumvent the limitations imposed by spherical symmetry, and to reproduce the actual structure of the lithosphere and of the upper mantle. Their main outcome is that laterally varying rheological structures may significantly affect various geophysical quantities related to glacial isostatic adjustment (GIA), and particularly post-glacial relative sea-level (RSL) variations and 3-D crustal velocities in formerly ice-covered regions. In this paper, we contribute to the ongoing debate about the role of lithospheric and mantle heterogeneities by new 3-D spherical Newtonian finite elements models and we directly compare their outcomes with publicly available global RSL data. This differs from previous investigations, in that have mainly focused on extensive sensitivity analyses or have considered a limited number of RSL observations from formerly glaciated regions and their periphery. In our study the lithospheric thickness mimics the global structure of the cratons based on geological evidence, and the upper mantle includes a low-viscosity zone beneath the oceanic lithosphere. We use two distinct global surface loads, based upon the ICE1 and ICE3G deglaciation chronologies, respectively. Our main finding is that using all of the available RSL observations in the last 6000 years it is not possible to discern between homogeneous and heterogeneous GIA models. This result, which holds for both ICE1 and ICE3G, suggests that the cumulative effects of laterally varying structures on the synthetic RSL curves cancel out globally, yielding signals that do not significantly differ from those based on the 1-D models. We have also considered specific subsets of the global RSL database, sharing similar geographical settings and distances from the main centres of deglaciation. When we consider the data from the margins of the Baltic region, a laterally varying lithospheric thickness improves significantly the agreement with the observations. This is not observed in other relevant situations, including the Hudson bay region. In the regions where the disagreement between predictions and observations is particularly evident, further investigations are needed to improve the geometry of the heterogeneous structures and of the surface ice-sheets distribution.

Ship Motion Computations Using a High Froude Number Time Domain Strip Theory

High-speed strip theories are discussed, and a time domain formulation making use of a fixed reference frame for the two-dimensional fluid motion is described in detail. This, and classical (low-speed) strip theory, are compared with the experimental results of Wellicome et al. (1995) up to a Froude number of 0.8, as well as with our own test data for a semi-SWATH, demonstrating the marked improvement of the predictions of the former at high speeds, while the need to account for modest viscous effects at these speeds is also argued. A significant contribution to time domain computations is a method of stabilizing the integration of the ship's equations of motion, which are inherently unstable due to feedback from implicit added mass components of the hydrodynamic force. The time domain high-speed theory is recommended as a practical alternative to three-dimensional methods. It also facilitates the investigation of large-amplitude motions with stern or bow emergence and forms a simulation base for the investigation of ride control systems and local or global loads.

Wavelet transforms and ice actions on structures

A wavelet cross-correlation analysis was applied to ice loads and corresponding acceleration signals measured at the Norstromsgrund lighthouse in the Gulf of Bothnia, Sweden. It was shown that the excitations of the lighthouse by actions from ice floes are influenced by structural responses with dominant excitation frequencies in the same range as the frequencies of the responses. The lengths of intervals with synchronized excitations and responses are predicted to be from 1.9 to 8.6 s. Local ice loads were found to be synchronized in the same intervals and for the same frequencies, but for a slightly shorter time. This synchronization causes simultaneous ice actions which can trigger higher global ice loads on the structure than has been obtained during brittle non-simultaneous crushing.

Global and slam loads for a large wavepiercing catamaran design

Prediction of design loads for small high speed commercial craft has traditionally been governed by classification society, rule based formulae. Larger high speed vessels, particularly those greater than 100 m in length, require validation of rule based design loads. Validation methods include model tests or hydrodynamic studies using computer based motion and loads software. This paper presents the results of studies into the calculation of global loads for a 112 m, wavepiercing catamaran design. In particular, the preliminary design approach for assessment of loads is discussed. Longitudinal bending and pitch connection moment loads are derived from linear and nonlinear motion and loads software packages FASTSEA, SWAN and BESTSEA. The computed results are compared with rule based loads and empirically derived loads based on full-scale measurements from similar craft. Local wet deck slamming loads are discussed with reference to current rule based formulae and results of full scale tests on similar vessels. The effect of high local slam loads is discussed and how these loads may influence the design process. Conclusions are drawn on the presented results concerning the interaction of global and local loads in the design of a 112 m wavepiercing catamaran for Incat shipbuilders in Hobart, Australia.

Damage tolerance assessment of composite sandwich panels with localised damage

The work described herein is part of a larger context in which the effect of damage in sandwich composite structures for marine applications has been investigated. The overall aim of this effort has been twofold: to develop and verify existing damage assessment models to be used to assess the effect of damage on marine sandwich structures, and to develop a damage assessment scheme to be used by shipyards, ship owners and navies. More specifically, this paper presents a sub-set of this overall effort looking at impact and indentation damage and its effect on the load carrying capacity of state-of-the-art carbon composite sandwich panels for marine applications. Damage types are modelled based on physical observations from tests. Testing is then performed on different scales in order to validate the models. The overall aim is to use such models to produce information that can be used for decision-making at two levels. The first is to evaluate the damage tolerance of ship structural components and thus to calculate the size and extent of damage that a component can have without risk of growth or failure at ultimate local or global loads on the entire ship. The second is to have information at hand to decide if, and when, a structural part needs to be repaired if damage has been detected. A scheme developed for this purpose is presented herein. Finally the paper will briefly describe a common framework for damage assessment in composite sandwich structures. Herein, models are used in conjunction with the design specifics and functional requirements to create a scheme for repair decisions.

Proper Orthogonal Decomposition-Based Modeling, Analysis, and Simulation of Dynamic Wind Load Effects on Structures

Multicorrelated stationary random processes/fields can be decomposed into a set of subprocesses by diagonalizing their covariance or cross power spectral density ~XPSD! matrices through the eigenvector/modal decomposition. This proper orthogonal decomposition ~POD! technique offers physically meaningful insight into the process as each eigenmode may be characterized on the basis of its spatial distribution. It also facilitates characterization and compression of a large number of multicorrelated random processes by ignoring some of the higher eigenmodes associated with smaller eigenvalues. In this paper, the theoretical background of the POD technique based on the decomposition of the covariance and XPSD matrices is presented. A physically meaningful linkage between the wind loads and the attendant background and resonant response of structures in the POD framework is established. This helps in better understanding how structures respond to the spatiotemporally varying dynamic loads. Utilizing the POD-based modal representation, schemes for simulation and state-space modeling of random fields are presented. Finally, the accuracy and effectiveness of the reduced- order modeling in representing local and global wind loads and their effects on a wind-excited building are investigated.