冰与柔性结构作用挤压破坏形成的动荷载

基于现场原型结构测量,研究了冰与柔性直立结构作用产生挤压破坏过程形成的交变冰力。结果发现:随着冰速的增加,可以出现三种不同冰力形式并分别引起结构准静态振动、稳态振动和随机振动响应。进一步对形成不同冰力模式的机理进行了分析,提出了随着冰速变化,冰与结构相对速率变化导致结构近场冰可以出现塑性破坏、裂纹损伤破坏与纯脆性破坏三种破坏模式,并解释形成交变荷载过程。     

Experiments on level ice loading on an icebreaking tanker with different ice drift angles

A series of tests was performed with a laboratory-scale model ship to simulate the effects of ice load parameters on an icebreaking tanker. A model of the icebreaking tanker Uikku was mounted on a rigid carriage and towed through an unbroken ice sheet in the ice tank of the Marine Technology Group at Aalto University. Two ice sheets and 11 different experimental configurations were used. The carriage speed, heading angle of the model ship, and ice thickness were varied, and the forces, accelerations, ice cusp sizes, carriage positions, and ice pile dimensions under the intact ice sheets were measured.This paper includes results for the measurements of ice rubble loads against the model hull in the horizontal plane. Phenomena such as ice failure modes and ice rubble accumulation on the upstream side of the hull beneath the ice sheet were observed in some tests. The icebreaking lengths and dimensions of ice rubble were analyzed for some tests. The effects of towing speed, heading angle under the intact ice sheet in front of the hull, and the accompanying ice loads on the formation and build-up of ice rubble were analyzed. In addition, the evolution of ice rubble geometry, in cross sections and the horizontal plane, was investigated. There was good agreement over several orders of magnitude between the measured and calculated values of the lateral ice forces. These results are relevant to the modeling of ice loading on hulls and the design of moored or dynamic positioned structures for operation in ice-covered waters. Some parameters obtained from these tests can be used as input for future numerical simulations.     

Mitigating ice-induced jacket platform vibrations utilizing a TMD system

Field observations conducted on oil platforms in the Bohai Sea revealed that the offshore jacket structures, which can withstand the maximum static ice force, might suffer from severe vibrations when ice sheets pass through the legs of the structure. It has been proved that, when the natural frequency of narrow or flexible structures is close to or coincides with the predominant frequency of cyclical ice forces, severe ice-induced vibrations will be evoked. Excessive and consistent vibration will cause the discomfort of working crew, failure of deck facilities even structural fatigue failure. Therefore, it is imperative to seek economical ways to reduce the vibrations to an acceptable level. Based on full-scale tests and technical feasibility, the strategies of vibration mitigation are investigated and compared in this paper. The primary studies demonstrate that adding a Tuned Mass Damper (TMD, also called a Dynamic Vibration Absorber, DVA) is a feasible and practical approach to reduce vibrations. The efficiency of optimally designed TMD and one-directional conceptual TMD device has been presented in this study. Field ice load time history was used as loading scenario to assess the performance of TMD system. The results show that the supplemental device can favorably reduce the dynamic response of platform.     

Ship resistance in thick brash ice

The mechanical properties of thick brash ice are considered, with special emphasis on the friction angle ф for a linear Mohr-Coulomb failure criterion. The static equilibrium of a uniform brash layer is discussed, and the internal stresses are given for neutral, active, and passive stress states. Horizontal displacement of brash by a smooth vertical plate is analyzed, and the crushing forces for plastic yielding in the passive stress state are deduced for cohesionless brash and for ice that has finite internal cohesion. Propagation of a crushing disturbance is treated, and the condition for localization of crushing is derived from consideration of mass and momentum conservation. Formation of pressure ridges by localized crushing against a plate is analyzed, and estimates are obtained for the limiting thickness of ice crushed against a plate. Horizontal thrusting of brash by a plate inclined from the vertical is dealt with by graphical derivations of initial passive pressure using the Poncelet construction, and it is argued that for large displacements the crushing forces against an inclined plate are the same as those for a vertical plate because of pressure ridge formation.In the analysis of low speed ship resistance, the emphasis is on resistance at the bow. Starting from analysis of penetration by a slender smooth wedge and taking account of interface friction in an approximate way, an expression is obtained for the resistance of “sharp” conventional bows, and comparisons are made with plasticity results for plane-strain penetration of wedges into semi-infinite media. Formation of a “false nose” of brash in front of a blunt bow is considered for Mohr-Coulomb and Von Mises failure criteria, and an expression for the resistance of blunt bows is derived. Frictional resistance aft of the bow section is controlled largely by the normal pressure of ice against the hull. After consideration of neurtral and active stress for the thickened ice alongside the hull, the normal force is expressed as a fraction of the passive state crushing force. Bow resistance and hull friction can be combined into a single expression of the form ABR, where A is a dimensionless function, B is the ship's width and R is the passive state crushing force (per unit width) for the brash layer. The limiting ice resistance for ship movement can be related to the ship's thrust, which in turn can be estimated from readily available data such as shaft power and thrust per unit power. Equating the theoretical expression for ship resistance to a suitable expression for limiting propeller thrust at an appropriate hull speed, an estimate of limiting ice thickness is obtained. For ships of a given type, which tend to have shaft power proportional to the cube of the beam, the limiting ice thickness is directly proportional to the beam. Using the very limited data on mechanical properties of brash ice and assuming that the propellers continue to function well, the estimates indicate that a Great Lakes bulk carrier would be near its limit of movement when the brash thickness in a wide channel is about 10% of the beam, while an icebreaker in saltwater brash could perhaps keep moving until the brash thickness reached about 20% of the ship's beam.     

A constitutive model for ice as a damaging visco-elastic material

A general three dimensional theory is proposed for describing constitutive properties for ice as a viscoelastic brittle material. The elastic and viscous properties of ice are assumed to be initially orthotropic. Initiation and growth of a multitude of small microcracks is described using continuum mechanics principles. The material damage caused by microcracks is described using independent vectorial fields of planar microcracks. The constitutive equations are derived from general thermodynamic principles combined with some results from invariant theory for tensors. The developed theory is tested on two simple loading cases for a bar with material properties selected in conformance with test results for S2-ice. Results obtained numerically are compared with results reported in the literature. The results show good correspondence with test results over a wide range of strain rates including regions of ductile and brittle as well as regions of transition from ductile to brittle behaviour. The proposed model can easily be implemented in FEM programs.     

Cyclic loading and fatigue in ice

Isotropic polycrystalline ice was subjected to cyclic loading in uniaxial compression at ?5°C, with stress limits 0–2 and 0–3 MPa, and frequencies in the range 0.043 to 0.5 Hz. Stress-strain records showed hysteresis loops progressing along the strain axis at non-uniform rates. The effective secant modulus, which was about half the true Young's modulus, decreased during the course of a test. The elastic strain amplitude and the energy dissipated during a loading cycle both increased with increase of time and plastic strain. Strain-time records gave mean curves which were identical in form to classical constant-stress creep curves, with a small cyclic alternation of recoverable strain about the mean curve. The inflection point of the “creep curve”, marking the transition from strain hardening to strain softening, occurred at a plastic strain of 1% (±0.1%), which is about the same as the “ductile failure strain” found in constant stress creep tests and in constant strain-rate tests on ice of the same type at the same temperature. The dissipation of strain energy up to this “failure point” was much higher for the cyclic tests than for corresponding quasi-static tests ? 100 to 600 kPa (or kN-m/m³) in comparison to about 30 kPa. The number of cycles taken to reach the “failure point” was of no direct significance, varying greatly with stress amplitude and with frequency. The results of the tests suggest that maximum resistance under compressive cyclic loading occurs at an axial plastic strain of about 1%, which is essentially the same as the failure strain for ductile yielding under constant stress and under constant strain-rate.     

Fracture in the compression of columnar gained ice

Failure behavior of fine-grained columnar ice is investigated under compression. A special experimental set-up is used to achieve a uniform stress (strain) state in the specimen. The strength is then determined at several constant temperatures and strain rates.Three failure mechanisms have been identified: cleavage fracture, plastic flow, and linkage of grain boundary cracks. The failure mechanism is believed to be a material property. The failure mode, however, is dependent on the specimen geometry and the loading condition. Thorough investigation is made on the failure modes of ice specimens (4 × 8 × 12 cm blocks).A major conclusion is that ice is an excellent material to be used in the investigation of the failure of brittle polycrystalline materials.     

Experimental investigation of high velocity ice impacts on woven carbon/epoxy composite panels

Spherical-shaped ice simulating hailstones were projected onto woven carbon/epoxy composite panels to determine the damage resistance of thin-walled composite structures to ice impact, and to observe the resulting damage modes that occur over a wide range of velocity. To study the behavior of ice in isolation from the complex response of the composite panel targets, impacts onto a dynamic force measurement device were first conducted. These experiments show a linear relationship between the peak measured force and the projectile kinetic energy, regardless of projectile size. Composite panel impact experiments show a linear relationship between the kinetic energy at which failure initiates, referred to as the failure threshold energy (FTE), and the thickness of the panel. Impacts at kinetic energies greater than the FTE produced a multiplicity of damage modes. Glancing impact tests on composite panels show that the FTE can be accurately estimated using a simple trigonometric scaling relationship.     

Fracture toughness of short-fibre reinforced thermoplastics

The failure process of short-fibre reinforced thermoplastics is characterized by different energy dissipation mechanisms, especially by mode II debonding along the fibre/matrix interface, sliding of debonded regions, brittle or ductile matrix fracture and pull-out. It is assumed that these failure processes are acting within a certain zone ahead of the notch tip— the dissipation zone. The modes of energy dissipation are mainly affected by the matrix fracture mode (brittle or ductile) which is mainly determined by the loading rate or temperature conditions. On the basis of an energy principle and relationships for the different energy dissipation mechanisms, we propose theoretical expressions for the static and dynamic fracture toughness and we compare these with experimental results.     

Machinability of Metal Matrix Composites Reinforced by 3-D Network Structure

Metal matrix composites reinforced by three-dimensional (3-D) continuous network structure reinforcement (3DCNRMMC) are difficult to machine due to serious tool wear and poor surface roughness caused by the brittle and hard reinforcement which interpenetrate into ductile matrix. In order to achieve the approach of low cost of 3DCNRMMC, the machinability of it needs to be understood. The influences of three cutting parameters and volume fraction of reinforcement on cutting force were analyzed in detail. The results indicate that: (1) Due to the brittle phase(s) introduced into ductile matrix of composites, there is a large fluctuation of cutting force causing deterioration of machinability. The fluctuation ranges of cutting forces, initially increase rapidly with the increase of volume fraction of reinforcement and then decrease finally, are largest at the range of the volume fraction of 55–65%; (2) The influence of cutting parameters on cutting force is obvious. With the increases of cutting speed, cutting force decreases gradually unless cutting speed exceeds the value of 209 m/min. Cutting forces increase with increasing feed rate and depth of cut; (3) Owing to the large fluctuation of cutting force, there were some cratered surfaces caused by Si₃N₄ reinforcement pulling-out and flaking-off. Some brittle phase protruding from the machined surface caused the deterioration of machined surface.     

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

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.     

海冰作用下平台结构自激振动的参数分析与响应的数值计算

本文依据Ｍａａｔｔａｎｅｎ的自激冰力模型,分析了冰和结构的各种参数对海冰作用下平台结构自激振动的影响;对不同动力特性的导管架平台结构简化模型,进行了海冰作用下结构自激振动响应的数值计算,发现了自激冰力和结构振动的一些特征,为进一步理解冰致平台结构自激振动现象,避免自激振动的产生及采取相应的控制措施提供了有益的结果。     

Study of the fracture of ferritic ductile cast iron under different loading conditions

This work is a continuation of the studies presented in a recent paper by the authors, where the fracture surfaces of pearlitic ductile cast iron under different loading conditions were exhaustively analysed. In this study, fracture surfaces of ferritic ductile cast iron (or ferritic spheroidal graphite cast iron) generated under impact, bending and fatigue loading conditions were characterised and compared. The fracture surfaces were characterised qualitatively and quantitatively from the observation under a scanning electron microscope. The fracture mechanisms in each case were identified. For impact tests, as test temperature increases, the dominant fracture mechanism changes from brittle to ductile. For bending tests, a fully ductile fracture micromechanism dominates the surface. In fatigue tests, the surface shows a mix of flat facets that appear to be cleavage facets and ductile striations, but the typical fatigue striations are not easily found on the fracture surface. Methodologies for the determination of the macroscopic direction of main crack propagation in both ductile and brittle failure modes are proposed. These allow identifying main crack propagation direction with good approximation. The results are potentially useful to identify the nature of loading conditions in a fractured specimen of ferritic spheroidal graphite cast iron. The authors believe that it is necessary to extend the methodologies proposed in samples with different geometry and size, before they can be used to provide additional information to the classical fractographic analysis.     

Failure analysis of RC shear walls with staggered openings under seismic loads

Reinforced concrete shear walls are used to design buildings located in seismic areas, because of their rigidity, bearing capacity and high ductility. Until now many theoretical and experimental tests on shear walls with or without openings have been made, therefore their failure modes have been analysed and are rather very well-known; the research results being confirmed by real failure modes of RC walls after earthquakes.Design codes and standards based on the knowledge of the failure modes of the reinforced concrete walls were developed in order to obtain the ductile failure mechanisms.A special case is the failure mode of the reinforced concrete shear walls with vertical staggered openings. If at coupled walls the elements must be designed so that the plastic hinges appear at the ends of the coupled beams and then in the pier, this thing is more difficult at shear walls with staggered openings.Theoretical and experimental studies on structural walls with staggered openings, lamellar walls and walls with bulbs at the end have been made recently. There have also been studied the followings: the degradation of the stiffness, the ductility function to the intensity of the seismic force, the presence of the vertical forces, the position and the size of the openings and the reinforcing ways.The article presents the results of the theoretical and experimental tests on failure modes of three types of reinforced concrete shear walls with staggered openings which are compared to those obtained from walls with vertical ordered openings as far as the seismic response is concerned.The failure modes of the structural walls under seismic stress have been identified using calculus programs and cyclic alternated experimental tests.The theoretical research on the failure modes was the basis for the elaboration of a simplified methodology for the calculus of the maximum theoretical seismic force that produces the concrete crushing in the ultimate limit stage. The results theoretically obtained with the help of the calculus programs have been confirmed experimentally. The analysis of the failure modes, obtained with the computing methodology proposed, contributed to the completion of the seismic design codes for shear walls with staggered openings.     

A panel method for modelling level ice actions on moored ships. Part 1: Local ice force formulation

Model tests of a moored ship in level ice have been performed. The experimental setup made it possible to measure interaction forces between the ice and the ship's bow area. These kinds of measurements are for the first time reported in the literature.Methods for constructing local ice forces based on observations from the model tests are introduced. The ice-hull interaction is separated into ice breaking, rotation and sliding phases. It is assumed that breaking and rotation occur in the vicinity of the waterline and that the main sliding phase occurs under water. A simplified formulation for forces near the waterline caused by breaking and rotation of ice floes is based on four random variables. An algorithm to extract these variables from the measured data is given. Probability distributions are fitted to the random variables and the distributions are analysed with respect to effects of the mooring stiffness and the ice drift speed. The difference between the constructed and the measured ice forces in terms of mean values and standard deviations are small.A Coulomb friction model is assumed for the friction forces on the wet part of the bow. The friction coefficient is assumed to be stochastic and is extracted from the measurements. Even though the assumed friction model is simplified, it closely resembles the measured forces.The model is applied to study the dynamic response of a moored realistic hull in a separate paper.     

The structure and strength of first-year ice ridges in the Baltic Sea

Sea ice ridges are typical features in the Baltic Sea ice pack accounting for on average up to one-third of the total ice mass. They are difficult obstacles to winter navigation and cause large forces on ships and offshore structures. However, the internal structure and strength properties of ice ridges are quite poorly known. This paper presents the results of an experimental project on the structure and strength of first-year ice ridges in the Baltic Sea carried through during 1987–1989. Altogether, six freely floating ridges were investigated. Their total thickness ranges from 4 to 17 m. Valuable field data about the size and shape of ridges, consolidated and unconsolidated parts, block size and porosity have been obtained by drilling and sampling. Divers and underwater video cameras have been used for observing the ridge keel structure. The totally consolidated layer within the ridges was 1–2 times the thickness of the surrounding level ice. The average porosity was 29%. The strength of the keel part has been measured with a full scale loading test. The force required to shear the keel was determined as a function of the normal force, the settlement rate, and the porosity of the keel. The shear strength of the ridge keels was from 1.7 up to more than 4.0 kPa. Also small scale tests were conducted in an ice tank giving results in agreement with the full scale tests.     

高延性混凝土加固钢筋混凝土柱抗剪承载力计算

设计了8个RC柱,采用高延性混凝土（HDC）和灌浆料进行加固,通过低周反复荷载试验,主要研究其破坏形态、加固层作用机理及抗剪承载力计算方法。试验结果表明:HDC加固层可对核心区混凝土形成良好的约束作用,破坏形态由脆性破坏向延性破坏转变,试件承载力和变形能力都得到显著提高;对HDC加固柱的加固层作用机理进行了分析;基于桁架-拱模型对试件受力进行了分析,推导出加固柱的抗剪承载力计算方法,计算结果与试验值吻合较好。     

Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

Both scientists and engineers are very much concerned with the study of ductile-to-brittle transition (DBT) in ferritic steels. For historical reasons the Charpy impact test remains widely used in the industry as a quality control tool to determine the DBT temperature. The transition between the two failure modes, i.e. brittle cleavage at low temperature and ductile fracture at the upper shelf occurs also at low loading rate in fracture toughness tests. Recent developments have been made in the understanding of the micromechanisms controlling either cleavage fracture in BCC metals or ductile rupture by cavity nucleation, growth and coalescence. Other developments have also been made in numerical tools such as the finite element (FE) method incorporating sophisticated constitutive equations and damage laws to simulate ductile crack growth (DCG) and cleavage fracture. Both types of development have thus largely contributed to modeling DBT occurring either in impact tests or in fracture toughness tests. This constitutes the basis of a modern methodology to investigate fracture, which is the so-called local approach to fracture. In this study the micromechanisms of brittle cleavage fracture and ductile rupture are firstly shortly reviewed. Then the transition between both modes of failure is investigated. It is shown that the DBT behavior observed in impact tests or in fracture toughness specimens can be reasonably well predicted using modern theories on brittle and ductile fracture in conjunction with FE numerical simulations. The review includes a detailed study of a number of metallurgical parameters contributing to the variation of the DBT temperature. Two main types of steels are considered : (i) quenched and tempered bainitic and martensitic steels used in the fabrication of pressurized water reactors, and (ii) modern high-toughness line-pipe steels obtained by chemical variations and optimized hot-rolling conditions. An attempt is also made to underline the research areas which remain to be explored for improving the strength-toughness compromise in the development of steels.     

Sliding friction of sea ice blocks

Sliding friction is known to be the largest energy sink during ridging events and it controls the force during rafting. During the 1994 Sea Ice Mechanics Initiative (SIMI), we conducted a set of sliding friction tests by pulling meter-size blocks of sea ice over the natural ice cover to determine the friction coefficients. The ice blocks were similar in size to those occurring in natural rafting and ridging events. Our simple tests were conducted by pulling the blocks using a boat trailer winch and cable. This system was definitely compliant, and elasticity of the cable was an important property that gave rise to stick–slip behavior comparable to that observed during rafting and ridging events.A simple model was introduced to analyze the data. It proved useful in understanding the stick–slip response, and exposed the proper way to interpret results. Static and dynamic friction coefficients are presented for 25 test cases for blocks of various sizes pulled over ice covered with no snow and a small amount of snow. The model shows how to interpret the friction coefficients in this compliant system correctly. Interestingly, static and dynamic friction coefficients do not occur at the times of maximum and minimum pulling forces, respectively.