Investigation Of Water Impact On Elastic And Plastic Structures

The water impact on various structures has garnered a great deal of research attention over recent decades.As a complex transient process,such impact can be considered as a three-phase coupling model encompassing solid,liquid,and gas.This phenomenon is common in the natural environment and across countless engineering applications.Water slamming is a phenomenon characterized by high pressure acting over a body surface for a very brief duration.The impact pressure could cause great structural deformation and even local yields;the elastic and plastic response also influence the flow of fluids.There are many factors that can influence the impact loads,such as structural dimension,impact velocity,hydroelastic effect,air cushion effect,etc.So it is very important to investigate the prediction of the hydrodynamic response and the corresponding influencing factors thoroughly and in-depth by analytic,numerical and experimental methods.As above mentioned,this thesis carried out the following works:(1)A new simplified analytical method of water impact on the elastic plate is proposed based on Verhagen's model.The analysis is focused on the initial stage during which the highest hydrodynamic loads are generated.The fluid-structure coupling results by simplified method are compared to numerical results from Arbitrary Lagrangian–Eulerian method in LS-DYNA code and existing experimental measurements to validate the feasibility and accuracy of the simplified method.It is indicated that the change rate of air cushion thickness will influence the impact loads,and the distribution of the speed of air in the cushion also affects the loads in the later stage.The water entry problems of plates with different thickness,elastic modulus and added mass are studied in the paper.It shows that as the structural bending rigidity is lower,the impact pressure decreases and the duration and plate deflection increases.As the added mass increases,the pressure peak and plate deflection are larger,and the duration is shorter.In addition,the proposed model is mainly suitable for the rectangle plate whose length-width ratio is larger than 1.5.(2)The kinematic equation of elastic plate is replaced with that of the elasticplastic plate in the proposed model,and the application of simplified method is expanded.The results by simplified method are compared with numerical results by ALE method to validate the accuracy of this approach.In the same working condition,the pressure peak of elastic-plastic plate is smaller than that of elastic plate,and the deflection is greater.Influences of material properties(e.g.,elastic modulus,hardening modulus and yield stress)and structural properties(e.g.,plate thickness and added mass)on the impact pressure and structural deformation are discussed in detail.It is indicated that the total deformation of plate decreases while the plastic deformation increases as the elastic modulus is larger.The pressure peak will rise first and then decreases,then fluctuates on a small scale,and finally turns to stability.The plate thickness,yield stress and hardening modulus have,respectively,positive relationship with the pressure peak,and negative relationship with the maximum vertical deflection and residual deformation.The pressure and deformation are larger as the added mass increases.(3)The water entry tests of a typical stiffened panel are carried out at different drop heights.The structure free falls from the drop heights of 0.5m,1.0m,1.5m and 2.0m,and impact happens on the stiffened side.The impact pressures and strains on the panel during water entry are measured.It shows that the pressure peak at the center and effective stresses on the stiffened panel increases linearly as the drop height increases.The great air cushion effect exists in the impact process,so the pressure curve is smooth,and the impact duration is between 80 and 100 ms.Due to the compression on both ends of girder,the elastic tripping may easily occur under the lateral pressure induced by air cushion,and cause huge deformation.(4)A numerical model is built and calculated by ALE method based on the experimental design.The numerical results are compared with the experimental measurements to validate the feasibility of this method.The impact process is simulated by numerical approach and two types of impact are summarized: the direct water impact on the outer panel and the stiffener flanges;the impact transmitted by air on the inner panel.It is also indicated that the influence of the width of water domain is a little greater than that of the depth on the hydrodynamic response by calculation of models with double-width and double-depth water domain.But their influences are still limited and can be ignored.(5)Through numerical approaches,relationships between hydrodynamic response and dimensions of stiffeners are analyzed.It is indicated that the girder mainly affect the pressure peak,start time and duration at the center of the panel,as well as its local loads.When the girder is higher,its effective stress is greater,and the overall stress will change due to the change of the loads.Lower stiffeners have no effects on the impact pressure,and will decreases its own stress when its web height increases.The widths of all the stiffener flanges have positive relationship with its stress.In addition,due to the strong cushion effect in the impact process,the effects of gas properties on impact pressures are discussed.Compared with the model in the vacuum condition,impact pressure at the center of the panel decreases to one third when affected by the cushion effects,and the impact begins earlier and lasts longer.In general,the growth rate and peak value of the pressure increases as the density and sound velocity of the gas increases.But when they are too small to a certain degree,the pressure peak may decrease in turn.The innovations of this thesis are listed as follows:(1)A new simplified method to predict the impact pressure and structural deformation of elastic and elastic-plastic plate is proposed based on Verhagen's model.The coupling relationship among structure,air and water is simplified as interactions between structure and air,as well as air and water by this method.Then the water entry of an elastic or elastic-plastic plate in initial stage can be described in a new angle.(2)The concept of minimum air cushion thickness and corresponding formula are defined in the calculation of the simplified model to fill the gap of no convergence during the later stage of impact in Verhagen's model.Through the quantitative analysis of parameters of air cushion during water entry process of flat-bottom structure,the change rate of air cushion thickness is determined as the key factor to influence the impact load.Also,influences of other parameters on the hydrodynamic response is studied.(3)The water impact on the stiffened side of stiffened panel is studied by experimental and numerical approach firstly.The whole impact process is revealed.The impact load and structural response at different drop heights are adopted.Relationships between the drop height,stiffener dimension,gas property and dynamic response are analyzed and summarized.