Elastic-plastic Responses Of Ship Structures Under Repeated Impact Loadings

The dynamic behavior of marine structures under repeated loadings has attracted wide attention in academic and industrial circles.Research work on the repeated load shows that material elasticity has a significant influence on the dynamic response of structures with the increase of the impact numbers.However,the existing theoretical studies on this topic often adopt a rigid-plastic material idealization,and the elastic-plastic theoretical analysis considering both the elastic and plastic deformations of structures is hardly found in the open literature due to its complexity.In this thesis,the theoretical models are developed to study the large deformation behavior of various ship structures under repeated impacts.The main focus is on the theoretical modelling of influence of the material characteristics including the material elasticity,plasticity strain-hardening and strain-rate sensitivity effects on the elastic-plastic dynamic behavior(in particular,the permanent deflections).Validation and comparisons are carried out against numerical simulations and small-scaled experiments.The main contents and conclusions are as follows:(1)The elastic-plastic analytical method and its verification to analyze the dynamic response of beam subjected to repeated impact loadings.First,the analytical formulae are derived for predicting the quasi-static force-deformation relations of an elastic-plastic beam during the loading,unloading and reloading phases.For a particular impact scenario in which the striker with a heavy mass impinges the midspan of the beam at a low speed,by employing energy method and taking the inertia effect into account,then the quasi-static characteristics are utilized to develop an elastic-plastic theoretical model of the beam under impact loadings so as to predict its overall dynamic responses.Validation and comparison are conducted against numerical simulations using the commercial software package ABAQUS and it is shown that good agreement between the theoretical and numerical results has been reached.(2)Parametric study on the dynamic behaviour of beam under repeated impact loadings.The dimensionless energy parameter ? is introduced to represent the influence of the factors including the impact velocity,impact mass,beam length and shape of the beam's cross section.The elastic-plastic analytical and numerical methods are used to calculate the following response parameters: maximum deflections,maximum acceleration,impact duration and coefficient of restitution during the four identical impacts,so that the variation trend of these characteristic response parameters with ? and its physical mechanism is investigated.(3)The rigid-plastic and elastic-plastic method to analyze the dynamic response of rectangular plate under two repeated impacts.On the condition of the low impact numbers and the large plastic deformations,the rigid-plastic method which can deduce simple analytical solutions is used to approximately analyze the dynamic behavior of the rectangular plate and make it easy to figure out the dangerous scenario where the structural deformation reaches the maximum.(4)The influence of the interaction between the stiffener and the plate on the large deflection dynamic behavior of stiffened plates under impact.Two analysis models for the stiffened plate are presented.The first model neglects any interaction between the stiffener and the plate,which means that the two structural components could be analyzed separately,while the second one considers part of the plate as a large flange of the stiffener,thus the plastic behavior of the stiffener is treated dependently of the plate.By comparing the theoretical predictions for the permanent deflections of the one-way stiffened plate in the analyzed impact cases,it is found that the two theoretical models differ slightly,which validates the first simplified model.(5)The improved rigid-plastic theoretical model to consider the material properties including the material elasticity,strain-hardening and strain-rate sensitivity effects.The rigid-plastic analysis is adopted to quantify the strengthening effects on the load-carrying capacity of structures under repeated impacts caused by the strain rate sensitivity and strain hardening.In addition,the effective energy absorption,which contributes to the plastic deformation of structure at each impact was modified in the rigid-plastic solution so as to develop an elastic-plastic semi-analytical model.(6)The repeated mass impact tests and numerical simulations on small-scale rectangular stiffened ship plates were conducted to obtain some detailed information about the rebound velocity of the striker and the permanent deformations of the stiffened plates.Meanwhile,the developed semi-analytical model is used to predict the permanent deflections against the number of impacts,and good agreement has been achieved between the theoretical predictions,numerical calculations and experimental results,which validates the theoretical method.