Investigations On The Scaling Method And Damage Features Of Box-shaped Structures Under Internal Blast Loading

Major and monumental buildings,bridges and ships might be targeted by terrorists because collapse of these structures can cause economic disaster,a large number of casualties,and great public panic.And internal blast attack is one of the main possible threaten to these structures for the reason of that these structures are consist of box-shaped rooms,girders and chambers.Hence,it is of vital significance to study the response mechanism and damage features of box-shaped structures under internal blast loading.Meanwhile,in order to obtain the damage features,understand the plastic behavior and verify the theoretical and numerical models of structures under blast loadings,full scale blast tests are required.However,full scale tests are limited due to security restrictions and lack of considerable resources required.Hence,scaled-down models are usually adopted to conduct blast tests for the purpose of predicting the physical phenomenon of a full-size structure.The present dissertation mainly focuses on two aspects of the problem of box-shaped structures subjected to internal blast loadings,the equivalent scaling method and the damage mechanism.With aspect to the equivalent scaling method,main achievements are as follows:?1?Proposed an equivalent experiment method for stiffened plate subjected to blast loading.Firstly,two equivalent ideas of mass equivalent and deformation energy equivalent were introduced.And the energy equivalent calculation method of simply supported and fully fixed stiffened plate were presented.Then,two sets of specimens with different dimensions under blast loads were conducted with two different scaled distances for the purpose of examining the equivalent effect.Numerical simulations were also conducted by using ANSYS/LS-DYNA and the results are in good agreement with the experiments.The results show that the deflection values of mass equivalent plate are greater than that of stiffened plates while the energy equivalent plate the opposite.Finally,the based on the experimental and numerical data,design equivalent method for stiffened plate under normal blast loads is obtained and the calculation of the equivalent thickness is discussed in detail.?2?Discussed the“imperfect similarity”of the traditional scaling law and analyzed the influence of structural dimension and strain rate.Firstly,three sets of internal blast experiments and numerical simulations of steel box model with a similarity scale of2:3:4 were designed according to the traditional scaling law,with three different explosive masses considered in each set.The results show that the deformation features of all tests are almost similar with outward bulging produced in all sides and inplane buckling in the central area of boundary plate.And imperfect similarity was observed through comparing the plate center deflections though the deformation shapes present good similarity.In addition,the reasons for the deviation of the traditional scaling law was discussed.And then,based on the experimental results,correction of the scaling law for steel box structures was conducted which considered both the scale-down factor?size effect?and the scale distance?strain-rate effect?.?3?Based on the Kirchhoff–Love thin plate theory and the governing equations of a fully clamped plate,a new dimensionless number(D_in)for the dynamic response of box-shaped structures subjected to internal blast loading was suggested.The dimensionless number has clear physical meaning and the parameters can be easily obtained in practice.It was also found that the structural response is inversely proportional to the material strength and the volume of the structure.And,the validity and efficiency of the dimensionless number were discussed.Then,according to the proposed dimensionless number,a new scaling method for box-shaped structures under internal blast loading was presented.The new method considered the influence of both the size effects of structural dimension and the strain rate sensitivity on material resistance.And,this method is convenient in engineering application.In addition,two steel multi-box specimens designed with the present scaling method were built to examine the effective of this method.Experimental results of the two tests were analyzed in detail.And the two different models present similar damage features according to the comparison of test results.The experimental results indicate that the present scaling method is effective and efficient.In the field of response mechanism and damage features,main findings are as follows:?1?The dynamic response mechanism of box-shaped structures under internal blast loading was studied by using digital image correlation?DIC?technique.Firstly,a series of steel chamber specimens with different size were designed to conduct internal blast tests,and numerical simulations were also conducted.The 3D deformation features were analyzed through both experimental and numerical results.Furthermore,plastic hinge lines along the two diagonals of the target side plate are observed.In addition,3D dynamic response processes were analyzed through both the DIC technique and numerical results.“Five mountains”deformation shape was observed during the response process because of the confinement of chamber corner to the blast wave.The dynamic response results indicate that the response process of the side plate includes two stages:the whole plate plastic response stage and the central area oscillating stage.?2?Based on the equivalent experiment method of stiffened plate and the scaling method proposed in the first aspect of the present thesis,two scaled-down simplified steel box girder models and a large scale?dimension:7.5m×2.9m×2.2m?multiple stiffened box-shaped model were designed and internal blast tests were conducted,respectively.The damage features and dynamic response of the specimen were analyzed.In addition,the dynamic response processes were recorded by using the high-speed photography system.And the response process of the whole model can be divided into two different stages,namely,localized damage response stage and integral vibration stage.?3?The accumulated damage of steel box under repeated internal blast loading was studied.Firstly,four series of internal blast tests were conducted using four box-shaped models with different dimensions.The damage features of every test were analyzed.The results show that the deflection of side plate increases with the increasing of blast loading times,and break or tearing may occur when suffered a specific loading time.While,the value of increased deflection decreases with the increasing of loading times.Then,an empirical formula for predicting the accumulated deflection of box-shaped structures subjected to multiple times internal blast loading was obtained according to the experimental data.?4?The failure modes of steel box-shaped structure under internal blast loading were analyzed systematically.Firstly,large numbers of numerical simulations were carried out.Then,through analyzing the numerical results,two damage forms of“+”-shaped damage and non-“+”-shaped damage form were observed with respect to multiple box-shaped structures.In“+”-shaped damage form,five failure modes were concluded for a single box:plastic deformation?mode 1?;break in plates center?mode2?;corner shear failure?mode 3?;edge tearing?mode 4?and shear failure over entire plate?mode 5?.The failure mode 2 can also be divided into two failure types–petalling and capping.As to the side plate of a box,there are also two different damage modes are observed,namely,convex-shaped?mode A?and concave-shaped?mode B?.Meanwhile,series of experiments were conducted with different dimension and different masses of explosive to examine the observed failure modes from simulations.And the failure process and mechanisms of some mode were also recorded and analyzed by using high-speed photography system.In addition,fast predictions of these failure modes were proceeded by using the dimensionless number D_inn and two-dimensional dimensionless number(L/H,D_in),and failure modes maps as well as the corresponding prediction expressions were obtained.?5?A function for predicting the damage range of multiple box-shaped structures under internal blast was established based on the dimensionless number.And the detailed prediction method was given according to large numbers of numerical simulations.Moreover,the applicability and accuracy of the prediction method was discussed by using experiment data.The achievements and results presented in the current thesis can be helpful in damage evaluation and anti-blast design for buildings,bridges and ship structures against possible internal explosion loads.