ISUM: Its Birth, Growth and Future

ISUM (The Idealized Structural Unit Method) was born in 1972 to efficiently and accurately analyze the behavior of large size structures up to and beyond their ultimate strength. In this method a structure is divided into large elements, basically its structural units (members). Geometric and material non-linear behavior inside the element is formulated and expressed at a limited number of nodal points at the element boundaries. In this way a large structure can be modeled using a coarse mesh while still being able to consider the nonlinear behavior until the collapse of the structure. Several ISUM elements have been formulated and used to analyze the non-linear behavior of large ship structures. In further developments, more elements with more accurate formulations have been developed and more types of structures have been analyzed using this method. The same ISUM concept has been applied to the analysis of welding deformation of large welded structures and to failure analysis of structural and mechanical components subjected to impact loads. In this paper, the basic ISUM concept is outlined, and several elements are presented. Examples of applications to ships and marine structures are presented demonstrating the effectiveness of the method. Recent developments are also reviewed and future potential is explored.

Prediction of Welding Deformation and Residual Stress in a Multiply-Stiffened Plate

In this paper, welding induced deformation and residual stress of a multiply-stiffened plate is studied by means of sequentially coupled thermal elasto-plastic finite element method. For the purpose of enhancing calculation efficiency, the FE model combining shell and solid elements is employed in the analysis. A transient moving heat source is used in the numerical analysis to consider important welding parameters such as heat input, welding speed and welding sequences. The welding processes of three stiffeners being attached to a base plate by single-side welds are simulated according to assembly configurations. The influence of three welding sequences and the position of weld foot on distortions and residual stress are discussed. The results demonstrate the different characteristics of residual distortion and stress in the multiply-stiffened plate by different welding sequences. The position of weld foot affects the local distortion of panel between two adjacent stiffeners.

A Study on the Simplified Calculation Method for the Residual Ultimate Strength of Damaged Hull Structures

In order to obtain the non-linear average stress-average strain relationships (σ-ε curves) of damaged structural members under both tensile and compressive loads, the systematical calculations are performed using the non-linear FE analysis (FEA) code, LS-DYNA, and the idealized σ-ε curves of damaged structural members are estimated from FEA results. In addition, by introducing the idealized σ-ε curves of damaged structural members to the simplified calculation program, which is developed by authors and based on the Smith’s method, the residual ultimate strength of damaged hull structures is calculated. The residual ultimate strength of damaged hull structures is also calculated utilizing FEA, the calculation results by the simplified calculation program are compared with the results obtained from FE analyses so as to examine the accuracy of simplified calculation method.

Crack Propagation and Local Failure Simulation of Reinforced Concrete Subjected to Projectile Impact Using RBSM

In this paper, numerical simulations of reinforced mortar beams subjected to projectile impact are conducted by using the proposed 3-D Rigid-Body-Spring Model (RBSM) in order to investigate mechanisms of crack propagation and scabbing mode of concrete members under high-velocity impact. The RBSM is one of the discrete-type numerical methods, which represents a continuum material as an assemblage of rigid particle interconnected by springs. The RBSM have advantages in modeling localized and oriented phenomena, such as cracking, its propagation, frictional slip and so on, in concrete structures. The authors have already developed constitutive models for the 3D RBSM with random geometry generated Voronoi diagram in order to quantitatively evaluate the mechanical responses of concrete including softening and localization fractures, and have shown that the model can simulate cracking and various failure modes of reinforced concrete structures. In the target tests, projectile velocity is set 200m/s. The reinforced mortar beams with or without the shear reinforcing steel plates were used to investigate the effects of shear reinforcement on the crack propagation and the local failure modes. By comparing the numerical results with the test results, it is confirmed that the proposed model can reproduce well the crack propagation and the local failure behaviors. In addition, effects of the reinforcing plates on the stress wave and the crack propagation behaviors are discussed from the observation of the numerical simulation results. As a result, it was found that scabbing of reinforced mortar beams subjected to high velocity impact which is in the range of the tests is caused by mainly shear deformation of a beam.

Evaluation of Ultimate Hull Girder Strength in Longitudinal Bending: Historical Review and Smith’s Method

Ultimate hull girder strength in longitudinal bending is the most important strength of ship structure. In the present paper, firstly, historical review is made regarding the research activities regarding the ultimate hull girder strength evaluation. Then, focusing on the Smith’s method, possibility of simplified method is discussed including what are the limitations of a simple method and how it can be extended to more general situation. A simple method is introduced to derive average stress-average strain relationships of stiffener elements with attached plating. At the end, results of some example calculation are introduced to demonstrate the effectiveness of a simple method for progressive collapse analysis of a ship hull girder.

Experimental Determination of the Ultimate Strength of Box Girder Specimens

The Finite Element Method (FEM) is a feasible tool to perform progressive collapse analyses of large structural systems. Despite enormous developments in finite element formulations and computer technologies the results of structural analyses should be validated against experimental results. In this paper the collapse behaviour of two identical box girder specimens is determined experimentally for the load case of pure longitudinal bending. The specimens are composed of stiffened plate panels and connected at either ends to a loading structure. Within a 4-point bending test a constant bending moment is applied to each specimen to determine the collapse behaviour even in the post-ultimate strength range. The results of the experimental determination of the ultimate strength are presented for the box girder specimens. To simulate the collapse behaviour a finite element model is used and validated against experimental results.

Effect of Welding Sequence on the Residual Stress Distribution in a Stiffened Plate

This work investigates the temperature distribution, deformation and residual stress in steel plates as a result of different sequences of welding. The single-pass gas tungsten arc welding process is simulated by a three dimensional nonlinear thermo-elasto-plastic approach. It is observed that the distribution of residual stress varies through the direction of plate thickness. It is concluded that the welding sequence affects not only the welding deformation but also the residual stress mainly in the lower layer of the plates. An in-depth discussion on the pattern of residual stress distribution is presented, especially on the width of the tension zone. Smaller residual tension zone and slightly lower compressive stress are found in thicker plate.

Numerical Simulation on Response of Foam Core Sandwich Panels Subjected to Underwater Explosion

Sandwich panel structures, which consist of two thin faces and low relative density cores, can significantly mitigate the possibilities of panel fractures. In the present paper, numerical simulations are conducted to study the deformation and fracture modes of sandwich structures under near-field underwater blasts and contact underwater blasts. Two different core materials are employed, namely aluminum foam and PVC foam. Main focus of this paper was placed to (i) study the failure mechanisms and energy absorption characteristics of sandwich structures in typical conditions, (ii) to demonstrate the benefits of such structures compared with solid plates of equal weight, and (iii) to obtain the properties of withstanding underwater explosion for single core material sandwich panels. In addition, the effects of panel thickness configuration and core height on deformation and energy absorption of the plates were explored. Results indicated that sandwich structures showed an effective reduction in the maximum panel deflection compared with a monolithic plate of same mass. The design parameters have great impacts on the results.

Progressive Collapse Analysis of a Medium-Rise Circular RC Building Against Blast Loads

This paper investigates the vulnerability of a typical medium-rise circular RC building against progressive collapse as a result of blast generated waves. The building is an eight storied (including one story basement) commercial complex. The likely blast threat scenario was identified by qualitatively assessing the vulnerability of the critical elements of the structure. LS-DYNA was used for the finite element modelling of the structure. The study presents local model analysis of one of its circular columns for which fluid-structure interaction through Alternate Lagrangian Eulerian (ALE) element formulation has been employed. The concrete volume in the columns was modeled using 8-node reduced integration solid hexahedron elements. The global model analysis was carried out to examine the overall response of the structure due to the failure of one of the critical columns. The building was modeled using beam and shell elements. The 2-node axial beam elements with tension, compression, torsion, and bending capabilities were employed to represent the RC beams and columns, whereas the four node quadrilateral and three node triangular shell elements were used to represent the core wall, floor slabs, retaining walls and facade. The column bases of the building were fixed at the level of raft slab. The results of the study are proposed to be used to control or prevent progressive collapse of RC buildings.

An Appreciation of Professor Norman Jones’ Contributions to Impact Engineering

This article summarises Professor Norman Jones’ academic career and his scholarly contributions to impact engineering. In the past 50 years, Professor Jones has performed profound research on a wide range of impact engineering problems, supervised postgraduate students, researchers and academic visitors from all over the world, initiated international research networks and conferences, and has played important roles in consulting government bodies and in generally serving the academic community. Due to his research excellence and achievements, Professor Jones has received numerous prestigious awards and titles including Fellowship of the Royal Academy of Engineering and Foreign Fellowship of the Indian National Academy of Engineering.