3D meso-scale modelling of concrete material in spall tests

Tensile strength is one of the key factors of concrete material that need be accurately defined in analysis of concrete structures subjected to high-speed impact loads. Dynamic tensile strength of concrete material is usually obtained by conducting laboratory tests such as direct tensile test, Brazilian splitting test and spall test. Concrete is a heterogeneous material with different components, but is conventionally assumed to be homogeneous, i.e., cement mortar only, in most previous experimental or numerical studies. The aggregates in concrete material are usually neglected owing to testing limitation and numerical simplification. It has been well acknowledged that neglecting coarse aggregates might not necessarily give accurate concrete dynamic material properties. In the present study, a 3D meso-scale model of concrete specimen with consideration of cement mortar and aggregates is developed to simulate spall tests and investigate the behaviour of concrete material under high strain rate. The commercial software LS-DYNA is used to perform the numerical simulations of spall tests. The mesh size sensitivity is examined by conducting mesh convergence tests. The reliability of the numerical model in simulating the spall tests is verified by comparing the numerical results with the experimental data from the literature. The influence of coarse aggregates on the experimental test results is studied. The wave attenuation in concrete specimen is analysed, and empirical equations are proposed for quick assessment of the test data to determine the true dynamic tensile strength of concrete material. The contributions of aggregates to dynamic strength in spall tests are quantified for modifying the test results based on mortar material in the literature

Influence of corners in excavations on damage assessment

This paper provides guidance on quantifying the extent of corner effects in excavations and their impact on damage assessment. The corner effects’ extent is of great importance in making early decisions during project planning and preliminary design, particularly in relation to stakeholder engagement and placement of instruments. By using empirical relations, one is able to provide an equation, validated against the literature and additional numerical models, for estimating the extent of corner effects for a particular excavation geometry. Furthermore, two more equations for quantifying the damage of excavations to adjacent structures are presented and validated against two case studies in the literature. The proposed equations are also useful in the context of early stages of project development. Finally, a simple study shows the different effects of corners in sections parallel and perpendicular to a retaining wall. This highlights that corner effects may actually induce additional damage due to the introduction of a movement gradient, as opposed to the common previous perception that assumed that they were always conservative as they reduced absolute movements

Earthquake response of a multiblock nuclear reactor graphite core: experimental model vs simulations

The complex dynamics of a quarter-scale model of a graphite nuclear reactor core, representative of the second generation of British advanced gas-cooled nuclear reactors, is investigated numerically and experimentally. Advanced gas-cooled nuclear reactor cores are polygonal, multilayer, arrays of graphite bricks, with each brick allowed to rock by design relative to each other in accordance with the boundary conditions. A 35 000 DOF, nonlinear finite element model of the core created by Atkins Nuclear, was analysed on a high performance computing facility at the University of Bristol, and a corresponding 8 t physical model, equipped with 3200 data acquisition channels, was built and tested on the University of Bristol 6-DOF shaking table. In this paper, the two models are subjected to a series of (1) synthetic earthquake and (2) idealised harmonic input motions. The experimental data are used to compare and verify the two models and explore the dynamics of the core. A kinematic model of the response is also developed based solely on geometric constraints. The results are presented in the form of response maps and graphs. Important conclusions are drawn as to the dynamics and earthquake response of such systems, which inform numerical model validation. It is found that contrary to the case of a small number of rocking blocks that exhibit highly complex response patterns, the behaviour of the model at hand is both smooth and repeatable. An analogy between the response of the core and that of dense granular matter exhibiting particle interlocking and dilatancy is highlighted

About the preliminary design of a self-aligning energy absorber system for railway vehicles

A new impact energy absorber for a railway car, which maintains its optimal performances even when an impact with a vertical offset occurs, is described. Starting from the description of the concept design, the present paper moves on to the structural optimisation of its components, in respect of both the functionality of the entire system and the maximum deformation energy that it can absorb, even in the case of an offset. A numerical simulation of the impact between two absorbers with offset verifies the effectiveness of the design choices. The numerical model has been also developed to reproduce the behaviour of the absorber under a falling impact mass that is made by means of a full-scale drop test, achieving a support of the effectiveness of both the simulation and the proposed concept.Peer reviewe