Computational mesoscale modelling of concrete material under high strain rate loading

Cement-based composite materials are widely used in ngineering applications. The strength and damage patterns of such materials depe n upon the properties of the constituent components as well as the microstructur e. Three scale levels are generally recognized in the analysis of the mechani cal behaviour of composites, namely, macro-scale, meso-scale, and nanoor atomi stic scale. Modelling of the mechanical properties at the meso-level provides a powerful means for the understanding of the physical processes underlying the macroscopic strength and failure behaviour of the composite materials under various loading conditions. This thesis endeavours to develop effective and eff ici nt mesoscale models for cement-based composites, especially concrete, with a focus on dynamic analysis applications and in a three-dimensional stress-stra in environment. These models are subsequently applied to investigate the intrinsic m roscopic mechanisms governing the behaviour of such material under complex and hi gh rate loadings, such as those due to shock, impact and blast. To cater to the needs of dynamic analysis under com plex stress conditions, a general 2-dimensinal (2D) mesoscale modelling frame work is further developed with the incorporation of the 3-D effect. This framework integrates the capabilities of MATLAB programming for the generation of the mesosc ale geometric structure, ANSYS-CAE for finite element mesh generation, and t he hydrocode LS-DYNA for solving the dynamic response of the model. The 3D e ffect is incorporated via a novel pseudo-3D modelling scheme such that the crucial la teral confinement effect during the transient dynamic response can be realistically represented. With the above mesoscale model a comprehensive inve st gation is conducted on the dynamic increase factor (DIF) in the concrete s tr ngth under compression, with particular focus on the variation trend at differen t strain rate regimes, and the key influencing factors. The wave propagation effect un der high strain rate is scrutinised from a strip-by-strip perspective, and the correlat ion between the externally measured stress-strain quantities and the actual pr ocesses within the specimen is examined. The contribution of the material heteroge neity, as well as the structural effect (inertia), in the dynamic strength enhanceme nt is evaluated. The classical Brazilian (splitting) test for the dy namic tensile behaviour of concrete is also investigated with the aid of the m soscale model. Of particular interest here is the validity of such an indirect s e up in reproducing the tensile behaviour of the specimen under high strain rates, as well as the effect of the heterogeneity in the dynamic tensile strength. Comp lications are found to arise as the loading rate increases. The change of the damage pa tt rns with increase of the loading rate and the implications on the interpreta tion of the results are discussed. As an ideal solution to modelling of the 3-D effect s, a methodology for the creation of a complex real 3-dimensional mesoscale model is put forward in the last part of the thesis. A geometric concept, called con vex hull, is adopted for the representation of aggregates, and this makes it pos sible to utilize the relevant algorithms in computational geometry for the presen t purpose of generation of random 3-D aggregates. A take-and-place procedure i s employed to facilitate the generation of the complete 3-D meso-structure. Asso ciated techniques are developed for fast detection of particle inclusion-intersecti on. An example 3D mesoscale model is presented and representative numerical simulatio ns are carried out to demonstrate the performance of the 3-D mesoscale modelling sche me. Acknowledgements There are too many people I owe my gratitude to whe n t is thesis finally reaches its completion. Particularly, utmost thanks to my super visor Prof. Yong Lu. It is utterly impossible to finish this thesis without his skilfu l supervision: his in-depth insight to the field and broad range of knowledge ensured that my research always goes in the correct direction and that any problems encountered ar solved in an appropriate way. His logical way of thinking and a global persp ective in problem-solving have been a great value for me. I would like to thank Dr Zhenguo Tu, Dr Lei Mao and Dr Xiaoqin Li for their invaluable suggestions and constructive discussions during my study. I would also like to thank Prof. Michael Rotter, my second super visor, and the friends and colleagues at the University of Edinburgh for their help and support. I owe my loving thanks to my wife Sherlly Li and my son Ruoshui Song. They have sacrificed a lot due to my research abroad. A special gratitude is due to my parents and parents-in-law for their loving support . The financial support of the University of Edinburg h is gratefully acknowledged.