Multiple Pseudo-Plastic Appearance of the Dynamic Fracture in Quasi-Brittle Materials.

Gianmario Riganti,Ezio Cadoni

doi:10.3390/ma13214976

Abstract

Understanding and simulating the dynamic response of quasi-brittle materials still remains as one of the most challenging issues in structural engineering. This paper presents the damage propagation material model (DAMP) developed in order to obtain reliable results for use in structural engineering practice. A brief overview focuses on the differences between fracture mechanics studies, and engineering material modelling is presented to highlight possible guideline improvements. An experimental dynamic test performed on ultra-high-performance concrete specimens was used to obtain evidence of the physical behaviour of brittle materials with respect to specimen size variations and, consequently, to verify the reliability of the material equations proposed. Two widely used material models (RHT and M72R3), as representatives of the classical brittle material models for structural purposes, and the proposed material model are compared. Here, we show how: (i) the multiple structural strength of brittle materials arises from the damage propagation process, (ii) there is no contradiction between fracture mechanics and the engineering approach once the velocity of damage propagation is chosen as fundamental material property and (iii) classical dynamic material models are intrinsically not objective with related loss of predictive capability. Finally, the original material model equation and the experimental strategy, dedicated to its extended verification, will be shown in order to increase the design predictiveness in the dynamic range considering structure and specimen size variations. The dynamic stress increasing factor (DIF), experimentally observed and widely recognised in literature as a fundamental concept for quasi-brittle material modelling, has been reviewed and decomposed in its geometrical and material dependencies. The new material model defines its DIF starting from the physical quantities of the damage propagation velocity applied to the test case boundary conditions. The resultant material model predictiveness results improved greatly, demonstrating its ability to model several dynamic events considering size and dynamic load variations with a unique material property set without showing contradictions between numerical and experimental approaches.

Highlights

In the design and construction of critical infrastructures, there is a great and renewed interest in understanding the mechanical response of quasi-brittle material and structural components subjected to high dynamic loading including earthquakes, man-made or natural hazards, oil perforations, etc
The purpose of this paper is to present the damage propagation material model and to assess its suitability to describe the dynamic fracture of quasi-brittle materials by comparing it with the results obtained with two widely used models (M72R3 [4] and RHT [5,6])
The extensive experimental program was performed by means of the modified Hopkinson bar apparatus (MHB) based over the original idea of Albertini and Montagnani [31]

Summary

Introduction

In the design and construction of critical infrastructures (i.e., power plants, nuclear reactors, protective structures, dams, bridges), there is a great and renewed interest in understanding the mechanical response of quasi-brittle material and structural components subjected to high dynamic loading including earthquakes, man-made or natural hazards, oil perforations, etc. Current structural design is principally based on the knowledge of the material failure limits and on the capability of the material model to represent the actual behaviour of both material and structure. Materials 2020, 13, 4976 applied to quasi-brittle materials has brought about several open questions regarding the reliability and the predictive capability of the material models used. Better integration between the material model equations and the real physical behaviour of materials in a wider strain rate range is expected. The quasi-brittle material failure is characterised by a crack propagation process in which the final reduction of the structural element resistance is determined by the loss of its integrity. The process of brittle fracture has been investigated in different ways based on researchers’ scientific fields and practical interests. It is possible to distinguish different approaches, e.g., based on:

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Discussion

Conclusion