Photonics and fracture toughness of heterogeneous composite materials

Fracture toughness measures the resistance of a material to fracture. This fundamental property is used in diverse engineering designs including mechanical, civil, materials, electronics and chemical engineering applications. In spite of the advancements made in the past 40 years, the evaluation of this remains challenging for extremely heterogeneous materials such as composite concretes. By taking advantage of the optical properties of a thin birefringent coating on the surface of opaque, notched composite concrete beams, here we sense the evolution of the maximum shear stress distribution on the beams under loading. The location of the maximum deviator stress is tracked ahead of the crack tip on the experimental concrete samples under the ultimate load, and hence the effective crack length is characterised. Using this, the fracture toughness of a number of heterogeneous composite beams is evaluated and the results compare favourably well with other conventional methods using combined experimental and numerical/analytical approaches. Finally a new model, correlating the optically measured shear stress concentration factor and flexural strength with the fracture toughness of concretes is proposed. The current photonics-based study could be vital in evaluating the fracture toughness of even opaque and complex heterogeneous materials more effectively in future

Assessments of the Microstructural and Mechanical Properties of Hybrid Fibrous Self-Consolidating Concretes using Ingredients of Plastic Wastes

This paper focuses on the experimental investigation carried out on self-consolidating concrete (SCC) reinforced with micro-steel fiber and hybrid fibers (combination of micro-steel fiber and recycled high density polyethylene fiber derived from municipal wastes). The physical properties of fresh and hardened concrete including flowability, setting time and durability, the mechanical properties, namely, compressive strength and flexural strength, and microstructural analysis were studied. Micro-steel fiber addition was seen to enhance the flowability of concrete than the non-fibrous and hybrid fiber reinforced concretes. The setting time of SCC mixtures prolonged with the addition of fibers into concrete mixtures. Hybrid fiber reinforced SCC mixtures have displayed reduction in drying shrinkage. The compressive and flexural strengths of the fiber reinforced concretes show a marginal reduction in strength- when compared with the strength of unreinforced concrete. The results of the microstructure analysis clearly demonstrate that the hybrid fibers bond well with the cement matrix and stronger than the bonding between micro-steel fibers and cement matrix

Whole-Field Stress Sensing and Multiscale Mechanics for Developing Cement-Based Composites Containing Recycled Municipal Granular Wastes

Worldwide, there is a growing level of interest to develop sustainable cement-based products and processes in which the usage of natural resources such as sand and limestone are reduced from the current levels. One of the ways to achieve this is by replacing them with suitable inclusions of recycled granular materials from municipal wastes where possible. However, to understand the effects of such inclusions in concrete structures, research advancements are needed to sense and characterise the distribution of stresses (/strains) at the local scale and to establish their links with the fracture and bulk strength characteristics under external loading environments, which is the focus here. In this research, polyethylene (PE)-based granular materials derived from municipal wastes and fly ash obtained from the incineration of municipal solid wastes are used together as secondary raw materials in preparing the concrete mixtures. Photo stress analysis (PSA) is performed here, making non-contact and whole-field digital measurements of maximum shear stress distribution and the directions of the principal stresses at any point of interest on the surface of the samples under external loading. Their links with the fracture toughness and flexural strength of the samples cured at different times are presented. The novel PSA-based stresssensing helps to establish new understandings of the strength characteristics of composites across scales in the future