Eric N. Landis

Microstructure and fracture in three dimensions

Abstract A high resolution three dimensional (3D) scanning technique called X-ray microtomography was used to measure internal crack growth in small mortar cylinders under compressive loading. Tomographic scans were made at different load increments in the same specimen. 3D image analysis was used to measure internal crack growth during each load increment. Load–deformation curves were used to measure the corresponding work of the external load on the specimen. Fracture energy was calculated using a linear elastic fracture mechanics approach using the measured surface area of the internal cracks created. The measured fracture energy was of the same magnitude that is typically measured in concrete tensile fracture. A nominally bilinear incremental fracture energy curve was measured. Separate components for crack formation energy and secondary toughening mechanisms are proposed. The secondary toughening mechanisms were found to be about three times the initial crack formation energy.

Ultrasonic investigation of concrete with distributed damage

Bridge decks deteriorate due to many causes including low level fatigue cycling, thermal loading, chemical attack, and reinforcing steel corrosion. This deterioration takes the form of distributed microscopic damage that may evolve into large defects such as cracks, delaminations, spalling, and scaling. An experimental program was conducted to evaluate ultrasonic techniques for measuring distributed cracking in concrete structures. Distributed cracking refers primarily to microcracking and other high porosity regions that generally precede large cracks. An investigation of distributed cracking yields information on weaknesses in the materials that may ultimately lead to major cracking and failure, but also can be used to evaluate distress mechanisms that do not necessarily result in large cracks. Distributed cracking in concrete was induced by freeze-thaw cycling and salt-scaling. Ultrasonic tests were used to measure changes in attenuation, pulse velocity, and peak frequency of the ultrasonic waves due to the distributed damage. The ultrasonic measurements were correlated with damage observed using optical microscopy. It was found that ultrasonic pulse velocity was not very sensitive to changes caused by distributed microcracking. The change in signal amplitude (a measure of ultrasonic attenuation) was quite sensitive to changes caused by microcracking, although the measurements showed considerable scatter. The peak frequency of the ultrasonic signal was also quite sensitive to the condition of the concrete. These results must be considered in the development field tests for evaluation of concrete structures.

Micro–macro fracture relationships and acoustic emissions in concrete

Abstract In this work we are interested in how micromechanical phenomena affect bulk mechanical properties. Specifically we are interested in microfracture characteristics and how they influence damage evolution and fracture toughness. Toward this end, quantitative acoustic emission techniques were used to measure microfracture properties in an array of cement-based materials of varying microstructure. Microcracks were modeled using a seismic moment tensor, which could be estimated through deconvolution of the measured acoustic emission waveforms. Results of the experiments indicate that materials with higher bulk fracture toughness had larger numbers of sliding mode microcracks, while materials with lower bulk fracture toughness had fewer numbers of tensile mode microcracks.

Coupled experiments and simulations of microstructural damage in wood

In this paper, we explore ways to couple experimental measurements with the numerical simulations of the mechanical properties of wood. For our numerical simulations, we have adopted a lattice approach, where wood fibers or bundles of wood fibers are modeled as discrete structural elements connected by a lattice of spring elements. Element strength and stiffness properties are determined from bulk material properties. Damage is represented by broken lattice elements, which cause both stiffness and strength degradation. The modeling approach was applied to small specimens of spruce subjected to transverse uniaxial tension, and mode I transverse splitting. The model was found to be good at predicting the load-deformation response of both notched and unnotched specimens, including the post-peak softening response. In addition, the damage patterns predicted by the model are consistent with those observed in the experiments.

Three-dimensional work of fracture for mortar in compression

Abstract A high resolution three-dimensional scanning technique called X-ray microtomography was used to measure internal crack growth in small mortar cylinders loaded in uniaxial compression. Tomographic scans were made at different load increments in the same specimen. Three-dimensional image analysis was used to measure internal crack growth during each load increment. Load–deformation curves were used to measure the corresponding work of the external load on the specimen. Fracture energy was calculated using a linear elastic fracture mechanics approach, but using the actual surface area of internal cracks created. Preliminary results indicate fracture energies in the same range as those measured using traditional techniques.

Explicit representation of physical processes in concrete fracture

The utility of concrete as a cost-effective, durable structural material depends largely on its fracture properties. Improved understandings of the physical bases and scaling of concrete fracture are needed to meet the growing expectations and constraints on concrete usage in high-performance applications, and to develop alternative cementitious materials for reduced environmental load. This paper reviews relevant knowledge of fracture processes in concrete, with a particular focus on ways new 3D measurements may be coupled with discrete modelling approaches. The microstructure of concrete is briefly reviewed in the context of the physical processes that dictate fracture properties. We advocate a modelling approach where, to the extent possible, a direct correspondence is made between measured material structure and the structures explicitly represented by numerical models. This correspondence is made by utilizing x-ray microtomography, a high-resolution 3D imaging technique, and lattice models that mimic physical structure and processes. 3D image analysis provides us with quantitative measurements of internal damage progression. 3D lattice simulations offer the potential for extracting additional knowledge from these high-fidelity measurements.

The influence of microcracking on the mechanical behavior of cement based materials

Abstract Quantitative acoustic emission techniques were applied to basic problems of microfracture in cement based materials. Acoustic emissions in cement based materials result from microcracks and other dynamic phenomena in the fracture process zone. The goals of this research program were to characterize microcracking in various cement based materials, to track the evolution of damage in those materials, and to examine the relationships to overall mechanical behavior. Characterizations of the microcracks showed a dependence on the degree of inhomogeneity in the material. Fine-grained materials showed different microfracture characteristics than coarse-grained materials. Microcracks were characterized according to their fracture mode. The fine-grained materials tested showed primarily mixed-mode microfracture, whereas the coarse-grained materials showed primarily mode II (shear) microfracture. It is experimentally shown that there exists a relationship between the microcrack characteristics established through quantitative acoustic emission analysis and the overall fracture toughness of the material.

Nanocellulose and Microcellulose Fibers for Concrete

A study was conducted in which a reactive powder concrete was reinforced with a combination of nanocellulose and microcellulose fibers to increase the toughness of an otherwise brittle material. These fibers could provide the benefit of other micro- and nanofiber reinforcement systems at a fraction of the cost. An empirical investigation into the effects of several different reinforcement schemes on processing parameters and mechanical properties of a reactive powder concrete mixture was conducted. In particular, notched-beam tests were performed under crack-mouth opening displacement control to measure fracture energy under stable crack-growth conditions. Preliminary results show that the addition of up to 3% micro- and nanofibers in combination increased the fracture energy by more than 50% relative to the unrein-forced material, with little change in processing procedure. Splitting tensile tests were also performed for comparison with beam-bending tests. Current work focuses on applying high-resolution three-dimensional imaging techniques to better quantify the physical microstructures and the corresponding shifts in damage mechanisms that lead to higher toughness.

Finite element techniques and models for wood fracture mechanics

Numerical models for wood fracture and failure are commonly based on the finite element method. Most of these models originate from general theoretical considerations for other materials. This limits their usefulness because no amount of complexity in a model can substitute for lack of an appropriate representation of the physical mechanisms involved. As for other materials, wood fracture and failure models always require some degree of experimental calibration, which can introduce ambiguity into numerical predictions because at present there is a high degree of inconsistency in test methods. This paper explores avenues toward achieving models for wood fracture that are both appropriate and robust.

Mechanical resilience and cementitious processes in Imperial Roman architectural mortar

Significance A volcanic ash–lime mortar has been regarded for centuries as the principal material constituent that provides long-term durability to ancient Roman architectural concrete. A reproduction of Imperial-age mortar based on Trajan’s Markets (110 CE) wall concrete resists microcracking through cohesion of calcium–aluminum–silicate–hydrate cementing binder and in situ crystallization of platey strätlingite, a durable calcium-aluminosilicate mineral that reinforces interfacial zones and the cementitious matrix. In the 1,900-y-old mortar dense intergrowths of the platey crystals obstruct crack propagation and preserve cohesion at the micron scale. Trajanic concrete provides a proven prototype for environmentally friendly conglomeratic concretes that contain ∼88 vol % volcanic rock yet maintain their chemical resilience and structural integrity in seismically active environments at the millenial scale. The pyroclastic aggregate concrete of Trajan’s Markets (110 CE), now Museo Fori Imperiali in Rome, has absorbed energy from seismic ground shaking and long-term foundation settlement for nearly two millenia while remaining largely intact at the structural scale. The scientific basis of this exceptional service record is explored through computed tomography of fracture surfaces and synchroton X-ray microdiffraction analyses of a reproduction of the standardized hydrated lime–volcanic ash mortar that binds decimeter-sized tuff and brick aggregate in the conglomeratic concrete. The mortar reproduction gains fracture toughness over 180 d through progressive coalescence of calcium–aluminum-silicate–hydrate (C-A-S-H) cementing binder with Ca/(Si+Al) ≈ 0.8–0.9 and crystallization of strätlingite and siliceous hydrogarnet (katoite) at ≥90 d, after pozzolanic consumption of hydrated lime was complete. Platey strätlingite crystals toughen interfacial zones along scoria perimeters and impede macroscale propagation of crack segments. In the 1,900-y-old mortar, C-A-S-H has low Ca/(Si+Al) ≈ 0.45–0.75. Dense clusters of 2- to 30-µm strätlingite plates further reinforce interfacial zones, the weakest link of modern cement-based concrete, and the cementitious matrix. These crystals formed during long-term autogeneous reaction of dissolved calcite from lime and the alkali-rich scoriae groundmass, clay mineral (halloysite), and zeolite (phillipsite and chabazite) surface textures from the Pozzolane Rosse pyroclastic flow, erupted from the nearby Alban Hills volcano. The clast-supported conglomeratic fabric of the concrete presents further resistance to fracture propagation at the structural scale.