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Brittle Materials in Mechanical Extremes

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A special issue of Materials (ISSN 1996-1944).

Deadline for manuscript submissions: closed (31 October 2019) | Viewed by 31576

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Prof. Dr. Giovanni Bruno

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Bundesanstalt für Materialforschung und –prüfung (BAM), Berlin, Germany
Interests: neutron diffraction; residual stress; mechanical properties of materials; additive manufacturing; porous ceramics
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Dear Colleagues,

Brittle materials include a wide range of material classes: From polymers to metals, through classic glass, ceramics, and composites. They all share a supposed linear elastic behavior, but are often found to display non-linear stress–strain relationships, as well as high temperature dilation (or other properties). In this Special Issue, contributions describing and explaining this intriguing behavior, whether due to microcracking, interaction among constituent phases, or micro-structural features, are welcome. Advanced characterization techniques, challenging numerical and analytical models, as well unconventional experiments should be reported and spark the debate about the origin of the behavior of brittle materials under mechanical extremes.

The description of the mechanical behavior of brittle materials under operational (sometimes unconventional) loads, such as mechanical and temperature cycling, electric fields, corrosion environments, represents one of the focuses of the Special Issue. The discussion of analogies and differences between different materials, such as polymers and concrete, represent another focus of the issue (e.g., plasticity vs. microcracking). Modeling and rationalization of peculiar behaviors of brittle materials will be another focus of the issue.

All should be corroborated by advanced microstructural studies (microscopy, 3D imaging, etc.), leading to the identification of the microstructure–property relationships.

The grand goal is to provide an overview, through different aspects, of unconventional or unexpected reaction of brittle materials to external (mechanical, thermal, chemical, etc.) loads, at all temperatures of interest, including cryogenic.

Prof. Dr. Giovanni Bruno
Guest Editor

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Keywords

Ceramics
Concrete
Composites
non-linear behavior
anelasticity
microcracking
mechanical properties
high-temperature
micromechanical modeling
FEM
in-situ testing

Published Papers (11 papers)

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Editorial

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3 pages, 178 KiB

Open AccessEditorial

Brittle Materials in Mechanical Extremes

by Giovanni Bruno

Materials 2020, 13(20), 4610; https://doi.org/10.3390/ma13204610 - 16 Oct 2020

Viewed by 1188

Abstract

The goal of the Special Issue “Brittle Materials in Mechanical Extremes” was to spark a discussion of the analogies and the differences between different brittle materials, such as, for instance, ceramics and concrete. Indeed, the contributions to the Issue spanned from construction materials (asphalt and concrete) to structural ceramics, reaching as far as ice. The data shown in the issue were obtained by advanced microstructural techniques (microscopy, 3D imaging, etc.) and linked to mechanical properties (and their changes as a function of aging, composition, etc.). The description of the mechanical behavior of brittle materials under operational loads, for instance, concrete and ceramics under very high temperatures, offered an unconventional viewpoint on the behavior of brittle materials. This is not at all exhaustive, but a way to pave the road for intriguing and enriching comparisons. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

Research

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21 pages, 8758 KiB

Open AccessArticle

Experimental Investigation on the Quasi-Static Tensile Capacity of Engineered Cementitious Composites Reinforced with Steel Grid and Fibers

by Liang Li, Wenli Liu, Jun Wu, Wenjie Wu and Meng Wu

Materials 2019, 12(17), 2666; https://doi.org/10.3390/ma12172666 - 21 Aug 2019

Cited by 4 | Viewed by 2105

Abstract

An engineered cementitious composite (ECC) was reinforced with a steel grid and fibers to improve its tensile strength and ductility. A series of tensile tests have been carried out to investigate the quasi-static tensile capacity of the reinforced ECC. The quasi-static tensile capacities of reinforced ECCs with different numbers of steel-grid layers, types of fibers (Polyvinyl alcohol (PVA) fiber, KEVLAR fiber, and polyethylene (PE) fiber), and volume fractions of fibers have been tested and compared. It is indicated by the test results that: (1) On the whole, the steel grid-PVA fiber and steel grid-KEVLAR fiber reinforced ECCs have high tensile strength and considerable energy dissipation performance, while the steel grid-PE fiber reinforced ECC exhibits excellent ductility. (2) The ultimate tensile strength of the reinforced ECC can be improved by the addition of steel grids. The maximal peak tensile stress increase is about 50–95% or 140–190% by adding one layer or two layers of steel grid, respectively. (3) The ultimate tensile strength of the reinforced ECC can be enhanced with the increase of fiber volume fraction. For a certain kind of fiber, a volume fraction between 1.5% and 2% grants the reinforced ECC the best tensile strength. Near the ultimate loading point, the reinforced ECC exhibits strain hardening behavior, and its peak tensile stress increases considerably. The energy dissipation performance of the reinforced ECC can also be remarkably enhanced by such an increase in fiber volume fraction. (4) The ductility of the steel grid-PVA fiber reinforced ECC can be improved by the addition of steel grids and the increase of fiber volume fraction. The ductility of the steel grid-KEVLAR fiber reinforced ECC can be improved by the addition of steel grids alone. The ductility and energy dissipation performance of the steel grid-PE fiber reinforced ECC can be improved with the increase of fiber volume fraction alone. A mechanical model for the quasi-static initial and ultimate tensile strength of the steel grid-fiber reinforced ECC is proposed. The model is validated by the test data from the quasi-static tension experiments on the steel grid-PE fiber reinforced ECC. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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17 pages, 2821 KiB

Open AccessArticle

Evolution of Rheological Behaviors of Styrene-Butadiene-Styrene/Crumb Rubber Composite Modified Bitumen after Different Long-Term Aging Processes

by Yangsheng Ye, Gang Xu, Liangwei Lou, Xianhua Chen, Degou Cai and Yuefeng Shi

Materials 2019, 12(15), 2345; https://doi.org/10.3390/ma12152345 - 24 Jul 2019

Cited by 26 | Viewed by 2780

Abstract

In this study, a new type of composite modified bitumen was developed by blending styrene-butadiene-styrene (SBS) and crumb rubber (CR) with a chemical method to satisfy the durability requirements of waterproofing material in the waterproofing layer of high-speed railway subgrade. A pressure-aging-vessel test for 20, 40 and 80 h were conducted to obtain bitumen samples in different long-term aging conditions. Multiple stress creep recovery (MSCR) tests, linear amplitude scanning tests and bending beam rheometer tests were conducted on three kinds of asphalt binders (SBS modified asphalt, CR modified asphalt and SBS/CR composite modified asphalt) after different long-term aging processes, including high temperature permanent deformation performance, resistance to low temperature thermal and fatigue crack. Meanwhile, aging sensitivities were compared by different rheological indices. Results showed that SBS/CR composite modified asphalt possessed the best properties before and after aging. The elastic property of CR in SBS/CR composite modified asphalt improved the ability to resist low temperature thermal and fatigue cracks at a range of low and middle temperatures. Simultaneously, the copolymer network of SBS and CR significantly improved the elastic response of the asphalt SBS/CR modified asphalt at a range of high temperatures. Furthermore, all test results indicated that the SBS/CR modified asphalt possesses the outstanding ability to anti-aging. SBS/CR is an ideal kind of asphalt to satisfy the demand of 60 years of service life in the subgrade of high speed railway. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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16 pages, 3894 KiB

Open AccessArticle

Analysis of a Large Database of Concrete Core Tests with Emphasis on Within-Structure Variability

by Angelo Masi, Andrea Digrisolo and Giuseppe Santarsiero

Materials 2019, 12(12), 1985; https://doi.org/10.3390/ma12121985 - 20 Jun 2019

Cited by 24 | Viewed by 2901

Abstract

In reinforced concrete (RC) structures, the compressive strength of concrete can play a crucial role in seismic performance and is usually difficult to estimate. Major seismic codes prescribe that concrete strength must be determined essentially from in situ and laboratory tests. Mean values obtained from such tests are the reference design values when assessing existing structures under seismic actions. The variability of concrete strength can also play an important role, generally requiring that various homogeneous domains are identified in a single structure, in each of which a specific mean value should be assumed as representative. This study analyzes the inter- and intra-variability of the concrete strength of existing buildings using a very large database made up of approximately 1600 core tests extracted from RC buildings located in the Basilicata region (Southern Italy). The analysis highlighted that concrete strength variability was dependent on the structures’ dimensions as well as on the number of storeys. Moreover, the concrete strength of cores extracted from columns was found to be, on average, lower than that from beams, thus justifying the usual practice to extract cores mainly from columns, which results in a conservative approach as well as a more feasible one. Finally, some case studies were analyzed, specifically focusing on the effects of the within-storey variability. Conservative strength values, to be used especially in the case of vertical members subjected to high axial loads, are suggested. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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11 pages, 2396 KiB

Open AccessArticle

Quantifying the Adhesion of Silicate Glass–Ceramic Coatings onto Alumina for Biomedical Applications

by Francesco Baino

Materials 2019, 12(11), 1754; https://doi.org/10.3390/ma12111754 - 30 May 2019

Cited by 5 | Viewed by 2370

Abstract

Deposition of bioactive glass or ceramic coatings on the outer surface of joint prostheses is a valuable strategy to improve the osteointegration of implants and is typically produced using biocompatible but non-bioactive materials. Quantifying the coating–implant adhesion in terms of bonding strength and toughness is still a challenge to biomaterials scientists. In this work, wollastonite (CaSiO₃)-containing glass–ceramic coatings were manufactured on alumina tiles by sinter-crystallization of SiO₂–CaO–Na₂O–Al₂O₃ glass powder, and it was observed that the bonding strength decreased from 34 to 10 MPa as the coating thickness increased from 50 to 300 µm. From the viewpoint of bonding strength, the coatings with thickness below 250 µm were considered suitable for biomedical applications according to current international standards. A mechanical model based on quantized fracture mechanics allowed estimating the fracture toughness of the coating on the basis of the experimental data from tensile tests. The critical strain energy release rate was also found to decrease from 1.86 to 0.10 J/m² with the increase of coating thickness, which therefore plays a key role in determining the mechanical properties of the materials. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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16 pages, 7009 KiB

Open AccessArticle

In-Situ Damage Evaluation of Pure Ice under High Rate Compressive Loading

by Matti Isakov, Janin Lange, Sebastian Kilchert and Michael May

Materials 2019, 12(8), 1236; https://doi.org/10.3390/ma12081236 - 15 Apr 2019

Cited by 11 | Viewed by 2716

Abstract

The initiation and propagation of damage in pure ice specimens under high rate compressive loading at the strain rate range of 100 s⁻¹ to 600 s⁻¹ was studied by means of Split Hopkinson Pressure Bar measurements with incorporated high-speed videography. The results indicate that local cracks in specimens can form and propagate before the macroscopic stress maximum is reached. The estimated crack velocity was in the range of 500 m/s to 1300 m/s, i.e., lower than, but in similar order of magnitude as the elastic wave speed within ice. This gives reason to suspect that already at this strain rate the specimen is not deforming under perfect force equilibrium when the first cracks initiate and propagate. In addition, in contrast to quasi-static experiments, in the high rate experiments the specimens showed notable residual load carrying capacity after the maximum stress. This was related to dynamic effects in fractured ice particles, which allowed the specimen to carry compressive load even in a highly damaged state. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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15 pages, 5497 KiB

Open AccessFeature PaperArticle

Evolution of Thermal Microcracking in Refractory ZrO₂-SiO₂ after Application of External Loads at High Temperatures

by René Laquai, Fanny Gouraud, Bernd Randolf Müller, Marc Huger, Thierry Chotard, Guy Antou and Giovanni Bruno

Materials 2019, 12(7), 1017; https://doi.org/10.3390/ma12071017 - 27 Mar 2019

Cited by 8 | Viewed by 3828

Abstract

Zirconia-based cast refractories are widely used for glass furnace applications. Since they have to withstand harsh chemical as well as thermo-mechanical environments, internal stresses and microcracking are often present in such materials under operating conditions (sometimes in excess of 1700 °C). We studied the evolution of thermal (CTE) and mechanical (Young’s modulus) properties as a function of temperature in a fused-cast refractory containing 94 wt.% of monoclinic ZrO₂ and 6 wt.% of a silicate glassy phase. With the aid of X-ray refraction techniques (yielding the internal specific surface in materials), we also monitored the evolution of microcracking as a function of thermal cycles (crossing the martensitic phase transformation around 1000 °C) under externally applied stress. We found that external compressive stress leads to a strong decrease of the internal surface per unit volume, but a tensile load has a similar (though not so strong) effect. In agreement with existing literature on β-eucryptite microcracked ceramics, we could explain these phenomena by microcrack closure in the load direction in the compression case, and by microcrack propagation (rather than microcrack nucleation) under tensile conditions. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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14 pages, 2881 KiB

Open AccessArticle

Instrumented Indentation of Super-Insulating Silica Compacts

by Belynda Benane, Sylvain Meille, Geneviève Foray, Bernard Yrieix and Christian Olagnon

Materials 2019, 12(5), 830; https://doi.org/10.3390/ma12050830 - 12 Mar 2019

Cited by 5 | Viewed by 2793

Abstract

Highly porous silica compacts for superinsulation were characterized by instrumented indentation. Samples showed a multi-scale stacking of silica particles with a total porous fraction of 90 vol %. The two main sources of silica available for the superinsulation market were considered: fumed silica and precipitated silica. The compacts processed with these two silica displayed different mechanical properties at a similar porosity fraction, thus leading to different usage properties, as the superinsulation market requires sufficient mechanical properties at the lowest density. The measurement of Young’s modulus and hardness was possible with spherical indentation, which is an efficient method for characterizing highly porous structures. Comparison of the mechanical parameters measured on silica compacts and silica aerogels available from the literature was made. Differences in mechanical properties between fumed and precipitated compacts were explained by structural organization. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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16 pages, 4684 KiB

Open AccessArticle

Simulating Fiber-Reinforced Concrete Mechanical Performance Using CT-Based Fiber Orientation Data

by Vladimir Buljak, Tyler Oesch and Giovanni Bruno

Materials 2019, 12(5), 717; https://doi.org/10.3390/ma12050717 - 01 Mar 2019

Cited by 7 | Viewed by 2761

Abstract

The main hindrance to realistic models of fiber-reinforced concrete (FRC) is the local materials property variation, which does not yet reliably allow simulations at the structural level. The idea presented in this paper makes use of an existing constitutive model, but resolves the problem of localized material variation through X-ray computed tomography (CT)-based pre-processing. First, a three-point bending test of a notched beam is considered, where pre-test fiber orientations are measured using CT. A numerical model is then built with the zone subjected to progressive damage, modeled using an orthotropic damage model. To each of the finite elements within this zone, a local coordinate system is assigned, with its longitudinal direction defined by local fiber orientations. Second, the parameters of the constitutive damage model are determined through inverse analysis using load-displacement data obtained from the test. These parameters are considered to clearly explain the material behavior for any arbitrary external action and fiber orientation, for the same geometrical properties and volumetric ratio of fibers. Third, the effectiveness of the resulting model is demonstrated using a second, “control” experiment. The results of the “control” experiment analyzed in this research compare well with the model results. The ultimate strength was predicted with an error of about 6%, while the work-of-load was predicted within 4%. It demonstrates the potential of this method for accurately predicting the mechanical performance of FRC components. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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19 pages, 5641 KiB

Open AccessArticle

Influence of Activator Na₂O Concentration on Residual Strengths of Alkali-Activated Slag Mortar upon Exposure to Elevated Temperatures

by Tai Thanh Tran and Hyug-Moon Kwon

Materials 2018, 11(8), 1296; https://doi.org/10.3390/ma11081296 - 27 Jul 2018

Cited by 20 | Viewed by 3432

Abstract

The mechanical strength variation of ambient cured Alkali-activated mortar (AAS) upon exposure to elevated temperatures from 200 to 1200 °C was studied in this article. Slag was activated by the combination of sodium silicate liquid (Na₂SiO₃) and sodium hydroxide (NaOH) with different Na₂O concentrations of 4%, 6%, 8%, and 10% by slag weight. Mechanical properties comprising compressive strength, flexural strength, and tensile strength before and after exposure were measured. Thermogravimetric analysis (Thermogravimetric analysis (TGA) and Derivative thermogravimetric (DTG)), X-ray diffraction (XRD), scanning electron microscope (SEM), and energy-dispersive X-ray spectroscopy (EDS) were also used for strength alteration explanation. The results indicated that Na₂O concentration influence on strength variation of AAS mortar was observed clearly at temperature range from ambient temperature to 200 °C. The melting alteration of AAS mortar after exposed to 1200 °C was highly dependent on concentrations of Na₂O. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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14 pages, 3889 KiB

Open AccessArticle

Double Feedback Control Method for Determining Early-Age Restrained Creep of Concrete Using a Temperature Stress Testing Machine

by He Zhu, Qingbin Li, Yu Hu and Rui Ma

Materials 2018, 11(7), 1079; https://doi.org/10.3390/ma11071079 - 25 Jun 2018

Cited by 20 | Viewed by 3630

Abstract

Early-age restrained creep influences the cracking properties of concrete. However, conventional creep measurements require a large number of tests to predict the restrained creep as it is influenced by the combined effects of variable temperature, creep recovery, and varying compression and tension stresses. In this work, a double feedback control method for temperature stress testing was developed to measure the early-age restrained creep of concrete. The results demonstrate that the conventional single feedback control method neglects the effect of restrained elastic deformation, thus providing a larger-than-actual creep measurement. The tests found that the double feedback control method eliminates the influence of restrained elastic deformation. The creep results from the double feedback method match well with results from the single feedback method after compensation for the effects of restrained elastic deformation is accounted for. The difference in restrained creep between the single and double feedback methods is significant for concrete with a low modulus of elasticity but can be neglected in concrete with a high modulus of elasticity. The ratio between creep and free deformation was found to be 40–60% for low, moderate, and high strength concretes alike. The double feedback control method is therefore recommended for determining the restrained creep using a temperature stress testing machine. Full article

(This article belongs to the Special Issue Brittle Materials in Mechanical Extremes)

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