Double Torsion Test Research Articles

大理石におけるサブクリティカルき裂進展の周辺環境依存性

It is important to investigate time-dependent deformation and fracturing in rock for the purpose of ensuring the long-term stability of structures in a rock mass. Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. It is known that subcritical crack growth is influenced by not only stress but also surrounding environment. Studies of subcritical crack growth have been widely conducted for silicate rocks such as igneous rocks and sandstones. On the other hand, information of subcritical crack growth in carbonate rocks is not enough. Especially, influence of surrounding environmental conditions on subcritical crack growth in carbonate rocks has not been clarified yet. In this study, we investigated subcritical crack growth in carbonate rocks. Specifically, we investigated the influence of temperature and relative humidity on subcritical crack growth in air and in water. Macedonian marble was used as a rock sample. To measure subcritical crack growth, we used the load relaxation method of the double-torsion test. It was shown that the crack velocity increased when the temperature was higher at a constant relative humidity in air and in water. In addition, the crack velocity increased with increasing the relative humidity at a constant temperature. It is concluded that the temperature and relative humidity influence subcritical crack growth in carbonate rock in air and in water. From the results obtained in this study, it can be suggested that the condition with low temperature and low humidity is desirable for the long-term integrity of a carbonate rock.

In Situ Mechanical Testing Techniques for Real-Time Materials Deformation Characterization

In situ mechanical property testing has the ability to enhance quantitative characterization of materials by revealing the occurring deformation behavior in real time. This article will summarize select recent testing performed inside a scanning electron microscope on various materials including metals, ceramics, composites, coatings, and 3-Dimensional graphene foam. Tensile and indentation testing methods are outlined with case studies and preliminary data. The benefits of performing a novel double-torsion testing technique in situ are also proposed.

Effect of microstructure and grain boundary chemistry on slow crack growth in silicon carbide at ambient conditions

Silicon carbide (SiC) is being used increasingly as a room temperature structural material in environments where moisture cannot always be excluded. Unfortunately, there have been almost no reports on slow crack growth (SCG) in SiC at room temperature. To address this gap, SCG in SiC was studied using constant stress rate and double torsion tests in water. SiC based materials were produced with a wide range of grain boundary chemistries and microstructures, which may affect their slow crack growth behaviour. To clarify the role of chemistry and microstructure respectively, solid state (SS) sintering with carbon and boron along with liquid phase (LP) sintering using oxides additives were used to produce materials with fine and coarse grains. The LP-SiC was three times more sensitive to SCG than SS-SiC materials. Moreover, the larger grained material with a higher toughness was less sensitive to SCG than the materials with fine grains.

Test of subcritical crack growth and fracture toughness under water-rock interaction in three types of rocks

The subcritical crack growth and fracture toughness in peridotite, lherzolite and amphibolite were investigated with double torsion test. The results show that water-rock interaction has a significant influence on subcritical crack growth. With water-rock interaction, the crack velocity increases, while the stress intensity factor declines, which illustrates that water-rock interaction can decrease the strength of rocks and accelerate the subcritical crack growth. Based on Charlse theory and Hilling & Charlse theory, the test data were analyzed by regression and the correlation coefficients were all higher than 0.7, which shows the correlation is significant. This illustrates that both theories can explain the results of tests very well. Therefore, it is believed that the subcritical crack growth attributes to the breaking of chemical bond, which is caused by the combined effect of the tensile stress and the chemical reaction between the material at crack tip and the corrosive agent. Meanwhile, water-rock interaction has a vital effect on fracture toughness. The fracture toughness of samples under atmospheric environment is higher than that of samples immersed in water. And water-rock interaction has larger influence on fracture toughness in amphibolite than that in peridotite and lherzolite.

An experimental study of crack growth in claystones

An experimental study is presented for the characterisation of crack propagation in claystones. Three point bending (TPB) and double torsion (DT) tests are conducted in order to determine the material toughness and crack propagation velocity, respectively. The experimental data obtained from both TPB and DT tests show that the value of critical stress intensity factor is a function of loading rate and it is affected by the material structural anisotropy. The crack propagation velocity is determined as a function of applied stress intensity factor. The data obtained from the present work will provide an experimental background for the modelling of time-dependent deformation in claystones.

Three-dimensional Analysis of an In Situ Double-torsion Test by X-ray Computed Tomography and Digital Volume Correlation

The double-torsion (DT) test is commonly used to characterize the slow crack growth behavior of brittle materials. However, it relies on several mechanical hypotheses which still need to be deeply tackled. Measuring the full 3D displacement field experimentally through in situ experiments would improve the understanding of the mechanical behaviour. A dedicated experimental set-up was designed to perform in situ tests in a X-ray Computed Tomography (XCT) scanner on a porous brittle ceramic which is optically opaque. The analysis of the three-dimensional images with Digital Volume Correlation (DVC) allows a first study of the global kinematics and the stable crack growth propagation. This experimental and numerical methodology presents a strong potential for a future deep understanding of this complex mechanical test.

Determination of the subcritical crack growth parameters in rocks using the constant stress-rate test

Subcritical crack growth plays an important role in evaluating the long-term stability of structures in rocks. The characteristics of subcritical crack growth can be described by a relationship between the stress intensity factor and the crack velocity. This paper presents the results of studies conducted to validate the constant stress-rate test for determining subcritical crack growth parameters in rocks, compared with the conventional testing method, the double torsion test. Various types of specimens including the Brazilian disk, three-point bending, grooved disk, single edge notched bend, and compact tension specimens have been tested on Coconino sandstone under the constant stress-rate scheme and the subcritical crack growth parameters have been determined. The results of the constant stress-rate test are in good agreement with the results of double torsion test. More importantly, the stress-rate tests can determine the parameter A with a much smaller standard deviation than the DT test. Thus the constant stress-rate test seems to be a valid test method for rocks. The results show that the parameter n is almost constant regardless of the test method and specimen geometry, but the parameter A varies with the test method and specimen geometry.

Optical method for measuring slow crack growth in cementitious materials

The double-torsion (DT) test is commonly used to calculate slow or subcritical crack velocities in (quasi-)brittle engineering materials directly from the measured load relaxation of notched DT-specimens. In cementitious materials a significant part of the recorded load relaxation in the DT-test may be due to specimen creep deformation, and this would then lead to overestimated crack velocities. In this paper we describe a method to optically measure slow crack growth in cementitious materials by carrying out DT-tests under the optical microscope or inside the environmental SEM. Crack tip detection is facilitated by digital image correlation of the time-lapse microscope recordings. DT-tests at 10 % relative humidity in hardened cement paste (with w/c-ratio of 0.4, 0.5 and 0.6) showed that optically measured crack velocities were significantly lower than those calculated from the DT-specimen relaxation. In many experiments the subcritical crack growth rapidly stopped, while an ongoing specimen load relaxation was recorded. At 90 % relative humidity, load-relaxation in the DT-test was much stronger than at 10 % relative humidity, because subcritical cracking and creep-induced relaxation both increase with moisture content.

Development of a double-torsion fracture test to predict channelized crack behaviors of asphalt concrete

Fracturing of asphalt concrete paving surfaces and overlays is a significant cause of premature pavement deterioration. Fracture mechanics and the proper handling of crack propagation are much needed to understand the mechanisms that are relevant for increasing pavement life. The topic is still quite wide in the field of asphalt pavement design and analysis. This paper shows the experimental development of a double-torsion fracture test for predicting fundamental fracture properties of bituminous materials. Double-torsion test specimens were prepared from a model asphalt material using an automatic French slab compactor. Double-torsion tests were performed at low temperatures to investigate brittle and/or quasi-brittle behavior of bituminous materials. A constant displacement control method was applied to obtain reliable fracture properties of bituminous materials. Test geometry with different notch lengths was considered to obtain a standard specimen configuration without any unexpected failure. Characteristic fracture properties from double-torsion tests were determined based on the linear elastic fracture mechanics theory including compliance. Experimental results showed that the notch length was not a significant factor in the determination of stress intensity factor properties from double-torsion fracture tests on the model material used in this study.

Double torsion testing machine to determine the subcritical fracture index in rocks

This paper discusses the design methodology applied to build a testing machine to determine the sub-critical fracture index in a rock, based on double torsion testing in order to characterize naturally fractured formations such as those located in the Colombian Llanos Foothill Basin . These formations have been subjected to cyclic loads over time, causing fractures that trend to spread at sub-critical stress intensity values. Similarly, it presents the results of testing conducted on nine specimens of the Tambor Formation from 2 different outcrops to establish the testing traceability in the equipment.

An Evaluation of the Double Torsion Technique

Double Torsion (DT) is a powerful testing technique for fracture mechanics characterisation of brittle materials as, in principle, it provides a crack length independent test configuration. However, several corrections have been proposed to address variations of experimental results reported from various laboratories. These correction factors address the validity of the DT configuration and its crack length independent stress intensity. Never the less, there seems to be no consensus in literature on the various corrections and the reason of reported variations. This paper presents firstly a critical review of the DT technique, followed by proposed corrections through an experimental analysis using the proposed corrections, a Finite Element model of the geometry and the use of Digital Image Correlation to measure out-of-plane surface deformations. It focuses on the validity of the constant stress intensity regime and the independence of crack length in a critical evaluation using Polymethylmethacrylate test specimens. Assessment of three un-grooved specimen geometry configurations demonstrated the apparent regime of approximately constant stress intensity, although a small but clear dependence of the stress intensity on crack length was observed in all specimen configurations. This dependence is attributable to significant load-point deflections and out-of plane deformations that are not accounted for in the DT analysis. Revisions of the proposed analysis methodologies show that a crack length independent specimen geometry can be achieved, however at the cost of less accurate data. Reliable and accurate data can be achieved with a DT testing configuration using an optimum specimen configuration.

Effects of humidity and temperature on subcritical crack growth in sandstone

In order to ensure long-term stability of structures in a rock mass, the study of time-dependent fracturing is essential. The influences of the surrounding environmental conditions and rock fabric on subcritical crack growth in sedimentary rocks in air are yet to be clarified, while the nature of subcritical crack growth in igneous rocks has been studied well. In this study, the influences of temperature and relative humidity on subcritical crack growth in Berea sandstone, Shirahama sandstone and Kushiro sandstone were investigated in air. The load relaxation method of Double Torsion (DT) testing method was used to measure both crack velocity and stress intensity factor under a controlled temperature and relative humidity. Results show that the change of the crack velocity at a given stress intensity factor was unclear when the temperature increased under a constant relative humidity in air. On the other hand, we show that the crack velocity increased by several orders of magnitude when the relative humidity increased threefold or fourfold under a constant temperature at a given stress intensity factor. This increase is much larger than that expected from the conventional concept based on the theory of stress corrosion. It is therefore necessary to consider the additional mechanisms for subcritical crack growth in sandstone. The increase of the crack velocity was larger for sandstone which contained larger amount of clays. We conclude that subcritical crack growth in sandstone in air is affected remarkably by the relative humidity and the amount of clays in rock.

Numerical investigations of the stepped double torsion fracture toughness specimen

This study aims at investigating the fracture behaviour of double torsion specimens using the finite element method. Typical double torsion tests encompass a series of constant-thickness specimens to evaluate the material plane strain fracture toughness. In contrast, the concept of using a novel variable thickness stepped specimen aims at deducing the fracture toughness using a single specimen. In this work, the feasibility of this approach is examined and the effect of the number of steps and fracture thickness in a specimen upon the resulting conditional stress intensity factor is evaluated. The finite element models employed experimentally determined values of the fracture load to evaluate the conditional stress intensity factor of the specimen. Finite element predictions were compared with earlier experimental results using both cast aluminium silicon alloy and gray cast iron specimens and good matching was observed between experimental results and numerical predictions.

Processing and performance of self-healing materials

Two self-healing methods were implemented into composite materials with self-healing capabilities, using hollow glass fibres (HGF) and microencapsulated epoxy resin with mercaptan as the hardener. For the HGF approach, two perpendicular layers of HGF were put into an E-glass/epoxy composite, and were filled with coloured epoxy resin and hardener. The HGF samples had a novel ball indentation test method done on them. The samples were analysed using micro-CT scanning, confocal microscopy and penetrant dye. Micro-CT and confocal microscopy produced limited success, but their viability was established. Penetrant dye images showed resin obstructing flow of dye through damage regions, suggesting infiltration of resin into cracks. Three-point bend tests showed that overall performance could be affected by the flaws arising from embedding HGF in the material. For the microcapsule approach, samples were prepared for novel double-torsion tests used to generate large cracks. The samples were compared with pure resin samples by analysing them using photoelastic imaging and scanning electron microscope (SEM) on crack surfaces. Photoelastic imaging established the consolidation of cracks while SEM showed a wide spread of microcapsules with their distribution being affected by gravity. Further double-torsion testing showed that healing recovered approximately 24% of material strength.

Subcritical crack growth in rocks in an aqueous environment

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In the present study, we investigated subcritical crack growth in rock in distilled water (pH = 5–7) and in an aqueous solution of sodium hydroxide (NaOHaq, pH = 12), comparing the results to those in air. We also investigated the effect of the pH in an aqueous environment. We used andesite and granite for all our tests. We determined the relationship between the crack velocity and the stress intensity factor using the double-torsion test under conditions of controlled temperature. We showed that crack velocities in water were higher than those in air, in agreement with other research results indicating that crack velocity increases in water. When we compared our results for NaOHaq with those for water, however, we found that the crack velocity at the same stress intensity factor did not change even though the pH of the surrounding environment was different. This result does not agree with the accepted understanding that hydroxide ions accelerate subcritical crack growth in rocks. We concluded that the pH at the crack tip influences subcritical crack growth, and not the bulk pH, which has little effect.

Slow crack growth resistance and bridging stress determination in alumina-rich magnesium aluminate spinel/tungsten composites

The slow crack growth (SCG) resistance ( V–K I diagrams) of magnesium aluminate spinel and its tungsten composites with different metallic content (7, 10, 14 and 22 vol.%) is reported. It is found that tungsten plays a crucial role in the composite by increasing crack resistance: the higher the W content, the higher the stress intensity factor needed for crack extension at a given rate. The reinforcement is due to the bridging mechanism performed by metal particles, as it strongly affects the compliance of cracked specimens. Its magnitude is estimated by a compliance function Φ( a) from a double torsion test. From the compliance function, R-curve behaviour is predicted for the composite with highest tungsten content. It explains the effect of metal particles on SCG curves. The W–MgAl 2O 4 interface is believed to influence the reinforcement mechanism.

Study of the dimensions of double-torsion test specimens

Double-torsion tests were carried out to ascertain whether a thin test specimen would affect the results of this test. The tests involved two test specimen sizes, one with a thickness five-fold greater than the particle size of the material and the other with a three-fold greater thickness. The width of both test specimens was ten times that of their thickness and their length was double their width. The material utilized was an alumina-based refractory castable without cement. It is important to ascertain the influence of the thickness of double-torsion test specimens when materials with a coarse microstructure are involved. Otherwise, test specimens would have to be very large, rendering the experimental procedure difficult, as in the case of the size of sintering furnaces and the size of accessories for the test. This study shows that test specimens with lower thicknesses are representative when one analyzes the calibration curve of the compliance, the length of the crack at which the break occurred (critical crack length), and the shapes of the R-curve and of the load (P) vs. displacement curves. However, the analysis of the mean values of R (R), obtained by the arithmetic average of the R values in the section of stable crack propagation and the total fracture energy (γwof), showed that these values are dependent on the size of the test specimens, indicating that this subject deserves further investigation.

Experimental evaluation of the double torsion analysis on soda-lime glass

According to the classical model developed by Evans and co-workers on the double torsion test [(1972) J Mater Sci 7:1137 and (1973) J Testing Eval 1:264], the stress intensity factor is independent of the crack length. Recent applications and analysis question this independency (Chevalier et al (1996) Cer Inter 22:171, Ciccotti et al (2000) Inter J Rock Mech Min Sci 37:1103). This work consists of using samples with different lengths of a typical brittle material (a soda-lime glass) in order to discuss on the validity of the different equations proposed to analyse the DT technique. Experimental compliance tests always showed linear variations with crack length. Successive relaxation tests revealed, however, a clear dependency of the stress intensity factor on crack length. This dependency, observed through the non reproducibility of the V−KI diagrams, is reduced as the sample length increases. The corrections proposed by Chevalier and Ciccotti on Evans model revealed that their applications remain limited to the sample and the loading configurations used by the authors. The application of Evans model without correction is conditioned by the use of sufficiently long samples and advanced crack lengths.

Characteristics of the double-torsion test to determine the R-curve of ceramic materials

Double-torsion tests were carried out on a commercial ceramic floor tile to verify whether this test is suitable for determining the R-curve of ceramics. The instantaneous crack length was obtained by means of compliance calibration, and it was found that the experimental compliance underestimates the real crack length. The load vs. displacement curves were also found to drop after maximum loading, causing the stress intensity factor to decline. The R-curves were calculated by two methods: linear elastic fracture mechanics and the energetic method. It was obtained that the average values of crack resistance, R, and the double of the work of fracture, 2 <FONT FACE=Symbol>× g</FONT>wof, did not depend on notch length, a0, which is a highly relevant finding, indicating that these parameters were less dependent on the test specimen's geometry. The proposal was to use small notches, which produce long stable crack propagation paths that in turn are particularly important in the case of coarse microstructures.

Crack Propagation Behavior of Alumina with Different Grain Sizes under Static and Cyclic Fatigue

A compliance function is used to quantify the shielding capacity of grain bridging, the degradation of which is the main cyclic fatigue mechanism in alumina. Materials with different grain sizes were processed and the fatigue experiments were performed using the double torsion test. Significant degradation is observed in the coarse grain material and a marked sensitivity to the loading level is outlined. At moderate loads, bridging degradation occurs prior to fatigue crack growth during an incubation period. At low cyclic loads, the shielding capacity can be entirely degraded, leading to a cyclic fatigue threshold equivalent to that of the fine grain material.