Width Of Microcracks Research Articles

Residents' journal review

Residents' journal review

Quantitative damage evaluation of AAR-affected concrete by DIP technique

This paper presents a method to evaluate alkali–aggregate reaction (AAR) damage in concrete by digital-image-processing (DIP) technique. A vacuum fluorescent-epoxy impregnation technique is used to highlight microcracks in the AAR-damaged concrete and a Matlab-based software package is developed to quantitatively analyse the microcrack characteristics (area, length, width etc.) in the fluorescent microscopic images of concrete. Six microcrack pattern parameters – the total length, the length density, the total area, the area density, the maximum width and the average width of microcracks – in slices from the concrete specimens that suffered different degrees of AAR damage were obtained. Results show that the length density, the area density and the average/maximum width of microcracks all increase as the degree of AAR damage increases. Relationships between the mechanical properties and the microcrack characteristics are established and results show that both the relative flexural strength and relative axial tensile strength have quadratic correlations with the microcrack density. The degree of AAR damage has a good logarithmic correlation with the microcrack density and the expansion ratio has a linear correlation with the microcrack density. Quantitative microcrack analysis based on DIP techniques could be developed as a new powerful tool of quantifying AAR damage in concrete.

Micro and macrolevel properties of fly ash blended self compacting concrete

This investigation is mainly focused on the effect of class F fly ash on the micro and macrolevel properties of self compacting concrete (SCC) after 28, 56 and 112days of curing. The microlevel properties studied were the microcrack widths between aggregate and paste and atomic Calcium–Silica (Ca/Si) ratio. The macrolevel properties studied were compressive strength, modulus of elasticity and splitting tensile strength. A conventional concrete (CC) having an equivalent 28-day SCC compressive strength has also been examined at different ages. Scanning electron microscope (SEM) analysis was carried to examine the width of microcracks and energy dispersive X-ray analysis (EDAX) was carried out to determine the chemical elements of both SCC and CC. Studies revealed that pozzolanic action of class F fly ash improved the microlevel properties of SCC with age by reducing the microcracking width and Ca/Si ratio and thus enhanced the macrolevel properties.

Detection of Dentinal Microcracks Using Infrared Thermography

IntroductionIt is difficult to make a definite diagnosis of a cracked tooth solely based on an inspection within the root canal, especially in case of microcracks. At present, there seems to be no established method to detect dentinal microcracks in roots; therefore, the current detection techniques need to be improved. Vibrothermography (VibroIR) helps to detect microcracks by the friction heat generated from ultrasonic vibration. The purpose of this study was to establish a novel method using VibroIR to detect dentinal microcracks. MethodsThe root canals of 20 roots with cracks and control roots were prepared after removing the tooth crowns. A tapered indenter was inserted into the root canal and pressed until a microcrack was created under an optical microscope. Using VibroIR, the detection trials for dentinal microcracks were performed with an ultrasonic vibration power ranging from 0.43 to 1.48 W at an angle of 0°, 30°, 45°, 60°, and 90° between the ultrasonic vibration point and the microcrack line. After the detection test, the microcrack width was measured with an optical microscope. ResultsFrictional heat was detected in the microcracks with thermography at 0.89 to 1.48 W and at an ultrasonic vibration point angle less than 60° from the crack line for 10 seconds. Microcracks with a width of 4 to 35.5 μm were detected with this method. ConclusionsVibroIR may be an effective method for the diagnosis of root dentinal microcracks.

Evaluation of enamel micro-cracks characteristics after removal of metal brackets in adult patients

The purpose of this study was to evaluate and compare enamel micro-crack characteristics of adult patients before and after removal of metal brackets. After the examination with scanning electron microscopy (SEM), 45 extracted human teeth were divided into three groups of equal size: group 1, the teeth having enamel micro-cracks, group 2, the teeth without initial enamel micro-cracks, and group 3, control group to study the effect of dehydration on existing micro-cracks or formation of new ones. For all the teeth in groups 1 and 2, the same bonding and debonding procedures of metal brackets were conducted. The length and width of the longest enamel micro-crack were measured for all the teeth before and after removal of metal brackets. The changes in the location of the micro-cracks were also evaluated. In group 3, teeth were subjected to the same analysis but not bonded. The mean overall width of micro-cracks after removal of metal brackets was 3.82 μm greater than before bonding procedure (P < 0.05). Also, a significant difference was noticed between the width of micro-cracks in first zone (cervical third) and third zone (occlusal third) after debonding procedure (P < 0.05). New enamel micro-cracks were found in 6 of 15 (40 per cent) examined teeth. Greatest changes in the width of enamel micro-cracks after debonding procedure appear in the cervical third of the tooth. On the basis of this result, the dentist must pay extra care and attention to this specific area of enamel during removal of metal brackets in adult patients.

Changes of Pore Structures in Hardened Cement Paste Subjected to Flexural Loading and Wet-Dry Cycles in Seawater

The coupling effect of flexural loading and environmental factors has great influence on the pore structures in hardened cement paste. In this paper, Mercury intrusion porosimetry (MIP) and field emission scanning electron microscope (SEM) were used to analyze and observe the changes of pore structures in hardened cement paste subjected to flexural loading and wet-dry cycles in simulated seawater. The results show that the porosity greatly increases when the flexural loading level is raised from 0 f (the ultimate flexural loading capacity) to 0.8 f. Micro-cracks are observed and the connectivity, width and density of micro-cracks increase with the increment of flexural loading. The peaks position of pore size shifts toward greater micro-pores when the flexural loading was raised from 0 f to 0.8 f. The flexural loading and simulated seawater accelerate the degradation of pore structures.

Statistical Analysis of Micro-Crack Parameters for PPy-Coated Fiber Strain Sensors Applicable for Large Deformation

The conductive fibers have great potential as flexible sensors. In our previous study, based on the experimental studies on sensing behavior of PPy (polypyrrole)-coated Lycra fibers, the mechanism of the strain sensing behavior was identified as open-close mechanism of the cracks on the coating layer when experiencing large deformation. In order to modeling its strain sensing behavior and describe quantitively the development of the micro-cracks, statistical analysis is conducted by image processing technique in MATLAB software. It was concluded that the variation of resistance is related to the micro parameters of the surface cracks, suck as the number, width and length of micro-cracks. These will be used in our further modeling work.

Effect of pre-oxidation on microcracks in graphite foams

The formation mechanisms of microcracks in graphite foams were investigated by varying the preoxidation temperature of the precursor mesophase pitch. Results indicated that after the mesophase pitch was preoxidized, its quinolineinsoluble component became the dominant fraction, which leads to a decrease in heat stress gradient and as a result, a decrease in the amount, length and width of microcracks.

Preparation and thermomechanical characterisation of aluminum titanate flexible ceramics

Flexible ceramics may be useful, for example to process refractory materials with an improved resistance to thermal shocks. A natural flexible sandstone, itacolumite, is mainly constituted of interlocked quartz grains and contains microcracks. Its microstructure allows some free motion between grains that induces its flexibility. The aim of this study is to prepare and characterize flexible aluminum titanate ceramics by mimicking the microstructure of itacolumite. Aluminum titanate (AT) has a high thermal expansion anisotropy that induces grain boundary microcracking leading to flexibility. Here, the flexibility is the capacity of the material to endorse large strain-to-rupture level. This concept is also closely related to a low value of the stiffness induced by damage mechanisms. In this study, the flexibility has been estimated by the measurement of the deflection at fracture on three-point bending test. By preparing AT samples sintered according to different heating cycles, the correlations between the sintering cycle, the microstructure and the flexibility have been studied. Grain size and microcrack width have been observed by scanning electron microscopy. A major parameter for flexibility is the microcrack volume fraction within the sample. Three types of AT materials have been processed: non flexible (NF), flexible (F), and very flexible (VF). Their thermal and mechanical behaviors have been investigated and showed that NF has a brittle behavior while F and VF have a nonlinear ductile one. This was found to be due to grain boundary microcracks network and to the interlocking of grains. VF is more flexible than F because its microcracks are wider. Flexibility improves the thermal shock resistance: F and VF have a higher thermal shock resistance than NF. Moreover, thermal expansion measurements during thermal cycles showed anomalous effects induced by crack closure when heating and crack opening when cooling.

Simulation of Chloride Diffusivity for Cracked Concrete Based on RBSM and Truss Network Model

For concrete structures exposed to salt environment, the microstructure and cracks play a crucial role in the ingress of chloride ions into concrete. In this study, concrete is simulated on the meso scale as a three-phase composite, i.e., aggregate particles, mortar and the interfacial transition zone (ITZ). Because of the advantages in predicting cracks behavior in concrete, Rigid Body Spring Model (RBSM) is employed to carry out the mechanical analysis to simulate the distribution and width of microcracks. And then, the truss network model is adopted to evaluate the chloride diffusivity of the cracked concrete. On the basis of the statistics analysis of diffusion coefficients of concrete and mortar determined experimentally, the diffusivity of ITZ is analytically clarified. The range of diffusion coefficient of ITZ estimated in this paper is approximately 3-16 times of that of mortar depending on the different assumed thickness, which agrees well with that of the previous experimental results. With the aim to validate the effect of microcracks on the diffusivity of concrete, a series of the chloride ions penetrating analysis is numerically carried out on the concrete specimen under different stress levels. The axial compressive and tensile loading conditions are investigated respectively and the effects of stress level on chloride diffusivity of cracked concrete are examined. Results indicate that the chloride diffusivity is significantly dependent on the stress level, but only considering the effect of cracks predicted by RBSM is not sufficient. So an empirical equation which can account for the microstructure variation of concrete under loading is proposed. With it, a reasonable estimation for chloride diffusivity of cracked concrete is achieved.

Durability of mechanically loaded engineered cementitious composites under highly alkaline environments

Durability of mechanically loaded Engineered Cementitious Composites (ECC) is investigated in this paper. ECC offers significant potential for durable civil infrastructures, due to its high tensile strain capacity of more than 3%, and controlled micro-crack width of less than 80 μm. An experimental study was designed to investigate the durability of ECC material with regard to cracking and healing under combined mechanical loading and environmental loading conditions. ECC coupon specimens were firstly pre-loaded under uniaxial tension to different strain levels, and then exposed to an alkaline environment up to 3 months at 38 °C and reloaded up to failure. The reloaded specimens showed slight loss of ductility and tensile strength, but retained the multiple micro-cracking behavior and tensile strain capacity of more than 2% (about more than 200 times that of normal concrete and normal fiber reinforced concrete). The test results indicated strong evidence of self-healing of the micro-cracked ECC material, which can still carry considerable tensile stress and strain and restore nearly the original stiffness. The phenomenon of self-healing effectively closes the micro-cracks even after one month exposure period. In addition to coupon specimens, ECC bar specimens were also immersed in alkali solution at 80 °C in accordance to ASTM C 1260 to determine their length change due to alkali silica reaction (ASR). The ECC bar specimens did not show any expansion at the end of 30 days soaking period. Therefore, these test results indicated that ECC, both virgin and micro-cracked, remain durable despite exposure to a high alkaline environment. The risk of ASR in ECC is determined to be low.

Integrated structures and materials design

This paper introduces the concept of␣Integrated Structures and Materials Design (ISMD). ISMD combines materials engineering and structural engineering for the purpose of more effectively achieving targeted structural performance, by adopting material composite properties as the shared link. An application example, design of a bridge deck link-slab, is used to illustrate the essential elements of ISMD. It is shown that the composite hardened properties—tensile strain capacity, microcrack width, and Young’s Modulus, as well as composite self-consolidating fresh properties, are amongst the most important composite parameters that govern the targeted structural performance of safety, durability and ease of design and implementation. These are also properties that can be controlled in an Engineered Cementitious Composite—an ultra ductile concrete, by tailoring the ingredients for desired fiber, matrix and interface micromechanical parameters. Broad implications of ISMD on educational approach, research collaboration, and next generation infrastructure development, are briefly discussed.

Tensile creep behavior of notched two-dimensional-C/SiC composite

Tensile creep tests of two-dimensional-C/SiC specimens with double-edge arc notches have been carried out at 1100, 1300 and 1500 °C in vacuum. The matrix cracks on the surface and resonance frequency were examined at different creeping times. At 1100 °C, the creep strains of both smooth and notched specimens were concentrated at the transient stage and the steady creep rates were nearly zero, whereas steady creep rates of notched specimens and smooth specimens were similar at 1500 °C. It has been observed that the creep damage mainly concentrated at the area near the notches. Micro-cracks appeared in the area near the notches and on the cross-points of the woven fiber bundles, and the longitudinal fibers near the notches fractured easily. Both types of curves, namely quantity of micro-cracks vs. time and micro-crack width vs. time, were extremely similar as for the creep curves. In general, micro-cracks developed fast during the first 10 h. It has been noticed that within the first 2 h, the micro-cracks near the notches grow faster than those far from the notches, whereas the growth rate of micro-cracks far from notches was faster than those near the notches after 2 h. This phenomenon indicates the stress redistribution during creep. Damage curves at 1300 and 1500 °C have similar trend, though the damage and the quantity of micro-cracks at 1500 °C are higher than those at 1300 °C.

Fatigue Microcrack Plastic Zone Size Determination

The microcrack tip plastic zone sizes in austenitic steels are measured using REM and interference microscope. It is shown that the plastic zone size varies from 300µm to 350µm. The importance of determining this parameter is discussed. Based on the analysis of the conventional continuum equations of linear-elastic approach a simple formula is derived for calculation of plastic zone size, R=d E/2π σF, establishing relation between the plastic zone radius (R), microcrack width (d), elasticity modulus (E) and the yield strength of the material (σF). The measured values of plastic zone size are in a good agreement with those reported in literature, and calculated by the above formula.

High‐temperature seawater circulation throughout crust of oceanic ridges: A model derived from the Oman ophiolites

We describe here the high‐temperature (HT ∼ 700°C) and very high temperature (VHT ∼ 1000 °C) recharge and discharge systems which are responsible for HT‐VHT hydrothermal alterations at the oceanic ridge of origin in the crust of Oman ophiolites. The recharge system is a regular network of parallel microcracks, a fraction of a millimeter wide, which carries seawater to the walls of the magma chamber where hydrous melting is triggered. The discharge system is a network of clinopyroxene, pargasite gabbroic dikes issued from the hydrous melting; upsection, these HT‐VHT gabbroic dikes grade into the well‐documented low‐temperature (LT) (400–500°C) hydrothermal veins. Microcracks, gabbroic dikes, and veins are preferentially parallel to the diabase‐sheeted dikes and normal to magmatic lineations in enclosing gabbros. Thus the fractures of both the recharge and discharge systems are parallel to the paleoridge plane. In the brittle field, fracturing is induced by tensional stress related to seafloor spreading. In the ductile field and up to temperatures of ∼900°C, opening of microcracks is ascribed to the anisotropy of thermal contraction during gabbros cooling. Gabbro fabrics and anisotropy are such that the induced microcracks are also parallel to the ridge plane. Thermal contraction between 1200°C (temperature of magma chamber) and ∼700°C (brittle‐ductile limit) produces a differential strain of 0.1% comparable to that deduced from width and spacing of microcracks in the gabbros. Simple estimates on activity time, fluid velocity, and fluid flux in microcracks account for all available data and suggest that in microcracks, activity was short lived and episodic.

Study of the surface integrity of the machined workpiece in the EDM of tungsten carbide

A comprehensive study of the surface integrity of the machined workpiece in the electrical discharge machining (EDM) of tungsten carbide is presented in this paper. EDM tests on a tungsten carbide workpiece were conducted on a Charmilles Technologies Roboform 40 EDM Die-Sinking Machine, with the peak current and pulse duration varied. The EDMed surface morphology was examined with a scanning electron microscope (SEM) (JEOL JSM-5600LV) with energy dispersive spectrometers. Surface hardness was determined with a macro-hardness tester. It is found that there is no difference between the hardness of the EDMed surface and the original hardness of the workpiece for all EDM conditions. It is observed from the SEM micrographs that there is a clear EDM damaged layer on the workpiece, distinguished by the amount of WC grains and micro-cracks. The amount or concentration of WC grains decreases from the internal structure of the workpiece to the top surface layer. Another feature on EDMed surfaces is the abundance of cracks, especially at high peak current and pulse duration. It is also observed that the EDM conditions have no effect on the microstructures of the bulk workpiece material. This means that damage caused by EDM on the EDMed surface is limited to a certain depth only. However, it is observed that the depth of the damaged layer and the average length, width and number of micro-cracks increase with the peak current and pulse duration. The damaged layer and micro-cracks seem to disappear when the peak current and pulse duration were set at very low values.

Effects of Microcracks in CVD Coating Layers on Cemented Carbide and Cermet Substrates on Residual Stress and Transverse Rupture Strength

Microcracks were mechanically induced in the CVD coating layers on two types of cemented carbides with different thermal expansion coefficients, and one type of cermet. The microcracks were found to have beneficial effects on residual stress, transverse rupture strength, and chipping resistance during interrupted cutting. Residual stress in the coating on cemented carbide is tensile. Tensile residual stress decreases with increasing microcrack width and decreasing microcrack distance. Induction of 20 μm-distant and 0.025 μm-wide cracks relieves tensile residual stress by about 0.5 GPa, increases transverse rupture strength by about 0.70 GPa, and almost doubles the chipping resistance. Residual stress in the coating on cermet is compressive. Microcracks in the coating layer do not change residual stress or transverse rupture strength.

Internal stresses in zinc-chromate coatings

Zinc coatings immersed in a chromating solution acquire a mixed potential characteristic of zinc dissolution and the deposition of a chromium hydroxide layer. Zinc deposits are initially in a compressive stress but a zinc-chromate coating has a tensile stress which increases with chromate thickness. The stress in the chromate films after drying increases only slightly compared to a wet chromate film. Scanning electron and scanning probe microscopy reveals microcracks in the chromate coatings due to the tensile stress. As the internal stresses in the chromate film increase, scanning probe studies show an increase in microcrack width and depth.