Non-destructive evaluation of concrete mixtures for direct LNG containment

The suitability of six concrete mixtures for use in direct containment of liquefied natural gas (LNG) was assessed using nuclear magnetic resonance (NMR), X-ray computed tomography (XRCT) and acoustic emission (AE). The mixtures were prepared with river sand as fine aggregate using different coarse aggregates. The mixtures were cooled from ambient to cryogenic temperatures at a cooling rate of 3 °C/min. Proton NMR measurements and XRCT imaging were carried out before and after cooling to monitor changes in porosity and pore size distribution, and internal microstructure, respectively. AE sensors monitored damage evolution during cooling and warming. NMR results indicated porosity increases of 0%, 0.3%, 1.4% and 3.3% in the non-air-entrained trap rock aggregate, limestone aggregate, sandstone aggregate and lightweight aggregate concrete mixtures, respectively. The air-entrained trap rock and limestone mixtures showed porosity increases of 0% and 1.9%, respectively. There was a strong positive correlation between AE cumulative energy and NMR porosity change. XRCT imaging generally showed no frost-induced cracking in the concrete mixtures. Thus, pore structure changes and apparent damage were in the form of microcracks less than the XRCT resolution (22 microns). The results highlight the utility of trap rock aggregate in production of durable concrete for direct LNG containment.     

Evaluation of pore structure and its influence on permeability and moisture damage in asphalt concrete

This study evaluates the pore structure of asphalt concrete (AC) samples by measuring the different components of pore space and their contributions to permeability and moisture damage. Three components of the total pore space namely permeable, dead-end and isolated pores are quantified using tracer test, which is a combination of permeameter and salt concentration measuring meter. Permeable pores become impermeable if an AC sample is compacted below 5% air voids. Permeable pores increase with an increase in sample radius. It is observed from this study that the permeable pore has a good correlation with permeability, whereas total pore shows a poor correlation with permeability. The effects of permeable and dead-end pores on moisture damage of asphalt concrete are evaluated using a moisture induced sensitivity testing device and the AASHTO T 283 method. It is observed that permeable and dead-end pores do not contribute to moisture damage of AC samples.     

In-situ X-ray computed tomography characterisation of 3D fracture evolution and image-based numerical homogenisation of concrete

In-situ micro X-ray Computed Tomography (XCT) tests of concrete cubes under progressive compressive loading were carried out to study 3D fracture evolution. Both direct segmentation of the tomography and digital volume correlation (DVC) mapping of the displacement field were used to characterise the fracture evolution. Realistic XCT-image based finite element (FE) models under periodic boundaries were built for asymptotic homogenisation of elastic properties of the concrete cube with Young's moduli of cement and aggregates measured by micro-indentation tests. It is found that the elastic moduli obtained from the DVC analysis and the FE homogenisation are comparable and both within the Reuss-Voigt theoretical bounds, and these advanced techniques (in-situ XCT, DVC, micro-indentation and image-based simulations) offer highly-accurate, complementary functionalities for both qualitative understanding of complex 3D damage and fracture evolution and quantitative evaluation of key material properties of concrete.     

Diagnostic of fatigue damage severity on reinforced concrete beam using acoustic emission technique

Fatigue damage assessment using non-destructive testing on structures as well as reinforced concrete (RC) beam has become an attention for recent decades. In this paper, diagnostic of fatigue damage in RC beams using acoustic emission (AE) technique was investigated. Based on severity analysis of AE signal strength during service life of the beams, bath–tub curves derived from AE signal are presented and divided into three stages; burn-in, steady state and burn-out. At the same time, deflection in the RC beams also has been analyzed.     

Chloride permeability of concrete subjected to freeze-thaw damage

The effects of freezing and thawing cycling on the chloride permeability of normal weight and lightweight concretes were investigated by using the AASHTO T277 chloride permeability test method and the freeze-thaw test method similar to that recommended by ASTM C666. The results showed that the chloride permeability of the normal weight concretes having an air content of at least 5·3% changed little with the repeated cycles of freezing and thawing up to 618 cycles, irrespective of the presence and the type of mineral admixtures. It was also found that lightweight concretes made with fully-saturated expanded shale aggregates exhibited an extremely high chloride permeability at any air content, when they were subjected to a single freeze-thaw cycle. Furthermore, the type of microcracks developing in normal weight concretes exposed to the repeated freeze-thaw cycles was discussed.     

X-ray computed tomography proof of bacterial-based self-healing in concrete

Self-healing strategies are regarded as a promising solution to reduce the high maintenance and repair cost of concrete infrastructures. In the present work, a bacterial-based self-healing by use of hydrogel encapsulated bacterial spores (bio-hydrogels) was investigated. The crack closure behavior of the specimens with/without bio-hydrogels was studied quantitatively by light microscopy. To have a view of the self-healing inside the specimens, a high resolution X-ray computed microtomography (X-ray μCT) was used. The total amount and the distribution of the healing products in the whole matrix were investigated. This study indicates that the specimens incorporated with bio-hydrogels had distinct improved healing efficiency compared to the reference ones with pure hydrogel only. The healing ratios in the specimens with bio-hydrogels were in the range from 70% to 100% for the cracks smaller than 0.3 mm, which is more than 50% higher than for the ones with pure hydrogel; and the maximum crack bridging was about 0.5 mm (in 7 d), while pure hydrogels only allowed healing of cracks of about 0.18 mm. The total volume ratio of the healing product in the specimens with bio-hydrogels amounted to 2.2%, which was about 60% higher than for the ones with pure hydrogel (1.37%).     

Computations of permeability tensor coefficients and anisotropy of asphalt concrete based on microstructure simulation of fluid flow

Asphalt concrete is the most widely used material for building the surface layer of pavements. It is a porous material that consists of a non-uniform arrangement of asphalt binder, aggregate particles and air voids. One of the primary factors controlling pavement performance is the fluid flow characteristics within the surface asphalt concrete layer.

This paper focuses on the numerical simulation of fluid flow in the three-dimensional (3-D) microstructure of asphalt concrete, and the calculation of permeability from the flow field. The asphalt concrete microstructure was captured using the non-destructive X-ray computed tomography (CT) technique. X-ray CT images were processed in order to identify and retain interconnected air voids and eliminate isolated voids. This image processing enhanced the efficiency of the model as it does not have to solve for flow in isolated voids that do not contribute to fluid flow. The X-ray CT images were analyzed and the results were used to determine the relationship between air void distribution and permeability directional distribution or anisotropy.

The computed permeability values were found to have good correlation with the experimental measurements. The major and minor principal directions of the permeability tensor were found to correspond to the horizontal and vertical directions, respectively. The results indicated that the non-uniform spatial distribution of air voids created more open flow paths in the horizontal directional than the vertical direction, and hence was the much higher permeability in the horizontal directions.     

Spatial distribution of synthetic fibers in concrete with X-ray computed tomography

The strength and toughness prediction models for fiber-reinforced concrete (FRC) typically assume the spatial distribution of fibers is uniform. However, non-uniform dispersion can greatly affect the FRC’s mechanical properties. Several techniques have been used in the past to quantify the distribution and orientation of steel fibers within concrete. For quantifying dispersion of synthetic fibers within concrete, a non-destructive technique using X-ray computed tomography (CT) combined with a post-processing image analysis is proposed. Due to X-ray attenuation similarities, the synthetic fibers were resolved from air voids by shape and size-based filters. The described approach to determine the actual fiber content within FRC samples was verified to be accurate. The method can be used to determine the individual fiber spatial distribution inside the concrete. As expected, the actual volume fraction of fibers in a fracture sample was correlated with the measured total fracture energy of the sample.     

Using pore parameters to estimate permeability or conductivity of concrete

This study investigated the relationships between pore parameters and transport properties. Fourteen concrete mixtures were investigated for water permeability, conductivity for the pore solutions and bulk concrete, as well as total porosity and critical pore diameter. The measured parameters allowed comparison to the Katz–Thompson relationship as well as Archie’s Law. Using a low-pressure device, measured permeability from 1 to 28 days was found to be approximately an order of magnitude higher than that calculated using the Katz–Thompson relationship for the six mixtures examined with this technique. Better agreement between measured and predicted permeability was found using apparatus capable of higher applied pressure. Comparing the data to other published data, the Katz–Thompson relationship seems to be a useful technique for the approximation of water permeability. The exponential relationship between porosity and normalized conductivity (the inverse of the Formation factor) forming the basis of Archie’s Law was found to hold within each specific concrete mixture. However, no overall trend was apparent. The constants of the Archie’s Law vary over a wide range.     

聚乙烯醇纤维混凝土损伤演化声发射监测及其评价

通过对掺入聚乙烯醇（PVA）纤维混凝土试件进行三点抗弯的声发射试验,来研究它的损伤失效机理。首先,通过三点弯曲试验,得到了PVA纤维腐蚀前后混凝土的抗弯强度。然后,基于加载全过程中声发射特征信号,根据声发射幅值、计数和能量相关图,并通过比较有无腐蚀PVA纤维混凝土声发射信号,揭示了PVA纤维腐蚀混凝土的抗弯强度下降原因及其损伤机理。 最后,借鉴NDIS-2421定量评定标准分析声发射信号,对混凝土试件的损伤情况做出定性评价,分析PVA纤维在腐蚀前后混凝土中的作用;此外,通过计算声发射特征信号裕度指标,发现PVA纤维混凝土弯曲损伤明显分为三个阶段,未腐蚀PVA纤维混凝土试件的裕度值较均匀。     

An investigation into the damage development and residual strengths of open-hole specimens in fatigue

An extensive experimental program was carried out to investigate and understand the sequence of damage development throughout the life of open-hole composite laminates loaded in tension–tension fatigue. Quasi-isotropic carbon/epoxy laminates, with stacking sequence [45₂/90₂/−45₂/0₂]_S, [45/90/−45/0]_2S and [45/90/−45/0]_4S were examined. These were selected on the basis that under quasi-static loading the [45₂/90₂/−45₂/0₂]_S configuration exhibited a delamination dominated mode of failure whilst the [45/90/−45/0]_2S and [45/90/−45/0]_4S configurations showed a fibre dominated failure mode, previously described as “pull-out” and “brittle” respectively. Specimens were fatigue loaded to 1 × 10⁶ cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles, which were inspected using X-ray computed tomography (CT) scanning. A static residual strength program was carried out for run-out specimens of each configuration.     

X‐ray computed tomography quantification of damage in concrete under compression considering irreversible mode‐II microcracks

A method for X‐ray computed tomography quantification of damage in concrete under compression considering irreversible mode‐II microcracks is developed. To understand damage behaviour in concrete, a micromechanical analysis of damage under biaxial compression is conducted focusing on random micro‐defects in micro‐cells isolated from the representative volume element. Furthermore, for stress–strain response prediction, a quantification is developed concerning the behaviours of the dominant macrocracks in multiaxial compression. Specifically, two crack types are taken into account: mode‐I cracks and irreversible deformation cracks (including mode‐II microcracks). Furthermore, mode‐I cracks generate compression‐induced tensile load (transverse) area reduction and further stiffness degradation, whereas the latter contribute to the development of irreversible strains. Additionally, by investigating the development of gradually converging dominant cracks, the procedure for quantifying damage is competently executed. In addition, distinguished from other approaches, the quantified damage can be applied directly to constitutive models to produce stress–strain response highly agrees with experimental results.     

Dynamic modulus simulation of the asphalt concrete using the X-ray computed tomography images

The objective of this study is to predict the dynamic modulus of asphalt mixture using both two-dimensional (2D) and three-dimensional (3D) Distinct Element Method (DEM) generated from the X-ray computed tomography (X-ray CT) images. The 3D internal microstructure of the asphalt mixtures (i.e., spatial distribution of aggregate, sand mastic and air voids) was obtained using the X-ray CT. The X-ray CT images provided exact locations of aggregate, sand mastic and air voids to develop 2D and 3D models. An experimental program was developed with a uniaxial compression test to measure the dynamic modulus of sand mastic and asphalt mixtures at different temperatures and loading frequencies. In the DEM simulation, the mastic dynamic modulus and aggregate elastic modulus were used as input parameters to predict the asphalt mixture dynamic modulus. Three replicates of a 3D DEM and six replicates of a 2D DEM were used in the simulation. The strain response of the asphalt concrete under a compressive load was monitored, and the dynamic modulus was computed. The moduli of the 3D DEM and 2D DEM were then compared with both the experimental measurements results. It was revealed that the 3D discrete element models successfully predicted the asphalt mixture dynamic modulus over a range of temperatures and loading frequencies. It was found that 2D discrete element models under predicted the asphalt mixture dynamic modulus.     

Assessment of statistical responses of multi-scale damage events in an acrylic polymeric composite to the applied stress

Assessments of the statistics of damage ensemble are essential steps to develop accurate modeling and predictions of material failures. Events of random damage constitute a damage system that resides in the microstructures of the materials. Characterization and evaluation of such a system involve assessing the evolving the cascading damage events from hierarchical microstructures of the solids, and there currently lacks an experimental means to do so. To address this need, we established an approach to acquire the events of random damage (ERD) by employing a measureable multi-variate D_A defined in our previous work based on acoustic emission. It was found that the responsive events of random damage created by pure tension and three-point bending correlated strongly across all multiscale column vectors of D_A in spacetime. The correlation strength is much stronger under tension than that under bending, and much stronger in early loading stages across the column scale vectors of the D_A variate. ERD were found to be in clear distinct statistical populations by Andrews' exploratory data analysis plots under tension and bending, and in different stages of loading, which suggests that damage mechanisms are not only “physical”, but also “statistical”. Furthermore, our data showed that the strongly coupled multiscale column vectors of D_A can be transformed orthogonally to becoming decoupled principal components, PCs, which may facilitate the constitutive modeling. However, a PC indexes nearly evenly all scale vectors of D_A, which implicates, in conjunction with the findings of correlation and Andrews' plot, can be unidirectional, bi-directional, and or interwoven, but is a complicated index variable to describe the cascading multiscale damage events in evolving hierarchical microstructures of semicrystalline polymers.     

High-resolution 3D imaging of microvascular architecture in human glioma tissues using X-ray phase-contrast computed tomography as a potential adjunct to histopathology

Gliomas are rich in blood vessels, and the generation of tumor-associated vessels plays an important role in glioma growth and transfer. Histology can directly depict microvascular architecture in the tumor, but it just provides two-dimensional (2D) images obtained by destroying three-dimensional (3D) tissue specimens. There is a lack of high-resolution 3D imaging methods for observing the microvasculature throughout the entire specimens. X-ray phase-contrast computed tomography (PCCT) which is an emerging imaging method has demonstrated its outstanding potential in imaging soft tissues. Thus, this study aims to evaluate the potential of PCCT as an adjunct to histopathology in nondestructive and 3D visualization of the microvascular architecture in human glioma tissues. In this study, seven resected glioma tissues were scanned via PCCT and then processed histologically. The obtained PCCT data was analyzed and compared with corresponding histological results. Significant anatomical structures of the glioma such as microvessels, thrombi inside the microvessels, and areas of vascular proliferation could be clearly presented via PCCT, confirmed by the histological findings. Moreover, PCCT data also provided additional 3D information such as morphological alterations of the microvasculature, 3D distribution of the thrombi and stenosis severity of the vessels in glioma tissues, which cannot be fully analyzed in 2D histological slices. In conclusion, this study demonstrated that PCCT can offer excellent images at a near-histological level and additional valuable information in screening gliomas, without impeding further histological investigations. Thus, this technique could be potentially used as an adjunct to conventional histopathology in 3D nondestructive characterization of glioma vasculature.     

Three-dimensional assessment of low velocity impact damage in particle toughened composite laminates using micro-focus X-ray computed tomography and synchrotron radiation laminography

Results are presented studying the contribution of particle toughening to impact damage resistance in carbon fibre reinforced polymer materials. Micro-focus X-ray computed tomography and synchrotron radiation computed laminography were used to provide a novel, multiscale approach for assessing impact damage. Thin (1 mm thick) composite plates containing either untoughened or particle-toughened resin systems were subjected to low velocity impact. Damage was assessed three-dimensionally at voxel resolutions of 0.7 μm and 4.3 μm using SRCL and μCT respectively; the former being an innovative approach to the laterally extended geometry of CFRP plates. Observations and measurements taken from μCT scans captured the full extent of impact damage on both material systems revealing an interconnected network of intra- and inter-laminar cracks. These lower resolution images reveal that the particle-toughened system suppresses delaminations with little effect on intralaminar damage. The higher resolution images reveal that the particles contribute to toughening by crack deflection and bridging.     

Tentative application of computed tomography to study of otoliths and their responses to environment variations

As the carrier of biomineral aragonite, fish otoliths memorize various messages of environment throughout the fish’s life. In the past three decades, quite a few achievements have been made in the studies of fish otoliths, but no advances in research using medical instruments have been reported. The authors tentatively applied X-ray computed tomography (CT) to the studies of the internal structure of wild carp otoliths, with the CT value determined by variations in the sample density and element composition. The wild carps were collected, respectively, from the Baiyangdian Shallow Lake in Hebei Province and Miyun Reservoir in the Beijing metropolis, whose water environments are quite different. The former has suffered serious pollution and eutrophication, whereas the latter has nearly experienced no pollution. The primary result indicates that differences exists in CT values of otoliths for the carps from the above-mentioned waters. With in-depth studies, it is possible that X-ray computed tomography could serve as a useful tool in the study of fish otoliths, and the CT values can be taken as typomorphic parameters to distinguish the waters with different degrees of pollution.     

Health monitoring of carbon/carbon,woven reinforced composites. Damage assessment by using advanced signal processing techniques. Part I: Acoustic emission monitoring and damage mechanisms evolution

The present work, first of two parts, deals with three types of woven carbon/carbon (C/C) composites having differentiations during the manufacturing procedure, which influences their fibre/matrix interface. All material types were tested under tensile loading in a load–unload–reload configuration, with online acoustic emission monitoring. Unsupervised pattern recognition algorithms were utilized to classify the acoustic emission (AE) data recorded during the tests. The resulted clusters, concluded by the analysis of AE hits, are associated with the damage mechanisms of the material, activated at the different load levels, and significant remarks were extracted regarding the damage evolution and its differentiation according to the different fibre/matrix interfaces. Emphasis is given on the impact of the different interface types upon the total mechanical behavior and damage accumulation at the test coupons. A qualitative evaluation of the interfaces using non-destructive testing data is also attempted. This first part intends to propose methodologies and procedures to analyze data from online acoustic emission monitoring in order to extract useful information regarding the damage evolution within C/C materials.     

Stress and damage analysis of composite–aluminium joints used in electrical insulators subject to traction and bending

Numerical and experimental stress and failure studies are performed on metal/fibreglass-reinforced-plastic (GRP) joints used in composite electrical insulators consisting of an epoxy tube reinforced with E-glass fibres and of two aluminium fittings fastened to the tube with an adhesive. Subjected to traction and bending, the joints are modelled with axisymmetric 2D and solid 3D finite elements respectively. Numerical stress and strain distributions through the bond are calculated and the damage initiation in the composite is highlighted. The simulations are compared to experimental data obtained from several joint specimens tested on an experimental set-up equipped with strain gauges and an acoustic emission system. Good correlation between the numerical predictions and the experimental results is found.     

Investigation of the response to low velocity impact and quasi-static indentation loading of particle-toughened carbon-fibre composite materials

This work investigates damage caused by low velocity impact and quasi-static indentation loading in four different particle-toughened composite systems, and one untoughened system. For impact tests, a range of energies were used between 25 and 50 J. For QSI, coupons were interrupted at increasing loading point displacement levels from 2 to 5 mm to allow for monitoring of damage initiation and propagation. In both loading cases, non-destructive inspection techniques were used, consisting of ultrasonic C-scan and X-ray micro-focus computed tomography. These techniques are complemented with instrumentation to capture force–displacement data, whereby load-drops are associated with observed damage modes. Key results from this work highlight particular issues regarding strain-rate sensitivity of delamination development and an earlier onset of fibre fracture associated with particle-toughened systems. These issues, in addition to observations on the role of micro-scale events on damage morphology, are discussed with a focus on material development and material testing practices.