Internal Structural Damage Research Articles

Influence of carbon nano fibers (CNF) on the performance of high strength concrete exposed to elevated temperatures

Carbon nano fibers (CNFs) were added into high strength concrete (HSC) to evaluate the fire endurance of incumbent matrix. The residual mechanical performance was evaluated by conducting material property tests namely compressive strength, tensile strength, stress-strain response, elastic modulus, compressive toughness and mass loss made with 0.1% CNF and 0.2% CNF as cement replacement in the temperature range of 23 °C to 800 °C. The internal structure damage on exposure to fire was analyzed by ultrasonic pulse velocity (UPV) in macro-phase while scanning electron microscopy (SEM) is used for micro and nano forensics. Being exceptionally conductive and resilient, these fibers reduce spalling potential via low thermal inertia and crack bridging action as established in forensic analysis. The test results revealed better retention in mechanical and physical properties of high strength concrete containing CNFs. Mathematical relationships based on statistical analysis have been proposed for predicting mechanical, durability and energy related properties of HSC modified with varying percentages of CNFs at elevated temperatures.

Multi-physics safety model based on structure damage for lithium-ion battery under mechanical abuse

The failure of lithium-ion batteries under mechanical abuse is a multi-physical process involving mechanical failure, electrochemical degradation, internal electric short circuit, thermal runaway and structure damage. To gain a fundamental understanding of the failure mechanisms, in this work we propose a mechanical-electrochemical-thermal coupling model based on structure damage to study the failure behavior of lithium-ion battery under mechanical abuse in hard short-circuit stage from the real 3D structure and the whole battery level. The failure elements in the mechanical model are transferred into failure geometry and combined with the deformed cylindrical lithium-ion battery body. Then the short-circuit thermal power is calculated by the short-circuit model and sent to the thermal model. The electrochemical model outputs the terminal voltage and the electrochemical heat generation rate to the short-circuit model and the thermal model, respectively. The thermal model calculates cell temperature and feeds it back to the electrochemical model. The coupling model points out the fact that the internal short circuit heat is the main heat source during the short time of hard short-circuit stage under mechanical abuse. Furthermore, the coupling multi-physics model provides fundamental insights into the mechanisms of battery failure under mechanical abuse condition and can accelerate the analysis process which helps to understand the failure phenomenon in the future researches.

Crash Analysis of Aircraft Nose Prototype

This paper presents the comparison of structural deformation of generic metals and the new age composite materials on the aircraft nose during a crash. The analysis is conducted to be able to make more educated predictions of the internal structure damage caused when the airplane has a head on collision with a vertical obstacle (buildings) or when affected by a bird strike. Two nose profiles widely seen nowadays are spherically blunted tangent ogive and elliptical. These nose cones have been designed based on model to prototype ratio on NX CAD. CFD has been performed on the nose designs and solved on ANSYS Fluent for flow visualizations. Materials like Aluminum alloy (which is still widely used in fuselage frames) and Carbon Fiber Reinforced Polymer with epoxy resins, have been applied to the CAD models. These were analyzed for stress, strain and deformation on ANSYS 18.1 by simulating the crash of the nose on a thick structural steel plate. After the analysis, it was inferred that the elliptical nose made of Carbon Fiber Reinforced Polymer has less structural deformation on being crashed.

A Facile and Controllable Approach for Surface Modification of Wool by Micro-dissolution

Most modifications of wool focused on its surface due to the existence of the cuticle scales. For instance, chlorination, oxidation and protease treatments were used to break down or remove the scales. However, these modifications usually severely damaged the internal structure of wool. In this paper, a novel approach based on surface micro-dissolution was employed to modify the surface of wool to minimize the internal damage of wool, and improve the surface-related properties of wool. The micro-dissolution system used NaHSO3/urea to dissolve the scale layer of wool. After being micro-dissolved with NaHSO3/urea system, scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), fluorescence microscopy, Fourier transform infrared spectroscopy (FTIR), and the Allworden reaction were utilized to characterize the treated wool. Furthermore, felting shrinkage, strength loss and wettability of the treated wool were determined. The results showed that NaHSO3/urea system could dissolve the scale layer from outside to inside. The fluorescence microscopic images further indicated that low concentration of NaHSO3/urea system can dissolve the surface of wool without the serious damage of internal structure. Moreover, the micro-dissolution system can be controlled to dissolve the scale layer on the surface of wool slightly by adjusting the reaction conditions. The presented method is simple, easy to operate, and economical, thereby can open up new directions for the surface modification of wool.

Online structural state assessment for aerospace composite structures using an acousto-ultrasonics-based multi-damage index identification approach

The development of aerospace manufacturing has promoted the application of lightweight composite materials into aerospace structures. Although the aerospace composite structures possess numerous advantages, invisible internal structural damage such as delaminations induced by various external factors can significantly reduce the mechanical affordability, safety, and life-cycle of the structure. Therefore, it is of great significance to monitor and assess the health state and predict accurately the lifetime of aerospace composite structures. An acousto-ultrasonics-based multi-damage index identification approach is thus proposed in this study to identify and quantify possible multiple damage in thin-walled aerospace composite structures. In this approach, two indices for damage quantification were proposed: the energy and phase divergence indices. The energy index defines the reflected energy resulting from damage, and the phase divergence index defines the instantaneous phase variation of propagating waves due to damage. The two damage indices are obtained through the developed mode decomposition and spectral element analysis using sensor response signals collected by a transducer array placed onto the examined structure. Through a series of relevant experimental tests on the fabricated laminated composite panels with/without damage, the proposed acousto-ultrasonics-based multi-damage index identification approach was validated. The developed damage indices are competent to evaluate a structural health state in terms of damage quantification, and all of the validation results fell well in the prospected ranges. Moreover, it shows a linear and consistent trend between the variation of two damage indices and damage extents. Based on the particular relation, the linear regressive prediction functions were established separately regarding the two damage indices. They can be used to assess a structural health state due to the damage growth in real time. The proposed multi-damage index identification approach demonstrates its potential to serve as an online assessment tool to be aware of the reliability condition of a composite structure.

Effect of adding graphite nano/micro platelets on salt freeze-thaw resistance of nano-modificent concrete

Concrete has wide application in construction engineering but it has durability issues in extreme weather conditions. In this article, carbon based nano material, graphite platelets is introduced into cementitious matrix to make it more resilient in extreme weather condition. Four different percentages of graphite nano/micro platelets by mass of cement, 0.25%, 0.50%, 0.75%, 1.0% were incorporated in conventional concrete and its effect on salt freeze-thaw resistance in comparison to control sample were studied. Test results reveals that concrete were still durable after 7, 14, and 28 freeze-thaw exposures. Salt freeze-thaw response was evaluated in terms of surface scaling and internal structure damage which was significantly improved with the addition of nano graphitic plates. Scanning electron micrographs have verified the crack branching effects of induced platelets. The pore structure was refined as revealed in nitrogen absorption isotherms with the nano-modifications contributing in added resilience to subjected frost damages.

Radiation resistance of carbon fiber-reinforced epoxy composites optimized synergistically by carbon nanotubes in interface area/matrix

In order to investigate the effect of the addition of carbon nanotubes (CNTs) in carbon fibers reinforced epoxy matrix composites (CF/EP) on the ability of composites to resist γ radiation. We used γ irradiation and investigated composites, γ-CF/EP without CNTs; γ-CF/EP with CNTs in matrix (γ-CF/EP-CNTs); γ-CF/EP with CNTs in interface (γ-CF-CNTs/EP) and γ-CF/EP with CNTs both in the interface area and matrix (γ-CF-CNTs/EP-CNTs). The results show that the bending strength and bending modulus of γ-CF-CNTs/EP-CNTs are 28.59% and 41.44% higher than that of γ-CF/EP before fatigue testing, and 32.19% and 73.51% higher than that of gamma-CF/EP after fatigue testing. Ultrasonic C-scan microscopic examination after fatigue test showed that the internal structure damage of γ-CF-CNTs/EP-CNTs was the smallest, while γ-CF/EP was the largest. The storage modulus of the three modified composites are significantly improved compared with the unmodified composite, especially the storage modulus of γ-CF-CNTs/EP-CNTs is sharply improved from 2.53 GPa (γ-CF/EP) to 5.66 GPa (γ-CF-CNTs/EP-CNTs). X-ray spectroscopy shows changes in the surface and internal valence content of the composites, further revealing the crosslinking reaction and the secondary curing reaction. Combined with scanning electron microscopy and crack propagation models, the microstructure and enhancement mechanisms of CNTs-reinforced composites for anti-gamma radiation are discussed. The CNTs synergistic enhance the composite's anti-gamma radiation performance with excellent effects when CNTs are added together into matrix and interface area.

High-repetition-rate burst-mode-laser diagnostics of an unconfined lean premixed swirling flame under external acoustic excitation.

Lean premixed swirling flames are important in practical combustors, but a commonly encountered problem of practical swirl combustors is thermo-acoustic instability, which may cause internal structure damage to combustors. In this research, a high-repetition-rate burst-mode laser is used for simultaneous particle image velocimetry and planar laser-induced fluorescence measurement in an unconfined acoustically excited swirl burner. The time-resolved flow field and transient flame response to the acoustic perturbation are visualized at 20 kHz, offering insight into the heat release rate oscillation. The premixed mixture flow rate and acoustic modulation are varied to study the effects of Reynolds number, Strouhal number, and acoustic modulation amplitude on the swirling flame. The results suggest that the Strouhal number has a notable effect on the periodic movements of the inner recirculation zone and swirling flame configuration.

Sublethal Effects of Ionic and Nanogold on the Nematode Caenorhabditis elegans.

The nematode Caenorhabditis elegans is used as an ecotoxicological model species in both aqueous medium and solid substrates. It is easy and of low cost to maintain in the laboratory and it produces hundreds of offspring within a short period of time. It also has a small body size (1 mm), making it possible for in vivo assays to be conducted in 12-well plates. Engineered nanomaterials (ENMs) are a class of emerging pollutants. Nanogold (nAu) is used in many consumer products and in vivo drug delivery. These materials can be released into the aquatic environment during production or discarding of consumer products. As nAu is insoluble in water, the sediment would become the final depository for the materials. It has become increasingly important to use sediment dwelling organisms to screen for possible toxicity of these ENMs. In this study C. elegans was exposed to a range of concentrations of nAu and ionic gold in M9-media, acting as a substitute for pore water. After 96-hour growth, fertility and reproduction were determined. Internal structure damage and internalisation of particles in C. elegans were determined by using SEM and CytoViva® Darkfield Imaging. From these images the nanomaterials are distributed around the oocytes in the reproductive organs, as well as the pharynx. Results obtained indicate that nAu affects reproduction more than growth due to internal gonad damage, albeit at very high exposure concentrations, indicating no toxicity at environmentally relevant concentrations. Ionic Au is more toxic than nAu and effects fertility and reproduction due to ion release. These results give more information regarding the toxicity and in vivo uptake of nAu and form part of an environmental risk assessment of ENMs.

CO2 sorption induced damage in coals in unconfined and confined stress states: A micrometer to core scale investigation

Gas sorption in coal, in particular CO2, is known to cause swelling and internal structural change or damage. For instance, CO2 adsorption on coal causes swelling which in turn can close the fractures or cleats whereas its desorption can open pre-existing fractures as well as create new fractures, thus enhancing gas flow paths in coal seams. Gas release and the associated induced damage are relevant for several applications such as coal burst in coal mining operations and for geological sequestration of CO2 into coal seams to reduce both the carbon footprint and increase the efficiency of energy extraction. However, the sorption-induced internal structural damage in coal, in particular the link between the micro-scale damage phenomena to macro-scale processes in confined and unconfined stress states, is not yet fully understood.We carried out a series of experiments to study the potential sorption-induced damage in both confined and unconfined stress states at micro (μm) and macro (cm) scale. 3D images, obtained by X-ray microcomputed tomography (micro-CT) technique, showed that in the unconfined stress state, adsorption of CO2 closes some pre-existing fractures while new fractures form in the specimen of coal. This is well supported by high pressure CO2 adsorption analyses that were independently conducted. After CO2 release, the overall fracture intensity, defined as the area of fractures per volume of rock mass, was considerably increased by opening both pre-existing fractures together with new fractures. In the confined condition, the gas adsorption was significantly lower confirming the closure of the initial fractures by swelling without creation of induced fractures. However, the gas release under shearing stress was significantly higher showing that the shearing stress causes further damage assisted by gas release and its induced micro-structural damage.

Experimental study on the dynamic mechanical properties of concrete under freeze–thaw cycles

This study investigated the dynamic mechanical properties of concrete subjected to freeze–thaw cycles. Freeze–thaw cycle experiments were carried out using a freeze–thaw tester to vary the temperature from 20 ± 2 °C to −20 ± 2 °C, from 0 to 100 cycles. Uniaxial compression tests were performed with a tested strain rate ranging from 1.0 × 10−5/s to 1.0 × 10−2/s using a servo‐hydraulic testing machine. Dynamic compressive strengths were measured and the effects of freeze–thaw cycles and strain rates were investigated. The experimental results suggested that the dynamic compressive strength of specimens generally decreased as the number of freeze–thaw cycles increased, and the dynamic compressive strength was significantly affected by strain rates. The dynamic compressive strength of concrete and the dynamic increase factor both increased with increased strain rate. X‐ray computed tomography scanning was used to study the strain‐rate effects on internal structure damage in the specimens. Cracks propagated along two different paths, passing along the aggregate–mortar interfaces and passing through the aggregates along the failure surfaces.

Multi-scale structural design and biomechanics of the pistol shrimp snapper claw

The Arthropoda, the largest phylum of the Animal Kingdom, have successfully evolved to survive various ecological constraints under a wide range of environmental conditions. Central to this survival are the structural designs developed in their exoskeletons and their raptorial appendages for protection and hunting. One such example, the pistol shrimp, is a shallow-water crustacean that is well-known for its aggressive hunting behavior, using its snapper claw to trigger the nucleation of cavitation bubbles that strike targets. In this study, we conducted a multi-scale structural/nanomechanics relationship study of this biotool to analyze its mechanical response to contact stresses. We found that the pistol shrimp snapper claw, which exhibits the capacity to emit a high-velocity water jet during rapid closure actions, is more brittle than other mineralized biotools, exhibiting accelerated wear damage under contact stresses. However, due to an angular offset between the dactylus and pollex of the snapper claw, the appendage never engages in any mechanical contact during the snapping action. This feature is in stark contrast to that reported in other fast raptorial appendages of crustaceans, notably the mantis shrimp dactyl club, which is designed to shatter close range targets in contact mode and exhibits a superior resistance to contact damage and wear. These findings suggest that adaptation of hunting appendages goes beyond their macroscopic morphology, and that multi-scale structural design concomitantly adapted to function, with enhanced structural complexification for tools that are subjected to more intense contact stresses. Statement of SignificanceThe evolution success of crustaceans is largely due to the diversification of their mineralized exoskeletons and hunting appendages, which exhibit a large palette of morphometric characteristics that have adapted to meet particular functions. We explored the “snapper claw” of the pistol shrimp, which is used to generate cavitation bubbles and strike prey. Our multi-scale structure-property relationship study reveals that the snapper claw is more brittle than other fast raptorial appendages – such as the stomatopod dactyl club – because it is not directly subjected to direct contact forces during action. This study implies that when higher mechanical stresses are needed to meet the function, the internal structure is built of a more complex architecture that allows to mitigate internal structural damage.

Pneumatically Powered Drilling of Carbon Fibre Composites Using Synthetic Biodegradable Lubricating Oil: An Experimental Study

Carbon fibre composites are a key component of aircraft structures because of their enhanced material properties such as favourable strength to weight ratios when compared to metal alloys. During the assembly process of an aircraft, carbon fibre components are joined to other structures using rivets, bolts, and fasteners, and as part of the joining process, the components will need to be machined or drilled. Unlike metal alloys, composites are sensitive to heat and are vulnerable to internal structural damage from machining tools. They are also susceptible to a reduction in strength when fibres are exposed to moisture. In the machining process, carbon fibre composites may be drilled using oils to lubricate carbide machining tools. In this study, a description of the experimental apparatus is provided along with an investigation to determine the influence synthetic biodegradable lubricating oil has on drill rotational speed, drilling load, and drilling temperature when using a pneumatic drill to machine carbon fibre composite material.

The Relationship between Mechanical Properties and Gradual Deterioration of Microstructures of Rock Mass Subject to Freeze-thaw Cycles

Under freeze-thaw cycles, the relationship between rock microstructure deterioration and its macroscopic mechanical characteristics has drawn extensive attention from engineers. With the objective to incorporate freeze-thaw cycle experiment into headrace tunnel engineering, in the present study two groups of andesite rock samples in different states are tested under the conditions of the lowest freezing temperature of –40 ℃ and the thawing temperature of 20 ℃. Damage detection was performed by magnetic resonance imaging for the interior microstructure of rock samples subject to different freeze-thaw cycles, and the relationship between the sample mechanical properties and gradual deterioration of rock microstructures was discussed. The results demonstrate evident influence of freeze-thaw cycle on the damage and deterioration of internal pore structure in andesite, and the rock uniaxial compressive strength and elasticity modulus exhibit a decreasing trend with the increase of freeze-thaw cycles. After 40 cycles, the strength of naturally saturated rock samples decreases by 39.4% (equivalent to 69.4 MPa) and the elasticity modulus drops by 47.46% (equivalent to 3.27 GPa). For rock samples saturated by vacuum, 40 freeze-thaw cycles lead to a decrease of 36.86% (equivalent to 58.2 MPa) in rock strength and a drop of 44.85% (equivalent to 2.83 GPa) in elasticity modulus. Therefore, the test results quantitatively elucidate the substantial influence of freeze-thaw cycle on the damage and deterioration of internal structure in andesite.

Assessment of sulfur concrete properties for use in civil engineering

Sulfur concrete is a material that was introduced to the civil engineering industry as a substitute of a typical concrete in which cement is a binder. Sulfur concrete properties are provided by a sulfur polymer produced in the process of synthesis of the ordinary sulfur with modifiers that prevent a process of crystallization of sulfur what results in stabilizing sulfur polymer structure. The purpose of this investigation was to evaluate characteristics of sulfur concrete, which was designed with combustion products. It was established that mechanical parameters of sulfur concrete should correspond with properties of a cement concrete used in production processes of prefabricated elements of road and construction industry. Accurateness of parameters of a designed mixture was confirmed and rated by a series of tests of hardened concrete – physical, mechanical (compression strength), durability in harsh environment. The test results confirmed low water absorption, high resistance to wear abrasion, high freeze-thaw scaling resistance (surface weathering) in a salty environment and high compressive and tensile strength of individual compositions. However, due to the internal structural damage, the freeze-thaw-attack cube tests did not prove the freeze-thaw resistance of the sulfur concrete specimens. The reason was low freeze-thaw-attack resistance of sulfur binders.

Detection of structural changes and mechanical properties of light alloys after severe plastic deformation

Severe plastic deformation by equal channel angular pressing has been performed to produce light aluminum and magnesium alloy billets with ultrafine-grained structure. The physical and mechanical properties of the processed alloys are examined by studying their microstructure, measuring microhardness, yield strength, and uniaxial tensile strength. A nondestructive testing technique using three-dimensional X-ray tomography is proposed for detecting internal structural defects and monitoring damage formation in the structure of alloys subjected to severe plastic deformation. The investigation results prove the efficiency of the chosen method and selected mode of producing ultrafine-grained light alloys.

Development of PZT-excited stroboscopic shearography for full-field nondestructive evaluation.

Nondestructive evaluation using shearography requires a way to stress the inspection target. This technique is able to directly measure the displacement gradient distribution on the object surface. Shearography visualizes the internal structural damages as the anomalous pattern in the shearograpic fringe pattern. A piezoelectric (PZT) excitation system is able to generate loadings in the vibrational, acoustic, and ultrasonic regimes. In this paper, we propose a PZT-excited stroboscopic shearography. The PZT excitation could generate vibrational loading, a stationary wavefield, and a nonstationary propagation wave to fulfill the external loading requirement of shearography. The sweeping of the PZT excitation frequency, the formation of a standing wave, and a small shearing to suppress the incident wave were powerful controllable tools to detect the defects. The sweeping of the PZT excitation frequency enabled us to determine one of the defect-sensitive frequencies almost in real time. In addition, because the defect sensitive frequencies always existed in wide and plural ranges, the risk of the defect being overlooked by the inspector could be alleviated. The results of evaluation using stroboscopic shearography showed that an artificial 20 mm-diameter defect could be visualized at the excitation frequencies of 5-8 kHz range and 12.5-15.5 kHz range. This technique provided full field reliable and repeatable inspection results. Additionally, the proposed method overcame the important drawback of the time-averaged shearography, being required to identify the resonance vibration frequency sensitive to the defect.

Effects of ball milling on structural changes and hydrolysis of lignocellulosic biomass in liquid hot-water compressed carbon dioxide

Mechanical activation is an effective method for destroying the crystalline structure. Biomass, especially its hemicellulose, can be degraded in the green solvent of liquid hot-water compressed carbon dioxide. To improve the degradation of crystalline cellulose in liquid hot-water compressed carbon dioxide, pretreatment of camphorwood sawdust by mechanical activation with a stirring ball mill was studied. Ball milling parameters and their effects on structure were determined by SEM, XRD and FT-IR. The influence of milling parameters on cellulose conversion can be ranked as follows: ball milling speed>activation time>the mass ratio of ball to biomass. The optimum milling condition was obtained at ball milling speed of 450 rpm and mass ratio of 30: 1 of ball to biomass for 2 h. In this condition, cellulose crystallinity of sawdust decreased from 60.93% to 21.40%. The cellulose conversion was 37.8%, which was nearly four times of raw material (10.2%). The glucose yield in the hydrolysate was 1.49 g·L−1, which was nearly three times of that of raw material. It showed mechanical activation can destroy the crystalline structure of cellulose to promote degradation and the damage of lignocellulosic internal structure caused by ball milling is irreversible.

In-operando high-speed tomography of lithium-ion batteries during thermal runaway.

Prevention and mitigation of thermal runaway presents one of the greatest challenges for the safe operation of lithium-ion batteries. Here, we demonstrate for the first time the application of high-speed synchrotron X-ray computed tomography and radiography, in conjunction with thermal imaging, to track the evolution of internal structural damage and thermal behaviour during initiation and propagation of thermal runaway in lithium-ion batteries. This diagnostic approach is applied to commercial lithium-ion batteries (LG 18650 NMC cells), yielding insights into key degradation modes including gas-induced delamination, electrode layer collapse and propagation of structural degradation. It is envisaged that the use of these techniques will lead to major improvements in the design of Li-ion batteries and their safety features.

Substructural Identification of Flexural Rigidity for Beam-Like Structures

This study proposes a novel substructural identification method based on the Bernoulli-Euler beam theory with a single variable optimization scheme to estimate the flexural rigidity of a beam-like structure such as a bridge deck, which is one of the major structural integrity indices of a structure. In ordinary bridges, the boundary condition of a superstructure can be significantly altered by aging and environmental variations, and the actual boundary conditions are generally unknown or difficult to be estimated correctly. To efficiently bypass the problems related to boundary conditions, a substructural identification method is proposed to evaluate the flexural rigidity regardless of the actual boundary conditions by isolating an identification region within the internal substructure. The proposed method is very simple and effective as it utilizes the single variable optimization based on the transfer function formulated utilizing Bernoulli Euler beam theory for the inverse analysis to obtain the flexural rigidity. This novel method is also rigorously investigated by applying it for estimating the flexural rigidity of a simply supported beam model with different boundary conditions, a concrete plate-girder bridge model with different length of an internal substructure, a cantilever-type wind turbine tower structure with different type of excitation, and a steel box-girder bridge model with internal structural damages.