This article presents a systematic study on creep design of epoxy bonded anchor system. First, a theoretical model for prediction of long-term performance of an anchor system is suggested assuming epoxy layer following Maxwell visco-elastic behavior, which is then validated through an ABAQUS numerical model. A close match between the ABAQUS result and the theoretical model has been reached. Next, a general-purpose methodology for estimating creep capacity of an adhesive bonded anchor system is suggested following AC58 and two creep capacity equations are reached for Model Dry Epoxy and Model Wet Epoxy bonded anchor systems. The derived creep capacity is then used to re-examine the Boston D Tunnel anchor design and a safe design is reached through the suggested method considering the anchor system's long-term creep performance. Last, the ABAQUS model is extended to include the effect of curing temperatures, which shows significant influences of curing temperature on the capacity of an epoxy bonded anchor system.
In this study, the guided wave technique is applied to nondestructively assess the damage in various engineering materials, like alumina, laminated composites, and composite sandwiches. A combined theoretical, numerical and experimental investigation of the pulse-echo method using piezoelectric sensors and actuators is conducted. The dispersion effect of wave guides on these materials is first analyzed, and the transient propagation process of wave guides and its interaction with inside damages are then numerically simulated. The implementations of the pulse echo method are illustrated in experimental testing and damage detection of aluminum beams, carbon/epoxy laminated composite plates, and composite sandwich beams. In particular, the experimental results on damage detection of the composite sandwich beams are reported and discussed. As illustrated in this study, the pulse-echo method combined with piezoelectric material can be used effectively to locate damage in various engineering materials and structures.
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The chapter analyzes steel tube confined concrete (STCC) structure defects and proposes a method to avoid effects in the course of construction. The improved construction method of concrete about dumbbell-section STCC arch-bridges supplies construction criterion and has an important reference value. STCC is widely used in civil engineering because it has advantages of high loading capacity, good plasticity, convenient construction, good fire-resistance, and economic benefit. However, the inherent defects of combined material and its effect on performance of the structure loading behavior cannot be ignored. Mechanism of construction defect of steel tube confined concrete and material defect are presented in the chapter. A method to control defects in the course of perfusing concrete in dumb-bell STCC arc-bridges is also proposed in the chapter. It widens the new field of vision in the research and practice of STCC technology.
Natural frequency is an important parameter in the structural health monitoring (SHM) system. Any changes in this parameter indicate structural alteration due to damage. This study provides a neural network (NN) solution as an alternative to the finite element (FE) method to measure the natural frequencies of a cantilever beam with random multiple damage. It is based on a statistical dataset of a free vibration test obtained from the APDL (Ansys parametric design language) simulation using a MATLAB (matrix laboratory) script. The script can generate an unlimited number of possible damage combinations for any given parameters with the help of the Monte Carlo (MC) technique. MC helps to generate a random number of damages in random locations at each simulation. Damage conditions are controlled by three parameters including damage severity and damage size (in terms of the mean and standard deviation of damage). Moreover, the method proposes a curve-fitting equation to validate the predicted natural frequency for the first three modes obtained from the neural network model. Both methods are in good agreement with each other, having minimal errors in the range of 0.2-3% for each mode. The frequency result shows that the beam frequency is 8.6486 Hz if the area reduction is 10%, whereas it comes down to 7.2338 Hz if there is a 30% area reduction. A two-level factorial test shows that damage severity is the most impactful factor compared to the damage sizes on the frequency shift event. This indicates that damage alters the composition of the beam and has an impact on its frequency change with the assumed damage parameters. Therefore, the proposed NN model can estimate the frequency shift for various damage scenarios. It can be utilized in the vibration-based damage identification process to predict the frequency changes of the damaged beam without any computational burden.
In this study, a higher-order impact model is presented to simulate the response of a soft-core sandwich beam subjected to a foreign object impact. A free vibration problem of sandwich beams is first solved, and the results are validated by comparing with numerical finite element modeling results of ABAQUS and the solution by Frostig and Baruch [Frostig, Y., Baruch, M., 1994. Free vibration of sandwich beams with a transversely flexible core: a high order approach. Journal of Sound and Vibration 176(2), 195–208]. Then a foreign object impact process is incorporated in the higher-order model, and the contact force and deflection history as well as the propagation of transverse normal, shear, and axial stresses during the impact are analyzed and discussed. The validity of the model in the impact response predictions is demonstrated by comparing with finite element solutions of LS-DYNA. The calculated stresses caused by a foreign object impact are then used to assess failure locations, failure time, and failure modes in sandwich beams, which are shown to compare well with the available experimental results. The effects of impact mass, initial velocity, core stiffness, and core height on the impact stresses generated in the beams are discussed. The influences of impact mass and initial velocity on the contact force history are close to those by the linearized impact solution, but the proposed higher-order impact model captures the non-linear impact process and different generated stresses. Compared to the fully backed sandwich case, the core height shows a great influence over the impact process of a simply supported sandwich system, in which the global behavior of the sandwich is dominant; while the core stiffness shows minor effect over the impact process. The higher-order impact model of sandwich beams developed in the study provides accurate predictions of the generated stresses and impact process and can be used effectively in design analysis of anti-impact structures made of sandwich materials.
The turbulent flow of wind and snow creates a complicated interaction during snow-drifting. To study snowdrift distribution patterns around a snow fence, a two-dimensional model is developed using ANSYS fluent (a commercially available CFD software). K-epsilon and discrete phase model are applied for this multiphase simulation. The model can simulate the dynamic changes as well as snow distribution patterns around a porous snow fence under no concurrent snowfall condition. It also works for different bottom gaps, porosities, and fence surfaces. The numerical result shows that snowdrift accumulation increases on the leeward side of the snow-fence and the snow-phase volume fraction decreases behind the snow fence for a certain time. Results obtained from the numerical model are in good agreement with the experimental measurements. A comparison is also made between the performance of a solar snow fence and a traditional snow fence. Higher snow drifting on the leeward side of the fence is observed for the traditional snow fence; whereas the maximum height of drifting is closer to the fence for the solar snow fence than those of traditional snow fences do. Therefore, the proposed model could be a useful guide for any snow fence design and implementation.
The effects of the crystallographic orientation on the H2 gas sensing properties were investigated in highly oriented polycrystalline Pd-doped SnO2 films, which were obtained using rf magnetron sputtering of a Pd (0.5wt%)-SnO2 target on various substrates (a-, m-, r-, and c-cut sapphire and quartz). All the films had a similar thickness (∼110nm), root-mean-square (rms) roughness (∼1.3nm), surface area, and chemical status (O, Sn, and Pd). However, the orientation of the films was strongly affected by the orientation of the substrates. The (101), (002), and (101) oriented films were grown on ( 1 1 2 ¯ 0 ) (a-cut), ( 1 0 1 ¯ 0 ) (m-cut), and ( 1 1 ¯ 0 2 ) (r-cut) Al2O3 substrates, respectively, and rather randomly oriented films were deposited on (0001) (c-cut) Al2O3 and quartz substrates. In addition, the oriented Pd-doped SnO2 films were highly textured and had in-plane orientation relationships with the substrates similar to the epitaxial films. The (101) Pd-doped SnO2 films on ( 1 1 2 ¯ 0 ) and ( 1 1 ¯ 0 2 ) Al2O3 showed a considerably higher H2 sensitivity, and their gas response decreased with increasing sensing temperature (400–550°C). The films deposited on ( 1 0 1 ¯ 0 ) and (0001) Al2O3 showed the maximum sensitivity at 500°C. The comparison of the H2 gas response between undoped and Pd-doped SnO2 films revealed that the Pd-doping shifted the optimum sensing temperature to a lower value instead of improving the gas sensitivity.
Both the crushing and indentation behaviors of sandwich materials are important aspects in failure analysis and energy absorption. In this paper, the core crushing strength of honeycomb materials from the experiment is briefly introduced, and the indentation of sandwich structures is studied in detail. Using the beam on elastic-plastic foundation models, theoretical formulations for predicting indentation behavior of sandwich materials are proposed, from which the mechanical response of elastic-plastic sandwich beams are obtained. Three stages of failure are clearly depicted by the global stiffness changes in the load versus displacement curve of elastic-plastic beams. The models are compared with the available experimental data and numerical simulation, and relatively close agreements are achieved. The compliance and compliance gradient derived form the indentation models are then incorporated with the equation of motion of the projectile to study impact response of elastic-plastic beams, and the impact energy dissipation due to the plasticity of elastic-plastic sandwich beam is uniquely recovered from the derived damping ratio. The beam on elastic-plastic foundation models proposed can be used to predict the indentation behavior of honeycomb sandwich materials, and the unique incorporation of the compliance and compliance gradient of the elastic-plastic sandwich beams in the standard mass-spring model can be utilized to characterize the effect of core compression and plasticity in the impact process.
Due to increased traffic volumes and incidence of impact damaged to bridge structures, there is a pressing need to develop a collision protection system for highway bridges against over-height truck impact. In this study, a linear elastic model for the impact response of bottom face sheet-rigidly supported sandwich structures including shear-off effect is developed. The solutions for a discrete model and concentrated and distributed loading cases for collision protection I-Lam system are obtained, of which the contact force history and maximum deflection are predicted. Compared to the LS-DYNA finite element analysis (FEA), the relatively simple analytical methods proposed accurately predict the impact response. The discrete model shows a better correlation with FEA predictions, while the model with distributed forces overestimates the deflection of the top face sheet but under-evaluates the peak contact force. The developed models including the shear-off mechanism can be used to approximate the impact response of rigidly supported sandwich structures hit by an over-height truck. The two important impact factors, i.e. the maximal deflection and peak contact force, can be used in primary design and optimization of collision protection sandwich structures.
Most of the rocks surrounding deep underground construction consists of cracked rock. The deformation of the surroundings is complicated, and not only involves slipping, but also involves re-fracturing. Available results in conventional uniaxial and triaxial experiments of rock samples can be used to analyze deformations of supports and surrounding rocks on underground constructions, but they cannot reflect the evolution of the re-fracture processes and the mechanical characteristics of the cracked surrounding rock mass. The purpose of this paper is to study deformations and re-fracture characteristics of surrounding rock mass in deep underground engineering. First, deformations and cracking characteristics of intact rock samples are analyzed on the basis of uniaxial and triaxial experiments. The lateral constraint testing method is then applied to study deformation and re-fracturing characteristics of cracked rock mass. The lateral constraint test provides different lateral stiffness to the cracked rock samples and causes different failure modes in the cracked rock samples. Conditions for re-fracturing have been derived and validated through the lateral constraint tests, which can be used to study load capacities and stabilities of surrounding rock in deep underground engineering under structural constraints formed by different supports.
This paper reports an investigation of deep learning techniques in structural damage identification that can overcome the limitations of traditional visual inspection. First, a vibration-based deep learning model is established to locate the damage in a beam and a truss structure. Then an optical photo-based model is established and used to classify different defects. Based on the satisfactory outcomes of these two models, a new structural health monitoring technique is proposed through the infusion of optical photos and vibration data. Vibration signals and true structural images for a truss are used to demonstrate the capability of the proposed method. It was found that the infusion of vibration data and optical photos can enhance damage identification significantly and overcome the drawbacks in the existing deep learning models due to incomplete vibration signals or blurred optical photo inputs.
A systematic summarization of the alkali-aggregate reaction (AAR) effect of activators and aggregates in alkaline active cement concrete has been discussed in this chapter. High calcium and low calcium alkaline-activated cement concrete have been separately summarized and the effect of AAR on these two systems are compared. Effect of activator modulus, sodium oxide does, and percentage of calcium, silica, and aluminum content of aggregates on expansion of alkaline-activated concrete are lab tested and analyzed.
Pervious concrete has been widely used in parking lots and airport fields. However, past experience has shown durability and strength of the pervious concrete remains a challenge for adopting them in wider applications, as the binding material proportion is low and the use of fine aggregates is nearly zero. When loading in compression, the failure appears first in the weak concrete zone induced by the random distribution of voids. The process continues until the failure of the whole specimen occurs. Thus, the position, size and shape distribution of voids in the pervious concrete microstructure control the overall behavior of the material. In this work, the influence of the distribution of voids on the strength, stiffness, and permeability of pervious concrete microstructure is studied by 2D image analysis and finite element modeling. The position and size distribution of the microstructure voids are first summarized through Fast Fourier Transformation (FFT) analysis. A finite element analysis is then conducted on the microstructure models regenerated through the derived microstructure characteristics using ANSYS Parametric Design Language (APDL). The effect of the sample size of 2D microstructures is studied by varying the section size and a possible representative volume element (RVE) is therefore found. Predicted stress-strain plots are generated for the 2D specimen under compressive load and the obtained results, including stiffness, strength and permeability are compared with the results from the experiments conducted following ASTM standards. The comparison between models and physical experiments showed around or above 90% accuracy, which validates the suggested micromechanical analysis method of the previous concrete composite.
This paper studies the capacity of prestressed concrete bridge girders to withstand overheight truck impacts. The numerical model of the bridge was validated with the existing experimental data in the literature. The elastic-plastic modeling of concrete damage and quasi-static impact simulation of the bridge were considered. Parametric studies were then conducted with respect to the spacing and depth of intermediate diaphragms as well as girder types from which the role of intermediate diaphragms in protection and enhancement of prestressed concrete bridge girders from truck impacts is obtained. From the simulation results, it was concluded that the optimum spacing of intermediate diaphragms (that is, 25.0 ft [7.62 m]) is consistent with the results reported for previous studies. The depth of intermediate diaphragms should be extended to the top edge of the bottom flange of girders to provide maximum impact protection. Wide flanges in girders increase the bending stiffness and reduce the horizontal deflection and correspondingly decrease damage areas. However, deep webs in girders introduce a large rotation with respect to the deck, and thus increase the deflection under the truck impact. Design guidelines for typical parameters for intermediate diaphragms in prestressed concrete bridges are provided. The numerical finite-element parametric study provides a better understanding of placement and size of intermediate diaphragms in prestressed concrete bridges and their role in protecting girders from overheight truck impacts.
The addition of dry ice as an admixture to cement pastes has many advantages such as improving the compressive strength and sequestration of CO2. However, its effect on the hydration mechanism of cement pastes is still unknown and will be investigated using electrochemical impedance spectroscopy in this paper. Based on the linear correlation of cumulative hydration heat from the isothermal calorimetry test and the logarithm of high-frequency resistivity, a modified Knudson equation is derived. Corresponding hydration kinetic parameters are calculated by applying the boundary nucleation and growth theory and kinetics model, including Avrami, geometrical contraction, and diffusion equations. The boundary area per unit volume (OvB) for the control specimen and specimens with 0.2% and 0.5% cement weight dry ice addition is 0.1171, 0.1250, and 0.1562, respectively, indicating that the dry ice addition increases the surface area of the nucleation process. In addition, the results show that the Avrami equation for monitoring the nucleation process is limited. Regarding the hydration curves, the transition from the nucleation and growth stage to the interaction of phase boundaries (I) stage is delayed by adding dry ice to the mixture.
