The present study belongs to a broader investigation aiming to reduce noise emissions in nail guns. This noise reduction objective may be achieved by nail gun concept design improvements. However, modifying the tool design requires precise understanding of it dynamics. Therefore a dynamic model of the system including accurate predictions of the tribo-dynamic interactions at the wood-nail interface generating the penetration resistance forces ( PRF ) appears to be essential. Since different wood products possess different structural/material properties, PRF is first evaluated for various types of wood product individually. Ref. [1] develops the PRF modeling strategy and examines the nail penetration process for plywood samples. The present paper proposes an empirical model predicting PRF imposed on nails when penetrating particle board ( PB ) at quasi-static velocities (20–500 mm/min range). A universal testing machine (MTS) is used to drive the nails into the wood samples. Each wood sample is composed of five panels PB screwed together. The sample size is chosen to reduce the boundary influence on the penetration process and to avoid the complete perforation of the sample. To eliminate the possible acceleration influence, the penetration speed is maintained at constant amplitudes. The MTS machine measured PRF as a function of the position. The objective is to prepare a formulation predicting PRF as a function of nail position. In order to extend the prediction formula application range, the analysis reduces the studied factors to dimensionless parameters. The analysis shows that the PB fabrication process results in panels presenting three regions of different hardness modulus. As a result, at the region transitions the PRF curves show large amplitude fluctuations. This layered heterogeneity makes the development of a high precision prediction model representing various nail sizes very difficult. Nevertheless, the final model produces PRF evaluations with overall precision greater than 88% for most of the nail penetration.

Slender, lightweight structures are demanded to meet efficiency targets or to enhance vehicle system performance characteristics. Yet, when subjected to static stress for load-bearing purposes, the flexible structural members may buckle. Furthermore, additional dynamic excitations may activate adverse snap-through responses in such post-buckled components, which accelerates fatigue and failure. The severe nonlinearity associated with these phenomena challenges traditional forms of analysis and necessitates studious experimental methods for conclusive system characterization and model validation. This research builds upon state-of-the-art analytical and experimental strategies to examine the complex forced, dynamic behaviors of built-up structures that contain one or more post-buckled members. An analytical modeling and solution formulation is reviewed that is uniquely amenable to the study of multistable structures and permits experimentally-observable measures of impedance to be identified. Through theoretical and experimental studies, the efficacy of the impedance measures is evaluated towards their usefulness in identifying the onset of dynamic bifurcations in the multistable structural dynamics. For moderate amplitudes of input energy, the analysis is found to provide qualitatively accurate prediction of the drive point impedance changes observed prior to dynamic bifurcations from low to high amplitude of displacement.

Super elastic alloy (SEA) has good flexibility with moderate rigidity and then is widely used in eyeglasses, bras and so on. SEA’s flexibility is based on the phase transformation between austenite and martensite structures of material metallurgy, and it is known that the transformation can be controlled by heating and cooling the material. The active deformation of SEA is already reported as controllable by the heating and cooling under pre-stressed condition, but it is known that the deformation is on / off control because the material has very strong nonlinearity in their stress-strain relationship. Then the development of smooth control of SEA’s active deformation is studied to realize soft actuator in this report. The smoothness of the control is realized by the multiplying the wires of SEA in actuating unit of 1DOF actuator. The theory of SEA controller is formulated by using the constitutive equation of the metallo-thermo-mechanics which represents the kinetics of phase transformation between austenite and martensite structures. The developed controller with the formulation is applied to actuate 1DOF actuator with multiple wires of SEA for the verification of the realization. The qualitatively match between theory and experimental results is observed here but quantitatively mismatch has occurred in this report. In the discussion of this paper, it is shown that the mismatch is occurred because the formulation is based on the ideal condition without the deformation of actuator unit. Then this report shows that it is important to design the soft actuator not only their actuating element but also the other elements which affect the deformation behavior of the actuator as a total unit.

Topological interlocking is an effective joining approach in both natural and engineering systems. Especially, hierarchical/fractal interlocking are found in many biological systems and can significantly enhance the system mechanical properties. Inspired by the hierarchical/ fractal topology in nature, mechanical models for Koch fractal interlocking were developed as an example system to better understand the mechanics of fractal interlocking. In this investigation, Koch fractal interlocking with different number of iterations N were designed. Theoretical contact mechanics model was used to analytically capture the mechanical behavior of the fractal interlocking. Then finite element (FE) simulations were performed to study the deformation mechanism of fractal interlocking under finite deformation. It was found that by increasing the number of iterations, the contact area increases and the interlocking stiffness and strength also significantly increase. The friction coefficient of contact plays an important role in determining the mechanical properties of fractal interlocking.

High-speed and heavy-loaded rotating machinery require accurate prediction of rotor’s response and stability, which can be characterized by the static and dynamic coefficients of the bearing support. In this paper, a theoretical study has been done to investigate the performance of a fixed-tilting pad journal bearing with ball-in-socket pivot. The analytical model is established with the flexibility of the pad pivot and turbulent effect of the oil film both taken consideration. Under such situation, the pad pivot elastic deformation and its stiffness are calculated using Hertz Contact Theory for various operating points of the rotor-bearing system. The finite element method is adopted to simulate the static coefficients of the fixed-tilting pad bearing, obtaining its oil film pressure distribution varied with the bearing eccentricity ratio. The corresponding dynamic stiffness and damping of the oil film are solved using partial derivative method. In addition, a special interest is put in investigating the effect of the series complex stiffness of the oil film and pad pivot, according to which, the equivalent dynamic characteristics are obtained. The results show that the relation between these two factors are complex and interactive, both of which have a significant influence on the static and dynamic performance of the bearing.

Adhesive joint technology has been developed gradually, and the application fields of this type of joints have been expanded increasingly since they reduce the weight of the applications, provide uniform stress distribution across the joints, allow to bond similar, and dissimilar materials, and contribute to dampen the shock, and vibration. However, the performance of the adhesive joints under high loading rate such as blast or ballistic loading has been studied by few researchers. In this study, fully laminated plates consisting of 6061 aluminum plates (15” in diameter and 1/16” thick) and FM300K epoxy film adhesive were tested under shock wave loading. Full displacement field over the testing plates were obtained by TRC-SDIC technique, and the strain on the plates were computed by classical plate theory for large deflections. FEM model was analyzed and the results were compared with experimental results.

The objective of the current work is to conduct a systematic analysis on the effects of manufacturing induced defects such as random distribution of fibers and presence of voids in matrix on the damage initiation in polymeric composites. Upon infusing resin, the initial fiber configuration undergoes perturbation and results in a random distribution with pockets of resin rich areas and fiber clusters. In addition, this could result in micro voids (between the fibers in the bundle) and macro voids (between the fiber bundles). A novel methodology has been put forward to generate random distributions of fibers that would simulate different levels of perturbations in the manufacturing process resulting in different configurations of fiber clusters. An embedded Representative Volume Element (RVE) approach has been adopted in a finite element model to calculate the stress fields without artificial effects of the RVE boundary. Damage initiation is then analyzed using a previously proposed energy based criterion for cavitation in polymers.

It is estimated that more than 70% of failures in engineering components are associated with fatigue loading. Therefore, fatigue is a major design tool for mechanical components. These components are usually subjected to multiaxial cyclic loading. In fact, multiaxial state is very common as tension specimen is under triaxial strain state even though its stress state is uniaxial. There are three approaches to modeling fatigue damage: stress, strain and energy. Critical plane concept is established based on the fact that fatigue cracks initiate at specific plane(s), therefore, multiaxial fatigue damage parameter is evaluated at these plane(s). Critical plane fatigue models such as Fatemi-Socie is among the popular strain-based models. Because it was shown to provide estimation mostly within two factors of life for different materials and different multiaxial loading conditions. This paper presents a new method for analyzing critical plane damage parameters. Using plane stress-strain transformation, maximum values of normal and shear stresses and strains from hysteresis loops are obtained at 360 planes. Plotting these values on polar diagrams shows that multiaxial cyclic responses represent polar curves that can successfully be fitted with definitive known polar equations. In principle, this means that both critical plane and fatigue damage can be determined analytically for a given loading path. However, fitting constants must first be determined. A systematic analysis is performed on different experimental data that were obtained by testing two extruded magnesium alloys at proportional and 90° out of phase loading paths. A closed-form solution for Fatemi-Socie damage parameter is presented for these two loading paths.

Single Cantilever Beam (SCB) testing was conducted on foam and honeycomb core sandwich specimens to predict the fracture toughness and bonding strength. Adhesion characteristics can also be identified between facesheet and core materials conducting SCB test on precracked specimen. Disbonding under combined tensile and shear loading may involve failure modes such as kinking and cell buckling. Modified Beam Theory (MBT) method specifies required parameters to generate a least square plot based on experimentally determined compliance and disbond length obtained during the SCB test. Interfacial fracture toughness G c , enumerated by following recommended data reduction methodologies, required to predict minimum G c to initiate disbond growth. Finally, experimental compliance was compared with elastic foundation stiffness compliance to verify the performance of the test. Good agreement between two compliance solutions validates the accuracy of the SCB test.

A microstructure based model is developed to predict the effective electrical conductivity of open-cell metallic foams. A tetrakaidecahedral cell is adopted as the repeating unit, which comprises of 32 ligaments and 24 joints. Each ligament contains a straight section with a uniform cross section and two identical joint sections. Each joint is formed with four identical joint sections. The geometries of the ligaments are constructed based on the minimum surface energy reached during the foaming process. The electrical conductivity of a joint section is first numerically computed. It is then used, along with that of the straight section of the ligament, to give rise to the conductivity of the ligament for a given relative density. The effective electrical conductivity of the foam is then analytically determined along four typical orientations of the unit cell with the resistor network method and Kirchhoff’s current and voltage laws. The numerical results indicate that the electrical conductivity of 3-D open-cell foams exhibits some degree of anisotropy. The predictions of the foam conductivity along the four identified orientations well encompass the scattering of the experimental measurements reported in the existing studies.

In order to study the dry rough line-contact mechanism between two longitudinally rough metallic surfaces, the measured profile is mathematically described by quadratic functions for the application of the existing micro-contact models. The mechanical parameters are determined using the different approximating criteria. Next, based on these deterministic parameters, different micro-contact models are employed and extended to predict the characteristics of a line-contact. Comparison of different theoretical calculation results reveals that the greater maximum values of the contact deformation and the ratio of real to nominal contact area are predicted by the Hertz model as compared to the micro-contact models considering the elastoplastic deformation, and that the KE (Kogut and Etsion) and JG (Jackson and Green) models predict closer results. It is also found that when the rough surfaces are described by quadratic functions according to the same area criterion or same root mean square (RMS) criterion, the line-contact responses between them prescribed by any micro-contact models have the same tendency.

This paper presents an experimental investigation into the effects of the application of carbon nanotube (CNT) based nanopolymer, and thin film buckypaper, to the interface of stiffened carbon fiber reinforced polymer (CFRP) composite joints. Bonded CFRP composite T-joints, were manufactured with dispersed CNT epoxy nanopolymer mixture, and buckypaper films, applied at the joint interface, and tested under pull-off loading. The presence of the nanomaterial at the interface causes a localized out-of-plane reinforcement, which resists pull-off loads, leading to superior performance compared to composite bonded joints without nano-reinforcements, however, the introduction of substantial voids, in the case of the buckypaper samples, lead to faster structural failure. Digital image correlation (DIC) was used to map the strain contours of the T-joint specimen during testing, which revealed damage initiation and hot-spot zones. Fluorescent optical microscopy of the joint sections was also performed to investigate these hot-spot zones and damage initiation areas, at the mesoscale, to study the possible causal mechanisms of the failure process in the tested composite bonded joints.

In this work, molecular dynamics simulations have been used to study the brittle fracture behaviour of vitreous silica in mixed mode loading at room temperature. An implementation of the BKS potential with the coulombic term was used along with Lennard-Jones modification to model initial cristobalite. Ewald summation was used to obtain long-range coulombic contribution to the total potential energy of the system. A recipe (Huff et. al., Journal of Non-Crystalline Solids, 253, 133–142, 1999) was used to obtain the vitreous silica using a combination of different molecular dynamics runs which were done initially as NVT ensemble and at the end as NPT ensemble. Uniaxial tensile tests for uncracked specimen was carried out to validate the microscopic and macroscopic properties with that in the literature. Further, slit center cracks of different orientations were introduced in the vitreous silica and subjected to mixed mode loading by moving the boundaries slowly. Studies of mechanical behaviour were made to derive the variation of fracture stress and stiffness with the mode-mixity in amorphous solids.

Despite the significant progress in the development of modern alloys, low alloy steels continue to be the materials of choice for large structural components at elevated temperature for extended periods of time. The resistance of these alloys to deformation and damage under creep and/or fatigue at elevated temperature make them suitable for components expected to endure decades of service. The material 2.25Cr-1Mo is commonly applied in boilers, heat exchanger tubes, and throttle valve bodies in both turbomachinery and pressure-vessel/piping applications alike. It has an excellent balance of ductility, corrosion resistance, and creep strength under moderate temperatures (i.e., up to 650°C). In the present work, a life prediction approach is developed for situations where the material is subjected conditions where creep and fatigue are prevalent. Parameters for the approach are based on regression fits in comparison with a broad collection experimental data. The data are comprised of low cycle fatigue (LCF) and creep fatigue (CF) experiments. The form of the life prediction model follows the cumulative damage approach where dominant damage maps can be used to identify primary microstructural mechanism associated with failure. Life calculations are facilitated by the usage of a non-interacting creep-plasticity constitutive model capable of representing not only the temperature- and rate-dependence, but also the history-dependence of the material. For the inelastic response, both the Garofalo and Chaboche models for creep and plasticity are employed, respectively.

Understanding the transport of hydrogen within metals is crucial for the advancement of energy storage and the mitigation of hydrogen embrittlement. Using nanosized palladium particles as a model, recent experimental studies have revealed several highly nonlinear phenomena that occur over a long period of time. The time scale of these phenomena is beyond the capability of established atomistic models. In this work, we present the application of a new model, referred to as diffusive molecular dynamics (DMD), to simulating long-term diffusive mass transport at atomistic length scale. Specifically, we validate the model for the long-term dynamics of a single hydrogen atom on palladium lattice. We show that the DMD result is in satisfactory agreement with the result of the classical random walk model. Then, we apply the DMD model to simulate the absorption of hydrogen by a palladium nanocube with an edge length of 16 nm. We show that the absorption process is dominated by the propagation of a sharp, coherent α/β hydride phase boundary. We also characterize the local lattice deformation near the dynamic phase boundary using the mean positions of the palladium and hydrogen atoms.

The quality of grains and grain boundaries of polycrystalline copper thin films was analyzed by using image quality (IQ) value obtained from the observed Kikuchi pattern by applying electron back-scatter diffraction (EBSD) analysis. It is considered that the IQ value strongly correlates with the order of atomic configuration in the observed area, in other words, density of various defects, and thus, the area with high IQ value was defined as the area with high crystallinity. The yield strength of a grain was measured by using micro tensile test system in a scanning electron microscope. A bicrystal structure which had two grains with different IQ values was cut from a copper thin film by using focus ion beam (FIB) and the sample was fixed to a single-crystalline silicon beam and a micro probe, respectively, by tungsten deposition. Finally it was thinned to 1μm and stretched to fracture at room temperature. In this micro tensile test, however, the tungsten deposition on the side surface of the test samples caused serious error on the measured strength. Therefore, in this study, the experimental method was improved by the development of an effective method for elimination the excess tungsten deposition. During the tensile test, a mass of plastic deformation and necking phenomenon were obviously observed. Ductile fracture always occurred in the grain with higher Schmidt factor. It was found that the yield strength of a copper grain decreased monotonically with the increase in the IQ value when the IQ value at the grain boundary was larger than 3500.

Alike other polymer material, PolyVinyl Chloride (PVC) shows a clear creep behavior, the rate of which is influenced by temperature, load and time. Polyvinyl chloride bolted flange joints undergo relaxation under compression for which the material creep properties are different than those under tension. Since the sealing capacity of a flanged gasketed joint is impacted by the amount of relaxation that takes place, it is important to properly address and predict the relaxation behavior due to flange creep under compression and reduce the chances of leakage failure of PVC flange joints. The main objective is study the creep behavior of PVC flanges under the influence of normal operating conditions. This is achieved by developing a PVC creep model based on creep test data under various compressive load, temperature and time. A simulation of a PVC flange relaxation behavior bot numerically and experimentally is conducted on an NPS 3 class 150 bolted flange joint of dissimilar materials one made of PVC material and the other one by steel SA105. The study also provides a clear picture on how the compression creep data on Ring specimen may be utilized for predicating the flange performance under various operating temperatures with time.

The evaluation of the mutual effect of non-aligned multiple cracks is a prerequisite in applying fitness-for-service codes. For non-aligned parallel cracks, during on-site inspection, one needs to decide whether the cracks should be treated as coalesced or separate multiple cracks for Fitness-for-Service. In the existing literature, criteria and standards for the adjustment of multiple nonaligned cracks are very source dependent, and those criteria and standards are often derived from on-site service experience without rigorous and systematic verification. Based on this observation, the authors previously reported on the influence of an embedded crack on an edge crack in 2-D scenarios and, more recently, in 3-D scenarios of the influence of a surface crack on a quarter-circle corner crack. However, realistic crack configurations detected using non-destructive methods are generally 3-D in nature and their influences are mutual. Thus the SIF distribution characteristics along the surface crack is equally important as the SIF distribution of the corner crack when Fitness-for-Service rules are to be applied. Therefore, non-aligned flaws with different configurations and shapes and the SIFs along their crack fronts are deemed necessary in order to obtain more practical guidance in the usage of rules speculated in Fitness-for-Service codes. In this study, the characteristics of the SIF distribution along a semi-elliptic non-aligned surface crack is examined under the influence of a quarter-circle corner crack of various geometries in an infinitely large plate. For any given geometry of a quarter-circle corner crack, a pair of horizontal (H) and vertical (S) separation distances between the two cracks is chosen followed by a detailed analysis of the effect of the quarter-circle corner crack on the 3D SIFs of the surface crack at different ellipticities. The analysis is repeated for various combinations of separation distances S and H. The results from this study are collectively significant to the understanding of the correlation between the criteria and standards in Fitness-for-Service community and the consequence of their usage in engineering practice.

The variations in thermal conductivity of nanocomposites are found to depend not only the intrinsic properties of the fiber and matrix phases but also on the interfacial resistance of the reinforcing phase. As we go down the length scales, the interfacial thermal resistance due to size of the nanoparticle becomes significant. In order to address the effect of size (length and diameter) of nanotube on the thermal transport property of nanotube composites, thermal conductivity of different nanotube samples varying in length and diameter will be estimated first using molecular dynamic (MD) simulations with AIREBO potentials. This will be carried out using the ‘Heat-Bath’ method - non-equilibrium molecular dynamics (NEMD) approach. In the heat bath method, constant amount of heat is added to and removed from the hot and cold regions and the resulting temperature gradient is measured and the thermal conductivity is calculated using the Fourier Law. This will be followed by the study of interfacial thermal resistance of these nanostructures. These intrinsic properties are then used with continuum based mathematical formulations to study the effect of size of the nanoparticle on the overall thermal conductivity of the nanocomposite.

This research study highlights the testing method and relevant results for assessing impact performance of a carbon fiber composite front bumper crush can (FBCC) assembly subjected to full frontal crash loading. It becomes extremely important to study the behavior of lightweight composite components under a crash scenario in order to apply them to automotive structures to reduce the overall weight of the vehicle. Computer-aided engineering (CAE) models are extremely important tools to virtually validate the physical testing by assessing the performances of these structures. Due to lack of available studies on carbon fiber composite FBCCs assemblies under the frontal crash scenario, a new component-level test approach would provide assistance to CAE models and better correlation between results can be made. In this study, all the tests were performed by utilizing a sled-on-sled testing method. An extreme care was taken to ensure that there is no bottoming-out force for this type of test while adjusting the impact speed of sled. Full frontal tests on FBCC structures were conducted by utilizing five high-speed cameras (HSCs), several accelerometers and a load wall. Excellent correlation was achieved between video tracking and accelerometers results for time histories of displacement and velocity. The standard deviation and coefficient of variance for the energy absorbed were very low suggesting the repeatability of the full frontal tests. The impact histories of FBCC specimens were consistent and in excellent agreement with respect to each other. Post-impact photographs showed the consistent crushing of composite crush cans and breakage of the bumper beam from middle due to the production of tensile stresses stretched caused by straightening of the bumper curvature after hitting the load wall.