Polymeric substances have seen an unprecedented integration in countless applications and fields of life, subjecting them to a broad range of environmental conditions, which can cause a form of damage to the structural and mechanical properties of these components known as aging. In this study we investigate the effects of the degradation of polymeric materials due to the extended exposure to sunlight, which can be simulated through controlled photo-oxidative aging using ultraviolet (UV) radiation as the main catalyst of this phenomenon. The effects on mechanical behavior and chemical properties of four polymeric materials with different mechanical characteristics was accordingly monitored throughout the study. The materials range in hardness with the polyurethane adhesive being the hardest and acting plastically, followed by an acrylic which exhibits greater hardening with time, a polyurethane sealant, which behaves in an elastomeric manner, and a silicon sealant, which is the softest of the set. Samples were prepared and subjected to UV radiation at three different temperatures namely 45, 60 and 80°C, for different aging periods ranging from 1 day for higher temperatures to 150 days for lower ones. The samples were then subjected to the Fourier transform Infrared spectroscopy (FTIR) test to monitor the change in the chemical composition of the materials along the aging durations and conditions. The materials were then subjected to mechanical testing along two modes of mechanical loading, which are a simple tension until failure and a double-cyclic test with progressively increasing strain limits after every two successful cycles until the sample reaches failure. The samples are loaded at constant strain rates throughout both tests. These tests revealed the change of behavior of our materials along the aging periods and conditions. The changes in mechanical behavior vary across each individual material depending on the aging temperature and period, with the changes ranging from hardening to softening, embrittlement or an increase in the maximum strain the material can endure before failure. A general trend would be that most materials become harder after photo-oxidation, however due to the range of temperatures in the aging conditions, thermo-oxidation, which causes softening in some of the polymers, has an increasingly notable effect at higher temperatures. A visible correlation can be noted between the change in mechanical behavior and a change in absorbance of Infrared Radiation (IR) in the FTIR spectrum, as the greater the crosslinking throughout the polymer matrix, the lower the absorbance of IR radiation due to the increased stiffness of the overall macro-molecular structure.

Results of online acoustic emission (AE) monitoring during fatigue crack growth rate (FCGR) experiments on a stainless steel SS 316 LN are presented in this paper. Two specimen geometries — viz., standard compact tension (C(T)) specimens as well as side-grooved C(T) specimens were considered for experiments at ambient temperature and at 600°C (873K). There is a good correspondence between crack length increment and the increase in AE cumulative count and cumulative energy during the experiments. The side grove introduced on the thickness direction of the test specimen constrains the plastic zone ahead of the crack tip, thereby enforcing plane strain conditions at the crack. Reduced AE activity at initial stages of crack growth was observed for side grooved samples. The transition to Stage-II crack growth was observed using acoustic emission (AE) technique which otherwise was not visible from the fatigue crack growth plot. The work further attempts to correlate the AE parameters obtained during elevated temperature (873K) fatigue crack growth in stainless steel. For the purpose of acquiring AE signals outside the heated zone, a waveguide was used to transmit the acoustic waves from the specimen at high temperature. A correlation between crack advance and AE parameters was obtained from the elevated temperature tests.

The utilization of additively manufactured parts is gaining popularity in functional applications. Polymer-based additive manufacturing (AM) parts are utilized in a variety of engineering applications for automotive, aerospace, and energy. AM printed parts are however newer class of materials, and structural performance of these materials is not fully understood completely, and very limited exists currently on precisely performance of Polyjet printed parts and associated digital materials under fatigue loading. This paper investigates the stiffness degradation under tension-tension fatigue loading of digital polypropylene using homogenous 3-Dimensional test coupons formed using PolyJet printing. Homogeneous 3-Dimensional test configuration employed in the present study eliminates the process-induced limitations of traditional ASTM D638 2D fatigue test coupons for AM processed materials. Fatigue data is analyzed to present an empirical model of effective elastic modulus and an analytical model of the accumulated damage state, as defined on the basis of stiffness degradation during cyclic loading. Further, the actual damage accumulation due to cyclic loading with the predicted model is compared. Modeling of the S-N diagram provides a better estimation of fatigue life and fatigue life modeling of AM printed test coupons and is obtained via linear regression analysis of experimental data with high correlation coefficient R2 (0.9971). The analytical model of the accumulated damage state is based on the stiffness degradation and is derived from the regression analysis of experimental data of stiffness degradation at different loading percentages assuming a polynomial of degree 4. Present study provides insight into the fatigue damage state and cyclic performance of digital polypropylene from Polyjet printing.

Modeling tendon tensions for applications of tendon-actuated continuum robots under significant loading is necessary for sizing motors, tendons, and other components to ensure that the robot can safely support its mass during operation. While models exist that express tendon tensions as a function of continuum robot configuration, previously proposed models do not consider the effects of gravity on tendon tensions. In this paper, we discuss the addition of gravity to a static model previously developed for low-mass tendon-actuated continuum robots. Using the Euler-Lagrange methodology, the potential energy due to gravity is incorporated into the formulation of the equations that describe tendon tensions as a function of robot configuration. Preliminary experimental results reveal the potential of this nonzero-gravity tendon-tension model.

This study presents the testing and numerical modeling results of composite sandwich beams. The sandwich beams are constructed from balsa wood in the core and high strength steel wire and E-glass fiber reinforced polymer composite in the facings. The testing of these beams is performed using a monotonic static four-point loading to failure in accordance with ASTM C393-00. Local strain distribution in the mid-span of the beams is obtained using strain gauges. Mid-span deflections of the beams are real-time measured using linear variable displacement transducer (LVDT). From the experimental results, flexural properties of the beams are calculated, including bending stiffness, bending strength, core shear strength, and facing modulus, core modulus, etc. The experimental results have shown that the beams have all failed in the compression zone by local buckling of the top face and shear of the core. The bottom skin does not exhibit any type of premature failure or distress. No bond failure of the composite in the tension zone is observed in any of the tested beams. Finite element modeling of the beam has been conducted using ANSYS. The mechanical properties of the skin and core material used in finite element modeling have been determined by testing of coupons. The predicted results are compared to experimental results, with a reasonable agreement.

When tightening a large number of bolted joints, the calibrated wrench method is used. Since this method is indirect, the axial tension varies greatly in many cases. However, the calibrated wrench method is still widely used because of the simplicity of the tool and easy standardization. When the tightening torque and axial tension are considered to be two independent random variables, the axial tension (stress) is distributed within an elliptical confidence limit. Conventionally, it is thought that the shape of this distribution is a rhombus. Considering the permitted limit for a working load (stress) on a bolted joint, the elliptical variation has a larger margin to the yield point than that of a conventional rhombus. On the basis of this feature, we show in this paper that a higher tightening target torque and a higher axial tension can be set than before. By applying the elliptical confidence limit, one can obtain higher tightening torque and initial axial tension than the conventional values within a smaller range of variations. In this study, in the case of tightening a large number of bolted joints at factories and so forth, tightening reliability is considered as a problem associated with quality or process control and a probabilistic statistical method is investigated. Finally, we carry out analysis to establish the optimum tightening torque for bolted joints.

Axons as microstructural constituent elements of brain white matter are highly oriented in extracellular matrix (ECM) in one direction. Therefore, it is possible to model the human brain white matter as a unidirectional fibrous composite material. A micromechanical finite element model of the brain white matter is developed to indirectly measure the brain white matter constituents’ properties including axon and ECM under tensile loading. Experimental tension test on corona radiata conducted by Budday et al. 2017 [1] is used in this study and one-term Ogden hyperelastic constitutive model is applied to characterize its behavior. By the application of genetic algorithm (GA) as a black box optimization method, the Ogden hyperelastic parameters of axon and ECM minimizing the error between numerical finite element simulation and experimental results are measured. Inverse analysis is conducted on the resultant optimized parameters shows high correlation of coefficient (>99%) between the numerical and experimental data which verifies the accuracy of the optimization procedure. The volume fraction of axons in porcine brain white matter is taken to be 52.7% and the stiffness ratio of axon to ECM is perceived to be 3.0. As these values are not accurately known for human brain white matter, we study the material properties of axon and ECM for different stiffness ratio and axon volume fraction values. The results of this study helps to better understand the micromechanical structure of the brain and micro-level injuries such as diffuse axonal injury.

Damage initiation and propagation material models for carbon fiber composites can be categorized according to the loading applied to constituent components. An example of such categorization is fiber tension, fiber compression, matrix tension, and matrix compression material models. Of these, matrix compression has been by far the least studied based on amount of published literature. Recent work at Oregon State University (OSU) has begun to address this lack of study. OSU researchers have published several papers culminating in the specification of an effective test specimen for isolating matrix compression damage initiation and propagation in carbon fiber laminates. While providing compelling results indicating the effectiveness and usefulness of this test specimen, little or no information has been provided regarding its manufacture, usable notch lengths, and optimum loading rate during testing. Experience at OSU has shown that this information is critical and not trivial to obtain. The purpose of this paper is to provide specific guidelines and “lessons learned” needed for other researchers to efficiently and effectively use this specimen in a comprehensive study. Test specimens are manufactured in the OSU Composites Materials Manufacturing Laboratory using typical commercial pre-peg carbon fiber following the specified layup and curing procedures. Once the material was cured the carbon fiber plate was then water-jet cut into the desired geometry and notch length. Usable notch length and optimum loading rate was determined by testing a series of specimens. All testing was conducted at an OSU lab using a universal testing machine with Digital Image Correlation (DIC) data collected. Specimens were preloaded and matrix compression initiation and propagation data collected until tensile failure occurred on the back edge of the specimen. Testing showed that shorter notch lengths result in inconsistent data and longer in effective initiation but limited propagation due to reduced ligament length. Testing suggested that a speed less than 5 mm/min gave the best results as faster displacement rates caused less crack propagation to occur, while increasing the likelihood of the specimen to fail in tension along its back edge. Through the use of these guidelines, researchers are able to manufacture and use an effective test specimen for the investigation of matrix compression damage initiation and propagation.

The widespread use of sodium aluminosilicate glass in many critical applications due to its hardness, weight, density and optical properties (transparency, dielectric etc.), instead of metals or plastics has become common in recent years. However, glass which is known to be a brittle material has its own vulnerability to fracture. Processes such as heat treatment (heat tempering) or chemical strengthening, through ion-exchange have been deployed to create residual stress profile on the glass, in a bid to improve its strength for applications such as in the automobile windshield design, consumer electronics mobile communication devices e.g. smartphones and tablet etc. However, failure still occurs which is mostly catastrophic and expensive to repair. Therefore, understanding, predicting and eventually improving the resistance to damage or fracture of chemically strengthened glass is significant to designing new glasses that would be tougher, while retaining their transparency. The relationship between the glass residual stress parameters, compressive stress (CS), depth of layer (DOL), center tension (CT) and fracture strength was investigated in this study using a grit particle blast plus ring on ring test method, based on IEC standard for retained biaxial flexural strength measurements. This technique can be used to measure both the surface and edge fracture strength of the glass. Preliminary results showed that for a reasonable level of CS, and CT, high DOL are beneficial to resisting fracture due to severe surface damage, while a high CS and low CT are beneficial to resisting fractures due to shallower flaws. The correlation of critical stress intensity factor versus DOL and CT for various level of CS were also determined and discussed. These results provide a valuable piece of information in the design of a more robust glass in engineering applications.

In the present work, a two-dimensional finite element analysis is carried out to understand the influence of contact geometry and surface treatment on the fretting behavior of a flat with round edge-on-flat plate contact. The fretting pad and plate are modeled using isotropic elastic material properties of Ti-6Al-4V. The mating pair was subjected to a constant normal load followed by a tangential displacement. The effect of contact geometry was studied by independently varying length of the central flat region and radii of corners. Parameters important from the context of fretting viz. contact pressure and normal stress (in tangential direction) were extracted. The effect of surface treatment was studied by modeling two layers of different elastic modulus and yield strength on the mating surfaces. It is found that addition of intermediate layers of lower elastic modulus and yield strength than the parent material leads to a reduction in both contact pressure and peak tensile stress; the influence was more on the peak tensile stress than contact pressure. Further, the addition of softer and less stiff layers on the pad is noted to be less advantageous than adding it on both pad and substrate or substrate only case. The study suggests that contact geometry should be taken into account while carrying out surface modifications of contact pairs.

Synthetic fibers are used for critical performance applications of marine rope and cable industries, apart from military applications. The high strength-to-weight ratio and corrosion resistance offered by polymeric synthetic fibers makes them superior alternatives to steel wire ropes particularly in marine applications. These marine ropes and cables are subjected to a complex history of static and cyclic mechanical loading during service, leading to sudden and unexpected failure. The presence of corrosive sea water medium during service adds to the complexity of the problem. Thus, it is essential to study the tensile and fatigue performance of these synthetic fibers in the presence of sea water. In this study, experiments were conducted to examine the effect of marine environment on Vectran (Liquid Crystal Polymer) yarns. The first set of experiments analyzed the tensile strength degradation of Vectran yarns when exposed to simulated sea water for two months. The experiments were performed on Parallel continuous filament yarns and Twisted continuous filament yarns (0.5 twists per centimetre) and the results compared. In the second set of experiments, Vectran yarns were subjected to load controlled fatigue in dry state and continuously wetted state under tension-tension loading at a nominal frequency of 0.1–0.5 Hz. Stress vs. Number of cycles graphs were plotted to compare the fatigue performance in dry and wet conditions. Fatigue experiments were performed on Parallel and Twisted yarns to study the combined effect of twisting and wetting. Scanning Electron Microscopy was used to observe filament surface and failure mechanism. The results indicate that, twisting the continuous filament yarns improves both tensile strength and fatigue performance. Sea water exposure degrades the tensile strength of Parallel yarns. Twisted yarns show no such degradation. The fatigue performance of Parallel yarns appears to be higher in dry state compared to wet state. The fatigue performance of Twisted yarns seems to increase with wetting. SEM images show that failure of filaments is by severe fibrillation.

This study evaluates the interfacial fracture strength and toughness of an epoxy resin/aluminum alloy based on two different tests of impact loading and quasi-static loading. In the impact test, we carried out the Laser Shock Adhesion Test (LaSAT). This method uses a strong ultrasonic wave induced by pulsed laser irradiation to induce interfacial fracture. Due to the grease layer ablation caused by laser irradiation, strong elastic wave generates and propagates. The tensile stress acts on the Al/epoxy resin interface, inducing an interfacial delamination. This delamination is further extended by an additional laser irradiation in order to evaluate the interfacial fracture toughness. By simulating the experimental delamination progression in FEM (Finite Element Method), we evaluated the dynamic fracture toughness of the Al alloy/epoxy resin interface. On the other hand, a quasi-static test for the toughness evaluation was conducted using a uniaxial tensile test. Before the tensile test, we produced an initial crack (initial delamination) at the interface by using laser ablation. Subsequently, this sample having initial crack is loaded by uniaxial tension. It is found that the interfacial crack progresses, resulting in unstable interfacial fracture. Furthermore, we conducted FEM simulation, in order to estimate the stress distribution near the delamination. By deriving a stress intensity factor from the stress distribution, we evaluated the quasi-static fracture toughness. To compare the dynamic and quasi-static fracture toughness of Al alloy/epoxy resin interface, we clarified the loading rate dependency of interfacial fracture toughness.

Based on the well-known nonlinear hyperelasticity theory and by using the Carrera Unified Formulation (CUF) as well as a total Lagrangian approach, the unified theory of slightly compressible elastomeric structures including geometrical and physical nonlinearities is developed in this work. By exploiting CUF, the principle of virtual work and a finite element approximation, nonlinear governing equations corresponding to the slightly compressible elastomeric structures are straightforwardly formulated in terms of the fundamental nuclei, which are independent of the theory approximation order. Accordingly, the explicit forms of the secant and tangent stiffness matrices of the unified 1D beam and 2D plate elements are derived by using the three-dimensional Cauchy-Green deformation tensor and the nonlinear constitutive equation for slightly incompressible hyperelastic materials. Several numerical assessments are conducted, including uniaxial tension nonlinear response of rectangular elastomeric beams. Our numerical findings demonstrate the capabilities of the CUF model to calculate the large-deformation equilibrium curves as well as the stress distributions with high accuracy.

This paper presents a novel design methodology, which combines topology and shape optimization to define material distribution in the structural design of a truss. Firstly, in order to identify the best layout, the topology optimization process in the design domain is carried out by applying the BESO (Bidirectional Evolutionary Structural Optimization) method. In this approach, the low energy elements are eliminated from an initial mesh, and a new geometry is constructed. This new geometry consists of a set of elements with a higher elastic energy. This results in a new division of material providing different zones, some subjected to higher stress and others containing less elastic energy. Moreover, the elements of the final mesh are re-arranged and modified, considering the distribution of tension. This new arrangement is constructed by aligning and rotating the original mesh elements coherently to the principal directions. In the Shape Optimization stage, the resulting TO (Topology Optimization) geometry is refined. A process of replacing the tabular mesh is performed by rearranging the remaining elements. The vertices of the mesh are set as control polygon vertices and used as reference to define the NURBS (Non-Uniform Rational B-Spline) curves. This provides a parametric representation of the boundaries, outlining the high elastic energy zones. The final stage is the optimization of the continuous and analytically defined NURBS curve outlining the solid material domain. The Shape Optimization is carried out applying a gradient-based optimization method.

Compliant mechanisms are highly preferred in applications demanding motion with high precision. These mechanisms provide friction-less, backlash-free precise motion obtained through deformation of flexible members. The double parallelogram compliant mechanism (DPCM) is one the most important compliant mechanisms to obtain highly precise straight-line motion. DPCM when operated in horizontal plane yield high precision straight-line motion (even with large deformations) useful in several engineering applications. However, constraints such as space, dead loads, etc. may demand DPCMs to be used in the vertical plane. For DPCMs operating in a vertical plane, the axial load due to gravity causes tension and compression in flexible beams which get coupled to bending under large deformations. This ultimately affects the parasitic error of straight-line motion. This paper presents a coupled analysis, along with experimental validation, of DPCM operating in vertical plane considering gravity effects with large deformation.

To support the design of a mechanism with two opposing, underactuated, multi-segmented feet that enables a small UAV to grasp and perch upon a branch or similar structure, a hybrid empirical-computational model has been developed that can be used to predict whether the mechanism can kinematically grasp structures with a range of cross-section shapes and sizes in various orientations and to quantify the forces exerted by the grasp. The model, based on experimentally-determined parameters, relates the curvature of the feet to the displacement and tension of the cable tendon which is related in turn to the weight of the UAV. The working principle of the landing gear follows the anatomy of birds that grasp and perch as tendons in their legs and feet are tensioned. Results demonstrate how the model can be used to simulate and evaluate grasping in order to determine the size and weight of a UAV for landing and perching upon a range of target structures.

This research is intended to create a prototype to generate controllable finger movement of a robotic prosthetic hand using Electromyography (EMG) signals. The instrumentation used in this project includes a Bitalino bio-signal sensor kit, skin electrodes, Arduino Uno microcontroller and a prosthetic hand. The Bitalino’s primary function is to serve as a means to obtain the EMG signal. The Arduino Uno’s function is to implement the control algorithm and actuate the robotic hand to move as intended. Using an EMG signal based counter, the method of control deemed fairly reliable since there was proportional control over the hand but it was based on the duration of the muscle in tension rather than how tense the muscle was. The overall control of the hand was generally responsive to the biological signal.

The effect of the pre-bending operation on ductility can be significant in determining the limit strains of the final product. The strain path experienced in straight tube bulging is significantly different from that of elbow (tube with pre-bending), leading to a reduction in bulge height. The Tube bending introduces strain gradient both along the tube and across the tube. In this work the effect of pre-bending on limit strains during tube bulging process is predicted — and the results are compared to the limit strains of bulged tubes without pre-bending. The Finite Element (FE) model of the bending operation is developed which utilizes an explicit dynamic finite element formulation. The PAMSTAMP 2G code is used to perform the numerical pre-bend (and bulging) simulations. Tension side of bend tube axial strain is found to be positive and hoop strain as negative and vice versa along the compression side. During the bulging, the neck usually develops perpendicular to major strain direction. During bend tube bulge test with fixed expansion and axial feed expansion of bend tubes, in both cases the crack is found to be in the axial direction.

Robotic ultrasonic welding monitored in realtime with a force sensor and infrared thermal imaging camera is developed as an instructional project and laboratory component for undergraduate engineering education. The broad aim is to integrate robotics, plastic component manufacture, and thermal imaging with a unified theme. A secondary aim is to adapt ultrasonic welding for rapid prototyping as a tool for student design projects. Plastic parts for microsystems, such as lab-on-a-chip devices, made by laser cutting or 3d printing, are ultrasonically welded and tensile-stress tested and leaked checked.

Sudden concrete failure is due to inelastic deformations of concrete subjected to tension. However, synthesizing nanomaterials reinforcements has significant impact on cement-based composites failure mechanism. Nanomaterials morphology bridges cement crystals as homogeneous and ductile matrix. In this experiment, cement matrix with water to cement ratio of 0.5 reinforced by 0.2–0.6 wt% of functionalized (COOH group) multi-walled and single-walled carbon nanotubes were used. After sonication of carbon nanotubes in water solution for an hour, the cementitious nanocomposites were casted in cylindrical molds (25 mm diameter and 50 mm height). Failure mechanism of cementitious nanocomposite showed considerable ductility throughout splitting tensile test compared to cement mortar. Additionally, the failure pattern after developing the initial crack provided additional time before ultimate failure occurred in cement-based nanocomposites. The evolution of crack propagation was assessed until ultimate specimen failure during splitting-tensile test on cementitious nanocomposite surface. The deformation of cross section from circle to oval shape augmented tensile strength by 50% in cementitious nanocomposite compared to conventional cement mortar.