Simplified Mechanics Research Articles

Fatigue life prediction of friction-stir-welded aluminum bridge decks – Experimental tests and numerical framework

This paper presents the results of experimental tests and proposes a numerical framework for the prediction of the fatigue behaviour of friction stir welded (FSW) butt-lap joints of aluminium bridge deck extrusions. Fatigue test specimens made of welded real roadway bridge deck extrusions were subjected to cyclic bending under different loading conditions. The fatigue failure modes were thoroughly analyzed. A numerical framework incorporating a 2D finite element (FE) model based on the theory of critical distances (TCD) and a simplified linear elastic fracture mechanics (LEFM) model was developed to predict the fatigue failure initiation location and the fatigue life of the specimens under the test conditions. The location of the failure initiation played a key role in the choice of the numerical approach adopted in the prediction of fatigue life. For fatigue failures initiating from the base metal, the framework utilizes the TCD formulations while for failures in the weld joint, the LEFM model was applied. The proposed numerical framework showed a good agreement with the experimental data.

Research on launching, water exiting, and river crossing of an amphibious vehicle

The main focus of this paper is the amphibious vehicle's water-land trans-media capability, specifically its ability to efficiently carry out transportation tasks in rivers and nearshore areas. This capability relies on three key processes: launching, water exiting, and river crossing. To study these processes, hydrodynamic numerical simulations are conducted. The Reynolds-averaged Navier–Stokes) equation, simplified terra mechanics model, and body force method are adopted to analyze the trans-media and self-propulsion processes. Results indicate that the optimal launching speed is around 15 km/h, with a stable trim and heave, and a launching angle range of 15°–25° for insubmersibility and stability. Furthermore, high-speed water exiting enhances insubmersibility and imposes lower requirements on road adhesion conditions, outperforming low-speed water exiting. In terms of self-propelled river crossing, higher heading angles and faster river currents improve hydrodynamic lift, with the fastest crossing achieved at a 10° heading angle for a current speed of 2.5 m/s and a forward speed of 30 km/h.

Extraction protocol in Class II malocclusion with a straight profile: Description of simplified mechanics in two growing patients

Extraction protocol in Class II malocclusion with a straight profile: Description of simplified mechanics in two growing patients

Nanoparticle Imprint Lithography: From Nanoscale Metrology to Printable Metallic Grids.

Large scale and low-cost nanopatterning of materials is of tremendous interest for optoelectronic devices. Nanoimprint lithography has emerged in recent years as a nanofabrication strategy that is high-throughput and has a resolution comparable to that of electron-beam lithography (EBL). It is enabled by pattern replication of an EBL master into polydimethylsiloxane (PDMS), that is then used to pattern a resist for further processing, or a sol-gel that could be calcinated into a solid material. Although the sol-gel chemistry offers a wide spectrum of material compositions, metals are still difficult to achieve. This gap could be bridged by using colloidal nanoparticles as resist, but deep understanding of the key parameters is still lacking. Here, we use supported metallic nanocubes as a model resist to gain fundamental insights into nanoparticle imprinting. We uncover the major role played by the surfactant layer trapped between nanocubes and substrate, and measure its thickness with subnanometer resolution by using gap plasmon spectroscopy as a metrology platform. This enables us to quantify the van der Waals (VDW) interactions responsible for the friction opposing the nanocube motion, and we find that these are almost in quantitative agreement with the Stokes drag acting on the nanocubes during nanoimprint, that is estimated with a simplified fluid mechanics model. These results reveal that a minimum thickness of surfactant is required, acting as a spacer layer mitigating van der Waals forces between nanocubes and the substrate. In the light of these findings we propose a general method for resist preparation to achieve optimal nanoparticle mobility and show the assembly of printable Ag and Au nanocube grids, that could enable the fabrication of low-cost transparent electrodes of high material quality upon nanocube epitaxy.

Nonlinear Impact Force Reduction of Layered Polymers with the Damage-Trap Interface

In this paper, a damage-trap material interface design of polymeric materials was proposed. Towards that, baseline and layered Polymethyl methacrylate (PMMA) and Polycarbonate specimens were fabricated with a Loctite 5083 adhesive layer between the interfaces. Out-of-plane impact experiments were conducted and found that the maximum impact force was reduced in layered polymers with so-called “damage-trap material interfaces”. At the impact energy of 20 J, the maximum impact force of the layered PMMA specimens with the 5083 adhesive was reduced by 60% compared to the identical specimens without any adhesive bonding. For the layered Polycarbonate specimens with the 5083 adhesive bonding, the maximum impact force was reduced by 20% and energy absorption was increased by 130%. Simplified contact mechanics analysis showed that the low Young’s modulus of the 5083 adhesive layers was a key parameter in reducing impact force and damage. Therefore, a simple and effective way to design layered materials with improved impact resistance was proposed.

Estimation of ice loads on offshore structures using simulations of level ice-structure collisions with an influence coefficient method

During their service, ice-class vessels are exposed to ice collision loads. Precise estimation of such ice loads is essential to the design of ice-class vessels. Due to difficulties encountered in real measurement of ice loads, a simple yet logically straightforward method, namely the influence coefficient method (ICM), has been widely employed to convert strain data to ice pressures. However, the method's accuracy and reliability depend on how strain gauges are arranged and load cells are divided accordingly. This study investigated those aspects in order to enhance the accuracy and reliability of the ICM in estimating ice loads incurred in collisions of ice with offshore structures. It began by developing a numerical model of level ice-structure interactions. For this, an ice-material model applying the Drucker-Prager (DP) plasticity model combined with an element erosion technique's simplified damage mechanics was developed. The developed numerical model was verified by comparing its results with the relevant model test data and corresponding numerical results available in the open literature, and good agreement was achieved. Next, the present study performed, based on the numerical simulation strategy thus validated, simulations of level ice collisions with actual offshore structures showing inflexible and deformable behaviors. Subsequently, a finite element (FE) model of the structures’ instrumented area, which is the same as the scantling used in the ice collision simulation, was developed to construct an influence coefficient matrix for the ICM. The structural responses, including stress/strain data and ice-contact pressure observed numerically, were then applied to an investigation of the ICM capability. The effects of the strain gage arrangements and corresponding load cells on the estimation of the local ice pressures acting on actual offshore structures were demonstrated. The proposed procedure is expected to provide insights into how the ICM can be improved in reliably and cost-effectively estimating ice loads on ice-class offshore structures.

Simplified indentation mechanics to connect nanoindentation and low-energy impact of structural composites and polymers

Nanoindentation (nanometer scale, extremely small) and impact (microsecond scale, extremely fast) experiments are two important techniques for characterizing modern material systems. However, these two experiments were often studied individually. In this pilot study, a multiscale indentation mechanics approach is proposed to correlate these two very different mechanics events acting on the same target materials using a spherical indenter and a projectile. The contact stiffness of nanoindentation of a target material is fitted using Hertz’s contact law, and then the contact stiffness of impact is obtained using a simplified multiscale relation. Therefore, the maximum impact force of a projectile impact can be predicted by inputting the impact energy and the contact stiffness of impact. The above new approach was validated by drop-weight impact experiments of polymers and structural composite materials subjected to low-energy impact. Results show that only a few minutes are needed to predict the maximum impact force.

Assessing effects of exoskeleton misalignment on knee joint load during swing using an instrumented leg simulator

BackgroundExoskeletons are working in parallel to the human body and can support human movement by exerting forces through cuffs or straps. They are prone to misalignments caused by simplified joint mechanics and incorrect fit or positioning. Those misalignments are a common safety concern as they can cause undesired interaction forces. However, the exact mechanisms and effects of misalignments on the joint load are not yet known. The aim of this study was therefore to investigate the influence of different directions and magnitudes of exoskeleton misalignment on the internal knee joint forces and torques of an artificial leg.MethodsAn instrumented leg simulator was used to quantify the changes in knee joint load during the swing phase caused by misalignments of a passive knee brace being manually flexed. This was achieved by an experimenter pulling on a rope attached to the distal end of the knee brace to create a flexion torque. The extension was not actuated but achieved through the weight of the instrumented leg simulator. The investigated types of misalignments are a rotation of the brace around the vertical axis and a translation in anteroposterior as well as proximal/distal direction.ResultsThe amount of misalignment had a significant effect on several directions of knee joint load in the instrumented leg simulator. In general, load on the knee joint increased with increasing misalignment. Specifically, stronger rotational misalignment led to higher forces in mediolateral direction in the knee joint as well as higher ab-/adduction, flexion and internal/external rotation torques. Stronger anteroposterior translational misalignment led to higher mediolateral knee forces as well as higher abduction and flexion/extension torques. Stronger proximal/distal translational misalignment led to higher posterior and tension/compression forces.ConclusionsMisalignments of a lower leg exoskeleton can increase internal knee forces and torques during swing to a multiple of those experienced in a well-aligned situation. Despite only taking swing into account, this is supporting the need for carefully considering hazards associated with not only translational but also rotational misalignments during wearable robot development and use. Also, this warrants investigation of misalignment effects in stance, as a target of many exoskeleton applications.

A Mechanistic Lumped Parameter Model of the Berlin Heart EXCOR to Analyze Device Performance and Physiologic Interactions.

The Berlin Heart EXCOR (BH) is the only FDA-approved, extracorporeal pulsatile ventricular assist device (VAD) for infants and children with heart failure. Clinicians control four settings on the device-systolic and diastolic drive pressures, device pump rate, and systolic time as a percentage of the pump cycle. However, interactions between BH pneumatics and the native circulation remain poorly understood. Thus, establishing appropriate device size and settings can be challenging on a patient-to-patient basis. In this study we develop a novel lumped parameter network based on simplified device mechanics. We perform parametric studies to characterize device behavior, study interactions between the left ventricle (LV) and BH across different device settings, and develop patient-specific simulations. We then simulate the impact of changing device parameters for each of three patients. Increasing systolic pressure and systolic time increased device output. We identified previously unobserved cycle-to-cycle variations in LV-BH interactions that may impact patient health. Patient-specific simulations demonstrated the model's ability to replicate BH performance, captured trends in LV behavior after device implantation, and emphasized the importance of device rate and volume in optimizing BH efficiency. We present a novel, mechanistic model that can be readily adjusted to study a wide range of device settings and clinical scenarios. Physiologic interactions between the BH and the native LV produced large variability in cardiac loading. Our findings showed that operating the BH at a device rate greater than the patient's native heart decreases variability in physiological interactions between the BH and LV, increasing cardiac offloading while maintaining cardiac output. Device rates that are close to the resting heart rate may result in unfavorable cardiac loading conditions. Our work demonstrates the utility of our model to investigate BH performance for patient-specific physiologies.

Droplet size distributions in cryogenic flash atomization

Flash atomization can occur in rocket thrusters operated at near vaccum conditions just prior to ignition when cryogenic propellants are injected into the reaction chamber. Although the morphology of flashing sprays has been widely studied, empirical observations of the primary breakup mechanisms at the micro-scale are currently not feasible. Still, further analysis of the micro-scales is necessary for development of closure models in both empirical and numerical models. In this work direct numerical simulations are performed to provide insight into the microscopic processes that dominate the dynamics of the primary atomization. We use a multiphase solver (FS3D) based on the volume of fluid method and PLIC reconstruction to track the liquid-vapour interface with high fidelity, fully resolving viscous and capillary effects. Thermodynamic effects are introduced by calibration of the fluid properties and evaporation rate to a target range of thermodynamic conditions and nuclei number density. The use of an affordable incompressible scheme allows for highly resolved simulations comprising up to thousand bubbles across a range of Weber and Ohnesorge numbers. Using a randomized bubble arrangement it is generally observed that the qualitative characterization of the breakup dynamics corroborates previous results using regular bubble arrays. However, these differ substantially from the simplified mechanics hypothesised in the literature. A temporal analysis of the breakup provides estimates for the rate of change in the spray surface area and volume fraction. The droplet size distributions are compared in terms of droplet number, surface area and volume fraction. A model based on the thermodynamic conditions is proposed to estimate the SMD for different Weber numbers.

Study on mechanical behaviors of loose mortise-tenon joint with neighbouring gap

The neighbouring gaps at the mortise-tenon joint in traditional timber structure, which leads to the complexity of the joint, are considered to impair the mechanical performance of the joint. In this paper, numerical simulation of loose joint was conducted to examine the deformation states, stress distributions, and bearing capacities, which was verified by full-scale test. On the basis of the experimental and numerical results, a simplified mechanics model with gaps has been proposed to present the bending capacity of the loose joint. Besides, the gap effects and parameter studies on the influences of tenon height, friction coefficient, elastic modulus and axial load were also investigated. As a result, the estimated relationship between moment and rotation angle of loose joint showed the agreement with the numerical results, demonstrating validity of the proposed model; The bending bearing capacity and rotational stiffness of loose joint had a certain drop with the increasing of gaps; and the tenon height may be the most important factor affecting the mechanical behaviors of the joint when it is subjected to repeated load; Research results can provide important references on the condition assessments of the existing mortise-tenon joint.

Biomechanical evaluation of total ankle arthroplasty. Part I: Joint loads during simulated level walking.

In total ankle arthroplasty, the interaction at the joint between implant and bone is driven by a complex loading environment. Unfortunately, little is known about the loads at the ankle during daily activities since earlier attempts use two- or three-dimensional models to explore simplified joint mechanics. Our goal was to develop a framework to calculate multi-axial loads at the joint during simulated level walking following total ankle arthroplasty. To accomplish this, we combined robotic simulations of level walking at one-quarter bodyweight in three cadaveric foot and ankle specimens with musculoskeletal modeling to calculate the multi-axial forces and moments at the ankle during the stance phase. The peak compressive forces calculated were between 720 and 873 N occurring around 77%-80% of stance. The peak moment, which was the internal moment for all specimens, was between 6.1 and 11.6 N m and occurred between 72% and 88% of the stance phase. The peak moment did not necessarily occur with the peak force. The ankle joint loads calculated in this study correspond well to previous attempts in the literature; however, our robotic simulator and framework provide an opportunity to resolve the resultant three-dimensional forces and moments as others have not in previous studies. The framework may be useful to calculate ankle joint loads in cadaveric specimens as the first step in evaluating bone-implant interactions in total ankle replacement using specimen specific inputs. This approach also provides a unique opportunity to evaluate changes in joint loads and kinematics following surgical interventions of the foot and ankle.

Dynamic Model Testing of Low-Gravity-Center Cable-Stayed Bridges with Different Girder-to-Tower Connections

Abstract To investigate the seismic response characteristics of low-gravity-center cable-stayed bridges with twin towers, a simplified mechanics model was established based on the transmission path...

Time to cracking in concrete cover length due to reinforcement corrosion via a simplified fracture mechanics approach

Determination of the concrete cover cracking time is one of the main parameters in estimating service life of concrete structures exposed to chloride-laden and other aggressive environmental conditions. The corrosion of reinforcing steel embedded in concrete occurs as a result of chloride attack or carbonation, and thus the increase in the volume of corroded reinforcing steel cracks the cover. In this paper, a theoretical model is proposed to calculate the cracking initiation and propagation time within the concrete cover. Increase in the volume of corroded reinforcing bar induces an internal pressure on the concrete around the bar, which brings on the cracking initiation and propagation through the cover. The internal pressure caused by corrosion is expressed in terms of steel mass loss, and then Faraday's law is implemented to account for the time in relationships. The time corresponding to creation of the first crack is obtained based on crack opening definition, and the crack propagation is approximated on the basis of fracture mechanics relationships. The merit of using fracture mechanics is that the crack propagation time can be calculated thru any desired length of the cover. In this model, the time required for corrosion products to fill the porous zone between concrete and steel bar, as well as the time needed to fill the void within the crack are also accounted for. Validity of the proposed model is investigated via comparing the results achieved in this model with those of experiments published in the literature.

New Methodology for Aircraft Performance Model Identification for Flight Management System Applications

This paper presents the validation results of a study conducted at the Laboratory of Applied Research in Actives Controls, Avionics, and Aeroservoelasticity to develop a modeling technique for determining a performance model of a particular aircraft using a limited amount of data. This technique was applied to the well-known business jet aircraft, Cessna Citation X. All the reference data used to design the model were generated using an in-house in-flight performance program. These data were subsequently combined with simplified flight mechanics equations in order to estimate various performance and aero-propulsive characteristics of the aircraft. An original identification algorithm was next developed in order to determine a mathematical model describing the fuel flow, as well as the aircraft thrust and drag aerodynamic coefficients. Validation of the study was accomplished by comparing trajectory data predicted by the model with trajectory data measured with a research aircraft flight simulator (RAFS) of the Cessna Citation X. The results show a very good agreement for the flight time, the ground distance traveled, and fuel consumption.

Fracture studies on carbon steel straight pipes having off-centred circumferential through-wall crack under finite compliance

The level-3 Leak-Before-Break (LBB) assessment of nuclear piping system is based on simplified fracture mechanics calculations. A postulated through-wall circumferential crack is shown to be stable even under extreme loading conditions. An important aspect of this fracture assessment, which is generally not considered, is the reduction in load at the cracked section that occurs due to the local and global compliance of the piping system. In this study, the effect of piping system compliance, crack size and crack location on the load carrying capacity of SA-333 Gr. 6 carbon steel pipes was analysed. Experimental investigations were performed on 8״ NB Schedule 100 pipes (Outer diameter 219 mm and wall thickness of approx. 12.6 mm) under quasi-static four point bending loads. Out of the six fracture tests, one specimen had a through-wall circumferential crack at the centre (along the length of pipe) and the remaining five tests had off-centred cracks. While the initial crack size showed a weak influence on the load carrying capacity of a stiff piping system, the effect of crack location was, however, quite significant. Calculations based on twice-elastic slope (TES) method and limit analysis of uncracked piping system (in some cases) for evaluation of load carrying capacity of pipes under finite compliance may lead to large safety margins.

Foreign Object Impact Damage in Ceramic Matrix Composites: Experiments and Computational Predictions

Abstract Foreign object impact of ceramic matrix composite (CMC) materials and components in a gas turbine engine environment could be detrimental to engine performance and hence must be accounted for in the design of such components. This paper is concerned with experiments and computational modeling of foreign object impact phenomenon in silicon carbide (SiC)-based CMC. Controlled impact experiments were conducted on the CMC material using a gas-gun apparatus with spherical hardened steel projectile. The internal damage state within the CMC specimens was assessed using X-ray computed tomography scan technique. The computational modeling involved explicit dynamic finite element (FE) simulation of the impact process wherein either delamination mechanism is modeled or both ply damage and delamination mechanisms are modeled in a coupled manner. The delamination mechanism is modeled explicitly using cohesive-zone (CZ) fracture mechanics approach, whereas, the ply-damage mechanisms are modeled implicitly using simplified continuum damage mechanics approach. The simulation results were found to be in reasonable qualitative and quantitative agreement with the experimental results. Furthermore, it is shown that modeling both the ply damage and delamination mechanisms are essential to predict the correct delamination pattern even for intermediate velocity impacts that leads to predominantly delamination damage. The predictive nature of the modeling approach is demonstrated and approaches to enhance the models are also discussed.

Hole enlarging anchorage technology of surrounding rock of roadway in weakly consolidated formation

Aiming at the problem of surrounding rock deformation control of the roadway in weakly consolidated formation and combined with the calculation method of anchoring force of anchor bolt, this paper analyzes the feasibility of hole enlarging and anchoring and proposes a calculation method for the anchoring force of hole enlarging and anchoring; this paper establishes a simplified mechanics model of cylinder table enlarging, standing circular table enlarging and circular table enlarging with the method of theoretical mechanics and material mechanics and finds after analysis that the anchoring effect is best when the angle between the busbar and the axis of the circular table is θ < 45°. It draws a two-variable function relation curve for the surrounding rock stress σb and the hole enlarging parameters at the supporting region of the enlarged part of the standing circular table and obtains the optimal anchor length L is 130 mm, the optimal hole enlarging angle θ is 15°, and the optimal hole diameter D is 100 mm. It verifies the theoretical analysis results with numerical calculation, and the results show that the theoretical derivation is consistent with the simulation results. The research results make up the inadequacies of the accurate calculation method for the optimal hole enlarging parameters of the surrounding rock hole of roadway in the weakly consolidated formation, and provide a theoretical basis for the popularization and application of the anchoring technology in the roadway support of weakly consolidated formation.

A simplified continuum damage mechanics based modeling strategy for cumulative fatigue damage assessment of metallic bolted joints

Fatigue damage and consequent failure account for the majority of the failure in metallic bolted joints of aerospace and mechanical engineering applications. In this paper, we present a cumulative fatigue damage evaluation approach for metallic bolted joints based on a combined continuum damage mechanics (CDM) theory and the critical plane approach. The analysis approach is applicable to both proportional and non-proportional multiaxial loading conditions. A cycle jumping based algorithm with a fixed damage evolution rate is adopted for efficient numerical approximation. To further increase the computational efficiency, a simplified modeling strategy is developed to avoid the expensive calculation of the full loading-unloading cyclic stress history. The proposed approach is established based on the assumption that a linear relation can be established between the minimum and maximum stress tensors in high-cycle fatigue analysis and is thus only dependent on the maximum stress states. A set of examples including a notched specimen and a metallic bolted joint with pre-tension are simulated to demonstrate the simplified approach.

Enhancing the pressure limitation in large-volume Bridgman-anvil cell used for in situ neutron diffraction

ABSTRACTWe analyzed the compression process of the centrally caved Bridgman-anvil cell by a simplified mechanics model and proposed that the pressure limitation for a given configuration is governed by the compressibility and compression ratios (V0/V) of the gasket and sample. Then we replaced the middle part of the sample that is prone to undergo plastic deformation with a diamond plate, which will obviously change the stress state of the sample and gasket. The results of ex situ and in situ pressure testing experiments show that this way doubles the efficiency of the pressure generation and largely expands the pressure limitation. The pressure of sample Ni can reach 29.4 GPa in the double toroidal sintered-diamond anvil cell with a sample volume of 17 mm3 even at the loading force of 1500 kN.