Tip Loading Research Articles

Nonlinear deformation of a tapered elastica cantilever due to a tip load

The nonlinear large deformation of a tip loaded tapered cantilever is studied. Different cross sectional shapes, deformation directions, tapers, base inclinations and end loads are considered. Explicit stability characteristic equations are given. Asymptotic analysis gives good approximations for large tip loads ( p > 20). Deformation properties of the tapered cantilever are discussed.

Actuation characterization of the lightweight unimorph piezo-composite actuator for different loading cases

In this study, we focused on the actuation behavior of the lightweight piezo-composite unimorph actuator (LIPCA). Common loading configurations found in real applications of LIPCA have been studied including the center loading and tip loading cases. The domain switching effect that has strong influence on the performance of LIPCA was recorded. While a longitudinally compressed stress state within the PZT (lead zirconate titanate, Pb[Zr(x)Ti(1–x)]O3) layer in LIPCA is preferable to avoid failure during operation, experimental results showed that, for the transverse loading cases, the actuator should be arranged in a manner such that the stress state within the PZT is in as much tension as possible to encourage the non-180° domain switching. With investigated loading cases in this paper, suitable arrangements could help improve the actuation performance. The improvement can be up to almost 33% in simply-supported with center loading and 140% in clamped tip loading case.

Dynamic Fracture of a Zr-based Bulk Metallic Glass

In the present study, dynamic fracture experiments are performed on fully amorphous Liquidmetal-1 (LM-1), a Zr-based BMG, to better understand fracture initiation and propagation in notched specimens. Experiments are conducted on notched (110 μm notch radius) four-point bend specimens using an instrumented modified split-Hopkinson pressure bar apparatus. The results of these experiments suggest that the critical dynamic stress intensity factor achieved by the notched LM-1 specimens is ~110 MPa m1/2, which is similar to the fracture toughness determined from previous quasi-static fracture experiments. This insensitivity of the fracture toughness to crack tip loading rate suggests negligible loading-rate sensitivity on the dynamic fracture initiation toughness in LM-1. In situ high-speed camera images of the notched sample during the dynamic loading process show multiple fracture initiation attempts and subsequent arrests prior to catastrophic fracture initiation. Controlled stress wave loading experiments designed to induce sub-critical levels of damage in the notched specimens show extensive deformation banding extending 150 to 200 μm outward from the notch. The deformation bands, nominally perpendicular to each other, run along the direction of the notch and perpendicular to it. They are consistent with slip-line fields in notched samples of elastic perfectly plastic materials. Subsequent loading of the damaged specimen again shows several attempts at crack initiation followed by blunting; the initial sub-critical damage in the region around the notch is understood to increase the energy required for catastrophic specimen failure and is consistent with an increase in the effective notch radius due to preexisting damage.

Continuous stiffness mode nanoindentation response of poly(methyl methacrylate) surfaces

Indentation is a comparatively simple and virtually nondestructive method of determining mechanical properties of material surfaces by means of an indenter inducing a localized deformation. The paper present experimental results of the load-displacement curves, the hardness and the elastic modulus data, and associated analysis for poly(methyl methacrylate) (PMMA) surfaces as a function of contact displacement. The experimental results include continuous stiffness indentations performed using constant loading rate and constant displacement rate experiments. The continuous stiffness indentation involves continuous calculation of a material stiffness, and hence hardness and elastic modulus of surfaces, during discrete loading-unloading cycles, as in a conventional indentation routine, and in a comparatively smaller time constant. The dependence of the compliance curves, the hardness, the elastic modulus and the plasticity index upon the imposed penetration depth, the applied normal load and the deformation rate are described. Tip area and load frame calibrations for the continuous stiffness indentation are also reported. The paper includes practical considerations encountered during indentation of polymers specifically at low penetration depths. The experimental results show a peculiarly harder response of PMMA surfaces at the submicron (near to surface) layers.

Optimization of capillary-channeled polymer (C-CP) fiber stationary phase extractions of proteins from MALDI-MS suppressing media

Solid phase extraction (SPE) is an essential component in many proteomic analyses involving matrix-assisted laser desorption/ionization mass spectrometry detection (MALDI-MS). Parametric variables affecting the efficiency of protein extractions from buffer (Tris–HCl) and urine media using a capillary-channeled polymer (C-CP) fiber stationary phase in a tip-based format are evaluated. Proteins are immobilized on to polypropylene (PP) C-CP fibers and eluted in organic solvents conducive to high analytical performance using a simple benchtop centrifuge to drive the fluids. Experimental variables, including elution solvent strength, aqueous wash volume, the interactive aspects of fiber tip length (2.5–10 mm), and loading and elution volumes, were evaluated to gain insights into fundamental processes and to optimize protein recoveries/analytical signals. Based on the MALDI-MS responses of the test proteins cytochrome c, lysozyme, and myoglobin, the optimized elution solvent was determined to be a 60 : 40 ACN–H2O mixture with 0.07% trifluoroacetic acid (TFA). It was determined that different aqueous wash volumes were required to flush test solution remnants, depending on the buffer concentration, concluding that a 100 μL wash was effective at concentrations of up to 1 M. Fiber tip length was explored to determine the limits of loading volume/protein mass for each bed size while efficiently extracting from the buffer environment. Interestingly, the shortest C-CP fiber tips provided the best efficiency, isolating nanogram levels of protein from 1 μL aliquots of sample. After optimization, a proof of practice experiment was performed to extract the three-protein suite (<5 μM of each) from a synthetic urine matrix. The previously undetected proteins could be readily distinguished with high spectral clarity due to the SPE procedure utilizing C-CP fiber packed micropipette tips.

Crack growth in elastic materials with internal boundaries and interfaces

AbstractThis work presents numerical methods used for predicting crack paths in technical structures based on the theory of linear elastic fracture mechanics. The FE‐method is used in combination with an efficient remeshing algorithm and a post processor to calculate crack tip loading. The interaction of cracks and internal boundaries and interfaces is investigated. (© 2012 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Dynamic modeling and robust control of an L-shaped microrobot based on fast trilayer polypyrrole-bending actuators

In this article, analytical dynamic model derivation and robust position control of a microrobot based on fast trilayer polypyrrole-bending actuators employing quantitative feedback theory is presented. The conjugated polymer actuators based on polypyrrole can be employed to achieve microscale precision positioning, having a wide range of application including biomimetic robots and biomedical devices. There has been extensive research on modeling the electrochemical dynamics of polypyrrole bending actuators. However, the mechanical dynamics modeling of actuator remains to be unexplored. In this article, we first develop a proper mechanical dynamics model for the fast-conducting polymer actuators that matches well with the existing experimental results and then extend it to a microrobot. In the controlling part, the robust control quantitative feedback theory will be used to control the microrobot with variable tip loading. The numerical simulation results show that the quantitative feedback theory controller has consistent and robust-tracking performance.

Real-space mapping of nanoplasmonic hotspots via optical antenna-gap loading

Plasmonic hotspots located in the nanogaps of infrared optical antennas are mapped in the near-field. The enhanced evanescent field resonance is shown to depend strongly on excitation wavelength, the excitation and detection laser polarization, and gap size. In addition, we demonstrate that in nanogap hotspot imaging using scattering probes, the probe tip can be considered as a load in the gap of the antenna, and the impedance of the load can then be tuned from inductive to capacitive or vice versa by changing the dielectric value of the tip load. Experimental results are in agreement with finite-difference time-domain simulations.

Indentation response of a NiTi shape memory alloy: modeling and experiments

The indentation response of a pseudoelastic nickel-titanium based shape memory alloy (SMA) has been analyzed. Indentation tests have been carried out at room temperature using a spherical diamond tip and indentation loads in the range 50-500 mN in order to promote a large stress-induced transformation zone in the indentation region and, consequently, to avoid local effects due to microstructural variations. The measured load-displacement data have been analyzed to obtain information on the pseudoelastic response of the alloy. To aid this analysis numerical simulations were performed, by using a commercial finite element (FE) softwarecode and a special constitutive model for SMAs, so as to understand better the microstructural evolution occurring during the indentation process. Finally, the FE model has been used to analyze the effects of temperature on the indentation response of the alloy. This analysis revealed a marked variation of both the maximum and residual penetration depths with increasing test temperature.

The Fined COD Transform Formula for CT Specimens to Investigate Material Fracture Toughness

For front-force compact tension specimen (FFCT), based on the refined results of the relationship between crack tip opening displacement and load line crack opening displacement from Finite Element Analysis (FEA), the influences of material properties and plastic deformation near the crack tip have been analyzed. A simplified and accurate transform formula for FFCT specimens is presented in this paper, and the error analysis is conducted.

Modeling, Force Sensing, and Control of Flexible Cannulas for Microstent Delivery

This paper presents a kinetostatic modeling framework for flexible cannulas (concentric tubing robots) subject to tip loads. Unlike existing methods that allow fast computation of the beam tip position, this modeling framework provides fast computation of both the tip position and the entire shape of the deflected robot. A method for online force sensing based on inverse kinetostatic solution is also proposed and assistive telemanipulation control methods for microstent delivery are presented. The modeling framework uses polynomial approximation and linear interpolation based on elliptic integral solutions to the deflection of lightweight beams. To date, there are no systems capable of stent delivery in retinal vasculature. The modeling and control frameworks of this paper are validated experimentally on pilot studies for microstent delivery. We believe that the methods presented in this paper open the way for robot-assisted retinal microvascular stenting that may potentially revolutionize the treatment of blinding retinal vasculature diseases.

Robust control of conjugated polymer actuators considering the spatio-temporal dynamics

Conjugated polymers are emerging materials which have a wide range of applications including biomimetic robots and biomedical devices. These novel materials attract considerable attention owing to their interesting sensing and actuating behavior. However, complicated electro-chemo-mechanical dynamics is a drawback for their application in functional devices. Although extensive research has been conducted on static modeling of conductive polymers, the dynamic modeling is not well understood. In this paper we focus on dynamic modeling and robust control of a fast trilayer polypyrrole actuator as a distributed parameter system. The Euler–Bernoulli beam model with inclusion of Kelvin–Voigt damping and tip loading is utilized for vibration analysis of the smart actuator. Considering the spatio-temporal dynamics of the actuator, a one-point feedback controller based on quantitative feedback theory is designed that can suppress arbitrary external disturbances and consistently track desired inputs.

쏘일네일링과 앵커가 결합된 하이브리드 공법의 하중전이 메커니즘

본 논문에서는 쏘일네일링 공법, 앵커 공법, 또한 이 두공법의 장점을 결합한 하이브리드 공법의 하중전이 메커니즘에 대한 연구를 수행하였다. 하이브리드 공법은 철근과 PC강연선을 보강재로 하며, 프리스트레스를 가함으로써 능동적인 주면마찰력 양상을 유도하고자 한다. 본 논문에서는 주면마찰력 산정 이론과 하중전이 이론을 바탕으로 각 공법의 하중전이 메커니즘을 파악하였다. 또한 현장인발시험을 실시하여 측정된 계측결과의 분석 및 하중전이 이론과의 비교 검토를 통해서 하이브리드 공법의 하중전이 메커니즘을 규명하였다. 쏘일네일링 공법은 인발거동 시 전면에서부터 선단으로 인장력이 전이되는 양상을 보이는 반면, 앵커 공법은 내하체가 위치한 선단에서 프리스트레스를 가함에 따라 선단에서 전면으로 압축력이 전이된다. 이 두 공법을 결합시킨 하이브리드 공법은 프리스트레스를 가할 때에는 앵커공법과 같이 선단에서 압축력이 발생하지만 동시인발 시에는 선단에서는 압축력이 전면에서는 인장력이 발생하게 된다. 따라서 하이브리드 공법의 하중전이 메커니즘은 쏘일네일링의 거동과 앵커의 거동이 모두 나타난다. The load transfer mechanism of hybrid model of soil-nailing and compression anchor is studied in this paper. The hybrid model is composed of an anchor bar (installed at the tip) with two PC strands and a steel bar. It can make active behavior of skin friction by applying the pre-stress. In this paper, the load transfer mechanisms of soil-nailings, compression anchors, and hybrid models, respectively, are obtained from skin friction theory and load transfer theory. Field pullout tests are performed to identify the load transfer mechanism and experimental results are compared with analytical solution. In case of soil-nailings, the tension load is transferred from face to tip, however, in case of compression anchors, the compression load is transferred from tip to face. The experimental behavior of the hybrid model is similar to that of compression anchor when only pre-stress is applied. If the pullout test is performed by simultaneously pulling out the anchor and the nail, the compression load is dominant at the tip and tension load is dominant at the face. The load transfer mechanism of the hybrid model shows the combined behavior of soil-nailings with compression anchors.

Hybridization in nanostructured DNA monolayers probed by AFM: theory versus experiment

Nanografted monolayers (NAMs) of DNA show novel physico-chemical properties that make them ideally suited for advanced biosensing applications. In comparison with alternative solid-phase techniques for diagnostic DNA detection, NAMs have the advantage of combining a small size with a high homogeneity of the DNA surface coverage. These two properties favour the extreme miniaturization and ultrasensitivity in high-throughput biosensing devices. The systematic use of NAMs for quantitative DNA (and protein) detection has so far suffered from the lack of a control on key fabrication parameters, such as the ss- or ds-DNA surface coverage. Here we report on a combined experimental-computational study that allows us to estimate the surface density of the grafted DNA by analyzing the sample mechanical response, that is the DNA patch height vs. applied tip load curves. It is shown that the same analysis scheme can be used to detect the occurrence of hybridization with complementary strands in solution and estimate its efficiency. Thanks to these quantitative relationships it is possible to use a single AFM-based setup to: (i) fabricate a DNA NAM, (ii) control the DNA surface coverage, and (iii) characterize its level of hybridization helping the design of NAMs with pre-determined fabrication parameters.

Fracture Mechanical Analysis of Cracks in Ice Shelves using the Finite Element Method and Configurational Forces

AbstractIce shelves are important elements of the climate system and sensitive to climate changes. The disintegration of large Antarctic ice shelves is the focus of this fracture mechanical analysis. Ice is a complex material which, depending on the context, can be seen as a viscous fluid or as an elastic solid. A fracture event usually occurs on a rather short time scale, thus the elastic response is important and linear elastic fracture mechanics can be used. The investigation of the stress intensity factor as a measure of crack tip loading is based on a 2‐dimensional analysis of a single crack with a mode‐I type load and additional body loads. This investigation is performed using configurational forces. Depth dependent density and temperature profiles are considered. The relevant parameters are obtained by literature, remote sensing data analysis and modeling of the ice dynamics. The criticality of wet surface cracks is investigated. (© 2011 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Effect of axially non-uniform rotor tip clearance on aerodynamic performance of an unshrouded axial turbine

Tip leakage flow is responsible for a significant amount of aerodynamic losses in a turbine stage, and the mixing of leakage flow with the rotor passage secondary flow causes losses and reduces turbine stage efficiency. This article presents a numerical investigation of the effect of axially non-uniform tip clearance on the aerodynamic performance of an unshrouded axial turbine at design and off-design conditions, in an attempt to seek an optimal tip clearance chordwise distribution to control the interaction between tip leakage and passage secondary flows, and then reduce the total losses in turbines. Different types of axially uniform and non-uniform rotor tip clearances were used in this investigation, which include uniform, expanding, shrinking, and back- and front-step tip clearances. Numerical results show that, for the axially uniform tip clearances, the interaction mechanism between tip leakage and passage secondary flows is different in the relatively small and large tip clearance heights. With the tip clearance height gradually increasing, tip passage vortex at the passage exit is first enhanced, and then it becomes weak. Axially non-uniform tip clearance changes the chordwise distribution of over tip leakage mass flow. The front-step and shrink tip clearances are able to efficiently utilize the interaction of tip leakage flow with passage secondary flows, and then reduce the total losses in turbines. For different axially non-uniform tip clearances, loading distribution near the tip has a great impact on the tip leakage flow. It will result in great losses when the maximum tip clearance and loading positions are in the same position along the blade.

Nano-structure fabrication of GaAs using AFM tip-induced local oxidation method: different doping types and plane orientations

In this study, we have fabricated nano-scaled oxide structures on GaAs substrates that are doped in different conductivity types of p- and n-types and plane orientations of GaAs(100) and GaAs(711), respectively, using an atomic force microscopy (AFM) tip-induced local oxidation method. The AFM-induced GaAs oxide patterns were obtained by varying applied bias from approximately 5 V to approximately 15 V and the tip loading forces from 60 to 180 nN. During the local oxidation, the humidity and the tip scan speed are fixed to approximately 45% and approximately 6.3 μm/s, respectively. The local oxidation rate is further improved in p-type GaAs compared to n-type GaAs substrates whereas the rate is enhanced in GaAs(100) compared to and GaAs(711), respectively, under the identical conditions. In addition, the oxide formation mechanisms in different doping types and plane orientations were investigated and compared with two-dimensional simulation results.

A finite deformation nonlinear thermo-elastic model that mimics plasticity during monotonic loading

We present and study a nonlinear thermo-elastic constitutive model that under monotonic loading closely reproduces the response seen in plasticity, showing the initial stiff elastic response, kneeing as if yielding, and then showing response resembling post-yield hardening. The proposed large deformation thermo-elastic response model is constructed based on four physically identifiable mechanical parameters, that are closely related to the parameters used to construct plasticity models, thermal expansion parameters and two thermodynamic parameters. The four mechanical parameters are the initial elastic shear and bulk moduli, the yield point in shear, the hardening slope in shear. The thermodynamic parameters are the heat capacity at a reference temperature and its rate of change with changes of temperature. The model can be considered an alternate to deformation plasticity models currently used and, as such, can be used as a lightweight substitute for plasticity modeling in certain analysis. Since the proposed model is thermodynamically based, not only thermal effects are integrated into the model, but also the stress is calculated in terms of the applied deformation, allowing the model to be integrated with other models when conducting numerical analysis. We study the response of the proposed model under simple shear, uniaxial extension, confined compression, partially-confined compression, and biaxial extension. We incorporate the elastic model into ABAQUS using its UMAT subroutine for solid elements and using UHYPER for shell elements. We compare the large deformation response from the proposed elastic model with J2-plasticity, and with plasticity and deformation plasticity models implemented in ABAQUS. The model in most cases compares very favorably to all such models. This comparison is done for both homogeneous and non-homogeneous problems including the case of a cantilever beam under tip loading. We show that for the problems that it applies to, the models run in approximately one tenth the computational time and with one tenth the number of iterations needed to conduct the analysis using the plasticity model in ABAQUS.

Dynamic fracture toughness of high strength metals under impact loading: increase or decrease

An elusive phenomenon is observed in previous investigations on dynamic fracture that the dynamic fracture toughness (DFT) of high strength metals always increases with the loading rate on the order of TPa·m1/2·s−1. For the purpose of verification, variation of DFT with the loading rate for two high strength steels commonly used in the aviation industry, 30CrMnSiA and 40Cr, is studied in this work. Results of the experiments are compared, which were conducted on the modified split Hopkinson pressure bar (SHPB) apparatus, with striker velocities ranging from 9.2 to 24.1 m/s and a constant value of 16.3 m/s for 30CrMnSiA and 40Cr, respectively. It is observed that for 30CrMn-SiA, the crack tip loading rate increases with the increase of the striker velocity, while the fracture initiation time and the DFT simultaneously decrease. However, in the tests of 40Cr, there is also an increasing tendency of DFT, similar to other reports. Through an in-depth investigation on the relationship between the dynamic stress intensity factor (DSIF) and the loading rate, it is concluded that the generally increasing tendency in previous studies could be false, which is induced from a limited striker velocity domain and the errors existing in the experimental and numerical processes. To disclose the real dependency of DFT on the loading rate, experiments need to be performed in a comparatively large striker velocity range.

Stability analysis of composite columns and parameter optimization against buckling

The stability analysis of a vertically standing or hanging composite column under end force and distributed axial load is made. The composite column has varying cross-section and variable material properties. The integral equation method is formulated to deal with this problem. Critical buckling load can be evaluated by seeking the lowest eigenvalue of the resulting integral equation. A characteristic equation is derived and it is a polynomial equation. The effects of self-weight and taper ratio on the buckling load are discussed for clamped-free prismatic and non-prismatic columns. As an application, two optimum design problems of freestanding tapered columns against buckling are considered to enhance the load-carrying capacity of cantilevered non-uniform columns. One is devoted to the parameter optimization of given shape profile for a homogeneous heavy column subjected to gravity load and tip load simultaneously under constant weight or volume constraint, and obtained results are very close to the exact ones of the strongest columns; the other is devoted to material tailoring such that the ratio of buckling load to weight reaches maximum for an axially graded inhomogeneous column made of two constituents with uniform cross-section.