Stiffness Material Properties Research Articles

Comparative Study on Complete Characterisation of Elastic–Ductile Compressive Behaviour of Aged and New Norway Spruce Timber

ABSTRACT Material properties of historical timber structures may change over time due to decay, change of chemical composition of material, biological agents, climatic conditions, etc. Adequate bending performance requires stiffness and strength both in tension and compression, as well as sufficient ductility in compression. While the focus of most research is on the former quantities, the latter one is generally disregarded. This study gives a comparative analysis on complete characterization of compressive behaviour of recent and 60-years-old aged spruce timber (Picea abies). Parallel-to-grain compressive tests on small-sized specimens were conducted to obtain stress – strain diagrams and to determine failure modes. All material properties of stiffness, strength, and ductility were calculated for each specimen. It was concluded that elastic stiffness, elastic limit stress, and compression strength are virtually invariable after sixty years of service while the ductility decreased and the post-peak behaviour became more brittle. It is also found that end-rolling failure mode can yield the same elastic and stiffness properties as the standard kinkband failure. Climatic changes increase the risk of material deterioration prompting for more thorough monitoring and study of historical timber structures.

Analysis of different geometrical features to achieve close-to-bone stiffness material properties in medical device: A feasibility numerical study

Background and objectiveIn orthopedic medical devices, elasto-plastic behavior differences between bone and metallic materials could lead to mechanical issues at the bone-implant interface, as stress shielding. Those issue are mainly related to knee and hip arthroplasty, and they could be responsible for implant failure. To reduce mismatching-related adverse events between bone and prosthesis mechanical properties, modifying the implant's internal geometry varying the bulk stiffness and density could be the right approach. Therefore, this feasibility study aims to assess which in-body gap geometry improves, by reducing, the bulk stiffness. MethodsUsing five finite element models, a uniaxial compression test in five cubes with a 20 mm thickness was simulated and analyzed. The displacements, strain and Young Modulus were calculated in four cubes, each containing internal prismatic gaps with different transversal sections (squared, hexagonal, octagonal, and circular). Those were compared with a fifth full-volume cube used as control. ResultsThe most significant difference have been achieved in displacement values, in cubes containing internal gaps with hexagonal and circular transversal sections (82 µm and 82.5 µm, respectively), when compared to the full-volume cube (69.3 µm). ConclusionsThis study suggests that hexagonal and circular shape of the gaps allows obtaining the lower rigidity in a size range of 4 mm, offering a starting approach to achieve a “close-to-bone” material, with a potential use in prosthetic devices with limited thickness.

Example-based damping design

To date, material modeling in physically based computer animation has largely focused on mass and stiffness material properties. However, deformation dynamics is largely affected also by the damping properties. In this paper, we propose an interactive design method for nonlinear isotropic and anisotropic damping of complex three-dimensional solids simulated using the Finite Element Method (FEM). We first give a damping design method and interface whereby the user can set the damping properties so that motion aligned with each of a few chosen example deformations is damped by an independently prescribed amount, whereas the rest of the deformation space follows standard Rayleigh damping, or any viscous damping. Next, we demonstrate how to design nonlinear damping that depends on the magnitude of the deformation along each example deformation, by editing a single spline curve for each example deformation. Our user interface enables an art-directed and intuitive approach to controlling damping in solid simulations. We mathematically prove that our nonlinear anisotropic damping generalizes the frequency-dependent Caughey damping model, when starting from the Rayleigh damping. Finally, we give an inverse design method whereby the damping curve parameters can be inferred automatically from high-level user input, such as the amount of amplitude loss in one oscillation cycle along each of the chosen example deformations. To minimize numerical damping for implicit integration, we introduce an accurate and stable implicit integrator, which removes spurious high-frequency oscillations while only introducing a minimal amount of numerical damping. Our damping can generate effects not possible with previous methods, such as controllable nonlinear decaying envelopes whereby large deformations are damped faster or slower than small deformations, and damping anisotropic effects. We also fit our damping to videos of real-world objects undergoing large deformations, capturing their nonlinear and anisotropic damping dynamics.

Quick and inexpensive paraffin-embedding method for dynamic bone formation analyses

We have developed a straightforward method that uses paraffin-embedded bone for undemineralized thin sectioning, which is amenable to subsequent dynamic bone formation measurements. Bone has stiffer material properties than paraffin, and therefore has hereforto usually been embedded in plastic blocks, cured and sectioned with a tungsten carbide knife to obtain mineralized bone sections for dynamic bone formation measures. This process is expensive and requires special equipment, experienced personnel, and time for the plastic to penetrate the bone and cure. Our method utilizes a novel way to prepare mineralized bone that increases its compliance so that it can be embedded and easily section in paraffin blocks. The approach is simple, quick, and costs less than 10% of the price for plastic embedded bone sections. While not effective for static bone measures, this method allows dynamic bone analyses to be readily performed in laboratories worldwide which might not otherwise have access to traditional (plastic) equipment and expertise.

Alginate-Chitosan Hydrogels Provide a Sustained Gradient of Sphingosine-1-Phosphate for Therapeutic Angiogenesis.

Sphingosine-1-phosphate (S1P), a bioactive lipid, is a potent candidate for treatment of ischemic vascular disease. However, designing biomaterial systems for the controlled release of S1P to achieve therapeutic angiogenesis presents both biological and engineering challenges. Thus, the objective of this study was to design a hydrogel system that provides controlled and sustained release of S1P to establish local concentration gradients that promote neovascularization. Alginate hydrogels have been extensively studied and characterized for delivery of proangiogenic factors. We sought to explore if chitosan (0, 0.1, 0.5, or 1%) incorporation could be used as a means to control S1P release from alginate hydrogels. With increasing chitosan incorporation, hydrogels exhibited significantly denser pore structure and stiffer material properties. While 0.1 and 0.5% chitosan gels demonstrated slower respective release of S1P, release from 1% chitosan gels was similar to alginate gels alone. Furthermore, 0.5% chitosan gels induced greater sprouting and directed migration of outgrowth endothelial cells (OECs) in response to released S1P under hypoxia in vitro. Overall, this report presents a platform for a novel alginate-chitosan hydrogel of controlled composition and in situ gelation properties that can be used to control lipid release for therapeutic applications.

Infarcted Left Ventricles Have Stiffer Material Properties and Lower Stiffness Variation: Three-Dimensional Echo-Based Modeling to Quantify In Vivo Ventricle Material Properties.

Methods to quantify ventricle material properties noninvasively using in vivo data are of great important in clinical applications. An ultrasound echo-based computational modeling approach was proposed to quantify left ventricle (LV) material properties, curvature, and stress/strain conditions and find differences between normal LV and LV with infarct. Echo image data were acquired from five patients with myocardial infarction (I-Group) and five healthy volunteers as control (H-Group). Finite element models were constructed to obtain ventricle stress and strain conditions. Material stiffening and softening were used to model ventricle active contraction and relaxation. Systolic and diastolic material parameter values were obtained by adjusting the models to match echo volume data. Young's modulus (YM) value was obtained for each material stress-strain curve for easy comparison. LV wall thickness, circumferential and longitudinal curvatures (C- and L-curvature), material parameter values, and stress/strain values were recorded for analysis. Using the mean value of H-Group as the base value, at end-diastole, I-Group mean YM value for the fiber direction stress-strain curve was 54% stiffer than that of H-Group (136.24 kPa versus 88.68 kPa). At end-systole, the mean YM values from the two groups were similar (175.84 kPa versus 200.2 kPa). More interestingly, H-Group end-systole mean YM was 126% higher that its end-diastole value, while I-Group end-systole mean YM was only 29% higher that its end-diastole value. This indicated that H-Group had much greater systole-diastole material stiffness variations. At beginning-of-ejection (BE), LV ejection fraction (LVEF) showed positive correlation with C-curvature, stress, and strain, and negative correlation with LV volume, respectively. At beginning-of-filling (BF), LVEF showed positive correlation with C-curvature and strain, but negative correlation with stress and LV volume, respectively. Using averaged values of two groups at BE, I-Group stress, strain, and wall thickness were 32%, 29%, and 18% lower (thinner), respectively, compared to those of H-Group. L-curvature from I-Group was 61% higher than that from H-Group. Difference in C-curvature between the two groups was not statistically significant. Our results indicated that our modeling approach has the potential to determine in vivo ventricle material properties, which in turn could lead to methods to infer presence of infarct from LV contractibility and material stiffness variations. Quantitative differences in LV volume, curvatures, stress, strain, and wall thickness between the two groups were provided.

Thermal Stress Comparison on Copper and Gold Wire Bonded on Aluminium Bondpad

This paper reports on the thermal stress comparison between gold (Au) and copper (Cu) wires in wire bonding. The objective of this study is to examine the stress induced during different operating temperatures. ANSYS 11 has been deployed as the simulation tool for this study. This simulation was performed using a three dimensional (3D) non-linear finite element model. The gold and copper wires were attached to an aluminium bondpad on a silicon die. The results showed that when the highest stress concentrates at the area between the ball bond and the bondpad. Moreover, increasing temperature will increase the von mises stress. As for this study, the Cu wire display the greater thermal stress compared to Au wire due to its harder and stiffer material properties.

Physical mechanisms of megahertz vibrations and nonlinear detection in ultrasonic force and related microscopies

Use of high frequency (HF) vibrations at MHz frequencies in Atomic Force Microscopy (AFM) advanced nanoscale property mapping to video rates, allowed use of cantilever dynamics for mapping nanomechanical properties of stiff materials, sensing μs time scale phenomena in nanostructures, and enabled detection of subsurface features with nanoscale resolution. All of these methods critically depend on the generally poor characterized HF behaviour of AFM cantilevers in contact with a studied sample, spatial and frequency response of piezotransducers, and transfer of ultrasonic vibrations between the probe and a specimen. Focusing particularly on Ultrasonic Force Microscopy (UFM), this work is also applicable to waveguide UFM, heterodyne force microscopy, and near-field holographic microscopy, all methods that exploit nonlinear tip-surface force interactions at high frequencies. Leveraging automated multidimensional measurements, spectroscopic UFM (sUFM) is introduced to investigate a range of common experimental parameters, including piezotransducer excitation frequency, probed position, ultrasonic amplitude, cantilever geometry, spring constant, and normal force. Consistent with studies of influence of each of these factors, the data-rich sUFM signatures allow efficient optimization of ultrasonic-AFM based measurements, leading to best practices recommendations of using longer cantilevers with lower fundamental resonance, while at the same time increasing the central frequency of HF piezo-actuators, and only comparing results within areas on the order of few μm2 unless calibrated directly or compared with in-the-imaged area standards. Diverse materials such as Si, Cr, and photoresist are specifically investigated. This work thereby provides essential insight into the reliable use of MHz vibrations with AFM and provides direct evidence substantiating phenomena such as sensitivity to adhesion, diminished friction for certain ultrasonic conditions, and the particular benefit of UFM and related methods for nanoscale mapping of stiff materials.

The influence of the mechanical environment on remodelling of the patellar tendon

An understanding of the remodelling of tendon is crucial for the development of scientific methods of treatment and rehabilitation. This study tested the hypothesis that tendon adapts structurally in response to changes in functional loading. A novel model allowed manipulation of the mechanical environment of the patellar tendon in the presence of normal joint movement via the application of an adjustable external fixator mechanism between the patella and the tibia in sheep, while avoiding exposure of the patellar tendon itself. Stress shielding caused a significant reduction in the structural and material properties of stiffness (79%), ultimate load (69%), energy absorbed (61%), elastic modulus (76%) and ultimate stress (72%) of the tendon compared with controls. Compared with the material properties the structural properties exhibited better recovery after re-stressing with stiffness 97%, ultimate load 92%, energy absorbed 96%, elastic modulus 79% and ultimate stress 80%. The cross-sectional area of the re-stressed tendons was significantly greater than that of stress-shielded tendons. The remodelling phenomena exhibited in this study are consistent with a putative feedback mechanism under strain control. This study provides a basis from which to explore the interactions of tendon remodelling and mechanical environment.

Estimation of complex modulus using wave coefficients

The paper presents a new, efficient and accurate method for experimentally determining structural damping properties of stiff materials in flexural vibration. The estimation method seeks a wavenumber that forces data at all measurement points to conform with the general forced-vibration solution for a beam, and, thus, to have the same wave coefficients. The method does not depend on beam boundary conditions, making it relatively simple to implement in any laboratory setting. The paper shows results of using the method to estimate the complex modulus of aluminum and of polymethyl methacrylate (PMMA) beams in the frequency range from 30 to 800 Hz.Results compare very well with those obtained by more conventional methods and with those previously published over most of the frequencies of interest. Discrepancies in results for frequencies less than 80 Hz are attributed to the difficulty in measuring differences of velocity between points that are near each other at low frequency, and a technique for avoiding such discrepancies is outlined. Evidence shows that the error in the results near 350 Hz is due to a resonance in the scanning laser vibrometer that was used to take the experimental measurements.

Measurements of stiff-material compliance on the nanoscale using ultrasonic force microscopy

Ultrasonic force microscopy (UFM) was introduced to probe nanoscale mechanical properties of stiff materials. This was achieved by vibrating the sample far above the first resonance of the probing atomic force microscope cantilever where the cantilever becomes dynamically rigid. By operating UFM at different set force values, it is possible to directly measure the absolute values of the tip-surface contact stiffness. From this an evaluation of surface elastic properties can be carried out assuming a suitable solid-solid contact model. In this paper we present curves of stiffness as a function of the normal load in the range of 0–300 nN. The dependence of stiffness on the relative humidity has also been investigated. Materials with different elastic constants (such as sapphire lithium fluoride, and silicon) have been successfully differentiated. Continuum mechanics models cannot however explain the dependence of stiffness on the normal force and on the relative humidity. In this high-frequency regime, it is likely that viscous forces might play an important role modifying the tip-surface interaction. Plastic deformation might also occur due to the high strain rates applied when ultrasonically vibrating the sample. Another possible cause of these discrepancies might be the presence of water in between the two bodies in contact organizing in a solidlike way and partially sustaining the load.

Elastic mapping of heterogeneous nanostructures with ultrasonic force microscopy (UFM)

Ultrasonic force microscopy (UFM) is an atomic force microscopy (AFM)-related technique originally introduced to study the surface elastic properties of stiff materials. We report elastic images of heterogeneous nanostructures with a lateral resolution of the order of a few nanometres. One of the main intentions of this paper is not only to show the capability of UFM to allow one to image surface elastic properties of stiff materials but also to show that UFM can be applied to relatively soft materials with reproducible and interpretable results. The samples presented were chosen over a wide range of stiffness values (with Young's modulus E = 0.1-400 GPa): very stiff silicon carbide fibres embedded in a mullite matrix, less stiff carbon fibres embedded in an epoxy matrix and relatively compliant rubber inclusions in a polymethylmethacrylate matrix. A discussion of the conditions required to obtain unambiguous data is also provided. Results obtained using the more traditional force modulation mode are also presented and compared with the UFM images of the same samples.