Micromechanical testing of SU-8 cantilevers

SU‐8 is a photoplastic polymer with a wide range of possible applications in microtechnology. Cantilevers designed for atomic force microscopes were fabricated in SU‐8. The mechanical properties of these cantilevers were investigated using two microscale testing techniques: contact surface profilometer beam deflection and static load deflection at a point on the beam using a specially designed test machine. The SU‐8 Young's modulus value from the microscale test methods is approximately 2–3 GPa.     

Uniaxial compression of SiO2 glass cylinders: Analysis using stress‐dependent material coefficients

The present publication extends the prior analysis and discussion of data from the uniaxial compression (upsetting) of hot glass cylinders of vitreous silica that clearly revealed nonlinearity in viscoelastic parameters, i. e., Young's modulus and viscosity. While the simple Maxwell model is retained, the assumption of constant material coefficients in the course of an experiment is dropped. Two approaches, supported by both numerical integration and FEM simulations are applied to unravel the stress dependence of Young's modulus and the viscosity. Thus, nonlinearities are manifested by a non‐Hookean elasticity and a non‐Newtonian viscosity. The stress relaxation behaviour has also been analyzed disregarding any particular model. The relaxation ability is characterized by thermorheological simplicity depending on both the temperature and the stress attained.     

Time and temperature dependent mechanical characterization of polymer‐based materials in electronic packaging applications

Abstract

The thermo‐mechanical testing of high performance polyimide films Type HPPST supplied by Dupont^® was conducted at different strain rates and in different temperature environments. The stress‐strain behavior of materials was investigated, and the dependence of Young's modulus on temperature and strain rate is reported. In view of the uncertainty of the Young's modulus determination, the specimens were tested with unloading‐reloading to verify the test results. Constant strain rate uniaxial tensile tests and long‐time creep tests at various temperatures were performed to characterize the time‐temperature‐dependent mechanical property precisely. Cyclic loading tests were also implemented on specimens to investigate cyclic stress‐strain behaviors. This research is expected to enhance finite‐element‐modeling accuracy and characterize material properties precisely.     

Transverse elasticity of rabbit sciatic nerves tested by in vitro compression

Abstract

The Young's modulus of a rabbit sciatic nerve under stepwise in vitro linear compression test was estimated. Segments of rabbit sciatic nerves were compressed in the transverse direction with a custom‐made parallel compression apparatus. Digital images of the cross‐sectional face were taken simultaneously by using a camera mounted on a microscope. The applied force and the gap between the parallel plates of the compression apparatus were measured. Digitized images of the nerve crosssections were used to construct two‐dimensional models of the nerves. The parallel compression results showed that the mean Young's modulus of rabbit sciatic nerves was 41.6±5.0 kPa. By direct visual inspection and by comparing the finite element model simulation results and experimental data, we may suggest that the large fascicle is the main load‐bearing component while the small fascicle and the loose connective tissues like the epineurium bear less load in the parallel compression process.     

On the measurement of pile-up corrected hardness based on the early Hertzian loading analysis

Nanoindentation has been used to gain insight into the elastic/plastic contact responses of material at very small scales. The Oliver and Pharr's analysis (W.C. Oliver and G.M. Pharr, J. Mater. Res. 7 (1992) 1564) on the nanoindentation curve, however, can be meaningless when plastically deformed material piles around the indented points. This study suggests a measuring methodology of the real contact area enlarged by the material pile-up and its corresponding mechanical properties; the pile-up corrected contact area can be calculated inversely from the reduced modulus formulation with input information of the independently determined Young's modulus based on the Hertzian loading analysis. This contact correction relaxed overestimates in the elastic modulus and hardness interpreted from the nanoindentation curve and yielded actual mechanical properties comparable to the literature values of a (100) tungsten monocrystal. In addition, theoretically estimated upheaval amount of the contact boundary in this study was nearly consistent with the average pile-up height measured from an atomic-force microscope.     

Topology optimization with incompressible materials under small and finite deformations using mixed u/p elements

This paper focuses on topology optimization utilizing incompressible materials under both small‐ and finite‐deformation kinematics. To avoid the volumetric locking that accompanies incompressibility, linear and nonlinear mixed displacement/pressure (u/p) elements are utilized. A number of material interpolation schemes are compared, and a new scheme interpolating both Young's modulus and Poisson's ratio (E ‐ν interpolation) is proposed. The efficacy of this proposed scheme is demonstrated on a number of examples under both small‐ and finite‐deformation kinematics. Excessive mesh distortions that may occur under finite deformations are dealt with by extending a linear energy interpolation approach to the nonlinear u/p formulation and utilizing an adaptive update strategy. The proposed optimization framework is demonstrated to be effective through a number of representative examples.     

Uniaxial compression of SiO2 glass cylinders: analysis using stress‐independent material coefficients

Glass cylinders made of the vitreous silica type Suprasil 1 were exposed to axial stress at nominal compressive strain rates from –10^–5 to –10^–2 per second in a servohydraulic press at constant temperatures ranging from 1273 K to 1648 K. Subsequently, the stress was allowed to relax. True viscoelasticity is applied for evaluation of the experimental results and closed‐form solutions demonstrate that the interpretation as a single‐element Maxwell model renders Young's modulus readily measurable along with the tensile viscosity. The significant contribution of elasticity is found to be inherent in glass even at elevated temperatures. This very distinct property did not receive general recognition before and has been neglected in the majority of earlier studies on glass upsetting. The analysis reveals that the Young's modulus decreases with a rise in temperature if the nominal strain rate is held fixed, and with a reduction in nominal strain rate at constant temperature. The viscosity can be characterized as a function of the temperature either by a Vogel‐Fulcher‐Tammann‐Hess equation or by an Arrhenian fit. The findings when fed into a FEM programme reproduce the recorded force histories quite well. However, the present study reveals that the experimental data of Young's modulus depend on the stress. The results prove unambiguously the failure of linear viscoelasticity for this particular loading case. The full implications are reserved for a subsequent publication dealing with important consequences for glass rheology.     

Ultrastiff,Strong, and Highly Thermally Conductive Crystalline Graphitic Films with Mixed Stacking Order

A macroscopic film (2.5 cm × 2.5 cm) made by layer‐by‐layer assembly of 100 single‐layer polycrystalline graphene films is reported. The graphene layers are transferred and stacked one by one using a wet process that leads to layer defects and interstitial contamination. Heat‐treatment of the sample up to 2800 °C results in the removal of interstitial contaminants and the healing of graphene layer defects. The resulting stacked graphene sample is a freestanding film with near‐perfect in‐plane crystallinity but a mixed stacking order through the thickness, which separates it from all existing carbon materials. Macroscale tensile tests yields maximum values of 62 GPa for the Young's modulus and 0.70 GPa for the fracture strength, significantly higher than has been reported for any other macroscale carbon films; microscale tensile tests yield maximum values of 290 GPa for the Young's modulus and 5.8 GPa for the fracture strength. The measured in‐plane thermal conductivity is exceptionally high, 2292 ± 159 W m^?1 K^?1 while in‐plane electrical conductivity is 2.2 × 10⁵ S m^?1. The high performance of these films is attributed to the combination of the high in‐plane crystalline order and unique stacking configuration through the thickness.     

Computational study of the effect of carbon vacancy defects on the Young's modulus of (6, 6) single wall carbon nanotube

Most molecular dynamics (MD) simulations for single wall carbon nanotubes (SWCNT) are based on a perfect molecular material structure. The presence of vacancy defects in SWCNTs could lead to deviations from this perfect structure thus affecting the predicted properties. The present paper investigates the effect of carbon vacancy defects in the molecular structure of SWCNT on the Young's modulus of the SWCNT using MD simulations performed via Accelrys and Materials Studio. The effect of the position of the defects in the nanotube ring and the effect of the number of defects on the Young's modulus are studied. The studies indicate that for an enclosed defect with the same shape in a SWCNT structure, its position did not cause any change in the Young's modulus. However, as the number of defects increased, the predicted Young's modulus was found to decrease. For a 10 ring (6, 6) SWCNT, six vacancy defects (corresponding to a defect percentage of 2.5%) reduced the Young's modulus by 13.7%.     

Y3−xErxAl5O12 Aluminate Ceramics: Preparation,Thermal Properties and Theoretical Model of Thermal Conductivity

Rare‐earth aluminate ceramics for thermal‐barrier coatings (TBCs) are synthesized. The Young's modulus and thermal properties decrease with erbium additive increasing. The Y_3?xEr_xAl₅O₁₂ ceramics (x = 1, 3) possess a much‐lower thermal conductivity compared with 8YSZ. The lower Young's modulus and thermal‐expansion coefficient are due to the larger atomic weight of the Er substitutional atom. Additional phonon‐scattering effects also contribute to the lower thermal conductivity. The results indicate that Y_3?xEr_xAl₅O₁₂ can be explored as a candidate material for TBC systems. A theoretical model that describes the influence of point defects on the thermal conductivity is discussed.     

Mechanical characterization of single crystal silicon and UV-LIGA nickel thin films using tensile tester operated in AFM

This paper describes the mechanical characteristics of microscale single crystal silicon (SCS) and UV‐LIGA nickel (Ni) films used for microelectromechanical systems (MEMS). A compact tensile tester, operated in an atomic force microscope (AFM), was developed for accurate evaluation of Young's modulus, tensile strain and tensile strength of microscale SCS and UV‐LIGA Ni specimens. SCS specimens with nominal dimensions of 20 μm in thickness, 50 μm in width and 600 μm in length were prepared by a conventional photolithography and etching process. UV‐LIGA Ni specimens, with a thickness of 15 μm, a width of 50 μm and a length of 600 μm in nominal dimensions, were also fabricated by electroplating using a UV thick photoresist mould. All specimens have line patterns on their specimen gauge section to measure axial elongation under tensile loading. The SCS specimens showed a linear stress–strain response and fractured in a brittle manner, whereas the UV‐LIGA Ni specimens showed elastic–inelastic deformation behaviour. Young's modulus of SCS and UV‐LIGA Ni specimens obtained from tensile tests averaged 169.2 GPa and 183.6 GPa, respectively, close to those of bulk materials. However, the tensile strength of both materials showed a larger value than the bulk materials: 1.47 GPa for the SCS and 0.98 GPa for the Ni specimens. Yield stress and breaking elongation of UV‐LIGA Ni specimens were also quite different from those of the bulk Ni because of the specimen size effect on inelastic properties.     

A compensation of Young's modulus in polysilicon structure with discontinuous material distribution

This paper is concerned with the effect of material discontinuities, such as porosity, on the modulus of elasticity and attempts to simulate the effects of porosity in polysilicon using a finite element model. The proposed Young's modulus model is applied to the simulation of a polysilicon MEMS gyro. When the compensated Young's modulus is used in this simulation, we can reduce the difference between the simulation and the actual experimental result.     

Beyond the Hybridization Effects in Plasmonic Nanoclusters: Diffraction‐Induced Enhanced Absorption and Scattering

It is demonstrated hererin both theoretically and experimentally that Young's interference can be observed in plasmonic structures when two or three nanoparticles with separation on the order of the wavelength are illuminated simultaneously by a plane wave. This effect leads to the formation of intermediate‐field hybridized modes with a character distinct of those mediated by near‐field and/or far‐field radiative effects. The physical mechanism for the enhancement of absorption and scattering of light due to plasmonic Young's interference is revealed, which we explain through a redistribution of the Poynting vector field and the formation of near‐field subwavelength optical vortices.     

A hybrid material-point spheropolygon-element method for solid and granular material interaction

Capturing the interaction between objects that have an extreme difference in Young's modulus or geometrical scale is a highly challenging topic for numerical simulation. One of the fundamental questions is how to build an accurate multiscale method with optimal computational efficiency. In this work, we develop a material-point-spheropolygon discrete element method (MPM-SDEM). Our approach fully couples the material point method (MPM) and the spheropolygon discrete element method (SDEM) through the exchange of contact force information. It combines the advantage of MPM for accurately simulating elastoplastic continuum materials and the high efficiency of DEM for calculating the Newtonian dynamics of discrete near-rigid objects. The MPM-SDEM framework is demonstrated with an explicit time integration scheme. Its accuracy and efficiency are further analyzed against the analytical and experimental data. Results demonstrate this method could accurately capture the contact force and momentum exchange between materials while maintaining favorable computational stability and efficiency. Our framework exhibits great potential in the analysis of multi-scale, multi-physics phenomena.     

Effect of fibre volume fraction on fatigue behaviour of glass fibre reinforced composite

The aim of this paper is to study the fatigue behavior of GFRP composites manufactured by vacuum bagging process by varying the volume fraction. Constant‐amplitude flexural fatigue tests were performed at zero mean stress, i.e. a cyclic stress ratio R=?1 by varying the frequency of the testing machine. The relationship between stiffness degradation rate and fibre volume fraction, was observed, and the influence of volume fraction on the tensile strength was also investigated. The results show that, as the volume fraction increases the stiffness degradation rate initially decreases and then increases after reaching a certain limit for the volume fraction. Graph between volume fraction and Young's modulus shows that as the volume fraction increases Young's modulus also increases and reaches a limit and then it decreases with further increase in volume fraction, due to the increase in fibre content which changes the material properties of the composite material. The obtained results are in agreement with the available results.     

Experimental Investigation of Strain Behaviour of Heated Cement Paste and Concrete

The material degradation of concrete subjected to fire events has a severe influence on the load‐carrying capacity of support structures. Spalling of concrete layers, exposing the reinforcement bars and degradation of the material properties (Young's modulus, compressive strength) may lead to significant damage of the reduced cross‐section and, therefore, cause failure of the structure. In order to understand the stress build‐up at the heated surface caused by thermal expansion due to fire loading, finally leading to damage and spalling of concrete, the strain behaviour of cement paste and concrete exposed to combined thermo‐mechanical loading is the focus of this work. Hereby, the evolution of thermal strains, Young's modulus and Poisson's ratio with increasing temperature are investigated experimentally. For this purpose, the specimens are loaded uniaxially while the temperature is increased up to 800 °C. The obtained results provide the proper basis for the development of realistic material models, allowing more sophisticated simulations of structures exposed to fire.     

Effect of UV‐Radiation on the Packaging‐Related Properties of Whey Protein Isolate Based Films and Coatings

The high oxygen barrier properties of whey protein based films and coatings means these materials are of great interest to the food and packaging industry. However, these materials have poor mechanical properties such as the tensile strength, Young's modulus and elongation at break. Up until now, the influence of ultraviolet (UV) radiation on whey protein films has not been reported in the literature. This study thus investigates the influence of UV‐radiation on the properties of whey protein based films. UV‐irradiated films showed increased tensile strength and a yellowing that was dependent on the radiation time. After irradiation, the films showed no significant change in the barrier properties, Young's modulus or elongation at break. In addition, a protein solubility study was undertaken to characterize and quantify changes in structure‐property relationships. The significant decrease in protein solubility in buffer systems which break disulfide and non‐covalent bonds indicates that additional molecular interactions arise with increasing radiation dose. This study provides new data for researchers and material developers to tailor the characteristics of whey protein based films according to their intended application and processing. Copyright © 2015 John Wiley & Sons, Ltd.     

Experimental Investigation of Bond Stress and Deformations in LWAC Ties Reinforced with GFRP Bars

The structures' durability is an engineering concern for a long time but has been increased in the last years. Lightweight aggregate concrete (LWAC) combined with glass fibre reinforced polymer bars allows to create structures with high performance in terms of durability. The glass fibre reinforced polymer (GFRP) bars have different ribs from those of steel bars, and consequently, its bond to concrete is affected. Moreover, the Young's modulus of GFRP is much below compared with that of steel, and this influences significantly the behaviour of structural elements reinforced with this material. This paper presents an experimental study focused on bond between LWAC and reinforcing bars of GFRP. Thirty‐six pull‐out tests were carried out using steel and GFRP bars. These reinforcements were combined with three types of concrete, all with the same design density 1900 kg m^?3 but with different values of compressive strength: 35, 55 and 70 MPa. Furthermore, 12 reinforced ties were tested, combining different types of bars (steel and GFRP), two different diameters (12 and 16 mm) and the three types of LWAC. Based on experimental results, several relations were established to understand the behaviour of LWAC structures reinforced with GFRP bars, mainly in the serviceability conditions. These results point out that ties deformation and crack width are very affected by the reduced Young's modulus of GFRP: deformations and crack width of ties reinforced with GFRP are significantly higher, approximately three times greater, compared with those of ties reinforced with steel. The tension stiffening effect was also analysed in detail, and it was found that it is slightly influenced by the concrete compressive strength but is highly dependent of the Young's modulus of the reinforcing material.     

Unloading‐Based Stiffness Characterisation of Cement Pastes During the Second,Third and Fourth Day After Production

The stiffness evolution of binder ‘cement paste’ is triggering the stiffness of concrete. In the engineering practice, concrete formworks are typically removed 24 h after production. This underlines that knowledge on mechanical properties of cementitious materials during the second, third and fourth day after production is of high relevance for the ongoing construction process. This provides the motivation to perform early‐age stiffness characterisation on hydrating cement pastes, by means of the following three test methods. Unloading modulus is determined using a novel setup for non‐destructive uniaxial compression testing including overdetermined deformation measurements. Dynamic Young's moduli are obtained from ultrasonics experiments. Isothermal differential calorimetry allows for linking the observed temporal evolution of early‐age stiffness to the hydration degree of cement. Pastes with three different compositions are investigated, defined in terms of the initial water‐to‐cement mass ratio w/c and the initial water‐to‐solid (binder) mass ratio w/s. Pure cement pastes exhibit w/c = w/s = 0.50 and w/c = w/s = 0.42, respectively. A fly ash‐blended cement paste refers to a cement mass replacement level of 16%, and this is related to w/c = 0.50 and w/s = 0.42. Both unloading moduli and dynamic Young's moduli of all three materials increase practically linearly with increasing hydration degree, in the investigated regime of hydration degrees ranging from 40 to 60%. Fly ash does not contribute significantly to the early‐age hydration of the material, i.e. it represents a quasi‐inert part of the material's microstructure, exhibiting a significant stiffening effect.     

Ultra‐High‐Speed Full‐Field Deformation Measurements on Concrete Spalling Specimens and Stiffness Identification with the Virtual Fields Method

Abstract: For one decade, spalling techniques based on the use of a metallic Hopkinson bar in contact with a concrete sample have been widely employed to characterise the dynamic tensile strength of concrete at strain rates ranging from a few tens to hundreds of s^?1. However, the processing method based on the use of the velocity profile measured on the rear free surface of the sample (Novikov formula) remains quite basic. In particular, the identification of the whole softening behaviour of the concrete material is currently out of reach. In the present paper, a new processing technique is proposed based on the use of the virtual fields method (VFM). First, a digital ultra‐high‐speed camera is used to record the pictures of a grid bonded onto the specimen. Then, images of the grid recorded by the camera are processed to obtain full‐field axial displacement maps at the surface of the specimen. Finally, a specific virtual field has been defined in the VFM equation to use the acceleration map as an alternative ‘load cell’. This method applied to three spalling tests with different impact parameters allowed the identification of Young's modulus during the test. It was shown that this modulus is constant during the initial compressive part of the test and decreases in the tensile part when microdamage exists. It was also shown that in such a simple inertial test, it was possible to reconstruct average axial stress profiles using only the acceleration data. It was then possible to construct local stress–strain curves and derive a tensile strength value.