Micro-scale Testing Research Articles

Anisotropic Elastic Properties of Chiral Sculptured Thin Films at Micro-Scale Evaluated by Resonance Frequency Spectra

Chiral sculptured thin films (STFs) Glancing-angle deposition (GLAD) thin films are nanoengineered to meet the requirements of a variety of applications such as micro filters, sensors, and waveguides due to their unique frequency characteristics which cannot be achieved by conventional solid materials. For the design, it is necessary to understand the elastic properties of STFs. To facilitate this, we report on our newly developed advanced micro-scale vibration testing process. In the testing, specially designed micro-specimens with surface areas of tens by tens of microns are excited using a piezoelectric (PZT) actuator and the resonance frequencies are detected by a laser device in the vertical or lateral directions successfully. The anisotropy elastic modulus of STFs composed of helical nanosprings are identified on the basis of vibration testing. The thin film shows strong characteristic anisotropy that the solid one hardly can attain. The micro-scale testing technique can be extended to other materials and microstructures.

On measuring fracture toughness under load control in the presence of slow crack growth

The influence of slow crack growth in ceramics on the measurement of their fracture toughness under steadily increasing controlled load (as is generally practiced in microscale testing) is analyzed to identify conditions under which the phenomenon can cause error. The analysis considers the two regimes classically observed, namely a power-law variation of crack growth rate on the stress intensity factor K, and, at higher K values, a regime of fixed crack tip velocity up to rapid fracture at K=Kc. Results are presented for standard test specimens in a convenient graphical form and also in the form of upper bound expressions for loading rates above which measurements should be valid. It is shown that, when subcritical crack growth is present, chevron-notched samples present advantages over more conventional precracked specimens.

Micro-scale testing and micromechanical modelling for high cycle fatigue of CoCr stent material

This paper presents a framework of experimental testing and crystal plasticity micromechanics for high cycle fatigue (HCF) of micro-scale L605 CoCr stent material. Micro-scale specimens, representative of stent struts, are manufactured via laser micro-machining and electro-polishing from biomedical grade CoCr alloy foil. Crystal plasticity models of the micro-specimens are developed using a length scale-dependent, strain-gradient constitutive model and a phenomenological (power-law) constitutive model, calibrated from monotonic and cyclic plasticity test data. Experimental microstructural characterisation of the grain morphology and precipitate distributions is used as input for the polycrystalline finite element (FE) morphologies. Two microstructure-sensitive fatigue indicator parameters are applied, using local and non-local (grain-averaged) implementations, for the phenomenological and length scale-dependent models, respectively, to predict fatigue crack initiation (FCI) in the HCF experiments.

Nanoindentation Characterization of Moisture Damage in Different Phases of Asphalt Concrete

Abstract Traditional microscale testing cannot be performed on asphalt binder, mastic, or aggregate, while they are an integral part of asphalt concrete (AC). Recently, nanoindentation has created an opportunity to characterize mastic and asphalt binder while they reside in an AC sample. In the study, laboratory nanoindentation testing is carried out to characterize moisture-induced damage in different phases of AC. A moisture-induced sensitivity testing (MIST) device is used for moisture conditioning of AC. In the MIST device, an AC sample is fully submerged under water and all-around cyclic pressure is applied through the pores inside an AC sample to cause damage. Damaged AC samples are indented for an extended dwell time of 200 s using an unloading rate of 0.02 mN/s to minimize viscous effects of asphalt on test results. The indentation load-displacement curve is analyzed by the Oliver–Pharr method to obtain elastic modulus and hardness. When comparing wet and dry sample indentation test results, it is observed that the modulus of the wet mastic reduces to 60 % of dry mastic modulus. Overall moisture conditioning reduces the modulus of AC by 70 %. In addition, the creep response of the mastic phase is modeled by a viscoelastic Burger model. The creep compliance value of wet mastic is 42 % higher than that of dry mastic.

Anisotropic elastic properties of chiral sculptured thin films at micro-scale evaluated by resonance frequency spectra

Abstract Chiral sculptured thin films (STFs) are nano-engineered to meet the requirements of a variety of applications such as micro filters, sensors, and waveguides due to their unique frequency characteristics which cannot be achieved by conventional solid materials. For the design, it is necessary to understand the elastic properties of STFs. To facilitate this, we developed an advanced micro-scale vibration testing process. In the testing, specially designed micro-specimens with surface areas of tens by tens of microns are excited using a piezoelectric (PZT) actuator and the resonance frequencies are detected by a laser device in the vertical or lateral directions successfully. The anisotropy elastic modulus of STFs composed of helical nano-springs are identified on the basis of vibration testing. The thin film shows strong characteristic anisotropy that the solid one hardly can attain. The micro-scale testing technique can be extended to other materials and microstructures.

Stochastic effects in plasticity in small volumes

Recent studies of micro- and nano-scale metallic structures have exposed considerable statistical distribution, in addition to significant size dependencies, in the yield strength. This intrinsic statistical variation is particularly evident in the micro-compression and microtension thin film tests. This work investigates the relationship between the initial dislocation density, the heterogeneous initial spatial dislocation distribution, and the resulting localized deformation with multiscale discrete dislocation dynamics simulations. This relationship is examined separately from commonly reported external factors affecting observed strength, such as variations in specimen geometry and base support. Towards this end, we performed multiscale dislocation dynamics simulations of geometries commonly employed in micro-scale testing techniques, including micro-pillar compression, microtensile thin film, and microbulge tests. The statistical variation of yield strengths from all three simulation geometries is in agreement with experimental data from the corresponding loading techniques. We show that the onset of plasticity is stochastic in small volumes containing a small density of dislocations: a contrast to classical deterministic plasticity theory. The yield stress in these small volumes is stochastic, not deterministic, because of statistical variation of the initial dislocation content. The numerical results exhibit a localized deformation process and demonstrate a strong dependence of the yield stress on the initial dislocation density, the initial dislocation spatial distribution, and the specimen geometry size. Leveraging nucleation theory, a stochastic model for the onset of plasticity in micro- and nano-scale structures is developed based on these results.

Micro-scale testing of ductile and brittle cantilever beam specimens in situ with a dual beam workstation

Micro-scale cantilever beam specimens created by focused ion beam milling have been mechanically loaded in situ at room temperature to observe the deformation and fracture of single crystal silicon, nanocrystalline nickel and thermal barrier coatings with a multilayer structure. The micro-scale preparation technique allows cantilever beams to be selected from preferred positions in the samples so that specific mechanical properties can be evaluated. As a consequence these microstructural specific properties can be combined with direct observation of the response of the test specimen throughout the period of the test. The measured mechanical properties and response for the materials given above are discussed and compared with previously published data.

Micro-Scale Cantilever Testing of Linear Elastic and Elastic-Plastic Materials

t is increasingly a requirement to be able to determine the mechanical properties of materials: (i) at the micro-scale, (ii) that are in the form of surface coatings and (iii) that have nanoscale microstructures. As a consequence micro-scale testing is an important tool that has been developed to aid the evaluation of the mechanical properties of such materials. In this work cantilever beam specimens (typically 2μm by 2μm by 10μm in size) have been prepared by gallium ion milling and then deformed in-situ within a FEI Helios Dual Beam workstation. The latter is achieved using a force probe with a geometry suitable for loading the micro-scale test specimens. Thus force and displacement can be measured together with observing the deformation and fracture of the individual specimens. This paper considers the evaluation of the mechanical properties in particular elastic modulus, yield strength and fracture strength of materials that result in relatively large deflections to the micro-scale cantilever beams. Two materials are considered the first is linear elastic single crystal silicon and the other elastic-plastic nanocrystalline (nc) nickel. The results are discussed with respect to the reproducibility of this method of mechanical testing and the evaluated properties are compared with those derived by alternative procedures.

Demonstration of an in situ microscale fatigue testing technique on a titanium alloy

Under high cycle and very high cycle fatigue (HCF and VHCF) conditions, scatter in fatigue lifetimes is substantial; often 2–3 orders of magnitude. Characterization of fatigue crack initiation sites in laboratory scale fatigue specimens has led to the identification of characteristic initiation sites and microstructural arrangements. Despite these observations, in some cases, it is still unclear how apparently similar initiation sites exhibit such different total fatigue lifetimes. Differences in crack-initiation mechanisms can be further revealed if specific microstructural arrangements are isolated within a micro-specimen. Towards this end, an in situ microscale tension testing technique was adapted to enable microscale fatigue testing on tensile dog-bone specimens. Microscale tensile fatigue specimens with approximate gage diameters of 20μm were prepared with a focused ion beam (FIB) microscope. Initial tensile experiments were conducted to characterize the mechanical behavior of microscale specimens for this microstructure. The microscale tensile specimens were observed to exhibit reduced yield and flow stresses in comparison to bulk tensile specimens. However, in this feasibility demonstration of in situ microscale fatigue testing, microscale specimens exhibit enhanced fatigue properties relative to conventional specimens, which may result from the differences in strain rate and, potentially, the method of test control (load vs. displacement) between these two testing methods. Both tensile and fatigue specimens have been characterized with electron backscatter diffraction to identify the neighborhood and the specific slip systems that were activated under local deformation conditions within the microstructure. Both basal and prism slip were observed, although prism slip was more prevalent. Results of the tension and fatigue experiments are discussed in the context of the growing body of literature on microscale testing, and avenues for future work are highlighted.

Nanoindentation of human meniscal surfaces

Menisci are crescent shaped fibrocartilaginous structures which support load distribution of the knee. The menisci are specifically designed to fit the contour of the femoral condyles, aiding to disperse the stresses on the tibial plateau and in turn safeguarding the underlying articular cartilage. The importance of the meniscal superficial layer has not been fully revealed and it is suspected that this layer plays a pivotal role for meniscal function. In this study, both femoral (proximal) and tibial (distal) contacting meniscal surfaces were mechanically examined on the nano-level among three distinct regions (anterior, central and posterior) of the lateral and medial menisci. Nanoindentation testing showed no significant differences among regions, surfaces or anatomical locations, possibly elucidating on the homogeneity of the meniscal superficial zone structure (Einstantaneous: 3.17–4.12MPa, Esteady-state: 1.47–1.69MPa). Nanomechanical moduli values were approximately an order of magnitude greater than micro-scale testing derived moduli values. These findings validate the structural homogeneity of the meniscal superficial zone, showing that material properties are statistically similar regardless of meniscal surface and region. Understanding the mechanical behavior of meniscal surfaces is imperative to properly design an effective meniscal replacement.

Comparison of baking tests using wholemeal and white wheat flour

Production of whole-grain products can provide more variety in bakery products and might impact positive health effects due to fermentable soluble fibers. To assess technological quality, micro-scale methods using 10 g of white flour are usually carried out, in particular if amount of material or additives is limited. Using wholemeal flour for micro-scale testing might enhance use efficiency of raw material. Up to date, there is no information, whether micro-scale testing of wholemeal flour provides the same information on techno-functionality as classical methods. Two wheat varieties with the same protein concentration but different bread-making performance were used to analyze this aspect. Micro-scale baking test was optimized for wholemeal flour concerning dough consistency and fermentation time. Wholemeal flour reduced loaf volume, but optimization was successful. Although all processing steps of wholemeal baking test were adapted, differences in loaf volume between the two varieties were only present with white flour. Coarse particles in wholemeal flour may be responsible for reduced gas retention. Decreasing particle size of bran material might reduce adverse effects on loaf volume. A micro-scale baking test using wholemeal flour would have several advantages after correct implementation: lower price, less time and material required, direct measurement of quality aspects.

Mechanical assessment of ultrafine-grained nickel by microcompression experiment and finite element simulation

Abstract

Laser heating: a minimally invasive technique for studying fire-generated heating in building stone

Due to the irreparable damage it can cause, fire is one of the major risks to buildings. Recent studies on the effects of fire tend to focus on micro-scale analysis, addressing questions such as micro-cracking or mineralogical or chemical changes that are particularly relevant to listed buildings. The fire simulation techniques employed to date (convection heating laboratory furnaces and real flame tests) are subject to a series of limitations including non-repeatability, the lack of combustion by-products and, most importantly, the need to work with large samples. In this final context, techniques must be found that require only minimal sample sizes. This paper introduces laser irradiation as a technique suitable for simulating the “combustion” of building materials, since it mimics the physics of fire, is repeatable, yields combustion products and, most importantly, can be applied to small specimens. Indeed, as laser irradiation concentrates high amounts of energy in small areas, it can be used for micro-scale testing.

Biomaterial microarrays: rapid, microscale screening of polymer–cell interaction

The identification of biomaterials that induce optimal gene expression patterns and allow for appropriate levels of cellular attachment is of central importance in tissue engineering and cell therapy. Herein, we describe the creation of cell-compatible, biomaterial microarrays, that allow rapid, microscale testing of biomaterial interactions with cells. As proof of principle, we simultaneously characterized over 3456 human mesenchymal stem cell (hMSC)–biomaterial composite interactions, and describe preliminary studies on the utility of these arrays with a neural stem cell line (NSC), and primary articular chondrocytes.

Micromechanical testing of SU‐8 cantilevers

ABSTRACTSU‐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.

Microscale testing of the strut in open cell aluminum foams

This paper presents the results of micro-tensile testing of the individual struts that were extracted from open cell aluminum foams in the as-fabricated (F), annealed (O) and T6-strengthened (T6) conditions. The results reveal that foam struts are generally stronger and more dutile than the corresponding solid alloys under the same heat treatment conditions. The distinguished strut properties are attributed to the unique grain structures of the struts typically consisting of one grain or having bamboo-like grain structure. The measured strut strengths are also incorporated into mechanics models to estimate foam strengths. A comparison of the predicted and measured foam strengths shows that direct measurement of strut properties is critical to make precise estimation of foam strengths.

Micro-scale abrasion testing of PVD TiN coatings on conventional and nonledeburitic high-speed steels

Micro-scale abrasion testing has been used to evaluate the wear resistance of PVD TiN and Ti/TiN coatings on conventional and nonledeburitic high-speed steels. The experimental data were analysed with a model employing only the inner diameter of the wear scar and independently measured wear coefficients for the substrates. A new simplified method involving the measurement of a single crater and the evaluation of coating thickness by SEM and image analysis was also used. Both approaches were found to be equally accurate and yielded results which were not statistically different. The simplified method can be recommended for initial and/or rapid assessment of coating performance, or for small samples or small coated areas on which only a single test is possible. The results were augmented by SEM observations of the wear mechanisms.

Use of a scanning thermal microscope to examine corrosion protective coatings in exposure

As the capabilities for control of coatings properties at the nanoscale advance, characterization of these properties at these same length scales is needed. One set of organic coatings properties that is very important to understand and control at these length scales are the thermomechanical properties of the coatings. A new hybrid instrument, a scanning thermal microscope was used to examine the morphology and the thermal property simultaneously for two component polyurethane coatings under exposure to corrosive environments. Through the morphology measurement, the surface appearance on the micro-level was monitored and the influence of different corrosive factors such as wet/dry cycling, thermal cycling and condensation on the appearance was studied. At the same time, the micro-scale thermal measurements and concurrent FTIR testing provided the information on the relationship between the change of the structure and the properties of the coating. The information gained from these measurements on the changes in local coatings properties will be presented and correlated to other studies in our lab on nanoscale properties as well as global coating property changes due to exposure.

Mixing Properties, Baking Potential, and Functionality Changes in Storage Proteins During Dough Development of Triticale‐Wheat Flour Blends

ABSTRACTFlours from advanced lines or cultivars of six triticales and two prime hard wheats, along with triticale‐wheat blends, were investigated for mixing, extension (excluding blends), and baking properties using microscale testing. Percentage total polymeric protein (PPP) and percentage unextractable polymeric protein (UPP) of flours and doughs, including blends, mixed to optimal dough development were estimated using size‐exclusion HPLC to determine the changes in protein solubility and association with blend composition (BC), mixing properties, and loaf height. Each triticale was blended with flours of each of the two wheat cultivars (Hartog and Sunco) at 0, 30, 40, 50, 60, 70, and 100% of wheat flour. Nonlinear relationships between BC and mixograph parameters (mixing time [MT], bandwidth at peak resistance [BWPR], and resistance breakdown [RBD]) were observed. A linear relationship between BC and peak resistance (PR) was predominant. PPP of triticale flours was mostly higher than PPP of wheat cultivars. UPP of all triticales was significantly lower than wheat cultivars. PPP of freeze‐dried doughs was mostly nonsignificant across the blends and showed a curvilinear relationship with BC. The deviations from linearity of MT and PPP were higher in triticale‐Sunco blends than in triticale‐Hartog blends. UPP of blends was closer to or lower than the lower component in the blend. The deviations from linearity for MT and UPP were greater in triticale‐Hartog blends than triticale‐Sunco blends. A highly significant correlation (P < 0.001) was observed between BWPR and loaf height. This suggested that BWPR in triticale‐wheat flour blends could be successfully used for the prediction of loaf height. Triticale flour could be substituted for wheat flour up to 50% in the blend without drastically affecting bread quality. Dough properties of triticale‐wheat flour blends were highly cultivar specific and dependent on blend composition. This strongly suggested that any flour blend must be tested at the desired blend composition.

The energetics of heterogeneous deformation in open-cell solid foams

Compressed open–cell solid foams frequently exhibit spatially heterogeneous distributions of local stretch. The theoretical aspects of this deformation habit have not been clearly elucidated. Here we propose a simple nonlinear model aimed at illustrating the most salient features of the micromechanics of uniaxially stretched solid foams. Then we study the energetics of the model to show that the stretch heterogeneity observed in experiments stems from the lack of convexity of the governing energy functional, which favours two characteristic values of local stretch. These characteristic values are independent of the applied overall stretch and define two configurational phases of the foam. The predicted stretch distributions correspond to stratified mixtures of the phases; stretching occurs in the form of a phase transformation, by growth of one of the phases at the expense of the other. We also compare the predicted mechanical response with experimental data for a series of foams of different densities and discuss the analogy between the stretching of foams and the liquefaction of van der Waals gases. Lastly, we perform displacement field measurements using the digital image correlation technique and find the results to be in agreement with our predictions.