Microsample Tensile Testing Research Articles

A Microsample Tensile Test Application: Local Strength of Impact Welds Between Sheet Metals

Microsample tensile testing was conducted to evaluate the quality of impact welds created by vaporizing foil actuator welding. Tensile test samples with a gauge length of 0.6 mm were electro-discharge machined out of welds created between 1-mm-thick aluminum alloy type 6061 (AA6061) sheets and 6-mm-thick copper (Cu110) plates. Aluminum sheets were used as flyers, while copper plates acted as targets. Flyer sheets in T6 as well as T4 temper conditions were utilized to create welds. Some of the welds made with T4 temper flyers were heat treated to a T6 temper. It was found that the welds made with T4 temper flyers were slightly stronger (max. of 270 MPa) than those produced with T6 temper flyers. Generally, failure propagated in a brittle manner across the weld interface; however, elemental mapping reveals material transfer on either member of the welded system. This work proves the feasibility to apply microsample tensile testing to assess impact welding, even when conducted with flyer sheets of 1 mm or less, and provides insight that is complementary to other test methods.

Fracture strength, elastic modulus and Poisson's ratio of polycrystalline 3C thin-film silicon carbide found by microsample tensile testing

This paper reports the measurement of elastic modulus, fracture strength, and Poisson's ratio of thin-film 3C polycrystalline silicon carbide determined by microsample tensile tests. The tensile specimens had a gage width of 600 μm, gage length of 4 mm, and gage thicknesses of 20–40 μm. A custom-built microsample tensile testing machine was used to conduct the tests and strain was measured with a laser-based direct strain measurement system. Twenty-three tests were completed on two sets of material with two different grain textures. Results of the microsample tensile tests show an average elastic modulus of 428 GPa, Poisson's ratio of 0.24 and fracture strength of 0.49 GPa for batch 1. Batch 2, a material with a different texture, resulted in an average elastic modulus of 448 GPa, Poisson's ratio of 0.17 and fracture strength of 0.81 GPa. In addition to differences in average values, variations were also seen in the Weibull characteristics of the two differently textured materials.

Mechanical properties of epitaxial 3C silicon carbide thin films

Microscale tensile specimens of epitaxial 3C silicon carbide (3C-SiC) thin films were fabricated on Si substrates and tested to provide measurements of strength and elastic modulus. Samples were fabricated using both micromolding and reactive ion etching (RIE) processes to pattern the 3C-SiC films. All specimens were on the (100) plane with a <110> tensile direction. Testing was performed on a microsample tensile testing machine previously used on other materials. The samples had a thickness of 0.5 to 2 /spl mu/m, a gauge length of 4 mm, and a minimum width of 600 /spl mu/m. Testing results show an average strength of 1.19 /spl plusmn/ 0.53 GPa and 1.65 /spl plusmn/ 0.39 GPa for micromolded and RIE patterned specimens, respectively. The elastic modulus was measured to be 424 /spl plusmn/ 44 GPa, which was consistent with but slightly lower than the elastic modulus calculated with single crystal elastic constants found by ab initio calculations.

Evolution of a diffusion aluminide bond coat for thermal barrier coatings during thermal cycling

The thermal cyclic durability of a TBC is thought to be strongly dependent on the physical and mechanical properties of the bond coat layer. A novel high temperature microsample tensile testing technique has been employed to characterize the mechanical behavior of a platinum modified nickel aluminide bond coat at 0% and 28% of cyclic life in the temperature range of 25 to 1150 °C. Values for the coefficient of thermal expansion and the Young’s modulus are reported. The bond coat exhibits a ductile to brittle transition temperature at approximately 600 °C, and above this temperature the yield and creep strength decreases rapidly with temperature. A power law description of elevated temperature stress relaxation is developed. The intermediate temperature strength was found to increase with thermal cycling, while the high temperature strength remained the same. This evolution in properties has been related to the development of a martensitic transformation that occurs during each thermal cycle.

Application of fourier analysis to the laser based interferometric strain/displacement gage

The laser based interferometric Strain/Displacement Gage (ISDG) measures the in-plane surface deformation between two small reflecting surface markers. When illuminated with a coherent beam of light, the reflected beams from the two markers form an interference pattern, and monitoring the shift of the fringe pattern allows strain in the gage section of a specimen to be directly measured. A minimum on the fringe pattern can be isolated and tracked as the test proceeds, but this technique utilizes only a small part of the optical signal and often requires a complex programming scheme. This paper presents the application of Fourier transform and phase shifting techniques to the use of the ISDG during microsample tensile testing. The Fourier transform samples the entire fringe pattern and greatly improves the optical signal to noise ratio, and the phase shifting fringe pattern analysis has proven to be more robust and less affected by speckle or optical noise than fringe pattern minimum tracking. This results in the ability to measure larger deformations with a system resolution of ∼5 microstrain and an uncertainty of ±15.5 microstrain. An example involving the microsample tensile testing of a MEMS related LIGA Ni specimen is included to demonstrate the utility of these new techniques.

High temperature microsample tensile testing of γ-TiAl

Dislocation activity in two-phase commercial TiAl alloys occurs most readily in the γ-TiAl phase, and measurements of the physical and mechanical properties γ-TiAl are needed to provide a solid foundation for modeling the mechanical performance of these alloys. The recent development of elevated temperature microsample tensile testing is outlined and the application of this technique to elevated temperature tensile testing of γ-TiAl described. Microsample single crystal specimens of Ti–55.5% Al have been tested in tension at temperatures ranging from 400 to 1000 °C. Measurements of the Young's modulus, coefficient of thermal expansion, and flow strength have been made as a function of temperature and crystal orientation. The Young's modulus and coefficient of thermal expansion measurements are compared with the limited data set that is available in the literature. The flow strength data evidences a clear violation of Schmid's law and highlights a significant tension/compression asymmetry, both of which appear to be related to the underlying dislocation mechanisms that govern plasticity in γ-TiAl.

Microsample tensile testing of LIGA nickel for MEMS applications

Electro-deposited LIGA Ni components are being considered for use in a number of microelectromechanical systems (MEMS) and applications. The metrology of these components and their non-equilibrium microstructures play an important role in determining the mechanical response of these structures. Microsample testing has proven to be a reliable way of measuring the elastic and plastic tensile properties of these 100–200 μm thick films. Measured values of the in-plane Young's modulus (180±24 GPa) are significantly lower than the modulus for bulk coarse grained Ni (207 GPa), but can be explained and modeled in terms of the crystallographic texture in the as-deposited films. Variations in strength associated with post-processing heat treatments are highlighted and explained in terms of microstructural coarsening. The effect that coarsening and the related drop in strength have on the performance of LIGA Ni spring-like structures is discussed, and the need for further high temperature testing is highlighted.

Deformation mechanisms and microtensile behavior of single colony Ti–6242Si

The constant strain rate deformation behavior of individual α–β colonies of the titanium aeroengine alloy Ti–6Al–2Sn–4Zr–2Mo–0.1Si (compositions in wt.%) has been investigated through microsample tensile testing techniques. The mechanical behavior and deformation mechanisms observed were found to depend strongly on colony orientation. A near-Burger's orientation relationship (OR) has been observed for the α (hcp) and β (bcc) phases in these colonies, grown utilizing a vertical float zone technique. The Burger's OR is believed to be responsible for the observed anisotropy in the critical resolved shear stress for the three a/3〈11-20〉{10-10} prism slip systems. A more pronounced anisotropy was observed in the critical resolved shear stress (CRSS) for the three a/3〈11-20〉{0001} hcp basal slip systems. Scanning Electron Microscopy (SEM) investigations of the slip line morphology of the deformed samples indicates an inhibition of β-lath shearing for colonies oriented for basal slip, while frequent shear offsets of the β phase by slip lines were observed in the samples oriented for prism slip. The effects of the Burger's OR, slip line spacing and elastic interactions stresses arising from the α–β interface are considered in explaining the observed anisotropy in CRSS.

Development of high-temperature microsample testing

Microsample tensile testing has been established as a means of evaluating the room temperature mechanical properties of specimens with gage sections that are tens to hundreds of microns thick and several hundred microns wide. The desire to characterize the mechanical response of materials at elevated temperatures has motivated the development of high-temperature microsample testing that is reported here. The design of specially insulated grips allows the microsamples to be resistively heated using approximately 2 V DC and currents ranging between 2 to 6 A. An optical pyrometer with nominal spot size of 290 μm and 12 μm diameter type K thermocouples was employed to measure and verify the temperature of the microsamples. The ability of the pyrometer to accurately measure temperature on microsamples of different thicknesses and with slightly different emissivities was established over a temperature range from 400°C to 1100°C. The temperature gradient along the length and thickness of the microsample was measured, and the temperature difference measured in the gage section used for strain measurements was found to be less than 6.5°C. Examples of elevated temperature tensile and creep tests are presented.

Measuring the Properties of Diffusion Aluminide Bond Coat Layers with Microsample Tensile Testing

ABSTRACTThe bond coat layer plays an important role in the TBC system by providing oxidation resistance and a foundation for the ceramic top coat. The thermal cyclic durability of a TBC is thought to be strongly dependent on the physical and mechanical properties of the bond coat layer. Attempts to measure these properties for as-deposited and thermally cycled diffusion aluminide bond coats have been greatly inhibited by the limited thickness (∼60 μm) of these coatings. In the present study, novel high temperature microsample tensile testing technique is employed to measure the Young's modulus (E), coefficient of thermal expansion (CTE), yield strength and stress relaxation behavior of an as-deposited (Ni, Pt)Al bond coat in the temperature range of 25 oC to 1150 °C. Preliminary results for these material parameters are reported here. They are being used as material inputs for an independent finite element analysis (FEA) study of the development of stresses in the TBC layers during thermal cycling.

Mechanical behavior of a μm-sized single crystal silicon structure with sharp notches

The mechanical behavior of μm-sized single crystal silicon structures is investigated in this paper. A new apparatus for microsample tensile testing is used to determine the mechanical properties of single crystal silicon microbars. Force and elongation are measured independently with high accuracy. Elastic constants as well as critical loads are determined. Failure of the sample occurs at the extremities of the microbar at a sharp notch. The stress field in the notch tip vicinity is therefore analyzed to characterize the load capacity of the structure. For this purpose, finite element computations are combined with approximate analytic solutions based on the Stroh formalism. Both the case of plane stress and two-dimensional displacements are considered. Comparison with the FEM solution shows that the plane stress assumption is better, leading to the determination of the critical stress intensity factor F cr for this case. Because F cr is dependent on the geometry, a new scalar failure criterion—i.e. a critical radius R cr—is introduced and derived from the calculated stress field at the notch. It is based on a comparison of stored elastic energy and surface energy at the critical location. Because it is a scalar criterion, it can be applied to single crystal microstructures with notches of arbitrary notch angle. For the case considered, experiments yield R cr = 0.8 nm. The proposed criterion can be considered as a step towards the definition of a design rule for sharp notches in single crystal structures, which is required for the optimized design of micromechanical devices.

Effect of As-Processed and Annealed Microstructures on the Mechanical Properties of Liga Ni MEMS

AbstractMicrostructure – mechanical property relations of 200 μm thick LIGA deposited Ni films have been investigated with a combination of optical cross-sectional microscopy and microsample tensile testing. Measurements of the grain size and morphology of nine different asdeposited films evidenced a predominantly columnar microstructure with significant variations in grain size. These as-deposited microstructural variations resulted in a range of Young's modulus, yield strength, and ultimate tensile strength values that are consistent with previously reported measurements, but these values did not appear to scale with the as-deposited microstructures. By contrast, significant changes in both the microstructure and mechanical properties were observed when these films were annealed at 800'C for 1 hour. Recrystallization and grain growth transformed the grain morphology from columnar to equiaxed, which resulted in a dramatic decrease in the tensile strength and corresponding increase in the ductility of the annealed films.

MEMS Material Microstructure and Elastic Property Modeling

AbstractResults from initial efforts to develop a quantitative understanding of the material microstructure – elastic properties relationship for electroplated LIGA nickel will be presented. Microsample tensile testing and microstructural characterization results including grain size and grain orientation measurements obtained using orientation imaging microscopy (OIM) are discussed. The use of these data in the development of a finite element modeling approach to represent the mechanical response of “meso-scale” LIGA nickel MEMS material will be highlighted.

Microstructural Observations of LPCVD Double Layer Polysilicon Thin Film Tensile Specimens

ABSTRACTThe vast majority of micro electro-mechanical systems fabricated today depend on polycrystalline silicon thin films for structural support. Studies involving the mechanical performance of these thin films have progressed to the point where the elastic properties and tensile strength of the films can routinely be measured using a specially designed microsample tensile testing machine. However, a fundamental understanding to predict the mechanical behavior of the polycrystalline silicon films requires that these experimental measurements be complemented with detailed observations of the underlying thin film microstructure. This paper describes some of the plan view and cross-section transmission electron microscope observations that have been performed on different deposition runs of double layer polycrystalline films obtained from the Microelectronics Center of North Carolina. The emphasis has been placed on determining the flatness and dimensions of the polycrystalline films, grain morphology and distribution, texture, and dislocation substructure and microtwinning in the undeformed films.