Tensile and Tensile-Mode Fatigue Testing of Microscale Specimens in Constant Humidity Environment

A tensile and tensile-mode-fatigue tester has been developed for testing microscale specimens in high humidity environments in order to investigate the fracture mechanisms of microelectromechanical materials. A humidity control system was installed on a tensile-mode fatigue tester equipped with an electrostatic force grip. A specimen and a griping device were inserted into a small chamber and the humidity was controlled by air flow from a temperature and humidity chamber. The humidity stability was within ±2%RH for humidities in the range 25–90%RH for eight hours of testing. Fatigue tests were performed on single-crystal silicon (SCS) specimens in constant humidity environments and laboratory air for up to 10⁶ cycles. The gauge length, width, and thickness of the SCS specimens were 100 or 500 μm, 13.0 μm, and 3.3 μm, respectively. The average tensile strength was 3.68 GPa in laboratory air; this value decreased in high humidity environments. Fatigue failure was observed during cyclic loading at stresses lower than the average strength. A reduction in the fatigue strength was observed at high relative humidities. Different fracture origins and fracture behaviors were observed in tensile tests and fatigue tests, which indicates that the water vapor in air affects the fatigue properties of SCS specimens.     

Strain Measurements of Silicon Dioxide Microspecimens by Digital Imaging Processing

Silicon dioxide thin film is a common component in electronic devices and in MEMS, but its mechanical properties have rarely been studied. Techniques have been adapted and developed to conduct tensile tests on 1.0 μm thick silicon dioxide specimens that are 100, 150, and 200 μm wide and either 1 or 2 mm long. One end of the specimen remains fastened to the substrate, and the other is glued to a silicon carbide fiber attached to a 30 g load cell mounted on a piezoelectric translation stage. Strain is measured by digital imaging of two gold lines applied to the gage section of the transparent specimen. Twenty-five tests yield a Young’s modulus of 60.1 ± 3.4 GPa and a fracture strength of 364 ± 57 MPa.     

An Investigation of Fatigue Characteristics of Copper Film

Tensile and fatigue behaviors of the copper film coated by tin (Sn) were investigated considering S-N relationships and scanning electron microscope (SEM) observation of fracture surfaces. The fatigue behavior was investigated considering the effect of load ratio, R. The specimen of 2000 μm width, 8000 μm length and 15.26 μm thickness was fabricated by etching process. Tensile properties were measured using the micro-tensile testing system and in-plane electronic speckle pattern interferometric (ESPI) system for measuring the tensile strain during the test. The fatigue tests of the film were carried out in load-control mode with 40 Hz at three different stress ratios of 0.05, 0.3 and 0.5. The S-N curves, including the slope of the curve and fatigue limit, at the respective stress ratios were obtained. These curves were dependent on the load ratio. Empirical relationships indicating the dependency of the fatigue limit and S-N curve on the load ratio were suggested in this study. SEM observation of the tensile fracture surface showed that the cross-sectional area of the testing section was necked in the direction of the film thickness (i.e. parallel to the substrate surface normal) and some ductile dimples in the fracture surface were present. The fracture of the copper film under cyclic loading was progressed in the transgranular fracture mode.     

Mini-Tensile Experiments of Clock-Rolled Zirconium Plate

We present our efforts to measure the tensile strength of clock-rolled pure zirconium in the through-thickness (TT) direction of the plate. Although the plate is too thin to produce standard ASTM tensile samples in the TT orientation, such measurements are relevant to benchmarking our constitutive models of hardening and texture evolution. We have designed a fixture and sample to perform tensile tests on our 9 mm thick plate. The sample is a double-ligament mini-tensile sample: 8 × 8 × 1 mm overall; each ligament has a gage section of 1 × 1 × 3 mm. In contrast, our standard “macro” tensile sample is a flat dogbone with a gage section of 3 × 1.5 × 25 mm. We validate our design by comparing the results of mechanical tests performed on samples of both geometries. Although the hardening response is nearly identical, the flow stress of the miniature samples is offset by +25 MPa at the onset of plastic yield. We present our efforts to resolve the origin of this offset.     

Prediction of bond failure and deflection of carbon fiber-reinforced plastic reinforced concrete beams

Carbon fiber-reinforced plastic (CFRP) reinforced concrete beams can fail due to interface debonding, due to the high tensile strength of such rebars. A set of 16 concrete beams reinforced with different amounts of CFRP reinforcement was subject to static three-point bending. The beam dimensions and CFRP reinforcements used were selected to demonstrate a transition from compression failure to bond failure with decreasing reinforcement ratio. It is shown that accurate bond strength data to predict such failures can be obtained from a “hinged-beam” test configuration, rather than the conventional direct “pull-out” tests. Deflection under service loads can also be predicted more accurately using a proposed equation that includes the reinforcement ratio and the elastic modulus of the reinforcement.     

Pattern evaluation for in-plane displacement measurement of thin films

Ultra-lightweight spacecraft incorporating “gossamer” structures are extremely compliant, which complicates control, design, and ground testing in full scale. One approach to model the behavior of a full-scale gossamer structure is to construct a small-scale model test article that can be used to verify a corresponding small-scale computer model. Once the predictions of the computer model have been verified by measurement of the physical test article, it can be scaled up to allow computation of the full-scale structure behavior. As model verification requires accurate deflection measurements at multiple points along the surface of the structure, a sensing system that provides full-field data without changing the dynamic response of the structure must be developed. Hence, an optical approach is taken. Since the thin films used in gossamer space structures are typically smooth and featureless, targets must be incorporated into the film surface to enable tracking of both in-plane and out-of-plane displacements. A krypton fluoride excimer laser system was used to etch 35 μm wide linear features approximately 0.1 μm into the surface metallization of both 50.8 μm polyester and 127 μm polyimide films. These optically diffuse surface features, designed mainly to investigate the precision of the laser etching method, were used as targets for ultra-close-range photogrammetry, the method chosen for displacement tracking. A force applied to the surface of the etched mirror (test article) produced in-plane and out-of-plane deformations that were resolved via ultra-close-range photogrammetry. To measure the in-plane tracking resolution, 1.5 and 3.0 mm circular dots were added (using ink) to the surface of the thin film, and some of these targets were tracked as the test article was translated on a precision linear stage. In-plane tracking resolution using ultra-close-range photogrammetry was related to the ground sample distance of the camera, which in this case was 51.25 μm pixel⁻¹ (equal to the ratio of sample dimension to number of pixels in the field of view). Using a manual technique to identify features of the etched pattern for tracking, the mean tracking error was about 13 μm (σ=43 μm). Using an automated, subpixel marking technique to identify the 1.5 mm circular targets, the mean tracking error was 22 μm (σ=13 μm). Neither of these methods achieved the desired 10 μm tracking resolution.     

Full Field Strain Measurement in Compression and Tensile Split Hopkinson Bar Experiments

The 3D image correlation technique is used for full field measurement of strain (and strain rate) in compression and tensile split Hopkinson bar experiments using commercial image correlation software and two digital high-speed cameras that provide a synchronized stereo view of the specimen. Using an array of 128 × 80 (compression tests) and 258 × 48 (tensile tests) pixels, the cameras record about 110,000 frames per second. A random dot pattern is applied to the surface of the specimens. The image correlation algorithm uses the dot pattern to define a field of overlapping virtual gage boxes, and the 3-D coordinates of the center of each gage box are determined at each frame. The coordinates are then used for calculating the strains throughout the surface of the specimen. The strains determined with the image correlation method are compared with those determined from analyzing the elastic waves in the bars, and with strains measured with strain gages placed on the specimens. The system is used to study the response of OFE C10100 copper. In compression tests, the image correlation shows a nearly uniform deformation which agrees with the average strain that is determined from the waves in the bars and the strains measured with strain gages that are placed directly on the specimen. In tensile tests, the specimen geometry and properties affect the outcome from the experiment. The full field strain measurement provides means for examining the validity and accuracy of the tests. In tests where the deforming section of the specimen is well defined and the deformation is uniform, the strains measured with the image correlation technique agree with the average strain that is determined from the split Hopkinson bar wave records. If significant deformation is taking place outside the gage section, and when necking develops, the strains determined from the waves are not valid, but the image correlation method provides the accurate full field strain history.     

Design of Inserts for Split-Hopkinson Pressure Bar Testing of Low Strain-to-Failure Materials

In the present study a new insert design is presented and validated to enable reliable dynamic mechanical characterization of low strain-to-failure materials using the Split-Hopkinson Pressure Bar (SHPB) apparatus. Finite element-based simulations are conducted to better understand the effects of stress concentrations on the dynamic behavior of LM-1, a Zr-based bulk metallic glass (BMG), using the conventional SHPB setup with cylindrical inserts, and two modified setups—one utilizing conical inserts and the other utilizing a “dogbone” shaped specimen. Based on the results of these computational experiments the ends of the dogbone specimen are replaced with high-strength maraging steel inserts. This new insert-specimen configuration is expected to prevent specimen failure outside the specimen gage section. Simulations are then performed to validate the new insert design. Moreover, high strain-rate uniaxial compression tests are conducted on LM-1 using the modified SHPB with the new inserts. An ultra-high-speed camera is employed to investigate the changes in failure behavior of the specimens. Additional experiments are conducted with strain gages directly attached to the gage section of the specimens to determine accurately their dynamic stress–strain behavior.     

Dynamic torsion testing of nanocrystalline coatings using high-speed photography and digital image correlation

The strength and ductility of microcrystalline and nanocrystalline tungstsen carbide-cobalt (WC-Co) cermets have been evaluated by employing a stored energy Kolsky bar apparatus, high-speed photography and digital image correlation. The test specimens were thin-walled tubular AI7075-T6 substrates 250 μm thick, coated with a 300 μm thick microcrystalline or nanocrystalline WC-Co layer with an average grain size of about 3 μm and 100 nm, respectively. Dynamic torsion experiments reported in this paper reveal a shear modulus of 50 GPa and a shear strength of about 50 MPa for both microcrystalline and nanocrystalline WC-Co coatings. The use of high-speed photography along with digital image correlation has shown that damage to the coating coincides with a significant softening on the stress-strain curve. The coating failed in mode III, and strong interactions between the crack faces were probably responsible for the increase in load after failure of the coating. The overall failure of the coating-substrate system was not brittle but rather progressive and controlled by the ductility of the aluminum substrate. A methodology for investigating damage kinetics and failure has been established. This methodology can be applied to examine the behavior of other advanced materials that can be manufactured as coatings on ductile substrates. Manufacturing coatings free of initial microcracks remains a significant challenge. Research on optimization of the spray deposition parameters should be pursued to produce high-quality nanostructured coatings that can fully exploit the benefits of nano-size grains.     

Dynamics of a “collective” bubble in a magma melt flow behind the decompression wave front

Specific features of the dynamics of the wave field structure and growth of a “collective” bubble behind the decompression wave front in the “Lagrangian” section of the formed cavitation zone are numerically analyzed. Two cases are considered: with no diffusion of the dissolved gas from the melt to cavitation nuclei and with the diffusion flux providing an increase in the gas mass in the bubbles. In the first case, it is shown that an almost smooth decompression wave front approximately 100 m wide is formed, with minor perturbations that appear when the front of saturation of the cavitation zone with nuclei is passed. In the case of the diffusion process, the melt state behind the saturation front is principally different: jumps in mass velocity and viscosity are observed in the vicinity of the free surface, and the pressure in the “collective” cavitation bubble remains unchanged for a sufficiently long time interval, despite the bubble growth and intense diffusion of the gas from the melt. It is assumed that the diffusion process (and, therefore, viscosity) actually become factors determining the dynamics of growth of cavitation bubbles beginning from this time interval. A pressure jump is demonstrated to form near the free surface.     

Tensile testing of polysilicon

Tensile specimens of polysilicon are deposited on a silicon wafer; one end remains affixed to the wafer and the other end has a relatively large paddle that can be gripped by an electrostatic probe. The overall length of the specimen is less than 2 mm, but the smooth tensile portion can be as small as 1.5×2m in cross section and 50m long. The specimen is pulled by a computer-controlled translation stage. Force is recorded with a 100-g load cell, whereas displacement is recorded with a capacitance-based transducer. Strain can be measured directly on wider specimens with laser-based interferometry from two small gold markers deposited on the smooth portion of the specimen. The strength of this linear and brittle material is measured with relative ease. Young's modulus measurement is more difficult; it can be determined from either the stress-strain curve, the record of force versus displacement or the comparison of the records of two specimens of different sizes. Specimens of different sizes—thicknesses of 1.5 or 3.5 m, widths from 2 to 50 m and lengths from 50 to 500 m—were tested. The average tensile strength of this polysilicon is 1.45±0.19 GPa (210 ±28 ksi) for the 27 specimens that could be broken with electrostatic gripping. The average Young's modulus from force displacement records of 43 specimens is 162±14 GPa (23.5 ±2.0×10³ ksi). This single value is misleading because the modulus values tend to increase with decreasing specimen width; that is not the case for the strength. The three methods for determining the modulus agree in general, although the scatter can be large.     

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.     

Combined shear/compression structural testing of asymmetric sandwich structures

Asymmetric sandwich technology can be applied in the design of lightweight, non-pressurized aeronautical structures such as those of helicopters. A test rig of asymmetric sandwich structures subjected to compression/shear loads was designed, validated, and set up. It conforms to the standard certification procedure for composite aeronautical structures set out in the “test pyramid”, a multiscale approach. The static tests until failure showed asymmetric sandwich structures to be extremely resistant, which, in the case of the tested specimen shape, were characterized by the absence of buckling and failure compressive strains up to 10,000 μ strains. Specimens impacted with perforation damage were also tested, enabling the original phenomenon of crack propagation to be observed step-by-step. The results of the completed tests thus enable the concept to be validated, and justify the possibility of creating a much larger machine to overcome the drawbacks linked to the use of small specimens.     

Mechanical properties of ultrananocrystalline diamond thin films relevant to MEMS/NEMS devices

The mechanical properties of ultrananocrystalline diamond (UNCD) thin films were measured using microcantilever deflection and membrane deflection techniques. Bending tests on several free-standing UNCD cantilevers, 0.5 μm thick, 20 μm wide and 80 μm long, yielded elastic modulus values of 916–959 GPa. The tests showed good reproducibility by repeated testing on the same cantilever and by testing several cantilevers of different lengths. The largest source of error in the method was accurate measurement of film thickness. Elastic modulus measurements performed with the novel membrane deflection experiment (MDE), developed by Espinosa and co-workers, gave results similar to those from the microcantilever-based tests. Tests were performed on UNCD specimens grown by both micro and nano wafer-seeding techniques. The elastic modulus was measured to be between 930–970 GPa for the microseeding and between 945–963 GPa for the nanoseeding technique. The MDE test also provided the fracture strength, which for UNCD was found to vary from 0.89 to 2.42 GPa for the microseeded samples and from 3.95 to 5.03 for the nanoseeded samples. The narrowing of the elastic modulus variation and major increase in fracture strength is believed to result from a reduction in surface roughness, less stress concentration, when employing the nanoseeding technique. Although both methods yielded reliable values of elastic modulus, the MDE was found to be more versatile since it yielded additional information about the structure and material properties, such as strength and initial stress state.     

Surface deformation measurements of a cylindrical specimen by digital image correlation

Planar digital image correlation has been extended to measure surface deformations of cylindrical specimens without physical contact for high-temperature situations. A single CCD camera acquires the surface image patterns of a section of a specimen in the undeformed and deformed states to determine two-dimensional displacements on a projection plane. Axial, circumferential and shear deformations are determined through curvature transformation on the two-dimensional projection displacement field. The resolution of this technique was determined for a cylinder of 22.23-mm diameter to be 3.5 μm for the axial displacement, 0.05 percent for the axial and shear strains and 0.08 percent for the circumferential strain when correlation computations are carried out over a field of 5 mm×5 mm.     

Rheological properties of fresh and cryopreserved human arteries tested in vitro

Cryopreservation is widely used to preserve blood vessels for a while but is controversially suspected to affect the mechanical behavior of these allografts. The aim of this study was to determine whether differences in the three-dimensional mechanical behavior exist or not between fresh and cryopreserved arteries. Using a previously developed experimental system, in vitro inflation tests were performed on twenty segments of human fresh and cryopreserved arteries, in static conditions. Opening angles were also measured from images of rings in zero-stress state. The initial reference state was chosen as the unloaded state and tests were performed on specimens stretched at natural “in vivo” length. Mechanical measures calculated are “natural” (Hencky) strains (finite deformations), “true” (Cauchy) stresses in radial, circumferential, and longitudinal directions as well as strain energy per unit volume. Tangent moduli are derived from radial and circumferential stress-strain characteristics using non-linear curve fitting. Values of incremental and pressure-strain elastic parameters, wall stiffness, and compliance per unit length are also calculated. Results are presented in terms of characteristics of stresses and strains in the three directions, axial force, tangent moduli vs strains or stresses, and energy per unit volume, for both types of artery, with reference to transmural pressure. Detailed numerical results are given at mean transmural pressure or in the physiological range. Significant differences are indicated by statistic Student T-tests. Results obtained show that significant differences exist between rheological properties of fresh and cryopreserved segments of human artery. Strains, stresses, axial force, strain energy, and wall stiffness values highlight those differences whereas elastic parameters, compliance, and opening angle do not. The usefulness of some parameters to compare the mechanical behavior existing between fresh and cryopreserved arteries is therefore underlined. Received: 3 January 2000 Revision received: 12 April 2000 Accepted: 8 May 2000     

Formation Mechanism of Cracks in Saturated Sand

The formation mechanism of “water film” (or crack) in saturated sand is analyzed theoretically and numerically. The theoretical analysis shows that there will be no stable “water film” in the saturated sand if the strength of the skeleton is zero and no positions are choked. It is shown by numerical simulation that stable water films initiate and grow if the choking state keeps unchanged once the fluid velocities decrease to zero in the liquefied sand column. The developments of “water film” based on the model presented in this paper are compared with experimental results.The project supported by the National Natural Science Foundation of China (40025103 and 10202024) and Key Laboratory of Mountain Hazards and Surface Process, Chinese Academy of Sciences. The English text was polished by Keren Wang.     

Evolution of perturbations of a charged interface between immiscible inviscid fluids in the interelectrode gap

Perturbations of the interface between two immiscible ideal fluids of finite thickness (the lower and upper fluids are the conductor and the dielectric, respectively) located in the gap between two electrodes are considered. In the cases of the “shallow” and “deep” upper fluid the dispersion relations of linear waves and their longwave expansions are found. The methods of determining the space-time evolution of an initial surface perturbation are developed on the basis of the linear approximation. In the cases of the “shallow” and “deep” upper fluid examples of the development of an initial perturbation of the “step” type are given. The development of an initial perturbation of the “step” type are also considered in the near-critical electric fields and in the case of degeneration of cubic dispersion.     

A Tensile Split Hopkinson Bar for Testing Particulate Polymer Composites Under Elevated Rates of Loading

A tensile split Hopkinson bar apparatus is developed for testing high strain rate behavior of glass-filled epoxy. The apparatus uses a specimen gripping configuration which does not require fastening and/or gluing and can be readily used for castable materials. Details of the experimental setup, design of grips and specimen, specimen preparation method, benchmark experiments, and tensile responses are reported. Also, the effects of filler volume fraction (0–30%) and particle size (11–42 μm) are examined under high rates of loading and the results are compared with the ones obtained from quasi-static loading conditions. The results indicate that the increase in the loading rate contributes to a stiffer and brittle material response. In the dynamic case lower ultimate stresses are seen with higher volume fractions of filler whereas in the corresponding quasi-static cases an opposite trend exists. However, the absorbed specific energy values show a decreasing trend in both situations. The results are also evaluated relative to the existing micromechanical models. The tensile response for different filler sizes at a constant volume fraction (10%) is also reported. Larger size filler particles cause a reduction in specimen failure stress and specific energy absorbed under elevated rates of loading. In the quasi-static case, however, the ultimate stress is minimally affected by the filler size.