Mechanical Properties Of Bulk Material Research Articles

Bend testing of micro-scale bulk metal specimens using a chip-scale test instrument

We describe bend testing on micro-scale specimens of 302 stainless steel, using a MEMS test instrument. Bend testing is a common way of measuring the flexural stiffness of structural materials across many size scales, from thin laminate sheets to large weldments. Whereas the stiffness of a material under tensile loading is given by the Young's Modulus, the flexural stiffness, or the stiffness in bending, is much lower. In the past two decades, conventional materials testing machines and the specimens themselves have undergone miniaturization for the purpose of evaluating the mechanical properties of miniaturized mechanical components such as sensors and biomedical implants, for which the smallest specimen dimension is typically around 1 mm [2]. Another driver for miniaturizing the testing apparatuses is to test materials with inherently small form factors such as wires and thin films [3]. Now the emerging 3D printing technology is creating another need for material property measurement at micrometer size scales, to accurately capture the property gradients resulting from the layered manufacturing. However, with ever increasing miniaturization comes increasing difficulty in specimen handling, gripping, and alignment. Concurrently, MEMS technology has been used over the past 2 decades to fabricate small actuators and sensors for mechanical testing of materials of thin films [4] or nanoscale materials such as nanowires. We seek to use the advantages of MEMS to study the mechanical properties of bulk materials rather than thin films, but at the micrometer scale. We believe this will result in greater accuracy and spatial resolution of property measurements of structural materials used in civil infrastructure, aerospace, transportation and energy industries, as well as characterizing manufacturing processes that lead to steep property gradients such as 3D printed components. Our approach is to use MEMS actuators as chip-scale re-useable test instruments into which small specimens sectioned from bulk materials can be inserted and tested [5], to reduce the cost and time to obtain large data sets and to allow the measurements to be done in-situ in harsh environments. We will describe the design of a micro-size 302 stainless steel specimen, and the use of a MEMS test instrument for performing the bend testing on the specimens. The specimen's gage section was 350 um long, 65 um wide and 25 um thick, and was made by lithographic etching of a foil. The MEMS test instrument was fabricated from silicon and glass wafers. The specimens were inserted into the MEMS test chip and the silicon actuator applied static bending loads to the specimen. Displacements were measured from optical microscope images, and the force was calculated from the applied voltage and the known (measured) stiffness of the silicon actuator. The applied force from the MEMS actuator was measured directly, without any specimen, using a custom table-top force probe and load cell apparatus, and was in agreement with the force calculated from the applied voltage. The flexural stiffness of the micro specimens were measured, using the MEMS test device, at 90 – 130 N/m. These values were validated by independently testing the specimens with the much larger table-top force probe. We thus show that our MEMS test chip can be used to perform bending tests on micro scale specimens of bulk materials, but with a 1000-fold reduction in size compared to table-top force-measuring apparatuses.

Novel correction methods on a miniature tensile device based on a modular non-standard layout

A novel in situ tensile device with a large output load–volume ratio was developed for testing the mechanical properties of bulk materials. A major characteristic of the device was the modular non-standard layout, as the specimen was placed on the top plane of the device to approach the lens of an optical microscope or the electron gun of a scanning electron microscope. Accordingly, to investigate the effects of non-standard layout on tensile properties, displacement and load correction methods were given by formulas based on theoretical calculations to describe the specimen's actual tensile displacement and load. Based on in situ observation, the feasibility of the correction method was verified by comparing it with the data from metallographic microscope images. The bending effects on the specimen's vertical displacements and tensile load due to the installation mode were also discussed. This paper presents a modularized correction method for a horizontal-type tensile device with a non-standard layout design.

Fast determination of parameters describing manufacturing imperfections and operation wear of nanoindenter tips

Nanoindentation is an effective technique for determining mechanical properties of bulk materials and thin films. Prevailing measurement uncertainties in nanoindentations by Vickers or Berkovich diamond pyramids are commonly caused by manufacturing imperfections of the indenter's side angles and tip sharpness. Moreover, the tip geometry changes over the indenter operation time, due to diamond wear.The paper presents a fast method for estimating diamond indenters' tip nano and micro geometry. This method is based on a combination of nanoindentations on Si(100), used as reference material, with FEM supported calculations of Martens hardness. The hardness calculations are conducted using an equivalent indenter tip geometry with geometrical characteristics, which may vary for a specific set of parameters, describing the real indenter with manufacturing imperfections. These parameters are varied in successive iterations until the calculated hardness converges with that of the reference material.For a quick determination of these parameters, the software package “TIDE” (TΙp Deviations Estimation) has been developed. “TIDE” is based on numerous FEM supported simulation results of nanoindentations, conducted on the reference material, varying the indenter tip geometry. By this software package, a quick prediction of nanoindenters' tip equivalent geometry is facilitated, also for anticipating changes due to wear of the diamond over time.

A Novel Tensile Device for In Situ Scanning Electron Microscope Mechanical Testing

To facilitate the study of deformation mechanisms and mechanical properties of bulk materials with feature size of centimeter level, a novel tensile device compatible with scanning electron microscope (SEM) was designed and built. Integrating the servo motor and three-stage reducer, the device could realize quasistatic loading mode with a loading speed of 10 nm/s. The device also presents broad compatibility with various types of SEMs due to its miniaturized dimensions and larger volume–output load ratio compared with existing and commercial tensile instruments. A small lead precise ball screw with left- and right-hand thread was adopted to keep the center of the specimen remain stationary without moving during the tensile testing. A novel gripping method was carefully taken into account to guarantee the alignment issues. The closed-loop control mode of the tensile process was developed. The displacement resolution of 40 nm was tested to verify the driving performance of the device. Furthermore, correction method on testing displacement was investigated based on the calibration experiments of the load sensor and displacement sensor, and the comparison tests based on stress–strain curve were carried out between the self-made device and the commercial tensile instrument (Instron 3345) to verify the feasibility and universality of the correction method.

Finite element model based elastoplastic contact model and some aspects on indentation response

Indentation tests are useful in evaluating the response of materials. They can be applied to determine the mechanical properties of bulk materials, thin films and coatings. Control of some factors that affect the test and the resulting property evaluation is important. Finite element model based elastoplastic contact analysis of a spherical indenter on a flat half space has revealed some interesting behaviours, which highlight certain limitations associated with the response. Under elastoplastic behaviour, it is observed that the constraint factor value sometimes defies the limit set for the validity of the analysis, and the variation is sensitive to yield strength, with a decreasing effect on the critical depth of deformation. Failure to acknowledge such effects will lead to erroneous evaluation of the response. The present work highlights the observations of the numerical simulation results and some experimental data on static indentation response evaluation of metallic and sprayed ceramic composite coatings.

Finite element and dimensional analysis algorithm for the prediction of mechanical properties of bulk materials and thin films

In this work, the applicability of a new algorithm for the estimation of mechanical properties from instrumented indentation data was studied for thin films. The applicability was analyzed with the aid of both three-dimensional finite element simulations and experimental indentation tests. The numerical approach allowed studying the effect of the substrate on the estimation of mechanical properties of the film, which was conducted based on the ratio h max /l between maximum indentation depth and film thickness. For the experimental analysis, indentation tests were conducted on AISI H13 tool steel specimens, plasma nitrated and coated with TiN thin films. Results have indicated that, for the conditions analyzed in this work, the elastic deformation of the substrate limited the extraction of mechanical properties of the film/substrate system. This limitation occurred even at low h max /l ratios and especially for the estimation of the values of yield strength and strain hardening exponent. At indentation depths lower than 4% of the film thickness, the proposed algorithm estimated the mechanical properties of the film with accuracy. Particularly for hardness, precise values were estimated at h max /l lower than 0.1, i.e. 10% of film thickness.

Failure of Wheels Due to Improper Manufacturing Process

Premature cracking of wheels was observed in the prime movers used for logistic services. The fractures were located between the stud-holes and between stud-holes and hand-holes. A detailed study was carried out to investigate root cause(s) of these failures. Although chemical composition and mechanical properties of bulk material were the same, there was a difference in microstructure and hardness at the edges of stud-holes. In addition, metallography revealed the presence of burr at the circumference of the stud-holes. A number of hairline cracks and a severely distorted structure were also observed at this location. Fractography, using an SEM, confirmed the presence of sharp cracks at the root of the burr. The hardness at the edge was also higher than the bulk material. The problem was found to be due to the manufacturing process which may be inducing more work hardening and deformation than permissible at regions near the holes edges. This deformation and hardening created a material condition that led to fine cracks in this region. The resulting fine cracks grew during service under fatigue and caused premature failure of the wheels.

From the bottom up – measuring bulk mechanical properties via microtechnology

The micromechanics community has developed a large set of tools for measuring the properties of microand nano-scaled materials. These new tools have been successfully applied to a range of materials, most often in thin-film form, to determine the critical length scales below which bulk properties no longer describe the behaviour of materials, and to understand the underlying mechanisms behind the divergent behaviour. These tools, essential for the characterization of smallscale materials, have recently been applied in a new strategy to evaluate the mechanical properties of bulk materials through microsized testing in situations where conventional testing is challenging such as in extreme or harsh environments (e.g., temperature extremes, radiation exposure, corrosive chemistries, etc.). In this work, the initial results from a microtechnology-based tensile technique developed for the evaluation of bulk materials properties via microscale test specimens sectioned from bulk materials are described.

Effect of resin impregnation on the prevention of crack propagation for bulk Y–Ba–Cu–O superconductors

We studied the mechanical properties of bulk Y–Ba–Cu–O superconductors by applying cyclic loads using a three-point bending test setup. X-ray tomography was used to monitor the presence of cracks inside the sample. We also evaluated the adhesive forces between the epoxy resins and bulk Y–Ba–Cu–O superconductors using the tensile test. X-ray tomographic observations showed that cracking was mainly formed along the cleavage plane, although the stress was applied perpendicular to this plane. We confirmed that a strong adhesive force between the resin and bulk materials was responsible for the improvement of the mechanical properties of bulk materials.

An evaluation of mechanical properties of YBaCuO and (Sm,Gd)BaCuO bulk superconductors using Vickers hardness test at cryogenic temperatures

Vickers hardness properties of bulk superconductors have been investigated to estimate the mechanical properties of bulk materials. However, the test at cryogenic temperatures had not been conducted at all. Then we conducted Vickers hardness test on (001) plane of YBaCuO and (Sm,Gd)BaCuO bulk superconductors from 40 to 293 K. The hardness of YBaCuO and (Sm,Gd)BaCuO increased with decreasing temperature up to 19 000 MPa and 10 000 MPa, respectively. Furthermore, the cracks caused by the indentation were observed with SEM, and the fracture toughness K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IC</sub> at cryogenic temperatures was calculated. The K <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">IC</sub> of YBaCuO and (Sm,Gd)BaCuO decreased with decreasing temperature to 0.6 MPa√m and 0.8 MPa√m, respectively.

Resin-impregnated bulk YBCO current leads for Maglev

Recent advancement in the fabrication techniques has allowed the production of large grain bulk RE–Ba–Cu–O (RE: rare earth elements) superconductors with large critical current density. The current leads can be machined from such large disks. However, the stress produced by refrigeration or electromagnetic induction sometimes causes cracking. It is therefore essentially important to improve mechanical properties of bulk materials to enhance their current carrying capability. In this study, we report on a novel technique to dramatically improve the mechanical properties of the current lead made of bulk superconductors, in which epoxy resin is impregnated into the bulk materials. At 77 K, a large current of 1000 A could be passed along the RE–Ba–Cu–O current lead without the transition from superconducting to normal state. The heat generation was also found to be much smaller than the current lead employed in the present Maglev model, showing that bulk RE–Ba–Cu–O is promising for current lead applications for Maglev.

Influence of He implanted surface layer on the mechanical properties of Fe-3%Si alloy

Change in the mechanical properties of single and polycrystals of Fe-3%Si alloys implanted with 100 keV He + ions is investigated at room and liquid nitrogen temperatures. The main results are as follows: the implanted single crystals exhibit the increase in yield stress with dose; Coarse slip bands are formed at a dose of 3 × 10 20 ions m −2; the implantation to a dose of 4 × 10 21 ions m −2 leads to cleavage fracture at liquid nitrogen temperature, which results from surface cracks associated with deformation twins; the implanted polycrystals show stress-strain curves similar to those of the non-implanted ones at room temperature, while surface cracks are formed at grain boundaries as well as in grains at 4 × 10 21 ions m −2; Surface cracks at grain boundaries cause cleavage at liquid nitrogen temperature. These results indicate that the hardening and embrittlement of He-implanted layers markedly affect the mechanical properties of bulk materials.

Nanoindentation, microscratch, friction and wear studies of coatings for contact recording applications

A nanoscale monolithic slider-suspension produced by photolithography is used for contact recording. The contact pad consists of a multilayered structure consisting of SiC, amorphous hydrogenated carbon (a-C:H), Al 20 3, Si, and CoNbZr films. In this study, we have compared hardness, Young's modulus of elasticity, and scratch resistance or adhesion of various coatings deposited on a single-crystal silicon wafer by nanoindentation and microscratch techniques and friction and wear performance by sliding against a diamond tip and sapphire ball in reciprocating mode. SiC coatings exhibit the highest hardness, about 27 GPa, and the highest elastic modulus, about 255 GPa. Microscratch data indicate that SiC and a-C:H coating exhibit the highest resistance to scratching or debonding from the substrate. During scratching, an Al 20 3 coating deforms like a ductile metal rather than like a ceramic. Si and CoNbZr coatings exhibit ploughing of the tip into the sample surface and debris generation right in the beginning of the scratch. SiC coatings exhibit the best wear performance against a diamond tip as well as a sapphire ball. For comparisons, we also made mechanical property measurements on bulk materials used in conventional recording: NiZn ferrite, Al 20 3-TiC, and SiC (under development). The bulk Ni-Zn ferrite sample was found to be damaged by grain pull-out during scratching even at a low load of 3 mN. Bulk Al 20 3-TiC exhibits unexpected ploughing of the sample right from the beginning of the scratch. Bulk SiC did not exhibit any signs of significant damage up to a normal load of about 15 mN. Overall comparison of mechanical properties of bulk materials and coatings suggest that SiC is the most desirable coating in the contact pad for low wear. An SiC coating is also recommended as an overcoat for thin film magnetic disks.