Tensile Apparatus Research Articles

Stress Wave Attenuation in Aluminum Alloy and Mild Steel Specimens Under SHPB Tensile Testing

Investigations on the effect of intensity of incident pressure wave applied through the striker bar on the specimen force histories and stress wave attenuation during split Hopkinson pressure bar (SHPB) tensile testing are presented. Details of the tensile SHPB along with Lagrangian x–t diagram of the setup are included. Studies were carried out on aluminum alloy 7075 T651 and IS 2062 mild steel. While testing specimens using the tensile SHPB setup, it was observed that the force calculated from the transmitter bar strain gauge was smaller than the force obtained from the incident bar strain gauge. This mismatch between the forces in the incident bar and the transmitter bar is explained on the basis of stress wave attenuation in the specimens. A methodology to obtain force histories using the strain gauges on the specimen during SHPB tensile testing is also presented. Further, scanning electron microscope images and photomicrographs are given. Correlation between the microstructure and mechanical properties is explained. Further, uncertainty analysis was conducted to ascertain the accuracy of the results.

Reducing drag force on polyester fabric through superhydrophobic surface via nano-pretreatment and water repellent finishing

Energy saving is a very important factor for athletics that works in high speed and award winning competition. Drag force is of prime importance for determining the speed of objects that moves in the fluid. Nanotechnology is of great potential among the methods for reducing drag force. Enhancing the fabric speed in the fluids could be achieved using superhydrophobic surfaces to reduce the drag force. A simple and versatile method was utilized for the fabrication of superhydrophobic surfaces via pad-dry-cure and in situ synthesis of ZnO nanoparticles as a pre-treatment. The treated samples were examined by SEM for assessing the structure and size of nanoparticles and mapping and EDX analysis was carried out for further characterization. A novel method was designed based on the tensile tester apparatus for studying the changes in drag force. The results indicated a reduction of 80% in drag force of the treated fabric in the optimum conditions of pre-treatment with 10% zinc nitrate (Zn(NO3)2) and 5% sodium hydroxide (NaOH) at 130 °C for 1 h.

A consistent experimental protocol for the strain rate characterization of thermoplastic fabrics

AbstractThis paper introduces an experimental procedure aiming at performing a consistent mechanical characterization of textile composites behavior, under static and dynamic loadings, and particularly their macroscopic strain rate‐dependent mechanisms. The procedure includes the design and validation of a reduced specimen adapted to dynamic testing, which guarantees the consistency of the identified macroscopic properties under a wide range of strain rates. An interrupted high speed tensile apparatus was also developed to investigate the strain rate sensitivity of the nonlinear constitutive behavior from = 10 − 4 s − 1 to 102 s − 1. The originality of this experimental device lies in its ability to stop high speed loadings before ultimate failure, at adjustable, accurate strain levels. Finally, an extensive characterization campaign was performed. Results reveal a high strain rate dependency of the linear and nonlinear behavior. The procedure therefore generates consistent characterization data that may trustworthily be used for modeling purposes.

Stress relaxation and stress-strain characteristics of porcine amniotic membrane

Recently, amniotic membrane (AM) as scaffold is accumulating much more attention in tissue engineering. It is well-known that the mechanical properties of the scaffold inevitably affect the biological process of the incorporated cells. This study investigates the stress relaxation and stress-strain characteristics of AM, which have not been sufficiently elucidated before. Porcine AM samples were prepared at four different AM regions and at three different directions. Ramp-and-hold and stretch-to-rupture tests were conducted on a uniaxial tensile apparatus. A nonlinear viscoelastic model with two relaxation coefficients is proposed to fit the ramp-and-hold data. Rupture strain, rupture stress, and elastic modulus of the linear portion of the stress-strain curve are used to characterize the strength properties of the AM. Sample direction has no significant effect on the mechanical properties of the AM. Samples at the ventral region has the maximum rupture strength and elastic modulus, respectively, 2.29±0.99MPa and 6.26±2.69MPa. The average of the relaxation coefficient for the fast and slow relaxation phases are 12.8±4.4s and 37.0±7.7s, respectively. AM is a mechanically isotropic and heterogeneous material. The nonlinear viscoelastic model is suitable to model the AM viscoelasticity and potential for other biological tissues.

Direct three-dimensional imaging of the fracture of fiber-reinforced plastic under uniaxial extension: Effect of adhesion between fibers and matrix

The three-dimensional morphology and a mechanical property of a fiber-reinforced plastic (FRP) have been investigated under uniaxial extension. A custom tensile apparatus for X-ray computerized tomography was developed for this purpose. The two FRPs used in the present study consisted of nylon 6 with glass fibers. In one of the FRPs, the fiber surfaces were treated to adhere to the nylon 6. It was observed that the fibers tended to align along the extension axis, and that cavitation occurred simultaneously during the extension. In the case of the FRP with glass fibers without any surface modification (hereafter, referred as “neat” glass fibers), void formation was dominant over the whole extension range. In the case of the FRP with surface treatment, fiber alignment occurred first and was followed by void formation. A numerical simulation was carried out to study the stress concentration around a fiber with such morphological changes during extension. Through quantitative measurements of fiber orientation and void volume, together with predictions from the numerical simulations, the effect of fiber/matrix adhesion on the morphological developments and mechanical properties of the FRPs was discussed.

Nano-structuration effect on the mechanical behavior of gold thin films studied by 2D synchrotron x-ray diffraction

Thin film technology is pervasive in microelectronics, and in optical, magnetic or micro-mechanical devices. The mechanical performance of such nanoscale structure is crucial for applications since it is related to device lifetime. Furthermore, the mechanical behavior of nanostructured materials is still not well known. In this work, nanostructured gold thin films with different thicknesses have been elaborated by sequenced ion beam sputtering to study size effect. The grain size is controlled by stopping the grain growth during the thin film growth. Using the biaxial tensile setup developed at the DiffAbs beamline of the French synchrotron facility SOLEIL, x-ray diffraction (XRD) measurements have been performed during controlled biaxial deformation tests on gold thin films deposited on Kapton substrate. Strain analysis of the gold thin films with different grain size and architecture has been achieved for a non equi biaxial loading with a force ratio of 0.8. XRD allows measuring the intra-granular strains using the so-called sin2ψ method while the macroscopic in-plane strains are measured simultaneously thanks to Digital Image Correlation. By analyzing the mechanical response of the different films, we conclude that gold thin films follow a plastic deformation mode whatever the grain size or thin film architecture.

Evaluation of tensile load-strain characteristics of geogrids through in-soil tensile tests

This paper evaluates in-soil tensile load-strain characteristics of geogrids with the help of a custom designed and developed in-soil tensile setup in the laboratory. Displacement controlled in-soil tensile tests were carried out to evaluate the effect of normal stress, soil type, and presence of sand-sandwiched layer, on the tensile load-strain characteristics of geogrid. Confinement of geogrid within the soil and application of normal stress were found to increase the mobilized tensile load and secant tensile stiffness of geogrid. Secant stiffness improvement factors were determined to quantify the improvement in tensile load-strain characteristics of geogrid under confinement, on comparison to in-isolation values. Geogrid was observed to exhibit lower secant tensile stiffness when embedded in marginal soil, moist-compacted at wet of optimum. However, the concept of sand-sandwiched geogrid was found to improve the tensile load-strain behaviour of geogrids embedded in marginal soil compacted at wet of optimum.

Experimental studies on mixed‐mode dynamic fracture behaviour of aluminium alloy plates with narrow U‐notch

AbstractMixed‐mode dynamic fracture behaviour of cast aluminium alloy ZL205A thin plates with narrow U‐notch was studied by split Hopkinson tensile bar apparatus. Specimens with different loading angles were designed to realize different fracture modes. The same loading condition was maintained during the tests. Recovery specimens show that crack propagates along the notch direction. Force–elongation relations show that with the loading angle increasing, the fracture force increases while the final elongation decreases. Deformation and fracture process was observed by a high‐speed camera. Displacement distribution around the crack was calculated through digital image correlation technique. Based on the photos and displacement results, initiation time of the crack was derived. Besides, two stress components (normal stress and shear stress) applied on the fracture surface were investigated. Results show that crack initiation stresses at different loading angles satisfy the ellipse equation. Pure mode I and II fracture stresses are 425.3 and 236.7 MPa, respectively. Furthermore, specific fracture energy of different specimens was calculated. The energy data vary with loading angle and located on an approximate upward parabolic curve. From the curve, the minimum specific fracture energy of the thin plate specimen is 42.0 kJ/m2 under loading angle of 76.3°.

A preliminary technical study on sodium dodecyl sulfate-induced changes of the nano-structural and macro-mechanical properties in human iliotibial tract specimens

IntroductionAcellular scaffolds are frequently used for the surgical repair of ligaments and tendons. Even though data on the macro-mechanical properties related to the acellularization process exist, corresponding data on the nano-structural properties are still lacking. Such data would help identify target proteins of the formed extracellular matrix that are chemically altered by the acellularization. In this study we examined the altered structure by comparing molecular properties of collagens from native and acellular iliotibial tract samples to the macroscopic stress–strain behavior of tract samples. Material and methodsMatched pairs of five human iliotibial tract samples were obtained from five donors (mean age 28.2±4.7 years). One of each pair was acellularized using 1vol% sodium dodecyl sulfate (SDS) for 7 days. 13C magic angle spinning nuclear magnetic resonance spectroscopy (13C CP MAS NMR) was utilized to compare the collagen overall secondary structure and internal dynamics of collagen-typical amino acid proteins. The resulting data was compared to age-matched stress–strain data of tract samples obtained in an uniaxial tensile setup and histologically. ResultsTypical and nearly identical collagen 13C CP MAS NMR spectra were found in the tract samples before and after acellularization with SDS. The characteristic collagen backbone remained intact in the native and acellular samples. Collagen molecular composition was largely unaltered in both conditions. Furthermore, a similar dynamic behavior was found for the amino acids Hyp γ, Pro α/Hyp α, Ala α, Gly α and Ala β. These minute alterations in the collagens’ molecular properties related to acellularization with SDS were in line with the similarly minute changes in the macro-mechanical tensile behavior, such as the elastic modulus and ultimate stress. Histology showed intact type I collagens, minute amounts of elastins before and after acellularization and evidence for acellularization-induced reductions of proteoglycans. DiscussionNano-structural properties of collagens are minutely affected by SDS treatment for acellularization, indicated by the molecular composition and dynamics. The lacking acellularization-related changes in the molecular structure properties of collagens in iliotibial tract samples are in line with the small alterations in their macro-mechanical tensile behavior. Though the given setup approaches soft tissue mechanics from both scaling extremes of mechanical testing, further structural analyzes are needed in a larger sample size to substantiate these findings.

Influence of epichlorohydrin content on structure and properties of high‐ortho phenolic epoxy fibers

ABSTRACTThe high‐ortho phenolic epoxy fibers (HPEFs) were prepared by the crosslinking of heat‐meltable spun filaments derived from melt‐spinning of the novolac epoxy resins copolymerized among phenol, formaldehyde, and epichlorohydrin (ECH) in the presence of zinc acetate and sulfuric acid catalyst, and cured in a combined solution of formaldehyde and hydrochloric acid. The resulting fibers were heat‐treated in N2 at elevated temperature. Infrared (IR) spectrometer, thermogravimetric analysis (TGA), scanning electron microscope (SEM), and electrical tensile strength apparatus were employed to characterize the change of functional groups, thermal performance, microstructure of fibers, and mechanical properties. The results show that the addition of ECH in the precursor resin can increase the content of long alkyl ether linkage, and gain the peak of thermal stability and mechanical strength. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43375.

New Development to Measure Mode I. Fracture Toughness in Rock

Tensile fracture toughness is one of the dominant characteristics of rocks that play an important role in fracture mechanics of rock structures. In spite of the substantial amount of work that has been conducted as suggested methods to determine the fracture toughness of rock in Mode I., research on new methods are still demanded. The compact tension (CT) specimen is widely used to determine the fracture toughness of metals and is a standard method in accordance with ASTM standard. To conduct similar direct tests in rock, preparation of rock specimen a difficult task and needs some special tools to create notched sample. To address these issues, experimental techniques based direct tensile apparatus have been developed. Developed experimental procedure involves the use of the specimens having a central hole and two notches or cracks at both inner side of proposed specimen. The stress intensity factor formula at the crack tip of a CT specimen can be availed through the literature. However a new expression of stress intensity factor is needed for the developed test in order to account geometry and loading configuration. To develop a new stress intensity factor expression, the numerical models of suggested method were worked out. The focus of the current work is on development of the experimental technique, which involves determining the optimum sample size and shape, loading procedure, fracture toughness for different rock types, and verification of suggested method based on CT test which has been carefully considered in this investigations.

Mechanical properties of porous ITM alloy

Abstract Mechanical characteristics of porous ITM are assessed for the use as metallic substrate in solid oxide fuel cell stacks. Elastic modulus and Poisson's ratio are determined using an impulse excitation technique, thermal expansion is measured with an optical dilatometer, and ultimate tensile strength and creep are measured in a tensile set-up. Data are compared and discussed with respect to the properties of dense ITM. The results indicate that the coefficient of thermal expansion is not affected by the porosity, while the Poisson's ratio is only affected to a minor extent. The coefficient of thermal expansion and elastic modulus appear to be influenced by a ferromagnetic-paramagnetic transition. The elastic modulus of the porous material is, as expected, reduced due to smaller effective area of the porous specimens. The largest change compared to the behavior observed for the dense ITM is seen for the ultimate tensile strength. The large creep stress exponent of the dense and porous material over a wide stress range is a result of the dispersed nano-sized Y-rich phase, however, a change in creep mechanism is indicated for the porous ITM for the low-stress region.

Influence of packing density and surface roughness of vertically-aligned carbon nanotubes on adhesive properties of gecko-inspired mimetics.

We have systematically studied the macroscopic adhesive properties of vertically aligned nanotube arrays with various packing density and roughness. Using a tensile setup in shear and normal adhesion, we find that there exists a maximum packing density for nanotube arrays to have adhesive properties. Too highly packed tubes do not offer intertube space for tube bending and side-wall contact to surfaces, thus exhibiting no adhesive properties. Likewise, we also show that the surface roughness of the arrays strongly influences the adhesion properties and the reusability of the tubes. Increasing the surface roughness of the array strengthens the adhesion in the normal direction, but weakens it in the shear direction. Altogether, these results allow progress toward mimicking the gecko's vertical mobility.

A New Method for Testing Formability in Sheet Metal Forming at Biaxial Tensile State

This paper presents a new method concerning testing formability in sheet metal forming, especially focuses on clarifying the divergence of the experiment and a variety of theoretical predictions on biaxial tensile state. Up to now, there are many different fracture criteria appeared. All researches have presented their experimental data which could justify the criterion they presented. However, the experimental results and predictions in the first quadrant of the forming limit diagram (FLD) often diverge. Today, limiting dome height test is commonly used for FLD experiment, but specimens are rubbed and bended during the test, both influencing the experimental results.In order to provide for convincible experimental data, this paper presents a new experimental method to establish the first quadrant of FLD. In this method, cruciform biaxial tensile specimen and biaxial tensile apparatus have been developed. The proposed specimen has the feature of thickness reduction and contour design to ensure the fracture location is in the central region, so that accurate biaxial tensile state can be obtained. Through this method, there is an opportunity to obtain the whole FLD using uniaxial tensile testing machine, which is a low-cost alternative in compared with limiting dome height test. Besides, the experimental results can be utilized to clarify the divergence between various theoretical predictions and experimental results in the first quadrant of the FLD.

Determination of Johnson–Cook Parameters for Cast Aluminum Alloys

A series of experiments were conducted to determine the Johnson–Cook parameters for three different cast aluminum alloys, namely, A356, A357, and F357. Room temperature compression experiments were performed under varying rates of loading ranging from 10−3 s−1 to 5000 s−1. High temperature compression (235 °C and 435 °C) experiments were performed at an average strain rate of 5000 s−1. A split Hopkinson pressure bar (SHPB) apparatus was utilized in conjunction with an induction coil heating system for applying dynamic loading at elevated temperatures. In addition, experiments were performed under high strain rate tensile loading using tensile SHPB apparatus, and the fractured specimens were examined under scanning electron microscope (SEM) to understand the failure modes in these alloys. High-speed photography was used to capture the chronological progression of the deformation under dynamic tensile loading. The results indicated that all the three cast aluminum alloys were sensitive to strain rate and temperature. A356 exhibited the least value of flow stress under both static and dynamic loading conditions, and the highest elongation before break under dynamic tensile loading. The SEM images of the fractured specimens under dynamic tensile loading showed characteristics of transcrystalline ductile fracture in these cast aluminum alloys.

Influence of stress triaxiality and strain rate on the failure behavior of a dual-phase DP780 steel

To better understand the in-service mechanical behavior of advanced high-strength steels, the influence of stress triaxiality and strain rate on the failure behavior of a dual-phase (DP) 780 steel sheet was investigated. Three flat, notched mini-tensile geometries with varying notch severities and initial stress triaxialities of 0.36, 0.45, and 0.74 were considered in the experiments. Miniature specimens were adopted to facilitate high strain rate testing in addition to quasi-static experiments. Tensile tests were conducted at strain rates of 0.001, 0.01, 0.1, 1, 10, and 100s−1 for all three notched geometries and compared to mini-tensile uniaxial samples. Additional tests at a strain rate of 1500s−1 were performed using a tensile split Hopkinson bar apparatus. The results showed that the stress–strain response of the DP780 steel exhibited mainly positive strain rate sensitivity for all geometries, with mild negative strain rate sensitivity up to 0.1s−1 for the uniaxial specimens. The strain at failure was observed to decrease with strain rate at low strain rates of 0.001–0.1s−1; however, it increased by 26% for an increase in strain rate from 0.1 to 1500s−1 for the uniaxial condition. Initial triaxiality was found to have a significant negative impact on true failure strain with a decrease of 32% at the highest triaxiality compared to the uniaxial condition at a strain rate of 0.001s−1. High resolution scanning electron microscopy images of the failure surfaces revealed a dimpled surface while optical micrographs revealed shearing through the thickness indicating failure occurred via ductile-shear. Finite element simulations of the tests were used to predict the effective plastic strain versus triaxiality history within the deforming specimens. These predictions were combined with the measured conditions at the onset of failure in order to construct limit strain versus triaxiality failure criteria.

Effect of discrete fibre reinforcement on soil tensile strength

The tensile behaviour of soil plays a significantly important role in various engineering applications. Compacted soils used in geotechnical constructions such as dams and clayey liners in waste containment facilities can suffer from cracking due to tensile failure. In order to increase soil tensile strength, discrete fibre reinforcement technique was proposed. An innovative tensile apparatus was developed to determine the tensile strength characteristics of fibre reinforced soil. The effects of fibre content, dry density and water content on the tensile strength were studied. The results indicate that the developed test apparatus was applicable in determining tensile strength of soils. Fibre inclusion can significantly increase soil tensile strength and soil tensile failure ductility. The tensile strength basically increases with increasing fibre content. As the fibre content increases from 0% to 0.2%, the tensile strength increases by 65.7%. The tensile strength of fibre reinforced soil increases with increasing dry density and decreases with decreasing water content. For instance, the tensile strength at a dry density of 1.7 Mg/m3 is 2.8 times higher than that at 1.4 Mg/m3. It decreases by 30% as the water content increases from 14.5% to 20.5%. Furthermore, it is observed that the tensile strength of fibre reinforced soil is dominated by fibre pull-out resistance, depending on the interfacial mechanical interaction between fibre surface and soil matrix.

Strain rate sensitivity and mechanical anisotropy of selective laser melted 17-4 PH stainless steel

Quasi-static and dynamic tensile testing was performed upon machined tensile specimens fabricated from bulk primitives produced by the consolidation of water atomized, 17-4 precipitation hardened stainless steel powder by layer-based, selective laser melting. Such mechanical evaluation was performed by a screw-driven uniaxial tension testing machine and a split-Hopkinson tensile bar apparatus. Strain rates evaluated include 10-3, 10-1 and 103 s-1. Prior to tensile testing, specimens underwent additional thermal processing in accordance with industry standards. Evaluations of the solution heat treatment and peak-age conditions were made alongside similarly prepared, but traditionally processed specimens meeting the same material standard for chemistry (drawn rod). Tensile strength across all strain rates is higher for these selective laser melted (SLM) specimens as a result of microstructure refinement through rapid melt, solidification and cooling during processing. Ultimate tensile and yield strengths increase with increasing strain rate and show no preferential direction relative to the building direction. Elongation anisotropy is observed as a consequence of directional porosity stemming from pores limited to within individual layers. Specimens loaded normal to the SLM building plane commonly rupture at small elongation because of this mechanical fibering from the selective laser melting process.

A simple constrained uniaxial tensile apparatus for in situ investigation of film stretching processing

A simple constrained uniaxial tensile apparatus was designed and constructed to obtain stress-strain curve during stretching and subsequent structural evolution of polymeric films. Stretch is carried out through two motor driven clamps in the machine direction and scissor-like clamps in the transverse direction keeping the sample width constant. The force information during film stretching process is recorded by a tension sensor and structural evolution can be obtained by in situ X-ray scattering technique. All parameters related to film stretching manufacturing, such as temperature, draw ratio, and stretching speed can be set independently, making the apparatus an effective method to explore the relationship between processing parameters and structure.

Dynamic and static failure behavior of notched CFRP laminate investigated by digital image correlation

Static and dynamic fracture behavior of notched CFRP laminates are investigated experimentally. The dynamic tensile loading apparatus is developed based on a split Hopkinson bar. Impact tensile tests for notched (open-hole) CFRP specimens are performed. The full-field strain around the hole in the specimen is evaluated employing digital image correlation method using photographs obtained using a high-speed camera at a frame rate of 1,000,000 frames/s. Far-field strain at the failure is obtained from the specimen-mounted strain gauge. Increased strain around the hole is observed over time. A static tensile test for the notched CFRP specimen is then conducted as well. The strain distribution and far-field strain at failure are obtained similarly. The results show that the far-field strain at failure varies with the strain rate. The strain distributions obtained from two tests are compared and a mechanism causing the difference is discussed. The results show that high strain is distributed along the transverse direction to the loading direction from the hole for the impact (high-strain-rate) tests, although high strain exists on the side hole in loading direction for the static tests. The difference is possibly attributable to the difference of internal damage, which depends on the strain rate.