Output Bar Research Articles

The Acute Effect of Various Doses of Caffeine on Power Output and Velocity during the Bench Press Exercise among Athletes Habitually Using Caffeine.

Background: Previously studies confirm ergogenic effects of caffeine (CAF); however there is no available scientific data regarding the influence of acute CAF intake on power output in athletes habitually consuming CAF. The main goal of this study was to assess the acute effect of 3, 6, 9 mg/kg/b.m. doses of CAF intake on power output and bench press bar velocity in athletes habitually consuming CAF. Methods: The study included 15 healthy strength-trained male athletes (age = 26.8 ± 6.2 years, body mass = 82.6 ± 9.7 kg; BMI = 24.8 ± 2.7; bench press 1RM = 122.3 ± 24.5 kg). All participants were habitual caffeine consumers (5.2 ± 1.2 mg/kg/b.m.; 426 ± 102 mg of caffeine per day). This study had a randomized, crossover, double-blind study design where each participant performed four different experimental sessions, with one week interval between each trial. In every experimental session participants performed bench press, three sets of five repetitions at 50% 1RM. The power output and bar velocity assessments under four different conditions: a placebo (PLAC), and three doses of caffeine ingestion: 3 mg/kg/b.m. (CAF-3), 6 mg/kg/b.m. (CAF-6) and 9 mg/kg/b.m. (CAF-9). Results: The statistical significance was set at p < 0.05. The repeated measures ANOVA between PLAC and CAF-3; CAF-6; CAF-9 revealed no statistically significant differences in power output and velocity of the bar during the bench press exercise. A large effect size (ES) in mean power-output was found between PLAC and CAF-9 in Sets 1 and 2. A large ES in peak power-output was found between PLAC and CAF-6 in Set 2, and between PLAC and CAF-9 in Sets 1 and 2. A large ES in peak velocity was found between PLAC and CAF-9 in Sets 1–3. Conclusion: The results of the present study indicate that acute doses of CAF before exercise does not have a significant effect on power output and bar velocity in a group of habitual caffeine users.

A new testing method for the dynamic response of soft cellular materials under combined shear–compression

This paper presents a new method based on the split Hopkinson pressure bar (SHPB) to perform dynamic combined shear–compression tests on cellular materials. For this purpose, the input bar is designed to have a beveled end at the specimen side, and the output bar is rotatable to form an angle with the input bar axis and keep perpendicular to the beveled end of input bar. The specimen is glued onto the input and output bars to avoid slippage during loading. Different shear–compression loading states can be realized by changing the beveled end of the input bar and accordingly rotating the output bar. Such a testing method is checked by simulating the whole loading process with FE model. It shows that this combined shear–compression method provides a quite accurate measurement on the specimen forces and deformation in both shear and normal directions. At last, this loading system is applied to test the shear and normal behaviors of 5052 aluminum honeycomb in TW plane at different loading angles. Good reproducibility is achieved for the applied material in terms of normal stress/normal displacement and shear stress/shear displacement curves. The experiment results reveal that the normal strength of the honeycomb will be weakened with an increasing shear component at high strain rate loading, which is consistent with our previous conclusions.

Determination of length to diameter ratio of the bars in torsional Split Hopkinson bar

Split Hopkinson bar is a well-known device for mechanical characterization of materials under dynamic loading. The device can be compressive, tensile, and torsional or shear. The effect of dimensions of SHB in particular the length to diameter ratio, L/D, of the bars has been the subject of some investigations over the past several decades. However, no work has so far been performed on the forgoing ratio in torsional Split Hopkinson bar (TSHB). In this work, the length to diameter ratio of the input or output bar, L/D, is studied by numerical simulation, theory through Lagrange diagram of the TSHB and experiment. It has been found that for L/D>80, the incident, reflected and transmitted wavelengths obtained from simulations and Lagrange diagram are the same. This value of L/D is quite consistent with that usually recommended for compressive and tensile versions of SHB. The numerical model of TSHB is validated by Lagrange diagram and experiment.

Hybrid Split Hopkinson Pressure Bar to Identify Impulse-dependent Wave Characteristics of Viscoelastic Phononic Crystals

There has recently been a rising interest in the nonlinear wave transmission behavior of phononic crystals. However, experimental studies focusing on the nonlinear wave transmission behavior of phononic crystals have been predominantly performed on 1-D granular crystals using customized impact apparatus. In this study, we explore split Hopkinson pressure bar (SHPB) apparatus as a tool to study the nonlinear wave characteristics of a 1-D continuum viscoelastic phononic crystal. In order to resolve experimental challenges relating to signal-to-noise ratios and input impulse magnitudes, we propose a hybrid SHPB system composed of an aluminum input bar and a nylon output bar. For a considered viscoelastic phononic crystal, the application of the hybrid SHPB apparatus enabled us to observe some low transmission frequency zones, which were not identified from the linearly perturbed settings such as the analytical solution and the electrodynamic shaker tests. We further conducted a series of additional FE simulations to ensure the appearance of impulse-dependent low transmission frequency zones of the considered viscoelastic phononic crystal specimen. The additional sets of simulations evidently illustrate the impulse-dependent evolution of wave transmission coefficients, and demonstrate that the impulse-dependent wave transmission behavior can be experimentally investigated by adopting the hybrid SHPB apparatus. Thus, this study shows that the conventional SHPB apparatus can be employed effectively to study the emerging research field of nonlinear wave characteristics of phononic crystals.

An SHPB Test Study on Stress Wave Energy Attenuation in Jointed Rock Masses

A series of dynamic tests were carried out to investigate the effect of joint roughness on the wave energy attenuation in rock masses and estimate the relation between joint roughness and seismic quality factor of rock masses. The modified split Hopkinson pressure bar (SHPB) apparatus was adopted in this study, where the loading, input and output bars were made of gypsum. The propagation coefficient of the gypsum bars was measured from trial tests. According to the propagation coefficient of the gypsum bars, the strain, stress and particle velocity on the contact surfaces between the specimen and input/output bars were obtained from the test data recorded by the strain gauges. The specimens were prepared by a three-dimensional printer with plaster and binder. Each specimen modeled a rock mass with one joint with different roughness. The seismic quality factor of specimen is also estimated from the proposed approach of wave energy dissipation. The effects of joint roughness on the seismic quality factor of rock mass and the wave energy attenuation across the rock mass are analyzed from test results.

Effects of Geometrical Parameters on Stress Wave Propagation across the Rough Joint

The contact surface of the rough joint has geometrical parameters as the joint matching coefficient (JMC) and the distribution pattern of the contact segments. The specimen used in the modified SHPB test got the artificial joint by sawing some notches on the surface contacted to the output bar. Different assemblies of the notches formed various contact areas and distribution patterns. Using the modified SHPB tests data, the altered thin-layer interface model was used to analyze the effects of geometrical parameters on one-dimensional stress wave transmission and energy dissipation across a single rough joint. It revealed that the transmission coefficient decreased with the diminution of JMC, and it increased with the scattered distribution pattern as the similar trend for each JMC set. As for the energy coefficients, with the decrease of JMC, the transmission energy coefficient reduced sharply, but it increased very slowly with the reflection energy coefficient and irreversible energy coefficient. It revealed that the JMC of 0.5 was the critical point of the energy dissipation. More energy transmitted across the joint rather than reflecting back and dissipating, when JMC &gt; 0.5. Nevertheless, the irreversible energy coefficient was much larger than the transmission and reflection coefficients, when JMC &lt; 0.5.

Impact response of low-density foam impinging onto viscoelastic bar: A theoretical analysis

The Taylor-Hopkinson test is widely used in predicting the dynamic strength of materials. For low-density foam materials, the output bar in the Taylor-Hopkinson test is often made from much weaker materials like polymethylmethacrylate (PMMA) to reduce the wave impedance mismatch between the foam sample and the output bar. These low impedance materials exhibit viscoelastic properties and may affect the dynamic stress prediction of foam samples. In this paper, the shock wave theory for the foam sample in conjunction with the viscoelastic wave propagation in the output bar is employed to illustrate the influence of the viscoelastic output bar on the dynamic response of foam samples in the Taylor-Hopkinson impact. The theoretical model presented in this paper can be degenerated into previous elastic target bar model and rigid target bar model. Finite element simulations are further performed to verify the proposed theory. The influence of material parameters on the final deformation of the foam sample, impact duration and residual velocity of the foam sample are investigated. The results in this paper not only help in understanding the effect of the viscoelastic bar on the dynamic response of foam samples, but also set up guidelines to perform the Taylor-Hopkinson test more precisely.

High speed impact test on rock specimens with a compressive stress pulse

In exploration for a methodology to analyze fast deformation of a thick specimen where a state of equilibrium cannot be assumed, high-speed impact tests on pumice-stone and granite sample specimens are conducted with Hopkinson bar test apparatus. A compressive stress pulse from the input bar is applied to the specimens ranging from 8 mm to 30 mm in thickness. The stresses on the front (input-bar) side and the rear (output-bar) side of the specimen are evaluated separately based on the strain gauge signals from the input and output bars. Optical emission from the specimen is captured with a photomultiplier. Analyses on the time variation of the strain gauge and photomultiplier signals lead to the following observations. (a) The front-side stress plays an important role in the deformation and fracture, (b) the transition from the elastic to plastic regime can be estimated from oscillatory features observed in the front-side stress and other quantities derived from the strain gauge signals, (c) the optical emission is highly correlated with the front side stress and step-wise energy loss in the specimen, and (d) the main frequency components of the stress wave associated with the energy-loss are found in the same frequency band as conventional acoustic emissions observed in rock fractures. A question remains, and provides an interesting subject for future study, regarding the step-wise energy loss observed in the strain gauge signal unaccompanied by the optical emission. In view of a recent fracture mechanical theory that states the evolution of fracture is accompanied by a reduction in frequency on the emission spectrum, it is possible that these energy loss is associated with emission in frequency ranges outside the sensitivity of the photomultiplier.

Sensor calibration of polymeric Hopkinson bars for dynamic testing of soft materials

Split Hopkinson pressure bar (SHPB) testing is one of the most common techniques for the estimation of the constitutive behaviour of metallic materials. In this paper, the characterisation of soft rubber-like materials has been addressed by means of polymeric bars thanks to their reduced mechanical impedance. Due to their visco-elastic nature, polymeric bars are more sensitive to temperature changes than metallic bars, and due to their low conductance, the strain gauges used to measure the propagating wave in an SHPB may be exposed to significant heating. Consequently, a calibration procedure has been proposed to estimate quantitatively the temperature influence on strain gauge output. Furthermore, the calibration is used to determine the elastic modulus of the polymeric bars, which is an important parameter for the synchronisation of the propagation waves measured in the input and output bar strain gate stations, and for the correct determination of stress and strain evolution within the specimen. An example of the application has been reported in order to demonstrate the effectiveness of the technique. Different tests at different strain rates have been carried out on samples made of nytrile butadyene rubber (NBR) from the same injection moulding batch. Thanks to the correct synchronisation of the measured propagation waves measured by the strain gauges and applying the calibrated coefficients, the mechanical behaviour of the NBR material is obtained in terms of strain-rate–strain and stress–strain engineering curves.

Improving data interpretation from SHTB tests on ductile metals

This work presents a methodology to calibrate a strength model for ductile metals, based on dynamic tension tests of relatively long Dog-Bone specimens conducted on a Split Hopkinson Tension Bar (SHTB). We address the main difficulties involved in conducting and interpreting such tests, namely the duration of the loading pulse needed to deform long specimens and the non-uniform stress and strain distributions along the specimen due to neck formation. The first issue is addressed by using the waves‘ reflections within the output bar, as explained below. When the first loading (tension) wave does not cause failure of the specimen, a reflected compression wave travels from the specimen‘s bar end to the free bar‘s end. Upon reaching the free end this latter compression wave is reflected again as a second tension wave, which travels back along the bar until it reaches the specimen and loads it the second time. This enables further deformation of the specimen, practically doubling the loading pulse duration without changing the striker‘s length. The second issue is addresses by using full numerical simulations of the experimental setup, including the striker, the bars and the specimen. This way, the full dynamic behaviour of the specimen is taken into account, eliminating the need to consider specimen equilibrium and taking into account the current strain rate in the specimen as it deforms. Hence, model calibration can be done from the very start of plastic deformation and without the need to keep the strain rate constant during deformation. As a result, it is possible to reliably calibrate the strength model considering necking and neck location, as well as plastic heating which is a significant factor in the plastic deformation of ductile metals.

A Constant Acoustic Impedance Mount for Sheet-Type Specimens in the Tensile Split-Hopkinson Bar

This paper addresses a problem well-known amongst practitioners of the Split-Hopkinson Tension Bar method: attaching a flat test specimen made from sheet material to the cylindrical input-and output bars. To date, slotting the bar ends and gluing the specimens into these ends with high-strength adhesives is the gold standard. However, this approach is not universally applicable because some materials are difficult to bond, and the adhesion surface is limited by the bar diameter, meaning that only small width specimens can be tested. In contrast, the hitherto published mechanical clamping mechanisms typically introduce excessive additional mass into the Split-Hopkinson system which detrimentally affects wave propagation and thus causes errors in the stress-strain signals. We circumvent this problem by designing a mechanical clamping device which has the same acoustic impedance as the bar material and is suitable to securely attach specimens with a width larger than the bar diameter. The benefits of our new clamping device are demonstrated by reporting tensile stress-strain data for Polycarbonate at high strain rates. The data is free from unwanted oscillations and enables accurate determination of dynamic strength and stiffness.

Effect of a viscoelastic target on the impact response of a flat-nosed projectile

Taylor impact is a widely used strategy in which a flat-nosed projectile is fired onto a rigid anvil directly to determine the dynamic strength of rod specimens. Nowadays, the rigid anvil is often replaced by an output target bar to ensure the accuracy of measurement via recording strain signals in the output bar. For testing the dynamic strength of low-density materials, a low-impedance target bar, which exhibits viscoelastic characteristics is often employed. In this paper, an extended Taylor model is proposed to improve the idealization of treating the target bar as perfectly rigid material in the classic Taylor model, and the viscoelastic effect of the target bar is incorporated. The viscoelastic target bar is depicted by two elastic springs and one dashpot. Based on the plastic shock wave theory in the flat-nosed projectile associated with the viscoelastic wave analysis in the target bar, the viscoelastic effect of the target bar on the impact response of the flat-nosed projectile is investigated. The finite element simulation is also carried out to verify the theoretical model, and good agreement is found. The present theoretical model is also called the Taylor-cylinder Hopkinson impact, which provides a more accurate way to identify the dynamic material parameters. The dynamic responses of the present model are further compared with previous elastic and rigid target bar models. It is found that the viscoelastic effect of the target bar should be taken into consideration in the Taylor-cylinder Hopkinson impact test for low-impedance materials.

Redistributing load using wearable resistance during power clean training improves athletic performance

A popular method to improve athletic performance and lower body power is to train with wearable resistance (WR), for example, weighted vests. However, it is currently unknown what training effect this loading method has on full-body explosive movements such as the power clean. The purpose of this study was to determine what effects WR equivalent to 12% body mass (BM) had on the power clean and countermovement jump (CMJ) performance. Sixteen male subjects (age: 23.2 ± 2.7 years; BM: 90.5 ± 10.3 kg) were randomly assigned to five weeks of traditional (TR) power clean training or training with 12% BM redistributed from the bar to the body using WR. Variables of interest included pre and post CMJ height, power clean one repetition maximum (1RM), peak ground reaction force, power output (PO), and several bar path kinematic variables across loads at 50%, 70%, and 90% of 1RM. The main findings were that WR training: (1) increased CMJ height (8.7%; ES = 0.53) and 1RM power clean (4.2%; ES = 0.2) as compared to the TR group (CMJ height = −1.4%; 1RM power clean = 1.8%); (2) increased PO across all 1RM loads (ES = 0.33–0.62); (3) increased barbell velocity at 90% 1RM (3.5%; ES = 0.74) as compared to the TR group (−4.3%); and (4) several bar path kinematic variables improved at 70% and 90% 1RM loads. WR power clean training with 12% BM can positively influence power clean ability and CMJ performance, as well as improve technique factors.

An SHPB test study on wave propagation across rock masses with different contact area ratios of joint

The Split Hopkinson Pressure Bar (SHPB) apparatus was adopted to investigate the influence of joint contact area and spatial geometry of joint surface on the dynamic property of rock joint and wave propagation. The specimen was comprised of two rock cylinders. The top surfaces of the two cylinders contacted with each other, and their bottom surfaces contacted with the input and output bars of the SHPB apparatus, respectively. One top surface of one cylinder was sawn to shape a number of notches, while the other surfaces of the two cylinders were smooth and flat. The artificial rock joint was modeled as the contacted top surfaces of the two cylinders. The area ratio of contact between joint surfaces equals to the joint matching coefficient (JMC). The incident, transmitted and reflected waves were recorded from the strain gauges mounted on the input and output bars. Then, the transmission and reflection coefficients for strain wave propagation across the rock specimen were obtained. Based on the basic theory of SHPB tests, the stress on the specimen, the deformation of rock specimen and the stress-closure relation of joint were analyzed. The experimental results show that the JMC and the spatial geometry of joint surface affect not only the dynamic behavior of joint but also the stress wave propagation.

Impact Combined Shear-Compression Testing of Honeycombs Using a Rotatable Hopkinson Bar

This paper presents a new method based on the split Hopkinson pressure bar (SHPB) to perform impact combined shear-compression test for cellular materials. For this purpose, a bevelled end is cemented to the input bar and the output bar are rotatable to be parallel to the inclined plane of the bevelled end. The system uses the friction between the specimen and the pressure bars to apply the combined shear compression loading on the honeycomb specimen. Such a testing method is validated by the simulation of the whole loading system (split bar + specimen) using ABAQUS code. It shows that this combined shear-compression test provides a quite accurate measurement. Tests on the 5052 aluminium honeycombs are performed. The shear stress-strain behaviour and the compressive behaviour are separated. The experiment result confirms previous testing results and reveals that the shear component will weaken the compressive strength of the honeycomb at high strain rate.

A Further Study on Wave Propagation Across a Single Joint with Different Roughness

As a key dynamic feature of the rock mass, joint results in wave attenuation when a stress wave propagates across it. An experimental study has obtained the relation between the transmission coefficient and the contact area ratio of joints with different thicknesses (Chen et al. (doi: 10.1007/s00603-015-0716-z)). However, the spatial geometry of the contact surface also determines the dynamic behavior of the discontinuous masses. Thus, the present study is focused on the effects of distribution and dimension of the sawn notches on the contact surface of the joint, which lead to the different spatial geometry. The joint matching coefficient (JMC) was used in this study to define the contact condition. Using a modified SHPB apparatus, all the bars and specimens were aluminum, and the artificial joint formed a rough surface of the specimen contacted to the output bar. Based on the wave separation method, the incident, reflected and transmitted waves across the joint were acquired from the records of the strain gauges on pressure bars. Then the transmission coefficient and specific stiffness of the joint were obtained. Comparisons of them were made on different JMCs, spatial geometries and thicknesses of the joint.

Experimental Characterization and Modified Constitutive Modeling of the Strain Rate Dependent Compressive Behavior of Adhesives

Predicting the impact performance of adhesively bonded structures requires the accurate characterization and constitutive formulation of the compressive behavior of the adhesive at dynamic strain rates. In this work, the stress–strain curves of Sikaflex adhesives are measured in uniaxial compression tests at quasi‐static and dynamic strain rates. The split‐Hopkinson pressure bar technique is modified with the hollow output bar to effectively characterize the dynamic response of the adhesive. It is found that the Sikaflex adhesive is incompressible and hyperelastic; the compressive behavior is significantly influenced by the strain rate. The strain rate sensitivity is not constant but varies as a function of the strain and the strain rate. The strain rate effect on compressive behavior is quantified and incorporated into the phenomenological hyperelastic constitutive model based on strain energy density. The modified constitutive equation accurately represents the strain rate dependent compressive behavior of Sikaflex adhesives. image

Novel approach to computational simulation of cross roll straightening of bars

A new finite element formulation, aimed at the modeling of cross roll straightening of circular bars is presented. It is based on Eulerian description of elastoplastic material flow through a stationary finite element mesh, which is fixed in space and coupled with the multi-roller straightening machine. This approach leads to extremely effective computational algorithm which provides the relation between input parameters like curvature, diameter and material of the bar, straightening rollers geometry and intermeshing on one hand and output bar curvature and residual stress distribution on the other. Using this strategy, multiple repetition of the straightening process analysis for variable intermeshing scheme is a feasible task, aimed to optimization of roller intermeshing. Basic features of the algorithm are described, some verification results presented and compared with real experiments realized in industrial conditions. The industry was also a motivation for the optimization study, presented in the last part of paper. The results prove that the suggested solution strategy is a mighty tool for effective analysis of cross roll straightening and show a great potential for other types of straightening processes as well.

Application of photon Doppler velocimetry to direct impact Hopkinson pressure bars

Direct impact Hopkinson pressure bar systems offer many potential advantages over split Hopkinson pressure bars, including access to higher strain rates, higher strains for equivalent striker velocity and system length, lower dispersion, and faster achievement of force equilibrium. Currently, these advantages are gained at the expense of all information about the striker impacted specimen face, preventing the experimental determination of force equilibrium, and requiring approximations to be made on the sample deformation history. In this paper, we discuss an experimental method and complementary data analysis for using photon Doppler velocimetry to measure surface velocities of the striker and output bars in a direct impact bar experiment, allowing similar data to be recorded as in a split bar system. We discuss extracting velocity and force measurements, and the precision of measurements. Results obtained using the technique are compared to equivalent split bar tests, showing improved stress measurements for the lowest and highest strains in fully dense metals, and improvement for all strains in slow and non-equilibrating materials.

Understanding the anomalously long duration time of the transmitted pulse from a soft specimen in a kolsky bar experiment

The reason for the observation of an anomalously long pulse duration time of a signal transmitted from a soft specimen in a Kolsky bar experiment is investigated. A systematic numerical experiment based on explicit finite element analysis has been carried out using elastic specimens with varying elastic moduli. The soft specimen is in a compressive state even after the passage of the pulse because it cannot push the bars, such that a fairly long transmitted pulse is monitored at the gage position of the output bar. The arrival of the second pulse (tensile) from the right-end surface of the output bar releases the first transmitted signal (compressive) coming from the compressed state of the specimen, which puts an end to the fairly long transmitted pulse. Because the observation of a prolonged transmitted pulse is a natural phenomenon, the prolonged transmitted pulse should not annoy researchers in the process of confirming the validity of experiments on soft specimens. Our concern in verifying the validity of experiments can be limited to other aspects such as the process of measurement and the amplification of weak transmitted signals coming from soft specimens.