The transient force exerted by a low-speed liquid droplet impinging onto a flat rigid surface is investigated experimentally. The measurements employ a high-sensitivity piezo-electric sensor, along ...with a high-speed camera, and cover four decades in droplet Reynolds number and greater than two decades in Weber number. Across these ranges, the peak of individual force profiles span from 3 mN to over 1300 mN. Once normalised, the force–time profiles support the existence of an inertially dominated self-similar regime. Within this regime, previous numerical and theoretical studies predict a
$\sqrt{t}$
dependence of impact normal force during the initial pre-peak rise. While our measurements confirm this finding, they also indicate that, after the peak force the profiles exhibit an exponential decay. This long-time decay law suggests treatment of the momentum transport from the droplet using a lumped model. An observed linear dependence between the force and force decay rate supports this approach. The reason for the efficacy of treating this system via a lumped model apparently connects to the physics right at the surface that limit the rate of momentum transport from the droplet to the surface. This is explored by estimating the momentum transfer by solely using the deforming droplet shape, but under the condition of negligible momentum gradients within the droplet. The short- and long-time solutions are combined and the resulting model equation is shown to accurately cover the entire force–time profile.
In this paper, an analytical model is used to design a coil, called a Uniform Pressure Actuator (UPA), for use during electromagnetic forming (EMF) and magnetic pulsed welding (MPW) by combining and ...extending past efforts by other researchers. The UPA offers increased forming efficiency and repeatability, as well as a robust design. Magnetic pressure applied to the workpiece and workpiece velocity are predicted to ensure impact conditions are sufficient for MPW. The UPA is constructed and tested experimentally to validate the accuracy of the analytical model, as well as verify assumptions made during modeling. The coupling coefficient introduced in the magnetic analysis is experimentally determined and compared to previous researchers’ values. Workpiece velocities for various energy levels, workpiece thicknesses, and materials with various conductivities and densities are compared to analytical predictions and show good agreement for the initial acceleration in the process. Workpiece velocity measurements were obtained using a Photon Doppler Velocimetry (PDV) system, which provides a robust method for measuring velocities with submicron displacement and temporal resolution in the nanosecond range. Uniformity of the workpiece deformation is also examined, which is an advantage of the UPA.
The application of electrical current to conductive metals during deformation processes has been shown to reduce the flow stress required, increase the potential elongation prior to failure, and ...decrease the springback observed. These are believed to be beyond what can be explained by elevated temperatures due to resistive heating alone, i.e., an additional electro-plastic effect is supposed to be relevant. Further experimental data (including force, strain distribution, and temperature) for a standard deformation process would provide insight into the effects involved. In this paper, results from electrically-assisted (EA), three-point bending of C260 (brass) sheet metal with a 0.5mm thickness are presented. The strain distributions are measured using a stereomicroscope system with digital imaging correlation (DIC) while temperature was measured using a thermal imaging camera. Results show that the strain distribution through the thickness was more uniform for the EA case (in particular for coarse grain structures) which has not been shown in past literature. Also, the bending force decreased only during a temperature spike at the beginning of the process. Therefore for this specific application, an electroplastic effect was not observed with 40A/mm2 being applied to the specimens, and variations observed were simply caused by temperature effects.
Hertzian contact of a rigid sphere and a highly deformable soft solid is investigated using integrated photoelasticity. The experiments are performed by pressing a styrene sphere of 15 mm diameter ...against a 44 × 44 × 47 mm3 cuboid made of 5% wt. gelatin, inside a circular polariscope, and with a range of forces. The emerging light rays are processed by considering that the retardation of each ray carries the cumulative effect of traversing the contact-induced axisymmetric stress field. Then, assuming Hertz's theory is valid, the retardation is analytically calculated for each ray and compared to the experimental one. Furthermore, a finite element model of the process introduces the effect of finite displacements and strains. Beyond the qualitative comparison of the retardation fields, the experimental, theoretical, and numerical results are quantitatively compared in terms of the maximum equivalent stress, surface displacement, and contact radius dimensions. A favorable agreement is found at lower force levels, where the assumptions of Hertz theory hold, whereas deviations are observed at higher force levels. A major discovery of this work is that, at the maximum equivalent stress location, all three components of principal stress can be determined experimentally and show satisfactory agreement with theoretical and numerical ones in our measurement range. This provides valuable insight into Hertzian contact problems since the maximum equivalent stress controls the initiation of plastic deformation or failure. The measured displacement and contact radii also reasonably agree with the theoretical and numerical ones. Finally, the limitations that arise due to the linearization of this problem are explored.
The plasticity and formability of a commercially-pure aluminum sheet (AA1100-O) is assessed by experiments and analyses. Plastic anisotropy of this material is characterized by uniaxial and ...plane-strain tension along with disk compression experiments, and is found to be non-negligible (e.g., the r-values vary between 0.445 and 1.18). On the other hand, the strain-rate sensitivity of the material is negligible at quasistatic rates. These results are used to calibrate constitutive models, i.e., the Yld2000-2d anisotropic yield criterion as the plastic potential and the Voce isotropic hardening law. Marciniak-type experiments on a fully-instrumented hydraulic press are performed to determine the Forming Limit Curve of this material. Stereo-type Digital Image Correlation is used, which confirms the proportional strain paths induced during stretching. From these experiments, limit strains, i.e., the onset of necking, are determined by the method proposed by ISO, as well as two methods based on the second derivative. To identify the exact instant of necking, a criterion based on a statistical analysis of the noise that the strain signals have during uniform deformation versus the systematic deviations that necking induces is proposed. Finite element simulation for the Marciniak-type experiment is conducted and the results show good agreement with the experiment.
A ubiquitous experiment to characterize the formability of sheet metal is the simple tension test. Past research has shown that if the material is repeatedly bent and unbent during this test (
, ...Continuous-Bending-under-Tension, CBT), the percent elongation at failure can significantly increase. In this paper, this phenomenon is evaluated in detail for AA-6022-T4 sheets using a custom-built CBT device. In particular, the residual ductility of specimens that are subjected to CBT processing is investigated. This is achieved by subjecting a specimen to CBT processing and then creating subsize tensile test and microstructural samples from the specimens after varying numbers of CBT cycles. Interestingly, the engineering stress initially increases after CBT processing to a certain number of cycles, but then decreases with less elongation achieved for increasing numbers of CBT cycles. Additionally, a detailed microstructure and texture characterization are performed using standard scanning electron microscopy and electron backscattered diffraction imaging. The results show that the material under CBT preserves high integrity to large plastic strains due to a uniform distribution of damage formation and evolution in the material. The ability to delay ductile fracture during the CBT process to large plastic strains, results in formation of a strong fiber texture throughout the material.
There is extensive evidence in the literature that plastic deformation of metals is associated with an increase in Acoustic Emission (AE) activity. Thus, AE measurement techniques have the potential ...to monitor a forming process in real time and provide a signal for feedback control, to exploit optimum formability. In this work, custom-made AE sensors employing piezoelectric crystals are implemented to measure the emitted acoustic signal during uniaxial tension and cup drawing tests of an AA6013-T4 aluminum sheet (1.5 mm thick). The uniaxial tension tests are conducted with two AE sensors clamped to each end of the specimen gage section, along with full-field surface strain measurement using Digital Image Correlation (DIC) techniques. The AE signals along with the interrogation of the DIC images reveal that the maximum AE amplitude corresponds to the onset of diffuse necking, i.e., when the strain field starts to become spatially inhomogeneous. Interestingly, this onset occurs before the maximum force is attained. Comparing these observations to a model of dislocation activity supports the notion that dislocation is the main driver of AE activity. With these findings, AE measurements are performed in a cup drawing process where a custom-made Marciniak-type punch incorporates three AE sensors. These sensors are used to triangulate and determine the location of necking and eminent fracture based on the time difference of arriving signals to each sensor. The results from the cup drawing tests show that AE signals can identify the onset of necking and accurately predict the location of necking and fracture.
This paper reports the main results from an experimental investigation into the improved elongation-to-fracture (ETF) of the advanced high strength steel (AHSS) dual-phase (DP) 1180 by subjecting the ...material to the continuous-bending-under-tension (CBT) process. The investigation is carried out using a recently developed testing apparatus, where the specimen flows in a reciprocating fashion through a set of three rollers while it is continuously pulled in tension. The process parameters such as the roller depth defining the amount of bending and wrapping around the rollers and crosshead velocity applying the tensile force to the specimen are varied to maximize the ETF of the material. From the recorded force vs. displacement curves along the rolling direction (RD), 45°, and transverse direction (TD), ETF of DP 1180 is found to improve with the crosshead velocity and with the bending depth up to a certain level, after which it decreases. The optimal parameters of 1.35 mm/s for the crosshead velocity and 3.5 for the normalized bending depth improve ETF of the material over five times in every testing direction relative to simple tension tests. The role of CBT in preserving high integrity of the material to large plastic strains is discussed.
Display omitted
•Elongation-to-fracture of DP 1180 steel under continuous-bending-under-tension is investigated.•Over five-times improved elongation of DP 1180 is achieved by continuous-bending-under-tension relative to simple tension.•Continuous-bending-under-tension facilitates uniform depletion of ductility preventing a localized flow followed by necking.•Strength of the material can be significantly improved by continuous-bending-under-tension processing.
In earlier contributions, we discussed continuous-bending-under-tension (CBT) experiments on AA6022-T4. We found that CBT significantly enhanced the elongation-to-fracture and strength, over uniaxial ...tension. In the present paper, our understanding of CBT is expanded beyond these experimental observations, with the aid of material modeling and numerical simulations of the process. Cyclic tension-compression experiments were performed on this material, using strain histories that are expected to replicate the loading during CBT, i.e., different combinations of constant strain amplitude and linearly increasing mean value, to failure. During these experiments, a limited but not negligible amount of kinematic hardening was discovered. Some of these experiments are used for calibration of a combined isotropic-kinematic hardening model, while the rest are used for experimental validation of the model. The modeling framework is based on a rate-independent, associated flow rule with the von Mises yield criterion as the plastic potential. Isotropic hardening is introduced by a simple, exponential-decay model of the growth of the yield surface with plastic deformation. Non-linear kinematic hardening is introduced by a 4-term, Chaboche-type model. The large strain hardening curve is identified by extrapolation, an approach that is validated later in the work and contrasted with alternative options. This material modeling framework is introduced in finite element models of the CBT process. The model is meshed with linear, reduced-integration elements, with 7 elements through the thickness. It is found that the numerical model reproduces the experimental force-displacement curve, including the succession of spikes and plateaus typical of CBT, very closely. The model also replicates the development of strain on the surface during CBT, and compares well with post-test strain measurements. After these validations, the model is used to probe the mechanics of the CBT process, e.g., the development of stress and strain through the thickness and per cycle, the location and onset of failure, as well as the failure angle, which in CBT differs from the localized neck angle found in a typical uniaxial tension experiment.
This paper presents an experimental setup for continuous-bending-under-tension (CBT) of thin strips and sheets, aimed at the investigation of the observed elongation-to-fracture (ETF) enhancements ...under such conditions. In particular, the kinematics of the process are described and correlated with the evolution of axial force measured during testing. The main results of the effect of process parameters, such as crosshead velocity and bending depth, on the ETF and the reduction of axial force are presented for the aluminum alloy AA6022-T4. The force vs. displacement response is not found to be a strong function of the crosshead velocity, which is explained by the near rate-independence of the material in the strain-rate range that occurs in CBT. Similarly, ETF does not show a clear dependence on the bending depth or the crosshead velocity. On the other hand, the axial force decreases linearly with the bending depth and slightly increases with the velocity. From the detailed analysis, the optimal parameters for enhancing ductility of AA6022-T4 under CBT are 1.2 mm/s for crosshead velocity and 2.0 for normalized bending depth. Under these conditions, a significant finding is that the strain level achieved throughout the CBT specimen is comparable to that in the necked region of a specimen tested to rupture in uniaxial tension.
PDF
