Experimental Techniques最新文献

The bake hardening effect in steel, characterized by a boost in yield strength resulting from initial deformation and subsequent heat treatment, holds significant promise for the automotive sector. Bake hardening is characterized by the Bake Hardening Index (BHI), as outlined in the test standards EN 10325:2006 and JIS G 3135:2018. The suggested test procedure involves pre-straining the dogbone samples and subjecting them to thermal cycling. However, the current standards lack specificity regarding the pre-straining procedure and the estimation of elastic recovery in metals, leading to inconsistencies in results across different materials and laboratories. This study addresses this gap by proposing a standardized and fully automated method utilizing in-situ strain measurements to precisely pre-strain uniaxial tension samples to any desired level of plastic strain. This method offers precise control over strains and accurate estimation of strain overcompensation, which accounts for elastic recovery during unloading. In this paper, the proposed method is validated using optical strains using digital image correlation, however the proposed method is independent of the choice of strain measurement method and works with clip-on extensometers, optical extensometers and even the crosshead transducers in the load frame. The foundation and effectiveness of the proposed method are demonstrated through statistical data obtained from testing 11 different steel grades, including bake hardenable steels and Advanced High Strength Steels commonly used in the automotive industry. The proposed method is independent of the choice of steel grade and consistently delivers satisfactory results for all tested steel grades, producing test samples with plastic strains that adhere to the specifications of the testing standards.

Structures can exhibit non-linear behavior due to the presence of free-play. To analyze this behavior, different numerical and analytical methods like the substructure synthesis method or the harmonic balance method are used. To determine the effect of non-linarites caused by free-play, descriptive functions must be identified. The restoring force approach is frequently used in systems where mass and stiffness matrices are known, such as finite element models. Another approach involves frequency response functions obtained through vibration tests.

This article discusses the second approach, which involves extracting the free-play function using frequency response functions obtained from tests. The tests include a modal test with random excitation to determine the frequency response functions for the linear mode of the modes with low excitation amplitude. Additionally, sinusoidal excitation is used with increasing amplitude for the frequency response functions in the nonlinear state of the modes. The laboratory method used to determine the nonlinear modes involves increasing the amplitude of sinusoidal stimulation in the frequency range of each resonance. This article presents a practical method for analyzing the dynamic behavior of structures related to the folding wing of a flying system. Using simple techniques and vibration test results, it deals with extracting the free-play function of the structure.

The majority of ballistic experiments in granular media in the literature involve horizontally launching projectiles. Notwithstanding the significant scientific findings resulting from these studies, the depth-dependence of geostatic stresses is not captured in a horizontal configuration. The design and performance of a vertical ballistic range is described herein. The range is capable of launching projectiles at impact speeds of up to 900 m/s into soil targets. A pluviator is employed to prepare sand targets with precise and highly repeatable bulk densities. Use of a photon Doppler velocimeter (PDV) and other instrumentation to track projectile velocity both in-flight and during penetration into the soil target are elucidated. A relationship is found between the muzzle velocity and chamber pressure. Launcher performance is quantified by comparing measured muzzle velocities with theoretical velocities calculated from isentropic expansion of gas behind the projectile in the launcher barrel. It is found that the launcher efficiency is in the range of 70 to 90%, with efficiency increasing for heavier projectiles. The PDV instrumentation developed for the range successfully resolves projectile velocities in flight and during penetration into the soil target.

Simultaneous measurement of the temperature in different regions during machining operations presents many limitations. Currently, only orthogonal cutting using a infrared camera allows the simultaneous measurement of temperature in different regions. Additionally, temperature measurement in certain regions is a challenge, for instance, in the chip/tool interface and inside the chip. The application of advanced sensors and the adaptation of well-established techniques in regions of difficult access, such as the chip-tool interface and the chip itself, have been the subject of research to allow the better understanding of the heat generation and temperature evolution during machining operations. This work investigates the application of the inserted thermocouple method to measure the temperature inside the chip during its formation, together with the tool-workpiece thermocouple method to compare the effect of the cutting parameters on both the chip-tool interface and chip temperature. Orthogonal cutting of AISI 1020 steel was performed using cemented tungsten carbide bits. The findings indicated that both methods were able to assess the influence of the investigated parameters and that temperature presented the same behavior, in spite of the differences in absolute values (higher temperatures were recorded using the tool-workpiece thermocouple). Temperature increased with cutting speed, decreased with the elevation of the undeformed chip thickness and was not affected by width of cut. The highest temperature (668 °C) was observed at the tool-workpiece interface using a cutting speed of 120 m/min, undeformed chip thickness of 0.1 mm and width of cut of 1.5 mm.

Shear force at the seat interface has been known to increase the risk of pressure injury (PI) formation in wheelchair users. Understanding the frictional interaction between pant fabric and seat cover fabric in the presence of deformable material (human tissue and seat cushion) is essential to select optimal fabrics to reduce shear force and decrease PI risk. The objective of this work was to evaluate the coefficients of friction between five pant fabrics and two seat covers using human subjects and a novel experimental setup. This experimental set up created a system where participants were able to slide across a seat pan. Shear forces required to pull the participants as well as the normal force on the seat pan were collected for twelve participants. The coefficient of friction was calculated using the force balance from these experimental data sets. The coefficient of friction was higher with the vinyl seat cover common on wheelchairs (0.503–0.572) compared to the office seat cover (0.336–0.448) with all pant fabrics. Furthermore, with the office cover, a substantial drop was observed in the coefficient of friction after sliding, which differed from the vinyl seat cover. This study developed a methodology to study friction behavior at the human-seat interface and quantified coefficients of friction between pant fabrics and two common types of seat covers. It provides a method by which realistic situational shear forces can be studied, as they are a risk factor for PI formation in wheelchair users.

Shaft design assumes that the end supports of the shaft are simply supported that is not entirely correct. This article investigates the effects of simply supported ends and fixed-fixed supported ends on the bending moment developed in shafts. The bending moments and hence bending stress are life limiting parameters of shafts. Moreover, the effects of transverse loading inclination, loading spacing, and loading variation on the bending moment developed in shafts are studied. Analytical, numerical, and experimental approaches were adopted. Notched steel rods were used in fatigue experiments. The fatigue lives of those rods were measured and recorded. The bending moment applied to the rod specimen was calculated and compared to those obtained from the analytical and numerical approaches. The studies revealed that the simply supported end conditions will result in a shaft diameter that is 88% larger. However, the fixed-fixed end condition will result in a shaft diameter that is 67% smaller. The average bending moments of the simply supported and the fixed-fixed end conditions will result in the most accurate shaft diameter. Moreover, the maximum bending moment occurred when the load inclination angle θ = 0.0. It also increased with increasing the load ratio P₁/P₂ and the load spacing ratio l₁/L, where P₁, P₂, l₁, and L are respectively the left-hand load, the right-hand load, the position of P₁ from the left-hand support, and the total length of the shaft.

This paper presents a novel control strategy to improve the performance of a low-frequency vibration calibration system. The low-frequency vibration calibration system based on a linear motor is first developed. The amplitude sensitivity and phase sensitivity are then derived. Subsequently, the composite control strategy of the iterative learning control (ILC) and Luenberger observer is established to improve the performance of the linear motor vibration generator. Finally, the frequency stability of the linear motor is measured, and a tri-axial accelerometer is calibrated. Experimental results indicate that the linear motor controlled by the proposed composite strategy can fulfill accelerometer calibration with high precision, and maintain the performance in the full frequency band.