A new model to determine contact angles on swelling polymer particles by the column wicking method

The contact angle determination on swelling polymer particles by the Washburn equation using column wicking measurements may be problematic because swelling occurs during the wicking process. The objective of this research was to develop a new model to more accurately determine contact angles for polymer particles that undergo solvent swelling during the column wicking process. Two phenomena were observed related to the swelling effect during the wicking process: (1) a temperature rise was detected during the wicking process when the swelling polymer particles interacted with polar liquids, and (2) a smaller average capillary radius (r) was obtained when using methanol (polar liquid) compared to using hexane (non-polar liquid). The particle swelling will induce both particle geometry changes and energy loss which will influence the capillary rise rate. The model developed in this study considered the average pore radius change and the energy loss due to the polymer swelling effect. Contact angle comparisons were conducted on wood with formamide, ethylene glycol, and water as test liquids, determined by both the new model and the Washburn equation. It was shown that the contact angles determined by the new model were about 4-37° lower than those determined by the Washburn equation for water, formamide, and ethylene glycol. Todetermine whether the polymer particles are swelling, two low surface tension liquids, one polar (methanol) and the other non-polar (hexane), can be used to determine the average pore radius (r values) using the Washburn equation. If the same r values are obtained for the two liquids, no swelling occurs, and the Washburn equation can be used for the contact angle calculation. Otherwise, the model established in this study should be used for contact angle determination.     

Peel adhesion to paper—interpreting peel curves

Peel experiments involving three PSA tapes and three paper substrates were employed to develop a general approach for the analysis of peeling from paper. Plotting the logarithm of the peak peel force (i.e. the maximum value) versus the logarithm of the peel rate yielded a generalized peel curve which illustrated transitions from interfacial to paper failure. The general peel curve consisted of linear segments (on the log/log plots) which intersected when the failure mode changed from interfacial to paper failure. The influence of paper type, PSA type, and peel angle on the generalized peel curve was determined.     

A new method to measure adhesion and surface forces using a closed-loop accelerometer

A force-balanced MEMS sensor is developed to measure the forces between two surfaces with controllable distance. Its mechanical structure is like a pendulous micromachined accelerometer, and it is designed as a closed-loop system with electrostatic force feedback. The surface force on the sensor probe is balanced by the electrostatic force and the probe operates without displacement. This method avoids the displacement of the conventional cantilever, and the distance between the two surfaces is precisely controlled during the measurement. Experiments on surface force measurements between a probe surface and a ball surface are performed, and the attractive force and adhesion force between the two surfaces are measured under a precise distance controlled by a nanopositioner.     

Static and Dynamic Aspects of Liquid Capillary Flow in Thermally Bonded Polyester Nonwoven Fabrics

The weight gain method is employed to study the vertical capillary flow of wetting liquids in polyester nonwoven fabrics with different basis weights. The quantity of liquid absorbed by capillarity in the nonwoven is recorded as a function of time, until saturation. The liquid retention capacity of the nonwovens has been studied from their “saturation level”, i.e. the fraction of pore volume effectively filled with liquid. It is found that this saturation level varies greatly with the type of nonwoven, and generally decreases with nonwoven thickness. Moreover, the expected 100% value is rarely attained even when the sample height is smaller than the Jurin equilibrium height. These observations are attributed to the more heterogeneous pore sizes in very thin nonwovens, where the interconnection of large and small pores inhibits the continued capillary rise of liquid front. The other part of the study concerns the kinetics of liquid capillary flow which has been analyzed by taking into account the contribution of the meniscus in filling the pores. By subtracting this contribution from the mass of liquid absorbed, the new absorption mass is found to vary linearly with the square root of time, in agreement with the Washburn theory. For the thinnest nonwovens, the very small and unrealistic values of Washburn radii deduced from the experimental results do not correspond to the real physical pore sizes, but reflect slow capillary kinetics. This phenomenon is, however, less important when the thickness of the sample increases.     

Delamination growth during fatigue of advanced polymer matrix composites

Abstract

Advanced polymer matrix composites such as carbon fibre reinforced polymers (CFRP), offer many advantages over more traditional materials such as metals. Usually, CFRP have greater strength/weight and stiffness/weight ratios than traditional engineering materials, which makes them ideal for use in many weight sensitive applications, especially in the aerospace sector. To maximise the use of these materials there is a need to gain a better understanding of how CFRP, and more generally FRPs, behave under fatigue load conditions. This work investigates the fatigue response and damage mechanisms found in a CFRP. Previous work has highlighted that fatigue with a compressive element is more damaging than pure tensile fatigue and that delamination is the dominant damage mechanism in both cases. However, in the tensile fatigue tests the primary delamination was on a different interface from the primary delamination found in the compression fatigue tests. The cause of this trend to delaminate along a particular interface has been investigated using mixed mode bend tests. These tests have been used to investigate the response of the interface to both static and fatigue loads. Initial tests have been carried out on the 0°/45° interface. Delamination growth was monitored at three levels of mode mixity, ratios of MI/MII of 1:1, 1:3 and 3:1. PRC/1848     

New technique to control welding buckling distortion and residual stress with non-contact electromagnetic impact

Abstract

A new technique using non-contact electromagnetic forces has been proposed for controlling welding buckling distortion and residual stresses in welded thin plates. The experimental results show that the method can successfully eliminate the buckling distortion and reduce the residual stresses. Three-dimensional finite element modelling has been developed to study the evolution of the stress and strain throughout the welding and electromagnetic impacts. The predicted welding distortion and residual stresses are in good agreement with the experimental results. The numerical analyses show that the reduction in distortion and stress is a result of the change of the plastic strain field in the weld region: electromagnetic impacts reduce longitudinal compressive plastic strain in the local region near the weld, and even produce the tensile plastic strain. Moreover, it is found that the residual stress can promote the changes of the longitudinal plastic strain state under electromagnetic impact.     

Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding

Abstract

The effect of welding parameters on the distribution of wire feeding elements has been investigated during CO₂ laser and pulsed gas metal arc hybrid welding process. The molten metal flow on the pool surface and inside of the samples was observed by a high speed video camera and an in situ X-ray transmission imaging system respectively. The results indicate that the fluid flow towards the inside of keyhole, namely inward flow, improves the homogeneity of weld metal. The distribution of alloying elements is more homogeneous in leading laser compared with leading arc, since both of the drag force of the plasma jet and momentum of droplet promote the inward flow in leading laser. Almost homogeneous distribution of alloying elements can be attained if the oxygen content in the shielding gas is more than 2%, since the Marangoni flow direction changes from outward to inward with increasing the oxygen content.     

Position-based impedance control of a biped humanoid robot

This paper describes position-based impedance control for biped humanoid robot locomotion. The impedance parameters of the biped leg are adjusted in real-time according to the gait phase. In order to reduce the impact/contact forces generated between the contacting foot and the ground, the damping coefficient of the impedance of the landing foot is increased largely during the first half double support phase. In the last half double support phase, the walking pattern of the leg changed by the impedance control is returned to the desired walking pattern by using a polynomial. Also, the large stiffness of the landing leg is given to increase the momentum reduced by the viscosity of the landing leg in the first half single support phase. For the stability of the biped humanoid robot, a balance control that compensates for moments generated by the biped locomotion is employed during a whole walking cycle. For the confirmation of the impedance and balance control, we have developed a life-sized humanoid robot, WABIAN-RIII, which has 43 mechanical d.o.f. Through dynamic walking experiments, the validity of the proposed controls is verified.     

Force-guided motions of a 6-d.o.f. industrial robot with a joint space approach

This article deals with the interaction between humans and industrial robots, more specifically with the new design and implementation of an algorithm for force-guided motions of a 6-d.o.f. robot. It may be used to comfortably teach positions without using any teaching pendant or for some assistance tasks. For this purpose, from readings of the force/torque sensor mounted in the robot wrist, the gravity forces and torques first have to be eliminated. To control the robot in joint space, it is then convenient to transform the external force and torque values from Cartesian space into joint space using the manipulator transposed Jacobian. This is why with the present approach the Jacobian matrix of the robot used was calculated. Now, from the computed joint torques, suitable position commands of the robot arm can be generated to obtain the desired behavior. A suggestion for this desired behavior is also included in this article. It is based on the impedance control approach in joint space. The proposed algorithm was implemented with the standard Stäubli RX90B industrial robot.     

Robust force control strategy based on the virtual environment concept

The main goal of this paper is to present a force control strategy based on the virtual environment concept. This concept is a way to increase the robustness of force control schemes with respect to a variation of the environment characteristics. We first propose this approach, then we analyze it and, finally, we adapt it to a classical external force control scheme. Experimental results with a DELTA fast parallel robot are presented to prove the efficiency of this method.