EFFECT OF TRIAXIAL STRESS CONSTRAINT ON THE DEFORMATION AND FRACTURE OF POLYMERS

One purpose of this paper is to give a brief overview on the research status of deformation,fracture and toughening mechanisms of polymers,including experimental,theoretical and numerical studies.Emphasis is on the more recent progresses of micromechanics of rubber particle cavitation and crazing,and the de- velopment of fracture criteria for ductile polymers. The other purpose is to study the effect of triaxial stress constraint on the deforma- tion and fracture behavior of polymers.Polycarbonate(PC),acrylonitrile-butadiene- styrene(ABS)and PC/ABS alloy are considered in this investigation.A series of circumferentially blunt-notched bars are used to experimentally generate different tri- axial stress fields.The fracture surfaces of specimens with different notch radius are examined by scanning electron microscope(SEM)to study the fracture and tough- ening mechanisms of polymer alloy.It is shown that the triaxial stress constraint has a significant effect on the deformation,fracture and toughening of PC,ABS and PC/ABS alloy.We will also discuss the extent to which a micromechanies criterion proposed by the first author can serve as a fracture criterion for ductile polymers. A new ductile fracture parameter is emphasized,which can be employed to evaluate the fracture ductility of polymers.Stress state independence of the parameter for the PC,ABS and PC/ABS alloy has been experimentally verified.

Development of a Leg Mechanism for Soft Landing Based on Biological Motion

With jumping mechanisms,soft landing motion is important to protect loads and the mechanisms.This study proposes a leg mechanism for soft landing based on biological motion.Human jumping motion with a load suggests a unique motion for soft landing.The landing model consists of two periods.Jerk is minimized in the first period and force is minimized in the second period.In comparison with other landing models,this model is specialized for soft landing motion protecting an objective part.Given all mechanisms have mass,such model is useful in practical application.For the purpose of realizing soft landing motion,this study proposes a new leg mechanism.The mechanism achieves quick variable transmission with cam and wire.Design process of the cam is explained with dynamics and computation.With the calculated cam shape,the leg mechanism can be driven by constant input voltage for simple control.Robustness against height change is also verified with landing simulation.With 50mm falling experiment,prototype leg mechanism performed soft landing without bounce motion and large sound.The acceleration profile of the body also agrees with the proposed soft landing model.

Driving force and structural strength evaluation of a flexible mechanical system with a hydrostatic skeleton

The purpose of this study was to build a flexible mechanical system with a hydrostatic skeleton.The main components of this system are two type flexible bags.One is a structural bag with constant inner pressure.The other is an actuator bag with controlled inner pressure.To design the system,it was necessary to estimate both structural deformation and driving force.Numerical analysis of flexible bags,however,is difficult because of large nonlinear deformation.This study analyzed structural strength and driving force of flexible bags with the nonlinear finite element analysis (FEA) software ABAQUS.The stress concentration dependency on the bag shape is described and the driving force is calculated to include the large deformation.From the analytical results,this study derives an empirical equation of driving force.The validity of the equation was confirmed by condition-changed analyses and experimental results.

Evaluation of temperature dependent vortex pinning properties in strongly pinned YBa_(2)Cu_(3)O_(7-δ)thin films with Y_(2)BaCuO_(5)nanoinclusions

The pinning of quantized magnetic vortices in superconducting YBa_(2)Cu_(3)O_(7-δ)(YBCO or Y123)thin films with Y_(2)BaCuO_(5)(Y211)nanoinclusions have been investigated over wide temperature range(4.2-77 K).The concentration of Y211 nanoinclusions has been systematically varied inside YBCO thin films prepared by laser ablation technique using surface modified target approach.Large pinning force density values(Fp∼0.5 TNm^(−3)at 4.2 K,9 T)have been observed for the YBCO film with moderate concentration of Y211 nanoinclusions(3.6 area%on ablation target).In addition,uniform enhancement in critical current density(J_(c))was observed in the angular dependent J_(c)measurement of YBCO+Y211 nanocomposite films.Y211 nanoinclusions have been found to be very efficient in pinning the quantized vortices thereby enhancing the in‐field J_(c)values over a wide range of temperature.Increasing the concentration of Y211 secondary phase into Y123 film matrix results into agglomeration of Y211 phase and observed as increased Y211 nanoparticle size.These larger secondary phase nanoparticles are not as efficient pinning centers at lower temperatures as they are at higher temperatures due to substantial reduction of the coherence length at lower temperatures.Investigation of the temperature dependence of J_(c)for YBCO+Y211 nanocomposite films has been conducted and possible vortex pinning mechanism in these nanocomposite films has been discussed.

Evaluation of interfacial strength by an instrumented indentation method and its application to an actual TBC vane

The thermal fatigue behaviour of an air plasma sprayed thermal barrier coating was investigated. And also the interfacial strengths of thermal barrier coated specimens subjected to thermal fatigue, as well as a retired TBC vane were also evaluated by means of an instrumented indentation machine. The results indicated that, （1） the TGO grew at the interface during thermal fatigue cycle as a function of the exposure time at elevated temperature; （2） the microcracks were initiated in the top coating and at the interface after thermal cycle tests; （3） the interfacial strength of TBC, which was evaluated by the indentation method, increased with the thermal cycles; （4） the interfacial strength of the retired TBC vane was almost equal with that of the as-sprayed TBC specimen.

Quantitative Analysis of the Silk Moth＇s Chemical Plume Tracing Locomotion Using a Hierarchical Classification Method

The silk moth （Bombyx mori） exhibits efficient Chemical Plume Tracing （CPT）, which is ideal for biomimetics. However, there is insufficient quantitative understanding of its CPT behavior. We propose a hierarchical classification method to segment its natural CPT locomotion and to build its inverse model for detecting stimulus input. This provides the basis for quantitative analysis. The Gaussian mixture model with expectation-maximization algorithm is used first for unsupervised classification to decompose CPT locomotion data into Gaussian density components that represent a set of quantified elemental motions. A heuristic behavioral rule is used to categorize these components to eliminate components that are descriptive of the same motion. Then, the echo state network is used for supervised classification to evaluate segmented elemental motions and to compare CPT locomotion among different moths. In this case, categorized elemental motions are used as the training data to estimate stimulus time. We successfully built the inverse CPT behavioral model of the silk moth to detect stimulus input with good accuracy. The quantitative analysis indicates that silk moths exhibit behavioral singularity and time dependency in their CPT locomotion, which is dominated by its singularity.

Round Robin Study on the Thermal Conductivity/Diffusivity of a Gold Wire with a Diameter of 30μm Tested via Five Measurement Methods

Since first establishing thermal measurement techniques for micrometer-scale wires,various methods have been devised and improved upon.However,the uncertainty of different measurements on the same sample has not yet been discussed.In this work,a round robin test was performed to compare the thermal conductivity and thermal diffusivity measurement methods for a fine metal wire.The tested material was a pure gold wire,with a diameter of 30μm.The wire was cut into certain lengths and distributed to four institutions using five different measurement methods:the direct current(DC)self-heating method,the DC heating T-type method,the 3ωmethod for thermal conductivity,the scanning laser heating alternating current(AC)method,and the spot periodic heating radiation thermometry method for thermal diffusivity.After completing the measurements,the reported thermal conductivity and thermal diffusivity at room temperature,i.e.,317 W·m^(-1)·K^(-1)and 128×10^(-6)m^(2)·s^(-1),respectively,were adopted as references for comparison with the measurement results.The advantages and disadvantages of each method are described in terms of the effect of electrical and thermal junctions fabricated on a wire,such as an electrode,a thermocouple,and a heat bath.The knowledge obtained from the tested methods will be useful for selecting and designing a measurement technique for various wire-like materials.

Multi-dimple phenomena in TEHL point contacts

Theoretical analysis and experimental measurement of the multiple dimples in thermal elastohydrodynamic lubrication (TEHL) point contacts have been carried out. Good agreement is found between the theoretical and experimental results. A thermal multi-spike theory is proposed to explain the multi-dimple phenomena.