Mitsuo Notomi

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 development of fracture criteria for ductile polymers.The other purpose is to study the effect of triaxial stress constraint on the deformation and fracture behavior of polymers. Polycarbonate (PC), acrylonitrile-butadienestyrene (ABS) and PC/ABS alloy are considered in this investigation. A series of circumferentially blunt-notched bars are used to experimentally generate different triaxial stress fields. The fracture surfaces of specimens with different notch radius are examined by scanning electron microscope (SEM) to study the fracture and toughening 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 micromechanics 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.

Effects of moisture absorption on fracture behaviors of acrylonitrile-butadiene-styrene resin

The effect of moisture absorption on fracture behaviors of acrylonitrile-butadiene-styrene (ABS) resin has been studied. For comparison, polystyrene (PS) and styrene-acrylonitrile (SAN) resin have been tested. The fracture toughness of PS and SAN resins is determined by the ASTM standard test method for brittle polymers. The fracture toughness of ABS resin is obtained on the basis of the multiple specimens method. The fracture toughness of PS and SAN resin decreases with the increase in moisture absorption. On the other hand, the fracture toughness of ABS resin slightly decreases despite enormous moisture absorption. On the specimens absorbing moisture, a bright whitening region and a milky coloring region are distinguished in the stress-whitening region, and the milky coloring region expands around the bright whitening region at the crack tip. From the transmission electron microscopic observation, the precraze formation can be recognized in this region. The crack-tip shielding effect induced by this formation compensates the fracture toughness decrease due to moisture absorption.

Musculoskeletal computational analysis of the influence of car-seat design/adjustment on fatigue-induced driving

Main causes for fatigue and discomfort experienced by vehicle drivers during city driving were investigated computationally using a musculoskeletal modeling simulation method. Key adjustments of car seat (i.e., the seat-pan and back-rest inclination) together with various values of accelerator pedals spring stiffness were analyzed in the present work. A public-domain rigid-body model of a seat together with the detailed full-body musculoskeletal model was used to study biomechanics of seated drivers. Interactions between the drivers and vehicle in various combinations of seat-pan/back-rest inclinations and pedal spring stiffness were analyzed using an inverse dynamics approach. To deal with the muscle redundancy problem, (i.e. the problem with the human-body containing more muscle than necessary to drive its degrees of freedom) the “minimum-fatigue” criterion [1] was utilized. The results show that seat-pan/back-rest inclinations and pedal spring stiffness have complex influences on the muscle activation and spinal joint forces of the human body. From the result, it may be suggested that a slight backward inclination of the seat-pan (approx. −20deg) and back-rest (approx. −20deg) may reduce the muscle fatigue of a driver. In addition, adding a spring (stiffness around 20Nm/rad) to the accelerator pedal does help in minimizing the muscle activity and spinal joint forces.

Mixed-mode fracture behavior of silica particulate filled epoxide resin

In the present paper, fracture behavior of the epoxide resin, containing 70 wt% silica particles, have been studied at room temperature and high temperature, that is, 250 C. The three-point and four-point bending tests were performed to obtain Mode I and mixed-mode fracture toughness, respectively. The specimen fractured in a brittle manner under room temperature conditions, and the results obeyed the maximum hoop stress criterion. On the other hand, the results at high temperature showed a relatively low dependency on Mode II stress intensity factors. Fracture surface were observed in a scanning laser microscope. Finite element analysis, taking into account the microstructure of the material, have been conducted in order to investigate the influence of temperature on mixed-mode fracture behavior. Local fracture criteria were considered for the silica particles and the epoxide resin. The material constants in criteria were estimated from the bending strength and the Mode I fracture toughness of the material. Good agreement between experimental and numerical results was obtained for the fracture boundary curves on the Mode I and Mode II stress intensity diagram.

Musculoskeletal analysis of driving fatigue: The influence of seat adjustments

Main causes for discomfort experienced by vehicle drivers during driving were investigated using a rigid-body model originally developed in the AnyBody Modeling System [. The interactions between the human body and the car-seat in various combinations of seat-pan/backrest inclinations and the effect of pedal spring stiffness were analyzed using an inverse dynamics approach. To deal with the muscle redundancy problem, (i.e. the problem with the human-body containing more muscles than necessary to drive its degrees of freedom) a minimum-fatigue criterion [ was utilized. The results show that various seat adjustments (e.g., seat-pan and backrest inclinations) and the pedal spring stiffness have complex influences on the muscle activation and spinal joint forces of the human body. From the results, an optimal adjustment for the car-seat is proposed, i.e. the backrest inclination is 10° and the seat-pan inclination is between 0o to 5 o. This study can in general capture the overall interactions between human body and environment (i.e. the maximum muscle activity and spine forces), which is thought to be the factors of driving fatigue.

Musculoskeletal model of awkward carrying postures

Abstract Improper posture of carrying loads can cause low back disorders. This study investigates the impact of using a footstool in spinal force and muscle activity when: (1) pushing/pulling load farther/nearer from the body and (2) twisting the trunk while carrying load. A whole body musculoskeletal model carrying a light load of 5, 7.5 and 10 kg is developed and inverse dynamics analyzes are conducted. Electromyography activities are also recorded to compare to the results from analyzes. Analyzes demonstrated that using a footstool when carrying a light load can reduce the intradiscal compression force. The results from the analysis are found to be consistent with the electromyogram measurement. This study suggests that load should be positioned closer to the body and footstool of 5 cm height should be used to reduce spinal forces and muscle activity on the lumbar region.

Finite Element Analysis of Rubber Particles Size Distribution on Fracture Toughness of Rubber-Modified Polymer

A numerical study on the effects of the distribution of rubber particles size on the fracture toughness of rubber-modified polymer alloys was computed. FEM analyses were conducted on the deformation field near the crack tip under mode I for small scale yielding condition. Near the crack tip is modelled as composite of matrix materials and rubber particles. On the other hand, outer region is modelled as homogeneous material whose constitutive equation has been obtained by analysing unit cell model of matrix and rubber particle. Perfect bonding or partial debonding of the interface is assumed in the computation. Matrix and rubber particles are treated as Mises and Mooney-Rivlin materials, respectively. It is shown that energy flux into fracture process zone; Ĵ -integral is smaller for bimodal type than monomodal one. This behavior largely occurred on the partial debonding case. These results imply that the screening effects occurred in the bimodal type larger than monomodal one.

Effects of Temperature on Fracture Toughness of Silica-Particulate-Filled Epoxide Resins.

Silica-particulate-filled epoxide resins are used as encapsulant materials in semiconductor packaging. In the present paper. the effects of temperature on the fracture toughness of two types of epoxide resin containing 70 wt.% silica particles have been studied. The bridge indentation method was employed in pre-cracking of the specimens, and three point bending tests were carried out to obtain fracture toughness. The fracture toughness of both resins was almost constant at the temperatures from room temperature to the glass transition temperature of the resin. The fracture toughness decreased remarkably above the glass transition temperature. Fracture surface observation by scanning electron microscope shows that epoxide resin matrix predominates in the fracture surface at temperatures above the glass transition temperature while cracking of silica particles is seen at room temperature. This suggests that differences in fracture toughness due to temperature are attributable to differences in crack propagation of the resin due to softening of the resin matrix above the glass transition temperature.

Special Issue on Fracture Mechanics. Effets of High Humidity and High Temperature Exposure on Fracture Behaviour of Polycarbonate and Cellulose-Acetate.

Effects of high humidity and high temperature exposure on polycarbonate (PC) and cellulose-acetate (CA) were investigated. After the specimens were exposed under 85°C 40% or 85°C 85% for 8 days, tension tests and fracture toughness tests were carried out. The fracture behavior of the specimens was studied by fractography using a video microscope. The results were summarized as the tensile strength-failure strain diagram and the fracture toughness-crack extension resistance diagram. The failure strain decreased because a mechanical scratch at the corner of the specimen had grown during the exposure. Specimens which were finshed smoothly after exposure showed an increase in failure strain. The appearance of fracture surface in CA specimens was influenced by the location of initial defects which causes them fracture. The change in tensile strength of PC was small while that of CA decreased with the increase of exposure humidity. For the case of PC, the exposure increased the fracture toughness but diminished the crack extension resistance. For the case of CA, the exposure did not so much influence the fracture toughness while it diminished the crack extension resistance significantly. The difference in fracture behavior between two materials was discussed by use of the crack extended model in an ideally plastic solid.

Effects of Humidity on the Mechanical Behavior of Polyamide-6 and Cellulose-Acetate.

The effects of humidity on the mechanical behavior of polyamide-6 (PA) and cellulose-acetate (CA) were investigated. The specimens were exposed in a chamber at constant temperature and humidity for 4∼16 days, and measurements were made on the variations of weight and tensile properties. In addition infrared analysis was conducted using the attenuated total reflectance attachment with a KRS-5 prism. The result are as follows : ( 1 ) In PA, there exists a linear relationship between the weight change and the maximum stress ; i. e. it becomes heavier and more ductile when it is exposed to high humidity and temperature. The main factor of the weight change is moisture absorption and/or desiccation, and this relationship also holds for the specimens under dry-wet cyclic exposure. ( 2 ) The variation of yield stress of CA is correlated well with its weight change but not with the exposed humidity. IR spectroscopy shows that the weight change of CA occurrs by dispersion of plasticizer as well as by absorption of water. Due to dispersion of plasticizer, the variation of the yield stress under dry-wet cyclic exposure is nonreversible.