Kyung-Mox Cho

Adiabatic shear band formation during dynamic torsional deformation of an HY-100 steel

Abstract Dynamic torsion experiments have been conducted on thin-walled tubular specimens of a tempered martensitic HY-100 steel, causing adiabatic shear bands to form. The strain rates imposed were ∼ 10 3 s −1 and local temperature increases up to 600°C within the shear bands were measured. The shear band microstructure was examined by transmission electron microscopy (TEM), revealing two distinct microstructures. In some regions, highly elongated narrow subgrains extended in the shear direction, while in other regions, fine equiaxed cells were characteristic. The proportions of the two microstructures varied for different specimens, and the observations were interpreted to indicate that a process of dynamic recovery accompanying large deformation and a high temperature rise occurred within the shear band. Although thermal effects were apparent, there was no evidence to support a phase transformation to austenite followed by martensite formation. On the basis of present findings, it appears that the thermodynamic stability of the original microstructure can influence the tendency toward shear localization under dynamic loading conditions.

Effects of morphology on toughening of tetrafunctional epoxy resins with poly(ether imide)

Abstract Tetraglycidyl-4,4′-diaminodiphenyl methane based resin with 30 phr diaminodiphenyl sulfone as curing agent was toughened with poly(ether imide) (PEI). The effects of morphology on the fracture toughness of modified epoxy resins were investigated. Morphology was controlled by changing the curing conditions. The co-continuous structure and morphology of the PEI spherical domain dispersed in the epoxy matrix were obtained. Phase-inversed morphology with PEI matrix was also obtained with 30 phr PEI content. The cured resin with phase-inversed morphology showed the highest fracture toughness. The modified epoxy resins with enhanced fracture toughness exhibited other improved mechanical properties such as flexural strength, flexural modulus and strain at break.

Real time in situ X-ray diffraction studies on the melting memory effect in the crystallization of β-isotactic polypropylene

The melting memory effect during the crystallization and heating of semi-crystalline polymers was clearly demonstrated using β-isotactic polypropylene (β-iPP). Differential scanning calorimetry and real-time in situ X-ray diffraction using a synchrotron radiation source were employed to investigate the role of the newly formed α-form crystals via phase transformation from the metastable β-form during the melting process, and to elucidate the memory effect of these new α-form crystals during the crystallization process. The evolution of the memory effect in β-iPP during the crystallization and melting processes is ideally based on the existence of locally ordered α-form in the melt. We monitored the role of this local order by preparing the melt state using a range of hold temperatures and hold times. It was found that the final melt temperature and hold time greatly affect the crystallization behavior during cooling and the phase transformation behavior during heating. Lower hold temperatures and shorter hold times lead to samples rich in α-modification, whereas longer hold times generate samples rich in β-modification during crystallization. At higher hold temperatures even a short hold time is sufficient to destroy the local order in the melt, and the resulting sample exhibits more β-modification. The results are explained on the basis of the existence of local order in the amorphous melt along with external nucleating agent during the crystallization process.

Adhesion improvement of epoxy resin/polyethylene joints by plasma treatment of polyethylene

Abstract Low density polyethylene (LDPE) and high density polyethylene (HDPE) were plasma-treated with N 2 and O 2 plasma. The wettability and polar component of surface free energy of plasma-treated polyethylene were investigated by contact angle measurement. The concentration of functional groups formed by plasma treatment such as hydroxyl and carbonyl groups was measured using attenuated total reflection Fourier transform infrared spectroscopy (ATR FT i.r.). The concentration of polar functional group increased rapidly with 5–10s of plasma treating time and then very slowly after that. The adhesion strength of epoxy resin/plasma-treated polyethylene joints was examined by a 90° peel test. The increase of the adhesion strength was similar to that of concentration of polar functional groups. The higher adhesion strength of epoxy resin/plasma-treated HDPE joints was observed than that of epoxy resin/plasma-treated LDPE joints since HDPE deformed more during the peel tests and had more polar functional groups on the surface.

Toughening of cyanate ester resins with cyanated polysulfones

Abstract Bisphenol-A dicyanate (BADCy) resin was toughened by incorporating polysulfone (PSF) and cyanated polysulfone (CN-PSF). CN-PSF was synthesized by the bromination of PSF and cyanation. The effects of PSF content and interfacial adhesion between the matrix and the domain were investigated on the fracture toughness and morphology of BADCy/PSF blends. PSF formed a matrix phase when more than 20 parts per hundred of resin (phr) of PSF was incorporated. The particle size of BADCy decreased with increasing numbers of cyano groups in PSF. There was an optimum cyano group content for maximizing the fracture toughness of BADCy/CN-PSF blends with 30 phr of CN-PSF content. Toughening mechanisms were examined using scanning electron microscopy and transmission optical microscopy.

Adhesion improvement of epoxy resin/polyimide joints by amine treatment of polyimide surface

Abstract Polyimide (PI) surfaces were modified to improve the adhesion strength of epoxy resin/PI joints by immersing in amine solutions. Adhesion strength of epoxy resin/amine-treated PI joints were measured depending on structure, molecular weight, concentration, treatment time and drying temperatures of amines. There was an optimum drying temperature for maximum adhesion strength after amine-treatment of PI surface. The optimum drying temperature and the maximum adhesion strength increased with increasing the molecular weight of diamines or polyamines. Poly(amic amide) was formed by the reaction of primary amine of diamines and imide group of PI, and the other primary amine of poly(amic amide) reacted with the imide groups of adjacent PI chains to form cross-linked structure. In this way, adhesion strength of epoxy resin/PI joints was improved by reinforcing the weak PI surface layer. Another additional adhesion mechanism could be the chemical reaction of epoxide in the epoxy resin and unreacted amine of poly(amic amide). Adhesion strength decreased at above the optimum drying temperature since poly(amic amide) was imidized. The adhesion mechanisms and existence of optimum drying temperature were investigated using FT i.r., contact angle goniometer, X-ray photoelectron spectroscopy and rheometric dynamic spectroscopy.

Phase separation behavior of cyanate ester resin/polysulfone blends

The composition of the blends and the curing temperature affect the morphology of the blends and the phase separation mechanism. The phase separation mechanism depends on the viscosity of medium at the initial stage of phase separation determined by the amount of thermoplastics and the curing temperature, and is closely related with the final morphology. When the homogeneous bisphenol A dicyanate (BADCy)/polysulfone (PSF) blends with low content of PSF (less than 10 wt %) were cured isothermally, the blends were phase separated by nucleation and growth (NG) mechanism to form the PSF particle structure. On the other hand, with more than 20 wt % of PSF content, the BADCy/PSF blends were phase separated by spinodal decomposition (SD) to form the BADCy particle structure. With about 15 wt % of PSF content, the blends were phase separated by SD and then NG to form a combined structure having both the PSF particle structure and the BADCy particle structure.

Dynamic fracture behavior of SiC whisker-reinforced aluminum alloys

This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2, and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, KI,, of about 2 × 106 MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,KId, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness.

Retardation of the surface rearrangement of O2 plasma-treated LDPE by a two-step temperature control

The effects of the specimen temperature of low-density polyethylene (LDPE) in O2 plasma treatment were studied to enhance the amount of hydrophilic functional groups introduced and to reduce the aging effect. The specimen temperature was varied from 25°C to 100°C. The smallest water contact angle was obtained with the 45°C specimen and the largest amount of hydrophilic functional groups was introduced with the 100°C specimen, as determined by X-ray photoelectron spectroscopy (XPS). Therefore, a two-step plasma treatment with two different specimen temperatures, i.e. 100°C followed by 45°C, decreased the water contact angle and reduced the aging effect. It appears that the hydrophilic functional groups introduced were located at the specimen surface (about 0.5 nm) at low temperature and that the aging effect was reduced due to the hydrophilic functional groups formed inside (0.5-8 nm) at high specimen temperature. The aging rate and the diffusion coefficient were also estimated, depending on the specimen temperature, using the experimental aging data.

Kinetics of Cr/Mo-rich precipitates formation for 25Cr-6.9Ni-3.8Mo-0.3N super duplex stainless steel

The amount and composition of Cr-rich (σ) and Mo-rich (χ) precipitates in super duplex stainless steels was analyzed. An isothermal heat treatment was conducted at temperatures ranging from 700 °C to 1000 °C for up to 10 days. A time-temperature transformation (TTT) diagram was constructed for the mixture of σ and χ phases. The mixture of the σ and χ phases exhibited the fastest rate of formation at approximately 900 °C. Minor phases, such as Cr2N, M23C6, and M7C3, were also detected using a transmission electron microscopy (TEM). Also, a continuous cooling transformation (CCT) diagram was constructed for the mixture of σ and χ phases using the Johnson-Mehl-Avrami equation. Compared with the known CCT diagram of the σ phase, this study revealed faster kinetics with an order of magnitude difference and a new CCT diagram was also developed for a mixture of σ and χ phases. The calculated fraction of σ and χ phases obtained at a cooling speed of 0.5 °C/s was in good agreement with the experimental data.