Joung-Man Park

Improvement of interfacial adhesion and nondestructive damage evaluation for plasma-treated PBO and Kevlar fibers/epoxy composites using micromechanical techniques and surface wettability

Comparison of interfacial properties and microfailure mechanisms of oxygen-plasma treated poly(p-phenylene-2,6-benzobisoxazole (PBO, Zylon) and poly(p-phenylene terephthalamide) (PPTA, Kevlar) fibers/epoxy composites were investigated using a micromechanical technique and nondestructive acoustic emission (AE). The interfacial shear strength (IFSS) and work of adhesion, Wa, of PBO or Kevlar fiber/epoxy composites increased with oxygen-plasma treatment, due to induced hydrogen and covalent bondings at their interface. Plasma-treated Kevlar fiber showed the maximum critical surface tension and polar term, whereas the untreated PBO fiber showed the minimum values. The work of adhesion and the polar term were proportional to the IFSS directly for both PBO and Kevlar fibers. The microfibril fracture pattern of two plasma-treated fibers appeared obviously. Unlike in slow cooling, in rapid cooling, case kink band and kicking in PBO fiber appeared, whereas buckling in the Kevlar fiber was observed mainly due to compressive and residual stresses. Based on the propagation of microfibril failure toward the core region, the number of AE events for plasma-treated PBO and Kevlar fibers increased significantly compared to the untreated case. The results of nondestructive AE were consistent with microfailure modes.

A study of interfacial aspects of epoxy-based composites reinforced with dual basalt and SiC fibres by means of the fragmentation and acoustic emission techniques

Abstract Fibre/matrix interfacial properties and interfacial shear strengths (IFSS) in epoxy composites reinforced by dual basalt and SiC fibres (DFC) were investigated by the fragmentation method combined with acoustic emission (AE) analysis. Statistical analysis of fibre tensile strength was performed in terms of statistical parameters. The tensile strength and elongation of basalt and SiC fibres decreased with increasing gauge length because of the size effect. Fibre tensile strengths above an optimum concentration decreased because of the stress concentrations at lumps in the coating. When an amino–silane coupling agent was used, the IFSS showed significant improvements of more than three times under dry conditions. The IFSS was also considerably improved under wet conditions. This environmental effect is probably due to chemical and hydrogen bonds as well as to interdiffusion effects in two different interphases in the fibre/silane-coupling-agent/epoxy-matrix system. In situ monitoring of AE during the straining of DFC specimens showed the sequential occurrence of two distinct groups of AE data. The first group may have come from fibre breakages, and the second mainly from cracking of the epoxy matrix. Characteristic frequencies coming from the different failure modes of the fibres and epoxy matrix were investigated by fast Fourier transform (FFT) analysis. By setting an appropriate threshold level, a one-to-one correspondence between the number of AE events and fibre breakages was established. This AE method could be correlated well with the fragmentation technique of obtaining the IFSS value. It can be also be applied to semi- or non-transparent composites where visual observation is not possible.

Interfacial evaluation and microfailure mechanisms of single carbon fiber/bismaleimide (BMI) composites by tensile and compressive fragmentation tests and acoustic emission

Abstract Interfacial properties and microfailure modes of carbon, SiC, and glass fibers/bismaleimide (BMI) composites were evaluated using both tensile fragmentation and compressive Broutman tests with acoustic emission (AE). Since brittle BMI is a difficult matrix to apply for the conventional fragmentation test, dual matrix composites (DMC) were applied due to their too brittleness and high modulus. By curing further from two- to three-stage, the interfacial shear strength (IFSS) between the fiber and the BMI matrix increased under both tension and compression, whereas the number of the vertical crack spacings of the BMI matrix decreased under tension. It was considered that the IFSS increased due to further progressed curing at the three-stage. The typical microfailure modes including fiber break, matrix crack, and interfacial failure were observed in tension. Due to transverse tensile stress at the interface in compression, many overlapped diagonal slippages were observed only at the broken carbon fiber ends without a BMI vertical crack. In tension, SiC and glass fibers were never broken due to their rather longer elongation to failure than carbon fiber. AE amplitudes of the brittle BMI matrix fracture were even higher than those of carbon fiber fracture in tension because of high modulus of the BMI matrix to fracture and geometrical effect due to thick rod shape. The waveform of signals coming from BMI matrix fractures in tension was consistent with AE amplitude result.

Interfacial evaluation of single Ramie and Kenaf fiber/epoxy resin composites using micromechanical test and nondestructive acoustic emission

Interfacial shear strength (IFSS) of environmentally friendly natural fiber reinforced polymer composites plays a very important role in controlling their overall mechanical performance. The IFSS of various Ramie and Kenaf fiber/epoxy composites was evaluated using the combination of micromechanical test and nondestructive acoustic emission (AE) to find the optimal conditions for desirable final performance. Dynamic contact angle was measured for Ramie and Kenaf fibers and correlated the wettability properties with interfacial adhesion. Mechanical properties of Ramie and Kenaf fibers were investigated using single-fiber tensile test and analyzed statistically by both unimodal and bimodal Weibull distributions. The effect of clamping on the real elongation for both Ramie and Kenaf fibers was evaluated as well. Two different microfailure modes, axial dedonding and fibril fracture, coming from fiber bundles and single fiber composites (SFC) were observed under tension and compression. They were evaluated optically and also determined by AE and their FFT analysis nondestructively.

Interfacial properties and microfailure degradation mechanisms of bioabsorbable fibers/poly-l-lactide composites using micromechanical test and nondestructive acoustic emission

Abstract Interfacial properties and microfailure degradation mechanisms of the bioabsorbable composites for implant materials were investigated using micromechanical technique and nondestructive acoustic emission (AE). The tensile strength of absorbable fibers with hydrolysis was analyzed statistically using either uni- or bimodal Weibull distribution. As hydrolysis time increased, the tensile strength, the modulus and the elongation of poly(ester-amide) (PEA) and bioactive glass fibers decreased, whereas those of chitosan fiber almost did not change. Interfacial shear strength (IFSS) between bioactive glass fiber and poly- l -lactide (PLLA) was much higher than PEA or chitosan fiber/PLLA systems using dual matrix composite (DMC) specimen. The decreasing rate of IFSS was the fastest in bioactive glass fiber/PLLA composites whereas that of chitosan fiber/PLLA composites was the slowest. Work of adhesion, Wa between bioactive glass fiber and PLLA was the highest, and the wettability results were consistent with the IFSS. AE energies of PEA fiber decreased gradually, and their distributions became narrower than those in the initial state with hydrolysis time. In case of bioactive glass fiber, AE energies in tensile failure were much higher than those in compression. In addition, AE parameters at the initial state were much higher than those after degradation under both tensile and compressive tests. Interfacial properties and microfailure degradation mechanisms can be important factors to control bioabsorbable composite performance.

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.

A new method of evaluating the interfacial properties of composites by means of the gradual multi-fiber fragmentation test

A new method for evaluating the interfacial properties of fibrous composites based on a fragmentation technique has been proposed by use of the gradual multi-fiber composite, in which the inter-fiber spacing is gradually changed. The results showed that as the inter-fiber distance increased, the aspect ratio of broken fibers decreased while the fibre/matrix interfacial shear strength increased. When the reciprocal of the inter-fiber distance was taken for the above relationships, both the aspect ratio and interfacial shear strength were found to show a saturated value. This means that the gradual multi-fiber composite indicates an upper bound in aspect ratio and a lower bound in interfacial shear strength, while the single-fiber composite shows a lower bound in aspect ratio and an upper bound in interfacial shear strength. It was concluded that this fragmentation test could be a new method for composite evaluation, since reducing the difference between these two bounds is effective for composite strengthening. In addition an elasto-plastic finite-element analysis was carried out to relate the above results with the fiber stress distribution around fiber breaks. It is proved that the bound obtained in the gradual multi-fiber composite test is closely concerned with stress concentrations caused by a group of multi-fiber breaks.

Properties of interfacial adhesion for vibration controllability of composite materials as smart structures

The performance of smart structures depends on the quality of the bonding along the interface between the main structure and the attached sensing and actuating elements. By using interfacial adhesives providing optimum bonding conditions between them, vibrational properties of the composite beams with attached sensor and actuator were investigated. Three different adhesives, i.e. ethyl-2-cyanoacrylate adhesive (type A), toughness and stiffness controlled epoxy adhesives (types B and C) were compared with each other. An optimal type adhesive was chosen, based on its mechanical and wetting properties. Under severe flexure loading condition, type C was found to be most suitable for the vibrational structure system, assuming that the composite beam/adhesive layer/PZT system can deform with the same curvature. In addition, type C adhesive can provide the optimized bonding for manufacturing the smart structure and transfer the elastic behavior precisely. In addition, the beam with type C adhesive provided the fastest response time for the vibration control. This means that the optimum bonding between composite beam and PZT was formed comparatively using this interfacial adhesive.

Load transfer from fiber to polymer matrix, studied by measuring the apparent elastic modulus of carbon fiber embedded in epoxy

Load transfer from a single carbon fiber to the surrounding epoxy matrix was studied by measuring the apparent tensile modulus of the fiber while the fiber was embedded in epoxy and comparing the apparent modulus (1650 GPa) with the real modulus (230 GPa). Thus, it was found that 87% of the tensile load applied to the fiber was transferred to the epoxy.

Surface control and cryogenic durability of transparent CNT coatings on dip-coated glass substrates

Transparent carbon nanotube (CNT) coatings were deposited on boro-silicate glass substrates by dip-coating. Ultraviolet-visible (UV) spectra, surface resistance measurement, and the wettability tests were used to investigate the optical transmittance and electrical properties of these CNT coatings. The changes in electrical and optical properties of these coatings were observed to be functions of the number of dip-coating cycles. The surface resistance of the CNT coated substrates decreased dramatically as the number of dip-coatings was increased, whereas the increases in the CNT layer thickness beyond that for the first dipping cycle had little effect on the transparent-properties. Static contact angle measurements proved to be an effective means for evaluating the surface morphology of CNT coatings. The interfacial durability of the CNT coatings on a glass substrate was much better than that of ITO coatings over the temperature range from -150°C to +150°C.