Dong-Yeul Song

Fatigue life prediction of composites under two-stage loading

In order to predict the fatigue life under two-stage loading by formulating a cumulative fatigue damage rule for composite materials, the fatigue process in SiC–Al and glass-fibre reinforced plastics were investigated. A microcrack occurred within the composites which resulted in cumulative fatigue damage that increased linearly with the number of cycles. The mechanical conditions of damage growth and failure were determined by characterizing the microdamage governing the fatigue. The ultimate failure is shown to occur when the product of the stress amplitude ratio and microdamage density is beyond a critical value and an expression for the remaining fatigue life is derived.

Effects of temperature and water content on impact properties of injection-molded glass nylon-6 composites

Instrumented drop-weight impact tests were conducted for injection-molded short glass fiber reinforced nylon-6 thermoplastic composites. The effects of fiber surface treatments, specimen temperatures and water contents on the impact fracture properties were studied. The first peak load P, or the corresponding absorbed energy E1 was found an appropriate parameter to evaluate the first cracking on specimen surfaces and also to compare the impact performance of thermoplastic composites. The effects of specimen temperatures and drying conditions were also found important in evaluating these materials. Microscopic observations revealed the microfracture mechanisms to explain the above macroscopic mechanical results. Static in-plane fracture toughness properties corresponded well with the impact properties.

A method of stress analysis for interfacial property evaluation in thermoplastic composites

An analysis of stress transfer between the fiber and matrix in a single fiber reinforced composite which is useful to evaluate the effect of the properties of the interphase layer between the fibers and matrix is presented. The model is based on a concentric cylinder and considers an interphase surrounding the fiber to simulate a surface-treated layer as the product of processing conditions of short fiber reinforced thermoplastic composites. Stress and displacement fields in a single fiber composite under uniaxial tension in fiber direction are analyzed. The analysis presented here is used to predict the deformation of a short fiber in a thermoplastic composite. The analysis is compared with the experimental results obtained by using the micro-grid method.

Approximate estimation of fatigue strength of polymer matrix composites by material properties

To estimate the fatigue strength (life) of polymer matrix composites (PMC) by material properties, the fatigue process of satin woven carbon fabric laminate composites with two different matrix resins (CF/epoxy, CF/PEEK) was investigated from a microscopic viewpoint and the failure mechanism was discussed in relation to the local stress states and fatigue strength. It was found that fatigue strength can be approximately estimated by the function of material properties, κ. Moreover, it was revealed that the main fatigue damage changes from transverse crack in transverse fiber bundle to fiber fracture in longitudinal fiber bundle in dependence on the value of κ.

Approximate analysis of the stress state near the fibre ends of short fibre-reinforced composites and the consequent microfracture mechanisms

An approximate stress analysis is performed to evaluate the microfracture behaviour near a fibre end in short fibre-reinforced thermoplastic composites. The model presented here predicts the axisymmetric stress distribution in a concentric cylinder assemblage containing the fibre, the matrix and the interphase, taking into account the stress transfer across the fibre end. Then, the calculated stresses are used to explain the damage initiation and growth behaviour of injection-moulded short fibre-reinforced thermoplastic composites with different fibre surface treatments.

Microscopic fatigue failure process in interleaved and toughness-improved CFRP cross-ply laminates

Microscopic damage progress in CFRP cross-ply laminates under tensile fatigue loading was measured by the replica technique. Material systems were interleaved CFRP, T800H/3631-FM300 and toughness-improved CFRP, T800H/3900-2, with selectively toughened interlaminar layers. The damage parameter, the transverse crack density, was presented. A Paris law model was proposed, which related the cyclic strain range and the number of cycles at the transverse crack initiation. Based on the experimental data, a variational stress analysis combined with the modified Paris law was conducted to model the transverse crack multiplication.

Corrosion, oxidation and strength properties of Nicalon SiC fibre under loading

Abstract Hot corrosion and oxidation behavior of Nicalon fibres in air under loading were investigated and compared. Degradation of Nicalon fibres exposed at temperatures over 900°C in the presence of Na 2 SO 4 was extremely severe compared with that in oxidation environment without Na 2 SO 4 . This is attributed to the rapid oxidation-dissolution of the fibres due to the formation of the liquid Na 2 SiO 3 . Room temperature strength of Nicalon fibre after oxidation decreased with increasing exposure temperature. Reduction of tensile strengths was most significant under the combined condition of exposure temperature and applied loading. This evident strength reduction is caused by the combination of applied load and the reaction between the inherent SiO 2 and free carbon in the fibre. Fracture surfaces of oxidized fibres exhibited mirror-hackle regions, whereas the fracture surfaces of fibres corroded were planar.

Fatigue life prediction of cross-ply composite laminates

Abstract To predict the fatigue life of fiber reinforced composites, fatigue process of CFRP laminates of [0°/90°] s is investigated and the influence of damages occurring at fiber, matrix and fiber/matrix interface on the various critical strengths and the relationship between residual critical strength and failure are discussed. As a result, it was shown that fatigue strength (i.e. fatigue life) consisted of residual critical strength and stresses occurring at each layer (0° and 90° layers) and interlayer. Moreover, the fatigue failure occurred because the residual critical strength of each layer and interlayer decreased with dependence of their microdamage densities, so that the fatigue life can be predicted by evaluating microdamage behavior in fatigue process.

Fatigue Damage Evaluation of PMC by Time Series Model

To detect quantitatively transverse cracks, the new technique was developed from the viewpoint of the time series model. The transfer function of PMC (polymer matrix composites) was determined using the mechanical model and the time series model. The response of the system depending on the internal damage was examined from the change of the time series signal passing through PMC. Moreover, the relationships among ARMA (auto regressive moving average) parameter, transverse crack density and mechanical constants were discussed. As a result, it was found that the increase of transverse cracks can be evaluated by using AR (auto regressive) parameter. It is also suggested that this detecting technique is applicable to the health-monitoring.