Fiber Protrusion Research Articles

Fiber orientation and rheological properties of short fiber-reinforced plastics at higher shear rates

Fiber orientation and rheological properties of short fiber-reinforced polystyrene at high shear rates were investigated by scanning electron microscopy (SEM) and rheometry. This is the first attempt to observe fiber orientations in samples taken from inside the capillary as well as taken from the extrudate by employing SEM. The fiber orientations near the center of the capillary are similar to those near the wall. Also, the fiber orientations inside the capillary changed very little, even if the shear rate was varied from 25.4 to 3110 s−1. This is because most fibers near the center as well as near the wall of the barrel are already aligned toward the flow direction before entering the capillary. The possible sources to induce surface roughness of the extrudate or fiber protrusion are discussed. It appears that the surface roughness depends mainly upon the velocity rearrangement at the capillary exit. Interestingly, fiber flexing was seen in the filament taken from the extrudate as well as from inside the capillary, which suggests that this is not likely a dominant source for inducing surface roughness.

Evaluation of the mechanical properties of PMC interface using slice compression test — Analysis of transfer mechanism of interfacial shear stress

Properties of the fiber/matrix interface in SiO2/epoxy and SiC/epoxy composite are investigated using the slice compression test (SCT) for the single fiber, where the specimen is loaded and unloaded between a plate which has different mechanical properties. It is found that the interfacial debonding proceeds from the polished surface at a soft plate side and that the fiber protrusion occurs after unloading. The fiber-protrusion length is directly measured at each applied stress level using a scanning electron microscope. Interfacial shear-sliding stress is obtained based on the constant shear-sliding stress analysis employing the obtained protrusion length. It is demonstrated that the value of interfacial shear-sliding stress shows good agreement with that obtained from another technique, the push-out test, on the same system. The relation between the fiber-protrusion length and applied stress is proportional to a certain extent. From this result, it is analytically pointed out that the applied stress has a limiting value in this SCT because of Poisson's effect. Also, two interfacial debonding criteria, which are determined analytically for the PMC, are discussed.

An experimental study of large scale model composite materials under thermal fatigue

Abstract The effects of temperature cycling on the structural integrity of polyester matrix composites were investigated using a large scale model composite. The damage of composites during the heating and cooling phases was observed when subjecting the composites to thermal cycling using an especially designed temperature cycling apparatus. Results of the tests revealed that cracks were initiated at the fiber/matrix interface near the free surface as early as after one thermal cycle. Deterioration of the composites was found to occur only during the cooling phase of the thermal cycle. Fiber protrusion was also observed, with fibers protruding from the surface of the matrix at both ends of the specimen. Fiber/matrix debonding, initiated at the same time as fiber protrusion, progressed with increasing number of thermal cycles, and was found to arrest at approximately the same number of cycles as the number at which the progression of protrusion was found to terminate. The magnitude of the resulting fiber/matrix separation when the protrusion is stabilized was found to be equal to approximately three times the diameter of the fiber.

Experimental and theoretical aspects of slice compression tests

A basis for evaluating the interfacial properties of aligned ceramic matrix composites from slice compression tests is provided in the present study. Specifically, experimental results of tests performed on unidirectional Tyrrano SiC fiber-reinforced lithium aluminosilicate composites are analyzed. Precautions in experimental procedures are discussed, and the analytical solutions required for data analysis are summarized. To extract the interfacial properties, the measured relation between the residual fiber protrusion after complete unloading and the peak loading stress is fitted to the theoretical analysis.

Analysis of fiber pullout or push-in with frictional sliding at the fiber-matrix interface

The detail stress fields caused by fiber pullout or push-in while the fiber-matrix interface is under compressive thermal residual stress and undergoing frictional sliding are studied. The dominant stress fields in the local region at the immediate vicinity of the fiber protrusion point are solved using Muskhelishvili-Kolosov complex potential theory and asymptotic analysis. Parameteric studies determined the effects cast on the local fields by fiber-matrix material property combination, coefficient of friction, and the direction of relative fiber sliding. Reversing fiber sliding from pullout to push-in completely changes the nature of the local field. Using two specific composite systems as examples (fiber push-in Nicalon/calcium aluminosilicate composite, and fiber pullout in AVCO-SCS-6/borosilicate composite), the germane features of the local fields are independently verified by finite element global analyses.

A local-global matching method for the single fiber pullout problem with perfectly bonded interface

This paper presents a local-global matching method to effectively determine the detail spatial structure and magnitude of the locally singular stress field as well as the complete global stress distribution in the single fiber pullout model. The motivation to solve for the local stress field is the belief that these accentuated stresses, strains and energy are likely to induce damage. The local-global matching method consists of three components: a local analysis, a global analysis, and proper matching of the local asymptotic field to the global complete stress field. The method as developed is applicable for various fiber-matrix interfacial conditions: namely perfectly bonded interface, partially debonded interface with interfacial crack, or debonded interface with frictional interfacial sliding. In this paper, results for perfectly bonded fiber-matrix interface are presented to illustrate key features of the local-global matching method. For this problem, the local stress field is asymptotically singular at the location where the fiber protrudes from the matrix. A local analysis at the fiber protrusion point reveals that, when the fiber is stiffer than the matrix, the most dominant singular stresses are of the from: $$\sigma _{ij} (\rho ,\theta ) = K\rho ^\lambda \tilde \sigma _{_{ij} } (\theta )$$ where the exponent λ is real-valued and λ<0. The local analysis can solve for the spatial structure of the local field: its radial dependence ρλ and angular variations $$\tilde \sigma _{ij} (\theta )$$ . The actual magnitude of the local stress field is scaled by the amplitude factor K which depends upon externally applied load and global boundary conditions. The global analysis, performed using a finite element model, can be subjected to arbitrary fiber-pulling load and/or thermal load. With solutions from both the local analysis and global analysis, a local-global matching method based on angular variation of stresses is developed to accurately determine K. In local-global matching, a proper region is selected in which the angular variation of stresses of the local field is scaled to match the angular variation of the finite-element computed full-field stresses. Several monitoring parameters are developed to measure the quality of the matching and to determine the region of dominance of the local asymptotic field. The local analysis shows that in many composite material systems, there are two singular terms in the local field: $$\sigma _{ij} K = \rho ^{\lambda _1 } \tilde \sigma _{ij}^1 (\lambda ;{\text{ }}\theta ) + K_2 \rho ^{\lambda 2} \tilde \sigma _{ij}^2 (\lambda ;{\text{ }}\theta ), - 1 < \lambda _1 < \lambda _2 < 0$$ . Hence, local-global matching procedures have been developed for both one-term field and two-term field. The matching method is further generalized to determine complex-valued K for composites having complex-valued λ. The local-global matching method may also be applied to problems with material nonlinearity.

Analyses of slice compression tests for aligned ceramic matrix composites

The application of slice compression tests to evaluate the interfacial properties of aligned ceramic matrix composites is addressed. A slice of the aligned composite, which is cut normal to the fiber, is subjected to a constant compressive stress and a constant compressive strain at the top and the bottom surfaces, respectively. The fiber protrusion length vs the applied stress relation during the loading-unloading cycle of the test is analyzed. The interfacial bonding, Coulomb friction at the debonded interface, Poisson's effect, and residual stresses are included in the analysis, and closed-form analytical solutions are obtained. Based on the analytical solutions, a methodology is established to extract the interfacial properties from experimental results. The roles of interfacial bonding, Poisson's effect, and residual stresses on the residual fiber protrusion after complete unloading are also addressed in the present study.

Microstructural evaluation and fiber debonding in aligned ceramic-matrix composites

The slice compression test (SCT) represents a new approach for the estimation of the interfacial shear stress and debond length in fiber-reinforced ceramics. The technique is based upon multiple measurements and simultaneous loading of a large number of fibres in the test specimen. The specimen slice is compressed between a hard ceramic anvil and a ductile metal to produce interfacial debonding and slippage as a result of the fiber/matrix elastic mismatch. The extent of fiber protrusion under maximum load and the residual protrusion of the fibres after load relaxation are the experimentally determined parameters and have been measured for a laminate reinforced with Nicalon SiC fiber (LASIII). The minimum applied stress for the initiation of debonding was estimated. In addition, the microstructure of the ceramic composite was quantitatively characterized to determine numerical values of the microstructural parameters which could affect the interfacial shear stress. The fiber diameters and spatial fiber distribution were determined to obtain the degree of order of the fibers within the composite.

A high-performance single-mode multifiber connector using oblique and direct endface contact between multiple fibers arranged in a plastic ferrule

A novel type of high-performance single-mode multifiber connector was developed using oblique and direct endface contact. This was obtained by butting a pair of obliquely polished plastic ferrules with a slight fiber protrusion. The fabricated connector offers a low insertion loss of 0.16 dB and a high return loss of 59 dB for single-mode four-fiber ribbons without index-matching. It enables easy reconnection and high performance for fiber ribbon cable joints. >

Surface characteristics of the cardiac prostheses in vivo.

The pseudoneointima (PNI) deposited onto a cardiac prosthesis surface reflects many factors of biocompatibility, surface morphology, flow distribution, design, animal's physiological condition, and duration. In the evaluation of any prosthesis, the PNI is one of the prime considerations from both material and functional standpoints. Historically, Dacron fabric has been used as an internal lining for cardiac prostheses. However, we have observed cracks on the Dacron fibers, fiber fracture, fiber protrusion, and poor attachment to the diaphragm, which can cause potentially disastrous complications. In addition, there are basic differences in the PNI formation on aldehyde-treated pericardium and natural aortic valves as compared to the Dacron fabric. 1) Minimal degeneration takes place on the chemically treated natural tissue compared with the fabtic surface. Intact cells on the tissue suggest a greater compatibility. In later specimens (13 and 24 days), there is active cell infiltration onto the pericardium structure with capillary formation. 2) The deposits on natural tissue are mostly fibrin, with minimum cellular involvement and a trend toward reduction in thickness. 3) Fibroblast cells are found on the natural tissue as early as 7 days but were not observed on the Dacron fabrics. Based on these findings, the Dacron fabric-covered diaphragm studied was not favorable for use in long-term implantation of cardiac prostheses.