László Mihály Vas

Modeling and Testing the Fracture Process of Impregnated Carbon-fiber Roving Specimens During Bending: Part I – Fiber Bundle Model:

A summarizing description of a statistical modeling method partly based on earlier publications is given to predict the total loading and breaking process of fiber bundles generated by tensile tests. This method uses some types of idealized fiber bundles, the so-called fiber bundle cells (basic types: E, EH, ES, and ET) to model the structure of real fibrous structures. Using one of these bundle cells or a composite bundle made of some bundle cells connected in parallel, the expected value and standard deviation of the whole damage process of this bundle can be calculated up to the breakage of the last intact fiber during a mechanical test. As a new application the fracture process of unidirectional composite beam is modeled during the 3P bending test considering the beam is built up of elementary embedded-fiber layers considered as E-type fiber bundle cells in the first step. Formulae for calculating the expected value and standard deviation processes of the bent specimen are elaborated assuming that the fiber breakages determined the failure of layers.

Strength Modeling of Two-component Hybrid Fiber Composites in case of Simultaneous Fiber Failures

The strength of unidirectional short fiber hybrid structures and composites is strongly influenced by the length of the constituent and reinforcing elementary fibers. Based on our earlier fiber bundle-theoretical results, we have developed a simple statistical model and determined the strength of hybrid structures for elementary fibers, which simultaneously fail or debond from the environment in a brittle manner, and the dependence of the strength on the length of the elementary fibers. In the case of constant elementary fiber length, a simple analytical relation is obtained between the average tensile strength and the length of the elementary fiber. Based on these results, formulae have been developed to estimate the strength of unidirectional short hybrid fiber reinforced composites as a function of fiber length and fiber content. Practical applicability of the results has been demonstrated for basalt fiber hybrid composites taking into account fiber fragmentation, imperfect interfacial adhesion, and fiber orientation by using suitable correction factors.

Thermochemical stabilization and analysis of continuously electrospun nanofibers

Carbon nanotube (CNT)-loaded and neat polyacrylonitrile nanofibers were produced by a needleless continuous electrospinning method as carbon nanofiber precursors. The details of the stabilization, which is a crucial issue during carbon fiber production, were investigated as these nanofibers are especially sensitive to degradation. In order to determine the optimal parameters, the nanofibers were stabilized at different temperatures. The stabilized samples were analyzed by Fourier-transform infrared spectroscopic and differential scanning calorimetric (DSC) measurements and by the determination of the color changes. The chemical changes during the stabilization (the formation of the so-called ladder-polymer) can be followed by infrared spectrometry, while the conversion can be monitored by DSC. The formation of the ladder-polymer occurs according to the Gaussian distribution function, where the temperature of the stabilization is the statistical parameter, which was also determined. In the case of CNT-loaded samples, the range of stabilization temperature was wider, which provides better controllability of the process. Based on the established models, an appropriate multi-step heat-treatment program could be determined, which led to completely stabilized nanofibers, suitable for carbonization.

Creep failure strain estimation of glass fibre/polypropylene composites based on short-term tests and Weibull characterisation

In order to study the short- and long-term creep behaviours of injection moulded, unreinforced and glass fibre reinforced polypropylene composites, tensile and creep measurements were carried out and analysed. Based on the linear viscoelastic behaviour and variable transformations as well as Weibull-based distribution characterisation, estimations for the tensile strength parameters and creep failure strain were determined and fitted to the measurements. These mathematical estimations and the relationships between the material model czonstants and the fibre content, determined by fitting, give a possibility for designers’ calculations at arbitrary creep load levels and fibre contents.

Investigation of fiber/matrix adhesion: test speed and specimen shape effects in the cylinder test

The cylinder test, developed from the microdroplet test, was adapted to assess the interfacial adhesion strength between fiber and matrix. The sensitivity of cylinder test to pull-out speed and specimen geometry was measured. It was established that the effect of test speed can be described as a superposition of two opposite, simultaneous effects which have been modeled mathematically by fitting two parameter Weibull curves on the measured data. Effects of the cylinder size and its geometrical relation on the measured strength values have been analyzed by finite element method. It was concluded that the geometry has a direct influence on the stress formation. Based on the results achieved, recommendations were given on how to perform the novel single fiber cylinder test.

Strength of Unidirectional Short Fiber Structures as a Function of Fiber Length

The strength of super-oriented polymer fibers or unidirectional short fiber composites strongly depends on the length of the molecule chains or the reinforcing fibers, respectively. A simple statistical model is developed based on the theory of fiber flows and the strength of the structure and its dependence on fiber length are determined in case of fibers broken or damaged owing to parting rigidly from the environment simultaneously. Later, two new special and modified versions of the so-called ES-bundle and one of the idealized statistical fiber-bundle-cells developed earlier, are introduced and applied to model the breaking process of the fibrous structure at different damage modes and its dependence on the fiber length is analyzed. In case of constant fiber length, a simple analytical relationship between the mean tensile strength and the fiber length is obtained which is valid for all the cases discussed. On the basis of the results, formulas are developed to estimate the strength of short fiber composites as a function of fiber length and fiber content, as well as to model the effect of the reduction in the fiber adhesion in case of a very small matrix content. The practical applicability of the results is demonstrated by identifying the relationship between tensile strength and molecule mass of PP fibers.

Analysis of the Creep Behavior of Polypropylene and Glass Fiber Reinforced Polypropylene Composites

In this paper tensile and creep tests were performed on polypropylene (PP) and its glass fiber reinforced composites. The tensile tests were carried out on 6 different glass fiber content reinforced PP composites (0, 5, 10, 20, 30 and 40%) while the creep tests were performed on the unreinforced and 30% and 40% fiber reinforced ones of industrial importance. 50 N/s constant force rate was used until the specimen failed (tensile test) or the preset load level was reached (creep test). The applied load levels for the creep experiments were determined as given ratios of the average breaking force. The tensile breaking strain and tensile strength versus fiber content relationship were analyzed and described by empirical formulas based on the correction and averaging procedure developed.

Testing and Modeling the Tensile Strength Behavior of Glass Fibers, Fiber Bundles and Fiber Mat

In this paper glass fibers, chopped roving pieces, and fiber mats were tested in case of emulsion bonded glass fiber mat samples. Relationships between the tensile strength properties of the different structural levels of the fiber mats were studied and fiber bundles as structural elements of fiber mats were modeled by idealized statistical fiber bundles developed by the Department of Polymer Engineering, BUTE. The geometrical and mechanical measurements were carried out by image processing methods and a computer aided tensile tester. The results proved that the modeling method gave a tool in understanding the failure behavior of the fiber mat samples and studying the effect of the structural parameters. The applicability of the modeling method is demonstrated by the good agreement with some measurements.

Relationship between bending modulus and test sizes of laminated glass/polyester composites

The effect of span-to-thickness ratio (L/h) on the bending modulus was investigated in warp, 45°, 67.5° and weft directions in woven glass fiber reinforced polyester laminates. Using classical beam theory the results of the bending tests carried out for L/h = 5,..,50 were extended to the total range of definition (0≤L/h<∞) by applying rational fractional and exponential fractional functions. The extensions take the asymptotic behavior of the bending test for L/h = 0 and L/h = ∞ in account while providing the best fitting to the measured data. It has been shown that the exponential fractional estimations give better results regarding the form of functions that is independent from the directions.

INVESTIGATING DAMAGE PROCESSES OF GLASS FIBER REINFORCED COMPOSITES BY USING IMAGE PROCESSING

The local strength of a glass fiber mat and a glass fiber mat reinforced unsaturated polyester (UP) composite sheet were tested by using tensile tester and image processing systems. In addition, the damage process was analyzed by calculating fiber orientation histograms determined on the basis of grabbed images.