J. Andersons

Adhesion of silicon oxide layers on poly(ethylene terephthalate). I: Effect of substrate properties on coating's fragmentation process

Reference LTC-ARTICLE-1997-008View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

Adhesion of silicon oxide layers on poly(ethylene terephthalate). II: effect of coating thickness on adhesive and cohesive strengths

Reference LTC-ARTICLE-1997-007View record in Web of Science URL: http://www3.interscience.wiley.com/cgi-bin/jhome/36698 Record created on 2006-06-26, modified on 2016-08-08

Glass fibre strength distribution determined by common experimental methods

The tensile strength of brittle fibres is routinely described by the Weibull distribution. The parameters of the distribution can be obtained from tests on single fibres and fibre bundles or from model composite tests. However, there is growing evidence that the distribution parameters obtained by different experimental techniques differ systematically. In order to investigate the possible causes of such discrepancies, single-fibre tension, fibre bundle, and single-fibre fragmentation tests are employed in this study to obtain strength distribution of commercial E-glass fibres. The results reveal parameter dependence on the approach used to extract the distribution parameters from experimental data. Particularly, in the case of single-fibre tension tests, the shape parameter obtained from average fibre strength vs. length data is larger than that obtained at a fixed gauge length. It is assumed that the apparent fibre strength scatter is caused by both the inherent flaw structure along a fibre and by the fibre-to-fibre strength variability within a batch, due to slightly differing processing and handling history of the fibres. Fibre fragmentation test results are used to derive the Weibull distribution parameters applicable to the fibre batch. The strength distribution obtained is compared with strength data for the single fibres, and reasonably good agreement is observed.

Model of delamination propagation in brittle-matrix composites under cyclic loading

A model of interlaminar fatigue crack growth based on damage accumulation ahead of the crack is proposed. Linear cumulative assumption is used for damage estimation, and a quadratic failure criterion is applied for complex interlaminar loading. Model parameters are determined from mode I and mode II fatigue tests, and used to predict mixed-mode delamination propagation rate. Comparison of theoretical prediction with mixed-mode test results for different brittle graphite FRP at several mode- and load ratios show reasonable agreement.

Analysis of the initial fragmentation stage of oxide coatings on polymer substrates under biaxial tension

Crack patterns of 100-nm thick silicon oxide coatings on poly(ethylene terephthalate), polypropylene and polyamide films are analyzed under equibiaxial stress loading. The loading is achieved by means of a bulging cell mounted under an optical microscope with stepwise pressurization of film specimens. The evolution of the coating fragment area distribution in the early stages of the fragmentation process is modeled using Weibull statistics to describe the coating strength. Fragment areas are found to follow the exponential distribution at low strains close to the cracking onset. With the increase of equibiaxial strain, the distribution changes reflecting stress transfer and crack propagation features.

Strength and Damage of Elementary Flax Fibers Extracted from Tow and Long Line Flax

Flax fibers possess high specific strength and stiffness, and thus are competitive in terms of mechanical properties with traditional reinforcing fibers used in polymer-matrix composite materials. For environmental and economical benefit, it would be preferable to apply nontextile grade fibers in composites provided their mechanical characteristics are acceptable. Elementary fibers have been extracted from long line flax, used as high-quality raw material for textile industry, and flax tow, and their strength distribution and damage level determined. It is shown that the elementary flax fibers coming from short flax fiber are not inferior to those of textile-quality flax in terms of strength and damage.

Modeling the nonlinear deformation of flax-fiber-reinforced polymer matrix laminates in active loading

In an attempt to fully utilize the mechanical properties of bast fibers in polymer-matrix composites, unidirectional (UD) or quasi-UD flax-fiber-reinforced composites are being developed and characterized. Their response in tension is markedly nonlinear both in on- and off-axis loading. A semiempirical tensor-linear model is applied to describe such deformation nonlinearity in active combined loading. The deformation model of UD ply, combined with an elementary laminate theory, is used to predict the stress–strain curves of laminated composites in tension. Reasonable accuracy of prediction is demonstrated for fiber-dominated layups.

Mechanical performance of thermoplastic matrix natural-fibre composites

Publisher Summary Natural-Fibre Composites (NFCs) are being considered as high-tech materials meant to replace conventional composites in high-performance applications. There are a number of advantages that natural fibres can offer compared with synthetic materials. Thermoplastic natural-fibre composites can be classified by the polymer used as a matrix as well as by the type of the fibre reinforcing the polymer. Currently, the main areas of application of thermoplastic NFCs are packaging, transportation and building industries. The packaging industry requires materials that possess specific properties, but that can also be easily discarded after use. This chapter discusses the mechanical properties of thermoplastic natural-fibre composites and grades their performance with respect to their synthetic counterparts. Tensile, flexural and compressive properties of NFCs are rather competitive compared with glass fibre composites, especially if comparison of specific properties is made. However, long-term performance, highly non-linear behaviour, low-impact strength and extreme sensitivity to temperature and moisture are weak points of thermoplastic matrix natural-fibre composites in terms of mechanical performance.

Strength-length scaling of elementary hemp fibers

The application of hemp fibers as a reinforcement of composite materials necessitates the characterization of fiber strength scatter and the effect of fiber length on its strength. With this aim, elementary hemp fibers were tested in tension at two different gage lengths. Due to the similar morphology of hemp and flax fibers, the probabilistic strength models derived and verified for the latter were applied to the former. The fiber strength was found to agree with the modified Weibull distribution. The modeling approaches developed for describing the variability of the strength and failure strain of elementary flax fibers are shown to be also applicable to hemp fibers.

Markov model of fatigue of a composite material with the poisson process of defect initiation

As a development of the model where only one weak microvolume (WMV) and only a pulsating cyclic loading are considered, in the current version of the model, we take into account the presence of several weak sites where fatigue damage can accumulate and a loading with an arbitrary (but positive) stress ratio. The Poisson process of initiation of WMVs is considered, whose rate depends on the size of a specimen. The cumulative distribution function (cdf) of the fatigue life of every individual WMV is calculated using the Markov model of fatigue. For the case where this function is approximated by a lognormal distribution, a formula for calculating the cdf of fatigue life of the specimen (modeled as a chain of WMVs) is obtained. Only a pulsating cyclic loading was considered in the previous version of the model. Now, using the modified energy method, a loading cycle with an arbitrary stress ratio is “transformed” into an equivalent cycle with some other stress ratio. In such a way, the entire probabilistic fatigue diagram for any stress ratio with a positive cycle stress can be obtained. Numerical examples are presented.