Size effect on compression strength of fiber composites failing by kink band propagation

The effect of structure size on the nominal strength of unidirectional fiber-polymer composites, failing by propagation of a kink band with fiber microbuckling, is analyzed experimentally and theoretically. Tests of novel geometrically similar carbon–PEEK specimens, with notches slanted so as to lead to a pure kink band (not accompanied by shear or splitting cracks), are conducted. They confirm the possibility of stable growth of long kind bands before the peak load, and reveal the existence of a strong (deterministic, non-statistical) size effect. The bi-logarithmic plot of the nominal strength (load divided by size and thickness) versus the characteristic size agrees with the approximate size effect law proposed for quasibrittle failures in 1983 by Bažant. The plot exhibits a gradual transition from a horizontal asymptote, representing the case of no size effect (characteristic of plasticity or strength criteria), to an asymptote of slope -1/2 (characteristic of linear elastic fracture mechanics, LEFM). A new derivation of this law by approximate (asymptotically correct) J-integral analysis of the energy release, as well as by the recently proposed nonlocal fracture mechanics, is given. The size effect law is further generalized to notch-free specimens attaining the maximum load after a stable growth of a kink band transmitting a uniform residual stress, and the generalized law is verified by Soutis, Curtis and Fleck's recent compression tests of specimens with holes of different diameters. The nominal strength of specimens failing at the initiation of a kink band from a smooth surface is predicted to also exhibit a (deterministic) size effect if there is a nonzero stress gradient at the surface. A different size effect law is derived for this case by analyzing the stress redistribution. The size effect law for notched specimens permits the fracture energy of the kink band and the length of the fracture process zone at the front of the band to be identified solely from the measurements of maximum loads. The results indicate that the current design practice, which relies on the strength criteria or plasticity and thus inevitably misses the size effect, is acceptable only for small structural parts and, in the interest of safety, should be revised in the case of large structural parts. This revised version was published online in July 2006 with corrections to the Cover Date.     

Swelling of acrylic microgels in aliphatic ketones

The paper deals with the swelling behaviour of hydroxyl-functionalised microgels in the presence of aliphatic ketones. The copolymer microgels with different cross-linking densities were synthesized by the semi-batch emulsion copolymerisation of methyl methacrylate, butyl methacrylate, 2-hydroxyethyl methacrylate and allyl methacrylate. The extent of microgel particles swelling was evaluated using the dynamic light scattering, the potentiometric titration of accessible hydroxyl groups and the solvent-uptake measurements. It was found that the swelling ability of microgels decreased with growing degree of cross-linking. Microgels comprising copolymerised butyl methacrylate swelled less in utilised aliphatic ketones than microgels without this comonomer. Among all the investigated solvents, acetone was found to be the strongest solvent, while 5-methyl-2-hexanone (methyl isopentyl ketone) was shown to be the weakest one. Further, the microgels were investigated as reactive network precursors in a commercial thermosetting solvent-borne acrylic binder. It was shown that the application of functionalised microgels that were redispersed in acetone did not affect the surface appearance and transparency of coatings. Moreover, the presence of microgel network precursors accelerated film curing at ambient temperature and improved final hardness of coatings.     

RBS, XPS, and TEM study of metal and polymer interface modified by plasma treatment

We performed a study of the diffusion of Ag and Au atoms in polyethyleneterephtalate (PET). Thin metal layers were deposited using a diode-sputtering technique on polymer foils at room temperature. Simultaneous post-deposition annealing and plasma treatments were used to induce metal-polymer intermixing. Rutherford back-scattering spectrometry and X-ray photoelectron spectroscopy were used to determine the integral amount of metal and chemical structure in the surface layer. After plasma treatment Ag thin films exhibit dramatic changes of chemical composition and an integral amount of metal compared to Au thin films. Transmission Electron Microscopy shows the differences in the size and the depth distribution of metal particles, depending on the annealing temperature at the metal-polymer interface.     

NO – oxygen scavenger or reaction intermediate in the decomposition of N2O?

Nitric oxide and nitrogen dioxide were found during the thermal desorption of surface species left on Fe-ferrierites after the decomposition of nitrous oxide. This demonstrates the formation of surface NOx species during N₂O decomposition. Repeated decomposition and subsequent desorption of surface species confirm the active role of surface NOx species. Addition of NO up to a fraction of 0.1 times the amount of N₂O increased the decomposition of nitrous oxide as well as the amount of surface NOx species. The use of nitrous oxide labeled with ¹⁸O demonstrated that the zeolite oxygens participate in the reaction and that the presence of NO enhances this participation.     

Effect of Heat Treatment on the Microstructure and Properties of HVOF-Sprayed Co-Cr-W Coating

Co-Cr-W HVOF-sprayed protective coatings are used for their high oxidation and wear resistance. Apart from the oxidation resistance, the stability of their mechanical properties in relation to thermal loading is crucial with respect to the most common high-temperature application areas. This work is focused mainly on evaluation of the heat-induced changes in the phase composition and related mechanical properties. It was shown that the original powder, composed fully from face-centered cubic Co-based alloy, partly changes its phase composition during spraying to a hexagonal close-packed (hcp) structure. The annealing further increases the ratio of the hcp phase in the structure. The heat-induced phase changes are accompanied by an increase in the coatings’ hardness and cohesion strength. The abrasive and adhesive wear behavior was evaluated. While the coatings’ heat treatment had a positive effect on the coefficient of friction, the abrasive and adhesive wear resistance of annealed coating was lower compared to as-sprayed coating.     

Sensitivity assessment of steel members under compression

The objective of the paper is to analyse the influence of initial imperfections on the behaviour of a steel member under compression. The influence of the variability of initial imperfections on the variability of the load-carrying capacity studied has been calculated by sensitivity analysis. The advantages of Sobol’s sensitivity analysis and the most important properties of Sobol’s sensitivity indices are described. The Sobol’s first order sensitivity indices are evaluated in dependence on the nondimensional slenderness. The Sobol’s sensitivity indices are supplemented with a lucid elaboration based on the Monte Carlo method. Material and geometrical characteristics of a steel member IPE 220 were considered to be random quantities the histograms of which were obtained from experiments. Imperfections that have a dominant influence on the load-carrying capacity are identified.     

Temperature of connections during fire on steel framed building

To study the global structural and thermal behaviour of buildings in fire, a research project was conducted including a fullscale test on a three storey steel frame building at Mittal Steel Ostrava before demolition. The main goal of the experiment was to verify the method for predicting joint temperatures and to improve it for the cooling phase. The fire compartment extending over a floor area of 24 m² was built on the second floor. The fire load was 140 kg/m² of wood and the ventilation was limited to an opening of 1,400 × 1,970 mm. This paper presents the time-temperature curves showing the development of the fire in the compartment and in the primary and secondary beams and its header plate connections. Comparisons are made between the test results and the temperatures predicted by the structural Eurocodes. The sensitivity of the connection behaviour to the estimated temperatures and associated degradation in material properties during the fire is demonstrated.     

An analysis of the non-preemptive mixed-criticality match-up scheduling problem

Many applications have a mixed-criticality nature. They contain tasks with a different criticality, meaning that a task with a lower criticality can be skipped if a task with a higher criticality needs more time to be executed. This paper deals with a mixed-criticality scheduling problem where each task has a criticality given by a positive integer number. The exact processing time of the task is not known. Instead, we use different upper bounds of the processing time for different criticality levels of the schedule. A schedule with different criticality levels is generated off-line, but its on-line execution switches among the criticality levels depending on the actual values of the processing times. The advantage is that after the transient prolongation of a higher criticality task, the system is able to match up with the schedule on a lower criticality level. While using this model, we achieve significant schedule efficiency (assuming that the prolongation of the higher criticality task rarely occurs), and at the same time, we are able to grant a sufficient amount of time to higher criticality tasks (in such cases, some of the lower criticality tasks may be skipped). This paper shows a motivation for the non-preemptive mixed-criticality match-up scheduling problem arising from the area of the communication protocols. Using a polynomial reduction from the 3-partition problem, we prove the problem to be \(\mathcal {NP}\)-hard in the strong sense even when the release dates and deadlines are dropped and only two criticality levels are considered.     

Hardness of equivalence checking for composed finite-state systems

Computational complexity of comparing behaviours of systems composed from interacting finite-state components is considered. The main result shows that the respective problems are EXPTIME-hard for all relations between bisimulation equivalence and trace preorder, as conjectured by Rabinovich (Inf Comput 139(2):111–129, 1997). The result is proved for a specific model of parallel compositions where the components synchronize on shared actions but it can be easily extended to other similar models,   to labelled 1-safe Petri nets. Further hardness results are shown for special cases of acyclic systems.