Institute for Polymer Mechanics

Studies on structure and properties of natural vegetable fibers (NVF) show that composites made of NVF combine good mechanical properties with a low specific mass. The high level of moisture absorption by the fiber, its poor wettability, as well as the insufficient adhesion between untreated fibers and the polymer matrix lead to debonding with age. To build composites with high mechanical properties, therefore, a surface modification of the fibers is necessary. The existing physical and chemical NVF modification methods—e.g., plasma treatment or graft copolymerization—which are used for the development of NVF–polymer composite properties is discussed. It is shown that modified cellulose fiber–polymer interaction mechanisms are complex and specific to every definite system. By using an coupling agent, like silanes or stearin acid, the Young's modulus and the tensile strength increases, dependent on the resin, until 50%. Simultaneously, the moisture absorption of the composites decreases for about 60%. With other surface modifications, similar results are obtained. © 1996 John Wiley & Sons, Inc.

Abstract Problems concerning the processing of thermoplastics reinforced with wood fillers are discussed. The high level of moisture absorption by the filler, its poor wettability, as well as the insufficient adhesion between untreated filler and the polymer matrix are reasons for the low tensile strength and high moisture sorption of composites. These shortcomings of composites can be prevented by the modification of the interface. The fiber-matrix compatibility and the composites properties can be improved by using some physical (e.g., steam explosion, corona, cold plasma) and chemical (cross-linking and acetylation of cellulose, grafting, use of coupling agents) methods. Modified wood-polymer interaction mechanisms are complex and specific for each definite system and processing conditions. Cellulose cross-linking and acetylation reduce hygroscopicity and swelling of wood and wood composites. Grafting of styrene to wood is effective for wood-polystyrene systems. The best coupling agent for wood-thermoplastics is polymethylenepolyphenyl isocyanate. Silanes coupling effect can be increased with additives to the polymer matrix. Optimizing of technological parameters of wood-thermoplastics processing is necessary.

Fragmentation tests in the uniaxial mode were performed on poly(ethylene terephthalate) (PET) films coated with a 100 nm thin silicon oxide layer. The coating's fragmentation process was analyzed in light of the mechanical behavior of the polymer substrate. It was shown that, upon unloading samples strained to less than 4% nominal strain, strain recovery leads to the closure of coating cracks. The usual fragmentation diagram, which shows the crack density (CD) versus applied strain, was used to identify the various energy dissipation mechanisms controlling the fragmentation process. An alternative presentation of CD versus true stress provided accurate measurements of both fragmentation and saturation onsets. The interfacial strength was modeled from the CD at saturation according to the Kelly-Tyson approach, including a Weibull distribution of the coating strength. The prediction was compared to the substrate shear stress at saturation. Effects of substrate yield, temperature, and molecular orientation are discussed. It was shown that the coating deposition by evaporation on the PET substrate did not induce structural changes at the polymer interface, whereas heat treatments increased the polymer crystallinity in the interfacial zone, resulting in higher interfacial strength. © 1997 John Wiley & Sons, Inc. J. Polym Sci B: Polym Phys 35: 1449–1461, 1997

Fiber reinforced polymers (FRPs) are considered to be a promising alternative to steel reinforcement, especially in concrete structures subjected to an aggressive environment or to the effects of electromagnetic fields. Although attempts to develop effective reinforcement have been followed, the application of FRPs remains limited by the solution to simple structural problems that mainly appear due to the absence of design codes, significant variation in the material properties of FRP composites and limited knowledge gained by engineers as regards the application aspects of FRP composites and structural mechanics of concrete elements reinforced with FRPs. To fill the latter gap, the current state-of-the-art report is dedicated to present recent achievements in FRPs applying practice to a broad engineers’ community. The report also revises the manufacturing process, material properties, the application area and design peculiarities of concrete elements reinforced with FRP composites. Along the focus on internal reinforcement, the paper overviews recent practices of applying FRP reinforced concrete (RC) elements in structural engineering. The review highlights the main problems restricting the application of FRPs in building industry and reveals the problematic issues (related to the material properties of the FRP) important for designing RC following the formulation of targets for further research.

Fragmentation tests in the uniaxial mode were performed on poly (ethylene terephthalate) (PET) films coated with a silicon oxide layer of thickness ranging from 30 to 156 nm. The coating's fragmentation process was investigated to reveal the crack onset strain and the crack density at fragmentation saturation. Adhesive strength was modeled from the Kelly-Tyson approach, including a Weibull distribution of the coating strength. The prediction was found to be independent of coating thickness, and equal to the substrate shear stress at saturation. The cohesive strength of the coating was characterized from the crack onset strain. The measured decrease in crack onset strain with coating thickness increase was modeled by means of Weibull and fracture mechanics theories, the latter providing the best predictions. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1463–1472, 1997

Abstract Flax fiber composites with thermoset and thermoplastic polymer matrices have been manufactured and tested for stiffness and strength under uniaxial tension. Flax/polypropylene and flax/maleic anhydride grafted polypropylene composites are produced from compound obtained by coextrusion of granulated polypropylene and flax fibers, while flax fiber mat/vinylester and modified acrylic resin composites are manufactured by resin transfer molding. The applicability of rule‐of‐mixtures and orientational averaging based models, developed for short fiber composites, to flax reinforced polymers is considered. POLYM. COMPOS. 27:221–229, 2006. © 2006 Society of Plastics Engineers

Studies on the effect of the foams’ polymeric matrix’ properties on the tension and compression properties of pour rigid polyurethane (PUR) foams, apparent core density 65—70 kg/m 3 , at 296 and 77 K were carried out. PUR foams were produced by the hand mixing method from polyol systems that comprised polyether, polyester polyols, and chain extenders. To produce PUR foams, crude MDI was used, and Solkane 365 mfc/227 ea was used as a blowing agent. The molecular weight per branching unit (M c ) of the polymeric matrix of PUR foams was varied in the range 300—1150. Cohesion energy densities of the blocks forming the polymeric matrix were calculated. The effect of M c on the formation of hydrogen bonds between the urethane groups was estimated from FTIR spectroscopy data and ratio NH bonded /NH free . It has been found that, with increasing polymeric matrix’ M c , the tensile strength and elongation at break of PUR foams at 296 and 77 K increases, while Young’s modulus decreases. The increase in the parameter M c promotes the decrease in the compressive strength and modulus of elasticity of PUR foams at 296 K, while compressive strength indices at 77 K are higher for the foams, whose polymeric matrix has the highest M c . With increasing polymeric matrix’ M c , the concentration of the urethane groups bonded with hydrogen bonds increases. Structural and mechanical properties of layered spray polyurethane foams, apparent core density approx. 48 kg/m 3 , having two layers and polymeric matrix’ M c = 740 were investigated.

The effect of stress raisers in the form of a slit-like notch and an open circular hole on the tensile strength of a quasi-UD flax-fiber-reinforced composite is studied experimentally. A finite fracture mechanics approach is applied to determine the intralaminar fracture toughness of the composite and to predict the strength in the presence of stress concentration. Reasonably good agreement of the notch effect predicted using finite fracture mechanics with a coupled strength and toughness fracture criterion and test results is demonstrated.

The results obtained on the problem of the interaction between a large crack and an array of microcracks or other microdefects are reviewed. The following problems are considered: interaction of main crack with microcracks in the two-dimensional case at tensile, shear or combined stress state; a closure of macro or microcracks as a result of their interaction, and the influence of this phenomenon on the stress intensity factor; the thermal cracking of an elastic solid caused by the macro-microcracks interaction and cracks closure; the interaction of a crack with an array of small pores or rigid inclusions; three-dimensional problems of the interaction of a penny-shaped crack with small penny-shaped microcracks. Discussed analytical results are based on the asymptotic analysis and the series solution to systems of singular integral equations describing the interaction of the macrocrack and microdefects. The series solutions were obtained with respect to the small parameter representing the ratio of micro- to macrocrack sizes. Throughout the review, the known solutions on the crack interaction are surveyed. The comparison with solutions to other relevant problems such as an interaction of semi-infinite crack with an array of finite cracks is given. The impact of a close crack location, and a comparison with relevant results of the continuum model approach are discussed. This review article includes 332 references.

Polyols with the hydroxyl value OHV from 290 to 318 mg KOH/g are synthesized from rapeseed, sunflower seed, flaxseed, or coconut oils by the way of their: (1) transesterification with triethanolamine or (2) amidization with diethanolamine. The influence of water as the blowing agent on the storage stability of the polyol premix system is assessed from values of the polyol premix system pH, acid value, and variations in such parameters as cream and gel time. Water-blown polyisocyanurate (PIR) foams (150 · II × 300; II—isocyanate index) characterized by good mechanical characteristics, dimensional stability at 70°C and at relative humidity RH = 95% as well as by a very low water absorbance, are obtained. The optimal physical and mechanical properties of water-blown PIR foams are achieved at the isocyanate index values 150—200. The values of the solubility parameter δ and normalized cohesion energy E coh.norm . for the groups and blocks, incorporated in the polymer matrix, are calculated. A detailed experimental analysis has proved that the water-blown PIR foams from vegetable oil polyols possess competitive physical and mechanical properties to those exhibited by the traditional petrochemical origin foams. The optimal physical and mechanical properties suitable for practical applications, are achieved at the isocyanate index values 150<II<200.

Studies on structure and properties of natural vegetable fibers (NVF) show that composites made of NVF combine good mechanical properties with a low specific mass. The high level of moisture absorption by the fiber, its poor wettability, as well as the insufficient adhesion between untreated fibers and the polymer matrix lead to debonding with age. To build composites with high mechanical properties, therefore, a surface modification of the fibers is necessary. The existing physical and chemical NVF modification methods—e.g., plasma treatment or graft copolymerization—which are used for the development of NVF–polymer composite properties is discussed. It is shown that modified cellulose fiber–polymer interaction mechanisms are complex and specific to every definite system. By using an coupling agent, like silanes or stearin acid, the Young's modulus and the tensile strength increases, dependent on the resin, until 50%. Simultaneously, the moisture absorption of the composites decreases for about 60%. With other surface modifications, similar results are obtained. © 1996 John Wiley & Sons, Inc.

The polymer liquid crystal PLC is the PET/0.6PHB copolymer; PET=poly(ethylene terephthalate), PHB=ρ-hydroxybenzoic acid (LC): 0.6=the mole fraction of PHB. This is a multiphase system with PHB-rich islands in a PET-rich matrix. Tensile creep compliance was measured isothermally from 20 °C to 160 °C in 10 °C intervals. Master curves were determined using the time–temperature superposition for 20 °C and for the glass transition temperature of the PET-rich phase TgPET=62 °C. Experimental values of the temperature shift factor aT as a function of temperature T agree in the entire T range with those from Eq. (7) relating aT to the reduced volume ṽ and the Hartmann equation of state Eq. (10). Values of aT(T) calculated from the Williams–Landel–Ferry (WLF) formula give very large errors below Tg. A control 14 months creep experiment agrees with the theoretical predictions from Eq. (7). Stress relaxation experiments were performed under the constant strain of 0.5% from 20 °C to 120 °C, again master curves were determined for 20 °C and for TgPET and aT(T) values calculated. The stress relaxation aT(T) results agree with those from creep and with those from Eq. (7).

This study investigates the mechanical behavior of steel fiber-reinforced concrete (SFRC) beams internally reinforced with steel bars and externally bonded with carbon fiber-reinforced polymer (CFRP) sheets fixed by adhesive and hybrid jointing techniques. In particular, attention is paid to the load resistance and failure modes of composite beams. The steel fibers were used to avoiding the rip-off failure of the concrete cover. The CFRP sheets were fixed to the concrete surface by epoxy adhesive as well as combined with various configurations of small-diameter steel pins for mechanical fastening to form a hybrid connection. Such hybrid jointing techniques were found to be particularly advantageous in avoiding brittle debonding failure, by promoting progressive failure within the hybrid joints. The use of CFRP sheets was also effective in suppressing the localization of the discrete cracks. The development of the crack pattern was monitored using the digital image correlation method. As revealed from the image analyses, with an appropriate layout of the steel pins, brittle failure of the concrete-carbon fiber interface could be effectively prevented. Inverse analysis of the moment-curvature diagrams was conducted, and it was found that a simplified tension-stiffening model with a constant residual stress level at 90% of the strength of the SFRC is adequate for numerically simulating the deformation behavior of beams up to the debonding of the CFRP sheets.

For damage indication needs in polymer composites, three smart acrylic coatings based on microencapsulated crystal violet lactone leuco dye with embedded developer silica gel and methyl 4-hydroxybenzoate were elaborated. Damage visualization ability upon duralumin substrates was tested. Indentation tests were conducted in the interval of 0.0005–0.55 J of mechanical work. Digital image analysis was used for evaluation of the visual response in the form of “bruise” spots. The measure of response intensity was relative excess of blue color above achromatic gray in the scanned images of the “bruise” spots. This technique allowed detecting influence of the coating formulations onto the substrate damage visualization.

STUDY QUESTION: What are the parameters of semen quality in Baltic men? SUMMARY ANSWER: Combined parameters of sperm concentration, motility and morphology revealed that 11-15% of men had low semen quality, 37-50% intermediate and 38-52% high semen quality. WHAT IS KNOWN ALREADY: Previous studies have revealed regional differences in semen parameters, and semen quality of Baltic men has been suggested to be better than that of other European men. STUDY DESIGN, SIZE, DURATION: This was a cross-sectional study of 1165 men aged 16-29 years from Estonia (N = 573), Latvia (N = 278) and Lithuania (N = 314) conducted in 2003-2004. PARTICIPANTS/MATERIALS SETTING METHODS: Men from the general population, median age 19.8 years, provided one semen sample each, had blood samples taken, had testis size determined, and provided information on lifestyle. Based on combined data of sperm concentration, sperm motility and morphology the cohort was classified into three categories: low, intermediate or high semen quality. Comparisons between groups (including subgroups of Estonian men of Russian versus Estonian ethnicity) were tested, adjusting for ejaculation abstinence and age. MAIN RESULTS AND THE ROLE OF CHANCE: The median sperm concentration of the Estonian, Latvian and Lithuanian populations of Baltic men was 63 mill/ml. Low semen quality was detected in 11-15% of the men, intermediate in 37-50% and high in 38-52%. No crucial differences between national subgroups were detected, except that a higher percentage (9.6%) of the subgroup of Russian Estonians reported having had cryptorchidism compared to the other men (2.5-3.6%, P < 0.001). Smoking had an adverse impact on both sperm concentration and total sperm counts (P < 0.001). LIMITATIONS REASONS FOR CAUTION: The semen quality data were collected >10 years ago. Thus, a recent change in semen quality cannot be excluded. Owing to the study design, it is assumed, but unproven, that the men were representative of the general populations. Some men were very young (16 years), however, this was also the case for other European studies of similar populations. Assessment of sperm motility is associated with inter-observer variation, and no quality control was undertaken for sperm motility assessment to account for that. Thus, estimates of sperm motility should be interpreted with caution. WIDER IMPLICATIONS OF THE FINDINGS: Analysis of the semen variables separately did not identify that a considerable percentage of Baltic men had low semen quality. The combined analysis, however, showed that more than one out of nine men had semen quality at a level indicating reduced fertility chances. We suggest that future studies of semen quality should be carried out reporting both results of single semen parameters and estimates that combine the most frequently assessed variables. STUDY FUNDING/COMPETING INTEREST(S): The study was funded by the EU fifth framework project Number QLK4-1999-01422 'Envir.Repro.Health' extension to Baltic countries Number QLRT-2001-02911; Estonian Science Foundation, grant numbers 2991 and PUT181. There are no competing interests. TRIAL REGISTRATION NUMBER: N/A.

Basalt polymers are rather new materials for civil engineering; therefore, identification of peculiarities and limitations of application of such polymers in concrete structures (particularly bridges) is of vital importance. This paper experimentally investigates deformation behaviour and cracking of flexural elements, which are predominant parameters governing serviceability of the bridges. Unlike a common practice, the present study is not limited by the analysis of concrete beams reinforced with the polymer bars; it also considers effectiveness of basalt fibre reinforced polymer sheets for repairing the beams. The analysis has revealed that a combination of the high strength and elasticity polymer materials governs the effective repair of the beams by significantly increasing (up to 40%) the structural stiffness.

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.

Abstract The material studied is a longitudinal polymer liquid crystal (PLC). The creep behavior of the PLC is examined in the region of nonlinear viscoelasticity. The creep compliance D curves at nine different stress σ levels, from 10 to 50 J.cm −3 at a constant temperature are determined and shifted along the log time axis for σ ref = 10 J · cm −3 to produce the D versus t/a σ master curve. A fairly general formula for stress shift factor a σ based on free volume v f and the chain relaxation capability (CRC) derived by one of the authors is applied. The formula predicts values that agree with the experimental ones within the limits of the experimental accuracy. Thus, experiments at several stress levels can serve for prediction of long‐term behavior from short‐term tests. The same value of the Doolittle constant B is obtained separately from temperature shift and stress shift experiments for the PLC.

The effect of through-thickness reinforcement by thin 1 mm steel needles (z-pins) on the static tensile strength of double-lap joints of a carbon/epoxy composite was investigated. Two types of joints-z-pinned and hybrid (including glued ones)-were considered. The joints were reinforced in the overlap region with 9, 25, or 36 z-pins. Comparing mechanical properties of the double-lap joints with the corresponding characteristics of their unpinned counterparts, the z-pins were found to be highly effective: the strength and stiffness of the pinned joints increased up to 300% and 280%, respectively. These improvements were due to a transition in the failure mechanism from debonding of the joint in the absence of z-pins to pullout or shear rupture of z-pins or to the tensile failure of laminate adherends, depending on the volume content of the pins.

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.