Is there any contradiction between the stress and energy failure criteria in micromechanical tests? Part III. Experimental observation of crack propagation in the microbond test

A direct observation of crack propagation in the microbond test was carried out for five different fiber/polymer matrix systems. This technique appeared to be a very effective tool for interface characterization. Experimental plots of the force required for further crack propagation as a function of debond length were analyzed using both energy-based and stress-based models of debonding. The fracture mechanics analysis was used to construct families of crack resistance or R-curves which showed the variation of energy release rate, G, with the debond length, and included the effect of interfacial friction in debonded regions. For the first time, analogs of the R-curves were created within the scope of the stress-based model to present the local shear stress near the crack tip, τ, as a function of crack length. In both models, the behavior of the interfacial parameter (G or τ) strongly depends on the assumed value of the interfacial frictional stress (τ_f). However, for each matrix/fiber system there exists such a τ_f value for which the investigated parameter is nearly constant over the whole region of stable crack propagation (70–90% of the embedded length). Moreover, these best-fit τ_f values for each specimen appeared to be practically the same for both energy-based and stress-based approaches. Thus, both interfacial toughness, G _ic, and local interfacial shear strength, τ_d, adequately characterize the strength of a fiber/matrix interface. Extrapolation of R-curves and their analogs to zero crack length allows measurement of the interfacial parameters with good accuracy.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength

The techniques aimed at adhesion strength measurement between reinforcing fibers and polymer matrices (the pull-out and microbond tests) involve the measurement of the force, F _max, required to pull out a fiber whose end is embedded in the matrix. Then, this maximum force value is used to calculate such interfacial parameters as the apparent bond strength, τ_app, and the local interfacial shear strength (IFSS), τ_d. However, it has been demonstrated that the F _max value is influenced by interfacial friction in already debonded regions, and, therefore, these parameters are not purely 'adhesional' but depend, in an intricate way, on interfacial adhesion and friction. In the last few years, several techniques for separate determination of adhesion and friction in micromechanical tests have been developed, but their experimental realization is rather complicated, because they require an accurate value of the external load at the moment of crack initiation. We have developed a new technique which uses the relationship between the maximum force and the embedded length ('scale factor') to separately measure fiber-matrix interfacial adhesion and friction. Using the equation for the current crack length as a function of the applied load, based on a stress criterion of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force value versus the embedded length. By varying τ_d and interfacial friction, τ_f, to fit experimental plots, both interfacial parameters were estimated. The micromechanical tests were modeled for three types of specimen geometries (cylindrical specimens, spherical droplets, and matrix hemispheres in the pull-out test) with different levels of residual thermal stresses and interfacial friction. The effect of all these factors on the experimental results is discussed, and the importance of specimen geometry is demonstrated. One of the most interesting results is that the 'ultimate' IFSS (the limiting τ_app as the embedded length tends to zero) is not always equal to the 'local' bond strength.     

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part II: Critical energy release rate

A new approach to experimental data treatment in the pull-out and microbond tests has been developed. It uses the relationship between the maximum force recorded in these tests and the embedded length ('scale factor') to separately determine adhesional interfacial parameters (critical energy release rate, local bond strength) and interfacial friction in debonded regions. The new method does not require the measurement of the debond force, which corresponds to interfacial crack initiation, and is, therefore, much more convenient and simpler than 'direct' techniques involving continuous monitoring of crack growth. Using the equation for the current crack length as a function of the load applied to the fiber, based on a fracture mechanics analysis of interfacial debonding, we modeled the pull-out and microbond experiments and obtained the maximum force versus the embedded length. By varying the critical energy release rate and interfacial frictional stress to fit experimental plots, both interfacial parameters were determined for several fiber-polymer pairs. Effects of specimen geometry, residual thermal stresses, and interfacial friction on the measured values are discussed. The results are compared with those obtained with our similar stress-based approach. The energy criterion works when the embedded length is not very short, and in this range of embedded length it is better than the stress criterion. Both criteria can be complementarily used for interface characterization.     

Effects of atmospheric pressure helium/air plasma treatment on adhesion and mechanical properties of aramid fibers

In order to investigate the effect of atmospheric pressure plasmas on adhesion between aramid fibers and epoxy, aramid fibers were treated with atmospheric pressure helium/air for 15, 30 and 60 s on a capacitively-coupled device at a frequency of 5.0 kHz and He outlet pressure of 3.43 kPa. SEM analysis at 10 000× magnification showed no significant surface morphological change resulted from the plasma treatments. XPS analysis showed a decrease in carbon content and an increase in oxygen content. Deconvolution analysis of C1s, N1s and O1s peaks showed an increase in surface hydroxyl groups that can interact with epoxy resin. The microbond test showed that the plasma treatment for 60 s increased interfacial shear strength by 109% over that of the control (untreated). The atmospheric pressure plasma increased single fiber tensile strength by 16-26%.     

Curing Kinetics and Effects of Fibre Surface Treatment and Curing Parameters on the Interfacial and Tensile Properties of Hemp/Epoxy Composites

The curing kinetics of neat epoxy (NE) and hemp fibre/epoxy composites was studied and assessed using two dynamic models (the Kissinger and Flynn–Wall–Ozawa Models) and an isothermal model (the Autocatalytic Model) which was generally supported by the experimental data obtained from dynamic and isothermal differential scanning calorimetry (DSC) scans. The activation energies for the curing of composites exhibited lower values compared to curing of NE which is believed to be due to higher nucleophilic activity of the amine groups of the curing agent in the presence of fibres. The highest tensile strength, σ was obtained with composites produced with an epoxy to curing agent ratio of 1:1 and the highest Young's modulus, E was obtained with an epoxy to curing agent ratio of 1:1.2. Alkali treated hemp fibre/epoxy (ATFE) composites were found to have higher σ and E values compared to those for untreated hemp fibre/epoxy (UTFE) composites which was consistent with the trend for interfacial shear strength (IFSS) values. Composites σ and E were found to be higher for a processing temperature of 70°C than for 25°C for both UTFE and ATFE composites, but were found to decrease as the curing temperature was increased further to 120°C.     

Characterization of Surface Modification of Polyethersulfone Membrane

Surface modification of polyethersulfone (PES) membrane surfaces using UV/ozone pretreatment with subsequent grafting and interfacial polymerization on membrane surface was investigated in order to improve the resistance of membrane surface to protein adsorption. The surface modifications were evaluated in terms of hydrophilicity, chemical composition of the surface and static protein adsorption. In both methods, poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG) and chitosan were chosen as hydrophilic polymers to chemically modify the commercial virgin PES membrane to render it more hydrophilic as these materials have excellent hydrophilic property. Modified PES membranes were characterized by contact angle and XPS. Contact angles of modified PES membranes were reduced by 19 to 58% of that of the virgin PES membrane. PES membrane modified with PEG shows higher wettability than other hydrophilic materials with the highest contact angle reduction shown for UV/ozone pretreated, PEG grafted PES membrane surface. In general, XPS spectra supported that the PES membranes were successfully modified by both grafting with UV/ozone pretreatment and interfacial polymerization methods. The results of the static protein adsorption experiments showed all surface modifications led to reduction in protein adsorption on PES membranes; the highest protein adsorption reduction occurred with membrane modified by UV/ozone pretreatment followed by PES grafting, which corresponded to the highest contact angle reduction. However, there seems to be no clear correlation between contact angle reduction and reduction in protein adsorption in the case that involved chitosan. Nevertheless, membranes modified with chitosan do show higher reduction in protein adsorption than membranes modified with other materials under the same conditions.     

Prediction of the tensile load-bearing capacity of a co-cured single lap joint considering residual thermal stresses

In this paper, stress distributions in a co-cured single lap joint subjected to a tensile load were investigated using the finite element analysis. Residual thermal stresses, which resulted from the curing process of the co-cured single lap joint, were also considered. Since the adhesive layer in the co-cured single lap joint was about 10 μm thick, very thin compared with the thickness of both adherends, the interface between the steel and composite adherends was assumed to be perfectly bonded. The co-cured single lap joint was analyzed with respect to several bond parameters such as the bond length and stacking sequence of the composite adherend. The failure mechanism of the co-cured single lap joint was partial cohesive failure in the composite material, which was significantly affected by the interfacial tensile stress at the free edge of the co-cured single lap joint. Interfacial tensile stress was a primary factor that caused interfacial delamination between the steel and composite adherends in the co-cured single lap joint. Finally, tensile load-bearing capacities calculated from the Ye-delamination failure criterion were compared with the experimental results, and relatively good agreement was found.     

Effect of Glycerol Coating on the Atmospheric Pressure Plasma Treatment of UHMWPE Fibers

In order to investigate how coatings of glycerol affects atmospheric pressure plasma treatment, ultra high molecular weight polyethylene (UHMWPE) fibers were first pretreated with 0.2 and 0.6 mol/l glycerol solutions, respectively, and then were modified by an atmospheric pressure plasma jet (APPJ) using helium as the carrier gas with a flow rate of 20 l/min, discharge power of 30 W and a radio frequency of 13.56 MHz. After the plasma treatment, scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis revealed that the glycerol coated-APPJ treated samples possessed smoother surface than the APPJ directly treated samples. The X-ray photoelectron spectroscopy (XPS) analysis indicated that the changed content of oxygen containing groups on the surface of the glycerol coated groups compared with the non-glycerol coated group was mainly due to the remaining glycerol on the fiber surfaces. The water contact angle test revealed that the wettability of the glycerol coated-APPJ treated fibers decreased slightly in comparison with the APPJ directly treated fibers. Furthermore, the microbond pull-out test indicated that the interfacial bonding of the fiber to epoxy resin decreased when the fiber was pretreated with glycerol before plasma treatment. Therefore, it was concluded that the presence of glycerol on fiber surface weakened the effectiveness of APPJ treatment of UHMWPE fibers in improving the interfacial bonding to epoxy. This was mainly attributed to the consumption of plasma energy in etching the glycerol layer on the fiber surface and a weak interfacial layer due to the presence of residual glycerol.     

On the Effect of Time-Dependent Deformations on the Interface Behaviour of RC Beams Strengthened by FRP Plates

In this paper, the effect of time-dependent deformations (such as shrinkage and creep) on the interfacial stresses between a concrete beam and a fibre reinforced polymer plate is presented. The analysis given here involves a closed-form solution for such stresses and includes creep and shrinkage effects. The adherend shear deformations have been included in the present theoretical analysis by assuming a parabolic shear stress through the thickness of both concrete beam and fibre reinforced polymer panel. Contrary to some existing studies, the assumption that both the concrete beam and the fibre reinforced polymer panel have the same curvature is not used in this investigation. The influence of creep and shrinkage effect relative to the time of the casting and the time of the loading of the beams is taken into account. Numerical examples of a typical concrete beam strengthened with an externally bonded fibre reinforced polymer plate are discussed with the emphasis on the shear and normal stresses at the edge of the plate.     

The role of joint viscoelastic function in the adhesion of low-density polyethylene to thermoplastic starch

Measured adhesion energies of low-density polyethylene (LDPE) to thermoplastic starch (TPS) joint and of joints in presence of poly(ethylene-co-vinyl acetate) (EVA), polyethylene grafted with maleic anhydride (PE-g-MAH) and styrene-ethylene-butadiene-styrene grafted with maleic anhydride (SEBS-g-MAH) compatibilizers were investigated. The compatibilizers were introduced to the interface via their pre-mixing with the adherend (PE) or adhesive (TPS). The results showed adhesion energy improvement from 41 J/m² for PE/TPS to 118, 151 and 272 J/m² by adding 3.3 wt% of EVA, SEBS-g-MAH and PE-g-MAH to the PE adherend, respectively. On the other hand, by raising the compatibilizers to 5.75 wt%, similar joint adhesion energies of about 250 J/m² were found for all studied systems. The measured adhesion energy increments were attributed to the migration of the compatibilizers to the interface during high temperature joint preparation. In addition, the observed efficacies of various compatibilizers were ascribed to their interfacial stress transfer capabilities. Joint viscoelastic function, i.e., joint adhesion energy divided by its thermodynamic work of adhesion, showed similar dependence on adherend tan δ divided by the adhesive tan δ (A) divided by the compatibilizer tan δ as the measured adhesion energy. This interesting finding supports the hypothesis that the main viscoelastic loss effects in joints with stiff adherends are localized in the interphase adjacent to the crack tip.