Anastasios P. Vassilopoulos

Fatigue Strength Prediction under Multiaxial Stress

A multiaxial fatigue failure criterion for composite materials is presented in this paper along with an assessment of the capability it offers for design under multiaxial constant or variable amplitude stresses. The applicability of this criterion, based on the well known quadratic failure tensor polynomial criterion for static loading, is validated through comparisons with uniaxial and multiaxial fatigue experimental data. Static and fatigue tests were carried out during this study on glass/polyester specimens cut off-axis from a multidirectional laminate at different angles. The agreement between experimental values and theoretical predictions is good. The proposed criterion is also compared to existing fatigue criteria and an overall assessment of their performance is given. Some theoretical design considerations for the case of irregular stress spectra, introducing the concept of multiaxial Miner coefficient, are finally presented.

Complex stress state effect on fatigue life of GRP laminates. Part I, experimental

In structural applications of thin-wall, box beam constructions with composites, the effect of complex stress states is not properly taken into account in determining operational life and fatigue response in general, due either to misconceptions or lack of experimental data and theoretical models. Results from a recent research project, presented here in two parts, aim to contribute to the better understanding of fatigue behaviour of GRP laminates under complex in-plane stress states. An initial estimate on the effect of neglecting shear and transverse normal stresses in fatigue life calculations is provided, based on experimental data and theoretical considerations. It is concluded that in structural GRP laminates, shear and transverse normal stresses have an important contribution in reducing operational life, irrespective of their magnitude, usually small compared to axial normal stresses.

Fatigue of composite laminates under off-axis loading

Abstract Results from an experimental program consisting of static and fatigue tests on flat coupons, cut at different off-axis directions from a multidirectional, (MD), Glass/Polyester, (GRP), laminate are presented in this paper. The material is similar to those used by GRP wind turbine rotor blade manufacturers, i.e. hand lay-up and room temperature curing. The stacking sequence of the MD laminate under consideration is [0/(±45)2/0]T. Specimens were cut at five different off-axis directions from that laminate and over one hundred and forty tests were conducted under static and cyclic loading. Based on the test results the effect of off-axis loading on static and fatigue behaviour of the MD laminate is studied. A simple empirical model is used to predict the observed stiffness degradation and to determine stiffness based S–N curves by means of a limited number of test data. For the materials investigated in this program it is shown that E-modulus variation depends on the off-axis loading as much as on the applied cyclic stress level. Stiffness based S-N curves corresponding to 5–20% stiffness reduction are more conservative than standard S-N allowables of 95% reliability.

Fatigue design allowables for GRP laminates based on stiffness degradation measurements

Abstract The fatigue behaviour of a multidirectional, [0(±45) 2 /0] T glass/polyester laminate under various loading conditions is studied in this paper. Series of coupons cut on- and off-axis, at seven different orientations, were uniaxially loaded at constant amplitude over a wide range of R ratios. Test data were subjected to statistical analysis and S / N curves at specific reliability levels were calculated. Stiffness changes were continuously monitored during fatigue testing, and design curves corresponding to a certain level of stiffness reduction, and not to failure, were determined. It is shown, for the material studied in this work, that stiffness-based and reliability S ./ N curves are correlated and therefore determination of fatigue design allowables bearing information on both stiffness loss and strength can be formulated.

Complex stress state effect on fatigue life of GRP laminates. Part II, Theoretical formulation

The synergistic effect of in-plane stress tensor components on fatigue strength is not traditionally considered in the design of thin-wall box-beam structures, e.g. composite rotor blades in general. Fatigue life calculations account only for the normal stresses due to bending and centrifugal forces, neglecting the contribution of shear and transverse normal stresses. The theoretical formulation of a life prediction methodology accounting for all in-plane stress tensor components, through the use of a multiaxial fatigue strength criterion, is presented here. Comparison of theoretical predictions with experimental results from constant amplitude, uniaxial, off-axis tests demonstrates the drastic effect of shear and transverse normal stresses, besides that of axial normal stress, in reducing operational life of a GRP structural laminate.

Thermomechanical behavior of multifunctional GFRP sandwich structures with encapsulated photovoltaic cells

The feasibility of encapsulating solar cells into the glass fiber-reinforced polymer (GFRP) skins of load-bearing and thermally insulating sandwich elements with foam cores has been evaluated. Exposure of the encapsulated cells to artificial sunlight led to a significant temperature increase on the top sandwich surface, which almost reached the glass transition temperature of the resin. Mechanical loading up to serviceability limit loads did not cause any damage to the solar cells. Stresses of less than 20% of the material strength arose in the face sheets due to thermal and mechanical loading up to failure. Composite action through the face sheets with encapsulated cells was maintained and no debonding between face sheets and foam core was observed. Thanks to the superior mechanical and thermal sandwich behavior, thin-film silicon cells are more appropriate than polycrystalline silicon cells for use in multifunctional GFRP sandwich structures, although they are less efficient.

Shear Buckling Resistance of GFRP Plate Girders

Thin webs of glass-fiber-reinforced polymer (GFRP) girders are sensitive to shear buckling, which can be considered an in-plane biaxial compression-tension buckling problem, according to the rotated stress field theory. An extensive experimental study was performed, which shows that an increasing transverse tension load significantly increases the buckling and ultimate loads caused by a decrease in the initial imperfections and additional stabilizing effects. The stacking sequence also greatly influenced the buckling behavior. Higher bending stiffness in the compression direction increased the buckling and ultimate loads, while higher bending stiffness in the tension direction changed the buckling mode shape. The general solution obtained using the Fok model accurately modeled the experimental results, while the simplified solution (modified Southwell method) provided accurate results only at higher tension loads.

Modeling the constant amplitude fatigue behavior of adhesively bonded pultruded GFRP joints

The constant amplitude fatigue behavior of adhesively bonded pultruded glass fiber reinforced polymer double-lap joints were modeled by a number of conceptually different phenomenological S–N (cyclic stress vs. number of cycle) formulations. An extended database containing constant amplitude fatigue data under tension (R = 0.1), compression (R = 10), and reversed loading (R = −1) were analyzed in order to investigate whether or not there exists an appropriate fatigue formulation for accurate modeling of the behavior of the examined joints throughout their lifetime, from the very low-cycle fatigue to the high-cycle fatigue regions. Based on an extensive review, appropriate fatigue formulations that take into account the probabilistic nature of lifetime measurements were selected and their fundamental assumptions were examined. The validity of the statistical assumptions of these models was found to be influenced by the applied loading conditions. The modeling results were similar for all selected fatigue formulations with the derived S–N curves exhibiting differences mainly in the low- and high-cycle fatigue regimes. The formulations insensitive to the scatter in the experimental data were found to be the most appropriate models.

Contribution to Shear Wrinkling of GFRP Webs in Cell-Core Sandwiches

Glass-fiber-reinforced polymer (GFRP) cell-core sandwiches are composed of outer GFRP face sheets, a foam core, and a grid of GFRP webs integrated into the core to reinforce the shear load capacity. One of the critical failure modes of cell-core sandwich structures is shear wrinkling, a local buckling failure in the sandwich webs because of shear loading. The shear wrinkling behavior of GFRP laminates with different laminate sequences, stabilized by a polyurethane foam core, was experimentally and numerically investigated. Shear wrinkling was simulated by a biaxial compression–tension setup. The results show that an increasing transverse tension load significantly decreases the wrinkling load. The decreasing effect of tension is explained by the lateral contraction because of Poisson’s effect, which causes an increase in the initial imperfections and subsequent accelerated bending.

Effect of Natural Weathering on Durability of Pultruded Glass Fiber–Reinforced Bridge and Building Structures

After 17 and 15years of use, respectively, a detailed inspection of a pedestrian bridge and a 5-story building structure composed of partly adhesively bonded pultruded glass fiber-reinforced polymer (GFRP) profiles was conducted accompanied by full-scale serviceability and destructive coupon testing on exchangeable profiles. The bridge is exposed to a harsh Alpine climate and the building to a mild plateau climate. The system and material stiffness of the bridge remained unchanged during the 17years. The material strength of the bridge, however, was significantly affected by combined freeze-thaw cycles and ultraviolet (UV) irradiation. The latter accelerated the strength degradation by uncovering the fibers (fiber blooming) and thus inducing humidity ingress into the material by wicking effects. A large majority of the small-area adhesive bonds in the bridge were intact, although a few small cracks were observed in some joints at the surface. The large-area bonds of the building did not show any damage. Based on results of the inspection of the bridge, the application of an appropriate coating on pultruded GFRP structures exposed to harsh environments is recommended