1,720,977 research outputs found
Fatigue life prediction of composite materials under realistic loading conditions (variable amplitude loading)
No full textTwo of the most widely used methods for the fatigue life prediction of composite materials under variable amplitude (VA) loading patterns are presented in this chapter. The first method is based on the theoretical formulation and use of a damage summation rule to predict life under VA loading without recourse to experimental observation of the damage accumulation process. An alternative to this classic fatigue life prediction methodology are the residual strength fatigue theories, where residual strength is used as the damage metric. Comparison of the remaining strength of the material to the static strength allows the estimation of the fatigue cycles until failure. The basic fatigue modeling introduced in previous chapters of this book for interpretation of the fatigue data ( Chapter 2), residual strength theories ( Chapter 3), and constant life diagrams ( Chapter 6) is combined here to establish fatigue life prediction methodologies.CCLA
Mode I fatigue and fracture behavior of adhesively-bonded pultruded glass fiber-reinforced polymer (GFRP) composite joints
No full textThe fatigue/fracture behavior of adhesively-bonded pultruded glass fiber-reinforced polymer (GFRP) joints is significantly affected by the loading ratio. This effect is analyzed in this chapter through a complete fatigue/fracture database, derived during recent years, and correlated with the exhibited failure processes of the examined joints. A phenomenologically based criterion is then used for the simulation of the exhibited behavior and the prediction of the fatigue/fracture behavior of the examined joints under different loading conditions. It is shown in this chapter that, upon accurate estimation of the model parameters, it is possible to reliably predict fatigue crack growth curves for several unknown loading conditions, thereby assisting the development of methodologies for the fatigue life prediction of joints under realistic loading conditions.CCLA
Novel computational methods for fatig life modeling of composite materials
No full textNovel computational methods such as artificial neural networks, adaptive neuro-fuzzy inference systems and genetic programming are used in this chapter for the modeling of the nonlinear behavior of composite laminates subjected to constant amplitude loading. The examined computational methods are stochastic nonlinear regression tools, and can therefore be used to model the fatigue behavior of any material, provided that sufficient data are available for training. They are material-independent methods that simply follow the trend of the available data, in each case giving the best estimate of their behavior. Application on a wide range of experimental data gathered after fatigue testing glass/epoxy and glass/polyester laminates proved that their modeling ability compares favorably with, and is to some extent superior to, other modeling techniques.CCLA
Fatigue and fracture behavior of adhesively-bonded composite structural joints
No full textConstant amplitude fatigue loading spectra stand for only a small percentage of the real loading of a structure, although they can be used to investigate the fatigue and fracture behavior of structural elements. The fatigue and fracture behavior of adhesively-bonded structural joints under different constant amplitude loading patterns has been experimentally investigated and the results are presented in this chapter. Double-lap joints composed of pultruded glass-fiber-reinforced polymer-matrix laminates bonded by an epoxy adhesive were examined. The dominant failure modes are described and the stiffness degradation trends under tensile or compressive fatigue loads are presented. The results show a significant effect of the loading pattern on the lifetime and fracture behavior of the joints examined.CCLA
Mixed-mode fatigue and fracture behavior of adhesively-bonded composite joints
No full textThe mixed-mode fatigue and fracture behaviors of adhesively-bonded pultruded glass fiber-reinforced polymer joints are presented in this chapter. These behaviors are based on experimental investigations using asymmetric mixed-mode bending specimens. In such specimens, the crack propagated along paths outside the symmetry plane, and therefore mode partitioning could not be performed in the standardized way as for symmetric specimens. Existing techniques for the characterization of the mixed-mode fracture behavior of adhesively-bonded joints, the partitioning of the fracture mode components, and the modeling of the fiber bridging that affects the total fracture energy are presented in this chapter.CCLA
Biaxial Wrinkling of Thin-Walled GFRP Webs in Cell-Core Sandwiches
Get PDFFiber-reinforced polymer (FRP) sandwich structures offer several advantages compared to structures made of traditional materials, such as high specific strength, good corrosion resistance, low thermal conductivity and rapid component installation. In this context, glass fiber-reinforced polymer (GFRP) cell-core sandwiches composed of outer GFRP face sheets, a foam core and a grid of GFRP webs integrated into the core to reinforce shear load capacity are well suited for load-bearing applications in civil engineering i.e. in bridge deck and roof construction. Despite the great potential of these structural concepts, the use of heterogeneous materials in FRP sandwiches results in more complex failure mechanisms compared to conventional structural components and lack of knowledge regarding the prediction of failure modes makes the design of structural components difficult. This is one of the major disadvantages limiting the acceptance of cell-core sandwiches in civil engineering applications. One of the critical failure modes of cell-core sandwich structures is wrinkling in the webs. A great deal of information exists concerning the phenomenon of skin wrinkling failure of sandwich laminates loaded in compression but comparatively little on wrinkling in the webs of sandwich structures where the pure compression loading is complicated by supplementary transverse tension. The purpose of this research is to develop an appropriate model for the prediction of wrinkling in the webs of cell-core sandwich structures. Two new approaches were developed to predict the wrinkling loads of webs. The first approach examines the wrinkling behavior in webs as an in-plane biaxial compression-tension buckling problem according to the rotated stress field theory. In this regard, extensive experimental, numerical and analytical studies were performed to investigate the interaction between the compression and tension stress tensors during the buckling/wrinkling instability phase of GFRP plates and sandwich panels subjected to biaxial compression-tension loading. The investigations demonstrated that the transverse tension in the biaxial compression-tension set-up induced two simultaneous counteracting effects: a stabilizing and a lateral contraction effect. The stabilizing effect tends to push the plate back to the median plane and thereby delays the onset of buckling/wrinkling instability. In contrast, lateral contraction accelerates the bending of the plate, which leads to a significant decrease in buckling/wrinkling loads. In composite plates, the first effect predominates and increases the buckling loads while in sandwich panels the second effect is dominant and decreases the wrinkling loads. Using the second approach, the wrinkling behavior of foam-filled web-core panels was modeled by applying an improved mixed-mode interaction formula in which two approximate models are developed based on the energy method in order to determine the critical loads when the pure shear and bending stresses act independently on the web. The application of both approaches to a real case study, the GFRP cell-core sandwich roof of the Novartis Campus Main Gate Building proved that they are sufficiently accurate to be used as valid tools assisting the optimum design of sandwich structures whereas existing models result in too conservative predictions.CCLA
Block and variable amplitude fatigue and fracture behavior of adhesively-bonded composite structural joints
No full textThe fatigue behavior of adhesively-bonded glass fiber-reinforced polymer (GFRP) joints is affected by the loading sequence. Analysis of several experimental data for composite materials and adhesively-bonded composite joints showed that the loading sequence effect is a function of the loading type, the applied loading levels and the material under investigation. Exhibited failure modes, related to the loading pattern, significantly affect the development of damage in adhesively-bonded composite joints. The aim of this chapter is to investigate the load sequence effect of both block and variable amplitude loading conditions on the fatigue behavior of adhesively-bonded pultruded GFRP joints.CCLA
Introduction to the fatigue life prediction of composite materials and structures: past, present and future prospects
No full textThis chapter aims to provide an overview of the fatigue life prediction methods for composite materials and structures, recalling methods used in the past, discovering the present status and attempting to foresee future trends.CCLA
Enhanced anti-impact performance of composite sandwich panels with modified polyurethane foams, exploiting phase transition occurrence of non-newtonian polymer.
Get PDFLightweight, flexible and low-cost materials which possess great energy absorption properties are in great demand in composite sandwich panels, and in body protective equipment used in sports and military. One of the most widely used materials in these applications is polyurethane foam (PUF), showing good energy absorption and shape restorability under low-rate dynamic loading conditions. However, their response to high-rate dynamic loading conditions can lead to cell wall damage significantly hindering their performance. In this work, a non-Newtonian polymer with high energy dissipation efficiency was selected as an excellent candidate for the enhancement of the mechanical properties of PUFs. Low Velocity Impact tests were employed directly on foams, composite sandwich panels and post-impact for the evaluation of their residual properties. Results indicated that the presence of the non-Newtonian polymer led to an improvement of the energy dissipation of the modified foams, averting cell wall damage, constituting these as ideal core structure candidates
Thermophysical and Thermomechanical Behavior of Cold-Curing Structural Adhesives in Bridge Construction
No full textThe use of the structural adhesive bonding technique is well established in the aircraft and automotive industries where, in most cases, joints can be fabricated indoors under controlled conditions. In the civil engineering domain, however, load-bearing structures are normally erected on-site in outdoor conditions, i.e. joint fabrication is exposed to varying outdoor temperature and humidity profiles and particularly in winter, when long periods of very low temperatures are frequent, the on-site joining of structural components must remain possible. Consequently, and due to the generally large scale of the structural components, cold-curing adhesives are used, unlike in other fields where hot-curing adhesives are used. Although it offers many potential advantages in bridge construction, structural adhesive bonding is not yet widely used in civil engineering structures. This can be attributed, amongst other things, to the lack of knowledge regarding adhesive behavior during exposure to varying environmental conditions. Despite significant research efforts concerning the characterization of structural adhesives, the question of the changes that occur in the thermophysical and thermomechanical properties of cold-curing structural adhesives during their service life, and particularly at early age and low temperatures, is yet to be addressed. The objective of this work was therefore, based on experimental and analytical investigations, to understand the thermophysical and thermomechanical behavior of cold-curing structural adhesives from the time of the mixing of the different components and throughout the long-term service life. Investigations of curing at early age and low temperatures (during winter) showed that curing already takes place at temperatures slightly above 0 °C. The process is very slow however and several days of curing are required to attain significant curing degrees. The glass transition temperature, Tg, develops even more slowly due to early initiation of vitrification. A new and practical experimental method was developed to establish the relationship between glass transition temperature and curing degree for different types of cold-curing adhesives. Furthermore, cure kinetics (developed for hot-curing adhesives) proved its applicability for cold-curing adhesives at low temperatures. In contrast to thermophysical properties, mechanical properties start developing significantly only after the onset of material vitrification. Lower curing temperatures also significantly decelerate the development process. An empirical model to predict strength and stiffness as a function of simple or complex curing temperature profiles particularly for curing at low temperatures was developed, taking the temperature-dependent vitrification into account. During summer, the temperature at specific locations of certain joints (e.g. joints below the asphalt) may exceed Tg, which may lead to a significant drop in mechanical properties. The investigations showed that when cooled to temperatures below Tg, the mechanical properties fully recovered and even a significant increase in properties is achieved due to post-curing. An existing model to predict temperature-dependent mechanical properties was extended to predict the change in stiffness and strength resulting from the exceeding of Tg and subsequently after recovery. Over the long term and during ambient curing, a significant increase in mechanical and thermophysical properties occurs over the years and decades due to the completion of curing. A model based on the similar long-term increase of concrete properties was revised to also predict the long-term strength and stiffness of cold-curing structural adhesives. Finally several case studies demonstrate how the outcome of this research can be applied, particularly to predict mechanical properties at early age and low temperatures. Such results can be used to plan construction stages or estimate the required waiting periods prior to a bridge being brought into service.CCLA
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