BUCKLING AND POSTBUCKLING OF LAMINATED THIN CYLINDRICAL SHELLS UNDER HYGROTHERMAL ENVIRONMENTS

The influence of hygrothermal effects on the buckling and postbuckling of composite laminated cylindrical shells subjected to axial compression is investigated using a micro-to-macro-mechanical analytical model. The material properties of the composite are affected hy the variation of temperature and moisture, and are hosed on a micromechanical model of a laminate. The governing equations are based on the classical laminated shell theory, and including hygrothermal effects. The nonlinear prebuckling deformations and initial geometric imperfections of the shell were both taken into account. A boundary layer theory of shell buckling was extended to the case of laminated cylindrical shells under hygrothermal environments, and a singular peturbation technique was employed to determine buckling loads and postbuckling equilibrium paths. The numerical illustrations concern the postbuckling behavior of perfect and imperfect, cross-ply laminated cylindrical shells under different sets of environmental conditions. The influences played by temperature rise, the degree of moisture concentration, fiber volume fraction, shell geometric parameter, total number of plies, stacking sequences and initial geometric imperfections are studied.

Effect Mechanisms of Hygrothermal Environments on Failure of Single-Lap and Double-Lap CFRP-Aluminum Bolted Joints

The high demands for load-carrying capability and structural efficiency of composite-metal bolted joints trigger in-depth investigations on failure mechanisms of the joints in hygrothermal environments.However,few studies have been presented to exhaustively reveal hygrothermal effects on the failure of CFRP-metal bolted joints,which differ from CFRP-CFRP or metal-metal bolted joints because of the remarkably different material properties of CFRPs and metals.In this paper,hygrothermal effects on tensile failures of single-lap and double-lap CFRP-aluminum bolted joints were experimentally and numerically investigated.A novel numerical model,in which a hygrothermal-included progressive damage model of composites was established and elastic-plastic models of metals were built,was proposed to predict the failures of the CFRP-metal bolted joints in hygrothermal environments and validated by corresponding experiments.Different failure mechanisms of single-lap and double-lap CFRP-aluminum bolted joints,under 23°C/Dry and 70°C/Wet conditions,were revealed,respectively.It follows that both the collapse failures of the single-lap and double-lap bolted joints were dominated by the bearing failure of the CFRP hole laminate in the two conditions,indicating that the hygrothermal environment did not change the macro failure modes of the joints.However,the hygrothermal environment considerably shortened the damage propagation processes and reduced the strength of the joints.Besides,the hygrothermal environment weakened the load-transfer capability of the single-lap joint more severely than the double-lap joint because it aggravated the secondary bending effects of the single-lap joint obviously.

Influence of local stiffeners and cutout shapes on the vibration and stability characteristics of quasi-isotropic laminates under hygro-thermo-mechanical loadings

The perforated stiffened panel is generally found as a sub-component of sophisticated structures.The fundamental purpose of this panel is to withstand against buckling under complicated loading and environmental conditions.Hence,an accurate knowledge of critical buckling behaviour of stiffened panels is very much essential for a reliable and lightweight structural design.In this paper,the focus is on quasi-laminated panels with different cutout shapes of various sizes and their responses to hygrothermal environments under nonlinearly varying edge loads and is compared with the locally stiffened panels.Towards this,the modelling of the panel and stiffener is done by adopting nine-noded heterosis plate elements and three noded beam elements respectively.The stiffener formulation is suitably modified in order to take the torsional effect also into consideration along with the effect of shear deformation.Initially,the plate and the stiffener elements are treated separately,and then the displacement compatibility is maintained between them by using the transformation matrix.For a given loading and geometric discontinuity,the stress distribution within the perforated panel is highly non-uniform in nature and hence a dynamic approach has been used to calculate buckling loads by adopting two sets of boundary conditions,one set for pre-buckling stress analysis and the second set for buckling analysis.Four different quasi-isotropic stacking sequences are deliberated in this work by varying different ply-orientation in each scheme.The study also addresses the effect of various parameters such as nonlinear loads,hygro-thermal loads,cutout size and shapes,position of cutout,stiffener parameters,stacking sequences,thickness of plate and boundary conditions.

Study on Compressive Properties of Z-pinned Laminates in RTD and Hygrothermal Environment

Compressive tests of [0]t2 and [90]t2 unidirectional laminates and [45/0/-45/90] 2s quasi-isotropic laminates are accomplished in both room-temperature and dry （RTD） and hygrothermal environment. And simulation studies on the compressive strength of Z-pinned laminates of [0112 and [45/0/-45/90] 2s are conducted by using finite element analysis （FEA）. A microstructural unit cell for FEA is created to simulate a representative laminates unit with one pin. Within the unit cell, the first directions of the elements＇ material coordinate systems are changed to simulate the fibres＇ deflecting around the pin. The hygrothermal effect is simulated by the material properties＇ adjustments which are determined by the compressive tests of non-pined laminates. The experimental results indicate that the percentage of reduction in the compressive modulus of Z-pinned laminates caused by Z-pin becomes smaller with the percentage of 0° fibres decreasing in the laminates; the compressive strength of quasi-isotropic laminates reduces and the percentage of the reduction in the compressive strength declines with Z-pin volume content increasing, and the moisture absorption ratio of the Z-pinned specimens is greater than that of the non-pinned specimens, because the cracks around Z-pin increase the moisture absorption. In addition, the simulations show that the deflection of fibres around Z-pin is the main factor for the reduction in the compressive strength of Z-pinned unidirectional laminates, the dilution of fibre volume content caused by resin-rich pocket is the principal factor for the decline in the compressive strength of Z-pinned quasi-istropic laminates, and the compressive strength of Z-pinned specimens in hygrothermal environment reduces as the result of superimposition of some factors, including the changes in material properties caused by hygrothermal environment, the deflection of fibres and the resin-rich pocket caused by Z-pin.

Flexural Performance of Reinforced Concrete Beams Strengthened with Carbon Fiber-Reinforced Polymer(CFRP)under Hygrothermal Environment Considering the Influence of CFRP-Concrete Interface

As an important component,the bond behavior of carbon fiber-reinforced polymer(CFRP)-concrete interface for a reinforced concrete(RC)beam is very significant.In this study,a theoretical model was established to analyze the flexural behavior of CFRP-strengthened RC beams,and the CFRP-concrete interfacial bond-slip relationship under hygrothermal environment was unified into one model.Two failure criteria corresponding to two types of failure modes,i.e.,concrete crushing and intermediate crack(IC)-induced debonding,were developed.Through the theoretical model,the flexural behavior of deflection,interfacial shear stress distribution and ultimate load of a CFRP-strengthened RC beam under hygrothermal environment were obtained and predicted.Moreover,the theoretical model was verified by test results.The results showed that the hygrothermal environment had a significant impact on the CFRP-concrete interface behavior.Compared with the control beam without hygrothermal environment pretreatment,the deflection and ultimate load of the strengthened RC beam decreased by 51.9%and 20%,respectively.

Experimental Study of Hygrothermal and Ultraviolet Aging on the Flexural Performance of Epoxy Polymer Mortar

This paper studies the effects of hygrothermal environment at different temperatures and ultraviolet(UV)radiation on the bending properties of epoxy polymer mortar(EPM).The microstructure changes of EPM during aging were studied by scanning electron microscopy,and the bending properties of EPM were predicted by the Arrhenius law.The results showed that the bending properties of EPM were greatly affected by the temperature in the hygrothermal aging,but not evidently affected by ultraviolet radiation in UV aging.The prediction of Arrhenius model shows that the EPM will steadily retain 92.8%,89.1%and 79.4%of the original flexural strength after long-term hygrothermal aging at 40∘C,60∘C and 80∘C,respectively.

Wave propagation in graphene reinforced piezoelectric sandwich nanoplates via high-order nonlocal strain gradient theory

Αn analytical method is developed to explore the wave propagation characteristics of piezoelectric sandwich nanoplates in the present work.The sandwich nanoplates are composed of a graphene reinforced composite core layer with two piezoelectric surface layers exposed to electric field.The material properties of the nanocomposite layer are given by the Halpin–Tsai model and mixture’s rule.The Euler–Lagrange equation of the nanoplates is obtained by Hamilton's principle and first-order shear deformation theory.Then,combining the high-order nonlocal strain gradient theory with the hygrothermal constitutive relationship of composite nanoplates,the nonlocal governing equations are presented.Finally,numerical studies are conducted to demonstrate the influences of scale parameters,applied external voltage,temperature variation,moisture variation,graphene size,and weight fraction on wave frequency.The results reveal that low-order and high-order nonlocal parameters and length scale parameters have different effects on wave frequency.The wave frequency can be reduced by increasing temperature and the thickness of graphene.This could facilitate the investigation of the dynamic properties of graphene nanocomposite structures.