Search

A sandwich assembly with core material enhances the bending stiffness of a structure and achieves lightweight requirements. Resin flowing behavior inside a sandwich assembly consisting of fiber layers and a segmented core is more complex than that for a fibrous assembly consisting of only fiber layers. This study presents infusion experiments on sandwich assemblies in the VARTM process, and compares the results to simulations from a 3D sandwich model including circular pipe elements to substitute for core grooves. Using the concepts of equivalent thickness to define the porosity of a sandwich assembly and porous space helps derive permeability trends and predictive equations to simplify permeability calculation. The deck infusing experiment in this study not only proves the permeability rationality of a sandwich assembly, but also provides concepts and suggestions for infusion arrangements. Consequently, the permeant characteristics in the sandwich assembly are applicable to manufacturing sandwich structures with good saturation.

With experiments, this study discussed the characteristics of composite curvature shells with constrained layered damping (CLD) subjected to low-velocity impact as well as compared the reactions of the curvature shells with/without CLD. In addition to the experiments, this study proposed the quasi-static simulation, which could rapidly obtain the results with different impact velocities and drift masses with the force–displacement curve of static crushing test, to receive the results subjected to low-velocity impact under various impact conditions. The shell specimens were laminated with two stacking sequences collocating two different drift masses and impact velocities. The experiment of low-velocity impact applied Dynatup-GRC8250 and IDA system to measure the impact force at various time-points in the process of impact and, with the conservation of energy, changes of velocity and strain energy in the impact process could be calculated with the initial velocity of the impact. Finally, the quasi-static simulations were compared with the experiments to verify the accuracy of simulations, and results were found satisfactory.

Utilizing a sandwich structure enables FRP assembly to increase bending stiffness and achieve lightweight requirements. Core surfaces are often cut and infused together with fiber laminates in the VARTM process. Nevertheless, resin infusion in the sandwich structure differs from pure fibrous laminates because resin flows rapidly in the grooves and then saturates into the laminates. This research performed sandwich structure infusion experiments under the VARTM process, and defined four resin saturation stages inside the sandwich assembly by observing and explaining the nonlinear experimental flowing fronts. This study also discussed the race-tracking phenomenon of the sandwich structure in detail. This research executed infusion simulations in the 3D sandwich model, including fiber layers and grooves replaced by the circular pipe elements, to compare with the experimental results. Realization of the permeant characteristics in the sandwich structure establishes a base to ensure complete saturation and to analyze the manufacture of large sandwich structures.

This investigation presents a quasi-static simulation method that adopts energy conservation and the results from the static crush to simulate the impact behavior of carbon-epoxy composite laminate plates. These plates consist of adhered visco-elastic material, and formed constrained layered damped (CLD) components. The damping force is considered in the quasi-static simulation. Comparison with experimental results indicates that the proposed method yields accurate results and produces significant saving in computing resources.

Plate and shell specimens were formed from carbon–epoxy prepregs. Then, attaching 3 M damping foil SJ-2552 to specimens formed constrained layered damped (CLD) components with sandwich-like structures. A steel indentor was used to compress the plate and shell specimens until they completely collapsed. The crushing strength of the CLD components is examined experimentally, and the finite element software ABAQUS is used to simulate the experimental crushing behavior using the user-defined material (UMAT) subroutine. UMAT allows users to define the progressive stiffness modification and analyze the fracturing of composite components. The ultimate strengths of the CLD components are accurately estimated using the UMAT, and the failure observed in the experiments closely corresponds to that in the numerical simulations.

The material and ply angles of a fiber-reinforced plastic (FRP) rotor blade of a helicopter are optimized using a genetic algorithm (GA) with local search. The variation of the structural stiffness with material and ply angle is investigated. The aim of the optimization is to find an optimal stacking sequence that minimizes twist deformation. An optimum rotor blade is obtained using the ply angles as variables; failure, natural frequency, and minimum moment of inertia are used to constrain the optimization. The GA is employed as an optimization tool. A simplified local search is implemented in the final stage of optimization to reduce the searching time.

This study examines the progressive failure of marine glass fiberreinforced plastic (GFRP) laminates subjected to out-of-plane water pressure. First, experimental data concerning laminates that consist of a single type of fiber are used to establish the finite element model by separating the section into fiber layers and matrix layers. After a suitable failure criterion has been specified and the stiffness modification factor of different materials obtained experimentally, a numerical model is used to simulate the failure behavior of marine GFRP laminates. This numerical model is then applied to analyze the failure of GFRP ship bottom structures under water pressure. The water pressure failure experiment is designed to verify the numerical results. The experimental system includes a watertight cabin, which comprises the wall of the fixture, the specimen, and a movable piston, filled with water. Then, the water pressure is increased by moving the piston to compress the water-sealed zone, until the specimen reaches ultimate failure. After the numerical model has been validated, the abilities of testing and analyzing the failure behavior of laminates under static water pressure are established. More information is presented about the capacity to resist out-of-plane water pressure, the proper use of the material, and improving the efficiency of the design.

This work investigates the impact behavior of carbon-epoxy composite-laminated shells with three different curvatures. For the low-velocity impact cases, a quasi-static method is introduced to reduce the need for dynamic approaches. The static crushing information (force-displacement data) of the shell is critical to the quasi-static simulation. Based on the crushing data extracted from experiments or numerical simulations, numerous dynamic impact results for distinct impactor masses and velocities can be obtained immediately from a single static result, as long as these impacts are classified as low-velocity conditions. Static experiments and simulations are always more effective than dynamic ones. This investigation concludes that the quasi-static method delivers excellent exactitude, provided the static force-displacement data, the dominator, are accurately acquired.

The crush behavior of carbon-epoxy composite laminated shells with three different curvatures is investigated. The FEM package ABAQUS and the user-defined material subroutine (UMAT) based on the concept of progressive stiffness degradation were employed to simulate crushing until the specimens totally collapsed. The 2-D delamination propagation proposed by Davies et al. is considered to be an additional failure mechanism for some curved shells whose contact crushing forces are large. The ultimate strengths of the shells are accurately predicted by the presented method, and all experimental cases are accurately simulated and explained.

A steel impactor was used in drop tests to impact composite laminated plates constructed using carbon-epoxy prepregs. The impact force and strain histories were recorded as well. Damage incurred from a low-velocity impact much closely resembled that from static crushing. This investigation creates a user-defined material subroutine (UMAT) of the FEM software ABAQUS, using our method of stiffness progressive modification, such that extreme failure is accurately simulated until the plates are thoroughly crushed. Using the results from the static crush simulation and the aspect of energy conservation, the dynamic impact force given with different impactor masses and initial velocities can be rapidly predicted while making significant savings in computing resources.

Plate specimens were formed from carbon-epoxy prepregs. Thereafter a steel indentor was used to compress the specimens, with the reaction forces were recorded until the plates thoroughly collapsed. The FEM software, ABAQUS, was employed to simulate the crushing of the plates. This investigation creates a user-defined material subroutine to enhance the damage simulation which includes Hashing and Yeh failure criteria. The ultimate strengths of the plates are accurately estimated using the present method of progressive stiffness modification, and the failure observed in experiments closely corresponds to that of the numerical simulations.

Marine glass fiber-reinforced plastic (GFRP) laminates, such as M-R laminates, have various tensile and flexural mechanical properties. This phenomenon holds not only for laminates composed of various fibers, but also for laminates composed of a single type fiber. Therefore, this paper will use both a theoretical and a numerical model to calculate the tensile and flexural Young's moduli, to elucidate the mechanism that explains the discrepancy. According to this, the numerical model of GFRP laminates made from a single type of fiber can be established from the data of material experiments, which are easily performed in the laboratory. The mechanical properties of laminates composed of different types of fibers can then be predicted precisely by this numerical model without further material experiments.

The present study employs a numerical approach to investigate the post-buckling behavior of laminates connected by rivets. In order to reduce the calculation time, the rivets are replaced with springs, while the stiffness of the springs is induced from the study that analyzes the bending behavior and the interaction between the rivet and the laminate. The finite element software ABAQUS is used to inspect the post-buckling behavior of composite laminates connected by rivets. Several parameters are considered like the stacking sequence, the arrangement of the rivets, and the span between the stiffener. In all these cases, springs can fully present the behavior around the rivets. Finally, the validity of this method is verified by experiment.

A nonlinear finite element code, DELAM3D, with a three-dimensional layered solid element based on the updated Lagragian formulation, is developed to simulate the compressive response of a laminated composite plate with multiple delaminations. An analytical model is established to characterize the mechanical behaviors such as postbuckling, contact of the delaminating interface, delamination growth and fiber-matrix failure. Double cantilever beam (DCB) and end notched flexure (ENF) tests, with T300/976 graphite/epoxy, are performed to verify the energy release rate of the material. Experiments with various crack numbers, sizes, locations and layer orientations have been conducted and compared with the numerical solution. Good agreement is expected.

The low-velocity impact behavior of sandwich beam subjected to the cylindrical impactor is investigated numerically. Finite element software ABAQUS is used, and maximum stress failure criterion and a modified stiffness degradation method are adopted to analyze the failure behavior of sandwich beam, including initiation of failure, and failure modes. The two failure modes ; shear cracks in foam (Mode I), and crushed in core and damage in the top face sheets right underneath the impactor (Mode II) can be simulated well. At the range of impact velocity on this study, it can be shown that the the dynamic responses of sandwich beam are nearly quasi-static, therefore, the dynamic behavior of sandwich beam such as impact force history can be approximately obtained from static results.

The impact behavior of composite sandwich panels subjected to a cylindrical impactor is investigated numerically and experimentally. Three kinds of facesheets and two different core densities of sandwich panels are considered in the impact analysis. During the experiment, the impact force and strain history are measured, and the failure is inspected by visual observations. The experimental results show that the impact behavior is mainly controlled by core material. The ABAQUS finite element software is used to analyze the impact responses of sandwich panels. Because of line load conditions, the sandwich panels impacted by the cylindrical impactor is modeled as a 2-D plane strain problem. The numerical results of impact force and strain history are compared with those measured in experiment, it is shown that the experimental results are in good agreement with the finite element solutions. Further, the stresses of sandwich panels are examined. The maximum stress failure criterion is used to estimate the fracture initiation of sandwich panels. Then the experimental and numerical results are compared and discussed.

This paper is to investigate the failure process and pin-bearing strength of laminated composites under different temperatures. An incremental finite element code, based on the updated Lagrangian approach, was developed for determining the strains dis tribution of laminates under the consideration of the temperature dependent material prop erties. The quadratic failure criterion is adopted for failure predictions. Numerical results include the failure process, failure mode and ultimate bearing strength for two differ ent laminates, S-500 glass/polyester and graphite/epoxy, with three different lay-ups, [0°/45°/90°/-45°]s, [0° 2/ ±45°]s and [90°2/ ±45°]s, as well as with different end distances (e = d-4 d) and widths (W = 1.5 d-6 d). In order to verify the accuracy of the numerical calculations, the bearing strength model test is also made for understanding the effect of temperature on the failure behavior of the laminated composites with pin-loaded hole.

Contact stresses and residual bearing strength of pin-loaded composite laminates at different temperatures are investigated. An incremental finite element code, based on the updated Lagrangian approach, was developed for determining the contact stresses near a pin-loaded hole in composite laminates under combined thermal and mechanical loadings. Based on the progressive damage model, the quadratic strength theory combined with a proposed failure simulation are adopted for failure predictions. In the stress and strength analyses, mechanical properties of the laminates are assumed to be temperature dependent. Numerical results include the distributions of initial (thermal) as well as total contact stress around the pin-loaded hole, residual bearing strength, and failure mode for a single-fastener clearance-fit joint with an elastic pin. The bearing strain model test has been performed at elevated temperature for the verification of present calculations.

The strength of glass fiber woven roving composites containing a circular hole was examined. Two types of circular hole — drilled, and moulded-in — were considered. Experiments were conducted on four different sizes of hole diameter. Experimental results show that specimens with a moulded-in circular hole exhibit failure at a higher strength than those with a drilled hole and exhibit different failure mode. A strength enhancement of 28–77% as compared with drilled laminates was observed. In addition to explaining the experimental results, a simple finite element model which used the concept of the stream line to simulate the fiber configuration was used in the stress analysis of laminates with moulded-in circular holes. The numerical results may explain why the laminates with moulded-in holes are stronger than those with drilled holes.

A combined numerical and experimental study is conducted to investigate the failure behaviour of bolted connections in vessel made of GFRP laminates. An incremental 3D FEM code, based on the progressive damage model, is developed for determining the stress distributions, damage propagation and ultimate bolted joint strengths of the FRP laminates with different stacking sequences and degrees of lateral clamping. Next, the bearing strength model test is performed to investigate the failure behaviour of the S-500 glass/polyester laminate specimens. The results show that the stacking sequence significantly affects the interlaminar stress concentration on the hole boundary as well as the failure behaviour of pin joint in GFRP laminate. Furthermore, the introduction of a suitable lateral clamping generally improves the normal interlaminar stress state at a bolted joint and also increases the initial delamination strength as well as the ultimate strength of bolted connections in GFRP laminates.

A study of the impact damage of laminated plates and cylindrical shells was conducted by experimental and numerical analysis. The curvature effect on the impact damage of composites was investigated. Laminating sequences of [0°5/90°5/0°5] and [90°5/0° 5/90°5 were considered. Detailed presentation of impact damage was provided through photographs and schematic diagrams. It is found that there are matrix cracks in the lower layer and delamination in the lower interface in plates, while the fiber crack in the upper layer, shear crack in the middle layer and delamination in the upper and lower interfaces are observed in cylindrical shells. A numerical analysis to predict and explain the crack initiation of laminated plates and cylindrical shells was also established.

The present paper investigates the ultimate compressive strength of a simply-supported wide panel reinforced longitudinally by multiple stiffeners. It was clarified in the previous paper that in the case of one stiffener, the ultimate behaviors depend upon the equivalent flexural rigidity γe of the stiffener and they are classified in the three modes by the specific values of γe : γ1 and γ2. The same tendencies are observied in the case of multiply stiffened panels ; weak stiffeners (γe γ2) are effective to be panel breakers and only local deformations are dominent even at the ultimate strength. If γ1

The buckling behavior of composite laminated square plates is analyzed with the aid of the finite-element method in which a modified isoparametric linear shell element is used. The in-plane and bending displacements are used together in the formulation of the finite-element method in the case of asymmetric laminates. A new method of modifying the constitutive law of composite materials to circumvent the shear-lock effect is also proposed. This new method has the useful property of removing the shear-lock effect in the case of very thin plates. Composite laminates of symmetric and asymmetric stacking sequences, with and without a central circular hole are analyzed. Using the developed program, various laminated cases are analyzed and discussed. The present results show an improvement over those found analytically when compared with experimental results for asymmetric laminates.

It is reported that several cargo ships have suffered from buckling failures of deck or bottom plates stiffened transversely. The ultimate compressive strength of rectangular plates is studied by analyzing a simplified model which clarifies the effects of yielding on the strength.Approximate formulas for prediction of the buckling strength are proposed in terms of the aspect ratio and the ratios of yield stress to elastic buckling stress and initial deflection to thickness. The accuracy of the formulas is verified by the detailed analysis using the combined technique of the finite-element and energy methods.