John Tomblin

A phenomenological design equation for FRP columns with interaction between local and global buckling

Abstract A design equation for fiber reinforced plastic columns is presented in this paper, based on the interaction between local (flange) and global (Euler) buckling observed during testing of the FRP columns included in this investigation. An existing interaction equation is adapted to account for the modes of failure observed in columns made of fiber reinforced composite materials. Experimental data generated during this investigation are presented and used to validate the interaction equation and to obtain the interaction constant. A slenderness ratio is proposed and used to present a plot of buckling for all sections and column lengths (short, long, and intermediate). An expression for the optimum column length to be used in the experimental determination of the interaction constant is proposed.

Local buckling experiments on FRP columns

Abstract In this paper, local flange-buckling of thin-walled pultruded FRP columns is investigated. Experimental data are presented and correlated with theoretical predictions. Good agreement between theoretical and experimental results is found. Possible explanations for slight deviations in the experimental data are advanced. The experimental and data reduction procedures used to obtain the local buckling loads are presented. A new data reduction technique using Southwells method is developed to interpret local buckling test data. The usefulness of the data reduction technique is demonstrated for various column sections and experimental conditions.

Elastic-plastic model of adhesive-bonded single-lap composite joints

An analytical model for determining adhesive stress distributions within the adhesive-bonded single-lap composite joints was developed. ASTM D3165 ‘‘Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies’’ test specimen geometry was followed in the model derivation. In the model derivation, the composite adherends were assumed linear elastic while the adhesive was assumed elastic-perfectly plastic following von Mises yield criterion. Laminated Anisotropic Plate Theory was applied in the derivation of the governing equations of the bonded laminates. The adhesive was assumed to be very thin and the adhesive stresses are assumed constant through the bondline thickness. The entire coupled system of equations was determined through the kinematics relations and force equilibrium of the adhesive and the adherends. The overall system of governing equations was solved analytically with appropriate boundary conditions. Computer software Maple V was used as the solution tool. The developed stress model was verified with finite element analysis using ABAQUS by comparing the adhesive stress distributions.

Euler buckling of thin-walled composite columns

Abstract Pultruded composite structural members with open or closed thin-walled sections are being extensively used as columns for structural applications where buckling is the main consideration in the design. In this paper, global buckling is investigated and critical loads are experimentally determined for various fiber reinforced composite I-beams of long column length. Southwells method is used to determine the critical buckling load about strong and weak axes. The experimentally determined buckling load is compared with theoretical predictions. A number of observations about testing methodology and data reduction techniques are presented.

Impact Damage Resistance and Tolerance of Honeycomb Core Sandwich Panels

The damage resistance and tolerance of flat [(0/45),/core/(45/0), ] sandwich plates with honeycomb core subjected to low-velocity impacts using hemispherical steel impactors has been investigated experimentally. The effects of impactor diameter on the impact behavior, resulting impact damage states, and residual strength under in-plane compressive loading was of particular interest. The impact responses characterized in terms of peak impact force was observed to be dependent on the facesheet type, core thickness, and impactor size, but was found to be independent of the boundary support conditions. The smaller impactor produced damage states characterized by residual dent depths that were comparable to the core thickness, accompanied by visible facesheet fractures. The larger diameter impactor produced damage states with large core damage regions but with dent depths less than the facesheet thickness. Under in-plane compressive loading, depending on the impact damage state, contrasting failure mechanisms involving net-section fracture and buckling failure were observed. A reduction in compressive strength up to 60% of the undamaged strength has been observed.

Energy Absorption Characteristics of Stitched Composite Sandwich Panels

The energy absorption characteristics of sandwich panels with through-the-thickness stitches, under edgewise compression, was investigated by conducting static crush tests. The effects of stitch row spacing on the sustained crush load were of particular interest. Also, the effects of the stitch spacing on the instability preceding the crush failure mode was investigated using finite element analysis. The analysis indicated that the local instability mode was influenced by the stitch row spacing only when the spacing was less than the semi-wavelength of wrinkles that would develop in the absence of stitches. The static crush tests showed that the average sustained crush load increases with reduced stitch row spacing and thereby increasing the total energy absorbed. The other parameters of interest that were investigated were the load ratio and the load uniformity index.

Experimental determination of the compressive strength of pultruded structural shapes

Abstract An existing coupon compression fixture was modified for testing cylindrical coupon samples of a pultruded material in compression. Then a new fixture was developed for testing full-sized structural shapes, which presents all the advantages of the coupon fixture. Particularly, splitting of the end of the sample is prevented while reducing the stress concentration factor at the ends, yielding compression failures at the center of the specimen. All the fiber reinforcements of structural shapes (CSM, ±45, and roving) were tested individually and combined to support the development of a simple model for compressive strength of structural shapes. A simple formula is developed for the prediction of the compressive strength of pultruded structural shapes. Comparisons between experimental data and predicted values are presented.

Characterization of Bondline Thickness Effects in Adhesive Joints

Growing application of composite materials in airframe structures tends to make significant use of bonded constructions. Despite the large work that has been done on adhesive characterization over previous years, various certification-related issues arise in these applications. The available test methods for determining the in situ properties of an adhesive joint for use in the design must be clarified for thick bondlines, where there is a lack of readily available data. Results from three test methods indicated a decrease in the apparent shear strength as the bondline thickness was increased. The apparent shear strength given by the test methods investigated was found to be highly dependent on the adherend bending stiffness, which directly effects the peel stress distributions in the adhesive layer. In the adhesive strength comparisons as a function of test method, it was found that the thin-adherend tests gave different comparative results than the thick-adherend tests, which were primarily a function of the high peel stresses in the thin-adherend joints. This combined state of stress does not give a true view of the apparent shear stress of the adhesive, but rather an indication of the adhesive behavior under this type of combined loading. It is recommended that thick adherends be used when comparing different adhesive systems for apparent shear strength, and that thin adherends should be used for qualitative tests only.

A damage mechanics model for compression strength of composites

A compression strength model based on damage mechanics and a Gaussian distribution of fiber misalignment is presented. The model uses only three material parameters that can be measured by well established methodologies. The compression strength and all the parameters that enter in the model were measured for the same materials. Predicted values are compared to experimental data for eleven different E-glass reinforced pultruded composites. Theoretical arguments are provided to support the use of damage mechanics and very good correlation is found between theory and experiments.

A Semi-analytical Method for Determining the Strain Energy Release Rate of Cracks in Adhesively-bonded Single-lap Composite Joints

A semi-analytical method for determining the strain energy release rates due to a prescribed crack in an adhesively-bonded, single-lap composite joint subjected to axial tension is presented. The field equations in terms of displacements within the joint are formulated by using first-order shear deformable, laminated plate theory together with kinematic relations and force equilibrium conditions. The stress distributions for the adherends and adhesive are determined after the appropriate boundary and loading conditions are applied and the equations for the field displacements are solved. Based on the adhesive stress distributions, the forces at the crack tip are obtained and the strain energy release rates of the crack are determined by using the virtual crack closure technique (VCCT). Additionally, the test specimen geometry from both the ASTM D3165 and D1002 test standards are utilized during the derivation of the field equations in order to correlate analytical models with future test results. The system of second-order differential field equations is solved to provide the adherend and adhesive stress response using the symbolic computation tool, Maple 9. Finite element analyses using J-integral as well as VCCT were performed to verify the developed analytical model. The finite element analyses were conducted using the commercial finite element analysis software ABAQUSTM. The results determined using the analytical method correlated well with the results from the finite element analyses.