Jack E. Helms

Buckling load analysis of grid stiffened composite cylinders

Stiffened cylindrical shells are the major components of aerospace structures. In this study global buckling load for a generally cross and horizontal grid stiffened composite cylinder was determined. This was accomplished by developing an analytical model for determination of the equivalent stiffness parameters of a grid stiffened composite cylindrical shell. This was performed by taking out a unit cell and smearing the forces and moments due to the stiffeners onto the shell. Based on this analysis the extensional, coupling and bending matrices (A, B and D matrices, respectively) associated with the stiffeners were determined. This stiffness contribution of the stiffeners was superimposed with the stiffness contribution of the shell to obtain the equivalent stiffness parameters of the whole panel. Making use of the energy method the buckling load was solved for a particular stiffener configuration. Buckling test was also performed on a stiffened composite cylinder and compared with analytical results. Finally, using the analytical model developed, parametric analysis of some of the important design variables was performed and based on these results conclusions were drawn.

Investigation into FRP repaired RC columns

Due to the aging of the infrastructures in the US, repair and rehabilitation of damaged steel reinforced concrete structures using fiber reinforced plastics (FRP) are increasingly becoming a topic of interest in the infrastructure community. In this study, a finite element analysis using ANSYS® was utilized to conduct a parametric analysis. Experiments were also conducted to justify the finite element analysis results. A reasonable agreement was found between the finite element analysis and the test results. The effect of the thickness, stiffness, and fiber orientation of the FRP layers as well as the interfacial bonding between the FRP wraps and the concrete on the strength and stiffness of the repaired columns was evaluated using the finite element modeling.

Fast repair of laminated beams using UV curing composites

In this study, laminated beam samples were predamaged by low velocity impact. The damaged samples were repaired using either ambient environment curing epoxy, heat activated curing prepreg, or ultraviolet (UV) curing resin. Environmental conditioning using UV radiation and a seawater bath was conducted to investigate the durability of the repaired samples. A uniaxial tension test was conducted on a total of 45 effective samples to evaluate the strength recovered by the repair materials and the strength lost by environmental conditioning. A finite element analysis was conducted to understand the failure modes of the repaired samples. The test results show that fiber reinforced UV curing resin is a fast, strong, durable, and cost effective method to repair low velocity impact damaged composite laminates.

Stiffness Degradation of FRP Strengthened RC Beams Subjected to Hygrothermal and Aging Attacks

The past decade has witnessed an ever increasing interest in strengthening, repairing, retrofitting, andu pgrading of deterioratedco ncrete structures using fiber reinforcedplastics (FRP). Enhancedload carrying capacity by FRP strengthening has been observedby a large number of researchers through experiments at ambient environments. In a harsh environment, however, FRP will degrade. This may result in structural degradation of FRP strengthened concrete members. The possible structural degradation has become a major obstacle for the wide-spread acceptance of this new strengthening technique. In the present study, boiling water and ultraviolet (UV) radiation were used to study the structural degradation of concrete beams strengthened with GFRP and CFRP fabrics. A total of eighteen 170 7.62 15.2 cm steel reinforcedconcrete (RC) beams were prepared as control specimens, conditioned specimens, and unconditioned specimens. Finite element modeling was used to analyze stiffness degradation of FRP layers. The tested and calculated results showed that environmental attacks had a considerable effect on the structural degradation of FRP strengthened concrete beams. 57–76% of the strengthening efficiency and43–48% of the stiffness of FRP were lost after conditioning.

Impact response of a composite laminate bonded to a metal substrate

Based on a spring-mass model developed by Pang et al. (1995) for a single-lap joint under impact load, the low-velocity impact responses of a composite laminate adhesively-bonded to a metal substrate have been studied. In this quasi-static model, the impact response is derived for a time-dependent force. The target structure is modeled as a single plate with an equivalent stiffness. Using an energy method, an analytical model has also been developed to determine the equivalent stiffness of the composite laminate/metal substrate hybrid structure. Advanced structural mechanics were used in the derivation of the governing equations of the structure. A FORTRAN program was developed on an IBM compatible personal computer to implement the solution algorithm. With the spring-mass system and the developed equivalent stiffness model, a relationship between the impact response and the impact duration has been established. Impact tests have been conducted to verify the proposed model. The analytical model results and the experimental data match reasonably well.

Comparison of Different Methodologies for Stress Analysis of Reinforcing Pads

Finite element analysis is widely used to model the stresses resulting from penetrations in pressure vessels to accommodate components such as nozzles and man-ways. In many cases a reinforcing pad is required around the nozzle or other component to meet the design requirements of Section VIII, Division 1 or 2, of the ASME Pressure Vessel Code [1]. Several different finite element techniques are currently used for calculating the effects of reinforcing pads on the shell stresses resulting from penetrations for nozzles or man-ways. In this research the stresses near a typical reinforced nozzle on a pressure vessel shell are studied. Finite element analysis is used to model the stresses in the reinforcing pad and shell. The commercially available software package ANSYS is used for the modeling. Loadings on the nozzle are due to combinations of internal pressure and moments to simulate piping attachments. The finite element results are compared to an analysis per Welding Research Council Bulletin 107 [2].© 2003 ASME

Preliminary Analysis of Grid Stiffened Composite Cylinders

Stiffened cylindrical shells are major components of Aerospace structure application. Two models were developed for assessing the universal buckling load of a generally cross and horizontal stiffened composite cylinder. The first model uses a simple conservation of volume and direction of stiffener orientation, while the second model analyzes the force and moment interaction of the stiffeners and the shell. Based on these models the A, B and D matrix stiffness parameters were determined for the overall cylinder panel. The buckling load was solved for a particular stiffener configuration by using the energy method. Buckling test was also performed on a stiffened composite cylinder and compared with buckling load results of both analytical models, and conclusions were drawn on the degree of reliability of the models developed. Finally, parametric analysis of some of the important design variables was performed based on the ‘Force Smearing’ model.Copyright

Buckling Analysis of a Taper-Taper Adhesive-Bonded Composite Joint

An analytical model of the behavior of an adhesive-bonded taper-taper composite joint under axial compressive loading has been developed using the Ritz Method. The model was based on laminated beam theory. A Fourier series was used to represent the transverse displacement variable and the Ritz method was used to derive an eigenvalue equation for adhesively bonded taper-taper composite joint. The smallest eigenvalue is the critical buckling load. Finite element analyses were performed on two unidirectional laminated beam joints with various taper angles to verify the analytical model. The effect of varying the taper angle, adhesive thickness and adhesive modulus on the critical buckling load were investigated analytically.Copyright