Eyassu Woldesenbet

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.

Response of Syntactic Foam Core Sandwich Structured Composites to Three-Point Bending

Use of syntactic foams as core materials gives several distinct advantages over traditionally used core materials. Syntactic foams have an excellent combination of compressive strength, low density, low radar detectability and low moisture absorption coefficient among others. The present work aims at studying the behavior of sandwich-structured composites containing syntactic foam as core material under three-point bending conditions. Flexural and short-beam shear tests are conducted, where large (16: 1) and small (5: 1) aspect ratio (span length/thickness ratio) specimens are tested, respectively. It is observed that the specimen failure mode changes completely with the change in the aspect ratio. Specimens are found to fracture under the effect of shear stresses in the smaller aspect ratio specimens, whereas compressive stresses lead to the fracture in higher aspect ratio specimens. The observations of fracture features are correlated with the test data and the load-displacement curves obtained in the tests. A method of analysis is also presented for syntactic foams and the sandwich structures containing syntactic foam as core material.

Surface preparation effect on performance of adhesively-bonded graphite/epoxy composites subjected to impact

The influence of pre-bond surface preparation on performance of adhesively-bonded composites subjected to impact was investigated in this study. Impact test was carried out on adhesively bonded graphite/epoxy composite specimens at different low impact energies ranging from 5 to 20 J using the drop-weight impact test. Post-impact ultrasonic evaluation was performed in order to determine the resulting internal damage due to impact on the adhesive bondline. The ultrasonic C-scan of the gated ultrasonic wave signal was acquired and the ensuing debond area in the adhesive bond was determined quantitatively for specimens made from substrates with different surface preparations such as paper peel ply, sandblasting, and sandpaper abrasion. In order to determine the flexural load bearing capacity and stiffness reduction after impact, a three-point bending test was conducted on unimpacted and impacted specimens. A comparative study was performed to evaluate the performance of adhesively-bonded composites with different surface preparations. The results revealed that paper-peel ply performed the best in terms of resistance to debond area formation in the adhesive layer, as well as in terms of retention of flexural load bearing capacity and stiffness after impact.

Bond thickness effect on impact response and damage of adhesively-bonded graphite/epoxy composites

Adhesively-bonded composites are often subjected to impact during manufacturing or service. This impact may result in a reduction in strength and structural integrity of engineering components made of adhesively-bonded composite structures. In this study, an impact test was carried out on adhesively-bonded graphite/epoxy composite panels at different low impact energies, ranging from 5 to 20 J, using a drop-weight impact test equipment. The effect of bond thickness on impact response and the resulting damage was investigated. The resulting internal damage due to impact on the adhesive bondline was interrogated using ultrasonic nondestructive evaluation. The ultrasonic C-scan of the gated ultrasonic wave signal was acquired and the ensuing debond area was determined quantitatively for specimens joined by different thickness adhesive bonds. The results revealed that increasing the thickness of the bond increased the maximum contact force reached during impact. In addition, the shape and size of the debond area in the adhesive layer showed dependence on both bond thickness and impact energy.

Nanoclay Based Grid Stiffened Syntactic Foam Composites

In this research work, a novel nanoparticle based syntactic foam was employed to form a grid structure with conventional syntactic foams filled in the bay area. Initially, a big slab of the syntactic foam was fabricated and later on cut into squares. These squares were properly arranged in a mold to have an orthogrid arrangement with empty channels between them. These channels were in turn filled with nanoparticle incorporated syntactic foams and cured. Care was taken to properly orient the squares to attain the desired orthogrid geometry. K46 and S22 type of microballoons with volume fractions of 30% and 60% respectively were used in this study. The volume fraction of nanoclay used in the syntactic foams poured within the channels was 1%. Low velocity impact and flexural tests were conducted on these novel nanoparticle based grid syntactic foams. From the low velocity impact results, it is clear that the nanoclay based grid stiffened syntactic foam with S22 type microballoons in the bay area and K46 type microballoons in the grid channels performed well in terms of the load bearing capacity, elastic energy absorption and damage suppression characteristics. On the other hand, under flexural testing, it was clear that the strain at break values for the S2260K4660-1 configuration was close to that of the K4660S2230–1 configuration.© 2010 ASME

Ultrasonic Signal Attenuation in Syntactic Foams Filled With Rubber Particles

Ultrasonic imaging is a non-destructive evaluation technique, which is used to obtain density profile, phase distribution and three-dimensional profiles of cracks and defects in a material. Although this technique is used for a variety of metals and non-metals, it is difficult to use it for testing of porous materials and foams due to high attenuation of ultrasonic waves in these materials. Syntactic foams are hollow particle filled composites that have recently emerged as attractive material for use in applications requiring low weight, low moisture absorption and high insulation properties. The present paper focuses on determining the attenuation coefficient in syntactic foams and its correlation with porosity distribution. Eight types of foam samples are tested in the study. A combination of four types of microballoons and two types of rubber particles is used. Volume fractions of microballoons and rubber particles are maintained at 0.63 and 0.02, respectively, in all samples. Pulse Echo ultrasonic test method is used and results are compared to determine the effect of constituent particles on the ultrasound signal attenuation. Coefficient of attenuation is observed to increase with decrease in density of foam samples and with decrease in size of rubber particles.Copyright

Finite Element Analysis of Composite-Concrete Adhesion

The need for the rehabilitation of bridges and structures is becoming more apparent as the number of deficient civil structure grows and the cost of replacement is becoming prohibitive. These leads to the search of alternative methods, such as rehabilitation, to put the deteriorated structures back to normal operation with the least possible cost. One such method is the use of composite plates adhesively bonded to concrete as reinforcement and to prevent the propagation of crack within the concrete structure. In this study the load transfer and the resulting stress distribution in the composite-concrete adhesion system is investigated using the finite element method. The effects of the different bond parameters are studied using the finite element. In addition, results of the finite element analysis are proved to be in agreement with the analytical solution of shear stress distribution in the adhesion layer that was developed in previous studies by the authors.Copyright

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

Enhancement of Composite Sucker Rods for Use in the Oil Industry

Analysis of polymer-matrix composite sucker rod systems using finite element methods is performed. Composite sucker rods fail mainly due to fatigue loading. In majority of cases, the failure is in the region of the joint where the composite rod and the steel endfitting meet. 2D and 3D Finite Element Analysis and experimental tests are carried out in order to observe the stress distribution and to find the regions of stress concentrations inside the endfitting. The causes of fatigue failure of the composite sucker rods are identified. These are overloading of the rod causing high transverse compressive stress that results crushing of the rod, and high stress concentrations present at the grooves of the endfitting that initiate premature fatigue cracks. Based on the result of this study, enhanced design of the composite sucker rod system can be accomplished.Copyright