ABSTRACT PEEK matrix reinforced by carbon fibers as one thermoplastic composite material is studied. Thermoplastic composites have the advantages of high specific stiffness and strength, longer fatigue life, good resistance to moisture absorption and high temperature condition. The thesis is aimed to investigate the mechanical properties and fracture mechanism of the centrally notched AS-4/PEEK composite laminates subjected to tension-tension fatigue loading at elevated temperature. We use three common types of laminates, such as cross-ply , quasi-isotropic and angle-ply . After centrally notched, we first obtain the base-line data of mechanical properties by tensile tests at five different temperatures, such as 25¢J¡B75¢J¡B100¢J¡B125¢J¡B150¢J. Then, the fatigue tests are conducted, we receive the fatigue strength and life and establish the stress-life curves. The fatigue characteristics and fracture mechanism of a centrally notched composite laminate at elevated temperature are also recorded and observed. The empirical results can be concluded as follows. At the same temperature, the laminate of cross-ply possesses the largest ultimate strength and fatigue strength, quasi-isotropic the second angle-ply the smallest. As for the elastic modulus, the laminate of cross-ply is larger than that of quasi-isotropic. However, the large strain of angle-ply is within the plastic range that is out of the limit of the study. Thus, a further investigation is needed for angle-ply laminates alone. After centrally notched, the net area is reduced of the specimen, and then the elastic modulus is raised and the ultimate strength and fatigue strength of composite materials are lower. As the temperature increasing, the ultimate strength, fatigue strength and elastic modulus are all decreasing.

ABSTRACT AS4/PEEK (APC-2) is a thermoplastic composite materials consisting of polyether-ether-ketone (PEEK) reinforced with AS4 carbon fibers. APC-2 has been widely used in many weight critical applications because of high specific strength and stiffness, good corrosion resistance, good formability and high temperature durability. However, the susceptibility of composite materials to damage result from low-velocity impacts (for example, from dropped tools, runway stones or hailstones) is a major problem. Low-velocity impact induces internal damage in the composite laminate without any visible sign on the surface, but it is result in a loss of laminate strength. This paper is aimed to investigate the mechanism and mechanical performance of [0/90] and [0/+45/90/-45] laminates subjected to Drop-Weight Impact by a cylindro-conical, a cylindro-hemisphere and a cylindrical impactor tip at temperature of 25¢J, 75¢Jand 125¢J. The study of impact response and post impact strength of composite laminates subjected to low velocity impact shows that the failure mechanism is predominantly delamination and fiber breakage. Generalizing the results of experiment, we can conclude that an impactor with a small nose (cylindro-conical) induces a larger impact-induced damage than one with a large nose (cylindrical), as well as a greater degree of fiber breakage. But for the reduction of post-impact strength, the cylindro-hemisphere impcator induces the most reduction of strength than the cylindro-conical and the cylindrical impactors. The post-impact residual strength of [0/90] specimens is higher than [0/+45/90/-45] specimens. But, [0/+45/90/-45] specimens are better to resist the impact effect. As for the effect of elevated temperature, we found that when the temperature increases, the damage extent reduces slowly. Keywords: composite, low-velocity impact, temperature effect, young's modulus, ultimate strength

The finite element method is used to investigate the effects of welding-induced residual stress and distortion on the strength and behaviour of ship hull structures. A finite element welding simulation consisting of sequentially coupled transient thermal and nonlinear structural analyses is used to predict the three-dimensional residual stress and distortion fields in welded stiffened plates. Three types of stiffener commonly used in commercial and naval applications are considered. The welding simulation is followed by a 'shakedown' analysis to study the possibility of residual stress relief caused by cyclic loads. The strength and behaviour of stiffened plates under axial load is characterized by normalized plots of average axial stress versus axial strain, commonly referred to as load-shortening curves. These curves are used to evaluate the effects of welding-induced residual stress and distortion on stiffened plate behaviour with and without considering stress relief by shakedown. Load-shortening curves generated by finite element analysis are also compared with load-shortening curves produced using analytical methods including those prescribed in ship structural design standards published by the International Association of Classification Societies (IACS). To conclude, a hull girder ultimate strength analysis is carried out using Smith's method with load-shortening curves generated by several different methods. Results indicate that welding-induced residual stress and distortion decrease the ultimate strength of flat-bar, angle, and tee-stiffened plates investigated in this study by as much as 17%, 15% and 13%, respectively. Stiffened plate ultimate strength values calculated using IACS common structural rules agreed reasonably well with results from numerical models in most cases. There was however, a significant discrepancy between the numerical load-shortening curves and the IACS curves in the post-ultimate regime, where the IACS curves overestimated the post-ultimate strength of stiffened plates by as much as 30%. To investigate stress relief by shakedown, axial stresses of 25% and 50% of the yield stress were applied and residual stresses were reduced by approximately 20% and 40%, respectively. In some cases, these reductions in residual stress led to increases in stiffened plate ultimate strength as high as 7%. Analysis of a box girder using load-shortening curves from a finite element model including residual stresses and distortions predicted by welding simulation predicted a bending moment capacity within 2.7% of the experimentally measured value. Using load-shortening curves from the IACS common structural rules, the ultimate strength was overestimated by 17%. / Thesis .pdf/A

An efficient interaction formula and a semi-analytical method are developed for calculating the ultimate strength of steel-elastomer sandwich panels under combined in-plane compression and lateral pressure. By using the Galerkin method and extending the semi-analytical method to clamped sandwich panels, the governing equations of sandwich panels have been solved by the Galerkin method. The material nonlinearity is treated by iteration and a three-dimensional mesh. For the load case of pure lateral pressure, the ultimate strength from the semi-analytical method is similar to that from hinge line theory and finite element analysis (FEA). However, the semi-analytical method requires about as much computation as FEA, and it is therefore not suitable for design. Finite element modeling and nonlinear analysis are performed to calculate the ultimate strength of sandwich panels under combined load. The results agree with experimental results. This verifies the accuracy of the current finite element model. The verified finite element model is used to obtain the results for a large set of sandwich panels with various dimensions and load combinations. The FEA results for pure lateral pressure load cases are used to derive a correction factor for the hinge line formula. Statistical analysis confirms that the generalized hinge line formula gives accurate values of ultimate strength of sandwich panels under pure lateral pressure. Except for the pressure-only FEA data points, the other FEA data points are corrected so as not to count the in-plane load carried by the elastomer core. Based on the corrected FEA data points, a general expression is developed for an interaction equation. The resulting equation has a bias of -0.003 and a standard deviation of 0.029. Since the radius of the interaction curve is close to 1, this standard deviation is of the order of 3%, which shows that the ultimate strength given by the equation is very close to the FEA results. The interaction equation is so simple that the ultimate strength of clamped sandwich panels under combined in-plane compression and lateral pressure can be easily calculated. / Ph. D.

An efficient beam-column approach, using an improved step-by-step numerical method, is developed in the current research for studying the ultimate strength problems of stiffened panels with two load cases: 1) under longitudinal compression, and 2) under transverse compression. Chapter 2 presents an improved step-by-step numerical integration procedure based on (Chen and Liu, 1987) to calculate the ultimate strength of a beam-column under axial compression, end moments, lateral loads, and combined loads. A special procedure for three-span beam-columns is also developed with a special attention to usability for stiffened panels. A software package, ULTBEAM, is developed as an implementation of this method. The comparison of ULTBEAM with the commercial finite element package ABAQUS shows very good agreement. The improved beam-column method is first applied for the ultimate strength analysis of stiffened panel under longitudinal compression. The fine mesh elasto-plastic finite element ultimate strength analyses are carried out with 107 three-bay stiffened panels, covering a wide range of panel length, plate thickness, and stiffener sizes and proportions. The FE results show that the three-bay simply supported model is sufficiently general to apply to any panel with three or more bays. The FE results are then used to obtain a simple formula that corrects the beam-column result and gives good agreement for panel ultimate strength for all of the 107 panels. The formula is extremely simple, involving only one parameter: the product λΠorth2. Chapter 4 compares the predictions of the new beam-column formula and the orthotropic-based methods with the FE solutions for all 107 panels. It shows that the orthotropic plate theory cannot model the "crossover" panels adequately, whereas the beam-column method can predict the ultimate strength well for all of the 107 panels, including the "crossover" panels. The beam-column method is then applied for the ultimate strength analysis of stiffened panel under transverse compression, with or without pressure. The method is based on a further extension of the nonlinear beam-column theory presented in Chapter 2, and application of it to a continuous plate strip model to calculate the ultimate strength of subpanels. This method is evaluated by comparing the results with those obtained using ABAQUS, for several typical ship panels under various pressures. / Ph. D.

In this thesis improved expressions for elastic local plate buckling and overall panel buckling of uniaxially compressed T-stiffened panels are developed and validated with 55 ABAQUS eigenvalue buckling analyses of a wide range of typical panel geometries. These two expressions are equated to derive a new expression for the rigidity ratio (EIx/Db)CO that uniquely identifies ¡°crossover¡± panels ¨C those for which local and overall buckling stresses are the same. The new expression for (EIx/Db)CO is also validated using the 55 FE models. Earlier work by (Chen, 2003) had produced a new step-by-step beam-column method for predicting stiffener-induced compressive collapse of stiffened panels. An alternative approach is to use orthotropic plate theory. As part of the validation of the new beam-column method, ABAQUS elasto-plastic Riks ultimate strength analyses were made for 107 stiffened panels ¨C the 55 crossover panels and 52 others. The beam-column and orthotropic approaches were also used. A surprising result was that the orthotropic approach has a large error for crossover panels whereas the beam-column method does not. Some possible reasons for this are suggested. Collapse patterns for the crossover panels are studied and classified from von Mises stress distribution at collapse. The collapse mechanism and load-deflection diagrams suggest stable inelastic post collapse behavior for most panels and an abrupt drop in load carrying capacity in only nine of the 55. / Master of Science

The objective of this thesis is to biomechanically evaluate a novel Double bundle technique for UCL reconstruction designed to accelerate recovery time and minimize gap formation. Excluding UCL surgery, ligament reconstruction procedures typically require an average of 6 months of recovery time. UCL reconstructive surgery requires approximately 1-2 years of recovery time. Valgus instability of the elbow is characterized by attenuation, or frank rupture of the UCL from repetitive and excessive valgus loads. This research compared the valgus stability, gap formation, and ultimate strength that resulted from the cyclic valgus loading at 30 ° of flexion of 3 techniques for reconstruction of the UCL: the Jobe, Docking, and a novel Double bundle procedure. A servocontrolled materials testing machine applied a cyclic valgus load to white cortical Sawbones elbow complex models while a 3D electromagnetic motion tracking system recorded the valgus displacement of the UCL reconstructions. The valgus stability, gap formation, and ultimate strength were measured at 50, 100, 200 and 600 cycles or the cycle at which failure occurred. The mean peak load to failure was 30N for the Jobe reconstructions, and 50N for both the Docking and Double bundle reconstructions. Both the Docking and the Double bundle reconstructions sustained a higher load to failure than the Jobe reconstruction. None of the separate univariate ANOVAs of the biomechanical parameters of each reconstruction were statistically significant. Although there was no statistically significant difference, a small standard deviation in all measured values indicated consistency in testing methodology. The power or sample size is not high enough to state with confidence that statistically there is no difference.

<p>Buckling and postbuckling of plates and stiffened panels are considered. Computational models for direct calculation of the response are developed using large deflection plate theory and energy principles. Deflections are represented by trigonometric functions. All combinations of biaxial in-plane compression or tension, shear, and lateral pressure are included in the formulations. The procedure is semi-analytical in the sense that the incremental equilibrium equations are derived analytically, while a numerical method is used for solving the equation systems, and for incrementation of the solution.</p><p>Unstiffened plate models are developed both for the simply supported case and for the clamped case. For the simply supported case the material types considered are isotropic elastic, orthotropic elastic, and elastic-plastic. Two models are developed for analysis of local buckling of stiffened plates, one for open profiles and one for closed profiles. A global buckling model for stiffened panels is developed by considering the panel as a plate with general anisotropic stiffness. The stiffness coefficients are input from the local analysis. Two models are developed for combined local and global buckling, in order to account for interaction between local and global deflection. The first is for a single stiffened plate, and uses a column approach. The second is for a stiffened panel with several stiffeners.</p><p>Numerical results are calculated for a variety of plate and stiffener geometries for verification of the proposed model, and comparison is made with nonlinear finite element methods. Some examples are presented. For all models, the response in the elastic region is well predicted compared with the finite element method results. Also, the efficiency of the calculations is very high. Estimates of ultimate strength are found using first yield as a collapse criterion. In most cases, this leads to conservative results compared to predictions from finite element calculations. </p>

Buckling and postbuckling of plates and stiffened panels are considered. Computational models for direct calculation of the response are developed using large deflection plate theory and energy principles. Deflections are represented by trigonometric functions. All combinations of biaxial in-plane compression or tension, shear, and lateral pressure are included in the formulations. The procedure is semi-analytical in the sense that the incremental equilibrium equations are derived analytically, while a numerical method is used for solving the equation systems, and for incrementation of the solution. Unstiffened plate models are developed both for the simply supported case and for the clamped case. For the simply supported case the material types considered are isotropic elastic, orthotropic elastic, and elastic-plastic. Two models are developed for analysis of local buckling of stiffened plates, one for open profiles and one for closed profiles. A global buckling model for stiffened panels is developed by considering the panel as a plate with general anisotropic stiffness. The stiffness coefficients are input from the local analysis. Two models are developed for combined local and global buckling, in order to account for interaction between local and global deflection. The first is for a single stiffened plate, and uses a column approach. The second is for a stiffened panel with several stiffeners. Numerical results are calculated for a variety of plate and stiffener geometries for verification of the proposed model, and comparison is made with nonlinear finite element methods. Some examples are presented. For all models, the response in the elastic region is well predicted compared with the finite element method results. Also, the efficiency of the calculations is very high. Estimates of ultimate strength are found using first yield as a collapse criterion. In most cases, this leads to conservative results compared to predictions from finite element calculations.

The failure behavior of collagenous soft tissues is important for clinical problems of plaque rupture and trauma. Cyclic tests require high frequencies that may affect the strength properties of the soft tissues. Experimental results of mechanical response of blood vessels to physiologic loads can be used to model and predict plaque rupture and direct medical therapy or surgical intervention. The goal of the study is to measure the mechanical failure properties of arteries to determine if they are strain rate and cycle dependant and to measure the progressive damage of arteries with time dependent loading. Ring specimens of porcine carotid arteries were preconditioned and then pulled to failure. In all cases, the intima broke first. Ultimate stress increased as a weak function of increasing strain rates. The ultimate stress at 100 mm/s was 4.54 MPa, greater than the 3.26 MPa at 0.1 mm/s. Strain rates between 1 and 100 mm/s correspond to a cyclic frequency of 0.5 Hz to 5 Hz for fatigue testing. In contrast, ultimate strain in arteries was independent of strain rate over the range tested. The creep tests showed a logarithmic relationship between stress magnitude and stress duration for this soft tissue. The creep testing indicates that damage is accumulating above certain threshold stress levels. The values of ultimate strength showed a 35% increase after 10,000 cycling loading. In contrast, the ultimate strain had a 13% decrease after cycling and the difference was statistically significant with p=0.018. The testing results showed that there were no significant differences on strength among fresh arteries and arteries stored at 5¡ã C for up to two weeks. The test results may be useful for developing a mathematical model to predict the behavior of arterial soft tissues and may be extended to estimate fracture and fatigue in the atherosclerotic plaque cap.