Study On Mechanical Behavior Of Variable Stiffness Laminates And Its Manufacturing Technology With Fiber Placement

With the increasing applications in domestic aerospace industry, traditional resinbased composite materials manufacturing, mostly by hand making, can no longer satisfy the demanding of massive produce and complex components because of its low productivity and unsteady quality. Automated manufacturing is an irresistible trend in the developing of resin-based composite materials manufacturing industries. Automated fiber placement(AFP) is one of the most important methods on fiber re-enforced composite materials forming. Not only can it realize the automation of manufacturing, but it also can obtain laminates with constantly changing fiber angle and optimize the mechanical behaviors of the laminate basing on the special mechanical needs of the component. Focusing on the variable stiffness laminates manufactured by AFP, this thesis is aiming to study the mechanical properties and automatic forming technology of the variable stiffness laminates and provide fundamental support for the applications of AFP in aviation.This thesis proposed an expression of fiber placing reference path based on Bezier curve, obtained the change rule of the fiber angle and built up a mathematical model. Compared to the reference path with linearly changed angles, this model has advantages as the angels can change non-linearly and the designers can have more freedom. Advantages and disadvantages of shifted method and parallel method for constructing variable stiffness laminates were discussed, shifted distance with minimum overlapping area and without gap by shifted method was obtained and fiber laminates utilizing that method were constructed. Taking the expression of laminates with linearly changed angles as a reference, an expression of Bezier-curve variable stiffness laminates was proposed. In combination with the properties of the designed variable stiffness laminates and the input requirements of the ABAQUS analysis software, a software to generate the analysis model of the variable stiffness laminates was written by Matlab.Buckling analysis were made respectively on the steadiness of laminates constructed by variable stiffness plies and straight-line plies, the buckling load of variable stiffness laminates was founded to be obviously higher and the rule of the influence that initial geometric imperfections cast on buckling load was obtained. From the plane statics analysis made on the variable stiffness laminates, the distribution of deformation and stress on the laminates under compressive load were obtained and it could be noticed that the change of fiber angel rearranged the load and consequently influenced the mechanical performances of the laminates. By keeping the start angle of the reference path constant and changing the terminate angle, the influence that different fiber path acted on the steadiness was analyzed. It was found that first order buckling load of the laminates would be higher if the terminate angel was larger than the start angle. In addition, further research showed that hybriding straight-line plies with a certain angle would help improving the buckling load of the laminates variable.In order to obtain the basic material parameters needed by ABAQUS finite element analysis, the main process parameters of AFP were studied. Proper range of working temperature, pressure and velocity were selected, unidirectional laminates were manufactured and tensile tests were finished. Moreover, focusing on the constructed variable stiffness plies, processing code generating software was written, curve-line placing tests were performed, the causes of folding tows were analyzed, solutions were put forward and the limitation of reference path curvature during the working process was obtained. Finally, no-folds plane placement of fiber tows with angle changing constantly was realized, variable stiffness laminates with hybrid plies were manufactured and then the mechanical behaviors of the properly designed variable stiffness laminates were proved to be better than that of the laminates constructed by straight-line plies through compression tests.