Stability And Progressive Failure Behavior For Composite Grid Stiffened Structure With Delamination Considering Hygrothermal Effect

In this paper, a finite element method has been developed to investigate the stability and progressive failure of advanced composite grid stiffened structure (AGS) with delamination considering hygrothermal environment. The research work of this paper is the main subject of the project of the National Natural Science Foundation in China "The mechanism of advanced grid structure (AGS) with damage", Grant No. 10302004, and National Basic Research Program of China "New theory of optimum design for extra-light porous materials and their structures", Grant No. 2006CB601205.On the basis of the Mindlin first order shear effect theory and the Von-Karman non-linear deformation assumption, a composite laminates element and a beam element are adopted to model the skin and the stiffener, respectively. By coordinate transformation and the geometry continuum condition of the skin and the stiffener, the element tangent stiffness matrices for the AGS are deduced. A finite element numerical method has been developed to study the buckling, postbuckling of composite grid stiffened structures with embedded delamination in the skin in the hygrothermal environment. A progressive failure criterion is applied to describe the failure of the skin and the ribs including the damage of matrix, fiber break and fiber-matrix shear failure, and the progressive failure evolution process of the structures during the loading process is simulated. On the basis of the numerical methods, according to the numerical investigation of some typical examples, the effects of temperature fluctuation, hygroscopic concentration, size of the delamination and stiffness degradation caused by progressive damage failure on the buckling, postbuckling and delamination evolution of composite grid stiffened structure with embedded delamination subjected to a compressive load are discussed.Stability and progressive failure behavior in the different hygrothermal environment are investigated for composite grid stiffened plates and shells by the numerical method, and some useful conclusions are obtained:1. When the progressive failure behavior of material is considered, because of stiffness degradation phenomenon happened suddenly, the postbuckling path carve of AGS plates and shells has a significant fluctuation. As the load is increasing continuously, the stiffness of the structure is also losing gradually, and the stability of structures are being weakened obviously. Therefore, progressive failure behavior is of great significance to estimate the structures' stability in the engineering.2. Three different kinds of critical buckling modes will occur in composite grid stiffened structure containing a delamination with different delamination sizes in skin, namely global buckling, mixed buckling and local buckling. On the basis of geometrical nonlinearity analysis, mixed buckling and local buckling in delamination are considered as key factors to the load carrying capacity of such structures.3. To stability analysis, temperature and humidity of environment will affect buckling critical load of structure in different degrees. It has relation with buckling mode of structure. In most cases, hygrothermal effect, material progressive failure and delamination have to be considered in one analysis procedure, as a result stability and progressive failure behavior are extremely complicated. Therefore, investigation in depth is necessary.4. To composite grid stiffened structure with a delamination, the results of stability computation from eigenvalue buckling are too conservative. We should apply postbuckling analysis based on large-deflection theory to estimate the structure's stability. The progressive damage and hygrothermal effects should be taken into account when design and make a more accurate and reliable predict of the load carrying capacity of AGS structure.