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1、Vcct (Virtual crack closure technique)Interlaminar fracture mechanics characterizes the onset of delaminations in composites. Shear loading causes the panel to buckle and the resulting out-of-plane deformations initiate skin/stringer separation at the location of an embedded defect.METHOD: Finite El
2、ement AnalysisELEMENT: the panel and surrounding load fixture were modeled with shell elements. A small section of the stringer foot and the panel in the vicinity of the embedded defect were modeled with a local 3D soil model.1. BACKGROUNDAerospaces structures are made of flat or cured panels with c
3、o-cured or adhesively bonded frames and stiffeners computational stress analysis to determine the location of first matrix cracking.An artificial defect was placed at the termination of the center stiffener. The stiffened panel is subjected to pure shear loading which causes the panel to buckle.ANAL
4、YSIS: nolinear finite element analysisStrain energy release rates and mixed made ratios were computed using the virtual crack closure technique.2. METHODOLOGY2.1 Interlaminar fracture mechanicsThe total strain energy release rate The mode I component due to interlaminar tension The mode II component
5、 due to interlaminar sliding shear The mode III component due to interlaminar scissoring shear Purpose: to predict delamination onset or growth for two-dimensional problems, theses calculated G components are compared to interlaminar fracture toughness properties measured over a range of mode mixiti
6、es from pure mode I loading to pure mode II loading.A quasi static mixed-mode fracture criterion is determined by plotting the interlaminar fracture toughness,versus the mixed-mode ratio, , determined from data generated using pure Mode I () Double Cantilever Bending (DCB) pure Mode II () four point
7、 End Notched Flexure (4ENF), and Mixed Mode Bending (MMB) tests of varying ratio for IM7/8852 carbon epoxy material. and are the fracture toughness data for mode I and II is a factor determined by the cure fit. Shown in figure 3 in this article.Failure is expected when, for a given mixed mode ratio,
8、 the calculated total energy release rate, exceeds the interlaminar fracture toughness, .2.2 . Analysis Tools2.2.1. Virtual Crack Closure TechniqueVCCT requires force and displacement input, which is obtained from continuum (2-D) and solid (3-D) finite element analyses of the cracked (2-D) or delami
9、nated (3-D) component. and are calculated for four-noded elements is the length of the elements at the crack front; and are the forces at the crack tip (nodal point i);The relative displacements behind the crack tip are calculated from the nodal displaces For geometric nonlinear analysis where large
10、 deformations may occur, both forces and displacements obtained in the global coordinate system need to be transformed into a local coordinate system () which originates at the crack tip.For the two-dimensional eight-noded quadrilateral element with quadratic shape functions this yieldsThe total ene
11、rgy release rate is calculated from the individual mode components aswhere for the two-dimensional case discussed.In a finite element model made of three-dimensional solid elements the delamination of length a is represented as a two-dimensional discontinuity by two surfaces. (why does it is discont
12、inuity?)The model I, mode II, and mode III components of the strain energy release rate,and are calculated as. Here is the area virtually closes, is the length of the elements at the delamination front, and b is the width of the elements.A local crack tip coordinate system is needed.2.2.2. A Global/
13、Local Shell 3D Modeling TechniqueComputed mixed mode strain energy release rate components depend on many variables such as element order and shear deformation assumptions, kinematic constraints in the neighborhood of the delamination front, and continuity of material properties and section stiffnes
14、s in the vicinity of the debond when delaminations or debonds are modeled with plate or shell finite elements.3. FINITE ELEMENT MODELING3.1. Global Shell Model of Stringer Stiffened PanelThe global model includes the steel load frame and attachments, the panel made of graphite/epoxy prepreg tape and
15、 the stringers made of graphite/epoxy fabric. The outer steel load frame and the attachment bolts were modeled with beam elements available in the element software ABAQUS. The inner steel load frame which overlaps the panel edge was modeled with standard shell S4 elements. The shell elements ate con
16、nected by beam elements designed to enforce plate theory constraints. In the sections containing the artificial defects the beam elements were replaced by gap elements. In preparation for the global/local modeling approach shell elements representing the foot of the stiffener and the panel were removed fro
