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 Element Analysis
ELEMENT: 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. BACKGROUND
Aerospaces structures are made of flat or cured panels with co-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.
ANALYSIS: nolinear finite element analysis
Strain energy release rates and mixed made ratios were computed using the virtual crack closure technique. 2. METHODOLOGY
Interlaminar fracture mechanics
The total strain energy release rate GT
The mode I component due to interlaminar tension GI The mode II component due to interlaminar sliding shear GII The mode III component due to interlaminar scissoring shear GIII
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 mixities 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,GC,versus the mixed-mode ratio, GII/GT, determined from data generated using pure Mode I (GII/GT?0) Double Cantilever Bending (DCB) pure Mode II (GII/GT?1) four point End Notched Flexure (4ENF), and Mixed Mode Bending (MMB) tests of varying ratio for IM7/8852 carbon epoxy material.
GC?GIC??GIIC?G??GIC???II?
?GT??GIC and GIIC 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 ratioGII/GT, the calculated total energy release rate,GT, exceeds the interlaminar fracture toughness, GC.
. Analysis Tools
2.2.1. Virtual Crack Closure Technique
VCCT 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 delaminated (3-D) component.
GI and GII are calculated for four-noded elements
1?Zi?wl?wl*2?a
1GII???Xi?ul?ul*2?aGI???????a is the length of the elements at the crack front;
Xi and Zi are the forces at the crack tip (nodal point i);
The relative displacements behind the crack tip are calculated from the nodal displaces