Testing and Analysis of Composite Skin/Stringer Debonding Under Multi Axial Loading
[Technical Report, Memorandum Report]
US Army Research LaboratoryNASA
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The objective of this work was to investigate the damage mechanisms in composite bonded skinstringer constructions under uniaxial and biaxial in-planeout-of-plane loading conditions as typically experienced by aircraft crown fuselage panels. The specimens for all tests were identical and consisted of a tapered composite flange, representing a stringer or frame, bonded onto a composite skin. Tests were performed under monotonic loading conditions in tension, three-pointbending, and combined tensionbending to evaluate the debonding mechanisms between the skin and the bonded stringer. For combined tensionbending testing, a unique servohydraulic load frame was used that was capable of applying both in-plane tension and out-of-plane bending loads simultaneously. Specimen edges were examined on the microscope to document the damage occurrence and to identify typical damage patterns. The observations showed that, for all three loadcases, failure initiated in the flange, near the flange tip, causing the flange to almost fully debond from the skin. A two-dimensional plane-strain finite element model was developed to analyze the different test cases using a geometrically nonlinear solution. For all three loading conditions, principal stresses exceeded the transverse strength of the material in the flange area. Additionally, delaminations of various lengths were simulated in two locations where delaminations were observed. The analyses showed that unstable delamination propagation is likely to occur in one location at the loads corresponding to matrix ply crack initiation for all three load cases. However, the current two-dimensional plane-strain finite element model may not fully account for the complex three-dimensional damage pattern observed. A detailed investigation of this damage pattern may require a local three-dimensional analysis of the damaged area.
- Laminates and Composite Materials