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Determining the Fatigue Disbond Growth Rate of a CFRP/ Nomex® Sandwich Structure Using the Single Cantilever Beam Test – Comparison of Experimental Approaches

Thanks to their superior strength-to-weight ratio carbon-epoxy facesheet and aramid honeycomb core sandwich structures are widely used in aerospace engineering for components such as rudders, flaps, or fairings. Due to their design, however, sandwich structure is susceptible to damage from low-velocity impact loads which can initiate a local disbond of facesheet and core. This can pose a threat to the overall structural integrity of a component. Disbond propagation can be assessed by means of fracture mechanics methods. Numerous studies demonstrated the suitability and robustness of the Single Cantilever Beam (SCB) test method to determine the static disbond fracture toughness GIC of the facesheet/core interface under dominant fracture mode-I.

To study the fatigue…

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Thanks to their superior strength-to-weight ratio carbon-epoxy facesheet and aramid honeycomb core sandwich structures are widely used in aerospace engineering for components such as rudders, flaps, or fairings. Due to their design, however, sandwich structure is susceptible to damage from low-velocity impact loads which can initiate a local disbond of facesheet and core. This can pose a threat to the overall structural integrity of a component. Disbond propagation can be assessed by means of fracture mechanics methods. Numerous studies demonstrated the suitability and robustness of the Single Cantilever Beam (SCB) test method to determine the static disbond fracture toughness GIC of the facesheet/core interface under dominant fracture mode-I.

To study the fatigue service life of such components, consequently, sandwich coupons have been analyzed under SCB dynamic test. Characterization of the crack propagation at the facesheet/core interface under fatigue is required, but it is not straightforward to perform. Indeed, during peeling under conventional load control test the acting load at the crack tip would inevitably increase with increment in the crack length, and the test could become unstable. Moreover, fatigue loading at constant energy release rate over a certain range of crack extension is desired to measure reliable material property.

This work presents two different approaches to solve this problem: 1- The G-constant method and, 2- the limited Fmax-constant method. Regarding method 1, a computer script autonomously controls the test apparatus by permanently monitoring the compliance response of the specimen and continuously readjusting the applied force to ensure a constant ∆G, so that the crack can grow at a constant rate. To generate the control script the relation between force, displacement and crack length must be known. For that purpose a static SCB test was performed in advance. Method 2 uses a more simple setup. The load amplitude is kept constant over a certain ∆a while a minor increase of ∆G is accepted. A stop criterion based on the specimen stiffness loss is implemented, preventing the disbond propagation from becoming unstable. The procedure easily can be repeated for another cycle. In return of a simpler setup and a more versatile test, in this case a continuous description of the crack is not possible. Experimental results are presented for both approaches, highlighting the strengths and disadvantages of each method.

Reference
LCF9-2022-075

Title
Determining the Fatigue Disbond Growth Rate of a CFRP/ Nomex® Sandwich Structure Using the Single Cantilever Beam Test – Comparison of Experimental Approaches
Author(s)
N. D'Antrassi, R. Schäuble, R. Schlimper
DOI
10.48447/LCF9-2022-075
Event
LCF9 - Ninth International Conference on Low Cycle Fatigue
Year of publication
2022
Publication type
conference paper (PDF)
Language
English
Keywords
Honeycomb sandwich structure,Fatigue disbond growth,SCB fatigue test,Constant energy release rate