Experimental study on damage evolution mechanism in bolted joints of CFRP laminates with toughened interlayers
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School of Engineering |
Master's thesis
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Date
2010
Department
Major/Subject
Lentotekniikka
Mcode
Kul-34
Degree programme
Language
en
Pages
v + 78 s. + liitt. 4
Series
Abstract
Fiber reinforced composites with toughened interlayer's are being used in aerospace applications in increasing amounts. The objective of this thesis is to experimentally observe how the toughened interlayer's affect the damage evolution mechanism in bolted joints of carbon fiber reinforced composite laminates with toughened interlayer's. Especially attention is paid on the formation of fiber micro buckling and kinking. In these experiments damage evolution in CFRP laminates toughened by thermoplastic polymer particles added to the interlayer's are observed. A double lap test joint rig with the specimen coupon is assembled and loaded in a tensile testing machine. Damage evolution is then explained by observing the damaged specimen coupons visually, damaged cross-sections by optical microscope and inner damage by soft X-ray radiography. Results are compared with similar data retrieved using untoughened materials. In the load-displacement curves for the interlayer toughened laminates, load drops presenting damage occurred very randomly when compared with untoughened laminates. Bearing failure mode was mainly observed until final failure, when on the other hand in untoughened laminates the failure mode often changed to shear-out failure. In toughened laminates, fiber kinking occurred mainly in the in-plane direction. The interlaminar layers stayed intact even after significant damage had occurred. Due to the toughness of the interlaminar layers, delaminations in the interlaminar region were quite rare, and instead longitudinal cracks propagated inside the plies. Also, due to the toughening, transverse shear cracks did not often propagate from ply to ply and buckling of whole plies was often observed.Description
Supervisor
Saarela, OlliThesis advisor
Takeda, NobuoKeywords
CFRP, bolted joint, toughened interlayer, damage evolution