ADHESIVE BOND INSPECTION FOR COMPOSITES WITH ULTRASONIC GUIDED WAVES

Open Access
Author:
Ren, Baiyang
Graduate Program:
Engineering Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 30, 2011
Committee Members:
  • Clifford Jesse Lissenden Iii, Thesis Advisor
Keywords:
  • ultrasonic guided wave
  • nondestructive evaluation
  • adhesive bond
  • carbon-fiber-reinforced polymer
Abstract:
Composite materials are increasingly used in aerospace industries as primary structure because of their low weight, high strength and stiffness, as well as resistance to corrosion. Adhesively bonded joints are one of the best options for joining composite materials. However, adhesive bonds are subject to damage such as adhesive defects, e.g. disbonds, and cohesive defects, e.g. porosity. Ultrasonic guided wave techniques are regarded as promising methods for bond condition inspection. Ultrasonic guided waves can travel long distances without too much attenuation. The large number of propagating modes that correspond to different displacement distributions provides the possibility to characterize different types of defects. Adhesive defects are the focus of this thesis. The objective of this thesis is to extend recent research on inspection of isotropic adhesive bonds to composite adhesive bonds. The mode conversion that occurs at waveguide transitions will be studied. Analysis methods that are similar to those used with isotropic materials are employed and their performance will be evaluated. The similarities and differences when selecting effective modes for inspection of isotropic and anisotropic materials will be studied. Challenges associated with inspection of bonded region can be summarized as finding a mode sensitive to adhesive defects and successfully exciting that mode. Mode selection takes both of these aspects into consideration. A simplified skin-stringer system is considered. Both skin and stringer are carbon fiber reinforced polymer with a [0/45/90/-45]S2 stacking sequence. Dispersion analysis is conducted on skin and bonded region waveguides. A wave structure based method gives the candidate modes and frequencies that are sensitive to adhesive defects. Wave structure matching and the normal mode expansion method are utilized to study the mode conversion at waveguide transitions. Mode selection is carried out based on the requirements of sensitivity and excitability. Further optimization is necessary for practical reasons. The attenuation and skewing propagation direction could affect the inspection considerably. The choices for mode selection have been further narrowed down by choosing modes in the low frequency range and having small skew angles. Finite element simulation performed to demonstrate the excitation of an effective mode shows that the resultant displacement field in the bonded region satisfies the requirement of having a large in-plane displacement at the interface. Experiments have been conducted on the adhesively bonded skin-stringer with artificial defects. Defects are simulated by inserting Teflon film between the skin and the adhesive layer. Experimental results show that the effective modes have good sensitivity to the adhesive defects while another mode that is expected to have poor sensitivity to the defect does not show the capability to detect the defects. One optimal mode detected bonds in terms of frequency content. The other optimal mode could distinguish the good bond from defected bonds by their peak amplitudes. The results from a mode with poor sensitivity does not show the ability to tell good bonds from defected ones considering either frequency content or peak amplitude.