This paper presents an initial study on Lamb wave propagation characteristics in z-pin reinforced, co-cured
composite pi-joints for the purposes of structural health monitoring (SHM). Pi-joint test articles were designed
and created to replicate a co-cured, all composite skin-spar joint found within a typical aircraft wing structure.
Because pi-joints exhibit various complex damage modes, formal studies are required if SHM systems are to be
developed to monitor these types of joints for potential damage. Experiments were conducted on a undamaged
(healthy) and damaged test articles where Lamb waves were excited using one lead zirconate titanate (PZT)
transducer. A three-dimensional (3D) scanning laser Doppler vibrometer (LDV) was used to collect high-density
scans of both the in-plane and out-of-plane velocity measurements. In the damaged test article, where delamination,
matrix cracking, and fiber breakage can clearly be seen, changes in both the fundamental antisymmetric
A<sub>0</sub> and symmetric S<sub>0</sub> Lamb wave modes are apparent. In both test articles, the effects of narrow geometry,
discontinuity due to the attachment of the web, and thickness has detectable effects on Lamb wave propagation.
From the comparisons between Lamb waves propagating through the undamaged and damaged test articles, it is clear that damage can be detected using Lamb waves in z-pin reinforced, co-cured composite pi-joints for this case of extensive damage.
This experimental research investigates the effects of adding z-pins to a carbon fiber reinforced plate (CFRP)
on Lamb wave propagation, such as mode conversion and reflections. The motivation for this study is derived
from the current and expected future use of z-pins in aircraft structures coupled with the requirement to design
structural health monitoring (SHM) systems for detecting damage in regions of composite structures with z-pins.
This experimental study is conducted on two 4.8 mm thick CFRP test articles, where one plate has a 20 by 279
mm2 band of z-pins and the other does not. The z-pins have an average diameter of 0.28 mm and are inserted
through the thickness of the panel with an area density of 4% before curing. A three-dimensional (3D) laser
Doppler vibrometer (LDV) was employed to collect velocity measurements over a 1 mm uniformly-spaced grid
of 17,899 scan points. Time-sequenced 3D LDV scans are presented to show that adding this relatively small
amount of z-pins to a 4.8 mm thick CFRP has few measureable effects on Lamb wave propagation.
The goal of this research is to establish a methodology for damage detection in unreinforced and z-fiber reinforced cocured
composite pi- joints using lamb wave based structural health monitoring technique. Because of the lack of natural
reinforcement in the thickness direction, delamination has been a predominant failure mode besides other failure modes
in laminated composites. Z-fiber reinforcement is one of the ways of controlling or delaying delamination and thus,
delaying the failure. Here, DCB (Double Cantilever Beam) and Pi-Joint specimens, with and without z-fiber
reinforcement, are considered for experimental analysis. Damage is experimentally induced in the specimen under static
loading. Lamb wave propagation based structural health monitoring is performed using PZT sensors in a pitch-catch
arrangement. Amplitude vs. time and amplitude vs. frequency response are plotted for various excitation frequencies. At
lower frequencies (particularly at 20 KHz), pure A0 mode is generated, which is confirmed by out of phase response of
simultaneous PZT sensors. From the response data analysis, presence of damage in unreinforced, z-fiber reinforced DCB
and pi-joint specimens is confirmed.