An in-process cure monitoring technique based on “guided wave” concept for carbon fiber reinforced polymer (CFRP) composites was developed. Key parameters including physical properties (viscosity and degree of cure) and state transitions (gelation and vitrification) during the cure cycle were clearly identified experimentally from the amplitude and group velocity of guided waves, validated via the semi-empirical cure process modeling software RAVEN. Using the newly developed cure monitoring system, an array of high-temperature piezoelectric transducers acting as an actuator and sensors were employed to excite and sense guided wave signals, in terms of voltage, through unidirectional composite panels fabricated from Hexcel® IM7/8552 prepreg during cure in an oven. Average normalized peak voltage, which pertains to the wave amplitude, was selected as a metric to describe the guided waves phenomena throughout the entire cure cycle. During the transition from rubbery to glassy state, the group velocity of the guided waves was investigated for connection with degree of cure, Tg, and mechanical properties. This work demonstrated the feasibility of in-process cure monitoring and continued progress toward a closed-loop process control to maximize composite part quality and consistency.
Molecular dynamic (MD) simulations were performed to compute the mechanical properties of off-stoichiometric epoxy resins as a function of hardener/epoxy mixture ratio (r). Properties were characterized in relation to their microscopic structures. Such resins have been used recently for adhesive-free bonding of large-scale composite structures using the co-curing-ply method. In this process, two partially precured composite panels with hardener-poor (HP) off-stoichiometric resins are coupled with ply(ies) of complementary hardener-rich (HR) formulations and then cured simultaneously. This bonding process has the potential to produce reliable and certifiable composite joints without the need for additional fasteners, which are often required for many conventional bonding methods because even small amounts of contamination can cause a weak bond. The reflow and mixing of the HP/HR resin in this bonding process result in a joint with no discernable interface that should not be susceptible to surface contamination. However, incomplete mixing of the two offset resins may result in chemical heterogeneity of the cured polymeric joint. Thus, different r values may be obtained across the joint. Classical MD simulations were performed to compute the Young’s modulus of polymers with different r values and correlate their properties to network structures. High stiffness was associated with molecular packing due to chemical crosslinking, leading to a single network structure. Moreover, the networks became denser as the ratio approached the stoichiometric value r = 1. Thus, the r = 1 systems were single clusters, with high stiffness, high molecular weight, and a high degree of crosslinking. Structural properties such as radius of gyration and mean square displacement were determined to investigate the variation in the stiffness with respect to r. This MD simulation study was validated with experimental measurements.
This study exploits the feasibility of imaging zones of local porosity/voids simulated by introducing microspheres during layup of a unidirectional carbon fiber–reinforced polymer composite panel. A fully non-contact hybrid system primarily composed of an air-coupled transducer and a laser Doppler vibrometer was used for imaging the local porosity/void zones from the guided wave response. To improve image resolution, several preprocessing techniques are performed. The wavefield reconstructed from the laser Doppler vibrometer measurements was first “denoised” using a one-dimensional wavelet transform in the time domain followed by a two-dimensional wavelet transform in the spatial domain. From the total wavefield, the much weaker backscattered waves were separated from the stronger incident wave by frequency–wavenumber domain filtering. In order to further enhance the signal-to-noise ratio and sharpen the image, the attenuation of incident wave propagation to the damage site was compensated through two proposed weight functions. Finally, a zero-lag cross-correlation was performed for imaging the zone where the compensated incident and backscattered waves were in phase. This improved imaging condition, the “denoised” weighted zero-lag cross-correlation, was proposed and tested for defect imaging in the composite panel with eight intentionally introduced zones of high porosity/voids of varying diameters (1.59–6.35 mm) and depths (0.36–1.08 mm). As expected, the sensitivity of the non-contact air-coupled transducer/laser Doppler vibrometer hybrid system was limited by the wavelength of the excitation signal. The system incorporated with the denoised weighted zero-lag cross-correlation imaging condition for guided wave interrogation gave similar image quality in comparison with that by the immersion C-scan.
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