This paper presents the development of a delamination detection framework for integrated circuit packages aiming at quantitative detection of sealant delamination between integrated heat sink and substrate, which is one of the potential failure mechanisms in integrated circuit packages. This method is expected to overcome the destructive nature of most existing techniques and maintain a relatively low cost of development. Ultrasonic guided waves are used as the interrogation method due to their sensitivity to small-size damage and capability of through-thickness penetration. The complexity of the received ultrasonic signals, caused by the geometric heterogeneity, is resolved and interpreted using a time-frequency signal processing technique. The extracted ultrasonic information, including time-of-arrival and amplitude of wave modes received from different sensing paths under multiple excitation frequencies, is used to construct the feature space for training. An unsupervised learning method, multivariate Gaussian model, is implemented as an information fusion and delamination detection tool. The multivariate Gaussian model efficiently investigates the distribution of feature space including correlations between features and flag the outliers without labeled examples. Results from the developed model are compared with two existing evaluation methods, including pullout test and a metric indicating the extent of delamination, which indicates that the developed method possesses a similar level of accuracy.
Physics-based computational models play a key role in the study of wave propagation for structural health monitoring
(SHM) and the development of improved damage detection methodologies. Due to the complex nature of guided waves
(GWs), accurate and efficient computation tools are necessary to investigate the mechanisms responsible for dispersion,
coupling, and interaction with damage. In this paper, a fully coupled electromechanical elastodynamic model for wave
propagation in a heterogeneous, anisotropic material system is developed. The final framework provides the full three
dimensional displacement and electrical potential fields for arbitrary plate and transducer geometries and excitation
waveform and frequency. The model is validated theoretically and proven computationally efficient. Studies are
performed with surface bonded piezoelectric sensors to gain insight into the physics of experimental techniques used for
SHM. Collocated actuation of the fundamental Lamb wave modes is modeled over a range of frequencies to demonstrate
mode tuning capabilities. The effect of various actuation types commonly used in numerical wave propagation models
on Lamb wave speed are studied and compared. Since many studies, including the ones investigated in this paper, are
difficult to perform experimentally, the developed model provides a valuable tool for the improvement of SHM
techniques.
This paper presents a preliminary study of the effects residual plastic strains have on Lamb wave velocities and time of flight measurements. The potential application of this research is non-destructive evaluation and structural health monitoring, particularly reconstructing plastic strain fields. The finite deformation of a semi-infinite plate due to residual plastic strain is used to accommodate the changes in plate thickness and elongation. The results show that the S0 mode exhibits significant variations in group velocity in the highly dispersive regions, as much as a 2% increase in velocity with a 1% plastic strain. However, for time of flight measurements, the plate elongation had an order of magnitude effect rather than showing velocity changes. By exploiting time delay measurements, it may be possible to use wave speed measurements to determine plastic zones through Lamb-like waves.
Woven fiber composites are currently being investigated due to their advantages over other materials, making them
suitable for low weight, high stiffness, and high interlaminar fracture toughness applications such as missiles, body
armor, satellites, and many other aerospace applications. Damage characterization of woven fabrics is a complex task
due to their tendency to exhibit different failure modes based on the weave configuration, orientation, ply stacking and
other variables. A multiscale model is necessary to accurately predict progressive damage. The present research is an
experimental study on damage characterization of three different woven fiber laminates under low energy impact using
Fiber Bragg Grating (FBG) sensors and flash thermography. A correlation between the measured strain from FBG
sensors and the damaged area obtained from flash thermography imaging has been developed. It was observed that the
peak strain in the fabrics were strongly dependent on the weave geometry and decreased at different rates as damage area
increased due to dissimilar failure modes. Experimental observations were validated with the development of a
multiscale model. A FBG sensor placement model was developed which showed that FBG sensor location and
orientation plays a key role in the sensing capabilities of strain on the samples.
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