Boyang Chen

A vibro-acoustic modulation method for the detection of delamination and kissing bond in composites

In this paper we employ a nonlinear acoustic method, namely, the vibro-acoustic modulation method, for the detection of delamination and kissing bond in composites. Both a (large-amplitude and low-frequency) pumping wave and a (low-amplitude ultrasound) probing wave are used to vibrate the structure. Permanently bonded piezoceramic transducers are used for both excitation and measurement. A side-band ratio is used to estimate the extent of nonlinear interactions between the two input waves in the structure. Current results show that the proposed method successfully differentiates the intact specimens from both the delamination and the kissing bond specimens used in this study.

Finite element modeling of nonlinear acoustics/ultrasonics for the detection of closed delaminations in composites.

HighlightsA simple easily replicable contact model for nonlinear acoustics.The modeling can be done without the need of any additional nonlinearity parameters.The model provides explanation of harmonics and subharmonics in the frequency spectrum.The results compare well with nonlinear spring damper model available in literature. &NA; Linear ultrasonics methods based on the principle of reflection, transmission, dissipation of sound waves have been traditionally used to detect delaminations in composite structures. However, when the delamination is in very early stages such that it is almost closed, or closed due to a compressive load, the linear methods may fail to detect such cases of delaminations. Nonlinear acoustics/ultrasonics have shown potential to identify damages in composite structures which are difficult to detect using conventional linear ultrasonic methods. The nonlinear method involves exciting the structure with a sinusoidal signal of certain (or multiple) frequency and observing the vibrations of the structure. The vibrations of the damage region differ significantly from intact regions and can be used to identify the damage. However due to the complex and varying nature of the nonlinear phenomena created by the interaction between the exciting signal and the damage, there are many variables at play which can lead to success or failure of the method. While experiments lead to the establishment of the method to be used as a damage detection technique, numerical simulations can help to explain the various phenomena associated with nonlinearity. This work presents a quick approach to model the nonlinear behavior caused by closed delaminations. The model is validated with a previously available approach for nonlinear vibrations modeling and a comparison is made between the two. The local nature of the nonlinearity enables to map out the area of damage in the structure. Additionally, a few parametric studies are performed to study the effect of various parameters related to the nonlinear phenomenon.

Progressive Failure Analysis of Scaled Double-Notched Carbon/Epoxy Composite Laminates

This article presents progressive failure analysis of double-notched carbon/epoxy composite laminates with different scales. A numerical analysis strategy based on material property degradation method (MPDM) and cohesive elements (CE) is developed to model progressive failure of scaled double-notched composite laminates, where the material property degradation method is used to model the intralaminar failure and the cohesive elements are employed to account for the delamination at the interfaces. Different failure theories are considered in the material property degradation method–cohesive element approach and a comparative study of these failure theories is presented. The mesh dependency of the material property degradation method–cohesive element approach is investigated with different notch and element types for the double-notched composite laminates. Size scaling effects are also studied by traditional fracture models and the material property degradation method–cohesive element approach, significantly revealing a trend in strength reduction of notched composites with increasing specimen size. The predictions are compared with the experimental results and reasonably good agreement is observed.

Progressive damage simulation of open-hole composite laminates under compression based on different failure criteria:

The strength prediction of open-hole fibre-reinforced composite laminate under compression is very important in the design of composite structures. The modelling of fibre, matrix damage and delamination plays an important role in the understanding of the damage mechanics of laminate under open-hole compression. In this article, a progressive damage model for open-hole compression that is based on continuum shell elements and cohesive elements is established to model in-plane damage and delamination, respectively. The damage mechanics of sublaminate-scaled laminates with ply sequence [45/0/−45/90]ms and ply-level-scaled laminates with ply sequence [45n/0n/−45n/90n]s are investigated by our proposed model. The Tsai-Wu and Hoffman failure criteria are employed for the determination of matrix damage initiation. Compared with the experiments, the numerical results using the Tsai-Wu criterion exhibit better accuracy regarding open-hole compression strength prediction and failure modes simulation.

A theoretical and numerical study on the mechanics of vibro-acoustic modulation

Vibro-acoustic modulation (VAM) is a form of a non-destructive testing technique used in nonlinear acoustic methods for the detection of defects. It comprises of exciting the structure with a dual frequency sinusoidal signal and studying the interaction of this wave with the underlying defect. In this work a theoretical study on the mechanics of VAM is presented for a generic material body. The roles of different types of defect on the response of the material are analyzed. The theoretical analysis shows the origins of the nonlinear frequencies in the form of higher harmonics and sidebands commonly observed in the output response of VAM excitation. In addition, the analysis provides insights on the relationships between the magnitudes of the nonlinear responses and those of the input vibrations, and on the physical origins of the nonlinear responses. For a physical visualization of the nonlinear vibrations associated with the theory a finite element analysis of VAM is also performed. The model looks into the plausibility of using VAM for the mapping of damage in physical structures. The model is also used to investigate the effects of the defect size and defect depth on the nonlinear mechanism of VAM.

Material orthotropy effects on progressive damage analysis of open-hole composite laminates under tension

The sizes effects on the strengths of open-hole fibre-reinforced composite laminates subjected to tensile loading (OHT) have been investigated widely. However, little attention has been paid to the influence of material orthotropy. This paper presents a progressive damage model for the model failure of notched laminates under tensile loading based on continuum damage mechanics and cohesive elements. The effects of orthotropy on the failure of notched laminates with seven different ply sequences are investigated by our proposed model. The prediction results adopting the Hoffman and Pinho failure criterions to determine matrix damage initiation are compared with the results of experiments. Our proposed models are able to predict the strong influence of orthotropy on strengths of open-hole laminate under tension, and model using Pinho criterion can predict the open-hole tension strength most accurately.

Progressive damage simulation of scaling effects on open-hole composite laminates under compression

The progressive damage simulation and strength prediction of open-hole fibre-reinforced composite laminates subjected to compressive loading is quite difficult due to numerical stability issues. Here, we proposed a progressive damage model for open-hole fibre-reinforced composite laminates subjected to compressive loading based on open-hole fibre-reinforced composite laminates under a tensile load. The proposed model is divided into two parts: the in-plane damage and delamination are modelled by continuum shell elements and cohesive elements, respectively, and the damage mechanics of sublaminate-scaled laminates with ply sequence [45/0/–45/90]ns and ply-level scaled laminates with ply sequence [45m/0m/−45 m/90 m]s are investigated by our proposed model. The complete failure process and mode of notched composite laminate under compressive load are also modelled in our model. The numerical results show good agreement with the experimental results.

Detection of Defects in Composites Using Vibro-acoustic Modulation

This paper employs a nonlinear acoustic method, namely, the Vibro-Acoustic Modulation method (VAM), for the detection of delamination and kissing bond in composites. Both a large-amplitude/low-frequency pumping wave and a low-amplitude ultrasound probing wave are used to vibrate the structure. Permanently bonded piezoceramic transducers are used for both excitation and measurement. Current results show that VAM as implemented in this work successfully differentiates the intact specimens from both the delamination and the kissing bond specimens used in this study. doi: 10.12783/SHM2015/315