In an article recently published in the open access journal Scientific reportsresearchers discussed a non-destructive method to assess the quality of polymer-metal laminates using digital holographic vibrometry and Lamb waves.
Study: A non-destructive technique using digital holographic vibrometry and Lamb waves for determining the quality of polymer-metal laminates. Image Credit: Joshua Sanderson Media/Shutterstock.com
Background
Polymer-metal laminates have become increasingly common in recent years. Automakers are increasingly trying to lighten their products to increase operational efficiency and reduce production costs. The materials of both partners of the laminate should be selected based on certain characteristics.
Nowadays, the assembly of the two components is carried out directly during processing without the need for additional steps, using standard techniques such as injection molding with metal inserts. However, rushed production cycles of metal-polymer gaskets could lead to connection failures.
Although production processes for polymer-metal laminates are becoming increasingly common, a significant concern remains the vulnerability to the development of hidden or barely perceptible defects. While the internal defects of these materials, the existence of which can have a substantial impact on the strength of the finished product, are difficult to detect, the surface defects of these materials can be identified very simply. Non-destructive testing (NDT) is a useful technique for examining individual parts for possible hidden defects. For large objects and high stimulation frequencies, conventional and well-known non-destructive testing techniques using Lamb waves are applied.
Sideview of a sample with an indicated defect; sizes are in mm. Image Credit: Nowak-Grzebyta, J et al., Scientific Reports
About the study
In this study, the authors used a non-destructive technique called digital holographic vibrometry (DHV) to find disbond points in aluminum and polymer laminates. The amplitude and phase patterns of the sample vibration were recorded simultaneously for the metal and polymer side of the laminate at low frequencies down to 30 kHz using A0 Lamb waves. These models have been used to locate delamination points in laminates. The frequency range over which regular Lamb waves have been observed, Lamb wave amplitudes, and frequency-dependent Lamb wave propagation velocities have all been studied for transmission qualities at low frequencies.
The team demonstrated that when a laminate defect occurs, these attributes are also altered. It was possible to determine if a sample was damaged based on the behavior of the Lamb waves even when the problem was not localized. The methodology was extended and antisymmetric Lamb A0 waves at different vibrational frequencies in rolled aluminum samples made from three different polymers were observed and their behavior was used to search for hidden defects. For high frequencies of 100 kHz and above, the propagation of lamb waves in an isotropic medium was well defined. For laminates and a relatively low frequency range down to 30 kHz, this was not the case.
The researchers focused on the impact of the defect on the laminates in the low frequency range for vibrations that occur in daily life, for example, in frequently used gadgets like ultrasonic animal repellents or machines. washing, as well as in public spaces such as stations or libraries. Digital holographic vibrometry (DHV) was used to track the simultaneous propagation of Lamb waves on the metal and polymer sides of the laminate. Especially for laminates, it was very useful to observe both sides. In this way, the effectiveness of bonding laminates formed from mixtures of polymers and aluminum alloys was evaluated.
Bottom view of a sample with position of the two types of piezo transducers used: 1: sample, 2: piezo transducer; left: KingState KPE-827, right: PI P-010.00P; sizes are in mm. Image Credit: Nowak-Grzebyta, J et al., Scientific Reports
Comments
The observed Lamb wave frequency (LFOR) range for which a laminate defect could be observed depended more on the magnitude of the defect than on the type of polymer used. Lamb wave propagation in the polymer followed in the metal for all defect-free aluminum-polymer laminates. This suggested strong adhesion between the two layers and demonstrated the viability of using any of the polymers examined to create a strong laminate. The proposed configuration required access to both sides of the sample since its metal and polymer surfaces were observed simultaneously. A bonding defect could manifest itself in different ways. Either way, the flaw reduced the LFOR.
The speed of the antisymmetric Lamb A0 wave as a function of the frequency curves has also been changed considerably. At frequencies above 10 kHz, the differences between the curves for metal and semi-crystalline polymers were apparent. The polymer part of the polyvinylidene fluoride (PVDF) Al D laminate with a defect had a Lamb wave velocity which was about 40% lower at a frequency of 15 kHz than that of the aluminum layer. In the case of semi-crystalline polymers, a laminate defect widened the gap between the vibration amplitudes of the metal and the polymer.
This deviation went from 40 to 50% for PVDF Al to 60 to 70% for PVDF Al D, in particular for PVDF, which showed the least transmission of vibrations among the materials studied. However, even for the laminates with a defect, there was little difference in the vibration amplitudes for the amorphous PC Al. Therefore, in the case of laminates with polymers that had good vibration transmission in the low frequency test range, the amplitude criterion was not useful.
Comparison of Lamb wave phase velocities for pure polymer and aluminum samples. Image Credit: Nowak-Grzebyta, J et al., Scientific Reports
conclusion
In conclusion, this study used two semi-crystalline polymers and one amorphous polymer for the development of laminates.
The authors mentioned that the test samples tested using the proposed technology had suitable dimensions for further strength testing. This has been useful for standard component testing in a lab environment as well as in an industrial production environment.
The team pointed out that the CND approach described in this study requires little time to assess how well two layered materials connect. They pointed out that it is a useful extension of typical laminate tests that can also be performed on a manufacturing line to use DHV to detect Lamb waves.
References
Nowak-Grzebyta, J., Stachowska, E., Meijer, F., et al. (2022) A non-destructive technique using digital holographic vibrometry and Lamb waves for the determination of the quality of polymer-metal laminates. Scientific reports, 12, p. 18041. https://www.nature.com/articles/s41598-022-22853-2
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