It is well known that laminated composite materials are prone to impact damage caused by foreign objects and exhibit poor damage resistance in through the thickness direction. By drawing inspiration from naturally occurring impact resistant structures, such as dactyl clubs of mantis shrimp, enhanced damage tolerance and impact energy absorption can be achieved with traditional CFRP layers by creatively arranging them into bio-inspired configurations, called helicoidal or Bouligand structures. Through an extensive numerical analyses of low velocity impact (LVI) supported by the experimental results, a further insight into the possibilities that these structures can offer in terms of damage resistance was attained. By comparing the results of three square plates with different planar sizes, it was shown that the helicoidal layups are more effective at absorbing energy while minimising through the thickness failure than standard quasi-isotropic and cross-ply laminates. Although the helicoidal composites generally exhibited a higher degree of delamination in LVI tests, the standard lamination schemes displayed higher degree of perforation, which resulted in a reduced residual strength in the compression after impact (CAI) testing of a quasi-isotropic laminate compared to several helicoidal ones subjected to 40 J and 80 J impact energy. Furthermore, using advanced finite element analysis (FEA) code LS-DYNA® for simulating LVI and CAI events, it was revealed that the helicoidal arrangement endured the least amount of fibre damage.
Nonlinear ultrasound techniques have shown greater sensitivity to microcracks and they can be used to detect structural damages at their early stages. However, there is still a lack of numerical models available in commercial finite element analysis (FEA) tools that are able to simulate the interaction of elastic waves with the materials nonlinear behaviour. In this study, a nonlinear constitutive material model was developed to predict the structural response under continuous harmonic excitation of a fatigued isotropic sample that showed anharmonic effects. Particularly, by means of Landau's theory and Kelvin tensorial representation, this model provided an understanding of the elastic nonlinear phenomena such as the second harmonic generation in three-dimensional solid media. The numerical scheme was implemented and evaluated using a commercially available FEA software LS-DYNA, and it showed a good numerical characterisation of the second harmonic amplitude generated by the damaged region known as the nonlinear response area (NRA). Since this process requires only the experimental second-order nonlinear parameter and rough damage size estimation as an input, it does not need any baseline testing with the undamaged structure or any dynamic modelling of the fatigue crack growth. To validate this numerical model, the second-order nonlinear parameter was experimentally evaluated at various points over the fatigue life of an Aluminium (AA6082-T6) coupon and the crack propagation was measured using an optical microscope. A good correlation was achieved between the experimental set-up and the nonlinear constitutive model.
Traditional non-destructive evaluation (NDE) and structural health monitoring (SHM) systems are used to analyse that a structure is free of any harmful damage. However, these techniques still lack sensitivity to detect the presence of material micro-flaws in the form of fatigue damage and often require timeconsuming procedures and expensive equipment. This research work presents a novel "nonlinear ultrasonic stimulated thermography" (NUST) method able to overcome some of the limitations of traditional linear ultrasonic/thermography NDE-SHM systems and to provide a reliable, rapid and cost effective estimation of fatigue damage in isotropic materials. Such a hybrid imaging approach combines the high sensitivity of nonlinear acoustic/ultrasonic techniques to detect micro-damage, with local defect frequency selection and infrared imaging. When exciting structures with an optimised frequency, nonlinear elastic waves are observed and higher frictional work at the fatigue damaged area is generated due to clapping and rubbing of the crack faces. This results in heat at cracked location that can be measured using an infrared camera. A Laser Vibrometer (LV) was used to evaluate the extent that individual frequency components contribute to the heating of the damage region by quantifying the outof-plane velocity associated with the fundamental and second order harmonic responses. It was experimentally demonstrated the relationship between a nonlinear ultrasound parameter (βratio) of the material nonlinear response to the actual temperature rises near the crack. These results demonstrated that heat generation at damaged regions could be amplified by exciting at frequencies that provide nonlinear responses, thus improving the imaging of material damage and the reliability of NUST in a quick and reproducible manner.
Thermosonics, also known as ultrasonic stimulated thermography, is a rapid non-destructive evaluation technique that uses an infrared camera to visualise material defects by detecting the frictional heating at crack surfaces when a part under inspection is vibrated. These vibrations are usually produced by an ultrasonic horn being pressed against the surface of the test sample, which result in uncontrolled generations of frequency components and excitation amplitude. This makes thermosonics highly non-reproducible and unreliable. This paper presents a novel thermographic method, here named as nonlinear ultrasound stimulated thermography (NUST), for the detection and imaging of real material defects such as impact damage on a complex composite stiffener panel. This technique combines nonlinear ultrasonic techniques with thermography. A nonlinear ultrasonic approach was used as signature for a reliable frequency-selective excitation of material defects, while an infrared camera was employed to reveal the damage location and severity. A nonlinear narrow sweep excitation method (NNSM) was employed to efficiently excite the local resonance frequencies of the damaged region in order to give rise to the highest nonlinear harmonic response in the material leading to a high heat generation at the crack surface. The experimental tests were carried out with a laser vibrometer in order to better understand the interaction of elastic waves with nonlinear scattering. An ad-hoc nonlinear thermal-structural finite element and crack model was developed to study the heat generation caused by the movement of the crack surfaces when elastic waves with a particular frequency impinges on the crack interphase with good agreement with the experimental results. The proposed new method allows to detect single and multiple barely visible impact damage in a quick, reliable and reproducible manner and overcomes the main limitations of classical thermosonics.
Nonlinear ultrasonic non-destructive evaluation (NDE) methods can be used for the identification of defects within adhesive bonds as they rely on the detection of nonlinear elastic features for the evaluation of the bond strength. In this paper the nonlinear content of the structural response of a single lap joint subjected to ultrasonic harmonic excitation is both numerically and experimentally evaluated to identify and characterize the defects within the bonded region. Different metallic samples with the same geometry were experimentally tested in order to characterize the debonding between two plates by using two surface bonded piezoelectric transducers in pitch-catch mode. The dynamic response of the damaged samples acquired by the single receiver sensor showed the presence of higher harmonics (2 nd and 3 rd ) and subharmonics of the fundamental frequencies. These nonlinear elastic phenomena are clearly due to nonlinear effects induced by the poor adhesion between the two plates. A new constitutive model aimed at representing the nonlinear material response generated by the interaction of the ultrasonic waves with the adhesive joint is also presented. Such a model is implemented in an explicit FE software and uses a nonlinear user defined traction-displacement relationship implemented by means of a cohesive material user model interface. The developed model is verified for the different geometrical and material configurations. Good agreement between the experimental and numerical nonlinear response showed that this model can be used as a simple and useful tool for understanding the quality of the adhesive joint.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.