In-plane local defect resonances for efficient vibrothermography of impacted carbon fiber reinforced plastics (CFRP)

It is well known that the efficiency of the vibrothermographic non-destructive testing (NDT) technique can be enhanced by taking advantage of local defect resonance (LDR) frequencies. Recently, the classical out-of-plane local defect resonance was extended towards in-plane LDR for enhanced efficiency of vibrometric NDT. This paper further couples the concept of this in-plane LDR to vibrothermography, on the basis of the promising potential of in-plane LDRs to enhance the rubbing (tangential) interaction and viscoelastic damping of defects. Carbon fiber-reinforced composites (CFRPs) with barely visible impact damage (BVID) are inspected and the significant contribution of in-plane LDRs in vibrational heating is demonstrated. Moreover, it is shown that the defect thermal contrast induced by in-plane LDRs is so high that it allows for easy detection of BVID by live monitoring of infrared thermal images during a single broadband sweep excitation. Thermal and vibrational spectra of the inspected surface are studied and the dominant contribution of in-plane LDR in vibration-induced heating is demonstrated

Investigation of fatigue damage growth and self-heating behaviour of cross-ply laminates using simulation-driven dynamic test

Structural integrity of aerospace assets is paramount for both the safety and economy of aviation industry. The introduction of composites into the design of aero-structures generated several economic benefits but also led to several challenges, including fatigue damage growth and self-heating behaviour. Fatigue of metals is widely managed by calculations of damage accumulation and prediction of residual life. These techniques do not always apply to the fatigue of composites, where the onset and propagation of damage are still under investigation. Furthermore, vibration-induced fatigue is even less understood because of a handful of failure criteria available and, also, because it is biased by the self-heating conditions of the material itself. The authors have underpinned one failure criterion for vibration fatigue and mapped that against self-heating and environmental temperatures. Despite the advances, several research questions were left open because of the complex multiphysics behaviour of fatigue which outreached the experimental capacity. Therefore, this research suggests a Simulation-Driven Dynamic Test (SDDT) framework that deconstructs vibration fatigue experiments into step-wise steady-state analyses. This novel approach will enable (a) investigating the failure mode mixity of the underlying failure criterion, and (b) simulating the surface temperature during the delamination growth under vibration conditions.</p

Data Processing Scheme for Laser Spot Thermography Applied for Nondestructive Testing of Composite Laminates

This paper proposes a data processing scheme for laser spot thermography (LST) applied for nondestructive testing (NDT) of composite laminates. The LST involves recording multiple thermographic sequences, resulting in large amounts of data that have to be processed cumulatively to evaluate the diagnostic information. This paper demonstrates a new data processing scheme based on parameterization and machine learning. The approach allows to overcome some of the major difficulties in LST signal processing and deliver valuable diagnostic information. The effectiveness of the proposed approach is demonstrated on an experimental dataset acquired for a laminated composite sample with multiple simulated delaminations. The paper discusses the theoretical aspects of the proposed signal processing and inference algorithms as well as the experimental arrangements necessary to collect the input data

An Experimental and Numerical Investigation to Characterize an Aerospace Composite Material with Open-Hole Using Non-Destructive Techniques

[EN] In this study, the open-hole quasi-static tensile and fatigue loading behavior of a multidirectional CFRP thick laminate, representative of laminates used in the aerospace industry, is studied. Non-destructive techniques such as infrared thermographic (IRT) and digital image correlation (DIC) are used to analyze the behavior of this material. We aim at characterizing the influence of the manufacturing defects and the stress concentrator through the temperature variation and strain distribution during fatigue and quasi-static tests. On the one hand, the fatigue specimens were tested in two main perpendicular directions of the laminate. The results revealed that manufacturing defects such as fiber waviness can have a major impact than open-hole stress concentrator on raising the material temperature and causing fracture. In addition, the number of plies with fibers oriented in the load direction can drastically reduce the temperature increment in the laminate. On the other hand, the quasi-static tensile tests showed that the strain distribution around the hole is able to predict the crack initiation and progression in the external plies. Finally, the experimental quasi-static tests were numerically simulated using the finite element method showing good agreement between the numerical and experimental results.This research was funded by the FEDER programme and the Spanish Ministerio de Ciencia, Innovacion y Universidades, projects DPI2017-89197-C2-1-R and DPI2017-89197-C2-2-R. The funding of the Generalitat Valenciana, Programme PROMETEO 2016/007 is also acknowledged.Feito-Sánchez, N.; Calvo, JV.; Belda, R.; Giner Maravilla, E. (2020). An Experimental and Numerical Investigation to Characterize an Aerospace Composite Material with Open-Hole Using Non-Destructive Techniques. Sensors. 20(15):1-18. https://doi.org/10.3390/s20154148S118201

Exploration of Thin Film Polymers for Phosphor Thermometry

Abstract Phosphor thermometry has been investigated in recent years as means to explore instantaneous remote thermometry. Many different types of thermographic powders exist. In most cases work has been performed on these materials while in a fine powder form. There are significant challenges when working with fine powders (nm-µm particle size) and these challenges have limited the range of characterizations and potential applications of these materials. Among the different types of thermographic phosphors La2O2Su:Eu is one of the most sensitive types that is currently available. Lanthanum oxysulfide doped with Eu has been reported to have a sensitivity of 0.01 °C with a wide temperature range of response-cryogenics to 1500 °C. In this work two types of La2O2S:Eu (0.1 % and 1.0% Eu concentration) have been fully characterized at cryogenic temperatures and at elevated temperatures while embedded in an elastomeric sleeve. The flexible optically transparent elastomer Sylgard 184 was chosen as the encapsulant for this study. Samples with increasing levels of La2O2S:Eu powder (from 1 wt % to 25 wt%) were prepared and studied. Both single layer (single concentration) and multilayer (concentration gradient) sample types were prepared using spin-coating techniques and fully characterized. The effect of the La2O2Su:Eu particles on the thermal, mechanical, and luminescence behavior of the composite materials was fully investigated and reported here. While the percentage of the Eu (0.1 versus 1.0%) did not have a detectable effect on the emission characteristics of the composites, it did indeed affect the thermal and mechanical properties of the composites. SEM investigation suggests that the differences in the granular structure of the two powders has influenced the properties of the composite polymers prepared in this study. Furthermore, the emission behavior of the La2O2Su:Eu+Sylgard184 composites showed a strong non-linear temperature dependence in the range of -40 °C to 75 °C and little dependence on the powder concentration level

Vibrothermographic spectroscopy with thermal latency compensation for effective identification of local defect resonance frequencies of a CFRP with BVID

Vibrothermography using sinusoidal vibration excitation at the resonance frequencies of a defected area (so-called local defect resonance, or LDR) is a promising technique to boost the defect's deformation and its interfacial interactions and as such enhance resultant vibration-induced heating. Contrary to the classical high-power vibrothermography, low power excitation at an LDR frequency results in a reproducible thermal response and adequate quantification of the corresponding damage features. However, the technique is mainly limited by the fact that it requires a priori knowledge of the LDR frequencies (e.g. obtained from prior vibrational measurements). To overcome this limitation, a stand-alone vibrothermographic spectroscopy procedure is introduced in this paper. The proposed technique applies two consecutive broadband sweep vibrational excitations with ascending and descending frequency modulation rates to the sample. The surface of the excited sample is monitored with an IR camera. Both time derivative analysis and superposition of the recorded thermal responses are performed in order to compensate for the thermal latency of the defect-induced heating. This compensation approach enables proper identification of the actual LDR frequencies based on the apparent LDR frequencies of the thermal response. The method is applied on a carbon fiber reinforced polymer (CFRP) with barely visible impact damage (BVID), and multiple LDR frequencies are readily identified. The identified LDR frequencies are also individually evaluated by both lock-in vibrothermography and 3D scanning laser Doppler vibrometry, confirming the competence of the proposed technique for extracting LDR frequencies in a proper and fast way

Fabrication of a Multi-Physics Integral Structural Diagnostic System Utilizing Nano-Engineered Materials

ABSTRACT Composites present additional challenges for inspection due to their heterogeneity and anisotropy, and since damage often occurs beneath their surface. Currently successful laboratory non-destructive methods, such as X-ray and C-scans, are impractical for inspection of large integrated structures. It is clear that new approaches for inspection of composites need to be developed. During the present research, multiple carbon nanotube (CNT) based NDE &amp; SHM techniques were investigated to resolve these issues. Aligned CNTs offer excellent mechanical toughness improvements for traditional composite laminates, and additionally enable multifunctional capabilities through piezoresistive properties and greatly enhanced electrical and thermal conductivity. This paper introduces the fabrication of fuzzy-fiber reinforced plastic (FFRP) composite laminates uses CNTs, and presents results for multiple CNT-enhanced laminates that were electroded using direct-write techniques and subject to impact damage. A resistive-based method was used to create a detailed damage map of the effected zone. Finally, demonstrations are also described for multiple alternative CNT-based NDE approaches that were explored

Fabrication of a Multi-Physics Integral Structural Diagnostic System Utilizing Nano-Engineered Materials

ABSTRACT Composites present additional challenges for inspection due to their heterogeneity and anisotropy, and since damage often occurs beneath their surface. Currently successful laboratory non-destructive methods, such as X-ray and C-scans, are impractical for inspection of large integrated structures. It is clear that new approaches for inspection of composites need to be developed. During the present research, multiple carbon nanotube (CNT) based NDE &amp; SHM techniques were investigated to resolve these issues. Aligned CNTs offer excellent mechanical toughness improvements for traditional composite laminates, and additionally enable multifunctional capabilities through piezoresistive properties and greatly enhanced electrical and thermal conductivity. This paper introduces the fabrication of fuzzy-fiber reinforced plastic (FFRP) composite laminates uses CNTs, and presents results for multiple CNT-enhanced laminates that were electroded using direct-write techniques and subject to impact damage. A resistive-based method was used to create a detailed damage map of the effected zone. Finally, demonstrations are also described for multiple alternative CNT-based NDE approaches that were explored