Precursor to damage state quantification in composite materials (Conference Presentation)

Subsurface damage in composite materials is difficult to detect using visual techniques, and other current inspection methods lack the ability to perform quick, wide-area inspections without the need for reference signatures or baseline measurements. This paper presents a method for detecting and locating subsurface damage in composite materials without historical reference measurements by considering the nonlinear behavior of the material in the vicinity of damage. Nonlinear behavior is identified by comparing frequency response functions measured at different input amplitudes. It will be shown that the nonlinear behavior of the material is most evident in the areas nearest to the damage. The proposed inspection method is demonstrated both analytically and experimentally. First, a finite element model of a sandwich beam is developed using Bernoulli–Euler beam elements to represent each layer of the beam and springs to represent the interface between the layers. A bilinear stiffness nonlinearity is simulated to represent disbond damage between the top and core layers of the beam. The simulated disbond damage is localized by identifying degrees of freedom which indicate significant nonlinear response through a comparison of frequency response functions measured at various input amplitudes. Next, the method is demonstrated experimentally by identifying disbond damage in a fiberglass sandwich panel. A three-dimensional scanning laser vibrometer is used to measure the forced frequency response of the panel in its damaged state as it is excited at two or more amplitudes of excitation by a piezoelectric actuator. Comparisons of the frequency response functions measured at different input amplitudes show that the subsurface damage introduces nonlinear behavior which resembles a bilinear stiffness nonlinearity, and the differences in the frequency response functions are largest in the vicinity of the damage location. In addition, it was found that improved localization of the damage is achieved by investigating the response at higher frequencies. This work has application as a nondestructive method for detecting and locating subsurface damage in composite materials and, by using a laser vibrometer for noncontact measurement, allows for quick, wide-area inspection of composite materials without the need for reference signatures or baseline measurements.

The paper presents a theoretical and experimental study of the influence of damage accumulation in composite material on the elastic fourth-rank tensor and on the strength under quasi-static, cyclic and dynamic loads. Layered polymer composites (glass or carbon fiber-reinforced plastic) are considered. This work aims to develop a mathematical model that accounts for the impact of damage in the composite material on the fourth-rank tensor of elastic properties. The constitutive relations for an orthotropic composite material are proposed, taking into account the accumulation of damage during tensile or shear loading. A numerical simulation using the finite element method is done for a periodicity cell for composite variants (unidirectional, layered, etc.) to determine the effective elastic properties. Quasi-static loading experiments were conducted on composite samples (carbon plastic) to validate the model and the degradation of elastic properties was determined by measuring the longitudinal sound speed. The full-scale experiments confirmed that damage accumulation in carbon fiber-reinforced plastic leads to the degradation of its effective elastic properties. In this connection, a technique was developed to take into account irreversible damage in an orthotropic composite material through the components of the ductility tensor, which may be helpful for the design and analysis of composite structures. The findings of the study could help with the creation of new materials with enhanced mechanical characteristics, as well as with raising the quality of already-existing materials. The developed mathematical model of orthotropic composite material can be used to enhance the structural strength properties and boost the application safety in a variety of industry.

The main purpose of the present investigation was to determine the damages generated by the low velocities with the help of the experimental method (Impact by a drop test) and the finite element method. The commercial transient finite element package LS-dyna used to model the effect of slug impactor and circular notch induced damage in composite material subjected to low velocity impact. The studied composite material was T800S/M21 made of carbon/epoxy. The effect of circular notch was examined. The composite material was subjected to stainless steel slug impactor in the transverse direction dropping the composite laminate at the center with three different velocities (2.85 m/s, 3.47 m/s and 4.17 m/s). The analysis was carried out using the model MAT59 for laminate and a rigid body model MAT20 for the slug impactor. The automatic_surface to_surface has been used to model the contact between the impactor and the laminate and the contact_automatic_surface_to_surface_tiebreak to simulate delamination between layers. A good agreement had been shown between the finite element results and the experimental values obtained from the drop tower test. The impact load, impact energy, displacements and the maximal were reported as function of the impact time. The delamination area and the layer impact energy were represented as function of layer orientations. By increasing the impact velocity, the displacement, the delamination area, the contact load and the impact energy increases respectively. The circular notch had an effect on the displacement values; this values increases in respect of the impact velocities. At the reverse, the contact load decreases respectively in function of the velocities. The main purpose of the present investigation was to determine the damages generated by the low velocities with the help of the experimental method (Impact by a drop test) and the finite element method. The commercial transient finite element package LS-dyna used to model the effect of slug impactor and circular notch induced damage in composite material subjected to low velocity impact. The studied composite material was T800S/M21 made of carbon/epoxy. The effect of circular notch was examined. The composite material was subjected to stainless steel slug impactor in the transverse direction dropping the composite laminate at the center with three different velocities (2.85 m/s, 3.47 m/s and 4.17 m/s). The analysis was carried out using the model MAT59 for laminate and a rigid body model MAT20 for the slug impactor. The automatic_surface to_surface has been used to model the contact between the impactor and the laminate and the contact_automatic_surface_to_surface_tiebreak to simulate delamination between layers. A good agreement had been shown between the finite element results and the experimental values obtained from the drop tower test. The impact load, impact energy, displacements and the maximal were reported as function of the impact time. The delamination area and the layer impact energy were represented as function of layer orientations. By increasing the impact velocity, the displacement, the delamination area, the contact load and the impact energy increases respectively. The circular notch had an effect on the displacement values; this values increases in respect of the impact velocities. At the reverse, the contact load decreases respectively in function of the velocities.

A comparison is made between conventional (i.e. uniform) and chirped optical fibre Bragg gratings (FBGs) for the detection of matrix cracking damage in composite materials. Matrix cracking damage is generally the first type of visible damage to develop under load in the off-axis plies of laminated composites and is generally the precursor of more serious damage mechanisms, particularly delamination. The detection of this type of damage is thus important, particularly in aerospace applications. Using a uniform FBG, characteristic changes develop in the reflected spectrum which can be used to identify crack development in the composite. The additional advantage of using a chirped grating is that the crack position can also be located.

Currently, one of the greatest challenges for the areas of Material Sciences and Structural Engineering is to perform accurate analysis for prediction of damage in composite materials, the evolution of damage and failure. Although several models and failure criterion already exist for the simulation of damage in composite materials, most models do not produce acceptable results for detailed designs. The models currently in use often under or overestimate loads required for the degradation and failure. This occurs as most of these models is based upon phenomenological or semi-empirical data, which adjust failure surfaces or failure envelopes to experiments. This approach neglects the inherent anisotropy and heterogeneity of composite materials, which cause several failure mechanisms to occur simultaneously in different materials scales and phases. One possible solution to this problem is to use and/or develop new damage and failure models based on multiscale approaches and physical failure mechanisms based on Continuum Fracture Mechanic. In this scenario, the main objective of the present work consists on studying and developing multiscale based damage models applied to composite structures manufactured with unidirectional fibers under different load cases: pure tensile, pure bending, mixed tensile-bending and multiaxial. For the development of these models, works found in the literature were critically evaluated and new formulations were studied, adapted and improved upon. The basic methodology is based on using homogenization techniques to obtain degenerated elastic properties from damaged Representative Volume Elements (RVEs); the damage profile of the RVE is defined as intralaminar cracks parallel to the fiber directions and is calculated using a multiscale approach. The multiscale approach comprehends three separate models, one in the macroscale for the calculation of accurate stress/strain states in the critical points, and two in the microscale for the prediction of intralaminar damage (matrix cracking). These models interact between themselves, as the results from each one are used as boundary conditions for the other in a computational analysis loop over load steps via an iterative process. The developed models were implemented either stand-alone Python codes or into the finite element analysis package AbaqusTM using its automatization capabilities with Python scripts, as well as subroutines in Fortran (UEL - User Element Subroutine) linked to commercial finite element package AbaqusTM

Composite materials play important roles in multifunctional applications, and thus, the diagnosis of damage patterns in composite materials becomes crucial to avoid critical events such as structural or functional failures. The impact of an individual damage in composite materials has been extensively studied, however, the interaction of defects/cracks, which leads to critical fracture paths, has not been understood well. In this paper, we develop a Bayesian estimation based statistical analysis technique that estimates the damage pattern of a composite material, in particular, the relative positions of defects in the material, by measuring its through-thickness dielectric properties. We first explain the fundamental dielectric principle that leads to the detection of defect patterns. A capacitance model is then built to measure the material permittivity, and the relationship between the dielectric permittivity and relative positions are found using COMSOL Multiphysics. The interaction effects between defects observed in the simulation are interpreted using the fundamental dielectric principle. A Bayesian estimation based statistical analysis model is then developed to estimate the relative positions of defects in composite materials from the measured global dielectric properties.

Video-thermography, the time-resolved observation of infrared radiation, is a nondestructive inspection technique which offers many potential applications in the study of material behavior. The present work emphasizes detection of damage in both homogeneous and composite materials. This work differs from most others because the materials are subjected to some steady-state mechanical energy, such as fatigue loads or low-amplitude vibrations, that activates heat sources near the damaged regions. Experimental observations are discussed for a variety of materials, including boron/aluminum, boron/epoxy, graphite/epoxy, aluminum, and plastics. Thermography has been used to investigate initiation and progression of subsurface damage caused by fatigue; vibrothermography has been used to locate delaminations and similar damaged regions. Discussed are several analytical studies of the relation between evolution of heat and the stress field in the region of the damaged zone, and the expected surface heat pattern from a subsurface heat source in an anisotropic material.

: The major part of the scientific work reported here, has been concerned with quantification of the concept of imperfect interface, the modifications of the theory of composite materials for imperfect interface and evaluation of the effect of various kinds of imperfect interface on various properties of various composite materials. In addition a study of the effect of matrix nonlinearity and imperfect interface on the critical compressive stress of unidirectional fiber composites has been performed. The work included the following: (1) Thermoelastic properties of fiber composites with imperfect interface; (2) Composite materials with viscoelastic interphase; (3) Micromechanics aspects of damage in composite materials; (4) The spherical inclusion with imperfect interface conditions; (5) Thermoelastic properties of particulate composites with imperfect interface; and (6) Effect of matrix nonlinearity and imperfect interface on compressive strength of fiber composites.

1. The rules of the change in the supporting capacity, strength, and degree of damage of carbon composite materials with a ceramic matrix were determined in relation to the time of action of the high temperature oxidizing atmosphere. 2. It was established that oxidation of such materials starts from the fibers of the carbon filler and there is a linear relationship between the thickness of the oxidized layer and heating time. 3. A relationship was proposed for describing the change in strength of the investigated carbon composites in relation to the time of action of the high temperature oxidizing atmosphere based on the experimentally established rules of the degree of damage of materials, which in the case of bend tests may be satisfactorily described by a second power polynomial relative to the thickness of the layer of material damaged by oxidation.

Structural damage classification in composite materials using the Wigner-Ville distribution and convolutional neural networks

Carbon fiber epoxy laminates are extensively used in the aerospace industry as they offer a combination of high performance with relatively low weight. However, damage may occur in these materials, either at the manufacturing level or during use, which may affect their structural integrity. Among the type of damage which may occur, delamination is a common defect which causes a reduction in compressive strength and thus limits the life of laminated composites. If undetected, delamination may propagate during use due to mechanical loads and in the worst case may cause fracture of the material. Therefore, delamination in composite materials needs to be efficiently detected and accurately quantified for safety and economic reasons. Because of their heterogeneity, multi-layered structure and anisotropy, the inspection of composite materials remains difficult with conventional NDT methods. A method making use of Foucault currents, and relying on the principle of electromagnetic induction, can be adapted for the detection and characterization of damage in composite materials. The use of Foucault currents in characterizing delaminated areas at interfaces between plies in carbon fiber/epoxy laminates is addressed in this paper. It is demonstrated that Foucault currents can accurately detect and quantify delamination in CFRP materials. Furthermore, the potential of this technique for monitoring defect propagation in composite materials is discussed.

first_page settings Order Article Reprints Font Type: Arial Georgia Verdana Font Size: Aa Aa Aa Line Spacing:    Column Width:    Background: Open AccessAbstract Structural Health Monitoring Using Carbon Nanotube Yarns: Sensing Concept and Applications in Composites † by Jandro L. Abot Department of Mechanical Engineering, The Catholic University of America, Washington, DC 20064, USA † Presented at the 5th International Symposium on Sensor Science (I3S 2017), Barcelona, Spain, 27–29 September 2017. Proceedings 2017, 1(8), 852; https://doi.org/10.3390/proceedings1080852 Published: 4 December 2017 (This article belongs to the Proceedings of Proceedings of the 5th International Symposium on Sensor Science (I3S 2017)) Download Download PDF Download PDF with Cover Download XML Versions Notes Non-destructive evaluation and structural health monitoring techniques can provide frequent or immediate feedback of the condition of a structure including potential damage. However, these techniques cannot detect initiating damage in composite materials with high compaction or multifaceted construction. More critically, they are unable to achieve damage detection without altering the microstructure of the composite material. An alternative method of strain monitoring and damage detection that may offer the advantages of structural health monitoring without their drawbacks consists of using piezoresistive-based carbon nanotube (CNT) yarns integrated in polymers and composite materials. The concept is that the CNT yarns form a continuous sensor circuit and their inherent piezoresistive sensitivity detects strain within the host material through resistance measurements without adding much weight or altering the integrity of the host material. This presentation includes a summary of the piezoresistive response of CNT yarns and the concept and latest experimental results on damage detection in laminated polymeric composite materials and distributed and localized strain measurement. Experimental results also show the ability of a combination of different yarn sensors to detect the exact location and extent of delamination in real time. CNT yarn sensors may provide an adaptive, practical, and sensitive structural health monitoring technique. © 2017 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/). Share and Cite MDPI and ACS Style Abot, J.L. Structural Health Monitoring Using Carbon Nanotube Yarns: Sensing Concept and Applications in Composites. Proceedings 2017, 1, 852. https://doi.org/10.3390/proceedings1080852 AMA Style Abot JL. Structural Health Monitoring Using Carbon Nanotube Yarns: Sensing Concept and Applications in Composites. Proceedings. 2017; 1(8):852. https://doi.org/10.3390/proceedings1080852 Chicago/Turabian Style Abot, Jandro L. 2017. "Structural Health Monitoring Using Carbon Nanotube Yarns: Sensing Concept and Applications in Composites" Proceedings 1, no. 8: 852. https://doi.org/10.3390/proceedings1080852 Find Other Styles Note that from the first issue of 2016, MDPI journals use article numbers instead of page numbers. See further details here. Article Metrics No No Article Access Statistics Multiple requests from the same IP address are counted as one view.

With the large-scale application of composite materials in military aircraft, various composite material detection technologies with infrared nondestructive and ultrasonic nondestructive testing as the core have played an important role in detecting composite material component damage in military aircraft. At present, the damage of composite materials is mostly recognized manually, which is time-consuming, laborious, and inefficient. It can effectively improve detection efficiency and accuracy by using intelligent detection methods to detect and recognize damage. Moreover, the effect of infrared detection is significantly reduced with increasing detection depth, while ultrasonic detection has shallow-blind areas. A cascade fusion R-CNN network is proposed in order to comprehensively identify composite material damage. This network realizes the intelligent fusion recognition of infrared and ultrasonic damage images of composite materials. The network is based on a cascade R-CNN network, using fusion modules and BiFPN for improvement. For the infrared image and ultrasonic C-scan image data set established in this paper, the algorithm can identify the type and location of damage detected by infrared and ultrasonic testing. Its recognition accuracy is 99.3% and mean average precision (mAP) is 90.4%. In the detection process, the characteristics of infrared and ultrasonic images are used to realize the recognition of damage depth. Compared to SSD, YOLOv4, faster R-CNN and cascade R-CNN, the network proposed in this paper is better and more effective.

This work presents numerical modeling-based investigations for detecting and monitoring damage growth and material nonlinearity in plate structures using topological acoustic (TA) and sideband peak count (SPC)-based sensing techniques. The nonlinear ultrasonic SPC-based technique (SPC-index or SPC-I) has shown its effectiveness in monitoring damage growth affecting various engineering materials. However, the new acoustic parameter, "geometric phase change (GPC)" and GPC-index (or GPC-I), derived from the TA sensing technique adopted for monitoring damage growth or material nonlinearity has not been reported yet. The damage growth modeling is carried out by the peri-ultrasound technique to simulate nonlinear interactions between elastic waves and damages (cracks). For damage growth with a purely linear response and for the nonlinearity arising from only the nonlinear stress-strain relationship of the material, the numerical analysis is conducted by the finite element method (FEM) in the Abaqus/CAE 2021 software. In both numerical modeling scenarios, the SPC- and GPC-based techniques are adopted to capture and compare those responses. The computed results show that, from a purely linear scattering response in FEM modeling, the GPC-I can effectively detect the existence of damage but cannot monitor damage growth since the linear scattering differences are small when crack thickness increases. The SPC-I does not show any change when a nonlinear response is not generated. However, the nonlinear response from the damage growth can be efficiently modeled by the nonlocal peri-ultrasound technique. Both the GPC-I and SPC-I techniques can clearly show the damage evolution process if the frequencies are properly chosen. This investigation also shows that the GPC-I indicator has the capability to distinguish nonlinear materials from linear materials while the SPC-I is found to be more effective in distinguishing between different types of nonlinear materials. This work can reveal the mechanism of GPC-I for capturing linear and nonlinear responses, and thus can provide guidance in structural health monitoring (SHM).

The purpose of this paper is to introduce an energy-based anisotropic damage model for predicting fatigue life and evaluation of progressive damage in composite materials. In this work we postulate that the damage state is sufficiently described by the second-rank symmetric damage tensor. The model is based on the assumption that the damage growth rate in composite materials depends on the value of elastic strain energy. The model parameters, which determine the damage accumulation rate, were established by the results of fatigue experiments of glass fabric laminates STEF-1. It is shown that the proposed theory reflects well fatigue damage accumulation process and fracture of glass fiber composite.