Lamb Wave-based BVID Imaging for a Curved Composite Sandwich Panel

Composite sandwich structures, consisting of a low density core sandwiched between two laminated facesheets, have been widely used in various aerospace structures. A new Lamb wave-based imaging condition, which will be referred to as the inverse energy imaging criterion, is proposed in this paper to resolve the situations where the incident wave energy weakly penetrates into the damaged area on the top facesheet region. Current imaging conditions by analyzing wavefield reconstructed from laser Doppler vibrometer (LDV) scanning have been proven to be adequate for imaging damage in layered composite laminates. In this research, those current imaging conditions were found to be less effective in the composite foam structures for barely visible impact damage (BVID). A piezoelectric wafer was used to excite Lamb waves into the structure and a LDV was used to scan the potential damaged areas in the front facesheet of the panel. A few sites of BVID in a curved composite sandwich foam aileron were inspected using various wavefield analysis methods and the damage images were compared with C-scan images. The results show that the proposed imaging condition performs better when the incident waves have difficulty penetrating into the damaged region

Detection and Quantification of Composite Manufacturing Defects Using Guided Waves

A guided wave defect detection technique for cured carbon fiber reinforced polymer (CFRP) composites was investigated in this paper. This technique may be extended to perform in-process cure monitoring, defect detection and sizing, and ultimately as a closed-loop process control approach to maximize composite part quality and consistency. The predominant types of manufacturing defects associated with automated fiber placement/laminate layup followed by thermal cure include porosity, tow gaps, tow overlaps, through-the-thickness fiber waviness, and in-plane fiber waviness. The present study focused on detection methods for two classes of defects: (i) providing a metric to infer porosity formed due to variation in the matrix curing process, and (ii) imaging the overlap and gap of adjacent plies intentionally introduced during layup of the prepreg. In the first set of experiments, three 24-ply unidirectional epoxy composite panels were manufactured such that each subsequent panel had a higher degree of cure and different values of porosity by intentionally terminating the cure cycle prematurely. The average porosity was determined by acid digestion and qualitatively confirmed by ultrasonic Cscan and optical micrographs. These panels were also investigated by guided waves. It was demonstrated that the group velocity, propagating normal to the fiber direction, of the guided waves increased monotonically from the first to the third panel. Therefore, group velocity may be utilized as a metric for degree of cure and as a means to infer porosity. In the second set of experiments, a fully non-contact guided wave hybrid system composed of an air-coupled transducer and a laser Doppler vibrometer (LDV) was used for imaging gap and overlap defects in adjacent tows. By transforming the transient wave response in a region covering tow gap and tow overlap from the time-space domain to the frequency-wavenumber domain, the total wavefield was separated into the incident and backscatter waves. The gap and overlap region was imaged by using a denoised weighted zero-lag cross-correlation (DW-ZLCC) imaging condition

A Numerical and Experimental Study of Damage Growth in a Composite Laminate

The present study has three goals: (1) perform an experiment where a simple laminate damage process can be characterized in high detail; (2) evaluate the performance of existing commercially available laminate damage simulation tools by modeling the experiment; (3) observe and understand the underlying physics of damage in a composite honeycomb sandwich structure subjected to low-velocity impact. A quasi-static indentation experiment has been devised to provide detailed information about a simple mixed-mode damage growth process. The test specimens consist of an aluminum honeycomb core with a cross-ply laminate facesheet supported on a stiff uniform surface. When the sample is subjected to an indentation load, the honeycomb core provides support to the facesheet resulting in a gradual and stable damage growth process in the skin. This enables real time observation as a matrix crack forms, propagates through a ply, and then causes a delamination. Finite element analyses were conducted in ABAQUS/Explicit(TradeMark) 6.13 that used continuum and cohesive modeling techniques to simulate facesheet damage and a geometric and material nonlinear model to simulate core crushing. The high fidelity of the experimental data allows a detailed investigation and discussion of the accuracy of each numerical modeling approach

Detection of CFRP Composite Manufacturing Defects Using a Guided Wave Approach

NASA Langley Research Center is investigating a guided-wave based defect detection technique for as-fabricated carbon fiber reinforced polymer (CFRP) composites. This technique will be extended to perform in-process cure monitoring, defect detection and size determination, and ultimately a closed-loop process control to maximize composite part quality and consistency. The overall objective of this work is to determine the capability and limitations of the proposed defect detection technique, as well as the number and types of sensors needed to identify the size, type, and location of the predominant types of manufacturing defects associated with laminate layup and cure. This includes, porosity, gaps, overlaps, through-the-thickness fiber waviness, and in-plane fiber waviness. The present study focuses on detection of the porosity formed from variations in the matrix curing process, and on local overlaps intentionally introduced during layup of the prepreg. By terminating the cycle prematurely, three 24-ply unidirectional composite panels were manufactured such that each subsequent panel had a higher final degree of cure, and lower level of porosity. It was demonstrated that the group velocity, normal to the fiber direction, of a guided wave mode increased by 5.52 percent from the first panel to the second panel and 1.26 percent from the second panel to the third panel. Therefore, group velocity was utilized as a metric for degree of cure and porosity measurements. A fully non-contact guided wave hybrid system composed of an air-coupled transducer and a laser Doppler vibrometer (LDV) was used for the detection and size determination of an overlap By transforming the plate response from the time-space domain to the frequency-wavenumber domain, the total wavefield was then separated into the incident and backscatter waves. The overlap region was accurately imaged by using a zero-lag cross-correlation (ZLCC) imaging condition, implying the incident and backscattered waves are in phase over the overlap boundaries

Probabilistic Fatigue Damage Prognosis Using a Surrogate Model Trained Via 3D Finite Element Analysis

Utilizing inverse uncertainty quantification techniques, structural health monitoring can be integrated with damage progression models to form probabilistic predictions of a structure's remaining useful life. However, damage evolution in realistic structures is physically complex. Accurately representing this behavior requires high-fidelity models which are typically computationally prohibitive. In the present work, a high-fidelity finite element model is represented by a surrogate model, reducing computation times. The new approach is used with damage diagnosis data to form a probabilistic prediction of remaining useful life for a test specimen under mixed-mode conditions

Postbuckling Analysis of Delaminated Composite Plates Under Compression (Composite Laminates)

202 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1986.Failure mechanics of delamination problems in composite laminates under compression is investigated. The failure modes in general may include prebuckling, postbuckling, delamination growth and final failure. Buckling load is determined by solving an eigenvalue problem using a subspace iteration technique. The Riks-Wempner iterative scheme is then used to determine the associated postbuckling load-displacement path. Owing to local buckling of the delaminated ligament, interlaminar stress transfer and strong material anisotropy, the singular stress field near the crack-tip must be considered for crack stability study. A rotational, singular hybrid finite element approach is taken for studying crack-tip stresses during postbuckling. Formulation of the element stiffness matrix is based on the assumption of small strain, large rotation and the recently developed laminate elasticity solution. The variational principle of a modified hybrid functional is employed for derivation of equilibrium equations in each incremental step. Comparisons of the results obtained from the present method and conventional finite elements are made to demonstrate the accuracy and efficiency of the present approach. Influences of eigenfunction truncation, and size of the rotational singular hybrid element on solution accuracy and convergence are studied. An analysis of symmetric graphite-epoxy composite laminates subjected to in-plane compression is attempted. Effects of fiber orientation and crack length on buckling instability and postbuckling behavior are studied. Mixed-mode stress intensity factors in delaminated (theta)/-(theta)/-(theta)/(theta) composite systems are calculated during postbuckling. Based on a critical mode-I fracture criterion, delamination stability is also investigated.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Lamb Wave-based BVID Imaging for a Curved Composite Sandwich Panel

Composite sandwich structures, consisting of a low density core sandwiched between two laminated facesheets, have been widely used in various aerospace structures. A new Lamb wave-based imaging condition, which will be referred to as the inverse energy imaging criterion, is proposed in this paper to resolve the situations where the incident wave energy weakly penetrates into the damaged area on the top facesheet region. Current imaging conditions by analyzing wavefield reconstructed from laser Doppler vibrometer (LDV) scanning have been proven to be adequate for imaging damage in layered composite laminates. In this research, those current imaging conditions were found to be less effective in the composite foam structures for barely visible impact damage (BVID). A piezoelectric wafer was used to excite Lamb waves into the structure and a LDV was used to scan the potential damaged areas in the front facesheet of the panel. A few sites of BVID in a curved composite sandwich foam aileron were inspected using various wavefield analysis methods and the damage images were compared with C-scan images. The results show that the proposed imaging condition performs better when the incident waves have difficulty penetrating into the damaged region.</p