Dale E. Chimenti

Unified Life Cycle Engineering: An Emerging Design Concept

Current engineering practice for a component such as a jet engine disk almost always involves a serial approach. The disk is first designed for maximum performance (e.g., minimum weight). The design is then sent to the manufacturing department which defines appropriate production approaches and may request some redesign to facilitate manufacturing. Finally, product support considerations such as inspectability and repair receive attention, but by now it is often too late to impact the design. This, of course, is an overstatement but is not too far from the truth for most components. The emergence of computational plenty offers opportunities to change the present practice towards a concurrent approach which has been called Unified Life Cycle Engineering (ULCE). [1]

Signal Processing of Leaky Lamb Wave Data for Defect Imaging in Composite Laminates

Inspection of composite laminates with Leaky Lamb waves (LLW) has been shown to hold promise of improved reliability and increased sensitivity to important defects [1]. Conventional scanning with the LLW has the possible disadvantage that the method is sensitive not only to internal structure, but also to small variations in plate thickness, which are indistinguishable from elastic property changes. To circumvent this potentially irrelevant sensitivity, a technique has been developed [2] whereby such variations can be selectively ignored, while retaining sensitivity to important defects or material property variations. The method consists of applying frequency modulation to the usual tone burst RF signal and exploiting detailed knowledge of the Lamb wave spectrum of composites [3] to discriminate between significant defects or property changes and small thickness variations in the plate. The current work extends and expands this analog signal processing scheme by performing the analysis on digitized data, permitting a much more general and flexible approach which will be described.

Swept Frequency Ultrasonic Imaging in Composite Plates

Conventional ultrasonic C-scan imaging normally employs focussed transducers excited by high-voltage impulsive signals. The reflected wave train, containing information about the internal features of the test piece, is translated, either through analog or digital means, to an intensity (or color) and plotted as a function of transducer position on the sample. While this method is certainly effective for many inspections, it is not the only way, or perhaps even the best way, to obtain this kind of information. The purpose of this paper will be to describe an alternate means to acquire ultrasonic C-scan data using a swept-frequency tone-burst imaging technique which presents several important advantages over more conventional means.

Microstructural Rayleigh Wave Dispersion on a Fluid-Coupled Anisotropic Surface with Vertical Lamination

In a recent paper Nayfeh et al. [1] presented theoretical and experimental results for the propagation of longitudinal waves in a composite whose microstructure was large enough to cause observable velocity dispersion. Only wave propagation along the fiber axis of a uniaxial laminate was considered. A reflection coefficient was also derived for the case of normal incidence and parallel to the fibers. For ultrasonic inspection applications, what is required is the ability to analyze situations in which the wave is incident at arbitrary angles. Analysis of such general situations are, however, difficult to treat. A relatively simpler two-dimensional composite, which has been analyzed for an off-normal incident angle [2], consists of a bilaminated model with layers bonded and stacked normal to the x3-direction. The structure occupies the half-space x2 ≥ 0 as illustrated in Fig. 1. The composite is immersed in water such that the x2-direction is normal to the fluid-composite interface and the wave is incident from the fluid in the x1-x2 plane. For this model the reflection coefficient and the characteristic equation for the propagation of fluid-composite interfacial waves was calculated. The results reported in [2] are also restricted such that the individual composite components are isotropic.

Ultrasonic Reflection and Wave Propagation in Multilayered Composite Plates

This paper presents selected results of numerous ultrasonic reflection measurements on plates of laminated composites in which the angle of incidence of the acoustic wave and its frequency have been varied. These measurements have been carried out on biaxially laminated graphite-epoxy specimens utilizing water as a fluid coupling medium. The stacking sequence of the samples investigated is restricted to the case of 0–90° lamination. The data are compared to the results of a recent theoretical analysis based on an exact analytical treatment of wave propagation in a fluid-coupled orthotropic plate in conjunction with a transfer matrix approach. Reflection and transmission coefficients are derived for the multilayer anisotropic laminate from which the characteristic behavior of the system is identified. In general, very good agreement is found between prediction and experiment. Moreover, significant changes in the reflection spectra are observed and predicted, depending on the orientation of the composite plates.

Leaky Plate Wave Inspection of Biaxial Composites

The utilization of leaky plate waves in a scanning arrangement has exhibited improved reliability and increased sensitivity to important defects in unidirectional material [1,2,3].The application of frequency modulation to the usual tone burst signal used to generate plate waves has also been shown to enhance defect discrimination [2,3].In the current work, leaky plate wave techniques have been applied to the inspection of biaxially laminated graphite-epoxy composites.Test samples having 8, 16, and 24 plies, respectively, are studied.Test specimens contain several types of defects — simulated delaminations, porosity, and ply cuts.In addition, a series of impact-damaged samples are examined to study the method’s sensitivity to this type of delamination.All simulated defects were detected, and comparisons with conventional normal-incidence C-scan measurements have shown that the plate wave technique is more sensitive to both porosity and ply cuts, consistent with our observations on uniaxial composites [3]. Novel gating methods have been applied to the plate wave spectra to improve defect detection in biaxial composites.

Mechanical Modeling and Measurements on Fibrous Composites

The mechanical behavior of fiber-reinforced composite has been investigated theoretically and experimentally. As an example, ultrasonic leaky Lamb wave propagation has been studied in detail to assess the validity of the modeling. From measurements of the reflected acoustic field from composites plates over a range of incident angles, experimental velocity dispersion curves for Lamb wave modes have been constructed. Because of the high degree of elastic anisotropy of composite laminates, these modes display behavior not typically found in isotropic plates. The theoretical model, based on a continuum-mixture approach, is found to account remarkably well for the data.

Ultrasonic Dispersion in Fluid-Coupled Composite Plates

Fiber-reinforced composite materials have excited significant interest among industries needing to fabricate structures which are both light in weight and high in stiffness. Therefore, much attention has been paid by researchers over the past decade to composite materials and their properties. One active area of endeavor has been the topic of wave propagation studies [1–8]. Several theoretical approaches have been attempted to render tractable the complicated problem of wave propagation in an anisotropic material with microstructure. Many of these theories are quite useful in their region of applicability. We have reviewed briefly this earlier work in a previous paper [9] and will not recapitulate those comments here. The specific problem with which we are now concerned centers on the role of the structure itself and the influence of its surrounding media on dispersive behavior in guided wave propagation. Since all our measurements have been conducted in a frequency regime where the sound wavelength is much larger than the fiber diameter, we adopt a continuum mixture approach to account for the combined fiber-matrix mechanical properties of the composite. A detailed description of this model is found elsewhere in these Proceedings [10].

Continuum Modeling of Ultrasonic Behavior in Fluid-Loaded Fibrous Composite Media with Applications to Ceramic and Metal Matrix Composites

Elastic wave propagation in fibrous composite materials has been the subject of numerous investigations in recent years. However, the morphology of fiber-reinforced composites can seriously complicate the calculation of their wave propagation properties. Since it is clearly not practical to attempt a solution of the completely general elastic-wave problem, most prior work [1–4] has employed various approximations to render the calculations tractable. Our own approach [5,6] to interacting continua offers an alternative procedure for modeling the response of composites, where in particular, a rational construction of the mixture momentum and constitutive-relation interaction terms is given. This theory leads to simple wave propagation equations which potentially contain the full influence of the microstructure, that is, the distribution of displacements and stresses within individual constituents of the composite.