外文原文-復(fù)合材料無損檢測技術(shù)

259 The 10th International Conference of the Slovenian Society for Non-Destructive Testing Application of Contemporary Non-Destructive Testing in Engineering September 1-3, 2009, Ljubljana, Slovenia, 259-265 NONDESTRUCTIVE TEST TECHNOLOGY FOR THE COMPOSITES Keynote lecture B. Boro Djordjevic Materials and Sensors Technologies, Inc. 798 Cromwell Park Drive; Suite C; Glen Burnie, MD 21061 USA ABSTRACT When manufacturing composite structure, material and structural components are created concurrently. Thus, for composite materials in critical structural applications, it is more important than ever to independently assure structural integrity. Complexity of the advanced composite materials manufacturing and composite in service maintenance represents challenges in developing optimized nondestructive tools and tests. Traditional metals based NDT methods are inappropriate and often misleading when applied to anisotropic and inhomogeneous composite materials. In advanced technology applications such as aerospace and with industrial emphasis on economics and safety, it is critical to use and develop robust and practical composites NDT methods. Composite NDT encompasses a range of modified traditional and new tools including ultrasonic, x- ray, acoustic emission, thermal, optical, electrical and a variety of hybrid methods. This paper provides overview of the current use of the NDT tools in the composite applications. Key words: NDT, NDE, composites 1.Introduction There are enormous mechanical advantages for using composite materials. Table 1 and Figure 1 illustrate the specific properties benefits of the composites structural use over traditional industrial materials. Fiber reinforced organic matrix composite materials specific-properties can double or triple the load carrying capacity over the traditional metals. This materials benefit enables structural designs that outperform the conventional application limitations commensurately improving system performance such as reducing weight, increasing fuel efficiency or increasing speed. 1,2,3 260 Table 1: Illustration of specific strength values for the composite materials in comparison to traditional materials. Fig. 1: Graphic comparison of the composite materials properties to traditional materials. Additionally, composite have better specific stiffness and their anisotropic character can be customized to the structural load requirements. The use of composites is acceleration and now spans transportation industry applications including next generation aircraft such as new Boeing 787.Composites are in wide use for marine applications and have been revolutionary in sporting applications such as skiing, tennis rackets or golf clubs. 261 2. Composite Materials and Testing Background Composite structures are often complex and formed by layers of dissimilar materials. Figure 2 illustrates complexity of the composite cross-sections. For weight-performance sensitive applications such as aerospace, composite materials are now common in critical structural components.3 Fig. 2: Typical cross-sections of the composite materials. Composite mechanical damage is typically in the form of delaminations or disbonds (laminate-to- laminate or laminate-to-core), broken fibers due to impact, fatigue damage that affects the zone of composite material via micro cracking, fiber delaminations, fiber breaks and overall loss of mechanical modulus, or can be caused by thermal damage from prolonged exposure to heat above resin cure temperatures as well as combination of effects due to extreme operational conditions. The detection and evaluation of damage in composites is compounded by the fact that damage is not visible to the naked eye and can occur in many different forms Table 2 shows a list of possible defects and damage found in composite materials. It should be noted that although the composite materials have been used for a long time, including in critical structural applications, the effects of defects, damage mechanisms, fatigue and failure mechanisms are not mature and well understood. Connection between NDE/NDT/NDC information and mechanical performance is also not well established. Table 3 is a listing of the nondestructive testing and evaluation methods that are applicable to composite materials and structures. 262 Table 2: Listing of typical defects and damage found in the composite materials. Table 3: Listing of the nondestructive testing and evaluation practices for the composite materials. Composite materials structural integrity can be compromised via many mechanisms including presence of discontinuities or loss of mechanical properties. Because of composite materials complexity, complexity of the part geometry and often a limited part access, materials damage and materials condition sensing cannot be achieved via conventional NDT/NDE/NDC methodology. Of all nondestructive methods, only ultrasonic methods are directly sensitive to mechanical changes 263 and can be used to directly assess the mechanical condition and integrity of the composite structure. Majority of NDT methods are based on and originated from metals experience. Many current test procedures are inadequately developed to properly and directly tackle the composite structural issues. The composite testing still requires fresh look at availability of new and advanced test methods better adapted to the unique composite material requirements. 3. Examples of Composite Testing methods Since introduction of bonding and composites into the airframe structures, extensive Nondestructive Inspection (NDI) is often performed to assure structural integrity. Composite ultrasonic inspection process is usually limited to a single point measurement across the thickness of the composite material and component coverage is achieved via scanning. Ultrasonic imaging C-scan as shown in Fig. 3 is dominant inspection tool in many aerospace composite manufacturing operations. Modern computerized C-scan has enormous advantages over manual testing in reproducibility and presentation of the ultrasonic tests. However, even for manufacturing subassemblies, ultrasonic C- scan, is often difficult to execute and is not practical in many complex structural configurations such as the leading edges or other complex sections of the flight control surfaces. Point-by-point ultrasonic tests crate composite material sound attenuation maps. Shown in Figure 3, these so called ultrasonic C-scan maps can potentially locate mechanical discontinuities such as delaminations, impact damage or fiber fatigue damage 4,5. Fig. 3 Ultrasonic C-scan of the composite plate outlining disbond areas with increased ultrasonic attenuation. The ultrasonic C-scan maps are very effective in finding mechanical discontinuity but cannot determine material mechanical state. Interpretation of the individual pixel crated from gated ultrasonic signal can be difficult and misleading. This technology originally adapted for testing adhesive bonding is being further refined and adapted to composite applications. By use of advanced transducers, better data collection via many signal gates and aided with digital signal processing, new C-scan tests are very effective NDT tool for composite structures.5 264 Although extensively used in metals for detection of voids and cracks, X-ray technology had undergone extensive modifications for the composite use. Most new system use real time imaging that enables effective imaging of the geometrical features in the composite structures. Shown in Figure 4 is a sequence of real time micro-focus X-ray images of the composite honeycomb. In field applications, X-rays can detect liquid intrusion in cells or impact damaged cell walls. Fig. 4: Sequence of the magnified X-ray images of the composite honeycomb. X-rays provide contrasting images of filed cells and very detail geometry of the cells walls. Optical methods are used for rapid large area screening of the composite materials. Subsurface composite damage can be often sensed via Sherography. Image in Figure 5 * is a result of this optical method sensitivity to minute surface changes due to structural defects inside the honeycomb part. Advanced Techniques such as X-ray tomography, laser ultrasonic, holography, laser-optical, vibro- thermography, acousto-ultrasonic, D-sight, neutron radiography, microwaves or thermal time- resolved methods are in development and will contribute to future NDT composite capabilities. Emerging NDT Technologies such as in-process monitoring, in-situ sensors, remote sensors, or embedded sensors are becoming part of the composite use. In-process organic matrix composites cure monitoring and control can assure better final products 4,6,7. Combination of the NDT methods and a need for continuous monitoring of the composite material structural condition, supports a rapid developments in health monitoring applications and eventual prognostics of the structural degradation. 265 Fig. 5: Shearography image of stabilized honeycomb part showing disbonds and potted honeycomb repairs (Data provided by the Material Physics & NDE Laboratory at The Aerospace Corporation, S. Kenderian) 4. Conclusions There is a long list of NDT methods and sub techniques that are applicable for composite testing. No one method currently has ability to meet all the needs for the composite integrity assessment. Historically focused on defect, emerging technology work is in areas of health monitoring and materials mechanical properties characterization. As critical composite structures become part of commercial use such as new Boeing 787, additional developments will be needed to enable economical, full mechanical integrity characterization of these systems. 5. Referen