Composite Materials And Testing Background

Published Date: 02 Nov 2017

Abstract: 3

Introduction: 4

Composite Materials and Testing Background: 5

Examples of Composite Testing methods: 7

Detecting debonds: 9

Repair of composites 12

Application of NDE 13

Conclusion: 15

Reference: 16

Abstract:

In this essay I have made use of three different articles which was mainly concentrated on Non destructive testing over a aerospace composite materials. In the first article some of the some of the non destructive testing techniques for composites have been explained. [B. Boro Djordjevic]1 In the Second article author [J Prasad]2 has given some different types of composites used in aircraft and NDE evaluation techniques on aircraft composites. In the third author [K. Dragan and W. Swiderski]3 has explained the efficiency of the non destructive evaluation in composites such as detection of debonds and delaminations using different NDT techniques.

Introduction:

In the manufacture of composite structures, materials and structural components to be created simultaneously. Therefore, the key structural applications of composite materials, it is more important to ensure the independent structural integrity than ever. Complication in manufacturing advanced composite materials and composite in service maintenance represents difficulties in developing improvised non destructive testing tools. Anisotropic and non-uniform composite materials, applicable to traditional metal-based non-destructive testing is inappropriate and often misleading. In the application of advanced technologies, such as aerospace and industrial focus on economic and security, the key is to use and the development of a strong and practical composite NDT methods. Composite non-destructive testing, including a series of improvement of traditional and new tools, including ultrasound, X-ray, acoustic emission, thermal, optical, electrical and a variety of hybrid methods. Development of strong and stiff reinforcements like glass, aramid and carbon along with advances in polymers to produce high performance resin as matrix has accelerated application of composites in military and civil aircraft. Composite materials, however, have their own complexities in design, analysis, fabrication and Non destructive test and evaluation (NDE). Inherent weakness of composite materials owing to high anisotropy and large number of manufacturing variables, invariably introduce defects and deficiencies. This necessitates careful control, of material, evaluation and certification of material and component through NDE methodologies. Further, during service and environmental constraints composites get damaged and need repair and Non destructive evaluation to ensure restoration of strength, stiffness, contour and dimensions. This paper examines defects and deficiencies introduced during fabrication and service and relevance of NDE in evaluating composites for "fitness for use".

Composite structures were becoming more popular in the aerospace sectors due to their unique features like excellent strength to weight ratio, corrosion resistance and it is easy to manufacture samples of difficult structures. these days, usage of composites in Aerospace components has become a real fact; for example, nearly 50% of structures in the Boeing 787 aircraft is made of composite materials [3]. nowadays, components such as main rotor blades (MRB) of helicopters are manufactured with composite materials. There are number of failure problems which may appear in composites during manufacturing and operating stages. those failures may affect residual strength and structural integrity of the composites including disbonds, delaminations. classification of such failures are not easy to detect using single non-destructive testing techniques. In order to conduct a complete damage inspection, multimode non destructive evaluation (NDE) of composites should be introduced.

Composite Materials and Testing Background:

Composite structures are often complicated and formed by layers of non uniform materials. Picture 1 shows the difficulties in the composite cross-sections For weight-performance sensitive applications such as aerospace, composite materials are now common in critical structural components.[3] In composite mechanical damage is usually in the form of delaminations or debonds, broken fibers due to impact, fatigue damage will affect the zones of composite material through micro cracking, fiber delaminations and fiber breaks or it can be caused by thermal damage from prolonged exposure to heat above resin cure temperatures. The detection and evaluation of damage in composites is combined by the fact that damage won't visible to the naked eye and it can take place in many different types.

Pic:1 Typical cross-sections of the composite materials.

Following are some of the possible defects and damage found in composite materials. It should be noted in the materials have been used for a long time, including in critical structural applications, damage mechanisms, the effects of defects, fatigue and failure mechanisms are not matured.

Delaminations

Debonds

Porosity

Contamination

Improper Cure

Damaged Fiber

Voids

Cracks

Loss of Properties

Wavy Fibres

Impact Damages etc...

Following is a listing of the non-destructive testing and evaluation methods that are applicable to composite materials and structures.

Ultrasonic

Acoustic Emission

Tap Test

X-Ray

Visual

Optical

Thermal

The integrity of the composite structure through a number of mechanisms, including the presence of discontinuities, damage or loss of mechanical properties. Due to the complexity of the geometry of the complexity of the composite material, parts, usually is a limited part of the connection, damage to the material, and the material conditions of detection cannot be achieved by conventional NDT / NDE / NDC method. All non-destructive method, an ultrasonic wave is directly sensitive to mechanical changes can be used to directly assess the status and integrity of the composite structure of the machine. Most of the non-destructive testing methods are based on experience derived from metal. Many of the current test program is not developed enough to properly directly address the issue of composite structure. The composite tests still need to re-examine the unique composite material requirements to better adapt to the availability of new and advanced testing methods.

Examples of Composite Testing methods:

Self-adhesive and composite materials introduced into the airframe structure, in order to ensure the integrity of the structure often a wide range of non-destructive inspection (NDI)is used. Composite ultrasonic inspection process is typically composite materials and components, coverage is achieved by scanning a limited number of single point measurement of the thickness direction. Picture 2 shows the C-scan ultrasound test. as is a the dominant detection tool in many aerospace composite materials manufacturing business. Modern C-scan ultrasonic testing has a huge advantage, reproducibility over the manual testing. However, even for the manufacture of sub-components, ultrasonic C scan is often difficult to perform the leading edge or other complex flight control surface configuration, as in many complex structure is not practical.

Pic:2 C-scan ultrasound test

To find the mechanical intermittent of a material ultrasonic C-scan is very effective, but it cannot determine the mechanical state of the material. The interpretation of a single pixel of the ultrasonic signal packing can be difficult and misleading. This technology was first available for bonding test is to further improve and adapt composite applications. Through the use of state-of-the-art sensor, the better data collected through a variety of signal gate and supplemented with digital signal processing, the new C-scan NDT testing tool is very efficient for composite structures. The voids and cracks in metal detection, although widely used in X-ray technique has been carried out extensively revised composite use. Most of the new system using real-time imaging, enable effective imaging geometric characteristics of the composite structure. Figure 3 shows a sequence of real-time micro-focus X-ray image of the composite honeycomb[1]. In field applications, X-ray detection liquid intrusion in the cells or impact damage to the cell wall.

Pic 3 Sequence of the magnified X-ray images of the composite honeycomb

In the Rapid screening of large area composite material optical method is used. Subsurface composite damage can often be found through Sherography. Picture 4 shows the result, sensitivity to minute changes in the surface of the optical method of the honeycomb portion of the internal structural defects.

State-of-the-art technology, such as X-ray tomography, laser ultrasound, holography, laser optics, vibration infrared thermography, ultrasonic sound, D-sight, neutron radiography, a microwave or a thermal time-resolved methods are being developed and it will help non-destructive testing composite capabilities.

Emerging non-destructive testing techniques, such as process monitoring, in situ sensors, remote sensors, or embedded sensors are becoming a part of the composite used. Organic matrix composite cure monitoring and control, and it can ensure a better final product.

Combination of the non-destructive testing methods and a need of continuous monitoring in composite material structural condition, supports a immediate developments in applications like health observation and future indication of the structural reduction.

Pic 4 Shearography image of stabilized honeycomb part

Detecting debonds:

Bonded joints are commonly used in the manufacture of aerospace components. In the specific case, this joining technique is considered to be more effective than normal, such as bolts, rivets, etc. In addition, it can be used to integrate different materials, as well as for the possible future maintenance. However, this joints may be collapsed due to manufacturing defect, the operational cycle of the load and impact damage. Different according to a specific key lines and different structure may be accompanied by some problems, the inspection of adhesive joints. In the maintenance phase, due to lot of stress distribution and operating system loading cycle, the critical region may spread from the edge of the adhesive line, as shown picture 5. [3] To apply non-destructive testing technology to check debonds, some standard specimens, with ultra-light aircraft, gliders and helicopters rotor blade are manufactured. The first sample was rudder Poland glider PW-5 (picture 6). [3]

Pic 5 Stress distribution in the bondline.

Pic 6 Rudder of the PW-5 glider.

The ultrasonic inspection technique is the one which is used to monitor bond quality. This technique enables the generation of elastic waves in structures under test. The bondline of the joint can be clearly seen in the scanned image. Ultrasound evaluation bondline can be done using the index of 6 dB standard or other signals, such as signal-to-noise ratio (SNR)

Non-destructive technique for the next inspection has been infrared (IR) thermography and it can be achieved in active and passive mode. Passive infrared thermographic test is applied to the target at object which is different from ambient temperature, due to its natural or mining reasons. Under normal circumstances, the evaluation of the defects of the material may be in the active mode, the transient heat flux by means of an external heat source in the stimulation of the test sample[4]. Several detection techniques have been developed, just to mention a pulse heat, the heating step, the lock-in thermography, vibro-thermographic imaging, etc. In all of these procedures, whether it is a reflection or transmission inspection procedures can be achieved. In the reflection process, the heat source and an infrared camera is located on the same side of the test object, in the transmission process, the two opposing surfaces, respectively, heating and monitoring. in this the quality of casual glue joints, can be easily evaluated.

Another detection solutions, X-ray radiography. Its advantage is the direct imaging of a sample internal geometry. In picture 7, the adhesive layer and the internal ribs integral part can be seen. Necessity to both sides of the shortcomings of the X-ray radiography the sample and insensibility of certain types of defects, such as delaminations [3].

Pic 7 internal ribs of the PW-5 rudder

In addition, the kissing bonds can be difficult to assess, through the use of ultrasonic technology. In order to detect such defects, other non-destructive testing technology should be used, such as mechanical impedance analysis and Sherography [2]. Future research, some representatives of the thick aircraft the adhesive joined structure (beams, ribs).

Using an ultrasonic non-destructive testing the thicker composite material is difficult to check due to its problem related to the attenuation of ultrasonic waves. In order to characterize this phenomenon, some attenuation measurement techniques can be used on the basis of measurements: the overall damping or ultrasound attenuation (the transmission double) (dB); damping mm (dB / month), the use of pulse-echo ultrasonic attenuation. As always, thick or high-dispersed composite material inspection should reach a compromise between the frequency and sensitivity.

Pic 8: Wing skin sample with a disbond

Picture 8 showing the images collected by using X-ray radiography (a) and IR thermography (b) lock-in technique, phasegram. The debond in this work can be easily detected with the ultrasonic inspection. For thicker structures, focused ultrasound, Or phased array technology can be applied to improve the material is introduced into the ultrasonic power spectrum. In many aerospace structures, especially in the small size of the aircraft, foaming and curing of the core material of the sandwich panels are frequently used. The debonding of different sizes and shapes have been introduced to the sample in order to allow evaluation of the efficiency of the test.

The low density of the core material is the main reason for low mechanical impedance. The framework of the ultrasonic sensor using a single pulse - echo technique has proved to be inefficient. in order to improve the detection efficiency, the ultrasonic phased array have been used. Successful experiment results in the inspection of the above-described structure is obtained by focusing the light beam, and defining the aperture. picture 9a, B having the data collected through the use of ultrasonic phased array system. Visualization techniques based on colour, the depth of the defect, as well as a description of the size and shape of the evaluation. The same sample to check all defects and heat Graph shows a good visibility.

Pic 9 Ultrasonic inspection results. (a) Top (b) Bottom

Repair of composites

The composite structure may damage during its production, transportation, assembly or services at any stage of its life cycle and it needs a suitable repair action. The role of NDE in pre repair stage is to determine the nature of damage and demarcate extension of damage and ensure satisfactory removal of defective areas. In post repair stage, NDE ensures freedom from harmful defects and in-homogeneities. Further, restoration of thickness, strength and stiffness is validated through NDE methods. Fixed honeycomb structures are being evaluated through the transmission of ultrasound or X-ray method. All maintenance activities need to record, design requirements, maintenance procedures, evaluation and acceptance criteria.

Application of NDE

The fact that the composite material is susceptible to multiple defects as shown above, it always need to produce evidence of materials and structures by non-destructive testing methods to establish a complete

homogeneity of material and Elastic properties

deviation in Dimensions

Level of defects

As the property of composites is highly process dependent, evaluation of these materials through NDE involves:-

Evaluation of repeatability of fabrication in achieving strength and stiffness, resulting from design of layup and cure operation.

Ensuring thickness variation as per design stipulations.

Detection, evaluation and acceptability of discrete and volume dispersed defects.

Complexity of geometry, size of components and state of assembly, dictate choice of testing facility as well as technique of testing. Usually a combination of complimentary NDT facilities is required to test and evaluate composites.

Most widely used and preferred tool for test and evaluation for composites is Ultrasonic [2] .Both, through transmission and pulse echo methods with capability of presentation of information in A, B, and C-scan modes are used. Ultrasonic system with capability to measure attenuation and velocity can effectively detect and evaluate various defects mentioned above. A few examples of effectiveness of Ultrasonics in detecting and evaluation defects is shown abow.

Among other important NDE methods that have been used successfully are - Low energy radiography for detection and evaluation of voids, fiber damage, inclusions, and core damage in honeycomb structures. Acoustic emission methods have been used for evaluating integrated effect of flaws produced during microscopic failure related to resin fracture, fiber fracture or failure of resin to fiber bond. High frequency eddy current methods have been used to detect and evaluate surface and subsurface and present the information in C-Scan mode. Method is reliable for inspection of composite skins during service.

Acoustic impact methods are used to detect and evaluate large defects like debond, and delamination. To reduce dependence on human factors robot based automatic NDE devices are finding increasing application.

Conclusion:

This paper describes some of the assessment of diagnostic issues related to the integrity of the composite structures, which was discussed in three different articles. There are a wide variety of defects may occur in the composite. Some of them, such as debonding and delamination is more typical, while others (porosity, inclusions) will appear randomly, in particular, in the manufacturing process. The composite material is a very critical impact damage, which may produce so it is called Barely Visible impact damage. Under normal circumstances, is accompanied by impact damage, but no visible surface marker, it may significantly affect the remaining strength of the material. In the aerospace industry, there are a bunch of non-destructive testing solution used for the characterization of composite integrity. A type of material, the access check component design and determines the specificity of an appropriate non-destructive technique. In this study, debonding, stratification and the inclusion of the foreign body detection method were analyzed. In particular, the three non-destructive testing techniques have been explored Namely, ultrasonic, infrared thermal imaging and X-ray photography. . The limits of the ultrasonic method are given by attenuation of elastic waves in particular materials; also, this technique cannot be used for the detection of kissing bonds.. The infrared thermography works good when it comes to the debonding. However, the efficiency of this technique is limited by the type of material being tested and the availability of a suitable heat source. We believe that the most promising results generated by the light flash and the recent emergence of the ultrasonic type thermal stimulation.

usually X-ray radiography is not very successful in determining the delamination, this technique requires both sides of the test sample to inspect, which may not be possible in inspecting aerospace components.