Title: A REVIEW OF NON DESTRUCTIVE TESTING METHODS FOR COMPOSITE MATERIALS
Authors: I.V.KAUSHAL & H.SAI KIRAN, 3rd Year,BTech, Production Engineering Department
College: Chaitanya Bharathi institute of Technology, Gandipet, Hyderabad
Abstract—Today Composites are receiving much attention not only because they are on the cutting edge of active material research field but also because there is a great deal of promise for their potential applications in various industries ranging from aerospace to construction due to their various outstanding properties. The nondestructive testing and inspection of composite structures, both for manufacturing quality assurance and for in-service damage detection, has prompted the development and adaptation of a number of methods and techniques over the years. In this paper, we review various NDT methods for inspecting composites, The present study covers an overview of the techniques like Ultrasonic Testing, InfraRed Thermography and Speckle Shearing Interferometry, known as Shearography, with emphasis given to techniques that demonstrated benefits to the practice of composite inspection.
Keywords- Composites, NDT Methods, Ultrasonic, Thermography, Shearography
By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical condition, without interfering in any way with the integrity of the material or its suitability for service. The technique can be applied on a sampling basis for individual investigation or may be used for 100% checking of material in a production quality control system. Whilst being a high technology concept, evolution of the equipment has made it robust enough for application in any industrial environment at any stage of manufacture – from steel making to site inspection of components already in service. This guide discusses the different Non destructive techniques to detect in defects in composite materials. Detection of flaws in composite materials requires a skilled work since the materials are often non-homogeneous and anistropic.
METHODS OF NDT
Visual, tap testing,microwave,magnetic particle ,thermography,acoustic emission,X ray,ultrasonic,fuel leakage,laser inferometry,eddy current,replication,liquid penetrant, magnetic particle,acoustic microscopy,magnetic measurements,ultrasonic,
USES OF NDT
- Flaw detection and evaluation
- Leak detection
- Location determination
- Dimensional measurements
- Structure and microstructure characterization
- Estimation of meachanical and physical properties
- Stress ( strain) and dynamic response measurements
- Material sorting and chemical composition determination
WHEN ARE THESE METHODS USED??
These NDT applications are used in at almost any stage in the production or life cycle of a component
To assist in product development
To screen or sort incoming materials
To monitor, improve or control manufacturing processes
To verify proper processing such as heat treating
To verify proper assembly
To inspect for in-service damage
SIX COMMON METHODS OF NDT
- liquid penetrant
- eddy current
- X ray
CONVENTIONAL METHODS OF NDT
This technique works on principle of ‘Differential Absorption of X rays’ and is suitable for the detection of internal defects. Material with internal voids is tested by placing the subject between the source of radiation and the film. The voids show as darkened areas, where more radiation has reached the film, on a clear background.
- This method can be successfully applied for thin sections of any material (i.e) Ferrous and non ferrous.
- The major disadvantages are we cannot detect surface defects and the depth of defect below the surface cannot be determined. It is also a possible health hazard.
MAGNETIC PARTICLE INSPECTION
This method is suitable for the detection of surface and near surface discontinuities in magnetic material, mainly ferrite steel and iron. It works on the principle that when magnetic flux is applied perpendicular to the defect, the crack edges act as magnetic attractive poles leading to attraction of finely divided metal particles like iron fillings.
- Operation and application are simple because of which it can be used to inspect large surface areas of complex parts rapidly.
- The major disadvantage being this method can be employed only for ferromagnetic materials. It is restricted to surface and sub surface flaws.
DYE PENETRATE INPECTION
This method is frequently used for the detection of surface breaking flaws in non ferromagnetic materials. In this method penetrate is applied to the chemically cleaned surface, the capillary action of the cracks leads to deposition of penetrate in it. This is studied by applying developer over the surface which form visual magnified indications in contrast to the background.
- Best method for detecting surface breaking cracks in non-ferrous metals. It is simple in operation and can be automated.
- It only detects the defects open to surface. Sub surface or deeper defects cannot be determined.
EDDY CURRENT AND ELECTRO MAGNETIC METHODS
In this method, Eddy currents are produced in the component by subjecting to an alternating magnetic field. The magnitude of the eddy currents generated in the product is dependent on conductivity, permeability and the set up geometry. Any change in the material or geometry can be detected by the excitation coil as a change in the coil impedance. The detected eddy current signals contain amplitude and phase information and which can be displayed on CRT type displays.
- This method not only helps us in detecting flaws but also helps in determination of a wide range of conditions of conducting material, such as composition, hardness, conductivity, permeability etc. in a wide variety of engineering metals.
- The wide range of parameters which affect the eddy current responses means that the signal from a desired material characteristic, e.g. a crack, can be masked by an unwanted parameter, e.g. hardness change.
III METHODS OF NDT FOR COMPOSITES
A. ULTRASONIC TESTING
This technique is often applied on composites. In ultrasonic testing, an ultrasound transducer connected to a diagnostic machine is passed over the object being inspected. The transducer is typically separated from the test object by a couplant (such as oil) or by water, as in immersion testing. There are two methods of receiving the ultrasound waveform: reflection and attenuation. In reflection (or pulse-echo) mode, the transducer performs both the sending and the receiving of the pulsed waves as the “sound” is reflected back to the device. Reflected ultrasound comes from an interface, such as the back wall of the object or from an imperfection within the object. In attenuation (or through-transmission) mode, a transmitter sends ultrasound through one surface, and a separate receiver detects the amount that has reached it on another surface after traveling through the medium. Imperfections or other conditions in the space between the transmitter and receiver reduce the amount of sound transmitted, thus revealing their presence.
This method can be successfully applied for detection of deeper parts with greater accuracy and this process is non hazardous. The major disadvantages are we cannot detect the non homogeneous materials and requires skilled work.
Thermographic methods are those in which the presence of flaws is determined by monitoring the heat of flow over the surface of the structure the presence of the flaw disrupts the normal pattern of the heat flow that would be expected in sound pattern. Modern thermographic methods mainly use infrared cameras to detect the radiated heat. Mainly there are two types of methods they are :-
- Passive infrared thermography: Thermal radiation emitted from object’s surface is scanned by infrared camera, and gives information about the surface temperature of the object in thermal equilibrium.
- Active infrared thermography: The object under test is thermally excited – usually irradiated by a source of infrared radiation. In this case the object is in non-equilibrium state and the transient temperature field measured by thermographic camera provides information about the thermo-physical properties, defects and in homogeneities inside the object
In the active thermography experiments special care should be taken if the emissivity of the surface is non-uniform, or the surface of the object is non-uniformly irradiated by the IR source.
This can introduce temperature gradients on the object’s surface similar to changes induced by subsurface defects, and therefore these non uniformities should be detected
A near surface flaw will decrease a composite’s local strength and therefore the surface will deform differently under loading. If a flaw is present these differences are very small so a technique based on optical interference is suitable to detect them .In this method the specimen is illuminated with a divergent laser beam and the scattered light is projected onto an image plane by a special shearing lens. Interference occurs between sheared and direct images. A stress distribution is then applied to the surface and a second interference pattern is recorded in the form of fringes. This allow the detection of sudden fringe deformations leading to greater detail and an image in which flaws show up as moon crater indications.
Shearography is used to scan the aerospace structures. This is also applied in GRP pressure vessels where shearography is used to detect inclusion
INSPECTION OF RAW PRODUCTS
- Extrusiuons etc
INSPECTION FLOWING SECONDARY PROCESSING
- Heat treating
- Plating etc
IN SERVICE DAMAGE PROTECTION
- Heat damage etc
OTHER COMMON APPLICATIONS
- Power plant inspection
- storage tank inspection
- Wire rope inspection
- Air craft inspection
- Jet inspection
- Pressure vessel inspection
- Rail,bridge inspection
- Pipline inspection
- Special measurements
Crash of united flight 232-(sioux city,lowa july19,1989)-A defect that went undetected in an engine disk was Responsible for the crash united flight 232
Hence this review paper has discussed all existing conventional Techniques of Non Destructive testing and latest techniques used to test composite materials. Hence the need for Non destructive testing in today’s industries is growing which calls for further Research and Development in the field of NDTs.
Special thanks to Mrs.Jyothirmayi, Professor-CBIT for her timely support and advice.
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- Blitz, Jack; G. Simpson (1991). Ultrasonic Methods of Non-Destructive Testing. Springer-Verlag New York, LLC