D N SHARMA – TRAINING OFFICER

Training Programme for CoE Instructors -

FOREMEN TRAINING INSTITUTE, DGE & T , Ministry of Labour, Govt. of India, Bangalore

Training

4/13/2009
NON DESTRUCTIVE TESTING


1.0 INTRODUCTION

Nondestructive testing is a branch of the materials sciences that is concerned with all aspects of the uniformity, quality and serviceability of materials and structures. The science of nondestructive testing incorporates all the technology for detection and measurement of significant properties, including discontinuities, in items ranging from research specimens to finished hardware and products. By definition, nondestructive techniques are the means by which materials and structures may be inspected without disruption or impairment of serviceability.

Nondestructive testing (NDT) has been defined as comprising those test methods used to examine an object, material or system without impairing its future usefulness.

It is very difficult to weld or mold a solid object that has the risk of breaking in service, so testing at manufacture and during use is often essential. During the process of casting a metal object, for example, the metal may shrink as it cools, and crack or introduce voids inside the structure. Even the best welders (and welding machines) do not make 100% perfect welds. Some typical weld defects that need to be found and repaired are lack of fusion of the weld to the metal and porous bubbles inside the weld, both of which could cause a structure to break or a pipeline to rupture.

During their service lives, many industrial components need regular nondestructive tests to detect damage that may be difficult or expensive to find by everyday methods. For example:

·  aircraft skins need regular checking to detect cracks;

·  underground pipelines are subject to corrosion and stress corrosion cracking;

·  pipes in industrial plants may be subject to erosion and corrosion from the products they carry;

·  concrete structures may be weakened if the inner reinforcing steel is corroded;

·  pressure vessels may develop cracks in welds;

·  the wire ropes in suspension bridges are subject to weather, vibration, and high loads, so testing for broken wires and other damage is important.

(Source: Hellier, 2001) Note the number of An industrial product is designed to perform a certain function. The user buys a product with expectation that it performs the assigned function well and gives trouble – free service for a stipulated period of time. Trouble free service given by any product may be termed as “reliability”. Reliability comes through improving the quality level of the component. The quality of products, components or parts depends upon many factors. Quality is related to the presence of defects and imperfections in the finished product which impair the performance level. In the correct sense, a defect is a rejectable discontinuity or flaw of rejectable nature. Certain flaws acceptable in one type of product need not be of acceptable nature in another product. A defect is definitely a discontinuity but a discontinuity need not necessarily be a defect. Acceptance standards dictate the type of inspection and testing the weld is subjected to.

Nondestructive testing (NDT), also called nondestructive examination (NDE) and nondestructive inspection (NDI), is testing that does not destroy the test object. NDE is vital for constructing and maintaining all types of components and structures. To detect different defects such as cracking and corrosion, there are different methods of testing available, such as X-ray (where cracks show up on the film) and ultrasound (where cracks show up as an echo blip on the screen). This article is aimed mainly at industrial NDT, but many of the methods described here can be used to test the human body. In fact methods from the medical field have often been adapted for industrial use, as was the case with Phased array ultrasonic and Computed radiography.

While destructive testing usually provides a more reliable assessment of the state of the test object, destruction of the test object usually makes this type of test more costly to the test object's owner than nondestructive testing. Destructive testing is also inappropriate in many circumstances, such as forensic investigation. That there is a tradeoff between the cost of the test and its reliability favors a strategy in which most test objects are inspected nondestructively; destructive testing is performed on a sampling of test objects that is drawn randomly for the purpose of characterizing the testing reliability of the nondestructive test.

Non Destructive Testing (NDT) plays a significant part in maintaining the quality of the products at various stages of manufacture. Raw material stage i.e. before the start of manufacture, intermediate stages, and final stage after manufacture all utilizes Nondestructive testing techniques.

Since the 1920s, nondestructive testing has developed from a laboratory curiosity to an indispensable tool of production. No longer is visual examination the principal means of determining quality. Nondestructive tests in great variety are in worldwide use to detect variations in structure, minute changes in surface finish, the presence of cracks or other physical discontinuities, to measure the thickness of materials and coatings and to determine other characteristics of industrial products.

Modern nondestructive tests are used by manufacturers (1) to ensure product integrity, and in turn, reliability; (2) to avoid failures, prevent accidents and save human life; (3) to make a profit for the user; (4) to ensure customer satisfaction and maintain the manufacturer's reputation; (5) to aid in better product design; (6) to control manufacturing processes; (7) to lower manufacturing costs; (8) to maintain uniform quality level; and (9) to ensure operational readiness

2.0 METHODS AND TECHNIQUES

NDT is divided into various methods of nondestructive testing, each based on a particular scientific principle. These methods may be further subdivided into various techniques. The various methods and techniques, due to their particular natures, may lend themselves especially well to certain applications and be of little or no value at all in other applications. Therefore choosing the right method and technique is an important part of the performance of NDT. Below are many NDT methods and they are merely mention to grasp the fact that NDT is more vast and the method to be applied is specific for each and every application.

·  Liquid penetrant testing (PT or LPI)

·  Radiographic testing (RT)

·  Digital radiography (real-time)

·  Computed radiography

·  SCAR (Small Confined Area Radiography)

·  Neutron radiographic testing (NR)

·  Computed tomography (CT)

·  Impulse excitation technique (IET)

·  Ultrasonic testing (UT)

·  Phased array ultrasonics

·  Time of flight diffraction ultrasonics (TOFD)

·  Time of Flight Ultrasonic Determination of 3D Elastic Constants (TOF)

·  Internal Rotary Inspection System (IRIS) ultrasonics for tubes

·  EMAT Electromagnetic Acoustic Transducer (non-contact)

·  laser ultrasonics (LUT)

·  Ellipsometry

·  Pipeline video inspection

·  Electromagnetic testing (ET)

·  Alternating Current Field Measurement (ACFM)

·  Alternating Current potential drop measurement (ACPD)

·  Direct Current potential drop measurement (DCPD)

·  Eddy-Current Testing (ECT)

·  Remote field testing (RFT)

·  Magnetic-particle inspection (MT or MPI)

·  Magnetic flux leakage testing (MFL) for pipelines, tank floors, and wire rope

·  Barkhausen testing

·  Acoustic emission testing (AE)

·  Positive Material Identification (PMI)

·  Hardness testing (Brinell) (HT)

·  Infrared and thermal testing (IR)

·  Thermographic inspection

·  Laser testing

·  Profilometry

·  Holographic interferometry

·  Electronic Speckle Pattern Interferometry

·  Shearography

·  Leak testing or Leak detection (LT)

·  Tracer-gas method testing Helium, Hydrogen and refrigerant gases

·  Bubble testing

·  Absolute pressure leak testing (pressure change)

·  Halogen diode leak testing

·  Mass spectrometer leak testing

·  Magnetic resonance imaging and NMR spectroscopy

·  Visual Inspection

TABLE 1. Nondestructive testing method categories
Basic Categories / Objectives
Mechanical and optical / color, cracks, dimensions, film thickness, gaging, reflectivity, strain distribution and magnitude, surface finish, surface flaws, through-cracks
Penetrating radiation / cracks, density and chemistry variations, elemental distribution, foreign objects, inclusions, microporosity, misalignment, missing parts, segregation, service degradation, shrinkage, thickness, voids
Electromagnetic and electronic / alloy content, anisotropy, cavities, cold work, local strain, hardness, composition, contamination, corrosion, cracks, crack depth, crystal structure, electrical and thermal conductivities, flakes, heat treatment, hot tears, inclusions, ion concentrations, laps, lattice strain, layer thickness, moisture content, polarization, seams, segregation, shrinkage, state of cure, tensile strength, thickness, disbonds
Sonic and ultrasonic / crack initiation and propagation, cracks, voids, damping factor, degree of cure, degree of impregnation, degree of sintering, delaminations, density, dimensions, elastic moduli, grain size, inclusions, mechanical degradation, misalignment, porosity, radiation degradation, structure of composites, surface stress, tensile, shear and compressive strength, disbonds, wear
Thermal and infrared / bonding, composition, emissivity, heat contours, plating thickness, porosity, reflectivity, stress, thermal conductivity, thickness, voids
Chemical and analytical / alloy identification, composition, cracks, elemental analysis and distribution, grain size, inclusions, macrostructure, porosity, segregation, surface anomalies
Auxiliary Categories / Objectives
Image generation / dimensional variations, dynamic performance, anomaly characterization and definition, anomaly distribution, anomaly propagation, magnetic field configurations
Signal image analysis / data selection, processing and display, anomaly mapping, correlation and identification, image enhancement, separation of multiple variables, signature analysis

Classification of NDT Methods

·  Each method can be completely characterized in terms of five principal factors:

·  energy source or medium used to probe the test object (such as X-rays, ultrasonic waves or thermal radiation);
·  nature of the signals, image or signature resulting from interaction with the test object (attenuation of X-rays or reflection of ultrasound, for example);
·  means of detecting or sensing resulting signals (photo emulsion, piezoelectric crystal or inductance coil);
·  method of indicating or recording signals (meter deflection, oscilloscope trace or radiograph); and
·  basis for interpreting the results (direct or indirect indication, qualitative or quantitative, and pertinent dependencies).

The objective of each test method is to provide information about the following material parameters:

·  discontinuities (such as cracks, voids, inclusions, delaminations);
·  structure or malstructure (including crystalline structure, grain size, segregation, misalignment);
·  dimensions and metrology (thickness, diameter, gap size, discontinuity size);
·  physical and mechanical properties (reflectivity, conductivity, elastic modulus, sonic velocity);
·  composition and chemical analysis (alloy identification, impurities, elemental distributions);
·  stress and dynamic response (residual stress, crack growth, wear, vibration); and
·  Signature analysis (image content, frequency spectrum, field configuration).

Out of the above, the most commonly used techniques are Liquid Penetrant Inspection (LPI), magnetic particle inspection, ultrasonic testing, Radiography Testing. In this article the basics of the testing techniques, equipment, methodologies and their limitations are covered in detail in the subsequent sections.

Over the past centuries, swordsmiths, blacksmiths, and bell-makers would listen to the ring of the objects they were creating to get an indication of the soundness of the material. The wheel-tapper would test the wheels of locomotives for the presence of cracks, often caused by fatigue — a function that is now carried out by instrumentation and referred to as the acoustic impact technique.

3.0 NOTABLE EVENTS IN EARLY INDUSTRIAL NDT

·  1854 Hartford, Connecticut: a boiler at the Fales and Gray Car works explodes, killing 21 people and seriously injuring 50. Within a decade, the State of Connecticut passes a law requiring annual inspection (in this case visual) of boilers.

·  1895 Wilhelm Conrad Röntgen discovers what are now known as X-rays. In hisfirst paper he discusses the possibility of flaw detection.

·  1880 - 1920 The "Oil and Whiting" method of crack detection is used in the railroad industry to find cracks in heavy steel parts. (A part is soaked in thinned oil, then painted with a white coating that dries to a powder. Oil seeping out from cracks turns the white powder brown, allowing the cracks to be detected.) This was the precursor to modern liquid penetrant tests.

·  1920 Dr. H. H. Lester begins development of industrial radiography for metals.1924 — Lester uses radiography to examine castings to be installed in a Boston Edison Company steam pressure power plant

·  1926 The first electromagnetic eddy current instrument is available to measurematerial thicknesses.

·  1927 - 1928 Magnetic induction system to detect flaws in railroad track developed by Dr. Elmer Sperry and H.C. Drake.

·  1929 Magnetic particle methods and equipment pioneered (A.V. DeForest and F.B. Doane.)

·  1930s Robert F. Mehl demonstrates radiographic imaging using gamma radiation from Radium, which can examine thicker components than the low-energy X-ray machines available at the time.

·  1935 - 1940 Liquid penetrant tests developed (Betz, Doane, and DeForest)

·  1935 - 1940s Eddy current instruments developed (H.C. Knerr, C. Farrow, Theo Zuschlag, and Fr. F. Foerster).

·  1940 - 1944 Ultrasonic test method developed in USA by Dr. Floyd Firestone.

·  1950 J. Kaiser introduces acoustic emission as an NDT method.

4.0 LIQUID PENERANT INSPECITON

Dye penetrant inspection (DPI), also called liquid penetrant inspection (LPI), is a widely applied and low-cost inspection method used to locate surface-breaking defects in all non-porous materials (metals, plastics, or ceramics). Penetrant may be applied to all non-ferrous materials, but for inspection of ferrous components magnetic-particle inspection is preferred for its subsurface detection capability. LPI is used to detect casting and forging defects, cracks, and leaks in new products, and fatigue cracks on in-service components.

Location of surface flaws and sub-surface flaws is essential for many of the industrial components since the failure to do so may lead to catastrophic situations. For locating gross surface flaws, visual inspection with the aid such as magnifying glasses is adequate. However, location of minute cracks may not be possible even with the aid of magnifying devices. In such cases, Liquid Penetrant Inspection can come to the assistance in the location of defects. For locating only surface flaws, Liquid Penetrant Inspection is sufficient Penetrant examination is generally considered to be one of the easiest methods of surface inspection to locate discontinuities that are open to the surface. Consequently the need for an accurate application and competent personnel may be under-estimated. To obtain optimum results, the method should be applied with care and accuracy. PT can be used on any material except when it is extremely porous.