Introduction

The field of non-destructive testing (NDT) is a very broad area. It plays a critical role in assuring that structural components and systems perform their function in a reliable and cost effective fashion. NDT is a part of the quality control process. Non-destructive testing (NDT)/non-destructive inspection (NDI)/non-destructive evaluation (NDE) is a wide group of analysis techniques used in science and industry to detect surface or internal flaws of a material, component or system without causing damage. Since NDT does not permanently alter the article being inspected, it is a highly valuable method that can save both money and time in product evaluation, troubleshooting and research. NDT is not just a method for rejecting sub-standard materials; it is an assurance that the supposedly good is really good. NDT is a commonly used tool in forensic engineering, mechanical engineering, electrical engineering, civil engineering, systems engineering, aeronautical engineering, metallurgical engineering, electronic engineering, medicine, and art. In this chapter, we shall discuss the basic principle involved in NDT and very briefly, the methods of NDT.

Objectives of NDT

NDT provides an excellent balance between quality control and cost-effectiveness. There are NDE applications at almost any stage in the production or life cycle of a component.

i. To assist in product development.

ii. To screen or sort out incoming materials.

iii. To assist in product development.

iv. To monitor, improve or control manufacturing processes.

v. To verify proper processing such as heat treatment.

vi. To verify proper assembly.

vii. To inspect in-service damage.

NDE may be used to determine material properties such as fracture toughness, ductility, conductivity, and other physical characteristics. NDT are used in

i. Flaw detection and evaluation.

ii. Leak detection.

iii. Location determination.

iv. Dimensional measurements.

v. Structure and microstructure characterization.

vi. Estimation of mechanical and physical properties.

vii. Stress/Strain and dynamic response measurements.

viii. Material sorting and chemical composition determination.

Methods of NDT

The NDT technique uses a variety of scientific principles. The list of NDT methods that can be used to inspect components and make measurements is large and continues to grow. Researchers continue to find new ways of applying principles of physics and other scientific disciplines to develop better NDT methods. However, there are six NDT methods that are used most often.

Introduction

The concept of nano structures and nano technology dates back to the history of the Nobel Laureate Richard Feynman's famous lecture in 1959 where he speculated the possibility of manoeuvring things atom by atom. He proudly said “There is plenty of room at the bottom”. By this sentence, he strongly pointed out that there is a lot of scope and application of materials at very small scale, i.e., at atomic or molecular level. Why cannot we write the entire 24 volumes of the Encyclopaedia Britannica on the head of a pin? One may wonder what it is so special at the nano scale? The answer to these questions is against the very notion that specific physical properties of a given material are the characteristic of the material itself and are irrespective of their size. This is no longer valid at the nano scale. There will be a drastic change in properties which are counter-intuitive and unbelievable. Yellow metal gold is no more yellow in any size or dimension. It may be purple red and orange. Cds under colloidal solution may yield a rainbow spectrum of colour. Ceramic oxides are not always brittle. Nanoceramics are ductile and malleable. What sounded as scientific fantasy or myth at that time, today has turned into reality.

Nano science and technology cut across disciplines without boundary. For physicists and chemists, nano means in the range of dimension of a few atoms and molecules. For biologists, the visualization is to the dimension of the size of a DNA or to the scale of a cell. Scientists have now devised techniques to prepare tiny nano-particles which have peculiar properties. Nano technology is now a portal opening onto a new world. This chapter will provide the readers a brief look at: nano scale and its visualization, i.e., what nano is; nano science and nanotechnology, i.e., why one should care about nano; surface to volume ratio and quantum confinement, i.e., what makes nano so important; nano cluster and nano fabrication, i.e., how we prepare nano.

Introduction

Dielectric materials are materials that do not conduct electricity, but can sustain an electric field. All dielectric materials are insulators. Insulators resist electric current when electric potential difference is applied. Dielectric materials can store electric energy. The molecules of a dielectric material are either polar or non-polar. Polar molecules are defined as molecules in which the centre of gravity of positive nuclei and negative electrons are not at the same point. Non-symmetrical molecules such as H2O, N2O and the like are polar molecules. Non-polar molecules are defined as molecules in which the centre of gravity of positive nuclei and negative electrons are at the same point. Symmetrical molecules such as H2, N2, O2 and the like are non-polar molecules.

An Overview of Dielectric Polarization

The phenomenon of slight relative shift of positive and negative electric charges in opposite directions within a dielectric, induced by an external stimuli like electric field, temperature, pressure etc is called dielectric polarization. Under the influence of an external applied electric field, non-polar molecules in dielectrics are polarized and become induced dipoles. This is depicted in Fig. 9.1.

Under the influence of an externally applied electric field, the polar molecules in a dielectric [polar molecules are itself electric dipoles having inherent dipole moments] become aligned in the direction of the applied electric field. The stronger the applied electric field, the greater is the aligning effect. This is depicted in Fig. 9.2.

The net effect of applying an electric field to either polar or non-polar dielectrics in the form of a slab is more or less the same. When an electric field is applied to a slab of dielectric in a direction perpendicular to one of its surfaces, negative charges are developed on one surface and positive charges developed on the opposite surface as shown in Fig. 9.3. Thus, the dielectric as a whole becomes polarized. These positive and negative charges are called induced charges as these charges are induced by applied electric field. These induced charges are not free but bound to the molecules lying just inside the surfaces.

Introduction

Newtonian mechanics describes the macro-world correctly. However, when it is applied to describe atomic particles of very high energies, or travelling at very high speeds, it gives predictions which disagree with experiments. The absoluteness of mass, length and time were discarded by the theory of relativity – it was theorized that they depend on reference frames. Though variations of mass, length and time with speed are too small to be observed in day-to-day life, their variations are clearly observable in the world of high energy atomic particles. In the special theory of relativity, gravitational effects and accelerated motions are beyond consideration and they are dealt with in the general theory of relativity. The theory of relativity is beyond the perception of common sense. It is our duty as students of science and engineering to change our minds to fit nature, not to change nature to fit the preconceptions in our minds.

Frame of Reference

To describe a physical event, we must establish a frame of reference. The motion of a body can only be described relative to some other bodies, observers, or a set of space– time coordinates. These are called frames of reference. If the coordinates are not chosen properly, the laws of physics may be unnecessarily more complicated.

Inertial frame of reference

The reference frame in which the law of inertia (Newton's first law) holds good is called the inertial frame of reference. All inertial frames are in a state of constant, rectilinear motion with respect to one another; they do not accelerate. They may be at rest. No experiment can be performed to know whether an isolated inertial frame of reference is in motion or at rest. In an inertial frame of reference, an object is observed to have no acceleration when no forces act on it. Furthermore, any frame moving with constant velocity with respect to an inertial frame must also be an inertial frame. Measurements in one inertial frame can be converted to measurements in another by the Galilean transformation in Newtonian physics and the Lorentz transformation in special relativity. Earth is approximately taken to be an inertial reference frame. No preferred inertial reference frame exists and all inertial reference frames are equivalent.

Non-inertial frame of reference

Opposite of the inertial frame of reference is the non-inertial frame of reference or accelerated frame of reference.

Introduction

In a homogeneous medium, light travels in a straight line – if an object is placed in its path, it should cast a sharp shadow of the object. However, the shadow of a fine wire caused by sunlight is not observable on a screen and the shadow of a straight edge as shown in Fig. 3.1 is not sharp when observed minutely. Some light bends into the shadow of the straight edge with the intensity of the light decreasing rapidly as we move into the shadow. The amount of bending depends upon the wavelength of the light and the size of the obstacle. In the region of the fuzzy boundary of the shadow of the straight edge, alternate dark and bright fringes of unequal spacings are seen when viewed by a well-focused microscope against a good contrast background. This optical phenomenon of bending of light around the obstacle is called diffraction and the fringes are called diffraction bands.

The diffraction phenomenon was first discovered by Italian scientist Grimaldi in the year 1665 and was correctly interpreted by Fresnel. He combined the Huygens’ principle of secondary wavelets with the principle of interference and concluded that diffraction occurs due to the interference of secondary wavelets originating from various points of the wavefront which are not obstructed by the obstacle. The diffraction phenomenon occurs only when a part of the advancing wavefront is obstructed by some sharp obstacle.

Classification of Diffraction

The diffraction phenomenon is broadly classified into the following two general classes.

i. Fresnel's diffraction

In this class of diffraction, the light source and the obstacle are separated by finite distance. Therefore, the wavefront incident on the obstacle is either cylindrical or spherical.

ii. Fraunhofer's diffraction

In this class of diffraction, the light source and the obstacle are separated by infinite distance. Therefore, the wavefront incident on the obstacle is plane. In the laboratory, infinite distance is created by placing a lens in between the light source and the obstacle so that the light source is at the focus of the lens.

Fresnel's Explanation of Rectilinear Propagation of Light

The greatest difficulty encountered by the supporters of the wave theory of light was how to explain the observed fact that light propagates in a straight line.