Decay of Spin-One Particle into Two Photons in Presence of Uniform External Magnetic Field

Yang's theorem states that an initial J=1 state cannot decay into two photons. Because of this result some reactions relating to elementary particles or atomic transitions can be ruled out. The theorem is not valid in the presence of background electric or magnetic fields. In this work we show that the decay of a J=1 particle into two photons is permitted by Bose symmetry and rotational invariance when the background of the decay process is not pure vacuum but contains an external classical magnetic/electric field. We also discuss constraints on these amplitudes from {\bf CP} invariance.Comment: Tex fil

Microstructure of Welded Joints

Welding is a process of joining smaller pieces and components to build up bigger components. The joining is mostly done by creating a molten pool at the joints. Creation of molten pool is not always desired and thus joining in the solid state is sometimes done. Figure 1 shows the classification of the welding processes. In this paper, discussion will pertain only to microstructures in welded joints made by creating a molten pool which solid-ifies to give the final joint. The heat input during melting and when the molten pool solidifies giving off the latent heat, and the temperature gradients created due to the heat sink effects by the solid metal surro-unding the molten pool affect the microstructure and introduces internal strain in the heat affected zone (HAZ) of the weld joint. There is a large number of books/handbooks which have been published in welding metallurgy in which microstructural aspects have been treated adequately. A few examples are given in References 1-5. The participants are requested to go through them

Failures of welded joints

For a failure at a welded joint, the important factors are : (a) presence of defects in the form of physical discontinuities, (b) a bad microstructure, and (c) residual stresses. A few important examples of bad microstructures leading to failures in stainless steel and alloy steels are presented in the paper. Apart from the references cited in the text, a list of general references are included to help the reader to get a better perspective on the conditions under which the failure of a weld joint takes place

Magnetic NDT for the characterization of industrial materials

Microstructure and residual stresses (RS) are important attributes of an industrial material which determines their various properties. In this paper, first an overview is given on the information available on the relationships of the microstructural parameters and RS on one hand and the parameters related to magnetic hysteresis loop and Barkhausen noise on the other The relevance of the relation¬ships for microstructurel characterisations and RS hr magnetic nondestructive evaluation techniques is discussed. Discussion is also made on the development and the use of sensors for the detec¬tion and assessment of magnetic flux to characterise microstruc¬tures. Two case studies on the use of the techniques in industrial materials and components are presented

Amorphization of Vortex Matter and Reentrant Peak Effect in YBa2_2Cu3_3O7−δ_{7-\delta}

The peak effect (PE) has been observed in a twinned crystal of YBa$_2$Cu$_3$O$_{7-\delta}$ for H$\parallel$c in the low field range, close to the zero field superconducting transition temperature (T$_c$(0)) . A sharp depinning transition succeeds the peak temperature T$_p$ of the PE. The PE phenomenon broadens and its internal structure smoothens out as the field is increased or decreased beyond the interval between 250 Oe and 1000 Oe. Moreover, the PE could not be observed above 10 kOe and below 20 Oe. The locus of the T$_p$(H) values shows a reentrant characteristic with a nose like feature located at T$_p$(H)/T$_c$(0)$\approx$0.99 and H$\approx$100 Oe (where the FLL constant a$_0$$\approx$penetration depth $\lambda$). The upper part of the PE curve (0.5 kOe$<$H$<$10 kOe) can be fitted to a melting scenario with the Lindemann number c$_L$$\approx$0.25. The vortex phase diagram near T$_c$(0) determined from the characteristic features of the PE in YBa$_2$Cu$_3$O$_{7-\delta}$(H$\parallel$c) bears close resemblance to that in the 2H-NbSe$_2$ system, in which a reentrant PE had been observed earlier.Comment: 15 pages and 7 figure

Non-Destructive Techniques for Microstructural and Structural Characterisation

The functional quality of a material or a component is influenced by (i) microstructure (ii) flaws (physical discontinuities like cracks, pores,delaminations etc.), and (iii) the presence of stress. Non-destructive evaluation (NDE) techniques are used to control and enhance the quality at various stages of the life cycle of a material or component. In this paper, various NDE techniques are discussed. The techniques mainly discussed are ultrasonics,radiography, X-ray techniques, and acoustic emission. The principles,procedures, advantages, and limitations of each technique, as well as applications in various materials are considered