Elif Temiz, 030070195, 5th Week Definitons-Part2

2-SECTIONING METHOD (New)(Better) (Destructive Methods for Measuring Residual Stress)

The sectioning method combines several other methods to evaluate residual stresses within a given specimen. By choosing the combination and sequence of methods, a higly specific measurement can be made tailored to particular specimen. The sectioning method typically involves attachment of strain gauges and cutting out parts of the specimen. The strain reliefs measured as the various parts are progressively cut out provide a rich source of data from whicih both the size and location of the original residual stresses can be determined.

Figure 2. shows an example of the sectioning method, where a sequence of cuts was made to evaluate the residual stresses a welded pipe.

(Proulx, T.,Experimental and Applied Mechanics: Proceedings of the 2010 Annual Conference on Experimental and Applied Mechanics, p. 222)

Sectioning Method (Previous)

Sectioning method is one of useful measurement tecnique for 3D profile measurement. It is insensitive to the surface optical property of 3D objects, has scarcely any demand on surrounding. So it can hold good to complex profile 3D object, which other techniques are invalidation. We design a shape measurement system which has 3600 measurement range and multi-resolution measurement property on the blade of aviation engine measurement demand. It can measure whole profile and the entering and ending edge partial detail of the blade with high precision at the same time.

(Xiong C., Liu H., Huang Y., Xiong Y., Intelligent Robotics And Applications,2008, p.762)

3-CONTOUR METHOD (New)(Better) (Destructive Methods for Measuring Residual Stress)

The contour method is a newly developed technique for making full-filled residual stress measurements, typically within the cross-section of a prismatic specimen. It consists of cutting through the specimen cross-section using a wire EDM, and measuring the surface height profiles of the cut surfaces using a coordinate measuring machine or a laser profilometer. Figure 6 illustrates the process. The original residual stresses shown in Figure 6(a) are released by the cut and cause the material surface to deform (pull in for tensile stresses, bulge out for compressive stresses), as shown in Figure 6(b). The originally existing residual stresses normal to the cut can be evaluated from finite element calculations by determining the stresses required to return the deformed surface shape to a flat plane. In practice, to avoid any effects of measurement asymmetry, the surfaces on both sides of the cut are measured and the average surface height map is used.

The contour method is remarkable because it gives a 2-dimensional map of the residual stress distribution over the entire material cross-section. In comparison, other techniques such as layer removal and hole-drilling give one-dimensional profiles, while excision gives the residual stress only at one point. The contour method provides measurements of the stresses normal to the cut surface. If desired, stresses in other directions can also be determined by making additional cuts, typically perpendicular to initial cut.

 

(Proulx, T.,Experimental and Applied Mechanics: Proceedings of the 2010 Annual Conference on Experimental and Applied Mechanics, p. 224-225)

Contour Method (15:02 18.03.2011) (Previous)


In the approach of Contour method, the analysed specimen is carefully cut in two along a plane using a wire EDM machine. Residual stresses relax as free surface is created by the cut. After cutting , the contours of the two opposing surfaces created by the cut are measured using a CMM machine or by laser scanning. Assuming that the contours are caused by elastic relaxation of the residual stresses, a straightforward finite element calculation, in which the opposite averaged contour is applied as boundary conditions to the FE model, permits to reveal the original residual stresses component (sigma x) normal to the cut surface.


(E. E. Gdoutos, Experimental Analysis of Nano and Engineering Materials and Structures, page 635)


 

4-SEMI-CRYSTALLINE POLYMERS (New) (Material)

Symmetrical linear polymers such as polyethylene (PE) are semicrystalline. However the crystalline structure is quite different to that of metals and also   there is always a significant portion of amorphous material. The long molecular chains fold back on themselves to produce lamella crystals of the order of 10nm thick, as shown schematically in Fig. 10.11, which are joined together by regions of amorphous polymer. High density polyethylene is 70-80% crystalline while low density polyethylene is only 40-50% crystalline.


 (Cotterell, B., Fracture and life,p.316)

Semicrystalline polymers exhibit a melting transition temperature (T_m), a glass transition temperature (T_g), and crystalline order, as shown by x-ray and electron scattering. The fraction of the crystalline material is determined by x-ray diffraction, heat of fusion, and density measurements. Major structural units of semicrystalline polymers are the platelet-like crystalline, or lamellae. The dominant feature of melt crystallized specimens is the spherullite. The formation of polymer crystals and the spherulitic morphology in bulk polymers has been fully described by Ketih and Padden, Ward, Bassett, and many others. Bassets points out that knowledge of morphology is an essential part of the development of polymer materials and a complete understanding of their structure-property relationships.
 

(Linda C. Sawyer, David T. Grubb, Gregory Frederick Meyers, Polymer microscopy,p.5)

Semi-crystalline Polymers: (18.03.2011-13:44) (Old)

 Unlike amorphous polymers, semicrystalline polymers have some regions where the polymer chains are arranged in specific spatial patterns relative to the other polymer chains within their polymer matrix. Segments of polymer backbones, from either adjacent polymer molecules or within the same polymer molecule (polymer chain folded back on itself) are aligned in an orderly manner with the exact same spatial arrangement and inter-or intramolecular distance from one set group of atoms to the next. Semicrystalline polymers contain both amorphous and crystalline regions within the same polymer matrix.

 (Giles H.F. et al, Extrusion: The Definitive Processing Guide and Handbook, p.181)

Semi-crystalline Polymers (Old) (Better)

Long polymer chains consisting of identical monomeric units are, in principle, capable of being organized into crystalline arrays. Usually the chains are parallel bundles and the unit cell is based in some elementary way on the monomeric repeat unit. However, because of their great chain lengths it is kinetically difficult for polymers to form large crystals or to crystallize completely. In the case of quiescent crystallization from the melt the common morphology involves very thin lamellar crystals in which the crystallizing chains fold back and forth across the growth face. A given chain may be incorporated into several lamellae where the latter are organized in ribbon-like sheaves. It is inevitable that a considerable fraction of chain units will be constrained from being laid down on the growth faces. An appreciable fraction of uncrystallized material results. The local organization is that of stacked lamellae separated by amorphous layers. Thus crystallizable polymers are typically semi-crystalline two-phase systems. The degree of crystallinity, i.e., the volume of the crystal phase relative to the specimen volume can vary according to the crystallization conditions. Slow cooling versus rapid quenching, annealing etc. are typical variables. Higher degrees of crystallinity tend to be accommodated by thicker crystal lamellae with a relatively minor role for the amorphous interlayer thickness.

(Polymer Dynamics and Relaxation, By Richard Boyd University of Utah, By Grant Smith University of Utah)

5-X-RAY DIFFRACTION (XRD)(New) (Nondestructive Testing Method) (Better)

X-rays have wavelengths in the Angstrom range, and are sufficiently energetic not only to penetrate solids but also to probe their internal structure. XRD is used to identify bulk phases, to monitor the kinetics of bulk transformations, and to estimate particle sizes. An attractive feature is that the technique can be applied in situ. We will first discuss XRD as conducted in the laboratory, and then describe some of the newer applications of XRD as are available by using synchrotron radiation.

A conventional X-ray source consists of target that is bombarded with high-energy electrons. The emitted X-rays arise from two processes. Electrons slowed down by the target emit a continuous background spectrum of Bremsstrahlung. Superimposed on this are characteristic, narrow line; the Cu Kx line , with an energy of 8.04keV and a wavelength of 0.154 nm, arises because from the L-shell under emission of an X-ray quantum. Kβ radiation is emitted when the K-hole is filled from the M-shell, and so on. This process, which is called X-ray fluoresrence, is the basis for X-ray sources and is also encountered in electron microscopy, EXAFS, and XPS.

X-ray diffraction is the elastic scattering of X-ray photons by atoms in a periodic lattice. The scattering monochromatic X-rays that are in phase procude constructive interference. Figure 6.1 illustrates how diffraction of X-rays by crystal planes allows one to derive lattice spacings by using the Bragg relationship:

nλ = 2d sinθ;   n = 1,2,...

Where:

λ  is the wavelength of X-rays;

d is the distance between two lattice planes;

θ  is the angle between the incoming X-rays and the normal to the reflecting lattice plane;

n  is the integer called the order of the reflection.

 ( Niemantsverdriet, J. W., Spectroscopy in Catalysis: An Introduction, pp.148-149)

(XRD) X-Ray Diffraction: (13:54 - 18.03.2011) (Previous)

XRD involves monitoring the diffraction of X-rays after they interact with the sample. It is a crystallographic technique used for identifying and quantifying various crystalline phases present in solid materials and powders. In XRD, the crystal structure can be determined as well as the size of grains and nanoparticles. When X-rays are directed at a regular crystalline sample, o proportion of them are diffracted to produce a pattern. From such a pattern the crystal phases can be identified by comparison to those of internationally recognized databases (such as International Center of Diffraction Data - ICDD) that contain reference patterns. In sensing applications, XRD is generally used to correlate the properties of a material to its sensing performance.

XRD is one of the most utilized techniques for determining the structure of inorganic and organic materials. It is also widely used for studying nano-structured thin films and nanoparticles. However, the materials must have ordered structure, and it cannot be used directly to study amorphous materials. Another inherent limitation of XRD is that mixtures of phases that have kow symmetry are difficult to differentatiate between because of the larger number of diffraction peaks. Furthermore, organic materials such as polymers are never completely crystalline, therefore XRD is primarily used to determine their crystallinity.

In crystallography, the solid to be characterized by XRD has a space lattice with an ordered three-dimensional distribution of atoms. These atoms form a series of parallel planes seperated by a distance d, which varies according to the nature of the material. For any crystal, planes have their own specific d-spacing. When a monochromatic X-ray beam with wavelength λ is irradiated onto a crystalline material with spacing d, at an angle Θ, diffraction occurs only when the distance traveled by the rays reflected from successive planes differs by an integer number, n of wavelength to produce constructive interference. Such constructive interference patterns only occur when incident angles fulfill the Bragg condition such that:

n.λ = 2d. sinΘ

By varying the angle Θ, the Bragg Law condition is satisfied for different d- spacings in polycrystalline materials.

(Nanotechnology-enabled sensors, Kourosh Kalantar-zadeh,Benjamin Fry, p.237)