060070103-Rifat Yılmaz-10th Week Definitions

Mean time to failure (MTTF)-GROUP: Quality management term

There is no old definition

MTTF represents the expected value of a system's time to failure random variable. It is used as a measure of reliability for non-repairable items such as bulb, microchips and many electronic circuits. Mathematically, MTTF can be defined as:

Thus, MTTF can be considered as the area under the curve represented by the reliability function, R(t), between zero and infinity. If the item under consideration is repairable, then the expression (3.31) represents mean time to first failure of the item. Figure 3.16 depicts the MTTF of an item.

For many reliability functions, it is difficult to evaluate the integral (3.31). One may have to use numerical approximation such as trapezium approach to find MTTF value.

Applications of MTTF

1.     MTTF is the average life of a non-repairable system.

2.     For a repairable system. MTTF represents the average time before the first failure.

3.     MTTF is one of the popular contractual reliability measures for non-repairable systems.

 (Dinesh Kumar,U. Dinesh Kumar,  Reliability And Six Sigma, pages 89-90-91)




Shot Peening(Previous) GROUP: Manufacturing method

Shot peening is a very common method for increasing the fatigue strength and har frequently been used for aero engile applications for mnay years . Aero engine components are characterized by high-strengh material , high cyclic loads and high service temperature. In the past shot peening has been used for safety-critical parts mainly as a measure to increase safety margins rather than for extending service lives. However , with increasing understanding of the mechanisms of shot peening and increasing predictive capabilites , the opportunities of increasing service live by shot peening or other surface treatments are expected to grow in the future.

(Shot peening ; Lothar Wagner ; pg: 13 , 2003)

Shot Peening(New)(Better)

Shot peening is a process intended to improve the fatigue strength of a workpiece. It produces a residual compressive stress at the surface, a few thousandths of an inch deep, from the effect of many small steel balls that are thrown against the workpiece. The shot balls are impelled by air pressure through a nozzle, or by centrifugal force from a spinning wheel. Sometimes, cut steel wire or glass beads are used instead of steel shot. Masking may be used if the effect is wanted on only part of the exposed portion of the workpiece. Fig. 257 in  illustrates an application of the process where the compressive stress provided by peening is used to modify the form of the workpiece.

(James G. Bralla, Handbook of Manufacturing Process, page 355)

Conversion Coating GROUP: Coating method

There is no old definition

These processes expose the base metal to certain chemicals that form thin, nonmetallic surface films. Similar reactions occur in nature; the oxidation of iron and aluminum are examples. Whereas rusting is progressively destructive of iron, formation of a thin Al₂O₃ coating on aluminum protects the base metal. It is the purpose of these chemical conversion treatments to accomplish the latter effect. The two main processes are phosphate and chromate coating.

                           

Phosphate coating transforms lite base metal surface into a protective phosphate film by exposure to solutions of certain phosphate sails (e.g., Zn. Mg, and Cu) together with dilute phosphoric acid (H₃PO₄). The coatings range in thickness from 0.0025 to 0.05 mm (0.0001-0.002 in). The most common base metals are zinc and steel, including galvanized steel. The phosphate coating serves as a useful preparation for painting in the automotive and heavy appliance industries.

Chromate coating converts the base metal into various forms of chromate films using aqueous solutions of chromic acid, chromate salts, and other chemicals. Metals treated by this method include aluminum, cadmium, copper, magnesium, and zinc (and their alloys). Immersion of the base pail is the common method of application. Chromate conversion coatings are somewhat thinner than phosphate, typically less than 0.0025 mm (0.0001 in). Usual reasons for chromate coating are (1) corrosion protection. (2) base for painting, and (3) decorative purposes Chromate coatings can be clear or colorful: available colors include olive drab, bronze, yellow, or bright blue.

(Mikell P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, page 679)

AFM (atomic force microscopy) GROUP: Microstructure determination device


There is no old definition

Atomic force microscopy (AFM) or as it is also called, scanning force microscopy (SFM) is based on the minute but detectable forces - of the order of nano Newtons -between a sharp tip and atoms on the surface. The tip is mounted on a flexible arm, called a cantilever, and is positioned at a subnanometre distance from the surface. If the sample is scanned under the tip in the x-y plane, it feels the attractive or repulsive force from the surface atoms and hence it is deflected in the z-direction. The deflection can be measured with a laser and photo detectors as indicated schematically in Fig. 4.29. Atomic force microscopy can be applied in two ways.

In the contact mode, the tip is within a few Angstroms of the surface, and the interaction between them is determined by the interactions between the individual atoms in the tip and on the surface.

The second mode of operation is the non-contact mode, in which the distance between tip and sample is much larger, between 2 and 30 nm. In this case one describes the forces in terms of the macroscopic interaction between bodies. Magnetic force microscopy, in which the magnetic domain structure of a solid can be imaged, is an example of the non-contact mode operation.

A third mode, which has recently become the standard for work on surfaces that are easily damaged, is in essence a hybrid between contact and non-contact modes, and is sometimes called the tapping mode. In this case the cantilever is brought into oscillation such that the tip just touches the surface at the maximum deflection towards the sample. When the oscillating cantilever approaches the maximum deflection, it starts to feel the surface and the oscillation becomes damped, which is detected by the electronics and used as the basis for monitoring the topography when the sample is scanned. In tapping mode, shear forces due to dragging the tip horizontally along the surface ("scratching") arc avoided, while forces in a perpendicular direction are greatly reduced. It has become the favored way of imaging small particles on planar substrates used as models for catalysts.

The image is always a convolution of the topography of the surface and that of the tip, and the one with the least steep features determines the image. Flat surfaces are scanned with the conventional pyramidal tips, which have a wide opening angle and arc relatively blunt. If surfaces contain features that are sharper than the tip, one images the tip shape rather than the surface topography.

(Ib Chorkendorff, J. W. Niemantsverdriet, Concepts of Modern Catalysis and Kinetics, pages 164-165)

CAQ(computer-aided quality assurance) GROUP: Computer integrated inspection method

 There is no old definition

The assurance of production quality is often also described with the term Computer Aided Qualify Assurance (CAQ). An extended meaning of CAQ includes also the control of product quality during the design phase, quality inspection in the goods receiving department, maintenance or repair of the shipped equipment or machines at the customer's and the processing of complaints. Here, one gets closer to the term Total Quality Management (TQM).

Modern solutions make it possible that an application system triggers tests (e.g., electrical measurements, surface tests, physical, chemical or microbiological inspections). If not all products are tested continuously and the computer tests are based on sampling, one achieves an economization or streamlining and, also the possibly desirable surprise effect. This keeps working staff alert, as it is not possible for them to anticipate any control activities.

(Rolf T. Wigand,Peter Mertens,Freimut Bodendorf,Wolfgang König,Arnold Picot,Matthias Schumann, Introduction to Business Information Systems, pages 113-114)