Buğra Çetinkaya 030070094 9th Week Definitions

1) Coons Patch (Group: CAD)

Previous Answer

All the surface methods introduced thus far share one common philosophy; that is, they all require a finite number of data points to generate the respective surfaces. In contrast, a coons surface patch is a form of "transfinite interpolation" which indicates that the coons scheme interpolates to an infinite number of data points, that is, to all points of a cureve segment, to generate the surface. coons patch paticularly useful in blending four prescribed intersecting curves which form a closed boundary . (CAD/CAM Theory and Practice, İbrahim Zeid, McGrawHill, 1991. p304)

New/Better Answer

The more general surface forms—the sculptured surfaces—often involve interpolation across an intersecting mesh of curves that in effect comprise a rectangular grid of patches, each bounded by four boundary curves. A variety of techniques have been developed for interpolating between such boundary curves of which perhaps the simplest is the linearly blended coons patch. Figure 9.19 shows surface patches defined using this formulation. Linear blending has limitations, and higher order blending functions such as cubics are used for formulations that allow tangency continuity between adjacent patches. The term coons patch is used generically to include other patches which are blendings of arbitrary boundaries.

(Lalit Narayan Et Al., Computer Aided Design And Manufacturing, p. 20)

2) Endurance Limit (Group: Material Properties)

Previous Answer

For most material , the logarithmic plot of S-N curve is approximated by a straight line . for some alloys , including the ferrous metal , the logarithmic plotting method will generate an additional straight linet o account fort he endurance limit the semilogarihmic method is the most widely used in engineering applications.

The endurance limit or fatigue limit is not a constant adn varies with the stress ratio , R. For a given material , the endurance limit can be influenced by the type of cycling loading. Experimental data obtained in the laboratory Show that the endurance limit of material tested in uniaxial loading is lower than the endurance limit tested in reverse bending , provided that the two loading cases are sujected to the same stress ratio R.

(Fatigue and fracture mechanics of high risk parts ; Bahram Farahmand, George Bockrath, James Glassco ; pg : 33-34 ; 1997)

New/Better Answer

For most materials, the endurance limit or fatigue limit is not a constant and varies with the stress ratio, R. For a given material, the endurance limit can be influenced by the type of cyclic loading. Experimental data obtained in the laboratory show that the endurance limit of a material tested in uniaxial loading is lower than the endurance limit tested in reverse bending, provided that the two loading cases are subjected to the same stress ratio, R. Under axial loading, the stresses are uniform throughout the part, as compared to a non-uniform stress distribution where bending load is applied.

Other factors affecting the endurance limit are: degree of surface finish, heat treatment, stress concentration, and corrosive environment. Therefore, it is expected that the endurance limit will have a wide range of values depending on the conditions described above. Consider the case of high strength steel with little ductility exposed to a corrosive environment and containing stress concentration. In this case, it is possible to have an endurance limit value as low as 15% of its ultimate tensile strength. On the other hand, consider the case of stainless steel in the annealed condition and subjected to a noncorrosive environment. The endurance limit can be as high as 70% of its ultimate tensile strength.

Experimental data have shown that certain alloys, such as ferrous material, exhibit a clear fatigue limit. For the maximum applied stress below this limit, failure will not occur and, therefore, the material has infinite life.

(Bahram Farahmand,George Bockrath,James Glassco, Fatigue and Fracture Mechanics of High Risk Parts, p. 34)

3) Biofouling (Group: Pollution Control)
Previous Answer
Biofouling represents the attachment and metabolism of macroorganism or microorganisms like algae, bacteria and fungi on surfaces It plays a major role in systems, in which the surfaces are in contact with water or aqueous solutions. the hull of ship, heat exchangers or water tanks are exposed to biofouling deposits. Ion exchanges resins or membranes are futher examples, where biofouling poses severe problems. Methods of mechanical, physical, and chemical nature are used for controlling or preventing biofouling. the most common method is chemical one namely chorolination which is applied periodically or continiously.
(Fouling science and technology, L.F. Melo, p.233)

New/Better Answer

Biofouling in water supply wells and other types of wells is a common, recognized problem although it is not routinely monitored in water wells. Consequently, biofouling problems are usually discovered only when well and pump performance and water quality markedly deteriorate. The present paper is concerned with biofouling, problems which originate from the microbiological oxidation and subsequent precipitation of iron and manganese compounds in water wells and from associated biofilm formation. Several methods have been proposed for the analysis of biofouling problems in water wells but none appear to have gained wide utility for routine maintenance monitoring purposes. Bacteria responsible for biofouling phenomena typically occur in complex biofilm communities. These biofilms range in size and depth depending on a number of physical, chemical, and biological factors which are site-specific. Analysis of these biological communities is essential for understanding the underlying biological nature of the problem and for designing effective well maintenance and control procedures.

Sampling for biofouling can take the form of:

(i)                 pumping and collection of water flow

(ii)               collection of biofilm development on coupons and wellhead filtration devices.

(Peter Howsam, Water Wells: Monitoring, Maintenance, Rehabilitation ,p. 75)

4) Microvoid Coalescence (Group: Fracture Mechanism)

Previous Answer

Transgranular microvoid coalescence is the typical process by which slow stable tearing and unstable ductile fracture occur. The microvoids nucleate at various discontinuties. For steels and aliminium alloys the most important nucleation sites are large particles and dispersoids. In titanium alloys the voids nucleate at boundaries between alfa and beta phases.

(Janssen J., Zudema J., Wanhill R. J. H., Fracture Mechanics, , p.329-330 )

New/Better Answer

The fracture mechanism of microvoid coalescence is characterized by nucleation of microvoids and their growth and coalescence to form cracks. The nucleation of voids due to plastic straining is typically attributed to either particle cracking, interfacial decohesion between a particle and the surrounding matrix, or decohesion between grains. As the deformation continues, these voids enlarge, which consumes most of the energy required for fracture. The final step of the fracture mechanism is coalescence of the numerous voids in the direction of maximum shear stress and necking down of the ligaments between adjacent microvoids, resulting in the formation of localised cracks. The cracks propagate slowly because they only extend when additional stress is applied. The resultant fracture surface is of highly irregular appearance, and the site of the dimples visible on the fractured surfaces may vary widely because nucleation of microvoids depends on several different factors (including size, stress and strain levels, the amount of deformation and purity of the material, for instance). Microvoid coalescence is a typical failure mechanism of ductile fracture.

(Tomi Laurila Et All, Interfacial Compatibility in Microelectronics, p. 40)

5) Spot Drilling (in G-coding) (Group: Manufacturing Process)
-No previous answer

There are two major differences between a standard drill and a spot drill. One is in the tool design, the other in the way the tool is used. The design between the two drill types affects the flutes, the web thickness, the over-all length, and the tool point angle. The way how the two types of drill are used is a major consideration when programming. In the earlier section covering the tool selection, the details for using the spot drill for this example has already been covered, and the illustration is presented as a reference.

In a summary, the spot drill depth of each hole spotted will be Z-2.35, at the calculated XY location of each hole.

Apart from the depth of cut, another critical part of programming a spot drill is to calculate the XY coordinates of the six holes, which means a good calculator will be needed. Once these coordinates are established, they will also be used for the drilling and tapping of this bolt hole pattern. G82 fixed cycle has been used for the spot drill operation. This cycle is very similar to the G81 drilling cycle (see next operation), but it requires a dwell. The purpose of dwell is to pause at the bottom of the hole, before retracting to the clear position - the reason? In order to make sure the surface of the spot drilled hole is smooth, the tool has to rotate at least one spindle revolution, to make it clean. To achieve this goal, the following formula calculates the minimum dwell for fixed cycles - units are in milliseconds and there are no decimal places when milliseconds are used (1 sec = 1000 ms):

Minimum dwell (ms) = (60 x 1000) r/min

In the program, the spindle speed is 1693 r/min, so the minimum dwell in milliseconds is 35.44 ms. Although Mat is the minimum dwell mathematically, practically we have to look at another possible situation, and that is the status of the spindle override switch, located at the control panel. On the majority of CNC machines, this switch has the range of 50-120%. For the dwell calculation, the concern should be with the lowest setting. In order to guarantee at least one full revolution of the spot drill, even at 50% setting, the minimum dwell has to he doubled.

(Peter Smid, CNC Programming Techniques, p. 17)