Ramazan Rıdvan SEKMEN, 030080083, 6th week words

1- Sandwich structure ( Group: Material)

Sandwich structure: One of the main problems and limitations of the skin material in stressed skin is its lack of rigidity. Skins often have to be made thicker than they might otherwise need to be because of a tendency to crumple under some types of load. A strip of paper illustrates this problem very well; one can pull it but not push it. A way of providing thin sheets with rigidity is to make a sandwich with one very thin sheet, a layer of very light but fairly rigid core material, and another very thin sheet, all bonded together with an appropriate adhesive. As with conventional semi monocoque structures, wooden construction led the way with sandwich structures. The famous and elegant de Havilland Mosquito of 1940 was built with plywood skins either side of a balsa-wood core. In today's major structures, a metal core of honeycomb-like cells is reconginsed as the most suitable core for metal-faced sandwich. (John Cutler,Jeremy Liber, Understanding aircraft structures , p. 14)

New and better explanation

Prior attempts at using extensive sandwich structure in aerospace applications have resulted in costly manufacturing and supportability problems. These problems have resulted due to the integration of materials and concepts that had a high affinity for moisture, resulting in corrosion and mechanical degradation of skin to core bond. In addition, challenges associated with cost of fabrication, joining, and load transfer through hard points further restricted their usage. Trade studies have traditionally shown that the sandwich configurations demonstrate large improvements in weight efficiency. Unfortunately, these large weight savings are not exploited due to the lack of engineering confidence in sandwich that has evolved from past experiences. This lack of confidence has established a sandwich structure design paradigm, which has prevented the full use of current-day structures and materials technology to be applied to sandwich designs. Through the development of composite materials, novel core concepts, tougher adhesives, textile preforms, and Z-Fiber^TM reinforcement, a potential opportunity exists to reduce weight and costs through unitized sandwich structure without the historical maintainability problems.

 Structural configurations have recently been developed that feature the use of sandwich in highly loaded mid- and aft-fuselage structure. Integration of sandwich in the wing and fuselage will significantly reduce weight and cost, but the ability to accomplish this largely depends on robust joining concepts. The joints in a primary sandwich structure are required to react higher pull-off loads than in conventionally

stiffened structure. The internal wing pressure induces joint pull-off loads that must be reacted by fewer bulkheads; therefore, the joint running loads increase. The use of sandwich panel construction with low-cost joining to produce a one-piece component with complex joint intersections is demonstrated in Fig. 2.38. This manufacturing demonstration component includes the outer mold-line skin, rib, fuel-floor, and bulkhead intersection. The component was fabricated with precured sandwich panels cobonded with a preimpregnated resin textile preform.

 ( Noor, A.K. (2000) Sandwich Concepts. Structures technology for future aerospace systems (pp.67,68). )

2-Contact Molding ( Group: Manufacturing)

Contact molding is the simplest method used for the manufacture of compositestructures.
This method requires a minimum of equipment and consequently a minumum of invesment. These advantages were the origin of the success of glass fiber composites in both industry and crafts.
Contact molding can be used to manufacture both small and large composite structures such as boat hulls, vehicle bodies, building panels,tanks, etc., where small production runs are required. Although the proportion of fibers may vary it nevertheless stays low, usually between 30% and 35% by weight of reinforcements.
(Jean-Marie Berthelot,Composite materials: mechanical behavior and structural analysis,p.54 )

New and better explanation

CONTACT MOLDING

Contact molding is a fiberglass lay-up in or over a male or female mold or form without pres-sure being applied to the side of the laminate away from the mold or form surface—that is, other than the contact pressure. Simple forms of this method were introduced in Chapter 7. For example, a sheet of wax paper was used as a mold or form for a flat surface. The resins and fiberglass reinforcing materials were laid up over it. While air bubbles were worked out, it was basically the weight of the resins and reinforcing materials that held the mixture against the wax paper until it cured.

Contact molding is done by hand lay-up, with the resins being applied to the mold surface by brush, rollers, and squeegees; by spray up, with the resins and reinforcing material being sprayed from a chopper gun; or by some combination of the two methods. For either method, the gel coat resin is usually sprayed onto the surface of the mold.

Advantages and Disadvantages

One advantage of contact molding over pres-sure molding is that molds and other equipment are generally less expensive. Also, relatively inexperi-enced workers can do hand lay-up work. Another advantage is that, with contact molding, large fi-berglass structures can be molded that would be impractical to do using pressure molding.

A main disadvantage of contact molding is that the back side of the laminate will not be as smooth and fair as the front side because there is no way to mold or form the back side during the cure of the laminate. Other disadvantages are that contact molding is slower, involves hand labor, and is some-what less accurate when small parts with compli-cated shapes are required.

( Wiley, J.(1988). CONTACT MOLDING.The fiberglass repair and construction handbook (p.95)

3-Abrasive Wear ( Group: Material)

Abrasive wear is defined as wear due to hard particles or hard protuberances forced against and moving along a solid surface. This form of wear in metals is most frequently caused by non-metallic materials, but metallic particles can also cause abrasion. Generally, a material is seriously abraded or scrached only by a particle harder than itself.

Davis R.J., Surface Engineering for Corrosion and Wear Resistance, p.56

New and better explanation

Abrasive wear occurs whenever a solid object is loaded against particles of a material that have equal or greater hardness. A common example of this problem is the wear of shovels on earth-moving machinery. The extent of abrasive wear is far greater than may be realized. Any material, even if the bulk of it is very soft, may cause abrasive wear if hard particles are present. For example, an organic material, such as sugarcane, is associated with abrasive wear of cane cutters and shredders because of the small fraction of silica present in the plant fibres [3]. A major difficulty in the prevention and control of abrasive wear is that the term 'abrasive wear' does not precisely describe the wear mechanisms involved. There are, in fact, almost always several different mechanisms of wear acting in concert, all of which have different characteristics. The mechanisms of abrasive wear are described next, followed by a review of the various methods of their control.

Mechanisms of Abrasive Wear

It was originally thought that abrasive wear by grits or hard asperities closely resembled cutting by a series of machine tools or a file. I lowever, microscopic examination has revealed that the cutting process is only approximated by the sharpest of grits and many other more indirect mechanisms are involved. The particles or grits may remove material by microcutting, microfracture, pull-out of individual grains 14] or accelerated fatigue by repeated deformations as illustrated in Figure 11.1.

The first mechanism illustrated in Figure 11.1a, cutting, represents the classic model where a sharp grit or hard asperity cuts the softer surface. The material that is cut is removed as wear debris. When the abraded material is brittle, e.g., ceramic, fracture of the worn surface may occur (Figure 11.1b). In this instance wear debris is the result of crack convergence. When a ductile material is abraded by a blunt grit, then cutting is unlikely and the worn surface is repeatedly deformed (Figure 11.1c). In this case wear debris is the result of metal fatigue. The last mechanism illustrated (Figure 11.1d) represents grain detachment or grain pull-out. This mechanism applies mainly to ceramics where the boundary between grains is relatively weak. In this mechanism the entire grain is lost as wear debris.

( Stachowiak, G. W., Batchelor, A. W. (2005). Abrasive Wear. Engineering tribology (pp.501,502). )

4-Bead Weld ( Group: Manufacturing )

If you weld at a rapid pace, the penetration depth and bead with decreases, and the bead is dome shaped. If the speed is increased even faster, undercutting-producing a weld surface level lower than base metal- can ocur. Welding at too low a speed can cause burn- through holes. Ordinarily, welding speed is determined by base metal thickness and/or voltage of the welding machine.

(James E. Duffy, Robert Scharff, Auto Body Repair Technology, p. 192 )

New and better explanation

Bead welding is a method of using welding to cover the surface with a wear-resistant, heat-resistant or corrosion-resistant coating of a certain metal. The metallurgical process and thermo-physical process of bead welding are basically the same as the common welding process, but its purpose is to obtain the special properties of the surface. Therefore, it is not exactly the same as welding.

The commonly used bead welding methods include general head welding, arc bead welding. submerged arc bead welding, plasma bead weldin,. automatic bead welding protected by carbon dioxide gas and so on, as shown in Figure 13.6.

In common bead weld, oxygen-acetylene is used as the heat source. Because its flame temperature is low, generally, a uniform layer less than 1 mm thick can be obtained, which is suitable for smaller part surface protection.

The arc bead welding is of high production efficiency. However, because the protective effect of the arc zone is poor, sometimes pores or cracks can easily form on the surface. Spraying water vapor and carbon dioxide on the protected area may improve the quality of the bead welding coating.

Because the plasma are bead welding is of a high temperature, bead welding material is refractory. In addition, it has a very high speed, and high bead speed, but a low dilution rate so it has been widely used.

(  Wen, S.,Huang, P.(2012).Bead welding. Principles of Tribology (pp. 331,332). )

5-Discontinuous Chip(in Metal Machining) ( Group: Material)

When relatively brittle materials (e.g. cast irons) are machined at low cutting speeds, the chips often form into separate segments ( sometimes the segments are loosely attached). This tends to impart an irregular texture to the machined surface. High tool-chip friction and large feed and depth of cut promote the formation of this chip type.

(Fundamentals of Modern Manufacturing: Materials, Processes, and Systems, Mikell P. Groover, p.491)

New and better explanation

When brittle materials like cast iron are cut, the deformed material gets fractured very easily and thus the chip produced is in the form of discontinuous segments as shown in Fig. 2.5. In this type the deformed material instead of flowing continuously gets ruptured periodically. Discontinuous chips are easier from the view point of chip disposal. However, the cutting force becomes unstable with the variation coinciding with the fracturing cycle as shown in Fig. 2.6. Also they generally provide better surface finish. However, in case of ductile materials they cause poor surface finish and low tool life. Higher depths of cut (large chip thickness), low cutting speeds and small rake angles are likely to produce discontinuous chips.

( Rao, P.N. (2000). Discontinuous Chip. Manufacturing technology: metal cutting and machine tools (pp. 8,9). )