Mechanical Engineering, Mechanical Engineer, Material Engineer, Material Engineering,Manucaturing Engineering

Magnetically Soft Materials

nospam@example.com (mustafa caykoylu) — Sun, 26 Nov 2006 17:33:51 +0200

MAGNETIC MATERIALS are broadly classified into two groups with either hard or soft magnetic characteristics. Hard magnetic materials are characterized by retaining a large amount of residual magnetism after exposure to a strong magnetic field. These materials typically have coercive force, Hc, values of several hundred to several thousand oersteds (Oe) and are considered to be permanent magnets. The coercive force is a measure of the magnetizing force required to reduce the magnetic induction to zero after the material has been magnetized. In contrast, soft magnetic materials become magnetized by relatively low-strength magnetic fields, and when the applied field is removed, they return to a state of relatively low residual magnetism. Soft magnetic materials typically exhibit coercive force values of approximately 400 A · m-1 (5 Oe) to as low as 0.16 A · m-1 (0.002 Oe). Soft magnetic behavior is essential in any application involving changing electromagnetic induction such as solenoids, relays, motors, generators, transformers, magnetic shielding, and so on. Important characteristics of magnetically soft materials also include:

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Tin in Coatings

nospam@example.com (mustafa caykoylu) — Sun, 26 Nov 2006 17:04:06 +0200

Tinplate. The largest single application of tin worldwide is in the manufacture of tinplate (steel sheet coated with tin), which accounts for about 40% of total world tin consumption. Since 1940, the traditional hot dip method of making tinplate has been largely replaced by electrodeposition of tin on continuous strips of rolled steel. Electrolytic tinplate can be produced with either equal or unequal amounts of tin on the two surfaces of the steel base metal. Nominal coating thicknesses for equally coated tinplate range from 0.38 to 1.5 μm (15 to 60 μin.) on each surface. The thicker coating on tinplate with unequal coatings (differential tinplate) rarely exceeds 2.0 μm (80 μin.). Tinplate is produced in thicknesses from 0.15 to 0.60 mm (0.006 to 0.024 in.).

Over 90% of world production of tinplate is used for containers (tin cans). Traditional tinplate cans are made of three pieces of tin-coated steel: two ends and a body with a soldered side seam. Innovations in can manufacture have produced two-piece cans made by drawing and ironing. Tinplate cans find their most important use in the packaging of food products, beer, and soft drinks, but they are also used for holding paint, motor oil, disinfectants, detergents, and polishes. Other applications of tinplate include signs, filters, batteries, toys, and gaskets, and containers for pharmaceuticals, cosmetics, fuels, tobacco, and numerous other commodities.
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Tin and Tin Alloys

nospam@example.com (mustafa caykoylu) — Sun, 26 Nov 2006 17:02:36 +0200

Introduction TIN was one of the first metals known to man. Throughout ancient history, various cultures recognized the virtues of tin in coatings, alloys, and compounds, and the use of the metal increased with advancing technology. Today, tin is an important metal in industry even though the annual tonnage used is much smaller than those of many other metals. One reason for the small tonnage is that, in most applications, only very small amounts of tin are used at a time.

Tin Production and Consumption Tin is produced from both primary and secondary sources. Secondary tin is produced from recycled materials (see the article "Recycling of Nonferrous Alloys" in this Volume). Figure 1 shows the consumption of primary and secondary tin in the United States during recent years. Figure 2 shows 1988 data for the relative consumption of tin in the United States by application.

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Gas-Metal Arc Welding

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 23:36:32 +0200

GAS-METAL ARC WELDING (GMAW) is an arc welding process that joins metals together by heating them with an electric arc that is established between a consumable electrode (wire) and the workpiece. An externally supplied gas or gas mixture acts to shield the arc and molten weld pool.

Although the basic GMAW concept was introduced in the 1920s, it was not commercially available until 1948. At first, it was considered to be fundamentally a high-current-density, small-diameter, bare-metal electrode process using an inert

gas for arc shielding. Its primary application was aluminum welding. As a result, it became known as metal-inert gas (MIG) welding, which is still common nomenclature. Subsequent process developments included operation at low current densities and pulsed direct current, application to a broader range of materials, and the use of reactive gases (particularly carbon dioxide) and gas mixtures. The latter development, in which both inert and reactive gases are used, led to the formal acceptance of the term gas-metal arc welding.

The GMAW process can be operated in semi-automatic and automatic modes. All commercially important metals, such as carbon steel, high-strength low-alloy steel, stainless steel, aluminum, copper, and nickel alloys can be welded in all positions by this process if appropriate shielding gases, electrodes, and welding parameters are chosen. Advantages. The applications of the process are dictated by its advantages, the most important of which are:
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Arc Physics of Gas-Tungsten Arc Welding

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 23:33:14 +0200

THE GAS-TUNGSTEN ARC WELDING (GTAW) process is performed using a welding arc between a nonconsumable tungsten-base electrode and the workpieces to be joined. C.E. Jackson defined a welding arc as "a sustained electrical discharge through a high-temperature conducting plasma producing sufficient thermal energy so as to be useful for the joining of metals by fusion." This definition is a good foundation for the discussion that follows.

The physics of GTAW are fundamental to all arc processes and are more straightforward, because the complications of materials (for example, filler and flux) transferred through and interacting with the arc can be avoided. Geometrically, the arc discharge in GTAW is between a rod-shaped tungsten electrode and a planar-shaped electrode, that is, the workpiece. Pure tungsten electrodes are less expensive and, possibly, more environmentally compatible than those with rare earth or other oxide additions. They are used for lower-specification welds, where tungsten contamination that is caused by the molten electrode surface can be tolerated. They are also used for alternating current (ac) welding of aluminum, copper, magnesium, and thin sections of low-alloy and stainless steels.

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Heat Flow in Fusion Welding

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 23:30:01 +0200

DURING FUSION WELDING, the thermal cycles produced by the moving heat source cause physical state changes, metallurgical phase transformation, and transient thermal stress and metal movement. After welding is completed, the finished product may contain physical discontinuities that are due to excessively rapid solidification, or adverse microstructures that are due to inappropriate cooling, or residual stress and distortion that are due to the existence of incompatible plastic strains.

In order to analyze these problems, this article presents an analysis of welding heat flow, focusing on the heat flow in the fusion welding process. The primary objective of welding heat flow modeling is to provide a mathematical tool for thermal data analysis, design iterations, or the systematic investigation of the thermal characteristics of any welding parameters. Exact comparisons with experimental measurements may not be feasible, unless some calibration through the experimental verification procedure is conducted.

Welding Thermal Process. A physical model of the welding system is shown in Fig. 1. The welding heat source moves at a constant speed along a straight path. The end result, after either initiating or terminating the heat source, is the formation of a transient thermal state in the weldment. At some point after heat-source initiation but before termination, the temperature distribution is stationary, or in thermal equilibrium, with respect to the moving coordinates. The origin of the moving coordinates coincides with the center of the heat source. The intense welding heat melts the metal and forms a molten pool.

Some of the heat is conducted into the base metal and some is lost from either the arc column or the metal surface to the environment surrounding the plate. Three metallurgical zones are formed in the plate upon completion of the thermal cycle: the weld-metal (WM) zone, the heated-affected zone (HAZ), and the base-metal (BM) zone. The peak temperature and the subsequent cooling rates determine the HAZ structures, whereas the thermal gradients, the solidification rates, and the cooling rates at the liquid-solid pool boundary determine the solidification structure of the WM zone. The size and flow direction of the pool determines the amount of dilution and weld penetration. The material response in the temperature range near melting temperatures is primarily responsible for the metallurgical changes.
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Energy Sources Used for Fusion Welding

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 23:27:19 +0200

WELDING AND JOINING processes are essential for the development of virtually every manufactured product. However, these processes often appear to consume greater fractions of the product cost and to create more of the production difficulties than might be expected. There are a number of reasons that explain this situation.

First, welding and joining are multifaceted, both in terms of process variations (such as fastening, adhesive bonding, soldering, brazing, arc welding, diffusion bonding, and resistance welding) and in the disciplines needed for problem solving (such as mechanics, materials science, physics, chemistry, and electronics). An engineer with unusually broad and deep training is required to bring these disciplines together and to apply them effectively to a variety of processes.

Second, welding or joining difficulties usually occur far into the manufacturing process, where the relative value of scrapped parts is high.

Third, a very large percentage of product failures occur at joints because they are usually located at the highest stress points of an assembly and are therefore the weakest parts of that assembly. Careful attention to the joining processes can produce great rewards in manufacturing economy and product reliability. The Section "Fusion Welding Processes" in this Volume provides details about equipment and systems for the major fusion welding processes.

The purpose of this Section of the Volume is to discuss the fundamentals of fusion welding processes, with an emphasis on the underlying scientific principles. Because there are many fusion welding processes, one of the greatest difficulties for the manufacturing engineer is to determine which process will produce acceptable properties at the lowest cost. There are no simple answers. Any change in the part geometry, material, value of the end product, or size of the production run, as well as the availability of joining equipment, can influence the choice of joining method. For small lots of complex parts, fastening may be preferable to welding, whereas for long production runs, welds can be stronger and less expensive.
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Fatigue Failure in Metals

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 22:24:28 +0200

FATIGUE is the progressive, localized, and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that have maximum values less than (and often much less than) the static yield strength of the material. Fatigue may culminate into cracks and cause fracture after a sufficient number of fluctuations. Fatigue damage is caused by the simultaneous action of cyclic stress, tensile stress, and plastic strain. If any one of these three is not present, a fatigue crack will not initiate and propagate. The plastic strain resulting from cyclic stress initiates the crack; the tensile stress promotes crack growth (propagation). Although compressive stresses will not cause fatigue, compressive loads may result in local tensile stresses. Microscopic plastic strains also can be present at low levels of stress where the strain might otherwise appear to be totally elastic.

During fatigue failure in a metal free of cracklike flaws, microcracks form, coalesce, or grow to macrocracks that propagate until the fracture toughness of the material is exceeded and final fracture occurs. Under usual loading conditions, fatigue cracks initiate near or at singularities that lie on or just below the surface, such as scratches, sharp changes in cross section, pits, inclusions, or embrittled grain boundaries.
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Alloy Design for Fatigue and Fracture

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 22:21:57 +0200

FRACTURE MECHANICS is a very powerful tool for predicting the loads and crack lengths at which fracture can occur. Broken down to its essential form, it allows an engineer to predict the onset of fracture if the following information is available:

· Load/crack geometry (usually available from NDI)

· A formula for the so-called stress-intensity parameter, K, for the load/crack geometry of interest (the result of sophisticated mathematical analysis but available in handbooks, such as Ref 1)

· The numerical value of the fracture toughness (generally denoted KIc), which is determined experimentally through well-defined procedures (Ref 2)

· For fatigue crack propagation, knowledge of the crack growth rate as a function of the stress-intensity parameter With this information and the use of fracture mechanic methods (as briefly described in the next section for monotonic and fatigue fracture), it is possible to compute the life without any consideration of those processes that determine the values of the fracture toughness or the crack growth rates. Such procedures as are outlined below are obviously of great value in carrying out engineering calculations for existing or contemplated components.

However, in some instances the properties are insufficient to meet the engineering requirements. In such cases it is necessary to consider alternate materials or, in some instances, to develop alternate heat treatments and compositions that yield properties that allow the requirements to be met.
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Fatigue Properties in Engineering

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 22:19:17 +0200

FATIGUE PROPERTIES are an integral part of materials comparison activities and offer information for structural life
estimation in many engineering applications. They are a critical element in the path relating the materials of construction
to the components and must take into account as many influences as possible to reflect the actual product situation. In
application, fatigue is a detail analysis, trying to assess what will occur at a particular location of a component or
assembly under cyclic loading.

The topic of fatigue properties is very broad and is typically based on testing coupons. To be applicable, determined
properties must support one of the fatigue design philosophies that may be applied to the part. In this article the three
general approaches to fatigue design are stated, with discussion of their respective attributes, and their individual property
requirements are described. The intent here is not to present a comprehensive catalog of properties; that would take many
volumes this size. Instead, the purpose is to provide the basic insights necessary to examine those properties that can be
found, review some of the common presentation formats, and recognize their inherent characteristics. It is important to
review information critically for any use, to know when a direct "apples to apples" comparison can be made, and
potentially to know how to manipulate some of the data to put it on equal footing with information gathered from diverse
sources. The susceptibility of mechanical properties to variation through microstructural manipulation and structural
consideration can be substantial.
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Fracture and Structure

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 22:17:23 +0200

IT IS DIFFICULT to identify exactly when the problems of failure of structural and mechanical equipment became of critical importance; however, it is clear that failures that cause loss of life have occurred for over 100 years (Ref 1, 2). Throughout the 1800s bridges fell and pressure vessels blew up, and in the late 1800s railroad accidents in the United Kingdom were continually reported as "The most serious railroad accident of the week"! Those in the United States also have heard the hair-raising stories of the Liberty ships built during World War II. Of 4694 ships considered in the final investigation, 24 sustained complete fracture of the strength deck, and 12 ships were either lost or broke in two. In this case, the need for tougher structural steel was even more critical because welded construction was used in shipbuilding instead of riveted plate. In riveted plate construction, a running crack must reinitiate every time it runs out of a plate. In contrast, a continuous path is available for brittle cracking in a welded structure, which is why low notch toughness is a more critical factor for long brittle cracks in welded ships. Similar long brittle cracks are less likely or rare in riveted ships, which were predominant prior to welded construction.

Nonetheless, even riveted ships have provided historical examples of long brittle fracture due, in part, from low toughness. In early 1995, for example, the material world was given the answer to an old question, "What was the ultimate cause of the sinking of the Titanic?" True, the ship hit an iceberg, but it now seems clear that because of brittle steel, "high in sulfur content even for its time" (Ref 3), an impact which would clearly have caused damage, perhaps would not have resulted in the ultimate separation of the Titanic in two pieces where it was found in 1985 by oceanographer Bob Ballard. During the undersea survey of the sunken vessel with Soviet Mir submersibles, a small piece of plate was retrieved from 12,612 feet below the ocean's surface. Examination by spectroscopy revealed a high sulfur content, and a Charpy impact test revealed the very brittle nature of the steel (Ref 3). However, there was some concern that the high sulfur content was, in some way, the result of eighty years on the ocean floor at 6,000 psi pressures.

Subsequently, the son of a 1911 shipyard worker remembered a rivet hole plug which his father had saved as a memento of his work on the Titanic. Analysis of the plug revealed the same level of sulfur exibited by the plate from the ocean floor. In the years following the loss of the Titanic metallurgists have become well aware of the detrimental effect of high sulfur content on fracture.
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What is Fatigue ?

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 22:14:06 +0200

Fatigueis a technical term that elicits a degree of curiosity. When citizens read or hear in their media of another fatigue failure, they wonder whether this has something to do with getting tired or "fatigued" as they know it. Such is not the case.

One way to explain fatigue is to refer to the ASTM standard definitions on fatigue, contained in ASTM E 1150. It is difficult, if not impossible, to carry on intelligent conversations if discussions on fatigue do not use a set of standard definitions such as E 1150. Within E 1150, there are over 75 terms defined, including the term fatigue: "fatigue (Note 1): the process of progressive localized permanent structural change occurring in a material subjected to conditions that produce fluctuating stresses and strains at some point or points and that may culminate in cracks or complete fracture after a sufficient number of fluctuations (Note 2). Note 1--In glass technology static tests of considerable duration are called `static fatigue' tests, a type of test generally designated as stress-rupture. Note 2--Fluctuations may occur both in load and with time (frequency) as in the case of `random vibration'." (Ref 2). The words in italics (emphasis added) are viewed as key words in the definition. These words are important perspectives on the phenomenon of fatigue:
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Industrial Significance of Fatigue Problems

nospam@example.com (mustafa caykoylu) — Fri, 03 Nov 2006 22:12:20 +0200

THE DISCOVERY of fatigue occurred in the 1800s when several investigators in Europe observed that bridge and railroad components were cracking when subjected to repeated loading. As the century progressed and the use of metals expanded with the increasing use of machines, more and more failures of components subjected to repeated loads were recorded. By the mid 1800s A. Wohler (Ref 1) had proposed a method by which the failure of components from repeated loads could be mitigated, and in some cases eliminated. This method resulted in the stress-life response diagram approach and the component test model approach to fatigue design.

Undoubtedly, earlier failures from repeated loads had resulted in failures of components such as clay pipes, concrete structures, and wood structures, but the requirement for more machines made from metallic components in the late 1800s stimulated the need to develop design procedures that would prevent failures from repeated loads of all types of equipment. This activity was intensive from the mid-1800s and is still underway today. Even though much progress has been made, developing design procedures to prevent failure from the application of repeated loads is still a daunting task. It involves the interplay of several fields of knowledge, namely materials engineering, manufacturing engineering, structural analysis (including loads, stress, strain, and fracture mechanics analysis), nondestructive inspection and evaluation, reliability engineering, testing technology, field repair and maintenance, and holistic design procedures. All of these must be placed in a consistent design activity that may be referred to as a fatigue design policy. Obviously, if other time-related failure modes occur concomitantly with repeated loads and interact synergistically, then the task becomes even more challenging. Inasmuch as humans always desire to use more goods and place more demands on the things we can design and produce, the challenge of fatigue is always going to be with us.
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Basics of Metal Matrix Composites

nospam@example.com (mustafa caykoylu) — Sun, 29 Oct 2006 14:07:49 +0200

Introduction

Metal composite materials have found application in many areas of daily life for quite some time. Often it is not realized that the application makes use of composite materials. These materials are produced in situ from the conventional production and processing of metals. Here, the Dalmatian sword with its meander structure, which results from welding two types of steel by repeated forging, can be mentioned.
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Thermoplastic rubber Natural rubber Styrene-butadiene rubber

nospam@example.com (mustafa caykoylu) — Thu, 26 Oct 2006 09:14:09 +0300

Thermoplastic rubber

One of the more interesting developments in the past 30 years has been that of thermoplastic rubbers. One of the problems with conventional vulcanised rubber is that, once cross-linking or vulcanisation has taken place, the material is, like a thermosetting plastic. ‘set’ and cannot be melted and reprocessed. Thus material used in a process but not incorporated into the final product or defective products cannot be re-used as can a thermoplastic material. Thus many attempts have been made to produce a rubbery material in which effective cross-links exist at normal use temperature but which disappear (become heatfugitive) at elevated temperatures. To some extent entanglements and crystalline zones fulfil this role and rubbers have been used that are simply very high molecular weight polymers (but thus difficult to process) or slightly crystalline. Polymers have also been made in which ionic cross-links exist at low temperatures, but which lose their force at elevated ones. A number of true covalent-bonded systems have been devised in which these bonds break down at elevated temperature, but which reform at low temperatures. Such systems have been of limited use. Far more successful has been the use of block copolymers.

Such block copolymers differ from the more common random copolymers in that the monomers of each type are grouped together in one chain. One such material consists of a block of butadiene molecules (forming a segment of a rubbery polybutadiene block) set between two blocks of styrene molecules (forming glassy polystyrene blocks). Such as system is known as a styrene-butadiene-styrene (SBS) triblock copolymer. At room temperature the polystyrene ends congregate into domains effectively forming cross-links between many triblock molecules at the chain ends. However, above Tg these domains tend to break up and, because the overall molecular weight is quite low, the whole system melts and is capable of flow. When the melt is cooled the domain structures and thus the cross-links reform. Slightly different are the polyetherpolyester block copolymers in which amorphous polyether zones are separated by crystallisable polyester blocks. At room temperature these latter blocks do crystallise together to produce small crystal structures which act as cross-links. These latter materials are available in a variety of polyether/polyester ratios and thus vary in stiffness and rubberiness. Because of the high melting point of the polyester blocks these materials have good heat resistance, and because of their chemical nature they have good oil resistance. They have become important engineering rubbers.

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