The suitability of a material can be judged from experiments ( ASTM E206 ) in which a specimen is exposed to a present level of stress S until fracture occurs after N cycles. The results are reported in fatigue diagrams or S-N diagrams. In some environments, materials such as steel may sustain some minimum stress level indefinitely; this is called the fatigue limit or endurance limit. It is better, however, to specify the stress which can be sustained for a given number ( say, two million ) of cycles.
Because fatigue involves the propagation of cracks under an imposed tensile stress, the number of cycles to failure ( or the stress sustained for a given number of cycles ) is greatly reduced if there are preexisting cracks, internal defects, or inclusions of a brittle nature. The surface roughness produced in some processes acts similarly, therefore, fatigue strength is reduced if the surface is rough, especially in high-strength materials that are less ductile.
Repeated loading at elevated temperatures-such as is caused by differential expansion and contraction of the surface of a part-may lead to thermal fatigue. It is particularly troublesome in forging tools, casting dies, and glass molds because cracking (crazing) of the surface is reproduced on the surface of the part.
A special form of failure occurs when certain materials are exposed to a chemically aggresive (corrosive) environment. Surface cracks form and, in combination with the applied stress, lead to stress-corrosion cracking. If there are residule tensile stresses on the surface of the part, cracks develope in a part even in the absence of external loads.
Because of the scatter of data, fatigue life and limiting stress are often expressed as probabilities.
( Schey, John A. Introduction to Manufacturing Processes, pg. 100 )
Thermal Fatigue ( Old )
Thermal fatigue characterization has been the area of the most testing for tin – leas as well as lead – free solders. There have been numerous studies on this mechanism for lead – free solders, as seen in many journal and conference proceedings. The driving force for damage in thermal fatigue is the coefficient of thermal expansion (CTE) mismatch between the different materials that make up an electronics assembly, along with the fact that solder plastically deforms to accommodate a portion of that strain mismatch. Damage accumulates within the solder joint with each temperature cycle, and eventually a crack is nucleated that will propagate through the joint and create an electrical open.
(Bradley E., Handwerker C. A., Bath J., Lead-free electronics: iNEMI projects lead to successful manufacturing, 2007, pg.218)2-) Life Expectancy ( New ) - ( Better ) - ( Machine Elements Life )
Nowadays the calculation methods have been improved drastically, but the real reliability and service life of the machine parts remains an estimate. Now, as well as in the distant past, the failure at a safety factor of 5 is sometimes possible. The authors are going to discuss in detail the basics of calculation for strength and durability. They also want to inspire in the reader's mind and understanding of some conventions and, to some extent, the uncertainty of these calculations to urge him to be cautious about the results obtained.
"The world rests on three things," the ancient sages said. Each of them had his own version of what these were. The authors, being in agreement with their subject's requirements, can state unequivocally that the calculation for strength is based on three data units:
- The strength parameters of the material
- The stress magnitude and pattern of change with time
- The safety factor
Each of these data units deserves detailed discussion.
( Klebanov, Boris M. Barlam, David M. Nystrom, Frederic E. Machine Elements Life and Design, pg. 356 )
Life expectancy of products: ( Old )
The life of expectancies of product can vary significantly, depending on the materials and processes employed (hence the quality) and on many other factors - especially the frequency and quality of maintenance required. Numerous examples can be given where a choice has to be made between different processes and materials to manufacture a product. For example, consider the following: (a) sheet metal versus cast-iron frying pans, (b) carbon-steel versus stainless-steel exhaust systems for automobiles, (c) wood versus metal handles for hammers, (d) plastic versus metal outdoor furniture, and (e) aluminum versus reinforced-plastic ladders.
(Kalpakjian S. Schmid S.R.,Manufacturing Engineering and Technology, 5th Edition, p. 1244)
3-) Elastomer Friction ( New ) - ( Better ) - ( Material Kinematics )
Nature and Mechanism of Elastomer Friction
Molecular-kinetic concepts of external friction of rubbery materials ( vulcanized rubbers, natural rubbers, etc. ) were first examined theoretically by Bartenev. He obtained a theoretical relation of the friction force to sliding velocity and temperature. Let us examine the basic points of this theory.
On a polymer surface, there are chain segments that jump in a disorderly manner on a smooth rigid surface from one junction ( adhesive point ) to another. The time between two successive jumps is defined as the " relaxation time" of a chain and, according to Frenkel, can be expressed in the following form:
T = To exp ( U/kT ),
where U is the energy barrier, and To is a constant equal to 10^(-12) sec.
The measure of the energy barrier U for external friction is determined by adhesive forces in the polymer-solid pair. Chemical interaction forces are not taken into account in the theory. For a solid's surface to be considered smooth, the mean asperity must not exceed the mean quadratic length of a polymer chain, that is, 100 A'.
( Bartenev G.M. & Lavrentev V.V. Friction and Wear of Polymers, pg. 113-114 )
Elastomer Friction: ( Old )
Elastomers are usually formulated to have high friction coefficients against a wide variety of counterfaces but the friction coefficients can be very low in some cases. Table 4 shows friction coefficients for some common elastomers against hardened stainless steel; the coefficients are quite high. On the other hand, in water, the coefficient of polyurethane against most other solids is less than 0.2. The slippery nature of this elastomer became apparent when these materials came into vogue in the 70’s for floor finishes. They produced a very abrasion resistant surface, but when wet they were a safety hazard. Their use as tires and floor toppings has almost ceased but they are now used for seals and wear parts in pumps where their slipperiness and abrasion resistance have a synergistic effect.
The coefficient of friction of many elastomers against other solids is often 1 or more, but the use environment may significantly alter the friction characteristics.
(K. C. Ludema,Raymond George Bayer,ASTM Committee G-2 on Erosion and Wear,Tribological modeling for mechanical designers, p.113)
4-) Decision Trees for CAPP ( New ) - ( Better ) - ( Info Presenting Methods )
5-) Decision Tables for CAPP ( New ) - ( Better ) - ( Info Presenting Methods )
( Halevi, Gideon, Handbook of Production Management Methods, pg. 99-100 )
Decision Tables for CAPP ( Old )
Decision tables are tables that are divided by colums and rows.(simple decision table given by source)
Desicion tables are often used in process planning. There are several factors that are to be considered while developing the decision table, such as the accuracy, repetitiveness, consistency, size and completeness of the table, to ensure that the table helps effective decision making. the size of the decision table is important. If a decision table is too large, for example several pages of printing, it is difficult for a human to readand interpret. the discipline may fit a menu-driven interactive computer programme, although this will not only require excessive memory, but will also efficiency of decision making.
the table should contain the actual rules and conditions specified in the design. According to the rule representation, decision tables can be classified as follows:
limited entry decision tables
extended entry decision tables
mixed entry decision tables
(Hong-Chao Zhang,Leo Alting,Computerized manufacturing process planning systems,first edition page 142, 143)
Decision Trees for CAPP ( Old )
Decision Trees resemble a graph with a root, nodes, and braches. they are used to represent the result of actions. The root is the source of the tree, and each tree has a one single root, however a three can call another sub-tree, for example A DCLASS sub-tree can call up to 25 additional sub-trees. When apply the decision making, the branches carry values/ expressions that can be likened to an IF statement, While the branches in series can be likened to an AND statement. Branches can have only two values - true and false. ıf a branches is true, than it can be passed to next node. Nodes can be classified into excursive and non-mutually excursive (Allen1987). A non-mutually excursive node allows all its successive branches to be true.
(Hong-Chao Zhang,Leo Alting,Computerized manufacturing process planning systems, first edition, pg 143-144 )
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