Hardness Alterations (01.04.2011 17:43)
In general, hardness measurement are carried out with Vickers or Knoop diamond indenters using the average indented diagonal in the former or the longer diagonal in the latter. Due to the fact that hardness may vary considerably within a short distance, loads below 1 N (microhardness test) are often employed. However, in contrast to the high loads typically employed for hardness testing, microhardness value depends on the applied load. Tönshoff and Brinksmeier [30] compared Knoop and Vickers indenters and noticed that the former presents the following advantages: the longer diagonal reduces the risk of misreading, the influence of lack of homogeneity normal to the surface is reduced due to the smaller indentation depth, the small diagonal allows measurement near the machined surface and it can be used to measure anisotropy. On the other hand, the Vickers indenter is less affected by form errors on the surface and the indenter is cheaper. Additionally, the above-mentioned authors state that owing to the steep variation in microhardness values, especially near the machined surface, measuring should be performed using the slope method, in which the indentations are produced on an tapered surface, thus allowing the measurement of hardness close to the machined surface (not possible when the sample is cross-sectioned due to the fact that a distance of not less than 2.5 times the diagonal indentation must be kept from a disturbed area). Finally, the polishing method employed prior to microhardness testing may affect the results: higher hardness values and wider scatter are obtained when grinding the samples in comparison with electrolytic polishing, probably due to microstructure non-homogeneity and work hardening induced by the former procedure.
(Davim. J. P., Machining of Hard Materials, 2011, p. 127-128)
Fatigue Strength (01.04.2011 17:43)
The fatigue process consists of three stages: initial fatigue damage leading to crack nucleation and crack initiation in regions where the strain is most severe, progressive growth of the crack (crack propagation) and finally sudden fracture of the remaining cross-section [44]. Czyryca [45] states that many testing devices and specimen designs are available for fatigue testing according to the mode of loading: direct (axial) stress, plane bending, rotating beam, alternating torsion or combined stress. The selected loading mode should replicate, as accurately as possible, the actual service condition of the sample being tested.
(Davim. J. P., Machining of Hard Materials, 2011, p. 136-137)
Evolutionary Algorithms (01.04.2011 17:43)
Evolutionary algorithms are a set of heuristics simulating the process of natural evolution (Figure 6.10). Although the underlying mechanisms are simple, these algorithms have proven them as a general, robust and powerful search tool. In particular, they are especially convenient for problems involving multiple conflicting objectives and large and complex search spaces.
In spite of the wide diversity in the proposed approaches, an evolutionary algorithm can be characterized by three features:
• A set of candidate solutions is maintained.
• A competitive selection process is performed on this set.
• Several solutions may be combined in terms of recombination to generate new
solutions.
There are two main evolutionary heuristics: the German school of evolution strategies (ES), and the American school of GAs. The main differences between these two approaches are summarized in Table 6.1. Some papers have shown the application of evolutionary techniques in hardmachining optimization [26, 33].
(Davim. J. P., Machining of Hard Materials, 2011, p. 191)
Piezoelectric stack actuators (02.04.2011 16:45 )
In current work, piezoelectric stack actuators (PSA) have been studied as an alternatice solution to allow for satisfying all desirable characteristics without compromising between them. The main motivation for such study is actuator not only demonstrates all properties (advantages) of a single piezoceramic actuator, but it also provides a high actuation force partially by accumulating the produced force of each single piezoceramic in one unit. Other contributing factor of producing large actuation force in a piezoelectric stack lies in the large actuation force in a piezoelectric stack lies in the utilization of direct direction of polarization.
(Mehrdad R. Kermani,Mehrdad Moallem,Rajni V. Patel, Applied vibration suppression using piezoelectric materials, 2008, p. 90)
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