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1 IntroductionA major problem confronting the designer is to choose thecorrect materials for the design of an individual componentor structure; the material properties have to ensure that thecomponent performs the work for which it was designed with-out malfunctioning throughout its guarranted life, and can beproduced for a price acceptable to the customer.To enable this, the designer needs to know the loads thathis component or structure will be subjected to the environ-ment in which it will act, the service life expected, and the pro-duction cost. These factors limit the range of materials for ap-plication in the distinctive design. To finalize the selection ofmaterials, he needs to be aware of their characteristics undervarious loadings and in various environments (i.e. the mate-rial properties). Knowing these properties, he must be able tocorrelate them with the bearing capacity of his proposed com-ponent or structure. Fracture mechanics deals with thesequestions. Conventional design assumes that the material is aflawless continuum. However, we know that rational designand material evaluation require knowledge of flaws in materi-als. Many materials, components and structures either haveinherent defects as part of the production routine, or developthem at some phase of their life. Comprehension of how acracked body conducts itself under loading is fundamental tothe explanation of any fatigue problem. Since much of the in-formation on fatigue from both laboratory experiments andservice characteristics was obtained and expounded beforethe study of cracked-material behaviour was implemented, noestablished field of study correlated the particulars. Isolatedsets of data have practically become fatigue folklore, and theirrelation to other groups of data has not been clarified.2 Crack initiationSignificant publications on fatigue research, in the 20thcentury include Forsyth [1], on extrusions and intrusions inslip bands, see Fig. 1. Three basic observations are: the impor-tance of the free material surface, the irreversibility of cyclicslip, and environmental influences on microcrack initiation.For the most part, microcracks begin at the free materialsurface, and in unnotched specimens possessing a nominallyhomogeneous stress distribution loaded with cyclic tension.Thereislesspreventionofcyclicslipthaninsidethemate-rial for the free surface at one side of the surface material.Microcracks also start more easily in slip bands with slip dis-placements normal to the material surface [2]. There remainquestionsaboutwhycyclicslipisnotreversible.Asfarbackasthe 1950s, it was understood that there are two reasons fornon-reversibility. One argument is that (cyclic) strain harden-ing occurs, which implies that not all dislocations return totheir original position. Another important factor is the inter-action with the environment. A slip step at the free surfaceimplies that fresh material is exposed to the environment. Ina non-inert environment, most technical materials are rap-idly covered with a thin oxide layer, or some chemisorption offoreign atoms of the environment occurs. Exact reversibilityof slip is then obviated. Fatigue initiation is a surface effect.In the mid 20thcentury, microscopic investigations werestill made with an optical microscope. This implies that cracknucleation was observed on the surface, where it indeed oc-curs. As soon as cracks grow into the material away from thefree surface, only the ends of the crack front can be observedon that free surface. It is questionable whether this informa-tion is representative of the growth process inside the mate-rial, a problem that is sometimes overlooked. Microscopicobservations on crack growth inside the material requirecross-sections of the specimen are made. Several investiga-34© Czech Technical University Publishing House http://ctn.cvut.cz/ap/Acta Polytechnica Vol. 46 No. 3/2006 Czech Technical University in PragueMetal Fatigue FeaturesP. BrožThis paper presents a summary of fatigue, crack initiation and growth, and fractographic findings for metal materials. The purpose of thispaper is to consolidate and summarize some aspects of the fatigue of materials and structures.Keywords: fracture surface, fracture toughness, macro-and microscopic appearance, microcrack, slip band, small cracks, striation,subgrain, transcrystalline.Fig. 1: Slip geometry at the material surface according to Forsyth[1]tions employing sectioning were made in the 1950s andearlier. These showed that in most materials fatigue cracksgrow transcrystalline. Although the fatigue fractures ap-peared rather flat to the unaided eye, it turned out that thecrack growth path under the microscope could be ratherirregular, depending on the type of material. In materialswith low stacking fault energy (e.g. Cu- and Ni-alloys), crossslip is difficult and as a result the cyclic slip bands are narrowand straight. Crack growth on a microscale occurs in straightsegments along these bands. In materials with high stackingfault energy (e.g. Al-alloys) cross slip is easy. Moreover, in theAl crystal lattice there are many slip systems which can easilybe activated. As a consequence, the slip lines are wider andcan be rather wavy. Crack growth on a micro scale does notsuggest that it occurs along crystallographic planes. As aresult, fatigue on a microscale can be significantly differentfor different materials. The behavior is structure-sensitive,depending on the crystal structure (fcc, bcc, or hexagonal),the elastic anisotropy of the crystalline structure, grain size,texture, and dislocation obstacles (e.g. pearlite bands in steel,precipitated zones in Al-alloys, twins, and so on).The outset of the damage in a cyclically stressed metal istied in with a free surface. There is the following evidence thatdamage in a polycrystalline ductile metal is connected withgrains having a free surface rather than those within the body:(i) Surface grains are in intimate touch with the atmo-sphere; thus if the environment is a factor in the damageprocess, they are apparently more receptive. A surfacegrain is the only part of a polycrystal not fully supportedby adjoining grains. Because the slip systems in neigh-bouring grains of a polycrystal are not related to eachother, a grain having a free surface will be able to deformplasticallymoreeasilythanagraininthebodyofthemetal that is surrounded by other grains.(ii) It has been shown that if a fatigue test is stopped aftersome fraction (perhaps 20

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NIT-T MECH 786 - Metal Fatigue Features

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