Acta Polytechnica Vol 46 No 3 2006 Czech Technical University in Prague Metal Fatigue Features P Bro This paper presents a summary of fatigue crack initiation and growth and fractographic findings for metal materials The purpose of this paper 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 the 1950s it was understood that there are two reasons for non reversibility One argument is that cyclic strain harden ing occurs which implies that not all dislocations return to their original position Another important factor is the inter action with the environment A slip step at the free surface implies that fresh material is exposed to the environment In a non inert environment most technical materials are rap idly covered with a thin oxide layer or some chemisorption of foreign atoms of the environment occurs Exact reversibility of slip is then obviated Fatigue initiation is a surface effect 1 Introduction A major problem confronting the designer is to choose the correct materials for the design of an individual component or structure the material properties have to ensure that the component performs the work for which it was designed with out malfunctioning throughout its guarranted life and can be produced for a price acceptable to the customer To enable this the designer needs to know the loads that his 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 of materials he needs to be aware of their characteristics under various loadings and in various environments i e the mate rial properties Knowing these properties he must be able to correlate them with the bearing capacity of his proposed com ponent or structure Fracture mechanics deals with these questions Conventional design assumes that the material is a flawless continuum However we know that rational design and material evaluation require knowledge of flaws in materi als Many materials components and structures either have inherent defects as part of the production routine or develop them at some phase of their life Comprehension of how a cracked body conducts itself under loading is fundamental to the explanation of any fatigue problem Since much of the in formation on fatigue from both laboratory experiments and service characteristics was obtained and expounded before the study of cracked material behaviour was implemented no established field of study correlated the particulars Isolated sets of data have practically become fatigue folklore and their relation to other groups of data has not been clarified 2 Crack initiation Significant publications on fatigue research in the 20th century include Forsyth 1 on extrusions and intrusions in slip bands see Fig 1 Three basic observations are the impor tance of the free material surface the irreversibility of cyclic slip and environmental influences on microcrack initiation For the most part microcracks begin at the free material surface and in unnotched specimens possessing a nominally homogeneous stress distribution loaded with cyclic tension There is less prevention of cyclic slip than inside the mate 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 remain questions about why cyclic slip is not reversible As far back as Fig 1 Slip geometry at the material surface according to Forsyth 1 In the mid 20th century microscopic investigations were still made with an optical microscope This implies that crack nucleation was observed on the surface where it indeed oc curs As soon as cracks grow into the material away from the free surface only the ends of the crack front can be observed on 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 Microscopic observations on crack growth inside the material require cross sections of the specimen are made Several investiga 34 Czech Technical University Publishing House http ctn cvut cz ap Czech Technical University in Prague Acta Polytechnica Vol 46 No 3 2006 tions employing sectioning were made in the 1950s and earlier These showed that in most materials fatigue cracks grow transcrystalline Although the fatigue fractures ap peared rather flat to the unaided eye it turned out that the crack growth path under the microscope could be rather irregular depending on the type of material In materials with low stacking fault energy e g Cu and Ni alloys cross slip is difficult and as a result the cyclic slip bands are narrow and straight Crack growth on a microscale occurs in straight segments along these bands In materials with high stacking fault energy e g Al alloys cross slip is easy Moreover in the Al crystal lattice there are many slip systems which can easily be activated As a consequence the slip lines are wider and can be rather wavy Crack growth on a micro scale does not suggest that it occurs along crystallographic planes As a result fatigue on a microscale can be significantly different for 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 is tied in with a free surface There is the following evidence that damage in a polycrystalline ductile metal is connected with grains having a free surface rather than those within the body i ii Surface grains are in intimate touch with the atmo sphere thus if the environment is a factor in the damage process they are apparently more receptive A surface grain is the only part of a polycrystal not fully supported by adjoining grains Because the slip systems in neigh bouring grains of a polycrystal are not related to each other a grain having a free surface will be able to deform plastically more easily than a grain in the body of the metal that is surrounded by other grains It has been shown that if a fatigue test