Fine lines seen on a fracture surface where a fatigue crack has advanced. Their spacing corresponds roughly to the crack growth per cycle, and they are observed with an electron microscope.
Japanese name: ストライエーション
Fatigue striations appears in 1 practice question across 1 topic, including Fracture mechanics and fatigue.
In a fatigue crack in a ductile metal, the crack tip opens by slip and becomes blunt and rounded on each loading, sharpens again as it closes on unloading, and advances by one cycle's worth. This process leaves steps on the fracture surface, and the fine lines lying perpendicular to the direction of crack growth are fatigue striations. The lines follow the shape of the crack front in arcs, and the direction in which the arcs bulge usually indicates the direction of crack growth. The spacing is often below a micrometer, and they are observed with a scanning electron microscope. They appear clearly in highly ductile materials such as aluminum alloys and may be indistinct in high-strength steels.
In the intermediate ΔK region, the striation spacing agrees well with the macroscopically measured crack growth rate da/dN, so in failure analysis ΔK at that location can be estimated from the spacing. For example, for a steel with Paris-law constants C = 1.0×10⁻¹² (da/dN in m/cycle, ΔK in MPa·√m) and m = 4, a spacing of 0.50 µm gives ΔK of about 27 MPa·√m. If the crack length at that location is 5.0 mm and the geometry factor is taken as 1, the stress range is estimated at about 210 MPa. By measuring the spacing at several places on the fracture surface, the magnitude of the stress that acted on the part can be worked back.
Not seeing striations does not mean that the failure was not fatigue. Near the threshold, where growth is very slow, striations are not visible or one line forms over several cycles, so one line no longer corresponds to one cycle. In the region of fast growth near the fracture toughness, ductile dimples are mixed in. If the fracture surfaces rub against each other or corrode, the lines disappear. Conversely, some patterns resemble striations, such as machining marks and steps from slip bands, so they are distinguished by checking the regularity of the spacing and its relation to the direction of crack growth.
Ordinary striations form by plastic deformation at the crack tip and are called ductile striations. In fatigue in corrosive environments, by contrast, brittle striations are sometimes seen: on flat, cleavage-like facets, lines perpendicular to the growth direction cross river-like markings running along it. Which of the two types is seen is therefore a useful clue for judging whether the environment played a part. In failure analysis, observations of striations, beach marks, the origin and the final fracture zone are combined to reconstruct when and from where the crack grew and under what loading.
Answer: B. Striations roughly mark the crack advance in each cycle, while beach marks record history such as shutdowns
Striations are lines left as the crack tip opens and blunts, then closes and sharpens again in each cycle, and their spacing corresponds roughly to the crack growth per cycle, da/dN. The spacing is fine, at the µm level or below, and cannot be seen without an electron microscope. Beach marks are patterns that record positions of the crack front through shutdowns and restarts, changes in load level, differences in how corrosion progresses and so on, and each band contains many cycles. Both appear in the region where the fatigue crack advanced little by little, and the common center of the patterns shows the origin. The final fracture zone is where the remaining cross-section broke all at once, and it shows a ductile or brittle fracture surface.
In the intermediate growth-rate region, roughly one line corresponds to one cycle. In the region of very slow growth, however, one line may form over several cycles, and under variable loading only the large loads may leave distinct lines, so care is needed when taking the number of lines directly as the number of cycles.
By their size. Beach marks are widely spaced patterns visible to the naked eye or at low magnification, with many cycles between one mark and the next. Striations are often spaced below a micrometer and become visible only at magnifications of a few thousand times in a scanning electron microscope.
No. They are clear in highly ductile metals such as aluminum alloys, copper alloys and austenitic stainless steels, but often indistinct in high-strength steels and cast irons. Even if they are not visible, whether the failure was fatigue is judged together with other features such as the location of the origin and beach marks.
Striations are lines left as the crack tip opens and blunts, then closes and sharpens again in each cycle, and their spacing corresponds roughly to the crack growth per cycle, da/dN.
The fracture surface of a part that failed by fatigue shows…Energy release rate Griffith theory Miner's rule Modified Goodman diagram Paris law Stress concentration factor Stress intensity factor