Fatigue & Fracture

Editor's Notes

• #4: -- //Fatigue is the response of a material to dynamic loading conditions. Components fail under fluctuation stresses at stress magnitude which is lower than the ultimate strength of material.// Sometimes, it is lower than yield strength. Further it has been found that the magnitude of stress causing fatigue failure decreases as number of stress cycles increases. This phenomenon of decreased resistance of the materials to fluctuating stresses is the main characteristic of fatigue failure -- And //Fracture is a failure which is the result of a static overload. It is described as either ductile or brittle fracture.// As we can see in the figure that ductile fractures are characterized by extensive plastic deformation prior to and during crack propagation. On the other hand, brittle fracture takes place at stresses below the net section yield strength, with very little observable plastic deformation and a minimal absorption of energy.
• #5: History of Fatigue --The discovery of fatigue occurred //in the 1800s when several investigators in Europe observed that bridge and railroad components were cracking when subjected to repeated loading// -- //It is difficult to identify exactly when the problems// of failure of structural and mechanical equipment became of critical importance; //however, failures that cause loss of life have occurred for over 100 years.// Throughout the 1800s, stories could be heard of bridges fell, and pressure vessels blew up in UK and USA As the century progressed and the use of metals expanded with the increasing use of machines, more and more failures of components subjected to repeated loads were recorded. Even though much progress has been made, developing design procedures to prevent failure from the application of repeated loads is still unpredictable.
• #6: --//A spectacular example of this problem was the S. S. Schenectady, whose hull completely fractured,// while it was docked at its fitting-out time. The fractured ship is shown in Figure. In this case, the need for tougher structural steel was even more critical because welded construction was used in shipbuilding instead of riveted plate. In riveted plate construction, a running crack must reinitiate every time it runs out of a plate. In contrast, a continuous path is available for brittle cracking in a welded structure, which is why low notch toughness is a more critical factor for long brittle cracks in welded ships.
• #8: Literature Review for Fatigue So how this fatigue takes place ?? It takes place in 3 stages --In Stage I - crack initiation -Initial fatigue damage leads to crack nucleation and crack initiation --next is…..Crack propagation – In this stage, Progressive cyclic growth of a crack occurs until the remaining uncracked cross section of a part becomes too weak to sustain the loads imposed And In Final Stage III - sudden fracture of the remaining cross section takes place..
• #9: Lets understand crack initiation separately: --Fatigue cracks form at the points of maximum local stress and minimum local strength => usually, it is near the surface of the part. E.g. scratches, marks, burrs, and other fabrication flaws -- crack initiation will eventually occur due to the formation of persistent slip bands (PSB), it is so called because, traces of the bands persist even when the surface damage is polished away. -- Slip bands are a result of the systematic build-up of fine slip movements, on the order of only 1 nano meter. The back-and-forth movement of the slip bands leads to the formation of intrusions and extrusions at the surface, eventually this leads to the formation of a crack.
• #10: --The crack initially follows the slip bands at approximately 45° to the principal stress direction. --When the crack length becomes sufficient for the stress field at the tip to become dominant, the overall crack plane changes and becomes perpendicular to the principal stress, and the crack enters stage II. -The crack propagation rate during stage I is very low, on the order of 1 nm per cycle, and produces a practically featureless fracture surface. -In observing locations of crack nucleation, the possibility of environment-related mechanisms, including pitting corrosion, stress-corrosion cracking, and other effects of a hostile environment, must be considered. For example, In any structure having mechanically fastened joints with some relative motion, fretting provides a possible failure origin -Once a fatigue crack has been nucleated, its rate and direction of growth are controlled by localized stresses and by the structure of the material at the tip of the crack.
• #11: Phase II – Crack Propagation --In Stage II crack growth occurs when the stage I crack changes direction and propagates in a direction normal to the applied stress. --As we can see in figure (a), The transition is from one or two shear planes in stage I to many parallel platowz (plateaus) separated by longitudinal ridges in stage II. The platowz (plateaus) are usually normal to the direction of maximum tensile stress as we can seen in figure (b). A transition from stage I to stage II in a coarse-grained specimen of aluminum alloy 2024-T3 is shown in figure (c), In this image, the stage II area shows a large number of approximately parallel fatigue patches containing very fine fatigue striations that are not resolved at the magnification used.
• #12: --Crack growth proceeds by a continual process of crack sharpening followed by blunting, as shown in the Figure (a). Crack propagation during crack growth often produces a pattern of fatigue striations as shown in figure (b). Each striation represents one cycle of fatigue. Although striations are indicative of fatigue, fatigue failures can occur without the formation of striations. Striations are microstructural details that are best examined with a scanning electron microscope and are not visible to the naked eye.
• #13: --In final phase --Ultimate failure occurs when the fatigue crack becomes long enough that the remaining cross section can no longer support the applied load. Final fracture occurs when the crack has grown to the critical size for overload failure. --The size of the final fracture zone depends on the magnitude of the loads, and its shape depends on the shape, size, and direction of loading of the fractured part. The final fracture zone of a fatigue fracture surface is often fibrous. --In tough materials, with thick or round sections, the final fracture zone will consist of a fracture by two distinct modes: tensile fracture (plane-strain mode) extending from the fatigue zone and in the same plane, and shear fracture (plane-stress mode) at 45° to the surface of the part, bordering the tensile fracture --But In thin sheet metal pieces having sufficient toughness, final fracture occurs somewhat differently. As you can see this in figure(a), As the crack propagates from the fatigue zone, the fracture plane rotates around an axis in the direction of crack propagation until it forms an angle of approximately 45° with the loading direction and the surface of the sheet. And in figure (b) we can observe that, The fracture in plane inclined 45° to the load direction, can occur on either a single-shear or a double-shear plane.
• #14: Research Paper --This paper discuss “Small fatigue crack initiation mechanisms and growth behavior of 304 stainless steel at room temperature” [3] --304 austenitic stainless steel is an alloy that has been widely used in process and power generating industries as piping and structural material. --Introduction --We will came to know that most of the total fatigue life could be spent in the initiation and small crack growth stages. -Fatigue fracture is one of the main causes of failure for these components due to the dynamic or alternating stresses which they are often subjected to.
• #15: Experimental Procedure --They carried out a fatigue test which can be seen in figure. A single edge notch tensile (SENT) specimen, was used to produce naturally initiated cracks --The yield strength and ultimate tensile strength of the material were 297.55 MPa and 668.64MPa respectively. --They obtained quantitative information such as the initiation, growth and coalescence behavior of small cracks in 304 stainless steel via surface replicas and photomicrographs
• #16: Analysis --In order to characterize the fatigue crack initiation mechanism of 304 stainless steel, the replicas were observed at different time intervals when the specimen was tested at 370MPa. It was found that there were eight major cracks initiated on the specimen. --the initiation and propagation of small cracks depended on both the grain boundary and the localization of plastic deformation along the slip bands. After nucleating at the grain boundaries, and then they propagated in a way perpendicular to the loading axis -
• #17: --We can see in figure (a), shows the variation of the lengths of major crack, which may cause the ultimate fracture. --The slope of the major crack length curve almost did not change too much when the crack length was lower than 0.2 mm. Once the surface small crack length reached around 0.2 mm, the crack will propagate rapidly --it can be deduced that the critical small crack size for the transition to rapid crack growth was around 0.2 mm for 304 stainless steel. --Figure (b) showed the variation of the small fatigue crack growth rates along with crack lengths. It was easy to be seen that there was not a general increase in crack growth rate with increasing crack length, as was normally observed for long cracks. Based on the observation of the replicas, the fluctuations of crack growth rate were due to the blocking effect of grain boundaries
• #18: They have concluded that 1.Grain boundaries are not only the initiation sites of cracks for austenitic 304 stainless steel, but also are the main factor that influence the growth rates of microstructurally small cracks. 2. Once the surface small crack length reaches the critical crack size of 0.2 mm, rapid growth of this crack occurs, resulting in final specimen fracture.