Fatigue Crack Initiation
Fatigue crack initiation refers to the process by which cracks begin to form in a material due to repeated cyclic loading. This phenomenon is a common cause of failure in engineering structures and components, particularly in materials subjected to dynamic stress. Understanding and managing fatigue crack initiation is crucial for ensuring the reliability and safety of various mechanical systems.
7 Key excerpts on "Fatigue Crack Initiation"
eBook - ePub- R. Winston Revie, R. Winston Revie(Authors)
- 2011(Publication Date)
- Wiley(Publisher)
...Consequently, both cyclic frequency and stress waveform strongly influence crack development by corrosion fatigue, whereas for fatigue alone these factors are usually less significant. Fatigue damage can be divided into the following four stages: 1. Precrack Cyclic Deformation. Repetitive mechanical damage is accumulated in some local regions; dislocation and other substructures may develop; and persistent slip bands (PSBs, slip bands that develop on the sample surface during cyclic deformation and that reappear at the same locations during further cyclic deformation after polishing the surface), extrusions, and intrusions form. 2. Crack Initiation and Stage I Growth. Cracks initiate as a result of deepening of the intrusions; crack growth in this stage is within the planes of high shear stress. 3. Stage II Crack Propagation. Well-defined cracks propagate on the planes of high tensile stress in the direction normal to the maximum tensile stress. 4. Ductile Fracture. When the crack reaches sufficient length so that the remaining cross section cannot support the applied load, ductile fracture occurs. The relative proportion of the total cycles to failure that are involved in each stage depends on mechanical loading conditions and on the material. There is considerable ambiguity in deciding when a cracklike surface feature should be called a crack. In general, a larger proportion of the total cycles to failure are involved in propagation of stage II cracks in low-cycle fatigue than in high-cycle fatigue, whereas initiation and stage I crack growth comprise the largest segment for low-stress, high-cycle fatigue. The surface conditions of the material also influence the proportion of each stage in the total fatigue lifetime...
eBook - ePubPractical Plant Failure Analysis
A Guide to Understanding Machinery Deterioration and Improving Equipment Reliability, Second Edition
- Neville W Sachs, P.E.(Authors)
- 2019(Publication Date)
- CRC Press(Publisher)
...This results in a situation where there is a long incubation before the crack starts and then an extremely slow-growing fracture, where the crack propagation per stress cycle is almost infinitesimal.) • Stress or load variations often complicate the diagnosis of fatigue failures. For example, in a machine that sees varying loads, such as a shaft that may turn at 100 rpm, the thought is that it sees 144,000 fatigue cycles per day. But if it is heavily loaded for only 10% of the time and the rest of the time the is lightly loaded, it actually sees about 14,400 significant stress cycles/day. Before becoming involved with the analysis of fatigue fractures, it would be a good idea to better understand: • The concept of stress concentrations because of their importance in a very high percentage of fatigue failures. • The effect that metallurgy has on fatigue crack origins. Stress Concentrations Stress concentrations, or stress risers as they are frequently called, are features of the part that multiply the local stress. In effect, because of a change in shape or a change in metallurgy, the effective stress in the immediate area of the stress concentrator is greatly increased. Figure 5.2 shows a shaft and the chart below it lists the range of stress concentration factors that are caused by the changes in shape...
- Anthony Sofronas(Author)
- 2012(Publication Date)
- Wiley(Publisher)
...Chapter 12 Metal Fatigue 12.1 Metal Fatigue Defined Throughout this book there have been cases where the cyclic forces, moments, and torques have caused parts to fail. Before the parts failed there was usually vibration or noise to some degree. The failing machine was trying to tell the operator or engineer something. When a part fails in metal fatigue, the failure region is quite descriptive as to what the mode of failure was, as shown in Chapter 11. Although there are many technical descriptions for metal fatigue, my experience is that it is crack growth, due to a cyclic load, from some defect that progresses to the point that it affects the function of a part. Although not technically correct, it is useful when trying to determine why vibration caused a failure. This definition eliminates crack growth from creep, which is due to a steady load applied over a long period of time and brittle fracture due to the material properties. In both cases a crack can grow, and with creep (e.g., in furnace tubes) to the point of rupture, and it can take years for it to grow a substantial amount in a hot ductile material. On the other hand, brittle fracture can grow from a minuscule crack at a rate of 5000 ft/sec [1, p. 203] in a low-toughness material. This definition also eliminates a shock or impact load, which is a one-cycle event, unless it is repeated many times, such as in water hammer or hydraulic surge (see Section 7.3). Cyclic loads cause cyclic stresses; however, the defect can be of a number of types. For example, anything that causes pitting of a surface can be an initiation point for a crack. Corrosion of all types, such as stress corrosion cracking and fretting, cause surface defects and pitting. Oxidation of the surface, or rust, which is corrosion, causes pitting. Stress risers such as gouges, sharp radii, notches, holes, and machining marks are all points of crack initiation...
- Mavis Sika Okyere(Author)
- 2018(Publication Date)
- CRC Press(Publisher)
...8 Fatigue Analysis Fatigue is the weakening of a material caused by the repeated application and removal of stress. For example, if you bend a piece of metal back and forth repeatedly in the same spot, fatigue will result at the bend location and will weaken the metal until it eventually breaks. Technically, platforms experience fatigue as a result of periodic increases (application of stress) and decreases (removal of stress) in operating pressures. Because fatigue can cause a failure to occur at stress levels well below those that a material can withstand in a single, nonrepetitive loading, materials that must resist repeated stress cycles must be specially designed for this service. Durability is the ability of the material to resist fatigue. Fatigue analysis is done to ascertain the offshore platform structural response to continual wave loading. Fatigue analysis is carried out using the deterministic method or spectral method. Use the S – N curves to determine fatigue cycles. Also calculate the fatigue stress or fatigue damage. A detailed fatigue analysis should be performed for all structures as recommended in Section 5.2 of API RP 2A. The fatigue analysis is performed with input from a wave scatter diagram and from the natural dynamic response of the platform and the stiffness of the pile caps at the mud line by applying the Palmgren-Miner formula. Miner’s rule is probably the simplest cumulative damage model. It states that if there are k different stress levels and the average number of cycles to failure at the i th stress, S i, is N i, then the damage fraction, C, is C = ∑ i = 1 k n i N i (8.1) where n i = the number of cycles accumulated at stress S i C = is the fraction of life consumed by exposure to the cycles at the different stress levels...
eBook - ePubFatigue Testing and Analysis
Theory and Practice
- Yung-Li Lee, Jwo Pan, Richard Hathaway, Mark Barkey(Authors)
- 2011(Publication Date)
- Butterworth-Heinemann(Publisher)
...2 FATIGUE DAMAGE THEORIES YUNG-LI. LEE DAIMLERCHRYSLER 2.1 INTRODUCTION Predicting fatigue damage for structural components subjected to variable loading conditions is a complex issue. The first, simplest, and most widely used damage model is the linear damage. This rule is often referred to as Miner’s rule (1945). However, in many cases the linear rule often leads to nonconservative life predictions. The results from this approach do not take into account the effect of load sequence on the accumulation of damage due to cyclic fatigue loading. Since the introduction of the linear damage rule many different fatigue damage theories have been proposed to improve the accuracy of fatigue life prediction. A comprehensive review of many fatigue damage approaches can be found elsewhere (Fatemi and Yang, 1998). This chapter addresses (1) underlying fatigue damage mechanisms, (2) fatigue damage models commonly used in the automotive industry, and (3) postulations and practical implementations of these damage rules. 2.2 FATIGUE DAMAGE MECHANISM Fatigue is a localized damage process of a component produced by cyclic loading. It is the result of the cumulative process consisting of crack initiation, propagation, and final fracture of a component. During cyclic loading, localized plastic deformation may occur at the highest stress site. This plastic deformation induces permanent damage to the component and a crack develops. As the component experiences an increasing number of loading cycles, the length of the crack (damage) increases. After a certain number of cycles, the crack will cause the component to fail (separate). In general, it has been observed that the fatigue process involves the following stages: (1) crack nucleation, (2) short crack growth, (3) long crack growth, and (4) final fracture. Cracks start on the localized shear plane at or near high stress concentrations, such as persistent slip bands, inclusions, porosity, or discontinuities...
eBook - ePubFatigue of Materials and Structures
Application to Damage and Design
- Claude Bathias, André Pineau, Claude Bathias, André Pineau(Authors)
- 2013(Publication Date)
- Wiley-ISTE(Publisher)
...Chapter 6 Prediction of Fatigue Crack Growth within Structures 1 6.1. Prediction problems Predicting the evolution of cracks within a structure subjected to various loadings is basically a mechanical problem where the data are given by: the geometry of the structure; configuration of the initial crack; history of the forces applied; behavior law regarding the stress; and crack growth law of the material considered. The calculation consists of finding the relationship that connects the successive configurations of the cracks to the history of the loadings. In the case of fatigue propagation, the crack growth law gives the increase in crack length or surface per cycle, δ a/ δ N, as a function of intensive variables, such as the stress intensity factors, K i, or Rice integral, J, or the energy restitution level, G i : [ 6.1] K i or J or G depend on the length, a, of the cracks and on the possible variable loadings with the number of cycles, N. Therefore, the evolution of the crack at (N) is the solution to this differential equation, which is usually nonlinear for the initial condition, a o (N o). The decoupling property of the static and evolution problems makes things much easier. As these intensive variables are analytically connected to the external forces, P M : [ 6.2] The reduced magnitudes, K ri or J r or G r, can be calculated as functions of the geometry of the structure or surface of the crack for some unit forces, K r (a), J r (a) or G r (a). Some continuum mechanics calculation is involved, relying on the behavior law of the deformation of the material...
eBook - ePub- Rui Miranda Guedes(Author)
- 2019(Publication Date)
- Woodhead Publishing(Publisher)
...Sci. Technol. 1988;32:81–120. Quaresimin M., Carraro P.A. Damage initiation and evolution in glass/epoxy tubes subjected to combined tension-torsion fatigue loading. Int. J. Fatigue. 2014;63:25–35. Raghavan J. Fracture criteria to predict creep and creep rupture of polymers and polymer composites. Trends Polym. Sci. 1997;5(4):116–122. Raghavan J., Meshii M. Prediction of creep rupture of unidirectional carbon fiber reinforced polymer composite. Mater. Sci. Eng. A. 1995;197:237–249. Raghavan J., Meshii M. Time-dependent damage in carbon fiber reinforced polymer composites. Compos. Part A. 1996;27:1223–1227. Raghavan J., Meshii M. Creep rupture of polymer composites. Compos. Sci. Technol. 1997;57:375–388. Reifsnider K. Fatigue behavior of composite materials. Int. J. Fract. 1980;16:563–583. Richardson M.O.W., Branco C.M., Ferreira J.A.M. The effect of test conditions on fatigue properties of a phenolic composite. J. Compos. Mater. 1997;31:1826–1837. Rosen B.W. Tensile failure of fibrous composites. AIAA J. 1964;2:1985–1991. Rotem A. Stiffness change of a graphite epoxy laminate under reverse fatigue loading. J. Compos. Technol. Res. 1989;11:59–64. Rotem A. Load frequency effect on the fatigue strength of isotropic laminates. Compos. Sci. Technol. 1993;46:129–138. Sandhu R. Nonlinear behavior of unidirectional and angle ply laminates. J. Aircr. 1975;13:104–111. Schaefer J.D., Daniel I.M. Strain-rate-dependent yield criteria for progressive failure analysis of composite laminates based on Northwestern failure theory. Exp...
