Fatigue Chart
A fatigue chart is a graphical representation of the relationship between stress and the number of cycles to failure for a material. It is used to predict the fatigue life of a material under cyclic loading conditions, helping engineers and designers understand how materials will perform over time and enabling them to make informed decisions about material selection and design.
5 Key excerpts on "Fatigue Chart"
- 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)
...4 STRESS-BASED FATIGUE ANALYSIS AND DESIGN YUNG-LI. LEE DAIMLERCHRYSLER DARRYL. TAYLOR DAIMLERCHRYSLER 4.1 INTRODUCTION The fatigue damage theories in Chapter 2 indicate that fatigue damage is strongly associated with the cycle ratio (n i / N i, f), where n i and N i,f are, respectively, the number of applied stress cycles and the fatigue life at a combination of stress amplitude and mean stress levels. The cycle counting techniques for the determination of n i have been addressed in Chapter 3. This chapter focuses on how to establish baseline fatigue data, determine the fatigue life based on a stress-based damage parameter, and demonstrate engineering applications of this methodology. Since the mid-1800s, a standard method of fatigue analysis and design has been the stress-based approach. This method is also referred to as the stress–life or the S−N approach and is distinguished from other fatigue analysis and design techniques by several features: • Cyclic stresses are the governing parameter for fatigue failure • High-cycle fatigue conditions are present – High number of cycles to failure – Little plastic deformation due to cyclic loading During fatigue testing, the test specimen is subjected to alternating loads until failure. The loads applied to the specimen are defined by either a constant stress range (S r) or a constant stress amplitude (S a). The stress range is defined as the algebraic difference between the maximum stress (S max) and minimum stress (S min) in a cycle: (4.1.1) The stress amplitude is equal to one-half of the stress range: (4.1.2) Typically, for fatigue analysts, it is a convention to consider tensile stresses positive and compressive stresses negative. The magnitude of the stress range or amplitude is the controlled (independent) variable and the number of cycles to failure is the response (dependent) variable. The number of cycles to failure is the fatigue life (N f), and each cycle is equal to two reversals (2 N f)...
- Anthony Sofronas(Author)
- 2012(Publication Date)
- Wiley(Publisher)
...Engineers need to know what can be done to prevent these fatigue failures when modifications of the springs, masses, forcing function, or damping are not possible. Sometimes the necessary changes are obvious, such as increasing a radius or removing a notch; other times they are not. This is discussed from a practical viewpoint in this chapter. No calculations on how to determine the fatigue life are presented using traditional engineering approaches, as that is available from other sources [2, p. 22; 3], as are crack growth rates. Crack growth has been discussed briefly in several case histories in this book. 12.2 Reduction of a Component's Life When Subjected to Excessive Vibration Although we use the term excessive vibration here, this is now known to mean excessive cyclic stress due to cyclic loads. The endurance limit of a material, a steel alloy in this book, is the alternating stress for which a part will not fail in fatigue. If a paper clip is bent back and forth long enough, it may or may not fail in fatigue. It depends on how much it is bent: the torsional stress, the material properties, the surface condition, and the residual stress due to forming. A rotating beam test is one in which a polished sample with no stress rises or residual stresses is rotated and a reverse bending load of different magnitudes is applied (i.e., a cycling bending stress). The number of cycles until the beam fails are measured. When the beam does not fail after about 10 7 cycles at a given stress level, it is said to have reached its endurance limit and will not fail under that stress or lower stresses. When no other information is available, a rough estimate of the endurance limit for steels is one-half the tensile strength. The tensile strength of the steel in Figure 12.1 is 120,000 lb/in 2. Figure 12.1 Surface factors affecting the endurance limit. Figure 12.1 represents a typical steel with a tensile strength of 120,000 lb/in 2...
eBook - ePub- John Dwight(Author)
- 1998(Publication Date)
- CRC Press(Publisher)
...Testing may also be preferred even when method (1) would be possible. Testing may also be preferred even when method (1) would be possible. For example with a mass-produced component, built to closely controlled standards of workmanship, it may be found that fatigue testing of prototypes would indicate a better performance than that predicted from the standard endurance curves. Advice on fatigue testing appears in BS.8118. 12.3 CHECKING PROCEDURE (SAFE LIFE) 12.3.1 Constant amplitude loading The simplest type of fatigue calculation is when a single load is repeatedly applied to the structure, so that at any point there is a steady progression from minimum to maximum stress in each cycle without any intervening blips (Figure 12.1), referred to as constant amplitude loading. In such a case, the checking procedure at each potential fatigue site is as follows: Decide on the design life of the structure. Refer to Section 12.3.3. Calculate the number of load cycles n during the design life. Determine the pattern and variation of nominal (unfactored) loading on the structure in each cycle. Calculate the resulting stress range (f r) at the position being considered —generally taken as the difference between maximum and minimum stress in each cycle. Refer to Sections 12.3.4 and 12.4. Establish the class of the detail at the point considered. Refer to Section 12.5. Using the endurance-curve appropriate to the class, read off the predicted number of cycles to failure (N) corresponding to the stress range f r. Refer to Section 12.6. The fatigue resistance at the point considered is acceptable if N n. Figure 12.1 Constant amplitude loading. f r=stress range, f m=mean stress. 12.3.2 Variable amplitude loading The simple state of affairs covered in Section 12.3.1 is fairly rare. In most fatigue situations, the loading is more complex, leading to a spectrum of stress ranges at any critical position...
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 3 Fatigue of Composite Materials 1 3.1. Introduction The term “composite materials” covers a large range of materials. Obviously, it is not always easy to find a general approach to determine the fatigue behavior of some materials, including polymer matrix, metal matrix and ceramic matrix composites, along with elastomeric composites, Glare, short fiber-reinforced polymers and nanocomposites. For this reason, we have to set up some boundaries. In addition, it is quite useful for the engineer to carefully compare the fatigue of composite materials to the fatigue of metals, as the design of a component made of a composite material is often the result of a metal substitution. We should keep in mind what Professor Tsaï said: “Think composite”. As Bryan Harris mentioned in Fatigue in Composites [HAR 03]: “it is wrong to assume, a priori, that there are some universal mechanisms by which fluctuating loads will inevitably result in failure at stresses below the normal monotonic failure stress of the material”. Fracture within high-performance composite material structures is of a very different nature compared to that of metallic components. Failure due to fatigue is less feared in the first case than in the second. Metallic fatigue has been intensively studied for more than 150 years, since the pioneering work performed by Wöhler. Nowadays, metallic fatigue is studied using some well-established concepts: – low-cycle fatigue by Coffin and Manson; – megacycle fatigue by Wöhler; – gigacycle fatigue by Bathias; – crack propagation by Paris. The fatigue of composite materials is slightly different, as gigacycle fatigue of composite materials has not yet been studied in detail, as the S-N curves are usually plotted between 10 3 and 10 7 cycles and no further [BAT 04]. We should also bear in mind that the concept of damage tolerance can only be applied to metals and not to composite materials...
