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# Fatigue Strength Design - PowerPoint PPT Presentation

DESIGNING AGAINST FATIGUE Fatigue failure account for about 80 % of part failure in engineering Occurs subjected to fluctuating loads

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Presentation Transcript

• Fatigue failure account for about 80 % of part failure in engineering

• Occurs subjected to fluctuating loads

• Generally, fatigue fractures occurs as a result of crack which usually start at some discontinuity in the material, or at other stress concentration location, and then gradually grow under repeated application of load.

• As the crack grows, the stress on the load-bearing cross-section increase until it reaches a high enough level to cause catastrophic fracture of the part.

• Fracture surface which usually exhibits smooth areas which correspond to the gradual crack growth stage, and rough areas, which correspond to the catastrophic fracture stage.

• The smooth parts of the fracture surface usually exhibit beach marks which occurs as a result of changes in the magnitude of the fluctuating fatigue load.

• Fatigue behavior of materials is usually described by means of the S-N diagram which gives the number of cycles to failure, N as a function of the max applied alternating stress, Sa.

• Alternating stress

• Alternating tension – compression

• Stress ratio, R = min / max = -1

• Fluctuating stress

• Positive R value

• Greater tensile stress than compressive stress

• max = m + a

• max = m - a

• Many types of test are used to determine the fatigue life of material

• Small scale fatigue test – rotating beam test

• Which a specimen subjected to alternating compression and tension stresses of equal magnitude while being rotate

• Data from this result are plotted in the form of S-N curves

• Which the stress S to cause failure is plotted against number of cycles N

• Figure (a) – S-N curves for carbon steel

(b) - S-N curves aluminum alloy

• In the majority cases, the reported fatigue strength or endurance limits of the materials are based on the test of carefully prepared small samples under laboratory condition.

• Such values cannot be directly used for design purposes because the behavior of a component or structure under fatigue loading does depend not only on the fatigue or endurance limit of the material used in making it, but also an several other factors including :

• Size and shape of the component or structure

• Stress concentration

• Surface finish

• Operating temperature

• Service environment

• Method of fabrication

• Endurance-limit modifying factors

Se = kakbkckdkekfkgkhSe’

Where Se = endurance limit of component

Se’ = endurance limit experimental

ka = surface finish factor (machined parts have different finish)

kb = size factor (larger parts greater probability of finding defects)

kc = reliability / statistical scatter factor (accounts for random variation)

kd = operating T factor (accounts for diff. in working T & room T)

kf = stress concentration factor

kg = service environment factor (action of hostile environment)

kh = manufacturing processes factor (influence of fabrication parameters)

• ka = Surface finish factor

• The surface finish factor, ka, is introduced to account for the fact that most machine elements and structures are not manufactured with the same high-quality finish that is normally given tolaboratory fatigue test specimens.

DESIGNING AGAINST FATIGUE

• kb = Size factor

• Large engineering parts have lower fatigue strength than smaller test specimen

• Greater is the probability of finding metallurgical flaws that can cause crack initiation

• Following values can be taken as rough guidelines :

• kb = 1.0 for component diameters less than 10 mm

• kb = 0.9 for diameters in the range 10 to 50 mm

• kb = 1 – [( D – 0.03)/15], where D is diameter expressed in inches, for sizes 50 to 225 mm.

• kc = Reliability factor

• Accounts for random variation in fatigue strength.

• Published data on endurance limit, represent 50 % survival fatigue test.

• Since most design require higher reliability, the published data must be reduced by the factor of kc

• The following value can be taken as guidelines

• kc = 0.900 for 90% reliability

• kc = 0.814 for 99 % reliability

• kc = 0.752 for 99.9 % reliability

• kd = Operating temperature factor

• Accounts for the difference between the test temperature and operating temperature of the component

• For carbon and alloy steels, fatigue strength not affected by operating temperature – 45 to 4500C kd = 1

• At higher operating temperature

• kd = 1 – 5800( T – 450 ) for T between 450 and 550oC, or

• kd = 1 – 3200( T – 840 ) for T between 840 and 1020oF

• Accounts for the difference in loading between lab. test and service.

• During service – vibration, transient overload, shock

• From experience show that repeated overstressing can reduce the fatigue life

• ke = 1 for application involving bending

• kf = Stress concentration factor

• Accounts for the stress concentration which may arise when change in cross-section

• kf = endurance limit of notch-free part

• endurance limit of notched part

• Low strength, ductile steels are less sensitive to notched than high-strength steels

• kg = Service environment factor

• Accounts for the reduced fatigue strength due to the action of a hostile environment.

• kh = Manufacturing process factor

• Accounts for the influence of fabrication parameter

• Heat treatment, cold working, residual stresses and protective coating on the fatigue material.

• kh difficult to quantify, but important to included.

• Endurance limit/Fatigue strength

• The endurance limit, or fatigue strength, of a given material can usually be related to its tensile strength, as shown in table 2.2.

• The endurance ratio, defined as (endurance limit/ tensile strength), can be used to predict fatigue behavior in the absence of endurance limits results.

• From the table shows, endurance ratio of most ferrous alloys varies between 0.4 and 0.6

• Other fatigue-design criteria

• Safe-life or finite-life

• Design is based on the assumption that the component is free from flaws, but stress level in certain areas is higher than the endurance limit of the material

• Means that fatigue-crack initiation is inevitable and the life of the component is estimated on the number of stress cycles which are necessary to initiate crack

• Fail-safe design

• Crack that form in service will be detected and repaired before they can lead to failure.

• Employed material adapted with high fracture toughness, crack stopping features and reliable NDT program to detect crack.

• Damage-tolerant design

• Is an extension of fail-safe criteria and assume that flaws exist in the component before they put in service.

• Fracture mechanics techniques are used to determine whether such crack will grow large enough to cause failure before they are detected during periodic inspection.

• Selection of materials for fatigue resistance

• In many application, the behavior of a component in service is influence by several other factor besides the properties of the material used in its manufacture.

• This is particularly true for the cases where the component or structure is subjected to fatigue loading.

• The fatigue resistance can be greatly influenced by the service environment, surface condition of the part, method of fabrication and design details.

• In some cases, the role of the material in achieving satisfactory fatigue life is secondary to the above parameters, as long as the material is free from major flaws

• Steel and cast iron

• Steel are widely used as structural materials for fatigue application as they offer high fatigue strength and good processability at relatively low cost.

• The optimum steel structure for fatigue is tempered martensite, since it provide max homogeneity

• Steel with high hardenability give high strength with relatively mild quenching and hence, low residual stresses, which is desire in fatigue applications.

• Normalized structure, with their finer structure give better fatigue resistance than coarse pearlite structure obtained by annealing.

• Nonferrous alloys

• Unlike ferrous alloy, the nonferrous alloys, with the exception of titanium, do not normally have endurance limit.

• Aluminum alloys usually combine corrosion resistance, light weight, and reasonable fatigue resistance

• Fine grained inclusion-free alloys are most suited for fatigue applications.

• Plastics

• The viscoelasticity of plastics makes their fatigue behavior more complex than that of metals.

• Fatigue behavior of plastics is affected by the type of loading, small changes in temperature and environment and method of fabrication

• Because of their low thermal conductivity, hysteretic heating can build up in plastics causing them to fail in thermal fatigue or to function at reduces stiffness level.

• The amount of heat generated increases with increasing stress and test frequency.

• This means that failure of plastics in fatigue may not necessarily mean fracture

• Composite materials

• The failure modes of reinforced materials in fatigue are complex and can be affected by the fabrication process when difference in shrinkage between fibers and matrix induce internal stresses.

• However from practical experiences, some fiber reinforced plastics are known to perform better in fatigue than some metal, refer table 2.2.

• The advantage of fiber-reinforced plastics is even more apparent when compared on a per weight basics.

• As with static strength, fiber orientation affects the fatigue strength of fiber reinforced composite

• In unidirectional composites, the fatigue strength is significantly lower in directions other than the fiber orientation.

• Reinforcing with continuous unidirectional fibers is more effective than reinforcing with short random fibers.