1. Aircraft fatigue analysis aims to predict fatigue life under cyclic loadings. Fatigue occurs due to repeated stresses below the failure load and can cause catastrophic failure.
2. Approximately 75% of aircraft structural failures are due to fatigue. Fatigue cracks initiate from stress concentrations and propagate over many load cycles.
3. Fatigue life prediction methods include S-N curves from constant amplitude testing and Miner's rule for variable loading, though it has limitations and does not consider stress order effects.
1. Aircraft fatigue analysis aims to predict fatigue life under cyclic loadings. Fatigue occurs due to repeated stresses below the failure load and can cause catastrophic failure.
2. Approximately 75% of aircraft structural failures are due to fatigue. Fatigue cracks initiate from stress concentrations and propagate over many load cycles.
3. Fatigue life prediction methods include S-N curves from constant amplitude testing and Miner's rule for variable loading, though it has limitations and does not consider stress order effects.
Original Description:
describes fatigue process in aircraft structure and materials
1. Aircraft fatigue analysis aims to predict fatigue life under cyclic loadings. Fatigue occurs due to repeated stresses below the failure load and can cause catastrophic failure.
2. Approximately 75% of aircraft structural failures are due to fatigue. Fatigue cracks initiate from stress concentrations and propagate over many load cycles.
3. Fatigue life prediction methods include S-N curves from constant amplitude testing and Miner's rule for variable loading, though it has limitations and does not consider stress order effects.
1. Aircraft fatigue analysis aims to predict fatigue life under cyclic loadings. Fatigue occurs due to repeated stresses below the failure load and can cause catastrophic failure.
2. Approximately 75% of aircraft structural failures are due to fatigue. Fatigue cracks initiate from stress concentrations and propagate over many load cycles.
3. Fatigue life prediction methods include S-N curves from constant amplitude testing and Miner's rule for variable loading, though it has limitations and does not consider stress order effects.
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Aircraft fatigue analysis
Objective Able to predict fatigue life of aircraft structures under cyclic loadings
Introduction Fatigue is a very important area of concern which will affect the structural integrity. Approximately 75% of all aircraft structural failures are due to fatigue.
Definition: Fatigue is a process of progressive permanent structural change in a material subjected to repeated cyclic applications of stresses associated with operating loads. It is a failure mode that occurs as a result of large number of fluctuations. Prepared by :Dr. Dayang Laila Week 3
A single load will not harm the structure if below static failure load, but if repeated many times, fatigue failure can occur.
Loads applied on aircraft structures are seldom static (monotonic) but usually fluctuate either above some mean stress or with complete reversal in sign. Endurance limit (fatigue limit) the highest stress level which the material can withstand for an infinite number of load cycles without failure. Fatigue failure initiates small (micro) cracks in the material which eventually grow into large (macro) cracks. If not detected, will result in catastrophic failure.
Prepared by :Dr. Dayang Laila Week 2
Scope of problem 10% of all aircraft crashes are due to structural failure, but only 2-3% in civil aircraft. Approximately 2/3 of all structural failures are due to fatigue. Historical disasters: DeHavilland Comet aircraft in 1954. Fatigue cracks in the pressurized fuselage structure initiated a fuselage decompression failure at a high altitude.
Major cause of fatigue cracking
Design deficiencies (stresses too high, notch effects, etc)
Improper assembly (including damage by maintenance)
Corrosion added initiation (surface corrosion progressing inwards causing strength deteriorates) Defective material Fretting aided initiation (small scale rubbing movements and abrasion of adjacent parts) Thermal aided initiation (caused by thermal expansion and contraction) High frequency stress fluctuations due to vibrations excited by jet or propeller noise.
Why structures fail due to fatigue?
Aircraft loads are cyclic and complex, depending routes and types of aircraft. If the fracture of a component is the primary failure mode, it is unlikely to be due to a simple event.
-stress concentrations at holes, sharp corners, cut-outs, etc will increase the probability of fatigue failure. Fatigue cracks are most likely to initiate at these stress concentration sites. It is very important to get correct stresses in order to estimate fatigue lives of structural components.
Elastic stress concentration
Definitions for fatigue analysis
Stress cycle: it is the smallest section of the stress-time function which is repeatedly periodically and identically.
Stress amplitude, Sa=0.5(Smax Smin) Mean stress, Sm=0.5(Smax + Smin) S = Smax-Smin R = Smin/Smax
Other examples of simple constant amplitude loading cycles.
Endurance, N the number of stress cycles to failure for tests at constant amplitudes. Fatigue strength, Sam(N) the alternating stress at a specified mean stress that give rise to an endurance N. Example: Sa0(104) denotes that the alternating stress which under zero mean stress give rise to an endurance of 10000 cycles. Fatigue limit, Sam(), or Se the highest level of alternating stress for a given mean stress at which the endurance may be regarded as infinite. In other words, it is the highest level of specified character which may be applied for an infinite number of cycles.
Fatigue life the useful life as limited by fatigue. The criterion oflimitation maybe one of strength, performance or service ability. In aeronautics the life may be expressed as flying hours, number of flights, number of applied loading cycles, etc.
Fatigue analysis under constant amplitude loading
Fatigue test on actual components are often impossible - high cost and time consuming. Laboratory fatigue testing method:
From rotating bending tests, relationship was found between fatigue limit and ultimate tensile strength; Se/Sult = 0.5 (steel, where mean stress is zero)
Example of fatigue curve
Example of S-N curve as a result of a number of fatigue test
Mean stress effects on fatigue life
In most applications, cyclic stress applied to a component is seldom fully reversed.
Higher tensile mean stress will decrease/increase fatigue life?
Combined effects of alternating and mean stress on fatigue endurance
Gerber, Goodman and Soderberg diagrams
Observations of models Experience has shown that most test data lie between the Gerber and Goodman diagrams. 1. The Soderberg line provides a conservative estimate of fatigue life for most engineering alloys 2. Goodmans line matches experimental data quite closely for brittle metals, but is conservative for ductile alloys. 3. Gerbers parabola is generally good for ductile alloys.
In general, the most widely used design aid for estimating the effect of mean stress on the alternating stress amplitude is the Goodman diagram, which at its simplest is shown below. Note that the ratio OB/OA is a reasonable assessment of the
Reserve Factor or Safety Factor.
Fatigue analysis under variable amplitude loading
Constant amplitude loading not realistic.
Block loading spectrum
The load spectrum may be simplified by some multilevel stress patterns(block loading spectrum). The stress cycles is divided into groups characterized by stress magnitude and number of cycles.
Cumulative damage due to a variety of stress amplitudes
Best known and widely used method for estimating the cumulative damage is known as the Palmgren-Miners rule, or Miners law. Hypothesis of the law: if the structure is subjected to ni cycles at a stress amplitude of a for which the average number of cycles to failure is Ni, then the amount of damage (Di) which will be caused by this particular stress amplitude will be ni/Ni. In other words, for each stress level the fatigue damage is
appliedloadcycles(ni ) Di allowableloadcycles( N i)
When fatigue loading involves many levels of stress amplitudes, the total damage is a sum of the different damage ratios and failure should still occur when the ratio sum equals one. In general form:
ni Di 1.0 i 1 N i k Where k = number of stress levels in the loading spectrum
i = ith stress level ni = number of cycles applied at i Ni = fatigue life at i (from material S-N data)
Palmgren-Miners Linear Cumulative Damage Rule
Limitations of Palmgren-Miner method
These effects are not accounted for: 1. The effect of the order of the load applications (as shown below) 2. Notch effect (such as fasteners holes, etc)
