Material's Fatigue
Material's Fatigue
Material's Fatigue
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.
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.
-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.
Stress amplitude, Sa=0.5(Smax Smin) Mean stress, Sm=0.5(Smax + Smin) S = Smax-Smin R = Smin/Smax
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.
2. S-N curve
experimental tests Derived S-N curve Statistical nature of fatigue
From rotating bending tests, relationship was found between fatigue limit and ultimate tensile strength;
Se/Sult = 0.5 (steel, where mean stress is zero)
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
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)