ENGINES
Bearing Wear and Compressor Case Rub - Photos Courtesy of Vector Aerospace
is design geometry because stress concentrations can magnify the applied load locally to very high levels, from which cracks usually grow. As more is understood about a failure, the failure cause
evolves from a description of symptoms and outcomes (ef- fects) to a systematic and relatively abstract model of how, when and why the failure comes about (causes). The more complex the product or situation, the more vital an understanding of its failure cause is to ensuring its proper operation (or repair). Materials can be degraded by their environment by corrosion processes. Such processes can also be affected by load in the mechanisms of stress corrosion cracking and environmental stress cracking.
Metal Fatigue Metal fatigue is the progressive and localized structural dam- age that occurs when a material is subject to cyclic loading. The nominal maximum stress values are less than the ultimate tensile stress limit, and might be below the yield stress limit of the material. Fatigue occurs when a material is subjected to repeat loading and unloading. If the loads are above a certain threshold, microscopic cracks will begin to form at the stress concentrators such as the surface, persistent slip bands (PSBs) and grain interfaces. Eventually, a crack will reach a critical size and the structure will suddenly fracture. The shape of the structure will affect the fatigue life signifi- cantly. Square holes or sharp corners will lead to elevated local stresses where fatigue cracks can initiate. Round holes and smooth transitions or fillets are therefore important to increase the fatigue strength of the structure.
Fatigue Characteristics
(Gremlins Galore) • In metals and alloys, when there are no macroscopic or microscopic discontinuities, the process starts with disloca- tion movements, eventually forming persistent slip bands that nucleate short cracks.
• Macroscopic and microscopic discontinuities, as well as component design features which cause stress concentra- tion (keyways, sharp changes of direction, etc.), are the preferred location for starting the fatigue process.
• Fatigue is a process that often shows considerable scat- ter, even in controlled environments.
22
HelicopterMaintenanceMagazine.com October | November 2013
• Fatigue is usually associated with tensile stresses, but fa- tigue cracks have been reported due to compressive loads.
• The greater the applied stress range, the shorter the life. • Fatigue life scatter tends to increase for longer fatigue lives.
• Damage is cumulative. Materials do not recover when rested. • Fatigue life is influenced by temperature, surface finish, microstructure, presence of oxidizing or inert chemicals, residual stresses, contact (fretting), etc.
• Some materials (e.g., some steel and titanium alloys) exhibit a theoretical fatigue limit below which continued loading does not lead to structural failure.
• High cycle fatigue strength (about 103 to 108 cycles) can be described by stress-based parameters. A load- controlled servo-hydraulic test rig is commonly used in these tests, with frequencies of around 20–50 Hz. Other sorts of machines, like resonant magnetic machines, can also be used and achieve frequencies up to 250 Hz.
• Low cycle fatigue (typically less than 103 cycles) is associ- ated with widespread plasticity in metals; thus, a strain- based parameter should be used for fatigue life prediction in metals and alloys. Testing is conducted with constant strain amplitudes typically at 0.01–5 Hz.
Buckling In practice, buckling is characterized by a sudden failure of a structural member subjected to high compressive stress, where the actual compressive stress at the point of failure is less than the ultimate compressive stresses that the material is capable of withstanding. For example, during earthquakes, reinforced concrete members might experience lateral deformation of the longitudinal reinforcing bars. This mode of failure is also described as failure due to elastic instability. When load is constantly being applied on a member such as a column, it will ultimately become large enough to cause the member to become unstable. Further loads will cause significant and somewhat unpredictable deformations, possibly leading to complete loss of load-carrying capacity. The member is said to have buckled, to have deformed.
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48