Friday, May 19, 2017

Aging Equipment

Current aircraft are designed to last for 40 years. In some cases,the life of these aircraft have been extended well over this time period, and ways to extend life further are being considered. In the case of landing craft gear on the plane, this equipment encounters the most stressful conditions.  For instance,if the plane is to accomplish its mission, it must take off and land each time. Therefore, it is difficult to truly determine the pressurization cycles sustained.
In the case of landing gears, the plane lands at approximately 180 mi/hr. Depending on the weight of the aircraft, you can understand that the conditions for stress are high for each cycle of landing and take-off sustained. Therefore as the number of landings sustained increase the effect of any defect present in the material becomes the initiation for a fatigue crack. Fatigue starts normally at the surface. The effects increase with stress  associated with repeated landings. As the number of cycles of landing increase, so to does the stress or work hardening increase. If a defect is present, then when the stress increases to a certain level  a small crack, known as a microcrack occurs at the defect. Once micro- fracture relieves the sustained stress, then the microcrack is arrested and the sequence of events begins once more, until the stress level is such that further crack extension occurs. This concept is known as fatigue. Therefore these events will continue until the remaining non-stressed area cannot support the forces or stresses sustained in landing. At this point catastrophic failure occurs, resulting in severe danger to the pilot and the expensive aircraft.
In the present sense the flight crew watches progress. When defects in excess of about 0.040 inch are observed the landing gear components are inspected to determine the extent of the damage. Unfortunately, records can become lost and errors or misinformation can result.
The process used for manufacturing these critical parts begins with the manufacture of the steel selected for the component. All steels have defects called inclusions, porosity and perhaps surface corrosion. These defect can be extremely small and difficult to detect. Currently these defects are not often detected because of their small size. Techniques are currently available to permit detection of inclusions 0.040 in size. However, current efforts are attempting to designate inclusions less than 0.010 inch and as small as 0.001 inch. These would be difficult to accomlish using current maintenance.
Therefore a study and record of the total processing conditions becomes of importance. Once the steel is obtained and certified regarding defect content,  machining of the component begins. Generally this is under control, but often defects can occur from poor machining practice. Once the component is machined, then the part must be heat treated to achieve the proper core properties to sustain the stresses that will occur in service. This process to can result in the formation of microcracks, ultimately resulting in fatigue.Finally there is cause for concern of corrosion resulting from operation in harsh environments. Therefore,the parts are normally chrome plated  after heat treatment. This process is quite often a severe problem because of the different expansion rates between the steel and the chrome plate. Therefore the possibility of defects sustained in processing become the limiting factor in the life cycle of the component. MagnaTech intends to propose solutions to identify causes and minimization of these defects in a proposal currently being prepared. MagnaTech is looking for prospective partners to co-operate in resolution or minimization of the current problem.