DEFECTSDEFECTS

DEFECTS, FRACTURE MECHANICS, INSPECTION AND SERVICE LIFE
Despite the fact that every possible issue is considered to produce an aircraft component with the sort of perfection there are some defects with which the fracture of component and therefore the failure occurs upon its real use. each such defect is normally discovered throughout the time of part inspection with numerous techniques and the service life of the element is determined. Few defects, the fracture mechanics of those, inspection techniques and hence the determination of service life of component that causes these failures have been discussed in this section.
LOW CYCLE FATIGUE:
A data bank survey obtained by the Canadian forces reveals the fracture mechanics and the defects due to which failure occurred. The data bank record the primary landing truck beam that fractured on ground simply after refueling because of a circumferential crack formed due to the low cycle fatigue. Visual and the microscopic exam found out the fracture became due to a semicircular shaped crack with intergranular faces. The fracture mechanics evaluation performed showed the actual stress corrosion crack correspond to the theoretical crack size.

Fig. a. Fractured gear truck beam with axles. b. overview of the fractured surface. c. Overview of semicircular crack zone.

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With a view to prevent the prevalence of such a failure it was emphasized to detect any such defects in its use and prevent it. Many non destructive techniques are available to detect those defects. One such technique being the magnetic particle method, which employs the principle that magnetic flux in a magnetized object is distorted in the presence of discontinuity. The manufacturer had furnished a service life of one year on the product. which means the tests are to be conducted prior to the service life mentioned.

WHEEL CRACKS:-
From the records financial institution surveyed, that provides information of several defects of landing gears, wheel cracks from an important aspect in overcoming the topic of discussion i.e. prevention of failure. over the years the wheels of all aircraft types will fail catastrophically and were determined by deflated tires and by using non destructive inspection. most of those failures locate its roots in corrosion pitting that is exposed to the weather side.

Fig. a) Catastrophic wheel failure. b) Corrosion pits at the origin of cracks. c) Pitting followed by intergranular cracking. d)Transition from intergranular to fatigue cracking

From the examining element typically the ultrasonic or the radiography is used as wheels form the non metallic component of the part. Ultrasonic is a sensitive method that detects small flaws with only single surface accessibility, which also provides enough records at the location and length of the defects. Radiography is a veteran method employed in the detection of flaws and surface defects. This is basically the recording of the photograph produced by the passage of electromagnetic radiation such as gamma rays or the x-rays via the object of inspection onto the film.

CURRENT AND POTENTIAL APPLICATIONS OF LANDING GEARS
Subsequently, whatever that goes up must come down. Tracing the current and potential programs of landing gears, it is the part that bears the weight of the entire aircraft during the ground operations and much more vitally during the landing of the plane. over the years due to consistent research and development of several components of the aircraft landing gears such as the design, materials used, manufacturing process many kinds of landing gears have been evolved. The evolution of this component has also been influenced by means of the improvement of several other parts of plane. for instance to note, commonly, an aircraft constructed with composite material to reduce the overall weight of the aircraft do not need a conventional landing gear system used in a commercial aeroplane. also, a better and improvised landing gear system is needed for a military aircraft that needs higher landing speeds.

1.CASE STUDY-Failure of Nose Landing Gear Strut
PROBLEM IDENTIFICATION:- Three students from Covenant university and Federal university of technology, Nigeria performed a failure investigation of the nose landing gear of a training aircraft ABT-18 7. ABT-18 is training aircraft for Nigerian pilots, it’s a cantilever low wing monoplane with a single tractor engine. In 1994 many incidents had been reported for failure of nose landing gear strut and mostly it collapses during tough landing due to which excessive harm is caused to propeller, engine, airframe and human damage. Visual examination, hardness tests and impact test, microstructural examination, fractography analysis and chemical analysis are some various strategies were used for identifying primary cause and mechanisms that lead to failure and provides suggestions and appropriate corrective action so that the same failures may be prevented from occurring in the future. beach marks, failure initiation and propagation indicating fatigue mode of failure were found by doing fractograph analysis on failed sample.

Fig. Nose landing gear assembly of ABT-18 7

VISUAL INSPECTION:-In visual inspection we have to examine the failed gear strut with naked eyes, bending, fractured and buckled strut material V-shaped markings on surface indicating brittle fracture.

Fig. 3 Visual examination revealing crack initiation and crack propagation zones 1

MICROSCOPIC-INSPECTION:-When strut was tested with undamaged surface with the assist of microscope it magnifies the inclusions indicating a design defect. threat of failure will increase due to growth in fatigue. when we examine failed strut with the assist of microscope it confirmed that inclusion spreads and consequences in pitting. these inclusions will act as pressure raisers which results in cracks

Microscopic view of undamaged surface1 Microscopic view of damaged surface1

BRINELL HARDNESS TEST:- Collection of hardness test have been completed on fractured as well as in-fractured component and the conclusion was due to fracture there has been lack of strength of material which indicated that the hardness of the strut was insufficient to bear the harm due to fatigue. The figure below shows the hardness values after brinell test.

Fig Brinell hardness test results 7

CHEMICAL ANALYSIS: Spectrometer, a tool used for analysis of chemical composition was used to reveal the chemical composition of gear strut and was found that the tools strut became made of medium carbon metal (carbon content between 0.3 and 0.6%) instead of spring metallic which has the property of absorbing shock on impact load. therefore, the incorrect material selection was the predominant cause of failure for the nose wheel strut.