Cracks in all areas of the blade, including radialcracks in the tips. Cracks generally start at thecooling holes.FOD/DOD, including nicks and dents.Aluminum spattering that appears as metallicdeposits on the blade. This results fromcompressor tip rubs.HP turbine blade tip rubs. This results in coatingremoval and tip damage.TURBINE MIDFRAME DAMAGE.— T h efollowing paragraphs describe damage that you mayfind when inspecting the turbine midframe.Discoloration. — On low-time liners, the colorationis random and sometimes appears as a wavy surface.T h e c o l o r a t i o n i s r a n d o m b o t h a x i a l l y a n dcircumferentially. On high-time liners you may observesome axial carbon streaking. There are no service limitson discolorationLiner Cracking.— Initial deterioration of theturbine midframe liners occurs at the forward inner linerflange in the form of axial cracking. It is difficult todetermine the magnitude or length of a crack in this area.The area is immediately aft of the HP turbine stage 2blade platforms. Small tight cracks will probably not benoticed. Of primary interest is that there are no crackswith visible turned up edges. If cracking is observed inthe forward inner liner flange, you can use a fiberscopefor a closer look to establish the extent of the crack andthe adjacent area condition. Cracking can also occuraround the leading edge weld beads on the strut fairingsat both the inner and outer liner areas.Liner Distortion.— Turbine midframe distortionmost commonly occurs in the 10 to 12 o’clock area ofthe outer liner forward flange. The only relative gaugea v a i l a b l ef o rc o m p a r a t i v ea s s e s s m e n t(roundness/contour) is the HP turbine stage 2 blade tiparc and the stage 2 shroud contour. A fiberscope isrecommended for the final assessment of any suggesteddistortion of the liner. You will need a guide tube toposition the fiberscope.Power TurbineThe most common problem in the power turbinesection is usually a loss of the hard coat on the tip shroud.Notch wear and subsequent blade bending are directresults of tip shroud hard coat loss. Notch wear andblade bending will ultimately lead to fatigue failure ofthe airfoil. The actual loss of the hard coat cannot beconfirmed through the borescope. It can be confirmedby removal of the upper case and actual physicalinspection of the tip shrouds. You can seethe symptomsthrough the borescope by looking at the notch withprobe No. 1. Uneven notch wear may indicate loss ofthe hard coat. You should carefully inspect for anytransverse cracks in the blade airfoil around the 10percent span. Anycrackingiscauseforreplacementofthepowerturbine.The power turbine first-stage blades also have ahistory of deposit buildup that leads to rotor unbalanceand excessive vibration.For reference to partsnomenclature used in this section, refer to figure 2-11,sections F and G. Power turbine damage that you mayfind is described in the following paragraphs.CRACKS IN BLADES.— Inspect the total airfoil,platform, and tip shrouds for evidence of cracks. If yoususpect a specific area, use the high-magnificationprobe. You will see a limited amount of the stage 1blading when viewing aft from the turbine midframeliner inspection ports. You can see more detail with afiberscope or by viewing forward from the turbineexhaust duct. Cracks will show depth and undermagnification will show edge material definition. Besure to distinguish cracks from false indications such assmears and carbon streaks.NICKS AND DENTS.— Record these defects inrelation to the percent span and percent chord formagnitude and location on the blade. Record also thecondition of the blade material adjacent (at theextremities of the defect) to the observed defect. Recordany cracking or sharpness of nicks or dents. Investigatesmooth impact deformities to determine the origin ofdamage.WEAR.— Inspect LP turbine rotor blade tip shroudinterlocks or circumferential mating surface for wear atstage 1. Wear is observable and will appear as shownin figure 2-31.D I R T , C O L O R A T I O N , P I T T I N G , A N DCORROSION.— High-time LP turbine rotorassemblies may show airfoil surface irregularities thatcould be dirt accumulation, carbon buildup, surfacepitting from particles in the gas stream, or corrosion ofthe blade material. Dirt and coloration are of littleconcern; however pitting and corrosion may besignificant.2-24
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