viewed through wide angle probe No. 2. Whencircumferential cracking is observed, record the bandnumber and the span of the cracking relative to thenumber of cooling/dilution holes. Use the diameterof the cooling holes as a comparative measurementgauge.Axial Cracks. — Axial cracking usually starts atband No. 3 on the inner liners and propagates aft andforward. As operating time is accrued, these axialpanel cracks grow into three-legged cracks as seen infigure 2-23. The edges of these cracks will separateand the corners will lift into the flow path. Inspect theareas aft and forward of these cracks, recording theaxially separated cracks that show a tendency to growtogether.DOD is the primary cause of damage to the HPnozzle and turbine rotor elements. It is caused by piecesfrom the combustor liners cracking out of the paneloverhangs and impacting with the rotating turbineelements. The most serious problem is the separation ofa large section of liner that could cause significantdamage. This usually occurs as a result of axial andcircumferential cracks growing together as shown infigure 2-24. It is important to record the damage toadjacent areas of about 5 inches to either side of thedamaged area.These areas can grow together andl i b e r a t e l a r g e p i e c e s o f m a t e r i a l .T h e s ecircumferentially spaced, cracked areas are usuallyseparated at every other fuel nozzle spacing along withaxial color streaking.Missing Metal and Burn-Through.— Inspect forthe loss of metal at the panel overhang and the areabetween dimples (fig. 2-25). Bum-through of the linersis not common. What is common are the bluish-blackslag areas that show roughness and appear to beoxidized. Inspect these areas carefully for T cracksbecause they will propagate and open up.Figure 2-25.—Inner/outer liner burns and missing metal.2-19
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