Shuttle RCC (reinforced carbon-carbon) panels?

Question

Shuttle RCC panels (22 panels together) were used on shuttle wing leading edge, nose cone, 0.25-0.5 inch thick and tough like concrete. link https://www.nasa.gov/centers/kennedy/pdf/167473main_TPS-06rev.pdf .  Article pages 5-6 states that after each flight they went through three kind of simple inspections and if any damage was found they were removed and returned to the vendor for repair and refurbishment. Article doesn't specify how often must be replaced, but say that after specified number of mission they were sent back to the vendor for re-coating (or in other worlds needed surface treatment).  During processing for return to flight, all RCC panels underwent extensive nondestructive inspections (NDI) and nondestructive evaluations (NDE).  NDI inspections included the use of thermography and a CAT scan to detect imperfections or cracks in the structures on and below the surface. NDE methods included eddy current, ultrasound and X-ray inspections. In addition, several inspections of the metal components behind the RCC panels were performed.

So did anybody here know how often any of those 22 panels:

1. Were damaged during the flight and then must be replaced, repair?
2. After what exact number of specified mission (3,5,10, more) they were sent back to vendor for re-coating - needed surface treatment?
3. Were all those inspections necessary, how expansive they were?
4. Were they necessary during all existence of Shuttle or only after Columbia disaster?

Answer 1

1) Columbia's history of RCC panel replacements:

Columbia has only had three panels/Tee seals replaced over its history. Panels 12R and 10L were removed for destructive testing and pinhole evaluations. Panel 11L had fit problems and was sent to spares. Also, over Columbiaʼs lifetime, seven RCC panels and six seals on the left wing were repaired, and 11 panels and 12 seals were refurbished. All of the Columbia RCC panels were within their predicted mission life limits, and most were original panels.

2)

3) It is impossible to say if they were necessary and I do not know how to find the cost. No vehicle was destroyed because of an issue missed by inspection.

4) RCC panel inspections were performed throughout the program.

Each time a vehicle returns from space, the entire RCC and Thermal Protection System (TPS) are visually inspected to determine the extent of any damage. Inspections look for RCC impact damage and any indications of flow in the interface between the TPS (tiles) and adjacent RCC. There also exists a test method whereby the RCC panel is pressed with a gloved hand in the vicinity of RCC cracks to deter- mine the integrity of the panel and the existence of potential unacceptable subsurface oxidation. This same test is always performed on RCC panels 6 through 17 near each of the adjoining Tee seals. During each OMM, all RCC components are visually inspected including all the attachment hardware and underlying attachment structure.

During the inspections, a determination is made to either repair, refurbish, or replace the panel as necessary. Repairs are required when there is noticeable damage to the surface of the panel. Field repairs can be made at KSC or Palmdale unless the carbon substrate is exposed. In that case the panels must be sent to the vendor for repair. Refurbishment is required at regular intervals to recoat the panels to increase their resistance to oxidation and mass loss. Occasionally, complete replacement of RCC panels is necessary due to unrepairable damage. Each wing leading edge RCC panel is paired with an associated Tee seal and both of these components are generally replaced/refurbished as a unit.

All info from Chapter 10 Left Wing Processing and RCC Design of Appendix D7 of the CAIB Report. I suggest you read this entire section for details on the RCC.