Ceramic matrix composites (CMC), developed for the fierce temperatures and pressures inside aircraft engines, are the latest in composite evolution. GE Aviation has been using micro-computed tomography (CT) to achieve finer resolution in its CMCs, says Matt Dragovich, manager of nondestructive evaluation and metrology at GE’s CMC Lean Lab. The aim is to scrutinize CMCs very closely as GE tries new tooling concepts and designs for CMC components. Micro-CT can see deep between CMC plies.
Regular CT will be used in actual production of CMCs, first for shrouds in high-pressure turbines in the Leap engine, later for shrouds and combustor liners in GE9X. This approach moves parts through far faster than micro-CT, and speed is needed for production purposes.
GE also will look into infrared (IR) inspections for GE9X combustor liners. The technique introduces heat into CMCs and measures how quickly it dissipates. Dragovich says GE may use IR more in the future, as it can cover broader areas than CT. It may be especially suited for flat, thin components such as combustor liners. Size is not an obstacle: “It can do larger structures than CT.”
GE is working also on a suite of analytic tools for CT evaluators. The goal is to have CT inspections so automated that they just flash red or green. Short of that, though, it is desirable to have tools for assisted defect recognition (ADR) to help CT operators judge results consistently and accurately. Dragovich expects to have ADR in place by year-end.
All these techniques are for design and production of CMCs. The Lean Lab has not looked yet at NDT for maintenance and repair of CMCs. Dragovich says shops will probably use CT or IR but may want to look at ultrasound and fluorescent penetration. “That’s a little way off; we are not working on that intensely.”
So far, the biggest challenge in CMC inspections is the familiar one of how to right-size inspection for specific points. GE can afford micro-CT in developing tools and designs for CMC production, switching to regular CT in actual production. “Cost and time are the big challenges,” Dragovich says. The same basic variables will influence aftermarket CMC inspections.