CT Scanning of Turbine Blades, Composites and Castings

During inspection of complex turbine components such as turbine blades, most demands can be satisfied using computed tomography (CT) X-ray technology. It is important to examine several aspects of the blade to ensure quality and performance. Turbine manufacturers are usually concerned with the erosion, wear, and wall thickness of the blade.

Wall Thickness

A wall thickness analysis can be carried out in Volume Graphics software, color coordinating wall thickness to scale. A predefined inspection can be set up with inspect/reject requirements, which will indicate if the wall thickness is too thin.

Another important aspect is airflow. When in use, air is pumped through so the turbine blades are maintained in a cool state. Vent holes are drilled following the cast process. It is important to examine the drilling to ensure that no hot-spot is created by striking the back wall during the drilling process.

Erosion and Wear

Generally, composite manufacturers try to identify delamination, wrinkles, porosities, lack of material, and fiber orientation problems.

The composite material is made of layers, where fibers are distributed in three dimensions, which makes 3D X-ray scanning important. The material properties are anisotropic as they vary based on the material orientation.

Manufacturers are concerned about the delamination of the layers, as this leads to a reduction in reinforcement with the structure becoming compromised. Porosity detection is also important because porosity can turn into a stress point to form a crack or delamination. Fiber orientation is critical to the strength of the structure being assembled.

3D Inspection

The 3D rendering capabilities of North Star Imaging’s proprietary efX-CT software enables multiple virtual cross sections through the part in multiple axes, and the resolution often enables observing individual composite fibers that are only a few micrometers in size. Wrinkles and delamination are identifiable. Porosities are measurable and quantifiable.

CT allows focus to be placed on observing the true indication and its location. Using 3D images, the relationship of the actual indications can be observed, their size can be quantified, and the relationship to machine surfaces and edges can be seen.

Another dynamic feature of CT is the capability to create a surface rendering of the component. Creating a polygonal mesh to extract the surface information and the ability to define or surface items such as gas porosity within a weld increases measurement ability and accuracy.

Surface extraction can be very simple for products like tube welds or castings where the material is somewhat homogenous. In the case of North Star Imaging’s tubing weld, the porosity indications have a surface created around them, which allows the post- processing software to measure these features or create a porosity report as per threshold size settings for maximum and minimum indications.

At this stage, the interpreter has the full capability of reviewing the data in 3D. They can measure the indication, review spacing and orientation, and make decisions on part quality with data that was previously unavailable.

For more information: www.4nsi.com

This article first appeared in AZO Materials and has been reproduced with permission of North Star Imaging.