QMT Features: September 2015
3D CT scanning for aerospace composites
Today’s faster and better CT scanning technology is widening its area of application in NDT – including the inspection of composite aerospace components

Industrial CT technology is improving very quickly. While a few single CT slices could take hours to generate years ago, it is now possible to reconstruct complete 3D models with billions of Voxels (three dimensional Pixels), in just seconds. This opens the door for numerous new applications like 3D inline automatic defect recognition, 3D reverse engineering, rapid prototyping, 3D metrology, and so on, so it has now become a very competitive technology for 3D scanning.

The principal benefit of using 3D CT for scanning or digitisation is that a complete model with both external and internal surfaces of an object is obtained without destroying it.  Moreover, CT works with any surface, shape, colour or material (up to a certain density and/or thickness penetrable with X-rays). Generally, a modern start-to-finish CT scan can be as fast as a two seconds or take longer than an hour, depending on the resolution requirements and size and/or density of the object.  Overall, the resolution is excellent both internally and externally, which in turn can fulfill virtually any designer’s needs.

Computed Tomography in Use

Computed Tomography has proven to be an outstanding tool for many industries, especially aerospace.  The demand for CT continues to be tremendous; largely due its versatility and capabilities to do what other technologies cannot.  CT scans nondestructively provide excellent resolution internally and externally, which then allows for measurement on surfaces both inside and outside an object.  Also, due to the penetration of X-rays, CT scans are unaffected by certain object characteristics such as dark, reflective or transparent surfaces and/or shaded zones on the item that can cause difficulty with other 3D scanning methods. 

Furthermore, 3D CT reconstruction models can be directly compared to CAD models and/or other CT models in order to display differences or commonalities in measurements, densities, voids, etc.
The images at the top of the page show a 3D CT reconstruction of a wrinkled composite plate.  The CT model can be manipulated in real time 3D and it is also possible to slice through in any direction for internal inspection.

The reconstruction process consists of complex algorithms that transform the stack of 2D X-ray images to a 3D Voxel volume model.  This process uses a GPU (Graphics Processing Unit) based software which utilizes the new NVIDIA graphic card capabilities. NVIDIA’s graphic card employs hundreds of computation cores.  This massive number of cores accelerates the process and increases the speed of 3D CT reconstruction by a factor of up to 50x. Developed with a CUDA interface and the latest technology in graphic cards, this proprietary North Star Imaging software now makes it possible to perform very fast CT reconstruction, which in turn boosts the number of achievable scans.  Due to the high speed capabilities, inline CT scanning for 100% quality inspection or 3D metrology control is now attainable. 

The image left shows an example of a modern CT system layout.  It consists of a radiation shielded enclosure, which houses the X-ray tube, detector and rotational stage.  Adjacent to the enclosure is a computer workstation, consisting of a 2D X-ray console for the set up and acquisition steps, and a 3D CT supercomputer workstation for volume reconstruction and visualisation.

Many options are possible through the use of a 3D CT reconstruction volume model.  For basic 2D measurements, the Slice Window pictured above is generated from the cutting plane in the 3D volume.  From there a length, diameter, angle, etc… can be applied on the single image. The main benefit, of course, is that any feature, part or even defect inside a structure or an assembly can now be measured without destroying it.

The 3D CT reconstruction, which is made of several million or billion Voxels, can also be transformed to a surface model. The resolution of the 3D model depends on the number of Voxels generated from CT reconstruction. A threshold value of radiodensity is chosen by the operator and set using edge detection image processing algorithms.  From this, a 3-dimensional model can be constructed and displayed on screen. Multiple models can be constructed from various different radiodensity thresholds, therefore allowing different colours to represent each component of an assembly.  Typically, models are composed of thousands of polygons up to 50 million polygons.

The pictures below show a surface reconstruction (polygon mesh) of structural honeycomb and an aerospace assembly.  All the internal structural features are reconstructed as well since the CT reconstruction provides volumetric information. 

With the generated polygon mesh surface model, many different applications become available to the user. The output format (points cloud, STL, WRL …) is compatible with most CAD software for reverse engineering applications, rapid prototyping machines for modeling, finite element analysis software for simulations, etc.  
In most cases, the polygon mesh generated by the CT system can be used in the above applications without modification and typically, the resolution is higher than needed.  However, in order to modify or take measurements of the CT surface model with a CAD software, the CT model needs to be processed to make it editable. New generation modelling software (e.g. Geomagic, Rapidform, Polyworks) propose semi-automatic tools to transform the polygon mesh to Nurbs Surfaces and parametric CAD models. User intervention is still necessary for manual operation in transforming the scanning surface to real solid CAD.

Comparing 3D CT image to CAD model
Dimensional analysis is one key application available for model comparison. Since CT and especially microCT provides very accurate dimensions on surfaces, the technology is often used for metrology studies.  Measurements can be done either directly on the surface using any CAD or Metrology software, or it can automatically compare the CT model with the CAD model, or even the CT model with another CT model.
All in all, 3D CT is now accessible for most industries as a viable tool; user-friendly interfaces, increased scan speeds, and decreasing prices have all attributed to the rapid growth of this technology in the marketplace.  Having very accurate internal dimensions without destroying the item, along with the ability to compare to a reference model is entirely unique to CT.  There are no shaded zones, it works with all kinds of shapes and surfaces, there is no post-processing work needed and the resolution is excellent. Above all, the greatest benefit is the ability to nondestructively obtain the internal structure of the object, and CT is the only technology capable of achieving such performance.
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Rob Tremain Photographer
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