Micro-CT scanning is a novel alternative to conventional non-destructive testing
The micro computational tomography (CT) scanning of components is becoming an increasingly attractive option for the non-destructive testing of components both prior to and during service.
Micro X-ray computational tomography is a non-destructive testing technique used for generating both 2-D and 3-D representations of an object at a high resolution and contrast. An object is placed between an X-ray emitter and detector in the CT scanning machine. A portion of the object is then exposed to collimated X-rays, generated by the emitter. The X-ray detector subsequently measures how much the object attenuates the stream of projected X-rays, allowing for a representation of a cross-sectional slice of the object to be created in the form of an image. These images are acquired for hundreds of views of the scanned object at various angles, captured while the object rotates. A computer then synthesizes a stack of these virtual cross section slices using various algorithms, to produce a 3-D image of the scanned object. The algorithms used are also able to determine small differences in the relative density and atomic number within each scanned projection. These relative density differences account for variations in the structure of the scanned object (e.g. porosity, cracks, change in thickness etc.) and allow for the internal structure of the scanned object to be mapped accordingly.
Scanning is typically limited to sample sizes ranging from 10 mm to 300 mm. Scanning of larger samples is possible, however, scanning times become excessive, making the scanning of such samples better suited to medical CT scanning machines. Typical scan times range from between 30 minutes to 1.5 hours, depending on the resolution and quality requirements, with scanning resolution dependent upon the size of the area scanned. By reducing the scanned area, resolutions ranging from 40 µm to 500 nm are achievable.
The CT scanning of metallic components is widely used to find flaws, defects, cracks, pores or even inclusions, all of which can have a detrimental effect on a component’s mechanical properties. By integrating the CT scanning process into the final stages of component manufacture, the screening of components prior to their intended application is possible. This screening allows for both the component geometry and mechanical makeup to be determined, allowing for some of the in-service risks commonly associated with manufacturing flaws to be mitigated.
The accurate sizing of defects that CT scanning affords is also very beneficial in fracture mechanics assessments and failure investigations where the components are small enough to be scanned.
Published in Technical Tips by Origen Engineering Solutions on 1 September 2016