January_February_2022_AMP_Digital

1 7 A D V A N C E D M A T E R I A L S & P R O C E S S E S | J A N U A R Y / F E B R U A R Y 2 0 2 2 INDUSTRIAL CT SCANNING USES X-RAYS TO CREATE 3D REPRESENTATIONS OF AN OBJECT IN SOFTWARE. T he 3D computed tomography (CT) scan was invented in 1972 by Godfrey Hounsfield of EMI Laboratories in England and Allan Cor- mack of Tufts University in the U.S. [1] . The technology was initially commer- cialized for use in the medical field. Although it is unclear when the first application of CT was used for indus- trial, aerospace, or consumer prod- ucts, a review of the literature suggests it occurred in the early 1980s, with 3D quantitative CT applications occur- ring in the late 1990s involving volume and distance analysis [2] . Industrial mi- cro-computed tomography (µCT) and more recently sub-micron computed tomography [3] is increasingly being uti- lized by national laboratories, industri- al, additive manufacturing, aerospace, defense, airline, product manufactur- ing, and failure analysis laboratories. The use of CT in applications outside of the medical field has broadened the scope of fundamental scientific/engi- neering research in fields, including but not limited to mechanical engineering, materials science, biology, metallurgy, semiconductors, electrical engineer- ing, archaeological research, avian re- search, and biomimetic studies. Advances in reconstruction algo- rithms, improvements in image reso- lution, development of quantitative software modules including geometric dimensioning and tolerancing (GD&T) and void volume fraction analysis, and improving scan methods (helical CT versus cone beam CT) have made CT data acquisition more efficient. The continued dissemination of CT knowl- edge, applications, and training sem- inars have made the methodology accessible to forensic engineering lab- oratories and independent engineer- ing consultants. Laboratories are now employing qualified industrial CT tech- nicians (as compared to CT/x-ray techni- cians in the medical field) who perform scans routinely and efficiently as a ser- vice, which has contributed to making the technique a standard methodolo- gy for forensic failure analysis investiga- tions. The use of CT imaging as applied to industrial applications has allowed CT cabinet manufacturers to expand ranging from micron to meter-length scales. However, resolution and fea- ture interpretation are dependent on sample size and the part geometry. It should be noted there is a tradeoff between feature resolution and the scan volume of interest when per- forming a CT scan. The methodology has been applied to a range of prod- ucts including braze joints, welds, and electromechanical sensors. The CT scan has become the most sought- after method for identifying the manu- facturer of 18650 Li-ion batteries, es- pecially if involved in exploding e- cigarettes or batteries which may have been sold as counterfeit rewrap- ped batteries. HISTORICAL INFORMATION ON MICROSCOPY AND MATERIALOGRAPHY Traditional metallography was in- vented in 1863 by Henry Sorby. Met- allurgists have used metallography to examine alloy microstructure to aid in understanding how various casting, mechanical reductions, and heat treat- ments change microstructure, constit- uent morphology, and phases, which in turn greatly effect mechanical and material properties of a formed cast or wrought product. Metallography is the study of selected planar sections of metallic materials using a microscope that reflects light from a polished sur- face to the objective eyepiece, allow- ing the surface structure to be studied with typical magnifications between 50X – 1500X [4] . This allows one to resolve microstructural features of ~0.2 µm or larger [5] . Various etchants can be ap- plied to the polished surface to reveal grains, grain boundaries, constituents, or phases. Recall that metals and alloys are opaque, and this requires careful surface preparation to reflect impinging light from the specimen surface. Polishing techniques that have been applied to study the structure of nonmetallic materials are called ma- terialography. This method of study has been developed to examine struc- tures in polymers, elastomers, ceram- ics, composites, and the interfaces of various multilayered materials. Both sales internationally to laboratories that do a variety of engineering work: product design/R&D, reverse engineer- ing, and postmortem failure analysis. As a nondestructive testing inspection method (NDT), it has gained the atten- tion of practicing forensic engineers as a means of preserving and analyzing articles considered evidence in mat- ters that may be part of civil or crimi- nal litigation. Depending on resolution, cabinet size, and power, the price of an industrial CT scanner can vary signifi- cantly ranging from $175,000 to well over $2,000,000. X-ray sources typical- ly range from 160 to 450 kV, with x-ray spot sizes ranging from 1 to 80 micron depending on configuration and power. Geometric magnification can vary, but can be well over 150X. Industrial CT scanning uses x-rays to create 3D representations of an ob- ject in software. X-rays with varying en- ergy levels are accelerated toward an object and interact with the object. The energy of the attenuated x-rays that pass through the object are captured using a detector. Grayscale values of the x-ray image correspond to attenua- tion levels of the x-ray source. The de- tector signal is used to develop a digital x-ray radiographic image of all internal and external details of the object at that specific instance in time and object ori- entation. The CT scan rotates the object 360 degrees to capture the object in multiple orientations. A typical scan takes 45 minutes to 2 hours and cap- tures thousands of x-ray radiographs as the part rotates about an axis in the cabinet. The 2D x-ray radiographs are collected and utilized by software to construct a 3D rendition of the object. Software analysis of the 3D object al- lows one to examine the object in virtu- ally any “cut” plane and orientation. The development of engineered consumer products creates the need to examine and identify features