Circular Composite Materials and Textiles

Computed tomography (CT) is a fast and non-destructive method for material and component examination. It enables a three-dimensional representation of the internal and external structure of objects with a detail detectability which goes down into the micrometer range. CT analysis is of great value for the efficient new development and further development of materials and production processes. By means of material samples, it is possible to determine within a short time whether, for example, a good infiltration of the reinforcing fibers with the plastic matrix has taken place during the manufacture of organic sheets, and whether the material contains pores or other flaws. As a consequence, the processing temperatures and pressures, for example, can be adapted, or differing matrix polymers can be compared regarding their workability. The success of the procedure adaptation can, in turn, be verified by means of CT images.

As CT can be applied largely independent of the material, an almost unlimited range of applications exists beyond these examples. With CT, not only classical materials such as plastics, wood-based materials, building materials or metals can be investigated but also, in particular, hybrid materials, which are increasingly gaining in significance. For hybrid components made from fiber composite plastics and metal, the interface quality between plastic and metal can, for example, also be assessed, in addition to the fiber-matrix bonding in the composite material. Furthermore, CT is also suitable for geological, biological and archaeological samples.

Our computer tomograph can be very flexibly utilized, due to its being furnished with a large measuring chamber, two x-ray tubes and two detectors. On the one hand, large objects with a diameter of up to 500 mm can be detected and on the other hand, resolutions down into the lower micrometer range are possible.

Measuring principle

CT uses the X-ray characteristic of penetrating objects and thereby being weakened in dependence on the material and the path length. From a series of 2D X-ray images of the examined object from different camera angles, generally by gradual rotation of the test object, the 3D volumetric representation of the object is reconstructed with the assistance of a computer. The 3D volume is comprised of a large quantity of so-called voxels - the 3-dimensional analogs of pixels - with absorption-specific gray values. The edge length of the voxel thereby determines the detail detectability of the CT scan. The measurement duration is taskdependent and lasts between a half-hour and two hours.

Geometry ascertainment

Through a CT measurement, the geometry of molded parts can be accurately ascertained. This enables widely-varying dimensional measurements to be precisely performed, for example of:

• distances
• diameters
• radii
• angles

Internal structures such as cavities are also hereby detected. Furthermore, the wall thicknesses of a component can be automatically determined, color-coded and compared with nominal wall thicknesses. The nominal/actual comparison enables the comprehensive adjustment of the CT volume data of the measurement object with a reference object. This can be, for example, a CAD model or a reference measurement. In the reverse direction, a 3D model can be exported from a CT measurement, for example for reverse engineering.

Pore, cavity and inclusion analytics

The CT characteristic of showing the internal structure of the material enables defects such as cracks, cavities or pores to be detected and quantified. This includes the determination of:

• defect volume
• positions of the defects in the sample
• geometric properties (diameter, volume, sphericity) of the individual defects

Analog to this, these evaluation possibilities can be applied to inclusions such as foreign particles or filling materials. A further application example is the characterization of the structure of foams.

Fiber analytics

In fiber composites, the individual reinforcing fibers can be resolved by means of CT. This opens up a comprehensive characterization of the materials as regards, for example:

• fiber length and diameter, ratio length/diameter (aspect ratio)
• fiber distribution
• fiber volume proportion
• fiber orientation

Furthermore, morphological features, e.g. the surface structure of fibers, fiber undulations or the vascular structure in wood fibers, can also be detected.

In situ CT

Unlike classic CT applications, in-situ-CT examinations do not record solely the stationary state of a test object, but instead dynamically track the behavior of the object throughout several consecutive CT scans, during which the object is exposed to an external load. This can be e.g. a mechanical, thermal or corrosive load.

We are able to subject material samples to a series of complementary in-situ-CT examinations. These include tensile, compression and bending tests, which can be performed within a temperature range of between -20 and +160 °C and which are either linear or cyclic.

In contrast to frequently practiced tensile or compressive tests, in bending tests a complex loading case exists under the simultaneous influence of tensile and compressive forces, through which even more extensive information can be recorded concerning the failure mechanisms. We are the only institute with the possibility of performing 4-point bending tests on a number of scales. On the temperature-controlled in-situ-stage, the bending failure of small samples (approx. 24 x 10 mm) can be examined in high resolution (< 10 μm). In a further in-situ-bending test, which arose through cooperation with the Application Center for Computed Tomography in the Measurement Technology CTMT of the Fraunhofer Institute for Integrated Circuits IIS, the examination of larger and therefore more realistic samples (up to 150 mm x 25 mm) is possible in accordance with DIN EN ISO 14125.
 

Through the comparative analysis on a number of scales, the influences of the limited sample dimensions can be visualized for high-resolution scans and thereby included in the evaluation of failure processes.

Due to the modular structure of the in-situ-bending test, an adaption to sample geometries which deviate from DIN EN ISO 14125 is also possible. 

Equipment

The system Procon X-Ray CTAlphaDuo, with its large measuring chamber, two x-ray tubes and two detectors, can be very flexibly utilized. On the one hand, large objects with a diameter of up to 500 mm can be detected and on the other hand, resolutions down into the lower micrometer range are possible. 

Key data:

• 240 kV micro-focus and 225 kV high-power x-ray tube
• 4MP detectors
• Samples: Diameter max. 500 mm, height max. 400 mm, weight max. 25 kg
• In situ stage for 4-point bending tests
• Z-shaft for further in situ structures (e.g. fluids, pressure, etc.)
   

In-situ Laboratory:

• In-situ-CT tension test for samples approx. 30 mm x 20 mm
• In-situ-CT compression test for samples up to 39 mm diameter, 15 mm height
• In-situ-CT 3-point bending test for samples approx. 24 mm x 10 mm
• In-situ-CT 4-point bending test for samples approx. 24 mm x 10 mm
• linear or cyclic load
• temperature control of the  the measurement chamber with a temperature range of between -20 and +160 °C 
• In-situ-CT 4-point bending test on the basis of DIN EN ISO 14125 for samples up to 150 mm x 25 mm
   

Max. scan volume dependent on recording mode:

• 500 mm diameter, 250 mm height
• 250 mm diameter, 400 mm height
• Minimum voxel size: < 1 μm