Application Center for Wood Fiber Research HOFZET®

Research focus

Material and damage analysis by means of computed tomography and electron microscopy

Why has a component failed? Does the cause of the problem lie in the manufacturing process? How can the dimensional stability or strength of a component be optimized? We can find this out for you. We test materials and components non-destructively regarding quality characteristics and damage caused by, for example, undesired pores or unfavorable fiber distribution. The investigation offer includes, amongst other items, the digitalization and measurement of components, defect detection, fiber analysis and in situ examination by means of computed tomography and scanning electron microscope. The decisive advantage: We can look into the component without destroying it.

Computed tomography

© Photo Fraunhofer WKI | Florian Bittner

Computer tomograph

© Photo Fraunhofer WKI | Florian Bittner

Wall-thickness analysis of a plastic injection-molded component

© Photo Fraunhofer WKI

Orientation analysis of a long fiber-reinforced plastic

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

In contrast to conventional CT applications, for in situ CT examinations not only the steady state of an examination object is detected; several successive CT scans dynamically follow the object's behavior whilst it is subjected to an external load. This can be, for example, a mechanical, thermal or corrosive loading. 

 

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.)

 

Max. scan volume dependent on recording mode:

  • 500 mm diameter, 250 mm height
  • 250 mm diameter, 400 mm height
  • Minimum voxel size: < 1 μm
© Photo Fraunhofer WKI | Florian Bittner

Fiber volume content and fiber orientation in a wood fiber-reinforced injection molded component

Process optimization of fiber-reinforced injection molded components

In the production of wood fiber-reinforced bookmarks, CT analysis enabled the verification of the structure-relevant analysis of the reinforcing fibers, in particular in critical stressed areas. The envisaged fiber volume content was also confirmed.

The open jet, which is clearly recognizable in the CT analysis (area with few fibers, extending from the injection point), led to an optimization of the process parameters in the injection molding process.

© Photo Fraunhofer WKI | Florian Bittner

Internal structure of hybrid organic sheets before and after process optimization

Process optimization of hybrid organic sheets

In the production of organic sheets for lightweight construction applications, a good impregnation of the reinforcing fibers with the plastic matrix is pursued, in order to achieve the best possible bonding properties.

For a hybrid organic sheet developed at the HOFZET, which combines carbon fibers and natural fibers with one another, CT provided the decisive evaluation features required in order to adapt the manufacturing process for an optimal fiber-matrix bonding of both fiber types.

Whilst in the early stages of the process development numerous pores were present in the matrix and between individual fiber bundles, a further development of the processing procedure enabled a significantly improved impregnation for both types of fibers.

© Photo Fraunhofer WKI | Florian Bittner

Bonding quality between aluminum and organic sheet in a hybrid material

Bonding between aluminum foam and organic sheet

Within the framework of the ”FunTrog” cooperation project, a hybrid material was developed in which an aluminum foam was combined with a glass fiber organic sheet.

By means of CT, the individual constituents of the sample (glass fibers, plastic matrix, aluminum) could be simultaneously measured and displayed in high resolution. For the structurally important interface between the polymer matrix and the aluminum, a positive connection could be seen in the CT images.

Electron microscopy and element analysis

© Photo Fraunhofer WKI | Florian Bittner

SEM image of natural fibers on the fractured surface of a composite material

With the aid of scanning electron microscopy (SEM), the surface morphology of samples can be presented in high resolution (into the nanometer range) and with a high depth of field with only a negligible amount of preparation outlay. Application examples include the investigation of particle sizes and morphologies, fiber structures and fracture surfaces.

In combination with energy-dispersive X-ray spectroscopy (EDX), an accurate localized analysis of the elemental composition of samples, e.g. in the form of line scans or mappings, is possible.

The electron microscopy laboratory of the Application Center HOFZET is operated in conjunction with the Hanover University of Applied Sciences and Arts and has a modern scanning electron microscope with EDX detector and comprehensive facilities for sample preparation (incl. microtome, sputtering unit and ion etching). The electron microscope can also be operated in the ESEM mode under reduced vacuum, thereby enabling the examination of samples which are not high-vacuum-compatible.