Technology for Wood and Natural Fiber-Based Materials

Research project

Computed tomography shows material density distribution and particle structures

Materials and components can be tested nondestructively using computed tomography (CT): The surface and the internal topology of the complete sample is mapped with a resolution down to the micrometer scale and imaged in three dimensions. A CT evaluation can comprise several qualitative and quantitative analyses, e. g. the particle structure, the density distribution and the dimensions. For example particle boards, MDF, OSB, WPC, plywood, insulation materials and wood foams can be measured. Samples with a size sufficient to evaluate local variations of the material quality (A4 and bigger) can be scanned with the equipment at Fraunhofer WKI. 

Measurement principle

X-rays are attenuated when they pass objects depending on material properties and path length, and CT utilizes this property. A series of 2-dimensional X-ray images of the sample rotating stepwise is acquired from different angles, and then these data are reconstructed to achieve the 3-dimensional volume image of the sample. The gray values represent the local absorption of the material, and after a calibration the local density can be determined. The 3D volume data consist of a large number of volume elements (voxels) – for example, a sample with a size of 50 mm x 50 mm x 20 mm can be presented at a resolution of 30 μm per voxel. For a thorough material evaluation a resolution of 1 μm is possible.

The radiation energy and the measurement geometry can be scaled according to material, sample size and scope of the analysis. The scanning time depends on sample size and resolution.

© Fraunhofer WKI | Burkhard Plinke
Horizontal density distribution in an OSB, size 210 mm x 300 mm

Density distribution

CT measures local X-ray absorption for a sample in three dimensions. Density distribution in all directions can be derived after a calibration, e. g.

  • the density profile perpendicular to the board plane for small sections,
  • density variations in the board plane also for big areas (A4 and more).

Such measurements can help to minimize material weakness due to local density variations.

© Fraunhofer WKI | Burkhard Plinke
Particle structures in a particle board, size 50 mm x 50 mm (face layer left, core layer right)
© Fraunhofer WKI | Florian Bittner
Fiber volume fraction and orientation in a wood-fiber reinforced injection-moulded part

Particle structure

Included particles in wood-based materials are visible by CT if the density difference is sufficient. This applies e. g. to

  • mineral and metal inclusions,
  • wood particles in WPC or injectionmoulded parts.

Particles in a matrix can be characterized by

  • length, diameter, aspect ratio,
  • distribution,
  • volume fraction,
  • orientation.

Moreover, specific morphological features can be made visible, e. g. the surface structure of fibers, ondulations in fabrics and vascular structures of wood.

© Fraunhofer WKI | Burkhard Plinke
Left: Wall thickness analysis for a compression moulded component; Right: Fiber orientation in a peeled veneer

Dimension measurement 

CT scans also deliver geometric models of shaped components. Precise values of different dimensional features can be derived, e. g.

  • distance, diameter, radius, angle,
  • wall thickness,
  • volume of glue joints.

Also internal structures like pores, hollow spaces, cracks etc. can be detected and measured. The local wall thickness of a component can be determined automatically, color coded and compared with reference values. Nominal/actual comparisons of the CT volume data with a complete reference are possible, e. g. a CAD model or a reference sample.

Equipment

  • Computer tomograph Procon X-ray CT-AlphaDuo:
  • 240 kV mikrofocus and 225 kV highpower X-ray tube
  • 4MP detectors
  • Sample: Max. diameter 500 mm, max. height 400 mm, max. weight 25 kg
  • Max. scan volume, depending on acquisition mode:
  • Diameter 500 mm
  • Height 400 mm
  • Min. voxel size: < 1 μm
  • Evaluation:
  • VG Studio Max for graphical, geometrical and morphological evaluation
  • Proprietary software for analysis of density distributions

Particle structure

Included particles in wood-based materials are visible by CT if the density difference is sufficient. This applies e. g. to

  • mineral and metal inclusions,
  • wood particles in WPC or injectionmoulded parts.

Particles in a matrix can be characterized by

  • length, diameter, aspect ratio,
  • distribution,
  • volume fraction,
  • orientation.

Moreover, specific morphological features can be made visible, e. g. the surface structure of fibers, ondulations in fabrics and vascular structures of wood.

 

Equipment

  • Computer tomograph Procon X-ray CT-AlphaDuo:
  • 240 kV mikrofocus and 225 kV highpower X-ray tube
  • 4MP detectors
  • Sample: Max. diameter 500 mm, max. height 400 mm, max. weight 25 kg
  • Max. scan volume, depending on acquisition mode:
  • Diameter 500 mm
  • Height 400 mm
  • Min. voxel size: < 1 μm
  • Evaluation:
  • VG Studio Max for graphical, geometrical and morphological evaluation
  • Proprietary software for analysis of density distributions

Particle structure

Included particles in wood-based materials are visible by CT if the density difference is sufficient. This applies e. g. to

  • mineral and metal inclusions,
  • wood particles in WPC or injectionmoulded parts.

Particles in a matrix can be characterized by

  • length, diameter, aspect ratio,
  • distribution,
  • volume fraction,
  • orientation.

Moreover, specific morphological features can be made visible, e. g. the surface structure of fibers, ondulations in fabrics and vascular structures of wood.

 

Equipment

  • Computer tomograph Procon X-ray CT-AlphaDuo:
  • 240 kV mikrofocus and 225 kV highpower X-ray tube
  • 4MP detectors
  • Sample: Max. diameter 500 mm, max. height 400 mm, max. weight 25 kg
  • Max. scan volume, depending on acquisition mode:
  • Diameter 500 mm
  • Height 400 mm
  • Min. voxel size: < 1 μm
  • Evaluation:
  • VG Studio Max for graphical, geometrical and morphological evaluation
  • Proprietary software for analysis of density distributions