The blog article is a summary of the work "Analysis of New Concepts for the Consolidation Roller in Laser-Assisted Automated Tape Placement Processes" by wonderful authors Nils Widmaier and Lukas Raps.
What did the research find?
The work aimed to find new concepts for compaction roller designs to enable the use of the laser-assisted thermoplastic in-situ tape placement process on curved surfaces. Both experimental and simulative methods were used to explore new concepts of compaction rollers to improve adaptability to curved surfaces, pressure uniformity, and compaction pressure.
Experiments revealed that 5 mm and 10 mm silicone sheaths exhibit a 10%–15% higher pressure uniformity on curved, concave surfaces with a radius of 250 mm than on a flat surface with a continuing trend towards reduced pressure uniformity on convex surfaces. This phenomenon is likely caused by internal deformation mechanisms that lead to buckling of the sheath between the rim and the tool surface.
The experimental results showed that the best compromise for the counteracting mechanisms of higher compaction force at the expense of lower pressure uniformity can be achieved with a solid 10 mm Shore A60 (SS960) silicone sheath. Further experiments to validate the general work principle of this compaction roller configuration are currently ongoing with the objective of manufacturing a curved demonstrator structure.
Simulations showed that through-thickness perforations in a 25 mm silicone sheath can lead to a localized reduction of compaction pressure by a factor of 3 over the contact length on flat surfaces. Compaction roller designs that deviate from a pure solid material sheath are therefore not recommended.
What is the problem with present consolidation rollers?
The present consolidation rollers have several issues, particularly when dealing with complex part geometries and curved surfaces. Here are the main problems:
Pressure Uniformity and Adaptability: Traditional compaction rollers made out of metal are not flexible enough to conform to tool surfaces with radii smaller than the tool. This lack of contact can lead to a reduction in mechanical properties. The rollers need to have full contact with the tape and even pressure distribution for successful interfacial bonding.
Misalignment: Flexible compaction rollers offer the benefit of compensating for misalignment between the roller and the tool surface. However, even at 1° misalignment, significant differences in peel forces have been observed between adaptive and non-adaptive rollers.
Material Limitations: Typical materials for flexible compaction rollers, such as Polytetrafluoroethylene or Polyurethane, have their own limitations. For instance, holes through the compaction roller can lead to deviations of up to 50% in compaction pressure.
Pressure Peaks and Full Contact: Full contact is not achieved for concave tools with a lateral orientation of the compaction roller. Pressure peaks were noted at the edges of the roller (concave tool) and in the middle (convex tool). This indicates that the compaction roller and tool selection must be coordinated.
Pressure Variations: Any change in the compaction roller geometry, which deviates from a full material sheath, will result in a change of pressure uniformity over the length of the tape. Areas directly under a perforation exhibit up to 3 times less compaction pressure than areas without perforation.
How does the author approach this problem?
The author, Nils Widmaier, approaches the problem of current consolidation rollers by investigating new concepts for the consolidation roller to enable the successful use of in-situ placement technologies on complex part geometries. Here's a summary of the approach:
Experimental Investigation: Different sheath thicknesses and materials were investigated in experiments. The uniformity of the pressure distributions was evaluated by averaging the row of sensors in the middle of the contact length over the width of the compaction roller without a tape. The pressure was averaged over the contact area of the tape in the middle of the contact length.
Simulation Setup: A Finite Element Method (FEM) model was set up in the software "Ansys Workbench v19.2" to replicate the deformation behavior of the silicone roller and to explore more complex, perforated compaction roller geometries and a roller width extension from 30 mm to 60 mm. The compaction roller (silicone sheath and aluminium rim), aluminium tool and if applicable the carbon fibre UD tape were replicated in the simulation. The load of 547 N was introduced through the rim.
Validation of Simulation Against Analytical Model: The general setup of the simulation model was verified against the analytical calculations. The parallel contact between two convex cylinders was modeled in ANSYS with an isotropic material and compared to Hertz theory. The theory of Hertz and the simulations show better agreements for rollers with thicker sheaths.
Conclusion
The article addresses the challenges faced by current consolidation rollers used in the production of fiber composite materials. The main issues include pressure uniformity, adaptability to complex geometries, misalignment, material limitations, and pressure variations. These problems become particularly pronounced when dealing with curved surfaces.
To tackle these issues, the author employs a combination of experimental and simulative methods. Different sheath thicknesses and materials are investigated experimentally, while a Finite Element Method (FEM) model is set up to replicate the deformation behavior of the silicone roller and explore more complex, perforated compaction roller geometries. The simulation model is then validated against analytical calculations.
The study concludes that the best compromise for the counteracting mechanisms of higher compaction force at the expense of lower pressure uniformity can be achieved with a solid 10 mm Shore A60 (SS960) silicone sheath. However, compaction roller designs that deviate from a pure solid material sheath are not recommended due to the significant reduction in compaction pressure.
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