Effect of Defects on Part Made with AFP Using Digimat

Automatic Fiber Placement (AFP) is a fast and efficient deposition process of carbon prepreg for large component application. To accommodate the composite strips onto a double curved surface, the tows can be cut then restarted and slightly misoriented, yielding to the apparition of gaps between the tows. These two defects, i.e. the gaps and misalignment of the tows affect the mechanical performance of the final part.

Robots that execute the deposition of tows are driven by the control software. The latter can be considered as the link between the part as-designed and the part as-manufactured. To assess final mechanical properties of the part, with the effects of the presence of defects, an additional link between the part as-manufactured and the part as-designed must be introduced.

Digimat 2019.0 contain tools to predict the performance of the final part produced by AFP, using information directly from the control software. The development consists in reading information from control software such as the localization of gaps and the fiber orientation and to map such information onto the mesh used for structural analysis.

During a stress analysis of the component, Digimat uses a micromechanical model to compute the local material properties by considering the presence or absence of tows and the local fiber orientation.

BENEFITS

1. Quick computation of the knock factor on the stiffness of any components with complex geometry using as-manufactured fiber orientation and effect of gaps.
2. Support the placement strategy optimization
3. Capability to differentiate effect of hard tooling and soft tooling

 

DIGIMAT FOR AFP AT A GLANCE

Outstanding features of Digimat 2019.0 are:

• Import of the geometrical description of each single ply: Digimat can import the geometrical description of each tow belonging to a single ply and identify the orientation of the tows and the presence of gaps.
• Mapping of the gaps and orientation onto the structural mesh: Digimat can compute the content of material or gap for each finite element of the model along with the local fiber orientation (as-manufactured). That value of material content will then be used to modify locally the composite microstructure.
• Generation of the composite section card for FE analysis: Digimat can generate automatically the composite shell section card ready to use for a finite element analysis.
• Distinction between hard tooling and soft tooling: The difference between hard tooling and soft tooling can be done by considering the effect of gap on the ply thickness in the case of soft tooling or on the local fiber volume fraction of the ply in the case of hard tooling.
• Advanced features to smooth the effect of the mapping onto a discretized mesh

 

APPLICATION

The geometrical description of the file is imported within Digimat (1.), then the mapping operation onto the structural mesh can be performed (2.). As a first result, the thickness variation of each single ply can be verified (3.) and the local fiber orientation as well (4.). Finite element analysis can be performed, based on the option soft/hard tooling (5.) and then knock-down factor between healthy or flawed structure can be estimated.

 

EFFECT OF HARD TOOLING AND SOFT TOOLING

Digimat provides the possibility to consider the effect of a hard tooling or a soft tooling applied on the composite during reconsolidation or curing:

• In the case of a soft tooling, the cure or consolidation pressure is the same everywhere even if there is a local difference in the overall composite thickness. In this case, Digimat offer the possibility to modify the thickness of the ply in the vicinity of the gap, while the thickness of the ply far from the gap remain unchanged.
• In the case of a hard tooling, the consolidation pressure is spread equally, generating a resin flow towards the gap while the overall thickness of the laminate remains constant. Hence, Digimat offers the possibility to adjust the local volume fraction in the vicinity of the gap. In this context, a Digimat material is used to compute local material properties based on the fiber and resin properties and the local fiber volume fraction.

 

USING OPTICAL INSPECTION TO BUILD THE NUMERICAL MODEL

As e-Xstream Engineering and Apodius are part of the HEXAGON, collaborative work is going that consist is the combination of Online Inspection for automated lay-up processes and the numerical workflow illustrated in the previous sections.

In this context, Apodius AFP Inspection System will be used to control the quality of the tow deposition, the size and the location of each gaps, the fiber orientation and even the defects like twisted tows. Apodius provides solution in terms of Hardware and Software. That software is able to generate a report of inspection that will be imported directly in Digimat in order to transfer all this information onto the structural mesh. In this case, such information directly coming from the inspection can be used instead of data of coming from deposition software.

 

 

Digimat at NAFEMS World in Canada, June 17-20