A design algorithm to model fibre paths for manufacturing of structurally optimised composite laminates
Summary (3 min read)
- Fibre-reinforced composites are traditionally designed by stacking plies built with a discrete set of constant fibre orientation angles: 0°, ±45° and 90° .
- Recently, a new manufacturing technology called continuous tow shearing (CTS) has been developed, avoiding gaps and overlaps at the expense of thickness variation [16,17].
- In addition, to overcome this issue, many authors have employed a functional parametrisation to represent the fibre paths.
- This method reduces the number of design variables an ease the consideration of manufacturing constraints while modelling continuous paths.
- Hence, generic capabilities for the design of fibre-steered laminates and analysis of manufacturing features are required .
2 Tool to design variable stiffness laminates for
- A software tool for manufacturing analysis and optimisation of fibre steering named FIPAM (Fibre Paths for Manufacturing) has been developed.
- It provides 6 a post-processing of the design configurations from structural optimisation prior to manufacturing.
- This tool enables the automatic generation of fibre paths (i.e., machine trajectories), imposing manufacturing requirements.
- Structural approximations of the Finite Element (FE) response are used to reduce the required number of FE analyses .
- The loading condition was shear force (1N) at the top and bottom edges.
2.1 Modelling of continuous fibre paths
- The objective of this step is to generate continuous paths following the optimal discrete fibre orientations.
- This process is repeated iteratively until the segments reach the boundary of the part or ply.
- Assuming the orientation of a segment to be always equal to the interpolated orientation at the starting point of this section introduces some inaccuracy to the generated curve.
- Measure minimum radius of curvature (section 3.2) and smooth the curve in case it does not comply with the minimum turning radius, also known as 8. Curve smoothing.
- The selection of the starting points is done iteratively, by choosing first a point contained in a parallel curve to the previous reference with an offset equal to the course width.
2.2 Modelling of manufacture compliant fibre paths
- In a second step, new fibre paths for manufacturing are modelled approaching the previously defined paths.
- Choosing one curve as starting path, the method consists of defining a feasible region where the next path should be placed to comply with the specifications on course width, maximum gap and maximum overlap.
- The feasible region where the fibre path must be contained to comply with the manufacturing constraints is defined by: a parallel curve to the current fibre path with a distance equal to the course width minus 12 the maximum overlap allowance, and a parallel offset of the course width plus the allowable gap .
- Any coverage different from 100% will result in the appearance of triangular gaps in the ply.
- When the contours of two adjacent courses intersect, tows will be dropped.
3 Analysis of manufacturing features of variable stiffness
- For the implementation of manufacturing constraints in the algorithms discussed in section 2, tools to analyse these manufacturing features are required.
- Specifically, methods to compute the gaps and overlaps of a particular fibre path design and to calculate the minimum curvature radius are presented.
3.1 Analysis of gaps and overlaps
- Gaps and overlaps are automatically modelled in CATIA, which enables an evaluation of this design constraint and a visual representation in the model.
- Select two adjacent paths to start 3. Compute edges of the fibre paths o Create parallel path: Distance = CourseWidth/2 17 o Extend and split parallel with curvature continuity to cover the surface 4. Compute intersection points of adjacent fibre path 5. Sort intersection points.
- Identify whether area limited by intersection points and path boundaries represents a gap or an overlap (if there is no intersection, the whole area between the boundaries will be either a gap or an overlap) 7. Perform measures of the gap/overlap regions: area and maximum size.
- For curves on surfaces, further measures of curvature can be defined: the geodesic curvature (]b), the normal curvature (]!), and the geodesic torsion (τr).
- This induces a deflection of the fibres in the out-of-plane direction, which does not represent an issue.
4.1 Design of flat square plate with a hole
- The variable stiffness design of a plate with a circular cut-out loaded in tension and optimised for strength has been undertaken.
- Initially, tow-dropping is not allowed and a constraint to limit the maximum allowable angle deviation from optimal has not been imposed.
- The resulting maximum angle deviation is lower than 22° for all plies and the average angle deviation is inferior to 8°.
- For comparison, it includes the results for the reference paths (that correspond to a 0° maximum deviation constraint) and the optimal paths when the constraint is not imposed.
- The gaps and overlaps of each design are modelled in Figure 10.
4.2 Design of a windshield front fairing
- This structure has a double curved shape with reinforcement areas.
- It is an aircraft component designed with conventional straight orientations (0°, ±45° and 90°).
- The objective is to provide a fibre path design complying with all the manufacturing constraints.
- For the 90° ply, the reference paths do not yield large overlaps and they can be completely eliminated with angle deviations below 3°.
- The gap area increases as a result of the objective to minimise overlaps, although in a much inferior proportion than the overlap area reduction, and, in every case, respecting the maximum allowable gap size constraint.
- The potential of fibre steering is limited by current manufacturing constraints of fibre placement technologies and design specifications.
- A novel approach to automatically model fibre paths based on structurally optimised fibre angle distributions and considering manufacturing requirements is proposed.
- This approach enables to design variable stiffness laminates with curvilinear paths as well as conventional complex structures that require fibre steering.
- The algorithms are designed to minimise gaps, overlaps and angle deviation.
- As the manufacturing variables are captured in the design process, variance between designed and manufactured parts can be reduced.
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