Free-sizing Optimization

Create design concepts that utilize all the potential of a composite structure where both structure and material can be designed simultaneously.

By varying the thickness of each ply with a particular fiber orientation for every element, the total laminate thickness can change 'continuously' throughout the structure, and at the same time, the optimal composition of the composite laminate at every point (element) is achieved simultaneously. At this stage, a super-ply concept should be adopted, in which each available fiber orientation is assigned a super-ply whose thickness is free-sized. In other words, a super-ply is the total designable thickness of a particular fiber orientation. In addition, in order to neutralize the effect of ply stacking sequence, the SMEAR option is usually a good choice for this design phase unless it is intended to follow through with the stacking preference of the super-ply laminate model.

In OptiStruct, additional manufacturing constraints can be defined for free-sizing optimization. As a composite laminate is typically manufactured through a stacking and curing process, certain manufacturing requirements are necessary in order to limit undesired side effects emerging during this curing process. For example, one such typical constraint for carbon fiber reinforced composites is that plies of a given orientation cannot be stacked successively for more than 3 or 4 plies. This implies that a design concept that contains areas of predominantly a single fiber orientation would never satisfy this requirement. Therefore, to achieve a manufacturable design concept, manufacturing requirements for the final product need to be reflected during the concept design stage. For the particular constraint mentioned above, for instance, the design concept would offer enough alternative ply orientations to break the succession of plies of the same orientation if the percentage of each fiber orientation is controlled (for example, no ply orientation should drop below 15%). In addition, balancing of a pair of ply orientations could be useful for practical reasons. For example, balancing 45° and -45° plies would eliminate twisting of a plate under bending along the 0 axis. In order to address these needs, the following manufacturing constraints are made available for composite free-sizing:
  • Lower and upper bounds on the total thickness of the laminate
  • Lower and upper bounds on the thickness of individual orientations
  • Lower and upper bounds on the thickness percentage of individual orientations
  • Constant thickness of individual orientations
  • Thickness balancing between two given orientations

As for the constraints on laminate thickness, ply thickness and thickness percentage, these can be applied locally through the definition of element sets. Therefore, multiple instances of these constraints are supported. Advantages include being able to allow different constraints in different regions while preserving the continuity of plies. For example, different thickness requirements in critical regions, such as bolted areas, can be factored into the design process. Additionally, zone based free-sizing (parameter) optimization can be performed. Zones are defined through groups of elements and there can be elements that do not belong to any zone. Zones are typically defined to simplify the design interpretation process and improve manufacturability. Instead of having to interpret manufacturable zones from the solution of a free-sizing (parameter) optimization, the optimization is run based on pre-defined zones. While the interpretation process is simplified, there is a loss in design freedom as now the optimization is restricted to some extent due to the defined zones.

Refer to the DSIZE card for detailed information regarding composite free-sizing optimization and its associated manufacturing constraints.
Note: Other generic manufacturing constraints, such as pattern grouping or member size control, can be activated for composite free-sizing as well.

The standard result from a free-sizing optimization is the thickness contribution of each orientation defined on the PCOMP(G) or STACK card referenced by the DSIZE card in the optimal laminate design. But, using free-sizing as part of the three-phase composite design and optimization process, and the mechanism to automatically generate an input file for phase two of this process, an additional level of detail/results can be drilled down to in terms of the thickness contributions per orientation. The automatically generated input file for phase two contains ply bundle data that can be reviewed in HyperMesh.

A ply bundle is a continuous stack of plies of the same shape (or coverage area). Each super-ply results in the formation of 4 ply bundles. This is the default behavior and can be changed, that is a different number of ply bundles can be requested from the super-plies. However, in most cases, it is recommended to use the default approach.

As described above, multiple ply shapes per orientation (through ply bundles) can be determined and generated from a free-sizing (parameter) optimization.
Note: Automatic offset control is available in composite free-size and sizing (parameter) optimization wherein the specified offset values are automatically updated based on thickness changes. The offset values can be specified on the PCOMP(P/G) property entries or the CTRIA3/CTRIA6, CQUAD4/CQUAD8 element entries using the Z0 or ZOFFS fields.