Cadfil^® - Filament Winding Software & Technology

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Introduction

There is currently great interest in the design and manufacture of Composite pressurise vessels (COPVs), particularly for applications in the areas of energy storage and sustainable transportation using hydrogen. The only viable manufacturing process for such vessels is via the use of the filament winding process. This article discusses how in some circumstances the efficiency of COPVs can be improved by using a hybrid process of filament winding and localised reinforcement using an Automatic Fibre Placement (AFP) method. Reference is made to the Cadfil software system which can be used for the design and manufacture of components using both manufacturing technologies.

Design and Consideration of stresses from internal Pressure

A classic COPV consists of a cylinder with two end domes. In most cases the cylinder part supplies most of the storage volume, the domes are there to close the ends and provide an inlet and outlet. The stress regime in the cylinder due to internal pressure is for the most part quite simple, there is a hoop stress which is in effect trying to expand the diameter in all directions and there is an axial stress due to the net effect of the pressure on the domes trying to elongate the cylinder. The hoop stress/load is twice the axial stress/load so in this respect an efficient theoretical design for the cylinder would have one third of the fibres axial (0º) and two third hoop (90º) fibres. In practical terms process constraints lead to a low angle helical windings and a high angle hoop winds.

We have described the simplistic stress case but often for efficiency of storage volume versus vessel length a flatter dome with a large base or crown radius and a smaller corner or knuckle radius is used. There are many different specific names for such end caps but generically we will refer to these as torispherical domes.

The geometry of the dome when pressurised can lead to axial bending effects in the dome/cylinder interface zone which requires some additional reinforcement above what is provided by the required low angle helical winding for axial cylinder strength. Other certification requirements such as impact tests can also lead to additional reinforcement being needed in this zone. In a traditional winding system, intermediate high angle windings are made to provides reinforcement in this area. An effect of doing this is to make an intermediate angle helical layer over the full length of the cylindrical portion. An example of such a filament wound layer is shown below.

This is inefficient as the intermediate fibre angle provides only a very small contribution to the axial and hoop strengths in the cylinder zone which represents most of the material applied by this layer. Expensive material is being wasted. For vessels with a very short cylindrical section this wastage/inefficiency is small but it becomes more significant as the vessel becomes longer.

Local Reinforcements in a manufacturing sequence

A solution to the material inefficiency problem is to have an automatic fibre placement (AFP) head that can apply short length of fibre tape like the examples in the following figure. The fibre placed in this scenario is in effect just a segment of the full layer shown above but localised to a targeted area on the knuckle radius.

This allows for an efficient material usage optimised design. Like all things this efficiency does not come for free as you need a more complex hybrid winding machine (higher capital cost) and in general the overall manufacturing process time will increase a little. The economic case can be calculated but if the production run is many parts material efficiency of with expensive material will often be the winner. Similarly for very long cylinders the economic benefits of AFP will be greater. And additionally, if the COPV is being used in a transportation application increased material efficiency leads to a lower weight of the tank which also has an input to lower lifetime cost of the system.

In the first figure we have two sets of local reinforcements (+/- Ply angles) at each end of the cylinder. In the next two pictures we can see these local reinforcements inserted into a winding sequence in the Cadfil Visualisation with winding included before and after the local AFP. We can have any number of such local layers inserted into a winding sequence of many layers. The examples shown here do not represent an actual design suitable for any specific purpose and are just an illustration.

Applying locals to FEA Models

The winding sequence shown can be post processed into motion control for a dual function machine that has both winding and AFP heads. The Cadfil post-processor is highly configurable. The combined winding can also generate a winding sequence that can be used directly in our FEA model creation functions as can be seen in the following pictures.

Cadfil can create ready to run data decks (input files) complete with loads, constraints, material properties, fibre architecture and shell of solid elements. A full range of model types can be created for leading FEA systems such as Abaqus, Altair Optistruct, LS-Dyna, MSC Nastran, SimCenter Nastran. There is also support for systems such as ANSYS, ESACOMP and MYSTRAN.

AFP Machine Control using Cadfil

Cadfil is a long established system for creating motion control program for filament winding. The highly configurable post-processer is capable of creating motion programs for CNC Systems like Siemens and Fanuc, generic g-code controllers, and also for Robot Languages used by leading Robot Systems such as Kuka or ABB. In recent times Cadfil has added features for active fibre laydown of a mandrel surface. This can be customised and integrated with the features of the AFP head. A simple canned cycle for placing a local reinforming ply is shown in the animation below.

Further information on the Cadfil software system can be found at www.Cadfil.com and at FilamentWindingFEA.com

You could also try the contact us page.

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