Making sandwiches for tomorrow - Ex-Struct TM points the way
TWI Bulletin, May/June 2001
Paul Burling obtained his Higher National Certificate in Production Engineering from Cambridge College of Art and Technology. He has extensive experience in the project management of large commercial projects worldwide. These include in depth knowledge of composite material for military and commercial applications.
Nigel Smith has participated in many large collaborative projects involving automated processing technology. This has given him a broad knowledge and experience of manufacturing processes, design, CIM and automation used within different sectors. As a Senior Project Leader in the Manufacturing Support Group, he provides services and consultancy in discrete event and dynamic simulation, computer aided engineering and adaptive control systems. He joined TWI in 1994 with a BSc(Hons) in Physics and a MSc in Information Technology (Computer aided engineering). He has a Professional Diploma in Management, is a Professional Member of The Welding Institute and a Chartered Engineer.
In most heavy manufacturing industries; railways, construction, shipbuilding, military and off-highway vehicles, there is an ever increasing drive to remain competitive by producing higher value products. Organisations are competing by offering customers more efficient and effective operation and extended life of their products. As Paul Burling and Nigel Smith report, for manufacturers this means reducing maintenance and weight, whilst increasing the strength, fatigue, corrosion and wear resistance of their products.
Consequently, there is growing interest in the opportunities presented by fabricated sandwich structures in many applications. Sandwich structures consist of thin, high strength skins separated by core materials that are bonded, welded or mechanically locked together. They typically use plywood or ply-metal, but recently a number of thermoset thermoplastic sandwich boards have found a niche area in construction.
The fabrication of sandwich boards is simple, with many techniques borrowed from the woodworking industry. Finished panels can be cut with saws, routers and drills, and joined to other structures or themselves using adhesives, welding, inserts (such as AdhFAST®) or mechanics.
This flexibility has led to growth in the use of sandwich panels in a wide variety of products. Unfortunately in some applications there are potential design weaknesses, and this has allowed for improvement. TWI has recently designed a new sandwich structure (known as Ex-Struct TM) which overcomes many of these deficiencies.
Ex-Struct sandwich structure design
Central to the Ex-Struct TM concept are modular sandwich panels that can be slotted together to create larger structures. The foundation of the modular panels is an array of interlocking circular tubes. These tubes are made from an extruded length of material, the sectioned length of extrusion determining the height of the panel. The design of the extrusion die is such that standard male and female connections are created on the external circumference of the tube. These features could also be detailed on the internal circumference of the tube for specific applications. They allow the tubes to be mechanically locked together in the X-Y plane using the male-female sliding joint.
To complete the sandwich, external skins can be attached to the top and bottom of the tubular array by stake welding. This is possible using a CO 2 or Nd:YAG laser to penetrate through the skin from the outside and into the material below. These welds will lock the tubular array in the Z plane, creating a very rigid structure in all three dimensions.
The alternate male and female connections on the side of the finished panels can be used to slot the panels together to form larger structures.
There are several features that can be changed within the panel to modify its characteristics.
Fabrication of Ex-Struct
Materials specification
Because of the need to use symmetrical cross-section components such as extrusions, the types of material that can be used are limited. However a cost-effective way of machining sections can be incorporated. Aluminium alloys are obvious candidates, as they can be both easily extruded and welded using a laser. Thermoplastics and thermoplastic composites are also possible candidates.
Process specification and equipment
Stake welding of the aluminium panels could be performed easily on a flat bed laser cutting machine. The machine settings would need to be changed from a set-up for cutting, to protect against any back reflection of the laser light, and to use a welding nozzle specifically designed for welding. It would also be beneficial to weld at a slight angle. Thermoplastic/thermoplastic composites can be hot plate welded to the array. Hot plate welding, sometimes known as heated tool welding, is one of the simplest welding techniques. The parts to be welded are held in fixtures, which press them against a heated tool. The heating takes place in two stages; the heated tool melts the surfaces and material is displaced so that a smooth interface is obtained. Mechanical stops on the equipment or a reduction in pressure prevent further displacement. Parts continue to be heated by the tool until they soften. The fixtures are then opened and the heated tool is removed, the fixtures then force the skins on to the array. This welds the upper and lower skins on to the array.
Fig. 1. Plan view of the mechanical sleeve joint created by the extrusion
Fig. 2. Typical section showing the lay-up process of Ex-Struct TM
There are other possibilities for joining the skins to the array such as ultrasonic welding. This involves the use of high frequency mechanical sound energy to soften or melt the thermoplastic at the joint line. The skins to be joined are held together and subjected to vibrations at a frequency of 20-40kHz. A typical joint using ultrasonic welding should take less than one second, and therefore this method is considered to be fast and ideally suited to mass production.
Ultrasonic welding allows fast clean assembly without the use of consumables, and can be used with a variety of techniques such as:
- Inserting (embedding a metal component such as a threaded insert).
- Swagging/forming (capturing another component of an assembly by forming a ridge).
- Spot welding (assembly technique to join two thermoplastic components at localised points).
- Slitting (used for edge sealing of woven and non-woven thermoplastics).
Customisation of Ex-Struct
The Ex-Struct concept is even more versatile because most of the properties of the sandwich can be tailored not only across the whole structure, but also at local areas. This can be achieved by changing the filling material, and the thickness, height and spatial density of the tubes.
Density
The first variant is to change the way the array is packed. It is possible to make a simple cubic or closer packed triangular array of tubes.
Load paths
Load paths within the array can be designed into the structure to allow failure routes to by-pass sensitive areas, or components that cannot withstand the loads being transmitted via the load path.
Skins
It is possible to attach the outer skin using an internal sleeve joint consisting of smaller tubes which fit into the larger ones already welded to the other skin. This means that the outer skin can be quickly and easily replaced to provide the structure with new performance characteristics.
Filler materials
Filler materials include those which are energy absorbing, sound attenuating, fire resistant, heat insulating and air.
Repairing Ex-Struct
Repair of Ex-Struct panels is simple. If a whole section of a panel is damaged it can be replaced with another. Damaged skin can be cut off, damaged tubes routed out and replaced and another skin welded on.
Advantages of Ex-Struct and sandwich structures
Sandwich boards have been used extensively in many industrial sectors for applications where their stiffness/weight and strength are of considerable importance. Almost all modern aircraft designs have honeycomb or some foam core material which is used in the manufacture of wings, floors, or controlling surfaces. Ship interiors are increasingly being made using composite sandwich structures, for lower weight above the water line and in interior construction.
Development work on high speed trains and light rail vehicles, to improve efficiency and safety, is increasingly using sandwich board construction. Architects are also using these materials to increase cubic capacity and hence occupancy, of buildings. There are many applications for sandwich boards which have significant benefits over traditional materials.
Listed below are some of the advantages and disadvantages, some of which Ex-Struct will address.
Basic advantages of using sandwich boards over solid material
- Improved stiffness and flexural strength with same mass.
- Reduced mass for same stiffness and flexural strength.
- Versatility.
Additional advantages of Ex-Struct over traditional sandwich board
- Stiff in the X-Y plane.
- Improved ability to absorb energy in the Z plane.
- Easily modified to specific requirements.
- Easy to repair using simple tools.
- Various densities can be produced by filling the core.
- Reduced risk of delamination as welded (not bonded) structure.
- Good durability.
- Joints easily fabricated and machined.
- Load paths can be incorporated into panel construction.
The reason relatively low weight sandwich structures work so well is because if the board is supported at each end and loaded in the middle, the top side is in compression (being shortened) and the under side is in tension (being stretched).
The compression and tension forces are greatest at the board surfaces, but at the centre these forces are zero. Shear forces within the board and other properties relating to the end product have to be assessed and taken into account. Versatility is most important to address needs such as compressive strength, impact resistance, fire performance, acoustics, surface finish, etc.
The graph in Fig.3 shows a guide of the beam stiffness against load and deflection. This graph should only be used as an indication of Ex-Struct's properties.
Fig.3. Ex-Struct's configuration: 0.5mm aluminium skins stake welded on to the aluminium array. (Panel simply supported) NB Mid-point deflection characteristics will depend on the type of material and the density of the array.
Disadvantages of sandwich structures
The main disadvantage with sandwich structures is cost. The manufacturing route can be complicated and time consuming, especially if the sandwich structure is to be bonded.
Lightweight metal/composite sandwich structures are usually bonded together using a high performance adhesive. It is this that is seen by many engineers to be the weakness in the construction, in particular when the sandwich structure is exposed to fire.
It can be argued that the collapse of a building element, such as a sandwich structure, will not occur at an early stage of a fire. However, the softening point of the adhesive (glass transition temperature) used to bond the skin materials to the core may result in the sandwich structure walls and ceiling collapsing before flashover.
It is not just fire that affects the performance of bonded sandwich structures. Water can also cause serious delamination even when a high cost epoxy film adhesive has been used. There are a number of cases where water ingress has led to train floors being replaced, control surfaces on aircraft delaminating and cladding systems failing.
Many of these problems can be overcome by mechanically fixing the skins together so they do not fall off in a fire, or sealing the perimeter of the sandwich structure to stop water ingress. All add cost and manufacturing time to the final product.
Fig.4. Applications of Ex-Struct in military vehicles, railways, aerospace, shipbuilding and bridge construction
Developing markets and potential industrial applications
Faced with the challenge of building faster, safer and more efficient passenger carrying craft, marine architects are increasingly seeking new materials, Roll-on/roll-off car ferries require a stiff robust car deck which is relatively thin but can easily be incorporated into new or existing ships. Ex-Struct offers many advantages in this area, with the ability for marine architects to incorporate actuators within the array structure. (Traditional bonded or welded corrugated sandwich sections are limited in this area.) Ex-Struct can be friction stir welded to give a continuous skin, reducing the number of mechanically/bonded joints.
Ex-Struct could be used for armour on tanks as there could be benefits in filling the array with energy absorbing materials and the ease of interchangeability of the armour plate.
Lightweight bridges can also be easily manufactured using Ex-Struct. Hard wearing surfaces can be applied to the skins or achieved by changing the alloy or metal/composite.
Commercial and military aircraft can benefit from the use of Ex-Struct for loading bays and floors.
There are many applications, from rail to construction, that could benefit from using Ex-Struct.
Conclusions and summary
The Ex-Struct principle is simple and versatile. The recent developments in robotic and CNC machines allow Ex-Struct to be manufactured without human intervention. Clearly the capital investment is higher than using plattern presses, but if volume is reasonably high then it can be economically viable.
Installation costs can be reduced as sections can be easily welded to other components. Load paths can also be engineered to reduce fatigue and improve joint configurations. Providing large thick sandwich structures for the heavy manufacturing industries is a key advantage, as many traditional sandwich structures are limited to the thickness of the core material. Since Ex-Struct uses extruded shapes to make the array, the thickness is dependent on the way the extrusions are stacked together. This can be carried out by simple stacking equipment, which is independent of the dimensions of the array to be produced.
No adhesives are used to construct Ex-Struct thus minimising the risk of delamination. Ex-Struct is therefore very robust and can easily be repaired on site by cutting away the damaged area and re-installing a new array and welding to adjacent skins.
Cutting and shaping is also easy and can be performed using existing engineering equipment. Additional joints can be easily incorporated into the material making it easy to join to other components.
