What is Shock Absorbing Liquid (SALi) Technology^TM ?

 

SALi in a nutshell

SALi based cushions have been proposed for protecting people and property from violent impacts, vibrations and shock waves. They were first described by Bill Courtney in the 1990s. ^{[1, 6, 7, 8.]}

They consist of lots of small, compressible elastic capsules blended with an “incompressible” matrix fluid, with the mixture being retained in a strong package, which does not stretch significantly during an impact.

 

Figure 1.  The essential features of a SALi based cushion.

SALi based cushion  =  lots of compressible capsules
                                 +”incompressible” matrix fluid
                                 + low stretch packaging

SALi based cushions offer four damage mitigation mechanisms:

1                   The capsules absorb energy when they suffer bulk compression during an impact.

2                   Viscous damping converts impact energy into heat when the matrix fluid swirls round the compressing capsules. This reduces the “kickback” when the capsules spring back into shape in the milliseconds following the impact.

3                   The hydraulic characteristics of the matrix fluid distribute impact loading over the surface being protected.

4                   Shock waves are scattered at the capsule-fluid interfaces.

 

The compressive stiffness of the device depends on the type of capsules used. Published research papers ^{[2, 3, 4.]} describe the use of air capsules made from bubble packing to produce very soft cushions, with stiffer expanded polystyrene beads being the most versatile for use in protective clothing. For very violent crash impacts, expanded metal foam beads can be used.

Thick liquids having a viscosity similar to treacle offer good viscous damping, but silicone gel is usually preferred because this eliminates the problem of liquid leakage if the packaging is ruptured. The use of shear thickening (dilatant) liquids that stiffen up during impact has been proposed. ^{[1, 4, 6, 9.]}

 

The packaging used for the published research varies from stout cotton bags to piston and cylinder arrangements. ^{[1, 2,3, 4, 5.]} A key feature of the SALi concept is that stretching of the packaging must be minimised in order to maximise the capsule compression and viscous damping processes.

 Q. Why is the SALi concept referred to as a "technology"?

A. We want to emphasise that a broad collection of materials and ways of blending and packaging is involved. The global idea is to create a composite material that shares the impact protection characteristics of solids, liquids and gasses.
But, to absorb useful amounts of impact energy, some form of low stretch, variable volume packaging is required.

 

 

1     The basic SALi mechanism

Figure 2. During an impact the capsules shrink in size as they are compressed on all sides by the matrix fluid. The capsules, lubricated by the fluid, re-arrange themselves inside the package, so that the front face of the package takes up the shape of the impacting body.

The matrix fluid In principle any liquid or gel which allows the hydraulic transfer of pressure can be used as the matrix fluid. In experiments^[1-4] silicone oil, glycol anti-freeze, wall-paper paste, Vaseline, mastic sealant and a range of engineering greases have been used as the matrix fluid. In order to maximise viscous damping very thick liquids having a viscosity similar to treacle are preferred. Gooey mastics or silicone gels are good because there are no leakage problems if the packaging is damaged.

Shear thickening fluids allow the package to change shape easily when deformed slowly, but stiffen up during violent impacts. This feature is appealing in protective clothing, for example in pads to protect the spinal column of motorbike or equestrian riders.

The capsules Elastomeric capsules tested in the published research^[1-5] include hollow rubber balls, expanded polystyrene beads, polymeric microspheres, bubbles cut from bubble packing and narrow diameter, open ended, hollow tubes, with filaments of air trapped inside them. The inclusion of hollow glass microspheres have been proposed as a mechanism for adding shear thickening.^[9]

The packaging  If the packaging stretches significantly during an impact then some impact energy is absorbed but overall, energy absorbing efficiency is reduced because the capsules suffer less compression and the matrix fluid provides less viscous damping.

Figure 3. The correct packaging is a vital part of SALi Technology.

 

2       The load spreading benefits of SALi cushions

 

Figure 3. Early experiments to demonstrate the load spreading advantages of SALi used very simple equipment.^[1]  
Impact patterns produced in the surface of soft clay slabs were compared, when the slabs were protected by different types of cushion.

Three types of cushion were tested: (i) Sorbothane (a visco-elastic rubber), (ii) elastomeric foam and (iii) SALi filled bags. The Sorbothane and foam cushions both produced distinct impact craters under the impact zone. In contrast, the SALi cushion produced a very shallow indentation over most of the clay slab.

Figure 4. The steel sphere produced a clear impact crater in the Sorbothane. In contrast, the SALi based cushion produced a broad shallow impact pattern. Similar impact craters to those made by Sorbothane were produced when elastomeric foams were tested.

 

Load spreading occurs throughout the interior of a SALi cushion. Consequently, elastomeric foam based SALi capsules suffer bulk compression. In contrast, if a similar foam is used as the basis of a conventional cushion, the individual air cells are flattened under the impact zone, but cells to the sides of the impact zone are unaffected.

 

 

Figure 5. The hydraulic nature of SALi cushions produces a load spreading effect throughout the interior of the cushion.

 

3       Reducing the weight of SALi cushions

The matrix fluid is responsible for most of the weight of SALi based devices. So the key to weight reduction is to minimise the fluid fraction by close packing the capsules. The basic SALi formulation as illustrated in Figure 1 has a single size range of low density capsules, with the matrix fluid occupying about 34% of the volume. The fluid fraction can be reduced by using two size ranges of capsules, with small capsules occupying part of the void spaces between the larger capsules. This reduces the matrix fluid to about 12% of the volume.

The larger capsules could be expanded polystyrene beads and the smaller ones, polymeric microspheres. The matrix fluid fraction can be further reduced by introducing hollow nano-particles between microspheres.

If the smallest size of capsules is fairly rigid, compared with the larger ones, the smaller capsules will tend to bunch up during impact, producing a shear thickening effect.^[9]

 

4                 Some proposed applications

This is a short review. Please follow the links for more detailed information.

4.1    Body armour

Depending on the market requirements, design issues include low weight, soft feel, flexibility for articulated parts of the body, penetration resistance and temperature control.

Where flexibility is important packaging based on strong polycotton, Kevlar or Cuban fibre can be used.

Figure 7. Soft hats incorporating SALi cushioning.
(Designed by Sue Darby-Pankhurt).

For more illustrations visit our Sassy Hats page.

 

Weight reduction using SALi plus foam If the body armour includes a stiff outer shell, then SALi cushions can be used to protect the most vulnerable body parts with lighter compressible foam protecting other parts.

 

Figure 8. This prototype footballer’s shin pad incorporates SALi cushioning to protect the tibia (shin bone), but reduces weight by using compressible foam to the sides of the tibia.^[1]

 

The following diagram represents a vertical cross section through a shin bone receiving a kick.

Figure 9. The load spreading characteristics of SALi work in harmony with the load transmitting characteristics of the human bodies own soft tissue.

 

Keeping cool
To reduce heating problems, the use of a phase change wax as the matrix fluid has been proposed. ^{[1, 6.]}

 

4.2    Soft, pedestrian friendly car bumpers (fenders)

 

For details of SALi filled bumpers, please visit our PedSALi page.

 

4.3 Vibration isolation units (vehicle suspension systems)

 

Several SALi based designs have been proposed.^{[1, 5, 6, 7, 10]} A common characteristic is that the elastomeric capsules stiffen up as they are compressed. This allows the suspension unit to offer a soft ride when travelling over smooth roads, then automatically stiffening up when moving over rough ground. Here is one of the proposed designs:

 

Figure 10. Vibration isolators are discussed on our CrashSALi and  battery charging suspension web pages

 

4.4 Acoustic vibration (sound) reduction

Work by Valentin LeRoy at Paris Diderot University, France suggests that SALi type materials may employ a process known as Minnaert scattering to reduce sound transmission through walls. ^{[15, 16]}

This is in line with earlier unpublished SALi research carried out at Cranfield University Royal Military College of Science. Researchers under the supervision of Professor Horsfall have verified that SALi type materials have good blast wave mitigation properties.

Figure 11. The blast mitigation properties of SALi being tested at Cranfield University RMCS.
The SALi filled bag was ruptured by debris thrown up during the explosion. If the bag is covered by a floating steel under plate, it remains intact during the explosion.

Figure 12. SALi vehicle protection combines shock wave scattering with the other impact mitigation benefits discussed in this article.

 

5       Barriers to product development

Impact tests can be very expensive, with the test piece commonly being destroyed during the impact. To minimise costs, engineers need to have a good idea of the outcome before an impact test take place. In recent years the key to cost reduction has been to carry out a range of computer simulated impact tests, before carrying out a live test on the most promising design.

Computer predictions are only as good as the data fed into the computer model. SALi poses particularly difficult simulation problems because it’s a system of interacting materials. There are almost too many choices of capsules, matrix fluids and packaging designs. A more serious challenge is that the energy absorbing characteristics of the SALi material change rapidly, and in a complex manner, as the viscous fluid swirls round the shrinking capsules.

A third problem is that the “text book” techniques used for measuring the core characteristic properties of solid and foam based impact absorbers cannot be applied to SALi because of its novel, visco-elastic fluid nature. The limited number of SALi characteristics that have been published to date cannot be used in computer simulations because they fail to reflect SALi’s complexity.^{[11, 14]}. Techniques for producing valid SALi characteristics are discussed on our SALi Core Characteristics web page.

Other early investigations in Britain failed because they used inappropriate materials, such as elastic packaging and corrosive fluids which damaged the packaging.^{[12, 13]} Consequently, in spite of £300,000 research funding by the British Government, commercial interest declined after 2003.

More recent work in China^[5] and Britain^[4] has been far more successful. For example, after completing their study of a SALi based vibration isolator, researchers at Nanjing University concluded that it, “offers outstanding performance and a good prospect in engineering practice.” ^[5]

 

References

1                   Courtney, W. A. Preliminary investigations into the mechanical properties and potential applications of a novel shock absorbing liquid, MPhil Thesis, Manchester School of Engineering, University of Manchester (1998).

2                   Courtney W A and Oyadiji S O (2001). Preliminary investigations into the mechanical properties of a novel shock absorbing elastomeric composite. Journal of Materials Processing Technology 119 (2001) 379-386.

3                   Courtney W A and Oyadiji S O (2000). Characteristics and potential applications of a novel shock absorbing elastomeric composite for enhanced crashworthiness. International Journal of Crashworthiness 5:4 (2000) 469-490.

4                   Huw Davies et. al., Cardiff University School of Engineering, Pedestrian Protection using a Shock Absorbing Liquid (SALi) based Bumper System, ESV Conference, Stuttgart, June 2009, Paper Number 09-002.

5                   H. d. Teng, Q. Chen, Study on vibration isolation properties of solid and liquid mixture, Journal of Sound and Vibration, (2009) doi.10.1016/j.jsv.2009.04.036

6                   Courtney, W .A. Device incorporating elastic fluids and viscous damping, World Intellectual Property Organisation, WO 97/25551 (1997).

7                   Courtney, W. A. Improved impact absorber with viscous damping, World Intellectual Property Organisation, PCT/GB98/03594 (1998).

8                   Courtney, W. A. Impact absorbent building structures, British Intellectual Property Office GB9805887.8 (1998).

9                   Courtney, W. A.  Impact energy absorbing device incorporating bunching capsules British Intellectual Property Office, GB0907996 3 (2009).

10              Courtney, W.A. Improved vibration isolator, British Intellectual Property Office GB0915807.2 (2009).

11              S O. Oyadiji et. al., University of Manchester, Core property characterization for a shock absorbing composite, SAVIAC 75^th Symposium, 17-22 October 2004.

12              S O. Oyadiji et. al., University of Manchester, Characteristics of deformable cylindrical beams filled with a shock absorbing composite, SAVIAC 75^th Symposium, 17-22 October 2004

13              G. Georgiades et. al., Impact response of flexible cylindrical tubes filled with a shock absorbing composite, University of Manchester, SPIE Conference 7-10 March 2005.

14              G. Georgiades et. al., University of Manchester, Characterization of the Core Properties of a Shock Absorbing Composite, Journal of Engineering Materials and Technology, ASME, October 2007, Vol. 129, pages 497-504.

15              Leroy, V, Applied Physics Letters volume 95, p 17.

16              Cartwright, J, New Scientist, 26 February 2011, p 45.

 

Other SALi pages:
SALi Core characteristics  Sassy Hats   CrashSALi,  PedSALi