© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

High speed liquid impact of Nautilus nacre

C, F, Kennedy &J. E. Field Cambridge University, Cavendish Laboratory, United Kingdom

Abstract

One of the emerging areas of interest in materials for aerospace applications is mimicking natural structures with a view to creating them synthetically for use in environmentally tough environments. Rain erosion is a cause of strength loss in materials during high velocity flight. The Cavendish Laboratory’s Multiple Impact Jet Apparatus (MIJA) is able to simulate high velocity rain impact accurately and reproducibly under laboratory conditions using a series of discrete, carefully controlled water jets. This paper presents initial high speed liquid impact data for Nautilus nacre and shows that there are two main failure patterns: circumferential cracking, and stripping of layers. Single and multiple velocity threshold values are given for damage to Nautilus nacre and geological aragonite,

1 Introduction

Over the last few years interest has been re-kindled in the structure and composition of natural materials which display excellent mechanical properties tailored to the environments in which they exist[l-3], For example, different combinations of calcium carbonate, in the form of calcite or aragonite, and protein sheets which make up shell microstructure are reported to be between 3 and 23 times stronger in tension than the mineral component alone[4], The question is whether this improvement could be reproduced using the same structure but stronger starting materials. Currey and Taylor[4] identified seven distinct structures of mollusc shells. Nautilus shell has a calcitic outer layer and columnar nacre imer layer, where aragonitic blocks, sheathed in a fine organic matrix are stacked like bricks in a wall (figure 1). The whole thickness is only about 1 mm. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

4cj2 Designand Nature

I 1

I I I I I ~ I I I I I I I 1 I , 1 I I I 1 I I I I I . ...!.. 0,5.1 ym +------> Organic layer -40.60 nm 5.50)m T L 1

Figure 1: Diagram of a section through generic columnar nacre, with approximate dimensions,

Figure 2 shows (unpolished) cross sections through dry abalone and nautilus shells, both with columnar nacreous interior layers, Abalone shell tends to contain holes bored by marine organisms such as Boccardia, which can make the material unpredictable and difficult to test. Nautilus is not plagued in this way; nonethelesscaution must be exercised as shell strength is influenced by hydration levels, orientation etc.

Mountingwax Calcitic exterior [ /\

Holis Nacreous inkxior

Figure 2: Sections through abalone (A) and nautilus (B) shells.

Liquid impact (rain erosion) can cause damage to aerospace materials exposed during high-speed flight, This damage results in decreased performance, Consider an ideally spherical drop impacting a hard brittle target (figure 3), In the earliest stage of impact, the liquid behaves compressible and so-called ‘water hammer’ pressures are generated. These high pressures are maintained while the edge of the contact area between the impacting liquid and the solid moves supersonically with respect to the shock speed in the liquid, Eventually, the © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

DesignandNature 403 shock wave overtakes the contact edge, release waves enter the compressed region and incompressible stream line flow is established. The pressure on the central axis then falls to the much lower Bernoulli stagnation pressure[5-*],The escaping liquid - so-called ‘lateral jetting’ - is released at many times the original jet impact velocity and strips material protruding from the target surface in the vicinity of the impact axis, This is particularly important for composites and coated systems. -L2)- Dmp stattst. jet u contact ●* moves Ad of h Amck W,* inthatip d , AS* V/km dart,

Figure 3: Composite diagram of the impact of a liquid drop impinging the surface of a brittle rigid material including stress waves generated in the target,

The Cavendish Laboratory Multiple Impact Jet Apparatus (MIJA) simulates high speed liquid impact. It can be used to obtain Damage Threshold Velocity (DTV) curves which indicate the highest velocity at which a material remains undama ed for a given number of impacts. Full details of the liquid impact process[P‘, MIJA[lO’ll]and DTV evaluation[12”13]are given elsewhere.

2 Experimental method

Flat pieces (as far as possible!) of Nautilus shell were cut, The inner nacreous surfaces were polished mechanically prior to testing; the outer surface left ‘as received’, The polishing routine was: 800 SiC paper till planar; 1000 SiC paper, 2400 SiC paper, 9pm diamond on silk cloth, 1pm diamond on multicloth. The nacre was polished as near to the natural shape of the shell as possible, although it is accepted that atler preparation the aragonitic layers will almost certainly form a shallow angle with the test surface. The samples were mounted on thick glass plates using QuickStick 135 wax, perpendicular to the impinging water jet. After some preliminary testing at different velocities, impacts were placed on a given site at low velocities. These velocities were deemed sufficient to cause the onset of damage after a reasonable number of impacts but not sufficient to destroy the sample. Samples were inspected with an optical microscope and environmental scanning electron microscope (ESEM) and the surface topology determined using a Sloan Dektak II Profilometer. The advantage of the ESEM is that samples do not have to be coated with a © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

404 DesignandNature conducting layer prior to analysis. A 0,8 mm MIJA nozzle was used throughout and the samples were tested dry and perpendicular to the nacreous layers.

3 Results

Under the ESEM, the edges of the nacreous layers can be seen prior to testing; they are about 5 pm apart (figure 4). They are quite faint, underlining the tenuous nature of the the protinaceous ‘cement’,

Figure 4: ESEM image of a polished nautilus surface at 10.03° to beam.

Impacting nautilus nacre with high speed liquid jets produces two types of failure: at low velocities the initial change to the surface is stripping of aragonitic layers; at higher velocities, initial failure is in the form of circumferential cracks. These crac~s do not appear to extend more than a few layers into the surface. Examples of damage are shown in figures 5 and 6,

Figure 5:110 shots at 136 m/s; at these velocities, stripping of the aragonitic layers occurs in preference to cracking. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

DesignandNature 405

Figure 6: Five impacts at 400 m/s; at these velocities, the initial failure mechanism is cracking,

Figure 7 shows a series of ESEM images of part of a stripped region from the site pictured in figure 5. The damage site appears to be surrounded by rubble and pull-out of blocks.

Figure 7: ESEM images of part of the damage site in figure 5- 110 impacts, 136 ds; LEFT part of the stripped region shown in figure 5 (integrated image, surface 10.02° to beam); CENTRE: close-up of damage site. RIGHT: close-up of ‘rubble’ at the edge of the site in the central image,

A basic damage threshold curve was obtained for Nautilus nacre. This records damage observed on the sample o~ any type (z’.e, stripping, cracking or a combination of the two). Also presented is a DTV curve for geological aragonite © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

406 Designand Nature whose main failure mode is circumferential cracking. Table 1 summarises the key data.

Kc Nautilus nacre (polished, dr’j

. danlsge 350 ● . . . dmqe

305 i ●

0 loo 1 10 fcm ,% 1 10 !00 mm no. Imoacts N, q..

Figure 8: DTV curve for Nautilus nacre (lejl) and geological aragonite (right).

Table 1. Summary of basic threshold data for Nautilus nacre and geological aragonite.

Material DTv~,g(300), Ill/s SSTO,S,nis nautilus columnar nacre 70*5 325 * 5 geological aragonite 122 &5 230 k 5

4 Discussion

The division of failure mechanisms in nacre can be traced back to the organic component. At low velocities material is stripped away by extra-high velocity lateral jets generated during the impact process. Obvious planes of weakness exist in the structure at the interfaces between the layers of aragonite blocks, and material is readily detached. The pressure, Pw generated on the central impact axis can be estimated using the equation

P.= Cv (1) where = water density, C = shock wave speed in water and V = impact velocity[g]. Whilst this waterhammer pressure lasts for a very short time (<1us), it is suggested that for a suftic~ently high pressure on the central impact’ axis, the effect of any organic matrix is negligible and the damage mechanism is that of discrete brittle blocks laid on top of one another - generally, cracking. Below some critical limit, however, the organic matrix should be able to cushion the impact so that the blocks do not impinge on one another. Moreover, the matrix introduces flexibility into the system which may be exploited by the Rayleigh © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Design and Nature, CA Brebbia, L Sucharov & P Pascola (Editors). ISBN 1-85312-901-1

DesignandNature 407 surface wave created during the impact. In this case, the fust damage observed is likely to be stripping of layers rather than cracking, For an impact velocity of 100 rnJs, and assuming values of 103kg/m3 and 1500 mls for and C respectively, Pw= 0.15 GPa; for an impact velocity of 400 mls, P. = 0.6 GPa. The exact onset of damage at low velocities (stripping) is difficult to determine with precision so the DTV value of 70 mis must be taken with caution. Moreover, it shoud be noted that aragonite is a brittle solid and fails by cracking. Comparisons with nacreous the composite are only really suitable when it fails by cracking also, i.e. at higher velocities. In this regime, the nacre is tougher and this is reflected in a higher SST value, Further testing on this an other nacreous shells with thicker organic matrices (such as abalone) should elucidate the problem.

5 Acknowledgements

The authors would like to thank Mr Ray Flaxman for polishing the samples and Qinetiq Ltd for a grant to carry out the project, We are grateful to Mr Stewart Palmer and Mr Frank Baker for assistance with the ESEM studies.

6 References

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408 DesignandNature [11] Seward CR., Rain erosion testing of IR window materials, Ph.D. Thesis, Cambridge University, 1992 [13] Coad EJ, Pickles CSJ, Seward CR, Jilbert GH and Field JE, The erosion of infrared transparent materials, Proc. Roy. Sot. Lend. A 454, p2 13-238, 1998