ForenszC Science Zntc-rnottil, 56 (1992) 95 - 106 Elsevier Scientific Publishers Ireland Ltd.

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BREAKING OF FLAT GLASS-PART 2: EFFECT OF PANE PARAMETERS ON PARTICLE DISTRIBUTION

JOHN LOCKE and JOHN A. UNIKOWSKI

Centra1 Research and Support Establishment, Home O&ke Forensic Science Seruice, Aldermaston, Reading, Bedshire, RG7 .$PN (UK)

(Received June 12th, 1991) (Revision received October 18th, 1991) (Accepted February lOth, 1992)

Summary

The effect of changing the dimensions of window panes on the distribution of glass particles when they are smashed has been investigated. The parameters changed were the sise, thickness and whether the pane was plain, pattemed or wired. When comparing results from experimenta, allowance was made for the inherent variability of the breaking process. NO significant trends were identiiïed in the particle distribution which could be clearly associated with a change in the pane parameters.

Kq worols: Criminalistics; Glass; Windows; Fragmentation

Introduction

In a series of studies on the breaking of glass objects, a test rig was con- structed to permit the breaking of window panes under controlled conditions [ 11. The rig consisted of a frame to hold the window and a heavy, steel bal1 on a pen- dulum arm to break the pane. Glass debris, ejected ‘backwards’ towards the breaker, was collected in a series of trays at ground leve1 extending out to a radius of 4 m from the pane. Unwanted glass debris moving away from the breaker was trapped in a booth behind the pane.

Initial studies concentrated on establishing the reproducibility of breaking. Eight replicate panes, al1 of the same size and thickness, were smashed under conditions that were kept as constant as possible in terms of the force and direc- tion of the impacting steel bah. The debris scattered into the collecting trays was analysed according to direction, distance travelled and particle size. Com- parisons were made between replicate runs. As expected, variation in the results

Correspondence to: John Locke, Central Research and Support Establishment, Home Office Forensic Science Service, Aldermaston, Reading, Berkshire, RG7 4PN, UK.

0379-0738/92/$05.00 0 1992 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland

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for individual trays could be high but greater reproducibility was observed when larger aggregates of results were considered. When data from al1 replicate runs were combined and analysed, distinct particle distributions were observed. For each size range, there was a regular decrease in the number of particles per unit area with distance from the pane.

These replicate breakings established that, overall, the results were sufficient- ly reproducible that the broad effect of varying certain parameters could be in- vestigated. However, it was clear that, with the resources available, only large changes in particle distribution would be detectable: minor effects would remain unresolved unless an unacceptably large number of replicate experiments were conducted. Thus duplicate runs were considered adequate to characterise the distribution from a pane of a particular type. The most important parameters re- quiring study were considered to be size of pane, thickness and physical form (plain, wired or patterned). This paper analyses the results from a series of win- dows where these quantities were varied.

Experimental

Effect of changing the pane parameters Breaking operations have already been described [ll. For ease of reference,

diagrams of the breaking rig and the collecting trays are reproduced in Fig. 1. Where the area of the pane differed from that used originally, mounting was such that the centre of the pane was maintained at a height of 1.5 m above ground level. For the thicker panes, more force was required than that used previously, the additional force being supplied by raising the height of the steel bal1 before releasing it. It was not thought necessary to record the preçise force required to break each pane. In instances of criminal breaking this would depend strongly on the shape of the breaking implement and would be uncontrolled in terms of force and direction.

The following series of breakings was conducted: (a) Plain glass. Three different sizes of plain glass, al1 4 mm in thickness, of

size 1.0 x 1.0 m, 0.5 x 0.5 m and 0.25 x 0.25 m, were examined. Smaller sizes than this were not practical because the steel bal1 then became of similar size to the pane and obscured the backward scattering of fragments.

(b) Pluin gkw. Three thicknesses of plain glass at 4 mm, 6 mm and 10 mm were investigated. In each case the size of the pane was 1.0 x 1.0 m. Panes of thickness 2 mm were not used because of safety problems in handling large panes of such thin glass.

(c) Wired glass. Three sizes, 1.0 x 1.0 m, 0.5 x 0.5 m and 0.25 x 0.25 m were examined. Al1 were 6 mm thick.

(d) Pattwmd glass. Three sizes of Pilkington Reeded at 1.0 x 1.0 m, 0.5 x 0.5 m and 0.25 x 0.25 m were investigated. Al1 were of 4 mm nomina1 thickness.

Sieving and counting operations were conducted as previously described [ll.

Data analysis In order to minimise the effect of uneven distribution of particles across the

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hardboard witw

centre Of pan.3

Fig. 1. The breaking rig (above) and the arrangement of the collecting trays (below).

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collecting field, counts for trays at the same radial distance from the pane were combined. For example, for tray D (Fig. 1) each sector provided one tray, totall- ing four over the whole field. For a window of a particular size, thickness and type the breaking experiments were carried out in duplicate and the counts averaged. The distribution of particles over the collecting trays was represented graphically either as bar charts or as curves similar to those previously described [ll. With the curves, a mathematica1 refinement was used. Previously the distance of the particles from the point of breaking has been measured to the centre of each tray. For refining the data it is desirable to have a more accurate description of the average distance travelled by particles to a tray. For subse- quent calculations this wil1 be taken to be the distance (at floor level) from a point directly below the centre of the pane to the ‘centre of gravity’ of the tray. This adjustment allows for the trays being wedge shaped. In order to calculate the average particle density across a tray it was assumed for simplicity that the distribution of particles over a tray was uniform. There was, in fact, an ap- preciable gradient in particle distribution over some trays but the approximation sufficed for a broad assessment of the results.

Results and Discussion

Genera1 observations on pam breaking In the present work 24 panes were smashed and a brief photographic record

and a sketch were made of each broken pane. From these records, the genera1 sequence of events during breaking could be postulated. Certain characteristic features of the broken panes are illustrated by the four examples in Fig. 2.

In a typical breaking, where the impact is more or less central, the glass first deforms elastically until a limit is reached. Crack propagation then begins, radial tracks form and these frequently run to the edges of the pane (Fig. 2A, feature a). An associated series of concentric, circumferential tracks gives rise to a ‘spider’s web’ appearance. These latter tracks (feature b) are often less obvious after the break because the centra1 region of the pane, where they were most dense, may be dispersed as debris. These tracks were, however, clearly visible in certain of the wired glass samples (Fig. 2B, feature b). The spacing of these circumferential tracks was seen to increase as one moved outwards radially from the point of impact. As expected, the wire tended to hold the broken pane together and thus reveal the crack structure.

A third frequently occurring feature is surface flaking (Fig. 2A,B, feature c). This arises because, when large pieces of glass are projected away from the breaker on impact, immediately adjacent edges press against one another. As a result, many characteristic ‘beeswing’ flakes are generated from the side from which the impact came and these are ejected backwards towards the breaker. With wired glass the flaking from radial tracks can be seen to be extensive (Fig. 2B). In the last stages of pane disintegration large pieces or shards can be displaced backwards by residual forces or fall, under the influence of gravity, directly downwards. The loss of these pieces may be limited in extent (Fig. 2A, feature d) or involve large areas of pane (Fig. 2D). The rig was designed to

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Fig. 2. Examples of broken panes showing the following features: A, 6 mm plain glass (1.0 x 1.0 m) radial cracks; B, 6 mm wired glass (1.0 x 1.0 m) circumferential tracks; C, 4 mm patterned glass (0.25 x 0.25 m) regions of surface flaking and D, 4 mm plain glass (1.0 x 1.0 m) hole resulting from large pieces dropping vertically down. Note: the scale of C is different from the other three diagrams.

deflect such material away from the collecting field and thus minimise further fragmentation. Any secondary breaking process, generating additional par- ticulate material at ground level, must be minimised: it wil1 introduce a bias into attempts to estimate the quantity of glass falling upon a person breaking a window .

One simplistic view is that the number of particles produced could relate Purely to pane size. However, comparing Fig. 2A with 2C shows that the central highly fragmented region, where many of the smallest particles are generated, can be roughly equal in area for radically different pane sizes. (NB. Allowance must be made for differences in scale between the two figures.) It follows therefore that one cannot expect the total number of particles to relate simply to total pane area. The mechanism of surface flaking provides a further complication: the number of particles generated by this mechanism wil1 depend on the total amount of cracking and the degree of disintegration of the central region. Similarly, on first consideration it might be expected that thicker panes would yield more glass particles than thinner ones of equivalent area merely on the

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basis of the sheer bulk of glass involved. Surface flaking is one mechanism which would cause this simple relationship to break down.

This paper must therefore be seen as a simplistic introduction to the subject, an attempt being made initially to look for obvious trends in the total number of particles generated. Broadly, the term ‘particle distribution’ is used to signify the number of particles of each size recovered and the distance to which they travel. More detailed analyses involving, for example, the relative proportion of surface particles to bulk particles, are left to subsequent reports.

Effect of changing the pane parameters One method of displaying a particle distribution is as a bar chart. Figure 3

shows the particle counts for three sizes of pane, 1.0 x 1.0 m, 0.5 x 0.5 m and 0.25 x 0.25 m. The particle distribution is calculated as the number of particles per square metre of collecting surface. It was shown earlier that, when replicate panes were broken, a marked variability in particle distribution occurred over small, localised areas [ll. Taking this inherent variability into account, it is ap- parent that pane size has no significant effect on the subsequent particle distribution. This is despite the l&fold reduction in surface area on passing from the largest to the smallest panes.

Figure 4 displays a similar set of bar charts where the pane thickness was altered, the size being fixed at 1.0 x 1.0 m. The thicknesses studied were 4, 6 and 10 mm. Again, variability in the data is evident but the pattern of results generally establishes that pane thickness has limited influence on particle distribution. There is only a slight tendency for the thicker glasses to give rise to a larger number of particles.

The relative constancy of particle distribution with alteration of pane size was also observed with patterned glass (Fig. 5). There is some evidente for a decrease in the number of particles with reduction in pane size for trays D and E. However these trays are at the periphery of the collecting field where relatively few particles are deposited. As a result, it requires only a smal1 number of additional particles to fall in this region to have a large effect on the form of the bar chart. Bearing this in mind the conclusion reached with patterned glass was that pane size had virtually no influence on particle distribution. Comparison of Fig. 5 with the results from plain windows (Figs. 3 and 4) further suggests that particle distribution is independent of glass type, i.e. whether patterned or plain.

The behaviour of wired glass (Fig. 6) differed somewhat from the other types of pane. In general, larger numbers of particles were recovered. This can be seen by inspecting the figures bearing in mind that, for the wired glass (Fig. 6), the vertical scale of the histograms is 2.5 times greater than for non-wired glass. It could also be postulated that, for wired glass, particles are produced not only from radial and circumferential tracks but also from breaks between wires and the glass in which it is embedded. Hence the number of particles produced is in- creased.

Particle distributions can be displayed as curves of the type shown in an earlier report [ll. A logarithmic scale is used for the vertical axis in order that al1 parti-

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Fig. 3. Plain glass bar charts showing distribution of particles (no./m’) over collecting field for three sizes of pane (A, 1.0 x 1.0 m; B, 0.5 x 0.5 m; C, 0.25 x 0.25 m) al1 4 mm thick. Particle size ranges are: a, >5 mm; b, 5-1.0 mm; c, 1-0.5 mm and d, 0.5-0.25 mm.

cie counts can be displayed on a single scale. The distance from the pane to where the particles fel1 is displayed on the horizontal axis.

This method of displaying the data allows some representation of the variabili- ty inherent in the experiments. For comparison, Fig. 7 displays results for the

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Fig. 4. Plain glass bar charts showing distribution of particles (no./m2) over collecting field for three thicknesses of pane (A, 4 mm; B, 6 mm; C, 10 mm) of size 1.0 m x 1.0 m. Size ranges are: a, >5 mm; b, 5-1.0 mm; c,l.O-0.5 mm and d, 0.5-0.25 mm.

eight replicate runs that formed the initial stage of this series of studies [ll. The range of results for a particular tray of a specified particle size range is shown. It can be seen that the quantity of debris can differ between runs by a factor as large as ten times.

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Fig. 5. Bar charts for patterned glass showing distribution of particles (no.lm’) over collecting field for three pane sizes (A, 1.0 x 1.0 m; B, 0.5 x 0.5 m; C, 0.25 x 0.25 m) of patterned window of nomina1 thickness 4 mm. Particle size ranges are: a, >5 mm; b, 5- 1.0 mm; c, 1-0.5 mm and d, 0.5-0.25 mm.

Overall assessment of results Care must be taken when interpreting the results from panes where the

various parameters were altered. Although some differences in particle distribu- tion were seen this does not necessarily mean that fundamental changes had oc-

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Fig. 6. Bar charts for wired glass showing distrihution of particles (no./m’) over collecting field for three pane sizes (A, 1.0 x 1.0 m; B, 0.5 x 0.5 m; C, 0.25 x 0.25 m) of 6 mm wired glass. Particle size ranges are: a, >5 mm; b, 5-1.0 mm; c, 1.0-0.5 mm and d, 0.5-0.25 mm.

curred in the breaking process. The basic problem in assessing the effect of a change in pane parameters is that, despite careful experimental design to max- imise reproducibility, considerable variability can occur. Virtually the same breaking conditions can lead to a smal1 hole in one pane and a large one in

Fig. 7. Distribution according to particle size for eight replicate runs, pane dimensions were 1 m x 1 m x 4 mm. !lYhe range of values is indicated by a vertical bar.

another. It can be argued that the size of the hole does not necessarily relate to the quantity of fine debris produced. If a major mechanism in producing backward flying fragments is surface flaking then the total length of al1 of the tracks may have a significant influence: the actual size of the remaining hole may be less important. A further complication is that of secondary breaking: further disintegration may occur when large pieces of glass strike the ground. In the experiments described here this factor was reduced as much as possible but could not be eliminated entirely.

Some effects were seen that might be attributable to changes in pane parameters. These occurred with increase in pane thickness and with wired glass. The particle counts for thicker glasses increased some 2- to 3-fold com- pared with 4 mm plain glass. More particles could arise as a result of the greater force required to smash thicker or wired panes and hence more energy would be dissipated during the subsequent fragmentation. Alternatively, it could be that the broken edge in a thicker pane has to eliminate a larger volume of glass to pass behind its adjacent edge on breaking. Hence the number of fragments would increase, the energy for this being supplied by the greater force applied for breaking. However there was an insufficient number of runs to confirm either hypothesis.

The implications for casework are that essentially al1 of the window types studied gave broadly comparable results. Because of the natural variability be- tween breaking runs and the fact that the conditions existing at a crime scene wil1 always be somewhat different from those used here, it can be taken as an

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initial guide that al1 windows behave similarly. This can be regarded as true unless the panes are radically different in type to those described here. One such exception would clearly be toughened glass. Vehicle windscreens, either toughened or laminated, wil1 be considered in a later paper.

Conclusions

The purpose of the research reported here was to establish the extent to which the particle distribution was affected by a change in the nature of the window pane. The investigation, at this stage, was confined to changes in pane size and thickness and the effect of plain, patterned and wired glass. Toughened glass was not examined.

It was appreciated beforehand that the investigation would be constrained by the inherent variability of the breaking process: it would not be possible to observe smal1 effects on changing, say, pane size. The essential point was whether or not substantial systematic effects occur which might significantly alter the interpretation of results in operational casework. NO such effects were found. As a result, broad estimates are possible of the quantity of debris generated when a window is smashed.

References

1 J. Locke and J.A. Unikowski, Breaking of flat glass-Part 1: Size and distribution of particles from plain glass windows. Forensic Sci. Int., 51 (1991) 251-262.