Surface scratches on vehicle windscreens

Forensic Science International, 31 (1986) 253-260 Elsevier Scientific Publishers Ireland Ltd.

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SURFACE SCRATCHES ON VEHICLE WINDSCREENS

J. LOCKE and L.A. ROCKETT

Central Research Establishment, Home Office Forensic Science SerLqice, Aldermaston, Reading, Berkshire, RG7 4PN (United Kingdom)

(Received October 21, 1985) (Revision received January 27, 1986) (Accepted March 10, 1986)

Summary

A toughened vehicle windscreen, with a long service life, was found to have an extensive series of scratch marks over the whole area swept out by the windscreen wiper blades. Very little scratching was seen over the remainder of the screen. A collection of 25 other vehicle windows was examined and scratching was shown to be a general phenomenon, the oldest screens being the most scratched. The scratch marks, when present on a windscreen, could be observed on casework sized fragments either by using the specular reflection of white light or by interference microscopy.

Key words: Criminalistics; Glass; Windscreens; Scratches; Microscopy

Introduction

The mode of formation and physical appearance of surface particles generated during the breaking of glass objects is of considerable importance to the forensic scientist. Studies on the smashing of window panes [l] and bottles [Z] show that numerous fragments are generated and these can be studied by incident illumination [3], interferometry [4,5] and Nomarski differential interference contrast [3]. The surface characteristics of a range of sample types have been described [3--51 including flat glass, patterned material, containers and tableware. A short study has also been made in this laboratory on the breaking of vehicle windows and the appearance of the particles so generated [6]. The presence of scratches and chipped areas has been reported on the outer surfaces of windscreens with, in general, few imperfections on the innter surfaces. For the area swept by the wiper blades, the scratches were mainly parallel while samples from newer vehicles had fewer marks [3].

Published information concerning scratches on vehicle windows is some- what scant and therefore a more thorough study has been made of a collection of windscreens. Because it is difficult to inspect complete windscreens closely, examination was confined to the fragments collected after

0379-0738/86/$03.50 o 1986 Elsevier Scientific Publishers Ireland Ltd. Printed and Published in Ireland

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breaking. In order to relate the individual fragments to their position on an unbroken screen, a preliminary experiment was conducted in which an intact screen was completely covered with adhesive tape before smashing. The tape held the whole structure together and the removal of samples was a simple operation. The screen that was selected for this study was old and exhibited excessive scratching and hence the distribution of the scratch marks could be studied. This was followed by an examination of 25 vehicle windows which had been broken in the absence of adhesive tape. These experiments provided information concerning the frequency of occurrence of scratch marks on fragments of glass recovered from the debris of shattered vehicle windows.

Experimental

Sampling An intact Morris Minor windscreen was obtained which measured 112 cm

X 37 cm. It had been in service for about 20 years. This screen has also been used in another study to determine the number and form of particles generated when a toughened windscreen breaks, details of which are given elsewhere [6 1. The windscreen was taped over its whole surface on both sides, smashed and surface fragments recovered by removing selected portions of tape. For the present study fragments were removed from squares measuring 3 cm X 3 cm on both the inside and the outside surfaces. In all 18 squares were sampled on the screen, as shown in Fig. 1, with three distinct regions being represented. These were the region swept by the wipers (8 squares), the remainder of the outside of the screen (4 squares) and the inside of the screen (6 squares).

For the survey work a collection of 20 windscreens, derived from private motor cars, was collected with the help of a local firm of windscreen re-

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Fig. 1. Area swept by windscreen wipers and squares from which samples were removed,

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pairers. These samples have already been studied [7] and, with one exception, were obtained from vehicles manufactured between 1969 and 1978. The collection was stored as broken fragments in bags labelled according to the five regions of the screen from which they had been sampled, namely, upper right, upper left, centre, lower right, lower left. Five other vehicle glasses, of similar age to the main collection, were available for the current study and consisted of 4 sidescreens and a rear screen.

To cover the range of particle size encountered in casework, the com- bined collection of 25 vehicle windows was studied as both large and small fragments. Observations on each window were made on one of the large die shaped pieces that are characteristic of toughened glass and also on one small fragment of the size recovered from clothing in casework. The small fragments, typically 1 mm long, were cleaved from the surface of large fragments by applying pressure with a slightly blunt object against an edge. Six of the windscreens were, however, examined in particular detail and for each of these a large and a small fragment was studied from each of the five sampling regions described above. The remaining 19 items were examined in the central region only because samples of the full screen were not available.

The float surface was identified by observing the characteristic fluores- ence in short wave ultra-violet light. On receipt, many samples were coated with a thin greasy film which was removed, on samples required for microscopy, by rubbing with a freshly sharpened wooden cocktail stick. The wood had the effect of absorbing grease and gave rise to a bright specular finish. Permanent surface features such as scratches and pits were distin- guished from dirt by probing with the tip of a dissecting needle; dirt could be smeared out by this operation.

Surface examination Surface features were characterised by two methods based on equipment

developed to accompany the interference objective [ 41. The large fragments were placed on the transferable stage of the stereomicroscope, illuminated with incident white light and tilted until specular reflection was seen. Small samples were similarly treated and then further examined under the interference objective in the usual manner for casework sized particles.

Scratch marks were classified into four broad categories as follows:

A. No systematic scratch pattern. B. Just detectable series of parallel lines. C. Clearly observable series of parallel lines. D. Highly scratched, dense series of parallel lines.

A note was made of instances in which two series of lines were seen, one series crossing the other at a distinct angle.

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TABLE 1

CLASSIFICATION OF SCRATCH MARKS FOR SAMPLES REMOVED FROM MORRIS MINOR WINDSCREEN

The number of instances where a particular scratch pattern of type A, B, C or D is given. Four particles were removed from each sampling square. A total of 18 squares (= 72 particles) was examined.

A No parallel scratches

B C Just Clearly detectable detectable parallel parallel scratches scratches

D Dense series 0 f parallel scratches

Vehicle exterior, region swept by wipers, 8 squares

NIL NIL 6 26

Vehicle exterior, region not 13 3 NIL NIL swept by wipers, 4 squares

Vehicle interior, whole area of screen, 6 squares

24 NIL NIL NIL

Results and discussion

Observations on the in tat t screen As previously mentioned, particles from three regions were recovered by

sampling from small squares whose distribution is given in Fig. 1. The frequency with which a particular intensity of scratching was observed on recovered particles is given in Table 1. The pattern of results was quite distinct and not unexpected. For the region swept by the wiper blades, a dense series of parallel scratches was seen for 26 out of the 32 particles examined. For the remaining particles, i.e. 6 out of the 32, the parallel scratch pattern was still clearly observable but was slightly less dense. The area not swept by the wiper blades, however, was almost devoid of systematic scratching. Only three particles out of 16 examined showed faint marks with some degree of ordering which might be regarded as resembling abra- sion from wiper blades. The remaining 13 particles showed only an occa- sional scratch mark. Multiple scratches on a particle were observed infre- quently and, where they occurred, were randomly orientated to one another.

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Scratches were also very uncommon on the inside of the screen, 18 out of the 24 particles examined bearing no marks at all. The other six particles resembled those recovered from the unswept area on the outside of the screen; occasionally multiple scratches were observed but these were randomly orientated to one another.

Survey collection: large fragments The study on the intact screen established that a systematic scratch

pattern was essentially restricted to the area swept by the windscreen wiper blades. Attention was then given to examining a collection of 25 other vehicle windows to establish whether or not the phenomenon of scratch marks were general. The first stage was directed towards examining con- venient large fragments; small particles were then examined to assess how readily scratches would be detected on casework sized fragments.

Except for scratches and pits, all but one of the screens exhibited a smooth surface. The exception was from a Renault 12 which showed characteristic mouiding marks on one surface, indicating an unusual method of manufacture.

Single fragments from sixteen of the 20 windscreens showed, to varying extents, characteristic parallel scratches. The scratches generally formed series of parallel lines although additional randomly orientated lines were

TABLE 2

NUMBER OF WINDSCREENS SHOWING PARALLEL SCRATCH MARKS ON OUTER SURFACE AT VARIOUS INTENSITIES

Twenty windscreens examined as large fragments from centre of screen.

A No parallel scratches

B C Just Clearly detectable detectable parallel parallel scratches scratches

D Total Dense series of parallel scratches

Screens with float 3 2 6 1 12 surface facing exterior

Screens with float 1 2 4 1 8 surface facing interior

Total 4 4 10 2 20

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sometimes seen; in certain instances the latter were small in number. On the larger fragments, microscopic pits were frequently -observed and were attributed to impact from road gravel.

In order to establish how often scratches occur on other glass surfaces, ten household float windows, which had seen service for several years, were also studied. As expected, these did not show similar scratches or pitting. For the same reason, five other vehicle glasses, i.e. four sidescreens and a rear screen, were also examined. Systematic scratch marks were not observed in the samples but some pitting was present. This, however, does not mean that parallel scratch marks will not occur on sidescreens. Inspec- tion of a number of vehicles showed that side windows may also show deep scoring, produced by grit on the window seal, along the direction of move- ment of a window as it opens and closes. Because the driver’s window is generally operated much more frequently than the others, the occurrence of scratches will be more frequent at this site. Giass examiners should be aware of this phenomenon.

Table 2 summarises the scratch mark pattern observed on the outer surface of the windscreens. Four bore no marks at all, the remainder showed scratching to differing extents with four exhibiting just detectable parallel scratches, ten showing a clearly detectable scratch pattern and the remaining two screens being severely scratched. Only one screen showed marks on its inner surface in addition to the outer surface. In this instance the outer surface bore an extensive criss-cross pattern and inside a less dense and almost random arrangement of lines. The marks inside were attributed to harsh cleaning. This screen was from a 16-year-old Ford Cortina whereas the average age for all of the vehicles in the survey was 5 years. In fact, a rough correlation was seen between the age of a vehicle and the amount of scratching on the outer surface. In all, a total of 12 screens were classed as having a “clearly detectable” or a “dense series” of parallel scratches. The mean vehicle age for these was 7 years whereas the “unscratched” or “just detectably scratched” screens averaged only 3 years.

An examination was also made to establish whether the float surface was mounted facing outwards or towards the interior of the vehicle. It was found that 12 screens had the float surface of the glass facing outwards and eight had the float surface facing inwards. This suggests that the manu- facturer had no particular interest in the orientation of the float surface during the toughening process.

Six windscreens were selected for further more detailed study, three where scratch marks were weak or absent and three where scratches were clearly detectable. Large fragments from five positions over the full area of the screen were examined in the same manner as before. It was found that, where samples from the centre were not visibly marked or only lightly scratched, marks were not generally observed in samples from other areas of the screen. Conversely, where the centre was highly scratched, similar marks were usually observed over other areas of the screen.

B

Fig. 2. Examples of action of windscreen wipers showing region of overlap (as viewed from exterior of vehicle). A. 1987 Leyland Princess and B. 1981 Volvo.

The area of windscreen wiped depends on the location of the wiper arms. Some regions are not swept by the blades and, for a particular screen, the positioning of the wiper arms and the arcs they describe will dictate that unswept regions will lie in diagonally opposite corners. Examples are shown in Fig. 2 which indicate how the areas covered by the wiper blades overlap to some extent during their sweeps. Two instances were recorded in the survey where two sets of parallel lines were observed crossing one another at an angle of about 60” and these were assumed to have originated from this overlap region.

Survey collection: small fragments When the collection was examined as small particles, the same general

observations were made. The scratch pattern could be discerned on casework sized particles as readily as for large fragments. Observations using either specular reflection or the interference objective were suitable, the latter method being more sensitive for revealing very faint marks. An alter- native technique, which is excellent for revealing fine surface detail of this

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type, is Nomarski differential interference contrast (DIC). This technique was not available to the authors at the time the study was made but has been thoroughly described [ 31.

Conclusions

The action of wiper blades produces characteristic scratch marks on windscreens and these become more pronounced with time. Such marks can be detected on casework sized particles using either specular reflection, the interference objective or Nomarski DIC. Portions of the screen not swept by wiper blades will not normally show a systematic series of scratches unless some other factor is involved, e.g. poor cleaning techniques. All glass surfaces on the exterior of vehicles, including side screens and rear screens, may exhibit pitting by impact from road gravel. Some windows may show scoring arising from opening and closing operations.

References

1 J.A. Zoro, Observations on the backward fragmentation of float glass. Forensic Sci. fnt., 22 (1983) 213-219.

2 J.A. Zero and N. Hopkins, A short study of broken milk bottles. Forensic Sci. Znt. (1986) in press.

3 B.R. Elliott, D.G. Goodwin, P.S. Hamer, P.M. Hayes, J. Locke and M. Underhill, The microscopic examination of glass surfaces. J. Forensic Sci. Sot., 25 (1985) 459-471.

4 J. Locke and J.A. Zoro, The examination of glass particles using the interference objective: Part 1. Methods for rapid sample handling. Forensic Sci. Znt., 22 (1983) 221-230.

5 J. Locke and B.R. Elliott, The examination of glass particles using the interference objective: Part 2. A survey of flat and curved surfaces. Forensic Sci. Ink, 26 (1984) 53-66.

6 J. Locke, H.O. Badmus and A.C. Perryman, A study of the particles generated when toughened windscreens distintegrate. Forensic Sci. ht. (1986) in press.

7 M.M. Davies, R.J. Dudley and K.W. Smalldon, An investigation of bulk and surface refractive indices for flat window glasses, patterned window glasses and windscreen glasses. Forensic Sci. Znt., 16 (1980) 125-137.

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Surface scratches on vehicle windscreens