BULLET IMPACT ON BARE SHEET STEEL AND STEEL AUTOMOBILE BODY MATERIAL

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BULLET IMPACT ON BARE SHEET STEEL AND STEEL

AUTOMOBILE BODY MATERIAL

SREE VERSHA HARI - 11390832

Research Institute

Netherlands Forensic Institute

Supervisor Examiner

Wim Kerkhoff Dr. Maurice Aalders

Duration Credits

1-2-2018 to 30-7-2018 36EC

Suggested journal for publication

Journal Forensic sciences

30th July 2018

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1 CHAPTER

TITLE

PAGE NUMBER

1 1.1 Ricochet

2 1.2 Deflection after ricochet

3 1.3 Deflection after perforation

3 1.4 Lead in

3

2 2.1 Sheet metal

4 2.2 Car Bonnet

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3 3.1 Shooting Range setup

5 3.2 Shooting protocol- sheet metal

6 3.3 Shooting protocol- Car bonnet

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4 4.1 Sheet Metal

7 4.2 Car bonnets

8 4.3 Methods of comparison

9 4.4 Deflection after ricochet

10 4.5 Deflection after perforation

12 4.6 Accuracy and precision of the Lead in method

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5 5.1 Deflection after ricochet

14 5.2 Deflection after perforation

14 5.3 Accuracy and precision of the Lead in method

15

6 Conclusion

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7 Forensic Relevance

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8 Recommendations

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9 References

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10 Appendix

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11 Acknowledgement

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2 Abstract:

The primary focus of the study was to assess whether bare sheet metal is a suitable substitute for car body material in forensic/ballistic experimentation. To answer this question, we have investigated various factors that can influence the bullet behaviour on car bonnets. We have assessed these factors by statistically comparing the bullet deflection after perforation, ricochet on car bonnets and bare sheet metal. In addition, the accuracy and precision of the lead-in method was also studied. The results from statistical comparison showed that there was a significant difference in deflection angles for almost all the bonnets at 16° and 24° incident angles and almost all bonnets had no significant difference at 8° and 32°. These aberrations have been explained with the difference in critical angles between bonnets and sheet metal along with other factors. The results of this study will help researchers in unfolding the complete potential of bare sheet steel as a substitute for car parts in future.

Keywords:

FORENSIC SCIENCE, BALLISTICS, RICOCHET, DEFLECTION ANGLES, LEAD-IN, BONNET

Automobiles are an integral part of modern society; it is not a surprise that they are commonly found in crime scenes involving shootings. Shooting incidents involving vehicles must be investigated with caution, using rigid scientific methodologies, otherwise, the final reconstruction of events can be fallacious (1). There is a need to study the bullet behaviour on car body parts during a crime scene investigation to trace the trajectory of the bullet and to determine the direction from which the shot was fired. Using actual car parts for research can be tedious because parts like car doors have a smaller surface area, integrated reinforcements (insulation, plastic protective parts), which might lead to the miscalculating the true deflection of bullet after impact. This can lead to errors during the retracing of the bullet’s trajectory. So, there is a need for a substitute which can be used for laboratory research. Since bare sheet metal is flat, it can be ordered in desired dimensions (thickness, different standards), the results hence obtained are reliable and reproducible. In addition to this, it also allows easy inter comparing of different calibres, bullet types etc. Current studies are primarily focused on armoured cars used for military applications. Therefore, we aimed to assess the factors that can affect the bullet behaviour on the outer body parts of a car by comparing them to bare sheet steel, and to study the potential of bare sheet steel as a substitute for car outer body material. The main research question of the study is “Can standardised sheet

steel be used as a suitable substitute for automobile outer body parts for studying the bullet behaviour?” In

order to answer this question, we have compared the three aspects of bullet behaviour, namely: (1) deflection

after ricochet, (2) deflection after perforation and (3) the accuracy and precision of trajectory reconstruction from the lead-in part of the ricochet mark of a bullet hole. They have been studied using a Škorpion

sub-machine gun of calibre 7.65mm.

1.1 Ricochet

When a bullet hits a target at a low angle, it may deflect without perforating the target to a direction different from its initial one due to impact. This phenomenon of bullet deflection is called a ricochet (2). Ricochet is a surface phenomenon which typically occurs when there is deflection of the bullet without penetration or perforation. The line AB in figure 1 describes a ricochet. A bullet will ricochet when its incidence angle is less than its critical ricochet angle. (3). A critical angle is defined as “the incidence angle at which 50% of the fired bullets of a given ammunition type ricochet from a given object type.” (4). There are various factors that influence the bullet ricochet namely: velocity, shape of the bullet, nature of target material and the angle of incidence (3,5). Visible ricochet marks are usually present on almost all targets and can play a crucial role in shooting scene reconstruction.Reconstruction helps narrow down the different possible scenarios and decide what actually transpired at a crime scene.

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3 1.2 Deflection after ricochet

Thick hard steel plates are often considered non-yielding surfaces (the surfaces that causes deformation of the bullet after impact). Thickness and dimensions of our target sheet steel and the calibre of bullet used makes it behave like a yielding surface (the bullet creates a defect on the surface after impact). Another important phenomenon that occurs when a bullet ricochets from a yielding surface is horizontal and lateral deflection. Every bullet path can be resolved into two angular components: Vertical or lateral deflection angle which is the ascending or descending angle of a bullet projectile when it penetrates or perforates an object and the Azimuth angle or horizontal angle which the side angle, top angle (5). The angle between bullet’s plane of impact and trajectory after ricochet shown in figure 1. This can be used to estimate the trajectory prior to ricochet. The magnitude and direction of horizontal deflection can be affected by the twist rate of the bullet. (3,5).

1.3 Deflection after perforation

Perforation is a phenomenon that occurs when a bullet enters and exits an object which is shown in figure 2. When two objects are placed in front of one another in a straight line, a bullet that perforates the first object can cause secondary defects on the second object. The trajectory of the bullet prior impact can be calculated from the spatial orientation between primary and secondary defect (3,5,7). Every bullet path can be resolved into two angular components: Vertical deflection angle which is the ascending or descending angle of a bullet projectile when it penetrates or perforates an object and the Azimuth angle or horizontal angle which the side angle, top angle (5). Using these angular components, the bullet’s path can be re-traced during reconstruction. The accuracy and precision of the trajectory calculation can be influenced negatively if the bullet deflects from its original path after interacting with the first object (7).

FIG. 2 adapted from Mattijssen et al 2017 shows the vertical and horizontal deflection angle after perforation

1.4 Lead- In

Lead-in mark is a type of bullet wipe that occurs by dark, elliptical transfer of bullet material during its initial contact with a surface at a low angle of incidence (5). The incidence angle of the bullet prior to impact can be estimated using the lead-in portion of the primary bullet defect by aligning a probe with it (7). In addition to this, the entry side of a ricochet mark can also be established using lead-in marks. Only the first part of the lead-in mark should be used to estimate the angle of incidence because after the initial part there will be secondary effects like deformation, deflection, which can affect the accuracy of the method. Generally, the accuracy and precision of the lead-in method depends on the angle of incidence. When the angle of incidence increases, the accuracy and precision of the method decreases because the length of the initial part decreases (7).

2. Materials:

2.1 Sheet Metal

Selection of an ideal target material is an important aspect in any project. Since the project aims to compare the bullet impact on sheet steel with car bonnets, international and regional standards for automobile steel were studied and recorded, shown in Table 1. Finally, a particular metal namely DC01 under European Norms standard EN10130 was selected (8). DC01 was selected because of its international acceptance, easy availability from various suppliers, and it was used in an earlier study involving shooting on sheet steel (7). Figure 3 shows the range of tensile strength in different grades of steel used for automobile manufacturing and DC01 has a tensile strength similar to the grade of steel used for bonnet manufacturing. In the initial stage of the project, bare untreated sheets of steel plates (DC01), of dimension 36x48 cm and thickness of approximately 0.83mm, were employed to study the impact of a bullet on it. The mechanical and chemical properties of this sheet steel is mentioned in Table 2 and Table 3.

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STANDARD COUNTRY OF ACCEPTANCE PROPERTIES OF COLD ROLLED SHEET STEEL MECHANICAL CHEMICAL

SAE (Society of Automotive Engineers) / American Iron and

Steel Institute (AISI) UNITED STATES OF AMERICA

Tensile strength, Elongation rate, Yield strength and

Tensile test direction

Carbon, Manganese, Sulphur and Phosphorous

EN (Euro norm) EUROPE Tensile strength, Elongation rate, Yield strength and Tensile test direction

Carbon, Manganese, Sulphur, Aluminium, Silica

and Phosphorus

JIS (Japanese Industrial Standards) ASIA

Tensile strength, Elongation rate, Yield strength, Tensile test direction and Plasticity

strain ratio

Carbon, Manganese, Sulphur and Phosphorous

GOST (Gosudarstvennyy Standart) _{INDEPENDENT STATES}COMMONWEALTH OF strength, Elongation rate, Tensile strength, Yield Hardness

Carbon, Manganese, Sulphur and Phosphorous

ISO (International Organization for

Standardization) GLOBAL ACCEPTANCE

Tensile strength, Elongation rate, Yield strength, Tensile

strain hardening exponent and Plasticity

strain ratio

Carbon, Manganese, Sulphur, Titanium and

Phosphorus Table 1: Shows the different mechanical properties and chemical compositions that defines a standard for cold rolled sheet steel in

different regions

C%

(Max) (Max) Mn% (Max) P% (Max) S% (Max) Si% (Max) Al%

0.12 0.60 0.045 0.045 0.030 0.020

Table 2: Chemical properties of sheet steel used

Maximum Yield strength

(Mpa) Tensile strength (MPa) Minimum elongation (%)

280 270-410 28

Table 3: Mechanical properties of sheet steel used

FIG. 3: Shows the different grades of steel used for automobile body parts

2.2 Car Bonnet

For the second stage of the project, five bonnets from cars of different brands were used as the target material. The bonnets were selected as the target material because they can be easily removed from cars when compared to roofs or doors. They have larger surface area when compared to car doors, which is required for making 40 shots, with enough clearance between defects. In addition to this car bonnets have lesser integrated reinforcements (plastic components, insulation etc.) than car doors, so we can calculate the true deflection of

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the bullet without any interference. The bonnets used in the study were selected based on their brands and whether their surface area is large enough to make 40 shots. The thickness of each bonnet was calculated by cutting out 4 sample sections from each bonnet, scrapping of the paint coating (to prevent errors in measurement) and measuring the thickness at 3 different points on each sample section. The sample sections were removed only after completing the shooting tests, in order to make sure that the structural integrity of the bonnet was not compromised when shot at. The thickness of the sample sections was measured using an electronic micrometre (Hogetex) and the average thickness and standard deviation were calculated (mentioned in table 4).

BRAND MODEL OF THE CAR COUNTRY OF ORIGIN MEAN THICKNESS (mm)

STANDARD DEVIATION

Citroen Saxo France 0.800 0.04

Volvo 240 Sweden 1.055 0.04

Opel Astra Germany 0.910 0.03

Nissan Micra Japan 0.778 0.02

Ford Explorer United states of America

0.763 0.02

Table 4 Car bonnets collected from different brands of car

3. Methods:

3.1 Shooting Range setup

In this project the Škorpion sub machine gun of calibre 7.65mm, twist direction in right, barrel length of 115 mm and a twist rate of 1 twist in 305mm has been used, as it can be conveniently fixed on the mount during experimentation (9). An indicative double-check on the twist rate was performed by making and assessing a silicon casting of the barrels interior. The observed twist rate of the casting appeared consistent with the value derived from the literature. The ammunition was MAGTECH, with a 4.7g FMJ-RM bullet. The firearm was fixed in the mount. A stationary witness screen was placed at approximately 4 metres in front of its muzzle. The witness screen was suspended, ensuring a straight (in line with gravity) placement. To estimate the horizontal x- axis and vertical y- axis, two connected communicating vessels with water were employed. A floating device was used to indicate the water levels in both the vessels. At the expected point of bullet impact, a horizontal x axis and a vertical y axis were set up to the stationary witness screen using the communicating vessels and plum line, as shown in Figure 4. They were placed in such a way that they cross each other at the expected point of bullet impact. 5 shots were fired at the witness screen. The bullet must strike the point where the horizontal and the vertical axes meet. If it didn’t strike on the desired point, the mounted firearm was adjusted and then shots were made until the bullet struck the desired point.

FIG. 4: Shows the estimation of x- axis and y- axis using communication vessels

3.2 Shooting protocol for bare untreated steel plates

The target material was mounted on a cradle which was placed between the witness screen and the mounted firearm, approximately 2 metres from the muzzle. The cradle was adjusted to the required angle using a digital inclinometer (Mitutoyo Pro 360). An error of ±0.1° is accepted when setting the angleand the shot was fired. If the bullet bounces off the sheet steel, it was considered a ricochet, if it perforated the sheet it was considered

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a perforation. The X and Y values were measured from the location where the bullet hits the witness screen. The distance between witness screen and the exit hole on the end of the ricochet mark on the target material was noted for the Z value. The vertical deflection angle and horizontal deflection angle for each shot were calculated using their corresponding X, Y, Z values (see table 12 in Appendix). The vertical deflection angle was calculated using the formula θ = tan-1_{(Opposite / Adjacent) where Y values were substituted for opposite side and Z values}

for the adjacent side. From the calculated vertical deflection angles only for ricochets the incidence angles (8°, 16°) were subtracted, in order to prevent errors in true deflection calculation

.

The horizontal or azimuth deflection angle was calculated using the formula θ = tan-1_{(Opposite / Adjacent) where X values were}

substituted for the opposite side and Z values for the adjacent side. After five shots the angle of the cradle was adjusted again, and the above-mentioned protocol was repeated for angles from 10° to 90° with a 10° increment. From the results obtained from angles 10° to 90°, the critical angle of the sheet steel was estimated by repeating the above-mentioned protocol for angles 10° to 22° with a 2° increment. Five shots for each angle were fired. The critical region of the bare sheet metal for this ammunition type was found to lie between 16° to 20°. By combining these two results, four angles were selected; two that are expected to give ricochets consecutively: 8°, 16°and two that are expected to give perforations consecutively: 24°, 32°. The angles less than 40 ° were selected because when the angle of incidence is above 40°, the deflection angles were very low and insignificant

(7). With these four angles, 10 shots for each angle were fired at bare sheet steel and the acquired data was

used for comparison with car hoods. The shooting protocol followed is shown in figure 5

FIG. 5 shows the shooting range setup for shooting at the sheet metal mounted on a cradle

3.3 Shooting protocol for automobile body parts

A cradle cannot be used to mount a bonnet because of its irregular shape. Instead, an improvised structure with two height adjustable tables were used to mount it. The bonnet rested on three independently adjustable rests, which were mounted on to the height adjustable tables. The bonnets were placed with the front down and pointing towards the firearm. This structure was placed between the witness screen and the mounted firearm, approximately 2 metres from the muzzle. By fixing a laser to the muzzle, the point where the bullet strikes the bonnet was determined and a digital inclinometer (Mitutoyo Pro 360) was applied at this point. Then the whole set up was adjusted until the desired angle was obtained on the inclinometer (±0.1° was accepted when setting the angle). This procedure was repeated for each shot. The angle of the bonnet perpendicular to the shot was checked visually. If the bullet bounces off the bonnet, it was considered a ricochet, or it was considered a perforation if the bullet perforates the bonnet. The X and Y values were calculated from the location where the bullet hits the witness screen. The distance between witness screen and the exit hole on the end of the ricochet mark on the target material was noted for the Z value. The vertical deflection angle and horizontal deflection angle for each shot were determined using their corresponding X, Y, Z values (see table 12 in Appendix). The vertical deflection angle was calculated using the formula θ = tan-1_{(Opposite / Adjacent) where, Y values were}

substituted for opposite side and Z values for the adjacent side. From the calculated vertical deflection angles only for ricochets the incidence angles (8°, 16°) were subtracted, in order to prevent errors in true deflection calculation. The horizontal or azimuth deflection angle was calculated using the formula θ = tan-1_{(Opposite /}

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Adjacent) where X values were substituted for opposite side and Z values for the adjacent side. After 10 shots,

the angle was adjusted again, and the above-mentioned protocol was repeated for all the four angles 8°, 16°, 24°, 32°, selected from the results acquired from shooting at bare sheet steel. The shooting protocol followed is shown in figure 6.

FIG. 6 shows the shooting range setup for shooting at car bonnet with 2 height adjustable tables

4. Results:

4.1 Sheet Steel

After setting up the shooting range in accordance with the above-mentioned protocol, a total of 40 shots were fired. 10 shots for each angle namely: 8°, 16 °, 24 °, 32 °. After each impact, the bullet ricochet or perforation on the sheet metal was noted. Table 5 shows the trend observed after each bullet impact on sheet steel for the angles 8°, 16 °, 24 °, 32 °. The X, Y, Z values of each shot were also noted. The vertical and horizontal (azimuth) deflection angles were calculated for all the 40 shots using the X, Y, Z values. The calculated vertical and azimuth angles for all four incident angles are shown in Figure 7.

Angle of Incidence Perforation/ Ricochet

8 ° Ricochet

16 ° Ricochet

24 ° Perforation 32 ° Perforation

Table 5 shows the bullet effect on sheet metal for each incidence angle

4.2 Car Bonnet

After setting up the shooting range by following the above-mentioned protocol, 40 shots were fired in total for each bonnet, with 10 shots for each incident angle namely: 8°, 16 °, 24 °, 32 °. After each impact the bullet ricochet or perforation on the bonnet were noted. We observed that there were perforations (9 out of 10 shots) at incident angle of 16° with Citroen bonnets, when we expected ricochets. Likewise, for Volvo bonnets we had ricochets (for all 10 shots) instead of expected perforations at an incident angle of 24°. This means that the critical angle for ricochet on the Citroen bonnets is lower than the tested bare sheet metal and for the Volvo bonnet, the critical angle is higher. The deflection angles of these bonnets (Citroen and Volvo) calculated for the incident angles 16° and 24° were not included for comparison, because a comparison of deflection angles from ricochets and perforations will be fallacious. Table 6 shows the trend observed after each bullet impact on the five bonnets for the angles 8°, 16 °, 24 °, 32 °.

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Bonnet Brand

Incidence angle

8 ° 16 ° 24 ° 32 °

Citroen Ricochet Perforation Perforation Perforation Opel Ricochet Ricochet Perforation Perforation Volvo Ricochet Ricochet Ricochet Perforation Nissan Ricochet Ricochet Perforation Perforation Ford Ricochet Ricochet Perforation Perforation Table 6 shows the bullet effect on the 5 bonnets for each incidence angle

The X, Y, Z values of each shot were also noted down. The vertical and horizontal (azimuth) deflection angles were calculated for all the 40 shots using the respective X, Y, Z values. The deflection angles of sheet metal were compared with the deflection angles of the car bonnets. The deflection angles were compared with bonnets individually as well as the five bonnets together as one group. The calculated azimuth and vertical angle for each bonnet and sheet metal is shown in figure 7.

FIG. 7 Shows the Azimuth and Vertical angle of Citroen, Opel, Volvo, Nissan, Ford car bonnets and sheet metal respectively for an incidence angle of 8°, 16 °, 24°, 32°.

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9 4.3 Method of Comparison

Selection of the right statistical procedure for comparison of the obtained data is pivotal to this project. If an incorrect statistical procedure is used it can lead to fallacious results. After extensive research, Kruskal-Wallis test, which is anon-parametric test for comparing three or more samples of unpaired data (independent data which don’t affect each other) was selected. Table 7 shows the different statistical tests available for

comparison of parametric and non-parametric data (10). The requirements to be fulfilled for performing a nonparametric test are (11): 1) The sample should not be assumed to be from a normal distribution 2) For samples that contain a few outliers (data which are much larger or smaller than the normal range) 3) Sample size should be small (less than 15). Since our data fulfils these requirements the Kruskal-Wallis test was performed with a confidence interval of 95%.

WHAT IS COMPARED? TYPE OF DATA

PARAMETRIC NONPARAMETRIC One group with a hypothetical value t-test (one sample) Wilcoxon test

Two unpaired groups t-test (unpaired) Mann-Whitney test Two paired groups t- test (paired) Wilcoxon test Three or more unpaired groups ANOVA (one way) Kruskal-Wallis test

Three or more paired groups ANOVA (repeated measures) Friedman test Association between two groups Pearson correlation Spearman Correlation

Table 7 Shows the different statistical methods used for different types of data (Niroumand et al 2013) The steps involved in performing the Kruskal-Wallis test is:

I.Formation of null and alternative Hypothesis

For each test the H0: All medians are equal and H1: At least one median is different.

II. Calculate alpha value

The alpha value for confidence interval of 95% is 0.05

III. Calculate the degrees of freedom

The degree of freedom is calculated using the formula k-1, where k is the number of groups

IV. Decision rule

When the p value is less than 0,05 (alpha value) the null hypothesis is rejected.

V. Test Statistic calculation

The test statistic H is calculated using the formula

Where N is the total number of sample size, Ti is the sum of ranks.

VI. Result and Conclusion

Based on the p value and test statistic value the null hypothesis is rejected or not rejected.

The Kruskal-Wallis test using the above-mentioned protocol was performed for comparing deflection angles of sheet steel with deflection angles of car bonnets individually as well as together using the software MINITAB18.

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10 4.4 Deflection after ricochet

The vertical and azimuth angles of incident angles which gave ricochet in sheet steelwere compared with vertical and azimuth angles of incident angles which gave ricochet in car bonnets. For all five car bonnets the bullet ricochets at an incidence angle of 8 °. For incident angle of 16 ° except Citroen bonnet, the remaining four bonnets gave ricochet. For comparison Citroen bonnet was not used since it gave perforation instead of ricochet at an expected angle of incidence. Table 8 contains the results of Kruskal-Wallis hypothesis testing. It shows whether there was a significant difference between the deflection angles of sheet metal and car bonnets all together as one group, when they both ricochet the bullet at the same angle of incidence. The individual plot graph for vertical and azimuth angle of sheet metal vs bonnets all together as one group at angle of incidence 8° and 16°is shown Figure 8 and figure 9 respectively. Table 9 contains the results of Kruskal-Wallis hypothesis testing. It shows whether there was a significant difference between the deflection angles of sheet metal and car bonnets individually, when they both ricochet the bullet at the same angle of incidence. The individual plot graph for vertical and azimuth angle of sheet metal vs ´bonnets individually at angle of incidence 8 ° and 16 °is shown Figure 10.

Material

Deflection angles of sheet metal for incident angle

8 ° 16 °

Azimuth Vertical Azimuth Vertical All car bonnets as one group NSD NSD NSD SD

Where SD – Significant difference and NSD- No significant difference

Table 8 shows the comparison of deflections between sheet steel and all car bonnets as one group for angle of incidence 8 ° and 16 ° respectively.

FIG. 8 shows the Vertical and azimuth angles of sheet metal vs car bonnets as one group for angle of incidence 8 °

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Brand of Bonnet

8 ° 16 °

Azimuth Vertical Azimuth Vertical

Citroen NSD SD N/A N/A

Opel SD NSD NSD NSD

Volvo SD SD SD SD

Nissan SD NSD NSD NSD

Ford NSD SD SD SD

Table 9 shows the comparison of deflections between sheet steel and individual car bonnets for angle of incidence 8 ° and 16 ° respectively.

FIG. 10 shows the Vertical and azimuth angles of sheet metal vs car bonnets individually for angle of incidence 8 ° and 16 ° (except Citroen)

4.5 Deflection after perforation

The vertical and azimuth angles of incident angles which gave perforation in sheet steel was compared with vertical and azimuth angles of incident angles which gave perforation in car bonnets. For all five car bonnets the bullet perforates at an incidence angle of 32 °. For an angle of incidence of 24 °, every bonnet except the Volvo bonnet gave perforation. For comparison Volvo bonnet was not used since it gave ricochet instead of perforation at an expected angle of incidence. Table 10 contains the results of Kruskal-Wallis hypothesis testing, it shows whether there was a significant difference between the deflection angles of sheet metal and all car bonnets together as one group, when they both ricochet the bullet at the same angle of incidence. The individual plot graph for vertical and azimuth angle of sheet metal vs bonnets all together as one group at angle of incidence 8 ° and 16 °is shown Figure 11 and figure 12 respectively. Table 11 contains the results of Kruskal-Wallis hypothesis testing, it shows whether there was a significant difference between the deflection angles of sheet metal and car bonnets individually, when they both ricochet the bullet at the same angle of incidence. The individual plot graph for vertical and azimuth angle of sheet metal vs Bonnets individually at angle of incidence 8 ° and 16 ° is shown Figure 13.

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Material

24 ° 32°

Azimuth Vertical Azimuth Vertical All car bonnets as one group SD SD NSD NSD

Table 10 shows the comparison of deflections between sheet steel and all car bonnets as one group for angle of incidence 24 ° and 32 ° respectively.

FIG. 11 shows the Vertical and azimuth angles of sheet metal vs all car bonnets as one group for angle of incidence 32 °

FIG. 12 shows the Vertical and azimuth angles of sheet metal vs all car bonnets as one group for angle of incidence 24 ° (except Volvo)

Brand of Bonnet

24° 32 °

Azimuth Vertical Azimuth Vertical

Citroen SD SD SD NSD

Opel NSD SD NSD SD

Volvo N/A N/A SD NSD

Nissan SD SD NSD NSD

Ford SD SD NSD NSD

Table 11 shows the comparison of deflections between sheet steel and individual car bonnets for angle of incidence 24 ° and 32 ° respectively.

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FIG. 13 shows the Vertical and azimuth angles of sheet metal vs car bonnets individually for angle of incidence 32 ° and 24 ° (except

Volvo)

4.6 The accuracy and precision of trajectory reconstruction using the lead in method

Trajectory reconstruction using the lead-in position of the ricochet marks was performed only on car bonnets. The accuracy and precision of the lead-in method for trajectory reconstruction was studied because it is one the less exploited technique. The defects due to ricochet on bare sheet metal were not used because the rate of deformation is higher on bare sheet metal after bullet impact when compared to car body parts (7). So, comparing them is not necessary. The availability of bullet defects on the car bonnets which can be used for the lead- in methods was also very minimal. The ricochet marks on the bonnet were observed to be a continuing curve. As a result, there was less usable area for the lead in method to be applied. Initially the defect caused by the bullet for an incident angle of 8° on the Nissan bonnet was studied. The accuracy and precision of the lead-in method is higher for lower lead-incidence angles (7). The trajectory rod when placed approximately lead-in the first part of the defect, estimated the direction of the bullet prior impact. But this was not an accurate estimation, shown in figure 13. When an inclinometer was applied on the rod to check the incidence angle, the value was lower than expected. Using the lead in method on the defects due to bullet impact on car bonnets, the azimuth angle can be estimated but not the angle of incidence due to the shape of the defect. When rod was placed more than 2cm into to defect, it estimated that the direction of the bullet prior impact was lower than the actual direction. So the rod has to be placed in the first part of the defect cautiously. Since the results from the lead-in method even for lower incidence angle could not be estimated with accuracy or precision the method was not performed for the other incident angles. Similar unusable visible defects were observed on all the bonnets, so the lead-in method was not performed on the bonnet from other brands as well.

FIG. 14 shows lead in method to estimate the trajectory of the bullet from ricochet marks

5. Discussion:

We have attempted to assess the factors that can influence the bullet behaviour on car bonnets (from 5 different cars) by comparing them to bare sheet steel, and to study the potential of bare sheet steel as a substitute for

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car outer body material. The comparison was done statistically based on the deflection angles (azimuth, vertical) of car bonnets and sheet steel. In addition to this the accuracy and precision of the lead-in method in trajectory reconstruction were also attempted to be assessed

.

5.1 Deflection after ricochet

The results from the performed study show that the deflection angles of incident angles that gives a ricochet when compared with the deflection angles of all the bonnets together as one group had no significant difference at 8° for both azimuth and vertical deflection angles. But at an incident angle of 16°, the vertical deflection angles of all car bonnets together and sheet metal had a significant difference. But there was no significant difference between the azimuth angles at the same incident angle. In order to understand why these differences, arise, we compared the deflection angles of all bonnets individually with sheet metal at same incident angles. When compared individually with sheet metal, only the bonnets from the car brands, Opel and Nissan had no significant difference between them in vertical angles at an incident angle of 8°. But for the same incident angle, there was a significant difference in azimuth angle for all bonnets except Citroen and Ford. Similar to 8° Opel and Nissan had no significant difference in vertical deflection at 16° incident angle. But at 16°, the azimuth angle had a significant difference in all car bonnets except Opel and Nissan. From these results we can infer that the critical angles of the bonnets are different from the sheet metal. This might be the reason for witnessing perforations when ricochets were expected (Citroen). Around the critical region, predicting the bullet behaviour can be tedious, since it depends on the properties on the target material and cartridge types. It varies from material to material and ammunition to ammunition. So, this difference in critical angle might be the reason for the difference in deflection angles between bonnets. Even though Citroen bonnet’s thickness was the closest to sheet steel it had differences in deflection angles. This can be due to the difference in chemical and mechanical properties (Even after contacting various automobile companies we could not get information regarding the standard of steel used in this automobile). The curved, irregular shape of the bonnet creates more tension on the metal when compared to sheet steel which might influence the difference in deflection angles even at same thickness and incident angles. During a scientific experiment there can be a few observations which is not due to the sample analysed but can arise due to preparative methods used during the experimentations. This is called as an artefact. We checked the angle of the bonnet only in the direction of the shot, with an inclinometer. The angle of the bonnet perpendicular to the shot was checked only visually. Therefore, a bigger standard deviation and significant difference in azimuth angles between bonnets and sheet metal might be due to the experiment methodologyandnot from the inherent quality of the bonnet. The twist direction of the firearm can influence the deflection after bullet impact. The firearm used in the study had its twist direction as right, which could also be the reason for the difference in azimuth angles between bonnets and sheet metal.

5.2 Deflection after perforation

The results from the performed study shows that the deflection angles of incident angles that gives a perforation when compared with the deflection angles of all the bonnets together as one group had no significant difference at 32° and a significant difference at 24° for both azimuth and vertical deflection angles. In order to understand why these differences, arise, we compared all the bonnets individually with sheet metal at the same incident angles. At an incident angle of 24°, when compared individually with sheet metal, all the bonnets had a significant difference between them in vertical angles. But for the same incident angle, there was a significant difference in azimuth angle for all bonnets except Opel. Vertical angles of all car bonnets expect Opel had no significant difference in their vertical angle at 32°. For the same incident angle there was a significant difference in azimuth angle for all bonnets except Citroen and Volvo. From these results we can infer that the critical angles of the bonnets are different from the sheet metal. This might be the reason for witnessing ricochets when perforations were expected (Volvo). Around the critical region predicting the bullet behaviour can be tedious, since it depends on the properties on the target material and cartridge types. It varies from material to material and ammunition to ammunition. So, this difference in critical angle might be the reason for the difference in deflection angles between bonnets and sheet metal. Similar to deflection after ricochets, perforations also had artefacts. We checked the angle of the bonnet only in the direction of the shot with an inclinometer. The angle of the bonnet perpendicular to the shot was checked only visually. Therefore, a bigger standard deviation and significant difference in azimuth angles between bonnets and sheet metal might be due to the experiment methodology, not from the inherent quality of the bonnets. The twist direction of the firearm can influence the deflection after bullet impact. The firearm used in the study had its twist direction as right, which could also be the reason for the difference in azimuth angles between bonnets and sheet metal.

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15 5.3 The accuracy and precision of trajectory reconstruction using the lead in method

After visually and experimentally reviewing the results obtained from the lead in method on car bonnets, this method was proven to be inaccurate for trajectory reconstruction even for lower incidence angle. The main reason behind this aberration is the shape of the curve (continuous curve). The cylindrical rod used for the lead in method in this study seemed to not fit in the curve (due to bullet ricochet) on the car bonnets. Finding the first portion of the ricochet mark was difficult since it was a very small part of the whole mark and chances of human errors were high. We were unable to do a complete study on the accuracy and precision of the lead-in method for shooting on car body parts as we noticed that the method was less certain for the defects obtained.

6. Conclusion:

The difference in deflection angles for most bonnets at incident angles 16° and 24° can be explained using the difference in critical region of the bonnets employed in the study. The thickness, chemical and mechanical properties of the bonnet might have an effect on the deflection angles. The study of accuracy and precision of the lead-in method in estimating the bullet trajectory after impact with car bonnets was deficient due to the unavailability of usable lead-in marks and shape of the defects obtained after ricochet on car bonnets. The results are indicative for car outer body parts in general, but we must remember that the results can be different than with individual cars, especially near the critical region as it different for each material and ammunition type. The main research question of the study was “Can standardised sheet steel be used as a suitable substitute for

automobile outer body parts for studying the bullet behaviour?” From the results we can infer that the bare

untreated sheet steel has the competency to be used as a substitute for car body parts. The critical angle, chemical and mechanical properties of the car being tested have to be taken in toconsideration when choosing the properties of sheet metal. In conclusion, bare sheet metal has a promising potential to be used as a substitute for car outer body parts for studying bullet behaviour. In addition to this, it can also be used to inter-compare different calibres and bullet types. The results achieved from this current study can help in developing and choosing an appropriate substitute for car outer body parts which can produce reliable and reproducible results when tested under laboratory conditions.

7. Forensic relevance of the study:

• This is a ground-breaking research that involves cars that are not used for military and armour applications.

• We have used bonnets from 5 different car brands predominantly present in The Netherlands which facilitates our study to connect with the real life forensic setting.

• This study is the first step to develop a suitable substitute for car body part to study bullet behaviour to ease the practical difficulties.

• The results acquired from the current study with further research can ultimately be of a greater help in shooting scene reconstruction.

8. Recommendations:

• Caution during the design of the experimental setup and procedures to minimize artefacts, to prevent them from affecting the results of the study.

• Further research to test the accuracy and precision of other trajectory reconstruction methods such as probing method, ellipse method on car outer parts.

• The bonnets and sheet metal should be further tested with different cartridge types to check how it affects the deflection angles.

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16 9. References:

1. Shaler, Robert C. Crime Scene Forensics: A Scientific method approach. Boca Raton, FL: CRC Press; (2012).M. 2. Jauhari. Bullet Ricochet from Metal Plates. The Journal of Criminal Law, Criminology, and Police Science

(1969); 60(3): 387.

3. Mattijssen, E., Kerkhoff, W. and Bestebreurtje, M. Bullet Trajectory after Impact on Laminated Particle Board. Journal of Forensic Sciences (2017).

4. Kerkhoff W, Alberink I, Mattijssen EJAT. An empirical study on the relation between the critical angle for bullet ricochet and the properties of wood. Journal Forensic Science (2015); 60:605–10.

5. Haag MG, Haag LC. Shooting incident reconstruction, 2nd edn. San Diego, CA: Academic Press, (2011). 6. Kneubuehl BP. Das abprallen von geschossen aus forensischer sicht [dissertation]. Thun: IPSC, Universite de

Lausanne, (1999).

7. Mattijssen, E. and Kerkhoff W. Bullet trajectory reconstruction – Methods, accuracy, and precision. Forensic Science International (2016).; 262: 204-211.

8. European Standard EN 10130:2006, “Cold Rolled Steel Sheet for Drawing and Forming”.

9. X I.V. Hogg, ‘Jane’s Infantry Weapons, 1988-89’, 14th edition, Jane’s Information Group, UK. (1988) 10. Niroumand, Hamed, M.F.M. Zain, and Maslina Jamil. Statistical methods for comparison of data sets of

construction methods and building evaluation. Procedia - Social and Behavioral Sciences (2013); 89: 218-221.

11. Marius Marusteri, Vladimir Bacarea. Comparing groups for statistical differences: how to choose the right statistical test? Biochemia Medica (2010); 20(1):15–32.

10. Appendix:

Angle Vertical (sheet metal) Azimuth (Sheet Metal) Vertical (Citroen) Azimuth (Citroen) Vertical (Opel) Azimuth (Opel) Vertical (Volvo) Azimuth (Volvo) Vertical (Nissan) Azimuth (Nissan) Vertical (Ford) Azimuth (Ford) 8 6.29 0.31 8.55 1.22 6.81 1.15 5.05 -0.36 6.88 1.03 7.06 1.09 8 7.82 0.48 8.40 1.10 4.67 0.99 6.85 -0.37 7.17 0.69 7.90 0.91 8 7.76 0.62 7.08 1.22 8.47 1.58 6.21 -0.36 9.75 0.87 7.90 0.93 8 7.05 0.62 8.70 1.36 6.87 1.09 5.50 0.00 7.28 1.00 7.51 1.08 8 5.39 0.61 7.05 0.41 7.72 1.25 5.38 -0.18 7.26 1.01 8.53 1.07 8 7.15 0.77 8.05 0.54 7.99 1.42 5.86 -0.19 7.70 1.00 7.26 0.36 8 6.29 0.46 8.81 0.27 6.69 1.30 6.30 0.57 7.06 0.53 7.30 0.19 8 6.94 0.31 8.95 0.26 7.90 1.76 5.38 0.56 6.87 1.77 7.18 0.18 8 5.52 0.63 7.87 0.14 9.68 1.73 5.43 0.55 6.60 1.89 7.92 0.54 8 5.49 0.46 6.83 0.00 8.24 1.60 4.57 0.58 8.04 2.33 9.15 0.55 16 12.12 1.46 9.48 7.72 10.63 0.87 9.87 1.74 20.83 10.13 10.36 1.72 16 12.08 2.66 8.67 6.63 12.66 1.60 11.09 1.77 12.45 1.12 9.45 5.85 16 12.42 1.77 11.17 10.83 12.78 2.30 9.97 1.34 11.33 2.57 10.70 1.01 16 12.36 2.06 10.41 6.67 12.00 3.98 9.51 2.46 12.36 2.28 9.18 3.77 16 12.42 2.36 1.79 5.78 12.95 1.25 10.82 2.57 9.40 1.44 9.63 5.94 16 12.75 2.06 9.84 7.41 11.57 6.14 10.45 -1.16 13.64 3.87 10.57 5.68 16 12.55 2.08 -5.59 11.08 12.54 3.04 11.94 -0.43 12.32 0.64 8.66 6.67 16 12.13 1.77 -10.20 0.30 13.80 3.15 10.33 -1.16 12.59 3.33 11.43 2.50 16 12.54 1.92 -5.97 -4.90 12.52 2.48 9.37 -2.07 11.39 1.94 11.42 2.15 16 12.02 1.62 0.29 12.86 13.23 3.41 10.33 -1.18 13.63 2.53 11.03 2.80

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24 3.52 7.74 4.00 4.15 5.71 7.20 12.93 10.85 4.85 6.72 4.47 4.76 24 2.20 9.03 3.42 2.85 5.44 8.13 14.05 5.09 5.59 5.90 4.27 4.42 24 -0.88 11.90 3.87 3.16 4.63 10.07 14.17 7.09 5.47 5.79 4.54 4.11 24 3.81 6.15 3.29 2.15 9.32 8.47 13.19 12.86 5.68 5.68 4.85 4.56 24 3.24 7.47 4.18 4.18 5.25 6.69 13.50 10.38 4.56 6.02 5.13 4.25 24 4.43 5.61 2.63 2.63 5.93 4.90 13.80 5.58 4.50 5.46 4.04 3.47 24 2.35 9.61 3.95 3.37 5.41 6.16 16.00 7.18 4.59 8.47 5.51 3.68 24 2.36 3.98 3.63 2.76 6.64 7.39 14.92 6.49 6.61 7.40 4.84 4.09 24 2.80 9.20 3.62 4.68 5.45 5.45 14.92 4.65 4.83 4.19 5.18 5.03 24 1.03 11.20 3.64 2.73 6.36 6.20 14.81 4.35 2.02 5.88 6.19 3.72 32 3.90 1.16 3.67 0.59 7.23 3.04 4.22 12.48 4.00 2.34 3.77 1.02 32 5.47 3.17 3.52 0.29 7.95 4.40 0.14 13.33 5.33 2.84 4.02 0.43 32 5.55 5.13 3.23 0.29 8.08 3.74 -11.74 1.52 5.42 2.89 4.26 1.85 32 4.86 4.00 3.32 0.60 6.67 5.25 3.71 12.08 4.27 1.71 3.99 0.71 32 3.43 1.57 3.46 0.30 6.50 3.26 4.63 11.31 5.61 3.34 3.33 1.94 32 4.15 2.86 3.59 0.43 7.31 4.61 -4.30 14.35 4.64 2.49 3.68 1.23 32 3.28 1.14 3.44 1.10 7.08 3.31 2.55 14.57 5.01 2.34 4.08 1.77 32 3.00 1.29 3.75 0.47 6.84 4.85 4.64 18.47 5.84 4.71 35.87 1.91 32 4.72 3.29 3.44 0.78 6.21 2.95 5.12 15.05 4.97 2.32 3.69 1.37 32 5.27 3.28 3.75 0.47 0.00 0 0.54 14.67 4.94 2.55 3.69 2.05

Table 12 shows all the calculated vertical and azimuth angles for sheet metal and all the bonnets

FIG.14 Shows one of the target materials after shooting (sheet metal- RIGHT, Bonnet– LEFT)

12. Acknowledgements:

Research requires immense motivation, guidance, and assistance for its success. I would like to acknowledge the contribution of Erwin Mattijssen and Petra Pauw in this project. I thank Ruud Hes for helping us to collect the reiquired bonnets for shooting. The final outcome would not have been possible without their help and assistance.