Glass fracture analysis. A review

Forensic Science, 2 (1973) 0

l-21

Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

GLASS FRACTURE

ANALYSIS.

A REVIEW

STEVEN P. McJIJNKINS and JOHN I. THORNTON School of Criminology, Universityof California,Berkeley, California(U.S.A.)

SUMMARY a discussion

is presented

of the forensic interpretation

of annealed glass fracture. The

:,rocesses of glass fracture are given, the nature and formation of glass as a brittle isotropic solid is discussed, and the mechanical strength of glass is considered in terms of its atomic bond strengths

and external

flaws. Major modern theories of glass fracture propa-

gation are reviewed, and the relationships iies are developed.

of stress conditions

to fracture surface proper-

i. HISTORICAL APPLICATION OF THE FORENSIC INTERPRETATION OF GLASS FRACTIJRE The examination

and interpretation

benefit in providing useful information demonstrated

that the manner

of window glass fractures has historically in criminal investigations.

in which annealed

glass objects

been of

It has been convincingly are broken

is directly

related to the appearance of the fracture surfaces lm3. The appearance of these surfaces is often characterized by lines and shadowy figures which are found to assume a multitude 3f patterns and contours. By examining the nature of these patterns, the stresses that were extant within the breaking glass can be determined. The forensic scientist often liesires to go back a step further, caused the fracture. involving

the impact

translating

the stresses into the external

forces that

The forces of forensic interest causing glass fracture are chiefly those of blunt

objects

at relatively

Occasionally

thermal fractures can be of importance,

Low-velocity

impact fractures

low velocities,

and bullet

impacts.

as in arson investigations.

The earliest recorded interest in blunt-impact window fracture was expressed in an article published by the Russian criminologist Matwejeff 4. Matwejeff was called upon to solve a question which had arisen in a murder investigation and which directly played upon the guilt or innocence of the defendant. The issue was whether a particular window iad been broken from the inside or the outside. Matwejeff was unable to find published :eferences

to similar

problems

and was compelled

to conduct

his own experimental

2

S.P. McJUNKINS,J.I. THORNTON

investigations.

The results of his study were reported with clarity and simple explanations

and, for the major portion,

remain valid to this date.

Matwejeff noted the presence of arcing lines on the fracture surfaces of broken panes which bore a relationship broken.

It was noticed

in their appearance

edge to edge across the fracture while perpendicular struction

tures present

breaking of windows and subsequent

showed the occurrence

Matwejeff

made the important

their concave

observation

aspect to the side opposite

the application joined

that the perpendicular

portion

of the force in radial fractures;

the same side as the force in concentric

cations

could arise in the interpretation

distant

from the center

of fracture

that the arcs on radial frac-

the origin of the breaking force

window frame. These complications In offering an explanation concentric

fracture

some inherent

the perpendicular

portion

of the arcs

if one chose for study pieces

or close to restricting

structures

such as a

will later be discussed in greater detail.

for the reversed appearance

within

By the nature of

fractures. Matwejeff realized that compli-

surfaces, Matwejeff first concluded

property

process. He found

fractures.

of the arcs joined the side opposite

of these patterns

impact

recon-

of two major types of fractures, radial

while the opposite holds true for the arcs on the concentric the arcs this indicated

from

face. They were found to be nearly parallel to one edge

to the other. Controlled

of the fragments

and concentric.

to the side from which the pane was

that the arcs were not circular, but varied in their curvature

of the arcing lines on radial and that the arcs could not be due to

the glass, but were caused by some part of the fracture

that panes that were fractured

cutter did not have these arcing patterns.

after having been scored with a glass-

Matwejeff

theorized

that a certain sequence of

events occurs during blunt-impact fractures: 1. As the force pushes against the glass, the glass bends out elastically

until the elastic

limit on the far side of the glass is reached. While the far surface is in tension, the near surface is momentarily under compression. 2. Fractures occur first on the far side and radiate

outward

from a central

point

of

fracture origin. 3. The continued

motion

of the force pushes in on the pointed

segments between

the

radial fractures, placing tension on the front surface of these pieces. 4. The pointed between

segments break off with fractures starting on the near side and extending

the two adjacent

radial fractures.

These latter fractures

tend to form the

boundaries of a circle concentric about the fracture origin, and are therefore termed concentric fractures. Based on an assumed validity of this sequence of events in glass fractures, Matwejeff correctly concluded that the orientation of the arcs was determined by the free surface upon which the fracture crack originated. Shortly after the work of Matwejeff was reported, the Federal Bureau of Investigation 5 published a discussion of glass fracture analysis. The findings of Matwejeff were discussed and support for the validity of his conclusions was disclosed based on the results of independent FBI investigations. It was indicated that in over 200 glass fracture cases the FBI laboratory had no difficulty in establishing force direction after careful

GLASS FRACTURE

study

3

ANALYSIS

of the fracture

surfaces. The FBI cautioned

the examiner

tures which are caused not by the impact in question, falling from a fractured

6 published

tion as an aid to criminal investigation. to Tryhorn,

a detailed discussion of glass fracture interpretaSharp pointed objects striking a window point-on

cause only radial fractures

blunt or tapered objects will cause concentric Tryhorn concentric

noted

that if the blunt

around the point of impact, while

fractures in addition

force is insufficient

to the radial fractures.

to break out pieces of glass, the

fracture lines are likely to be absent.

In his explanation forces, Tryhorn release during reported

frac

forces, i.e. pieces

window pane and breaking anew upon striking the ground.

In the same year, Tryhorn will, according

against misleading

but by secondary

of the relationship

indicated

between

that his experimentation

the fracture

process,

a statement

the fracture

which is consistent

earlier by Matwejeff. The reverse relationship

iines on radial and concentric that the exact appearance

face lines and fracture

proved the lines to be caused by stress with conclusions

between the orientation

fractures is fully described.

of the arcing lines on concentric

In addition,

Tryhorn

of arcing claimed

fractures will depend on the

Impact Fig. 1. A window glass fracture diagram to show the interruption of conchoidai pattern on concentric fracture surfaces as being dependent on the sequence of fracturing. (After Tryhorn 6.) Tryhorn’s impact point is shown as a dotted arrow; a more accurate representation is shown by the solid arrow.

S.P. Mc.JUNKINS,

4

“relative moments

of occurrence

of the radial and concentric

Fig. 1, drawn after a figure in Tryhorn’s the essence of Tryhorn’s

explanation.

A to C and C to D are respectively point

C, where

concentric

The diagram in

The arcing lines on the concentric continuous.

fracture surfaces

The arcs tend to spread outward prior

to the formation

cracks C to D and C to A. The arcing lines across the concentric of concentric

radial fracture B be extended after impact,

the sequence

uous arcing pattern Tryhorn

of crack formation

against anomalous

at the ends of radial fractures, markings,

which appeared

fractures,

extended

Tryhorn

also mentioned structures

could be determined

by noting the contin-

at point B. fracture

face patterns

which he found to occur

that

usually

these are likely

orientation

in the neighborhood

that these of radial

of one-half inch.

to occur in radial fractures

near rigid,

like window frames (see Fig. 2). upon Tryhorn’s

pattern

should

ascribed to it by Tryhorn.

Nickolls

to note that should the

remote from the point of impact. He indicated

study of glass fracture, Nickolls 7 hints that the anom-

be considered

as being

of more

importance

Nickolls described an example of anomalous

in which their length was considerably in Tryhorn’s

surface C to A

fracture surface at some time

to be arcs reversed from the expected

over short distances,

confining

In commenting

crack C to A. It is interesting

to meet the C to A concentric

across the intersection

cautioned

alous fracture

from of the

by radial crack B, however, because the crack had not reached point B

prior to the formation

than

that

fracture markings

longer than the one-half inch “usual length” noted

article. suggested

glass bending

that the cause of these anomalous

in a wave form with the arc pattern

The reversal in direction

of bending

concluded

the fracture

able for determining

Fig. 2. Anomalous

markings could be due to the

reversal occurring

that only the first-order

center and the first concentric

of these complicating fracture

of plate glass. Note the reversal

fracture,

of ribbed

to the change in anomalous

pat-

surfaces, i.e. those occurring should be considered

the origin of breakage forces.

fracture

at the wave nodes.

at fracture would then correspond

of the arc lines. From the occurrence

terns, Nickolls between

fractures”.

original article, is offered as an aid to visualizing

the radial crack 0 to C was present

are uninterrupted

orientation

J.I. THORNTON

lines.

reli-

GLASSFRACTUREANALYSIS With the publication

of O’Hara and Osterberg’s

istics ‘, came a relatively

comprehensive

analysis. Notably,

it offered

nated

safety-glass,

automobile

surfaces. Osterberg presumably

5

a detailed

text,

An Introduction lo ~~i~i~d

and lucid discussion discussion

of forensic glass-fracture

of spiral fracture

and also of the formation

stated that the spiral fracture is most commonly because

of the extra

flexibility

phenomena

found in laminated

it possesses over nonlaminated

sequence of fracture causing spiral lines was explained

out against the

of the main body of glass by the initial impulse, a condition

apparently produced. 3. The glass rebounds back in the direction

glass,

glass. The

as follows:

1. The initial impact causes the usual radial cracks on the impact layer. 2. As the wedge-shaped sections between the radial cracks are pushed resistance

in lami-

of hackle marks on fracture

of torque is

of the force origin until the elastic limit of

the glass is reached, when a spiral fracture begins, crossing the radial cracks which were produced

on initial impact.

E-Iaward 9 is credited iractures

is dependent

by Osterberg as having shown that the number of discrete spiral on the nature of the impact force. A higher velocity of impact will,

to a limit, create a greater amount of spiral fracturing. Hackle marks were claimed by Osterberg 8 to be caused during a high shearing stress fracture. He states that their presence is indicative of a rapid explosive fracture force. It is also claimed that hackle will be found perpendicular to arcing lines on fracture surfaces. it is indicated

by Osterberg

that hackle can thus be of great aid in determining

force

direction when arcs are absent. Detailed instructions aid the criminal Mention

are given in an issue of the FBI Law Enforcement Bulletin I0 to

investigator

in determining

the direction

is made of a “3R” rule, indicating

right-angles

of force in window fractures.

that radial fracture surfaces possess arcs at

to the rear surface of the window. The complications

fracture lines were not cautioned Some interesting

contributions

Kirk 1’ with the publication usual introductory

against, nor mentioned to the picture

of his textbook,

remarks concerning

caused by anomalous

in the article.

of glass fracture

analysis were made by

Crime Investigation. In addition

the formation

to the

and sequence of radial and concen-

tric fractures, Kirk stated that glass rarely breaks “squarely across”. Rather, Kirk indicated that a sharp edge will remain on each piece broken apart. The larger piece, from which rhe smaller piece was broken,

reportedly

will bear the sharp edge on the side of impact,

-while the reverse will be true in the smaller piece. Interestingly, with forensic fracture comment upon it.

analysis

has made

this observation,

no other author dealing

or at least, has seen fit to

Hackle lines were only briefly discussed by Kirk and were described iines running

in the direction

of the fracture

front. It is indicated

under conditions of strong local shear stresses. The validity explored later in this article. Frye l2 published

a discussion

of his observations

as short parallel

that they only occur

of this statement

will be

of window fracture damage caused

6

S.P. McJUNKINS,

by medium-range

velocity missiles of small diameter.

After conducting

J.I. THORNTON

a series of experi-

or sling-shots, using steel ball-bearings and glass ments with air guns and “catapults”, beads as missiles, Frye concluded that the size of the hole and the diameter of the crater at the exit surface were relatively more closely connected particular

interest

independent

to the momentum

of the diameter

at impact.

because of the popularity

The earliest recorded

treatment

of the nature

of glass fractures

as an aid to criminal

is in the discussion of bullet fractures by Hans Gross ’3. Gross reported that of projectiles

of relatively

high velocity

window fracture having features dependent if the velocity out toward

are likely to occur and be

impact window fracture (Bullets)

investigation the impact

of Frye are of

of air guns and sling-shots with children and

the resultant frequency with which these types of fractures confused with bullet holes by the layman. High-velocity

of the missile, but were

The observations

is sufficiently

with window

panes will cause a

on the velocity and angle of impact. Notably,

high the window will exhibit a round, clean hole, bevelled

the exit side. As the velocity

creased discharge energy, the irregularity

is reduced

through

increased

distance

or de-

of the hole will increase. When the velocity

of

the bullet has been reduced beyond a point, the window no longer merely exhibits a hole, but is shattered.

In examining

that the bevelling instance,

if the bullet

predominantly pushing

occurs

the back or exit side of the bullet hole, Gross indicated

on the side opposite

strikes

the window

on the right side. He proposed

out the back “layers” denoting

of the bullet

origin. For is

that the bevelling is caused by the bullet

of glass as it passed through

observed to be in the form of shell-shaped conchoidal,

the direction

at an angle from the left, the bevelling

fractures

the pane. The bevelling was

described by the mineralogical

term

the curved markings on the conch shell.

These findings by Gross were later supported

by Matwejeff 4. Matwejeff was addition-

ally able to demonstrate that when a bullet passes through a window pane it takes on a rotational motion about its lateral axis, while previously it was rotating merely about its longitudinal

axis. While this vector change had no detectable

the hole, it was noted that subsequent of considerable

medicolegal

effect on the appearance

tissue damage caused by a tumbling

importance.

Although

specific citation

of

bullet could be

is absent, Matwejeff

reported being able to put his observations to use in the investigation of a homicide case involving the determination of the entry and exit side of a bullet hole in a window pane. Matwejeff further reported finding that the radiating fracture lines often found emanating from the edge of the bullet hole are formed early in the fracture process. The concentric-like fracture lines often cau+ng physical dislocation of the glass from the pane are created after the radial fractures. This was demonstrated by firing a firearm at close range into a pane of glass with a white cotton pad located immediately behind it. A smoke pattern of the fracture lines was recorded on the pad and remained after the glass had been shattered and had fallen out. It was observed that the circular cracks extended only between the radial cracks and did not cross over them. Matwejeff was of the opinion

7

GLASSFRACTUREANALYSIS that this proved

that radial cracks were present

before the formation

of the circular

cracks. In a discussion

of bullet window fractures

in their FBI Law Enforcemeiz:

the FBI drew an analogy between bullet-caused planks. Wood splinters

torn loose on the exit side of nail holes, in alignment

grain, were considered

to be analogous

bullet holes. It was considered condition

dissimilar

surfaces

‘,

with the

to glass chips knocked loose from the exit Side of that due to a lack of a directional

grain in glass, a

in wood, the glass particles are torn off as flakes

of the exit hole.

was also given to the appearance

glass. The FBI stated exterior

probable

to that occurring

from around the circumference Consideration

Bulletin

window fracture and nail holes in wooden

of bullet holes in laminated

safety-

that while both first and second panes may have chipping on the

around

panes. On the entrance

the hole, the chipping

is different

side the chips are approximately

in appearance

perpendicular

in the two

to the surface,

while on the exit side the chips form an angle with the surface. Through experimentation with automobile laminated safety-glass, the FBI claimed to be able to show that the caliber of the bullet can be approximated from the size of the bullet hole. The cohesive forces of the inner layer tend to hold the fractured place around tionality

the periphery

between

glass tightly in

of the hole. They claimed that there exists an inverse propor-

the bullet velocity

and the size of the hole. It was also claimed thai

high-speed bullet holes will have a number

of short, fine radiating fracture lines, a feature

absent, or present to only a limited extent,

in low-velocity

By examining

the intersection

and interruption

bullet-caused

holes.

of cracks resulting

from a series of

bullet holes in a single window, the FBI indicated the possibility of determining the order in which fractures were caused. This would give information concerning which bullets were fired first, and in conjunction the respective shots were fired. Tryhorn’s bullet-caused

with cratering observations,

article 6 on glass fracture contained fractures

a discussion

the direction of experimental

of windows. He observed that exit side chamfering,

present in bullet holes whether the window remains intact or is completely Tryhorn fractures

recognized present

that the processes in high-velocity,

problems

results of his examinations foilows:

distinct

short-duration

from those in low-velocity

impact

from which results un

or bevelling, is shattered out. impact window fractures.

of bullet impact fracture surfaces can be briefly summarized

The as

1. Chamfering, or bevelling, always occurs on the exit side of the “primary perforation”. 2. The diameter of the hole varies with the velocity. 3. Fracture surfaces from shattered windows show markings of a “confused” nature; some Similar in appearance to those in low-velocity impact fractures, but with unrelatable orientations. Lines appear to be “running across the glass normally to the faces”. 4. In all cases, at least one dulled region, concentric about the primary hole, is found on the radial fracture surfaces at which reversal of the arc-line orientation is observed. 5. Occasionally the characteristic lines on edges near the point of impact branch out in opposite directions

from a medial line running

through the thickness of the glass.

8

S.P. McJUNKINS,J.I. THORNTON Tryhorn

suggested

that the dulled areas occurring

hole could be due to the intersection glass by high velocity

of reflecting

impacts of extremely

concentrically

short duration.

At these intersection

large pressure would develop, possibly causing crystallization for a dull appearance

at these locations. orientation.

to run without

The relationship

areas, a

of the glass and accounting

Between these wave intersectional

areas of low wave pressure, allowing the fracture either radial or concentric

about the primary

mobile pressure waves set up in the

areas would be

causing arcing lines of

between low-velocity

crack mqve-

ment, low stress and faint rib markings to be discussed later in this article tend to support Tryhorn’s

explanation.

It occurs to the present writers that there exists a strong possibility

of a close relation-

ship between these areas of arc reversal and the anomalous fracture patterns observed in the extremities of radial fractures caused by low velocity impacts. The anomalous arcs usually

occur near confining

sure waves as Tryhorn which Tryhorn

structures

where the reflection

of vibration

waves, or pres-

calls them, is likely to cause the same type of wave interference

postulated

as occurring in high-velocity

needed into the cause of these phenomena

impact fractures. More research is

and any relationships

that may exist.

Turfitt ’4 wrote about the nature of bullet-caused fractures of safety-glass. His interests in the subject were inspired by involvement as a consultant in a criminal investigation matter.

The investigation

concerned

a particular

question regarding the cause of a hole in

a pane of tri-layer laminated auto safety-glass. The questions asked of Turtitt were: (1) whether the hole was bullet-caused; (2) if bullet-caused, what caliber; and (3) from what direction and distance was the bullet fired. In seeking explanations set of experiments

for the cause of the questioned

involving shooting

bullets at laminated

fracture,

Turfitt

conducted

a

safety-glass from varying dis-

tances and angles. Bullets of .22, .38 and .45 calibers were fired by Turfitt in his experimental research. Turfitt

observed

bullet holes reported

the same type of symmetrical

chamfering

around

the exit side of

by Gross and Matwejeff in shots fired normal to the pane. In shots

fired at the glass from an angle less than 90”, Turfitt observed the offset exit side chamfering reported by Gross. In addition, he found the lead bullets underwent shearing when striking the glass at these smaller angles. In a diagram, he illustrated body of the bullet passed on through the pane, having undergone

how the main

only slight deflection,

while the shaved-off fragment was deflected off at an angle back from the incident surface of the glass pane. As an example, a shot fired at an angle of 45” with the plane of the pane caused a fragment to fly off at a 15-20” angle from the pane. The range of discharge could not be determined closely, as Turfitt was able to find very little difference in bullet holes caused by discharge between 5 and 20 yards. Turfitt, in summarizing his results, saw fit to caution against problems caused by choosing laminated test windows of ages different from the window in question. It is explained that anomalous fracture can result due to the aging differences of the plastic interlayer. Turfitt suggested choosing test windows estimated to be of the same age as those in question.

GLASSFRACTUREANALYSIS

9

Svensson and Wendel l5 published Techniques

their textbook,

an interesting

account

of Crime Scene Investigation.

trasting bullet fractures with stone- and ball-bearing-caused provided.

Svensson

crater formation

and Wendel

An interesting

discussion

con-

fractures of window panes was

to the very irregular

caused by stone impacts in contrast

ance of bullet fractures. reported

drew attention

of glass fracture patterns in

radial crack and

to the more even, regular appear-

The total absence of radial fractures and a striking smoothness

as characteristic

of ball-bearing-

or round-shot-caused

when fired from air guns or sling-shots. A characteristic a hole, one or two millimeters

fractures

is

of windows,

of these fractures was stated to be

in diameter, located at the center of the crater, perforating

the glass at that point. Svensson and Wendel also discussed spontaneous fractures of glass objects apparently caused by stresses remaining in the glass from manufacture. Also mentioned is the possibility

of sonic vibration

experimental

airfields,

tures were reported

causing breaking of glass, a common or from aircraft flying at supersonic

to be similar in appearance

segments described as characteristic mobiles and glass doors.

of tempered

occurrence

in homes near

speeds. These types of frac

to the complete

fracture

into diced

glass. The latter is often used in auto.,

Thermal glass fracture Although fracture

a subject more often of minor importance

properties

may assume great importance

in criminal investigations,

in establishing

whether

a window was

broken by a fire or by mechanical

means in a suspected arson or burglary-arson.

mentioned

fracture

fractures

the nature

of thermal

therma.; Kirk ’’

surfaces only briefly by stating that thermal

are the result of shear stress rather than tensile stress, and characteristically

will

have smooth surfaces. While thermal cracks are known for their typical mirror surfaces, support

for the condition

by the present appearance subject

writers.

of shear stress as opposed to tensile stress could not be found Soderman

of thermal fractures

was provided,

and O’Connell ’6 discussed the characteristic

in window panes. Although

they mentioned

the occasional

these fracture surfaces. They drew attention to fly back from thermal

of conchoidal

of the hnes on

to the fact that small glass splinters are likely

fractures toward the source of the heat, providing an additional

means for proper interpretation Fracture line comparisons

only a brief treatment

appearance

wavy

of breakage causes.

to establish common origin

Stapleton 17 reported on a casting technique to illustrate the intricate patterns on glass fracture surfaces for the purpose of showing the origin of evidential glass chips. He describes the application of this method to a hit-and-run automobile investigation where eight chips of glass found at the scene of the impact were tested against the fracture of a suspect vehicle “windscreen”. While all eight glass fragments were placed in positions on the windscreen

in which they appeared to fit well, microscopic

comparison

of the frac-

S.P. McJUNKINS,.I.I. THORNTON

10

ture lines revealed that only six had markings supporting remaining

fragments

had fracture

tions in the windscreen. the fine fracture

Stapleton

Nelson I8 discussed criminal

commented

their respective placements.

with those on the prospective

on the implications

of the non-match

lines, stating that the case in point demonstrated

on a simple fit for establishing establishing

lines inconsistent

of

the dangers of relying

the sources of origin of large glass fragments.

the value of glass fracture

surface hackle markings as an aid to

the origin of the small glass flakes frequently

investigations.

The posi-

He suggested

encountered

as trace evidence in

the greater value of hackle marks over conchoidal

lines in comparison processes, as the visibility of the latter is highly dependent on the angle of illumination. Due to their curved nature, there is no single angle from which the specimen

can be illuminated

to show the entirety

of the curved lines. Hackle marks, being

straight and angular, do not present such problems. By placing the questioned fragment in its presumed

origin bed and slightly shifting it

out of position,

of hackle orientation

Nelson illustrated

could be compared the displacement

between the bed and the fragment. Nelson indicated

method in obtaining

In a recent article primarily photography

of glass fracture

information

into the structure

work from the forensic detail. Thompson

concerned

with the choosing of suitable illumination

surfaces, Thompson

l 9 provides illustrations

for

giving some

of hackle lines. The article is notable in that it is the only

was reportedly

of hackle markings in any

free surface to obtain a profile view of the chosen to surmount

two problems in visual-

of hackle lines which often occurs as they approach

and (2) the destructive

abrasive action

the

during the last stages of fracture

Later in this article it will be shown that this abrasion is actually a natural fine

forking of the hackle and splintering of the surfaces. From his profile study, Thompson observed that hackles structures,

and length

the usefulness of

display of a hackle mark match.

field which deals with the structure

izing hackle: (1) the fading-out separation.

a photographic

ground down a pre-existing

hackle marks. This technique free surface;

how the similarity

occur as varying stair-step

with a shelf often present at the top and base of the deeper steps. While the

upper shelves are parallel

to each other,

they are not necessarily

parallel

to the lower

shelves, which also lie in parallel planes. Thompson

indicated

that illuminating

the upper shelves from an angle will show them

up as narrow, bright bands which he claims to be the true observable hackle structures. By a slight change of illumination angle, Thompson found that the shelves at the bottom then became more prominent visually. Thompson pointed out that for these reasons lighting can be very important in comparing hackle lines. He advocated using a comparison microscope with equal, carefully controlled lighting to compare hackle in establishing common origin. While Thompson’s observations of hackle structure appear well illustrated, other references to the parallel plane shelving could not be found by the writers. A considerable number of hackle mark samples have been examined by the writers without revealing the presence of flattened peaks and valleys which Thompson associated with the hackle structure.

GLASS FRACTURE

The advancement characterized learned

11

ANALYSIS

of knowledge

about glass fracture patterns

as, at best, sporadic and in some instances

about

fracture

characteristics

from published

concerned

with brittle

fracture

problems.

properties

and fracture

surface markings,

in criminalistics

superficial.

research of those outlying

respectively,

in an attempt

to bring some bene-

in fracture theory to the field of criminalistics.

II. SOME FRACTURE-RELATED

PROPERTIES

OF GLASS

of glass is perhaps its most distinctive

physical

property.

readily shatter

or crack when struck a sharp blow or when placed in tension

stress beyond

its maximum

random

manner

characterize

Glass, a substance day through

Paradoxically, controversial

It is observed

known and used since ancient

times, has become ubiquitous

widespread

material

substance.

usage as a building

The technology

of glass formation

in our

and as an artistic medium.

standpoint,

and the focus of major research institutions

product

of glass

and its brittle fracture characteristics.

Glass has been defined by the American inorganic

or shear

that glass does not break in a

force. This section will discuss certain of the properties

however, it is, from the technological

entire industry

Glass will

as a result of a given force, but rather in a pattern which can be used to

the breaking

which relate to its brittleness present

endurance.

fields

Parts II and III discuss glass fracture-related

fit of the recent developments

The brittleness

can be

A great deal is to be

a poorly understood

and

has become the basis of an and professional

societies.

Society for Testing and Materials ” as “an

of fusion which has been cooled to a rigid condition

without

crystalli-

zation”. This is only one of many definitions of glass and of the physical state known as the vitreous or glassy state, but it will serve well here as a simple statement of one of the more important The physical

concepts of what glass is. properties

of commercial

glass have been reported

in notable

works by

Morey 2 ’, Shand 2 2, Stanworth ’ 3 and Stevels ’ 4. These works discuss glass properties related to the compositions of many .glass-forming inorganic mixtures.

as

Glass is primarily a noncrystalline solid. To be considered a glass, a solid must not contain any significant widespread regular crystalline lattice structures in its atomic array. This condition

includes some plastics, metals, organic glasses, and newer materials formed

by a rapid cooling, neutron imbardment and shockwave vitrification 25. Another fundamental but more complex property of glass lies in its relationship with the molten OI liquid state. Morey 2 ’ indicated in his definition of glass that the solid state is continuous with the liquid state, but has undergone for practical considerations

a reversible change in viscosity on cooling so that

it is rigid enough to be treated as a solid.

Processes in glass formation When a melt or liquid is allowed to cool into the stage of solidification, the solidifying process will primarily take place in one of two different ways. Either the atoms will arrange themselves

into repeating

three-dimensional

patterns

called lattices with a high

12

S.P. McJUNKINS, J.I. THORNTON

degree of order, or they will assume some randomness process is true crystallization

and depends primarily

slow rate of cooling and on the presence or formation successive layers of atoms can build. During the slow cooling in the crystallizing forcing

atoms

closer together.

thermal activity.

atoms begin choosing positions repulsive forces and minimum perature

These atoms

As cooling continues,

ing arrangement

process, the liquid contracts, normally

repel each other is eventually

in a neat and tight arrangement internal

of compounds

for a chemical compound

a

compounds

gradually

through

their

reached at which the

which provide minimum

energy, and thereby maximum

this temperature

The first

of suitable seed nuclei upon which

a temperature

at which this begins for chemical

point. For mixtures

in the solid structure.

on a high degree of melt purity,

stability.

The tem-

is the melting point or freezing

is called the liquidus

22. The result-

is called a crystal and the pattern

of arrange-

ment is a crystal lattice. The second

type of solidification,

duced, results in the formation melt being impure characteristic

atomic arrangement

of glass. This form of solidification

is primarily

is pro-

due to the

or to a cooling rate too rapid to allow the careful atomic arrangement

of crystals to take place. As the atoms lose thermal

liquid contracts. each lower

where a more random

During the contracting

temperature

the atoms

system, which is slightly contracted

energy in cooling, the

process the viscosity of the liquid increases. At

adjust their positions

so that, on the average, the

relative to its previous temperature,

least possible internal energy. This cooling, contraction and adjustment temperature is reached analogous to the freezing point of crystallizing

can achieve the continues until a liquids, at which

the atoms begin to arrange themselves into a nearly repetitive pattern approximating that of a crystal array. While the atomic arrangement of glass is somewhat lattice-like in structure, the precise and regular repetition is absent. As the viscosity increases on cooling, the atomic mobility adjustment

of the atomic system, or network,

decreases so that each new

requires more time. Finally, a temperature

is reached at which the adjustment is so slow that it becomes negligible over reasonable time-spans. This temperature is the fictive temperature. The fictive temperature of glass is significant

in the respect that practically

some extent, upon it. Under certain circumstances

all the properties

the atomic

arrangement

of the glass are dependent,

to

in glass will change or reverse

into a more perfect lattice, causing localized crystallization. This process, called devitrification, is troublesome for the glass industry as it causes impairment in the mechanical strength of the glass. Atomic arrangement in glass structure The structural aspects of glass are fuel for the greatest of controversies concerning the nature and properties of this material. Historically, a number of different theories have been proposed outlining the spatial arrangement of the atoms in the interstices of a glass body. Zachariasen 2 6 suggested that the atoms in glass comprise a three-dimensional

13

GLASSFRACTUREANALYSIS network.

He suggested that although this system lacks symmetry,

comparable criteria

to the network

of a corresponding

crystal.

it has an energy content

Zachariasen

which an oxide should meet to be a “glass-former”:

offered

(a) oxygen

a set of

atoms can be

linked to no more than two cations; (b) less than three or four oxygen atoms should surround any cation; (c) the oxygen polyhedra must share corners rather than faces or edges to form the three-dimensional

network;

and (d) three or more corners

must be

shared. Warren 27 confirmed

Zachariasen’s

glass bodies with X-ray diffraction. patterns appeared

on glass-forming

The early interpretation

fell primarily into two major opposing viewpoints. diffuse,

oxide-cation

resembling

those of liquids,

patterns

because the crystals comprising patterns

oxides by studying

of the resulting

diffraction

Because the patterns

obtained

one view was that the arrangement

units was not regular and repetitive

that the glass diffraction diffraction

postulations

were diffuse,

as in crystals, but random.

of

Others felt

not because of a random network,

but

glass were too closely spaced to yield the common X-ray

obtained

from normal

firmed the former view and, together network theory.

crystalline

material.

with Zachariasen’s

Further

concepts,

research con-

led to the random-

Glass composition The metal oxides which are found in commercial arbitrary formers

classes: glass-formers, are reported

might be considered

intermediates

capable

of forming

and modifiers a three-dimensional

25. Of these only the glassnetwork.

This network

the basic working matrix. The oxides in the modifier class form ionic

bonds with the anions in the glass network. properties

glass have been grouped into three

of glass. The intermediates

These modifiers

have the ability to alter the

can take up positions of glass-forming

oxides in the

basic matrix or they can be used as modifiers. A second arbitrary

grouping

and fluxes. The glass-formers are classed according given temperature

system divides the oxides into glass-formers,

to the function

to increase

which they serve. The fluxes lower the viscosity at a

the workability

mon fluxes are Na2 0, K2 0 and B2 0,. Stabilizers ty and prevent crystallization. It has been established

of the glass at lower temperatures.

Com-

are added to increase chemical durabih-

CaO, MgO and Al2 O3 are some common stabilizers. that there are approximately

systems with the elements presently cial interest. Silicon systems known.

stabilizers

are the same in the two systems. The stabilizers and fluxes

dioxide,

SiO,,

sixty-six possible glass-forming

known. However, only a few of these are of commeris by far the most important

of the glass-forming

The most important glass of forensic interest is soda-lime glass. This glass is economically produced and found in most inexpensive and commonplace glass objects such as windows,

containers

and light globes. The fluxing oxide soda, Na20,

is added to lower

the glass viscosity, as mentioned earlier. The stabilizing oxide lime, CaO, is added to decrease the solubility of the soluble sodium silicates, thus increasing the chemical dura-

14

S.P. McJUNKPNS,J.I. THORNTON

bility of the glass. A certain amount fluxing agent for economic

of magnesia, MgO, is used as a substitute

for lime as a

reasons. The most popular soda-lime glass composition

on the

basis of cost, durability and ease of manufacture is of the approximate composition 72% silica, 15% soda, 10% lime and magnesia, 2% alumina and 1% miscellaneous other oxides 25. The alumina durability

apparently

and preventing

Some mechanical properties

matrix

characteristic

structure

glass can be considered

as an isotropic,

is largely due to the random

which precludes

the possibility

elasticity of glass is limited primarily

to short duration prior to fracture.

in bending

ordering

elastic material.

of the atoms in the

of any directionality

permanent

deformation

factor for improving the chemical

of glass

For most practical purposes, The isotropic

serves as an additional

devitrification.

in behavior.

The

loads. Glass will undergo very little However,

it has been found that

glass will flow over a period of time under a load. For the violent fracture events usually encountered

in criminal

It should be noted materials

engineering,

investigations,

the glass can be considered

that the term “elastic” i.e. to indicate

as behaving elastically.

is being used as defined

a nonpermanent

deformation,

in the domain

of

and is not being used

in the everyday sense of being “stretchable”. Logically, the basic strength of glass should lie in its interatomic

bonds. As glass is a

heterogeneous solid of very irregular atomic structure, the interatomic bonds will have varying strengths. Some bonds will break more easily than others. Additionally, any accidental imperfections in the glass matrix or network will affect its strength. Some progress has been made in the study of the occurrence of flaws and microcracks in glass and their effect on glass strength. initiation

Due to a popular

association

of glass fracture

with flaws, research into the study of flaws has appeared justified

by materials

scientists. It has been realized for a number falls short of the theoretical to a concentration

of years that the practical tensile strength of glass 28 proposed that the loss in strength was due

value. Griffith

of stress at the boundaries

of minute

surfaces. These flaws have been given the name “Griffith

flaws occurring cracks”. Although

of these cracks has been widely accepted as the explanation study of their nature

remains a difficult

problem.

Ernsberger

on the glass the presence

for tensile strength loss, the 29 pointed

out that in the

forty years since their postulated presence, there had not been developed any “generally accepted” method for detecting, identifying or measuring the Griffith crack. Intentional altering of the strength of glass has become a common commercial practice, resulting in the production

of “disannealed”

or “tempered”

glass. The introduction

of a compression stressed condition in the outermost layers of a glass body has been found to add appreciable strength to the glass, resulting in a resistance to low-velocity blunt-impact fractures. Commercially, this stress is introduced into glass intended for doors and windows, and is the predominant type of finish for automobile window glass except for the front windscreen. The stress is introduced into the shaped glass window or

15

GLASSFRACTUREANALYSIS “‘lig;&>

as a last step in the fabrication

a tempering

process. The light is held in tongs and suspended in

furnace where the temperature

below the softening

of the glass is allowed to reach a point just

state. When the light reaches this condition,

it is removed and cooled

rapidly at its two surfaces. As the surfaces cool, they contract. The inside portions of the glass, still at the higher temperatures, adjust to the forces or stresses caused from the contraction

of the outer surface. This process proceeds until finally the exterior surfaces

reach a temperature considerations,

where continued

contraction

is negligible and they are, for practical

rigid. After the outside has cooled to a rigid condition,

the inside is still at

high temperatures where cooling and contraction processes are continuing. As the inside region continues to cool and contract, the rigid exterior cannot adjust to the new conditions. When the inner portions finally reach the temperature where they too are rigid, a condition exists where the outer surfaces are in compressive stress and the inner portions are in tensile stress. These stress lines run in planes perpendicular In the manufacture cause tempering-like

of molded

to the glass thickness.

or pressed glass objects, the cool surfaces of the mold

processes to occur, creating stress in the formed light. These acciden-

tal stresses increase the frangibility of the light. As this condition is often undesirable, the light must be thermally treated to reduce the stress. This treatment, called annealing, consists of heating until

the light to a specified temperature

and keeping it at this temperature

the stresses relax. Next, the light is allowed to cool at a controlled

mined to create the minimum of stress in the finished product. annealing process is given by Finn 30. Thus it becomes clear that glass is a brittle, practically tion, indeterminate

structure,

Because of these properties,

rate, predeter-

A good discussion of the

elastic solid of varied composi-

and having a marked absence of directionality glass exhibits the fracture characteristics

in behavior.

discussed in Part XII.

HKGLASS-FRACTURESURFACEMARKINGS As a crack travels through

a solid body, new surfaces are formed. These newly formed

surfaces have certain topographic features which are dependent on the behavior of the traveling crack. The relationship between fracture surface topography and crack behavior is the primary concern of this section. One of the most important early works on glass fracture theory was produced by Griffith 28. The Griffith theory of fracture propagation demands that a flaw be present in. the glass before a crack can grow or spread. These flaws are thought

to be microscopic

cracks. Stress placed on a brittle body, such as glass, possessing a tiny flaw, will concentrate at the tip of this flaw or microcrack. According to Griffith’s theory, the decrease of potential energy by the breaking of strained bonds between molecules and atoms around the crack tip may be partially offset by the increase in surface energy created by an extension of the crack. Until sufficient stress is applied to the glass to provide a quantity of potential strain-release energy sufficient to meet the free surface-energy requirement, the crack will not grow from the flaw. When this stress condition is met, the crack spreads very rapidly and continues until the stress falls below the necessary value, or until the

16

S.P. McJUNKINS,

body has been completely

traversed

by the crack. A major problem

theory is its failure in accounting for slow-moving cracks. Poncelet 3 ’ advanced an atomic approach to fracture propagation

J.I. THORNTON

with the Griffith theory. His theory

does not require the presence of a flaw for a crack to propagate, merely the application of a stress for a critical period of time. The normal equilibrium rate of bond breakage and reformation

is affected

by the stress. In the initiation

breakage exceeds the rate of bond formation,

of a fracture,

the rate of bond

thus opening a flaw at the surface.

Experimental studies of the behavior of cracks have not been able definitely to prove or disprove either the Griffith or Poncelet theory. Both theories predict the same result in fracture,

but via differing

electron

microscope,

Griffith

processes.

however,

Experimental

studies of glass using the scanning

have failed to reveal any indication

of the presence

of

Cracks 32. If they exist, it must be at the atomic, rather than at the molecular,

level. Glass-fracture surface characteristics The types of features

present

on the newly formed

surfaces of a crack are varied.

Preston ’ describes four types of fracture surface features: (1) a polished area; (2) micro-

Fig. 3. An automobile pling grid. The fracture punched locations.

tempered-glass was initiated

ventwing window at the intersection

prior to fracture, superimposed with a samof the axes. Segments were sampled from the

GLASSFRACTUREANALYSIS scopic hackle; basically mirror,

17

(3) coarse hackle;

and (4) ribbing.

only two types of markings: fine hackle,

Murgatroyd

’ claims that

there are

hackle and ribbing. Poncelet 3 3 describes ripples,

coarse hackle, undulations

and striations

as possible

fracture

face

features. Andrews 3 4 divides surface features into mirror, mist, hackle, conic hnes and Wallner Lines. Since many of the terms above denote the same things, the writers have chosen to discuss the fracture

surfaces in terms of the following characteristics:

ror; (2) mist or fine hackle;

(3) coarse hackle; (4) conchoidal

Wallner Lines. Some comments

concerning

Poncelet’s undulations

(1) mir-

lines; and (5) ripple, or and striations will also

be made. For the illustration

of fracture surface features of benefit to the discussion, the writers

chose to use automobile with a superimposed points and broken

disannealed

sampling

window glass. This window appears prior to fracture

grid in Fig. 3. The window was marked at the sampling

at the intersection

segments were carefully

of the axes with a hammer-driven

of impact. This allowed later recognition It is emphasized pretation

nail set. Sample

marked with arrows to orient their surfaces relative to the point

that the external

of radial and concentric force-stress-fracture

fracture faces.

relationships

used for inter-

of the cause and origin of annealed glass fractures cannot be practically

to disannealed

or tempered glass due to the inherent

Brittle-fracture

surface markings

applied

complex stress fields in the latter.

1. Mirror It is a commonly

held belief 1,2,33 that the optically flat fracture surface called mirror zone of a crack spreading from its origin.

is usually the first characteristic While proceeding velocity,

through

the mirror

but is rapidly accelerating.

region,

the crack is still at a relatively

Shand 3 states that the mirror will continue

slow

to grow

until a condition of limiting velocity and critical stress is reached. “The limiting velocity is related to the velocity of transverse elastic waves in the glass and the critical stress to the cohesive strength of the glass”. Shand claims that the mirror boundary fied with the onset of this limiting condition.

can be identi-

Preston * also claims, in his discussion of explosive crack propagation, that mirror is the first type of surface to form. Forming a somewhat circular zone around the crack origin, the mirror moves outward as the crack spreads. Preston states that as soon as the mirror zone reaches a certain size, probably associated with the limiting-boundary condition described

by Shand 3, a redistribution

of strain at the crack boundary

concentrates

enormous stresses there. These stresses violently rend the surfaces apart, giving rise to mist, the next type of surface to be discussed. Preston indicates that mirror fracture surfaces are generally characteristic of slowmoving cracks. He mentions this phenomenon as being characteristically slow-forming expansion and contraction fracture faces. This is consistent mirror surfaces observed in some thermal glass fractures, lined impact fracture surfaces.

in contrast

associated with with the typical

to the prominently

18

S.P. McJUNKINS,

Fig. 4. Mirror appearing

The

on a concentric

writers’ observations

characteristically

mirrored

fracture

surface

of deferred

concentric

agation.

These

rupture,

running between radiating fractures.

Poncele t 3 3 describes

glass segment.

tempered-glass

fractures

surfaces as shown in Fig. 4. These fractures

time-lag varying from seconds to several minutes deferred

of a tempered

fractures mirror

J.I. THORNTON

revealed

occurred after a

after the initial explosive fracture prop-

occur primarily

concentrically

as the first surface characteristic

about

the origin of

to form as the crack

spreads out from the origin. He claims that the crack velocity is sharply accelerating across the field of a mirror surface. This does not appear consistent with Preston’s association

of mirror formation

with relatively quiet, slow-moving crack fronts.

Preston has claimed that after the crack has left the mirror stage because of changing stress conditions, it may return to it and continue on for some distance. The stress, which was rapidly building at the mirror boundary, having peaked out, thereby greatly easing the strain, allows the crack to continue to grow at a slower pace, producing a second mirror region. 2. Mist

As the spreading crack reaches the stress and velocity condition at the mirror boundary, the spreading crack tip can no longer dissipate the accumulated stress fast enough. A multitude of additional cracks form at the crack tip. These tiny cracks are termed mist and are often found around the boundaries of mirror surfaces. According to Poncelet 3 3, crack velocity changes in the region of mist formation are imperceptibly small. Poncelet discusses the probability of mist forming at different angular distances from the stress at the crack tip. He claims that because the stress around the crack front

GLASSFRACTUREANALYSIS

Fig. 5. A radia:

fracture

ror surface as the fracture

surface on a tempered glass segment. progressed frond top to bottom.

Fig. 6. A shifted view of the radial in a zone of marked roughness.

surface

decreases

angular

with

the increasing

Note the growth

shown in Fig. 5. Note the increased

distance

from

size of hackle

mir-

resulting

the crack tip, the probability

minute cracks, or mist, forming at points on the crack boundary increasing angular distance from the crack tip. An shows ceeded surface

of mist out of the

of

decreases with an

illustration of mist growing from a mirror zone is offered in Fig. 5. This illustration a radial surface of a tempered-glass fracture segment, where the fractures prolengthwise traveling from top to bottom. Fig. 6 shows a shifted view of the in Fig. 5 to illustrate further growth of mist into hackle.

3. Hackle As the crack-tip

stress accumulation

causing mist formation

continues

to pile up, the

20

S.P. McJUNKPNS,

angular distance of small-crack

formation

J.I. THORNTON

sites increase and the size of the minute cracks

grow. As the cracks grow they become visible as long angular gouges, or hackle, on the surface. Preston ’ describes mist as microscopic

glass fracture

hackle. Poncelet

that even outer parts of the so-called mirror surfaces have ultramicroscopic

observes

hackle, visible

only with the aid of an electron microscope. Murgatroyd

’ states that hackle surfaces form when glass fractures under a shear stress,

as opposed to tensile stress, and that the shear stresses are relatively large. This does not appear to be consistent

with views held by Preston, Poncelet and Shand, who view hackle

as merely due to large excesses of released energy available at the crack boundary. Murgatroyd

offers a detailed account

ment-like behavior, which the fracture debr-is is observed

of hackle structure,

claiming it to be an escarp-

occurring at the unions of approximately parallel planes in each of front is simultaneously traveling forward. Sometimes splinter-like on the surface of hackle lines. Murgatroyd

being caused by an over-and-under

union

explains these splinters

as

of the same two planes (see Fig. 7). The criter-

ion of shear-stress fracture for hackle formation was determined by Murgatroyd as a result of his observation that hackle forms only on fracture surfaces obliquely angular to the free surfaces.

Fracture

shear-stress actions. The curved or undulated

surfaces of such angular form are generally associated fracture

surfaces marked with long striations,

Poncelet 3 3, appear very similar to the conditions for hackle occurrence. Murgatroyd

Although

are called by different

enon. The fact that Poncelet’s

the striations

Murgatroyd

of Poncelet

mentioned

with by

2 described as necessary

and the hackle described by

names, it appears that they may be the same phenom-

“hackle”

tends to grow gradually

in size from mist, while

Escarpment

Fig. 7. A diagram drawn to show a profile view of hackle tion of splinters by the over-and-under unions of fracture

as described planes.

by Murgatroyd.

Note the forma-

GLASS

FRACTURE

Fig. 8. Striated surface.

hackle

21

ANALYSIS

in compression

Note the branching

or bifurcation

Fig. 9. Striated hackle bordering mirror opment of hackle on the right side.

stress

zones

located

at edges of tempered-glass

radial-fracrure

of structures.

on a tempered-glass

fracture

segment.

Note the marked

deve:-

22

S.P. McJUNKINS,

striations,

or striated-hackle,

different

conditions

teristics.

A confusion

the same. Without fracture

tend to form abruptly,

of fracture

are involved,

in terminology

exception,

is apparent

the “hackle”

surfaces is consistent

indicates

producing

J.I. THORNTON

to the writers

two different

that two

types of charac-

if indeed hackle and striations

are not

described in forensic works concerning

with its being the striations

glass

described by Poncelet and the

“hackle” described by Murgatroyd. The writers have observed striated hackle on fracture surfaces, a condition

contrary

troyd. Fig. 8 shows striations glass. It should orthogonal

be noted

to the oblique-angle proceeding

that

the striations

stress field and are found

to the free

surface required by Murga-

across the edges of a radial segment of tempered at the edges lie on fracture

to the free surfaces and not on undulated

compression

surfaces orthogonal

fracture

consistently

faces nearly

or curved surfaces. These lie in the on radial fracture

surfaces of tem-

pered glass. Fig. 9 shows striations These striations associated suspect

with mirrored

that

beginning

were observed

only

tempered-glass

the explosive,

show these striations the delayed concentric

abruptly

bordering

from the boundary

to lie on slightly fracture.

early concentric

curved Although

contours

of a mirror often

as yet unlisted,

tempered-glass

fracture

surface.

found

to be

the writers surfaces will

a curved mirror surface. The writers have observed that

tempered-glass

fractures have mirror surfaces without

striations.

4. Conchoidal lines Conchoidal

lines are commonly

on the edges of window

seen as the curved shell-like fractures often occurring

glass. They are usually

caused by the sharp impact of a hard

object with the edge of the pane. Examples of this are shown in Figs. 10 and 11. Striated “hackle” can be observed oriented da1 and “hackle”

at right-angles

to the curved lines. It is these conchoi-

lines which have been put to use in forensic applications

by Nelson and

others as described in Part I. Another case of conchoidal fracture, also of great forensic significance, often occurs at the face of cracks in broken window panes. These curved lines present their concave sides to the fracture origin and their convex sides to the direction of crack propagation. As Matwejeff observed, the lines appear nearly perpendicular to the side on which the to the other. Preston ’ states that the tip of the curved line is at the location of maximum tensile stress, as the fracture always follows maximum tension. Preston claims that the lines represent hesitation points during an intermittent advance of the fracture. After the fracture begins to progress through the mirror stage, a redistribution of stresses allows the crack to hesitate until sufficient stress has again accumulated at the front to drive the crack forward. Preston states that in slow-moving cracks the rib marks probably correspond to dwelling positions of the crack front, the whole front dwelling simultaneously. Murgatroyd ’ claims confirmation of Preston’s intermittent advancement theory of conchoidal line formation. Having initiated a window glass crack, its termination was marked. By restartfracture

began and asymptotic

GLASS

FRACTURE

f’ig. 10. Shell fracture

Fig. 11. Shell fractures

23

ANALYSIS

of a plate glass surface.

on a plate glass surface.

Note the striated

hackle

Note the overlapping

perpendicular

rib lines,

to ribbing.

24

S.P. McJUNKINS,

ing the crack by tapping front, Murgatroyd

the pane and again marking

the termination

J.1. THORNTON

position

of the

was able to show that rib lines occur at crack rest positions.

5. Ripple marks (Wallner Lines) Ripple marks, also referred to as Wallner Lines, were discovered by Wallner 35. Preston claims that “ripple”

marks are caused by the intersection

of a progressing

crack front

Fig. 12. Ripple lines emanating from a central hackle zone on a tempered-glass radial-fracture segment. Note that the ripple lines are asymptotic to compression stress zone boundaries where striated hackle begins.

Fig. 13. A view of the same fracture surface shown in Fig. 12 with a slight change ination. Note the dependence of ripple visibility on the illumination angle.

in the angle of illum-

GLASS

FRACTURE

with an oscillation

ANALYSIS

25

of stress caused by a spreading stress pulse wave, and that transverse

stress pulses radiate from breaking bonds in a plane perpendicular transverse

to the broken bond. The

pulses travel faster than the crack front and overtake it, adding their stress to

the crack-tip

stress accumulation.

of a pulse from a point at the

When the orientation

crack front varies greatly from the pulses from neighboring a slight deflection

points, the fracture undergoes

at the crack front and pulse intersection,

resulting in the formation

of

ripples appearing to originate from the source of the divergent pulse. Poncelet

claims that the divergent

coarse hackle mark could potentially

wave pulses could originate in various ways. Any be the source of ripple. The writers have observed

curved lines on the radial surfaces of tempered-glass symmetrically

fracture segments, appearing to ;irise

from each side of a hackle zone in the center of the face. These ripples can

be seen in Figs. 12 and 13. Because of their fineness, illumination

sharply affects their appearance.

in Fig. 13 with only a slight variation in illumination

angle.

Andrews 34 stated that the Wallner Line-producing

Fig. 14. Kirchoff the lines.

a slight change in the angle of

The surface in Fig. I2 is the same surface as transverse

pulse waves are ultra-

Lines on a thermal-contraction

fracture

surface.

Note the exceptionai

Fig. 15. Kirchoff Lines on a thermal-contraction surface that appeared in Fig. 14.

fracture

surface.

Another

section

straightness

of

of the same fracru~e

26

S.P. McJUNKINS,

J.1. THORNTON

sonic waves and that they can be induced by external vibrating bodies in contact with the glass. An experimental investigation in support of this claim was made by the writers utilizing an ultrasonic cleaner bath as a transverse wave-pulse inducer. Clark and Irwin 3 5 pointed

out that the frequency

the velocity together fracture

of the fracture.

of the transverse pulse must not be too great in relation to In this event, the induced

to be easily resolved. was produced

Kerchoff

For this reason, a slow moving

by immersing

a heated microscope

Lines will be too close thermal-concentration

slide in a vibrating

ultrasonic

cleaner bath. The result can be seen in Figs. 14 and 15 as a series of parallel lines across the fracture face. These Iines indicate the shape of the fracture front as a result of the interaction

of the front with the waves propagated

by the ultrasonic

bath vibration.

CONCLUSIONS

The origin of a glass fracture sponse to a stress environment mination

of markings

glass and its propagational

can be ascertained

left on the generated

from such examinations vironments

in annealed

with reasonable

fracture

reliability

behavior in refrom an exa-

surfaces. The information

can be of twofold value to the forensic scientist:

available

(1) stress en-

can be related to the nature of forces acting on the glass when fracture occurs

during scenes of violence; and (2) fracture surfaces often provide a positive-negative viduality

to the separated

glass bodies whose common

origin can be established

indiwith a

high degree of certainty by matching the contour and character of the surface markings. Occasionally faced with the prospect of having to explain the cause of fracture characteristics on which conclusions

are based, the forensic scientist may draw on the advanced

body of fracture knowledge compiled by outlying development of an appreciation of a glass fracture realizing the maximum

information

fields of scientific endeavor. The surface will aid the criminalist in

from the glass evidence at his disposal.

REFERENCES

F.W. Preston, A study of the rupture of glass, J. Sot. Glass Technol., 10 (1926) 234. J.B. Murgatroyd, The significance of surface marks on fractured glass, J. Sot. Glass Technol., 26 (1942) 153. E.B. Shand, Experimental study of the fracture of glass. I. The fracture process, J. Am. Ceram. Sot., 37 (1954) 52. S.N. Matwejeff, Criminal investigation of broken window panes, Am. J. Police Sci., 2 (1931) 148. Federal Bureau of Investigation, Evidence of fractured glass in criminal investigations, FBI Law Enforcement Bulletin, October 1936, p. 2. F.G. Tryhorn, The fracture of glass, Forens. Sci. Circ., No. 2 (1936) 1. L.C. Nickolls, Anomalous fracture in glass, Forens. Sci. Circ., No. 3 (1937) 7. C.E. O’Hara and J.W. Osterberg. An Introduction to Criminalistics, MacMillan, New York, 1949, p. 239. R.N. Haward, The behavior of glass under impact and static loading, J. Sot. G/ass Tecknol., 28 (1944) 1.

GLASS FRACTURE

27

ANALYSIS

Glass fracture 10 Federal Bureau of Investigation, ment Bulletin, October 1936, p. 2.

examinations

aid the investigator,

FBI Law Enforce-

Interscience, New York, 1953, p. 239. 11 P.L. Kirk, Crime Investigation, 12 D.A. Frye, Unusual damage to plate-glass windows, Police J. (Land.), 30 (1957) 44. 13 H. Gross, Criminal Investigation, Carswell, Toronto, 1906, p. 119. 14 G.E. Turfitt, The fracture of glass by revolver bullets, Forens. Sci. Circ., No. 6 (1940) 12. 15 A. Svensson and 0. Wendel, Techniques of Cvime Scene Investigation, American Elsevier, New York, 1965, p. 189. 16 H. Soderman and J.J. O’Connell, Modern Criminal Investigation, Funk and Wagnalls, New York, 1962, p. 235. Evidence of fractured glass in the investigation of crime, Forens. Sci Circ., No. 6 17 D.J. Stapleton, (1940) 21. 18 D.F. Nelson, Illustrating the fit of glass fragments, J. Grim. Law 0iminol. Police Sci., 50 (1959) 312. 19 J.W. Thompson, The structure of hackle lines on glass, Int. Crim. Police Rev., (March 1969) 62. 20 American Society for Testing and Materials, Standard definitions of terms relating to glass products, ASTM civc. 162-56, 1955. 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

G.W. Morey, 7’he Properties of Glass, 2nd ed., Reinhold, New York, 1954. E.B. Shand, Glass EngineeringHandbook, Corning Glass Works, Corning, New York, 1955. J.E. Stanworth, PhysicalProperties of Glass, Clarendon Press, Oxford, 1950, p. 65. J.M. Stevels, Progress in the Theory of the Physical Properties of Glass, American Elsevier. New York, 1948. J.R. Hutchins, III and R.V. Harrington, Glass, in Encyclopedia of Chemical Technology, 2nd ed., Vol. 10, p. 533. W.H. Zachariasen, The atomic arrangement in glass, J. Am. Chem. Sot., 54 (1932) 3841. B.E. Warren, Basic principles involved in the glassy state, J. Appl. Phys., 13 (1942) 602. A.A. Griffith, Phenomena of rupture and flow in solids, Trans. R. Sot., A221 (1920) 163. F.M. Ernsberger, A study of the origin and frequency of occurrence of Griffith Microcracks on glass surfaces, Adv. Glass Technol., (1963) 511. A.N. Finn, The annealing of glass, J. Am. Ceram. Sot., 9 (1926) 493. E.F. Poncelet, Fracture and flow II, Glass Ind., p. 43. T.L. Hayes, Donner Laboratory, University of Calif., Berkeley, personal communications, E.F. Poncelet, The markings of fracture surfaces, Trans. Sot. Glass Technol., 42 (1958) 279. E.H. Andrews, Stress waves and fracture surfaces, J. Appl. Phys., 30 (1959) 740. H. Wallner, Linienstrukturen an Bruchflachen, Z. Phys., 114 (1939) 368. A.B.J. Clark and G.R. Irwin, Crack propagation behaviors, Exp. Mech., 6 (1966) 328.

1970.