Fractures Made With a Blunt Instrument or Object

Examination of glass fractures caused by the impact of a blunt instrument or object will reveal a pattern of radial and concentric fractures.

The impact side can be determined by careful examination of the stress lines on the edges of both the radial and concentric fractures.

As an initial step, at least partial reconstruction of the object should be made, so radial and concentric fractures can be positively determined.

On radial fracture lines, the portions of the stress lines on the rear side (the side opposite to the side of impact) are well developed and distinctly individual; whereas those on the front, or impact side, are much less so, tending to run together and lose their individuality. The 3-R rule applies.

On concentric fracture lines, the opposite condition will be found, with the well-developed and individually distinct portions of the fracture lines appearing on the front, or impact side.

This difference is a result of the glass bending away from the side of impact and the first (radial) fracture occurring on the rear side after the limit of stretching elasticity has been reached. This action produces the distinct stress lines on the stretching (rear) edge of the radial fractures. At the same time, the ensuing grinding action that takes place on the front side causes some chipping and flaking of the edge and a partial wiping out of the stress lines. (See fig. 16-10.)

Fractures Caused by Heat

Fractures caused by excessive exposure to heat can be distinguished from those caused by impact since those due to heat do not show a regular pattern of radial and concentric lines. Heat fractures are characteristically wave-shaped.

Heat fractures will also show little, if any, curve patterns (stress lines) along the edges. Expansion of the glass (stretching action) occurs first on the side exposed

Figure 16-10.-Chipping and flaking action due to grinding and fractured edges.

to the heat, and glass splinters will usually fall toward that side. Reconstruction of a glass object fractured by heat will disclose the wave-shaped fracture pattern. (see fig. 16-11.)

If the stress lines are smooth, or almost so, and no point of impact or penetration is present, these factors, together with other considerations such as the circumstances under which the fragments were found and their location, may indicate that fracture was due to excessive heat.

Reconstruction of Fractured Glass

In reconstruction of a glass object that has been fractured, one of several methods may be used, dependent upon the size and shape of the object. Reconstruction should not, however, be made on any fixed or permanent basis until all examinations have been completed. Neither should pieces or fragments be sent to a laboratory in a reconstructed form, due to possible damage in handling or transit.

In its simplest form, reconstructing a piece of fractured glass, such as a windowpane, is accomplished in much the same manner in which a jigsaw puzzle is reconstructed. The sizes of the fragments, their shapes, and particularly the fractured edges, are considered in their relation to each other, and moved around until fitting, or matching edges are found.

It will not always be possible to find exact and complete edges for all pieces, since some small fragments, chips, or flakes will either have been lost or will be too small to use in the reconstruction.

Figure 16-11.-Fractures due to heat.

For reconstruction of a flat piece such as a windowpane, lay the pieces on a sheet of cardboard or paper somewhat larger than the known or estimated size of the original piece. Perimeter edges can usually be identified by their straight lines and by the remains of putty, paint, or the scratches or marks left by nails or glazier's points. It is generally easier to place all of these pieces first, and to reconstruct from the outside in, than to work out from the inside.

Extreme care must be taken not to rub the fractured edges against one another, since this may cause further flaking or fracturing and destroy portions of the stress line marking. A recommended method is to keep the edges a pencil point's width apart. When all (or as many as possible) pieces are in place, the outlines may then be traced on the paper and properly annotated according to your markings on the pieces, for future reference and use.

If a permanent reconstruction is later desired, the pieces may be secured on a suitable base (plywood or heavy cardboard) with plastic tape or glue.

Reconstruction of a curved or irregular-shaped piece, such as a bottle or jar, presents more initial difficulties since it requires a determination, or approximation, of both the size and the shape of the _object. These difficulties are generally counterbalanced, however, because such pieces (such as automobile headlight lenses) frequently have patterns cast or cut into them that allow for comparisons and matching that are not as readily possible with flat or smooth glass surfaces.

In many cases, the pattern may be matched independently of the fractured edges, although the exact matching of the edges remains the most conclusive evidence of common source.

In preparing the reconstruction of a headlight lens, for example, the perimeter circumference size and the curvature of the spherical surface must first be determined. If sufficient pieces are available and matching edges or patterns can be found, the problem is simplified. The same is true if sufficient markings are available to identify the object as to make, type, size, and so forth and permit procurement of a duplicate.

A satisfactory method is to form a cast of linseed oil putty or similar material that will retain its elasticity in the size and shape of the inside surface. (If the lens can be identified and a duplicate obtained, a plaster cast may be made from the duplicate and the fractured pieces can be mounted on it, held in place by plastic tape.) Pieces and fragments, properly marked, can then be matched by their edges and pattern markings, placed on the cast, and pressed in only sufficiently to hold them in place.

If sufficient pieces are not available to identify the object and secure a duplicate, or to assemble enough pieces to indicate the circumference and spherical surface, a piece with sufficient arcs of the circumference and sphere can be measured to a rough, but sufficiently accurate, degree to permit formation of the putty cast. A spherometer, if one is available, or a Geneva gage, obtainable from an optician, can be used to determine the curvature of the spherical surface and the circumference of the lens.

Keep in mind that these are only rough approximations, and that lenses are made not only in round, but also in oval and other shapes, and that the spherical surfaces are not always completely regular in contour.

If neither the spherometer nor the Geneva gage is available, an approximation of measurements may be obtained by geometrical projection using the arcs of the circumference and sphere of the available fragments. By tracing the arc of an available fragment on a piece of paper and using standard geometrical construction to approximate the diameter, the circumference can be projected.