forensics exam 3

General knowledge of target surface impact on bloodstain patterns

The harder and less porous a surface upon which blood falls, the less spatter is seen

Irregular and more porous surfaces have protrusions and/or rough texture which disrupts the surface tension of blood, causing it to spatter more

directionality

The characteristic of a bloodstain that indicates the direction blood was moving at the time of deposition on the target surface.

90-degree directionality of bloodstains

90-degree blood drops will result in the blood being circular. there will be no directionality observed

directionality of bloodstains less than 90 degrees

bloodstain will be elongated or have an elliptical appearance

how to tell the direction of travel

the tail

tail will point in the opposite direction from where the drop originated

low velocity stain pattern

dripped blood

Gravity causes blood to fall from an object or person and impact a surface

Movement of the blood source (object or person) can result in a Drip Trail

low velocity stain pattern

splashed blood

a volume of blood is impact by a low velocity force

ex: pool of blood

Stain would have streaks skimming along the surface away from the impact point

Depending on volume and velocity of “stomp”, blood may also splash up and out

medium velocity

Size of stains are predominantly 1-4 mm in diameter (but there will be smaller stains as well)

Most often seen in beatings but also occur in some stabbings and other mechanisms of injury

medium velocity

back spatter

often (but not always) beating suspect will have blood on them or their clothing

cardinal rule for beatings

first blow brings blood to the surface

second and subsequent blows are what causes the spatter

high velocity

Almost universally associated with gunshot wounds (but also seen in any case involving explosives)

 Greater energy produces much finer droplets (can be almost mist like). Most will be under 1mm in diameter

There WILL be some larger drops as well

Mist size droplets can only travel 3-6 feet before air resistance and gravity cause them to fall to the floor

high velocity

forward spatter

blood projected from exit wound (much more than backspatter)

high velocity

backspatter

blood projected back towards the weapon from the entrance wound

cast off stain pattern

when a weapon covered with blood is swung, the force generated is enough to overcome blood surface tension

blood is “flung” off of the weapon

Depending on the shape of the weapon and swing arc, drops in the stain pattern can be “single file” linear path.

Individual linear paths may be able to be discerned to allow you to count the number of times the weapon was swung

If paths can be counted, remember to add one for the initial blow (that brought blood to the surface of the body) and one for the first impact that results in blood getting on the weapon

The cast-off patterns can be oriented vertically, horizontally, or even diagonally depending on how the assailant swung the weapon

If victim, assailant, or weapon moves during the course of an assault, complex cast off patterns may be produced that are not easy to read

transfer patterns

fingerprints or footprints

footwear shoe tread patterns

fabrics

weapons

hair

compression transfer pattern

object with wet blood on it is pressed against a surface with no movement

swipe

object with wet blood on it is pressed against a surface and there IS subsequent movement

Depending on object, as pattern diminishes there is a “feathering” effect (especially hair)

 Can help determine directionality of the swipe

wipe

an object moving through an existing bloodstain on a surface alters the character of the stain

blood spatter on clothing

often hard to see with naked eye

using some type of magnification or alternate light source helps to visualize the stains

close examination of stains can tell you if there was a transfer of blood (ex: clean clothing item pressed against blood clothing item)

it can also be determined if there was projected blood striking the clothing item (for example being present during a beating or shooting)

if blood is only on the “tops” or surface of the clothing weave, most likely it is a transfer stain

if blood is found “in” the weave, most likely it was projected

void patterns

When an object is between the origin of blood spatter and a target surface

Sometimes is clearly enough defined to determine the shape of the object