Lecture Notes on Wounding: Temporary Cavity and Projectile Behavior - lect 22

Ballistics Lab Write-Ups

• Complete all three components by next Wednesday (the 9th).

• Deadlines vary depending on when the lab was finished.

• Submit the Moodle quiz and two short reports by the deadline.

• No extension on the Moodle quiz.

Exam Timetable and Revision Session

• Exam timetable is available.

• Ballistics exam is potentially on Monday, May 19th.

• Revision session on the Wednesday before the exam.

• Timetabling will add the revision session to the timetable.

• Student input is welcomed to find out what to cover during the session for maximum effectiveness.

Wounding: Permanent vs. Temporary Cavity

• Permanent cavity: The physical path created by the projectile as it tears through human tissue.

• Visible on ballistic gelatin as the lines through the gel.

• Temporary cavity: A secondary cavity created as the bullet transits through the body; often temporary.

• More complex than the permanent cavity.

Temporary Cavity Dynamics

• Soft tissues are most severely damaged.

• The amount of energy transferred dictates the threat to life.

• Tissue elasticity plays a key role; more elastic tissues cope better.

• Examples of elastic tissues: muscle, skin, and lung.

• Examples of less elastic tissues/organs: liver and kidneys.

Elasticity and Plasticity

• Elasticity: The ability to stretch and return to the original shape.

• Plasticity: A material that deforms permanently or breaks without much deformation.

• Elastic Limit: The point beyond which a material is permanently deformed.

• Biscuits have a low elastic limit; elastic bands have a high one.

The Process of Temporary Cavity Formation

• The bullet tears tissue, creating the permanent cavity.

• Instantaneously behind the bullet, energy is imparted to surrounding tissues.

• The bullet pushes tissue sideways in three dimensions.

• This imparts radial acceleration to the tissue, forcing it out laterally.

• Energy is never destroyed but redistributed.

• Kinetic energy (bullet moving through tissue) becomes strain energy (elastic potential energy) stored in surrounding tissues.

Strain Energy and Tissue Response

• Tissues store potential energy and want to release it, springing back to their original position.

• If tissues spring back normally, they remain viable.

• Permanent cavity causes more physical damage.

• If tissues are stretched beyond their elastic limit, they are damaged.

• Cracks in muscle tissue are called fissures, leading to internal micro bleeds.

• Damage might heal or require surgery.

Young's Modulus (Modulus of Elasticity)

• Specific value relates to the ability of tissues to spring back.

• A higher value indicates the tissue is more resistant to temporary cavity formation and thus more susceptible to damage.

• High modulus means the tissue is not very stretchy.

• Muscle, skin, and lungs have a low modulus; liver, kidneys, and bones have a relatively high modulus.

Projectiles and Cavity Formation

• Pistol bullets: Generally stable; cause consistent cavity; size of cavity proportional to energy transfer.

• Energy transfer and aerodynamics work:

  • Dependent on density, and drag

  • Human muscle tissue is about 1,000 times denser than air

  • Increased density increases the drag force

• Drag on the projectile relates to the amount of energy transferred.

• Stable projectiles result in a smaller temporary cavity.

• Mushrooming bullets: Increased cross-sectional area increases drag and energy transfer, slowing the bullet down.

Rifle Bullets and Instability

• Fast rifle bullets can become unstable upon impact due to the sudden change in density, causing them to wobble and tumble.

• Tumbling increases the surface area presented to the tissue, leading to massive drag and energy transfer.

• Maximum temporary cavity occurs when the bullet travels sideways.

• This can lead to tissues being stressed beyond their elastic limit, resulting in an "explosive injury."

• The projectile may tear apart, creating multiple permanent cavities and causing significant internal damage.

Entry and Exit Wounds

• Exit wounds from rifle shots are often much larger due to bullet tumbling, causing greater damage.

• Instability creates more damage inside the body and at the exit point.

• Large exit wounds lead to more loss of bodily fluids, especially blood.

Cavity Closure and Vacuum Effect

• If the tissues aren't taken beyond their elastic limit, they close back up, causing the cavity to pulse.

• The opening of the cavity creates a temporary vacuum, sucking in air, bacteria, dirt, and grime through the entry hole.

• This can lead to easy infection of the wound.

• It's important to clean the wound effectively to remove these contaminants.

Generalizations and Projectile Types

• Low-velocity, large-caliber handguns generally create less significant temporary cavities.

• Rifle bullets lead to increased radial domain.

• Rifle bullets tend to stay stable.

• High-velocity, small-caliber rifle bullets cause bullets to be unstable.

• Smaller bullets leads to wider cavities when unstable.

Factors Influencing Wound Severity

• Shot placement is crucial for affecting gunshot mortality rate.

• If a bullet it's a major blood vessel or neurological center survival is less likely.

• Consideration must be given to factors impacting internal wounds.

• The scientist will always look for a definable answer to situations, but experimentalists have to rely on understanding situations piece by piece.

"Stopping Power" Myth

• The term "stopping power" is misleading when it comes to energy transfer and force acting on projectiles.

• It's more important to focus on impacting critical infrastructure within the body.

Documenting Wound Profiles

• High-speed videos can be used to document the radial acceleration created by the projectile within the body.

• Videos get translated into two-dimensional wound profiles.

• Looking at the scale is important.

AK 74 Wound Profile

• Firing a 5.45 millimeter rifle projectile details on here at about nine thirty five meters per seconds

• Instability upon entry

• Wound made on the gelatin with a block of gel simulating the body.

  • Black dark area shows the bullet passage, directly affecting tissue.

  • Light area is the temporary cavity.

• Internal wound can be seen approximately 20-25 centimeters indicating size of real world implications.

M4 Rifle Wound Profile

• 5.56 NATO standard assault rifle

• Entering and exit wounds can happen upon high speed upon entry.

7.62 Millimeter NATO Wound Profile

• Heavier bullets tend to be more stable.

• Historically a much larger exit wound than the 5.56

Sphere testing wound profile

• Shows what will happen upon radial wounds due to impact.

• You will typically see the greatest damage upon entry of the projectile.

• Energy transfer is the most is what is dictating the largest point of our cavities

Sphere high speed video clip analysis

• Gels allow to show the effect of what might happen due to a blast wave

• The projectile stops at the point of injection.

• Bone fragments tends to occur upon injection point.

• Open up a vacuum, pull air through, pull to where it came from.

• This could cause a pocket for infection if not cleaned properly.

Ballistics Lab Write-Ups

• Ensure all three components are completed by the specified deadline, which is next Wednesday (the 9th). Late submissions may incur penalties.

• Note that deadlines may vary depending on the actual completion date of the lab. Check the course schedule for specific details.

• Submit the Moodle quiz and two concise reports by the stipulated deadline. Ensure that all submissions adhere to the formatting guidelines provided.

• Be aware that there will be no extensions granted for the Moodle quiz. Plan accordingly to complete it on time.

Exam Timetable and Revision Session

• The exam timetable has been released. Review it carefully to note the date, time, and location of the ballistics exam.

• The ballistics exam is tentatively scheduled for Monday, May 19th. Confirm this date with the official timetable to avoid any discrepancies.

• Attend the revision session scheduled for the Wednesday preceding the exam. This session is designed to help consolidate your understanding of the material.

• Timetabling will formally add the revision session to the timetable. Keep an eye on official announcements for confirmation.

• Your input is highly valued in determining the content to be covered during the revision session to maximize its effectiveness. Please provide suggestions and feedback.

Wounding: Permanent vs. Temporary Cavity

• Permanent cavity: This refers to the physical path or channel created by the projectile as it penetrates and tears through human tissue. It represents the direct tissue damage caused by the bullet.

• Visible on ballistic gelatin as distinct lines traversing through the gel, indicating the projectile's path.

• Temporary cavity: This is a transient cavity that forms as the bullet passes through the body. It is secondary to the permanent cavity and is often short-lived.

• The dynamics of the temporary cavity are more complex than those of the permanent cavity, involving energy transfer and tissue displacement.

Temporary Cavity Dynamics

• Soft tissues are particularly vulnerable to damage from the temporary cavity due to their composition and structure.

• The amount of kinetic energy transferred from the projectile to the tissues largely determines the extent of the threat to life. Higher energy transfer typically results in more severe damage.

• Tissue elasticity is a critical factor; tissues with greater elasticity can better withstand the forces generated by the temporary cavity.

• Examples of elastic tissues include muscle, skin, and lung, which can stretch and return to their original shape more effectively.

• Less elastic tissues and organs, such as the liver and kidneys, are more susceptible to damage from the temporary cavity.

Elasticity and Plasticity

• Elasticity: This is the property of a material that allows it to deform under stress and return to its original shape once the stress is removed.

• Plasticity: This describes a material that undergoes permanent deformation or fractures without significant deformation. It does not return to its original shape.

• Elastic Limit: This is the point beyond which a material will experience permanent deformation. Once this limit is exceeded, the material will not fully recover.

• Biscuits have a low elastic limit, meaning they break easily, whereas elastic bands have a high elastic limit, allowing them to stretch significantly without permanent damage.

The Process of Temporary Cavity Formation

• Initially, the bullet penetrates and tears through tissue, creating the permanent cavity, which is the direct path of tissue disruption.

• Almost instantaneously behind the bullet, kinetic energy is transferred to the surrounding tissues, initiating the formation of the temporary cavity.

• The bullet pushes tissue sideways in three dimensions, causing it to expand rapidly.

• This action imparts radial acceleration to the tissue, forcing it outward laterally from the projectile's path.

• Energy is never destroyed but rather redistributed within the surrounding tissues.

• The kinetic energy of the bullet moving through tissue is converted into strain energy, which is stored as elastic potential energy in the surrounding tissues.

Strain Energy and Tissue Response

• Tissues store potential energy and naturally tend to release this energy, causing them to spring back toward their original position.

• If tissues spring back to their original position without exceeding their elastic limit, they generally remain viable and undamaged.

• The permanent cavity is responsible for more direct physical damage, while the temporary cavity can cause secondary damage if tissue limits are exceeded.

• If tissues are stretched beyond their elastic limit, they sustain damage, such as cracks in muscle tissue known as fissures, leading to internal micro bleeds.

• Depending on the severity, the damage may heal on its own, or it might necessitate surgical intervention.

Young's Modulus (Modulus of Elasticity)

• Young's Modulus is a specific value that quantifies the ability of tissues to resist deformation and spring back to their original shape. It is also known as the modulus of elasticity.

• A higher Young's Modulus value indicates that the tissue is more resistant to temporary cavity formation but, paradoxically, more susceptible to damage because it cannot stretch as much.

• High modulus values signify that the tissue is not very stretchy and is more prone to tearing or damage.

• Muscle, skin, and lungs have a low Young's Modulus, indicating they are more elastic, while liver, kidneys, and bones have a relatively high Young's Modulus, making them less elastic.

Projectiles and Cavity Formation

• Pistol bullets: Typically stable projectiles that create a consistent cavity. The size of the cavity is proportional to the energy transferred to the tissue.

• Energy transfer and aerodynamics are influenced by factors such as density and drag.
  -Human muscle tissue is approximately 1,000 times denser than air, which significantly affects projectile behavior.
  -Increased tissue density results in higher drag force on the projectile.

• Drag on the projectile is directly related to the amount of energy transferred to the surrounding tissues.

• Stable projectiles tend to produce smaller temporary cavities compared to unstable ones.

• Mushrooming bullets: These bullets are designed to expand upon impact, increasing their cross-sectional area, which enhances drag and energy transfer, thereby slowing the bullet down more rapidly.

Rifle Bullets and Instability

• Fast rifle bullets can become unstable upon impact due to the abrupt change in density as they transition from air to tissue, causing them to wobble and tumble.

• Tumbling increases the surface area presented to the tissue, which leads to greater drag and more rapid energy transfer.

• The maximum temporary cavity generally occurs when the bullet travels sideways through the tissue, maximizing the area of impact.

• This can result in tissues being stressed beyond their elastic limit, leading to an