Engineering applied toward the purposes of law. In forensic engineering, the majority of investigations are carried out in the context of civil litigation; for example, the cause of a plane crash is often due to defective design, which is a civil litigation consideration. However, the crash could be due to a bomb or terrorist activity in the cockpit, which is a criminal consideration. Often the cause of an accident is initially unknown or misattributed. See also: Criminalistics
Accident reconstruction is the a posteriori scientific process of using the facts of an accident and the appropriate natural laws of science to determine the possible circumstances and causes, consistent with the available data. The quality and reliability of the reconstruction depend on the amount and quality of the available data, as well as the skill and knowledge of the investigator.
Consider, for example, a two-vehicle, right-angle, intersection collision (see illustration). The two vehicles had proceeded through an intersection, and then an impact occurred. Each vehicle had skidded prior to impact, and the final positions of the vehicles are indicated relative to the point of impact. For an analysis, the input data required (distances and angles as shown along with vehicle weights) would have to be obtained from a field investigation and from databases of vehicle properties. The principle of conservation of linear momentum, which follows from a straightforward application of Newton's laws, where all calculation details are given, enables the engineer to calculate the speeds of both vehicles at impact (vi1, vi2) and at the onset of preimpact skidding (v1, v2), based solely on the data (independent of driver or witness statements). Depending on the actual input data and the calculated speeds, the results may be used in a variety of ways. Driver 2, for example, may claim that he stopped at the stop sign, and the accident occurred because driver 1 was speeding through the intersection. Based on the input data for the case illustrated, this claim cannot be true: the accident occurred because driver 2 did not stop at the stop sign, and driver 1 was not speeding. See also: Newton's laws of motion
In a product liability investigation, engineers are asked to determine whether a product is defective and whether the defect was causally related to any injuries that may have occurred. This is a challenging area of investigation and analysis. The three types of product defects recognized by law are (1) design defects (the product lacks those elements necessary for its safe and foreseeable uses), (2) manufacturing defects (the product was not made according to the manufacturer's specifications), and (3) failure to warn or instruct (the product was not accompanied by adequate warnings or instructions for its proper and safe use). In a lawsuit, if a forensic engineer has been retained by the plaintiff (the person bringing the lawsuit), the engineer's role is to objectively analyze the product for defects, which may or may not be found, and to causally relate the plaintiff's injuries to the product defect. If a forensic engineer has been retained by the defendant, the engineer's role will be to objectively defend the product, if it can be done, in view of the defect allegations made by the plaintiff. As part of the defense, the engineer may opine that the alleged defect does not exist or that the injuries sustained by the plaintiff are not causally related to the product design. Product liability investigations can be very intense and can involve considerable analysis and calculations, testing, and computer modeling, often requiring the expenditure of large sums of money by both the plaintiff and defendant.
As a simple but meaningful example of a design defect, consider cigarette lighters. Children are naturally inquisitive and may pick up lighters and try to light the flame. Some lighters have bright colors and shapes that make them particularly attractive to children. If the lighter does not have a child-proof or -resistant mechanism, which prevents children from lighting the flame, the lighter is defectively designed. The plaintiff's engineer in this case would show that at the time of manufacture it was practical and feasible to have provided a childproof mechanism, and that the device may have even been in existence on other lighters, which would have prevented the accident.
The fact that a person is injured using a product does not necessarily mean the product is defective. In addition, defective products do not cause injuries every time they are used. For example, a knife is designed to have a sharp edge in order to cut. If the user is cut, it does not necessarily follow that the knife is defective. However, suppose that a person is using a knife to carve a turkey and the blade fractures, causing the person to lose balance, to fall forward, and to be stabbed by the fractured blade. After the accident, a metallurgical analysis of the knife blade reveals that the steel was not properly heat-treated and that there were inclusions in the blade, which weakened it and caused the failure; this determination indicates that the knife was defectively manufactured.
Failure to warn
Car air bags can be life-saving devices if properly designed and used. When an air bag deploys in an accident, its front can move at up to 200 mi/h (320 km/h). A person sitting too close to the air bag can be seriously injured or killed by the impact to the head or upper torso. Small adults and children are particularly susceptible, as well as infants in rearward-facing child seats. Automobiles carry prominent warnings that occupants should not sit too close to air bags, that small children should sit in the rear, and that rearward-facing child seats should never be placed in the front seat of an air bag-equipped vehicle. Without such a warning, the vehicle would be defective by virtue of a failure to warn. The plaintiff's biomechanical engineer in this case would show by tests and/or calculations, or even using the automobile manufacturer's own data, that a person or child sitting too close to the deploying air bag would be subjected to injurious levels of impact forces.
Biomechanics of injury and death
Engineering science and methodology is being applied to injury and death investigation, a subject rooted in human biology. Biomechanical engineers working in the field of injury and death investigation are generally interested in applying engineering principles to the understanding of injury causation mechanisms in accidents, and in developing the criteria and conditions for when injury is likely to occur. For an accident, the kinematics of movement of the human body must be calculated and then correlated with any impacts and injuries that the body may have sustained. In an automobile accident, the movement of an occupant with respect to the vehicle (occupant kinematics) is often correlated with the acceleration-time history of the occupant, the occupant's change in velocity (delta-v), and the resulting injuries. This is a critical step if the occupant alleges an injury because of the accident or a vehicle defect. A very active research field in biomechanics is the determination of criteria that delineate the conditions in which injury may occur in a particular accident. See also: Biomechanics
This rapidly changing area of forensic engineering refers to methods for solving crimes committed using computer technology. Such crimes include extortion, violations of Internet security, hacking into supposedly secure Web sites and computers, computer theft of sensitive information and proprietary files (including national security data and financial data), and creation and propagation of computer viruses. Computer forensics uses techniques and procedures for recovering magnetic data, tracking hackers, and analyzing audio tapes, videotapes, and photographs for possible tampering and alteration. See also: Digital evidence; Internet
Cause and origin
This area refers to investigations to determine the cause and origin of fires and explosions, and to determine if the cause was accidental (electrical shorting, product defect, cooking mishap, and so on) or intentional (arson or planting an explosive device). An understanding of heat transfer, burn patterns, combustion rates, pressure-wave propagation, and analysis of explosive residues is essential for cause and origin investigators.
Structural collapse, blast loading
This area refers to the catastrophic failure of structures either during construction or after the structure is in service. For example, the Hyatt Regency Skywalk collapse in Kansas City, Missouri, on July 17, 1981, was caused by a simple design error and resulted in the loss of 114 lives. The collapse was caused by an improperly designed connection, which did not meet Building Codes, where the vertical hanging rods passed through the box beams that supported the pedestrian walkways. Forensic civil engineers are also concerned with understanding the effect of blast and seismic loading on structures in order to ensure that future structural designs are safer and more resistant to dynamic loading arising as a result of explosions and earthquakes. See also: Earthquake engineering
Significant structural lessons were learned from the bombing of the Murrah Federal Building in Oklahoma City on July 19, 1995, and its resulting collapse; these lessons (outlined in the Proceedings of the 1st Congress of the American Society of Civil Engineers in 1997) will be used to mitigate blast loading effects on future reinforced concrete building designs. Forensic engineers are becoming more actively involved in developing counterterrorism tactics and procedures, and this activity will likely grow as a separate area of forensic engineering. See also: Structural design