3.2       Principles of Occupant Injury Criteria To Rail Vehicle Interiors

In order to evaluate the severity of a particular type of injury an injury criterion is used with tolerance levels to determine the actual severity level.  Definitions of both injury criterion and tolerance levels are given overleaf.

‘An injury criterion is a mathematical relationship, based on empirical observation, which formally describes a relationship between some measurable physical parameter interacting with a test subject and the occurrence of injury that directly results from that interaction.’

                                                                        S. W. Rouhana 1993 (Ref 2.1)

‘The tolerance to injury can be defined as the value of some known injury criterion that delineates a non-injurious event from an injurious event.  Or, phrased another way, the tolerance is the minimum dose associated with a specified probability of producing injury of a specified severity.’

                                                                        S. W. Rouhana 1993 (Ref 2.1)

Direct and indirect impact load injury criteria have mostly been proposed by the automotive industry as methods for assessing the structural crashworthiness and restraint system performance of motor vehicles.  In order for an injury criterion to be determined, a repeatable and accurate measurement device is required and this has lead to the development of the Anthropomorphic Test Device (ATD), or crash test dummies.  Instrumentation within the crash test dummy measure the parameters which are defined in the injury criteria so that tolerance levels can be accurately analysed.  The crash test dummies have been developed for specific impact scenarios and for specific legislative tests and are shown below.

• HII, HIII at 50%, 95%, 5%, 6yr – Frontal Impact Dummies
• EuroSID, USSID and BioSID at 50% - Side Impact Dummies

The sophistication and variety of size of crash test dummies is increasing to reflect the full range of adults and children and more injury criteria.

However, the crash test dummies and injury criteria have been developed for the occupant body part impacts and injury types and mechanisms that are most frequently produced in car accidents.  These have then evolved to reflect the improvements in restraint systems used in cars, such as three-point lap and shoulder belts and airbags.  These originally concentrated on life threatening head, neck, chest and femur injuries and more recently potentially long-term disability injuries to the legs, knees and pelvis.

The tolerance levels defined for each injury criteria, used in automotive safety, vary according to the serious of the injury.  Most tolerance levels have been equated in some way to the Abbreviated Injury Scale (AIS), (Ref 2.2) that was produced to assist medical personnel to rapidly document an accident victim’s number of and severity of injuries. The Abbreviated Injury Scale, injury level classification system, adopted by the automotive industry compares favourably to the proposed injury severity levels for rail interiors.  AIS Levels 1 to 3 are basically non-life threatening injuries, ranging from very minor injuries to long-term disabilities, while Levels 4 to 6 are increasing severity of life threatening injuries, with Level 6 potentially non survivable.  The setting of a tolerance level of 1000 for the Head Injury Criteria (HIC), defined in FMVSS208 (ref 2.3), is the equivalent to a 14% probability of an AIS4 (life threatening) head or brain injury.

The dummies themselves have the correct masses and joints to give representative dynamic kinematics and also have the correct biomechanical impact properties for the head, neck, chest and knee, and more recently the lower legs.  It is those areas of the crash test dummies which are regularly calibrated to ensure that the injury criteria tolerance levels are repeatable.

Hence, the injury criteria used for occupants in rail vehicles, both in terms of type and mechanisms of injuries and tolerance levels, need not be the same used for the automotive industry.  Therefore for occupants in rail vehicles, each body part injury type and mechanism has been assessed individually and injury criterion proposed with tolerance levels based on the proposed injury severity levels.  Where applicable they use the same injury criterion as used in the automotive industry but with the 4 tolerance levels.

The main injury criteria tolerance levels used in the automotive industry have been generated for legislation or consumer crash tests.  These include:

• Federal Motor Vehicle Safety Standard (FMVSS) 208, Occupant Protection (Ref 2.3)
• European Frontal Impact Directive (96/79/EC) (Ref. 2.4)
• European Side Impact Directive (96/27/EC) (Ref. 2.5)
• European New Car Assessment Program (EuroNCAP) (Ref. 2.6)
• FMVSS 214
• USNCAP

Injury criteria used in these test protocols have been adopted and modified for occupant protection in the rail and aircraft industries as listed below:

• British Rail Rail Vehicle Interior Crashworthiness Code of Practice (BR/BCT 609) (Ref. 2.7)
• Performance Standard for Seats in Civil Rotorcraft, Transport Aircraft, and General Aviation Aircraft (SAEAS 8049)
• Approval of Seats of Large Passenger Vehicles (ECE 80) (Ref. 2.8)

In evaluating injury mechanisms, criteria and tolerance levels, five references have been used predominantly and provide a good background for biomechanics and the human response to impacts:

• Accidental Injury Biomechanics and Prevention
  Editors             A.M. Nahum                J.W Melvin       (Ref.2.1)

• Automotive Safety, Anatomy, Injury Testing and Regulation

  Editor             J.A. Pike                                              (Ref. 2.1.0)

• Hybrid III First Human-Like Crash Test Dummy

  Editors             S.H. Backaitis H.J Metz                   (Ref. 2.1.1)

• Advanced Anthropometric Test Device (AATD) Development program

  Phase 1. Reports: Concept Definition

                                 J.W. Melvin                                         (Ref. 2.1.2)

• Human Tolerance to Impact Condition as Related to Motor Vehicle Design

  (SAE J885)                                          (Ref. 2.1.3)                    

In proposing injury criteria each body part is evaluated individually for the mechanism and types of injury, a suitable injury criterion proposed with 4 tolerance levels to define the 5 levels of injury.