Head Injury Criteria Tolerance Levels

The head injury criteria are comprised of four different injury criterions.

Head Injury Criterion

The Head Injury Criterion has been devised to predict head injury for impact with flat objects combining both brain injuries, including consciousness, and linear skull fractures.  Obviously with increasing impact severity the probability and potential of both brain injury and skull fracture increase, but a sever brain injury may not be associated with a skull or a major skull fracture with brain injury.  However in assessing the ability for an occupant to egress a rail vehicle the degree of loss of consciousness is important.  Tyrell (Ref. 2.1.4) proposed levels of loss of consciousness in relation to HIC levels as shown Appendix 1, Table 13.

These will form the basis of the HIC tolerance levels.  The lowest level loss of consciousness must be a Level 2 injury as any, even momentary, loss would severely impede the ability of egress.

At a HIC level 900 major fractures with severe unconsciousness occurs, 2% probability of a AIS 4 head injury, producing a Level 3 injury while a HIC 1800 is potentially non-survivable with over 50% probability of a AIS 4 head injury.

Below these, tolerance levels have been correlated to the level of, firstly brain injury, and secondly skull fracture.  Also included for comparison are the Head Injury Criteria (or Head Performance Criteria if calculated over head impact contact time) for automotive and other safety legislation and an equivalent head resultant acceleration.  Proposed HIC tolerance levels correlated to brain injury are shown in Appendix 1, Table 14.  While the proposed HIC tolerance levels correlated to skull fracture in Appendix 1, Table 15.

Blunt Object Skull Fracture Injury Criterion

The Blunt Object Skull Fracture Injury Criterion is applied only for head (not facial) impacts with blunt (<40 mm radius) objects, which occur frequently in the form of grab handles and seat structures in rail vehicles.  The criterion is based on the force required to produce depressed skull fractures with blunt objects.  Skull fracture forces from actual blunt object impact tests have been compiled by D. Allsop  (‘Accidental Injury’ page 263 Ref 2.1), the lowest force being 2.2 KN with a 6.45 cm² impactor, while the average was approximately 5.5 KN.  Therefore using the same methodology as for Head Injury Criteria any skull fracture would be an injury Level 2, for which an impact force of 2.2 KN has been proposed as the tolerance level correlating to lowest probability of a skull fracture.  A major depressed skull fracture would related to an injury Level 3, for which a tolerance level of 5.5 KN has proposed, which would correlated to a 50% probability of a depressed skull fracture.

The Table 16 in Appendix 1, shows the proposed Blunt Object Skull Fracture tolerance levels compared with automotive and other safety legislation and equivalent resultant head acceleration, using F=ma and a head mass of 4.5 Kg.

Facial Injury Criterion

Facial injury criterion is based on facial stiffness (N/mm) and applies to blunt objects impacting directly on to the occupants face, for example chin to eyebrows.  It should be used in combination with the Head Injury Criterion to predict brain injuries, but not the Skull Blunt Object Injury Criterion which applies to impacts directly to the skull.  Facial bone fractures occur as a blunt object penetrates the face producing firstly minor fractures before major fractures, this will lead to impeding the ability of egress and long-term disfigurement.  A facial stiffness profile has been proposed by D. Allsop  (‘Accidental Injury’ page 263 Ref 2.1),  and has been used as the basis for the proposed facial injury tolerance levels.  Facial injuries have a maximum injury Level of 3, although they can be life threatening if respiration is inhibited.  The proposed tolerance levels are given in Appendix 1, Table 17 with the equivalent head resultant acceleration.  At present there is no comparative automotive or other safety legislation for facial injury.

Facial Laceration Criterion

Although not life threatening severe lacerations (cuts) to the face could impede egress as well as causing long-term disfigurement.  Techniques for quantifying and measuring levels of facial laceration are difficult as the strength of the skin varies greatly dependent on direction of laceration and depth.  The method most commonly used in automotive testing is to put 2 layers of ’chamois’ leather over the facial area of the HIII dummy and after testing observe which layers of the ‘chamois’ have been cut.  No cuts to the inner layer indicating a minor laceration, while moderate to major cuts in the inner layer a severe laceration.

Using this guide the proposed facial laceration injury criterion with proposed tolerance levels is shown in Appendix 1, Table 18.  The facial laceration criterion can be used with all other injury criteria.

Head Injury Criteria Summary

Table 19 in Appendix 1 summaries, all the injury criteria, which can be applied to the head and face of the rail occupant.