The dilemma is “When should a Seat Belt be replaced?” The concern has always been with regard to the continuing crash protection provided to the user after extended use of the Seat Belt. To answer this question, it is firstly important to understand the purpose of a seat belt and the design criteria applied.
The seat belt is intended as a device to restrain the occupant in an accident and thereby reduce the injury of that occupant. It is designed to operate on primary impact and is considered sacrificial in preference to injury. Because many accidents involve secondary impacts, the structural levels must be so designed that the system does not disintegrate after the first impact.

There are essentially three aspects of performance which need to be considered when assessing whether a seat belt should be replaced.

1. Structural Integrity
2. Locking Performance (Retractable Belts)
3. Webbing Elongation

1. Structural Intergrity
The structural design requirements of seat belts are such that in most cases they would be capable of sustaining the loads of a secondary impact in a subsequent crash if not replaced after the first. The mere fact that the belt may function after the first accident however, does not assure that structural damage has not occurred.

Unfortunately, the only true way of measuring the structural integrity is to conduct destructive testing, and this is self defeating.

2. Locking Performance (Retractable Belts)
The locking performance requirements defined in the ADR’s (Australian Design Rules) are quite specific and whilst these levels provide a safety margin, the deterioration due to an accident is not something that is easily measured in the car. With retractors in Australia being dual sensitive, they provide a failsafe situation. However, deteriorated locking performance could lead to excessive webbing payout during a second accident and this would almost certainly increase the injuries sustained by the restraint wearer.

3. Webbing Elongation
As part of the injury reduction, seat belts are made using a webbing designed to elongate when loaded so that the vehicle impact is translated to the restraint wearer at lower loads and over a longer period of time.

This elongation is permanent ie. not elastic, so if a seat belt is subjected to subsequent loading it’s ability to provide this “ride down” effect is reduced. Taken to the extreme, the webbing ultimately becomes a rigid member and then fails in tensile load.

If webbing has gone through it’s elongation stage, a second impact will induce significant injury to the wearer caused specifically by the belt.

As a percentage of overall length, the change in length experienced after an accident does not exceed manufacturing tolerances of length used in making the belt in the first case. Therefore, comparison of belt length before and after an accident is not possible.
Careful inspection of particular areas of the seat belt, however, can reveal particular witness markings that suggest that;

1. the belt has been in a severe accident or
2. the belt has deteriorated sufficiently through wear and tear and UV exposure, and in all cases, this would warrant replacement.

Proper inspection of a selected seat belt involves all parts of the belt. (A chain is only as strong as its weakest link), namely;

1. Webbing
2. Buckle and tongue
3. Retractor
4. Sash Guide or Running Loop
5. Anchor Bracket/Fittings/Sews

1. Webbing Conditions

1. Edge Fraying and Abrasion: Caused by complex vehicle geometrics and webbing paths, as well as severe wear and tear and bowing.
2. Door Lock Damage: Webbing trapped in between door jam/lock, possibly contributed to poor retraction.
3. Fading: Excessive exposure to extreme sunlight. Significant webbing strength reduction up to 50% can be contributed to webbing fading.
4. Bowing (Edge Bending): Bowing of webbing is more influenced by the seat belt installation attitudes or geometry. Excessive bowing is also contributed to by non-pivoting sash guides. The amount of bowing can vary from 10 to 100mm over the full length of the belt. The excessive bending of webbing although initially would not necessarily reduce webbing strength, it would impede retractability performance and render the webbing susceptible to edge abrasion and thus strength. It can also indirectly cause damage to other components that may be trapped in the door or under folding seats due to poor retractability.
5. Cuts: Various accidental possibilities.
6. Score Marks: Heavy abrasion usually on one edge caused by webbing bunching into one corner of a sash guide. This friction induces a high temperature within that local area and causes stress marks on the webbing. Sash guide also will show evidence of this.

2. Buckle/Tongue Condition

1. Buckle/Tongue Operation/Engagement: The buckle and tongue assemblies should securely latch every time. While an audible click is often heard, the click itself is not the criteria for secure latching.
2. Ejection: The tongue should actively eject from the buckle assembly when it is released.
3. Buckle/Tongue Corrosion: No base metal corrosion should be evident.
4. Wear or Damage to Covers or Tongues:
   • The Buckle - should show no signs of cover damage.
     Cable Buckle - should show no signs of cable fatigue or collapse. While a buckle with broken cable strands may restrain an occupant, a buckle with worn tubing will allow the wire cable strands to be exposed and prone to abrading against metal seat supports.
   • Convoluted Webbing Buckles similarly should show no sign of a broken or cracked convoluted sleeve as it will eventually allow the webbing to be exposed and prone to abrading against metal seat supports.
5. Tongue Protectors: Damage to some tongue protectors is a concerning factor. Most obvious causes are tongues being trapped in doors. The most severe consequence of this is that during the dynamic test, the protector breaks off and the webbing becomes exposed to a metal edge and breaks.
6. Intrusion of Foreign Matter

3. Retractor Condition

1. Retraction and Withdrawal should be smooth throughout its range without any binding or irregularities.
2. The Inertia Locking mechanism: This is the secondary locking mechanism. Should be operable throughout its range without any sticking or binding. It should lock if the webbing is pulled out suddenly (at greater than 2 'G's acceleration).
3. Noise: No irregular noise should be heard during extraction and retraction of the webbing.
4. Damage: No Damage or Cracks should be evident.
5. Corrosion: No red base metal should be evident.

4. Running Loop or Sash Guide Condition

1. Function: Should pivot freely where designed to do so.
2. Bunching: The sash guide should not exhibit signs of webbing burn or bunching. Basically, when the seat belt is in its functional geometry, a high load is applied to the system from the upper torso of the belt. The webbing running through the sash guide will try to push towards the applied load, thus causing the plastic of the sash guide to ‘bunch’ together. Most of the etched pattern will be removed.
3. Distortion or Cracking: Should not be evident on any of the metal or plastic over mould.
4. Corrosion: As before no red base metal should be showing, a particular area to check is between sash guide and spacers, which if severe enough may restrict the sash guide from pivoting and thus cause webbing bowing.

5. Anchor Bracket/Fittings/Sew Condition

1. Anchorage Points: Should be free from corrosion and securely fastened to the vehicle structure.
2. Belt Webbing: To be securely fastened to its end fittings.
3. Sews: Should be complete without signs of loose or cut threads.
4. Bolts: If removed should be checked to ensure they are not bent. Bent bolts usually imply high side loadings due to an accident.

(Bolt tongues: 35-40 Nm) (25-30 Ft. -= 1b)

Given the infinite variety of accidents, seating positions and occupant masses available, it is unfortunately not possible to define exactly what type of accident would necessitate the replacement of a seat belt. Our attitude and policy is; if in doubt, replace it.

We would also say that any belt more than 10 years old, whether subjected to an accident or not, has probably deteriorated sufficiently through wear and tear and exposure to warrant replacement.

In terms of both the human injury and the liability should someone be injured and claim damages, the risks involved in failing to replace an accident damaged seatbelt are quite high.

It is for this reason that we would continue to recommend that any seat belt restraining an occupant during an accident be replaced.

This recommendation takes into account the difficulty an assessing ‘damage’ to the seat belt and the fact that even in very low speed accidents, high ‘G’ forces can be present.

Please don’t wait for an accident to find out that your seat belt needs replacing.