Wherever we design new rigging equipment and rigging systems for heavy lift helicopter operations or performer flying the final step is to validate the claims made by OEM suppliers and our own calculated load limits.
We achieve this by destruction testing and proof load testing. What's the difference? Simply explained, destruction testing involves applying forces and measuring the observed yield point or point at which failure of the equipment occurs.
Proof Load Testing is a non destructive testing (NDT) method that is used to validate the data gained from destructive testing and the forces can be applied anywhere in the range of 100% of the rated value to twice the work load limit as defined by local standards and or codes of practice.
In the examples below, the rated work load limit or safe work load limit (SWL, WLL) is a percentage of the failure point or ultimate breaking load (UBL). This percentage is know as the safety factor (SF) and is applied to the UBL. In the case of the 85 tonne bow shackle and chain assembly the safety factor was 4:1 as claimed by the manufacture and was within the 4:1 SF, the point of failure was 4 times the rated WLL (greater than 300 Tonne).
Not all rigging equipment has an included SF and it is up to the professional rigger to apply one. Carabiners for example have a rated load limit stamped on them to identify the SWL/ WLL this is actually the UBL the point it which the carabiner will fail, and we apply a SF of 10:1 for human loads.
So you can see how important it is to know not only the correct gear selection but also what SF if any have been applied to a rigging system and equipment.