Tensile Strength versus WLL

[Ben Jensen]

Hello guys.

Have tried to find an answer thru' the search functiond, didn't really work. Can anyone tell me, what is, according to LOLER, the safety factor between tensile strength and WLL/SWL on the various types of rigging equipment. Must say, the answer is probably going to be one of the questions in an upcomming ETW-exam

Greetings Ben

[ukminch]

I wouldn't have said there is a relationship? Do you mean between MBS and swl?

[Ben Jensen]

Yes, well, without starting a feud, in my world, MBS and TS is the same

[ukminch]

I would argue that MBS is minimum breaking strength ie the limit up to which the product is guaranteed and will fail quite a bit above that. Tensile strength is the absolute maximum value of strength at which the material will fail.

 

10:1 ratio for fabric, 5:1 for metalwork although there are some exceptions such as certain slings

[Pete Mctree]

MBS - minimum breaking strain as declared by the manufacturer

SWL - safe working load. This is obtained by applying the relevant factor of safety (FOS) to the MBS

FOS is dependant upon usage & materials - eg metal work has a FOS of 5, so if a porty has a MBS of 10kn, then it's SWL is going to be 2kn

 

Clear as mud

[Loler UK]

In fact The Minimum Breaking Strength (MBS) is a statistically derived value and is poorly understood and should not be referred to as the tensile strength.

 

If 100 carabiners are submitted for destructive testing then the measured strengths will vary quite a bit. Inconsistencies in the manufacturing process may result in larger variations. Batch test results can be plotted on a graph showing the probability of breaking at particular strengths and this curve should approximate a “normal distribution” or “bell curve”.

 

The width of the curve is characterised by the “standard deviation” or σ (sigma). 3σ for a normal distribution indicates that 99.7% of samples should lie within the range of (mean – 3σ) to (mean + 3σ). Smaller values for σ indicate that samples are more likely to be close to the average, or mean value.

Some manufacturers state that they use “3-sigma” to determine their MBS values. This means that the MBS is actually the mean breaking strength less 3 times the standard deviation (3σ).

 

MBS = mean - 3sigma

 

Statistically, this also means that 0.3% of samples will lie outside this range and therefore half of these (0.15% or 1.5 in every thousand) may break below the MBS stamped on the karabiner.