Investigating the Influence of Pre-tension on Fatigue Performance of Large Bolts
TWI Industrial Member Report 1158-2022 [pdf / 2130 KB]
By Carol Johnston
Pre-tension is the tensile force in the bolt within a bolted joint and is applied for a number of reasons. It ensures that the joint faces remain in contact when tensile loads are applied to the joint, and is also used to protect the threaded fastener from applied fatigue loads. When pre-tension is used to protect fasteners against fatigue loads, BS 7608 recommends that a pre-tension of at least 1.5 times the applied load is used. Other design documents relating to bolted joints (eg ASTM F3125, DASt 021 and Eurocode 3), recommend that a pre-tension of 70% of the UTS or SMYS is used, irrespective of the applied load.
In practice, there can be variability in the magnitude of pre-tension in a bolt so users of preloaded bolts ask the question ‘what level of pre-tension loss can be tolerated before fatigue performance will be lower than that predicted by standard design S-N curves?’.
This report presents results from fatigue tests of M72 hot dip galvanised bolts which were intentionally pretensioned to less than the recommended level.
Strain gauges were used to measure the pretension applied to the bolts using the hydraulic tensioning tool, allowing the degree of ‘tool load loss’ to be measured for this tool and joint configuration. For this diameter to length (d/L) ratio, and pretensions of less than 70% of the 0.2% proof strength of the bolts, an ‘overtension’ of approximately two times the target tension to lock in the required target tension after removing the tool.
- Fatigue tests were performed on bolts with a range of pretensions (between 0 and 1675kN). The fatigue performance of pre-tensioned bolts was slightly higher than that of bolts tested without pre-tension but with a high mean stress, and the difference could be explained using the modified Goodman correction.
- Although the pre-tension applied to the bolts tested was intentionally lower than the values recommended in design standards, the fatigue performance of bolts which are pre-tensioned to a level lower than that recommended in standards was well described by the thickness corrected BS 7608 Class X-20%, or DNVGL Class G S-N curves.
- In order to predict the fatigue performance of a pre-tensioned bolt, the stress range actually experienced by the bolt needs to be known.
A relationship was derived which related the bolt load range to both applied pre-tension and applied load range for the set-up used in this test programme.
Strain gauges located in the bolt shank were used to measure strain and calculate the actual stress ranges experienced by the bolts during fatigue testing
Fracture face of a fatigue cracked bolt after testing