When a bolt yields, it will plastically/permanently deform. The bolt’s effectiveness as a spring clamp is impaired. Apart from being closer to tensile failure there is a resultant loss in tension/clamp force in the joint. The yield point is the bolt’s operational capacity and engineers must ensure that the design tension on installation and in service is below this value.
The yield point is determined from a tensile test. The traditional view is it is easily identified by the big change in the stress strain curve as shown in Fig A. Unfortunately this type of curve only happens with certain types of mild steel. Heat treated medium carbon steels display a typical stress strain curve as in Fig B. Here the actual yield point is not specific other than the end of the straight line. With this information coming from machined test pieces, taken either from the bolt itself or the material bar heat treated with the bolt, the yield versus ‘end of the elastic line’ relationship is not clearly defined. This gave rise to the 0.2% Proof stress value as the yield/bolt load capacity guide. It’s an arbitrary test value where a line is drawn parallel to the elastic portion of the stress strain curve; this line is displaced at a distance of 0.2% proof stress of the gauge length used in the tensile test. Where the 0.2% displacement line crosses the curve, the yield value is defined.
A more accurate assessment of true yield comes from the ‘Stress Under Proof Load’ test involving a load test on a full size and assessing any permanent deformation after loading to a minimum specified value. This test is under used despite being defined in key structural bolting specifications such as ISO 898 and SAE J 429. From curve b it can be seen that the 0.2%PS value is indeed a yield condition, however generally for hardened and tempered steels that exhibit this stress/ strain shape, the difference between the test value and end of the straight line is not big so the 0.2% PS is a reasonable capacity guide to the designer. When the bolt material has a stress strain curve shape similar to the curve in Fig C however, the 0.2% PS is ineffective for assessing a bolts maximum load limit. The end of the elastic line is substantially below the determined 0.2% PS value.
It has been found that bolts made from stainless steels, cupro-nickels and also some high nickel super alloys can have elastic limits 30-40 % below the 0.2% PS value.
In a previous blog we discussed tightening selection between torque or hydraulic tensioning. We debated how the need to hydraulically overload to compensate for load transfer losses can impact on the feasibility of selecting the tensioner especially where the design bolt load utilises a relatively high proportion of the bolt’s elastic/yield strength. Indeed, the feasibility of using hydraulic tensioners could be further curtailed with the ‘low elastic strength bolt materials’ mentioned above. Even lower design bolt stresses may not be achievable with the hydraulic jack.