Measures for improving the endurance strength of bolted joints

According to VDI 2230.
Improving Endurance Strength
Reduction of the thread diameter Distinct improvement possible in the range d < 40mm The increase of endurance strength with decreasing thread dia. is attributed to the influence of bolt size on mechanical properties (micro - structural effect).
Increase of preload Distinct improvement indirectly due to reduced additional bolt forces. No direct improvement due to an increased loading capacity. With an increased preload, the residual clamping forces in the joint also increase. Thereby the danger of one-sided opening of the interface is reduced. Thus, the bolt is subject to reduced additional stress. For bolts thread rolled after heat treating the endurance strength decreases with an increasing preload owing to the reduced effect of the bolts residual compressive stress. Not possible for bolts thread rolled before heat treatment, because there is no preload dependence. 
Bolt material and bolt material's strength Insignificant improvement by selection of material. Insignificant direct improvement by the bolt material's strength. Only indirect by higher preloads The dominating notch effect suppresses an influence of the material, once the material's flow capacity is sufficiently high to prevent damage during installation by tearing the highly stressed surface layer at the minor diameter. Similarly, the beneficial effect of higher strength is offset by high notch sensitivity.
Lubrication in the thread  Limited improvement possible Repeated movements between the screw thread and the nut thread flanks are less strongly resisted with a good lubricant. 
Thread rolling after heat treatment R.T.A.H.T Distinct improvement (up to 100%) especially for low preloads. For high preloads the positive affect decreases. Residual compressive stress induced by manufacturing has an increased effect on the endurance strength especially for low preloads. With an increase of the tensile preload, the influence of the residual compressive stress decreases: preload dependence of RTAHT bolts. For preloads in the range of the bolts yield load, the effect of the residual compressive stress can disappear almost totally. 
Surface carburisation or Surface decarburisation Limited improvement possible A brittle, carburised surface can lead to early damage owing to the reduced capacity for metal flow. A decarburised surface however, has no effect on the endurance Strength. 
Increase in thread pitch Limited improvement possible It is true that with an increasing pitch, the notch effect in the thread is reduced, owing to the greater root radius of the threads. On the other hand, the minor thread diameter decreases. Thereby the influence of the notch effect is offset. For relatively fine threads (d/P> 12) and for higher material strength the notch effect dominates. Here, an increase of the pitch causes an improvement in endurance strength. 
Increase of the thread clearance Improvement possible (e.g. tolerance location) An increase in the thread clearance brings about a better bending resilience of the threads. So, the load distribution in the nut becomes more homogeneous. 
Difference in pitch between the nut thread and the bolt thread Considerable improvement possible A reduced pitch of the screw thread, compared with the nut thread, modifies the load distribution over the engaged threads. With carefully controlled change in pitch, the load on the first bearing thread can be reduced substantially and thus the endurance strength can be increased. 
Increase the radius at the root of the thread No significant improvement possible An improvement of the endurance strength could be expected by decreasing the stress concentration by means of a larger thread root radius, and thus, increasing the minor diameter. However, this improvement is offset to a greater extent by the simultaneous effect of a more uneven stress distribution in the engaged threads. 
Reduction of the modulus of elasticity of the nut material Distinct improvement possible With a decrease in modulus of elasticity of the nut material, the bending resilience of the thread teeth of the nut increases. So, the load distribution becomes more even. The most stressed, first bearing screw thread carries a reduced load.
Nitriding, case-hardening Improvement possible for low preloads The improvement of the endurance strength caused by an increase in strength of the surface layer and especially by the residual compressive stress does not have an effect in the case of high preloads, because there, the relatively brittle surface layers would initiate fracture. 
Electroplating or hot galvanised zinc No improvement possible but rather a deterioration No impairment through relatively soft electroplated cadmium or zinc layers. The endurance strength can be reduced for nickel or chrome layers if residual tensile stresses are produced in the layer caused by the deposition conditions. Also, relatively brittle layers of iron-zinc alloy, which are produced during hot galvanising reduce the endurance strength in the order of 15%.
Manufacturing technique: cutting, non-cutting Improvement by non-cutting manufacturing method A smooth surface as is typical of threads manufactured with a non-cutting process can generally not be achieved by metal cutting techniques. (turning, milling, grinding - notch effect) In addition, it is possible that, for instance, on ground threads residual tensile stresses are induced, which are detrimental to the endurance strength.
Increase of thread engagement Improvement of the endurance strength with increased thread engagement With an increasing thread engagement, the distribution of the total bolt force on the individual nut threads is modified advantageously. Thereby the load on the first bearing bolt thread is reduced 
Form of the nut Distinct improvement possible The load distribution can be influenced effectively by the form of the nut. (e.g. tension nut with conical shape).
Reduction of the strength of the nut material Distinct improvement possible With a decrease in strength of the nut material, plastic deformation of the nut threads increases. Thereby the load distribution in the threads become more even. An overcritical nut height is a precondition for the loading capability of the joint.