In modern equipment, screws often operate under varying loads. For example, one type of internal combustion engine cylinder head bolt works in a harsh environment under repeated stress.
The structure does not allow an increase in screw size, requiring an increase in its strength and resistance to tensile fatigue.
In other words, there are greater demands on the tensile fatigue life of such bolts.
1. Fatigue standards for threaded fasteners
Given the diversity of user requirements and different operating environments for fasteners, it is essential to establish and select life expectancy indicators in standardized environments, where loading conditions are the most significant factor.
1.1 Loading Conditions
The load conditions referred to here are the maximum and minimum load values applied to the bolt during the fatigue test.
Currently, both ISO and our national standards for bolts rated σb≥1200MPa define the maximum load value as 46% of the bolt's minimum tensile failure load – the K value (load factor).
The standards specify minimum standard breaking load values for bolts of different diameters.
These values serve as the basis for accepting the static tensile strength and fatigue test load (maximum fatigue tensile test load = minimum tensile load × load factor K).
For example, for alloy steel hex head screws, the K value is set to 0.46.
The minimum load in the fatigue and tensile test is determined by the load ratio R. R = minimum load / maximum load, R = 0.1.
1. 2 life expectancy indices
According to the above-mentioned load stipulations, there is a unified service life index. That is, among the prescribed samples, the minimum cycle count is not less than 4.5×10 4 .
Any count greater than 13×10 4 in the samples is considered as 13×10 4 for averaging purposes.
2. Tensile fatigue life of threaded fasteners
2.1 Selection of Screw Materials and Heat Treatment
According to related standards in China (such as GB/T 3098.1—2000), fatigue performance requirements are only specified for bolts with σb≥1200MPa.
The main reason for imposing fatigue performance requirements on high-strength steel is that although its strength is increased, its material plasticity reserve is markedly lower than that of medium- and low-strength steel.
Comparing this requirement with nickel-based alloys and titanium alloys, which have greater strength and good plasticity reserve, is obviously inappropriate. For example, 40CrNiMo, 30CrMnSi, etc.
If we choose a higher strength alloy steel material such as American alloy INCONEL 718, which can have a strength of over 1600MPa, it will demonstrate high service life values during fatigue testing under typical loading conditions. Let's take the M6 screw as an example.
If the fatigue test load specified by the standard is 11.01kN, and the static tensile failure load is 23.93kN, while the actual static tensile failure load of INCONEL 718 alloy can reach up to 35kN.
If we still use 11.01kN as Pmax for the fatigue test, it would only be equivalent to 31% of the static tensile breaking load, naturally, its life value will be higher.
However, for high-strength materials like 30CrMnSiNi, its notch sensitivity is extremely high and the life values during tensile fatigue testing are very low. They are not suitable for use on threaded components requiring tensile fatigue resistance.
Although certain materials can match the static tensile breaking load of steel alloys such as 30CrMnSi, they do not meet the standard requirements in fatigue strength tests at the same load level as titanium alloy Ti6Al4V.
To align its fatigue life value with 30CrMnSi and other steel alloys, the load level should be reduced to 40% (i.e. taking the K value at 40%) and for other types of titanium alloys (such as Ti21523) , K must be reduced to 36%.
However, this approach is problematic: typically, titanium alloy screws with equivalent static strength have better fatigue performance than similar steel screws.
This is a basic understanding of the properties of different materials. In this case, the K value for titanium alloy screws can certainly be higher than 0.46 and definitely not as low as 0.36.
Therefore, for bolted joints that require high static tensile strength and longer tensile fatigue life, correct material selection should be given due attention.
Fatigue fracture and delayed fracture are two main reasons for the failure of mechanical components, which is a confusing concept. Delayed fracture in screws is often due to hydrogen-induced damage behavior caused by the surface coating, which is basically unrelated to fatigue fracture.
Generally speaking, when the tensile strength of steel is about 1200MPa, both fatigue strength and delayed fracture resistance increase with increasing strength and hardness.
However, when the tensile strength exceeds approximately 1200 MPa, the fatigue strength no longer continues to increase and the delayed fracture toughness drops sharply.
Most of the steel used in mechanical manufacturing is medium carbon alloy steel, used in a tempered state, with tensile strength mainly between 800 and 1000MPa.
Increasing your strength is not difficult, but the biggest challenge lies in solving the problem of short lifespan after increasing strength.
Fatigue failure and delayed fracture problems are the main barriers to the high strength and long service life of steel used in mechanical manufacturing.
Heat treatment is a critical factor, especially tempering during the hardening process of high-strength screws. In the high-temperature tempering zone, impurities such as sulfur and phosphorus are likely to form.
When these impurities accumulate at grain boundaries, they can lead to brittle fracture, especially when hardness exceeds 35 HRC, the tendency to brittleness increases significantly.
2.2 Techniques to improve fatigue life
Before reinforcement, the probability of tensile fatigue failure in threaded fasteners is as follows: 65% of failures occur at first engagement with the nut, 20% of failures occur at the transition between thread and rod (although this statement is widely precisely, it should be emphasized that the fundamental cause of fatigue failure at these points is still due to the high concentration of stresses), which is at the ends of the threads, and 15% of failures occur in the transition radius between the screw head and the rod, as shown in Figure 1.
It should be emphasized that these statistics are based on the condition that the metal flow lines of the entire fastener are not damaged.
To improve tensile fatigue life, measures can be taken on both the screw shape and the process, with the most effective methods currently being the following.
2.2.1 Use of MJ threads (i.e. reinforced threads)
The main difference between MJ threads and regular threads is in the smaller diameter (d1) and radius (R) of the external threads, as shown in Figure 2.
The main characteristic of MJ threads is a smaller diameter (d1) greater than normal threads, with an increased root fillet radius, reducing stress concentration in the screw.
Specific requirements for R are given (Rmax = 0.18042P, Rmin = 0.15011P, with P being the pitch), whereas regular threads have no such requirements and can even be straight. This significant change greatly increases the tensile fatigue performance of the smaller diameter.
Currently, MJ threads are widely used in aerospace screws.
2.2.2 Improving line fatigue performance
When using the thread rolling process, due to the effects of cold work hardening, there is residual compressive stress on the surface, allowing the directional flow of the internal metal fibers in the screw to be rational and uninterrupted.
Consequently, fatigue resistance can be 30% to 40% greater than that of threads machined by turning.
If the thread is rolled after heat treatment, it strengthens the surface of the part and creates a layer of residual stress, which can increase the surface fatigue limit of the material by 70% to 100%.
This process also has advantages such as high material utilization, high production rate and low manufacturing cost. Table 1 shows fatigue life values under different process methods.
The test bolt material is 30CrMnSiA, the bolt standard is GJB 121.2.3, and 6×26 (i.e. MJ6) is tested for tensile fatigue according to the test method, with test fatigue load: Pmax =10.1kN, Pmin=1.01kN . The results are shown in Table 1.
Table 1: Fatigue life (number of cycles) under different process methods
| Test no. | A | B | W | D |
| Before heat treatment, cold wind the threaded screw. | Before heat treatment, do not cold roll the threaded screw. | After heat treatment, cold wind the threaded screw. | After heat treatment, do not cold roll the threaded screw. | |
| 1 | 17800 | 13800 | 130,000 | 130,000 |
| two | 11900 | 11600 | 130,000 | 93700 |
| 3 | 13400 | 17400 | 130,000 | 70400 |
| 4 | 20100 | 8700 | 130,000 | 103300 |
| 5 | 15,500 | 18100 | 130,000 | 98600 |
| 6 | 18,000 | 15200 | 130,000 | 51300 |
| 1 | 14100 | 11300 | 130,000 | 95800 |
| 8 | 8400 | 12,000 | 130,000 | 88100 |
| 9 | 18200 | 17300 | 127600 |
From Table 1, it is evident that the tensile fatigue strength of the fillet r at the turning point of the cold-rolled threaded bolt, post-heat treatment, is ideal (see Figure 1). The requirements for the r value in cold extrusion are not strict. Technical specifications only stipulate an upper limit for deformation.
2.2.3 Strict control of final dimensions
As shown in Figure 1, the transition area between the screw thread and the smooth rod is one of the significant sources of fatigue. Strict control of the final dimensions to shape the transition area is a crucial measure to increase fatigue life in this region.
Therefore, during the design and manufacturing of line rolling mill wheels, it is imperative to strictly grind the ends according to standards and strictly control the position of the line rolling mill during the process.
Specific measures may include a larger transition fillet as shown in Figure 3a, creating discharge structures as shown in Figures 3b and 3c, and cutting a tool withdrawal groove in the end of the thread may also reduce concentration. of stress (the schematic diagrams in Figures 3b and 3c may be misleading. Increasing the fillet in the transition area actually helps to alleviate local stress concentration).
Cold extrusion of the fillet at the screw turning point, as shown in Figure 1, can increase the tensile fatigue life at this point. As shown in Table 1, if only the reinforcement measures in 2.2.1, 2.2.2 and 2.2.3 are taken, fatigue fractures will occur exclusively at the screw turning point.
Therefore, cold extrusion strengthening of fillet r is one of the important measures to improve the overall tensile fatigue life of the screw.
2.3 Avoid generating additional bending stress
Due to improper design, manufacturing and assembly, eccentric loading of bolts may occur. Eccentric loads can induce additional bending stresses in bolts, significantly reducing their fatigue strength. Therefore, appropriate structural and process measures must be taken to prevent the generation of additional torque.
(1) The screw countersink angle must be accurate, allowing only a positive deviation of 0° to 0.5°, no negative deviation is allowed.
(2) The screw supporting surface must be flat and perpendicular to the axis of the screw hole.
(3) For mounting holes on the workpiece, such as those for hexagonal heads, the hole chamfer must be in accordance with international standards.
2.4 Preload set
Precharge is one of the most critical problems in daisy chain connections. Theory and practice have shown that, with the stiffness of the bolt and joined parts held constant, appropriately increasing preload significantly increases tensile fatigue resistance. This is why the screw preload stress can reach up to 0.7 to 0.8 of the yield stress (σs).
Therefore, accurately controlling preload and maintaining its value are crucial. The magnitude of preload is controlled by a preset torque wrench or preload indicator washers.
The required preload varies under different conditions and commonly empirical formulas based on previous experience are used to estimate the preload.
For general mechanical preload: σ p = (0.5 to 0.7)σ is ; for high strength connections: σp = 0.75σs (this is the yield limit). This method of expressing preload contradicts the aforementioned 46% approach.
Recently, a new method of connecting screws has been developed, which involves preloading the screw to the yield point, allowing the screw to work within the plastic region. For more details, see the article “Plastic Screw Domain Connection” by Ichiro Maruyama, published in “Mechanical Research”, Volume 40, No. 12, 1988. For critical connections preloaded against fatigue, fatigue resistance tests under different preloads must be performed to determine correct and usable preload values.
3. Conclusion
Through experimental data and practical experience, the document proposes several specific measures to improve the fatigue and tensile strength of screws, addressing aspects of material selection, machining and assembly.
Some of these measures have proven their effectiveness in practical applications, while certain empirical data and conclusions await further exploration and theoretical validation.
In summary, comprehensive measures must be adopted to improve the fatigue strength of bolts; no single measure can satisfy the general need for fatigue resistance.
