There are various reasons for the fracture of fastener bolts. To sum up, the damage of general bolts is caused by stress factors, fatigue, corrosion and hydrogen embrittlement.
1. Stress factor
Exceeding normal stress (super stress) results from any one or a combination of shear, tension, bending and compression.
The first consideration for most designers is the combination of tensile load, preload, and additional utility load. Preload is essentially internal and static, and it compresses the joint assembly. Practical loads are external, typically cyclic (reciprocating) forces applied to the fastener.
Tensile loads attempt to resist the joint components apart. When these loads exceed the yield limit of the bolt, the bolt changes from elastic deformation to the plastic zone, resulting in permanent deformation of the bolt and therefore no longer able to return to its original state when the external load is removed. For similar reasons, if the external load on the bolt exceeds its ultimate tensile strength, the bolt will break.
The bolts are tightened by twisting with preload. During installation, excessive torque leads to over-torque, and at the same time, the fastener is subjected to over-stress and reduces the axial tensile strength of the fastener. The yield value is relatively low. In this way, the bolt may yield below the minimum tensile strength of the corresponding standard. A large torsional moment can increase the bolt pre-tightening force and reduce the joint slack accordingly. In order to increase the locking force, the preload force generally takes the upper limit. Thus, unless the number of differences between yield strength and ultimate tensile strength is small, bolts generally do not yield due to torsion.
The shear load applies a force perpendicular to the longitudinal axis of the bolt. Shear stress is divided into single shear stress and double shear stress. From empirical data, the ultimate single shear stress is about 65% of the ultimate tensile stress. Shear load is preferred by many designers because it takes advantage of the tensile and shear strength of the bolt, and it primarily acts like a pin, allowing sheared fasteners to form a relatively simple connection. The disadvantage is that the shear connection can be used in Small and shear connections cannot be used as often as they require more material and space. We] know that the composition and precision of the material also play a certain decisive role. However, material data to convert tensile stress to shear load are often not available.
Fastener preload affects the integrity of the shear joint. The lower the preload, the easier the joint layer will slide when in contact with the bolt. The shear load capacity is calculated by multiplying the number of transverse planes (one shear plane is commonly referred to as a single shear, two shear planes are referred to as a double shear), and these planes should be the cross-sections of the unthreaded bolts. We do not advocate designing for shear through threads because the shear strength of the fastener can be overcome by stress concentrations as the cross-section changes. When determining the shear strength of fasteners, some designers use tensile stress areas, while others prefer small diameter sections. If the bolt is twisted to the specified value in the shear connection, the mating surface of the contact layer cannot start to slide until the friction resistance is exceeded. Increasing the friction between the mating surfaces improves the integrity of the connection, sometimes limiting the number of bolts that must be used due to the size and design of the part.
In addition to tensile load and shear load, bending stress is another load experienced by the bolt, which is caused by external forces not perpendicular to the longitudinal axis of the bolt at the position of the bearing surface and the mating surface. The simpler the fastener connection, the greater its integrity and reliability.
2. Fatigue
At present, the relevant regulations of industrial fasteners, there is no specific legislation to instruct suppliers to purchase key components that meet industrial standards, especially without mentioning the main reason for the failure of fasteners - fatigue. Fatigue damage is estimated to account for 85% of all fastener failures.
Fatigue in bolts is the constant effect of cyclic tensile loads, so that bolts are subjected to relatively small preloads and alternating working loads. Under prolonged exposure to such double loads, bolts fail at less than their rated tensile strength. Fatigue life is taken from the number and amplitude of loading stress cycles. There may also be fatigue fractures in some compressed joints, such as presses, stamping equipment and molding machinery. Various compound stresses are generated between the power during operation and the preload. For example, in repeated stretching, the number of stress changes and the amplitude are affected by the degree of fatigue and damage.
Typical industrial fasteners, such as socket head cap screws, constantly elongate and return to their original shape within a certain elastic range. If subjected to more than normal stress, beyond the elastic range, they will produce permanent deformation until finally fracture. The act of elongation-recovery-elongation is called cycling.
Fasteners will eventually crack due to repeated peak-to-peak stress cycles. Fracture usually occurs at the fastener's most vulnerable point, what engineers call the "region of greatest stress concentration." Once the micro-cracks are generated at the stress concentration and continue to be stressed, the cracks will expand rapidly and the fasteners will be fatigued. Enterprises that manufacture industrial fasteners are constantly exploring new molding processes and designing and developing new manufacturing methods that can overcome the Achilles' heel.
The most common locations of fatigue failure include the joint (ie the first loaded thread), the root fillet, the thread, and the thread termination. As the processing industry has improved fatigue strength by developing better materials and production methods, the thread becomes the weakest point of a fastener and is currently the cause of the highest percentage of fatigue failure.
The interrelationship between design stress variables and fastener performance characteristics makes setting a criterion for fatigue strength a difficult task, which is currently determined by the number of "cycles to failure" and the relative strength of a series of fasteners, which is measured. is a complex process.
3. Corrosion
Another cause of bolt breakage is corrosion. Corrosion comes in many forms, including general corrosion, chemical corrosion, electrolytic corrosion, and stress corrosion. Electrolytic corrosion is: first, the fasteners are exposed to the outside and are eroded by various wet agents such as rain or acid mist. These are electrolytes, which will cause chemical corrosion of the fasteners; secondly, the materials of the fasteners are different, and their The electrolysis potential is different, and the potential difference is easy to produce "micro-battery". Designers should try to select materials with close electrolytic potential according to the compatibility of metals, and eliminate the conditions for the generation of electrolytes to prevent cracks from electrolytic corrosion.
Stress corrosion is relatively limited. Stress corrosion exists under the action of high tensile load, which mainly affects the fasteners of high strength alloy steel. Fasteners of alloy steel (especially those with high alloy composition) are prone to cracks under the action of stress. At first, crack pits are generally formed on the surface, and then further corrosion occurs. After corrosion, crack propagation is promoted, and the rate is determined by the stress on the bolt and the fracture toughness of the material. Fracture occurs when the remaining material is too functional to withstand the applied stress.
4. Hydrogen embrittlement
High-strength steel fasteners (generally Rockwell hardness above C36) are more prone to hydrogen embrittlement. Hydrogen embrittlement is the main cause of fastener fracture. Hydrogen embrittlement is the phenomenon when hydrogen atoms enter and diffuse throughout the material matrix. When hydrogen atoms enter the material matrix, the material matrix produces lattice: distortion, which destroys the original equilibrium state, so it is easy to crack under external force. When an external load is applied to the screw, the hydrogen atoms migrate to the highly stress-concentrated area, causing extreme stress between the crystal boundary edges, which leads to the fracture of the fastener crystal grains.
When the fastener contains hydrogen in a critical state before installation, it usually breaks within 24 hours. It is impossible to predict when the fracture will occur if hydrogen enters the fastener. Therefore, when using the relevant fasteners, the designer should specify the selection of suppliers with special process treatment and use of potential hydrogen embrittlement minimization.
5. Other factors
Joint failure is not always directly related to catastrophic fastener failure. Many fastener-related factors, such as loss of preload or fastener coupling fatigue, can cause wear; fastener center offset, noise, leaks in use, requiring unscheduled maintenance, or breakage . For example, vibration reduces the frictional resistance of threads, and fastener joints loosen after installation due to the application of working load, these factors and high temperature creep of the bolt can lead to loss of preload. Sometimes joint failure can be attributed to too large or too small through holes, too small load bearing area, too soft material, too high load. Neither of these conditions will result in direct failure of the bolt, but will result in a loss of joint integrity or eventual bolt failure.
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