Abstract
The fatigue behavior of bolted joints is critical to failure for many applications due to the high notch effect. Among other parameters, the lifetime is based on the influence of the surface system, consisting of corrosion protection and lubricant. With the intention of considering the surface system in the design and dimensioning process of bolted joints, experimental investigations are carried out systematically for an exemplary selected basecoat and various lubricant systems. The basis is given by fatigue tests supported by selected methods of material analysis for the fractographic evaluation. A reproducible method to determine the crack initiation as well as the crack propagation is developed and used for the evaluation of lubricant systems. Defined damage can be reproducibly assigned on the basis of a critical frequency change rate of the resonance test machine. A high durability of the friction in the load-bearing contact (e.g., greases) reduces the stress and delays the crack initiation. Lubricants which are able to creep (e.g., oil) slow down the crack propagation and increase the lifetime, which is proved by a higher number of cycles to failure and an increased fatigue strength.
Highlights
Bolted joints are often used in applications that are subject of cyclic loads, which is why fatigue failure is one of the most frequent causes in the event of damage [1]
The fatigue strength and the lifetime of bolted joints depend on a large number of overlapping factors [2–10], which are insufficiently considered in the relevant design methods and guidelines
The bolts provided with different lubrication were paired with standard steel nuts according to DIN EN ISO 4032 [26] on a resonance test system preloaded to a constant mean load and a sinusoidal operating load with constant amplitude was superimposed
Summary
The bolted connection is a frequently chosen technical solution used to establish a detachable and high-strength connection. Bolted joints are often used in applications that are subject of cyclic loads, which is why fatigue failure is one of the most frequent causes in the event of damage [1]. The durability under cyclic loading is reduced to less than 10% of the static strength and is determined by high local stresses in the thread root [2]. The fatigue strength and the lifetime of bolted joints depend on a large number of overlapping factors [2–10], which are insufficiently considered in the relevant design methods and guidelines. In addition to the design parameter, such as the depth of engagement, the nut material and the manufacturing process for the nut thread itself [3], the surface system, consisting of corrosion protection and a lubricant, has a significant influence on the fatigue strength [11–14]. In mechanical engineering and plant construction, the design process is carried out in accordance with the VDI guideline
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