# Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints
## 1 Introduction
### Detection of the Crack Initiation Point  
The result of a standard load controlled fatigue test is the number of load cycles until burst fracture at a certain load level, including the crack initiation and propagation phase. For cylindrical small-scale specimens without considerable imperfections, such as pores, the crack propagation phase is rather small compared to crack initiation. Therefore, the S-N curves obtained by such standard tests could be applied in fatigue life assessment concepts to estimate the time to crack initiation [14]. For welded components, on the other hand, the specimens should exhibit a certain width/sheet thickness ratio in order to ensure a steady state residual stress state and crack initiation from a plain weld rather than the specimen's edges [15]. In practice, a minimum width of three to five times the sheet thickness is suggested to avoid such effects [16; 17] causing welded specimens to be comparably large even in the case of thin sheets. Therefore, the crack propagation life of welds significantly contributes to the specimen's total fatigue life [18; 19; 20]. Hence, the knowledge about the technical crack initiation enables a split of the fatigue assessment to crack initiation and crack propagation phases. This is of great interest even for fatigue tests of welded specimens, as it makes a more precise assessment of a structure's fatigue life possible.  
A number of different methods exist for monitoring crack propagation. For welded structures, local strain measurement at hot spots [21; 22] is widely used due to its flexibility and easy handling. This procedure is expedient if the specimen has a low number of distinct hot spots, such as a longitudinal stiffener, where crack initiation will occur. Otherwise, a high number of strain gauges is necessary to cover the whole highly-stressed weld toe area.  
Another common technique is the application of a mixture of zinc oxide and glycerine on the weld toe [21; 23] giving a colour change from white to black when a crack initiates. The main disadvantage of this method lies in the fact that the weld is covered by a viscous fluid preventing any exchange of oxygen leading to a possible change of crack initiation and propagation behaviour.  
A purely optical tracking of crack propagation is performed in [24; 25]. Hereby, an industrial camera tracks the crack's surface extension using its contrast to the surrounding surface. This procedure achieves high accuracy but it requires a fine surface quality and special lighting.  
In [24], the optical tracking is compared to the crack propagation detected via crack gauges. For this procedure, a crack gauge is applied on the surface region where the crack is expected. During the fatigue test, constant current flows through the crack and the resistance based on voltage drop is measured. As the crack opens, the crack gauge is torn apart, thereby increasing the resistance of the gauge, which can be linked to the surface crack length. The comparison between optical measurement and crack gauges in [24] reveals a sound accordance.  
The eddy current method applied in [26; 27] enables non-contact measurement of surface cracks even below paint coatings. Thereby, eddy currents proportional to the conductivity of the material are induced by placing an AC-energized probe coil near the surface of the specimen. Discontinuities, such as cracks, affect the magnitude and phase of the induced current. The consequential magnetic field is then sensed by the probe indicating the presence and size of the surface crack.  
Digital image correlation (DIC) is used for estimating stress intensity factors in [28] and the consequential identification of crack propagation law parameters in [29]. In [30], DIC and crack opening displacement techniques are combined to monitor crack propagation tests. Therewith, the fatigue behaviour of thin cracked plates made of aluminium alloy in crack propagation mode I are analysed.