# Controlled Creation of Contact Cracks in Additive Manufactured Components
## 1 Introduction  
The aerospace and automotive sectors are two sectors that are beginning to integrate additive manufacturing (AM) into the production of critical components. In contrast to conventional manufacturing methods, AM offers the benefits of having low material waste and the ability to create highly optimized designs in a single stage [1]. However, these parts come with a unique range of defects and flaws which, combined with complex designs, presents challenges for the majority of conventional non-destructive testing (NDT) techniques. Therefore, with this shifting manufacturing paradigm comes a need for rapid and cost-effective NDT techniques at scale for AM components. This is compounded by the acknowledged general need for the creation of suitable NDT reference samples [2] for technique validation, optimization, and demonstration. In order to develop and certify these techniques, reference sample sets must be produced in statistically relevant quantities with controlled representative flaws. Flaw seeding in AM parts commonly focuses on creating delaminations, pores, or holes of an idealized geometry [3]. Flaws are achieved via non-optimal build parameters (such as the scanning speed or temperature), through machining, or by inclusion in the original model file. Thermal cracks can be produced, albeit without dimensional control, via variation in the constraints of the part during cooling, or the use of low-weldability alloys [2]. Outside of additive manufacturing, a common method of seeding flaws is by the use of a fatigue or creep-fatigue apparatus. Low-cycle, notched specimen fatigue is common, due to the controlled location of initiation as well as the requirement of relatively fewer cycles until failure. Notches such as those created using electrical discharge machining are commonly used as crack initiation sites [4]. Fatigue tests may monitor the material failure with extensometers or systems of strain gauges, but, in general, monitoring for the control of crack sizes is a challenge. To that end, [PERSON] et al. [5] measured the propagation of a crack by means of current monitoring over its area. This relies on the fact that the voltage measured over a crack relates to the area of material in the cross section. They concluded that there is a strong correlation between the predicted crack growth from current measurements and the observed dimensions of propagation after sectioning. [PERSON] et al. [6] explored the use of compliance measurements during conventional four-point fatigue cycling, successfully deriving a relationship between sample compliance and fatigue crack length. An alternative method of seeding cracks is through thermal fatigue, whereby a sample is thermally cycled in a specific region using an induction coil placed on an accessible sample surface. [PERSON] et al. [7] discussed such a technique, indicating a relationship between thermal cycling parameters and surface-breaking crack propagation. However, thermal fatigue cracks are known to be morphologically different from mechanical fatigue cracks in that there are more cracks at the surface and microbranching along the crack length or at the tip [8]. This renders this technique less suitable when the user wishes to study mechanical failure in parts. In previous work by [PERSON] et al. [9], controlled cracks were embedded in ASTM F519 type 1A1 tensile coupons using a combination of dynamic and static loading, achieved via resonance excitation and mass-loading with steel weights (\"bobs\"), respectively. This enabled the material to be locally fatigued very quickly, whilst retaining control over the propagation. This is in contrast to conventional fatigue cycling, where the requisite large applied forces limit the cycling frequency to the order of 10 Hz. The in situ monitoring of crack growth was achieved by tracking the shift in the resonance frequency of the assembly. That work demonstrated that cracks of controlled dimensions could be created in conventional metal components.