# Controlled Creation of Contact Cracks in Additive Manufactured Components
## 5 Conclusions  
This paper presents a method for the rapid and controlled generation of mechanically fatigue-induced cracks in additively manufactured test coupons. The creation of these reference flaws is crucial for the certification of NDT techniques, which will enable the adoption of AM for critical applications.  
To seed cracks in a controlled manner, a static tensile load is applied using steel bobs, whilst also exciting the system into a high-strain vibration mode. Utilizing resonance allows for the cracks to be seeded in a rapid, compact, and low-force setup. The bobs reduced the resonance frequency of the sample-bob assembly, increasing the strain at the tip of the notch, which acted as a stress raiser. Because this monitoring technique is non-invasive and utilizes the resonance of the entire sample-bob assembly, it does not require any probes in contact with the sample or near the vicinity of the crack, instead taking measurements with a laser Doppler vibrometer at an arbitrary location on the assembly. Through the judicious selection of the resonance mode shape and the location of stress-raising features, this technique provides much greater control over the crack location and morphology. Finally, this technique can be very quickly and inexpensively realized, taking a maximum of 15 min for the seeding of the largest crack in this study, and this timespan can be further reduced by increasing the excitation amplitude or the magnitude of the tensile load. The convenience of this technique makes it ideal for the creation of large sample sets, which can be used in statistically significant studies in both AM and NDT.  
The experimental work shows the successful creation of five reference samples with seeded cracks of lengths between 0.27 and 5.6 mm and openings between 14 mm and 79 mm. The crack lengths are controlled using a continuous, no-contact measurement of the resonance frequency, allowing for the creation of very small flaws.  
The cracks seeded in these samples were validated using Nonlinear Resonance and optical microscopy, with the former confirming the presence of cracking and the latter confirming the crack dimensions over the exposure process.  
**Author Contributions:** Conceptualization, J.W. ([PERSON]); methodology, D.P., A.A. and J.W. ([PERSON]); formal analysis, D.P.; investigation, A.A. and D.P.; resources, [PERSON] ([PERSON]); data curation, A.A.; writing--original draft preparation, A.A. and D.P.; writing--review and editing, D.S. and J.W. ([PERSON]); visualization, D.P.; supervision, [PERSON] ([PERSON]); project administration, D.P. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Data Availability Statement:** Graph datasets are available online at [[https://doi.org/10.6084/m9.figshare.24050781.v1](https://doi.org/10.6084/m9.figshare.24050781.v1)]([https://doi.org/10.6084/m9.figshare.24050781.v1](https://doi.org/10.6084/m9.figshare.24050781.v1)) (accessed on 20 October 2023).  
**Conflicts of Interest:** The authors are affiliated with a company that offers non-destructive testing and declare that this work does not constitute any competing interests.