## 5 Conclusions  
The fracture behavior and hydrogen-assisted cracking sensitivity of the lath martensite steel used to manufacture a new type of ultrahigh-strength tendon bar for construction has been explained and quantified from the cohesive crack model adapted to the SENT specimen configuration, as well as from the J-integral experimental values obtained by testing precracked SENT specimens.  
The complete agreement between the J-integral values measured and predicted allowed for the cracking resistance of the bar steel to be quantified through the cohesive resistance. The obtained values are fully consistent with the condition of the material constant attributed to the cohesive resistance in the model. The differences found when comparing the values in an inert medium and in a hydrogenating medium clearly illustrate and quantify the hydrogen embrittlement effect, since losses of almost 40% occur when an inert medium such as air is replaced by a hydrogen-promoting one.  
Despite these, the research has shown that the analyzed steel does not obey a fracture criterion based on a single toughness constant, such as K\({}_{\text{lc}}\) or J\({}_{\text{lc}}\), but its crack growth behavior is consistent with the cohesive model. Hence, a design application of this finding would consist of limiting the service tensile load of the bars as a function of environment aggressivity in order to assure their structural integrity in the presence of a postulated crack. The cohesive strength would be measured with SENT specimens, and the numerical implementation of the cohesive model for cracked bars would provide the designer with curves such as those resulting from the present research, i.e., load vs. CMOD. The combination of both results would allow for a safe load level to be adopted.  
In addition, it was found that the lath martensite microstructure is favorable to the hydrogen-embrittlement mechanisms like HELP and HEDE, since the high densities of dislocations and interfaces that it entails facilitate plastic deformation and decohesion which, in terms of the cohesive model, are equivalent to the loss of cohesive strength. The morphologies found on the fracture surfaces of the tested specimens confirm the hydrogen action ahead of the crack front by showing how they gradually transform from brittle to quasi-ductile from the onset of assisted cracking up to the unstable fracture process that causes the specimen collapse.  
**Author Contributions:** Conceptualization, M.I. and A.V.; methodology, P.S. and A.V.; validation, M.I. and P.S.; formal analysis, P.S.; investigation, P.S., M.I. and A.V.; resources, M.I. and A.V.; writing--original draft preparation, M.I. and P.S.; writing--review and editing, M.I. and A.V. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The authors gratefully acknowledge the RTI-2018-097221-B-100 grant funded by MCIN/AEI/ 10.13039/501100011033 and the \"ERDF A Way of Making Europe\", as well as PRE-2019-088263 grant funded by MCIN/AEI/10.13039/501100011033 and the \"ESF Investing in Your Future\".  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The data presented in this study are available upon request.  
**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.