## 1 Introduction  
The versatility and loading precision of ultrahigh-strength tendon rods for civil engineering are making them increasingly important in structural applications governed by durability and resilience criteria. This trend requires updated knowledge concerning structural integrity when service conditions entailing combined mechanical and environmental actions might promote the occurrence of failure by stress corrosion cracking or hydrogen embrittlement [1, 2, 3, 4, 5].  
Despite the relatively simple geometrical configuration of the high-strength steel bars used as tendon rods in modern structures, they are hardly compatible with most of the specimen configurations used in fracture mechanics tests. This complicates the experimental determination of steel bar resistance to hydrogen-assisted cracking, especially when deformation measurements are required on samples exposed to environments able to promote hydrogen uptake. Thus, the classical, single-edge bending (SEB) specimens and the varieties of compact tension (CT) specimens of the ASTM E1820 [6] and ASTM E1737 [7] standards for measuring crack growth resistance by means of the J-integral do not provide the optimal conditions. However, the recently adopted fatigue precracked single-edge notched tension (SENT) specimen by the BS-8571 [8] standard overcomes this issue. In particular, it has the advantage of not requiring displacement measurements along the load line to determine the strain energy absorbed by the specimen. This feature facilitates the experimental determination of the J-integral [9; 10; 11; 12; 13] and eases the use of digitalized video extensometry measurement systems [14; 15]. In addition to this, the standard explicitly considers the possibility of performing tests in liquid media, which makes it a suitable tool for assessing the hydrogen-assisted cracking resistance of the bar steel and its subsequent comparison with the steel under the hydrogen-free condition. Therefore, the BS-8571 standard [8] practically offers a laboratory method for assessing the effect of combining the mechanical action applied to a SENT specimen with the embrittlement action resulting from the steel's exposure to a hydrogen-promoting solution. The adopted solution is that prescribed in the recently updated standard, UNE-EN ISO 15630-3 [16], used to qualify high-strength steel bars for prestressing concrete according to resistance to hydrogen embrittlement.  
Accordingly, this work addresses, not only from an experimental, but also from a theoretical point of view, critical aspects regarding the mechanical behavior of an ultrahigh-strength tendon rod steel of lath martensitic microstructure, recently developed for highly demanding civil engineering applications related to structural integrity. Its improved mechanical properties are related to the hierarchical microstructure resulting from the prior austenite grains divided into the structural units of lath martensite, sub-blocks, blocks and packets [17; 18], as well as due to the presence of discrete inclusions coming from steel microalloying [19; 20; 21].  
The newly analyzed aspect of the present research is the resistance of the bar steel to non-assisted and hydrogen-assisted cracking, namely the resistance to crack growth caused by mechanical loading in both an inert medium and a medium capable of steel embrittlement by hydrogen uptake, on precracked SENT specimens. The experimental analysis is complemented by a theoretical approach, which involves the Dugdale cohesive model applied to the SENT configuration with the aid of the Green function proposed by [PERSON] [22] and the reciprocity theorem reformulated by [PERSON] for cracked Hookean solids [23]. The theoretical results regarding the applied load--CMOD (crack mouth opening displacement) and the J-integral--are compared with the experimentally obtained data, for both studied media in order to determine the cohesive resistance of the steel and its variation with hydrogen exposure.