PSI - Issue 77

P. Santos et al. / Procedia Structural Integrity 77 (2026) 339–347

346

8

P. Santos et al. / Structural Integrity Procedia 00 (2025) 000 – 000

Table 3. Cohesive strength values Y J and Y  of the bar steel.

a/W

 m [MPa]

dJ/d   [MPa]

Y J [MPa]

Y  [MPa]

0.170 0.186 0.190 0.219 0.297

836 822 842 815 661

935 937 920 948 821

1133 1159 1143 1224 1183

1213 1241 1282 1336 1337

1281 ± 4%

1169 ± 3%

The standard deviations of 3% and 4% of the two series of values obtained for the set of five tests carried out are in line with what was expected in tests designed to measure fracture toughness (ASTM E399, 1997). The respective mean values (1169 and 1281 MPa) are in perfect agreement with each other and with the physical meaning attributed to the cohesive resistance in the first versions of the cohesive model, that is, the yield strength of an ideally plastic material with no strain hardening. Besides, for applying theoretical formulations involving ideal plasticity to materials with low strain hardening capacity as the steel here examined, the tensile strength is often adopted as ideal yield strength. The tensile strength of the steel is 1138 MPa or 1268 MPa depending on whether it is expressed as engineering or true stress, which should be the respective values to be compared with the values of Y J and Y  because of the change from small to large strain regime occurred at the cohesive zone accompanying instability. The highest difference with respect to those given in Table 3 is 3%. 5. Conclusions The cohesive crack model has been particularized by analytical methods to the configuration of SENT specimen in order to determine the type of J-integral data resulting from applying the BS8571 standard to materials whose fracture behaviour fits this model. The analytical results have shown that the cohesive crack theory reproduces the experimental behaviour of an ultrahigh-strength, lath martensitic steel for construction bars when tested according to this standard to measure its resistance to crack propagation in inert environments. Fracture begins when the extension of the cohesive zone and the loading process become unstable, with the corresponding load being an asymptotic value determined by the crack size and the cohesive resistance. In addition, the J-integral value becomes a linear function of CMOD after the small- scale regime vanishes, with the corresponding slope being also determined by the crack size and the cohesive resistance. A full coincidence was found between the values of the cohesive resistance obtained from the series of five fracture tests performed. This occurs both with the values obtained from the maximum of the load-CMOD curve and with those derived from the constant slope part of the J-CMOD curve. Moreover, the two mean values are consistent with the physical meaning of the cohesive resistance since these fully coincide with the tensile strength of the steel, respectively expressed as engineering or true stress, because of the large geometry changes of the cohesive zone when instability develops. Acknowledgements The authors gratefully acknowledge receipt of the grants RTI-2018-097221-B-I00 and PRE 2019-088263 funded by MCIN/AEI/10.13039/ 501100011033, “ERDF A Way of Making Europe” and by “ESF Investing in Your Future”.