PSI - Issue 41

Hendrik Baarssen et al. / Procedia Structural Integrity 41 (2022) 183–191 Baarsen et al. / Structural Integrity Procedia 00 (2022) 000–000

184

2

the smallest value of the design ultimate resistance of the net cross-section, N u , Rd , and the design plastic resistance for the gross cross-section, N pl , Rd :

0 . 9 A net f u γ M 2

N u , Rd =

(1)

A gross f y γ M 0

N pl , Rd =

(2)

2 ] is the area of the gross cross-section, A

2 ] is the area of the net cross-section, f

2 ]

where A gross [ mm

net [ mm

u [ N / mm

is characteristic (nominal) value of the ultimate tensile strength, f y [ N / mm 2 ] is the characteristic (nominal) value of the yield strength, γ M 0 [-] is the partial factor for the resistance of cross-sections for every class, and γ M 2 [-] is the partial factor for the resistance of cross-sections in tension to fracture. In Equations 1-2 the partial factors ensure a predetermined safety level, and are statistically determined based on experimental data and may be di ff erent for each European country as specified in the national annexes. Omitting the partial factor allows the evaluation of the resistance function:

N u = 0 . 9 A net f u

(3)

The factor 0.9 in Equation 1 can be interpreted as an additional safety factor. The commentary to Eurocode 3 Sedlacek et al. (2008) reports that the reason for this factor is: (1) evaluation of tension tests with bolted connections, (2) con sistency with the resistance formula for bolts in tension from test evaluations of bolt tests, and (3) fracture mechanics safety assessments. For what concerns point (3), two main assumptions have been made in Sedlacek et al. (2008). The first one is that the toughness of the material is so high that Equation 2 ensures safety independent from the crack size, i.e. Equation 2 does not require the crack size as an input. The second assumption considers that when the first assumption is not verified for all the possible crack sizes, the load leading to brittle fracture is still larger than the load for the collapse of the net section for a certain permissible crack size, large enough to be timely detected. For a brief survey of the early literature on net cross-section resistance, the reader is referred to Rombouts et al. (2014). Recently, Snijder et al. (2017) statistically assessed Equations 1 and 2 based on a numerical model calibrated on the basis of experimental data on notched plate specimens. The conclusion is that a modified net cross-section design rule omitting the factor 0.9 is su ffi cient with a partial factor γ M 2 of 1.17 when the occurrence of cracks is not considered. This conclusion is similar to previous research of Mozˇe et al. (2007) who assessed low ductility failure mechanisms of specimens with holes and bolted connections. Similarly, Feldmann and Scha ff rath (2017) concluded that the reduction factor of 0.9 could be omitted for specimens with holes of all steel grades, if the existence of cracks could be excluded. Otherwise, the reduction factor should be applied. Mozˇe and Beg (2014) found that the design rule is applicable also to high strength steel. The assessment of acceptability of flaws in metallic structures can be carried out performing Engineering Critical Assessment using the Failure Assessment Diagram (FAD), as described in the BS7910 standard, among others BSI (2013). A special issue of the Journal of Pressure Vessel and Piping has been recently devoted to the latest improve ments of this guideline, as briefly described in Hadley (2018). The FAD is based on Elastic-Plastic Fracture Mechanics and it allows to consider the interaction between brittle and plastic failure. The present paper presents the results of a preliminary experimental investigation concerning the applicability of Equation 1 to rectangular plates with a single centered hole and cracks emanating from it. To the best author’s knowledge, no prior experimental investigations are done to determine the tensile strength of cracked steel plates with a hole for the purpose of assessing Equation 1. The experimental results are used to evaluate the performance of Equation 1 with respect to a S275JR steel. The monotonic strength properties of the material have been characterised.