PSI - Issue 75

Kevin Mouradian et al. / Procedia Structural Integrity 75 (2025) 616–624 Kevin Mouradian et al. / Structural Integrity Procedia (2025)

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1. Introduction Weld imperfection can occur during the fabrication process and may compromise the structural integrity of steel connections. One of the most prevalent imperfections is the undercut, defined as a groove at the edge of the weld bead that is not filled by weld metal. Undercuts typically result from high welding amperage, excessive welding speed, or incorrect electrode angles. In order to regulate the acceptability of such imperfections, most documents refer to the ISO 5817 (CEN 2023a) quality system, which specifies acceptance criteria and categorizes imperfections into three quality levels: B, C, and D, with quality level B representing the most stringent requirements. In Europe, steel structures subject to fatigue are designed according to prEN 1993 ‑ 1 ‑ 9 (CEN 2023b), while their fabrication has to comply with the quality, execution, and inspection requirements of EN 1090 ‑ 2 (CEN 2024). For welded connections under fatigue loading, prEN 1993‑1‑9 requires an execution class EXC3, which corresponds to ISO 5817 quality level B. However, the choice of quality level B was not originally based on scientific validation, as ISO 5817 was primarily developed to standardize the interpretation of non-destructive tests (NDT) results. Other documents such as EN 13001 ‑ 3 ‑ 1 (CEN 2018) for crane design permit quality level C contrary to prEN 1993‑1‑9 . In addition, documents like the International Institute of Welding guidelines (IIW 2016) on weld quality in relation to fatigue strength and XP ISO/TS 20273 (AFNOR 2017) are among the few references that provide guidance on the relationship between weld imperfections and fatigue strength. Few studies investigated the impact of undercut on the fatigue strength of welded connections. Bartsch & Feldmann (2022) performed fatigue tests on load-carrying cruciform connections with a 0.5-0.6 mm undercut height in a 15 mm plate thickness. The results indicate a substantial reduction in fatigue strength at the weld toe in comparison to other tested specimens. Steimbreger & Chapetti (2017) performed numerical simulations on 19 mm thick butt weld connections to establish a relationship between undercut dimensions and fatigue strength. Their findings highlight the substantial impact of undercut radius and height on the fatigue resistance. In a previous study, Mouradian et al. (2025) analyzed experimental tests from the background document of prEN 1993 ‑ 1 ‑ 9 (Sedlacek et al. 2014 ) and defined a group of test results labelled “filtered database”. The specimens selected for this group were based on different parameters, including dimensions, material properties, pre/post welding conditions, loading types, and stress ratios. Based on the findings of that study, the present work investigates the influence of undercut on the fatigue strength of butt weld connections. Section 2 provides an overview of the tested specimens, while Section 3 details the non-destructive testing used to characterize imperfections. Sections 4 and 5 present the fatigue test results and first conclusions.

Nomenclature r

Weld toe radius Weld toe angle Undercut height

α

h und h ali α ali

Linear misalignment between connected plates Angular misalignment between connected plates

Excess weld metal height

h w

Length of imperfection in longitudinal direction of the weld

l

Δ σ

Stress amplitude under cyclic loading Number of cycles until complete rupture

N

Δ σ c

Characteristic fatigue strength at two million cycles

2. Design and fabrication of specimens An experimental campaign was conducted to investigate the impact of undercut on the fatigue strength. 12 butt weld specimens, extracted from 4 larger workpieces using a milling process, were tested. Six specimens, designated as BW_TYPE, were welded to achieve ISO 5817 quality level B, thus serving as reference specimens. The remaining six specimens, designated as BW_UND, included undercuts. Each specimen featured a weld length of 40 mm and a