PSI - Issue 5

Lars Sieber et al. / Procedia Structural Integrity 5 (2017) 1019–1026 Sieber, Stroetmann / Structural Integrity Procedia 00 (2017) 000 – 000

1020

2

1. Introduction

Many steel structures from the 19th and early 20th century are still being used today although their expected useful life has been significantly exceeded. As well as bridge structures, structural steelwork makes up a significant part of existing buildings. Frame buildings and high storey buildings from the 20th century play a particular role. Due to heritage preservation and also economic reasons, it is of significant interest to ensure the safe usage of these buildings. Considering different cases of damage in old structural steelwork (see, e.g. Klinger et al. (2011)) the question of brittle fracture safety is important, as well as the safety of structural elements and connections concerning stability and strength failure. The procedure of choosing steel grade to avoid brittle fracture according to Eurocode (EN 1993 1-10) is used for the construction of current structural steels in welded structures. The particular properties of old mild steel which are produced in the Thomas-, Bessemer- or Siemens-Martin-procedure were not reflected. This is also the case concerning significantly lower notch effect of riveted connections. Nevertheless, it can be noticed in assessment practice that the limits of the Charpy impact energy according to EN 1993-1-10 are often used to evaluate old structural steels. This can lead to miscalculations of toughness specifications and unnecessary reinforcement measures or the precautionary dismantling of a structure. It is known from previous fracture mechanical material tests (e.g. by Hensen (1992) or Langenberg (1996)) that old mild steels can have sufficient toughness to withstand brittle component failure even at low temperatures. However, the evaluation of the safety against brittle fracture using fracture-mechanical methods is not widespread and has been up to now limited to selected structures in particular cyclically strained bridges. This can be attributed to the extensive experimental determination of the fracture toughness as well as the partially necessary complex FEM calculations.

2. Experimental determination of material toughness

2.1. Representative material samples and material investigations

It is already known from the investigation by Reiche (2000) or Helmerich (2005) that the chemical, metallurgical and mechanical characteristics of old structural steels may differ considerably. In order to capture a large range of mild steels, material samples from different structures and elements of different construction years were chosen for these investigations. Therefore, only material samples of structural steel buildings were analysed, since the steel quality has usually been lower than in bridge engineering. The results of the chemical analysis are concluded in Table 1 and contrasted with the average concentrations of typical mild steels by Reiche (2000). The test material includes Thomas steels with high content auf nitrogen (e.g. PA2) as well as Siemens-Martin steels (e.g. M3). All material samples are rimmed steels.

Table 1. Results of the chemical analysis

chemical contant [%]

sample (cross section)

C

Mn

Si

P

S

N

O

DT200 (I200) DT260 (I260) M31 (L120x13) M56 (L110x12) SGM21 (L80x8)

0,03 0,10 0,07 0,15 0,09 0,07 0,03 0,04 0,05 0,09

0,27 0,72 0,48 0,36 0,23 0,67 0,32 0,41 0,46 0,48

0,001 0,001 0,001 0,001 0,001 0,001 0,002 0,001 0,009 0,008

0,049 0,095 0,024 0,018 0,087 0,103 0,053 0,060 0,051 0,035

0,029 0,102 0,043 0,091 0,089 0,079 0,085 0,084 0,044 0,038

0,0135 0,0250 0,0115 0,0080 0,0210 0,0225 0,0250 0,0190 0,0140 0,0050

0,0110 0,0155 0,0385 0,0100 0,0160 0,0150 0,0710 0,0570

PA2 (L60x8)

DB_G1 (L100x65x11) DB_G3 (L100x65x11)

Thomas steel

- -

Siemens-Martin steel