Block Shear Strength Estimation of STS304L Stainless Steel Fillet-Welded Connection with Base Metal Fracture

1. Introduction

Stainless steel has excellent characteristics such as corrosion resistance, ductility, high strength, temperature properties, life-cycle cost benefit and recyclability. However, commonly used austenitic stainless steel (STS304¹ corresponds to ASTM 304L type²) exhibits intergranular corrosion in sensitized temperature range of 550^oC to 800^oC and is caused by the formation of chromium carbides at grain boundaries. Dissolved metals exhibit lower corrosion resistance than grain themselves because of chromium depleted line. Intergranular corrosion can be prevented using low carbon grade of stainless steel, STS304L (corresponds to ASTM 304L type,² which contains lower carbon and higher nickel compared to STS304).

As a basic research for establishing the design manual of stainless steel members, monotonic tensile experiments for cold-formed austenitic stainless steel (STS304L)¹ fillet-welded connection with the block shear fracture in base metal have been conducted. Recently, defect analysis and hardness test of cold metal transfer (CMT) welding coated steel have been conducted to provide the reliable data of welded connections between aluminum alloy(AL 6021) and galvannealed steel (SGARC340).³ Block shear is combination by failure surface that includes one or two longitudinal shear planes and one perpendicular tension plane. Block shear provision for carbon steel bolted connection in the 2010 AISC⁴ Specification and KBC 2016⁵ for Structural Steel Buildings considers a combination of net-section tension plane fracture and yielding (or fracture) of the gross (or net) section shear plane(s), with the capacity being taken as the lesser of the following two Eqs. (1) and (2).

Topkaya^6,7 proposed block shear failure equation of gusset plates with welded connections based on the experimental and numerical findings. In the development of this equation, triaxial stress is assumed that the gross section tension plane develops 25% higher stress in excess of F_u

Oosterhof and Driver⁸ suggested “unified equation” that provides more consistent block shear capacity predictions with net section tension plane than Eqs. (1) and (2).

However, most of previous studies and proposed equations stated above were focused on the block shear fracture of base metal in carbon welded connections. ASCE specification for the design of cold-formed stainless steel structural members⁹ provides the design fillet-weld strengths of tensile fracture and shear fracture according to loading directions against welding axis not including block shear strength of welded connections. Recently, experimental studies on the ultimate strength of austenitic stainless steel (STS304, corresponds to ASTM 304L type)² welded connection with weld metal fracture for three types according to loading directions against welding axis. As a result, modified equations were proposed and their validation for estimating the ultimate strength of welded connection was verified.¹⁰

In this paper, experimental research has been carried out to investigate the ultimate behaviors of cold-formed austenitic stainless steel (STS304L) fillet-welded connections by TIG (tungsten inert gas) welding with block shear fracture in base metal.

2. Experimental Plan and Material Properties

2.1 Experimental Plan

A total of seven specimens for austenitic stainless steel 304L fillet-welded connections was planned as depicted in Fig. 1. Table 1 shows the list and geometric information of the specimens. Two 3.0mm thick plane plates and two 10.0 mm thick plane plates were joined with weld of nominal fillet size, 3.0 mm. Electrode for TIG welding of STS304L plates was chosen as Y308L (minimum specified strength of F_xx = 580) with diameter of 3.2mm specified in KS D 7026 (corresponds to ER308 in AWS A5.9). The upper weld is deformable test part and the lower weld means fixed rigid part. Specimens were clipped by upper and lower fixtures of 2000 kN UTM. Tensile force was applied to the specimen with a displacement control mode in order load-displacement (stroke) curves of the specimens.

Fig. 1

Specimen configuration (SALT30L30) [Unit:mm]

Table 1.

List and geometry of specimens

2.2 Material Properties of Base Metal

Tensile coupon test for STS304L (austenitic stainless steel, corresponds to ASTM 304L type) 3.0 mm thick plane plates was conducted to obtain material properties and six coupons with two kinds (for example, 304L3T-T means coupon extracted from transverse direction and 304L3T-L means coupon extracted from longitudinal direction) were cut according to the rolling direction. Material test results for base metal are included in Table 2. Stress-strain curves for each type were exhibited in Fig. 2. Since JIS G 4321¹¹ definition may be reasonable to provide a sound deflection control at the serviceability limit state and more conservative value in estimating the width to thickness ratio for plate local bucking strength, 0.1% offset method in this paper was adopted for deciding the yield stress of stainless steels with no clear yield plateau based on JIS G 4321.¹¹ ASTM A240/A666² and KS D 3698¹ requirements for minimum yield strength in STS304L are 170 MPa and 175MPa, respectively. Minimum tensile strength requirements in STS304L are 485 MPa and 480 MPa, respectively. Mean elastic modulus for 304L3T-L coupon is 179.59 GPa, yield strength is 289.42MPa and tensile strength 753.34MPa. Mechanical properties of test coupons satisfy the requirements of ASTM and KS.

Table 2

Coupon test results for STS304L base metal

Fig. 2

Stress-strain curves for material (STS304L) for 3.0 mm thick plate

3. Experimental Results and Strength Comparison with Design Equations

3.1 Experimental Results

Figs. 3 and 4 display load-displacement curves and block shear fracture shapes of specimens obtained from test results. All of the specimens were tested under tension and fracture mode was determined at ultimate state. Structural behaviors of all specimens were similar. During the early stage of loading, slip took place at the base metal. Since austenitic stainless steel has a low yield ratio (F_y/F_u) and relatively high elongation, after the occurrence of slip, the load-displacement response was followed by a long yield state until specimens reached the ultimate state. A single fracture or two symmetrical fractures were observed in weld connection away from the toe of the transverse weld connection at the corner of the weld connection. All of welded connections failed by block shear fracture (tensile fracture → shear yielding or shear fracture) as shown in Fig. 4.

Fig. 3

Comparison of load-displacement curves

Fig. 4

Fracture shape of specimens and sequence

3.2 Strength Comparison

Table 3 shows test ultimate strength (P_ue), actual plate thickness (t_e), average weld size (S_e) and fracture mode. Actual weld leg sizes (S_e) were larger than nominal weld size (S_n = 3 mm) as can be seen in Table 3. Modified ultimate strength (P_uem) was calculated based on modification factor (m = t_n/t_e), which was obtained from compensation based on measured plate thickness (t_e) and nominal plate thickness (t_n = 3.0mm) to compare with ultimate strengths for specimens with identical plate thickness according to weld lengths.

Table 3

Test results

Strength ratios (P_uem/P_{uem_Lmin}) to specimen with the shortest transverse weld length ranged from 1.00 to 1.32 and ultimate strength got higher with the increase of transverse weld length.

Table 4

Strength ratio according to welding length

3.3 Development of Block Shear Strength

It is found that for fillet-welded connections with block shear fracture were underestimated by current design specifications with no consideration of triaxial stress effect and material property difference between stainless steel and carbon steel as shown in Table 5. As already discussed in the introduction, Topkaya^6,7 considered the block shear fracture in fillet-welded connection with carbon steel. Compared with carbon steel, stainless steel is lower to the yield ratio. The yield ratio (yield-to-tensile strength ratio) is important factor for estimating deformation resistance and the lower yield ratio reflects the higher resistance to deformation. Therefore, modified strength equation of Eq. (5) with tensile stress factor and proposal equations. (U_t = 1.35) for gross tension fracture provided strengths closest to test results was recommended in this paper.

Table 5

Comparison of design strength and test result

$\[ P_{ut}=\frac{F_u}{\sqrt{3}}{A}_{gv}+1.35F_u{A}_{gt} \]$

(5)

3.4 Comparison of Test Results and Design Strengths

AISC2010 specification⁴ and KBC 2016⁵ for hot-rolled carbon steel and existing literature provide the design equations^6-8 (Eqs. (1)-(4)) to predict the ultimate strength of block shear in fillet-welded connections with base metal fracture. Table 5 displays test strengths (P_ue) and design strengths (P_ut) predicted by AISC2010 specification⁴, existing design equations^6-8 by other researchers and proposal equations. It can be seen that design strengths (Eqs. (1) and (2)) predicted by AISC2010 specification¹ (P_ut) underestimated overly the ultimate strength (P_ue) of test results (mean strength ratio, P_ut/P_ue=0.46 with corresponding COV of 0.071). Topkaya’s equation^6,7 (Eq. (3)) and prediction by Oosterhof and Driver’s equation⁸ (Eq. (4)) also was conservative by 8%, 35% (the mean strength ratios, P_ut/P_ue are 0.92, 0.65 with corresponding COV of 0.017, 0.055, respectively). Proposed equation (Eq. (5)) provided more accurate block shear strength (mean strength ratio, P_ut/P_ue is 0.95 with corresponding COV of 0.023) for base metal fracture in stainless steel welded connection. Comparison of test ultimate strengths against design strengths calculated according to the design equations are plotted in Fig. 5. Data points appearing below the diagonal line indicate that design equations overestimate the strength of welded connection, while data points above the line indicate that design predictions are underestimated (see Fig. 5).

Fig. 5

Comparison of test results and design strengths

4. Conclusions

Experimental study for investigating the block shear behaviors (fracture shape and ultimate strength) fillet-welded connections fabricated with cold-formed austenitic stainless steel (STS304L, corresponds to ASTM 304L type, which exhibits high intergranular corrosion thanks to the content of low carbon) by TIG welding was conducted. All of specimens failed by block shear fracture in base metal not weld mental. Main variables of the specimens were weld lengths (longitudinal weld length and transverse weld length) according to welding direction against applied force. Block shear strengths got higher with the increase of weld length.

Test block shear strengths were compared with those by AISC2010 specification and equations suggested by other researchers (Topkaya and Oosterhof & Driver) for carbon steel welded connections. The existing equations underestimated the block shear strength of STS304L welded connection by 8%-64%. Block shear fracture for welded connections with base metal fracture was affected by stress triaxiality unlike that of the bolted connections. Proposed strength equation (Eq. (5)) with equivalent tensile stress factor (U_t = 1.35) for gross tension fracture and shear stress factor ($$ =1\sqrt{3}$$) for gross shear fracture provided the block shear strengths closest to those of test results. In the near future, more reasonable estimation of block shear strength of STS 304L stainless steel welded connections with base metal fracture are required to be made considering the stress triaxiality effect and shear factor for cold-formed stainless steel welded connection using parametric finite element analysis.

Acknowledgments

This paper was presented at ISGMA 2017

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (KRF-2015R1D1A3A01016603).

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