Fatigue Strength Of Quenched And Tempered Carbon Steel Plates And Welded Joints In Sea Water
Authors: Havens F.E. , Bench D.M. 
Published: October 2008

Fatigue tests were made on large welded and unwelded plate specimens of ASTM A537B steel in air and in aerated sea water. Welded joints and corrosive action of sea water each reduced fatigue strength Substantially but effects are not additive where both conditions exist simultaneously. However, fatigue strengths can be restored to nearly that of unwe1ded plate in each case. Grinding the weld reinforcement flush improves fatigue strength of welded joints by eliminating the stress concentration at the toe of the weld. Cathodic protection improves fatigue strength of unwe1ded plate in sea water by eliminating corrosion.

INTRODUCTION

Action of wind, waves, and sometimes ice floes produce alternating stress cycles in offshore drilling platform members that can cause fatigue failure at loads below static strengths of steels. How much the strength is lowered is dependent on (1) number of stress cycles, (2) stress ratio (minimum stress divided by maximum stress) and (3) stress concentrations. Economical fabrication necessitates welding of structures, but welding produces stress concentrations from three sources--mechanical notches, metallurgical notches, and residual stresses. Also, the corrosive action of sea water can further lower usable strength properties. This paper presents results of fatigue tests on 3/4n plates of ASTM A537B, a quenched and tempered carbon steel used extensively in construction of offshore drilling platforms where severe environmental conditions prevail. Large fatigue specimens were tested in air and aerated synthetic sea water to resemble structural members in actual service.

SPECIMENS

Chemical composition ranges and mechanical properties of ASTM A537B (Armco Super-Lo-Temp) steel are as follow:

Chemical Composition

Table of element and its range (available in full paper)

Mechanical Properties

3/16" to 1¼" Yield Strength, psi 60,000mmin. Tensile Strength, psi80-100,000 Elong. In 2", % 24 over 1¼ to 2" Yield Strength, psi 56,000 min Tensile Strength, psi75-95,000 Elong. In 2", %24

The fatigue specimen used for testing both butt welded and unwelded plate is shown in Figure 1. The specimen was 12" wide by 48" long by full plate thichness of 3/4", with the center section of the specimen reduced to 4" wide by mechining a 12-inch radius at each side. This specimen design produces fracture initiation at the narrowest point of the reduced section, assuring knowledge of stress at the point of fracture initiation. In butt welded specimens, welds were transverse to the direction of applied stress and located at the narrowest point of the reduced section, assuring knowledge of stress at the point of fracture initiation. In butt welded specimens, welds were transverse to the direction of applied stress and located at the narrowest point of the reduced section. Specimens were welded in the full 12" width and subsequently machined to the 4" wide reduced section. All welded specimens were X-rayed before testing to assure sound welds. Welding was donw manually using the shielded metal arc process. Details of welding are shown below:Joint: double vee, 60° included angle, .059" gap, a/16" land face