Experimental Programme

This section details the experimental programme carried out in this chapter. This includes details o f experimental test parameters used for each section of the investigation.

3 .

This part of the study was carried out on DILIMAX690E. The mechanical properties of this steel are shown in Table 3.1, while Table 3.2 shows its chemical composition. The data on mechanical properties and chemical composition were obtained from the steels supplier, Dillinger Hutte [3.3]. Two types of plates were tested; thick parent plates and flush ground welded plates. The specimen dimensions are shown in Figure 3.1. The welding procedures for the ground welded plates are in Appendix B. The objective was to

Chapter 3 Fatigue Testing o f High Strength Steels

study the fatigue performance o f the high strength steels in air and in seawater with CP and to compare results obtained from the two categories o f specimens. All of the eight tests in this part were carried out under constant amplitude loading. Two stress ranges were used, 350 MPa and 412 MPa with a cathodic protection level of -lOSOmV for the seawater tests. A summary of the test parameters is shown in Table 3.3.

3 .

The main objective in part 2 was to study the effects o f different levels o f CP on the fatigue life and crack growth behaviour in T-butt welded plates under variable amplitude loading. These tests were conducted using SE 702, a high strength steel with a yield strength o f over 700 MPa. The mechanical and chemical properties o f SE 702 are shown in Table 3.4 and Table 3.5 respectively. T-butt welded plates made from 40mm thick plates were used. Details of the specimen geometry are shown in Figure 3.1. Table 3.6 shows a summary o f the test parameters. The welding procedures for the T-butt welded plates can be found in Appendix C supplied by CLI. All the tests were conducted under variable amplitude loading having two equivalent stress ranges, 146 MPa and 172 MPa. Two levels of cathodic protection (-SOOmV and-1050mV) were used for the seawater tests.

3.1.3 Part 3: Effect of Sub-biock Duration

Part 3 studied the behaviour and effects o f different sea-state duration on the fatigue life and crack propagation of T-butt welded plates using the JOSH sequence. For most standard wave sequences, the generated sequence is made up from different sea-states with a fixed sub-block duration period for each state. The duration period is sometimes referred to as transition period. This is explained in greater detail in Chapter 5. Four tests were carried out at two equivalent stress ranges, 146 MPa and 172 MPa, for two different sub-block duration periods of 10 and 30 minutes. This was different from the test sequence employed in part 2, where a duration period o f 20 minutes was used. The test conditions were otherwise similar to the test conditions used in part 2 with a CP level of

Chapter 3 Fatigue Testing o f High Strength Steels

-1050mV. The main difference was in the sequence used. Two different variants of the JOSH sequence were generated with the required transition properties. A summary of the test parameters used is given in Table 3.7.

3 .

Metallurgical examination o f some of the fracture surfaces was carried out by Cranfield University. The sample specimens that were examined were the parent plate P3 and P4, the flush ground welded plates W1 and W2, and T-butt welded plates T04 and T08. These are summarised in Table 3.8. Hardness measurements, examination by optical microscopy and examination of the fractography by scanning electron microscopy were carried out.

3.1.5 Part 5 Investigation of Different Backing Conditions

In addition to the effects of different loading sequence and environmental conditions, the welding process can also affect weld quality and subsequently, the fatigue life. One such aspect is the use of a full penetration weld or in the case that was investigated here, different backing materials. A small number o f tests were carried out using SE702 to assess the performance o f two types o f T-butt welded plates; single sided full penetration weld with ceramic backing and single sided full penetration weld with metallic backing. The weld profile is similar to both type of plates, however for the metallic backing plate, the plate is still attached to the T-butt. The number o f tests and conditions are summarised in Table 3.9. Two equivalent stress ranges were chosen, 104 MPa and 146 MPa. Two seawater tests were also carried out at 146 MPa and at different protection levels.

Chapter 3 Fatigue Testing o f High Strength Steels

3.1.6 Long Life Tests (LLT)

In order to look at the long life VACF behaviour of high strength weldable steels, seven fatigue tests were carried out at different CP levels at UCL by Bowen to form part of the Health and Safety Executive Offshore Technology Report [3.2]. The material DILLIMAX690E-Z15, a Jack-up high strength steel was used to fabricated the 16 mm thick T-butt welded plates. Dillinger Hutte supplied the material in a different composition to the 85mm plate tested in part 1. The material properties are shown in Table 3.10. The different high strength steels material compositions from different sources are shown in Table 3.11. This is quite normal to achieve similar strength characteristically for different plate thickness. The 20 minutes sub-block duration variable amplitude sequence was employed for fatigue testing.