TESTING FACILITIES

All of the experimental testing was carried out in the Transport Systems Research Laboratory (TSRL) in the School of Engineering Sciences at the University of Southampton, the majority of which took place using the FORTRESS structural testing rig. In addition an Instron 8802 test machine with a 100 kN load cell was used to conduct some static tests.

4.2.1 FORTRESS

FORTRESS was developed as a multi-axial test facility in the early 1990’s specifically for testing large marine composite structures. In its present configuration a maximum of 209 kN can be generated using the current hydraulic pressure output connected to a hydraulic ram with a rod diameter of 110 mm. Load is acquired via an Instron 250 kN dynamic load

cell and position via a displacement transducer attached between the rig and the load cell. An Instron 8800 controller and associated software can control the loading of the test structure in either load or position control (i.e. in kN/min or mm/min). The controller is capable of producing linear and bilinear ramps of either load or displacement. In addition a sine, triangular or square wave function can be produced to perform fatigue tests. The control software can be used to acquire load, position and time data at a maximum frequency of 500Hz. In addition to the INSTRON data acquisition software, an 8-channel analogue to digital conversion card is used with the Wolfson Unit† software TURBO AD. This data acquisition system can acquire from the INSTRON test equipment and can also acquire data from additional sensors such as displacement transducers, temperature sensors, strain gauges, etc.

Incorporated within FORTRESS is an environmental chamber (FEARLESS) which allows specimens to be loaded while in a hot, humid environment. The chamber is capable of producing temperatures of 120°C and a relative humidity of 95 percent. Load can be transferred through the walls of the chamber by the use of stainless connecting rods. An adaptable specimen mounting and the load transfer system were developed specifically for testing of both the HSC and DLHC joints. Figure 4.1 shows a schematic representation of the test rig, specimen arrangement and the various fixtures used for each test.

The FORTRESS/FEARLESS testing facility was used to carry out the axial static strength, fatigue life and axial residual strength tests of both the HSC and DLHC joints.

4.2.1.1 FORTRESS parameters

A short study was made of the rig and its behaviour using the test fittings associated with both the DLHC and HSC configurations. The study involved instrumenting a solid steel bar with strain gauges and tested in FORTRESS using the fittings associated with the testing of the hybrid connections. The strain bar was loaded to a predetermined level and the displacement was acquired from the control transducer and from a transducer spanning the specimen. In addition, the signal from the strain gauges was acquired. Figure 4.1 shows schematically the position of the displacement transducers and strain gauges. From the strain gauges the displacement of the steel specimen was obtained. The difference between

the three displacements, control, span and strain, provides information on the flexibility of the rig, test specimen fixtures and specimen respectively. Figure 4.2 shows the difference between the control and span transducers, the relationship between these two curves is almost linear with respect to load. Therefore, it can be said that the relationship between the rig movement and applied load can be described as linear.

Based on the results obtained it is estimated that the rig has a stiffness of approximately 20 kN/mm and the immediate fixtures have a stiffness of 160 kN/mm. The strain bar has a stiffness of approximately 3000 kN/mm. The values obtained from the rig flexibility tests are applied most importantly to the numerical modelling for the definition of the boundary conditions and their application is described in detail in Chapter 7. However, the rig stiffness is important with respect to the experimental results and the determination of the global stiffness of the joint rather than the global stiffness of the joint plus the test rig. Due to the configuration, construction and size of FORTRESS and the magnitude of the applied loads it is inevitable that there will be some deformation of the rig and its fittings while subjected to possible loads in excess of 18 tonnes. Account of this movement must be made not only for the analysis of the experimental results but also for the numerical simulation in terms of boundary conditions. This became a factor when the HSC and DLHC were tested as the failure loads were in excess 100 kN and 160 kN respectively. For example, this resulted in rig movement in excess of 10 mm when testing the DLHC to failure.

4.2.2 INSTRON 100 KN TEST MACHINE (INSTRON A)

A standard servo-hydraulic 100 kN Instron test frame was used to perform a small number of tests either for material characterisation or for residual strength purposes. An Instron 8800 controller controls the actuator and data is acquired via a Strawberry Tree Data Shuttle and stored using the DasyLab suite of software. Out-of-plane static and residual strength tests were carried out using this test equipment.