La geografía y la época con las colocaciones

This phase of both the DLHC and HSC test programmes was undertaken in the FORTRESS test facility for axial tests and in Instron A for out of plane tests. Figures 4.3 and 4.4 show the test set-up for the DLHC and HSC specimens respectively for axial tests. The static tests were conducted in position control to observe any stress relief caused by joint damage during loading. The rate of loading for the static testing was 1 mm/min. Load and ram displacement were obtained for the HSC and additionally span displacement was acquired for the DLHC. The results were presented in the form of load-displacement curves.

Fatigue testing was conducted in the same configuration as that used for the static testing. However, the tests were carried out in load control to a percentage of the ultimate failure load of the specimen as found from the static tests.

In addition to the axial testing, 4-point static bend tests were conducted on the DLHC using Instron A. Figure 4.5 shows the test set-up for this phase of the testing.

4.3.1.1 Loading conditions for hybrid connections

The aim of this section is to justify the loading conditions applied to both the HSC and DLHC. In the present research three loading scenarios are used. For the HSC the axial load is compressive and the bending loads are applied using a 4-point bend test. For the DLHC the axial load is tensile and the bending load is a 4-point bend test.

The HSC has been previously defined as a likely candidate for a full-scale structural connection and therefore the envisaged loading on such a joint should be considered with respect to its location and function onboard a naval vessel. The location of such a joint was described in Chapter 3 as being suitable for naval superstructures particularly helicopter hanger structures. These are most often found on the after deck of naval platforms.

It is assumed that the helicopter hangar structure does not contribute to the global bending strength of the hull girder due to its length being considerably smaller than the overall length of the hull, and is decoupled from the hull girder due to the differences in elastic modulus of the materials used for the hull (steel) and hangar (GRP) as discussed in Chapter 3. Therefore only forces induced by the structures own weight and accelerations due to motion are considered significant. With the vessel at rest and in calm water, the joint will experience only a compressive force equal to the apportioned weight of the hangar structure and any ancillary equipment mounted on it. The generation of tensile forces on the other hand, whilst unlikely, are not inconceivable. When the vessel is underway and is sailing in seas rather than calm water, it will experience motions in all six degrees of freedom. These motions translate to the connection between the hull of the vessel and the hangar superstructure. Assuming the hangar is located on the after-deck of the ship, for the connection to experience a tensile load or at most a relief of the compression loading due to the hangar’s structural weight, the aft end of the ship needs to experience one negative g for weightlessness or greater for tensile load. This scenario would be possible if the aft end of the ship was in free fall from coming off the back of a wave but in reality this would be highly unusual.

It is feasible that either the port or starboard side of the structure would be subjected to one or more negative g’s during roll. The size of the tensile force would be related to the position of the neutral axis of the hangar structure and the roll rate but again it is highly unlikely that one negative g will be experienced in reality and only a combination of roll and heave may in extreme circumstances produce enough acceleration to cause the structure to be weightless.

From the discussion of realistic loading scenarios described above, the most likely force to be encountered by the HSC is therefore compressive and to this end the testing of the strength and durability of the HSC has been carried out in compression. Although this is the loading condition assumed in the present research it should be noted that depending on the configuration of the superstructure it may contribute to the global hull stiffness and therefore be subjected to stress transfer from the hull. This may change the loading condition. However, this is not considered in the present research.

For the DLHC it would have been advantageous if the specimen were also tested in compression, as this would keep the loading scenarios consistent. However, due to the test facility (FORTRESS) containing the environmental chamber (FEARLESS), load had to be transferred through the wall of the chamber via a stainless steel connecting rod 30 mm in diameter. Loading this system in compression would have resulted in the buckling of the connecting rod. Therefore all static and fatigue tests on the DLHC are in tension. Due to the DLHC being a symmetric joint the difference between tensile and compressive properties, provided that buckling did not occur in compression, would produce similar results.

Both the HSC and DLHC specimens were tested in 4-point bending as this load situation would represent an externally applied force. It is known that this test set-up will result in zero transverse shear across the joint between the rollers, however this joint represents a thin skin commonly found in marine structures, and assumed not to contribute to the shear strength of the structure, the surrounding support structure, transverse framing, will provide the transverse shear carrying capability.

4.3.1.2 Boundary conditions for HSC

The final boundary conditions used for the static and fatigue testing of the HSC were decided upon after the response of the specimen to compressive loading. Due to the HSC specimen’s asymmetry the compressive load path will be eccentric. This will influence the response of the joint by inducing a lateral deflection of the specimen under compression. Although due to the load path eccentricity this is the expected response of the joint, in full- scale application it is unlikely. The HSC, as described in Chapter 3, is designed for application to a helicopter hangar structure. It can be expected that such a structure will incorporate an internal transverse framing system. This internal structure should prevent lateral bending of the joint. Therefore it is deemed unacceptable to allow such deflections in the experimental study. The effect of the lateral deflection on ultimate joint strength is examined in Chapter 6.

In order to prevent lateral deflection experimentally, a system of anti-bending guides were designed and consisted of a set of rollers mounted perpendicular to the specimen plane. These were rigidly attached to the test machine and are shown in the HSC set-up in Figure 4.4. A check was made to obtain the lateral stiffness of the anti-bending guide system by

the use of a dial gauge while a specimen was loaded. A small amount of lateral deflection was observed. This small amount of lateral deflection is expected to have an influence on the global axial stiffness of the specimen under compressive load compared to a fully rigid structure. The anti-bending guides were found to have a stiffness of 2 mm at an axial load of 100 kN. This was measured using a dial gauge on the specimen measuring lateral deflection.

It is expected that although the anti-bending guide system provides a realistic reduction in the lateral bending of the specimen while under compressive load, its presence may influence the damage mechanisms within the joint. However, in order to realistically constrain the specimen it is felt that the presence of the anti-bending guides is more beneficial than any influence it may have on the progression of damage.

4.3.1.3 Boundary conditions for the DLHC

The DLHC specimen was tested in axial tension and 4-point bending. For the axial tests the specimen was clamped at either end in a similar manner to the HSC. No anti-bending guides were required due to the tensile loading. The 4-point bending configuration is shown in Figure 4.5.

4.3.1.4 Fatigue data analysis

Data was acquired at regular intervals (in most long term tests after every 100 cycles) during each fatigue test. The data was analysed via a FORTRAN routine, which calculated the stiffness, using a linear least squares fitting procedure, and the area within the hysteresis loop, using the trapezium rule. It was felt that due to the density of the data acquired, approximately 100Hz, the trapezium rule provides sufficient accuracy and is an extremely robust and easy to apply method for data with varying interval values.

The area within the hysteresis loop can be equated with the energy lost by the test system [77]. If there were no loop, i.e. the loading and unloading of the test system against displacement follows the same line, the system could be described as perfectly elastic (Hooke’s Law) – the energy input into the system during loading is exactly the same as the energy output by the system during unloading. However, if, while testing a specimen, a loop began to form after a given number of cycles, this can be defined as energy being

dissipated by the system. If the loading and unloading part of the system is said to be initially ‘perfect’, then any resulting energy dissipation over time must be originating from the specimen itself, provided that the loading and unloading of the specimen is purely adiabatic. Therefore, a change or development of energy dissipation can be considered a sign of damage within the specimen [77].

In the case of the present study, it is known from the rig stiffness tests, described in Section 2.1.1, that the testing system is not ‘perfect’. This will therefore lead to the generation of a hysteresis loop as energy is dissipated in the form of rig movement. Therefore, during testing the change in energy dissipation (area inside the hysteresis loop) is monitored as an indication of specimen damage due to fatigue cycling, assuming no failure in the test rig. Both the energy dissipation and the stiffness are plotted against number of cycles to help to identify when damage due to cyclic loading occurs.