Lateral loading phase

Lateral loading of the ROPS was performed gradually using a 50 tonne hydraulic jack that was securely mounted to the loading frame。 The load was applied to the upper portion of the right hand side post of the ROPS as seen in Figure 3。 A 100x200x20 section mild steel plate was used to spread the load and alleviate localised deformation。 A double ball and seat arrangement was incorporated into the loading system to prevent damage to the hydraulic jack。

During this testing phase, the energy absorption requirement of the standard was not reached after attainment of the minimum lateral load of 121 kN, as evident from the resulting load deflection profile in Figure 5。 At this stage, the ROPS was still predominantly in its elastic state with very little deflection taking place and therefore little energy absorption as well。 Loading was increased up to approximately the 200 kN mark, which saw the initiation of plastic hinges at the top and base of each post。 The load deflection response started to soften at this stage where the lateral deflection was about 35mm。 Further loading showed the formation of plastic hinges。 The load deflection profile of the ROPS plateaued for a short period of time and then began to fall gradually until it reached a peak deflection of 70mm at approximately the 175 kN mark。 At this level of deflection, the area under the  load deflection profile had equated to the code requirement of approximately 12100 J。 During the loading sequence, the ROPS had undergone significant plastic deformation at the top and base of both posts, which changed the cross sectional shape by inward folding at the compressions faces of the members。 This feature was also observed during Finite Element analysis of the ROPS under lateral load。 The load deflection response in Figure 5 indicates 60mm of permanent plastic deformation in the lateral direction。

The permanent plastic deformation sustained by the ROPS was monitored accurately with the strain rosette readings taken at consistent intervals throughout the test。 Visual inspection of the ROPS showed the presence of plastic hinge formation at the top and base of each post。 The strain gauge readings taken during the test, further emphasised the extent of yielding in these zones。 Figure 6 shows the variations with load of the Von Mises stress obtained from the strains recorded at gauge A, using the appropriate equation given in Moy (1981)。 It  has to be noted that this transformation requires accurate estimates of the Elastic modulus and Poisson's ratio。 Moreover, calculation of von Misses stress from strain gauge   measurements

is based on assumptions that the stress-strain relationship is linear and that the material is isotropic in it's elastic properties。 Severely cold worked material can deviate from isotropic conditions。 The results obtained from the strain gauges at the other locations were analogous (Clark, 2005)。

Vertical Loading Phase

A 150t hydraulic jack was employed to apply the required vertical load for this loading sequence。 Precautions similar to those in the previous loading phase were taken to alleviate localised deformation and prevent any eccentric loading of the jack。 Loading of the ROPS in its already pre-deformed position of approximately 5mm vertical deflection, was gradual and resulted in a load deflection response profile that is shown in Figure 7。 It is evident from this figure that the ROPS had exhibited a fairly stiff response with only 8mm of vertical deflection at maximum vertical load。 It is also clear from this figure that the residual amount of permanent vertical deflection in the ROPS was very small after release of the pressure in the jack。 The deflected shape of the structure showed little deformation in the vertical direction as the structure’s behaviour has been predominantly influenced by the previous lateral loading phase。 It is clear from the results of this loading phase, that a structure that had undergone significant plastic deformation was still able to withstand a further vertical loading。