Open Cut » Maintenance & Equipment
This project covered the preliminary investigation into using Sitecured Fibre Reinforced Plastics (FRP's) for strengthening structural members in dragline booms, more particularly tubular members in boom and mast structural system. Sitecure is a process developed by ICD to cure the prepregnated composite material on site using steam. Typically, strengthening or replacing any individual members of the structural system require the boom or mast to be lowered to the ground, then replacing the members by cutting off the old and welding back the new. Although this is the normal process, it involves excessive down time and repair costs. The strengthening of members is required to achieve Rated Suspended Load (RSL) upgrades or to restore the strength of a weakened member due to physical damage. The main purpose of this project was to investigate the feasibility of strengthening members with composite materials while the structure is in position, considerably reducing the downtime and costs.
The traditional method of replacing structural members has many disadvantages including:
- The boom having to be lowered for the procedure (7-14 days) causing significant operational disruption with lost opportunity cost of ~$10,000 per hour. These types of upgrades are generally postponed until major shut downs which generally take place in 4-5 year intervals.
- The heavier members added increase the total mass of the structure. This can be significant in the case of main chord replacements.
- As the lacing members are welded according to a particular sequence at each cluster, the replacements can not be undertaken without causing significant interruptions to the remaining welds at the cluster, which in turn lead to reduced fatigue life of the cluster.
In recent years strain gauging has been used to monitor stresses in critical structural members and individual lacings. It will be necessary to extend this methodology to monitor the critical members that are strengthened using FRP.
Prior to the development of Sitecure, there was no method by which resin pre-impregnated (prepreg) FRP's could be bonded to infrastructure. Prepregs, originally made for aerospace applications, are the most up to date, high strength and durable FRP's available on the market. They outperform traditional structural materials (concrete and steel) in terms of strength, stiffness and durability. By combining Sitecure with aerospace prepreg materials, carbon fibre can now be bonded to steel and concrete structures to provide an efficient, cost effective and long term method of strengthening and repair.
The following is a summary of the main outcomes of this project:
- Tests and the research focused on maximizing the effectiveness of the bond between the composites and the steel substrate. The bond test samples were made out of Grade 350, 6mm thick steel plates. The bond strength achieved in the samples was sufficiently high to bring the 6mm plate to yield stress and that became the limitation of the test. It may have been possible to further increase the bond strength if a thicker steel plate was used for the samples.
- Cyclic loading tests on bond strength were undertaken simulating the main chord and lacing stress levels in the steel plate. At the end of the cyclic loading the samples were tested to failure statically. The results showed that the bond between FRP and steel could maintain its integrity and strength for more than 2 million cycles. The tests were limited to 2 million cycles, i.e. equivalent to ~4 years of dragline operation, due to cost and time limitations of testing.
- The incorporation of glass fibre between the carbon fibre and the steel, whilst providing a significant increase in bond strength, decouples the carbon from the steel, eliminating possible galvanic corrosion problems.
- Tests undertaken with composite patches placed on a notch showed that the FRP can be used to prevent or reduce crack growth rates. The comparison of results for samples with and without FRP showed that in some cases the fatigue life could be increased by twelve fold by employing FRP.
- Ultrasonic and radiographic techniques can be easily used to detect steel cracks under composite patches, giving a NDT method that can be used to monitor cracks on-site.
- The limited number of column tests undertaken showed that the axial compressive load capacity of dragline lacing members can be improved by 57% for a 22% increase in mass using FRP, compared to possible 57% increase in mass for the conventional approach.
- Flexural stiffness of the lacing member can be increased by at least 31% for an increase in mass of 14%.
- The FRP strengthening increases the axial and bending stiffnesses of the column. ICD has argued that the stresses that cause fatigue at the two end clusters will be reduced due to this increase in stiffnesses. However, based on the knowledge, experience and the available literature, MTI do not believe that the increase in bending stiffness of the column will significantly change the load attracted to the column or the stresses that cause fatigue at the clusters. If at all, the increase in axial stiffness should increase the stresses that cause fatigue at the clusters, unless the cluster area itself is strengthened by similar means.
- The Finite Element modelling (FE modelling) and comparison with test results have shown the potential to simulate the general behaviour of the columns. However, the unloading part of the load deflection curve is not simulated well compared to the test results. The reason is that there are complex non-linear material and geometry aspects involved in localized areas of the column during the latter part of the collapse behaviour. Further work is needed to model these complex behavior aspects through finite element analysis.
- The analysis should also be extended to develop a methodology to theoretically predict the load carrying capacity (i.e. the peak value of load if the column is tested in the laboratory) of the column, similar to the AISC formula typically used for unreinforced columns. It should be noted that this prediction invariably involves elastic-plastic behaviour of the materials and the large deflection effects of geometry. This is an aspect that should be considered during the next stage of the project.
The excellent performance of FRP used in this project indicates its potential for other structural applications involving different member types. Although no investigations have been undertaken so far, the other applications that can be considered are:
- Using FRP at welded joints to improve fatigue life.
- Using FRP over areas with existing cracks to reduce crack propagation rates.
- Strengthening structural members affected by corrosion.
The results of this research show that high quality prepreg systems, combined with Sitecure, can be used to strengthen structural members in draglines in-situ. This method has the potential to significantly reduce the cost and downtime compared to the traditional method of replacing the members.