EWP Inspections

Acoustic Emission Monitoring Of Fibreglass

Booms On Elevating Work Platforms (EWP’s)

The Case for NDT on EWP’s

Fibreglass Reinforced Plastic (FRP) booms are found on many of the EWP’s in Australia and New Zealand.  In addition to being an electrical insulator, fibreglass has good strength.  ATTAR have found that EWP’s have been used as rests for tree limbs, lifting power poles or transformers in place, lifting cross arms to extract power poles from the ground and to support cables.  However, none of these applications meet the design requirements of an EWP.  Such applications have the potential for high dynamic loads and have damaged FRP booms.  In addition, lifting the boom without undoing restraining straps can also cause damage resulting in subsequent catastrophic boom failures and even fatalities.

The emergence of Non-Destructive Testing (NDT) techniques such as Acoustic Emission (AE) monitoring on FRP booms followed boom failures in the late 70’s on otherwise visibly viable booms. Such events were the trigger for Standards incorporating AE monitoring of FRP booms in EWP’s. Today, the applicable Standard in Australia for the application of AE monitoring of FRP booms is AS 4748-2001 Acoustic Emission Testing of Fibreglass-insulated Booms on Elevating Work Platforms. 

For many years now and as a result of Australian and USA experience and in-house research, many operators adopt regular AE monitoring of FRP booms to ensure structural integrity and ultimately, safety for operators. One of ATTAR’s first major clients was Auckland’s Electric Power Board that followed a Department of Scientific and Industrial Research report into a failed boom and a related fatality.  Today, ATTAR’s clients include EWP hire companies, EWP importers, power distribution, earthmoving and tree maintenance services.

Advantages Of AE Testing

1. AE monitoring of FRP booms detects all active defects in the fibreglass boom, as well as movement at the steel/FRP splice and assists in identifying hydraulic system leaks.  It does not damage the boom - unless it is already defective.

2. AE monitoring is quick and thorough, allowing examination of the entire FRP section during the test. It detects defects early in their life and provides the opportunity to repair rather than replace.

3. AE monitoring removes the burden of responsibility from personnel untrained in NDT of FRP booms, provided the AE test is conducted by a competent operator.

Boom Design

All FRP boom failures have been attributed to design deficiencies, poor quality fabrication, in-service abuse or lack of knowledge of the mechanical properties of FRP. The inclusion of an effective NDT program can isolate design and in-service deformities in FRP booms that would otherwise go un-noticed until an investigation follows a failure.

The application of NDT to FRP booms should be undertaken upon commissioning the vehicle and at regular in-service inspections intervals. This is reinforced by the relevant Standard AS 1418.10, which states, “The structural integrity of the boom is confirmed throughout its service life by a recognised non-destructive test.  The frequency of testing shall be commensurate with the intensity of use and should occur at intervals not exceeding 2 years or after the application of impact or accidental loads.  Such requirements shall be detailed in the operator’s manual,” [Clause 1.5.4.4 (c) of AS 1418.10].

AE monitoring is not only a sound NDT method for the determination of faults in FRP booms, it provides a higher level of safety and security than visual inspection alone. Visual inspection is neither an effective nor efficient method of inspection inside an FRP boom or within the layers of FRP used to make up the boom. 

Defects in FRP Booms

Defects can be produced in FRP structures at various stages including manufacture, in-service and maintenance. 

Defects in FRP booms created during manufacture and can be caused by lay-up and cure, such as voids, thermal stresses, foreign body inclusions and fibre kinks.  They can also arise from machining and assembly, such as ply splitting and delamination at holes and free edges, handling damage or damage due to assembly stresses. 

Defects produced in-service can arise from impact, cutting, abrasion, local heating or chemical attack or damage due to overloading as a result of a design fault or inappropriate use.

Defects during service can occur from sources such as poor repairs causing porosity in adhesive bond lines.

Early detection with AE monitoring identifies essential maintenance and repairs and increases the service life of the boom. 

Defects may occur in isolation but a number of defects could occur together, either of a single or several different types.  One type of defect may cause another to occur. Voids or inclusions can initiate intra-ply splitting or delamination.  Of particular concern are impacts that can produce splitting, multiple delamination and fibre damage. Moreover, multiple impact could produce such defects on adjacent sites.

Mechanical Behaviour of FRP

As part of their investigation into the failure of their boom, the Philadelphia Electric Company studied the effect of load and time at load on chopped strand mat laboratory specimens and found when the FRP was loaded to greater than 60% of its ultimate strength, glass fibre failure occurred and the ultimate strength of the FRP was reduced.  The amount of strength reduction was related to both the load amplitude and the time over which the load was sustained.  Higher loads and longer times resulted in greater strength reduction and earlier failure.

Predictably, the presence of defects in FRP booms result in localised stress concentrators which mean that lower load/Ultimate Tensile Strength ratios in FRP booms than those observed in the laboratory specimens. In other words, laboratory tests on FRP indicate that 60% load on an FRP boom would result in serious reductions in the ultimate strength of FRP structures on booms.

Flow-coats

Flow-coats are applied to the FRP structures to protect the weight bearing FRP from the damaging effects of the ultra violet light and moisture. While the flow coat does not provide structural strength, its condition can indicate the need for further investigation into the integrity of the underlying FRP structure.  Cracking of the flow-coat may be due to impact, overload, incompatibility of resin systems, incorrect mixing of flow-coat components or excessively thick flow-coats.

Critical flow-coat cracking such as that due to impact is characterised by a star-like appearance. In this case, the underlying FRP will be damaged.  If the cracks are due to overload, flow-coat cracks appear as nearly straight lines normal to the applied load and the underlying FRP may be damaged.  If the cracking is due to chemically incompatible resins or incorrect mixtures, there may be damage in the underlying FRP and if the resins are mechanically incompatible or the flow-coat is too thick, the visible surface cracks will probably be restricted to the flow-coat.

Non-Destructive Testing of FRP Booms

Traditional NDT inspection methods on FRP booms are dye penetrant and ultrasonics. The limitations of these methods and the adoption of AE monitoring as an alternative should be well understood so that an effective and efficient safety management of EWP’s with FRP booms is maintained.

Dye penetrant testing for FRP integrity is suitable only for detection of surface breaking defects such as cracks or porosity and cannot indicate sub surface cracks in the FRP without removing the flow-coat.  Moreover, dye penetrant testing cannot indicate crack depth nor the presence of delaminations and porosity within the FRP.

Ultrasonic inspection may be used to determine FRP thickness and will pick up delaminations but it cannot pick up tight through thickness cracks or be used to examine the steel/FRP interface area where many defects occur.

Acoustic emission monitoring has been successfully applied to the inspection of FRP structures around the world, particularly EWP booms and storage vessels.  On the basis of these tests, Standard test procedures such as AS 4748-2001 have been developed.  Its great advantage is that the entire FRP structure is interrogated during the test and only “active” or growing defects are detected.

Loading of FRP Booms

In Australia, the recommended procedure for ensuring structural integrity of EWPs is the application of 125%-150% of the SWL in the most critical position. In New Zealand, AE monitoring is a requirement of the Approved Code of Practice for Power Operated Elevated Work Platforms, set down by the Occupational Health & Safety Authority under the Health and Safety in Employment Act, 1995.  The code requires that fibreglass booms on EWP’s and buckets must be tested at least every 2 years or annually if they are in arduous service.

AS 2550 Cranes - Safe Use, Part 10, Elevating Work Platforms

Published in 1996, this Standard contains sections that are open to interpretation, particularly in relation to the visual assessment of severity of damage and subsequent fitness for work.

Cracks in an FRP boom resulting from damage due to overload may be detected on the top or bottom surface of the boom, particularly where the fibreglass insert changes to steel at the lower or elbow end.  Aside from cracks, damage can also take other forms, particularly fracture and delamination below the surface such as may arise when the boom is struck by falling objects anywhere on the boom.  Because this is usually blunt object damage it is often difficult to detect as indicated in the Standard, and the effect is usually greater than just the visible surface crazing. Blunt object damage can result in cracking and delamination within the FRP boom and its severity cannot be relied upon with visual inspection.  Its actual effect may be substantial and its classification from visual observation as “minor damage” in the Standard may result in an incorrect diagnosis.

Damage can also occur inside the boom at changes in sections or voids within the FRP boom itself.  All fibreglass booms contain voids due to the method of manufacture which cannot be detected visually.

EWP Testing In Australia And New Zealand

ATTAR has been performing AE monitoring of the FRP section of EWP’s since 1986 and collation of the results of our tests for the period from July 1986 to January 2010 show that just over 1% of booms were defective, representing a potential threat to the safety of the operator.  However, because many of the defects were detected early, most of the unsatisfactory booms were repairable, thereby avoiding potential costly consequences.  Booms that have been involved in accidents should be tested prior to undertaking repairs since testing could avoid unnecessary repairs or dismantling if, as has often been found in recent tests, some booms involved in accidents are still fit for use.  Some of those booms that were unrepairable were replaced after an insurance claim was supported by acoustic emission test results.

The AE test has also indicated areas of minor damage, reported as ‘Other Damage’ in the table. Other damage includes gouges, cracks in flow-coat, cracks in levelling rod support bar holes and at inspection holes, as well as movement between the FRP and steel interface.  These defects and any leakage in the hydraulic system are always reported as a part of ATTAR’s AE monitoring service.

Test Interval

AS 4748-2001 suggests the following testing interval for booms:

Figure 1 shows the load program that the test requires using a hydraulic ram and load indicator while monitoring the AE from the boom using sensors strategically located as shown in Figure 2. A dead weight is used to load the boom to a calculated load based on the vehicles SWL. A concrete block, transformer or a fork lift can be used for this purpose.  Each test takes around 2 hours so that vehicles are out of service for a short time with results available to the operator/owner immediately. Inspection intervals may be set by the owner who should have an understanding of his local work conditions and equipment use.  However, vehicles subject to a 10 year inspection should have their

booms tested with AE monitoring, as an AE test is the only way to guarantee structural integrity of the FRP section.

References

• AS 1418.10 – 1994 Cranes (including hoists and winches), Part 10: Elevating work Standards Australia, Sydney, NSW.
• AS 2550.10 – 1994 Cranes – Safe use, Part 10: Elevating work platforms, Standards Australia, Sydney, NSW.
• AS 4748 – 2001 Acoustic emission testing of fibreglass insulated booms on elevating work platforms, Standards Australia, Sydney, NSW.

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