Structures research at the DSTO Centre of Expertise in Structural Mechanics

Citation

Jones, R. (2010), "Structures research at the DSTO Centre of Expertise in Structural Mechanics", International Journal of Structural Integrity, Vol. 1 No. 4. https://doi.org/10.1108/ijsi.2010.43601dab.001

Structures research at the DSTO Centre of Expertise in Structural Mechanics

Article Type: Technical paper From: International Journal of Structural Integrity, Volume 1, Issue 4

Introduction

The Defence Science and Technology Organisation (DSTO) Centre of Expertise in Structural Mechanics (CoE-SM) was opened in 1996 by the Australian Chief Defence Scientist Dr R. Brabin-Smith. Since then, it has established a national and international reputation and is a key member of two Commonwealth Research Centres (CRCs), namely: the CRC Railway Innovation and the CRC for Integrated Engineering Assett Management (CIEAM). The CoE-SM has created facilities and a critical research mass that are unique within an Australia University and plays a key role within the wider Australian community. This is highlighted by Professor Jones’, the Head of the CoE-SM, role in co-ordinating the failure investigation into the mechanical causes associated with the failure, and subsequent explosion, at the ESSO gas plant for the Longford Royal Commission and by his work with SP AusNet for the Royal Commission into the Victorian Bushfires. In the case of the Longford Royal Commission, the failure analysis made extensive use of the tools developed for DSTO.

The CoE-SM works closely with both DSTO and the Royal Australian Air Force Directorate General Technical Airworthiness (DGTA) which has taken over funding responsibilities from DSTO and supports several research students at the CoE-SM. As a result, the CoE-SM research program is now managed directly from DGTA by Dr Madabhushi (Jana) Janardhana, who is the Technologies ASIP Manager (ASI3B). This work is complimented by research for Mr Lorrie Molent, Functional Head Structural Integrity (Combat and Trainer Aircraft) at DSTO.

There are currently more than 12 staff at the CoE-SM, of which several have had an extensive RAAF Air Structural Integrity experience, i.e. Group Captain John Baker, and Group Captain Ken Cairns, whilst Professor Jones was previously Research Leader Aircraft Structural Mechanics at the Australian DSTO’s Aeronautical Research Laboratory.

There are a number of research themes at the CoE-SM, some of these are as follows.

Structural integrity assessment

In conjunction with DSTO, the CoE-SM has created a unique capability for assessing aircraft structural integrity (Jones et al., 2007, 2008a, b, c). This couples advanced crack growth approaches, developed jointly with DSTO, with the NEi-Nastran finite element model of the aircraft. Unlike many other approaches, it can accurately predict life from small near micron-sized defects (Jones et al., 2007). This development was funded by DSTO as a result of inadequacies in the AFGROW, NASGRO and FASTRAN approaches, which generally gave errors in life of up to 400 per cent. In contrast, the tools developed by the CoE-SM were within 20 per cent for both the F/A-18 centre barrel and the F-111 wing test (Figure 1).

Figure 1 Comparison with FASTRAN II for the F/A-18 centre barrel test

Structural test laboratory

In December 1999, the CoE-SM bid for and won the BHP Rail and BHP Maintenance Research Group. This has resulted in a team of 20+ researcher engineers with skills ranging from materials engineering, instrumentation, failure analysis, to data analysis. As a result, the CoE-SM shares with the Rail group a unique structural test laboratory (Plate 1). This laboratory which has a range of servo-hydraulic test equipment with capacities that range from 50 to 500 kN, is being used for both defence and non-defence-related research.

Plate 1 Rail sidefame being tested for Bradken Rail

The CoE-SM structural thermography facility

The capabilities of this laboratory are complimented by a structural thermography facility with capabilities as presented in Table I that has been used to assess a wide cross-section of problems, namely, the assessment of incipient corrosion damage and paint degradation in a RAAF P3 C aircraft wing (Plate 2 and Figure 2) the stresses in the F/A-18 centre barrel under fatigue testing at DSTO, see Figures 3 and 4 and the stresses in a weld repair to a rail sideframe (Plate 3 and Figure 5).

Table I Capabilities available within the CoE-SM structural thermography facility

Plate 2 P3C wing examined for corrosion damage and paint degradation

Figure 2 Corrosion damage and paint degradation detected using infra-red thermography

Figure 3 Stresses in an F/A-18 bulkhead under test at DSTO

Figure 4 Stresses in Region I in the F/A-18 bulkhead

Plate 3 Undressed local detail showing the weld passes

Figure 5 Stresses (hot spots) in an undressed weld

Rail structural integrity research

Australia currently exports approximately 37 per cent of the worlds coal, most of which is carried by rail as are most of Australia’s mineral exports. Furthermore, the November 2002 Green Paper (Auslink) estimated that the amount of general freight traffic will double before 2020. Managing this freight task requires railways to run heavier and faster trains resulting in a higher working stress level on the railway infrastructure. The current rail infrastructure, both track and rolling stock, is already at its limit for operating capacity. The durability in rolling stock components and in structures such as bridges and locomotives is now showing up as a major problem for the Australian railway industry. A process for reducing the weight of rolling stock components while improving strength and fatigue resistance capacity and a technique to efficiently increase the capacity of railway bridges is now essential and the DSTO CoE-SM is playing a key role in this work.

Both Bradken Rail and AMSTED Rail International (US) have used this technology to design next generation railway bolsters and sideframes. These components will be used to transport a high proportion of Australia’s mineral exports. The Structural Thermography facility has been used to experimentally validate the critical stress points in several sideframes and bolsters. One unique feature of this facility is its ability to directly measure energy dissipation and thereby evaluate fatigue critical hotspots without the need to perform a full fatigue test. In this fashion, various proposed structural modifications can be evaluated and ranked on the basis of their fatigue initiation.

Composite repair technology

Staff at the CoE-SM have played a leading role in the composite repair program undertaken as part of the US FAA Aging Aircraft program (Atluri et al., 1993) and have pioneered the development of composite repair technology for both military and civil aircraft (Baker and Jones, 1988; Baker et al., 2002) (Figure 6). As part of this work, a simple test and analysis methodology was developed that enabled cracking in fuselage lap joint to be both predicted and reproduced in the laboratory (Jones et al., 2008b) (Figure 7).

Figure 6 Composite repairs applied to B-747 aircraft as part of the FAA Aging aircraft program

Figure 7 Comparison of la and flight cracking

Figure 8 Stresses in the SPD doubler at 11,100 cycles, units are in Mpa

Research into supersonic particle deposition for fatigue life enhancement

The recent law passed through the US Congress, namely: Public Law 107-314 Sec: 1067 entitled: “Prevention and Mitigation of Corrosion of Military Equipment and Infrastructure”, means that the effect of corrosion on the structural integrity of USAF, US Army, US Navy aircraft and other Navy weapons platforms must be now quantified and managed as per cracking, i.e. in a damage tolerance fashion. As a result of a subsequent detailed study of the problem of corrosion, the US DoD has focused its life-cycle corrosion research and development efforts on four primary areas (Office of the Secretary of Defense, 2007). One of these four areas was: repair processes that restore corroded materials to an acceptable level of structural integrity and functionality.

To meet this challenge, the CoE-SM, in conjunction with Rosebank Engineering, have extend the supersonic particle deposition (SPD) process (Karthikeyan, 2004; Sakaki, 2004; Pepi, 2006; Villafuerte, 2010). In one test study, it was shown that whereas crack growth from a small 2-mm edge crack in a 1-mm thick 2024-T3 aluminium alloy wing skin tested under constant amplitude fatigue loading with a maximum stress of 180 MPa and R=0.1 resulted in failure in approximately 30,000 cycles by using a 1-mm thick SPD doubler crack growth could be suppressed. Indeed, after 60,000 cycles, there was no growth or degradation of the SPD or damage to the SPD – 2024-T3 interface (Figures 8 and 9). This finding has been supported by the results of an extensive research program in the technology. This work opens the door for SPD to be used to restore the structural integrity of damaged or corroded parts.

The effects of corrosion preventative compounds on structural integrity

It is well known that metallic aircraft structural components are susceptible to corrosion, including general corrosion and localized corrosion, such as stress corrosion cracking, corrosion pitting and exfoliation, corrosion fatigue interaction, etc. This degradation can adversely affect structural integrity and significantly increase the maintenance costs. As a result, corrosion preventative compounds (CPCs) are now widely used with the intention of preventing, retarding and controlling corrosion, as well as for extending service life and reducing life-cycle costs. The CoE-SM, under funding from the Commonwealth Aviation and Safety Authority, has investigated the effect of CPCs on fatigue crack growth in a representative structural joint based on the specimen geometry developed for the FAA as part of the FAA Aging Aircraft Program. This specimen, which represents a typical Boeing 737 fuselage joint, was tested under constant amplitude loading, both with and without CPCs. Three commonly used CPCs were investigated, namely: Strikegold, which is widely used in the USA, ACF50, which is used within the Australian general aviation industry, and LPS3, which is used in the Royal Australian AirForce (Jones et al., 2010).

The results of this study have shown reveal that CPCs can have a marked detrimental effect on fatigue crack growth (Figure 10). Here, we see that, as first documented in Jones et al. (2008b), crack growth in fuselage lap joints without CPCs has a linear relationship between the log of the crack length (a) plus the radius of the fastener (r) and the number of cycles (pressurizations). However, for specimens treated with CPCs this relationship is bi-linear with a second slope that is much greater that that without CPCs. This study is ongoing and subsequent work will focus on CPCs in common use in the RAAF as well as on specimen geometries more representative of general aviation aircraft.

In situ structural health monitoring research

The CoE has also established a structural health monitoring test facility to investigate the propagation of stress waves in idealised flat plates and in representative aircraft structural components (Figure 11). Figure 12 shows the dispersive characteristics of stress waves propagating in a structural component (Ong and Chiu, 2010) that was measured with this test facility. There was an excellent agreement between the theoretical dispersion curve (solid lines) and the experimentally measurements. This test facility has recently been used to study the acoustic power distribution in a real structural component and thereby identify the optimal locations for sensor placement for structural health monitoring (Ong and Chiu, 2010) (Figure 13).

Figure 9 Stresses in the SPD doubler at 56,100 cycles

Figure 10 Comparison of the three CPC types with specimen results without CPC

Figure 11 Ultrasonic laser vibrometry scanning facility

Figure 12 Dispersion curves

Figure 13 Power distribution of the propagating stress wave in a component representative of the lower wing skin of F-111 aircraft in service with the RAAF

Rhys JonesDSTO Centre of Expertise Structural Mechanics, Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Australia