Detection of Bolt Load Loss in Hybrid Compositemetal Bolted Connections

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    Engineering Structures 26 (2004) 895906www.elsevier.com/locate/engstruct

    Detection of bolt load loss in hybrid composite/metalbolted connections

    Vincent Caccese a, Richard Mewer b, Senthil S. Vel a,

    a Department of Mechanical Engineering, University of Maine, 5711 Boardman Hall, Room 214, Orono, ME 04469-5711, USAb Naval Undersea Warfare Center, Newport, RI 02841-1708, USA

    Received 15 August 2003; received in revised form 17 November 2003; accepted 17 February 2004

    Abstract

    Hybrid composite/metal connections are susceptible to bolt load loss due to viscoelastic creep and/or environmental effects.Accordingly, the focus of this research is on experimentally quantifying changes in bolt load of composite/metal hybrid connec-tions. A proof-of-concept model was created consisting of a fiber reinforced composite panel bolted to a steel frame. A piezo-electric actuator bonded to the center of the composite panel was used to provide controlled vibration input. The response of theplate was measured using either shear accelerometers or dynamic strain sensors located at the four corners of the compositepanel. The load on an instrumented bolt was decreased and three different monitoring techniques were used to detect bolt loadloss, including (a) low frequency modal analysis, (b) high frequency transfer functions between the actuator and sensors and (c)high frequency transmittance functions between pairs of sensors. Experiments demonstrate that the transmittance functionapproach shows the most promise and was able to reliably detect a single bolt loosening. A damage index based on change intransmittance function is very sensitive to changes in bolt load.# 2004 Elsevier Ltd. All rights reserved.

    Keywords:Bolted connection; Composite joint; Hybrid connection; Bolt load loss; Structural health monitoring

    1. Introduction

    This paper describes the development of a system todetect bolt load loss in hybrid composite/metal con-nections. Determination of bolt load is critical to theevaluation of integrity of bolted connections. A fullytightened bolted joint can withstand orders of magni-tude more load cycles than an untightened joint due tothe mechanics of how the bolted joint withstands an

    applied load. Accordingly, estimation of bolt load lossis a vital information in assessing the condition of arepetitively loaded composite/metal bolted connection.Even so, little work has been done to develop real timemethods of detecting loss of bolt force. This workrepresents the development and evaluation of monitor-ing methods for bolted composite panels using readilyavailable equipment and recently developed monitoring

    techniques with a goal to detect small changes in bolt

    load.The development of structural health monitoring

    and damage detection systems is the focus of much

    current research. Monitoring systems can be used to

    substantially reduce maintenance costs or to prevent

    catastrophes. In developing a damage detection system,

    the objectives need to be clearly defined so that an

    optimum and robust detection scheme is the outcome.An accurate assessment of structural integrity depends

    on a proper evaluation of both the global structural

    response and the condition of the connections and

    interfaces. Accordingly, real time monitoring of con-

    nection integrity is an important challenge. Global

    damage typically produces large structural changes that

    usually can be detected by analyzing the lower

    vibration frequencies. This is often done in conjunction

    with numerical models of the structure. Localized dam-

    age mechanisms such as delamination, bolt loosening

    and cracking may be difficult to detect using the lower

    Corresponding author. Tel.: +1-207-581-2777; fax: +1-207-581-2379.

    E-mail address:[email protected] (S.S. Vel).

    0141-0296/$ - see front matter # 2004 Elsevier Ltd. All rights reserved.doi:10.1016/j.engstruct.2004.02.008

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    structural modes. Techniques that excite and analyzethe higher frequencies are typically more robust whenthe detection of local structural changes or damage isthe goal.

    Composite materials are used increasingly in engin-eering applications because of their high specific

    strength. A synergistic effect and economical design isoften achieved when composites are used in combi-nation with metal superstructure. Accordingly, com-bined composite/metal structural systems requirehybrid connections. More often than not structuralfailures occur at connections and interfaces, therebymaking it difficult to assess integrity using techniquesthat appraise only the global structure. Hybrid com-posite/metal connections in particular are susceptibleto metal fatigue, bolt loosening primarily due to viscoe-lastic creep of the composite, temperature effects dueto coefficient of thermal expansion mismatch andmoisture absorption causing differential strain between

    the metal and composite.The motivation for developing a system to detect

    bolt load loss came about through a project to designan efficient structural system for underwater bodies tobe used on ship hulls. This is a part of a joint effortbetween the University of Maine, the Navatek Ltd ofHonolulu, HI, and Applied Thermal Sciences (ATS) ofSanford, Maine called Modular Advanced CompositeHull-forms (MACH). The project is performed in con-junction with the Navy Surface Warfare Center at Car-derock, MD (NSWCCD). The mission of the MACH

    program is to develop fast efficient surface vessels thatuse additional underwater bodies attached to a moretraditionalhull-form. The goal is to deploy ships where more pay-load and/or higher speeds can be achieved at little orno additional power consumption and with excellent

    sea-keeping ability. Fig. 1 shows one example vesselcalled MIDFOIL where a hydrofoil and a paraboliclifting body shape are combined with a catamaran hullto achieve additional buoyancy and dynamic lift whichgreatly improves the performance and sea-keeping ofthe vessel. Relatively, inexpensive pilot tests onMIDFOIL and similar vessels have shown that thismethod has great advantage for fast military supportcraft and commercial vessels such as ferries. Recentstudies have shown that the addition of underwaterbodies can dramatically improve speed, reduce fuelconsumption and increase payload. These efforts havealso demonstrated that composite material construc-

    tion can bring about increased structural efficiency.The method proposed for construction of a com-

    posite/metal hybrid version of the underwater liftingbody is to use a skin made of composite materialsattached to a metal framework. Fig. 2 shows a sche-matic of this concept. This type of system will allow forease of maintenance of equipment housed inside thelifting body and it will provide a metal skeletonto facilitate attachment of propulsion equipment. Theskin designed is of monocoque, stiffened or sandwichconstruction depending upon structural requirements.

    Fig. 1. MIDFOIL craft with underwater lifting body (photo courtesy of Navatek Ltd).

    896 V. Caccese et al. / Engineering Structures 26 (2004) 895906

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    It is subsequently attached to the metal structure usinga hybrid connection. Detailing this connection to haveadequate strength and watertight integrity is imperative.

    Often, bolted connections are critical to the functionof the structure and their failure may have high associa-ted repair costs, or may endanger lives. It is essentialthat a minimum torque be maintained for a hybrid bol-ted composite/metal connection to respond properly,especially if watertight integrity is to be insured. Realtime detection of bolt load loss is a major benefit inevaluating the performance of such connections. How-ever, there has been little research on the detection ofbolt load loss in hybrid connections. Accordingly, theeffort summarized in this paper focuses upon real timedetection of bolt load loss. To the authors knowledge,

    a real time method for detection of bolt load loss due tostress relaxation has not been published to date. Parket al. [1] have developed an impedance-based healthmonitoring technique using piezoelectric wafers bondedto a structure to assess damage in bolted pipeline struc-tures. However, their method is not very sensitive tostress relaxation of a single bolt and one or more boltshave to be completely loosened in order to affect a sig-nificant change in the impedance signature. In addition,under the high frequency ranges used in this impe-dance-based method, the sensing region of the PZT islocalized to a region close to the sensor/actuator.

    The results presented in this paper represent a

    potential robust methodology based existing methodsthat were originally developed for other purposes. Toaddress this issue, a proof-of-concept test bed wasdeveloped that consists of a square plate made ofE-glass/vinyl ester composite. The hybrid connectionwas formed by bolting this plate to a steel frame usingsteel bolts, including an instrumented bolt. Severalinterrogation techniques were employed and comparedincluding: (1) low frequency modal analysis; (2) highfrequency transfer functions and (3) high frequencytransmittance functions. Each of these techniques useda piezoelectric actuator bonded to the panel to deliver

    a characterized disturbance in a controlled manner.The technique using transmittance functions to evalu-ate changes in bolt tensioning level shows the mostpromise and was able to detect small changes in boltload.