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· Use the results of the numerical studies to develop a method to relate the toughness from conventional fracture test specimens to the prediction of fracture behavior in structural components containing small defects. Duration 3,000 labor hours over 2 years lDL/9SM-R Fracture Methodology for Strength Mismatched Weldments (94M-D) Objective Expenmentally study the fracture behavior of mismatched weIdments to evaluate new analytical and numencal solutions of fracture parameters for improved safety and integrity assessment of ship structures. Benefit The fracture behavior in mismatched weldments can be more correctly evaluated with the development of a fracture parameter that includes the effects of the strength mismatch. Results from this project should contribute significantly to enhanced safety and integrity of ship structures and provide greater confidence of pollution control in the case of tankers. SSC National Goal Improve the safety and integers of manne structures. SSC Strategy Improved engineering analysis and evaluation Background Mismatch of strength in weIdments causes a problem for fracture prediction. The correct evaluation of the fracture parameters depends upon a known pattern of deformation in the fracture specimen. A mismatch of strength in a fracture specimen containing a weld disturbs this known pattern of deformation. An overmatched weld metal tends to spread the deformation, and an undermatched weld concentrates deformation. In either case, the fracture parameter evaluated from the external loading may not be correct. New analytical and numerical work has developed methods to evaluate the fracture parameters from external loading for mismatches] welds.t These methods should be subjected to experimental venfication. Specially prepared weldments with known strength mismatch should be tested to determine the influence of the mismatch on fracture behavior so that these new solutions can be evaluated. Recommendations Perform the following tasks: · Fabricate we]dments with known amounts of both overmatch and undermatch. · Test the fracture behavior of the specimens. · Evaluate the fracture toughness using the newly developed solutions for the fracture parameter. ~ Kirk, Mark. 1992. Studies to Evaluate Fracture Parameter for Mismatched Welds. Ph.D. dissertation. University of Illinois, Urbana. 93
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· Determine the appropriateness of these new solutions, and recommend improvements. Duration 3,000 hours over 2 years 95TC-A Post Yield Strength of Ship Structural Members Objective Assess the post-ye behavior of some typical ship structural elements with particular reference to any tendency to instability under loads and to its effect on structural performance. Benefit The assessment wall provide a basis for venfication or further development of structural design guides and regulations. SSC National Goal Improve the safely and integrity of marine structures. SSC Strategy Development of better design tools and infonnation systems Background The new Canadian Arctic Shipping Pollution Prevention Regulations (ASPPR) allow for plastic design to meet extreme ice loads; thus it is of great importance to vend that structural design practices and regulations incorporate the necessary details to maintain stabilizer as far as practical as Bend progresses under an extreme load, while also avoiding unnecessary weight in the structure and consequent economic penalties. An exploratory investigation has been carried out, in which some representative ship structures were finite-element modeled to study their post-ye behavior under a lateral ice-force load, with particular reference to any tendency to become unstable as areas of yield develop. The progress and degree of the resulting loss of structural performance was tracked, indicating the effectiveness of proposed rules (ASPPR) and showing the relative stability of a limited range of frame sections and related arrangements. Reporting is in progress. Recommendations Perform the following tasks: · Continue the exploratory investigation with physical modeling. · Build a representative ice-belt structure meeting the new proposed ASPPR nales at as large a scale as practical. · Test this mode] into the plastic range to study its post-y~eld behavior. · Compare the results with those for similar f~nite-element models, and make an evaluation of the effectiveness of the finite-element models to represent post-ye behavior ant] of the new rules for maintaining stability. · If the effectiveness of the finite-element methods in this type of investigation is confirmed, use the data developed by finite-element methods to establish design guides. 94 i
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-in Duration 1,000 labor hours over ~ year 95TC-C Fiber Optic Strain Gauge Objective Demonstrate and evaluate the application of a recent advance In strain gauging In a hulI-stress monitoring environment. Benefit Provide improved long-term reliability of response monitoring systems. Provide cost-effective long-term response-data acquisition. SSC National Goal Improve the safety and integrity of marine structures. SSC Strategy Structural monitoring of vessels in service Background A fiber-optic strain-gauge system is being brought to a stage of development ready for extensive use in a propeller blade instrumentation project, which is in its early stages. (The project is evolving with the cooperation of the U.S. Coast Guard, with the intention of instrumenting the propeller blade of a USCG POLAR class icebreaker.) The fiber-optica] system promises stable performance over a long period of time, without the sensitivities to moisture, electro-magnetic interference, etc., that plague eiectnc strain gauges. This relates to several SSC projects involving response monitonng. Recommendations The project would occur in two phases: Phase ~ Carry out a paper study to investigate how the optical strain gauge would fit into response monitonug systems which are the subject of recent SSC projects. Provide a specification and cost estimate for phase 2. Phase 2 Install fiber-optic gauges in a monitoring system, observe their effectiveness over an extended period, and evaluate and report. Duration Phase ~ 400 labor hours over 0.5 years Phase 2 labor hours and time to be determined in phase ~ 95M-C Intelligent Composite Structure Development for Marine Applications Objective Apply smart composite-structures technology to marine and offshore structures to provide enhanced productivity and improved safety through incorporation of integral power and signal transmission capabilities and structural and safety monitoring devices. Benefit The development of this technology wall provide new enabling capabilities to monitor the performance of future marine and offshore composite structures, and it will result in significant enhancement to manna-structure safety. 95
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SSC National Goal Improve the safer and integnty of marine structure. SSC Strategy Structural monitoring of vessels in service Background Composite structures are increasingly being applied in make and offshore applications ranging from complete ship hulls for mine sweepers to low-pressure piping for critical applications such as the fire water system on an offshore platfonn. The aerospace and mild industries have for the last two decades investigated and developed "smart composite structures," and some applications have been integrated into products. As used here, "smart structure" includes any technology in which a wire, ~Sber- optic, tube, or other device is integrated into the matenal dunng fabrication or construction for the purpose of transmitting an electncal, light, or fluid pressure- modulated signal, to transmit power to or from a remote region of the stn~cture, or to do work so as to deform the structure. Such integrally constructed systems may be used as the information line that links the remote region to a central data-processing system or in themselves as the mon~to~g device. The instruments may include sensors and instruments such as strain gages and thermocouples built into the composite structure and may also include extended sensing devices such as fiber-optic scopes and cameras or ar~alytica] instruments such as a miniature gas chromatographs. Structural and safety monitoring of remote regions can be accomplished by the use of such devices. Fiber optics can be used to transmit signals, and its state of stress or its failure can be used to monitor the state of stress or failure of the structure. In a project conducted by the National Institute of Standards and Technology in collaboration with a U.S. of} company, it was shown, for example, that an optical fiber placed in a "pultruded" carbon rod rope tether could be used to predict not only failure but the location of failure along the length of the rod. Many other sensing technologies may also find application here. The advantage of integrating the communication line into the composite structure wall is that it is thereby protected and uniquely located. Intelligent structures are also being explored for integration into bridges and other infrastructures to ensure the safely of the structure and to help predict the remaining life. Smart structure development is also the focus of advanced technology being developed by the Japanese. One Japanese company, for example, proposes to use the change In electnca] conductivity of embedded carbon fibers to monitor the safety of a concrete bridge structure. The common technology base for these diverse applications will permit considerable technology transfer. The opportunities are almost boundless but wild require imagination and development. Recommendations Perform the following tasks: · Survey the existing technology base for smart structures, and identify marine and offshore structure applications where smart composite structures would enhance productivity or improve safety. · Select one or more promising applications, and build prototype models to demonstrate their feasibility and practicality. 96
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Duration 4,000 labor hours over 2 years 95M-S Long-Term Durability of Polymer-Based Composites and Corrosion at Metal-Composite Interfaces (94M-K) Objective Determine the effects of long-term marine exposure on the mechanical properties of polymer-based composite materials, and assess corrosion at metal/polymer interfaces to ensure safety and integrity of vessels incorporating composite materials. Benefit This work will prevent the potential catastrophic failure or need for costly repair of composite structures used in the marine environment due to environmental degradation. SSC National Goal Improve the safety and integrity of marine structures. SSC Strategy Structural reliability engineering Background Increased use of polymer-based composite materials in marine structures raises questions concerning the effects of long-te~ manne exposure on the degradation of composites and also the effects of these composites on the corrosion of metals. When polymer-based composites were first used in aircraft, they absorbed water, which resulted In a decrease in their structural capability. Had exposure experiments been conducted prior to their use, some of the later problems that resulted could have been prevented. The National Aeronautics and Space Administration exposed a number of composite test specimens in low earth orbit prior to extensive usage of the material in such applications. These specimens were later retrieved and tested. In this manner, the sensitivity of these materials to atomic oxygen was discovered and accounted for in later designs, thus preventing potential problems Tom occurring. In the discussion at the National Conference on the Use of Composite Materials in Load-Bearing Manne Structures (SR-1311), it was noted that there are v~rtuaDy no published data on the environmental effects of immersion in seawater. This led to the development of a recommendation to establish a data base for the extension of environmental modeling to marine structures with respect to seawater, temperature, pressure, aging, salt spray, ultraviolet radiation, and marine organisms. A program sponsored by the National Science Foundation at Texas A&M University has begun to address this problem, but much more work stir] remains to be done In this area. The marine environment does have unique constituents to which composite structures have generally not been exposed for long periods microbes, marine growth, numerous ionic species, organisms, and others. It is necessary to determine what effects these factors may have on mechanical properties and thus provide a data base that wild allow for the ability to account for these effects in design. Of specific interest with regard to metal/composite interfaces is galvanic corrosion that can anse from the use of graphite-reinforced composites. Since graphite is a 97
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conductive matenal, its use in systems containing dissimilar metals could lead to unexpected coupling of adjacent metals and severe galvanic corrosion if any of the graphite fibers are exposed and in contact with these metals. Processing, wear, or degradation of the composite with tune can result In exposure of graphite fibers, which can significantly accelerate corrosion of metal in contact with the graphite. It is important to identify what metals and alloys are galvanically compatible with graphite. In addition, electrical isolation methods and design and fabrication processes neec! to be reviewed in order to make graphite-reinforced composites a viable design choice in situations where the composite wall come in contact with metals. Recommendations Perfo~ the following tasks: · Survey composite matenals, and make recommendations on those to be tested. Summarize available long-te~m exposure data. · Conduct experiments on the recommended matenals to expose standard composite specimens (i.e., tension, compression, shear, and interiaminar specimens) to marine environments (both simulated and natural seawater). Test the materials after varying exposure times of up to 30 months. Some of the exposed specimens should be tested under load. In addition, an acceleration procedure, such as high pressure, should be identified and employed for some specimens, and results should be evaluated and compared with the real-time data. · Perform microanalytical, fractographic, and other analyses judged to explain the mechanism of any environmentally influenced property degradation. · Based upon the information obtained, identify models that need to be developed to account for the observed behavior. · Review polymer-based composite-processing methods in order to determine the probability of graphite fibers being exposed at composite-metal mating surfaces. If possible, recommend modifications to current processing methods to ensure that fibers are not exposed at the surface of the composite. · Identify eng~neenng metals and alloys that are galvanically compatible with graphite, and investigate methods for electncally isolating graphite-reinforced composite/metal interfaces. Duration 3,500 labor hours over 3 years 95M-T Analysis and Design Technology Development for Marine Composite Structures (94M-N) Objective Adapt current, and develop as required, analysis and design techniques, methodologies, and practices to permit composite matenals to become a practical and cost-effective option for the construction of ships and offshore-platform structural components. 98
Representative terms from entire chapter: