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ELASTOMERIC CHARACTERISTIC AND PIANC-2002 FENDER TESTING GUIDELINES Indian company, IRM Offshore and Marine Engineers Pvt Ltd is a well known fender manufacturing company and puts in continuous effort for improving the products, processes and testing methods for fenders. In this article Dr Sujit Datta,Senior Technical Manager at IRM offers a commentary on the PIANC-2002 Testing Protocols.

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esting Protocols for evaluation of Fender Performance and reporting in PIANC2002 guideline can be briefly classified in two main categories, (A) Type approval testing where influence of berthing conditions and environmental factors like temperature are discussed and (B) Verification and quality assurance testing where the discussion is based on different compliance testing.Type approval testing mainly deals with “in the lab” simulation testing of a scaled down model of a fender of specific size under different conditions.These variables are: a) Variable Velocity - Velocity Factor b) Variable Ambient Temperature - Temperature Factor c) Variable contact / berthing angle - Effect of contact angle

Rubber - the visco-elastic material Solid rubber elements of different profiles are one of the integral components of fendering systems, used in berth side application for commercial and naval vessels. Rubber fenders are available with several variations of its material besides shape and sizes.The “visco-elastic” behavior of rubber is exploited in the selection of rubber fender, being an energy absorbing element. Completely elastic materials require the same stress, which was applied during the last moment of deformation, to maintain the deformation indefinitely and do not show any dependence on the rate of deformation. In the case of liquids, deformation in the static sense does not exist; progressive deformations are not recovered and energy cannot be stored in the liquid or regained. Instead, the process of deformation requires force, and the work done in this irreversible process is quantitatively dissipated as heat.The term “visco-elastic” has

come to refer to the dealing of those materials which show a behavior intermediate between those of liquids and solids. In rubbers, the elastic element forms the continuous phase but encompasses frictional viscous elements. Such materials are termed visco elastic.When they are deformed, the viscous element consumes energy and retard the elastic deformation; similarly energy is dissipated when the elastic phase returns in the process of strain recovery and releases its stored energy.The viscose element or internal frictions are thus responsible for the energy difference or hysteresis between work recovered and expended.The superimposed elastic and viscous behavior of rubbers is clearly demonstrated during cyclic deformation. Consider a rubber fender subjected to a sinusoidally varying deformation. During a deformation half cycle - from zero deformation, through a maximum, to zero deformation - the rate of deformation and hence the viscous force is a maximum at zero deformation and zero at maximum deformation.The force due to such cycling may be elastic owing to the deformation or viscous owing to the rate of deformation. Each of these force components will be in phase with its contributing process and therefore 900 out of phase with each other in the cycle; their vector sum is the resultant force which is parallel to the rate and extent of the deformation. In an ideally elastic material, all stress is due to strain and is in phase with the deformation; in an ideally viscose material, the stress is in phase with the rate of straining or deformation. In visco-elastic materials, the resultant stress magnitude lags behind the deformation by a phase angle; the more viscose the material, the greater the lag and hence phase angle.The response of rubber to imposed stresses is governed by a characteristic parameter with the dimension of time, which is called relaxation or retardation time, and defined as the ratio of viscous modulus to elastic modulus of a rubber vulcanizate.

Necessity of velocity correction Raw rubber consists of a large number of flexible long chain molecules possessing a structure which permits free rotation about

certain chemical bonds along the chain.The balanced vulcanization introduce cross links distributed randomly throughout the material and build up a three dimensional network structure. Controlled spatial distribution of network structure throughout the rubber vulcanizate helps to resist flow and thus balance the viscous component as well as partly overcomes the irreversible deformation.The elastic component (responsible for recovery from a given deformation) of rubber vulcanizate (visco elastic material) responds instantaneously and does not dissipate energy, but viscose flow or relaxation requires time in dissipating energy proportional to the deformation rate. At low deformation frequency the viscose element will operate but contribute little and the stress will be almost in phase with the deformation. As the deformation frequency is raised the dissipative effort will rise sharply and the stress will become increasingly out of phase with the deformation and largely dependent on the rate of straining or deformation. PIANC-2002 guideline for establishing the performance data corroborate the principles of visco elastic material such as rubber. It demonstrates the process of evaluation of performance parameters like reaction force and energy absorption at different strain rates or berthing velocities. Different initial velocities have been taken into consideration to evaluate the velocity factor on scaled down model fenders of specific size.Velocity factor thus evaluated in a laboratory simulation test on model fender can be applied on actual size fender to calculate corrected performance parameters in practical use. In most practical situations the frequency of deformation of actual fender is low and thus a pre-assumption is made in guideline that the deflection (berthing) frequency of not less than one hour i.e., maximum deformation cycle is one per hour. This assumption greatly reduces the effect of viscous component on overall performance of the fender and thus the resultant property tends to be more elastic at different deflection rates. As a result the magnitude of deviation of performance parameters at different deformation rates from that at RPD deformation

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rate is low. Further the balanced compounding and state of vulcanization can produce the rubber article with optimized elastic-viscous component, which perform within a close tolerance (e.g., ± 10% of the performance value at RPD velocity).

Velocity correction - an experiment

Figure 1: Modulus Factor vs. Strain Ratio

Figure 2: Reaction and Energy Factor vs Strain Ratio (scale down cell fender)

A laboratory evaluation has been tried to establish the variation of key property of well balanced rubber compound (e.g., modulus) with variable strain rates followed by the effect of strain rate on the properties of scaled down model fenders of specific size. In this experiment the modulus of elasticity of rubber compound at different strains has been measured at variable deformation rate e.g., 100, 200, 300, 400, 500 mm/min. Modulus factor and strain ratio have been calculated as follows: Strain Rate Strain ratio = Standard strain rate (A moderate strain rate - 300 mm/min is taken as standard strain rate) Modulus Factor = Modulus of Elasticity at a particular strain rate Modulus at 300 mm/min Figure 1 shows the variation of modulus factor of modulus at different strain (50, 100, 200 & 300%) with strain ratio.Very marginal variation of modulus factor with the strain ratio is observed for all four modulus. Scale down model fenders (Cell & Cone fender of 100mm height are manufactured from the same rubber compound and deformed up to 55% for cell fender and 72% for cone fender at different strain rate as stated above. In both the cases (Figures-2 & 3), the variation of both reaction factor as well as energy factor is marginal with the strain ratio. Reaction factor and energy factor are determined as follows: Reaction factor = Reaction at a particular strain rate Reaction at standard strain rate (300 mm/min) Energy factor = Energy absorption at a particular strain rate Energy absorption at standard strain rate (300 mm/min) The above laboratory evaluation demonstrates that the performance of rubber compound can predict the different performance parameters of actual fender if made up of same rubber compound at variable conditions.

Temperature effect Figure 3: Reaction and Energy Factor vs Strain Ratio (scale down cone fender)

A property of visco-elastic materials like rubber

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shop level the testing can be carried out at specified velocity, temperature and angle and the performance can be adjusted as per the corrections factors arrived at during the prototype testing.

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Figure 4: Service temperature range of different rubbers

is that they progressively stiffen as the temperature decreases. PIANC-2002 guideline for type approval test also considers this fact. It suggests the method of evaluation of temperature factor over a wide temperature range i.e. -300C to 500C. Figure 4 shows the service temperature range of different elastomer. Variation of berthing angle also has influence on performance parameter i.e. reaction force and energy absorption.Variation of contact angle has been taken into consideration over a wide range from 30 to 200. Studies prevail that both the reaction force and energy absorption value decrease with increase in contact angle.

Verification testing The second part of the guideline deals with verification / quality assurance testing of actual fenders as well as the importance of evaluation of physical properties of rubber compound. Evaluation of physical properties of rubber compound and the performance of production / actual fender is extremely important in view of real functioning. On site performance trial of actual fender under all possible real environmental conditions is probably more effective than test on scale down model fender under ideal laboratory simulated working as well as environmental condition. However at

☛ Established in 1964, having extensive experience of manufacturing entire range of Marine Fenders and Offshore Rubber Engineering Products. ☛ Becoming the first Fenders and Offshore Rubber Product manufacturing company in this part of the World having complete IMS (ISO 9001 : 2000, ISO 14001 : 2004, OHSAS 18001 :1999) certified by Germanischer Lloyd GmbH - Germany. ☛ Having valid Manufacturers Capability Certificate (MCC) certified by Germanischer Lloyds - Germany to manufacture any type and size of Solid and Pneumatic Fenders available in today’s Maritime World. ☛ A corporate Member of PIANC. ☛ Offers the widest range of Marine Fenders and Rubber Engineering Products for offshore platform installation and protection. ☛ Products and process conform to various international standards like ASTM/ BS/ ASME/ PIANC/ AWS/ ISO/ and DIN. ☛ Fully Integrated quality management system to ensure best quality of products and services. ☛ Offers the full range of services- from designing to manufacturing, testing, installation and maintenance. ☛ Well equipped Research, Development and Testing Laboratory approved by GL - Germany. ☛ Globally networked for sales and services. ☛ Worldwide acceptance. ☛ Impressive clientele and supply record.

About the Author

Compression testing of scale down model cone fender in high capacity computer controlled compression test equipment

Dr Sujit Datta (M.Tech, PhD in Polymer Science and Technology) is a Sr. Manager (Technical) of IRM offshore and Marine Engineers Pvt. Ltd - India, having substantial experience in research & development as well as manufacturing field. He has published many papers in International Journals and Conference. If you have any query related to content of this article, feel free to contact on [email protected]