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Effect of visco elastic properties of engineered textile materials

Key Words: ETM, Creep Curves, Visco-elastic.

Introduction:Before going into the study details, it is important to understand the key terms those are closely related to the assignment to be discussed. The “Engineered Textile Materials (ETMs)” under the title can be defined as the textile materials that are developed and/or designed for a special need and application where a very high-performance is required. Engineered textiles may combine fabrics with glass, ceramics, metal, or carbon to produce lightweight hybrids with required properties such as sophisticated finishes, silicone coatings and holographic laminates, transform color, texture and etc.

Visco elasticity: is the property of materials that exhibit both viscous and elastic characteristics when undergoing deformation. Viscous materials, like honey, resist shear flow and strain linearly with time when a stress is applied. Elastic materials strain instantaneously when stretched and just as quickly return to their original state once the stress is removed. Visco-elastic materials have elements of both of these properties and, as such, exhibit time dependent strain.[1]

Visco- elastic Creep: is subjected to a step constant stress, visco-elastic materials experience a time-dependent increase in strain. This phenomenon is known as visco-elastic creep. At a time t0, a visco-elastic material is loaded with a constant stress that is maintained for a sufficiently long time period. The material responds to the stress with a strain that increases until the material ultimately fails.

There are many composite engineered textiles those have the visco-elastic properties such as mesh sack, geo-textiles with fibrous membrane, protective flex, textile scaffolds, shape memory material, vascular graft, high performance Inflatable coated fabrics and etc.

Discussion: ETMs are fibers (main components) and fibers polymer. That’s why all polymers show the elastic, viscous and visco-elastic behavior. So, the effect of visco-elastic property in ETM is inevitable to  show. At the same time, it is to be noted that all the ETMs are also under temperature as well as time dependent characteristics [2]

In ETM, both viscous and elastic properties coexist. ETMs with increased elasticity (textiles made from elastane yarn as one of the components of composite) possess a wider region of time-dependent deformation or visco-elastic region [3]

The prediction of the mechanical behavior of elastomeric materials/ETM has been an active research area for many years. There have been numerous experimental studies addressing different characteristics of the elastomeric response. For example, it is well known that an elastomer undergoes significant softening during the first couple of load cycles and that the material response after that becomes repeatable [4].

Vascular graft, is one the ETMs, has sufficient visco-elastic properties similar to blood vessels so that it is sufficiently compliant but will not allow for over expansion or bursting. Vascular graft supports the patients having cardiovascular problems to live their normal life. PET (Polyehelenetereripthalate) and PTFE (Polytetrafluoroethylene) are chemically stable material after implant when compared with others. PTFE is most successful material for small diameter vascular graft. Good handling property of graft includes pre-clotting resistant during implant and it should have high breaking resistant along with high visco-elastic property [5].

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Mesh sack, a composite ETM (fabric) manufactured from warp knitted polyester to prevent heart enlargement for patients with degenerative heart failure in where the effect of visco- elastic property is easily understandable.

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Composite scaffolds show mechanical properties of the same order of magnitude as values for native articular cartilage, as measured by compressive, tensile and shear testing. Moreover, findings showed that porous composite scaffolds could be engineered with initial properties that reproduce the anisotropy, visco-elasticity and tension-compression nonlinearity of native articular cartilage [6].

Protective Flex is also an example, which can show the visco-elastic behavior by the action of restraining, cushioning and flexibility as required. PET (Polyethele Terephthalate) & PCL (Poly capro lactone) are the component of Protective Flex.The response of polymeric materials with stress or strain is dependent on, loading rate and time, difference between material behaviours, such as elastic, viscous and or visco-elasticity [7].

Visco-elastic property or behavior of Engineered Textile Materials are as follows:

  • At low temperature and high strain rate: Engineered Textile Materials demonstrate elastic behavior.
  • At low temperature and low strain rate: Engineered Textile Materials demonstrate viscous behavior.
  • At intermediate temperature and strain rate: Engineered Textile Materials demonstrate visco-elastic behavior.

Behavior of Elastic Materials:

  • Elastic behaviour is instantaneous.
  • The total deformation (or strain) occurs the instant stress is applied or release.
  • Upon release of the external stress-the deformation is totally recovered.
  • The specimen assumes its original deformation.
  • At low strain in conformity to Hooke’s law, strain is proportional with strain
  • The strain is independent of time.
  • Stress is not dependent with loading rate.

At time ta  Load Applied &  that has been unloaded by time tr

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Behavior of Viscous Materials:

  • Deformation or strain is not instantaneous.
  • In response to an applied stress- the deformation is delayed or dependent with time.
  • This deformation is not reversible or completely recovered after stress is released.
  • At low strain rate, behave according to the Newtonian relationship.
  • Totally dependent with time.
  • Stress being function of strain rate.
  • Stress independent of strain.

At time ta  Load Applied &  that has been unloaded by time tr

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Behavior of Visco-elastic Materials:
This behaviour is in between of elastic and viscous materials.

  • At low temperature & high strain rate, polymer demonstrates elastic behaviour.
  • At high temperature & low strain rate, polymer demonstrates viscous behaviour.
  • At intermediate temperatures & rate of strain, polymer demonstrates visco-elastic behaviour.
  • Instantaneously, elastic strain followed by viscous time dependent strain.

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characteristics of ETM (Engineered Textile Materials). This is a phenomenon of a sample to deform against time at particular temperature. Creep is the tendency of a solid material (Here, ETM) to move slowly or deform permanently under the influence of stresses. It occurs as a result of long-term exposure to levels of stress that are below the yield strength or ultimate strength of the material. Creep is more severe in materials that are subjected to heat for long periods, and near the melting point. Time & temperature dependent Creep Phenomenon is closely related with visco-elastic properties of polymers. Here, the phenomenon is applicable for composite engineered textile material as the engineered textile materials are polymeric components.

There are three stages of Creep as below:
1. Primary Creep: The slope of strain versus time decreased.

2. Secondary Creep: This is constant strain rate.

3. Tertiary Creep: The strain rate increased rapidly until rupture (formation of crack, yielding etc.) The creep rate of polymeric materials (Here, ETM) is dependent on loading time and temperature. The creep results can be explained by three types of widely used curves those are as follows:

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1. Isometric Curve: At the beginning, the stress is higher due to bonding forces between atoms are higher. After a few moments, slippage between atoms occurs and the polymer crystallization rate decreased than the strain was increased with time. By using this curve, long terms material behavior can be known or predicted.

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2. Modulus Curve: The elasticity of certain materials exists due to the materials decomposition of chain to become more order. This graph is widely used for the determination of materials rigidity and persistent based on the life span of the materials. Creep modulus decreased with increasing time showing the visco-elastic behavior.

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3. Isochronous Curve: The slope of the graph is equivalent to the Young modulus, E that is the determination of resistance towards the neighboring separation of the atoms. The high modulus value can be obtained from small strain changes due to the applied stress. By using this graph, the comparison can be done of various types of polymeric materials during design.

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Time & Temperature dependent creep phenomenon is closely related with visco-elastic properties of polymers. Here, the phenomenon is applicable for composite ETM as the engineered textile materials are polymeric components.

Conclusion:
From the study, the importance of visco-elastic properties understood not only in case of ETMs,but also the wide varieties of textile materials. If we can assess the impact of visco- elastic properties in advance during designing the engineered textiles, our campaign would be successful as well as we can easily meet our valued customers/buyers/clients’ requirements. Specially, the aspect of mechanical behavior of engineered textiles can be studied. Textile materials are polymers or fibers that are visco-elastic and their mechanical behavior can be adjusted by mechanistic model consisting of unidirectional friction, hook, spring etc., when correctly combine could produce the mechanical behavior under mechanical stress. It is more essential for assessing the material in advance to get the desired output. If the assessment is done properly, it is possible to create desired products by selecting proper material, technique, design,or modification of design if required. The below are the test names suggested for the real assessment:

1. Tensile strength test (Breaking strength, elongation, recovery, stretch etc.).
2. Tear Strength test.
3. Bursting Strength test.
4. Stretch & Recovery test.
Stress-Strain Curve is also one of the most important tools to study about the effect of visco-elastic property of ETMs.

REFERENCES:
[1]. Mayers and Chawla (1999); “Mechanical Behavior of Materials.” page: 98-103
[2]. Shutian Liu, Fengzhi Wang: “Temperature-dependent viscoelastic properties of unidirectional composite materials”, 16th International Conference on Composite Materials.
[3]. Dunja Sajn, Gorjanc, Vili Bokosek, “The bahivior of fabric elastane yarn during stretching”, Fibres and Textiles in Eastern Europe, Volume 16, No 3 (68 ).
[4]. J.S. Bergstrom and M.C. Boyce, “Consecutive modeling of the large strain time-dependent behavior of elastomers”, Vol 46, pp 931-954, 1998.
[5]. Chinta S.K. and Abhisek,  “Medical Textiles – Vascular Graft”, International Journal of Advanced Biological Research; Vo 2 (3): 2012, pp: 557 – 560.
[6] Franklin T. Moutos, Lisa E. Freed & Farshid Guilak,; “A biomimetic three-dimensional woven composite scaffold for functional tissue engineering of cartilage”, Nature Materials,  Vol 6, pp : 162-167 , 21 January, 2007.
[7] Dr. Azura A. Rashid, “Principle of Visco-elasticity”, School of Material & Mineral resource engineering. EBB 220/3

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