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1

Tensile and tearing properties of bi-axial warp knitted coated fabrics

Luo Y., Hong H., Fangueiro R.

Autex Research Journal

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2008

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Vol. 8, no. 1

17--20

EN

Based on uni-axial tensile testing, the performances of bi-axial warp knitted PET/PVC flexible composites has been analyzed in seven in-plane directions, i.e., 0°,15°,30°,45°,60°,75° and 90°. The crack propagation of each sample has been observed and analyzed by evaluating the influence of the crack length and direction under uni-axial testing. The results show that bi-axial warp knitted coated fabrics present a strong orthotropic behavior and that the tearing strength depends on the initial crack orientation and length. Regardless of the crack length and orientation, the propagation is always perpendicular to the tensile loading direction.

2

Modelling and simulation of the mechanical behaviour of weft-knitted fabrics for technical applications. Part IV: 3D FEA model with a mesh of tetrahedric elements

Araújo M., Fangueiro R., Hong H.

Autex Research Journal

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2004

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Vol. 4, no. 2

72--80

EN

This paper is in four parts. The first is related to general considerations and experimental analyses, and each of the others is related to different approaches to the theoretical analyses of the mechanical behaviour of weft-knitted fabrics and weft-knitted reinforced composites made of glass fibre. The objective is to find ways of improving the mechanical properties and simulating the mechanical behaviour of knitted fabrics and knitted reinforced composites so that the engineering design of such materials and structures may be improved. In Part IV the technologies for weft-knitted 3D complex shape preform development are surveyed and a third model is presented. This a 3D model based on FEA (finite element analyses). A solid representation of a 2D yarn is built up, and an MES (mechanical event simulation) is applied to obtain a 3D-shaped loop. The final knitted fabric geometry is obtained by interacting this loop with the adjacent loops, according to the dimensional properties of the knitted fabrics and using an MES. Finally, the geometry of the reinforcement inside the composite is built up, and the composite material is divided into small tetrahedric elements to obtain a mesh of finite tetrahedric elements (FEA). The average values of the mechanical properties are obtained with FEA and compared with the experimental ones.

3

Modelling and simulation of the mechanical behaviour of weft-knitted fabrics for technical applications. Part III: 2D hexagonal FEA model with non-linear truss elements

Araújo M., Fangueiro R., Hong H.

Autex Research Journal

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2004

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Vol. 4, no. 1

25--32

EN

This paper is in four parts. The first is related to general considerations and experimental analyses, and each of the others is related to different approaches to theoretical analyses of the mechanical behaviour of weft-knitted fabrics and weft-knitted reinforced composites made of glass fibre. The objective is to find ways of improving the mechanical properties and simulating the mechanical behaviour of knitted fabrics and knitted reinforced composites, so that the engineering design of such materials and structures may be improved. In Part III the second model is presented, and this is a 2D model based on FEA (finite element analyses). A plain weft-knitted fabric, based on the simple loop structure, is simplified and represented by a 2D hexagonal structure constructed by non-linear truss elements. The characteristics of the truss elements for FEA simulation are obtained from experimental results through an analytical method when a loop is converted to a FEA model. The elongation deformation is simulated in one, two and multiple directions. The model can also be used to calculate a planar knitted fabric for deformation to fit a 3D spherical mould.

4

A friction model of microbearings with thin metallic compliant coatings

Stiharu I., Bhat R., Hong H.

International Journal of Applied Mechanics and Engineering

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2003

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Vol. 8, no 2

317-329

EN

The paper presents experimental results of the investigation on the micro-tribological properties of Ti thin film layers deposited on metallic substrates of various hardness. The tests were carried out under standard dry friction conditions using a miniature pin on disk apparatus. The experimental results found for the friction coefficient developed between 52100 steel pin and Ti as a compliant layer deposited on various hardness substrates from soft to hard are consistent with other results presented in literature for the pair steel/Ti. A friction model that assumes the plowing of microasperities into the soft thin film layer supported by the elastic substrate as the main factor in the originating friction is further proposed. A simplified one-asperity model is assessed against the compliant layer assumed deposited on a rigid substrate. The analysis of the influence of the stress developed at the interface substrate-film is assumed in this model. Tests carried out qualitatively confirmed that the influence of the inter-laminar stress is significant to friction. Given the simplification assumptions, the proposed model exhibits reasonable agreement with the experimental results. It is further suggested that the lubrication of the hard surfaces with thin compliant metallic layers of Ti represents an alternative for the friction improvement in the small bearings belonging to miniature mechanical systems and the Micro-Electro-Mechanical Systems (MEMS).

5

Modelling and simulation of the mechanical behaviour of weft-knitted fabrics for technical applications. Part II: 3D model based on the elastica theory

Araújo M., Fangueiro R., Hong H.

Autex Research Journal

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2003

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Vol. 3, no. 4

166--172

EN

This paper is in four parts. The first is related to general considerations and experimental analyses, and each of the others is related to different approaches to the theoretical analyses of the mechanical behaviour of weft-knitted fabrics and weft-knitted reinforced composites made of glass fibre. The objective is to find ways of improving the mechanical properties and simulating the mechanical behaviour of knitted fabrics and knitted reinforced composites so that the engineering design of such materials and structures may be improved. In Part II the first model is presented, a 3D model based on the classic elastica theory which is used to calculate the load-extension curves of a plain weft-knitted fabric in the coursewise and walewise directions. Good agreement is obtained between theoretical and experimental results.

6

Modelling and simulation of the mechanical behaviour of weft-knitted fabrics for technical applications. Part I: General considerations and experimental analyses

Araújo M., Fangueiro R., Hong H.

Autex Research Journal

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2003

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Vol. 3, no. 3

111--123

EN

This paper is in four parts. The first is related to general considerations and experimental analyses, and each of the successive papers is related to different approaches to theoretical analyses of the mechanical behaviour of weft-knitted fabrics and weft-knitted reinforced composites made of glass fibre. The objective is to find ways of improving the mechanical properties and simulating the mechanical behaviour of knitted fabrics and knitted reinforced composites, so that the engineering design of such materials and structures may be improved. In Part I, general considerations, experimental analyses and ways of improving the mechanical properties of weft-knitted fabrics and knitted reinforced composites are discussed. In Part II the first model is presented, a 3D model based on the classic elastica theory, and it is used to calculate the load-extension curves of a plain weft-knitted fabric in coursewise and walewise directions. Good agreement is obtained between theoretical and experimental results. In Part III the second model is presented, a 2D model based on FEA (finite element analyses). A plain weft knitted fabric, based on the simple loop structure, is simplified and represented by a 2D hexagonal structure constructed by non-linear truss elements. The characteristics of the truss elements for FEA simulation are obtained from experimental results through an analytical method when a loop is converted to a FEA model. The elongation deformation is simulated in one, two and many directions. The model can also be used to calculate a planar knitted fabric to be deformed to fit a 3D spherical mould. In Part IV the technologies for the development of weft-knitted 3D complex shape preforms] are surveyed and a third model is presented, a 3D model based on FEA (finite element analyses). A solid representation of a 2D yarn is built up, and a MES (mechanical event simulation) is applied to obtain a 3D shaped loop. The final knitted fabric geometry is obtained by interacting this loop with the adjacent loops, according to the dimensional properties of the knitted fabrics and by using a MES. Finally, the geometry of the reinforcement inside the composite is built up, and the composite material is divided into small tetrahedric elements to obtain a mesh of finite tetrahedric elements (FEA). The average values of the mechanical properties are obtained with FEA and compared with the experimental ones.

7

Process control for total quality in circular knitting

Araújo M., Catarino A., Hong H.

Autex Research Journal

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1999/2000

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Vol. 1, no. 1

21--29

EN

This paper deals with a problem of major concern to the knitting industry, which is fabric defects. When a defect occurs, the knitting machine has to be stopped and the fault corrected, thus resulting in time loss which is uneconomic. Eventually, the knitted fabric may be rejected if quality requirements are not met. An effective monitoring of the knitting process is required in order to avoid or detect and locate a defect and its cause as soon as possible, avoiding productivity and quality losses. In circular knitting machines the yarn input tension (Ti) can be used as a means of process control, so that defects may be prevented or quickly detected. This was found to be a valuable approach to accomplish this task, since it reflects the general behaviour of the knitting machine. A measuring system is presented, along with the results obtained. Considerations are made around the problem of automatic detection and some approaches are suggested. Finally, some conclusions are drawn from the work developed.