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*Current Address: The University of Bolton, Deane Road, Bolton BL3 5AB, UK**Department of Metallurgy and Material Engineering, Katholieke Universiteit,Leuven, Kasteelpark Arenberg, 44, B-3001 Leuven, Belgium
A fabric consisting of an assembly of textile fibres (oriented in one direction or in a random manner) held together (1) by mechanical interlocking; (2) by fusing of thermoplastic fibres, or (3) by bonding with a rubber, starch, glue, casein, latex, or a cellulose derivative or synthetic resin.
Importance of Mechanical Properties in Nonwoven Structures
Thermal bonded structures undergo various modes of deformation during their end-use performance. For example, geotextiles when placed under the soil exhibit high levels of tensile and compressive modes of deformation. Characterisation of tension, shear, compressional and bending resistance is needed for accurate prediction of fabric draping during the formation of a three-dimensional (3D) shaped composite part for many automotive applications.
The main objective of the research work was to characterise the mechanical properties of thermal bonded nonwoven structures namely, tension, bending, shear and compression, including anisotropy in the properties, along with the fibre orientation distribution in the fabric. A simple micromechanical model has also been proposed to investigate the initial tensile behaviour of thermal bonded structures.
Measurement of Mechanical Properties• Tension: Fabric strips of 20x15 cm were tested
on an Instron tensile testing machine under uniaxial loading in a “grab” test. Poisson’s ratio was also determined by measuring the contraction in the centre of the specimen relative to the strain in the test direction.
• Shear: Picture frame was mounted on Instrontensile testing machine.
• Bending: KES-F B2 (Bending Tester) was used and the fabric was bent between the curvature of –2.5 and 2.5 cm-1. Three cycles were repeated for each sample in the machine (0°) and cross-machine (90°) directions.
• Compression: KES-F B3 (Compression Tester) was used and the pressure was increased gradually up to 50 g/cm2.
Ambiguities, Errors and Corrections in 2D Image Analysis
• Ambiguity: There are two possible values for fibre in the in-plane orientation angle ( ) as the fibres with orientations and + 180 have identical cross-sections.
• Correction: Assuming the orientation distribution to be symmetrical, and the out-of-plane orientation angle is randomised by using the following equation.
Ambiguities, Errors and Corrections in 2D Image Analysis (Cont.)
• Ambiguity: The probability of finding a fibre with defined orientation ( ) such that it has an elliptical cross-section on the sectioning plane.
• Correction: The “probability of intersection”between the fibre and the sectioning plane has been determined by dividing the fibre orientation distribution with cosine of the sectioning angle (Zak et al., 2001).
MT cc,
’
( , )
cos
XZi jXZ
ijcorrectedi
Q T MQ Tc c
Zak, G., Park, C. B. and Benhabib, B., Estimation of three-dimensional fibre-orientation distribution in short-fibre composites by a two-section method, Journal of Composite Materials, 35, 316-339 (2001).
Ambiguities, Errors and Corrections in 2D Image Analysis (Cont.)
• Error: Small errors in the measurement of elliptical axes for the fibres oriented nearly perpendicular ( ) to the sectioning plane can yield large errors in the measured value of (Mlekusch, 1999).
• Correction: Fitting a normal distribution to the measured values of in-plane fibre orientation angles .
T cT c
Mlekusch, B., Fibre-orientation in short-glass-fibre composites II: Quantitative measurements by image analysis, Composites Science and Technology, 59, 547-560 (1999).
A simple micro-mechanical model has been developed to predict the anisotropy of the tensile properties of through-air bonded structures. The model is based on the averaging schemes of bond distribution and fibre orientation proposed by Pan et al.(1993), Komori and Makashima(1977, 1978) and Lee and Lee (1985).
Assuming the centre of fibreB ( ) is brought into contact with the surface of fibre A ( ), then the parallelepiped consisting of two rhombuses of length l and height 2D, where D is the diameter of the fibre.
MT ,MT cc,
FMTMT sin2),,,( 2Dlv cc
where V is the total volumeP is the probability that fibre A will contact with fibre B
is the angle between the two neighbouring axes of fibres A and B
VDl
PFsin2 2
Contact of Fibre A with Fibre B (Komori and Makashima, 1977)
)cos(sinsincoscoscos MMTTTTF c�c�c
1. Komori, T., and Makishima, K., Numbers of fibre-to-fibre Contacts in General fibre Assemblies, Textile Research Journal, 47, (1977).
Consider a tension test in the direction D with respect to the machine direction. Here, volume ( ) of the sample, having a unit cross-sectional area (AD=1) that is limited by two planes normal to the test direction. According to Lee and Lee (1985),
Projections of a fibre length between two bonds
is the directional parameter indicating the length projections of the fibres on the test direction.
is the projection of the average length of the fibre between two bonds ( ) on the test direction
DK
Db
D�b
b
1�
�A
DdV
DDD KDLIV
Kbb2
b
T
MD
�
sinb
)sin(sinb ���
�
)cos(sinb �
�
/ 22
0 / 2
sin cos( ) ( , )K d d
� �
�
T T M D T M D M
�
� �
� �: �³ ³Assuming the fibres lying parallel to X-Y plane, therefore,
Tensile Properties of Through-air Bonded Structures
• The slope of curves decreases with the angle of test, showing high degree of anisotropy of the tensile resistance.
• The ratio of tensile strength in the machine direction to the cross-machine direction of thermal bonded structures TB1 and TB2 was found to be 3.3 and 3.5, respectively.
• TB1 has lower tensile strength in comparison to TB2 in all the test directions, although the fibres (4.4 and 12 dtex) used in the production of TB2 have high tensile strength in comparison to the fibres (2.2 and 3.3 dtex) used in TB1.
Shear Properties of Through-air Bonded Nonwoven Structures
• The first shear cycle describes as the sample installation on the frame rather than the actual material behaviour.
• Initial high shear stiffness (up to shear angles of 3°) represents bond resistance that is sufficient to prevent rotation of the fibres in the thermal bonded nonwoven structure. Later, bond resistance is overcome and the shear resistance is primarily dominated by the rotation of structural elements of the fabric.
• TB1 requires more shear force than nonwoven TB2 at higher shear angles although both fabrics have similar weight and thickness.
• The scatter of the local shear angle is high, i.e. 16°to 32°at a frame shear angle of 30°.
0
0.005
0.01
0.015
0.02
0.025
0 10 20 30 40 50
Shear Angle (o)
She
ar F
orce
per
uni
t wid
th (N
/mm
)
1st
2nd
3rd
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0 10 20 30 40 50
Shear Angle (O)
Sh
ear F
orce
(N/m
m)
TB1
TB2
Typical diagrams for the three shear cycles (TB1) Comparison of shear diagrams of TB1 and TB2
Bending Properties of Through-air Bonded Nonwoven Structures (Cont.)
• The fabrics are significantly (2-3 times) stiffer in bending in the machine (0°) direction than in the cross-machine direction, which is easily explained by the preferential orientation of the fibres in the machine direction.
• The bending hysteresis values are quite small, indicating reversibility of the bending deformation and hence strong and elastic bonds between the fibres.
• The bending diagrams are linear for the cross-machine direction and non-linear for the machine direction. This correlates well with the higher hysteresis values for the machine direction and indicates the presence of the frictional component of the bending resistance.
• The bending resistance of fabric TB1 is higher than TB2 in the cross-machine direction. The same arguments of higher density of the finer fibres and lower local porosities in the nonwoven TB1 can explain the observed difference.
Comparison of Theoretical and Experimental Results of Initial Tensile Response
Comparison between theoretical and experimental secant modulus of through-airstructures (a) TB1 (b) TB2 at 4.17 % strain in various test directions
a
0
5
10
15
20
0 22.5 45 67.5 90
Test Angle (o)
Sec
ant M
odul
us (M
Pa)
Experimental
Incl. Poisson’s Ratio
Excl. Poisson’s Ratio
b
0
5
10
15
20
0 22.5 45 67.5 90
Test Angle (o)
Sec
ant M
odul
us
(MP
a)
Experimental
Incl. Poisson’s Ratio
Excl. Poisson’s Ratio
There is a distinct difference between the theoretical and experimental values of secant modulus in test directions, 0 and 45°(TB1) and 22.5°(TB2). The deviations from the experimental observations may have been caused by the assumption that the distribution of in-plane orientation of the fibres to be normal. This may have exaggerated the number of fibres oriented in these directions.
• The mechanical properties namely, tension, compression, bending and shear of two through-air bonded nonwovens of similar weight and thickness have been investigated.
• The anisotropic characteristics of the properties in relation to the fibre orientation distributions have been studied.
• The observed behaviour correlates well with the directional anisotropy and with the peculiarities of the structural characteristics (different fineness of the fibres and observed porosity).
• The initial tensile response of through-air bonded nonwoven structures has been modelled based upon orientation averaging and simple fibre deformation scheme, taking into account the effect of Poisson’s ratio.
• Measurements of orientation distribution of fibres using the 2D image analysis of the fabric cross-sections are subjected to errors for fibres oriented normal to the section plane. These errors should be corrected using general hypothesis of the type of the distribution function.
• Anisotropy of tensile and bending behaviour of the through-air bonded nonwoven fabrics correlates well with the anisotropy of orientation distribution of the fibres. The tensile resistance can be reasonably estimated using simple orientation averaging approach.
• In all the mechanical tests (tension, bending and shear) the frictional losses are small, suggesting good stability and elasticity of the thermal bonds.
• The picture frame technique can be used to study the shear behaviour of nonwoven structures in the wide range of the shear angles. The second and third shear loading cycles should be used for the characterisation of the shear behaviour.
• Optical full field measurement of the strain is a promising instrument to study the unevenness of nonwoven structures.