The Influence of Structure of Multilayer Woven Fabrics on Their Mechanical Properties

Multilayer woven fabrics used for conveyor belts must be characterized by high mechanical strength. The design process of multilayer woven fabrics for such application requires taking into account the structural characteristics of the fabric, which allows to adjust the final product properties to the dedicated use. The geometry of warp threads—means stuffer and binding is the decisive aspect, which influences the strength properties of multilayer woven fabrics and materials made with their use as well. The aim of this work was to examine the possibility of shaping mechanical strength and bending rigidity of multilayer woven fabrics by changing the order of introducing weft threads into individual layers. The eight variants of multilayer woven fabrics were manufactured using laboratory harness loom. They were produced using different structural models in two weft variants, then tested. The mechanical features were determined, such as breaking force, recovered and unrecovered elongations in cyclic tensile test, stiffness rigidity. The analysis of the obtained results confirmed, that both the model and the order in which the weft threads were introduced into individual layers influence the mechanical strength and bending rigidity of multilayer woven. It was found, that the strength properties characterized by the above mentioned indicators are influenced by the number of threads weaved as both the stuffer and binding warp.


Introduction
Textiles are a group of products which, thanks to their properties and diverse structures, are used in many areas, including technical ones. The usage of textiles as structural elements, thanks to their ability to shape their structure and properties, allows to provide engineering constructions meeting of high strength requirements. The one of the group of textile materials, that are used in technical applications, are multilayer woven fabrics (MLW). The MLW are structures consist of a great number of layers connected to each other by an additional warp or arrangement of warp threads. Multilayer woven fabrics are most often used for drive belts, conveyor belts, structural composite reinforcements and others. Conveyor belts are usually used in coal mines, cement and lime industries, paper and sugar factories, agriculture, and power plants [1][2][3].
The right choice of the belt is crucial for the stability of the whole installation. The belt, regardless of its type and purpose, has solid elements, which consist of: spacers made of fabric impregnated with latex solution, carrying pulling force, and rubber covers, protecting the core against damage, affecting the life of the tape [4,5]. All materials used in the production of conveyor belts are characterized by a strong non-linear mechanical behaviour. During an operation on the conveyor, the belt is subjected to loads that change over time [6].
The safety coefficient of the conveyor belt system changes with the type and size of the material to be transported, the methods of material loading, the belt construction and installation of the conveyor. It is assumed that the working load is about 10-15% of the are bonded by means of the warp binding was analysed [30]. It was found that warp introducing order was responsible for load transfer characteristic and can influence tensile strength of products significantly.
The continuous development and new applications of such products and growing expectations of its end users cause the continuous development of MLW structures and dedicated them measurement methods [31].

Materials and Specimen Preparation
The four basic structural models of MLW are known, based on the geometry of binding threads. The course of the binding threads, i.e., the method of binding significantly influences the internal structure of the woven fabric. The distribution of the binding threads also affects the quantity and packing of the two remaining basic systems. The structures of multilayer woven were classified into four models taking into account the geometry of binding warp thread's course.
Structural models of multilayer woven [32] (Figure 1  The order of wefts to be inserted: k-subsequent-from the bottom to the top layer, presented in Figure 2; p-shifted-from lower to upper layer-from upper to lower layer, shown in Figure 3.   The order of wefts to be inserted: k-subsequent-from the bottom to the top layer, presented in Figure 2; p-shifted-from lower to upper layer-from upper to lower layer, shown in Figure 3. are bonded by means of the warp binding was analysed [30]. It was found that warp introducing order was responsible for load transfer characteristic and can influence tensile strength of products significantly. The continuous development and new applications of such products and growing expectations of its end users cause the continuous development of MLW structures and dedicated them measurement methods [31].

Materials and Specimen Preparation
The four basic structural models of MLW are known, based on the geometry of binding threads. The course of the binding threads, i.e., the method of binding significantly influences the internal structure of the woven fabric. The distribution of the binding threads also affects the quantity and packing of the two remaining basic systems. The structures of multilayer woven were classified into four models taking into account the geometry of binding warp thread's course.
Structural models of multilayer woven [32] (Figure 1  The order of wefts to be inserted: k-subsequent-from the bottom to the top layer, presented in Figure 2; p-shifted-from lower to upper layer-from upper to lower layer, shown in Figure 3.   are bonded by means of the warp binding was analysed [30]. It was found that warp introducing order was responsible for load transfer characteristic and can influence tensile strength of products significantly. The continuous development and new applications of such products and growing expectations of its end users cause the continuous development of MLW structures and dedicated them measurement methods [31].

Materials and Specimen Preparation
The four basic structural models of MLW are known, based on the geometry of binding threads. The course of the binding threads, i.e., the method of binding significantly influences the internal structure of the woven fabric. The distribution of the binding threads also affects the quantity and packing of the two remaining basic systems. The structures of multilayer woven were classified into four models taking into account the geometry of binding warp thread's course.
Structural models of multilayer woven [32] (Figure 1  The order of wefts to be inserted: k-subsequent-from the bottom to the top layer, presented in Figure 2; p-shifted-from lower to upper layer-from upper to lower layer, shown in Figure 3.   For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1).
The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.  14,1315 For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1). For each structural model, two versions of multilayer woven, differ in case of thread insertion order into the individual layers have been developed (Table 1).
The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The MLW fabrics were manufactured of 1060 dtex with 256 filaments, high strength continuous filament polyester yarn with reduced elongation. The yarn twist was 60 twists/m. The tensile properties of yarn were measured, results in Table 2 was shown.
The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant: The multilayer woven were produced on SW550 type laboratory weaving station, consisting of an automatic warping machine and SL8900S harness loom (Figure 4), equipped with two warp shafts while the following constant technological parameters remain constant:

Mechanical Tests
The basic structural parameters of produced MLW fabrics were tested. The mass per unit area according to PN-EN 12127:2000 [33] was determined thickness according to PN-EN 5084:1999. The crimp factor of stuffer and binding warp threads [34] following the

Mechanical Tests
The basic structural parameters of produced MLW fabrics were tested. The mass per unit area according to PN-EN 12127:2000 [33] was determined thickness according to PN-EN 5084:1999. The crimp factor of stuffer and binding warp threads [34] following the Equation (1) was calculated: where: w o -crimp factor, %; l rz o -the actual length of the warp thread taken from woven section, mm; l p o -the section length of woven fabric within the warp thread is present, mm. During the tensile tests of manufactured fabrics, the maximum force F max and elongation at this force were determined in accordance with PN-EN ISO 13934-1:2013-07 [35].
For further tests purpose it was assumed, that during use, the forces acting on the fabric structure constitute about 70% of the F max . The fabrics were subjected to five cycles of stress and relaxation up to 70% F max . The values of recovered and un-recovered elongations were determined in the 4th test cycle. Additionally, the values of elongations at the force of 35% F max were determined in three consecutive measuring cycles (fatigue test).
The samples were conditioned and tested in the atmosphere: the temperature of 20 ± 2 • C and relative humidity 65 ± 5%.
The bending rigidity of fabrics was performed using the fixed angle method according to PN-73/P-04631. The test method is based on the relationship developed by Peirce, between the length of woven sample from the fixed point to leading edge deformed under its own weight and bending rigidity of woven fabric.

Structural Parameters
The results obtained during mentioned test are presented in Table 3. In fabric I, in which the threads are introduced successively into the layers, the mass per unit area is almost 9% greater than in fabric Ia, in which the threads are introduced in an shifted order. It is related to the increase in the warp thread crimp factor. In case of fabrics: II, III, and IV, the area weights are lower in the variant of the subsequent introduction of threads compared to fabrics in which the threads are introduced in an shifted order, respectively, for IIa by 1%, IIIa by 2.9%, and IVa-by 2.8%.
The highest thickness in both weft variants, reaching 2.35 and 2.43 mm, respectively, for fabric III was achieved. Mentioned fabrics are characterized by the lowest values of the warp thread crimp factor (3-5 and 2-4%, respectively) and the lowest values of mass per unit area. The smallest thickness for fabric II was obtained, in both variants (II and IIa), achieving 1.72 and 1.70 mm, respectively. The crimp factor of stuffer warp threads and binding warps differs significantly from 3 to 19% (fabric II) and from 4 to 20% for fabric IIa, respectively. The crimp factor of weft threads is greater in the case of shifted weft variant, for all fabrics. The considerable differences occur in fabrics produced according to the O/T and O/L models, but in case of A/T and A/L fabrics models they are less significant.

Mechanical Properties of the Woven Fabrics
The test results obtained for mechanical properties of woven fabrics are presented in Table 4. The measurements were carried out using Instron 3367 tensile machine ( Figure 5) equipped with tensile sensors of following capacity: 10 and 30 kN.     (10,500 N). The reason, that Fmax values, recorded for fabric the lowest ones was the way of both warps inside A/T model. The stuffe The both variants of fabric I (I and Ia) showed the lowest values of maximum force F max , reaching 4982 and 4862 N, respectively. The highest values of F max were achieved by fabrics II and IIa (10,500 N). The reason, that F max values, recorded for fabric I and IIa were the lowest ones was the way of both warps inside A/T model. The stuffer and binding warps were fixed not enough firmly in woven structure, creating the long paths between the thread's jams in fabric. Therefore, the load was transmitted in the simply layout, warp threads were elongated along relatively long distances in fabric structure, being more fragile to damage. Typically, inside this structure only the part of warps works simultaneously, so if they broke, the fabrics failed. These woven fabrics were made according to the models in which the layers binding took place through the whole thickness of the fabric (i.e., T). On the other hand higher values of maximum force occur for the shifted warp variant in the case of fabrics performed in accordance with the L binding layer model, means III/IIIa and IV/IVa woven fabrics.

A/T/k A/T/p O/T/k O/T/p A/L/k A/L/p O/L/k O/L/p
The binding model together with warp order variant shown an opposite effect on elongation value at the force F max . In case of test result obtained for A model fabrics with angled binding, higher elongation values occur when subsequent warp order is used, compared to shifted one.
The fabrics III and IV in both warp order variants present the best elastic properties. The recovered elongation determined for fabrics III and IV reach up to 72%, compared to 58 and 61% obtained for fabric I. The warp order variant does not influence recovered elongation significantly. In structures designed for conveyor belts, which operates in continuous mode and under heavy load, it is important to evaluate the elongation in the fatigue test. The values of elongation at 35% F max in the 3rd tensile cycle marked 35, correspond to the fatigue characteristics of the fabric. The highest values of 35 was 24%, reached by fabric I and 21%-reached by fabric Ia. The lowest 35 values 11 and 12% were achieved by fabrics III and IIIa, respectively. The lowest increase in elongation values from first to third cycle shown fabrics I and Ia, 26 and 30%, respectively, while for the other fabrics this enlargement is higher, especially for subsequent weft order, rising to even 48% for fabric II.
It was found, that the insertion way of stuffer warp threads influences strength characteristics the most. Taking into account the relationship between the F max and the stuffer thread crimp factor (Figure 6), a firm negative correlation was found (correlation coefficient r = −0.93). The value of elongation at F max and the crimp factor of stuffer warp is characterized by a strong positive correlation (correlation coefficient r = 0.87). The values of recovered elongation at 70% F max in the 4th cycle and the crimp factor of stuffer warp (Figure 7) are also correlated (correlation coefficient r = −0.82).
Materials 2021, 14, 1315 9 of 12 correspond to the fatigue characteristics of the fabric. The highest values of Ɛ35 was 24%, reached by fabric I and 21%-reached by fabric Ia. The lowest Ɛ35 values 11 and 12% were achieved by fabrics III and IIIa, respectively. The lowest increase in elongation values from first to third cycle shown fabrics I and Ia, 26 and 30%, respectively, while for the other fabrics this enlargement is higher, especially for subsequent weft order, rising to even 48% for fabric II. It was found, that the insertion way of stuffer warp threads influences strength characteristics the most. Taking into account the relationship between the Fmax and the stuffer thread crimp factor (Figure 6), a firm negative correlation was found (correlation coefficient r = −0.93). The value of elongation at Fmax and the crimp factor of stuffer warp is characterized by a strong positive correlation (correlation coefficient r = 0.87). The values of recovered elongation at 70% Fmax in the 4th cycle and the crimp factor of stuffer warp (Figure 7) are also correlated (correlation coefficient r = −0.82).

Binding Rigidity
For MLW characterized by the lowest and highest values of elongation at 35% Fmax, i.e., fabrics I, Ia, III, and IIIa the bending rigidity was determined. In order to analyse the influence of the structural model, the bending rigidity was additionally determined for fabric II (Table 5).  It was found, that the insertion way of stuffer warp threads influences strength characteristics the most. Taking into account the relationship between the Fmax and th stuffer thread crimp factor (Figure 6), a firm negative correlation was found (correlation coefficient r = −0.93). The value of elongation at Fmax and the crimp factor of stuffer warp is characterized by a strong positive correlation (correlation coefficient r = 0.87). The values of recovered elongation at 70% Fmax in the 4th cycle and the crimp factor of stuffe warp (Figure 7) are also correlated (correlation coefficient r = −0.82).

Binding Rigidity
For MLW characterized by the lowest and highest values of elongation at 35% Fmax i.e., fabrics I, Ia, III, and IIIa the bending rigidity was determined. In order to analyse th influence of the structural model, the bending rigidity was additionally determined fo fabric II (Table 5).

Binding Rigidity
For MLW characterized by the lowest and highest values of elongation at 35% F max , i.e., fabrics I, Ia, III, and IIIa the bending rigidity was determined. In order to analyse the influence of the structural model, the bending rigidity was additionally determined for fabric II (Table 5). For fabrics dedicated for conveyor belts, the bending rigidity along the weft is an important parameter. The bending rigidity determines the structure ability to take the shape from a conveyor. The highest values of the rigidity of the tested fabrics along the weft were found in case of fabric I and Ia. Additionally, the difference between the bending rigidity along weft and warp directions in these fabrics was the greatest. The fabric III in both warp order variants has the lowest values of bending rigidity along the weft. Moreover, in this case, bending rigidity in both warp and weft directions are very similar. At the same time, there was no effect of warp order on fabric rigidity for fabric III. The compared fabrics I and III are manufactured according to model A (angle interlock) and significant differences in bending rigidity come from the method of binding (L or T). Binding type L (layer to layer) caused a decrease in bending rigidity along the weft.
Comparing the values of the analysed parameter determined for fabrics I and II (models A and O), it can be concluded that for model A (angle interlock) the bending rigidity along the warp is much lower, despite along the weft being at the same level.
The significant influence the crimp factor of binding warp on bending rigidity was found, the correlation coefficient reached r = 0.94 ( Figure 8). Moreover, the correlation between bending rigidity along weft direction and the crimp factor of stuffer warp was determined (r = 0.64).  For fabrics dedicated for conveyor belts, the bending rigidity along the weft is an important parameter. The bending rigidity determines the structure ability to take the shape from a conveyor. The highest values of the rigidity of the tested fabrics along the weft were found in case of fabric I and Ia. Additionally, the difference between the bending rigidity along weft and warp directions in these fabrics was the greatest. The fabric III in both warp order variants has the lowest values of bending rigidity along the weft. Moreover, in this case, bending rigidity in both warp and weft directions are very similar. At the same time, there was no effect of warp order on fabric rigidity for fabric III. The compared fabrics I and III are manufactured according to model A (angle interlock) and significant differences in bending rigidity come from the method of binding (L or T). Binding type L (layer to layer) caused a decrease in bending rigidity along the weft.
Comparing the values of the analysed parameter determined for fabrics I and II (models A and O), it can be concluded that for model A (angle interlock) the bending rigidity along the warp is much lower, despite along the weft being at the same level.
The significant influence the crimp factor of binding warp on bending rigidity was found, the correlation coefficient reached r = 0.94 ( Figure 8). Moreover, the correlation between bending rigidity along weft direction and the crimp factor of stuffer warp was determined (r = 0.64).

Conclusions
The multilayer woven fabrics were made according to different structural models in two warp order variants and was tested for of determination their structural parameters, tensile strength properties and bending rigidity. The analysis of the obtained results confirmed that both the model and the order in which the warps were introduced into individual layers affect the mechanical properties and bending rigidity of MLW. In terms of the weft bending rigidity values along recovered elongation and breaking force, the best results were obtained for fabrics III and IIIa. A slight improvement in the weft bending rigidity, the value of recovered elongation at 70% Fmax in the 4th cycle, as well as the maximum force (Fmax) by using shift weft order was obtained. It was found that the

Conclusions
The multilayer woven fabrics were made according to different structural models in two warp order variants and was tested for of determination their structural parameters, tensile strength properties and bending rigidity. The analysis of the obtained results confirmed that both the model and the order in which the warps were introduced into individual layers affect the mechanical properties and bending rigidity of MLW. In terms of the weft bending rigidity values along recovered elongation and breaking force, the best results were obtained for fabrics III and IIIa. A slight improvement in the weft bending rigidity, the value of recovered elongation at 70% F max in the 4th cycle, as well as the maximum force (F max ) by using shift weft order was obtained. It was found that the crimp factor values determined for both warp threads (stuffer and binding) have an influence on tensile properties of MLW, characterized by F max , recovered elongation and bending rigidity. In the individual models and variants of weft order, the crimp of warp threads varies and is related to the geometry of threads course. The evaluation of test results shown, that mechanical strength and bending rigidity of MLW could be shaped by their construction, especially by way in which the warps were interlaced. The design process of multilayer woven fabrics for technical applications requires understanding and evaluating the structural characteristics of the fabric, which allows the product properties to be adapted to the dedicated application. The results of carried out works were an input data for further search for optimal MLW properties. They include conveyor belts core structures with different areas desirable stiffness.