UDC 677.077.001.4:006.354 Group.409
INTERSTATE STANDARD
TECHNICAL FABRICS
Method for determining breaking load and elongation at break 29104 4 91
Industrial fabrics.
Method for determination of breaking stress and extension
MKC 59.0S0.30 OKSTU 8209, 8309
Date in command 01/01/93
This standard applies to technical fabrics and establishes a method for determining breaking load, elongation at break and standard load.
1. SAMPLING METHOD
Selection of spot samples - in accordance with GOST 29104.0 with the following addition: the length of the spot sample must be at least 500 mm.
2. EQUIPMENT AND MATERIALS
2.1. To carry out the test use:
tensile testing machines that provide a constant lowering speed of the lower clamp (pendulum type), or a constant rate of deformation, or a constant rate of increase in load with a relative error in the breaking load readings ± 1.0%, an absolute error in the elongation readings ± 1.0 mm, with an average breaking duration adjustable from ( 30 ± 15) to (60 ± 15) s.
If disagreements arise, tests are carried out on pendulum tensile testing machines.
metal measuring ruler according to GOST 427;
stopwatch according to TU 25-1S94.003.
2.2. Tensile testing machines must be equipped with clamps of the VNII "GG system (Fig. 1).
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2.3. To avoid slipping or cutting of the elementary sample, gaskets may be used in the clamps of tensile testing machines. In this case, the ends of the spacers must be at the level of the clamping planes that limit the clamping length of the sample.
3. PREPARATION FOR THE TEST
3.1. Before testing, spot samples are kept in climatic conditions in accordance with GOST 1068! at least 24 hours
The fabric is tested under the same conditions.
3.2. From each point sample, seven elementary samples are taken in the form of strips: three for the warp and four for the weft.
Elementary samples are preliminarily marked as follows. so that one sample is not a continuation of another. The longitudinal threads of the elementary sample must be parallel to the corresponding warp or weft threads of the spot sample. The first elementary sample in the direction of the base is marked at a distance of at least 50 mm from the edge of the point sample. Elementary samples in the direction of the weft are marked at a distance of at least 50 mm from the edge of the point sample, distributing them sequentially along the length.
The cutting diagram for elementary samples is shown in Fig. 2.
3.3. The dimensions of elementary samples are taken to be 50 x 500 mm or 80 x 500 mm. Permissible deviations in the size of elementary samples are set *mm.
Depending on the design of the clamping devices, it is allowed to use elementary samples with a length of more than 500 mm.
3.4. The working width of elementary samples should be 25 or 50 mm. The permissible deviation should not be more than 0.5 mm.
3.5. To obtain the working width of an elementary sample, the threads in the longitudinal directions are removed from both sides until until the load-bearing width becomes 25 or 50 mm.
3.6. When preparing elementary samples from fabrics with frayed outermost lobar threads, one of the following methods is used:
a) on an elementary sample with easily crumbling outer threads, mark the working width and insert the elemental sample into the clamps of the tensile testing machine. On both sides of the sample, perpendicular to the direction of stretching in the middle, cuts are made along the longitudinal threads to the lines indicating the working width. The samples cut on both sides of the threads are taken away, except for 2-4 threads bordering the marked lines;
b) on an elementary sample with little shedding outer threads, remove the threads from both
sides along the length of the elementary sample, leaving 2-4 threads on each side of the marked lines. In that part of the elementary sample that will be tucked into the upper clamp, these threads are pulled back and cut 25-30 mm larger than the length of the jaw of the clamp. The end of the prepared sample with the remaining threads is inserted into the lower clamp, and the other end into the upper clamp.
3.7. On the tensile testing machine, set the distance between the clamps equal to (200 ± 1) mm.
3.8. The load scale of the tensile testing machine should be selected so that the average breaking load of the tested point sample is from 20 to 80% of the maximum scale value.
3.9. The lowering speed of the lower clamp of the tensile testing machine is set so that the average duration of the process of stretching an elementary sample before breaking corresponds to (40 ± 25) s.
4. CONDUCT OF THE TEST
4.1. One end of the elementary sample is inserted into the upper clamp of the tensile testing machine without distortion and it is clamped lightly. The other end of the sample is inserted into the lower clamp and a preload weight is suspended. When the upper clamp is loosened under the action of a preload, the elementary sample drops slightly. Then firmly clamp first the upper and then the lower clamps. After this, the lower clamp is activated.
4.2. The preload value is selected depending on the surface density of technical fabrics in accordance with the table.
4.3. If an elementary sample breaks in the clamp or at a distance of 5 mm or less from the clamp, the test result is taken into account only if its value is not less than the minimum breaking load provided for in the regulatory and technical documentation for technical fabrics. Otherwise, additional elementary samples are subjected to rupture.
4.4. The values of breaking load and elongation at break are taken from the corresponding scales of the tensile testing machine after breaking the elementary sample.
4.5. When testing technical fabrics made from combined yarns, machine scale readings are taken at the moment the force meter needle stops for the first time.
4.6. The elongation of the fabric under a standard load is recorded at the moment the arrow of the force meter indicates the load, installed in accordance with the regulatory and technical documentation for a specific fabric, or according to the “load - elongation” diagram, which is obtained on the recording device of the tensile testing machine. The chart processing technique is given in Appendix 1.
In case of disagreement, elongation at standard load is determined using the load-elongation diagram.
5. PROCESSING RESULTS
5.1. The breaking load of the fabric is taken as the arithmetic mean of the results of all measurements on the warp or weft.
The calculation is carried out to the first decimal place and then rounded to the nearest whole number.
5.2. The elongation (/) of an elementary sample at break along the warp or weft in percent is calculated using the formula
where /1 is elongation at break, mm;
200 - distance between clamps of the tensile testing machine, mm.
The final result is taken as the arithmetic mean of all measurements on the warp or weft.
The elongation of the fabric under a standard load is taken as the arithmetic mean of all measurements along the warp or weft.
Calculations are carried out with an error to the second decimal place, followed by rounding to the first decimal place.
5.3. The test report is given in Appendix 2.
APPENDIX I Mandatory
The load-elongation diagram is taken on a non-menss scale M 1:1 and processed as follows:
1. From point a on the curve, a perpendicular is lowered onto the / axis. The length of the perpendicular ab corresponds to the value of the actual breaking load of the elementary sample. Using a measuring metal ruler, measure the length of the perpendicular ab in millimeters.
2. On the perpendicular ab, mark the segment cd, corresponding to the value of the load established in the regulatory and technical documentation for a specific fabric or from the actual breaking load of an elementary sample. Dishu of segment cb
where Pim is the load norm, for which it is necessary to determine the intermediate elongation value, daN (kg);
4a - length of perpendicular ab. mm;
/"p - actual breaking load of an elementary tissue sample, daN (kgf).
3. From point c parallel to the / axis, draw a straight line until it intersects with the curve (point d).
4. From point d, a perpendicular de is lowered onto axis I.
5. On axis I, measure the segments oe and oh.
6. The intermediate value of elongation (/t) as a percentage is calculated using the formula
/ - 1 L CM l *
where I is elongation at break. %;
1m - length of the segment oe. mm;
1ы> - length of segment ob, mm.
APPENDIX 2 Mandatory
TEST REPORT
The test report must contain:
name of fabric;
batch number;
type of tensile testing machine;
preload value, N (kgf);
breaking load of an elemental sample along the warp and weft, daN (kgf): arithmetic mean value of the breaking load along the warp and weft. daN (kgf); elongation at standard load along warp and weft, %: arithmetic mean value of elongation at break along warp and weft. %: elongation under standard warp and weft load. &;
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Breaking load is the greatest force that a material can withstand before failure and expresses its ability to bear load.
For fabrics, the breaking load (absolute) is usually expressed in newtons (N) or kilogram-force (kgf); 1 kgf" ~9.8 N.
This indicator is mandatory for most fabrics of various fiber compositions. Interest in it is explained by the comparative simplicity of its definition; In addition, the tensile load of fabrics makes it possible to indirectly assess the qualitative composition of the raw materials used to produce products, as well as the degree of damage to the material in the final finishing processes. For example, fabrics made from defective wool or insufficiently mature cotton have breaking load values that are lower than the norms. Overburning, overpainting, improper singeing, bleaching, or finishing with thermosetting resins (crease-resistant finishing) also reduce the tensile strength of the fabric. Therefore, despite the fact that fabrics, especially for household use, usually do not experience loads close to breaking during operation, the latter are widely used to characterize the mechanical properties of fabrics and are standardized in standards.
Breaking load is often used to evaluate the wear kinetics of fabrics. In Fig. Figure 3 shows typical curves of changes in the breaking load of fabrics during the operation of the latter. As you can see, a high initial value of the breaking load does not yet determine the behavior of the fabric in the sock. One fabric (curve) had a greater initial breaking load than the other fabric (curve). But during operation, the first fabric wears out faster, and after a certain period its breaking load is less than that of the second fabric. In this regard, the fabric to which the curve corresponds has a shorter wear life.
Elongation at break (absolute) is the difference between the length of the sample at the moment of rupture and its clamping length before rupture.
Fabrics with high elongation at break, such as wool and synthetic fibers, usually have good elasticity, wrinkle resistance, abrasion resistance, etc.
Like breaking load, elongation at break largely depends on the quality of the raw material from which the fabric is made. At the same breaking load, the best fabric in terms of mechanical properties is considered to be the one that has a higher elongation at break. The mechanical properties of the fabric to which the curve / corresponds are better than those of the fabric to which the curve corresponds, since due to the greater elongation at break, the work of rupture (shaded area) is greater. Since the work of rupture characterizes the amount of energy that must be expended to break the material, the first fabric can be considered more “strong” than the second.
The breaking load and elongation at break of fabrics are determined by testing three test strips for the warp and four for the weft. The dimensions of the test strips are indicated in table. 6. If disagreements arise, test strips measuring 50x100 mm for woolen fabrics and 50x200 mm for all other fabrics are tested. Blanks for test strips are cut from a fabric sample using special metal templates. The width of the workpieces is 30 or 60 mm, the length should be 150 mm greater than the clamping length. Longitudinal threads are removed from both sides of the workpieces until the working width of the test strips of fabric becomes equal to 25 or 50 mm.
According to GOST 3813-72, test strips are subjected to stretching to failure on three types of tensile machines: with a variable rate of increase in load and deformation, with a constant rate of increase in load, and with a constant rate of deformation. The difference between these machines lies in the nature of the loading or deformation of the material being tested.
In Fig. Figure 5 shows diagrams of load and deformation obtained on tensile testing machines of various types. Machines of the second and third types are considered more advanced, since the nature of the increase in load or deformation of the materials tested on them does not depend on the characteristics of the mechanical properties of the latter. This makes it possible to more correctly evaluate the mechanical properties of different materials in comparison. Machines of the first type lack this advantage. For example, a shows diagrams of the increase in load and deformation of two tissues. Despite the fact that the final test results of these fabrics (breaking load and elongation at break) are the same, it cannot be said that the mechanical properties of the fabrics are the same. At the same time, machines of the first type are simpler to design and operate.
A test strip of fabric is tucked into the clamps. The clamp is connected to a lever (pendulum). Therefore, the machines in question are sometimes called tensile testing machines with a pendulum force meter, or pendulum-type tensile testing machines. The clamp can be lowered at a constant speed; it receives movement from some kind of drive, usually electric. As the lower clamp moves, force is transferred through the sample to the upper clamp, and the load arm begins to deviate to the left. The load on the sample increases in proportion to the increase in the angle cp. At the moment of destruction of the test strip, the arrow of lever 2 stops and on the scale / shows the value of the breaking load. And on scale 3, the elongation at break is determined.
By changing the load on lever 2, you can change the range of loads obtained during testing.
In the USSR, the RT-250M tensile testing machine with a pendulum force meter is commercially produced, having a load range from 0 to 50 and from 0 to 250 kgf. We note here that the load scale of the tensile testing machine should be selected so that the average breaking load of the test sample is within 20-80% of the maximum scale value.
According to GOST 3813-72, when inserting test strips into the clamps of a tensile testing machine, they are given pre-tension by hanging special weights from the lower end of the test strip. The size of the pre-tension weights is selected depending on the size of the test strip and the surface density of the fabric being tested.
When testing, the lowering speed of the lower clamp of the tensile testing machine must be such that the average duration of stretching of the test strip before failure corresponds to 30 ± 5 s for fabrics with an elongation of less than 150% and 60 ± 15 s for fabrics with an elongation of 150% or more.
The arithmetic mean of all primary results is taken as the final result when determining the breaking load and elongation at break.
Tearing load is the force (kgf, N) required to break a specially cut test strip of fabric. This load characterizes the ability of tissues to withstand force, which is concentrated in a relatively small area, for example, during tears, when rigidly securing the edge of the fabric, etc.
When determining the tearing load (GOST 17922-72), test strips cut from the sample - three with a transverse arrangement of warp threads and four with a transverse arrangement of weft threads - are marked according to the diagram. An incision is made along the line and the resulting tabs are inserted into the clamps of the tensile testing machine along lines AB and AC. The distance between the clamps is set to 100 mm, the lowering speed of the lower clamp is 100 ± 10 mm/min. When the lower clamp moves, the load is transferred through the longitudinal threads to the transverse threads and they tear in the direction of the cut. The test strip is broken to line aa. The tearing load of the fabric is calculated as the arithmetic mean of the results of the primary tests for the warp and weft.
Typically, the tearing load of tissues is much less than the breaking load. For example, if according to GOST 5067-74 the tearing load of silk and semi-silk dress and suit fabrics is at least 0.8 kgf, then the breaking load is at least 20 kgf.
For cotton and silk fabrics with pile, the standards should standardize the strength of pile fixation.
The strength of the pile is characterized by the force required to pull out one lint from the pile fabric. When determining this indicator (GOST 3815.3 -77), five strips measuring 20X100 mm are cut from the sample along the base. Another strip of fabric 20 mm wide and 250 mm long is sewn to both ends of each strip. By folding the resulting tape in half, a number of fibers are isolated from the test strip of fabric, which are clamped in the upper clamp of a tensile testing machine for testing a single thread. The lower part of the tape under a tension of 25 gf is inserted into the lower clamp of the tensile testing machine. The distance between the clamps is 200 mm, the lowering speed of the lower clamp is 200 mm/min. At the moment of complete pulling out of the villi, the readings of the load scale are noted. The fibers remaining in the upper clamp are counted, after which the force required to pull out one fiber is determined.
Textile materials in clothing most often experience tensile deformation. This type of deformation is the most studied.
The classification of characteristics obtained by stretching a material is presented in Scheme 2.1.
Half-cycle discontinuous characteristics. These characteristics are used primarily to evaluate the ultimate mechanical capabilities of textile materials. By the indicators of mechanical properties obtained when the material is stretched to the point of rupture, the degree of resistance of the material to constantly acting external forces is judged; indicators of breaking load and breaking elongation are important standard indicators of material quality.
Uniaxial tension. Let us consider the main half-cycle tensile characteristics obtained with simple uniaxial tension.
Indicators of half-cycle characteristics are determined when the material is stretched using tensile testing machines.
A rectangular sample (Fig. 2.2, a) is accepted as standard for testing fabrics, knitted and non-woven fabrics. A test method based on the use of such a sample is often called the strip method. For fabrics, the following sample sizes are established: width 25 mm, clamping length 50 mm (in controversial cases, width 50 mm and clamping length 200 mm, and for woolen fabrics 100 mm). For knitted and non-woven fabrics, sample width 50 mm, clamping length 100 mm.
Samples, the shapes of which are shown in Fig. 2.2, b, c, widely used in research work. To test highly tensile materials (for example, knitted fabrics), samples in the form of a double spatula or in the form of a ring sewn from a strip of material are sometimes used (Fig. 2.2, d, d).
When testing textile materials for uniaxial tension, the following main characteristics of mechanical properties are obtained.
the force that samples of material can withstand when stretched to the point of rupture. The breaking load is expressed in newtons (N) or decanewtons (daN); 1 daN = 10 N = 1.02 kgf. „ Elongation at break (elongation at break) is the increment in the length of a tensile sample of material at the time of its rupture. The absolute value of elongation /p, mm, is obtained as the difference between the final L.K. and initial L0 sample lengths. The relative value of elongation of the material at the moment of its rupture kr is determined as the ratio of the absolute value of elongation /p to the original length /_„ and is expressed either in fractions of a unit:Where a and n are coefficients, the values of which depend on the type of material and its structure.
Other characteristics are also used to assess the strength properties of textile materials.
Specific breaking load PyR, N m/g, calculated by the formula
Rule = PR/BMs,
B- width of the material sample, m; Ms - surface density of the material, g/m2.Indicators of specific breaking load for some textile materials, given in table. 2.3, take into account the surface density of materials and allow one to compare their strength properties.
The mass t m2 of many fabrics contains different proportions of the mass of warp and weft threads. For such fabrics, the specific breaking load is calculated using the formula
PyR = Pp/(BMs50(Y)),
Where 5o(U) is the proportion of the mass of the warp (or weft) threads, calculated using the formulas given on p. (37.
N (daN or kgf), - breaking load per element of the material structure (per one warp or weft thread in fabric, per one loop row or column in knitwear, per one stitching line in non-woven fabrics):Where P - the number of threads in a fabric sample, rows or columns in a knitwear sample, stitching lines in a non-woven fabric sample along which the sample is stretched.
When stretching samples of materials, a certain amount of work is expended, which is spent on overcoming the energy of bonds in the material (between fibers and threads, between atoms and macromolecules in the fiber-forming polymer). If a load P is applied to the material and the material receives elongation (increase in length) Dl(De), then the meaning of elementary work D.R. is defined as the product of load (force) and length increment (Fig. 2.5):
D.R. = Pdl,
Where D.R. - elementary work, J.
The total work expended on the rupture is Rp, J
Where d) is the coefficient of completeness of the load-elongation diagram.
The initial stiffness modulus quite fully characterizes the resistance to deformation of low-extensibility materials. Resistance of easily extensible materials module E( characterizes roughly. According to Prof. A. I. Koblyakova, modulus values E] for knitted fabrics are very small and amount to 1 10~3-1 10"4 μPa. Moreover, when testing the fabric along the width, the value is 2-8 orders of magnitude less than when testing along the length.
Setting the initial stiffness module E1 allows us to describe the stress-strain relationship for a material: a = Z^c*. Calculation of the indicators of knitted fabrics using this formula indicates their good agreement with experimental data at stresses close to breaking. For the initial stretching period, significant deviations of the calculated data from the experimental ones are observed.
For easily extensible materials, when calculating the initial stiffness modulus, A. N. Solovyov proposed not to take into account the initial zone of the diagram (Fig. 2.10), since in this zone the rigidity of the material is practically not manifested. In this case, the initial stiffness modulus Ez + b Pa, for the second zone is calculated using the formula
Where dp is the stress at break, Pa; er- elongation at break, %; K2 - stiffness indicator that determines the nature of the stress-elongation diagram in the second zone:
112 = S2 ,
Where b1 is the area of the figure ACD (see Fig. 2.10); S 2 - area of the figure AFCD (dot A - the beginning of the departure of the stretching curve from the abscissa).
The stress-elongation relationship for the second zone of the diagram can be described as
0 = EZ+I(Јp-Z)K2 -
The current stiffness modulus e (at r = 0) allows you to evaluate the material’s resistance to deformation at any elongation value. Module /g is calculated as the first E, sk:
Derivativefrom a
The final stiffness of the material is estimated by the modulus of the current final stiffness E1K, calculated for the moment of rupture of a material sample (at r = 0 and c = e) according to the formula
ET, TO= ke4-1.
Strength properties of materials. Strength is an important property of materials, which constantly attracts the attention of researchers and is studied comprehensively. The main problem of strength is revealing the mechanism of destruction of materials, identifying the reasons for the discrepancy (underestimation) of the actual strength of Materials with its theoretical value.
I Several theories have been proposed to explain the process of destruction of bodies. Proponents of the critical nature of the rupture (theory of physical stress) - A. Griffith and his followers, "(considering strength properties, proceed from the assumptions that any real body, unlike an ideal one, does not have a "Perfect structure and contains a significant amount defects (microcracks) that weaken it. Destruction occurs when, as a result of the action of a load, the overstress at the top of at least one of the microcracks reaches a value corresponding to the theoretical strength determined by the forces of interatomic bonds. In this case, the microcrack begins to grow with the speed of propagation of elastic waves (at the speed of sound) and causes destruction of the material.
The hypothesis about the existence of defects (microcracks) was experimentally confirmed by Academician. A. F. Ioffe and his colleagues, who showed that the stress at the tip of a surface microcrack is many times higher than the stress value determined by the ratio of the effective load to the area
di cross section of the weakened sample sample. It was found that the development of microcracks is the result of the action not of average, but of maximum, critical stress. The work of A.F. Ioffe and his colleagues explained the difference between the theoretical and experimental values of strength.
However, such a purely mechanical approach to solving the problem of strength, based on the assumption of the critical nature of the rupture, does not reveal the essence of the phenomena occurring in loaded bodies during their destruction over time. From the perspective of this theory, it is impossible to explain the difference in the strength values of a material at different rates of its deformation.
Academicians A.P. Aleksandrov and S.N. Zhurkov proposed a statistical theory of strength, according to which the rupture of the material does not occur simultaneously over the entire fracture surface, but begins from the most dangerous defective area, where the overstress reaches a value close to the theoretical strength. Then the rupture occurs in a new dangerous area of a micro-crack, etc. As a result of the growth of cracks, the material is destroyed.
Thus, the statistical theory of strength considers destruction as a process that occurs over time. The main position of the statistical theory of strength is that the probability of the occurrence of the most dangerous defects is much less than that of less dangerous ones, and the most dangerous defect located on the surface determines the strength of the material. The practice of testing materials confirms this fact. Samples that are small in size (minimum cross-section) are characterized by increased strength. As the sample sizes of textile materials decrease, their strength increases.
When studying the strength properties, it was noticed that the process of material destruction, which is temporary, depends not only on the magnitude of the acting load, but also on the test temperature and the structure of the material.
Fundamental research in the field of strength properties carried out by S.N. Zhurkov and his colleagues led in the 1950s. to the creation of a kinetic theory of strength of solids. According to this theory, the destruction of materials occurs not so much due to the acting mechanical force, but due to the thermal movement (fluctuation) of structural elements (atoms).
An important role in interatomic interactions is played by the unevenness of thermal motion - energy fluctuations, which are a consequence of chaotic thermal motion. In this case, individual atoms acquire kinetic energy many times greater than the average. As a result of excess energy, thermal tensile forces in interatomic bonds also increase. The rupture of the material occurs mainly as a result of fluctuations in thermal energy and thermal decomposition of interatomic-dysfunctional bonds. The existing mechanical stress reduces the energy barrier, activates and directs the destruction process. Thus, the mechanical strength of materials, according to the theory of S. N. Zhurkov, is determined not by purely mechanical, A kinetic nature due to thermal movements of atoms.
From the standpoint of the kinetic theory of strength, the main factors influencing the strength of materials are the absolute temperature T, the effective stress a and the duration of the stress t. The fundamental characteristic of strength is durability. The basic durability equation has the form
T = t0 exp ---- -.
The parameter m0 does not depend on the nature and structure of the material. Its value is 10~12-10"13 s - the duration of one thermal vibration of atoms; UQ - energy of activation of destruction, i.e. the energy of bonds that must be overcome in order to destroy the material; y is a structure-sensitive coefficient that strongly depends on the structure of the material. The coefficient y characterizes the heterogeneity of stresses in the volume of the body and indicates how many times the true local stress, under the influence of which destruction practically occurs, is higher than the average stress; a is the constant voltage acting during the test; R - universal gas constant; T is the absolute test temperature.
Works by G. N. Kukin, A. A. Askadsky, L. P. Kosareva and other employees of the MIT. A. N. Kosygin confirmed the possibility of using the basic principles of the kinetic theory of strength to describe the destruction of textile threads.
Research by B. A. Buzov and T. M. Reznikova (MTILP) showed that the temperature-time dependence of strength is also suitable for such rather complex mesh systems as fabrics. The short-term and long-term strengths of cotton and nylon fabrics under uniaxial tension over a wide temperature range were studied. Fabric samples measuring 5x50 mm were tested over a time range (s) of five to six orders of magnitude. During the experiments, the actual time of destruction of the samples was recorded. Experiments have confirmed the possibility of using the basic durability equation to describe the process of tissue destruction, but with some modifications. As is known, fabric is a material of a complex structure, therefore, determining the value a for it - the constant Voltage acting during the test - presents significant difficulties. As a result, to calculate the long
Ig t Fig. 2.11. Dependence is permanent
Sti fabric art. 52188 from load at
Temperature, ° C: / - +60; 2 - +30; 3 - +20; 4 30.
For tissue eternity, instead of value a, an equivalent value was used - the pressure created by a constant 1 2 3 4 5 6 L MPa load P and determined by
A unit of cross-sectional area of fabric. The cross-sectional area was taken to be the initial cross-sectional area of the fabric sample along the system of loaded warp (weft) threads. The total cross-sectional area of the sample was determined as the product of the number of threads directly involved in tensile resistance and the average cross-sectional area of these threads. Thus, the durability of the fabric was studied under constant load, and its calculation was carried out using the formula
U0 ~ YP 1 = T° eХР RT "
The research results presented in Fig. 2.11 indicate that the main patterns of temperature-time dependence of strength are also characteristic of such complex mesh systems as fabrics. Received parameter values U0 p y are consistent with the parameter values of similar studies of fibers and threads;
Parameter Cotton Nylon
TOC o "1-3" h z Fabric art. 3/04 fabric
Art. 52188
U 0 , kJ/mol........................ 145 190
U, m3/kmol........................ 0.7 2.5
|
5Н 4 3 2- I - 0- -11- -12- |
Durability of fabrics. When uniaxially stretched along the warp or weft thread, the strength of fabrics, characterized by breaking load ppt, depends primarily on the strength and number of longitudinal threads of the test sample directly bearing the load. In fabric, threads due to mutual interweaving are connected by friction into a single system. Therefore the average
breaking load on one thread of a strip of fabric PP11t, located in the direction of the acting force, there may be greater breaking load for the same thread Рр, in a free state.
Fabric breaking load Prt calculated by the formula
Рр,= Ррп1П = РрмКгП,
Where P- the number of threads in the cross-section of a strip of fabric; TO- coefficient of utilization of the breaking load of the thread in the fabric, equal to 0.8-1.2; tj is the coefficient of thread heterogeneity for breaking load, equal to 0.85.
Coefficient TO the more frequent the connections and the greater the girth angles, which determine the friction area of mutually perpendicular systems of threads. As the length of thread overlap increases, the number of connections and the value of the coefficient decrease TO. Therefore, plain weave, which has frequent connections between the threads, all other things being equal, provides the greatest strength to the fabric.
With an increase in the number of threads per 10 cm of fabric, the angles of the threads and, consequently, the friction surface increase, the cohesion of the fabric elements increases, the force of mutual pressure of the warp and weft threads and the degree of adhesion of the fibers in the yarn become greater. As a result, the coefficient "L" and the strength of the fabric increase. Beyond the optimal number of threads per 10 cm, not only does the growth of strength stop, but also, due to overstressing of the threads, the fabric weakens.
Twisted yarn, the fibers of which are quite tightly twisted, is strengthened by the weave into the fabric less than weakly twisted single yarn.
Heterogeneity of threads in terms of breaking load reduces the strength of the fabric. The threads with the smallest elongation are the first to take the load and break, after which the load is redistributed to the remaining threads, as a result of which an increasing force is exerted on each of them, and the tissue rupture occurs earlier than with the simultaneous rupture of all threads.
Taking into account the distribution of forces acting on the threads in the fabric when it is stretched (Fig. 2.12), TO. I. Koritsky proposed to determine the load PpjlT according to the formula
Рр1„ = (Ррн +R)chsof,
Where F- load caused by the action of friction forces and a decrease in the sliding length of the fibers; p - angle of inclination
Rice. 2.13. Diagram of tearing: load /p and elongation of tissue when it is stretched in different directions (value RR and g:r but based on
Threads to the line of application of tensile force at the moment of rupture.
The value /" depends on the friction of the threads, the force of normal pressure and the deflection of the thread; it is calculated by the formula
Where p is the friction coefficient of the threads; pp. Msin p is the normal pressure force on one thread of the stretchable system; And is a value proportional to the deflection of the thread.
Thus, the breaking load of fabric, taking into account the parameters of its structure, can be determined by the formula
Ррт = Ррр.„(1 + И sin рЛ)г| cos p.
Fabrics are anisotropic bodies, so their strength in different directions is not the same (Fig. 2.13). When tensile forces are applied at an angle to the warp and weft threads, the strength of the fabric is less than when forces are applied in the longitudinal or transverse direction. This is explained primarily by the fact that when stretching samples cut at an angle to the warp and weft threads, only a portion of the sample threads are clamped by both clamps of the tensile testing machine. In addition, the strength of even this clamped part of the threads is not fully used, since the niches are located at a certain angle to the acting force.
Tissue elongation. In the direction of the warp or weft, the fabrics elongate due to the straightening and elongation of the tensions located along the acting force. Typically, straightening threads requires less effort than stretching them, which is associated with changing the inclination of spiral turns of twist, straightening and sliding of fibers. Therefore, the elongation of the fabric, especially at the beginning of its stretching, is directly dependent on the number of bends of the thread per unit length and the depth of the threads. In my
|turn, the number of bends of the thread is determined by the weave and density of the fabric, and the depth of the bend is determined by the thickness of the threads of the perpendicular system and the phase of the fabric structure. Therefore, other things being equal, plain weave fabrics have the greatest elongation. With increasing density, the elongation of the fabric increases to a certain limit, after which the cohesion of the filigree elements becomes so great that the ability to stretch decreases.
The phase of the structure has a great influence on the elongation of the fabric, especially at the beginning of loading, when the stretching of the fabric occurs mainly due to the straightening of the threads. Fabrics of the fifth phase of the structure can have similar elongation rates both at the warp and at the weft, since the curvature of their threads is the same. Fabrics of the remaining phases of the structure have a large elongation in the direction of the curved system.
1 Research carried out at MTILP by B. A. Buzov and
D. Alymenkova, showed that when a sample is stretched, the deformation of the fabric is complex: it depends on the direction [of stretching relative to the warp or weft threads. The mechanism of deformation is determined by the stretching and compression of the threads, their bending in the plane of the fabric, the change in the angle between the warp and weft threads, and the formation of longitudinal folds in certain areas.
The complex nature of the deformation causes uneven elongation of individual sections of the sample. In Fig. 2.14 shows graphs of tissue deformation in sample sections depending on the direction of stretching (angle<р) и величины полного удлинения пробы (в процентах от разрывного), схематически показан также характер изменения размеров и формы проб.
For the considered cases of stretching samples cut along the base (φ = 0°) and at an angle φ = 15°,<р = фпр, <р = 30° и ф = 45° к, основе, деформация крайних участков проб, примыкающих к зажимам, значительно больше, чем средних участков. Особенно заветна разница в степени деформации участков при растяжении, Проб под углом ф = 15° и ф = фпр (где <рпр - угол растяжения пробы, в которой все нити основы, расположенные в рабочей зоне раз - "рывной машины, закреплены только одним концом: одна поло - дана нитей - в верхнем зажиме, а другая половина - в нижнем [зажиме).
For samples cut at an angle of 45° to the base (<р = 45°), кривые растяжения ткани по участкам расположены почти рядом, что свидетельствует о более равномерном распределении общего удлинения по участкам пробы. Однако на первом этапе растяжения (примерно до 20 % удлинения пробы) больше деформируется средний Участок и немного меньше - крайние. При дальнейшем растяжении крайние участки начинают деформироваться больше, чем средний.
A - f = 0°; b - f = f,|p; V - f = 45°, g - f = 15°; E - f = 30°
The complex nature of the distribution of deformations is due to the fact. that the threads in the samples are located differently relative to the clamps and, therefore, perceive the applied load differently. This is clearly visible in the diagrams of changes in the size and shape of samples (see Fig. 2.14). When the fabric is stretched along the base (φ = 0°), the zone of greatest transverse contraction is located in the central part of the sample. When stretching fabric at angles of 15°, fpr and
A sharp change in the shape and size of the samples is observed. In the test (φ = 15°), two zones of greatest transverse contraction appear, which are located closer to the clamps; in samples (<р = <рмр, ф = 30°) зоны наибольшего поперечного сокращения смещаются к центральной части пробы, а сами пробы приобретают сложную конфигурацию. В пробе (ф = 45°) максимальное поперечное сокращение наблюдается в центральной зоне, а сама проба получает достаточно правильную форму. Выявленные закономерности деформации ткани по участкам пробы при ее растяжении и изменеNya sample forms are of significant interest to designers and sewing technologists.
Strength and elongation of knitwear. When calculating approximate Nykh values of the breaking load of knitwear Rtr take into account the number THREADP, resisting tensile forces in each loop row or column, the breaking load of the Yar thread, and the density of the fabric P - the number of loop rows (77,) or columns (D.) involved in the gap. The calculation is carried out according to the formula
Рtr = Рр11пИ
The horizontal breaking load for knitwear of the main weaves, in which /7 = 1, is calculated by the formula
P = P P
1 tr 1 "рн"-"п"
In knitwear of derivative weaves, there are two threads in each row, i.e. n = 2, so the calculation formula takes the form
P = 2P P
1 tr 1 ^ r. II "1 G.■
For cuff weave knitwear, in which there are two branches in each loop of the column, i.e. and = 2. The vertical breaking load is determined by the formula
_ WithDe^ = A
V~dt "dt L
When c = const
Dt N
Integrating this expression from 0 to T and from a0 to a, we get a =
Let us denote -^ = m, then a = a0exp --> where a0 is the initial
Dressing up; T - time; t is a constant characterizing the rate of stress relaxation over time or the time of stress relaxation in a material sample.
When t = T voltage a = a0e~", i.e. t is the time during which the initial voltage a0 will decrease by e times. At A= const
For textile materials with elastic deformation, more complex mechanical models have been proposed.
!< А. И.Кобляков для изучения механизма ^растяжения трикотажа использовал трех - Компонентную модель Кельвина -Фойгта Црис. 2.29), в которой первый элемент соответствует начальной фазе релаксации, второй - замедленной фазе и третий - фазе с ^Заторможенными процессами. Модель, использованная А. И. Кобляковым, хорошо |описывает процесс деформирования при напряжении в пробе материала, не превышающем 10% разрывного.
In general, the deformation equation for such an elastic (mechanical) mode is Lee looks like
1 Fig. 2.29. Three-compo-
£ = e T c7!,1)"1-net model Kel-
0 wine-Voigt
At constant voltage
Where t], t2, t3 (O, From, 03) - the average relaxation time (delay) of fast, slow and inhibited processes, respectively; ab a2, As - deformations with average relaxation time t2, t3.
After removal of external forces
- L
G = c, e 0| + c2e + r3e~"", (21)
Where c e2, £s> are deformations that disappear with the average delay time 9b 02, 03.
For the rest period, A.I. Koblyakov proposed the following graphic-analytical method for calculating the parameters of the equations. Equation (2.1) is written as
TOC o "1-3" h z E = £1e-a"" +C2e-^" +E3e-"1", (2.2)
A, = 1/0,; (2.3)
A, = 1/0.,. (2.5) The first boundary condition of the model at T ~ 0
C = c, + c2 + c3 = e0,
Where c0 is the deformation of the sample before unloading, or the complete deformation.
The second boundary condition at T = oo
£i = £, + e2 + c3 = 0.
The sequence of calculating model parameters using the method of A.I. Koblyakov is as follows.
1. Determine parameters c3, a3 and 03. To do this, components characterizing fast and slow processes are excluded from equality (2 2):
C„ =E, e-"""+c2e-u--". (2.6)
Then the relaxation process of inhibited elastic deformation will be described as
£ = g, e-"-". (2.7)
After taking the logarithm of this equality, we get the equation
Lge = lge3 - a3/lge.
This equation is a straight line equation of the form V= A +Bt. Where
A = lgfi3; (2.8)
B=-0.4343a3. (2.9)
According to the values of lge and T build a graph (Fig. 2.30, A), on which the section of the straight line MNU coinciding with the largest number of experimental points is marked. Next, the values are calculated using the least squares method A And IN:
« ZMZO2- "E"ChM2"
Model parameters g>„ a3, 83 is established using equalities (2.5, 2.8, 2.9).
2. Determine the parameters c2, a2 and 02. To do this, only the components of the rapidly reversible part of the deformation are excluded from equality (2.2). Then
Г-с3е-а-" = С2е-н"". (2.10)
Denoting r - r^e-0"" = t and taking the logarithm of expression (2.10), we obtain the equation of the straight line
Lge" = lge2 - (a2lge)/,
JtoiH y2 = C + Dt,1where
/> = -0,43430,. (2.12)
According to the values lge" and T build a graph (Fig. 2.30, b), on which a straight section is marked M2 N->. Then the parameters C and D.
F = -0.4343a,. (2.14)
According to the values lge" and T build a graph (Fig. 2.30, V), on which the straight segment M^N^ is marked. Then parameters 0 and F are calculated
"Z "MZ"f" " "Z"MZ"f"
Using equalities (2.3, 2.13, 2.14), we establish the parameters c, a, and 9,.
The considered graphic-analytical method for calculating the process of deformation of knitted fabrics ensures good agreement between the calculated values and the experimental data.
The use of this method by B. A. Buzov and D. G. Petropavlovsky revealed the possibility of using the three-link Kelvin-Voigt model for a quantitative description of tissue deformation (both in the creep mode and in the elastic recovery mode. However, the methodology for calculating the model parameters required clarification and adjustment. Experiments have shown that at the initial stage, which is 0.1-0.15 s, the magnitude of the deformation, as well as the rate of slowdown of its further development, depend on the level of load, type of material and direction of stretching.However, in all cases of the experiment it was noted that the deformation of the tissue at this stage it is predominantly the elastic component that develops linearly with time.Therefore, when determining fast processes, it is proposed to carry out calculations using the first two points of the experimental curve, which significantly reduces the error in calculating all model parameters.
Multi-cycle characteristics. During the manufacture and especially during use of clothing, the material experiences repeated stretching, which causes a change in the structure of the material and leads. to deterioration of its properties. This process is accompanied by a change in the size and shape of clothing, and the formation of swelling in certain areas of it (in the area of the elbow, knee, etc.).
Studying the behavior of a textile material when exposed to high-cycle stretching allows us to more fully evaluate its operational and technological properties. 1 The process of gradual change in the structure and properties of a material due to its repeated deformation is called fatigue. Fatigue occurs as a result of material fatigue - a disruption or deterioration in the properties of a material that is not accompanied by a significant loss of mass.
In the initial period of repeated exposure in accordance with the load-unload cycle (of the order of tens and hundreds of cycles), the material is deformed, but its structure, as a rule, stabilizes. At this stage of repeated stretching, a rapid increase in residual cyclic strain is initially observed. Then, as a result of some ordering of the structure of the material, the increase in slow deformation, which replenishes the residual part, practically stops, and the proportion of highly elastic reformation, which manifests itself during a time coinciding with the resting time in each cycle, increases. This is explained by the fact that in the initial period of the cycle, more mobile and weaker bonds are broken, the elements of the material structure are regrouped, neighboring threads and fibers come closer together, and new bonds arise. At the same time, the orientation of the fibers relative to the axes of the threads and molecular chains of the polymer occurs. As a result, the material is strengthened.
A further increase in the number of cycles of multiple stretching, not accompanied by an increase in load (strain) in each cycle, does not cause a noticeable change in the structure of the material and its properties. The fact is that the material, having undergone structural changes in the first period, subsequently adapts to new conditions. External and internal connections involved in resisting the action of load in each cycle, under the conditions of a steady-state stretching mode, manifest themselves in the form of elastic and elastic cyclic deformations with a short relaxation period. Under these conditions, the material is able to withstand tens of thousands of cycles without a sharp deterioration in properties.
In the final stage of multi-cycle exposure (tens and hundreds of thousands of cycles), due to fatigue of the material, fatigue occurs. The phenomenon of fatigue is observed in some of the weakest areas or in places that have any defects. During this period, intensive growth of residual cyclic deformation of the material and its destruction occur.
With multi-cycle stretching of the material, the following characteristics are obtained: endurance, durability, residual cyclic deformation and its components, endurance limit.
Endurance- the number of cycles that the material can withstand before failure at a given deformation (load) in each cycle.
Durability/p is the time from the beginning of multi-cycle stretching to the moment of failure at a given deformation (load) in each cycle.
Residual cyclic deformation e0)
