Standard tests to ascertain strength factors are performed according to ASTM or British standards, although a few countries adopt their own standard methods. In this section, a brief outline of the tests will be discussed. Readers may consult alternative sources for details of these tests (Saville, 1999). All tests are conducted in both the dry and wet states, as well as both warp- and weft-wise.
2.3.1 Tensile strength
A cut or ravelled strip specimen is prepared to a specific size, with excess length on either end for effective grip in the clamps, and excess width to avoid slippage of threads. This is clamped between the top and bottom jaws of a constant rate of extension (CRE) tester and is put under a progressively increasing load for nearly 20 s until it breaks. The breaking force (Newton) as well as elongation (%) are noted. The unidirectional strip test suffers from ‘waisting’ and the effects of fabric assistance, causing a fall in specimen strength. As such, it may be replaced by a bidirectional test to achieve precision (Clulow and Taylor, 1963).
In a grab test, the specimen is mounted lengthwise in the direction of the application of load, with a certain length projecting at each end beyond the clamps to ensure adequate grip of the yarns in the bottom and top jaws to avoid slippage.
The working width of the specimen is marked with a pencil, and must just fit in the jaws on both sides. The specimen is put under increasing load until it breaks. The load and extension at break are noted and calculated as:
Grab strength/tensile strength per inch = 1 + breaking extension %/40 [2.1]
For a 50 mm specimen width, the ratio of grab and strip strength lies between 1.0 and 2.0. However, use of the strip test is preferred in production, for precise results.
A modified grab test can be used for the testing of high-strength fabrics. Slits are produced at the centre line of the specimen on both sides, perpendicular to the direction of the application of force, and all of the threads are cut with the exception of those held by both jaws at the centre. The grab test is not recommended for knits, glass fibre fabrics and other high stretch fabrics, due to the inconsistent increase in test data these produce.
2.3.2 Tearing strength
The ‘tongue tear’ or ‘single rip’ method measures the force required to continue a tear already initiated in the fabric, preferably using a CRE tensile tester. Two tongues are produced by making a cut at the centre of the short edge of a rectangular fabric specimen, and a reference line is marked to indicate the point of tear propagation. The two tongues are gripped in the upper and lower jaws of the
Strength properties of fabrics 41
2.2 Propagation of tear during test.
machine. The jaws are then progressively separated by the application of force to propagate the tear at adjusted speed (Fig. 2.2). A specific full-scale force range of the machine is selected to enable the maximum force to act between 10 and 90%
of the full-scale force. The force to propagate the tear is calculated from the autographic chart recorder or micro-processor data collection system. The tearing force can then be obtained either by calculating the mean of the highest peaks, or by using the highest peak force for fabrics exhibiting less than five peaks.
The ‘falling pendulum method’ (Elmendorf type) assesses the force required to propagate a single rip tear starting from a cut in a fabric. A slit is produced at the centre of the longer side of a rectangular fabric specimen, prepared with a cutting die. Additional fabric is attached to the two tongues produced on both sides of the slit, to ensure tearing of the bottom portion of the specimen. A groove is formed in the opposite side of the slit at the middle, to enable a knife to enter inside the specimen through the slit, propagating the tear for the rest of the length, and exiting smoothly through the groove. With a pendulum at its initial rest position and ready to conduct a test, the two clamps are separated by a certain length and aligned such that the clamped specimen lies in a plane parallel to the axis of the pendulum, with identical tension on both clamps. A pointer is mounted on the same axis as the pendulum to register tearing force. Alternatively, a digital display or computer- interfaced calculating device can be used.
Of the two tear tests, the ‘tongue method’ is a static type, whilst the ‘falling pendulum method’ is a dynamic one. In practice, tearing occurs suddenly and the tests must be conducted at high speed to achieve the most realistic results. The
Fixed with upper jaw
Fixed with lower jaw
42 Understanding and improving the durability of textiles
tongue test is conducted at slow speed and hence results show sharp differences from the falling pendulum method, where the rapid action of the pendulum imposes the force at a rate quite close to the actual tearing force. The tongue test reports the tearing force in a graphical mode, making it a highly time-consuming test in the absence of attachment of an electronic device. In contrast, the dynamic tear test offers quick results through a direct reading on the device. In addition, the force calculated from the tongue test is not the average of the maximum loads. It is neither the top peak of the saw tooth diagram nor of the lower loads, but is somewhere in between, and hence it does not reveal the expected load needed to tear the fabric. Various modes of specimen tearing and the effect on accuracy of the subsequent test results have been discussed in the literature (Witkowska and Frydrych, 2004; Harrison, 1960; Booth, 1968).
2.3.3 Bursting strength
Resistance of textile fabrics to bursting is tested using a hydraulic or pneumatic diaphragm bursting tester. The specimen under test is clamped over an expandable diaphragm, which is expanded by fluid pressure until the specimen ruptures. The difference between the total pressure required to rupture the specimen and that required to inflate the diaphragm is reported as the bursting strength. In hand- driven testers, the specimen is inserted under the tripod, drawn taut across the plate and clamped. The hand wheel is rotated at a specific speed until the specimen bursts. Just after rupture, the clamping lever on the specimen and the strain on the diaphragm are released by turning the wheel anti-clockwise to the starting posi- tion. The pressure required to inflate the specimen (the tare pressure) and the total pressure to rupture the specimen are recorded. In the motor-driven tester, the specimen is clamped securely, and pressure is applied until it bursts. The tare pressure is found by distending the diaphragm to the same height as that used to burst the specimen, and, as with the hand-driven tester, the pressures required to both inflate the diaphragm and rupture the specimen are recorded. The bursting pressure can then be calculated by subtracting the tare pressure required to inflate the specimen from the total pressure to burst it.
2.3.4 Abrasion resistance
In a ‘uniform abrasion test’, the specimen is mounted in a holder and abraded uniformly in all directions. The testing instrument comprises an abrading mecha- nism, a specimen-supporting mechanism and a driving mechanism. The specimen-supporting mechanism allows tension mounting of thin, flexible mate- rials as well as rigid mounting of thick and stiff materials. The specimen under test is clamped and locked. Constant tension is applied to the specimen to stretch it uniformly over the pressure foot. The abradant is lowered down and the force placed on the specimen is adjusted before 1000 cycles (for low abrasion) and up to
Strength properties of fabrics 43
20 000 cycles (for higher abrasion) are imparted. The abrasion continues until the tester stops automatically at the time of failure, or at the destructive end point.
Abrasion resistance for wear is evaluated by one or more conditions of destruc- tion, including loss in wear (a defined surface damage), of mass, breaking strength or thickness of the specimen. The end point of the test is considered to be when two or more threads are broken in a woven fabric, a hole appears in a knitted fabric, or the abrader stops automatically due to destruction of the sample. Against a specific number of cycles, loss in mass is calculated as the difference in mass of the starting and the ruptured specimens. Loss in thickness is calculated as the difference in thickness of the starting and abraded specimens. Alternatively, it may also be expressed as the number of cycles required to reach a desired percentage loss in thickness. Loss of breaking strength is expressed as the difference between the breaking strength of the abraded and the unabraded specimens in grams.
In the Martindale abrasion test, only 16 cycles of the abradant are applied to the specimen under known conditions of pressure and abrasive action, to complete a Lissajous track. A piece of felt fabric with the piece of standard fabric on top are secured to a table, along with a mounting weight to flatten these. The holder is assembled, and the specimen placed face down into the specimen holder. The tester is started and the specimen is examined to assess progress towards the end point for Options 1, 2 or 3. In Option 1, the end point refers to the breaking of two or more yarns for a woven fabric, or the appearance of a hole for knits. Option 2 refers to a change in shade or appearance that would be adequate to incite customer complaint. Option 3 stands for the difference in the masses of the abraded and the unabraded specimens.
2.3.5 Yarn slippage and seam strength
Slippage of the seam occurs when fabric yarns parallel to the seam line move away from the line under transverse stresses, exacerbating potential damage. When the external force working on a seam during wear crosses the optimum limit, seam failure occurs as either a rupture of the sewing thread, rupture of the fabric, excessive yarn slippage adjacent to stitches, or a combination of these failures.
Even if a full rupture does not occur, excessive seam slippage causes a significant fall in seam efficiency and is detrimental to a garment’s appearance.
In the testing of seam strength, at least one clamp should be supported by a free swivel or universal joint to allow the clamp to rotate across the plane of the fabric.
The specimen is placed along the diagonal of the fabric, to allow testing of the full range of different warp and weft yarns, or machine and cross-direction areas in each specimen. The specimen is folded-in from one end, with the fold parallel to the short direction of the fabric, and a seam is applied. After seaming, the fabric is cut open; the test specimen carries a seam at a specific distance from one end. Each test specimen possesses sufficient material for one seamed and one fabric test (Fig.
2.3). The stitch density (stitch/cm) of the sewn samples is counted. The specimen
44 Understanding and improving the durability of textiles
2.3 Seamed specimen construction.
is placed into the clamp of a CRE tester, with the seam line centrally located between the clamp faces, and perpendicular to the pulling force. Seam slippage is calculated by comparing the load–displacement curve of the sewn seam with that of the fabric, after putting the specimen under continually progressive force.
During application of force to the sewn seam specimen, it is important to see whether the seam rupture has been effected by: (i) fabric yarn rupture, (ii) sewing thread rupture, (iii) sewn seam yarn slippage, or (iv) a combination of two or more of these. The maximum seam strength of an individual specimen with a seam assembly is calculated from the maximum force (Newton) required to rupture the specimen, as read from the instrument. Seam efficiency is calculated as the ratio of the sewn seam strength to the ‘fabric breaking force’. Seam slippage is measured for displacement by setting the dividers at one quarter of the distance of chart travel for a specific length of jaw travel.
2.3.6 Pile loss
Loss in pile tuff due to abrasion, and the extent to which cut-pile yarns remain secured and intact during wear (pile retention) are evaluated as follows. A rotary platform abrader and a round template are used for this test. The back and face of the specimen are separately exposed to a specific number of abrasion cycles and are evaluated by viewing over a light box and comparing with photographic standards.
Two specimens of a specific diameter, consisting of different warp and filling yarn, are mounted on the holder and attached with a plate and nut. The clamp ring is positioned to fit tightly over the specimen and holder, and the specimen is drawn
Specimen for test of fabric break
Specimen for test of seam break Seam
Clamps for fabric specimen
Clamps for stitched specimen
Strength properties of fabrics 45
taut by pressing the hold-down ring uniformly over the edge of the holder. The abrading heads are lowered carefully onto the surface of the specimen, and abraded particles are removed via an adjustable suction nozzle. Heavy and light duty procedures are carried out by altering the desired number of test cycles and wheel load. The abraded specimen is rated by placing it face up over the lighted viewing box, where it is viewed perpendicularly and compared with photographic stand- ards according to a six-level pile retention rating system as follows: 5.0 (excellent), 4.0 (good), 3.5 (fair to good), 3.0 (fair), 2.0 (poor) and 1.0 (very poor). This method assesses the extent of the pile pulled from the base fabric, rather than surface pile appearance attributes.