Abstract
The influence of bias angle of stitching on tensile characteristics of lapped seam parachute canopy fabric has been studied based on proposed test specimen dimension as reported in Part I of this paper. Test results obtained using the postulated test specimen dimension exhibit reliable tensile properties of canopy fabrics. It is also therefore possible for comparative analysis of tensile strength and elongation of unseamed and seamed fabric at different bias angle. During comparative analysis, the data obtained from the previous test dimension (as mentioned in Part I of this paper) and new test dimension show that there is significantly large difference in absolute values of tensile data obtained by these two methods. The trend of breaking elongation is same in both the cases, but the trend of tensile strength is somewhat different with these two dimensions. Besides these, the absolute values of tensile data obtained through proposed specimen dimension are found to be realistic as compared to the results based on previous test dimension.
Introduction
The tensile characteristics of unseamed and seamed fabric changes with the change of fabric bias angle [1–7]. Therefore tensile properties of fabrics along various directions besides principle direction are very important in many technical textiles [6–10]. In case of parachute, during opening it experiences high shock force and this shock force is shared through canopy fabric, stitches and seams, webbing and suspension line of parachute system. Out of many parameters, quality of seams and stitches are very important for effective performance of parachute [11–14]. As discussed in Part I of this paper that the seam angle directly related with fabric bias angle (an acute angle made between the direction of specimen length and its warp). Therefore measurement of tensile property at different bias angle becomes more important in parachute where the seam angle changes with the change of number of gores and type of constructions [15]. In this respect a study was accomplished for unseamed and seamed specimen at different bias angle using specimen dimension selected by Coplan and Bloch [16]. In our work, it is found that such dimension is not realistic in analysing the tensile strength and elongation of unseamed and seamed specimen at bias angle other than orthogonal and 45°. Due to specimen dimension, a large difference in absolute values of tensile data in unseamed and seamed fabric is found. Data also indicate more than 100% seam efficiency by default, which is a wrong indicator for any fabric as seam efficiency cannot be more than 100%. Further this method is also not effective because in most of the samples, breakage starts through slit line resulting fairy less values of tensile strength of materials. This results in rejection of tensile data which causes the wastage of samples and time.
Therefore by considering the above mentioned limitations of specimen dimension as suggested by Coplan and Bloch [16], an optimized specimen dimension without any slit was postulated (as mentioned in Part 1 of this paper [15]). Present paper mainly analyses the influence of bias angle of stitching (keeping two fabric components in parallel as mentioned in Part 1 of this paper [15]) on tensile strength and elongation of unseamed and lapped seam parachute canopy fabric using postulated specimen dimension. The findings from the study are expected to be realistic and important in designing and development of parachute fabric.
Experimental
Materials
A light weight nylon plain parachute fabric had been selected for the present study. The details of fabric and sewing threads have been mentioned in section 2.1 of Part 1 of this paper [15].
Methods
In present study, postulated specimen dimension of 7.5 cm gauge length and 8 cm width [15] is used for the measurement of tensile strength and elongation of unseamed and lapped seam parachute fabric (as shown in Figure 1(a) and (b), respectively) at different bias angle from warp direction.
(a) Dimension of test specimen for unseamed fabric, (b) dimension of test specimen for lapped seam fabric and its side view.
For unseamed fabric, a fabric dimension of 175 mm length (including 75 mm gauge length and a total of 100 mm length for both top and bottom jaw’s grip) and 80 mm width at different bias angle has been used. On the other hand for the testing of lapped seam, samples are first prepared by cutting a fabric of dimension 219 mm length (including 75 gauge length, extra length of 44 mm for folded part of lap formation and 100 mm length for both top and bottom jaw’s grip) and 80 mm width at different bias angle. This fabric sample is then cut at the middle across the length and then joined in parallel [15] by using LSC2 lapped seam with four stitches per cm. The distance between the two stitch lines are kept at 10 mm which is normally used in the joining of panels and gores of parachute canopy.
The measurement of breaking strength and elongation of both unseamed and seamed fabric at different bias angles are carried out at tensile tester through mounting the specimens between the jaws of 80 mm width and 50 mm height at constant rate of extension of 300 mm/min. For each bias angle, 10 tests are conducted and the average values of tensile strength and elongation and their standard deviation are computed. The experiment was conducted in standard atmospheric condition of 20 ± 2℃ temperature and 65 ± 2% RH. Before the tensile test all the samples were conditioned for 24 h in standard atmospheric conditions. The test results obtained has been compared statistically using t statistics. For the measurement of seam efficiency following equation has been used:
Results and discussion
The effect of bias angles on tensile strength and elongation of unseamed and seamed fabric has been studied. Generally the tensile property of the unseamed and seamed fabric depends on the numbers of parameters like strength and elongation % of warp and weft yarns, numbers of warp and weft yarns per unit length of fabric, friction and contact force between the yarns, cover of the fabric, numbers of interlacement point of warp and weft inside specimen, strength of sewing thread, stitch density, etc. In the present case, square fabric with plain woven structure has been selected and also the type of sewing thread and stitch density are kept constant for all samples, therefore the factors influencing tensile property at bias angle can be conceived as function of orientation and properties of the warp and weft yarns, the number of interlacement points made by warp and weft yarns and corresponding frictional and contact force between the yarns. In present case, these parameters are mainly influenced by bias angle as it not only changes the orientation of yarns inside the specimen but also the number of different category of warp and weft yarns and other associated parameters. The type of different category of yarn can be grouped in four categories: i.e. warp and weft yarns which are gripped by only top jaw, warp and weft yarns which are gripped by only bottom jaw (in case of unseamed fabric) or by only pseudo jaw (in case of seamed fabric), warp and weft yarns which are gripped by both jaws, and warp and weft yarns which are gripped by neither of the jaws.
The number of above mentioned group of yarns in both unseamed and seamed test specimen has been calculated using mathematical formula as mentioned in Part 1 of this paper [15] and shown in Figures 2 and 3. When the bias angle changes from 0° to 45°, the orientation of yarns inside the specimen varies from asymmetry to symmetry (as shown in Figure 4) which affects the load sharing behaviour of the specimen. It may be added that the seam line also participates in influencing the tensile strength and elongation of seamed fabric. In case of seamed fabric, the seam line acts as a pseudo jaw and grips some of those yarns which are unable to grip between the top and bottom jaw of unseamed fabric. The details of influence of bias angle on tensile characteristics of unseamed and seamed specimen have been discussed in the following sections.
Number of different category of yarns available between the top and bottom jaws in unseamed specimen at different bias angle. Number of different category of yarns available between the jaw and seam line. Increasing order of symmetric yarn orientation in the specimen with respect to tensile test direction when bias angle changes from 0° to 45°. (a) Perfectly asymmetric orientation of warp and weft yarns at 0°. (b) Correspondingly more symmetric orientation of yarns at 15°. (c) Perfectly symmetric orientation of yarns at 45°.
Effect of bias angle on tensile characteristics of unseamed fabric
The change in tensile strength and elongation of unseamed fabric with the change of bias angles can be observed from Figure 5. Such change in the load bearing capability of specimen at different bias angle is also reflected in its tensile behaviour which can be seen in Figure 6.
Breaking strength and breaking elongation of unseamed fabric. Load extension graph of unseamed specimen at different bias angle.
When the bias angle is at 0°, all warp yarns oriented vertically and gripped by both the jaw while all the weft yarns remain parallel to jaw width. Breaking strength and breaking elongation in this region are contributed mainly by the characteristics of warp and weft yarns, number of its interlacement points and corresponding frictional and contact forces between the yarns. In this case, the breakage of specimen occurs between the gauge lengths (Figure 7(a)). With the increase in bias angle from 0° to 45°, initially there is drop in breaking strength between 0° and 30° and in breaking elongation between 0° and 15°. In spite of same degree of asymmetry, the present trend is somewhat different than the trend observed in Part I of this paper (decreasing trend for both strength and elongation was observed only for 0° to 15° [15]). The aforesaid finding can be explained on the basis of number of threads which are under positively/indirectly controlled by jaw. In case of previous specimen, the number of yarns gripped by both jaws decreases till 15° but after that it soon reaches to zero and remains zero till 45° (as mentioned in Part I of this paper [15]). In present case, the aforesaid thread density decreases till 45° (as shown in Figure 2). However, strength of the material improves after 30°. This can be explained on the basis of increase in total number of thread under control of single jaw and cross over points besides the impact of increased symmetry of threads along load direction. The impacts of aforementioned factors are predominating over the effect of yarn gripped by both the jaws. It can be also observed from Figure 2 that with the increase in bias angle, in spite of increase in total number of yarns gripped by top/bottom jaw, numbers of unsupported threads are also increasing. The latter group of thread laying parallel close to the edge; this leads to unsupported edged yarns tend to slip out during loading. This leads to breakage of specimen nearer to jaws. Figure 7(b) exhibits the breakage pattern of unseamed fabric. It is to be noted that this type of breakage should not be considered as jaw breaks.
Breakage pattern of unseamed fabric after tensile test at two different bias angle (a) 0° and (b) 15°.
At 45° bias angle, although the total number of yarns gripped by both the jaws becomes almost zero (as shown in Figure 2), still high strength and elongation are observed mainly because of enhancement in higher number of warp–weft interlacement points and complete symmetric orientation of warp–weft yarns inside specimen. In addition to this while applying tensile load, shear force acting between warp–weft also tend to modify fabric strength and elongation. It is noticed that breakage of the specimen at 30° and 45° bias angles eventually take place nearer to the jaws as like in 15°. However breaking elongation of the specimen does not follow similar trend to that of strength. It may be due to the dominated factor of load imbalance till 15°, which also bring down the breaking elongation of specimen. After 15°, increase in breaking elongation of the specimen has been noticed. Besides the effect of load sharing among threads, the aforesaid trend will also be influenced by shearing which tends to increase with the increase in bias angle. It is found that the trend of breaking strength and elongation is almost symmetric to 45° as the bias angle further increases from 45° to 90°. This is because of the square plain structure of selected parachute fabric.
Effect of bias angle on tensile strength of seamed fabric
It can be seen from Figure 5 and Figure 8 that the tensile strength of seamed fabric is lower than unseamed fabric. The trend of change in ultimate tensile characteristics of seamed fabric with the change of bias angles can be observed from Figure 8 which is also reflected in its tensile behaviour (Figure 9). This trend is similar to that of unseamed fabric (except 0° to 15°) and have similar explanation as discussed in case of unseamed fabric. However, the initial increase in the strength of seamed specimen is due to the sharp reduction in the number of yarns gripped by neither of the jaws (from 146 to only 48 as shown in Figure 3) which is mainly because of the incorporation of seam line. As explained earlier that the seam line can be conceived as pseudo jaw which restricts the movement of aforementioned free yarn to slips out during tensile test of specimen. Initially this effect dominates (for bias angle 0° to 15°) over the load imbalance, thus increasing the strength of seamed specimen.
Breaking strength and elongation of seamed fabric. Load extension graph of lapped seam specimen at different bias angle.
But beyond 15° due to reduction in the numbers of yarns gripped by neither of the jaws (reaches zero at 30° and remains zero till 45° as shown in Figure 3) and comparatively less increase in the number of yarns gripped by only either of jaws, load imbalance and yarn symmetry dominates in this region and the corresponding change in strength and breaking elongation of the test specimen now occurs due to combination of these factors which has been explained earlier. The breakage of the specimen occurs nearer to the seamline for all bias angles (due to presence of sufficiently higher number of positively controlled yarns). The same pattern of breakage is also been observed at orthogonal direction (0°/ 90° bias angle) of testing. Figure 10 exhibits type of breakage of seamed specimen at two different degrees. As the bias angle further increases from 45° to 90°, the trend of breaking strength and elongation are almost symmetric to 45° because of the square plain structure of selected parachute fabric.
Breakage pattern of seamed fabric after tensile test at two different bias angle (a) 0° and (b) 15°.
Figure 9 shows the load extension graph of seamed specimen where a number of peaks are obtained. This is general trend for all seamed specimen except at 45° bias angles. Here the presence of peaks indicates that load distribution across the seam line is not uniform for asymmetric thread distribution leading to staggered kind of breakage. This results in fall in ultimate properties of the seamed specimen.
Comparative analysis of tensile characteristics of unseamed and seamed fabric through previous and current test methods
Comparative analysis of tensile characteristics of unseamed and seamed specimen by previous and new test method.
The values under parenthesis indicate standard deviation.
Conclusions
Determining test specimen size is an important issue while characterizing the ultimate behaviour at various bias angles of parachute canopy fabric. The present study is started with the postulated specimen dimension as mentioned in Part I of this paper. Investigation reveals that the postulated specimen dimension without any slit is working well in characterization of tensile characteristics of unseamed and seamed parachute canopy fabric.
Based on the studies, following conclusions have been made:
The angle of bias significantly affects the tensile strength and elongation of both unseamed and seamed specimen. Due to almost square plain structure of selected parachute fabric, the tensile characteristics of both seamed and unseamed specimen is almost symmetric to 45°. The main reason for the change in tensile characteristics at different bias angle is mainly because of change in yarn orientation and the change in number of different category of warp and weft yarns (the number of warp and weft yarns gripped by both the grips / either of the grips / neither of the grips) available between the grip lines. Yarns which are not under control either by jaw or intersecting points could lead to less fabric strength. It is to be noted that parachute canopy is made endless devoid of uncontrolled edge yarns. In present sample dimension, uncontrolled edge yarns are largely restricted. However, there is further scope to improve upon the above aspect by following grab test instead of strip test to eliminate uncontrolled threads. The tensile strength of unseamed fabric along the warp, weft and at 45° bias direction is almost equal and is highest as compared to other bias angles. But in case of seamed fabric, the maximum tensile strength was found at 45°. However, breaking elongation is always maximum at 45° in both unseamed and seamed specimens. Therefore the panels and gores of parachute joined at 45° bias angle will provide not only maximum tensile strength but also highest elongation %.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.