Investigation on the impact and compression-after-impact properties of warp-knitted spacer fabrics

Abstract

The paper reports an experimental study on the influence of spacer fabrics' structures, that is, different surface stitches, fabric thicknesses, and diameters of spacer yarns, on the impact and compression-after-impact properties of warp-knitted spacer fabrics. Low-velocity impact tests, damage deformation characterizations, and compression tests after impacts were performed on six spacer-fabric specimens knitted on a double needle bar Raschel knitting machine. The impact method was flat-sphere (a flat striker contacted with a fabric wrapped on a hemispherical steel anvil). Deformation processes of typical spacer fabrics, force–displacement/time and energy–time curves, impact damage, and residual strength were analyzed and discussed; moreover, peak force, absorbed energy, damage depth, and the drop-off rate of residual strength were involved to estimate impact and compression-after-impact properties of the spacer fabrics with different structures. The results show that with the closed surface structure and coarser spacer yarns, the fabrics have the characteristics of lower peak forces, higher energy-absorption ability, lower damage depths, and higher drop-off rates of residual strength. Fabrics with higher thickness are of the features above but they do have lower drop-off rates

Keywords

impact compression after impact warp-knitted spacer fabric residual strength damage

Warp-knitted spacer fabrics are three-dimensional textile structures consisting of two separate outer fabric layers joined together while being kept apart by spacer yarns.¹ As a cushion material, warp-knitted spacer fabrics have much better moisture transmission features,² better pressure relief properties, higher air permeability, and lower heat resistance than polyurethane (PU) foam;³ thus, they have been widely used in areas such as automobile textiles (cushions³ or car seats⁴), sports textiles,⁵ and foundation garments (bra cups, pads for swimwear), etc.⁶ Many studies have been conducted on warp-knitted spacer fabrics, which mainly focus on their static compression properties, as well as sound absorption behavior,⁷ pressure distribution,^3,8,9 air permeability, heat resistance,² etc.

Studies on static compression behaviors of warp-knitted spacer fabrics are as follows. Some effect factors, that is, material, pattern, and threading, as well as location angle and the number of spacer yarns, on the compression behaviors were investigated by Armakan and Roye.¹⁰ Lateral compressive behaviors of spacer fabrics were explored based on Van Wyk’s equation.¹¹ The indentation force deflection (IFD) properties of different warp-knitted spacer fabrics were studied in a paper by Miao and Ge.¹² Mechanical and the stress–strain model of single spacer yarn of warp-knitted spacer fabric were found by Chen Y¹³ and Chen HL.¹⁴ Energy efficiency² and non-linear compression behavior¹⁵ of spacer fabrics were investigated as well.

The static compression properties of warp-knitted spacer fabrics have been investigated in many researches, as mentioned above. However, few of the researches^6–15 focus on the impact behaviors of spacer fabrics. The impact method in the study is flat-sphere, which is quite different from the impact method (flat-flat) in a previous study.¹⁶ As a cushion material, spacer fabric is expected to be sewn into some parts of clothing, corresponding to injury-prone joints of the human body, such as the elbow, femoral greater trochanter, knee, and so on, in order to reduce fractures after falling, especially for elderly people while in motion.¹⁷ A simplified model of the event was that of spacer fabric wrapped on a hemispherical steel anvil (joints of human body) impacted with a flat striker (ground).

Experimental details

Samples

Six warp-knitted spacer fabrics were used in this study. They were produced on a double needle bar Raschel knitting machine (GE 296, E18) at Wuyang Textile Machinery Co., Ltd, China. Surface structures of spacer fabrics can be divided into two types: closed structures without meshes and open structures with small/large-size meshes according to their different shapes. As their typical representatives, pillar + weft insertion, rhombic mesh, and hexagonal mesh were used for the surface layers, as shown in Figures 1(a)–(c), respectively.

Figure 1.

Three different surface structures. (a) Pillar + Weft-insertion, (b) Rhombic mesh, (c) Hexagonal mesh.

It can be seen from Figure 1 that the fabric surface with pillar + weft insertion is plain, while there are smaller meshes on the fabric with rhombic mesh surface layers and larger meshes on that with hexagonal mesh surface layers. The chain notations and materials for these samples are shown in Table 1.

Table 1.

Chain notations and materials for fabrics with different surface structures

Structure	Details of the lapping movements
Pillar + weft insertion	GB1: 0-0, 0-0/5-5, 5-5//fully threaded A
	GB2: 1-0, 0-0/1-0, 0-0//fully threaded A
	GB3: 1-0, 3-2/3-2, 1-0//one B one empty
	GB4: 3-2, 1-0/1-0, 3-2//one B one empty
	GB5: 0-0, 1-0/0-0, 1-0//fully threaded A
	GB6: 5-5, 5-5/0-0, 0-0//fully threaded A
Rhombic mesh	GB1: 1-0, 0-0/1-2, 2-2/2-3, 3-3/2-1, 1-1//one A one empty
	GB2: 2-3, 3-3/2-1, 1-1/1-0, 0-0/1-2, 2-2//one A one empty
	GB3: 1-0, 3-2/3-2, 1-0//one B one empty
	GB4: 3-2, 1-0/1-0, 3-2//one B one empty
	GB5: 1-1, 1-0/0-0, 1-2/2-2, 2-3/3-3, 2-1//one A one empty
	GB6: 2-2, 2-3/3-3, 2-1/1-1, 1-0/0-0, 1-2//one A one empty
Hexagonal mesh	GB1: (1-0, 3-3/3-2, 1-1) × 2/1-0, 3-3/3-2, 4-4/(5-4, 3-3/3-2, 4-4) × 2/5-4, 3-3/3-2, 2-2//two A two empty
	GB2: (5-4, 3-3/3-2, 4-4) × 2/5-4, 3-3/3-2, 1-1/(1-0, 3-3/3-2, 1-1) × 2/1-0, 3-3/3-2, 4-4//one empty two A one empty
	GB3: 1-0, 3-2/3-2, 1-0//one B one empty
	GB4: 3-2, 1-0/1-0, 3-2//one B one empty
	GB5: (1-1, 1-0/3-3, 3-2) × 3/(4-4, 5-4/3-3, 3-2) × 3//two A two empty
	GB6: (4-4, 5-4/3-3, 3-2) × 3/(1-1, 1-0/3-3, 3-2) × 3//one empty two A one empty

Materials ‘A’ and ‘B’ represent 33.3 tex/96 f polyester multifilament and polyester monofilament, respectively.

Three different fabric thicknesses were involved in this study, and the distances between the two needle bars were 7.2 mm (F4), 10.2 mm (F1), and 13.2 mm (F5). The distances for the other specimens (F2, F3, and F6) were all 10.2 mm. In addition, two different diameters of spacer yarns, 0.16 mm (F6) and 0.2 mm (F1), were also included in the investigation. All the samples were heat set at 180℃ for 3 minutes. The resultant details of all the samples are listed in Table 2.

Table 2.

Details of all the spacer fabrics

Fabric	Both surface layers	Thickness (mm)	Diameter (mm)	Angle of spacer yarn (degrees)	Density
Fabric	Both surface layers	Thickness (mm)	Diameter (mm)	Angle of spacer yarn (degrees)	Weft-wise (wale/cm)	Warp-wise (course/cm)
F1	Pillar + weft insertion	7.38	0.20	82.27	7.03	5.42
F2	Rhombic mesh	6.96	0.20	80.33	7.07	5.84
F3	Hexagonal mesh	7.26	0.20	77.09	7.08	5.79
F4	Pillar + weft insertion	5.47	0.20	79.30	6.69	5.41
F5	Pillar + weft insertion	10.30	0.20	83.97	7.03	5.61
F6	Pillar + weft insertion	7.02	0.16	82.27	6.84	5.64

Angle of spacer yarn is defined as the acute or right angle between the spacer yarn and the fabric surface.

Despite the same distances between two needle bars (10.2 mm) in warping, actual thicknesses of F1, F2, F3, and F6 were slightly different, but they were all near 7 mm; thus, it was considered that thicknesses of these fabrics were close to each other. Similarly, the actual thicknesses of F4 and F5 were less than the theoretical ones on the machine.

Impact tests

The impact tests referred to BS EN1621-1:1998 (Motorcyclists' protective clothing against mechanical impact) and were conducted with an Instron Dynatup 9250HV, which is shown in Figure 2.

Figure 2.

Impact instrument structure.

The drop hammer falls freely and is automatically arrested by a secondary impact preventer after rebounding to keep it from a second strike. The striker face is made of polished steel with dimensions of 40 mm × 80 mm and 5 mm radius edges. The weight of the drop hammer is 6.3080 kg. A polished steel anvil, set on a firm plane support, is hemispherical with a radius of 50 mm. The warp-knitted spacer fabric was wrapped on the hemispherical steel anvil and four sides of the fabric are caught by eight clamps on the plane support. Incident impact energies used for the work were 3.51, 6.51, 10.49, and 15.44J. For each spacer fabric type, three samples were tested, and every sample was impacted one time. The ultimate value of each type was the average of the three tests. The samples were conditioned in an atmosphere with a temperature of (20 ± 2)℃ and a relative humidity (RH) of 65 ± 5%. After positioning a sample on the steel anvil well, the test was completed within 3 min.

An XSM-LC three-dimensional laser scanner was used to evaluate damage deformations and depths of all the specimens after impact. The working principle of the scanner is shown in Figure 3. A transmitter sent out a laser to the top surface of a fabric and collected the reflection to calculate the distance between the fabric and transmitter; thus, the height of each point on the fabric surface could be obtained. By a given scanning area, the laser beam, guided by the transmitter, moved along the X-axis and Y-axis directions alternately. The interval of the X-axis was 0.001 mm and that of the Y-axis was 0.1 mm.

Figure 3.

Working principle of XSM-LC three-dimensional scanner.

Compression-after-impact tests

The compression-after-impact tests were carried out using a TexLab Precision Instrument CT250, as shown in Figure 4. The compression curves were obtained from all the fabrics after conditioning at 20℃ and 65% RH for at least 16 h based on Chinese standard FZ/T01051.2-1998.

Figure 4.

Compression-after-impact test instrument.

The instrument was composed of a presser foot with a contact area of 20cm², which moved vertically at a speed of 12 mm/min. The maximum compressibility was 20%. The size of all the samples was a circle with an area of 50cm².

Analysis of significance tests

The values of peak force, absorbed energy, and damaged depth of the specimens in the tests were evaluated by using one-way analysis of variance (ANOVA, Matlab) for each dependent variable. Any difference for each dependent variable was considered to be significant if the p-value was equal to or less than 0.05.

Results and discussion

Impact behaviors of spacer fabrics

The deformations of fabric during an impact event are shown in Figure 5. Figure 5(a) presents the side elevation of an original sample: the top and bottom surfaces of the fabric are plane and all the spacer yarns bend slightly on their nature state. When it is wrapped on a steel anvil (Figure 5(b)), tensile and compressive stresses are produced on the top and bottom surfaces, respectively. During the loading stage of the impact process (Figure 5(c)), the spacer fabric stores and absorbs energy given by the striker, meanwhile, it produces elastic and plastic deformations so as to reduce the force transmitted to the steel anvil. Stress in the contact area is non-uniform due to the specific contact method (flat-sphere); in particular, the maximum stress σ_max occurs at the center of the contact zone (x = 0) and spacer yarns in the central zone are compressed more seriously than those in surrounding zone. In the unloading process (Figure 5(d)), the striker is rebounded and elastic energy stored in the spacer fabric is released, the spacer fabric recovers to some extent as well. After removing the specimen from the steel anvil (Figure 5(e)), under the compressive and tensile stresses on the top and bottom surface, an approximate circular pit is formed on the surface of the specimen and irrecoverable deformation occurs on spacer yarns in the contact zone, which is shown in Figure 5(f).

Figure 5.

Deformation of fabric during an impact process.

To analyze typical curves of samples during impact tests, F1 is taken as an example: its force–time and force–displacement curves are shown in Figures 6(a) and (b), respectively.

Figure 6.

Typical force-time/displacement curves.

It can be seen from Figures 6(a) and (b) that these curves vary by increasing incident energy levels both in magnitude and configuration. At peak forces (Figures 6(a) and (b)), the maximum displacement (Figure 6(b)) increases while contact duration (figure 6(a)) decreases with the rising of impact energy in the loading stage. Forces drop sharply and displacements decline to some degree in the unloading stage (Figure 6(b)). Accompanied by vibrations of the machine, force values fluctuate around zero and incline ultimately to their value. Moreover, the vibrations occur between the drop hammer and secondary impact preventer (Figure 2). Since they are both rigid bodies, the drop hammer cannot stop immediately after rebounding and being arrested by the preventer; instead, it will stop gradually with the reducing of vibrations or collisions (Figure 6(a)).

The change of force is related with two factors as follows. First is the variation of displacement. Compression strength increases with the increasing of displacement.² The change of contact area resulted from that of displacement, shown in Figure 7, also has effects on force values.

Figure 7.

Variation of contact area with displacement.

It can be seen from Figure 7 that when the striker falls from Pos. 1 to Pos. 2, its displacement increases from D1 to D2, then the contact area ascends from S1 to S2. Contact area and displacement satisfy the following relationship:

S = - π D^{2} + 2 π (R + T) D

(1)

where S is the contact area, D is the displacement of the striker, R is the radius of the steel anvil, and T is the thickness of specimen.

Thus, contact area increases with the increasing of displacement. That more spacer yarns exert force on the striker produces a rising force on the striker.

Second is failure of spacer yarns in the central zone. Due to the non-uniform stress on the contact zone, spacer yarns in the center are damaged more seriously than others. The declination of load-bearing capacities of these damaged spacer yarns leads to the decrease of the force. Normal spacer yarns and damaged spacer yarns impacted under lower (3.51J) and higher energy (15.44J) are shown in Figures 8(a)–(c), respectively.

Figure 8.

Failure of spacer yarns.

The two factors above have an effect on the total force exerted on the striker and the change of force depends on which factor plays a major role. A continual rise of force in the loading stage for fabric subjected to lower impact energy is the result of increasing of displacement and relatively slight failure of spacer yarns (Figure 8(b)). While spacer yarns are damaged more seriously when they are impacted under higher energy (Figure 8(c)), the force drops at some instants where the specimen is subjected to higher energy is due to the failure of spacer yarns resulted from the change of contact area during loadings.

The energy–time curves of F1 impacted under different energies are shown in Figure 9.

Figure 9.

Typical energy-time curves.

When the striker impacts with the specimen surface, the given impact energy by the striker can be classified into two quantities. One is rebounded energy (elastic energy), which is stored elastically in the specimen and transferred back to the striker. The other is absorbed energy, which is the sum of absorbed energy in the specimen by forming irrecoverable deformations, and the energy absorbed by the impact system in vibration, heat, inelastic behavior of striker, and supports; thus, the following relationship exists:¹⁸

E_{total} = E_{reb} + E_{abs}

(2)

where E_total is the total impact energy, E_reb is rebounded energy, and E_abs is absorbed energy.

There is an assumption that the energy absorbed by the impact system can be ignored; thus, the total energy is the sum of rebounded energy and absorbed energy by specimen. In particular, the energy-absorption rates (E_abs/E_total) of fabric subjected to different energies are also shown in Figure 9; the ratio decreases with the increase of impact energies, and the undamaged material still can absorb a substantial proportion of the impact energy for all energy levels.

With the aim of examining the effects of fabric structure, thickness, and yarn diameter on the impact behavior, force–displacement curves for all samples subjected to different impact energies are shown in Figure 10.

Figure 10.

Force-displacement curves for all samples.

It can be seen from Figure 10 that change of forces for each specimen impacted under different energies has a tendency of that force’s drop at some instants in the loading stage in higher energies. For different fabrics subjected to the same impact energy, an ideal spacer fabric is considered to absorb more energy, and thus has lower peak force. The values of peak force and absorbed energy for all samples impacted under different energies are shown in Figures 11(a) and (b), respectively. Significance test results are shown in Table 3.

Figure 11.

Peak forces and absorbed energies for all specimens.

Table 3.

Significance test results

Category	Specimen	Test	Impact energy
Category	Specimen	Test	3.51J	6.51J	10.49 J	15.44 J
Peak force	F1, F2, F3	F _{(2, 6)}	358.123	836.742	13.798	43.632
	F1, F2, F3	p	<0.001	<0.001	0.006	<0.001
	F1, F4, F5	F _{(2, 6)}	20.411	208.358	127.581	347.871
	F1, F4, F5	p	0.002	<0.001	<0.001	<0.001
	F1, F6	F _{(1, 4)}	3795.006	289.183	853.265	656.556
	F1, F6	p	<0.001	<0.001	<0.001	<0.001
Absorbed energy	F1, F2, F3	F _{(2, 6)}	111.026	401.060	44.239	143.820
	F1, F2, F3	p	<0.001	<0.001	<0.001	<0.001
	F1, F4, F5	F _{(2, 6)}	150.283	301.647	186.251	96.665
	F1, F4, F5	p	<0.001	<0.001	<0.001	<0.001
	F1, F6	F _{(1, 4)}	237.222	647.464	155.348	418.918
	F1, F6	p	<0.001	<0.001	<0.001	<0.001

F1, F2, and F3 represent spacer fabrics with the same needle bar distance and same fineness of spacer yarns, but different surface structures, which are pillar + weft insertion, rhombic mesh, and hexagonal mesh, respectively. It can be seen from Figure 11(a) that the F_max value of fabric with the large-size meshes on surface layers is higher than that of fabric with the small-size ones, so the former can be used to absorb less energy (Figure 11(b)). Two main factors can account for this phenomenon. Firstly, spacer yarns' angles of the three samples are A_F1 > A_F2 > A_F3, which can be seen from Table 2. The fabrics with higher spacer yarn inclination angles can be used to absorb more energy in compression behaviors.² Secondly, spacer yarns of fabric with large-size meshes are more inclined to collapse and fail to deform normally to absorb impact energies; in addition, these spacer yarns are easier to damage, since parts of them are exposed in the open air and are not covered by surface layers perfectly.

F1, F4, and F5 represent spacer fabrics with the same surface structure and same fineness of spacer yarns, but different needle bar distances, which are 10.2, 7.2, and 13.2 mm, respectively. It can be seen from Figure 11(a) that the F_max value of fabric with the larger thickness is lower than that with the smaller one, so the former can be used to absorb more total energy than the latter, which can be seen from Figure 11(b).

F1 and F6 represent spacer fabrics with the same surface structure and same needle bar distance, but different finenesses of spacer yarns, which are 0.2 and 0.16 mm, respectively. It can be seen from Figure 11(a) that the F_max value of fabric with the coarser spacer yarns is lower than that with the finer ones, so the former can be used to absorb more energy than the latter, which can be seen from Figure 11(b).

Table 3 shows that the effects of surface structure, fabric thickness, and spacer yarns' fineness of fabrics impacted under different energies on peak force and absorbed energy are all highly significant.

Damage deformation and depth

Although the same method (flat-sphere) was adopted for all impacts, the damaged surfaces of specimens and their depth values differed from each other. However, deformation and depth give an indication of a fabric's ability to resist impact: small damage depth demonstrates good impact resistance in the same conditions.

Typical damaged surface deformations of specimens examined with the XSM-LC three-dimensional scanner are shown in Figure 12.

Figure 12.

Typical damaged surface deformations.

Figures 12(a)–(d) show that surface deformations of fabrics with the same surface layers (F1) under four different impact energies resemble each other in shape but not in dimensions. The deformations of fabrics with different surface structures (F1, F2, and F3) impacted under the same energy (15.44J) vary from one another (Figures 12 (d)–(f)). In addition, there are many height lines around the damaged areas for F2 and F3. As the surface layers of F1 are plain and tight, while those of F2 and F3 have many meshes on the outer layers, laser beams derived from the transmitter could irradiate the closed top surface of F1, while parts of them irradiated directly to the inner or even the bottom of the fabrics F2 and F3 because of the open structures on outer layers. The damaged deformation of specimen F1 (Figures 12(a)–(d)) reveals that the damaged zone tends to become localized and is more or less a round pit in shape: the deepest point occurs in the center of the pit. Figures 12(d)–(f) show that the damaged area appearances of F1 and F2 are similar (a round pit); however, for F3, the hexagonal sides have been broken. To more clearly demonstrate this, the photographs of damaged fabrics (impact energy: 15.44J) are shown in Figure 13.

Figure 13.

Surfaces of damaged fabrics with different stitches. (a) Pillar + Weft-insertion, (b) Rhombic mesh, (c) Hexagonal mesh.

Figures 13(a)–(c) show that the outer layers of F1 and F2 are still fine, while the appearance of F3 shows the breakage of hexagonal sides and collapse of spacer yarns around them, since higher stress concentrates on the mesh sides and parts of spacer yarns are exposed in the open air.

Damage depths for all the samples can be calculated based on the laser scanner, the results of which are shown in Figure 14.

Figure 14.

Damage depths of all samples.

Depth values of different specimens impacted under the same energy (15.44J) and those of the same sample (F1) subjected to different impact energies are shown in Figures 14(a) and (b), respectively. Figure 14(a) shows that, for F1, F2, and F3, the depth of F1 is smaller than that of F2. This means that the impact resistance of fabric with a closed surface layer (F1) is better than that with small-size meshes on surface layers (F2) (significance test F(1, 4) = 71.525, p < 0.001). While the damage depth of F3 (with large-size meshes on the surface layers) has no comparability with that of F1 and F2, for their damage conditions are different, the surface yarns of F3 are broken while those of F1 and F2 are not (Figures 13(a)–(c)). Almost all absorbed energy is converted into irrecoverable deformation energy of spacer yarns. Thus, a pit is produced on the fabric surfaces for F1 and F2. While part of the absorbed energy of F3 is consumed on the breakage of surface yarns, the depth of damage zone for F3 decreases. Viewed from this perspective, fabric with hexagonal meshes is not a perfect protector for the surface is easy to be damaged, and that large-size meshes on outer layers cannot protect spacer yarns in the middle layer effectively. The depths of fabrics with larger thickness and coarser spacer yarns are less than those of their opposites (significance test F(2, 6) = 227.953, p < 0.001; significance test F(1, 4) = 32.272, p < 0.001). The results reveal that the former have better impact resistance.

Depth values of F1 subjected to different energies are shown in Figure 14(b); it can be concluded that depths increase with the increasing of impact energies. Moreover, depth values of fabrics under two lower impact energies (3.51 and 6.51J) approximate to each other, and those under the two others (10.49 and 15.44J) are similar, while the latter is larger than the former. The results show clearly that the damage of fabrics impacted under higher energies is more serious than those under the lower ones.

Compression-after-impact behavior

Typical pressure-compressibility curves before and after impacts for F1 are shown in Figure 15.

Figure 15.

Typical pressure-compressibility curves before-and-after impacts for F1.

In Figure 15, the ‘0J’ curve represents that of the sample before impacts (undamaged fabric), while the others indicate those of fabrics after impacts under different energies. It can be seen that the two types of curves, before and after impacts, have similar configurations but different magnitudes.

All curves can be divided into two stages, that is, initial stage (stage I) and elastic stage (stage II), according to slope changes of the curves. Moreover, compression resistances of fabrics before and after impacts are not obviously different in stage I, while in stage II, those of the damaged fabrics decline by a large margin. Since the compression in stage I derives from fastening the multifilament loops and monofilaments, while that in stage II is mainly due to bending of spacer yarns, unfortunately, the spacer yarns have been damaged and their load-carrying capacities were diminished after impacts.

Residual strength

The residual strength is normalized with respect to damaged and corresponding undamaged compression strength of each specimen type under the compressibility of 20%. This is graphically presented as a function of impact energy in Figure 16. All related test results are given in Table 4.

Figure 16.

Residual strength values of damaged and undamaged specimens versus impact energy.

Table 4.

Residual strength values of fabrics before and after impacts and drop-off rates

Samples	Residual strength before impacts (kPa)	Average residual strength after impacts (kPa)	Drop-off rate (%)
F1	96.442	66.090	31.471
F2	89.292	64.252	28.042
F3	68.054	51.150	24.840
F4	98.708	66.354	32.778
F5	60.315	42.843	28.969
F6	45.299	33.639	25.741

It can be seen from Figure 16 that residual strength values of fabrics before impacts (RB) are RB_F3 < RB_F2 < RB_F1, RB_F5 < RB_F1 < RB_F4 and RB_F6 < RB_F1. This demonstrates that undamaged fabrics with smaller size meshes on outer layers, smaller fabric thickness, and coarser spacer yarns have better compression resistance. However, residual strength values for each specimen under various impact energies (except 0J) vary within a small range. In other words, these values incline to be stable, and damage of a specimen impacted under relative smaller energies tends to saturation. Therefore, the average residual strength value of each sample in various impact energies (except 0J) is used to evaluate the compression resistances after impacts (Table 4).

It can be seen from Table 4 that the sequence of average residual strength after impacts (RA) is RA_F3 < RA_F2 < RA_F1, RA_F5 < RA_F1 < RA_F4, RA_F6 < RA_F1. The logics remain coherent with those of RB. Therefore, fabric with better compression resistance before an impact performs better after it as well. The drop-off rates (DO) of compressive strength for all the specimens can be calculated (DO = (RB – RA)/RB), as shown in the last column of Table 4, and the sequence is DO_F3 < DO_F2 < DO_F1, DO_F5 < DO_F1 < DO_F4, DO_F6 < DO_F1, which is identical to the two above. This demonstrates that fabrics with better compression resistance also have higher drop-off rates. For fabrics with the same thickness, higher drop-off rates of fabrics (closed surface structure and coarser spacer yarns) mean that these fabrics can absorb more energy in an impact event by forming irrecoverable deformation to reduce peak force. For fabrics with different thicknesses, those with higher thicknesses can absorb more energy and have lower drop-off rates.

Conclusions

As a cushion material, the impact behavior and damage characteristic after impacts of warp-knitted spacer fabrics were investigated in this study. Four parameters, including peak force, absorbed energy, damage depth, and drop-off rate of residual strength, were involved to evaluate the two behaviors above of spacer fabrics with different structures. A spacer fabric with good protection performance was considered to have lower peak force, more absorbed energy, lower damage depth, and a lower drop-off rate of residual strength. According to the experimental results and analysis, the following conclusions can be drawn.

The variation of force in an impact event is the result of the co-effect of two factors: displacement and failure of spacer yarns in the center zone. In addition, force increases with the increasing of the former and decreases because of the latter.

For fabrics with the same needle bar distance (different surface structures and finenesses of spacer yarns), fabrics with coarser spacer yarns and close surface structure have several satisfying results, such as lower peak force, larger absorbed energy, and lower depth value, but higher drop-off rate of residual strength. While considering the specific impact method (flat-sphere), fabric with a mesh structure is body-fitted, for meshes can open and shut more freely according to the shape of wrapped objects. However, meshes that are too large on surface layers makes the mesh sides inclined to collapse and means that spacer yarns cannot be protected perfectly.

For fabrics with different needle bar distances (same surface structure and fineness of spacer yarns), fabric with higher thickness performs perfectly in the experiments (lower peak force, more absorbed energy, lower damage depth, and lower drop-off rate), but a person’s body may feel uncomfortable if the protector is too thick. The balance between the protective performance and comfort should be taken into consideration by selecting a suitable thickness of fabric for the specific protective application.

For a fabric subjected to different energies, peak force, absorbed energy, and damage depth increase with the increasing of energy levels, while residual strengths approximate to each other.

Footnotes

Funding

This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

References

Liu

. Compression property and air permeability of weft-knitted spacer fabrics. J Textil Inst 2011; 102: 366–372.

Liu

Zhao

. Compression behavior of warp-knitted spacer fabrics for cushioning applications. Textil Res J 2012; 82: 11–20.

Feng

. Development of the warp knitted space fabrics for cushion applications. J Ind Textil 2008; 37: 213–223.

. Application of warp-knitted space fabric in car seats. J Textil Inst 2007; 98: 337–344.

Bruer

Smith

. Three-dimensionally knit spacer fabric: a review of production techniques and applications. J Textil Apparel Technol Manag 2005; 4: 23–25.

Yip

. Study of three-dimensional space fabrics: physical and mechanical properties. J Mater Process Technol 2008; 206: 359–364.

Liu

. Sound absorption behavior of warp knitted spacer fabrics. Textil Res J 2010; 80: 1949–1957.

Chen

Jiang

Chen

. Research on the pressure relief of warp knitted spacer fabric. Knitting Ind 2008; 4: 26–28.

Feng

. Research on pressure relief of warp knitted spacer fabric. J Donghua Univ 2006; 32: 88–91.

10.

Armakan

Roye

. A study on the compression behavior of space fabrics designed for concrete applications. Fiber Polym 2009; 10: 116–123.

11.

Sheikhzadeh

Ghane

Eslamian

. A modeling study on the lateral compressive behavior of spacer fabrics. J Textil Inst 2010; 101: 795–800.

12.

Miao

. Vibration behavior of cushioning warp knitted spacer fabric. J Textil Res 2008; 29: 57–60.

13.

Chen Y. Compression resistance of warp knitted spacer fabric. Dissertation, Jiangnan University, 2007, p.19.

14.

Chen HL. Development and research on two-bed Raschel spacer fabrics. Dissertation, Donghua University, 1998.

15.

Hou

Liu

. Nonlinear compression behavior of warp-knitted spacer fabric: effect of sandwich structure. Textil Res J 2011; 23: 119–134.

16.

Liu

Long

. Impact compressive behavior of warp-knitted spacer fabrics for protective applications. Textil Res J 2012; 82: 773–788.

17.

Douglas

Magaziner

Zimmerman

. Efficacy of a hip protector to prevent hip fracture in nursing home residents. J Am Med Assoc 2007; 298: 413–422.

18.

Tae

Cheol

. Energy-absorption mechanisms in Kevlar multiaxial warp-knit fabric composites under impact loading. Compos Sci Technol 2000; 60: 773–784.