JP4955798B2 - Optical fiber cable design method - Google Patents

Description

The present invention relates to an optical fiber cable, and more particularly to a method for designing an optical fiber cable excellent in durability.

An example of an optical fiber cable with improved waterproofness will be described with reference to FIG.
In the optical fiber cable 1 shown here, an assembly 4 of optical fiber ribbons is accommodated in a slot groove 3 formed in a slot core 2, and a water absorbing tape layer 5 is formed on the outer periphery of the slot core 2. In this configuration, a sheath 6 is provided.
As the water-absorbing tape constituting the water-absorbing tape layer 5, for example, a material obtained by adhering a water-absorbing powder made of a highly water-absorbing resin to a base fabric can be used. The water-absorbing powder has a function of swelling when it is immersed and filling the gap in the cable to prevent the expansion of the immersion.

However, since the swelling of the water-absorbing powder increases the pressure in the cable, it is necessary to prevent the sheath 7 from being damaged due to an increase in internal pressure.
Increasing the strength of the sheath is effective in preventing sheath breakage.
In order to increase the sheath strength, it is conceivable to form a thick sheath. However, if the sheath is thickened, the outer diameter of the cable increases, resulting in a problem of cost increase and handleability. In recent years, a thin cable is often required, so that it is problematic to form a thick sheath.

As technologies for improving the durability of tubular bodies, there are those shown in Patent Documents 1 to 3, but there are the following problems in applying these to optical fiber cables.
Patent Document 1 proposes a pipe material excellent in durability, but this material is not suitable for a sheath material. Patent Document 2 mentions a resin material for a tubular body exhibiting high durability. However, if this material is used for a sheath, the outer diameter of the cable becomes too large, which is disadvantageous in terms of cost. Patent Document 3 discloses a pipe reinforcement structure, but this also has problems in terms of cable outer diameter and cost.

JP 2000-109521 A JP-A-6-328577 Japanese Patent Publication No. 1-59474

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for designing an optical fiber cable that can prevent damage to the sheath at the time of flooding without increasing the cable outer diameter and that is free from cost. And

The present invention relates to an optical fiber cable design method in which a water-absorbing tape layer is formed on the outer periphery of a cable core and a sheath is provided on the outer periphery, and the maximum stress on the inner surface of the sheath when pure water is immersed on the inner surface of the sheath Provided is a method for designing an optical fiber cable , in which a sheath satisfying the relationship represented by the following formula (1) is used .
σ = p (Dt) / 2t <Yield point strength of sheath material (1)
(Where D: outer diameter of the sheath, t: sheath thickness, p: pressure on the inner surface side of the sheath)
The optical fiber cable design method of the present invention may be an optical fiber cable design method in which a tear string is provided on the inner surface side of the sheath.
The present invention relates to a design method of an optical fiber cable in which a water-absorbing tape layer is formed on the outer periphery of a cable core, a sheath is provided on the outer periphery, and a tear string is provided on the inner surface side of the sheath. The tear string is formed with a recess, and the tear string is made of a fiber that is not twisted , and the maximum stress σ ′ of the sheath inner surface when pure water is immersed in the sheath inner surface is expressed by the following equation: (2) Provided is a method of designing an optical fiber cable characterized by using a sheath satisfying the relationship shown in (3).
σ ′ = K · p (Dt) / 2t <Yield point strength of sheath material (2)
K≈50 × d / t (3)
(Where D: outer diameter of the sheath, t: sheath thickness, p: pressure on the inner surface side of the sheath, d: depth of the recess)
In the method for designing an optical fiber cable according to the present invention, it is preferable that the p is an internal pressure of the sheath when immersed in water, and satisfies the relationship represented by the following formula (4).

According to the present invention, the design based on the relational expression between the maximum stress on the inner surface of the sheath at the time of water immersion and the strength of the sheath material gives the sheath sufficient strength against the pressure increase due to water immersion, and the sheath at the time of water immersion Breakage can be reliably prevented.
In addition, based on the above relational expression, it is possible to design to prevent sheath breakage without increasing the outer diameter.
Furthermore, since the sheath material has only to be selected within a range that satisfies the above relational expression, the cost can be reduced.

It is sectional drawing which shows the optical fiber cable which is the 1st example of this invention. It is a schematic diagram which shows a test apparatus. It is a schematic diagram which shows the structure of an optical fiber cable. It is sectional drawing which shows the optical fiber cable which is the 2nd example of this invention. It is a graph which shows a test result. It is a figure which shows the analysis result by FEM (finite element method) about stress distribution of a sheath. It is a principal part enlarged view of FIG.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a cross-sectional view showing an optical fiber cable 1 which is a first example of the present invention.
In the optical fiber cable 1 shown here, an assembly 4 of optical fiber ribbons is accommodated in a slot groove 3 formed in a slot core 2 (cable core), and a water absorbing tape layer 5 is formed on the outer periphery of the slot core 2. The sheath 6 is provided on the outer periphery thereof.

As the water-absorbing tape constituting the water-absorbing tape layer 5, for example, a base fabric made of polyester or the like and a water-absorbing powder made of a highly water-absorbing resin adhered can be used. A tape made of a highly water-absorbing resin such as starch or polyacrylic acid may be used. Powders and tapes made of a highly water-absorbent resin have a function of swelling during water immersion to fill voids in the cable and prevent the water from spreading.
As the material of the sheath 6, a resin such as polyethylene or polyvinyl chloride can be used. The physical properties of the sheath 6 can be set by selecting materials and additives such as a plasticizer.

The internal pressure (pressure on the inner surface side) of the tubular body can be represented by the inner radius, the outer radius, the displacement amount of the outer diameter, and the tensile elastic modulus of the tubular body.
For this reason, as shown in FIG. 2, the internal pressure p of the sheath 6 (pressure on the inner surface side of the sheath 6) when the test body 8 obtained by cutting the optical fiber cable 1 is immersed in water in the water tank 9 is 3, based on the dimensions and physical properties of the sheath 6 shown in FIG. 3 (inner radius a, outer radius b, displacement u of the outer radius when applying internal pressure, and tensile modulus E of the sheath 6 (based on JIS K7161)), It can be estimated using equation (4).
For the amount of displacement when applying the internal pressure, the outer diameter of the specimen 8 measured after immersing the specimen 8 (length 30 cm) whose dimensions were measured in advance in water (temperature 80 ° C.) in the water tank 9 for 2000 hours was used. Can be calculated.

  In addition, the internal pressure of the sheath raised by immersing a general optical fiber cable in water is about 0.04 to 0.1 MPa.

The present inventor has found that the maximum stress σ (MPa) of the inner surface of the sheath when pure water is immersed in the sheath can be expressed by Equation (5) based on the hoop stress model of the thin cylinder.
σ = p (D−t) / 2t (5)
(P: internal pressure, D: sheath outer diameter, t: sheath thickness)

From equation (5), it can be seen that the maximum stress on the inner surface of the sheath increases as the internal pressure increases and the sheath 6 becomes thinner, and this facilitates sheath cracking.
From this, by selecting a sheath material that satisfies the following formula (6), it is possible to give the sheath 6 sufficient strength against the pressure increase due to water immersion, and reliably prevent the sheath 6 from being damaged during water immersion. Recognize.

σ = p (D−t) / 2t <Yield point strength of sheath material (6)
(D: sheath outer diameter, t: sheath thickness, p: internal pressure)

Based on Expression (6), it is possible to design the sheath 6 to have strength without increasing the outer diameter D (mm) and prevent breakage.
Moreover, since it is sufficient to select the sheath material within a range that satisfies this on the basis of the formula (6), the range of material selection is widened, and the cost can be suppressed.

FIG. 4 shows a second example of the optical fiber cable of the present invention, and has the same configuration as the first example shown in FIG. 1 except that a tear string 7 is provided.
The tear string 7 is provided on the inner surface side of the sheath 6, and is attached vertically in the longitudinal direction of the slot core 2.
On the inner surface of the sheath 6, a tear string 7 is formed by a tear string 7, which is a recess having an inner surface shape corresponding to the outer surface shape of the tear string 7.
The tear string 7 enables the sheath 6 to be cut in the longitudinal direction for the purpose of intermediate post-branching. As the tear string 7, an aramid fiber, a PBO fiber, or the like can be used.

  As a result of the study by the present inventor, it has been found that the tear string mark 10 formed on the inner surface of the sheath 6 greatly affects the sheath cracking. That is, it was found that when the internal pressure increases, the stress concentrates on the tear string mark 10, so that a sheath crack occurs even if the value of the stress σ shown in the equation (5) is small.

  Taking the stress concentration in the tear string mark 10 into account, the maximum stress σ ′ on the inner surface of the sheath can be expressed by Equation (7).

σ ′ = K · p (D−t) / 2t (7)
(D: sheath outer diameter, t: sheath thickness, p: internal pressure, K: stress concentration factor)

That is, when there is a tear string mark 10 on the inner surface of the sheath 6, stress concentration occurs in the tear string mark 10, and the conditions under which the sheath crack does not occur are as follows.
σ ′ = K · p (D−t) / 2t <Yield point strength of sheath material (8)

  The present inventor evaluated the stress concentration when the concave portion is formed on the inner surface of the sheath by FEM (finite element method). The stress concentration coefficient K in the above equation (7) is expressed by the following equation (9). We found that it can be approximated.

K≈50 × d / t (9)
(D: Depth of tear string mark, t: sheath thickness)

Therefore, by selecting a sheath material that satisfies the above formulas (8) and (9), even when the tear string mark 10 is present, the sheath 6 is given sufficient strength against the pressure increase due to water immersion, Breakage of the sheath 6 can be reliably prevented.
That is, based on the equations (8) and (9), it is possible to design the sheath 6 to have strength without increasing the outer diameter D and prevent breakage.
Moreover, since it is sufficient to select a sheath material within a range satisfying the equations (8) and (9) as a reference, the range of material selection is widened, and the cost can be suppressed.

FIGS. 6 and 7 are diagrams showing FEM (finite element method) analysis results on the stress distribution of the sheath 6 in a model in which pressure (hoop stress) is applied to the sheath 6 having the tear string mark 10 from the inner surface 6a side. is there. FIG. 6 is a diagram of a four-divided portion of the cross section of the sheath 6 including the tear string mark 10, and FIG. 7 is an enlarged view of the main part of FIG. 6.
6 and 7, the larger the stress generated when the internal pressure is applied, the deeper the stress is, and the smaller the stress, the lighter the screen is displayed.

When twisted fibers are used for the tear string 7, the sheath 6 can be torn without causing fraying. However, since the shape of the tear string 7 does not collapse, the tear string mark 10 formed on the inner surface of the sheath 6 is not broken. Deeply formed.
On the other hand, the tear string 7 made of untwisted fibers is reduced in thickness (flattened) when the sheath 6 is formed, so that the tear string mark 10 is formed shallow.
For example, when using a tear string 7 formed of a twisted fiber made of generally used aramid fiber, the tear string mark 10 becomes a concave part having a semicircular cross section with a depth of about 0.2 mm, When the tear string 7 formed of untwisted fibers made of PBO fibers is used, the tear string mark 10 is a concave portion having a circular arc shape with a depth of about 0.1 mm.

For this reason, it can be seen that by using the tear string 7 made of untwisted fibers, the tear string trace can be made shallow and the stress concentration factor K can be reduced (see equation (9)), thereby reducing the risk of sheath cracking.
In addition, when using the tear string 7 that does not contain a twist, there is a concern that the strength of the tear string 7 may decrease due to fraying when the sheath 6 is torn, but by using a high-strength fiber, the tear string Function can be ensured.

Example 1
The following verification test was performed using the optical fiber cable 1 having the structure shown in FIG. 4 (the outer diameter of the slot core 2 is 19 mm).
As shown in Table 1, the water-absorbing tape includes a water-absorbing tape A in which the internal pressure p when immersed in pure water is about 0.08 MPa, and the internal pressure p when immersed in pure water is about 0.00. Either of the water absorption tapes B used as 04 MPa was used. The water absorption tapes A and B both have a running length of 40 m or less in a water running test in seawater.

As the tear string 7, either a twisted tear string A made of an aramid fiber or a tear string B made of a PBO fiber having higher strength than the tear string A and not twisted was used.
When the tear string A was used, the depth d of the tear string mark 10 was 0.2 mm. When the tear string B was used, the depth d of the tear string mark 10 was 0.1 mm.
For the sheath 6, any one of four types of resins A to D having different tensile yield strength (80 ° C.) (yield point strength: conforming to JIS K 7113) was used. The thickness t of the sheath 6 was set to any one of 1.2 mm, 1.7 mm, and 2.0 mm with an index of obtaining sufficient mechanical strength as an optical fiber cable.

As shown in FIG. 2, the test body 8 (length 30 cm) obtained by cutting the optical fiber cable 1 was immersed in pure water at a temperature of 80 ° C. The immersion time was 2000 hours, and the presence or absence of a sheath crack was examined. The results are shown in Table 1 and FIG.
In Table 1, the case where the sheath crack did not occur was marked with ◯, and the case where it occurred was marked with ×. K was calculated based on Equation (9) (K = 50 × d / t). σ ′ was calculated based on Equation (7).
L1 in FIG. 5 is a line indicating “σ ′ = the yield point strength of the sheath material”.

  From Table 1 and FIG. 5, when the maximum stress σ ′ on the inner surface of the sheath satisfies the relationship shown in the above equations (8) and (9), that is, when “σ ′ <yield point strength of the sheath material”, the sheath crack occurs. If this relationship is not satisfied, it can be seen that a sheath crack has occurred.

(Example 2)
As shown in FIG. 1, Table 2 shows the results of a verification test similar to that of Example 1 using the optical fiber cable 1 having the same configuration as that of Example 1 except that there is no tear string 7.
For the sheath 6, any one of seven types of resins A to G having different tensile yield strength (80 ° C.) (yield point strength: conforming to JIS K 7113) was used. The thickness t of the sheath 6 was selected in the range of 0.5 to 2.0 mm, using as an index to obtain sufficient mechanical strength as an optical fiber cable. σ was calculated based on the formula (5).

  From Table 2, when the maximum stress σ of the sheath inner surface satisfies the relationship shown in the above formula (6), that is, when “σ <the yield strength of the sheath material”, the sheath crack does not occur, and this relationship is not satisfied. Shows that a sheath crack occurred.

1 Optical fiber cable 2 Slot core (cable core)
5 Water-absorbing tape layer 6 Sheath 7 Tear string 10 Tear string mark (recess)

Claims (4)

A method of designing an optical fiber cable in which a water absorbing tape layer is formed on the outer periphery of the cable core and a sheath is provided on the outer periphery thereof,
A design method for an optical fiber cable , wherein a sheath satisfying the relationship represented by the following formula (1) is used: a maximum stress σ of the sheath inner surface when pure water is immersed in the sheath inner surface side.
σ = p (Dt) / 2t <Yield point strength of sheath material (1)
(Where D: outer diameter of the sheath, t: sheath thickness, p: pressure on the inner surface side of the sheath)   The optical fiber cable design method according to claim 1, wherein a tear string is provided on an inner surface side of the sheath. A water-absorbing tape layer is formed on the outer periphery of the cable core, a sheath is provided on the outer periphery, and an optical fiber cable design method in which a tear string is provided on the inner surface side of the sheath,
On the inner surface of the sheath, a recess is formed by the tear string,
The tear string is made of untwisted fibers,
A method of designing an optical fiber cable , comprising using a sheath in which the maximum stress σ ′ of the inner surface of the sheath when pure water is immersed on the inner surface side of the sheath satisfies the relationship represented by the following expressions (2) and (3).
σ ′ = K · p (Dt) / 2t <Yield point strength of sheath material (2)
K≈50 × d / t (3)
(Where D: outer diameter of the sheath, t: sheath thickness, p: pressure on the inner surface side of the sheath, d: depth of the recess)   4. The optical fiber cable according to claim 1, wherein p is an internal pressure of the sheath when immersed in water, and satisfies a relationship represented by the following expression (4): Design method.

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