3.1

Payout Fiber

Stringent physical and optical performance requirements are placed on the optical fiber that is wound under tension onto a cylindrical bobbin to produce an operationally-stable fiber pack to be deployed at high speeds (> 150 m/s) following a 10-year storage life under adverse military environments. The optical fiber, which is employed as a vital part of the data link, has a size on the order of a human hair as shown in Fig. 4. The payout fiber provides for a long-range, non-jammable data link. Imagery data from the guided weapon is transmitted down the link to the operator control station and control data is transmitted up the link for man-in-the-loop operations. This design enables remote control of the weapon from launch to impact. Sensors aboard the guided weapon permit real-time targeting decisions.

Figure 4:

Fiber Size Comparisons

A special fiber with a duplex capability is required for the fiber optic data link to survive the harsh treatments during high-speed payout in military environments. Also, the payout fiber must exhibit resistance to bend-induced attenuation generated at the peel point (discussed in Section 4). The fibers most commonly used for payout applications are derived from telecommunications fibers.

The strength levels of payout fibers are established during the draw process by applying a specified “proof” tensile load to the fiber. The intrinsic or characteristic strength of the fiber is determined by conducting a series of tests where the fibers are pulled to failure at a specified rate. The intrinsic strength of high quality silica fiber is 1.4 times stronger than steel. However, the shear strength of the fiber is low.

The Government conducted programs to develop fibers of increased strength, greater resistance to bend induced attenuation, and stronger/thinner protective buffer layers. In the mid-70’s low loss, step index, multimode fibers, operating in the 850-nm regime, were used in the initial payout tests. In the late-70’s, efforts were conducted to reduce the fiber core diameter from the standard 50 to 35 µm for reduced optical attenuation at low temperature in wound fiber packs at increased lengths. The Integrated Fiber Optic Communications Link (IFOCL) program was initiated by CECOM with Hughes and ITT to develop the fiber optic data link. The IFOCL program led to the Enhanced Fiber Optic Communication Link (EFOCL) program, which was initiated to develop an advanced fiber optic data link. The initial fiber optic data link design, which was qualified for the FOG-M program was a 300 µm O. D., 35 µm core, and 200 kpsi proof-tested fiber. In the mid-80’s, efforts were conducted to develop, high strength, single mode fiber operating at longer wavelengths (> 1 µm) for extended range.

A standard measure of data link capacity is the length-bandwidth product. This length-bandwidth parameter for standard off-the-shelf telecommunication fiber is 100 GHz-km at the 1550 nm wavelength. The fiber optic data link used in FOG-M required only a 125 MHz bandwidth capability.

Dispersion shifted (DS) fiber was considered to obtain low attenuation at both the 1.3 and 1.55 µm wavelengths.¹¹ Pre-twist degrades the attenuation performance in DS fibers at the 1.55 µm wavelength. The DS fiber was replaced with a bend-insensitive fiber. Industry, AT&T and Corning Glass Works, developed microbend insensitive, high-Δ single mode fibers.^12,13 Δ is the relative index difference between the core and cladding. The Naval Ocean Systems Center (NOSC) in Hawaii initiated a program to develop a dual-mode optical fiber for long-haul transmission.¹⁴ The matched-clad and depressed-clad, pure-silica core designs were considered for applications in missile tethers and undersea surveillance cables.

Recently, a program was initiated at AMCOM, with contractors Spectran and Lucent Technologies, to develop a thin buffered (200 µm) fiber for long range FOG-V systems. Reducing the fiber diameter from 250 to 200 µm substantially reduces the number of layer in long-range fiber optic dispensers.

3.2

Fiber Winding Methods

The winding approaches considered for high-speed payout applications include: precision coil geometry, random winding, ortho-normal winding, basket-weave winding, flat coil geometry, hybrid coil geometry, and volumetric efficient winding.¹⁰ Since the winding dictates the payout characteristics, the winding must be accomplished such that dynamic pack instability is avoided. Winding defects lead to payout failures.

The precision coil geometry is used in most high-speed payout operations. The precision winding configuration involves winding the first layer of fiber onto a cylindrical bobbin in the form of a gentle helix which propagates with a pitch of one fiber diameter in the direction of the winding as shown in Fig. 5. Each fiber wrap is in contact with the preceding wrap. The direction of the pitch is changed during the transition from the first layer to the second layer when the direction of the winding is reversed as shown in Fig. 6. The fiber wraps of the second layer is nested in the grooves formed by the underlying wraps, except in the crossover regions. In cross section, the winding forms a hexagonal close pack pattern (See Fig 7). During the transition the fiber is stepped back over a specified number of underlying fibers, then settles into a new groove. There is a crossover event for each fiber wrap (See Fig. 6). Since the pitch of the winding is reversed, the fiber in the second layer is forced from one groove and obliquely crosses over two wraps of fiber in the first layer before settling into the proper groove for precision winding.¹⁰ This process continues for all layers wound above. The basic geometry for outside payout of the precision coil configuration is shown in Fig. 8.

Figure 5:

Winding Helix

Figure 6:

Stepbacks and Crossovers

Figure 7:

Close Pack Pattern

Figure 8:

Outside Payout

The precision winding method, which has been proven successful in high-speed payout deployment applications, produces crossovers as discussed above. These unavoidable perturbations are prime candidates for fiber microbending. The precision winding method also subjects the fiber to combined tensile, radial (lateral), shear, and bending stresses, not recognized in straight fibers. These stresses conspire with the imperfections at the crossovers to cause more microbending which in turn causes coupling of fiber guided modes to radiation modes, resulting in energy leakage in multimode fiber.

Inside payout winding is accomplished via precision winding the fiber onto a bobbin, which is removed following the winding. The free fiber pack is mounted into an external container for payout operations. The fiber is paid out from the innermost layer of the wound pack as shown in Fig. 9. Advantages of inside payout include: no radially inward forces to disrupt the pack as in the case of outside payout, reduced frictional drag, and no bobbin which can cause thermal mismatch instabilities. Inside payout is more successful for underwater payout applications. The fiber packs for underwater applications are wound from flange to flange with no stepbacks. The inner diameter is less than 1.5 inches. A pre-twist is introduced to ensure smooth payout, which is free of fiber kinks.

Figure 9:

Inside Payout

Volumetrically efficient spooling (VES) involves winding the fiber flush to a single flange with step-backs only at the aft end. Reduced diameter (135 µm) fiber is wound in a much smaller volume. Deep packs (> 50 layers) involve winding many layers of fiber in a compact configuration.

3.3

Fiber Cable Pack Mechanics Modeling

Theoretical fiber pack mechanics models were developed by the Fiber Optic Guided Vehicle (FOG-V) team, consisting of Government and industry personnel, to confirm fiber pack stress conditions following winding. The long-term reliability of fiber optic payout dispensers is directly influenced by the residual tension distribution within the fiber pack and by the bending stress in the fiber at the crossovers. The early models provided an adequate mathematical description of the mechanical state of precision-wound fiber packs for the very unique instant that the winding is completed. The effects of bobbin stiffness, fiber buffer spring parameter, and winding tension parameters on the bobbin compression, base layer tension, and residual tension throughout the wound pack were determined for a freshly-wound fiber pack. The principal parameters of the model are:

where E_b is the Young’s modulus of the bobbin, A_b is the surface area of the bobbin, E is the Young’s modulus of the glass fiber, A is the cross-sectional area of the fiber, k is the squeeze test spring constant, and R is the bobbin radius.

A set of determinate equilibrium equations for differential elements of the bobbin, base layer, and each fiber layer was solved via a tridiagonal computational process. Thus, the stress distribution in the fiber pack was determined. The early models predicted that stiffer bobbins, higher modulus fiber buffer coatings, and a wind tension profile were necessary to stabilize the pack during long-term storage and high-speed payout.

It is because of the role of the residual tension in resisting dynamic pack instability that a knowledge of pack mechanics is so important. A more complete set of factors influencing residual tension and pack stability are: 1) the winding tension profile, 2) the elastic and viscoelastic properties of the glass fiber, bobbin material, fiber buffer coating and adhesive materials, 3) the number of layers comprising the pack, 4) bobbin geometry, 5) the relative coefficients of thermal expansion (CTE) of glass fiber, bobbin, buffer coating and adhesive materials, 6) the temperature history and thermal state at the time of payout, and 7) degree of spacing between the fiber wraps.

The early aluminum bobbins contained a stainless steel base layer to provide grooves and dictate the spacing between the fiber wraps. These bobbins were inappropriate because of the CTE mismatch between the bobbin and the glass fiber. The thermal mismatch caused the inner layer fibers to loose tension, thus leading to much movement in the pack. The bobbin must be designed such that the CTE in the hoop (radial) direction match the CTE of the glass fiber and the CTE

in the axial direction match the CTE of the buffer coating. It was also found that a grooved base layer as shown in Fig. 10 provided proper fiber spacing control and more stability. The grooves are formed via winding a precision, stainless steel wire onto a smooth, uncured epoxy surface, removing the wire and curing the epoxy.

Figure 10:

Grooved Baselaver

Candidate materials to closely match glass include low expansion metal alloys, composites, sintered ceramics and injection molded plastics. The manufacturability of the bobbin with associated cost and physical properties such as compatibility with winding adhesives were considered in bobbin material selection. AMCOM-sponsored research produced several prototype bobbins of composite and metal alloy materials. These included graphite/epoxy, Kevlar-graphite/epoxy, and INVAR 36.

The early pack mechanics models suggested that the addition of more layers compress the lower layers, thus reducing their contribution to compression of the bobbin. Model validation techniques, described in the next section, were used to determine a proper tension profile to reduce or eliminate this effect.

The early fiber pack mechanics models described pack instability in a qualitative manner rather than in a quantitative manner. Analytical and experimental techniques were considered to determine the mechanical state of the precision-wound fiber packs following elevated temperature conditioning and storage. Results of laboratory experiments have shown that a heat conditioning and/or aging relieves a substantial portion of the residual stresses, especially in the outermost layers.¹⁵

The bending stresses at the crossover sites were not significantly reduced through a heat conditioning/aging process. A separate model was developed to investigate the stresses at the crossovers. The local inter-layer pressures and residual tension are input into the fiber crossover model. In the crossover region, the fiber is lifted from the groove in which it is nested between two rings of fiber in the layer beneath and crosses adjacent fibers at an angle, then becomes nested. The fiber tends to follow a helix on the fiber ring beneath. Since the fiber below is approximated as a torus, the configuration of the fiber in the crossover region behaves as a helix on a torus. A plane view of a typical crossover pattern in a fiber pack is illustrated in Fig. 6. The bending stress σ_b associated with the fiber geometry in the crossover region is expressed as¹⁶

where

is the fiber curvature, E_f is the Young’s modulus of the fiber, a is the radius of the glass fiber, r₀ is the radius of the torus, d is the diameter of the buffer coating, and θ_c is the crossover angle. The results of Eq. 5 are presented in Fig. 11 for a typical FOG-M fiber. The bending stress increases as the helix angle is increased. Abnormal crossovers can cause increased stress, thus reducing fiber pack reliability.

Figure 11:

Helix Angle Bending Stress

The fiber crossovers are the prime contributors to excess loss in precision-wound optical fibers. A model was developed to predict the excess loss in a precision-wound paroboloc-index fiber.¹⁵ The model utilizes the coupled mode theory¹⁷ along with perturbation techniques at the fiber crossovers. The results of the model showed that a reduced core diameter substantially reduced microbending loss in the wound pack. Furthermore it was determined that soaking precision-wound optical fibers at an elevated temperature for a proper period of time greatly reduces the initial excess signal loss. The elevated temperature soak, aimed at relaxing the internal shear stresses emanating from the winding process, brings the wound fiber pack to a stress equilibrium state in a short period of time (several hours). The model was extended to include the low temperature effects.¹⁸ Excess loss at low temperature is attributed to increased bending at the crossovers due to an increase in Young’s modulus of the buffer coating at low temperature and increased radial pressure due to loss in tension in the inner layers.

3.4

Adhesives Development

An adhesive is frequently used to augment the stability of the pack against internal movement between the fiber wraps. In some cases (where the cable has high transverse stiffness) the adhesive is applied to the “stepback” regions only. In the case of soft cables, adhesive must be applied throughout the pack.

An adhesive is necessary to hold the fiber wraps in place as it is laid down during the winding process. The adhesive must stabilize the fiber wound pack over the full military range of environmental exposure and during high-speed payout. The adhesive must be compatible with the buffer, i. e. does not cause any long-term change in buffer characteristics due to chemical interactions between the buffer coating and adhesive (solvents). The adhesive must be applied uniformly to prevent irregular transient adhesive-rupture forces, which can cause fiber failure during payout. The CTE of the adhesive must also match the CTE of the buffer coating.

If there is sufficient residual fiber tension throughout the pack, the intrinsic stability is adequate without any adhesive (except to stabilize the layer end transitions). The adhesive augments the stability of the wound pack. Anything that tends to destroy the integrity of the adhesive within the pack also tends to reduce the augmented stability of the pack. As the fiber breaks away from the wound pack during high-speed payout there is a radially inward force on the fiber due to the curvature of the winding. The fiber has a tendency to dig into the pack and dislocates other fiber wraps, which can lead to fiber breakage. This effect is most likely to occur in the stepback region at the aft end of the wound pack. An adhesive is required to minimize the occurrence of this dynamic instability. The adhesive stabilizes the fiber wound pack against this failure mode (discussed in Section 6) as well as a resistance to relative motion between the fiber wraps during thermal cycles in storage.

The primary physical properties of winding adhesives include viscosity, modulus, peel strength, shear strength, cure rate, cure mechanism, dimensional uniformity, ductility, ease of application, effect on microbend attenuation, compatibility with fiber buffer coating, coefficient of friction, operational temperature range, environmental aging, effect on cable pack stresses and manufacturability of the pack in a cost efficient process.¹⁹ A static peel force was used as a first indicator of a good adhesive formulation.

The use of adhesives results in increased fiber bending stresses at the peel point during payout. The adhesive strength must be bracketed between static and dynamic stability requirements. Inadequate adhesive strength results in dynamic instabilities (See Section 6). If the adhesive strength is excessive, peel point bending stress failures occur. This characteristic is illustrated in Fig. 12, where failure boundaries for these two failure mechanisms are plotted verses adhesive strength and payout speed. For successful payout, the operating point must be below the bending stress boundary and above the dynamic stability boundary. The distance from the operating point to either of the boundaries is a measure of the design margin against the particular failure mechanism. A sufficient design margin was incorporated into the fiber pack design for increased yield.

Figure 12:

Adhesive Boundary Values

It should be noted that the strength of most adhesives is strain-rate dependent. For the pack instability boundary, low-to-moderate strain rates are involved. For peel point bending, very high strain rates are present. As a result, for a j given adhesive material and quantity, the operating point for adhesive strength is different for the two failure boundaries. The ideal adhesive would have a high strength at low strain rates, providing high pack stability, but would have a low strength at high strain rates, resulting in low peel point bending stress. Unfortunately, for most adhesives, the high strain rate strength tends to be greater than that at low strain rates. This tendency tends to make the trade-off between these two failure modes even more difficult.

Through 1987, the types of adhesive investigated for fiber optic dispensers were neoprene, in the form of Polyfoam Rubber Cement and/or Lockbond 116 (Standard Brands).²⁰ Although successful payouts were conducted under ambient conditions, these adhesives exhibited poor aging characteristics when subjected to environmental temperature limits. Advanced adhesives, which include V-glue, Dow Corning Q2-7406, and UV-curable adhesives, were investigated for the long-range FOG-V systems. AMCOM initiated efforts in 1996 to develop a solventless adhesive material for use in EFOGM.²¹

3.5

Model Validation

Several instruments were investigated to measure strain in fiber optic dispensers. A Strain Sensing Optical Time Domain Reflectometer (SSOTDR) was developed by Georgia Tech to measure the total strain changes in a wound fiber pack.²² The SSOTDR was the first non-destructive measurement device, which was employed at AMCOM as a tool to validate fiber cable pack mechanics models via empirical determination of the residual energies (tensions) in wound fiber packs. The SSOTDR was used in concert with a rotary fiber coupler to obtain real-time fiber strain data during the precision winding process. The collective strain measurement yielded the sum of the fiber strains per layer and provided an indication of the equilibrium tensions generated by winding multiple layers of optical fiber. During thermal cycling, the viscoelastic response of the fiber allowed tensions within the pack to relax. The SSOTDR provided a direct method of capturing this relaxation phenomenon in wound fiber packs.

Since the SSOTDR did not provide fiber length-dependent strain capabilities, an advanced, non-destructive measurement capability was pursued to measure the distributed strain throughout the layers of the wound pack. A Brillouin Optical Time Domain Reflectometer (BOTDR), which is manufactured by Ando Corporation, was evaluated and found to be the leading candidate for measuring distributed strain in fiber optic dispensers.²³ The BOTDR utilizes stimulated Brillouin scattering (SBS), which is a nonlinear interaction between light-waves and acoustic waves. Distributed strain plots were generated for fibers wound under various configurations (fiber optic dispensers and fiber optic gyroscopes). Comparisons were made between model predictions and measured data for a number of winding scenarios.²⁴ Fiber layers, step-backs, winding anomalies, pack deformations and aging were discernable.

BOTDR measurements at cold temperature revealed that the outside layers of the fiber optic dispenser were in compression.²⁵ Furthermore, it was determined that the Brillouin Coefficient of the fiber buffer coating was not linear for cold temperatures.

4.1

Payout Dynamics Modeling and Validation

The FOG-V team adopted Padfield’s²⁶ model for the dynamics of a flexible string and developed computer models to predict the behavior of the optical fiber during high speed payout. Knowledge of the bend geometry of the peel point was necessary for designing the optimum payout fiber. The peel point is a sharp bend where the fiber being payed out leaves the wound pack. Additional stress is exerted on the fiber at the peel point during high-speed payout. This stress is maintained for less than a millisecond. The curvature and bend stress at the peel point is directly related to the fiber stiffness, adhesive bonding, and the friction caused by the fiber moving over the wound pack beneath.

The fiber shape at the peel point during high speed, steady-state payout was obtained analytically by applying Newton’s second law to a fiber, which moves on the surface of a cylindrical bobbin, without friction, in the presence of an adhesive force. The equations of motion were solved analytically to obtain a closed form solution, which describes the shape of the fiber from the peel point to the point where it leaves the bobbin. The resulting equation for the case of a concentrated adhesive force is^27,28

where

ϕ is the angle formed by a unit vector tangent to the fiber and the bobbin axis, α is the value of ϕ at the takeoff point, σ is the distance along fiber from peel point, T_to is the fiber tension, m_o is the lineal mass density of the fiber, V is the payout velocity of the fiber, and E_cI_c is the flexual rigidity of the fiber. The maximum bending stress can be expressed as

where E is the Young’s modulus of the composite fiber, d is the fiber diameter, and δ_o is the minimum radius of curvature.

AMCOM conducted a series of aircraft payout tests at the Naval Surface Warfare Center in China Lake to obtain data to validate the fiber payout dynamics model. A high-speed image capture system, described in the next section, was developed and implemented during the tests. Two-dimensional peel point images were obtained to provide a tool for understanding the dynamics of optical fiber payout. The photographic data were compared to the model for peel point shape for both hardway peel – the fiber is payed out from the forward end of the bobbin and easyway peel – the fiber is payed out from the aft end of the bobbin to the forward end of the bobbin as illustrated in Fig. 13.

Figure 13:

Peel Point

4.2

High Speed Payout Test Platforms/Instrumentation

Several high-speed payout machines were designed and constructed for the purpose of investigating payout dynamics. Instrumented laboratory and aircraft (Caribou, TA-7, C-130, F-16) were utilized to evaluate various fiber optic dispenser designs. Pinch-Wheels and P-Shoe Payout Machines were adopted for most of the laboratory payouts. The aerodynamic drag on the fiber in the laboratory represents the worse case scenario, since the fiber is pulled through the air for greater than 50 ft to damp out the helix before entering the payout machine eyelet. The laboratory instruments include a Force and Torque Sensing Spool Fixture (FTSF), a Fiber Turns Counter Device, a pulsed or continuous wave optical source coupled into one end of the fiber and a photodetector at the far end, a Bit Error Rate Tester, Strobe Photography, and High Speed Video.

The Force and Torque Sensing Spool Fixture, shown in Fig. 14, was used to measure the bobbin strains. The axial force F₁ in Eq. 1 varies (in a smooth manner) as the peel point moves back and forth along the wound pack as shown in Fig. 15. The minimum point on the trace represent the axial force when the peel point is at the aft end of the tapered fiber pack where there is no contact with the pack. The maximum point on the trace represent the axial force when the peel point is at the forward end of the fiber pack where the fiber is being dragged over the entire length of the fiber pack. Adjusting the adhesive and bobbin taper can alter the measurement.

Figure 14:

Force and Torque Sensing Fixture

Figure 15:

Axial and Torsional Forces

The Fiber Turns Counter Device, shown in Fig. 16, was used to provide a direct measure of the fiber payout velocity. A photointerrupter scheme is adopted where an optical device senses the fiber wraps as they are removed from the bobbin.²⁹ An infrared LED is mounted on the inner surface of the fiber optic dispenser and a photodetector is mounted on an extended arm alongside the fiber optic dispenser. The velocity is accomplished via counting the number of times the optical path is broken by the payout fiber and multiplied by the length of each fiber loop.

Figure 16:

Turns Counter

A high-speed image capture system was developed and implemented on a jet aircraft platform to obtain two-dimensional peel point images and instabilities.

6.1

Fiber Dispenser Aging Mechanisms

There are a number of aging mechanisms, which may potentially limit the storage and operational lifetime of the fiber optic dispenser.³⁵ An accelerated aging test plan was developed by the FOG-V team.³⁶ The aging mechanisms associated with fiber optic payout dispensers can be grouped into the following classifications:

The first category is subject to investigation through standard accelerated aging techniques based on the Arrhenus model. The chemical changes that may occur within a fiber optic dispenser include oxidation, cross-linking, hydrolysis, polymerization and depolymerization, outgassing, and chemical interaction between adhesive and buffer materials.

The second category includes normal mechanical degradation associated with shock and vibration. The visco-elastic effects within the fiber pack are included. The physical parameters of the adhesive and buffer materials, which influence the optical and mechanical performance of the fiber optic dispenser and are believed to be subject to aging-induced changes include: modulus, adhesion (buffer/glass and buffer/adhesive), elongation, friction, mechanical strength, and influences on fiber strength.

The third category relates to non-Arrenius processes, which may be accelerated, but the acceleration factors are subject to wide uncertainties. The fiber crossovers are prime sites for static fatigue in high humidity environments. It is well known that the intrinsic strength of optical fibers degrades when stored under a sufficient amount of stress in humid environments.

A number of “static” disturbances of the wound pack have been observed during simulated bobbin aging. Fiber pack “slumping” (See Fig. 18) is a well-known pack defect. Other static mechanisms observed include “earthquakes”, birch-bark, and fissures as shown in Fig. 19. Most of these static mechanisms involve significant straining of the adhesive in the vicinity of the disturbance. An adhesive rupture is obvious in the case of an “earthquake”. When the integrity of the adhesive is locally destroyed it creates “fissures” in which gaps have opened in the pack but subsequently closed. In the case of “birch-bark” it is likely that the adhesive integrity has been locally degraded.

Figure 18:

Pack Slumps

Figure 19:

Pack Fissures

6.2

Fiber Failure Mechanisms during Dispense

The mechanisms, which are responsible for fiber failure during high-speed payout, are briefly discussed here.¹⁴ 1) The force required to rupture the adhesive results in an axial force coupled into the fiber wraps adjacent to the fiber being payed out. This force, which is in the direction of the aft end of the wound pack, tends to axially compress the fiber wraps in the layers beneath. 2) The fiber being payed out contacts the downstream fiber in the layer beneath as it is lifted out of its nesting position in the pack for easy-way peel and as it is lifted over the adjacent fiber wrap for hard-way peel at the peel point. 3) The fiber tension at the peel point propagates for a short distance ahead of the peel point, decaying roughly exponentially due to friction and distortion of the adhesive. This region of increased tension coupled with the circular geometry of the fiber wrap results in a tendency for the fiber being payed out to knife down into the lower layers, generating both axial and radial force components in the fiber pack.

Fiber failure mechanism # 1 above leads to multiple fiber-wrap dispenses, which occasionally occurs simultaneously from more than one layer. Snarls and tangles can occur. This mode is triggered by the axial force disturbance trying to force the fiber wraps in the lower layer toward the aft end of the wound pack. One or more of the fiber wraps being forced up out of the fiber pack can relieve the axially compressive force. These disturbances, which usually extend only a short distance around the surface of the wound pack, are called “pop-ups”. These popped-up zones are subject to buffeting as the rotating helix of the fiber being payed out impacts them with every rotation. The disturbance can become knocked out of position and entangled with the rotating helix, resulting in multiple wrap payout, sometimes from more than one layer. The signature of this type failure mode is a region of local snarls and tangles, which is displaced down stream from the peel point.

Fiber failure mechanisms # 3 above leads to a catastrophic disturbance in the vicinity of the peel point. There is a tendency for the fiber wrap being payed out to be pulled down into the pack beneath. The resulting forces from the fiber trying to climb back up can explosively tear the pack apart in a local region. The signature of this failure mode is a snarl or tangle in the immediate vicinity of the peel point.

The mechanisms, which trigger the dynamic pack instability, are present in every highspeed payout. The movement of the fiber wraps, which triggers instability failures, tends to be abrupt and relatively large. Residual tension in the layers beneath tends to resist movement in the pack. The resulting stability is called intrinsic pack stability. An adhesive binder also augments the stability of the pack against unwanted movement.