Ruggedizing Coaxial and Fiber Optic Cable Assemblies from Mechanical Strain
From fiber optics to coaxial cables, common failure points in harsh environments call for various augmentations to enhance an interconnect’s operational lifetime. These modifications can range from the addition of armoring for crush/impact resistance to optimizing cable jacketing material for resistance to chemical/oil ingress. Mechanical strain is often a cause for premature failures of interconnects. Understanding the reasoning and methods behind ruggedizing commonly leveraged interconnects such as fiber optic and coaxial cables can be enlightening.
Coaxial Cable: Intrinsic and Extrinsic Sources of Signal Loss
Outside of transmission lines for microwave integrated circuit (MIC) assemblies (e.g., microstrip, coplanar waveguide, etc.), coaxial cables are arguably the go-to interconnect for high frequency transmissions. The desirable transverse electromagnetic mode (TEM) of propagation in coax offers broadband performance from DC to a specified cutoff frequency that is primarily determined from the inner cross-sectional dimensions.
Coaxial cables are a two-conductor transmission line with an inner conductor, dielectric, and shielding. The concentric inner conductor is typically constructed of either stranded or solid copper, where stranded variants typically do not function beyond 1 GHz. The dielectric material that spaces the two conductors can be a range of materials from air, to thermoplastics, to low density foams. The shielding material can be composed of a braided alloy or a foil; often both are used for added protection from electromagnetic and RF interference (EMI/RFI). Inherent attenuation of the signal is caused by the dielectric loss and resistive loss where resistive losses become more apparent at high frequencies due to the tendency of the signal to be pushed to the edges of the conductor (skin effect).
Outside of the intrinsic sources for signal loss, acquired signal loss can occur from inconsistencies in the inner geometry for both the connectors and cable. These inconsistencies are impedance mismatches that can ultimately cause a portion of the signal to reflect back at the source, degrading the signal received at the output of the coax. Inconsistencies can be generated by a large variety of stressors including excessive flexing, crushing, unwanted ingress, and poor handling.
Fiber Optic: Intrinsic and Extrinsic Sources of Signal Loss
Fiber optic assemblies are essential for high-speed, long-range data transfer and have uses in a huge range of military and industrial applications. Light-waves are sent from input to output via a glass waveguide, often fabricated from silica, as well as several layers of cladding, which is a coating with a lower refractive index that confines light within the core. In general, intrinsic attenuation is caused by the absorption, where light is converted into heat energy, as well as the scattering of light in directions outside of the waveguide.
Additional sources of signal loss occur in macro- and micro-bends in the fiber where the bend angle is too severe and causes light to pass through the interface. Similar to the coax, uniformity of the inner geometries is essential to prevent unwanted reflections of the signal. Fiber optic cables can become damaged in a number of ways including exposure to ice/wind, bending/pulling cable through ducts and under floors, and especially improper handling during the manufacturing process or any post-processing such as splicing. These failure modes, in essence, were cause for the slow propagation of cracks on the surface of the fiber core that reflect light back along the fiber.
There are generally five fundamental loading conditions: tension, compression, bending, shear, and torsion (Figure 1). Material properties such as tensile strength, elongation, and modulus of elasticity are essential parameters that determine how a cable responds to these various loads.
This basic mechanical concept can be applied to a dynamic loading scenario that would occur in real environments. A cable can be subject to a combination of all of these loads depending upon the type of stress-event, or “moment of force,” that occurs (Figure 2).
Shear: Impact and Crush Resistance
Transversal (shear) pressure can be exerted on a cable in a number of scenarios including a human stepping on a cable, a vehicle running over a cable, or even a rodent attempting to chew on a cable. A human step has an average ground pressure of around 16 psi; this number jumps up to 30 psi for a passenger car. The standard formula for pressure is as follows:
where P is pressure, F is the force perpendicular to the object, and A is the surface area of the contact. As made apparent by this equation, the pressure exerted onto the cable depends on whether the object it comes into contact with is in motion, as well as the surface area of that object. While this equation is rather basic, it illuminates the impact certain stressors can have on a cable—a rodent gnawing at a cable would produce a far more damaging shear force than a human stepping on a cable due to the smaller surface area of the contact. A more specific formula for shear stress is as follows:
where τ is torsional stress, G is the shear modulus of the material, r is the radius of the beam, L is the length under twist, and Θ is the degree of twist .
An improperly modified coaxial cable subject to shearing forces could cause either a kink in the inner conductor, dielectric, and shielding, or, a complete tear in the inner conductor and significant damage in the dielectric and shielding. Attenuation would instantly jump up and a failure would occur. More often than not, coaxial cable will handle torsional strains that come from daily use (e.g., torqueing connectors improperly when mating, accidental stepping, stored in tight bends, etc.). Outside of rodent damage, fiber optic cables are also generally not exposed to torsional and shear forces that cause irreversible failures. In most cases, coaxial and fiber optic cables will not experience enough torsional strain to cause an acute failure.
Crush, impact, and abrasion resistance can typically be achieved with a strong jacketing material that has an inherently higher abrasion and tear resistance such as polyurethane (PUR) or a UV resistant high-density Polyethylene (HDPE). In many cases, this level of protection is adequate. Additional resistance from shear forces can be obtained through “armoring” the cables with a stainless steel stripwound hose over the jacketing. Typically, a strip of steel is helically wound around a mandrel in a desired corrugation profile. This type of armoring is utilized in a wide range of components from flexible waveguides to garden hoses not just for its shear-, tear- and abrasion-resistance, but also for its resistance to cable kinking (Figure 3).
Bending, Flexing and Vibrations
Extensive bending and flexing of a coax can be necessary in automated test systems or systems that require high speed video transport such as aerospace surveillance where a cable will be subjected to flexing and vibrational strain. As stated earlier, fiber optic cables are particularly susceptible to long-term damage from excessive bending and even in cases when the bend is static since microfractures can spread and slowly deteriorate cable performance. From beam theory, the simplified equation for bending stress on a cylindrical beam of homogeneous material is :
where E is the elastic modulus, y is the distance to the neutral axis, and R is the bend radius. The distance to the neutral axis (center of the cable) is determined by the radius of the beam and the degree of compression within the cable. It is understood that coaxes are composed of a number of materials and this equation would not suffice as it does not take into account the interaction between those materials (e.g., coefficient of friction). It does, however, give a basis to work from.
For a coax under flexure, there are several points of weakness: the surface where the cable and connector head meet, the surface where the inner conductor and connector pin meet, and the point(s) where bends occur. Coaxial cables can be manufactured with crimpedon connector pins and outer ferrules where stranded wire can shift and loosen within the crimped or clamped connection (Figure 4). In space applications where frequent vibrations and flexures occur, the individual strands of metal within the inner conductor and braided shielding can become cold (vacuum) welded onto one another causing a localized point of constraint that causes a nonuniform distribution of stress over coax. This eventually causes fatiguing that unravels and breaks shielding, as well as the inner conductor.
Outside of space applications, much of the cable fatigue caused by bends can be undone by simply decreasing the diameter of the cable as it is directly proportional to bending stress (refer to Equation 3). Soldering the center pin and utilizing optimal tools for an exceptional crimp/compression would mitigate the risk of failing at the connector head. Moreover, strain relief boots, and in some cases overmolding, are added to distribute the stress between the crimp sleeve and coax. Overmolding, however, has the added benefit of a water-tight seal.
Stranded center conductors offer much better flexibility than solid copper center conductors due to the distribution of the bending stress over a number of thin wires. Stress increases as a linear function from the neutral axis to the surface. This means that jacketing and the shielding of a coax experience the most stress in particular. And, since the polymers used in jacketing have a far greater fatigue limit than the metal alloys used in braiding, shielding is typically the major concern for flexible coax. Oftentimes, ultra flex cables will incorporate non-metallic layers between the braided shield and the bonded aluminum foil underneath it, as well as between the braid and jacketing material.
This buffer generally reduces the coefficient of friction between the two interfaces and has the added benefit of being more viable for space applications (less risk of cold welds occurring). The dielectric is also a consideration for increasing the number of flex cycles coax can withstand. A material with a low elastic modulus and low coefficient of friction can be utilized for less overall bend stress on the cable.
There is a great deal of literature dedicated to understanding the behavior of cracks in optical fibers that go into detail on predictive lifetime models based on various applied stress models. It is beyond the scope of this article to inventory all the failure-predictive methods based on stress induced stimuli. Much of the material used in an optical fiber cable is meant to protect the delicate fiber and cladding (e.g., strength members, water-blocking gel, jacketing, buffered fibers, loose tubes, armor). Whereas, in a coax, the two conductor system separated by a dielectric is necessary for proper transmission and jacketing is often the only protective agent. Therefore there is no need to look at the strain caused furthest from the neutral axis, but rather, the strain that occurs at the optical fibers.
Although rare, flaws such as pores or bubbles can also appear on the silicate glass during the manufacturing process. More often than not, initial damage occurs due to improper handling where the relatively delicate fiber core is exposed. And — with the ever-increasing demand for fiber optic infrastructure to support the capacity and data rate requirements for 5G, Wi-Fi, and IoT — fiber installation best practices become more of a serious consideration to prevent early failures of massive fiber installations. Excessive flexure and bending are a source of concern for premature failures, causing further stress on an already potentially blemished fiber core. Typically, brittle fractures will occur in the “mirror” region of the fiber during splicing, termination, or connectorization. These fractures can cause dimples in the “mist” region and eventually progresses to multiple cracks in the “hackle” region (Figure 5).
A minor flaw in the mirror region generated during a splice may cause a slight drop in performance, but the propagation of the crack into the hackle region is what would cause a failure. This failure progression grows dramatically during tensile or bending stresses. Aside from following best practices, fiber optics that are predicted to endure flexure and vibrations are often fabricated in “loose tube” configurations where fibers are protected from moisture and mechanical stressors with water-blocking gels or tapes. High fiber count break-out and distributed configurations involve fiber cores further out from the neutral axis that would experience more strain from a bend. And since glass fibers can withstand far less surface strain before a break (~7.5%) than a high modulus fiber such as Kevlar (~100%), it is likely best to keep the fiber core as far away from the surface as possible (simplex configuration). In cases where this may not be viable, buffering is key. Tight buffered cables are often used to eliminate the damage due to shrinkage with a multilayer coating beginning with a tight buffered plastic jacket, followed by aramid strength elements and a color-coded thermoplastic jacket (Figure 6).
Premature failures and damage often caused by poor handling and excessive mechanical strain are common for both coaxial and fiber optic cables. It can be helpful to understand just how these stressors can damage the underlying materials used on these cables and how that, in turn, degrades electrical/optical performance. There are a myriad of methods for ruggedizing cable assemblies that buffer the critical conductors, dielectric, and optical fibers from deforming, cracking, or unraveling. Any flaws or discontinuities in either one of these media will cause unwanted mismatches, reflections and, ultimately, signal loss. The effects of these flaws are only exacerbated in environments with heightened mechanical strain.
This article was written by Peter McNeil, Marketing Manager, L-com (North Andover, MA). For more information, visit here .
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