ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
209   Conclusions 

Validation tests results have demonstrated the full adequacy of the high temperature high current contact technology implemented in the BepiColombo slipring assemblies. Moreover, the electrical dynamic resistance which has an important impact on the local thermal dissipation and consequently on the temperature distribution, appeared to be much lower than anticipated, validating the thermal simulation results with a comfortable margin.  Additionally, the demonstrated low friction and low wear properties of the composite brush technology are well above the required lifetime for the mission. Moreover, the selected material for the composite brush is well-adapted to space applications, as it behaves better in vacuum than in air.  Both validation campaigns (at component and at breadboard model levels), have helped in identifying important improvement points and lessons-learned. The production processes for the QM and FM manufacturing have been consequently enhanced. More specifically, the gluing and run-in processes, which are specific processes to the current application, have been updated and thoroughly detailed following these validation test campaigns.   Finally, it has been demonstrated that the selected composite brush assembly, including blade gluing, sintered flexible braids and heat shrink sleeves fulfills all mechanical, electrical and lifetime requirements of the BepiColombo mission.   References 

1. Heinrich B, Zemann J, Rottmeier F. "Development of the BepiColombo MPO Solar Array Drive  Assembly." Proceedings of the 14 th European Space Mechanisms & Tribology Symposium,  September 2011.
211 Lessons Learned to Avoid Coax Cable Failure   in Moving Mechanical Mechanisms 

Sheah Pirnack* 

    Abstract 

Several programs have experienced anomalies due to cold welding of coaxial cables within Moving Mechanical Assemblies. Cold welding occurs when similar clean adjacent metal surfaces (like silver on silver or gold on gold) molecularly bond to one another given sufficient cleanliness, time, and contact pressure; as the name implies, it can and will occur in the absence of heat. This paper addresses the effects of cold welding in coaxial cables, outlines specific test results, suggests how to test for cold welding, and how to avoid the problem of cold welding in the design and material selection phase of a program.  

Introduction   Within moving mechanical assemblies (MMAs), where any item is expected to move relative to another, cold welding can result in failure or severe performance degradation. Cold welds create a constraint where either the cold weld is strong enough such that the two surfaces are unable to move relative to each other or where movement / strain on the surfaces will cause early fatigue failure (such as local yielding, buckling or other low cycle fatigue damage) when the cold welded joint is flexed.   In a coaxial (or coax) cable, the condition of concern begins when cold welds develop a local constraint  within the multiple layers of the cable; then through subsequent motion the induced strain initiates low cycle fatigue failure in the proximity of the cold welds including tensile overload and buckling. Additional motion has the potential to result in severe damage to the conductors intended to carry the signal/data. Not every test specimen showed cold welding between the silver-coated copper layers  common to coax  cable design and cold welding does not always lead to a damage level that affects Radio Frequency (RF) performance. Within this paper the construction of coax cable will be introduced, photos of where cold welding has been found will be shared, and contributing factors for cold welding will be discussed. Additional comments will be given on specific application susceptibility. Figure 1 has photos of two different cables whose failure was initiated by cold welding.  

Figure 1.  Failed Coax Cable: failure initiated by cold welding                                                     * Lockheed Martin Space Systems, Denver, CO 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
212 Background  Within space flight Moving Mechanical Assemblies (MMA’s) the cabling design, construction, and  management should be an integral part of the design process: the cable should be chosen for its RF/data/power performance, space-compatible materials, flexing capability, and the cable management system should consider the cables’ capability for bending through life without fatigue or instability compromising the functional integrity of the cable. When a coaxial cable is part of a MMA, specific features of the cable design as well as the mechanical features that manage the flexing and containment of the cable must be carefully considered. Insufficient attention to these mechanical design details can cause cold welding and/or subsequent fatigue failures in space hardware.  Coaxial cables are used as transmission lines for radio frequency signals; their applications include feed  lines connecting transmitters and receivers with their antennas (data transmission / receipt). In coax cables the signal travels between the center conductor and the surface of the outer conductor separated by the dielectric. A typical flexible RF coax cable construction is shown and annotated in Figure 2.  

Figure 2.  Construction of a typical Flexible Coax Cable (Design 1)  Within a MMA application a coax cable must maintain its insertion loss / frequency performance throughout flexing and life. The construction details are critical to a flexible coax cable’s dynamic function. To maintain a high level of performance, the separation between the conductors has to be relatively constant through flexing/bending, the dynamic system (driving the motion) must be able to bend the cable, and the conductor’s have to maintain electrical continuity when flexed (no gaps). For high performance flexible cable the jacket and dielectric are plastic (low modulus/bend easily) and the metal conductive layers are multi-layer components themselves: the woven braided shield, the spirally wrapped outer-conductor foil wrap, and the stranded center conductor. The assembly is held together with the protective jacket sleeve to both protect the cable and ensure no radial gaps are generated at the outer conductor (maintaining cable insertion loss stability).   The majority of aerospace coax cable designs’ incorporates silver-coated copper center conductor  strands, outer conductor foil, and braid (Ag coated Cu). Recall that cold welding occurs when similar clean adjacent metal surfaces (like silver on silver or gold on gold) molecularly bond to one another given sufficient cleanliness, time, and pressure. Within the typical construction for flexible coax cable there are multiple opportunities for the various layers to cold weld to themselves or each other because of their similar material construction.
213 Results of Testing  Cold Welds Observed in Cable  In a series of cyclic cable tests performed in vacuum, post-test destructive physical analysis (DPA)  revealed evidence of cold welding in multiple specimens and multiple coax cable designs [Acknowledgments 1c]. Both silver to silver and copper to copper cold welds were observed. Cold welding was found between metal to metal surface layers within various coax cables - see Figure 3 and Figure 5 for evidence of cold welding within the cable constituents.   Cold welding in the center conductor can be identified by pulling individual strands apart; the evidence of  cold welding is one or more strands are attached to the strand that is being separated from the group. As can be seen in the photo in the upper right hand side of Figure 3 what appears to be two strands bonded together is actually two center conductor strands cold welded along the whole length of the center conductor. Cold welding in a stranded center conductor reduces fatigue life properties of this critical component effectively acting as a deeper beam section that must conform to the same cable bending radius, producing a higher stress state than a single strand would [Acknowledgments 1b].   In multiple cable cyclic test specimens fretting, plowing, and material transfer due to the outer conductor  foil layers cold welding and sliding relative to each other was observed [Acknowledgments 1c]. Not all cold welding affected the cable performance. When the foil was unwound from itself during DPA, intact cold welds were discovered in several specimens along the spirally wound length of the cable. The extent of the cold welding was dependent on the location along the cable, the type of cable, and the relative bending profile of that section of cable. An example of cold welding is pointed out in the lower right hand picture in Figure 3. Recall that an RF signals travel along the surface of the outer conductor. This type of wear and material transfer will not affect the cable’s function if the outer conductor layer remains intact and continuous.  

Figure 3.  Cold Welds and effects of Cold Welds observed on all metal layers of coax cable
214 The photo in the lower left hand frame of Figure 3 reveals braid distortion along the length of the cable.  The geometry and path of the seven (7) strand groups of woven braid are not uniform in their form; some strands are pulled away, or bent in an irregular shape, or, in an extreme case, an entire group of strands under local stress tensile yielded. The braid distortion is indicative of the braid strands sticking to each other or to the outer conductor foil layer underneath. With movement, the cold welding distorts or pulls the strands away from the woven group of strands that they originally followed. An X-ray of untested cable from the same lot cable is shown in Figure 4 (without distortion). An X-ray can be completed after test without DPA to look for evidence of cold welding in a cable by discerning if there is braid or conductor distortion.  

Figure 4.  CT Scan / X-Ray Image of undistorted cable (braid visible)  Figure 5 is a photo of the same section of braid distorted cable within Figure 3 with sections of braid peeled away to reveal the cold welds and outer conductor damage/stretching and tearing. Evidence of braid to outer conductor foil cold welding is clear in Figure 5. 

Figure 5.  Image of Cold Welds observed between the braid and outer conductor (foil wrap) during  DPA (some areas purposely cut away to make observation) [1c] 

Recall that RF signals travel between the center conductor and the inner surface of the outer conductor  layer with a precise separation set by the dielectric within coax cable. In the construction outlined above, the outer conductor foil is the main functional layer along with the center conductor. If either of these functional layers is damaged, a corresponding affect in insertion loss within the cable is realized: the
215 extent of insertion loss increase is dependent on the operational frequency range and the level of  damage.   Susceptibility of Cold Welds in Cable: Choosing a flexible cable design  Not every cable test sample showed cold welding between the silver coated copper layers, and not all  cold welding led to damage that affected the RF performance of the cable. So what is the potential for damage due to the layers cold welding? And what kind of cold welding is most damaging? How can it be adequately screened for? And if a cable management system was successfully qualified and life tested already, is that enough, regardless of seemingly small changes between cable production lots?  Multiple coaxial cables of different designs were tested in various conditions to determine the impact to  the performance of the cable. Development testing, fatigue testing, life testing, X-ray inspection, material analysis and various other techniques were used to characterize the cable; RF performance parameters were used to monitor test units during testing. Different manufacturers and types of cables were tested with a construction outlined in Figure 2- will be referred to as Design 1. Some cables with an additional layer between the outer conductor spiral wrap and the braided shield were also tested and designs with this feature will be referred to as Design 2. A schematic representation of a Design 2 cross section is shown in Figure 6.   

Figure 6.  Design 2 of cable with addition of an intermediate layer  Three specific tests are summarized in Table 1: each test was conducted in an equivalent strain cycle and strain rate profile that encompasses appropriate life test margins in a vacuum chamber.    Design 1 of cable went through multiple tests but one lot failed life testing (Lot C) and one lot successfully  completed life testing without degradation (Lot A). DPA of the cables revealed very subtle differences in material hardness, contamination levels, jacket pressure, and overall manufactured process / tightness but significant differences in cold welding and damage.   A key lesson learned from the results of the testing is a recommendation to complete vacuum life  testing on every material lot of cable to ensure that manufactured lot differences won’t have an impact on cable life performance. Subtle changes in contamination, material lot and manufacturing processes can impact the results of life testing even with the same design of cable.
217 Monitoring RF Parameters for indicators to screen for a problem during Coax Cable Vacuum Testing  During vacuum testing of the Design 1 lot C cable RF data (TDR, IL, & VSWR monitored continuously)  indicated that something was happening almost immediately after a test pause due to a chamber issue. Up until that point there was no indication of a cable performance change. Similar pauses occurred with the other cable lots/designs but the leading indicator (RF performance) was unaffected.   Soon after the restart of the test, the degradation of the Design 1 lot C cable was rapid. The initial  indicator was a time-domain reflectometer (TDR) test where a discontinuity / spike identified damage in the bending portion of a cable. Through additional cycling an insertion loss “suck-out” or increase started to form in the highest monitored frequency range and moved towards the lower frequency ranges with additional cycles (RF performance monitored from 100 MHz – 20 GHz). The test was stopped to determine the cause of RF degradation. The RF performance was cable motion and position dependent indicating opening and closing of functional layers (impedance changes affecting loss). An in-situ x-ray inspection was completed with the test unit inside the vacuum chamber to determine the condition of the cable without incurring the risk of moving it. One of the x-rays images taken (courtesy of Aerospace Corporation) [Acknowledgments / Reference 2, 3] is shown in Figure 7. This x-ray happens to be same cable as shown in Figure 1 (left photo). 

Figure 7.  In-Situ X-Ray inspection photo taken of  Test Unit in Chamber [2] reprinted with  permission of The Aerospace Corporation  At the onset of the degradation the RF indicators implied a gap forming on the outer conductor functional layer (impedance change showing up as a TDR spike/ reflectance). Additional cycling caused the gap to propagate to a full circumferential crack around the cable and after a full breach was realized, additional cycling increased the separation along the length of the cable (gap grew larger). As the gap began to expand around the circumference of the cable, the insertion loss became evident in the higher frequency range and moved to the lower frequency ranges as the gap expanded and grew larger.   Based on bench-top testing completed at Lockheed Martin and repeated by Aerospace Corporation,  (Acknowledgments 1a /References 2), the initiated crack would have had to have been at least 90 degrees circumferentially around the cable to show up in the highest frequency range (20 GHz). A crack development of 270 degrees about the circumference would impact the 10 GHz range. A full breach will
218 impact multiple frequency ranges with variable RF performance impacts dependent on the cable  geometry, conductor condition, bending requirements, size of the gap, etc.. The bench-top testing completed at Lockheed Martin [Acknowledgments 1a] and Aerospace Corporation [2] mapped the rapid physical degradation to the RF indicators.   Prior to the significant RF degradation resulting from the damage as shown in Figure 7, there was no  warning of an issue in the cable. Unfortunately this means if you can see a change in the RF, an issue is likely already present indicating some level of damage to the cable. There is no leading indicator of actual cold welding in the RF data until the damage is done. The effect of the cold welding condition occurs after a cable is moved and the damage is within the active cable layers. RF position dependent performance was due to the breach in the outer conductor foil layer opening (higher impedance) when bending the cable and closing (recovering) by reversing the bending / motion.   On a similar Design 1 Lot C cable, a partial breach of the outer conductor manifested itself in a similar  manner, but was in a much lower strain area. Prior to DPA, a CT scan of the break area (this cable did not have a full circumferential break) was completed. The CT scan revealed that the center conductor had failed. The center conductor strands appear to have failed due to accelerated fatigue caused by increased bending from a partial breach. A likely contributor to the accelerated fatigue in the cable is an adjacent stiffness increase due to cold welded cable layers. The CT scan image of the break area is shown in Figure 8 as a section view so that the center conductor is visible; this image is of the same cable as shown in Figure 1 (right photo). 

Figure 8.  CT Scan inspection photo taken of a partially breached cable with center conductor  condition shown (p art of outer conductor / shield taken out of visible frame)  As can be inferred from the photos of damaged cables and the trailing RF indicator, it is imperative to test cables well to screen for this condition.   Experience has shown that cold welding has led to a progressive damage process and, while not proven  experimentally, cumulative fatigue damage is expected to be a non-linear function of cyclic flexural amplitude times the number of cycles typical of metallic fatigue. Consequently, the design of the coax cable and the cable management system must be within proven fatigue capabilities for an expected mission angular movement profile [Acknowledgments 1b].
220 Given sufficient time and pressure, two metals of similar composition in clean contact will form cold welds.  Applications of concern  Depending on the amount of motion required of the cable, cold welds may or may not be a problem.  Depending on the geometry of the system, fatigue may or may not be an issue, but additional consideration to local low cycle fatigue of layers due to cold welding should be given. If cold welds form and the cables are not required to move during operation, then cold welding is not a concern. The following are application considerations regarding cold welding to keep in mind throughout the design, verification, or qualification life cycle of a program or MMA design:  Cables in or across single deployment mechanisms are generally NOT at risk:   Cable primary failure mode is fatigue (yielding and buckling). Cold welds reduce the number of cycles  / amount of motion required for local fatigue failure to occur. Single deployment mechanisms that  have to operate once will not generally have excessive cumulative damage from fatigue (even low cycle local fatigue).   Deployments generally occur at beginning of life (BOL). With relatively little time / motion relative to  last “successful on-ground” test, damage from cold welds likely will not have the chance propagate to a failure within a single motion / deployment.  However:    If the system has low force / torque margins there is some risk of higher force required to move / bend  cable relative to what was tested initially if cold welds formed post launch / pre-deployment.   If cold welds form prior to deployment and a very large motion/bending is required for the initial / first  and only motion some damage might occur at deployment. The likelihood of cold welds forming and  cumulative damage in one motion is low but should be considered in the context of the application.  Cables that are expected to operate and move over life (especially high cycle long life mechanisms) ARE at risk:   Dithering motion and vibration can exacerbate cold welds by abrading the surfaces and creating  atomically clean surfaces. A space vacuum environment also contributes to maintaining clean surfaces.    Pausing, idle periods and non-operation between operation / motion cycles can exacerbate cold  welds by allowing the two adjacent surfaces time to molecularly bond to one another and form stronger cold weld bonds.   Bending the cable through any angular motion has the potential to cause damage to the layers  within the cable particularly when the layers have cold welded or are sensitive to fatigue failure; but not all damage is life threatening - the cold welds’ strength, the forced strain on the layers and the fatigue properties of the individual cold welded cable materials will determine the extent of the damage.   Through multiple cycles of cold welded cables the extent of the damage becomes progressively  greater and can lead to a full break / breach in a cable.