SAE - ARP6881
Guidelines for the Use and Installation of Bonded Cable Harness Supports
|Publication Date:||1 October 2018|
This ARP provides recommended use and installation procedures for bonded cable harness supports.
The purpose of this ARP is to provide guidance for acceptable use of bonded cable harness supports.
The manufacturing and overhaul of an aircraft requires the use of studs and standoffs to secure a cable via harness supports. Rivets and through-bolts are the recommended technique and have traditionally been the method for securing this type of hardware. When this is not an option, such as on composite structure or where a penetrating hole compromises the structural integrity of the airframe structure, an alternative method is to secure the wire support hardware via structural adhesives or sealants. There can be significant advantages to the use of adhesive bonded fasteners on metal airframe structures, including: eliminating the stress concentration associated with the required hole(s), eliminating the rivet which is a potential source of corrosion (dissimilar metals), or reducing the potential for scrapping expensive airframe structure when holes are mis-drilled. There are also potential labor savings associated with the speed at which mechanics can install adhesive bonded fasteners compared to riveting. For pressure vessels, eliminating holes has obvious benefits, and in the case of fuel tanks (for example, wet wing applications), eliminating the rivets eliminates the possibility of fuel leaks associated with leaking rivets. Adhesive bonded fasteners also allow for aerodynamic flushness on the opposite side of the fastener, and, on thin airframe structures, may prevent knife edge conditions where countersunk rivets are required for aerodynamic flushness.
Although there are significant benefits to the use of adhesive bonded studs, standoffs, and cable tie mounts for wring applications, there is also a legitimate concern about the strength of the adhesive bonded fastener. The adhesive bonding is process dependent, and requires good adhesive selection, mixing, and application, as well as good surface preparation of the bonding surfaces. For that reason, training mechanics in proper adhesive bonding techniques is a key to successful use of adhesive bonded fasteners in aerospace applications. In most cases, with proper adhesive selection and surface preparation, the strength of adhesive bonded cable tie mounts can exceed the strength of the nylon zip ties that are used to attach the wiring to the adhesive bonded cable tie mount. For adhesive bonded cable tie mounts, the strength is such that adhesive failures are extremely rare; typically, if nylon zip ties are replaced by stainless steel wire ties, then the failure mode is usually a mechanical failure of the adhesive bonded cable tie mount. For adhesive bonded studs or standoffs used to attach p-clamps, the strength capabilities of these adhesive bonded studs and standoffs are even greater than for adhesive bonded cable tie mounts; however, overloading may be more likely, since the length of the stud or standoff may allow for a significant moment-arm making peel failures (or cleavage failures) more likely. In any case, the most likely cause for well-installed adhesive bonded fasteners is over-loading or abuse (bumping, kicking, or stepping on the adhesive bonded fastener, or pulling on, hanging from, or stepping on the wiring attached by adhesive bonded fasteners).
Another concern is the substrate material bonding surface for the fastener. In many aerospace applications, airframe structure is painted or primed, or has some other surface treatment, such as a chemical conversion coating (Alodine© or anodize). The most effective bond will occur with freshly abraded and properly cleaned bare substrates. If the paint is removed, then after the adhesive bonded fastener has been installed and the adhesive has cured, a touch-up of any unpainted surfaces is required. For other surface treatments, it would be prudent to test and evaluate the adhesive selected on the substrate to determine the optimum surface preparation. For well-adhered paints, scuff the glossy surface, followed by a solvent cleaning to remove any abrasive residue. Caution is advised when bonding to a treated surface, since some adhesives do not adhere well to conversion coatings (Alodine©). Although many adhesives have excellent strength properties on anodized surfaces, it is not uncommon to have a sealed anodize, which will likely result in inferior adhesive attachment strength.
One final concern is environmental exposure of adhesives or sealants to aerospace fluids (aviation fuels, hydraulic fluids, de-icing fluids, corrosion protective coating, etc.) or stresses such as extreme temperatures and changes, vibration, and high G-loads. Adhesive bonding has a long history in aerospace applications and it is critical that adhesives specifically qualified for aerospace applications be used and the appropriate adhesive be selected for the intended application. Selected adhesives need to have documented temperature ranges matching expected application temperature ranges, and resistance to typical aerospace fluids. Applying a sealant over the adhesive fillet and/or painting the entire adhesive bonded part is an effective method of providing additional protection to the adhesive attaching the adhesive bonded fastener.
Successful implementation requires:
1. Selecting the proper adhesive. Selection should be based on static and dynamic load requirements, operating temperature, and fluid vulnerability.
2. Verification of the bonded support substrate, material type, and thickness.
3. Selecting the proper bonded support. Consideration should be focused on application geometry, support material requirements, and bonded surface area based on static and dynamic load requirements.
4. Proper surface preparation of the substrate to ensure long-term durability.
5. Locating the fastener support correctly to minimize mechanical stresses.
6. Tooling to ensure positive pressure throughout adhesive cure and adhesive bond line control.