Download - Patch Size Optimization
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Adhesively
bonded composite patch have been shown to provide high
levels of bond durability under the operating conditions. It also offers
an efficient method for enhancing the structural integrity
which includes stiffening of under-designed regions, increasing
static strength, restoring strength or stiffness and reducing stressintensity factor (SIF).
Further, composite laminates have high
directional stiffness, high failure strain, durability under cyclic
loading, low density and excellent formability. Therefore, it is preferred
over isotropic patches which are predominantly made of
metal.
. the application
of bonded composite repairs to cracked primary structures isgenerally acceptable only on the basis of continuous monitoring
of patch performance to ensure that the load transfer is maintained
and also able to monitor damage growth in the parent/patch material. In case of
composite patch damages like delamination, matrix
cracking, and fiber pull out. should be detected and prevent further
catastrophic failures. Therefore, a suitable non-destructive testing
system should be devised for continuous online monitoring. From the
mechanical standpoint, two major challenges exist;
firstly, proper adhesion/bonding of the patch on the defective panel surface and
secondly selection of appropriate adhesive. For better
adhesion a good surface preparation is required and various techniquessuch as etching, shot peening or sand blasting. the choice of adhesive and itsthickness plays a major role on the integrity aspect and supposedly
it is the weakest link. Adhesives are generally classified as brittle, intermediate and ductile [15]. Depending upon the
combination
of parent and patch material one can choose the kind of adhesive. Lastly, the adhesive thickness influences the strength of thejoint as
the load is transferred from the base metal to the patch via the
adhesive. Thin adhesive layers are better than the thick layers becauseit gets the patch to be softer. But thickness provides good
durability to the entire patch as thick adhesive layer attracts lesser
strains. Good adhesive bonds are produced only in the range ofthickness 0.1250.25 mm
Patch repair is the most effective passive repair tecnique
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Patch repair may be done on one side of a damaged panel, or where possible,
symmetrically on both sides. The latter evidently produces a greater reduction in
stress intensity factor (SIF). This is
Sizes: economy of material
Service life extension: economic gain
Advancements will help move confidence of patches fromrepair of secondary to
primary repeair structures
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Perhaps A very large patch could be used instead of specialised shapes, but
economy of material is abandoned