Why composites need special fasteners: and when they don't
Laminates are weak through the thickness, so a blind fastener bearing on composite needs a larger blind-side footprint than a standard blind rivet forms. That footprint is what Composi-Lok and Cherry 1900 are built for.
Short answer
You need one when the blind side is composite: the handbook calls for a large blind-side footprint there (C-32). When the blind side is metal, that requirement doesn't apply, but composite in the joint still matters: the material pair sets the fastener alloy (galvanic), a composite head side sets the head style and countersink depth, and clamped composite limits installation torque.
The mechanism: it's about the blind-side footprint
Laminates are weak through the thickness. The composites handbook notes that this "led to specialized fasteners for composites and eliminated the use of rivets": the special fasteners feature larger tail footprints, which improve pull-through and bearing strength (MIL-HDBK-17-3F §6.3.1) [1]. For blind attachments to composite substructure it calls for a large blind-side footprint in titanium or A286 (C-32) [1].
In Composi-Lok II the sleeve "is drawn over the tapered nose portion of the nut. The sleeve is thereby expanded subsequently forming a head against the mating surface" (MBF2003 §7.4) [2]. At 3/16" the installed-dimensions table gives J dia. .300" minimum (Table 11) [2]; Monogram's comparison chart lists this as the footprint, ".300 minimum required", .315" typical, with preload "560 minimum required (40% of min UTS)", 1,000 lb typical [3]. Cherry rates the Cherry 1900's "large blind side footprint" as suitable for composite structures (CA-1035) [4].
But blind-side material is not the only driver
Galvanic compatibility depends on the material pair, not the side (MIL-STD-889C Table I) [5]. Pull-through is a head-side failure too: the handbook prefers 100° tension heads over shear heads in composite, cautions on 130° heads, and limits countersink depth to 70% of laminate thickness (§6.3.3.3, C-22, C-33) [1]. Preload relaxes in service, so design data assume finger-tight or light-torque installation (§6.3.3.3) [1]. The selector treats the blind-side footprint and the material pair / environment as separate inputs for this reason.
The rule the tool uses
A composite blind side drives the footprint requirement and rules out standard-footprint blind rivets. Solid rivets are ruled out whenever composite is in the stack. The material pair and environment independently drive the fastener alloy, the head style, and whether you wet-install with sealant. Each rule that fires shows up as a cited flag on your result.
Sources (5)
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