Mechanical sandwich panel spacer with improved torque resistant anchorage

Claim 1

A FASTENER FOR INSTALLATION IN A SANDWICH PANEL HAVING SPACED SKIN SHEETS JOINED BY A CORE OF LOW DENSITY, COMPRISING: A TUBULAR BODY INCLUDING A RELATIVELY THICK MAIN BODY PORTION, A WEAKENED SECTION DEFINED BY A COUNTERBORE IN ONE END OF SAID TUBULAR BODY AND JOINED TO ONE END OF SAID MAIN BODY PORTION BY AN ANNULAR OFFSET SHOULDER, AND A PREFORMED HEAD ON THE END OF SAID WEAKENED SECTION, PROJECTING RADIALLY THEREFROM, AND HAVING A FRUSTO-CONICAL ABUTMENT SHOULDER ENGAGEABLE IN A DIMPLE IN THE OUTER FACE OF ONE OF SAID SKIN SHEETS AROUND AN APERTURE THEREIN IN WHICH SAID FASTENER IS RECEIVABLE, SAID WEAKENED SECTION BEING ADAPTED TO BE BULGED OUTWARDLY UNDER COMPRESSION APPLIED TO SAID FASTENER, WHEREBY TO FORM A SECONDARY HEAD ENGAGEABLE WITH THE INNER FACE OF SAID ONE SKIN SHEET AT THE EDGE OF SAID APERTURE IN ADJACENT OPPOSED RELATION TO SAID PREFORMED HEAD SO AS TO CLAMP SAID EDGE BETWEEN SAID HEAD AND SECONDARY HEAD, SAID WEAKENED SECTION INCLUDING A BASE PORTION GRADUALLY INCREASING IN THICKNESS TOWARD SAID SHOULDER ITS INNER WALL BEING CONCAVELY CURVED AND SUBSTANTIALLY TANGENT TO THE CYLINDRICAL WALL OF SAID COUNTERBORE AND TO SAID SHOULDER, ON A RADIUS SUBTANTIALLY EQUAL TO THE DIFFERENCE BETWEEN THE RADII OF SAID COUNTERBORE AND OF THE INTERIOR OF SAID THICK-WALLED SECTION, SAID WEAKENED PORTION BEING OF UNIFORM THINNESS FULL LENGTH FROM ITS POINT OF TANGENCY WITH SAID BASE PORTION TO A JUNCTION WITH SAID ABUTMENT SHOULDER, SAID BASE PORTION PROVIDING A BUTTRESSING CHARACTERISTIC EFFECTIVE TO DIRECT THE OUTWARDLY BULGING FOLD OF SAID WEAKENED SECTION TOWARD SAID SKIN SHEET INNER FACE UNDER THE EFFECT OF COMPRESSION UTILIZED FOR COLLAPSING SAID WEAKENED SECTION AND EFFECTIVELY CLAMPING THE DIMPLE OF SAID ONE SKIN SHEET BETWEEN SAID HEADS, WITH SAID SECONDARY HEAD LEANING AXIALLY BEHIND SAID DIMPLE.