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Adhesive Bonded Scarf and Stepped Lap Joints

ABSTRACT
Continuum mechanics solutions are derived for the static load-carrying capacity
of scarf and stepped-lap adhesive—bonded joints. The analyses account for
adhesive plasticity and adherend stiffness imbalance and thermal mismatch. The
scarf joint solutions include a simple algebraic formula which serves as a close
lower bound, within a small fraction of a per cent of the true answer for most
practical geometries and materials. The scarf joint solutions are believed to
be the first such results ever obtained for dissimilar adherends. Digital
computer programs have been developed and, for the stepped-lap joints, the
critical adherend and adhesive stresses are computed for each step. The scarf
joint solutions exhibit grossly different behavior from that for double-lap
joints for long overlaps inasmuch as that the potential bond shear strength
continues to increase with indefinitely long overlaps on the scarf joints. The
stepped-lap joint solutions exhibit some characteristics of both the scarf and
double—lap joints. The stepped—lap computer program handles arbitrary (differ-
ent) step lengths and thicknesses and the solutions obtained have clarified
potentially weak design details and the remedies. Indeed, the program has been
used effectively to optimize the joint proportions.
KEYWORD DESCRIPTORS
Bonded Joints Scarf Joints
Adhesive Stresses and Strains Stepped-Lap Joints
Adherend Stiffness Imbalance Static Strength
Adherend Thermal Mismatch Elastic-Plastic Formulation
Computer Analysis Programs Advanced Composite Joints
TECHNICAL LIBRARY
FOREWORD
This report was prepared by the Doug1as Aircraft Company, McDonne11 Doug1as
Corporation, Long Beach, Ca1ifornia under the terms of Contract NAS1-11234.
One summary report (NASA CR 2218) and four technica1 reports (NASA CR 112235,
-6, —7, and —8) cover the work, which was perfonned between November 1971 and
January 1973. The program was sponsored by the Nationa1 Aeronautics and Space
Administration's Lang1ey Research Center, Hampton, Virginia. Dr. M. F. Card
and Mr. H. G. Bush were the Contracting Agency's Technica1 Monitors.