SAMPE Journal - May/June 2020 - 12

FEATURE / BONDED JOINTS
CTE LONGITUDINAL BONDED JOINT AND
SUB-ELEMENT TEST COUPON DESIGN
The CTE project leveraged joint trade studies for
longitudinal joints from past NASA projects, the
Advanced Composite Technology (ACT) Project
which ran from 2008-2010, the Lightweight Spacecraft Structures & Materials (LSSM) Project which
ran from 2010-2011, and the Composites for Exploration Upper State (CEUS) project which ran from
2014-2016. A double-lap composite bonded joint
was selected as the most advantageous longitudinal joint to advance for the CTE project. The joint
selection was based on an assessment of figures of
merit for mass, damage tolerance, inspectability,
cost, design/analysis uncertainty, and producability. The CTE Point Design as shown in Figure 2,
is a conical sandwich structure with a 35-degree
angle consisting of eight composite sandwich
segments joined together with a double lap composite joint. The down-selected composite double
lap bonded joint joins the segmented composite
sandwich panel sections together by bonding a
prepreg fabric doubler using FM209-1M adhesive.
The sections were affixed adjacent to one another
with a 2.54 mm gap between them and filled with
EA9396.6MD. The acreage panels chosen were a
sandwich construction with a 25.4 mm Plascore
49.66 kg/m3 aluminum core (5056) with 4.76 mm
hexagonal cells and IM7/8552-1 quasi-isotropic
graphite/epoxy face sheets. The doubler material
used was 5320-1/PW T650 3K epoxy and carbon fiber woven prepreg from Solvay Industries.
Longitudinal jointed flat panels were manufactured based on the design described above and
then manufactured into longitudinal joint sub-element test articles for several configurations to test
various critical loading conditions of the CTE Point
Design. The sub-element joint configurations included Axial Edge-Wise Compression (AEWC),
Hoop Edge-Wise Compression (HEWC), and Hoop
Edge-Wise Tension (HEWT) coupons. Their design
details are presented below.

Figure 2. CTE Point design joint.

stress concentrations of the core. The design of the
AEWC coupons was sized by linear static and buckling analysis with Nx Nastran7. The goal was to design the AEWC sub-element coupons to fail at the
joint location; however, multiple analysis iterations
were unable to find a coupon design that would fail
at the joint.
Hoop Edge-Wise Compression (HEWC) Coupons
The HEWC sub-element coupons were the same
size (198.1 mm long by 157.5 mm wide) as the
AEWC sub-element coupons as shown in Figure 3
(b). In preparation for testing, the load introduction ends of the bonded segments were designed to
be potted in order to better stabilize the face sheets
and to prevent local crushing. The load introduction ends consisted of aluminum frames to contain the potting material and to provide additional
lateral support during handling and testing. The
design of the HEWC coupons was sized by linear
static and buckling analysis with Nx Nastran. The
HEWC coupons were designed to fail at the joint
location according to preliminary analysis.
Hoop Edge-Wise Tension (HEWT) Coupons
The HEWT test specimens were designed to have
dimensions of 558.8 mm by 76.2 mm as shown in
Figure 4. The core at the load introduction ends of
the specimens was removed to a depth of 86.4 mm.

Axial Edge-Wise Compression (AEWC) Coupons
The AEWC coupons were sized to be 198.1 mm
long by 157.5 mm wide as shown in Figure 3a. In
preparation for testing, the load introduction ends
of the bonded segments were designed to be potted in order to better stabilize the face sheets and to
prevent local crushing. The load introduction ends
consisted of aluminum frames to contain the potting material and to provide additional lateral support during handling and testing. Edge relief was
included by removing some of the potting material
around the edges of the test article to alleviate high

SAMPE JOURNAL

M AY/J U N E 2 0 2 0

Figure 3. AEWC and HEWC coupon size and geometry (with potted ends).

w w w. s a m p e . o r g

http://www.sampe.org

SAMPE Journal - May/June 2020

Table of Contents for the Digital Edition of SAMPE Journal - May/June 2020