Shur-Lok Design - Manual PDF
Shur-Lok Design - Manual PDF
Shur-Lok Design - Manual PDF
Design Manual
Fasteners for Sandwich Structure
TABLE OF CONTENTS
Section Page
I INTRODUCTION
Why Use Sandwich? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Sandwich Structure Details . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Methods for Joining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Typical Sandwich Panel Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
II DESIGN CRITERIA
Transmitting Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Consider the Design Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
A Word About Standardization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Load Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Tension Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Shear Loading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
III APPLICATION METHODS
Molded-In Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Installation Hole Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Mechanical Inserts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Edge Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
IV MATERIALS
Selection of Potting Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Tailored Compounds Available . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Material Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
V TESTING
Methods of Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Recommended Test Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
VI TEST RESULTS
Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3
LIST OF ILLUSTRATIONS
Figure Page
1 Typical Honeycomb Panel Construction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3 Solid Section Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3 Typical Load Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4 Fastener Inserts for Honeycomb Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5 Sandwich Panel Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
6 Fastener Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7 Dimension Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8 Shur-Tab Potting Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
9 Typical Core Undercut for Molded-In Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10 Simple Mechanical Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11 Shur-Torq Self-Retaining Mechanical Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
12 Step Drill Provides Two Hole Sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
13 S h e a r T e s t . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
14 Flatwise Tension Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
15 Torque-Out Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
SANDWICH
PANEL
FASTENERS
DESIGN
MANUAL
PREFACE
1
SECTION — I
SECTION 1 — INTRODUCTION
WHY USE SANDWICH? face sheets attached to an inner core material; the core is
When design requirements demand superior strength- made of hexagonal cells having walls perpendicular to the
to-weight ratios, sandwich structure is indicated. In facesheet planes (See Figure 1). Many materials have been
addition to its high strength, inherent rigidity and minimum used successfully in honeycomb sandwich including
weight, sandwich provides the desirable side benefits of aluminum, steel, high-temperature alloys, paper, wood,
thermal and acoustical insulation. fiberglass and plastics. In some applications honeycomb
Sandwich, by its very nature, is generally used as cells are filled with a foam-in-place expanding plastic.
sheeting or flat panel form, applied to open framework as a Other forms of sandwich consist of face sheets bonded to
transverse web to carry shear loading. In other applications, homogeneous cores such as foamed plastics or wood. The
it acts as a support diaphragm. It serves as both a primary variety is limited only by the state of the art and the
and secondary load member. And, it is capable of imagination of the designer.
transmitting extremely high loads when properly attached
to the framework. METHODS FOR JOINING
Other applications take advantage of its favorably low Regardless of form, the methods for joining the two
weight-to-area ratio. Typically, these include curtain walls face skins and inner core into a rigid member are numerous.
for decoration or the baffling of sound and light. Such
applications do not generally consider the inherent load
capabilities of the structure.
Initially sandwich was used only in flat panel
applications-a logical step away from plywood and other
sheet panels. Recent improvements, however, in fabrication
techniques and growing industry awareness of sandwich
potential have spurred bolder forms. Today these include
compound curves, skeletonized sections and many complex
shapes previously considered impossible.
2
SHUR LOK
C O R P O R A T I O N
By far the most widely used is that of adhesives applied by TYPICAL SANDWICH PANEL APPLICATIONS
any of several techniques, and activated chemically or AIRCRAFT INDUSTRY
thermally. Floor Panels
1. One method consists of brushing or spraying the Interior Walls
adhesive film over one surface, and subsequently Food Handling Galley Assemblies
mating with the second part precoated with Wing Control Surfaces
activator. Passenger Storage Racks
Thrust Deflector Assemblies
2. A second approach use prefabricated sheet
adhesive. Adhesive, rolled into a thin sheet partially AEROSPACE INDUSTRY
cured to retain form, is stored between temporary Capsule Panels
non-adhering film, ready for use. Ablative Shields for Nose Cones
Instrumentation Enclosures & Shelves
3. Another bonding technique consists of applying the Bulkhead Panels
adhesive with rollers to a scrim or grid cloth, which Space Satellites
is then cut to size and applied between core and
ELECTRONICS INDUSTRY
face skins.
Electronic Radome Construction
4. Still another method simply calls for an even Large Antenna or Disk Reflectors
coating of adhesive on the face skin, which is Military Electronic Instrumentation Shelters
subsequently activated and applied before setup. Shipboard Electronic Deck Shelters
In all methods, development of optimum strength TRANSPORTATION INDUSTRY
depends on proper preparation of face skins and controlled Cargo Pallets
application of adhesive to form optimum fillets between Shipping Containers
mated ends or faces of the core structure. Such bonding Refrigeration Panels
optimization achieves even transmission of loads from face Rapid Transit Floor Panels
skins to core without bond rupture. Special Automobile Bodies
Other forms of sandwich structure which offer CONSTRUCTION INDUSTRY
excellent high temperature strength performance are Architectural Curtain Walls
composed of all steel honeycomb and face skin. These are Partitions & Divider Panels
most often resistance welded or of brazed construction. Expandable Hospital Shelters
3
SECTION — II
4
SHUR LOK
C O R P O R A T I O N
Basically there are two types of fastener inserts for application under study. Typically the designer must know
honeycomb application: the molded-in type and the mech- the structural arrangements of the joint involved as well as
anical type (See Figure 4). The molded-in type should the loads expected. The sandwich construction must
always be used if possible because it offers several distinct similarly be defined. The following two lists typify design
advantages. Among these is the ability to bond the insert, parameters which must be considered.
core and face skins into one rigid unit with the selected
potting medium. Another advantage is that molded-in
Sandwich (See Figure 5).
inserts are not particularly sensitive to manufacturing
variations within the sandwich structure. Furthermore, 1. Overall thickness of panel?
bonded parts necessitate a relatively short learning curve
for inexperienced installers. 2. Face skin thickness? Rigidized or plain?
3. Type of core?
4. Material of core and face skins?
CONSIDER THE DESIGN PARAMETERS
5. Flat, curved or tapered surface?
The first logical step in the selection of a fastener is to
determine exactly what is expected of it in the specific 6. Fastener molding compound?
Figure 4 — Fastener Inserts for Honeycomb Applications Figure 5 — Sandwich Panel Considerations
5
SECTION — II
6
SHUR LOK
C O R P O R A T I O N
A WORD ABOUT STANDARDIZATION very similar to the “Wagner Beam.” The face skins
Aside from the foregoing requirements, the duty of represent the cap strips. The vertical surfaces of cell walls
making standardization recommendations has also fallen to form individual shear panels, with the junctures of the cell
the fastener manufacturer. wall acting as vertical boundary stiffeners of each small
The best effort for standardization lies in the molded-in shear panel.
blind insert, because little is gained by increasing the In tension loading of a fastener, the bonding of core to
insert’s length once an optimum ratio of threads-to-bolt size face skin plays a major role. The whole area about the
has been established. Thus, while accepted design calls for fastener acts as an unsupported diaphragm, with load
a minimum of 1.5 diameters of thread engagement, 0.75 centered at the fastener. Design should strive to minimize
diameters is sufficient to develop full bolt and sandwich the unsupported diaphragm area and to obtain as much
strength. The additional provision of 0.75 diameters is support from the core material as possible. With molded-in
adequate for bolt overtravel resulting from tolerance inserts this support objective is dependent on the type of
buildup. A nominal insert length could be used for most potting material. Its ability to fill the core voids reduces the
sandwich applications. Obviously, ultra-thin sandwich loading per unit area and dependent upon adhesive strength,
panels must be treated differently when attempting provides the ultimate in load transfer between skins, core
sandwich insert length standardization. and insert. This support aspect is exhibited by the SL600
Series’ ability to provide a completely void-free fill.
LOAD ANALYSIS With mechanical inserts, strength is derived from a
Sandwich structures consisting of light gage face through part which can be secured to both face skins as in
materials and low density cores cannot be expected to carry the Shur-Lok SL500 Series. This method tends to reduce
any greater load than can be developed between the fastener dependency on the bond between core material and face
and the face skins. As mentioned earlier, all loads entering skin, making the unsupported diaphragm area less critical.
the sandwich panels are carried first through these skins.
The core member acts to stabilize and transmit transverse
stress across the panel. SHEAR LOADING
The chief design consideration, then, is to keep unit
Design for shear loading must take note of face skin
loading compatible with the load capabilities of and
thickness and bearing area. The insert must remain stable
through the thin face skins. This generally means
and in position in order to develop full bearing load in the
maintaining proper load distribution and a relatively low
face skin. In common single-shear applications, the load
load per fastener by using an adequate number of the proper
develops a relatively high upsetting moment, tending to
type and size fasteners.
cause the insert to turn over and lose continuity with the
Regardless of sandwich construction, it is necessary to face skin. In the case of molded-in blind type fasteners, a
make an analysis of total load vs. fastener size and quantity. complete distribution of the potting medium around the
The lighter the sandwich structure, the greater care required insert will achieve good stability. In cases of severely high
in selecting the proper fastener. unit loading, it may be necessary to undercut surrounding
cells in order to provide a sufficiently large bearing area for
TENSION LOADING stability. The core material plays only a secondary role in
An element analysis across any given section of single-shear loading; when the continuity between face skin
honeycomb in any direction will indicate a structure that is and insert is lost, failure is quite rapid.
7
SECTION — Ill
MOLDED-IN INSERTS the other hole thus insuring a completely uniform fill (See
With molded-in inserts, the key to proper design is the Figure 8).
selection of a head diameter which provides sufficient
bearing area in the face skin for the size of the bolt or screw INSTALLATION HOLE PREPARATION
to be used. The SL600 Series Shur-Tab system of fastening When drilling holes in honeycomb panel for blind
is available in both blind and through types. inserts, it is only necessary to drill deep enough to provide
For blind types, minimum insert length should provide a clear hole having a depth of 0.020 or 0.030 inches beyond
1½ diameters of thread engagement. It is not generally the length of the insert. When drilling holes in panel with a
necessary to use a different insert length for each homogeneous core such as foam material, it is
application, but certain special considerations may arise recommended to drill to the inside surface of the bottom
which will demand a longer length for thicker panels. skin for maximum performance. It is not necessary to clean
In these instances, however, the insert should not be up the inner surface when the core is honeycomb since any
allowed to touch the inner surface of the bottom skin. A roughness or remaining core will actually add materially to
minimum of 0.040 inches clearance is recommended to the bond efficiency by providing additional adhesion area.
allow the epoxy to flow under and around the insert. This The through type requires a single diameter hole through
assures a bond between the inside surface of the bottom both face skins. The core may be undercut to provide
skin and the fastener (See Figure 7). additional bearing surface or footprint area (See Figure 9).
For through types the insert length must be precise to Incidental to the drilling of installation holes in honey
assure flush installation across the overall thickness of the comb is a certain amount of cell opening. These opened cells
sandwich. To help position the insert accordingly, the Shur- become filled with epoxy which provides a bond between
Tab system provides a special adhesive-backed tab. Epoxy face skins, cell walls and the insert. Cell size and the number
is injected through one of the potting holes with a Semco of cells opened during drilling will thus determine the
sealant gun, or equivalent, which permits venting through amount of epoxy requires for a given installation.
8
SHUR LOK
C O R P O R A T I O N
MECHANICAL INSERTS
When loading requirements warrant, a simple
mechanical spacer such as the Shur-Lok SM or SN types is
recommended (See Figure 10). These spacers require only a
thru hole for installation and when flush mounting is
required the “D” type head style provides the advantage of
automatic dimpling of the face skins during installation.
Mechanical inserts, such as the SL500 Series, attach to
both face skins (See Figure 11). Therefore, two sizes of
drills or a step drill must be used to provide the installation
hole (See Figure 12).
There are two basic head forms on the SL500 type Figure 10 — Simple Mechanical Insert
mechanical inserts-the dimpled or flush type, and the flat-
head or standoff type. Flat or standoff types require control
only of hole size and overall length. The face skin gage,
type of material and effects of dimpling must be considered
when using the flush type.
When SL500 flush type inserts are used, the face skins
in most cases are depressed by the installation of the
fastener to provide a flush installation. Most prime
fabricators of missile or airframe assemblies have
established rules controlling the method of dimpling.
Some companies require predimpling of the skins for
all materials. Shur-Lok Corporation recommends that
predimpling be used on skins in excess of 0.020 inches. For Figure 11 — Shur-Torq Self-Retaining Mechanical Insert
skins thicker than 0.060 inches, we suggest countersinking.
Certain forms of sandwich facing will require special
consideration. For example, most fiberglass skins cannot be
dimpled but must be sufficiently thick to be countersunk
when flush heads are required. Hot predimpling is usually
required for 7075 aluminum alloy and like materials.
EDGE DISTANCE
Insert users frequently inquire about minimum required
edge distance for sandwich fastener installation. Best
practice is to maintain a minimum of two diameters from
the edge based on insert head diameter. In some cases a
closer edge distance may be required. The exact allowables
should be determined by testing. Figure 12 — Step Drill Provides Two Hole Sizes
9
SECTION — IV
SECTION IV — MATERIALS
10
SHUR LOK
C O R P O R A T I O N
11
SECTION — V
1.0 SCOPE
1.1 PURPOSE
The purpose of this document is to establish standardized procedures for the testing
and qualification of sandwich fasteners. Unless otherwise specified, all tests are
static tests to be conducted at room temperature.
12
SHUR LOK
C O R P O R A T I O N
13
SECTION — V
14
SHUR LOK
C O R P O R A T I O N
SCOPE
The following graphs and tables depict actual test results obtained with various
sandwich fasteners installed in different types of panel. This data is intended to
provide the engineer with comparative loads when different type fasteners are
installed in the same panel. It also illustrates that, as explained in the foregoing
sections of this manual, the selection of the type fastener is dependent upon the design
parameters and specific application.
The graphs illustrate the difference in loading curves between molded-in
fasteners installed with and without an undercut and mechanical fasteners. These
loading curves are the result of three test specimens for each condition of panel
variation, type of insert, undercut vs no undercut and potting compound. The
advantage of the undercut in the molded-in fastener is obvious. However, it must be
realized that the greater load-carrying capabilities are achieved at the expense of an
increase in weight. But in most cases the strength-to-weight ratio of the undercut
installation is still more attractive than without an undercut. It is emphasized that the
ultimate fastener selection must be made by the designer and that the information
provided in this section is to serve as an aid in making his selection an engineering
decision rather than simply guesswork. The higher load values which are obtained
with the mechanical fastener are attributed to the greater stability provided by this
type of part.
15
SECTION — VI
16
SHUR LOK
C O R P O R A T I O N
17
SECTION — VI
18
SHUR LOK
C O R P O R A T I O N
19
SECTION — VI
20
SHUR LOK
C O R P O R A T I O N
21
SECTION — VI
22
SHUR LOK
C O R P O R A T I O N
23
SECTION — VI
24
SHUR LOK
C O R P O R A T I O N
25
SECTION — VI
26
SHUR LOK
C O R P O R A T I O N
27
SECTION - VI
28
SHUR LOK
C O R P O R A T I O N
29
SECTION — VI
30
SHUR LOK
C O R P O R A T I O N
31
SECTION — VI
32
SHUR LOK
C O R P O R A T I O N
CONCLUSIONS
TOTAL ENVIRONMENT — Many individual factors have been discussed in this Design
Manual. It is important for each of these items to be measured in relation to the total
environment of fastener, sandwich structure and end use.
To aid design engineers in applying these factors to their specific requirements,
Shur-Lok Corporation offers two outstanding services, complete testing facilities and
engineering consultation.
IN-PLANT TESTING — To insure the validity of each step in the design process, Shur-
Lok maintains complete testing facilities staffed by qualified fastener specialists.
With this in-plant capability, Shur-Lok has documented a wide range of test data
beyond the examples presented in this manual. The test data is readily available to
designers.
This in-depth approach to problem solving has been greatly appreciated by
designers. It has been a major factor in Shur-Lok’s growth to pre-eminence in the
field of sandwich panel fasteners.
EXPERT CONSULTATION — Our highly skilled engineering staff is available at all times
to help in exploring each facet of a design problem. As pioneers in the development
of fasteners for sandwich structure, Shur-Lok has developed an impressive record of
successful solutions to complex problems. We welcome the opportunity to be of
service, whatever the challenge.
ADDITIONAL TEST DATA — Numerous tests have been conducted since the original
publication of the Design Manual. Be sure to contact our engineering staff for
information about testing with sandwich panel material or potting compounds not
included in this publication.
POTTING COMPOUNDS — For expert assistance concerning the aerospace industry’s most
advanced potting compounds you may also wish to contact:
Fiber-Resin Corporation
a subsidiary of H.B. Fuller Company
20701 Nordhoff Street
Chatsworth, CA 91311
TEL: 1-800-624-9487
FAX: 818/709-0399
33