ASTM D4896-95

NOTICE: This standard has either been superseded and replaced by a new version or discontinued.
Contact ASTM International (www.astm.org) for the latest information.
Designation: D 4896 – 95
Standard Guide for
Use of Adhesive-Bonded Single Lap-Joint Specimen Test
Results
This standard is issued under the fixed designation D 4896; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (e) indicates an editorial change since the last revision or reapproval.
INTRODUCTION
The true strength of an adhesive is a material property independent of the joint geometry, adherend
properties, and load, and is a good starting point for determining an allowable design stress. Allowable
stresses in shear and tension are needed to design safe, efficient, adhesively bonded joints and
structures. The true shear strength, however, cannot be easily determined using single-lap specimens.
Many factors affect the apparent shear strength of an adhesive when measured with a small
laboratory specimen, and in particular, with a single-lap specimen. For example, the failure of a typical
single-lap specimen, is usually controlled by the tensile stress in the adhesive, and not by the shear
stress. The factors that control the tensile stress in lap-joint specimen, and thus, the apparent shear
strength are the size and shape of the specimen, the properties of the adherends, the presence of
internal stresses or flaws, and the changes that take place in the specimen due to adhesive cure and the
environment. Similarly these factors affect the apparent tensile strength of an adhesive in butt-joint test
specimens.
Due to the effects of these factors, the apparent shear strength obtained through measurements on
small laboratory specimens may vary widely from the true shear- or tensile-strength values needed to
determine allowable shear and tension design stresses.
The objectives of this guide are: to develop an appreciation of the factors that influence strength and
other stress measurements that are made with small laboratory test specimens; to foster the acceptable
uses of the widely used thin-adherend single-lap-joint test; and, specifically, to prevent misuse of the
test results.
1. Scope Plywood Type Construction in Shear by Tension Loading
D 907 Terminology of Adhesives
1.1 This guide is directed toward the safe and appropriate
D 1002 Test Method for Apparent Shear Strength of Single-
use of strength values obtained from test methods using
Lap-Joint Adhesively Bonded Metal Specimens by Ten-
single-lap adhesive joint specimens.
sion Loading (Metal-to-Metal)
1.2 The discussion focuses on shear strength as measured
D 1144 Practice for Determining Strength Development of
with small thin-adherend, single-lap specimens. Many factors,
Adhesive Bonds
however, apply to shear modulus, tensile strength, and tensile
D 1151 Test Method for Effect of Moisture and Temperature
modulus measured by small laboratory specimens in general.
on Adhesive Bonds
This discussion is limited to single-lap specimens and shear
D 1183 Test Methods for Resistance of Adhesives to Cyclic
strength only for simplification.
Laboratory Aging Conditions
2. Referenced Documents D 1780 Practice for Conducting Creep Tests of Metal-to-
Metal Adhesives
2.1 ASTM Standards:
D 2294 Test Method for Creep Properties of Adhesives in
D 896 Test Method for Resistance of Adhesive Bonds to
Shear by Tension Loading (Metal-to-Metal)
Chemical Reagents
D 2295 Test Method for Strength Properties of Adhesives in
D 906 Test Method for Strength Properties of Adhesives in
Shear by Tension Loading at Elevated Temperatures
(Metal-to-Metal)
This guide is under the jurisdiction of Committee D-14 on Adhesives and is the D 2339 Test Method for Strength Properties of Adhesives in
direct responsibility of Subcommittee D14.80 on Metal Bonding Adhesives.
Two-Ply Wood Construction in Shear by Tension Loading
Current edition approved June 15, 1995. Published August 1995. Originally
D 2919 Test Method for Determining Durability of Adhe-
published as D 4896 – 89. Last previous edition D 4896 – 89.
sive Joints Stressed in Shear by Tension Loading
Annual Book of ASTM Standards, Vol 15.06.
Copyright © ASTM, 100 Barr Harbor Drive, West Conshohocken, PA 19428-2959, United States.
D 4896
product specifications, stress is calculated on the basis of the original
D 3163 Test Method for Determining Strength of Adhe-
dimensions of the cross section of the specimen.
sively Bonded Rigid Plastic Lap-Shear Joints in Shear by
Tension Loading
3.5.1 normal stress—the stress component perpendicular to
D 3164 Test Method for Determining the Strength of Ad-
the plane on which the forces act. Normal stress may be either:
hesively Bonded Plastic Lap-Shear Sandwich Joints in
3.5.1.1 compressive stress—normal stress due to forces
Shear by Tension Loading
directed toward the plane on which they act, or
D 3165 Test Method for Strength Properties of Adhesives in
3.5.1.2 tensile stress—normal stress due to forces directed
Shear by Tension Loading of Laminated Assemblies
away from the plane on which they act.
D 3166 Test Method for Fatigue Properties of Adhesives in
3.5.2 Discussion—In single-lap specimen testing, the plane
Shear by Tension Loading (Metal/Metal)
on which the forces act is the bondline. Tensile stress is
D 3434 Practice for Multiple-Cycle Accelerated Aging Test
sometimes used interchangeably, although incorrectly, with
(Automatic Boil Test) for Exterior Wet Use Wood Adhe-
peel or cleavage stress. Peel and cleavage involve complex
sives
tensile, compressive, and shear stress distributions, not just
D 3528 Test Method for Strength Properties of Double Lap
tensile stress.
Shear Adhesive Joints by Tension Loading
3.5.3 shear stress—The stress component tangential to the
D 3632 Practice for Accelerated Aging of Adhesive Joints
plane on which the forces act.
by the Oxygen-Pressure Method
3.6 Definitions of Terms Specific to This Standard:
D 3983 Test Method for Measuring Strength and Shear
3.6.1 allowable design stress—a stress to which a material
Modulus of Nonrigid Adhesives by the Thick Adherend
Tensile Lap Specimen can be subjected under service conditions with low probability
of mechanical failure within the design lifetime.
D 4027 Test Method for Measuring Shear Properties of
Structural Adhesives by the Modified-Rail Test
3.6.1.1 Discussion—Allowable design stress is obtained
D 4562 Test Method for Shear Strength of Adhesives Using
usually by multiplying the true shear strength of the material
Pin-and-Collar Specimen
(or close approximation thereof) by various adjustment factors
E 6 Terminology Relating to Methods of Mechanical Test-
for manufacturing quality control, load and environmental
ing
effects, and safety.
E 229 Test Method for Shear Strength and Shear Modulus
3.6.2 apparent shear strength—(in testing a single-lap
of Structural Adhesives
specimen) the nominal shear stress at failure without regard for
the effects of geometric and material effects on the nominal
3. Terminology
shear stress. Often called the lap-shear or tensile-shear
3.1 Definitions:
strength.
3.1.1 The following terms are defined in accordance with
3.6.3 average stress—(in adhesive testing) the stress calcu-
Terminologies D 907 and E 6.
lated by simple elastic theory as the load applied to the joint
3.2 creep—the time-dependent increase in strain in a solid
divided by the bond area without taking into account the effects
resulting from force.
on the stress produced by geometric discontinuities such as
3.3 shear strength—the maximum shear stress which a
holes, fillets, grooves, inclusions, etc.
material is capable of sustaining. Shear strength is calculated
3.6.3.1 Discussion—The average shear and tensile stresses
from the maximum load during a shear or torsion test and is
are denoted by t and s respectively. (See 5.3.1.1.)
avg avg
based on the original dimensions of the cross section of the
(Average stress is the same as the preferred but less common
specimen. (See apparent and true shear strength).
term, nominal stress, as defined in E 6.)
3.4 strain—the unit change due to force, in the size or shape
3.6.4 cleavage stress—(in adhesive testing) a term used to
of a body referred to its original size or shape. Strain is a
describe the complex distribution of normal and shear stresses
nondimensional quantity, but is frequently expressed in inches
present in an adhesive when a prying force is applied at one
per inch, centimeters per centimeter, etc. (Refer to E 6 for
end of a joint between two rigid adherends.
specific notes.)
3.6.5 peel stress—(in adhesive testing) a term used to
3.4.1 linear (tensile or compressive) strain—the change per
describe the complex distribution of normal and shear stresses
unit length due to force in an original linear dimension.
present in an adhesive when a flexible adherend is stripped
3.4.2 shear strain—the tangent of the angular change, due
from a rigid adherend or another flexible adherend.
to force, between two lines originally perpendicular to each
3.6.6 single-lap specimen—(in adhesive testing) a specimen
other through a point in a body.
made by bonding the overlapped edges of two sheets or strips
3.5 stress—the intensity at a point in a body of the internal
of material, or by grooving a laminated assembly, as shown in
forces or components of force that act on a given plane through
Test Methods D 2339 and D 3165. In testing, a single-lap
the point. Stress is expressed as force per unit of area
specimen is usually loaded in tension at the ends.
(pounds-force per square inch, newtons per square millimetre,
etc.).
NOTE 2—In the past this specimen has been referred to commonly as
the tensile-shear- or the lap-shear-specimen. These names imply that this
NOTE 1—As used in tension, compression, or shear tests prescribed in
is a shear dominated joint, and that the measured strength is the shear
strength of the adhesive. This is not true for most uses of such specimens.
Annual Book of ASTM Standards, Vol 03.01. (An exception would be where the adhesive being evaluated is so low in
D 4896
strength as not to induce any bending in the adherends.) It is recom-
4.3 The misuse of strength values obtained from such Test
mended that, henceforth, this specimen be referred to as a single-lap
Methods as D 906, D 1002, D 2339, D 3163, D 3164, D 3165,
specimen.
and D 3528, as allowable design-stress values for structural
3.6.7 stress concentration—a localized area of higher than
joints could lead to product failure, property damage, and
average stress near a geometric discontinuity in a joint or human injury.
member (such as a notch, hole, void, or crack); or near a
5. Considerations for the Analysis of Small Single-Lap
material discontinuity (such as a bonded joint or weld) when
Specimen Test Results
the joint or member is under load.
5.1 The true shear strength of an adhesive can be deter-
3.6.7.1 Discussion—In adhesive testing, the most common
mined only if normal stresses are entirely absent. These
and important discontinuities are the ends of the bonded
conditions can be approached under special conditions, but not
adherends and the interfaces between the adhesive and adher-
in single-lap specimens made with the thin adherends normally
ends.
used in manufacturing and in most standard test specimens. In
3.6.8 stress concentration factor—the ratio of the stress at a
most cases the tensile stress in the adhesive controls joint
point in a stress concentration to the average stress.
failure. As a consequence the single-lap specimen strength is
3.6.9 thick adherend—(in adhesive testing) an adherend
unrelated to, and an unreliable measure of, the true shear
used in a single-lap specimen that does not bend significantly
strength of an adhesive (1 and 2).
when a load is applied, resulting in relatively lower tension/
5.2 Changes in adhesive volume during cure, the size of the
normal stress at the ends of the overlap; and, more uniform
joint, the modulus of the adherends, and temperature or
normal and shear stress distributions in the adhesive compared
moisture shifts after cure, all affect the magnitude of the
to a joint made with thin adherends and placed under the same
stresses imposed on an adhesive in service. The thermal
load.
conductivity and permeability of the adherends affect the
3.6.9.1 Discussion—A thick adherend for a typical epoxy
extent of thermal or moisture softening and the rate of chemical
adhesive and steel joint is at least 0.25 in. (6.36 mm) thick
degradation of the adhesive in service. Therefore, in addition to
when the overlap is 0.50 in. (12.7 mm), based on finite element
the problems stated in 5.1, the average stress at failure of small
analysis and mechanical tests (1 and 2). Objective criteria for
single-lap specimens after a given exposure is an unreliable
determining whether or not an adherend is thick are given in
measure of an adhesive’s environmental resistance in any other
D 3983.
joint, especially a much larger structural joint.
3.6.10 thin adherend—(in adhesive testing) an adherend
5.3 Factors Affecting Apparent Shear Strength:
used in a single-lap specimen that bends significantly, causing
5.3.1 Specimen geometry, material properties, and load are
significant tension/normal stresses in the adhesive at the ends
factors affecting apparent shear strength. The shear and normal
of the overlap and nonuniform shear and normal stress distri-
stresses at any point in a single-lap specimen are described
butions in the adhesive when a load is applied.
mathematically in the classic linear-elastic analysis of Goland
3.6.10.1 Discussion—The bending of the adherends, the
and Reissner (3). Modern finite element analysis has proven
tension-normal stresses, and the nonuniform stress distribu-
the Goland and Reissner analysis to be accurate except at the
tions are continuous functions of the adhesive modulus and
very ends of the overlap (1). Both the Goland and Reissner and
thickness, the adherend modulus, and the joint overlap length
finite element analyses show that both the normal and shear
as described more fully in Test Method D 3983. An adherend
stress concentration factors increase toward the ends of the
thickness to overlap length ratio of less than 1:5 is a reasonable
overlap (Fig. 1). Usually the tensile stress concentration is
approximation of a thin adherend for epoxy-steel joints (1 and
higher and is the dominant factor in failure. This means that
2).
peak stresses, and in particular the peak tensile stresses cause
3.6.11 true shear strength—the maximum uniform shear
failure, not the average shear stress across the bonded area.
stress which a material is capable of sustaining in the absence
Thus t
...