ASTM D4896-01(2008)e1
(Guide)Standard Guide for Use of Adhesive-Bonded Single Lap-Joint Specimen Test Results
Standard Guide for Use of Adhesive-Bonded Single Lap-Joint Specimen Test Results
SIGNIFICANCE AND USE
Single-lap specimens are economical, practical, and easy to make. They are the most widely used specimens for development, evaluation, and comparative studies involving adhesives and bonded products, including manufacturing quality control.
Special specimens and test methods have been developed that yield accurate estimates of the true shear strength of adhesives. These methods eliminate or minimize many of the deficiencies of the thin-adherend single-lap specimens, but are more difficult to make and test. (See Test Methods D 3983, D 4027, D 4562, and E 229.)
The misuse of strength values obtained from such Test Methods or Practices as D 906, D 1002, D 1144, D 1151, D 1183, D 1780, D 2294, D 2295, D 2339, D 3163, D 3164, D 3165, D 3434, D 3528, D 3632, and D 5868, as allowable design-stress values for structural joints could lead to product failure, property damage, and human injury.
SCOPE
1.1 This guide is directed toward the safe and appropriate use of strength values obtained from test methods using single-lap adhesive joint specimens.
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.
1.3 The discussion focuses on shear strength as measured with small thin-adherend, single-lap specimens. Many factors, however, apply to shear modulus, tensile strength, and tensile modulus measured by small laboratory specimens in general. This discussion is limited to single-lap specimens and shear strength only for simplification.
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´1
Designation: D4896 − 01(Reapproved 2008)
Standard Guide for
Use of Adhesive-Bonded Single Lap-Joint Specimen Test
Results
This standard is issued under the fixed designation D4896; 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 (´) indicates an editorial change since the last revision or reapproval.
ε NOTE—The units statement in subsection 1.2 was corrected editorially in April 2008.
INTRODUCTION
The true strength of an adhesive is a material property independent of the joint geometry, adherend
properties,andload,andisagoodstartingpointfordetermininganallowabledesignstress.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
laboratoryspecimen,andinparticular,withasingle-lapspecimen.Forexample,thefailureofatypical
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.Similarlythesefactorsaffecttheapparenttensilestrengthofanadhesiveinbutt-jointtest
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.
Theobjectivesofthisguideare:todevelopanappreciationofthefactorsthatinfluencestrengthand
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 This discussion is limited to single-lap specimens and shear
strength only for simplification.
1.1 This guide is directed toward the safe and appropriate
use of strength values obtained from test methods using
2. Referenced Documents
single-lap adhesive joint specimens.
2.1 ASTM Standards:
1.2 The values stated in inch-pound units are to be regarded
D896 Practice for Resistance of Adhesive Bonds to Chemi-
as standard. The values given in parentheses are mathematical
cal Reagents
conversions to SI units that are provided for information only
D906 Test Method for Strength Properties of Adhesives in
and are not considered standard.
Plywood Type Construction in Shear by Tension Loading
1.3 The discussion focuses on shear strength as measured D907 Terminology of Adhesives
D1002 Test Method for Apparent Shear Strength of Single-
with small thin-adherend, single-lap specimens. Many factors,
however, apply to shear modulus, tensile strength, and tensile Lap-Joint Adhesively Bonded Metal Specimens by Ten-
sion Loading (Metal-to-Metal)
modulus measured by small laboratory specimens in general.
D1144 Practice for Determining Strength Development of
Adhesive Bonds
ThisguideisunderthejurisdictionofASTMCommitteeD14onAdhesivesand
is the direct responsibility of Subcommittee D14.80 on Metal Bonding Adhesives. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved April 1, 2008. Published April 2008. Originally contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
approved in 1989. Last previous edition approved in 2001 as D4896 – 01. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D4896-01R08E01. the ASTM website.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
D4896 − 01 (2008)
D1151 Practice for Effect of Moisture and Temperature on based on the original dimensions of the cross section of the
Adhesive Bonds specimen. (See apparent and true shear strength).
D1183 Practices for Resistance of Adhesives to Cyclic
3.4 strain—the unit change due to force, in the size or shape
Laboratory Aging Conditions
of a body referred to its original size or shape. Strain is a
D1780 Practice for Conducting Creep Tests of Metal-to-
nondimensional quantity, but is frequently expressed in inches
Metal Adhesives
per inch, centimeters per centimeter, etc. (Refer to Terminol-
D2294 Test Method for Creep Properties of Adhesives in
ogy E6 for specific notes.)
Shear by Tension Loading (Metal-to-Metal)
3.4.1 linear (tensile or compressive) strain—the change per
D2295 Test Method for Strength Properties of Adhesives in
unit length due to force in an original linear dimension.
Shear by Tension Loading at Elevated Temperatures
3.4.2 shear strain—thetangentoftheangularchange,dueto
(Metal-to-Metal)
force, between two lines originally perpendicular to each other
D2339 Test Method for Strength Properties of Adhesives in
through a point in a body.
Two-Ply Wood Construction in Shear by Tension Loading
D2919 Test Method for Determining Durability ofAdhesive
3.5 stress—the intensity at a point in a body of the internal
Joints Stressed in Shear by Tension Loading
forcesorcomponentsofforcethatactonagivenplanethrough
D3163 Test Method for Determining Strength ofAdhesively
the point. Stress is expressed as force per unit of area
Bonded Rigid Plastic Lap-Shear Joints in Shear by Ten-
(pounds-force per square inch, newtons per square millimetre,
sion Loading
etc.).
D3164 Test Method for Strength Properties of Adhesively
NOTE 1—As used in tension, compression, or shear tests prescribed in
Bonded Plastic Lap-Shear Sandwich Joints in Shear by
product specifications, stress is calculated on the basis of the original
Tension Loading
dimensions of the cross section of the specimen.
D3165 Test Method for Strength Properties of Adhesives in
3.5.1 normal stress—the stress component perpendicular to
Shear by Tension Loading of Single-Lap-Joint Laminated
the plane on which the forces act. Normal stress may be either:
Assemblies
3.5.1.1 compressive stress—normal stress due to forces
D3166 Test Method for Fatigue Properties of Adhesives in
directed toward the plane on which they act, or
Shear by Tension Loading (Metal/Metal)
D3434 Test Method for Multiple-Cycle Accelerated Aging 3.5.1.2 tensile stress—normal stress due to forces directed
Test (Automatic Boil Test) for Exterior Wet Use Wood
away from the plane on which they act.
Adhesives
3.5.1.2.1 Discussion—In single-lap specimen testing, the
D3528 Test Method for Strength Properties of Double Lap
plane on which the forces act is the bondline. Tensile stress is
Shear Adhesive Joints by Tension Loading
sometimes used interchangeably, although incorrectly, with
D3632 Test Method for Accelerated Aging of Adhesive
peel or cleavage stress. Peel and cleavage involve complex
Joints by the Oxygen-Pressure Method
tensile, compressive, and shear stress distributions, not just
D3983 Test Method for Measuring Strength and Shear
tensile stress.
Modulus of Nonrigid Adhesives by the Thick-Adherend
3.5.2 shear stress—the stress component tangential to the
Tensile-Lap Specimen
plane on which the forces act.
D4027 Test Method for Measuring Shear Properties of
3.6 Definitions of Terms Specific to This Standard:
Structural Adhesives by the Modified-Rail Test
3.6.1 allowable design stress—a stress to which a material
D4562 Test Method for Shear Strength of Adhesives Using
can be subjected under service conditions with low probability
Pin-and-Collar Specimen
of mechanical failure within the design lifetime.
D5868 Test Method for Lap Shear Adhesion for Fiber
3.6.1.1 Discussion—Allowable design stress is obtained
Reinforced Plastic (FRP) Bonding
usually by multiplying the true shear strength of the material
E6 Terminology Relating to Methods of Mechanical Testing
(or close approximation thereof) by various adjustment factors
E229 Test Method for Shear Strength and Shear Modulus of
for manufacturing quality control, load and environmental
Structural Adhesives (Withdrawn 2003)
effects, and safety.
3. Terminology
3.6.2 apparent shear strength—(in testing a single-lap
3.1 Definitions:
specimen) the nominal shear stress at failure without regard for
3.1.1 The following terms are defined in accordance with
the effects of geometric and material effects on the nominal
Terminologies D907 and E6.
shear stress. Often called the lap-shear or tensile-shear
3.2 creep—the time-dependent increase in strain in a solid
strength.
resulting from force.
3.6.3 average stress—(in adhesive testing) the stress calcu-
3.3 shear strength—the maximum shear stress which a
lated by simple elastic theory as the load applied to the joint
material is capable of sustaining. Shear strength is calculated
dividedbythebondareawithouttakingintoaccounttheeffects
from the maximum load during a shear or torsion test and is
on the stress produced by geometric discontinuities such as
holes, fillets, grooves, inclusions, etc.
3.6.3.1 Discussion—The average shear and tensile stresses
The last approved version of this historical standard is referenced on
www.astm.org. are denoted by τ and σ respectively. (See 5.3.1.) (Average
avg avg
´1
D4896 − 01 (2008)
stress is the same as the preferred but less common term, tions are continuous functions of the adhesive modulus and
nominal stress, as defined in Terminology E6.) thickness, the adherend modulus, and the joint overlap length
as described more fully in Test Method D3983. An adherend
3.6.4 cleavage stress—(in adhesive testing) a term used to
thickness to overlap length ratio of less than 1:5 is a reasonable
describe the complex distribution of normal and shear stresses
approximation of a thin adherend for epoxy-steel joints (1 and
present in an adhesive when a prying force is applied at one
2).
end of a joint between two rigid adherends.
3.6.11 true shear strength—the maximum uniform shear
3.6.5 peel stress—(in adhesive testing) a term used to
stress which a material is capable of sustaining in the absence
describe the complex distribution of normal and shear stresses
of all normal stresses.
present in an adhesive when a flexible adherend is stripped
from a rigid adherend or another flexible adherend.
4. Significance and Use
3.6.6 single-lap specimen—(in adhesive testing) a specimen
4.1 Single-lap specimens are economical, practical, and
made by bonding the overlapped edges of two sheets or strips
easy to make. They are the most widely used specimens for
of material, or by grooving a laminated assembly, as shown in
development, evaluation, and comparative studies involving
Test Methods D2339 and D3165. In testing, a single-lap
adhesives and bonded products, including manufacturing qual-
specimen is usually loaded in tension at the ends.
ity control.
NOTE 2—In the past this specimen has been referred to commonly as
4.2 Special specimens and test methods have been devel-
the tensile-shear- or the lap-shear-specimen. These names imply that this
oped that yield accurate estimates of the true shear strength of
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. adhesives. These methods eliminate or minimize many of the
(An exception would be where the adhesive being evaluated is so low in
deficiencies of the thin-adherend single-lap specimens, but are
strength as not to induce any bending in the adherends.) It is recom-
more difficult to make and test. (See Test Methods D3983,
mended that, henceforth, this specimen be referred to as a single-lap
D4027, D4562, and E229.)
specimen.
4.3 The misuse of strength values obtained from such Test
3.6.7 stress concentration—a localized area of higher than
Methods or Practices as D906, D1002, D1144, D1151, D1183,
average stress near a geometric discontinuity in a joint or
D1780, D2294, D2295, D2339, D3163, D3164, D3165,
member (such as a notch, hole, void, or crack); or near a
D3434, D3528, D3632, and D5868, as allowable design-stress
material discontinuity (such as a bonded joint or weld) when
values for structural joints could lead to product failure,
the joint or member is under load.
property damage, and human injury.
3.6.7.1 Discussion—In adhesive testing, the most common
and important discontinuities are the ends of the bonded
5. Considerations for the Analysis of Small Single-Lap
adherends and the interfaces between the adhesive and adher-
Specimen Test Results
ends.
5.1 The true shear strength of an adhesive can be deter-
3.6.8 stress concentration factor—the ratio of the stress at a
mined only if normal stresses are entirely absent. These
point in a stress concentration to the average stress.
conditions can be approached under special conditions, but not
3.6.9 thick adherend—(in adhesive testing) an adherend
in single-lap specimens made with the thin adherends normally
used in a single-lap specimen that does not bend significantly
used in manufacturing and in most standard test specimens. In
when a load is applied, resulting in relatively lower tension/
most cases the tensile stress in the adhesive controls joint
normal stress at the ends of the overlap; and, more uniform
failure. As a consequence the single-lap specimen strength is
normal and shear stress distributions in the adhesive compared
unrelated to, and an unreliable measure of, the true shear
to a joint made with thin adherends and placed under the same
strength of an adhesive (1 and 2).
load.
5.2 Changes in adhesive volume during cure, the size of the
3.6.9.1 Discussion—A thick adherend for a typical epoxy
joint, the modulus of the adherends, and temperature or
adhesive and steel joint is at least 0.25 in. (6.36 mm) thick
moisture shifts after cure, all affect the magnitude of the
when the overlap is 0.50 in. (12.7 mm), based on finite element
stresses imposed on an adhesive in service. The thermal
analysis and mechanical tests (1 and 2). Objective criteria for
conductivity and permeability of the adherends affect the
determining whether or not an adherend is thick are given in
extentofthermalormoisturesofteningandtherateofchemical
Test Method D3983.
degradationoftheadhesiveinservice.Therefore,inadditionto
3.6.10 thin adherend—(in adhesive testing) an adherend
the problems stated in 5.1, the average stress at failure of small
used in a single-lap specimen that bends significantly, causing
single-lap specimen
...
This document is not anASTM standard and is intended only to provide the user of anASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
e1
Designation:D4896–95 Designation: D 4896 – 01 (Reapproved 2008)
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.
e NOTE—The units statement in subsection 1.2 was corrected editorially in April 2008.
INTRODUCTION
The true strength of an adhesive is a material property independent of the joint geometry, adherend
properties,andload,andisagoodstartingpointfordetermininganallowabledesignstress.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
laboratoryspecimen,andinparticular,withasingle-lapspecimen.Forexample,thefailureofatypical
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.Similarlythesefactorsaffecttheapparenttensilestrengthofanadhesiveinbutt-jointtest
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
1.1 This guide is directed toward the safe and appropriate use of strength values obtained from test methods using single-lap
adhesive joint specimens.
1.2The discussion focuses on shear strength as measured with small thin-adherend, single-lap specimens. Many factors,
however, apply to shear modulus, tensile strength, and tensile modulus measured by small laboratory specimens in general. This
discussion is limited to single-lap specimens and shear strength only for simplification.
1.2 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical
conversions to SI units that are provided for information only and are not considered standard.
1.3 The discussion focuses on shear strength as measured with small thin-adherend, single-lap specimens. Many factors,
however, apply to shear modulus, tensile strength, and tensile modulus measured by small laboratory specimens in general. This
discussion is limited to single-lap specimens and shear strength only for simplification.
2. Referenced Documents
2.1 ASTM Standards:
This guide is under the jurisdiction of Committee D-14 on Adhesives and is the direct responsibility of Subcommittee D14.80 on Metal Bonding Adhesives.
Current edition approved June 15, 1995. Published August 1995. Originally published as D4896–89. Last previous edition D4896–89.
This guide is under the jurisdiction of ASTM Committee D14 on Adhesives and is the direct responsibility of Subcommittee D14.80 on Metal Bonding Adhesives.
Current edition approved April 1, 2008. Published April 2008. Originally approved in 1989. Last previous edition approved in 2001 as D 4896 – 01.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
e1
D 4896 – 01 (2008)
D896Test Method for Resistance of Adhesive Bonds to Chemical Reagents
D 896 Practice for Resistance of Adhesive Bonds to Chemical Reagents
D 906 Test Method for Strength Properties of Adhesives in Plywood Type Construction in Shear by Tension Loading
D 907 Terminology of Adhesives
D 1002 TestMethodforApparentShearStrengthofSingle-Lap-JointAdhesivelyBondedMetalSpecimensbyTensionLoading
(Metal-to-Metal)
D 1144 Practice for Determining Strength Development of Adhesive Bonds
D1151Test Method 1151 Practice for Effect of Moisture and Temperature on Adhesive Bonds
D1183Test Methods 1183 Practices for Resistance of Adhesives to Cyclic Laboratory Aging Conditions
D 1780 Practice for Conducting Creep Tests of Metal-to-Metal Adhesives
D 2294 Test Method for Creep Properties of Adhesives in Shear by Tension Loading (Metal-to-Metal)
D 2295 Test Method for Strength Properties of Adhesives in Shear by Tension Loading at Elevated Temperatures
(Metal-to-Metal)
D 2339 Test Method for Strength Properties of Adhesives in Two-Ply Wood Construction in Shear by Tension Loading
D 2919 Test Method for Determining Durability of Adhesive Joints Stressed in Shear by Tension Loading
D 3163 Test Method for Determining Strength of Adhesively Bonded Rigid Plastic Lap-Shear Joints in Shear by Tension
Loading
D 3164 TestMethodforDeterminingtheStrengthPropertiesofAdhesivelyBondedPlasticLap-ShearSandwichJointsinShear
by Tension Loading
D 3165 Test Method for Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated
Assemblies
D 3166 Test Method for Fatigue Properties of Adhesives in Shear by Tension Loading (Metal/Metal)
D3434Practice 3434 TestMethodforMultiple-CycleAcceleratedAgingTest(AutomaticBoilTest)forExteriorWetUseWood
Adhesives
D 3528 Test Method for Strength Properties of Double Lap Shear Adhesive Joints by Tension Loading
D3632Practice 3632 Test Method for Accelerated Aging of Adhesive Joints by the Oxygen-Pressure Method
D 3983 Test Method for Measuring Strength and Shear Modulus of Nonrigid Adhesives by the Thick-Adherend Tensile-Lap
Specimen
D 4027 Test Method for Measuring Shear Properties of Structural Adhesives by the Modified-Rail Test
D 4562 Test Method for Shear Strength of Adhesives Using Pin-and-Collar Specimen
D 5868 Test Method for Lap Shear Adhesion for Fiber Reinforced Plastic (FRP) Bonding
E 6 Terminology Relating to Methods of Mechanical Testing
E 229 Test Method for Shear Strength and Shear Modulus of Structural Adhesives
3. Terminology
3.1 Definitions:
3.1.1 The following terms are defined in accordance with Terminologies D 907 and E6 and E 6.
3.2 creep—the time-dependent increase in strain in a solid resulting from force.
3.3 shear strength—the maximum shear stress which a material is capable of sustaining. Shear strength is calculated from the
maximum load during a shear or torsion test and is based on the original dimensions of the cross section of the specimen. (See
apparent and true shear strength).
3.4 strain—the unit change due to force, in the size or shape of a body referred to its original size or shape. Strain is a
nondimensional quantity, but is frequently expressed in inches per inch, centimeters per centimeter, etc. (Refer to Terminology
E 6for specific notes.)
3.4.1 linear (tensile or compressive) strain—the change per unit length due to force in an original linear dimension.
3.4.2 shear strain—the tangent of the angular change, due to force, between two lines originally perpendicular to each other
through a point in a body.
3.5 stress—the intensity at a point in a body of the internal forces or components of force that act on a given plane through the
point. Stress is expressed as force per unit of area (pounds-force per square inch, newtons per square millimetre, etc.).
NOTE 1—Asusedintension,compression,orsheartestsprescribedinproductspecifications,stressiscalculatedonthebasisoftheoriginaldimensions
of the cross section of the specimen.
3.5.1 normal stress—the stress component perpendicular to the plane on which the forces act. Normal stress may be either:
3.5.1.1 compressive stress—normal stress due to forces directed toward the plane on which they act, or
3.5.1.2 tensile stress—normal stress due to forces directed away from the plane on which they act.
3.5.2 Discussion—In single-lap specimen testing, the plane on which the forces act is the bondline. Tensile stress is sometimes
For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
, Vol 15.06.volume information, refer to the standard’s Document Summary page on the ASTM website.
e1
D 4896 – 01 (2008)
used interchangeably, although incorrectly, with peel or cleavage stress. Peel and cleavage involve complex tensile, compressive,
and shear stress distributions, not just tensile stress.
3.5.3 shear stress— The—the stress component tangential to the plane on which the forces act.
3.6 Definitions of Terms Specific to This Standard:
3.6.1 allowable design stress—a stress to which a material can be subjected under service conditions with low probability of
mechanical failure within the design lifetime.
3.6.1.1 Discussion—Allowable design stress is obtained usually by multiplying the true shear strength of the material (or close
approximationthereof)byvariousadjustmentfactorsformanufacturingqualitycontrol,loadandenvironmentaleffects,andsafety.
3.6.2 apparent shear strength—(in testing a single-lap specimen) the nominal shear stress at failure without regard for the
effects of geometric and material effects on the nominal shear stress. Often called the lap-shear or tensile-shear strength.
3.6.3 average stress—(in adhesive testing) the stress calculated by simple elastic theory as the load applied to the joint divided
by the bond area without taking into account the effects on the stress produced by geometric discontinuities such as holes, fillets,
grooves, inclusions, etc.
3.6.3.1 Discussion—The average shear and tensile stresses are denoted by t and s respectively. (See 5.3.1.1.)5.3.1.)
avg avg
(Average stress is the same as the preferred but less common term, nominal stress, as defined in Terminology E 6.)
3.6.4 cleavage stress—(in adhesive testing) a term used to describe the complex distribution of normal and shear stresses
present in an adhesive when a prying force is applied at one end of a joint between two rigid adherends.
3.6.5 peel stress—(in adhesive testing) a term used to describe the complex distribution of normal and shear stresses present
in an adhesive when a flexible adherend is stripped from a rigid adherend or another flexible adherend.
3.6.6 single-lap specimen—(in adhesive testing) a specimen made by bonding the overlapped edges of two sheets or strips of
material, or by grooving a laminated assembly, as shown in Test Methods D 2339 and D3165 and D 3165. In testing, a single-lap
specimen is usually loaded in tension at the ends.
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 is a
sheardominatedjoint,andthatthemeasuredstrengthistheshearstrengthoftheadhesive.Thisisnottrueformostusesofsuchspecimens.(Anexception
would be where the adhesive being evaluated is so low in strength as not to induce any bending in the adherends.) It is recommended that, henceforth,
this specimen be referred to as a single-lap specimen.
3.6.7 stress concentration—a localized area of higher than average stress near a geometric discontinuity in a joint or member
(such as a notch, hole, void, or crack); or near a material discontinuity (such as a bonded joint or weld) when the joint or member
is under load.
3.6.7.1 Discussion—In adhesive testing, the most common and important discontinuities are the ends of the bonded adherends
and the interfaces between the adhesive and adherends.
3.6.8 stress concentration factor—the ratio of the stress at a point in a stress concentration to the average stress.
3.6.9 thick adherend—(in adhesive testing) an adherend used in a single-lap specimen that does not bend significantly when a
load is applied, resulting in relatively lower tension/normal stress at the ends of the overlap; and, more uniform normal and shear
stress distributions in the adhesive compared to a joint made with thin adherends and placed under the same load.
3.6.9.1 Discussion—Athick adherend for a typical epoxy adhesive and steel joint is at least 0.25 in. (6.36 mm) thick when the
overlap is 0.50 in. (12.7 mm), based on finite element analysis and mechanical tests (1 and 2). Objective criteria for determining
whether or not an adherend is thick are given in Test Method D 3983.
3.6.10 thin adherend—(in adhesive testing) an adherend used in a single-lap specimen that bends significantly, causing
significant tension/normal stresses in the adhesive at the ends of the overlap and nonuniform shear and normal stress distributions
in the adhesive when a load is applied.
3.6.10.1 Discussion—The bending of the adherends, the tension-normal stresses, and the nonuniform stress distributions are
continuous functions of the adhesive modulus and thickness, the adherend modulus, and the joint overlap length as described more
fully inTest Method D 3983.An adherend thickness to overlap length ratio of less than 1:5 is a reasonable approximation of a thin
adherend for epoxy-steel joints (1 and 2).
3.6.11 true shear strength—the maximum uniform shear stress which a material is capable of sustaining in the absence of all
normal stresses.
4. Significance and Use
4.1 Single-lap specimens are economical, practical, and easy to make. They are the most widely used specimens for
development, evaluation, and comparative studies involving adhesives and bonded products, including manufacturing quality
control.
4.2 Special specimens and test methods have been developed that yield accurate estimates of the true shear strength of
adhesives. These methods eliminate or minimize many of the deficiencies of the thin-adherend single-lap specimens, but are more
difficult to make and test. (See Test Methods D3983, D4027, D4562, and E229D 3983, D 4027, D 4562, and E 229.)
Annual Book of ASTM Standards, Vol 03.01.
The boldface numbers in parentheses refer to the list of references at the end of this guide.
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