Two structural adhesives can reach a similar maximum load and still behave very differently before failure.
One may remain relatively stiff through most of the test. The other may begin to deform significantly at a much lower shear stress. If the laboratory reports only the peak load, that difference disappears from the result.
ASTM D5656 is designed to answer this second question.
The method uses a thick-adherend metal lap-shear specimen to characterize the shear stress-strain behavior of an adhesive under tensile loading. Instead of focusing only on how much load a bonded joint can withstand, it examines how the adhesive responds as shear stress increases.
That difference also changes the equipment requirement.
A universal testing machine and load cell can measure force. They cannot, by themselves, tell you how much deformation occurred locally across the adhesive layer. For a useful D5656 test system, specimen geometry, alignment, local deformation measurement and data acquisition all matter.
What Does ASTM D5656 Measure?
ASTM D5656 is titled:
Standard Test Method for Thick-Adherend Metal Lap-Shear Joints for Determination of the Stress-Strain Behavior of Adhesives in Shear by Tension Loading
ASTM currently lists ASTM D5656-10(2017) as an active standard.
The method is intended to determine the shear stress-strain properties of adhesives using thick-adherend metal lap-shear joints. ASTM also identifies determination of the proportional limit of the stress-strain relationship as an application of the method.
There is an important distinction here.
D5656 is not simply asking:
How much load did the joint carry before it failed?
It is asking:
How did the adhesive deform as the applied shear stress increased?
The basic measurement chain can be viewed as:
TENSILE LOAD → LOCAL SHEAR DEFORMATION → SHEAR STRESS + SHEAR STRAIN → STRESS-STRAIN CURVE
Nominal shear stress is related to the applied force and bonded area:
where:
τ = nominal shear stress
F = applied force
A = bonded area
Force is relatively straightforward to obtain from the testing machine.
Accurate deformation measurement is where the test becomes more demanding.
There is another limitation worth understanding before choosing the method. ASTM states that D5656 was developed and applied using bonded aluminum adherends, and applicability to non-aluminum adherends should not simply be assumed.
That detail is easy to overlook, but it matters when a laboratory is deciding whether D5656 actually fits a particular material system.
Why Thick Adherends Matter
A conventional single-lap joint has an unavoidable geometric problem: the two adherends are offset.
As tensile force is applied, that offset can introduce bending and peel components in addition to shear. If the adherends themselves deform significantly, the measured displacement becomes increasingly difficult to interpret as adhesive deformation.
D5656 addresses part of this problem by using thick adherends.
The increased stiffness helps limit unwanted adherend deformation and provides a more useful configuration for characterizing the shear response of the adhesive layer.
It does not turn the specimen into an ideal state of perfectly uniform pure shear. That would be an oversimplification.
The practical goal is to reduce mechanical effects that would make adhesive deformation more difficult to characterize.
This has an important consequence for equipment selection:
A high-accuracy load cell cannot compensate for excessive specimen bending or deformation measured at the wrong location.
For D5656, the specimen and deformation measurement method are part of the measurement system—not just accessories attached to the machine.
Why Crosshead Travel Cannot Represent Adhesive Shear Strain
This is one of the most important practical points in a D5656 setup.
Most universal testing machines already record crosshead displacement. It can therefore be tempting to use that displacement directly as adhesive deformation.
The problem is that crosshead travel represents movement through the entire load train.
Suppose the machine reports:
Crosshead displacement = 0.80 mm
That does not mean the adhesive layer has sheared by 0.80 mm.
The measured movement may include:
machine compliance
grip and fixture movement
adherend deformation
adhesive deformation
If the entire 0.80 mm is treated as deformation occurring in the adhesive layer, the calculated stress-strain response can be distorted.
The useful question in D5656 is therefore not:
How far did the crosshead move?
It is:
How much relative movement occurred across the bonded region?
That is why local deformation measurement becomes such an important part of the test.
How the ASTM D5656 Measurement Chain Works
A useful way to understand the equipment is to separate the test into two simultaneous measurements.
On one side is force.
The load cell measures the tensile force applied to the specimen. Combined with the bonded area, this provides the nominal shear stress.
On the other side is local deformation.
A suitable displacement or extensometer arrangement measures relative movement associated with the bonded region. In D5656-type measurement systems, measurements on opposite sides of the specimen can be used to help account for bending and obtain a more representative local deformation value.
The two data streams must correspond to each other throughout the test:
LOAD CELL
↓
FORCE
↓
SHEAR STRESS
and:
LOCAL DISPLACEMENT MEASUREMENT
↓
RELATIVE BOND-LINE MOVEMENT
↓
SHEAR STRAIN
Then:
SHEAR STRESS + SHEAR STRAIN
↓
STRESS-STRAIN CURVE
This is why a request such as:
“We need a 20 kN tensile machine for ASTM D5656.”
is incomplete.
It tells the equipment supplier something about expected force, but almost nothing about how the customer intends to measure the quantity that makes D5656 valuable: local adhesive deformation.
From Specimen to Stress-Strain Data
The detailed specimen dimensions, preparation, conditioning, loading and calculation requirements should always be taken from the applicable edition of ASTM D5656 rather than from a blog article.
From an equipment perspective, however, a D5656 test follows a fairly clear sequence.
The bonded specimen is prepared and its relevant geometry is measured. It is then installed in the loading system with attention to alignment.
The local deformation measurement device is positioned around the bonded region before loading begins.
Once the setup has been checked, tensile loading is applied while the system records force and local deformation simultaneously.
Those measurements are converted into shear stress and shear strain values and used to construct the stress-strain curve.
In practice, this means a laboratory needs to control more than the machine's crosshead speed and maximum force.
A useful result depends on the complete chain:
SPECIMEN → ALIGNMENT → LOAD → LOCAL DEFORMATION → SYNCHRONIZED DATA → ANALYSIS
An error early in that chain can remain hidden inside a curve that otherwise looks perfectly smooth.
Reading the Shear Stress-Strain Curve
A maximum force is one number.
A stress-strain curve tells a story.
Consider two adhesives that eventually reach similar maximum shear stresses. If only the peak value is compared, they may appear almost equivalent.
Their curves may show otherwise.
At lower stresses, one adhesive may develop substantially less strain than the other. As loading increases, one may depart from its initial response earlier, while the other maintains its initial behavior over a larger range.
A simplified curve can be considered in several regions.
Initial response
The early part of the curve describes how stress and strain develop together at relatively low loading.
Proportional limit
ASTM D5656 identifies determination of the proportional limit of the stress-strain relationship as one of the uses of the test.
Nonlinear response
Beyond the initial region, deformation may increase in a way that is no longer proportional to stress.
Failure
Continued loading eventually leads to the final failure condition of the bonded specimen.
This is why two adhesives with similar ultimate results should not automatically be considered mechanically equivalent.
Their behavior before failure may be quite different.
ASTM D5656 vs ASTM D1002: Similar Specimens, Different Questions
ASTM D5656 and ASTM D1002 are easy to group together because both involve adhesively bonded metal joints subjected to tensile loading.
They should not be treated as interchangeable tests.
| Test Question | ASTM D1002 | ASTM D5656 |
|---|---|---|
| What is the main objective? | Apparent shear strength | Shear stress-strain behavior |
| What specimen concept is used? | Single-lap metal joint | Thick-adherend metal lap-shear joint |
| Is maximum load important? | Yes | Yes, but it is not the whole result |
| Is local deformation central to the test? | No | Yes |
| Is stress-strain behavior the main focus? | No | Yes |
| Proportional limit | Not the primary objective | Relevant result |
| Typical question | How strong is the lap joint? | How does the adhesive respond under shear? |
A simple way to separate the two is:
ASTM D1002 asks mainly how strong the lap joint is.
ASTM D5656 asks how the adhesive responds as shear stress increases.
That distinction is useful before buying equipment.
A laboratory performing D1002 may be primarily concerned with load capacity, specimen alignment, grips and failure mode.
For D5656, local strain measurement becomes a central part of the system specification.
Where Is ASTM D5656 Useful?
D5656 is most relevant when an adhesive is being treated as an engineering material whose deformation behavior matters, rather than when the laboratory simply needs a pass/fail bond-strength number.
| Application | Why Stress-Strain Data Matters |
|---|---|
| Aerospace adhesive R&D | Characterize deformation behavior before final failure |
| Automotive structural adhesive development | Compare stiffness and nonlinear shear response |
| Adhesive formulation development | Distinguish formulations that may have similar peak strength |
| Material laboratories | Study the effects of curing, aging or environmental conditioning |
There is an important scope limitation here.
ASTM D5656 was developed and applied using bonded aluminum adherends. ASTM states that applicability to non-aluminum adherends should not be assumed without evaluation.
This matters in automotive, composite and multi-material research.
An application may involve structural adhesives, but that does not automatically mean ASTM D5656 is the appropriate standard for every adherend combination.
Important Distinction
D5656 also should not be interpreted simply as a test of:
“How well does this adhesive stick to this substrate?”
The method is intended to characterize adhesive shear stress-strain properties that can be relevant to bonded-joint design and analysis.
That is a different question from adhesion-to-substrate evaluation.
How to Configure an ASTM D5656 Test System
A useful D5656 equipment discussion starts with the measurement requirement—not with a machine model.
There are five questions to answer.
1. What load will the specimen generate?
The expected force determines the appropriate machine and load-cell range.
A rough machine-selection estimate can begin with:
Suppose a laboratory expects approximately 18 MPa and the bonded area relevant to its preliminary capacity estimate is 400 mm²:
or approximately:
That tells us immediately that a 5 kN system would not provide enough capacity for this assumed case.
This is only an equipment-sizing example. It is not a replacement for the specimen geometry, calculations or data reduction required by ASTM D5656.
And capacity should not be selected from the estimated maximum alone. The expected working range and required load measurement performance also matter.
2. How will the specimen be loaded?
The gripping and loading arrangement must suit the thick-adherend specimen while maintaining a suitable load path.
Poor alignment can introduce additional bending and complicate interpretation of the deformation measurement.
3. Where will strain be measured?
This is the question that often separates a basic tensile machine from a D5656-oriented system.
The measurement arrangement needs to capture local relative displacement associated with the bonded region rather than simply relying on crosshead travel.
4. Do force and deformation channels work together?
The system must acquire force and deformation in a synchronized manner if a meaningful stress-strain curve is to be constructed.
5. Is temperature part of the research question?
If testing is only required under standard laboratory conditions, a conventional universal testing machine may provide the appropriate loading platform.
If the research objective includes adhesive behavior at elevated or reduced temperature, the loading system, fixtures and strain measurement method all need to function under those environmental conditions.
Only after answering these questions does machine selection become useful.
ITM-LAB Equipment for ASTM D5656 Applications
RS-8000 — Room-Temperature Testing Platform
For D5656 applications that fall within its load range, the ITM-LAB RS-8000 Servo Control tensile testing machine can be considered as the mechanical loading platform.
The RS-8000 is available in 10–50 kN capacities and has a specified load accuracy of ±0.25%.
For a D5656-oriented configuration, however, the machine should be considered one part of the system:
RS-8000
Thick-Adherend Specimen Loading / Gripping Arrangement
Suitable Local Shear Strain Measurement
Synchronized Data Acquisition
Stress-Strain Analysis
This wording is deliberate.
The presence of a tensile frame and adequate load capacity does not, on its own, establish a complete ASTM D5656 configuration.
The specimen, strain measurement method and required data outputs should be reviewed before the final system is specified.
ITM-LAB RS-8000 Servo Control Tensile Testing Machine
When the Research Question Includes Temperature
Not every D5656 project needs environmental temperature control.
But adhesive mechanical behavior can be temperature-dependent, and some R&D programs need to compare stress-strain response under different thermal conditions.
For this type of requirement, the ITM-LAB RS-8000GDW High and Low Temperature Tensile Testing Machine provides a possible platform that combines mechanical loading with a controlled-temperature test space.
The selection logic is therefore straightforward:
ROOM-TEMPERATURE MECHANICAL CHARACTERIZATION
→ RS-8000
TEMPERATURE-DEPENDENT MECHANICAL CHARACTERIZATION
→ RS-8000GDW
There is one practical point that should not be overlooked.
Moving the test into an environmental chamber does not mean only the machine changes.
The fixture and strain measurement system must also be suitable for the target temperature range.
A room-temperature extensometer should not automatically be assumed suitable for a high- or low-temperature D5656 configuration.
ITM-LAB RS-8000GDW High and Low Temperature Tensile Testing Machine
Common Sources of Bad ASTM D5656 Data
When D5656 results look inconsistent, changing the testing machine is not necessarily the first solution.
The problem may be elsewhere in the measurement chain.
Using crosshead movement as adhesive deformation is an obvious example. Machine, grip and specimen deformation can all contribute to crosshead travel.
Poor alignment is another. A thick adherend helps control unwanted deformation, but it cannot correct a badly installed specimen.
The location and installation of the local displacement measurement system also deserve attention. A high-resolution sensor does not produce useful adhesive strain data if it is measuring the wrong movement.
Then there is machine selection itself.
Choosing a 50 kN frame simply because it offers the highest available capacity is not a complete engineering decision. The expected working force, load-cell range, resolution, specimen configuration and strain measurement requirement should be considered together.
When troubleshooting questionable results, it is useful to work through the system in order:
SPECIMEN
→ ALIGNMENT
→ GRIPPING
→ LOAD MEASUREMENT
→ LOCAL DEFORMATION MEASUREMENT
→ DATA PROCESSING
A smooth curve does not prove that every part of this chain was correct.
What Should You Send Us for System Selection?
If you are requesting a D5656 test system, sending only the standard number usually leads to another round of questions.
The following information is much more useful:
Adhesive type
Adherend material
Specimen drawing or dimensions
Bonded area
Bond-line thickness
Expected maximum force or expected shear stress
Room-temperature or controlled-temperature requirement
Required strain measurement
Required test curves and report outputs
For example:
“We need ASTM D5656 testing.”
does not define enough information to configure the measurement system.
A much more useful inquiry would be:
“We are testing an epoxy adhesive with aluminum thick-adherend specimens. Expected maximum load is approximately 8 kN. We need room-temperature shear stress-strain curves and local strain measurement.”
That gives an equipment engineer something concrete to work with.
FAQ
Is ASTM D5656 a shear strength test?
Not primarily. The method uses a lap-shear configuration, but its main purpose is to characterize adhesive shear stress-strain behavior, rather than simply report a maximum apparent shear strength.
What is the main difference between ASTM D5656 and ASTM D1002?
ASTM D1002 focuses on the apparent shear strength of adhesively bonded metal single-lap joints. D5656 uses thick adherends and focuses on the adhesive's shear stress-strain response.
Why does ASTM D5656 use thick adherends?
Greater adherend stiffness helps reduce unwanted adherend deformation and provides a more useful configuration for measuring adhesive shear behavior.
Can I use crosshead displacement for ASTM D5656 shear strain?
Crosshead travel includes movement and compliance from the broader testing system. A suitable local deformation measurement approach is important when the objective is adhesive shear strain characterization.
What equipment is needed for ASTM D5656?
A practical system requires more than a universal testing machine. The complete configuration should consider the load frame and load cell, specimen loading arrangement, local shear deformation measurement, synchronized data acquisition and analysis.
Which ITM-LAB machine should I use for ASTM D5656?
For room-temperature applications within its force range, the RS-8000 10–50 kN platform is the primary machine to evaluate. Where the research program requires controlled high- or low-temperature mechanical characterization, RS-8000GDW can be considered. Final configuration depends on specimen geometry, expected load, strain measurement and environmental requirements.
Build the Test Around the Measurement
ASTM D5656 is easy to underestimate if it is viewed as simply another lap shear test.
The tensile machine is important, but the real measurement problem is more specific:
Can the system distinguish adhesive shear deformation from movement elsewhere in the test setup?
That question affects the specimen, alignment, extensometry, data acquisition and ultimately the stress-strain curve.
For this reason, selecting an ASTM D5656 system should begin with the specimen and the required measurement—not with the maximum capacity printed on the machine specification sheet.
For room-temperature applications, the ITM-LAB RS-8000 can provide the mechanical loading platform. Where temperature-dependent adhesive behavior is part of the research objective, RS-8000GDW offers a route to combine mechanical loading with environmental control.
In either case, the final configuration should bring together:
appropriate load capacity, controlled specimen loading, local shear strain measurement and synchronized force-deformation acquisition.
That is what turns a tensile test into useful adhesive stress-strain characterization.
Request an ASTM D5656 Testing Solution
Send ITM-LAB your specimen drawing, adhesive type, adherend material, bonded dimensions, expected load, required test temperature and strain measurement requirement.
We can evaluate the appropriate machine capacity, specimen loading configuration, local deformation measurement and environmental testing options for your application.
