A plastic part does not need to break in tension to fail in service. Housings, brackets, electrical components, and molded structural parts are often exposed to bending loads, where one side of the material is under tension while the opposite side is under compression.
ASTM D790 is one of the most commonly referenced methods for evaluating this behavior.
The method uses a three-point bending setup to measure how a plastic specimen responds as load and deflection increase. From the resulting data, laboratories can determine properties such as flexural strength, flexural stress, strain, and flexural modulus.
For buyers, however, knowing the standard name is only the beginning. Specimen thickness, support span, test speed, load-cell range, fixture geometry, and deflection measurement can all affect the final result.
This guide explains both the test method and how to configure a practical ASTM D790 testing system.
Quick Answer: What Is ASTM D790?
ASTM D790 is a test method used to determine the flexural properties of unreinforced and reinforced plastics, including certain high-modulus composites and electrical insulating materials.
The specimen is supported at two points and loaded at the midpoint.
During the test, the system records force and deflection as the specimen bends.
Typical reported properties include:
- Flexural Strength
- Flexural Stress
- Flexural Strain
- Flexural Modulus
- Flexural Offset Yield Strength
- Load-Deflection Behavior
ASTM D790 is primarily intended for rigid and semi-rigid materials.
One limitation is easy to overlook: if a material does not break or yield within the strain limit defined by the method, flexural strength should not simply be extrapolated beyond that limit and reported as an ASTM D790 result.
How the ASTM D790 3-Point Bending Test Works
The test specimen acts as a simply supported beam.
Two lower supports carry the specimen while an upper loading nose moves downward at the center of the span.
As the load increases:
- the upper surface is mainly in compression;
- the lower surface is mainly in tension;
- the highest bending stress occurs around the center loading region.
The machine continuously records:
Force + Deflection
From these measurements, the software can generate a force-deflection or stress-strain curve and calculate the required flexural properties.
The physical setup looks simple.
Getting repeatable results is less simple.
Small differences in specimen dimensions, span, alignment, test speed, or displacement measurement can noticeably change the calculated result.
What Does ASTM D790 Measure?
Flexural Strength
Flexural strength is the maximum flexural stress reached during the test under the applicable test conditions.
It is commonly used to compare materials intended for components that experience bending during service.
Examples include:
- Molded plastic housings
- Structural brackets
- Electrical enclosures
- Automotive plastic components
- Consumer electronics parts
- Reinforced polymer components
For a rectangular specimen under conventional three-point bending conditions, flexural stress is commonly expressed as:
[
]
Where:
σf = flexural stress
P = applied load
L = support span
b = specimen width
d = specimen thickness
This equation also shows why accurate specimen measurement matters.
Thickness appears as d², so even a small thickness error can noticeably affect the calculated flexural stress.
Flexural Modulus
Flexural modulus describes the stiffness of the material during bending.
A higher modulus generally means the specimen resists bending deformation more strongly.
Flexural modulus is especially useful when stiffness matters more than ultimate failure.
Typical applications include:
- Electronic housings
- Rigid polymer panels
- Electrical insulation components
- Structural plastic supports
- Automotive interior components
For modulus measurement, the quality of the initial portion of the force-deflection curve becomes particularly important.
Fixture seating, machine compliance, and displacement measurement can all affect this region.
Flexural Strain
Flexural strain describes the nominal strain at the outer surface of the specimen during bending.
Stress and strain together provide a more useful picture of material behavior than maximum force alone.
ASTM D790 Specimen Dimensions and Support Span
ASTM D790 typically uses a solid rectangular specimen.
Depending on the material and manufacturing method, specimens may be injection molded, machined from sheets, or cut from plates, molded parts, or extruded shapes.
Before setting the machine, check the applicable material specification. A material-specific ASTM specification may define specimen preparation, conditioning, dimensions, or other test parameters that take precedence over the general D790 method.
What Is the ASTM D790 Span-to-Depth Ratio?
A commonly used support span-to-depth relationship is:
[
L/d=16:1
]
Where:
L = support span
d = specimen thickness
For example, with a 3.2 mm thick specimen:
[
]
A common starting support span would therefore be approximately:
51.2 mm
However, 16:1 is not a universal setting for every ASTM D790 specimen.
Material behavior and the purpose of the measurement matter. Certain highly anisotropic materials and modulus measurements may require larger span-to-depth ratios.
The correct setting should always follow the applicable test procedure and material specification.
Engineer Note: Thickness Is Easy to Underestimate
When a flexural result looks inconsistent, laboratories often look first at machine accuracy.
Specimen measurement should also be checked.
Because thickness is squared in the basic flexural-stress equation, even a relatively small error can noticeably shift the calculated result.
This becomes particularly relevant when specimens have dimensional variation, surfaces are not perfectly parallel, or different operators use different measurement positions.
For repeatable comparison, specimen measurement should be treated as part of the test—not just preparation before the test.
ASTM D790 Test Speed: Why It Is Not a Fixed Number
One of the most common ASTM D790 questions is:
What test speed should I enter into the universal testing machine?
There is no single crosshead speed for every specimen.
The speed depends on specimen thickness, support span, and the required outer-fiber strain rate.
A commonly used relationship is:
[
]
Where:
R = crosshead speed
Z = outer-fiber strain rate
L = support span
d = specimen thickness
For Procedure A, ASTM D790 uses a nominal strain rate of:
0.01 mm/mm/min
Procedure B uses a higher nominal strain rate of:
0.10 mm/mm/min
Procedure A is the normal starting point for many flexural-property measurements, particularly modulus, while Procedure B addresses materials that may not break or yield under Procedure A conditions within the permitted strain range.
Worked Example: Calculate ASTM D790 Crosshead Speed
Assume:
Specimen thickness, d = 3.2 mm
Support span, L = 51.2 mm
Strain rate, Z = 0.01 mm/mm/min
Then:
[
]
[
]
So the machine would be set close to:
1.37 mm/min
for this example.
The important point is not the number itself.
The important point is:
Crosshead speed should be calculated from the specimen and test conditions instead of copied from another test.
Procedure A vs Procedure B
| Item | Procedure A | Procedure B |
|---|---|---|
| Nominal Strain Rate | 0.01 mm/mm/min | 0.10 mm/mm/min |
| Relative Speed | Lower | Higher |
| Typical Purpose | General flexural properties and modulus | Materials that may not yield or break under Procedure A |
| Modulus Work | Commonly preferred | Not the usual first choice |
| Strain Limitation | Applies | Applies |
The procedure should not be selected simply because a faster test reduces test time.
It should be selected according to material behavior and the applicable standard requirements.
Engineer Note: Maximum Machine Speed Is Rarely the Deciding Specification
Buyers often compare universal testing machines by maximum crosshead speed.
For ASTM D790, that number rarely determines whether the machine is suitable.
A machine capable of 1000 mm/min is not automatically better than one capable of 500 mm/min for a flexural test that may run at only a few millimeters per minute.
More useful questions are:
- Can the machine maintain stable motion at low speed?
- Is speed control repeatable?
- Can test methods be saved?
- Is force measurement appropriate at the expected load?
For ASTM D790, control quality matters more than headline speed.
Worked Example: How Much Force Might an ASTM D790 Test Need?
Many buyers know the approximate flexural strength of their material but do not know how that translates into testing-machine capacity.
Starting from:
[
]
we can estimate load as:
[
]
Assume:
Estimated flexural strength = 100 MPa
Width = 12.7 mm
Thickness = 3.2 mm
Support span = 51.2 mm
Then:
[
]
The estimated load is approximately:
[
]
or only:
0.169 kN
This simple example explains why selecting a 50 kN or 100 kN machine simply because it is “stronger” may make little sense for ordinary plastic flexural specimens.
Engineer Note: Bigger Load Capacity Is Not Automatically Better
For a specimen expected to generate only a few hundred newtons, machine capacity should not be the first purchasing priority.
The better question is:
What load range will the specimen actually generate?
A 5 kN machine may already provide substantial capacity margin for many plastic ASTM D790 applications.
More importantly, the load cell should be selected so the expected forces fall within a useful measurement range.
A practical selection sequence is:
Material → Specimen → Expected Load → Load Cell → Machine
rather than:
Largest Machine Available → Fixture Added Later
Five Sources of ASTM D790 Test Variation
Machine accuracy matters, but it is only one part of repeatable flexural testing.
1. Specimen Thickness Error
Because thickness has a strong influence on flexural calculations, dimensional variation can significantly affect the reported result.
Measure specimens consistently and use the actual dimensions in the calculation.
2. Incorrect Support Span
Do not assume the previous fixture setting is correct.
Changing the support span changes both specimen loading and calculated stress.
3. Loading Nose and Support Misalignment
The loading nose should be centered between the supports.
Poor alignment can create uneven contact, specimen twisting, or an abnormal initial curve.
4. Fixture Seating and the Initial Curve
Small fixture movements or specimen seating at the beginning of the test can create an artificial toe region.
This matters particularly when modulus is calculated from the initial linear portion of the curve.
5. Deflection Measurement
Crosshead movement and true specimen deflection are not always exactly the same.
Machine-frame deformation, fixture compliance, and specimen seating can contribute to measured crosshead displacement.
Crosshead Displacement or Deflectometer?
For routine production QC, crosshead displacement may be practical.
Advantages include:
- Simpler setup
- Lower equipment cost
- Faster operation
For more demanding flexural modulus measurements, direct specimen deflection deserves closer attention.
A dedicated deflectometer can be useful for:
- R&D laboratories
- Material qualification
- More accurate modulus determination
- Comparisons requiring tighter displacement control
Engineer Note: Crosshead Travel Is Not Always Specimen Deflection
If the crosshead moves 1.00 mm, this does not automatically mean the center of the specimen deflected exactly 1.00 mm.
Part of that movement may come from:
- Machine compliance
- Fixture deformation
- Specimen seating
- Contact movement
For routine QC, this difference may be acceptable.
For modulus work, it deserves closer attention.
When ASTM D790 May Not Be the Right Test
ASTM D790 is widely used, but it is not appropriate for every plastic material or bending application.
Very Flexible Materials
ASTM D790 is mainly intended for rigid and semi-rigid materials.
Highly flexible materials may deform substantially without producing the required failure or yield behavior.
Materials That Do Not Yield or Break Within the Permitted Strain Range
If a specimen continues bending without reaching the required condition within the applicable strain range, the result should not simply be extrapolated beyond the method.
Applications Requiring Four-Point Bending
ASTM D790 uses three-point bending.
Applications specifically requiring four-point loading should consider an appropriate four-point flexural method such as ASTM D6272.
Material-Specific Standards
Some plastics, laminates, and composite systems have dedicated material specifications.
Always check whether those requirements modify the general ASTM D790 procedure.
ASTM D790 Testing Machine: 6 Specifications That Matter
1. Machine Capacity
Estimate the expected specimen load first.
For many standard plastic specimens, a low-force testing machine is more practical than a large floor-standing system.
2. Load Cell Range
Select the load cell around the expected specimen force rather than the maximum machine capacity.
A machine that supports different load-cell ranges provides greater flexibility.
3. Low-Speed Control
Check:
- Minimum test speed
- Speed accuracy
- Speed stability
- Programmable test methods
4. Three-Point Bending Fixture
Before purchasing, confirm:
- Adjustable support span
- Support geometry
- Loading-nose geometry
- Maximum specimen width
- Fixture alignment
- Required span range
5. Deflection Measurement
Determine whether the application requires:
Crosshead displacement
or
Dedicated specimen deflection measurement
before finalizing the system configuration.
6. Software
The software should be able to:
- Record force and displacement
- Generate test curves
- Enter specimen dimensions
- Calculate required results
- Save test methods
- Generate reports
- Export data
Why the RS-8010A Fits Typical ASTM D790 Applications
Most conventional ASTM D790 plastic specimens do not require a very high-capacity test frame.
That makes a low-force universal testing machine a logical starting point.
Within the ITM-LAB universal testing machine range, the RS-8010A Single Column Universal Testing Machine can be configured for many conventional plastic flexural applications with the appropriate load cell and three-point bending fixture.
Key specifications include:
Capacity Range: 50 N–5 kN
Force Accuracy: ±0.25%
Test Speed: 0.001–500 mm/min
Displacement Resolution: 0.001 mm
The same machine platform can also be configured for:
- Tensile testing
- Compression testing
- Peeling
- Shearing
- Other low-force mechanical tests
This can be useful for plastics laboratories that need multiple mechanical tests but do not need a separate machine for every application.
Do You Need a 5 kN or 10 kN Machine?
There is no ASTM D790 rule saying every laboratory needs a 5 kN, 10 kN, or 50 kN machine.
Calculate or estimate the specimen load first.
If the expected maximum force is only 200 N, a 5 kN frame already provides substantial capacity margin. The more important decision may be selecting an appropriate load cell.
A higher-capacity system may make sense if the laboratory also needs to test:
- Thicker plastic sections
- Stronger reinforced materials
- Larger specimens
- Higher-force compression
- Structural components
Machine selection should reflect the complete testing program—not one headline capacity value.
ASTM D790 vs ISO 178
ASTM D790 and ISO 178 both address flexural testing of plastics.
They are often shown together on universal testing machine product pages because the same machine and flexural fixture can frequently be configured for both methods.
That does not mean the standards are technically equivalent.
Differences can exist in specimen requirements, conditioning, support-span conditions, test procedure, and calculation or reporting requirements.
| Item | ASTM D790 | ISO 178 |
| Test Category | Plastic flexural testing | Plastic flexural testing |
| Loading | 3-point bending | 3-point bending |
| Specimen Rules | ASTM requirements | ISO requirements |
| Procedure | ASTM-specific | ISO-specific |
| Direct Result Equivalence | No | No |
The practical approach is:
One machine can support both methods, but each standard should have its own test setup and procedure.
ASTM D790 vs ASTM D6272
ASTM D790 uses a:
Three-Point Bending Configuration
One loading nose applies force at the center of the specimen.
ASTM D6272 uses:
Four-Point Bending
The loading geometry and resulting stress distribution are different.
The two methods should not be substituted for one another without considering the material specification and test objective.
Practical ASTM D790 Test Workflow
A routine laboratory workflow can be summarized in eight steps:
1. Identify the Material
Confirm the material type and applicable specification.
2. Prepare and Condition the Specimen
Follow the required preparation and conditioning procedure.
3. Measure the Specimen
Record actual width and thickness.
4. Set the Support Span
Calculate and set the required span.
5. Select the Load Cell and Fixture
Choose the load cell around the expected force and install the three-point bending fixture.
6. Calculate Test Speed
Determine crosshead speed from the specimen and required strain rate.
7. Run the Test
Apply the center load while continuously recording force and deflection.
8. Review and Report
Check the curve, calculate the required flexural properties, and record the test conditions.
What Information Should You Send an Equipment Supplier?
You do not need to know every machine specification before requesting a quotation.
It is more useful to provide information about the actual test.
Prepare:
Material
For example:
ABS / PC / PA / PP / PEEK / Reinforced Plastic / Composite
Specimen Dimensions
Provide:
Length × Width × Thickness
Expected Flexural Strength or Maximum Force
An approximate value is already useful.
Required Standard
For example:
ASTM D790
or:
ASTM D790 + ISO 178
Required Results
Do you need:
- Flexural strength?
- Flexural modulus?
- Stress-strain curve?
- Production Pass/Fail?
- Comparative QC data?
Deflection Measurement
Specify whether crosshead displacement is acceptable or a dedicated deflectometer is required.
Additional Tests
If the same machine must also perform tensile, compression, peel, or other mechanical tests, include those requirements from the beginning.
With this information, the supplier can select the machine capacity, load cell, fixture, and measurement method around the actual application.
Common ASTM D790 Equipment Purchasing Mistakes
Buying More Capacity Than the Test Needs
Higher capacity does not automatically improve ASTM D790 testing.
Estimate the actual specimen force first.
Ignoring the Load Cell
The test frame and load cell are not the same specification.
Ask which sensor will actually be used for your specimens.
Ordering the Machine Before Confirming the Fixture
Support span and loading geometry are part of the test.
Confirm specimen dimensions before finalizing the bending fixture.
Assuming One Speed Works for Every Specimen
Crosshead speed depends on the test configuration.
Do not automatically reuse a speed from another specimen.
Treating ASTM D790 and ISO 178 as Interchangeable
The same equipment may support both standards.
The test procedures remain different.
Focusing Only on Breaking Force
If modulus is required, initial-curve quality and deflection measurement become much more important.
Frequently Asked Questions
What is ASTM D790 used for?
ASTM D790 is used to evaluate the flexural properties of rigid and semi-rigid plastics, reinforced plastics, selected composites, and electrical insulating materials using three-point bending.
Is ASTM D790 a three-point bending test?
Yes. The specimen is supported at two points and loaded at the midpoint by one loading nose.
What is the ASTM D790 span-to-depth ratio?
A 16:1 span-to-depth ratio is commonly used for conventional specimens, but it is not universal. The required ratio should be confirmed from the applicable procedure and material specification.
What is the ASTM D790 test speed?
There is no single speed for every specimen. Crosshead speed depends on support span, specimen thickness, and the required outer-fiber strain rate.
What does ASTM D790 measure?
Depending on the material and procedure, results can include flexural stress, flexural strength, flexural strain, flexural modulus, offset yield strength, and load-deflection behavior.
Is a 5 kN universal testing machine enough for ASTM D790?
For many conventional plastic specimens, yes. The final capacity and load-cell selection should still be based on specimen dimensions, expected flexural strength, and predicted maximum force.
Do I need a deflectometer?
Not always. Crosshead displacement may be sufficient for routine QC. More demanding flexural modulus measurements may benefit from direct specimen deflection measurement.
Is ASTM D790 the same as ISO 178?
No. Both standards cover plastic flexural testing, but their technical requirements differ. The same testing machine may support both standards, but separate test methods should be used.
What is the difference between ASTM D790 and ASTM D6272?
ASTM D790 uses three-point bending, while ASTM D6272 uses four-point bending.
Can the same testing machine perform ASTM D638 and ASTM D790?
Yes, when the machine capacity and accessories are appropriate. Tensile grips can be used for ASTM D638, while a three-point bending fixture can be installed for ASTM D790.
Conclusion
ASTM D790 looks like a simple three-point bending test, but reliable results depend on much more than placing a specimen on two supports.
The complete test chain is:
Material → Specimen Dimensions → Support Span → Test Speed → Expected Force → Load Cell → Bending Fixture → Deflection Measurement → Data Analysis
For many common plastic specimens, the required force is relatively low. A properly configured low-force universal testing machine can therefore be more practical than selecting a much larger machine purely on maximum capacity.
The ITM-LAB RS-8010A, with capacities from 50 N to 5 kN, can be configured with an appropriate three-point bending fixture for conventional ASTM D790 plastic flexural testing while also supporting tensile, compression, peel, and other low-force mechanical tests.
When selecting equipment, start with the specimen and the required result.
Configure the testing machine around the test—not the test around the machine.
