Self-identifying
Load Cells
A low-range cell. Rigid foam in tension fails at a small fraction of what a solid plastic of the same section carries, so resolution at the bottom of the range decides whether the result is usable.
SpecificationsTesting standard
ASTM D1623
Standard Test Method for Tensile and Tensile Adhesion Properties of Rigid Cellular Plastics
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D1623 pulls rigid cellular plastic apart in tension. Load is introduced through the specimen's end faces rather than by gripping its sides, because foam cannot be clamped without being crushed. The method reports the tensile strength of the foam itself and, using a different specimen, the tensile adhesion strength of the foam to a substrate. The current edition is D1623-17(2023).
At a glance
- Test type
- Tensile — the specimen is pulled apart
- Published by
- ASTM
- Edition
- D1623-17(2023)
- Material
- Plastics, polymers & films
- Runs on
- Series 7200 and Series 9000
What the test does
A specimen of rigid cellular plastic is pulled apart in tension along one axis under controlled temperature, humidity and machine speed. Because foam cannot be gripped like a plastic bar without being crushed, the load is introduced through the specimen's end faces rather than through its sides: the ends are held or bonded to loading blocks, and the blocks are held by self-aligning grips so that the specimen sees axial tension and not bending. The method also runs a second, closely related test in which the foam is bonded to a substrate, and what fails is the interface rather than the foam.
What it measures, and why it matters
Two different things, which is why the title names both. The tensile test gives the strength of the cellular material itself in the direction pulled. The tensile adhesion test gives the strength of the bond between the foam and whatever it is stuck to — a sandwich facing, or a single substrate.
The failure mode carries as much information as the force. A break through the foam says the bond was stronger than the material and reports the foam's strength; a clean separation at the glue line says the joint was the weak point and reports the bond. Confusing the two turns a good result into a false one: an adhesion figure that is really a cohesive foam failure says nothing useful about the adhesive. Rigid foams are also strongly anisotropic, because the cells elongate in the rise direction during foaming, so a through-thickness figure and an in-plane figure from the same board are not interchangeable.
The three specimen types
Which type you use is decided by how much material you have and by which of the two properties in the title you are after. The method names all three by their preferred use rather than by a hierarchy.
- Type A
- The preferred specimen for tensile propertiesUsed wherever there is enough sample material to make it. This is the default for the strength of the foam itself.
- Type B
- For smaller available specimensPreferred where only smaller pieces can be obtained, as from a sandwich panel.
- Type C
- For tensile adhesionThe cellular plastic bonded to a substrate — between two facings, or to a single substrate.
- Preparation
- Machined to the standard shapeA lathe specimen cutter is named among the required apparatus, which tells you the geometry is expected to be cut on a machine, not sawn by hand.
- Conditioning
- Defined temperature and humidityThe test runs under the same conditions the specimens were conditioned in.
- Record the direction
- Rise direction, or in-planeDakFoam cells elongate as the material rises. A through-thickness figure and an in-plane figure from the same board are different properties.
- Inspect the cut faces
- Before bonding, every timeDakTorn or smeared cell walls are a stress raiser in a material that has almost nothing to give. Non-parallel faces load one edge first.
Exact dimensions and tolerances for the three types are tabulated in the standard itself, and the type used must be reported with the result — a Type B figure and a Type A figure on the same foam are not interchangeable.
Test conditions
The method fixes temperature, humidity and machine speed together, which is why its scope names all three. Two laboratories running the same foam under different conditions will disagree, and both will be right.
- Machine type
- Constant rate of crosshead movement
- Speed
- As specified in the method for the specimen usedTake it from the current edition rather than from a laboratory's habit; report the value used alongside the result.
- Atmosphere
- The defined temperature and humidity, held through the test
- Let the adhesive cure fully
- DakA bond still gaining strength is the weakest thing in the load path, and it will be what fails.
Calculations
The arithmetic is trivial. What the number means depends entirely on where the specimen broke, which is why the failure mode is recorded as part of the result rather than as an observation.
Tensile strengthσ
σ = P / A
- σ
- tensile or tensile adhesion strength, kPa or MPa
- P
- maximum force carried, N
- A
- cross-sectional area of the specimen, mm²
The same expression serves both properties. Which one it produced is decided by where the specimen failed, not by the equation.
Failure mode—
Cohesive in the foam, or adhesive at the interface
A break through the foam reports the material's strength. A clean separation at the glue line reports the bond. Recorded and reported, always.
How the test runs
- 01Choose the specimen type from what the sample allows and which property is wanted.
- 02Machine the specimens to the standard shape, keeping the loaded faces flat and parallel.
- 03Note and record the direction the specimen was taken relative to the foam rise.
- 04Bond or fit the specimen to the loading blocks, or to the substrate for Type C.
- 05Allow the adhesive to reach full strength before loading anything.
- 06Condition the assembled specimens at the specified temperature and humidity.
- 07Fit the assembly into self-aligning grips so that no bending is imposed on the specimen.
- 08Zero the force with the specimen in place and unloaded.
- 09Pull at the specified constant crosshead rate, recording force throughout.
- 10Record the maximum force and inspect the fracture surface immediately.
- 11Classify the failure as cohesive in the foam or adhesive at the interface.
- 12Report the mean, the standard deviation and the failure mode distribution across the set.
Adhesive wicking into open cells reinforces the foam next to the block and moves the break away from the region being measured. It looks like a valid cohesive failure and is not one.
The fixture this method needs
What the report has to contain
- Full designation and edition, and which specimen type was used
- Complete material identification, including density and cell structure where known
- Orientation of the specimen relative to the foam rise direction
- For Type C, the substrate, the adhesive and the bonding procedure
- Conditioning temperature, humidity and duration
- Crosshead speed used
- Cross-sectional area used in the calculation
- Maximum force and calculated strength for each specimen
- Failure mode for every specimen — cohesive in the foam, or adhesive at the interface
- Number of specimens, the mean and the standard deviation
- Any specimen discarded, with the reason
What the machine must be capable of
Low force, well resolved, and dead straight. Rigid foam in tension fails at a small fraction of what a solid plastic of the same section would carry, so the load cell must be chosen for the specimen and not for the frame's capacity — resolution at the bottom of the range is the whole game. The method specifies a constant-rate-of-crosshead-movement machine, self-aligning grips, and an extension indicator.
Self-aligning is not a preference here. Foam has almost no capacity to redistribute a bending stress, so any misalignment between the two loading blocks puts one edge of the specimen into tension well before the other and lowers the result without leaving evidence. Alignment, bond quality and cutting are the three things that decide whether the number means anything.
What goes wrong in practice
The bond failing when the foam was the subject, which is a wasted specimen rather than a low result. Adhesive wicking into open cells and locally reinforcing the material next to the block, so the break moves away from the region being measured. Misaligned blocks. Cutting the specimen from the wrong direction of the board, which measures anisotropy rather than the material. And reporting a strength without the failure mode, which leaves the reader unable to tell which of the two properties in the title was actually determined.
Where D1623 sits among the cellular and sandwich methods
Foam is far more often specified in compression than in tension, so D1623 usually appears beside a compression method rather than instead of one.
| ASTM D1623 | ASTM D1621 | ISO 844 | ASTM C297 | |
|---|---|---|---|---|
| Loading | Tension | Compression | Compression | Flatwise tension |
| Subject | Rigid cellular plastic, and its bond to a substrate | Rigid cellular plastic | Rigid cellular plastic | A whole sandwich construction |
| Specimen | Types A, B and C | Block specimen | Block specimen | Panel section bonded to loading blocks |
| Load introduced | Through bonded or held end faces | Between platens | Between platens | Through bonded loading blocks |
| Typical use | Adhesion and tensile strength of foam | Acceptance of insulation board | Acceptance of insulation board | Core-to-facing bond in a panel |
ASTM C297 asks the panel-level version of D1623's Type C question. Where the item under test is a finished sandwich rather than a foam sample, C297 is usually the method a specification means.
Questions we are asked about this test
What is ASTM D1623?
It is the ASTM test method for the tensile and tensile adhesion properties of rigid cellular plastics. A foam specimen of standard shape is pulled in tension under defined temperature, humidity and machine speed, either to measure the strength of the foam itself or, using a different specimen, the strength of its bond to a substrate. The current edition is D1623-17(2023).
What is the difference between tensile and tensile adhesion in this method?
The tensile test measures the strength of the cellular material along the axis pulled. The tensile adhesion test measures the strength of the interface between the foam and whatever it is bonded to — a sandwich facing or a single substrate. The Type C specimen is the one defined for adhesion. Which property the test actually produced is decided by where the specimen broke.
Which specimen type should I use?
Type A wherever there is enough sample material, which makes it the default for the foam's own tensile strength. Type B when only smaller pieces are available, such as from a sandwich panel. Type C when the tensile adhesion of the foam to a substrate is the point. The type used has to be reported, because results from different types are not interchangeable.
Why do the grips have to be self-aligning?
Because foam has almost no capacity to redistribute a bending stress. Any misalignment between the two loading blocks puts one edge of the specimen into tension before the other, and the specimen fails early at a load that looks like a genuine result. The method names self-aligning grips among its required apparatus for exactly that reason.
What if the bond fails instead of the foam?
Then the test has measured the bond, not the foam. If the foam's tensile strength was the objective, that specimen is discarded and the bonding procedure improved — a stronger adhesive, better surface preparation, or a longer cure. If tensile adhesion was the objective, an adhesive failure is the result you wanted and a cohesive foam failure is the one that tells you nothing about the adhesive.
Does the direction the specimen was cut matter?
Considerably. Rigid foams are anisotropic because the cells elongate along the rise direction during foaming, so a specimen cut through the thickness of a board and one cut in-plane measure different properties of the same material. The orientation must be recorded and reported, and figures from different orientations should never be averaged.
Is there an ISO equivalent to ASTM D1623?
ASTM notes that no ISO equivalent to this test method is known. ISO 1926, Rigid cellular plastics — Determination of tensile properties, is the nearest international document covering foam in tension, but it is not declared an equivalent and results should not be assumed to transfer between the two.
What machine capacity does ASTM D1623 need?
Very little capacity and a great deal of resolution. Rigid foam fails in tension at a small fraction of the load a solid plastic of the same section would carry, so the load cell is chosen for the specimen rather than for the frame. A small universal testing machine with a properly sized cell and a well-aligned load train covers this method comfortably.
Running ASTM D1623 on the Series 7200 and Series 9000
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Rigid foam in tension fails at a small fraction of what a solid plastic of the same section carries, so the useful working range is low — tens to a few hundreds of newtons on typical insulation-grade specimens. The cell is sized to the specimen, never to the frame. | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | not specified — the method references D638, D883 and E691 only, and names no force accuracy class | ISO 7500-1 Class 0.5, verified to ASTM E4, DIN 51221 and BS 1610 |
| Strain measurement | An extensometer to not specified — the method names an extension indicator without assigning a class, gauge length not specified — set by the specimen type, whose dimensions are tabulated in the standard | Certified to ISO 9513 Class 1 and ASTM E83 — non-contact video, clip-on and high-elongation |
| Gripping | Self-aligning grips holding loading blocks bonded or fitted to the specimen end faces; a lathe specimen cutter for preparing the geometry | Our a fixture built for this method, built to the specimen |
| Environment | Defined temperature and humidity, with the test run under the conditions used for conditioning | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.