Compression Testing Guide

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 17, 2026

Compression testing pushes a specimen between two platens or inside a fixture while a universal testing machine records force and deformation. From the curve come compressive strength, yield strength, modulus, and for foams and rubbers the force at a fixed deflection and the set that remains afterward. It is the test that qualifies rigid plastics, composite laminates, foams, seals and gaskets, wood products, concrete and masonry, packaging and catalyst pellets, and it needs more care in fixturing than tension does, because a column in compression buckles, brooms or crushes under the platen long before the material itself fails unless the geometry and alignment are right.

This guide covers the compression methods Universal Grip equips most often, with the specimen, speed and result each one calls for, the platens and compression fixtures that run them, how to size the machine and load cell, how to measure strain in compression, the alignment and safety details that decide whether a result is valid, and the mistakes that produce low or scattered numbers. Each standard has its own detailed guide linked in its section; this page is the map.

In this guide: Who This Guide Is For · What a Compression Test Measures · Rigid Plastics: ASTM D695 and ISO 604 · Composites: D6641, D3410, D6484 and D7137 · Foams: ASTM D1621 and D3574 · Rubber and Sponge: ASTM D395, D575 and D1056 · Sandwich Panels, Wood, Masonry and Packaging · Configuring a Machine for Compression Testing · Specimen Preparation and Conditioning · Common Mistakes · Recommended Equipment Packages · FAQ

Who This Guide Is For

Compression testing is spread across more industries than any other mechanical test. Plastics compounders certify rigid resins to ASTM D695. Aerospace and automotive composite shops run combined-loading compression (D6641), shear-loaded compression (D3410), open-hole compression (D6484) and compression after impact (D7137) for allowables and quality control. Foam producers run D1621 on rigid foam and D3574 on flexible foam. Rubber and gasket labs run D395 compression set and D575 compression properties, and sponge-rubber producers D1056. Sandwich-panel makers run C364 edgewise compression. Wood-product labs run compression parallel and perpendicular to grain (D143, D198, D1037). Brick and masonry labs run C67. Packaging labs run box and edge crush. Catalyst and pharmaceutical labs crush pellets and tablets one at a time.

What they share is a universal testing machine with a compression-rated load cell, a pair of platens or a method-specific fixture, and a way to measure deformation that is not the crosshead. The sections below are organized by material because the fixture follows the material; the machine sizing and strain-measurement sections apply to all of them.

What a Compression Test Measures

Compressive strength: the maximum stress carried before fracture or, for materials that do not fracture, the stress at a defined strain or deformation (10 % for rigid foam in D1621, 5 % offset or a specified strain in other methods).
Compressive yield strength: the first point at which stress stops rising with strain, or an offset yield, for ductile plastics and metals.
Compressive modulus: the slope of the initial linear portion of the stress-strain curve, which needs an extensometer or compressometer on the specimen, not crosshead displacement.
Force at a given deflection: compression force deflection (CFD) for foams and sponge rubber, and indentation force deflection (IFD) for cushioning, reported as force to reach 25 % or 50 % deflection.
Compression set: the permanent deformation left after a specimen has been held compressed for a set time and temperature, reported as a percentage, the key durability property for seals and gaskets.
Crush strength and energy: the peak load and the area under the curve for honeycomb, boxes, pellets and cellular materials.

Because compression specimens are short and stiff, machine compliance is a larger share of the measured displacement than in a tensile test. Strength values are fine from the load cell alone; anything reported as modulus or strain must come from a device on the specimen or between the platens, or be corrected for the compliance of the load train. Our clip-on extensometers and compressometers and the video extensometers cover both routes.

Rigid Plastics: ASTM D695 and ISO 604

ASTM D695

ASTM D695 is the compressive properties test for rigid plastics. Strength is measured on a prism 12.7 × 12.7 × 25.4 mm or a cylinder 12.7 mm in diameter by 25.4 mm long (a 2:1 length-to-width ratio); modulus needs a longer specimen with a slenderness ratio of 11 to 16, which for 12.7 mm stock means about 50.8 mm long. The crosshead runs at 1.3 ± 0.3 mm/min, the specimen ends must be flat and parallel to within 0.025 mm, at least five specimens are tested, and the specimens are conditioned per D618. Sheet thinner than 3.2 mm is tested as a waisted specimen in the D695 support jig, which prevents buckling while leaving the gauge length free. Our D695 fixture is a subpress with hardened, parallel platens that keeps the load axial on a short specimen, plus the support jig for thin sheet; the complete guide covers the formulas, the modulus specimen and the worked example.

ISO 604

ISO 604 is the international equivalent, with separate specimens for modulus (50 × 10 × 4 mm at 1 mm/min) and strength (10 × 10 × 4 mm) and a chord modulus between 0.05 % and 0.25 % strain. Values are close to D695 but not interchangeable; a lab serving both markets normally keeps both specimen sets and one subpress.

Composites: D6641, D3410, D6484 and D7137

Composite laminates fail in compression at loads that depend as much on how the load is introduced as on the material, which is why there are four main methods and each has its own fixture. All four run on a stiff benchtop or floor-standing frame, usually at 1.3 mm/min, with strain from back-to-back gauges or a video extensometer, and all require the specimen to be machined flat, parallel and square.

ASTM D6641, combined loading compression (CLC): an untabbed 140 × 13 mm specimen is clamped in a four-block fixture that introduces the load partly by shear through the clamp faces and partly by end loading. The simplest of the composite compression methods to run and the one most labs now use for strength and modulus; our CLC fixture is covered on the compression fixtures page.
ASTM D3410, shear loading (wedge fixture): the load enters entirely by shear through wedge grips in a heavy fixture, for tabbed or untabbed specimens with a short unsupported gauge length. Preferred when end crushing is a problem or when a specification requires it.
ASTM D6484, open-hole compression (OHC): a laminate with a central hole is end-loaded in a support fixture to measure the notched compressive strength that governs bolted-joint design.
ASTM D7137, compression after impact (CAI): a plate that was first impacted per D7136 is compressed edgewise in a fixture with knife-edge side supports to measure residual strength. Note that D7136 itself is the drop-weight impact test, not a compression method.

Loads are high: a unidirectional carbon specimen in CLC can exceed 50 kN, so these fixtures live on 100 to 250 kN frames with alignment verified. For the shear and tensile companions, see the composite test fixture buyer's guide.

Foams: ASTM D1621 and D3574

ASTM D1621, rigid cellular plastics

ASTM D1621 compresses a block of rigid foam (polyurethane, polystyrene, PVC and structural core foams) between parallel platens at a speed proportional to its thickness, 2.5 mm/min for each 25 mm of specimen thickness, and reports compressive strength at yield or at 10 % deformation, whichever comes first, together with modulus from the initial slope. Specimens are at least 25.8 cm² in cross-section with the thickness as supplied, and deformation is taken from a deflectometer or the crosshead with the machine compliance corrected. Spherically seated upper platens are the norm so that a slightly out-of-parallel block is loaded evenly.

ASTM D3574, flexible cellular materials

ASTM D3574 is a suite of tests for flexible polyurethane foam. The compression tests are Test C, compression force deflection (the force to compress a 50 × 50 mm or larger specimen to 50 % of its thickness, read after the dwell the standard specifies), Test B1 and B2, indentation force deflection (a 323 cm² circular indentor pressed into a large cushion to 25 % and 65 % deflection), and Test D, compression set. These need large, flat platens or the IFD indentor, a low-capacity load cell (foam forces are tens to hundreds of newtons) and a frame with enough daylight for a 100 mm thick cushion. Our foam testing page covers the full suite.

Rubber and Sponge: ASTM D395, D575 and D1056

ASTM D395, compression set

ASTM D395 measures how much of a compression a rubber specimen fails to recover. Method B, the common one, clamps Type 1 discs (12.5 mm thick × 29 mm diameter) or Type 2 discs (6 × 13 mm) between plates with spacers that hold a 25 % deflection for 22 or 70 hours at the test temperature, typically 70 °C or higher, then measures thickness after 30 minutes of recovery. Method A applies a constant 1.8 kN spring force instead. Our compression set fixtures hold several specimens per plate; Method A can be run on a tester with platens. The complete guide gives the formulas and the Method C variation.

ASTM D575, compression properties of rubber

ASTM D575 compresses a cylindrical rubber specimen between platens on a testing machine and reports either the force at a given deflection (Method A) or the deflection at a given force (Method B), the properties a gasket or mount designer needs. The platen faces must be polished and parallel, and the specimen is cycled before the recorded run.

ASTM D1056, sponge and expanded rubber

ASTM D1056 grades cellular rubber by compression deflection, the force to compress a specimen to 25 % of its thickness, into its classification grades, alongside compression set and other properties. The same flat platens used for D3574 run it, at a load cell capacity to suit the grade.

Sandwich Panels, Wood, Masonry and Packaging

ASTM C364, edgewise compression of sandwich construction: a panel section is loaded on its edges between platens with the facings supported to measure the compressive strength of the facings and the core's ability to stabilize them. Flatwise compression of the core alone is C365, which uses plain platens on a core block.
Wood, ASTM D143, D198 and D1037: compression parallel and perpendicular to grain on small clear specimens, full-size members and wood-based panels; spherically seated platens and a safety cage are standard, and the full setup is on our wood testing guide.
ASTM C67, brick and structural clay tile: compressive strength of capped brick units between platens on a frame of 250 kN and up, with a spherically seated upper platen.
ASTM E9, metals: compression of short, medium and long metal specimens with a subpress or alignment fixture; the method for metals compressive yield and modulus.
ASTM C39, concrete cylinders: compressive strength of capped cylinders on high-capacity compression frames, normally a dedicated concrete machine.
Packaging, TAPPI T 811 and T 839 and box compression: edge crush of corrugated board and compression of filled or empty boxes between large platens; these use wide platens and low to moderate forces.
Catalyst pellets, tablets and ceramics: single-pellet crush strength on a small platen set or our catalyst crush fixture, reported as force per pellet, with a sensitive load cell.

Configuring a Machine for Compression Testing

Capacity and load cell

Choose the frame and the load cell separately. The frame must take the highest-load test you run; the load cell must resolve the lowest. A plastics and foam lab is served by a benchtop universal testing machine of 5 to 50 kN with two load cells, one at the frame capacity for rigid plastics and composites and a 500 N or 1 kN cell for foam and sponge. Composite CLC, wood, brick and concrete move up to floor-standing machines of 100 to 2,000 kN. Keep every result between about 1 % and 90 % of the cell's range, and prefer a load cell rated for compression as well as tension; most are, but check the certificate.

Speed

Compression speeds are slow and defined by the method: 1.3 mm/min for D695, D6641 and most composite methods, 2.5 mm/min per 25 mm of thickness for D1621, and 50 mm/min or more for foam CFD and IFD. The machine must hold the set speed as the load rises, which is a matter of frame stiffness and drive power at the top of the range; a frame near its capacity slows down under load and the recorded modulus drifts.

Platens and fixtures

Fixed platens for specimens that are machined flat and parallel (D695 prisms in a subpress, composite fixtures, capped cylinders).
Spherically seated upper platens for anything that may be slightly out of parallel: foam blocks, wood, brick, sponge, rubber cylinders. The seat lets the platen settle on the specimen at first contact and then locks under load. Our compression platens come in both styles up to 1,000 kN.
Method-specific fixtures where the standard draws one: the D695 subpress and thin-sheet jig, the CLC and shear-loading fixtures, OHC and CAI fixtures, compression set plates, the catalyst crush fixture and fiber compression fixtures, all on the compression fixtures page.
Platen size: larger than the specimen with margin on every side; for IFD the standard indentor, for box compression platens that cover the whole box.

Strain and deflection measurement

Strength needs only the load cell. Modulus and any deflection-based result need one of: a compressometer or clip-on extensometer mounted on the specimen, an LVDT or deflectometer measuring platen-to-platen travel, or a video extensometer tracking marks on the specimen or the platen edges. Crosshead displacement alone includes the compliance of the frame, load cell, platens and fixture, which on a 25 mm plastic specimen can be larger than the specimen's own deformation. If crosshead data must be used, run a compliance calibration with a steel block and subtract it. For composites, back-to-back strain gauges or a two-sided video measurement also check for bending, which the methods limit.

Alignment

Compression is unforgiving of misalignment. The two platen faces must be parallel and centered on the load axis; a subpress or alignment fixture guarantees this independently of the machine's crosshead. Verify with a soft-metal or pressure-paper check between the platens, and for composite allowables with a strain-gauged alignment specimen per ASTM E1012. Short specimens of ductile material barrel, long ones buckle; keep to the slenderness the method allows and center the specimen on the platen every time, with a stop or template rather than by eye.

Safety

Brittle materials in compression fail explosively: ceramics, brick, wood, glass-filled plastics and composites throw fragments. Use a polycarbonate or Lexan safety enclosure on the frame, keep the test area clear, and set a load or displacement limit in the software so the crosshead stops after fracture rather than driving on into the platens.

Specimen Preparation and Conditioning

Flat, parallel, square ends. For D695 the ends must be parallel within 0.025 mm; for most methods the practical rule is that the ends are machined, not sawn, and checked with a square. An out-of-square end loads one corner first and the specimen fails in bending.
Correct length-to-width ratio. Strength specimens are short (2:1 for D695); modulus specimens are longer (slenderness 11 to 16 for D695) but still short enough not to buckle. Follow the method's dimensions rather than what is convenient to cut.
Measure the cross-section on each specimen with a micrometer; strength is force divided by original area, and a 0.1 mm error on 12.7 mm is 1.6 % in stress.
Condition per the method: D618 (23 ± 2 °C, 50 ± 10 % RH, 40 hours) for plastics and foams unless the material specification says otherwise; D5229 for composites, including hot-wet states; D1349 standard temperatures for rubber. Test in an environmental chamber when the specification calls for temperature, with the platens at temperature as well as the specimen.
Cap or face brittle specimens (concrete, brick, some ceramics) so the platen bears evenly; use the capping or pad method the standard specifies.
Pre-cycle rubber and foam where the method requires it (D575, D3574) so the recorded run is on a conditioned specimen.

Common Mistakes

Modulus or strain reported from crosshead displacement with no compliance correction.
Fixed platens used on a specimen that is not parallel, so one edge carries the load.
Specimen too long for its width, failing by buckling and reported as compressive strength.
Load cell far too large for the material (a 100 kN cell on foam), so the result is noise.
Speed set as a fixed value instead of scaled to thickness (D1621) or as the method defines.
Composite specimens tabbed or untabbed contrary to the method, or end-crushing reported as strength.
Compression set fixtures assembled slowly, so the specimens are in the oven late, or measured before the 30 minute recovery.
Test run to a fixed displacement without a stop limit, driving the platens together after a brittle fracture.
Mixing D695 and ISO 604 values, or D6641 and D3410 values, as if they were the same test.

Recommended Equipment Packages

Plastics and foam lab: a 5 to 25 kN benchtop UTM, a second low-capacity load cell, fixed and spherically seated platens, the D695 subpress and thin-sheet jig, large foam platens and an IFD indentor, a compressometer or video extensometer, and compression set fixtures with an oven.
Composites lab: a 100 to 250 kN frame with alignment verified, CLC and shear-loading fixtures, OHC and CAI fixtures, strain gauging or two-camera video strain, an environmental chamber for hot-wet work, and the composite fixture guide as a starting point.
Rubber and gasket lab: a 2.5 to 10 kN frame with polished platens for D575, compression set plates for D395, cutting dies for discs, and an oven; see the elastomer testing guide.
Wood, masonry and construction lab: a 100 to 2,000 kN floor-standing machine, large spherically seated platens, a safety enclosure and a deflectometer; see the wood testing guide.
Any lab: adapters that fit the platens and fixtures to your existing Instron, MTS, ZwickRoell, Shimadzu, Tinius Olsen, Mark-10 or Galdabini frame, so a compression upgrade does not require a new machine.

Related application guides: tensile testing, bend testing and shear testing.

Compression Testing FAQ

What is a compression test?

A test in which a specimen is pushed between two platens or inside a fixture on a universal testing machine while force and deformation are recorded. It gives compressive strength, yield strength, modulus, force at a given deflection, and compression set, depending on the method.

What equipment do I need for compression testing?

A universal testing machine with a compression-rated load cell sized to the material, platens or the fixture the standard specifies, and a deflectometer, compressometer or video extensometer if modulus or strain is reported. A safety enclosure is advisable for brittle materials.

What is the difference between fixed and spherically seated platens?

Fixed platens are rigidly mounted and suit machined, parallel specimens. A spherically seated platen pivots on a ball seat to settle on a specimen that is slightly out of parallel, then locks under load, so foams, wood, brick and rubber are loaded evenly.

Can I use crosshead displacement for compressive modulus?

Not directly. Crosshead displacement includes the compliance of the frame, load cell and fixture, which on a short, stiff specimen can exceed the specimen's own deformation. Use an extensometer, compressometer, deflectometer or video system on the specimen, or correct the crosshead data with a measured compliance curve.

What speed is used for compression testing?

It depends on the method: 1.3 mm/min for ASTM D695 and D6641, 2.5 mm/min per 25 mm of thickness for D1621 rigid foam, and 50 mm/min or more for flexible foam CFD and IFD. Always use the rate in the standard you are reporting.

What is the ASTM D695 specimen size?

12.7 × 12.7 × 25.4 mm prisms or 12.7 mm diameter × 25.4 mm cylinders for strength, and a longer specimen with a slenderness ratio of 11 to 16 (about 50.8 mm for 12.7 mm stock) for modulus. Sheet under 3.2 mm thick uses a waisted specimen in a support jig.

Which compression test is used for composites?

ASTM D6641 (combined loading compression) is the most common for strength and modulus; D3410 (shear loading) is the alternative; D6484 covers open-hole compression and D7137 compression after impact. Each has its own fixture.

How is compression set measured?

Per ASTM D395 Method B, rubber discs are held at 25 % deflection between plates with spacers for 22 or 70 hours at the test temperature, allowed to recover for 30 minutes, and the thickness not recovered is reported as a percentage of the original deflection.

What load cell capacity do I need for foam?

Foam and sponge compress at tens to a few hundred newtons, so a 500 N or 1 kN load cell is appropriate even on a 10 kN frame. Rigid plastics need 5 to 50 kN; composites and wood 100 kN and up.

Can compression fixtures be fitted to my existing machine?

Yes. Platens, subpresses, composite fixtures and compression set plates are supplied with adapters for Instron, MTS, ZwickRoell, Shimadzu, Tinius Olsen, Mark-10, Galdabini and other frames.

Need platens, a compression fixture or a complete compression testing system? Contact Universal Grip with the standard, specimen size, expected load and your machine, and we will quote the right setup.