ISO 1798
Flexible Cellular Polymers: Tensile Strength and Elongation

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 24, 2026

ISO 1798 is the international standard for the tensile strength and elongation at break of flexible cellular polymeric materials: polyurethane, latex, polyethylene and other soft foams used in seating, mattresses, automotive trim, packaging, gaskets and medical products. A dumbbell test piece 10 to 15 mm thick is stretched at 500 mm/min until it breaks, and the maximum force and the gauge length at break give a tensile strength in kilopascals and an elongation in percent. It is the ISO counterpart of ASTM D3574 Test E, and the automotive foam specifications of most European makers cite it.

This guide covers the current edition and its status; the Type 1 and Type 1A test pieces with the narrow widths the calculation actually uses; the thickness, gauge-mark and cutting rules; the 72-hour and 16-hour conditioning timeline; the apparatus classes, the preload and the rule on extensometer mass; the step-by-step procedure with the five-valid-breaks requirement; the calculation with a worked example; the common errors; a numeric comparison with ASTM D3574 Test E; and the grips, dies, extensometers and machines Universal Grip supplies, including why foam needs a different clamping approach from rubber.

In this guide: What ISO 1798 Measures · Edition and Status: ISO 1798:2008, Confirmed · Test Pieces: Type 1 and Type 1A · Apparatus: Machine, Force, Grips and Extensometer · Conditioning: the 72-Hour and 16-Hour Rules · Step-by-Step Procedure · Calculation and Worked Example · Common Errors and Their Effect · ISO 1798 vs ASTM D3574 Test E · Equipment for ISO 1798: Grips, Dies, Extensometers and Machines · FAQ

What ISO 1798 Measures

ISO 1798 determines two properties of flexible cellular materials by extending a test piece at a constant rate until it ruptures: the tensile strength, the maximum tensile stress reached before rupture, expressed in kPa on the original cross-section; and the elongation at break, the increase in gauge length at rupture as a percentage of the original gauge length. Neither a modulus nor a yield point is defined, because foam has no linear region and does not yield; the stress-strain curve rises continuously to the break.

The values are low by the standards of solid materials. A 30 kg/m³ polyurethane seating foam has a tensile strength of roughly 80 to 150 kPa and an elongation at break of 100 to 250 %; a high-resilience or viscoelastic foam can be 60 to 300 kPa; cross-linked polyethylene packaging foam 200 to 800 kPa. On a 13 mm wide, 12 mm thick test piece those stresses correspond to maximum forces of 10 to 125 N, which sets the load-cell range and makes the clamping force, not the machine capacity, the engineering problem.

The standard applies to flexible cellular materials only. Rigid cellular plastics are tested under ISO 1926; solid rubber under ISO 37; foam tear strength under ISO 8067, which uses the trouser and angle dies of ISO 34-1. Our foam testing page lists the compression, indentation and fatigue methods that usually accompany ISO 1798 in a foam laboratory.

Edition and Status: ISO 1798:2008, Confirmed

The current edition is ISO 1798:2008, the fourth edition, published by ISO/TC 45/SC 4. It was reviewed and confirmed in 2021 and remains current; a 2008 date on a standard does not mean it is obsolete, only that it has not needed technical change. The European adoption EN ISO 1798:2008 (DIN EN ISO 1798, BS EN ISO 1798) is identical. The 2008 edition replaced the 1997 edition and its 2003 corrigendum; its main technical change was the addition of the smaller Type 1A test piece alongside the original Type 1, with an informative annex comparing results from the two.

The standard references four others that a laboratory needs on hand: ISO 1923 for measuring the dimensions of cellular materials under a defined contact pressure, ISO 7500-1 for the force verification of the testing machine, ISO 9513 for extensometer calibration and ISO 23529 for conditioning and standard laboratory conditions.

Test Pieces: Type 1 and Type 1A

Both test pieces are dumbbells cut with a sharp die of the shape in the standard's Figure 1, with a rectangular cross-section, with or without the moulded skin, and free of visible defects. The dimension that enters the calculation is the width of the narrow (central) portion; the tab width and overall length only have to accommodate the grips.

• Type 1: overall length 152 mm, tab width 25 mm, narrow portion width 13 mm. Gauge marks 25 to 50 mm apart, placed symmetrically in the narrow portion.

• Type 1A: overall length 120 mm, tab width 25 mm, narrow portion width 10 mm. Gauge marks up to 40 mm apart. The smaller piece suits moulded parts and small samples and, because the narrow portion is shorter, tends to give fewer breaks outside the gauge length.

• Thickness: 10 to 15 mm for both types, cut from the slab or moulding with the test axis at right angles to any predominant cell orientation (the rise direction in slab foam). If the material does not allow this, the sampling direction and location are reported.

Cutting. Foam dumbbells are die-cut in a single stroke on a press, or cut with a band knife where thick sections are needed. The cut must be square to the surface and the edges clean; a ragged or compressed edge starts the break early. Our die press with an ISO 1798 die cuts Type 1 and 1A pieces from slabs up to 15 mm; for moulded foam parts the die is pressed through the skin on one side, and whether the skin is left on is reported.

Measuring. Width and thickness of foam depend on how hard the gauge presses, so the standard requires ISO 1923, which measures under a defined, very light contact pressure. Thickness is measured at five evenly spaced points along the narrow portion; if any reading differs from the others by more than 2 %, the piece is rejected. The cross-section used in the calculation is the mean thickness multiplied by the nominal narrow width, 13 mm for Type 1 and 10 mm for Type 1A.

Gauge marks. Two parallel ink marks, perpendicular to the axis and measured to within 1 %, are placed on the narrow portion: 25 to 50 mm apart on Type 1, up to 40 mm on Type 1A. Choosing the longer gauge on Type 1 reduces the number of breaks that fall outside the marks and improves the resolution of the elongation measurement, which must be made to ±1.25 mm at break. The sample marking guide covers inks and marking jigs that work on open-cell foam.

Apparatus: Machine, Force, Grips and Extensometer

• Machine and speed: a tensile machine with a grip separation rate of 500 ± 50 mm/min, uniform through the test. Foams stretch to 100 to 300 %, so a Type 1 piece with 25 mm tabs and a 50 mm gauge needs around 250 mm of crosshead travel per test; check that the frame has it with the grips fitted.

• Force measurement: Class 0.5 or Class 1 of ISO 7500-1, with the maximum force read to within 1 %. Because the forces are 10 to 125 N, a 100 N, 250 N or 500 N load cell is needed; a 5 kN cell reading 25 N is at 0.5 % of range and outside its verified band.

• Grips: not specified in detail by the standard beyond holding the piece symmetrically without slipping, which is where foam testing goes wrong. Foam has no resistance to clamping: a jaw pressure that merely holds rubber crushes foam to a fraction of its thickness, cuts it at the jaw edge and causes a break at the grip rather than in the gauge length. The grips must open at least 20 mm, close evenly, and apply a controlled, low pressure over a wide flat face. The equipment section below compares the options.

• Extensometer: optional. The gauge length at break may be read from the marks by eye or by a mechanical or optical extensometer calibrated to ISO 9513. If a contact extensometer is used, its mass and inertia must not change the measured tensile strength by more than 1 %, which in practice rules out spring-loaded clip-on devices on soft foam; the usual choices are a video extensometer or a lightweight sensor-arm long-travel device. Crosshead travel is not an acceptable substitute for gauge-mark elongation, because the dumbbell neck is not of uniform section.

Conditioning: the 72-Hour and 16-Hour Rules

Foam continues to cure and to change its cell structure for days after manufacture, so ISO 1798 is stricter on time than on climate. No test is made earlier than 72 hours after manufacture, unless it has been shown that results at 16 or 48 hours fall within ±10 % of the 72-hour mean, in which case the shorter time may be used. For production quality control, with the customer's agreement, pieces may be tested not less than 12 hours after manufacture after 6 hours of conditioning.

Before testing, the test pieces are conditioned for at least 16 hours at one of the two ISO 23529 standard climates: 23 ± 2 °C and 50 ± 5 % relative humidity, or 27 ± 2 °C and 65 ± 5 % for tropical countries. The test itself is run at the conditioning temperature and humidity, since humidity affects the cell walls of polyether and polyester polyurethanes. Keep the pieces free of load and distortion during conditioning: a stack of dumbbells compresses the lower ones and changes their thickness.

Step-by-Step Procedure

• 1. Wait 72 hours after manufacture (or the agreed shorter time) and condition the cut pieces 16 hours at 23 ± 2 °C / 50 ± 5 % RH.

• 2. Measure thickness at five points along the narrow portion under ISO 1923 contact pressure; reject pieces with any reading more than 2 % from the others; record the mean. Confirm the nominal narrow width of 13 mm (Type 1) or 10 mm (Type 1A).

• 3. Apply two gauge marks 25 to 50 mm apart (Type 1) or up to 40 mm (Type 1A), centred on the narrow portion, and measure the distance to within 1 %.

• 4. Fit a load cell such that the expected maximum force is within its verified range; set 500 ± 50 mm/min; set the grip pressure or screw torque to the value established for the foam.

• 5. Zero the force. Mount the piece symmetrically with the tabs fully in the jaws and the axis in line with the load. Apply no more than a 0.1 kPa preload or 0.5 % extension to take out slack, then zero the extensometer or note the gauge mark distance.

• 6. Run at 500 mm/min to rupture. Record the maximum force to within 1 % and the gauge-mark distance immediately before break to within ±1.25 mm.

• 7. Reject any piece that breaks outside the gauge marks, at the jaw or from a visible defect, and test further pieces until five valid breaks within the gauge length have been obtained.

• 8. Calculate tensile strength and elongation at break for each valid piece and report the median of each, with the test-piece type, thickness, skin condition, sampling direction and conditioning.

Calculation and Worked Example

Tensile strength in kilopascals is TS = 10³ × F / A, where F is the maximum force in newtons and A is the mean initial cross-section in square millimetres (mean thickness × 13 mm for Type 1, × 10 mm for Type 1A). The factor 10³ converts N/mm², which is MPa, to kPa. Elongation at break is Eb = 100 × (L − L0) / L0, where L0 is the initial gauge length and L the gauge length immediately before rupture, both in millimetres. The reported result is the median of the five valid pieces for each property.

Worked example. A Type 1 piece of polyurethane seating foam has thickness readings of 12.1, 12.0, 11.9, 12.0 and 12.0 mm (all within 2 %), mean 12.0 mm. A = 12.0 × 13 = 156 mm². Gauge marks are 50.0 mm apart. The maximum force is 25.0 N and the gauge length just before break is 130 mm. TS = 10³ × 25.0 / 156 = 160 kPa. Eb = 100 × (130 − 50) / 50 = 160 %. If the same foam were cut as Type 1A at the same thickness, A = 12.0 × 10 = 120 mm², and a maximum force of about 19 N would give the same 160 kPa; the stress is the property, the force is not.

Resolution check. With a 500 N load cell of Class 1, the 1 % requirement is met at 25 N only if the cell is verified down to that force, which is typically 2 % of range; a 100 N cell reading 25 N at 25 % of range is the safer choice. On the elongation side, ±1.25 mm on an 80 mm extension is ±1.6 % of the elongation value, so a longer gauge on Type 1 improves the result.

Common Errors and Their Effect

• Over-clamping. Pneumatic grips at the pressure used for rubber crush foam at the jaw line and cause grip breaks; every grip break is a rejected test. Reduce the regulated pressure until the piece holds without slipping, usually 0.5 to 1.5 bar on a 25 mm face, or use screw grips set by feel.

• Using the tab width in the calculation. The cross-section uses the narrow width, 13 or 10 mm, not the 25 mm tab. Using 25 mm underestimates the strength by nearly half.

• Thickness under too much gauge pressure. A standard thickness gauge compresses foam by 10 to 30 %; a 12 mm piece read as 10 mm gives a result 20 % high. Use the ISO 1923 contact pressure.

• Elongation from crosshead travel. The tabs and shoulders stretch too, so crosshead elongation overstates the gauge-length elongation, often by 30 % or more, and the standard requires gauge marks or an extensometer.

• Contact extensometer on soft foam. A clip-on with spring-loaded knife edges deforms the neck locally and lowers the measured strength; the standard limits the extensometer's effect on strength to 1 %.

• Testing too soon. Foam tested at 24 hours can read 5 to 15 % different from the same foam at 72 hours; the 72-hour rule exists for this reason.

• Wrong load cell. A 5 kN cell reading 25 N is outside its verified range; the force may be in error by several percent and the noise can hide the true maximum.

• Fewer than five valid breaks. Three pieces with two jaw breaks do not meet the standard; test until five breaks fall between the gauge marks.

• Cell orientation ignored. Slab foam is anisotropic in the rise direction; pieces cut parallel and perpendicular to rise can differ by 20 to 40 % in strength. Cut at right angles to the rise direction unless the specification says otherwise, and report it.

ISO 1798 vs ASTM D3574 Test E

ASTM D3574 collects the flexible-foam test methods in one standard, and its Test E is the tensile test that corresponds to ISO 1798. The principle and the speed are the same, but the test pieces, conditioning and specimen count differ, and the two standards state that results are not directly comparable.

• Test piece: ISO 1798 Type 1 (152 mm, 13 mm narrow width) or Type 1A (120 mm, 10 mm narrow width), 10 to 15 mm thick. D3574 Test E uses a die-cut dumbbell of its own figure, nominally 140 mm long, from sheet 12.5 ± 1.5 mm thick.

• Speed: 500 ± 50 mm/min in both.

• Number of pieces: ISO 1798 requires five valid breaks within the gauge length; D3574 Test E tests three specimens.

• Conditioning: ISO 1798 72 hours after manufacture, then 16 hours at 23 ± 2 °C / 50 ± 5 % RH; D3574 allows testing 12 hours after manufacture with its own conditioning period at the same climate.

• Units: kPa and % in both; D3574 also reports psi (1 psi = 6.895 kPa).

• Tear companion: ISO 8067 (trouser and angle dies shared with ISO 34-1) for ISO, D3574 Test F for ASTM.

A laboratory serving both European and North American customers normally runs both with one set of grips and one low-force machine, keeping separate dies. The foam testing page covers the D3574 compression, indentation and fatigue tests we equip alongside Test E.

Equipment for ISO 1798: Grips, Dies, Extensometers and Machines

The test is undemanding on force and demanding on handling. The right equipment holds a 12 mm slab of foam without crushing it, cuts a clean dumbbell, measures 80 mm of extension without touching the piece, and reads 25 N accurately. Universal Grip supplies each element and configures them as a set.

• Grips for foam. Vise grips with wide, flat, smooth or lightly serrated faces and a 20 mm or larger opening are the traditional ISO 1798 grip: the operator sets the clamp by feel so the foam is held, not crushed, and the flat face spreads the pressure over the whole tab. Pneumatic grips give the same clamp every time and free the operator's hands, but must be run at a low regulated pressure, typically 0.5 to 1.5 bar, with flat or rubber faces; at rubber-testing pressures they cut the foam at the jaw. Pinch grips with a wide flat jaw suit thin or skinned foam. Self-tightening roller grips hold without a preset clamp and are a good choice for stiff, high-density or cross-linked foams where the force is higher.

• Cutting dies and press. ISO 1798 Type 1 and Type 1A dies in hardened or ribbon steel, cut on our die press; a band-knife slitter for bringing slab to 10 to 15 mm. The same press cuts D3574 Test E and ISO 8067 pieces.

• Extensometers. A video extensometer is the ideal instrument for foam: it reads the ink gauge marks without touching the piece, follows 300 % extension, and reports the gauge length at the instant of break to well inside the ±1.25 mm requirement. Our long-travel XL extensometers are the lightweight contact alternative where video is not available. Standard clip-on extensometers are not suitable for soft foam.

• Machines. A Mark-10 test frame with a 100 N or 250 N load cell, 500 mm/min and enough travel runs ISO 1798 on a bench; a 2.5 kN universal testing machine with a low-capacity cell adds ISO 7500-1 Class 0.5 force accuracy, software with median reporting and the capacity for the D3574 compression tests on the same frame. Check crosshead travel: 250 mm per test for Type 1 with a 50 mm gauge.

• Thickness gauge. An ISO 1923 gauge with a large foot and a defined light contact pressure; an ordinary dial gauge reads foam thin.

Send us the foam type and density, the thickness you cut, and the machine the grips must fit, and we will quote grips with the right face and pressure range, the die, an extensometer and a load cell as one package. The tensile grip selection guide explains the clamping choices in more depth.

ISO 1798 FAQ

What is ISO 1798?

ISO 1798 is the international standard for the tensile strength and elongation at break of flexible cellular polymeric materials (soft foams). A Type 1 or Type 1A dumbbell 10 to 15 mm thick is stretched at 500 mm/min to rupture; tensile strength is reported in kPa and elongation in percent, as the median of five valid breaks.

Is ISO 1798:2008 still current?

Yes. The 2008 fourth edition was reviewed and confirmed in 2021 and remains the current edition; EN ISO 1798:2008 is identical.

What are the ISO 1798 test piece dimensions?

Type 1: 152 mm long, 25 mm tabs, 13 mm narrow width, gauge marks 25 to 50 mm apart. Type 1A: 120 mm long, 25 mm tabs, 10 mm narrow width, gauge marks up to 40 mm apart. Both are 10 to 15 mm thick.

What is the test speed for ISO 1798?

500 ± 50 mm/min grip separation rate, the same as ASTM D3574 Test E.

How is tensile strength calculated in ISO 1798?

TS (kPa) = 10³ × F / A, with F the maximum force in N and A the mean thickness times the narrow width (13 mm for Type 1, 10 mm for Type 1A) in mm². Example: 25.0 N on 12.0 × 13 mm = 156 mm² gives 160 kPa.

How is elongation at break calculated in ISO 1798?

Eb (%) = 100 × (L − L0) / L0, where L0 is the initial gauge-mark distance and L the distance immediately before rupture, measured to ±1.25 mm. A 50 mm gauge stretching to 130 mm gives 160 %.

What is the conditioning for ISO 1798?

No testing before 72 hours after manufacture (unless 16 or 48 hour results are shown to be within ±10 % of the 72 hour mean), then at least 16 hours at 23 ± 2 °C and 50 ± 5 % RH (or 27 °C / 65 %). A QC option allows testing 12 hours after manufacture with 6 hours of conditioning.

How many test pieces does ISO 1798 require?

Enough to obtain five breaks within the gauge length. Pieces that break at the jaw or outside the marks are rejected and replaced.

What grips are used for foam tensile testing?

Wide-faced vise grips set by feel, or pneumatic grips at a low regulated pressure of about 0.5 to 1.5 bar with flat or rubber faces, opening 20 mm or more. Rubber-testing clamp pressures crush and cut foam and cause grip breaks.

Can I use a clip-on extensometer for ISO 1798?

Not normally. The standard limits a contact extensometer's effect on tensile strength to 1 %, and spring-loaded clip-ons deform soft foam. Use a video extensometer, a lightweight long-travel device, or read the gauge marks directly; crosshead travel is not acceptable.

What is the difference between ISO 1798 and ASTM D3574 Test E?

Same speed (500 mm/min) and units, but different dumbbells (ISO Type 1/1A with 13/10 mm narrow width, 10 to 15 mm thick; D3574 its own die at 12.5 ± 1.5 mm), five valid breaks versus three specimens, and 72 hours versus 12 hours after manufacture. Results are not directly comparable.

What load cell is needed for ISO 1798?

Maximum forces are typically 10 to 125 N on a Type 1 piece, so a 100 N to 500 N load cell of ISO 7500-1 Class 0.5 or 1 is right; a 5 kN cell reading 25 N is outside its verified range.

Need grips, cutting dies, an extensometer or a complete low-force system for ISO 1798 foam tensile testing? Contact Universal Grip with your foam type, thickness and machine connection and we will quote the right setup.