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ASTM D624
Elastomer Tear Strength Test Fixture

Universal Grip ASTM D624 specimen cutter for universal testing machines

Downloads: Standard Test Method for Tear Strength of Conventional Vulcanized Rubber and Thermoplastic Elastomers - PDF · ASTM D624 Specimen Cutter – Universal Grip Data Sheet (PDF)

Cutting dies for ASTM D624: Die B · Die C (hardened or ribbon steel, with data sheets)

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

ASTM D624 is the standard test method for tear strength of conventional vulcanized rubber and thermoplastic elastomers. A die-cut specimen with a built-in stress concentration (a crescent, a 90° angle, a nick or a trouser slit) is pulled in a tensile machine until a tear starts and runs across it, and the force is divided by the specimen thickness to give tear strength in kN/m. Tire, seal, hose, belt, footwear, glove and medical-device labs run it alongside ASTM D412 tensile because a compound that is strong in tension can still fail by tearing from a cut or a molding flaw.

Most descriptions of D624 stop at Die C and 500 mm/min. This guide covers all five specimen geometries and what each one measures, the speeds, the thickness and conditioning rules, which specimens are nicked and which are not, how the dies must be made and kept, how to grip rubber without slipping, the calculation with a worked example and the median-of-three reporting rule, the precision you can expect, and how D624 maps to ISO 34-1. It also lists the dies, grips and machines Universal Grip supplies for the test.

In this guide: What ASTM D624 Measures · The Five Specimen Types: Dies A, B, C, T and CP · Which Die to Use · Nicking: Dies A and B Only · Cutting Dies and Die Condition · Test Speeds · Grips and Slippage · Conditioning · Step-by-Step Procedure · Calculation, Worked Example and the Median Rule · Precision: What Scatter to Expect · ASTM D624 vs ISO 34-1 and ISO 34-2 · Common Errors · Equipment for ASTM D624 · FAQ

What ASTM D624 Measures

Tear strength in D624 is defined as the force per unit thickness needed to rupture, initiate or propagate a tear in a specimen of defined geometry. Note per unit thickness, not per unit width: the specimen width is fixed by the die, and the tear runs through the thickness direction, so the thickness at the tear path is the only dimension that normalizes the force. Several published guides divide by width; that is wrong and gives numbers that cannot be compared with anything.

The standard separates three things that are often lumped together as tear strength:

  • Tear rupture (Die A): the force to rupture a nicked crescent specimen, where the nick removes the initiation stage and the result reflects resistance to tear growth at a high rate.

  • Tear initiation (Die C): the force to start a tear at an unnicked 90° angle, dominated by the compound's resistance to crack formation at a sharp corner.

  • Tear propagation (Dies B, T and CP): the force or energy to keep a tear running, measured from a nick (B) or along a defined path (T and CP) at a lower rate.

Because these are different properties, results from different dies are not interchangeable, and a specification that calls out tear strength without naming the die is incomplete. The method applies to vulcanized rubber and thermoplastic elastomers; tear of plastic film is ASTM D1938 and ASTM D1004, and tear of textiles and coated fabrics uses D2261 and D5587.

The Five Specimen Types: Dies A, B, C, T and CP

D624 defines five specimens. The dimensions below are nominal values from the figures in the standard; die makers hold them to the figure tolerances, and the die itself is the reference, so verify a new die against Figure 2 rather than against a catalog number.

  • Die A (nicked crescent): a crescent about 57 mm long with a narrow waist and short tabs, nicked 0.50 mm at the concave edge. The short tabs make it the hardest specimen to grip; it survives mostly in older specifications.

  • Die B (nicked crescent with tabs): the same crescent waist as Die A but with long, wide tabs, about 110 mm overall, nicked 0.50 mm. Introduced because Die A slips in the grips; results are not identical to Die A.

  • Die C (unnicked 90° angle): about 102 mm (4.0 in) long with 19 mm (0.75 in) wide tabs and a 90° apex on one edge. The apex of the die must be sharpened to a true corner, not a radius. The most common D624 specimen and the default in most rubber specifications.

  • Die T (trouser): a rectangular strip about 150 mm by 15 mm with a 40 ± 5 mm slit cut along the centre of one end, the last millimetre by razor blade. The two legs are gripped and pulled apart so the tear runs along the strip.

  • Die CP (constrained path): a molded specimen 125 mm long, 28.5 mm wide and 5 mm thick with a longitudinal groove that confines the tear to a defined path, started with a 60 ± 5 mm cut. Covered in the standard's non-mandatory appendix for high-extension compounds whose tears wander in a plain trouser specimen.

Thickness. For Dies A, B, C and T the standard calls for molded sheet 2.3 ± 1.0 mm (0.09 ± 0.04 in) thick; the older D624-98 wording gave the same range as 1.3 to 3.2 mm. The 2.0 ± 0.2 mm figure that circulates online is the ISO 34-1 requirement, not the ASTM one. Measure thickness per ASTM D3767: three readings across the width near the centre and one at the apex or nick, and use the median. For Die CP the groove geometry sets the tear path thickness, and the insert height of 3.60 mm is subtracted from the measured thickness.

Grain direction. Calendered and milled sheets tear differently along and across the grain. Cut specimens so the tear runs in a known direction, test both directions when the specification requires it, and report the direction; an unexplained bimodal data set is very often a mix of with-grain and cross-grain specimens.

Which Die to Use

If a material or customer specification names the die, that decides it. When it does not, the following table is the practical logic:

  • Die C for routine quality control, compound comparison and most specifications: unnicked, fast, easy to grip, and the one with the most published data.

  • Die B when a specification calls for a nicked specimen, when tear growth from an existing cut is the service concern, or when Die A slips out of the grips.

  • Die A only when an existing specification or a historical data set requires it.

  • Die T when you need tear propagation force or energy at a low rate, for example for research on filler systems or for compounds where initiation at a sharp corner is not representative of service.

  • Die CP for very soft or high-extension compounds whose tear leaves the intended path in a trouser specimen, so the tear stays on a defined line.

Never compare a Die C number with a Die B or Die T number from a different lab or data sheet. A typical carbon-black-filled compound may give 40 to 60 kN/m on Die C and under 20 kN/m on Die B because the nick removes the initiation stage; both numbers are correct for their die.

Nicking: Dies A and B Only

Specimens cut with Dies A and B must be nicked at the concave edge of the crescent with a cut 0.50 ± 0.05 mm (0.020 ± 0.002 in) deep, made with a sharp blade in a guided nicking fixture so the cut is perpendicular to the major axis of the specimen and square through the thickness. Wet the blade with water or soapy water so it cuts rather than drags, and verify the nick depth at 10× magnification or better to the nearest 0.025 mm; a nick that is 0.6 mm instead of 0.5 mm lowers the result measurably, and a ragged nick scatters it.

Die C and Die T specimens are not nicked in D624. Nicked angle specimens are ISO 34-1 Method B (with a 1.0 ± 0.2 mm nick), and some die suppliers sell a combined D624-C / ISO 34-B nicking jig; using it on an ASTM Die C specimen turns an initiation test into a propagation test and gives a far lower number. The trouser specimen is started with the 40 mm slit, not a nick; the last millimetre of the slit is finished with a razor so the tear begins from a sharp crack rather than a die-cut edge.

Timing matters: cut and nick specimens at least 3 hours after they reach room temperature, nick after any aging or conditioning, and test within 24 hours of nicking. The nick tip blunts as the rubber relaxes, and a nick made a week before testing is not the nick the standard describes.

Cutting Dies and Die Condition

The die is a measuring instrument in this test. A dull or burred edge leaves a rough cut surface with micro-tears along the specimen edge, each of which competes with the apex or nick to start the tear; the result is a low and scattered tear strength that has nothing to do with the compound. D624 requires that the cutting edge be sharp, free of nicks and honed, that the inside faces of the die be polished for at least 5 mm back from the edge, that the walls be perpendicular to the cutting plane, and that the specimen be cut in a single stroke on a hard, flat surface such as a leather, cardboard or plastic block.

  • Cut with a die press or arbor press in one smooth stroke; a hammer blow or a second stroke doubles the edge and shifts the apex.

  • Inspect the Die C apex under magnification; a rounded apex is the most common cause of a sudden rise in Die C results, because the stress concentration is gone.

  • Resharpen with a die sharpening tool when the cut edge shows fuzz or pull-out, and keep a reference compound to check the die after sharpening.

  • Lubricating the die lightly with water or a soap solution helps on tacky compounds; wipe the specimen afterwards.

  • Segmented Die C tooling must keep at least 25 mm of straight cutting edge on each side of the apex.

Universal Grip stocks ASTM D624 cutting dies in Die A, B, C and T geometries, nicking fixtures for Dies A and B, and presses for them; the dies are hardened, ground and honed to the figure dimensions of the standard.

Test Speeds

  • Dies A, B and C: 500 ± 50 mm/min (20 ± 2 in/min) crosshead speed.

  • Dies T and CP: 50 ± 5 mm/min (2 ± 0.2 in/min).

The ten-fold difference is deliberate. Rupture and initiation tests run fast so that the tear propagates quickly once it starts and the maximum force is a clean peak. Propagation tests run slowly so that the force trace along the tear path can be read and averaged. Running a trouser specimen at 500 mm/min gives a noisy, rate-inflated trace that cannot be reduced correctly, and running Die C at 50 mm/min gives lower values than any published data because tear strength of most elastomers rises with rate. The machine must hold the speed under load; a small tester that slows as the force rises needs its speed checked at the expected load, and the force measurement must be accurate to within 2 % per ASTM D412, whose machine requirements D624 adopts.

Grips and Slippage

Rubber specimens pull out of ordinary grips. The specimen thins under load, the gripping force drops, and the specimen creeps out of the jaws, which shows on the load trace as a plateau or a slow fall instead of a sharp rupture. D624 therefore requires self-tightening grips that apply uniform pressure across the width, with the specimen held within its parallel tab portion and inserted symmetrically so the apex or nick sits centred between the jaws.

  • Pneumatic grips: constant air pressure maintains the clamping force as the tabs thin. The preferred grip for Dies B, C and T in most labs; set the pressure high enough to prevent creep but not so high that the jaws cut the tabs.

  • Roller grips (eccentric roller): self-tightening by geometry, so clamping force rises with the pull. Good for soft compounds and for Die A's short tabs, and the usual choice on small testers without an air supply.

  • Vise grips with rubber-faced jaws: adequate for harder compounds at low force; the clamping force is operator-dependent, so expect more scatter.

  • Jaw faces: 25 mm wide for Dies A, B, C and CP and 30 mm for Die T, so the jaw does not overhang the tab or grip only part of it. Flat rubber-faced or lightly serrated faces for thin, soft sheet; diamond-serrated faces for thick, hard compounds.

  • Alignment: line the top and bottom tab edges up with the jaw edges so the gauge section is the same on every specimen, and make sure the apex of a Die C specimen points sideways, not toward a jaw.

For the trouser specimen, grip one leg in each jaw with the legs bent back so the uncut portion hangs vertically between them; the legs must be long enough to grip without the jaw touching the tear front. Our tensile grips and elastomer testing page cover grip and jaw options for every rubber specimen, and our adapters fit them to Instron, MTS, ZwickRoell, Shimadzu, Mark-10 and other frames.

Conditioning

  • Test at 23 ± 2 °C (73.4 ± 3.6 °F). Humidity is controlled at 50 ± 5 % RH for at least 24 hours only when the compound is known to be moisture-sensitive.

  • Allow at least 16 hours between vulcanization and testing, and at least 3 hours at 23 ± 2 °C before cutting or nicking.

  • Test within 24 hours of nicking and within 72 hours of buffing, if specimens were buffed to thickness.

  • Protect specimens from light during storage; ozone and UV crack the surface and give false low tear values.

  • For tests at other temperatures, condition per ASTM D1349 with at least 3 hours of soak at test temperature in an environmental chamber and test inside it; a rubber specimen returns to room temperature within a minute of removal.

Step-by-Step Procedure

  • 1. Cut at least three specimens per sample with the specified die in a single press stroke, noting grain direction. Inspect edges; discard any with roughness or voids.

  • 2. Nick Die A and B specimens 0.50 ± 0.05 mm at the centre of the concave edge in a guided nicking fixture and check the depth under magnification. Do not nick Die C or T. For Die T, finish the 40 mm slit with a razor.

  • 3. Measure thickness with a D3767 gauge: three readings across the width near the centre and one at the apex or nick; record the median.

  • 4. Mount the specimen symmetrically in self-tightening grips, gripping only the tabs, with jaw edges aligned to the tab edges and the apex or nick centred.

  • 5. Set speed: 500 mm/min for A, B and C; 50 mm/min for T and CP.

  • 6. Run until the specimen tears through completely. Record the maximum force for A, B and C and the full force trace for T and CP.

  • 7. Inspect the tear path. For Die C the tear must start at the apex; for A and B at the nick; for T and CP along the intended path. A tear that starts at a jaw, an edge flaw or leaves the path invalidates the specimen; replace it.

  • 8. Calculate Ts for each specimen and report the median of three (see below).

Calculation, Worked Example and the Median Rule

Ts = F ÷ d, where Ts is tear strength in kN/m (numerically equal to N/mm), F is the force in newtons and d is the median thickness in millimetres. For Dies A, B and C, F is the maximum force. For Dies T and CP, F is the mean or median force over the propagation portion of the trace, and the standard allows several reductions (mean of the trace, median of peaks and troughs, or peak and trough values separately); state which one you used. Round Ts to the nearest 0.1 kN/m. Inch-pound results in lbf/in convert to kN/m by multiplying by 0.175.

Worked example (Die C). A Die C specimen with thickness readings of 2.08, 2.11, 2.10 mm across the width and 2.10 mm at the apex has a median thickness of 2.10 mm. It tears at a maximum force of 85.0 N. Ts = 85.0 N ÷ 2.10 mm = 40.5 kN/m (or 40.5 N/mm; 231 lbf/in). Two more specimens give 38.9 and 42.7 kN/m. The median of the three is 40.5 kN/m, and all values lie within 20 % of it, so 40.5 kN/m is the reported tear strength.

The median-of-three rule. D624 reports the median of three specimens, not the mean, because tear data are skewed by occasional low values from edge flaws. If any of the three values differs from the median by more than 20 %, test two more specimens and report the median of five. For referee tests, five specimens are the minimum. For Die T, a smooth trace may be reduced to a median force; a trace with distinct stick-slip peaks is usually reported as both the mean peak and the mean trough, which tells the reader whether the tear ran steadily or in jumps.

Typical values. Unfilled gum compounds tear at a few kN/m on Die C; general-purpose filled compounds at 20 to 60 kN/m; high-tear compounds such as reinforced natural rubber, polyurethane and some silicones at 60 to well over 100 kN/m. On a 2.3 mm specimen that corresponds to forces of roughly 10 to 300 N, which is why a 500 N to 2 kN load cell suits almost every D624 test.

Precision: What Scatter to Expect

D624 includes an interlaboratory precision statement, and it is sobering. For Die B the repeatability (within one lab) is about 14 to 18 % and the reproducibility (between labs) about 58 to 86 %; for Die C, repeatability 8 to 24 % and reproducibility 30 to 66 %. The standard's own wording is that repeatability is not good and reproducibility is poor. Two consequences follow. A 10 % difference between two compounds tested in one lab on the same day may be real; the same difference between two labs is not evidence of anything. And a specification limit on tear strength needs a margin that reflects the method, not just the compound.

Most of the within-lab scatter comes from the specimen, not the machine: die sharpness, nick depth, grain direction, thickness measurement and grip slippage. Controlling those five items is the entire difference between a lab at the good end of the precision range and one at the bad end.

ASTM D624 vs ISO 34-1 and ISO 34-2

ISO 34-1 covers the same property with different nomenclature, and mixing the two is a common source of errors. ISO 34-1 Method A is the trouser specimen, Method B the angle specimen, run either unnicked or with a 1.0 ± 0.2 mm nick, and Method C the crescent specimen with a 1.0 mm nick. Speeds are 100 mm/min for the trouser test and 500 mm/min for angle and crescent, and the sheet thickness is 2.0 ± 0.2 mm. ISO 34-2 is the small-specimen (Delft) method for limited material.

  • D624 Die C ≈ ISO 34-1 Method B unnicked, at the same 500 mm/min but on a thinner (2.0 mm) sheet.

  • D624 Die B ≈ ISO 34-1 Method C, but with a 0.50 mm nick instead of 1.0 mm, so values differ.

  • D624 Die T ≈ ISO 34-1 Method A, but at 50 mm/min instead of 100 mm/min.

  • ISO 34-1 Method B nicked has no D624 equivalent; a nicked Die C is not an ASTM test.

Because the nick depths, thicknesses and rates differ, do not treat ISO and ASTM values as interchangeable even for the matching geometries. Report the standard, the method or die, and the nick condition with every result.

Common Errors

  • Dividing force by width instead of thickness.

  • Nicking a Die C specimen, or running an unnicked Die B.

  • Dull or burred die edge, or a rounded Die C apex, starting competing tears at the specimen edge.

  • Nick depth not verified; a 0.6 mm nick on Die B lowers the result.

  • Running Dies A, B or C at 50 mm/min or the trouser at 500 mm/min.

  • Grip slippage in non-self-tightening grips, visible as a plateau in the trace.

  • Jaw faces wider than the tab, or the specimen gripped beyond the parallel tab section.

  • Thickness measured away from the apex or nick, or only once.

  • Reporting the mean instead of the median, or not re-testing when a value is more than 20 % from the median.

  • Specimens tested within 16 hours of vulcanization or more than 24 hours after nicking.

  • Mixing grain directions in one data set.

Equipment for ASTM D624

See the elastomers testing page for a full rubber lab configuration, and the Mark-10 D624 setup article for a small-tester example.

ASTM D624 FAQ

What is ASTM D624?

The ASTM standard test method for tear strength of vulcanized rubber and thermoplastic elastomers. A die-cut specimen with a stress concentration is pulled in a tensile tester and the maximum (or propagation) force divided by the thickness gives tear strength in kN/m.

What are the ASTM D624 specimen types?

Five: Die A (nicked crescent), Die B (nicked crescent with tabs), Die C (unnicked 90° angle), Die T (trouser with a 40 mm slit) and Die CP (molded constrained-path specimen). Die C is the most common.

What is the test speed for ASTM D624?

500 ± 50 mm/min (20 in/min) for Dies A, B and C, and 50 ± 5 mm/min (2 in/min) for Dies T and CP.

How thick should an ASTM D624 specimen be?

Molded sheet 2.3 ± 1.0 mm (0.09 ± 0.04 in) thick for Dies A, B, C and T. The 2.0 ± 0.2 mm figure belongs to ISO 34-1. Die CP is a 5 mm molded specimen.

Is the Die C specimen nicked in ASTM D624?

No. Only Dies A and B are nicked, 0.50 ± 0.05 mm deep. Die C is an unnicked tear-initiation test; a nicked angle specimen is ISO 34-1 Method B, not ASTM D624.

How is tear strength calculated in ASTM D624?

Ts = F ÷ d, with F the maximum force (Dies A, B, C) or the mean or median propagation force (T and CP) in newtons and d the median thickness in millimetres, giving kN/m. Multiply lbf/in by 0.175 for kN/m.

How many specimens does ASTM D624 require?

Three per sample, reported as the median. If any value is more than 20 % from the median, test two more and report the median of five. Referee tests use at least five.

What grips are used for ASTM D624?

Self-tightening grips that hold the tabs with uniform pressure: pneumatic grips or eccentric roller grips with 25 mm faces (30 mm for Die T), rubber-faced for thin soft sheet and serrated for thick hard compounds.

What are typical ASTM D624 tear strength values?

Roughly 20 to 60 kN/m on Die C for general-purpose filled compounds, under 10 kN/m for unfilled gum stocks, and 60 to over 100 kN/m for high-tear natural rubber, polyurethane and reinforced silicone. Die B values are lower because of the nick.

What is the conditioning for ASTM D624?

Test at 23 ± 2 °C, at least 16 hours after vulcanization and 3 hours after reaching room temperature, within 24 hours of nicking. Humidity control (50 ± 5 % RH for 24 hours) applies only to moisture-sensitive compounds. Other temperatures follow ASTM D1349.

What is the difference between ASTM D624 and ISO 34-1?

ISO 34-1 Method A is the trouser test at 100 mm/min, Method B the angle test (unnicked or with a 1.0 mm nick) and Method C the crescent with a 1.0 mm nick, on 2.0 ± 0.2 mm sheet. D624 uses a 0.50 mm nick on crescents, runs the trouser at 50 mm/min and uses 2.3 ± 1.0 mm sheet, so values are not interchangeable.

How repeatable is ASTM D624?

Not very. The standard's interlaboratory data give within-lab repeatability of about 8 to 24 % and between-lab reproducibility of 30 to 86 % depending on the die. Die sharpness, nick depth, grain direction and grip slippage drive most of the scatter.

Need a fixture, grips or a complete system for ASTM D624? Contact Universal Grip with your specimen details, capacity and machine connection and we will quote the right setup.

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