ISO 15024
Mode I Interlaminar Fracture Toughness (DCB)

Downloads: ISO 15024 Mode I DCB Load Block Clevis Set – Universal Grip Data Sheet (PDF)

Written by Chris Thomas, Universal Grip Company · Reviewed by Frank Bertini · Updated October 27, 2026

ISO 15024 is the international test method for the mode I interlaminar fracture toughness, GIC, of unidirectional fiber-reinforced plastic composites. A double cantilever beam (DCB) specimen with a thin starter film at its mid-plane is pulled open at one end, and the force, opening displacement and delamination length are used to calculate the energy needed to grow a delamination, in J/m².

Delamination is the weak link of laminated composites, so GIC is a core property in resin and prepreg development, aerospace material qualification and damage-tolerance analysis. Universal Grip makes a mode I DCB load block clevis set for ISO 15024 (part ref. UGC-ISO15024-TF): clevises that accept bonded load blocks so the arms rotate freely, with quick-release pins and a compact layout that keeps the crack front visible. This guide covers the specimen, loading hardware, precracking, data reduction by CBT and MCC with a worked example, common errors and the differences from ASTM D5528.

In this guide: What ISO 15024 Measures · Current Edition and Scope · Specimen Dimensions and Plate Preparation · Load Blocks, Hinges and Fixtures · Test Procedure Step by Step · Initiation Criteria: NL, VIS and 5 %/MAX · Calculating GIC: CBT and MCC · Interpreting Results and Common Errors · ISO 15024 vs ASTM D5528 and Related Standards · Equipment for ISO 15024 from Universal Grip · FAQ

What ISO 15024 Measures

ISO 15024 measures GIC, the critical strain energy release rate for a delamination growing under pure opening (mode I) load. The standard defines it as the resistance to initiation and propagation of a delamination under mode I loading, expressed in J/m². Physically, it is the energy dissipated per unit of new crack area as the two plies are pried apart.

The test produces two kinds of result. Initiation values describe the start of growth, either from the starter film (insert) or from a short mode I precrack. Propagation values are measured as the delamination grows, and plotted against delamination length they form a resistance curve (R-curve). In unidirectional laminates the R-curve usually rises because fibers bridge the crack behind its tip, so propagation values are often well above initiation values.

Note: Engineers often use the initiation value from the insert as the conservative design number, because fiber bridging seen in a 0° DCB specimen may not occur in a multidirectional structural laminate.

Current Edition and Scope

The current edition is ISO 15024:2023 (second edition, published 27 February 2023, prepared by ISO/TC 61/SC 13, Composites and reinforcement fibres). It cancels and replaces ISO 15024:2001. The main technical change is a third way of loading the DCB specimen: a flat insert hinge (Figure 1 c and Annex D), alongside the established load blocks and piano hinges.

• Materials: unidirectionally reinforced fiber-reinforced plastics, carbon, glass and other fibers, with thermoset or thermoplastic matrices

• Lay-up: limited to 0° unidirectional lay-ups (Annex B.1 explains why multidirectional lay-ups cause crack migration and unreliable values)

• Loading: displacement control at a constant rate, with three permitted load-introduction methods

• Result: GIC at initiation (NL, VIS and 5 %/MAX points) and during propagation (PROP points), in J/m²

Specimen Dimensions and Plate Preparation

Specimens are cut from flat plates molded per ISO 1268 with an even number of unidirectional plies and a non-adhesive insert film placed at the mid-thickness during lay-up. Recommended plate thickness is 3 mm for about 60 % by volume carbon fiber and 5 mm for about 60 % by volume glass fiber, so that the arms stay elastic and deflections stay moderate.

Preferred specimen (Table 1)

• Width b: 20 mm ± 0.5 mm (carbon and glass)

• Length l: at least 125 mm

• Thickness 2h: 3 mm ± 0.1 mm for carbon fiber, 5 mm ± 0.1 mm for glass fiber

• Uniformity: thickness and width kept within ± 1 % of the specimen mean

• Alternative specimens: other thicknesses using the guidance in Annex B.3, and widths from 15 mm to 30 mm; shorter specimens are not recommended

Starter film (insert)

• Film thickness: no more than 13 µm, because thicker films leave a blunt resin-rich pocket that raises the initiation value

• PTFE film: for epoxies cured below 180 °C

• Polyimide film: for cure temperatures above 180 °C; coat it with release agent and bake twice for 30 min at 130 °C before use

• Insert tip position: at least 45 mm from the near ends of the load blocks or hinges, so the crack starts away from the load-introduction stiffening

• Record the insert length on both edges; note in the report if the two edges differ by more than 1 mm

Measurement and marking

Measure width at three evenly spaced points and thickness at three points on the centerline plus two near-edge points at mid-length. Most labs coat one or both edges with a thin white brittle layer so the crack tip is easy to see. Mark the edges every 5 mm to at least 55 mm beyond the insert tip, with the first 10 mm and the last 5 mm marked every 1 mm.

Number of specimens and conditioning

Test at least five specimens; specimens that fail the validity checks are discarded and replaced. Dry specimens after bonding the load blocks or hinges, using the temperature and time recommended by the resin supplier, store them in a desiccator for no more than 24 h, and test them at 23 °C ± 2 °C per ISO 291. The dry condition is recommended for baseline data; hot/wet testing needs an environmental chamber.

Load Blocks, Hinges and Fixtures

The opening force is introduced through hardware bonded to the two arms at the insert end. ISO 15024:2023 shows three arrangements:

• Load blocks (Figure 1 a): metal blocks bonded to each arm and pinned into clevises; blocks at least as wide as the specimen, with block length l3 no more than 15 mm and the pin hole at the center of l3

• Piano hinges (Figure 1 b): hinges bonded to the arm surfaces, which rotate with negligible stiffening of the arms

• Flat insert hinges (Figure 1 c, Annex D): the new option in the 2023 edition

Annex A (normative) sets the surface preparation and bonding of blocks and hinges. A debonded block ends the test. Prepare both bonding surfaces carefully, choose an adhesive whose cure temperature will not affect the laminate, and align the blocks squarely with the specimen axis. Because load blocks stiffen the arms close to the load line and change the moment arm as they tilt, their geometry enters the calculation through the correction factor N.

Testing machine and measuring tools

• Force measurement: ISO 7500-1 Class 1

• Displacement measurement: ISO 9513 Class 2 over the range used

• Speed: constant displacement rate held within ± 20 %

• Width and thickness: micrometer and calipers to the resolutions in clause 5.3

• Crack length: optional travelling microscope with no more than ×70 magnification, readable to 0.05 mm, or a camera system

DCB forces are low (typically tens of newtons up to a few hundred newtons for 3 mm carbon specimens), so use a low-capacity load cell on an electromechanical frame for good force resolution, and keep the crack front in clear view.

Test Procedure Step by Step

ISO 15024 runs in two loadings: an initial loading from the insert, which also creates a natural mode I precrack, and a re-loading from that precrack, during which propagation values are taken.

Initial loading (from the insert)

• 1. Mount the specimen with the pins through the load blocks, the arms horizontal and aligned with the load axis, and zero force and displacement

• 2. Load in displacement control at a constant crosshead rate in the low mm/min range; 1 mm/min is the slowest rate ISO 15024 allows and is widely used

• 3. Watch the edge for the first movement of the delamination from the insert tip and mark that point (VIS) on the load-displacement record

• 4. As soon as the delamination has grown a short increment from the insert, stop and unload the specimen at a constant rate

• 5. Mark the precrack tip on both edges

Re-loading (from the precrack)

• 6. Re-load at the same constant rate

• 7. Record VIS again at the first visible growth from the precrack

• 8. As the delamination passes each marked increment, mark the load-displacement record (PROP points): every 1 mm for the first 10 mm, then every 5 mm, then every 1 mm for the last 5 mm

• 9. Continue until the delamination reaches the end of the marked region, then unload and record the final crack length

• 10. Measure the crack length on both edges; skip any points where the crack arrested and jumped

The standard permits a wedge-driven precrack only in prescribed circumstances and does not recommend it, because a wedge can produce a mixed-mode or damaged crack tip.

Initiation Criteria: NL, VIS and 5 %/MAX

Initiation values of GIC are reported from three points on the load-displacement curve, both for the insert and for the precrack:

• NL (non-linear): the point where the load-displacement trace first deviates from linearity. It is usually the lowest and most conservative value, and corresponds to growth starting in the center of the specimen width

• VIS (visual): the point at which the delamination is first seen to move at the specimen edge, observed with a microscope or camera

• 5 %/MAX: whichever comes first of the point where compliance has increased by 5 % over the initial compliance C0, or the maximum load point

NL is sensitive to the curve-fitting method and to noise; VIS depends on the operator and the edge coating; 5 %/MAX is objective but tends to give the highest initiation value. Reporting all three gives a fair picture, and the insert values are usually treated as the reference initiation result.

Calculating GIC: CBT and MCC

ISO 15024 gives two data reduction methods. Both use, at each initiation and PROP point, the force P (N), the load-point displacement δ (mm), the delamination length a (mm, measured from the load line), the width b (mm) and the compliance C = δ/P (mm/N). Values in N/mm are multiplied by 1,000 to give J/m².

Method A: corrected beam theory (CBT)

• GIC = 3Pδ / [2b(a + |Δ|)] × F/N

• Δ: crack length correction for the root rotation of the arms; find it by plotting the cube root of compliance against delamination length for the PROP points and taking the x-axis intercept of the straight-line fit

• F (large displacement correction) = 1 − (3/10)(δ/a)² − (3/2)(δ·l1/a²)

• N (load block correction) = 1 − (l2/a)³ − (9/8)[1 − (l2/a)²](δ·l1/a²) − (9/35)(δ/a)²

• l1: distance from the pin center to the mid-plane of the arm; l2: distance from the pin center to the edge of the load block

Method B: modified compliance calibration (MCC)

• Plot a/(2h) against C^(1/3) for the PROP points and take the slope A1

• GIC = 3P²C^(2/3) / [2·A1·b·(2h)], with 2h the specimen thickness

CBT also yields a back-calculated flexural modulus, E1f = 64(a + |Δ|)³P / [δ·b·(2h)³]. It should stay nearly constant along the crack; a strong trend with crack length points to a measurement or crack-length reading problem. CBT and MCC usually agree closely, and simple beam theory without the Δ correction overestimates GIC.

Worked example (CBT)

• Carbon/epoxy DCB, b = 20.0 mm, 2h = 3.0 mm, load blocks with l1 = 6.0 mm and l2 = 7.5 mm

• At a PROP point: P = 50.0 N, δ = 5.0 mm, a = 55.0 mm; regression gives Δ = 3.0 mm

• Compliance C = 5.0 / 50.0 = 0.100 mm/N

• Uncorrected GIC = 3 × 50 × 5 / [2 × 20 × (55 + 3)] = 750 / 2,320 = 0.3233 N/mm = 323.3 J/m²

• F = 1 − 0.3 × (5/55)² − 1.5 × (5 × 6 / 55²) = 1 − 0.00248 − 0.01488 = 0.98264

• N = 1 − (7.5/55)³ − 1.125 × [1 − (7.5/55)²] × 0.009917 − 0.2571 × (5/55)² = 1 − 0.00254 − 0.01095 − 0.00213 = 0.98439

• F/N = 0.99823, so corrected GIC = 323.3 × 0.99823 = 322.7 J/m²

The corrections are small at this short crack length and modest opening, but they grow as δ/a rises; with tough materials and long cracks they can change GIC by several percent.

Interpreting Results and Common Errors

Report the initiation values (NL, VIS, 5 %/MAX) from the insert and from the precrack, the propagation values, and the R-curve of GIC against delamination length for each specimen, plus mean, standard deviation and coefficient of variation across at least five specimens. As a rough guide, brittle carbon/epoxy systems sit at a few hundred J/m², toughened epoxies run higher, and thermoplastic-matrix composites can reach well over 1,000 J/m²; always compare within one method and initiation criterion.

Common errors

• Thick or creased insert film: a blunt, resin-rich starter raises the insert initiation values

• Load block debonding or slipping: a sudden load drop unrelated to crack growth invalidates the test; follow Annex A bonding practice

• Crack misreading: the edge crack tip can lag or lead the center of the width; use a thin brittle edge coating and good magnification, and check both edges

• Crack migrating out of the mid-plane: a sign of a non-0° lay-up, ply drop or asymmetric plate, outside the standard's scope

• Stick-slip growth: arrest points are excluded from the PROP set

• Arm plasticity or very large deflections: the F correction grows and results become questionable; thicker specimens help

• Using machine crosshead without allowing for frame and load-train compliance on stiff, low-load tests

• Comparing values from different reduction methods, initiation criteria or standards as if they were the same number

ISO 15024 vs ASTM D5528 and Related Standards

The ASTM counterpart is ASTM D5528, mode I interlaminar fracture toughness of unidirectional fiber-reinforced polymer matrix composites. Both grew out of the same international round-robin work on the DCB specimen, so the hardware and specimen are interchangeable:

• Specimen: both use specimens at least 125 mm long with an insert film no thicker than 13 µm; ISO 15024 prefers 20 mm width with 3 mm (carbon) or 5 mm (glass) thickness, while D5528 allows 20 mm to 25 mm wide and typically 3 mm to 5 mm thick, with a starter crack length of about 50 mm

• Data reduction: ISO 15024 uses corrected beam theory (CBT) and modified compliance calibration (MCC); D5528 offers modified beam theory (MBT, the same root-rotation correction as CBT), compliance calibration (CC) and MCC

• Loading hardware: both accept load blocks or piano hinges; ISO 15024:2023 adds the flat insert hinge

• Number of specimens: at least five in both

• Reporting: check which initiation criterion and data reduction method your customer specification calls for, because different editions and programs report different values

Related tests: mixed-mode I/II delamination by ASTM D6671, interlaminar shear strength by short beam per ISO 14130 or ASTM D2344, flexural properties per ISO 14125, and compression per ISO 14126. Together these give a full picture of a laminate's out-of-plane and interlaminar performance.

Equipment for ISO 15024 from Universal Grip

The Universal Grip ISO 15024 Tensile Fixture, Mode I DCB Load Block Clevis Set (part ref. UGC-ISO15024-TF), is a clevis fixture set for mode I double cantilever beam testing of composites. Load blocks bonded to the specimen arms are pinned into upper and lower clevises, so the arms rotate freely as they are pulled apart and the delamination grows under pure opening mode. Per the data sheet:

• Test type: opening-mode tension via load blocks, for unidirectional DCB specimens with bonded load blocks

• Load capacity: 1 kN standard; higher capacities on request

• Materials: stainless steel clevises; steel or aluminium load blocks, supplied in sizes to suit specimen width

• Connection: pinned clevis connection to the load blocks, with knurled quick-release pins for fast specimen loading

• Compact design leaves the crack front visible for length measurement

• Machine connection: pin with knurled retainer, or threaded adapter to suit your frame

• Compatible frames: electromechanical and servo-hydraulic universal test machines

• Applications: carbon and glass fiber laminates, toughened resin development, aerospace composite qualification and delamination research

When ordering, specify specimen width, load block size and pin diameter, required capacity and machine connection. A benchtop universal testing machine with a low-capacity load cell suits most DCB work; add a video extensometer or camera for crack-length tracking, software that tags PROP points against the load-displacement curve, and adapters for your load train. Contact us for a quote.

ISO 15024 FAQ

What is the current edition of ISO 15024?

The current edition is ISO 15024:2023, the second edition, published 27 February 2023 by ISO/TC 61/SC 13. It replaced ISO 15024:2001 and added a flat insert hinge as a third loading option alongside load blocks and piano hinges.

What does GIC mean?

GIC is the critical strain energy release rate in mode I, the opening mode. In ISO 15024 it is the resistance to initiation and propagation of a delamination under opening load, reported in J/m².

What are the ISO 15024 specimen dimensions?

The preferred DCB specimen is 20 mm ± 0.5 mm wide and at least 125 mm long, 3 mm ± 0.1 mm thick for carbon fiber or 5 mm ± 0.1 mm thick for glass fiber, with an even number of unidirectional plies. Widths from 15 mm to 30 mm are permitted as alternatives.

How thick can the starter film be?

The insert film must be no thicker than 13 µm. PTFE is used for resins cured below 180 °C and polyimide for cure temperatures above 180 °C.

What are the NL, VIS and 5 %/MAX points?

NL is where the load-displacement curve first deviates from linearity, VIS is where delamination growth is first seen at the specimen edge, and 5 %/MAX is the first of either a 5 % increase in compliance or the maximum load. GIC is reported at all three for initiation.

Why is the specimen precracked?

The film insert leaves a blunt, resin-rich tip that is not a natural delamination. ISO 15024 grows a short mode I precrack from the insert during the first loading, unloads, and then re-loads so propagation values come from a natural crack front.

Which data reduction methods does ISO 15024 use?

Method A is corrected beam theory (CBT), GIC = 3Pδ / [2b(a + |Δ|)] × F/N, and Method B is modified compliance calibration (MCC). F corrects for large displacements and N for the stiffening effect of load blocks.

How many specimens does ISO 15024 require?

At least five specimens are required. Specimens that fail the validity checks are discarded and replaced so that five valid results are obtained.

What test speed is used for ISO 15024?

The test runs in displacement control at a constant crosshead rate held within ± 20 %, in the low mm/min range. 1 mm/min is the slowest rate the standard allows and is commonly used; confirm the full permitted range for loading and unloading in clause 9 of your copy.

Is ISO 15024 the same as ASTM D5528?

They test the same DCB specimen with the same kind of hardware, and results are close when the same initiation point and reduction method are used. The main differences are in preferred widths, the reduction methods offered and reporting details; see our ASTM D5528 guide.

Can ISO 15024 be used for woven or multidirectional laminates?

The standard is limited to 0° unidirectional lay-ups. In multidirectional or woven laminates the crack tends to wander between plies and fiber bridging changes, so values are not comparable; Annex B discusses this.

What fixture does Universal Grip offer for ISO 15024?

The UGC-ISO15024-TF load block clevis set pins bonded load blocks into stainless steel clevises with knurled quick-release pins, at 1 kN standard capacity. Load blocks come in sizes to suit specimen width; contact us with width, pin diameter and frame connection for a quote.

Need a fixture, grips or a complete test system for ISO 15024? Contact Universal Grip with your material, specimen size and machine connection and we will quote the right setup.