Double Cantilever Beam Delamination Test

Specimen & Material

Ply system Carbon / Epoxy

Initial crack length a₀ 35 mm

Arm thickness h 1.5 mm

Crosshead opening rate 0.30 mm/s

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Fracture State

G_I0 / 350 J/m²

Crack length a

35.0 mm

Opening δ

0.00 mm

Applied load P

0.0 N

Energy release G_I

0 J/m²

Load–displacement curve (P vs δ)

How it works

A double cantilever beam (DCB) specimen is two bonded composite laminate arms with a pre-implanted delamination of length a at one end. Pulling the arms apart opens the crack in pure Mode I (opening mode). Each arm behaves as a cantilever beam fixed at the crack tip, so beam theory (ASTM D5528 "modified beam theory") gives the compliance:

C = δ/P = 2a³ / (3·E₁₁·I),   I = b·h³/12

Energy release rate (Irwin–Kies):
G_I = (P²/2b)·dC/da = 3·P·δ / (2·b·a)

The crack is stationary as long as G_I is below the material's Mode I interlaminar fracture toughness, G_Ic — the energy per unit new crack area the interface can absorb. As the crosshead keeps opening the specimen, P and δ rise together (compliance-controlled), and G_I rises with them. The instant G_I reaches G_Ic, the Griffith energy criterion is satisfied and the crack advances just far enough to bring G_I back down to G_Ic (resistance-controlled growth) — solving the compliance relation for the equilibrium crack length at the current opening:

a_eq(δ) = ( 9·E₁₁·I·δ² / (4·b·G_Ic) )^(1/4)

This produces the classic saw-toothed DCB load trace: load climbs, drops as the crack jumps, then climbs again on the freshly exposed (still bonded) interface ahead. Real interlaminar toughness is what keeps a wing spar, a wind-turbine blade, or a pressure-vessel wall from peeling apart ply-by-ply under service loads — this is the exact test aerospace and wind-energy labs run to measure it.

Ply system — swaps E₁₁ (fiber-direction modulus) and G_Ic for three real material systems.
a₀, h — initial crack length and arm thickness set the starting stiffness.
Opening rate — how fast the virtual crosshead pulls the grips apart.

Deflection and arm separation are visually amplified for legibility (true openings are a few millimeters over a 140 mm specimen); the underlying δ, P and G_I readouts are the real computed values.

Delaminated (open) region Bonded laminate Crack tip

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