Damage in which the resin or interface between plies of a laminate cracks and the plies separate. It occurs because the through-thickness strength is low, and when caused internally by impact it is hard to detect from outside.
Japanese name: 層間はく離
Delamination appears in 1 practice question across 1 topic, including Composite materials.
Field: Composite materials
In-plane loads in a laminate are carried by the fibers, but there are no fibers between plies; they are connected only by resin and by the fiber–resin interface. As a result, the strength against through-thickness tension (peeling forces) and interlaminar shear is lower than the fiber-direction strength by a factor of several tens. In laminates, interlaminar stresses arising from the differences in ply orientation concentrate at free edges, around holes, at ply drops and in curved sections, and delamination starts there. Voids and foreign matter introduced during molding also act as starting points.
Delamination is treated by fracture mechanics as the growth of a crack between plies. The interlaminar fracture toughness in the opening mode (mode I), G_Ic, is measured by the double cantilever beam (DCB) test, and that in the sliding mode (mode II), G_IIc, by tests such as the end-notched flexure (ENF) test. For carbon fiber laminates with brittle epoxy, G_Ic is roughly 100–300 J/m², orders of magnitude smaller than the fracture toughness of metals expressed in the same energy terms. Toughened epoxies containing thermoplastic particles, or thermoplastic matrices, can raise G_Ic substantially.
Even an impact as minor as a dropped tool or a flying stone can spread many delaminations of different orientation, ply by ply, in a cone shape inside a laminate. Only a slight dent remains on the surface, so the damage can hardly be found visually (BVID: barely visible impact damage). Delaminated regions buckle easily as thin sublaminates under compression, so compressive strength drops much more than tensile strength. Aircraft design therefore uses compression after impact (CAI) strength, measured on laminates that have been deliberately impacted, as one of its criteria.
Internal delaminations are usually found by ultrasonic inspection (C-scan), which examines the waves reflected from delamination surfaces; infrared thermography and tap testing are also used. To prevent them, besides using toughened resins, there are methods that put fibers in the through-thickness direction to tie the plies together, such as stitching with thread, inserting fine Z-pins and three-dimensional weaving. However, through-thickness fibers can disturb the in-plane fibers and reduce in-plane properties, so the choice is a trade-off between the two. Separation at interfaces in semiconductor packages and coatings is likewise called delamination, from the same idea of layers coming apart.
Answer: C. The Young's moduli in the 0° and 90° directions are nearly equal, but in the 45° direction in-plane shear dominates and the modulus is lower
In [0/90]ₛ, loading in the 0° direction is carried along the fibers of the 0° plies and loading in the 90° direction along the fibers of the 90° plies, so the moduli in the two directions are nearly equal. Loaded at 45°, however, the fibers of every ply are 45° off the load direction, and the deformation becomes mainly in-plane shear supported by the resin, so the modulus drops considerably. To approach in-plane isotropy, ±45° plies are added to make a quasi-isotropic layup such as [0/±45/90]ₛ. Saying the 45° direction is the same overlooks this point. In the 90° direction the fibers of the 90° plies carry the load, so the modulus is not that of the resin alone. Note that in any layup the through-thickness direction is supported by the resin, so susceptibility to delamination remains.
No. Besides laminated composite plates, the word is used for interface separation in anything built up in layers, such as semiconductor packages, paint films and laminated glass.
Because internal damage often cannot be seen from outside, it is usually examined by ultrasonic inspection. Ultrasound reflects from delamination surfaces, so their position and depth can be imaged.
Small delaminations are sometimes repaired by injecting resin to rebond them. For larger damage, the damaged area is machined away and new plies are laid up and bonded, as in doubler or scarf repairs.
Note that in any layup the through-thickness direction is supported by the resin, so susceptibility to delamination remains.
Which statement about the in-plane stiffness of a laminate …Quasi-isotropic laminate Ceramic matrix composite CFRP (carbon-fiber-reinforced plastic) Critical fiber length GFRP (glass-fiber-reinforced plastic) Metal matrix composite Silane coupling agent