Delamination

Delamination is a mode of failure where a material fractures into layers. A variety of materials, including laminate composites[1] and concrete, can fail by delamination. Processing can create layers in materials, such as steel formed by rolling[2][3] and plastics and metals from 3D printing[4][5] which can fail from layer separation. Also, surface coatings, such as paints and films, can delaminate from the coated substrate.

In laminated composites, the adhesion between layers often fails first, causing the layers to separate.[6] For example, in fiber-reinforced plastics, sheets of high strength reinforcement (e.g., carbon fiber, fiberglass) are bound together by a much weaker polymer matrix (e.g., epoxy). In particular, loads applied perpendicular to the high strength layers, and shear loads can cause the polymer matrix to fracture or the fiber reinforcement to debond from the polymer.

Delamination also occurs in reinforced concrete when metal reinforcements near the surface corrode.[7] The oxidized metal has a larger volume causing stresses when confined by the concrete. When the stresses exceed the strength of the concrete, cracks can form and spread to join with neighboring cracks caused by corroded rebar creating a fracture plane that runs parallel to the surface. Once the fracture plane has developed, the concrete at the surface can separate from the substrate.

Processing can create layers in materials which can fail by delamination. In concrete, surfaces can flake off from improper finishing. If the surface is finished and densified by troweling while the underlying concrete is bleeding water and air, the dense top layer may separate from the water and air pushing upwards.[8] In steels, rolling can create a microstructure when the microscopic grains are oriented in flat sheets which can fracture into layers.[2] Also, certain 3D printing methods (e.g., fused deposition) builds parts in layers that can delaminate during printing or use. When printing thermoplastics with fused deposition, cooling a hot layer of plastic applied to a cold substrate layer can cause bending due to differential thermal contraction and layer separation.[4]

## Contents

* 1 Inspection methods
* 2 Delamination resistance testing methods
* 2.1 Coating delamination tests
* 2.2 Interlaminar fracture toughness testing
* 2.2.1 Mode I interlaminar fracture toughness
* 2.2.2 Mode II interlaminar fracture toughness
* 2.3 Interlaminar shear strength testing
* 2.3.1 Asymmetric four-point bending
* 3 References

## Inspection methods

There are multiple nondestructive testing methods to detect delamination in structures including visual inspection, tap testing (i.e. sounding), ultrasound, radiography, and infrared imaging.

Visual inspection is useful for detecting delaminations at the surface and edges of materials. However, a visual inspection may not detect delamination within a material without cutting the material open.

Tap testing or sounding involves gently striking the material with a hammer or hard object to find delamination based on the resulting sound. In laminated composites, a clear ringing sound indicates a well bonded material whereas a duller sound indicates the presence of delamination due to the defect dampening the impact.[9] Tap testing is well suited for finding large defects in flat panel composites with a honeycomb core whereas thin laminates may have small defects that are not discernible by sound.[10] Using sound is also subjective and dependent on the inspector's quality of hearing as well as judgement. Any intentional variations in the part may also change the pitch of the produced sound, influencing the inspection. Some of these variations include ply overlaps, ply count change gores, core density change (if used), and geometry.

In reinforced concretes intact regions will sound solid whereas delaminated areas will sound hollow.[11] Tap testing large concrete structures is carried about either with a hammer or with a chain dragging device for horizontal surfaces like bridge decks. Bridge decks in cold climate countries which use de-icing salts and chemicals are commonly subject to delamination and as such are typically scheduled for annual inspection by chain-dragging as well as subsequent patch repairs of the surface.[12]

## Delamination resistance testing methods

### Coating delamination tests

Main article: Engineering:Paint adhesion testing

ASTM provides standards for paint adhesion testing which provides qualitative measures for paints and coatings resistance to delamination from substrates. Tests include cross-cut test, scrape adhesion,[13] and pull-off test.[14]

### Interlaminar fracture toughness testing

Fracture toughness is a material property that describes resistance to fracture and delamination. It is denoted by critical stress intensity factor

or critical strain energy release rate

.[15] For unidirectional fiber reinforced polymer laminate composites, ASTM provides standards for determining mode I fracture toughness

GIC

and mode II fracture toughness

GIIC

of the interlaminar matrix.[16][17] During the tests load

and displacement

is recorded for analysis to determine the strain energy release rate from the compliance method.

in terms of compliance is given by

G=P22BdCda (1)

where dC is the change in compliance C (ratio of δ/P), B is the thickness of the specimen, and da is the change in crack length.

#### Mode I interlaminar fracture toughness

Schematic of deformed double cantilever beam specimen.

ASTM D5528 specifies the use of the double cantilever beam (DCB) specimen geometry for determining mode I interlaminar fracture toughness.[17] A double cantilever beam specimen is created by placing a non-stick film between reinforcement layers in the center of the beam before curing the polymer matrix to create an initial crack of length

. During the test the specimen is loaded in tension from the end of the initial crack side of the beam opening the crack. Using the compliance method, the critical strain energy release rate is given by

GIc=3PCδC2Ba (2)

where

and

are the maximum load and displacement respectively by determining when the load deflection curve has become nonlinear with a line drawn from the origin with a 5% increase in compliance. Typically, equation 2 overestimates the fracture toughness because the two cantilever beams of the DCB specimen will have a finite rotation at the crack. The finite rotation can be corrected for by calculating

with a slightly longer crack with length

a+Δ

giving

GIc=3PCδC2B(a+Δ) (3)

The crack length correction

can be calculated experimentally by plotting the least squares fit of the cube root of the compliance

C1/3

vs. crack length

. The correction

is the absolute value of the x intercept. Fracture toughness can also be corrected with the compliance calibration method where

GIc

given by

GIc=nPCδC2Ba (4)

where n is the slope of the least squares fit of log⁡(C) vs. log⁡(a).

#### Mode II interlaminar fracture toughness

Schematic of edge notch flexure test.

Mode II interlaminar fracture toughness can be determined by an edge notch flexure test specified by ASTM D7905.[16] The specimen is prepared in a similar manner as the DCB specimen introducing an initial crack with length a0 before curing the polymer matrix. If the test is performed with the initial crack (non-precracked method) the candidate fracture toughness GQ is given by

GQ=3mPmax2a022B

where B is the thickness of the specimen and Pmax is the max load and m is a fitting parameter. m is determined by experimental results with a least squares fit of compliance C vs. the crack length cubed a3 with the form of

C=A+ma3.

The candidate fracture toughness GQ equals the mode II fracture toughness GIIc if strain energy release rate falls within certain percentage of GQ at different crack lengths specified by ASTM.

### Interlaminar shear strength testing

Interlaminar shear strength is used as an additional measure of the strength of the fiber-matrix bond in fiber-reinforced composites. Shear-induced delamination is experienced in various loading conditions where the bending moment across the composite changes rapidly, such as in pipes with changes in thickness or bends.[18] Multiple test architectures have been proposed for use in measuring interlaminar shear strength, including the short beam shear test, Iosipescu test, rail shear test, and asymmetrical four-point bending test.[19] The goal of each of these tests is to maximize the ratio of shear stress to tensile stress exhibited in the sample, promoting failure via delamination of the fiber-matrix interface instead of through fiber tension or buckling.[20] The orthotropic symmetry of fiber composite materials makes a state of pure shear stress difficult to obtain in sample testing; thin cylindrical specimens can be used but are costly to manufacture.[21] Sample geometries are thus chosen for ease of machining and optimization of the stress state when loaded.

In addition to manufactured composites such as glass fiber-reinforced polymers, interlaminar shear strength is an important property in natural materials such as wood. The long, thin shape of floorboards, for example, may promote deformation that leads to vibrations.[22]

#### Asymmetric four-point bending

(top-bottom) The point forces, shear stresses, and moments acting on an asymmetric four point bend test sample

Asymmetric four-point bending (AFPB) may be chosen to measure interlaminar shear strength over other procedures for a variety of reasons, including specimen machinability, test reproducibility, and equipment availability. For example, short-beam shear samples are constrained to a specific length-thickness ratio to prevent bending failure, and the shear stress distribution across the specimen is non-uniform, both of which contribute to a lack of reproducibility.[19] Rail shear testing also produces a non-homogeneous shear stress state, making it appropriate for determining shear modulus, but not shear strength.[19] The Iosipescu test requires special equipment in addition to the roller setup already used for other three- and four-point flexural tests.[22]

ASTM C1469 outlines a standard for AFPB testing of advanced ceramic joints, and the method has been proposed to be adapted for use with continuous ceramic matrix composites.[23][24] Rectangular samples can be used with or without notches machined at the center; the addition of notches helps to control the position of the failure along the length of the sample, but improper or nonsymmetrical machining can result in the addition of undesired normal stresses which reduce the measured strength.[24] The sample is then loaded in compression in its test fixture, with loading applied directly to the sample from 4 loading pins arranged in a parallelogram-like configuration. The load applied from the test fixture is transferred unevenly to the top two pins; the ratio of the inner pin load P and outer pin load Q is defined as the loading factor λ, such that

PQ=S2S1=λ,

where S1 and S2 are the lengths from the inner pin to the applied point load and from the outer pin to the applied point load, respectively. The normal stress in the sample σxx is maximized at the locations of the inner pins, and is equivalent to

σxx=6(λ−1)FL(1+λ2)bt,

where F is the total applied load on the sample, L is the sample length, b is the sample width (into the page as seen in a 2D free-body diagram), and t is the sample thickness. The shear stress σxz in the sample is maximized in between the inner span of the pins and is given by

σxz=3(1−λ)F2(1+λ)bt.

The ratio of normal to shear stress in the sample C is given by

C=σxxσxz=4L(1+λ)t.

This ratio is dependent both on the loading factor of the sample and its length-thickness ratio; both of these quantities are important in determining the mode of failure of the sample in testing.[18]

## References

1. ↑ Cantwell, W.J.; Morton, J. (1991). "The impact resistance of composite materials — a review" (in en). Composites 22 (5): 347–362. doi:10.1016/0010-4361(91)90549-V.
2. ↑ 2.0 2.1 Bramfitt, B. L.; Marder, A. R. (1977). "A study of the delamination behavior of a very low-carbon steel". Metallurgical Transactions A 8 (8): 1263–1273. doi:10.1007/bf02643841. ISSN 0360-2133. Bibcode: 1977MTA.....8.1263B.
3. ↑ Dogan, Mizam (2011). "Delamination failure of steel single angle sections" (in en). Engineering Failure Analysis 18 (7): 1800–1807. doi:10.1016/j.engfailanal.2011.04.009.
4. ↑ 4.0 4.1 "Layer Separation and Splitting" (in en-US). 2019-01-04. https://www.prusa3d.com/layer-separation-and-splitting/.
5. ↑ Barile, Claudia; Casavola, Caterina; Cazzato, Alberto (2018-09-18). "Acoustic Emissions in 3D Printed Parts under Mode I Delamination Test" (in en). Materials 11 (9): 1760. doi:10.3390/ma11091760. ISSN 1996-1944. PMID 30231488. Bibcode: 2018Mate...11.1760B.
6. ↑ Wisnom, M. R. (2012-04-28). "The role of delamination in failure of fibre-reinforced composites" (in en). Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 370 (1965): 1850–1870. doi:10.1098/rsta.2011.0441. ISSN 1364-503X. PMID 22431760. Bibcode: 2012RSPTA.370.1850W.
7. ↑ Li, C. Q.; Zheng, J. J.; Lawanwisut, W.; Melchers, R. E. (2007). "Concrete Delamination Caused by Steel Reinforcement Corrosion" (in en). Journal of Materials in Civil Engineering 19 (7): 591–600. doi:10.1061/(ASCE)0899-1561(2007)19:7(591). ISSN 0899-1561.
8. ↑ "CIP 20 - Delamination of Troweled Concrete Surfaces". May 4, 2019. https://www.nrmca.org/aboutconcrete/cips/20p.pdf.
9. ↑ "DOT/FAA/AR-02/121: Guidelines for Analysis, Testing, and Nondestructive Inspection of Impact- Damaged Composite Sandwich Structures". March 2003. http://www.tc.faa.gov/its/worldpac/techrpt/ar02-121.pdf.
10. ↑ "The Limitations of Tap Testing". http://carbonbikerepair.com.au/articles/82-the-limitations-of-tap-testing.
11. ↑ ASTM ASTM D4580/D4580M - 12: Standard Practice for Measuring Delaminations in Concrete Bridge Decks by Sounding, West Conshohocken, PA: ASTM International, 2018
12. ↑ Ahmadi, Hossein (December 2017). Concrete Bridge Deck Aging, Inspection and Maintenance (Master of Science thesis). University of Toledo. Archived from the original on 2019-05-16. Retrieved 2019-05-16.
13. ↑ ASTM D2197 - 98: Standard Test Method for Adhesion of Organic Coatings by Scrape Adhesion, West Conshohocken, PA: ASTM International, 1998
14. ↑ ASTM D4541 - 17: Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, West Conshohocken, PA: ASTM International, 2017
15. ↑ Zehnder, Alan (2012). Fracture mechanics. Springer. ISBN 9789400725959. OCLC 905283457.
16. ↑ 16.0 16.1 ASTM D7905/D7905M - 14: Standard Test Method for Determination of the Mode II Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites, West Conshohocken, PA: ASTM International, 2014
17. ↑ 17.0 17.1 ASTM D5528 - 13: Standard Test Method for Mode I Interlaminar Fracture Toughness of Unidirectional Fiber-Reinforced Polymer Matrix Composites, West Conshohocken, PA: ASTM International, 2014
18. ↑ 18.0 18.1 Theotokoglou, E.E.; Sideridis, E. (July 2011). "Study of composite beams in asymmetric four-point bending". Journal of Reinforced Plastics and Composites 30 (13): 1125–1137. doi:10.1177/0731684411417199.
19. ↑ 19.0 19.1 19.2 Spigel, Barry Stuart. An Experimental and Analytical Investigation of the Iosipescu Shear Test for Composite Materials (Thesis). doi:10.25777/1f0a-4934.
20. ↑ Spigel, B. S.; Prabhakaran, R.; Sawyer, J. W. (March 1987). "An investigation of the losipescu and asymmetrical four-point bend tests". Experimental Mechanics 27 (1): 57–63. doi:10.1007/BF02318864.
21. ↑ Prabhakaran, R.; Sawyer, Wayne (January 1986). "A photoelastic investigation of asymmetric four point bend shear test for composite materials". Composite Structures 5 (3): 217–231. doi:10.1016/0263-8223(86)90004-8.
22. ↑ 22.0 22.1 Yoshihara, Hiroshi; Kubojima, Yoshitaka (February 2002). "Measurement of the shear modulus of wood by asymmetric four-point bending tests". Journal of Wood Science 48 (1): 14–19. doi:10.1007/BF00766232.
23. ↑ Jadhav, Prakash; Lande, Chhaya (27 July 2023). "Asymmetric Four Point Bend Test Method for Interlaminar Shear Strength in Ceramic Matrix Composites". Materials Science Forum 1094: 19–24. doi:10.4028/p-zN31vp.
24. ↑ 24.0 24.1 ASTM C1469 - 10: Standard Test Method for Shear Strength of Joints of Advanced Ceramics at Ambient Temperature, West Conshohocken, PA: ASTM International, 2015

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