*4.1.1 Characterization of self-healing polymers and polymer matrix composites*

Over the last few years, several testing methods have been used to assess self-repairing in polymers (thermosets, thermoplastics and elastomers) and polymer-based fiber-reinforced composites before and after repairing at macro-, micro- and nano/molecular levels. Even computational and/or predictive approaches have been attempted for deeper understanding of self-healing processes in polymer systems.

Macroscale healing evaluation leverages on fracture mechanics test procedures. Evaluation of healing requires inflicting some form of controlled crack/damage that resembles the mode of damage during utilization on the virgin polymer [73, 75, 76] by application of mechanical loads. This is accompanied by applying similar mechanical

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*Exploits, Advances and Challenges in Characterizing Self-Healing Materials*

load of damage to the healed polymer and evaluation of recovery of the polymer from

Depending on the mode of fracture, fracture evaluation loads could be in form of impact, fatigue, quasi-static fracture, tensile, compressive and flexural loads for mode I (opening) or mode III (tearing) fracture processes [71]. Also based on the type of polymeric system and the type of crack it develops under stress, the test samples have special geometries such as tapered double cantilever beam (TDCB); compact tension (CT) test specimens and width-tapered double-cantilever beam (WTDCB); double-cleavage drilled compression (DCDC); double-cantilever beam (DCB) and others [71]. These geometries and their suitability for evaluating the mechanical healing efficiencies in different polymers and polymer matrix compos-

The extent of recovery of various material properties (healing efficiency) is estimated using various forms of Eq. (1) after subjecting the damaged and undamaged samples to any or combination of the above mechanical loads as shown in Eqs. (2)–(6). The recovery of fracture toughness, flexural, tensile, impact and tear strengths has been quantified in different polymer healing systems with healing

For instance, crack healing efficiency (*η*) for mode I type of healing can be

 = 100% *healed IC virgin IC <sup>K</sup> <sup>X</sup>*

where *healed KIC* is the fracture toughness of a healed fracture specimen and *virgin KIC*

 = 100% *healed IC healed IC <sup>G</sup> <sup>X</sup>*

Crack healing efficiency has also been defined in terms of fatigue life-extension

<sup>−</sup> = *healed Ncontrol*

*N*

where *Nhealed* and *Ncontrol* are the total number of cycles to failure for a self-

For elastomeric self-healing materials, the recovery of tear strength is used to

 = *healed* 100% *virgin <sup>T</sup> <sup>X</sup>*

where *Thealed* is the tear strength of the healed material and *Tvirgin* is tear strength

Healing efficiency has also been estimated based on change in stiffness and

*control*

where *healed GIC* and *virgin GIC* are the critical energy release rate from testing the

An alternative expression for healing efficiency based on fracture energy

*<sup>K</sup>* (2)

*<sup>G</sup>* (3)

*<sup>N</sup>* (4)

*<sup>T</sup>* (5)

η

η

η

healing sample and *for* similar sample without healing, respectively.

η

*d*

*DOI: http://dx.doi.org/10.5772/intechopen.93031*

ites are described in detail in Ref. [71].

efficiencies ranging from 23 to 100% [7].

is the fracture toughness of the virgin specimen.

healed fracture and virgin specimens, respectively.

recovery in a damaged and healed polymer [79] as

estimated using Eq. (2) [7, 77].

[74, 77] is

[77, 78] as

define healing efficiency as

of virgin material.

the fracture.

*Exploits, Advances and Challenges in Characterizing Self-Healing Materials DOI: http://dx.doi.org/10.5772/intechopen.93031*

*Advanced Functional Materials*

material properties developed, repairing mechanism and its intended application as shown in **Figure 2b** adapted from Ref. [6]. This subchapter considers quantification

*(a) Steps in characterization of self-healing materials, and (b) interrelationship among material properties,* 

of healing efficiencies at different length scales in the above material classes.

*self-healing mechanism, characterization method and application adapted from Ref. [6].*

*4.1.1 Characterization of self-healing polymers and polymer matrix composites*

micro- and nano/molecular levels. Even computational and/or predictive approaches have been attempted for deeper understanding of self-healing pro-

Over the last few years, several testing methods have been used to assess self-repairing in polymers (thermosets, thermoplastics and elastomers) and polymer-based fiber-reinforced composites before and after repairing at macro-,

Macroscale healing evaluation leverages on fracture mechanics test procedures. Evaluation of healing requires inflicting some form of controlled crack/damage that resembles the mode of damage during utilization on the virgin polymer [73, 75, 76] by application of mechanical loads. This is accompanied by applying similar mechanical

**4.1 General self-healing characterization techniques**

**222**

**Figure 2.**

cesses in polymer systems.

load of damage to the healed polymer and evaluation of recovery of the polymer from the fracture.

Depending on the mode of fracture, fracture evaluation loads could be in form of impact, fatigue, quasi-static fracture, tensile, compressive and flexural loads for mode I (opening) or mode III (tearing) fracture processes [71]. Also based on the type of polymeric system and the type of crack it develops under stress, the test samples have special geometries such as tapered double cantilever beam (TDCB); compact tension (CT) test specimens and width-tapered double-cantilever beam (WTDCB); double-cleavage drilled compression (DCDC); double-cantilever beam (DCB) and others [71]. These geometries and their suitability for evaluating the mechanical healing efficiencies in different polymers and polymer matrix composites are described in detail in Ref. [71].

The extent of recovery of various material properties (healing efficiency) is estimated using various forms of Eq. (1) after subjecting the damaged and undamaged samples to any or combination of the above mechanical loads as shown in Eqs. (2)–(6). The recovery of fracture toughness, flexural, tensile, impact and tear strengths has been quantified in different polymer healing systems with healing efficiencies ranging from 23 to 100% [7].

For instance, crack healing efficiency (*η*) for mode I type of healing can be estimated using Eq. (2) [7, 77].

$$\eta = \frac{K\_{IC}^{healthy}}{K\_{IC}^{vipu}} X \mathbf{a} \mathbf{o} \mathbf{o} \,\%\tag{2}$$

where *healed KIC* is the fracture toughness of a healed fracture specimen and *virgin KIC* is the fracture toughness of the virgin specimen.

An alternative expression for healing efficiency based on fracture energy [74, 77] is

$$\eta = \frac{\mathbf{G}\_{IC}^{headed}}{\mathbf{G}\_{IC}^{headed}} \mathbf{X} \mathbf{a} \mathbf{o} \mathbf{o} \mathbf{y} \tag{3}$$

where *healed GIC* and *virgin GIC* are the critical energy release rate from testing the healed fracture and virgin specimens, respectively.

Crack healing efficiency has also been defined in terms of fatigue life-extension [77, 78] as

$$\eta\_d = \frac{N\_{healthy-N\_{causal}}}{N\_{court}} \tag{4}$$

where *Nhealed* and *Ncontrol* are the total number of cycles to failure for a selfhealing sample and *for* similar sample without healing, respectively.

For elastomeric self-healing materials, the recovery of tear strength is used to define healing efficiency as

$$\eta = \frac{T\_{\text{heated}}}{T\_{\text{võgiu}}} \mathbf{X} \mathbf{1} \mathbf{o} \mathbf{o} \mathbf{y} \mathbf{6} \tag{5}$$

where *Thealed* is the tear strength of the healed material and *Tvirgin* is tear strength of virgin material.

Healing efficiency has also been estimated based on change in stiffness and recovery in a damaged and healed polymer [79] as

$$\eta = \frac{E\_{\text{headed}}}{E\_{\text{trig}}} X \mathbf{1} \mathbf{o} \mathbf{o} \mathbf{y} \mathbf{6} \tag{6}$$

Besides the macroscale methods used for mechanical performance evaluation of self-healing materials, evaluation at smaller length scale (micro- and nanoscale levels) is necessary to reveal the underlying healing mechanisms in polymers and deepen understanding of self-healing [6]. Characterization techniques at this scale enable the monitoring of the whole process of self-healing from stage of inflicting damage to identifying interactions and confirmation of healing functionality at molecular/nanolevels [16, 80]. The techniques include imaging, spectrometric, scattering, rheological and thermal techniques [6].

#### **4.2 Characterization of self-healing metals**

Self-healing efficiency quantification of metals based on the bulk material properties can also be carried out by subjecting pre-cracked samples to various mechanical tests and self-healing efficiency determined using Eq. (1). For instance, the self-healing efficiency of metallic system based on precipitation-induced approach can be evaluated by subjecting the age-hardened alloy (virgin alloy) and its pre-cracked counterpart but filled low-melting alloy to tensile loading to fracture. Alaneme and Omosule [81] used this method to determine the self-healing efficiency of underaged Al-Mg-Si alloys and 60Sn-40Pb alloy-reinforced aluminum metal–metal composites. The self-healing efficiency, ɳ, was estimated using relation based on the tensile strength criterion given by Eq. (7):

$$
\eta\_{\text{tenôle}} = \frac{\sigma\_{\text{healed}}}{\sigma\_{\text{vegin}}} X \mathbf{1} \mathbf{o} \mathbf{o} \,\%\tag{7}
$$

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*Exploits, Advances and Challenges in Characterizing Self-Healing Materials*

copper (Fe-Cu) alloy closing nano-voids in the system.

**4.3 Characterization of self-healing ceramics**

and nitrogen (N) maximally accelerates the precipitation of copper (Cu) in iron-

The current fracture mechanic tests used to assess self-healing capabilities in hard materials are also applicable to advanced ceramics, but the difficulty in creating controlled cracks in ceramic materials limits this application. Unlike other hard materials that exhibit some level of plasticity, ceramic materials are very brittle in nature and are prone to unwanted fast fracture under mechanical and thermal stresses [71]. Another limitation is that it is difficult achieve crack closure in ceramics at low temperatures. The most common procedure for creating controlled micro-cracks and quantifying healing efficiency is indentation method [84]. The healing efficiency is evaluated in terms of crack closure by comparing the control

For instance Nam and Hwang [40] investigated crack healing behavior of ZrO2/SiC composite ceramics with TiO2 additive. Cracks of about 100 μm were made on the sample surfaces using Vickers indenter. Since self-healing in ceramics is a high-temperature process, the indented samples were heat-treated to stimulate healing, but the test was conducted at room temperature. This was followed by observing the pre- and post-healed indents with X-ray diffractometer (XRD). The strength of crack closure was determined using three-point bending test. Li et al. [85] also used flexural test to evaluate the multiple healing of titanium aluminum carbide (Ti2AlC) ceramic damaged by indentation in terms of crack

Besides using bending tests, tensile and the biaxial ball-on-three balls (B3B) tests have been used to study healing efficiencies in ceramics at room temperatures. Gao and Suo [86] assessed adhesion healing efficiency in a ceramic coating by performing tensile tests and correlating the healing time and residual stress while Harrer et al. [87] studied the healing of surface defects induced by different machining

All these tests are usually performed at room temperature while the healing process takes place at elevated temperatures >1000°C with the attendant oxidative atmosphere and thermal stresses. At high temperatures, the internal structure and mechanical performance of the ceramics could change due to the accompanying local melting and phase transformations [88]. Since mechanical performance under the above atmosphere will be different from that at low temperatures, there is the need to develop a more suitable method for the quantification of self-healing efficiency in ceramics [71] that takes into consideration the real service conditions. Attempts in this direction have been made by Ando et al. [89, 90], who determined *in situ* crack healing ability by conducting mechanical tests on some ceramics at

The major cause of mechanical failure in concrete is cracks. A crack not only lowers strength, but exposes the reinforcing steel components to corrosion. Selfhealing targets crack closure or prevention of crack propagation to retain strength and reduce water permeability in order to maintain durability. Self-healing efficiency of concrete in hardened form has been determined by conducting tests at macro-, micro-, and nanoscale levels. Majority of the researchers evaluated the self-healing efficiency at macrostructural level, some at microscale level and very

conditions on silicon nitride ceramic using biaxial ball-on-three balls.

*DOI: http://dx.doi.org/10.5772/intechopen.93031*

samples with the healed specimens.

elevated temperatures [89, 90].

**4.4 Characterization of self-healing concrete**

few authors at nanostructure level [42].

propagation.

where σ *virgin* is the tensile strength of the virgin specimen and σ *healed* is the tensile strength of the healed specimen.

Other experimental techniques used to characterize solid materials subjected cyclic or creep loads can also be adapted to evaluate bulk metallic material systems.

Mechanical evaluation of materials using micro-indentation techniques is a widely accepted tool to reveal information on the surfaces of bulk hard materials [4] and it can be readily applied to study healing at micron scale. Self-mending at this length scale can be studied by inducing mechanical damage through micromachining accompanied by imaging of the repairing process. The imaging can be done using low and high imaging equipment such as optical microscope, scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS) and environmental scanning electron microscope (ESEM) or X-ray micro-tomography instrument to provide details concerning crack propagation arrest [31] and evolution of self-healing reactions and to reveal evolved microstructures and morphology [4]. The results obtained at micro- and nanolevels are used to buttress results at macrolevels.

Most research conducted on self-healing metals focused on either solid-state diffusion healing of micro-cracks, or shape memory alloy (SMA)-reinforced "off-eutectic" matrices. It is worthwhile to conduct tests at nanolevels to elucidate the bonding at the interface between the diffusing species and the metal matrices. Laha et al. [82] applied nanoscale investigation using Auger spectroscopy to show that boron (B) atom acts as a solute healing agent in 347-austenitic stainless steel by diffusing to the nano voids and precipitating at the void surfaces. He et al. [83] equally used positron annihilation spectroscopy to confirm that the addition of B

and nitrogen (N) maximally accelerates the precipitation of copper (Cu) in ironcopper (Fe-Cu) alloy closing nano-voids in the system.
