Nondestructive testing method for Kevlar and natural

Nondestructive testing method for Kevlar and natural fiber and their hybrid composites

16

Siti Madiha Muhammad Amir 1, 2,4 , M.T.H. Sultan 1, 3, 4 , Mohammad Jawaid 3 , Ahmad Hamdan Ariffin 1 , Shukri Mohd 2 , Khairul Anuar Mohd Salleh 2 , Mohamad Ridzwan Ishak 1 , Ain Umaira Md Shah 3 1 Aerospace Manufacturing Research Centre (AMRC), Faculty of Engineering, Universiti Putra Malaysia, Serdang, Malaysia; 2Industrial Technology Division, Malaysian Nuclear Agency, Bangi, Selangor, Malaysia; 3Laboratory of Biocomposite Technology (BIOCOMPOSITE), Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Malaysia; 4Department of Aerospace Engineering, Universiti Putra Malaysia, Serdang, Malaysia

16.1

Introduction

Composite materials are anisotropic and inhomogeneous materials. Composite material is made by combining a minimum of two or more materials, often with different properties. Composite materials usually present unique properties in which the strength-to-weight ratio is high. Another advantage of composite material is that it provides flexibility in design because the composites can be molded into complex shapes. There are many types of composite materials such as carbon-reinforced fiber plastic, glass fiberereinforced aluminium, composites with carbon nanotubes, and many more. Other types of composite include metal-matrix and ceramic-matrix composites. Composites have vast usage in engineering applications. Currently, laminated composite is becoming very popular in the area of aeronautics, wind energy, as well as in the automotive industry [1]. Extensive reviews of the application of composites in the automotive industry can be found in the literature [2,3].

16.2

Hybrid composites

Hybrid composites are widely used with many applications [4]. Currently, most hybrid composites are made only with synthetic fibers such as Kevlar with carbon, Kevlar with fiberglass, carbon with fiberglass, etc. [5]. However, recently the world has shown wide interest in hybrid composites involving synthetic and natural fibers [6]. Fig. 16.1 shows a synthetic fiber and Fig. 16.2 shows a natural fiber. According to the literature [6],

Durability and Life Prediction in Biocomposites, Fibre-Reinforced Composites and Hybrid Composites https://doi.org/10.1016/B978-0-08-102290-0.00016-7 Copyright © 2019 Elsevier Ltd. All rights reserved.

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Durability and Life Prediction in Bio-, Fibre-Reinforced, and Hybrid Composites

Figure 16.1 Synthetic fiber e Kevlar.

Figure 16.2 Natural fiber e oil palm empty fruit bunch.

fiber-reinforced polymer composites developed using synthetic fiber have many advantages such as high strength, high stiffness, long fatigue life, adaptability to the function of the structure, corrosion resistance, and environmental stability. There are also drawbacks to this type of material, which are their high cost, high density, poor recycling capability, and nonbiodegradability. For these reasons, the choice of fiber is moving away from synthetic fiber toward natural plant fiberereinforced polymer


369

composites; materials with natural fiber have satisfactorily high specific strength and modulus, light weight, low cost, and biodegradability. Begum [6] also studied the environmental aspects, the socioeconomic impacts, and the potential of Southeast Asia for contribution to natural fiberereinforced polymer composite production such as jute, rice husk, bamboo, coconut, banana, flax, hemp, pineapple, sisal, and wheat husk. There are various types of hybrid composites such as hybrids between syntheticsynthetic fibers, synthetic-natural fibers, and natural-natural fibers. The synthetic fibers normally used for hybrid composites are Kevlar, carbon, and glass fibers. Generally, synthetic fibers are manufactured through energy intensive processes that produce toxic by-products. The reinforced composites made from synthetic fibers are difficult to recycle and they are resistant to biodegradation. Besides, with increasing governmental pressure, as well as consumer and industrial awareness of the long-term effects of environmental pollution due to noncompostable polymeric products, this situation has led numerous research studies around the world to show an interest in developing greener composites by either eliminating or minimizing the usage of nondegradable synthetic polymeric resin and fibers. Table 16.1 displays hybrid composites made from combinations of synthetic-natural fibers, natural-natural fibers, and synthetic-synthetic fibers. Table 16.1 Hybrid composites made from various fibers Author

Syntheticsynthetic fiber

Natural-natural fiber

Natural-synthetic fiber

Rashid et al. [7]

e

e

Coir e Kevlar

Yahya et al. [8]

e

e

Kenaf e Kevlar

Suhad et al. [9]

e

e

Kenaf e Prepreg Kevlar

Tshai et al. [10]

e

e

Polylactide acid composite with empty fruit bunch e chopped glass strand

Al-Mosawi et al. [11]

e

e

Palms e Kevlar

Bachtiar et al. [12]

e

Sugar palm e Kenaf

e

Jawaid et al. [13]

e

Jute e Oil Palm

e

Ahmad et al. [14]

e

e

Jute e Glass

Radif et al. [15]

e

e

Kevlar e Rame polyester

Jawaid et al. [16]

e

Coir e Oil Palm

e Continued

370


Table 16.1 Hybrid composites made from various fibersdcont’d Author

Syntheticsynthetic fiber

Natural-natural fiber

Natural-synthetic fiber

Sharba et al. [17]

e

e

Kenaf e Glass

Warhbe et al. [18]

e

e

Kevlar e Jute

Alavudeen et al. [19]

e

Banana e Kenaf

e

Asaithambi et al. [20]

e

Banana e Sisal

e

Sfarra et al. [21]

e

Jute e Wool felt

e

Jusoh et al. [22]

e

e

Glass e Flex Glass e Jute Glass e Basalt

Sahu et al. [23]

e

Sisal e Pineapple

e

Madhukiran et al. [24]

e

Banana e Pineapple

e

Kumar et al. [25]

e

Bamboo e Banana e Pineapple

e

Bhoopathi et al. [26]

e

e

Glass e Hemp e Banana

Karina et al. [27]

e

e

Oil palm e Glass fiber

Rimdusit et al. [28]

Kevlar e Polycarbonate/ acrylonitrilebutadienestyrene

e

e

Al-Jeebory et al. [3]

Carbon e Kevlar e Araldite matrix

e

e

Guru Raja et al. [29]

Kevlar e Glass

e

e

Randjbaran et al. [5]

Kevlar e Carbon e Glass

e

e

16.3

Damage and defects in composites

Damage is defined as changes to the material and/or geometric properties of the systems, including changes to the boundary condition and system connectivity, which adversely affect the system performance. There are many causes of structural damage, such as moisture absorption, fatigue, wind gusts, thermal stress, corrosion, fire, lightning strikes, or even impacts from various sources.


371

The damage due to impact varies with the material. Impact damage can occur in many types of materials regardless of whether it is made from metal or composite material. The impact damage in a composite is usually more severe, hence it raises more concern. The difference between the damage impact in metal and that in a composite is that in metal, the damage is easily detected as the damage starts at the impacted surfaces. However, the damage in composite material often starts at the nonimpacted surface or is in the form of internal delamination. Impact damage is not a threat in metal due to its ductile characteristics. In contrast, composites are more brittle, and most of the time the damage is barely visible, especially with lowimpact damage [30]. Low-velocity impact damage causes internal damage to the structure with very minimal visual detectability, and thus nondestructive testing (NDT) is needed to detect the internal damage. Since hybrid composites between synthetic and natural fibers have gained wide interest, conventional NDT methods are not able to perform an inspection of the hybrid composites, so advanced NDT techniques are required instead. Amongst the defects that are induced by low-velocity impact damage are delamination, which is the debonding between stackable sheets, matrix cracking, and fiber failures [31]. Fiber failures will influence the residual tensile strength of the material. There are other types of defect in composite materials. The defects in composites are different from the defects in metals due to the characteristics of the composite materials, i.e., their nonhomogeneous, anisotropic, and multilayered structures. Table 16.2 tabulates the types of defects in composites and metals. The defects usually occur at the interfaces of the structures [32]. Several other defects that occur in composite materials are voids, porosity, cracks, etc. Due to the extensive usage of composites in various structural and engineering applications, there exists an increasing demand for their integrity and safety. This situation has resulted in the rapid development of NDT methods over the years.

16.4

Nondestructive testing

Due to the advancements in composite materials, conventional NDT methods need to be adapted to the new materials. Regardless of the material, impact damage is one of the problems that composite structures will experience during the lifetime of the structure. The damaged zones in a composite structure, in general, consist of fiber breakage, delamination, and matrix cracking, and they are complex in nature. Without due care, Table 16.2 Table of defects in metals and composites Material

Manufacturing defects

In-service defects

Composites

Void, Delamination

Delamination, Lightning impact, Hailstones [33]

Metals

Inclusion, Porosity, Lack of Fusion, Lack of Penetration

Crack

372


Figure 16.3 Digital radiography.

these defects, especially those caused by low-impact damage, will cause a hazardous situation. Thus, NDT is needed to detect the internal damage. Figs. 16.3 and 16.4 show the NDT technique applied in detecting discontinuities. NDT is applied in many industrial applications such as in the marine industry, the oil and gas industries, wind turbines, etc. Table 16.3 shows the applications of NDT in various industries. Table 16.4 summarizes the applicability of various NDT methods in detecting defects [32].

Figure 16.4 Ultrasonic induced thermography.

Table 16.3 NDT applied in various industries NDT methods

Industry/Field of study

Acoustic emission [34]

Wind turbine

Ultrasonic C-scan, thermography [35]

Marine industry

Ultrasonic thickness gauge [36], Computed tomography [37,38]

Pipeline industry

Automated radiography technique [39], Eddy current testing [40]

Welding

Ultrasonic phased array, thermography, shearography [41]

Aerospace industry

Ultrasonic [42]

Railway industry

Acoustic emission [43,44]

Material characterization

Imaging X-ray and Computed Tomography [45]

Food industry

Acoustic emission [46]

Damage assessment

Ultrasonic [47], X-ray Computed Tomography [48]

Powder metallurgy

Eddy current testing and metal magnetic memory testing [49]

Automotive industry

Ultrasonic and eddy current testing [50]

Thermal power plant

Pulsed eddy current [51]

Nuclear power application

Ultrasonic testing [52,53]

Nonmetallic material

Table 16.4 NDT for detecting defects Nondestructive testing methods

Types of defects

Ultrasonic C-Scan

• • • • • •

Laser Ultrasonic

• delaminations • disbonds • impact damage

Acoustic Emission

• • • • • • • • •

Acousto-Ultrasonics

• porosity content • fiber alignment • condition of resin

delaminations disbonds voids inclusions (contaminants in the sample) resin-rich areas porosity

cracking growth of delaminations fracture of fibers fracture of matrix fiber-matrix disbonding fiber pull-out relaxation of fibers after failure large flaws (e.g., interlaminar defects) fracture of brittle interfacial layers

Continued

Table 16.4 NDT for detecting defectsdcont’d Nondestructive testing methods

Types of defects • • • •

impact damage fatigue damage thermal shock adhesive bonds

Laser Testing

• • • • • • • • • • • • • •

delaminations disbonding unbonded area impact damage planar defects erosion voids inclusion excess resin or lack of resin gas bubbles or porosity near-surface imperfections in fiber layout local environmental ingress excess adhesive bond strength

Eddy Current Testing

• • • • •

impact damage heat damage significant fiber breakage lack of fiber and conversely excess resin localized fiber wrinkling and waviness

X-Radiography

• • • • •

matrix cracking cracks inclusions voids porosity

X-ray Tomography

• planar or crack-like defects • voids • porosity

X-ray Backscatter

• voids • porosity

Transient Thermography

• • • • • • • •

Vibrothermography

• cracks • kissing disbonds • corrosion

Acoustic Impact

• voids • inclusion • delaminations

delamination adhesive disbond impact damage density of porosity voids inclusion water ingress corrosion


375

There have been various papers on NDT methods. Gholizadeh [54] provides the advantages and disadvantages of several NDT methods such as visual testing, ultrasonic testing, thermography, radiography testing, electromagnetic testing, acoustic emission (AE), and shearography. Djordjevic [55] provides an overview of the current use of NDT in composite applications. The methods that were discussed in the paper were the ultrasonic C-scan mapping method, optical methods such as shearography, advanced techniques (for instance, X-ray tomography), laser ultrasonic, holography, laser-optical, vibrothermography, acousto-ultrasonic, D-sight, neutron radiography, microwaves, as well as the use of sensors such as the embedded sensor. Kumar et al. [56] provide a literature survey of NDT methods for various engineering applications. However, the literature survey provided is limited as it only goes up to 2013. The examples of applications that use NDT provided in the literature cover the area of composites, concretes, architecture, railroad wheels, pipelines, tunnel lining grouting, aerospace, etc. Jolly et al. [57] provided a review of NDT techniques for a thick-wall flywheel made of carbon fiber with approximately 30 mm thickness. In their review, only three methods were discussed: the radiation technique using X-ray and computed tomography (CT), ultrasound, and thermography. From the discussion, it was concluded that the CT method is proven to show high reliability and a much better traceability in detecting delamination defects. There is also a review paper on various NDT methods developed by the Center for Nondestructive Evaluation at Iowa State University, which addresses the inspection issues of different types of composite structures [58]. The paper emphasized the capability of various NDT methods, such as water and air coupled ultrasonic, bond testing, manual and automated tap testing, thermography, and shearography.

16.4.1 Nondestructive testing versus structural health monitoring NDT is a broad group of analytical techniques that are widely used in science and industry to evaluate the properties of materials, components, or systems without causing damage to the materials. The evaluation of the properties may include the characterization of the material and detection of damage in materials. NDT is crucial in heavy and high-risk industries, such as nuclear and offshore structures, gas and oil pipelines, the aircraft industry, and the automobile industry. Common NDT methods include radiography, ultrasonic, eddy current, dye penetrant, thermography, etc. There is no single best method in NDT to conduct an evaluation; these methods are complementary to each other. Besides NDT, structural health monitoring (SHM) also plays a vital role in detecting initial internal defects. It is a tool for condition monitoring in structures. SHM can be carried out in situ using various techniques such as vibration measurements, strain measurements using strain gauges of fiber Bragg gratings and acoustic emission. In SHM, sensors are placed at the load-bearing parts of a structure, and they will

376


Table 16.5 Nondestructive testing versus structural health monitoring [59] Nondestructive testing (NDT)

Structural health monitoring (SHM)

Ultrasonic, Radiography, Eddy Current, Magnetic Particle, Liquid penetrant, Laser Shearography

Acoustic emission

Need a priori knowledge of the damage location

Do not need a priori knowledge of the damage location

Detection and evaluation of flaw geometry and orientation

Information is gathered from the NDT results to evaluate the remaining lifetime of the structure and provide flaw assessment

Carried out off-line in a local manner after the damage has been located

Associated with online-global damage identification in a structural system

continuously evaluate the material for initial damage and subsequent propagation. Table 16.5 shows the differences between NDT and SHM. Over the years, many research studies on the use of NDT for composites have been conducted. Various NDT methods have been used to detect damage in composite materials. Samant et al. [60] used an ultrasonic imaging technique to study the damage from ballistic impact. The ultrasonic C-scan method was performed on different composite armors by the normal incidence immersion-type pulse echo method. The time and frequency domain features were extracted from the digitized wave form at each point using code that had been developed in-house. Using this method, the striking and residual velocity during the impact were also measured. Theodoros Hasiotis et al. [61] performed the same method to detect delamination in a carbon/epoxy system and a marine-type glass/polyester system. It was concluded that the ultrasonic C-scan method is able to give predictions of the position and shape of the defects embedded in the carbon fiberereinforced polymer (CFRP) material as compared to the glass fibere reinforced polymer (GFRP) material. Ultrasonic phased array [31] is also one of the possible NDT methods used to detect damage from low-velocity impact. According to Perez [31], ultrasonic inspection provides thickness damage information in a practical and efficient way. From this work, a good correlation between the incident impact energy and the delaminated area was noted. However, a prior knowledge of the damage location is required; otherwise more time is required to complete the inspection. Perez [31] suggested that vibration testing is an effective and fast method to detect impact damage in composite plates. Ultrasonic methods have been used for many years for inspecting defects in composite materials. However, over the years, many improvements and modifications have been made to suit the applicability of the ultrasonic methods to various types of composites and structures. For example, Ramzi et al. [62] used the immersion ultrasonic method for detecting artificial holes of various sizes, such as 2 and 4 mm, in fiber glass composite laminates.


377

Acoustic emission has also been used to characterize materials such as composite materials. Rosa et al. [63] used acoustic emission to study mechanical testing. The materials used in this work were hybrid composites reinforced with glass and jute fiber, celery fiberereinforced composites, and phormium tenaxereinforced epoxy composites. From the results, it was concluded that the acoustic emission technique is suitable to analyze the mechanical behavior of natural fiberereinforced composites. Besides the ultrasonic method, the radiography technique in NDT has advanced, especially in detector systems. The radiography technique started with using a film as the detector. But due to the rapid development in computers, as well as the knowledge of image processing, the detector used in the conventional radiography technique has shifted from films to digital detectors such as imaging plates and flat panel detectors; these are used in the computed radiography system (CR) and the digital radiography system (DR), respectively. One of the other radiation applications using imaging is known as CT. Currently, the techniques mentioned herein have been used in metals and composites, but mostly with synthetic types of composite. Imielinska et al. [64] used the X-radiography technique to detect impact damage in thin carbon fiber/epoxy composite plates. According to the literature, X-radiography can be used to assess the impact damage area in carbon fiber/epoxy composites. However, to observe the delaminations in X-radiography, a penetrant opaque to X-rays must be injected into the damage zone. Durao et al. [65] also used the radiography method for damage extension in the damage measurement of composites. In the study, the laminates were immersed in di-iodomethane to give a good contrast between the waste rubber particles and the sugarcane bagasse fibers. A different technique that uses radiation is the laminography method. Kurfiss [66] combined the principle of laminography and CT to investigate a large-sized object using a 600 kV X-ray tube for planar geometry objects. The size of object for this work is: length of 1600 mm, height of 750 mm, and weight of about 250 kg. From this work, it was concluded that using the laminography principle, it is possible to scan a small structure with a three-dimensional location. Besides the laminography method, the computed tomography method is also currently used in assessing the damage impact for composites using radiation. Bull et al. [67] assessed the intra- and inter-laminar cracks in CFRP plates with a thickness of 1 mm. Rique et al. [68] employed an X-ray imaging inspection system to study the voids that were present in bonded joints of GFRP. This work used the CR method, a DR flat panel system, and the microtomography method. From the work, the microtomography method has the potential for detecting the lack of adhesion between the adhesive and the pipe wall as compared to the CR and DR methods. Besides this, the microtomography technique is also able to quantify defects such as the voids that are present in the bonded joints. The terahertz wave has gained the attention of many researchers. Hsu et al. [69] conducted the nondestructive evaluation of composite materials and structural applications. In this work, various discontinuities in composites were investigated, such as foreign material inclusions, simulated disband and delamination, mechanical impact

378


damage, heat damage, and hydraulic fluid ingression. In this paper, the effectiveness and limitations of this technology were also discussed. Optical NDT methods have become more popular in recent years. The NDT methods categorized under the optical method are shearography, interferometry, infrared thermography (IR), and digital image correlation. In composite application, several optical methods have been used. The main weakness in composites is the porosity defect, and this is usually introduced during the manufacturing process. Toscano et al. [70] used infrared thermography to evaluate the porosity distribution in carbon/epoxy composites. Two types of thermography approach were conducted: flash thermography and lock-in thermography. The findings showed that the infrared thermography method can be used to obtain the material’s characteristics, especially the porosity amount, the distribution of the porosity, and the presence of defects. Hybridization of NDT methods is becoming essential, especially in the need for the continuous monitoring and eventual prognosis of structural degradation. A combination of two NDT techniques can sometimes offer a sounder comparison of the performance of the materials. Rosa et al. [43] combined both the acoustic emission method and the IR pulse thermography method to analyze the level of damage resulting from low-velocity impacts. The AE method is used for characterization, which is recorded during the postimpact mechanical test, while the IR pulse thermography method is for damage visualization. The outcome of the study helps in designing the specific layup of the particular hybrid laminates for better impact performance. In combining global monitoring and local assessment, Alexander Maier et al. [1] used the thermography method for quick assessment and an ultrasonic technique to further investigate the local damage. Cuadra et al. [71] performed a hybrid NDT methods approach in quantifying the damage in GFRP composites. Mechanical testing, such as tensile testing and fatigue testing, were conducted in this work. The data fusion method approach was used to investigate structural damage detection, identification, and remaining life estimation. The NDT methods involved were acoustic emission, digital image correlation, and infrared thermography.

16.4.2

NDT for Kevlar (synthetic fiber) and its hybrid composites

Kevlar is a type of aramid fiber. It is woven into textile materials and is extremely strong and lightweight, with resistance toward corrosion and heat. It is used in vast applications such as aerospace engineering (such as the body of the aircraft), body armor, bulletproof vests, car brakes, and boats. It is usually made into composites. Kevlar can also be combined with other fibers to produce hybrid composites. The hybrid composites of Kevlar are tabulated in Table 16.1. Since Kevlar is used in many structural applications, NDT plays an important role in determining the integrity of the structure. One of the methods used to inspect the integrity of structures made from Kevlar is CT. CT is commonly used in the inspection of metals. However, this method is now being used for composites. Fidan [72] utilized microcomputerized tomography to visualize the internal damage impact on glass


379

fiberereinforced and glass fiber þ aramid fiberereinforced polyester composites. Low-velocity impacts were tested at 80 J energy. From the investigation, the microCT showed that the 3D-delamination pattern defect in glass-reinforced composites is more visible due to the nature of the glass fiber. However, the delamination pattern lost its effectiveness when aramid fiber was added to the glass fiber. Another method used to detect defects is the eddy current method [73]. However, in the work of Grimberg and Savin [73], the eddy current method used is for composites made from synthetic fiber, which are carbon and Kevlar. Eddy current microscopy, involving a micro-focus transducer, was used to investigate the individual fiber breakage in the specimen after it was impacted by a 7.62-mm-caliber bullet. Woo et al. [74] investigated the failure process and characteristics of carbon/Kevlar hybrid woven composites under a high strain-rate impact. In this work, a destructive-nondestructive coupled impact test methodology was applied. The AE technique was used as the nondestructive method in this work. The parameters of AE signal cumulative counts and amplitude were interpreted to obtain results for the plastic deformation and fibrillation of fibers, matrix cracking and propagation, and fiber breakages. The AE technique can also be used to provide valuable information on the structural changes in a stressed material. Juroslav et al. [75] applied the AE technique on a tensile test on composite materials reinforced with carbon and aramid fibers. The AE results provide information on the separation of the matrix, the extraction of fiber from the matrix, and the breaking of the fibers. The root mean square is observed in this work. Maleki [76] introduced the liquid crystal thermography method for the inspection of delaminations and air bubbles in a hybrid of Kevlar/resin as the skin and glass/resin as the core. According to the literature [76], this method can be applied to detect delamination. However, there are limitations in the preparation procedure, and access to both sides during the inspection is required. Further work was also suggested concerning the detection of small cracks using this method. Destic et al. [77] conducted NDT using the THz imaging setup on Kevlar fibers. Even though the samples tested were not real cases, since they had very thin damage, the results seemed to be very promising. From the output, using the setup, the delamination defect in Kevlar can be detected, and a break in a carbon/epoxy sample was also detected.

16.4.3 NDT for natural-synthetic fiberehybrid composites In characterizing the postimpact, low-velocity impact damage (5, 7.5, 10, 12.5, 15J) of hybrid jute/glass polyester composites, AE and IR thermography methods were used [43]. In this research, IR thermography was used to observe the damaged area after the specimens had been impacted, while AE was used to monitor the postimpact flexural test. Wood plastic composite is a relatively new family of composite materials. Effective NDT is necessary to evaluate the materials. Najafi et al. [78] studied the effect of wood flour content, compatibilizers, glass fiber content, and specimen length on ultrasonic velocity in wood flour/E-glass fiber hybrid polypropylene composites.

380


Yoon and Han [79] applied AE to locate the source of internal damage or foreign impact, as well as to evaluate the degree of damage of a wind turbine blade made from hybrid composites of glass fiber plastic and balsawood fiber. In this work, the focus was on enhancing a source location with an energy contour and developing a new damage index for better damage identification. Another way to characterize the damage impact in a hybrid composite of synthetic and natural fiber is by using infrared vision and optical techniques [80]. The infrared vision technique applied was the thermography technique, and the optical methods applied used digital speckle photography and holographic interferometry.

16.4.4

NDT for natural-natural fiberehybrid composites

The hybridization of NDT methods is becoming essential, especially in the need for the continuous monitoring and eventual prognosis of structural degradation. In Sfarra et al.’s [21] work, the integrated NDT methods of infrared vision and optical nondestructive testing were applied to evaluate the emerging defects at the surface and subsurface of natural fiber composites after being impacted. Combination methods were applied, i.e., speckles and speckle contrast techniques, near infrared reflectography and transmittography, ultraviolet imaging, pulsed thermography and square pulse thermography, and X-ray fluorescence spectroscopy. The combination results obtained could minimize false alarms and, simultaneously, more detailed results could be retrieved. In this work, the natural fiber composites were made of hybrid wool felt/ jute fiberereinforced epoxy resin. Pure wool felt/epoxy composites were also fabricated for comparison purposes. Table 16.6 summarizes the applications of NDT in fibers.

16.5

Conclusion and future perspective

Among the NDT methods described herein, the best approach is always the method that produces the most efficient results for a given application. However, under certain circumstances, a combination of techniques offers the best way to achieve the greatest benefits. There is still a lack of methods that can produce material information from continuous monitoring and local assessment inspection even though various NDT methods and subtechniques have been developed over the years for composite testing. Currently, no single method has the ability to meet all of the needs for composite integrity assessment. As well, as critical composite structures become part of commercial use in many industries, and the rapid development of hybrid composites of natural and synthetic fibers continues, additional developments will be needed to enable us to characterize or examine the integrity of such materials. From the review, there exists a long list of NDT methods for composite materials. However, such composite materials are limited to synthetic composites such as carbon fiber, glass fiber, and Kevlar fiber. There are very limited resources on NDT methods for composites that are made of natural fiber, regardless of whether they are hybridized

Composite material Method

Synthetic fiber

Natural fiber

Jolly et al. [57]

Computed tomography, Ultrasonic, Thermography

Carbon

Rique et al. [68]

X-ray

Glass

Sfarra et al. [81]

Interferometric, infrared thermography

Glass

Hasiotis et al. [61]

Ultrasonic C-scan technique

Carbon, glass

Delamination

Perez et al. [31]

Ultrasonic phased array

Carbon

Delamination

Pieczonka et al. [82]

Vibrothermography

Carbon

Delamination

Ambu et al. [83]

Holography and ultrasonic testing

Graphite

Delamination

Maier et al. [1]

Thermography

Carbon

Delamination

Meola et al. [84]

Infrared thermography

Glass, carbon, aluminium

Low velocity impact damage

Ambrozinski et al. [85]

Ultrasonic lamb wave

Carbon

Impact damage

Obdulia Loey et al. [86]

Line scanning thermography

Carbon

Delamination

Bullinger [87]

X-ray refraction topography

Carbon

Impact damage

Aymerich et al. [88]

Vibroacoustic

Carbon

Impact damage

Samant et al. [60]

Immersion ultrasonic C-scan

Kevlar, polypropylene

Impact damage

Ruzek et al. [89]

Ultrasonic, shearography

Carbon

Impact damage

Defect/Flaw Delamination

Basalt fiber

Continued

381

Author name (year)


Table 16.6 NDT techniques for different fibers

382

Table 16.6 NDT techniques for different fibersdcont’d Composite material Method

Synthetic fiber

Natural fiber

Defect/Flaw

Rosa et al. [43]

Acoustic emission, infrared thermography

Glass

Jute

Impact damage

Bull et al. [67]

X-ray computed tomography, laminography

Carbon

Delamination

Toscano et al. [71]

Infrared thermography

Carbon

Porosity

He et al. [90]

Scanning pulsed eddy current

Carbon

Impact damage

Amaro et al. [91]

Shearography, ultrasonic testing, X-radiography

Carbon prepreg

Delamination

Durao et al. [65]

Digital radiography

Sugarcane bagasse, rubber particles

Delamination


Author name (year)


383

with synthetic fiber or only natural fiber. The NDT method for this type of composite is very important because each material has its own properties, e.g., the attenuation factor. This results in the modification or development of state-of-the-art NDT techniques for the inspection of such material composites. NDT is not merely concerned with detecting discontinuities or defects. The relationship between the NDT results and the mechanical performance is also important. However, the understanding of the effects of the defects, the damage mechanisms, and the failure mechanisms are still immature. Hence, the relationship between the NDT information and the mechanical performance is yet to be improved.

Acknowledgments This work was supported by UPM under GP-IPB/9490602. The authors would like to express their gratitude and sincere appreciation to the Aerospace Manufacturing Research Centre (AMRC) AND Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia UPM (HiCOE).

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