Evaluation of Different Non-destructive Testing Methods to Detect Imperfections in Unidirectional Carbon Fiber Composite Ropes
Online monitoring of carbon fiber reinforced plastic (CFRP) ropes requires non-destructive testing (NDT) methods capable of detecting multiple damage types at high inspection speeds. Three NDT methods are evaluated on artificial and realistic imperfections in order to assess their suitability for online monitoring of CFRP ropes. To support testing, the microstructure and electrical conductivity of a carbon fiber rope is characterized. The compared methods are thermography via thermoelastic stress analysis, ultrasonic testing with commercial phased array transducers, and eddy current testing, supported by tailor-made probes. While thermoelastic stress analysis and ultrasonics proved to be accurate methods for detecting damage size and the shape of defects, they were found to be unsuitable for high-speed inspection of a CFRP rope. Instead, contactless inspection using eddy currents is a promising solution for real-time online monitoring of CFRP ropes at high inspection speeds.
KeywordsNon-destructive testing Ultrasonic Eddy current Carbon fiber Composite
The use of synthetic fiber ropes has significant potential to replace steel wire ropes in civil engineering and hoisting applications. The high strength-to-weight ratio and corrosion resistance are beneficial in stationary applications such as suspended bridges  and offshore anchoring as well as in cyclically loaded applications like hoisting, where the rope is coiled on a drum or bent over a sheave . Although the application of non-destructive testing (NDT) methods is mature and well established for metallic ropes , the same does not apply to synthetic fiber ropes. In addition to damage from in-service static and cyclic loading, these new kind of ropes could contain manufacturing defects and get damaged during transportation, installation or maintenance operations. Periodic visual inspection of ropes is tiresome, subjective, capable of detecting only surface defects and means downtime for the customer . The purpose of this study is to find an online monitoring method that would allow condition-based maintenance, remote assessment of data and full utilization of the life of carbon fiber ropes.
Several methods for inspection of fiber ropes have been suggested, such as methods based on the electrical resistivity of the fibres [4, 5] and optical methods monitoring modal parameters . X-ray inspection can give high-resolution volumetric information of the damage state of a rope , but inspection speeds are slow for online monitoring and there is a large volume to be inspected. Electrical resistivity works well for small samples , but it does not provide information on the location of damage. Furthermore, the relative increase in resistance caused by a small fraction of broken fibres is low compared to the total resistance of a long rope. It is possible to detect local changes by using embedded optical fibres , however, they provide only strain values, which are an indirect indication of damage. Moreover, matrix and interface damage, such as micro-cracking, delamination, environmental aging and debonding, does not cause an increase in strain although it can reduce the strength and service life of a rope . Matrix cracking could be observed using electromagnetic methods if the matrix is made conductive by adding carbon nanotubes . Like embedded sensors, this approach may be difficult to implement into existing products and production lines.
This paper aims to evaluate three NDT techniques, based on three different physical phenomena, with the intention of using them for continous in-service monitoring of carbon fiber ropes. The compared techniques are thermography, via thermoelastic stress analysis (TSA), ultrasonic testing and eddy current testing (ECT).
The TSA analysis is one typical method applied to polymer composites, including carbon fiber reinforced plastic (CFRP), to detect different kinds of damage, but most sensitive to delaminations [12, 13, 14]. The reflection or transmission of ultrasonic waves can be used to detect interfaces or heterogeneities in CFRP components . The principle of ultrasonic inspection is similar to the thermal waves of the TSA phase image and is therefore especially sensitive to delaminations . Eddy current testing  is typically used for inspecting undulations in carbon fiber reinforcement fabrics, quality control of stacking sequence, fiber orientation and curing effects . Delamination detection has been proposed  and shown with artificial delaminations made with interply release film . Artificial cracks made by slitting the fabric before lamination have been detected [17, 19, 20] as well as impact damage [16, 21]. Here, an approach is presented based on custom-made probes specifically designed with unidirectional CFRP material properties in mind. Furthermore, the special requirements of online monitoring of ropes have not been considered in the context of ECT. Namely inspection speed, contactless monitoring and sufficient distance to the monitored component. In this work, the potential of the ECT technique is enhanced by using custom-made probes.
Electrical resistivity measurements were made on the CFRP elements using different methods. The CFRP bulk resistivity was measured in three directions (XYZ) represented in Fig. 1 using a four-point probe with four tungsten carbide needles, arranged along a straight line and spaced 0.635 mm, d [m], from each other. A current source forced a constant electrical current I [A] through the external needles. A 2182A nanovoltmeter simultaneously measured the voltage V [V] produced between the inner needles. Assuming equally spaced contact points, the resistivity of a bulk material (semi-infinite in lateral dimension and with t ≫ d) is given by Eq. 1.
Electrical resistivity and conductivity of the CFRP sample for XYZ directions
Electrical resistivity [mΩ m]
Electrical conductivity [S m−1]
Artificial damage was produced, and quasi-realistic damage was induced, to both the individual uncoated CFRP elements and the CFRP rope system, which comprises of four parallel CFRP elements completely surrounded by a protective polymer coating. Samples with artificial damage were produced by machining, via drilling of flat-bottomed holes in the individual uncoated CFRP elements and partial through-thickness sawing of the CFRP rope system. Internal defects can not be produced by cutting operations and therefore quasi-realistic damage was also created with mechanical loading to the individual rods, via three-point bending, inducing delamination damage, and to the whole rope system, via low-velocity drop-weight impact, inducing multiple (complex) damage types.
Defects in the samples: A and B are uncoated CFRP elements; C and D are rope specimens
Type of defect
Ø 2 mm flat-bottomed hole defect produced via drilling
Delamination induced via three-point bending
In-depth cut with penetration of 1/3 thickness produced via sawing
Multi-damage induced via low-velocity drop-weight loading (impact damage 15 J)
Coated rope samples were subjected to low-velocity drop-weight loading according to American Society for Testing and Materials (ASTM) D5628-10 . A weight of 1530 g with a hemispherical 16 mm diameter impact head was dropped from a height corresponding to a potential energy of 15 J. The samples were clamped to a steel frame with an Ø 76 mm opening during the impact test, making it essentially a constrained three-point bending setup. In addition, large fiber breaks were produced by sawing a transverse cut in the sample approximately 1 mm deep. The width of the cut was 2 mm.
4 Thermography via Thermoelastic Stress Analysis (TSA)
5 Ultrasonic Testing
Parameters of the ultrasonic inspection
Olympus Omniscan MX
64 elements in banks of 16 with 1 element (0.5 mm) pitch
Line scan resolution
Distance to sample and focal distance
Skew angle relative to Z
6 Eddy Currents Testing (ECT)
Thermography via TSA and ultrasonic testing are sensitive to interfaces interfering with wave propagation, but ECT presented here relies on using alternating current in an excitation coil, which causes a primary magnetic field, leading to induction of eddy currents in the material under inspection. The eddy currents cause a secondary magnetic field, which is measured using a sensing coil. Damage in the conducting constituents, namely carbon fibers, can cause changes in the eddy current paths, which alters the secondary magnetic field when compared to undamaged material. However, the damage does not necessarily have to be in the conductive constituents, because the quasi-UD carbon fibers are in contact with each other, leading to transverse bulk conductivity. Therefore, delaminations could, in theory, be detected as well, although the transverse bulk conductivity is much lower than the longitudinal conductivity.
6.1 Numerical Simulations
The carbon fiber material properties used for the simulation were the measured electrical conductivity (Table 1) and a relative magnetic permeability of μr = 1.
6.2 Probe Design
6.3 ECT Experimental Implementation
The experimental implementation of the custom-made ECT probes was performed by means of an automated scanning device  responsible for the carbon fiber rope system movement while the probe remains stationary. Each one of the four CFRP elements was inspected one at a time. The movement, as well as the signal acquisition, were controlled and programmed in LabVIEW environment. Between each acquisition point, the sample moved ΔZ = 500 μm. The equipment responsible for the excitation and signal processing was an Olympus Nortec 500. The sampling rate of the equipment is 6 kHz. Absolute commercial pencil probes from various manufacturers were tested under the same inspection conditions without success. The distance between the probe and the carbon fiber rope system was about 3 mm.
The requirements for online monitoring of carbon fiber composite ropes are challenging because several types of damage need to be detected and localised with fast and contactless inspection. In addition, information on damage size and morphology would help estimating the remaining service life, e.g., via modelling.
Thermography via TSA is able to detect the size and location of both fiber damage and delaminations. However, accuracy of the method is based on a large number of loading cycles of the same field-of-view, making it slow for online monitoring purposes. In addition, TSA works only for bare CFRP rods whereas the protective polymer coating, covering the rope, blocks infrared radiation and thus prevents monitoring rapid changes in surface temperature of the structural CFRP element.
Ultrasonic testing using phased-array probes can be used for inspection of coated ropes and it gives information on damage size and location. However, the coating causes significant attenuation making it crucial to obtain good coupling between the probe and rope surface. In practice, water coupling is needed which makes ultrasonic testing cumbersome to apply in field conditions.
Eddy current testing (ECT) can, however, detect various types of fiber damage at high inspection speeds, high signal-to-noise ratios and provides some indication of the extent of damage via signal amplitude. It requires no contact and is insensitive to non-conductive coatings. ECT is more sensitive to defects located near the surface, since current density is largest at the surfaces.
As commercially available ECT probes were insufficient for inspecting the highly anisotropic CFRP material, two new dedicated probes were developed based on results from numerical simulations. The probe technology is relatively inexpensive and opens up opportunities for developing application-specific probes. Future research will focus on optimising these ECT probes based on the physical properties of unidirectional CFRP. The high data acquisition rate and inexpensive probe construction allows high-speed inspection of multiple ropes simultaneously during operation. It also produces an abundance of simple impedance data, which can be automatically filtered and analysed without the need of human interpretation. Eddy current testing is therefore a promising technique for real-time online monitoring of carbon fiber ropes while other techniques can provide complementary information once the damage location is known.
Open access funding provided by Aalto University. KNA would like to acknowledge the Finnish Funding Agency for Technology and Innovation (TEKES). MAM acknowledge Fundação para a Ciência e a Tecnologia (FCT-MCTES) for its financial support via the PhD scholarship FCT-SFRH/BD/108168/2015. TGS acknowledge FCT-MCTES for the financial support in the scope of PEst-OE/EME/UI0667/2014 (UNIDEMI).
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