C-scan Results Research Articles

A comparative study on glass and carbon fibre reinforced laminated composites in scaled quasi-static indentation tests

This paper investigates the effect of fibre properties of composite structures on the mechanical performance and formation of low-velocity impact damage. Quasi-static indentation tests were conducted on a comprehensive set of scaled Quasi Isotropic (QI) S-glass/8552 epoxy and QI IM7-carbon/8552 epoxy laminates, comparing changes in both in-plane dimensions and fully three-dimensionally scaled cases. Due to the higher thickness of the S-glass laminates, the mechanical results were normalized by a thickness scaling rule to have a fair comparison between the mechanical behaviour. The results demonstrated that the shape of the load-displacement of the S-glass/epoxy laminates is similar to that of the IM7-carbon laminates, with evident changes in rigidity appearing due to the onset and propagation of delamination and final failure caused by fibre breakage. The S-glass/8552 epoxy laminates had smaller load drops, higher deflection and higher mechanical energy absorption before failure compared to the IM7-carbon/8552 epoxy laminates. X-ray computed tomography scanning revealed that delamination is the dominant failure mode for the investigated laminates, and the shape of delamination was influenced by the ply angles at the interfaces. Comparing the glass and carbon laminates, ultrasonic C-scan results indicated similar delamination damage size for the initiation stage, however the damage size was found to be dependent on the fibre properties and layup sequence in the propagation stage.

Detecting small flaws in two-phase Ti-6Al-4V with rough surfaces

Surface roughness degrades the performance of ultrasonic methods when detecting sub-wavelength flaws. In this work, a roughness-modified doubly-scattered response (DSR) model is developed to enhance the flaw detection method for two-phase Ti-6Al-4V with rough surfaces. Extreme value statistics are used to calculate the confidence bounds of grain noise, then the bounds are treated as a time-dependent threshold for segmenting flaws in C-scan images of two-phase Ti-6Al-4V with rough surfaces. Three Ti-6Al-4V samples with different surface roughness are designed and manufactured for validating the present method; ultrasonic C-scan results show that it can distinguish sub-wavelength flaws (about 1/3 wavelength) in two-phase Ti-6Al-4V with rough surfaces, which can be extended to industry applications.

Interfacial fracture behavior of a stainless/carbon steel bimetal plate in a uniaxial tension test

The interfacial fracture behavior of a stainless/carbon steel bimetal plate in a uniaxial tensile test was investigated in this paper. The ultrasonic C-scan results showed that the internal fracture had already appeared in the necking area before the tensile sample snapped, and the internal fracture occurred in the middle of the necking area and propagated with the tensile strain increase. The fractography showed that the internal fracture occurred at the interface between the diffusion layer and carburization stainless steel. The contribution of the internal stress to the interface fracture was analyzed by finite element method. The FEM results showed that, due to the different material mechanical properties, the large stress difference between the diffusion layer and the carburized stainless steel induced the interface shear stress. The shear stress value was related to the location and tensile strain, and when the stress difference was larger than 500 MPa, the interface would crack.

Eddy current O-scan and C-scan imaging techniques for macrocrack detection in silicon solar cells

ABSTRACTCrack detection is important in the quality control of silicon-based solar cells (SSC). Silicon wafers are semi-conductive and cracks disturb the local conductivity and even induce anisotropy in conductivity of SSC. Eddy current method, which can detect the change of conductivity in the material, is a potential tool for detecting cracks in SSC. In this study, an experimental set-up is proposed to conduct both eddy current O-scan and C-scan imaging to the SSC. The eddy current O-scan was firstly used to investigate the anisotropy in the conductivity of SSC. Through comparing the inherent and crack-induced anisotropy in impedance (amplitude ratio and phase lag), macrocracks could be detected. The changes in the measured directional impedance could roughly reflect the shape of the cross-type macrocrack. In the C-scan mode, multiple slit defects were prepared to simulate macrocracks. Though the lateral resolution of the sensor was limited, the crack detection ability of the sensor could be evaluated with the C-scan imaging results. A slit defect with a width of 0.5 mm and a length of 1 mm was detected. A new criteria, the distance between feature zero-crossing points in the first derivative of the phase lag curve accurately reflected the actual length of the slit.

Preparation of CFRP with modified MWCNT to improve the mechanical properties and torsional fatigue of epoxy/polybenzoxazine copolymer

This study proposes a novel method for preparing a high-performance laminated nanocomposite that simultaneously offers low weight, high strength, long material life, high weatherability, and high environmental friendliness. 4,4-Methylene diphenyl diisocyanate (MDI) was grafted onto multiwall carbon nanotube (MWCNT) surfaces, and hot-compression exhaust method was used to prepare nanocomposites and carbon-fiber-reinforced polymer (CFRP) composites, to improve the dispersivity of MWCNTs in the matrix while reducing interlaminar defects in CFRP, and thereby, enhance the mechanical strength and torsional fatigue life of the material. The experimental results showed 11% increase in storage modulus, with the glass transition temperature increasing from 102 °C to 158 °C. The drop-weight impact test and C-scan results indicated that the hot-compression exhaust method effectively constricted the expansion caused by delamination and damage through denser and tighter bonding between laminates of prepreg cloth. The torsional fatigue strength was also enhanced, with the average fatigue life increased more than 20 times.

Comparison of the impact damage resistance of non-hybrid and intra-ply hybrid carbon/E-glass/polypropylene non-crimp thermoplastic composites

This study aims to compare the low-velocity impact and post-impact properties of intra-ply hybrid carbon/E-glass/polypropylene non-crimp thermoplastic composites with non-hybrid carbon/PP and E-glass/PP non-crimp thermoplastic composites. Impact test was performed at four energy levels as 15 J, 30 J, 45 J and 60 J. Post-impact properties of hybrid thermoplastic composites were tested by compression after impact method for each energy level to understand the impact damage tolerance of intra-ply hybrid carbon/E-glass/PP non-crimp thermoplastic composites. The effect of hybridization on energy absorption of composites was not significant, while C-scan results showed that the intra-ply hybrid non-crimp thermoplastic composites had smaller impact damage areas in comparison to the non-hybrid samples. Compression and compression after impact tests results confirmed that the intra-ply hybridization increased the toughness of the composite laminates. Also, the residual compression strength/modulus increased with hybridization which indicated to damage tolerance.

Quantitative characterization of disbonds in multilayered bonded composites using laser ultrasonic guided waves

A quantitative method for evaluating the sizes of defects has been proposed to be applied to laser ultrasonic guided waves testing multilayered bonded composites. The finite element model was built for studying on laser-generated ultrasonic waves in a bonded composite. A laser ultrasonic inspection system using lasers for generation and detection of ultrasonic waves was established to be utilized to conduct a series of experiments for verifying the simulation results. The lowest guided wave modes generated by a pulsed laser were used to characterize a bonded joint. The interaction of laser ultrasonic guided waves with disbonds was investigated to determine a reasonable source-receiver distance to reach a compromise between detection accuracy and efficiency. The −8 dB method has been proposed to measure the sizes of disbonds based on the scanning laser source technique, and the relative errors between the measurement sizes and the actual sizes are not more than 8%. The C-scan results based on laser ultrasonic guided waves jointed with the proposed quantitative method not only facilitate the visualization of disbonds, but also provide precise information on the bonded quality, which has great potential for on-line inspection of adhesively bonded composites.

High-speed infrared thermal imaging during ballistic impact of triaxially braided composites

Ballistic impact experiments were performed on triaxially braided polymer matrix composites to study the heat generated in the material due to projectile velocity and penetration damage. Triaxially braided (0/+60/−60) composite panels were manufactured with T700S standard modulus carbon fiber and two epoxy resins. The PR520 (toughened) and 3502 (untoughened) resin systems were used to make different panels to study the effects of resin properties on temperature rise. The ballistic impact tests were conducted using a single stage gas gun, and different projectile velocities were applied to study the effect on the temperature results. Temperature contours were obtained from the back surface of the panel during the test through a high speed, infrared thermal imaging system. The contours show that high temperatures were locally generated and more pronounced along the axial tows for the T700S/PR520 composite panels; whereas, tests performed on T700S/3502 composite panels, using similar impact velocities, demonstrated a widespread area of lower temperature rises. Nondestructive, ultrasonic C-scan analyses were performed to observe the failure patterns in the impacted composite panels and correlate the C-scan results with the temperature contours. Overall, the impact experimentation showed temperatures exceeding 252℃ (485°F) in both composites which is well above the respective glass transition temperatures for the polymer constituents. This expresses the need for further high strain rate testing with measurement of the temperature and deformation fields in order to fully understand the complex behavior and failure of the material and to improve the confidence in designing aerospace components with these materials.

Detection of small-flaw in carbon brake disc (C-C) using air-coupled ultrasonic C-scan technique

The purpose of this paper is to conduct the study on detection identification and assessment of internal defects for the C-C brake disc composite for aircrafts by applying non-contact air-coupled ultrasonic testing. The Non-contact Air-coupled Ultrasonic Testing (NAUT) used in this paper is the most advanced ultrasonic testing technique. It is a nondestructive testing that complements the energy loss occurred due to the acoustic impedance difference in the air with two transmitting/receiving non-contact ultrasonic probes and enables the non-contact ultrasonic testing through high-powered ultrasonic focusing by using high-frequency band ultrasonic Pulser- Receiver. In this paper, non-contact air-coupled ultrasonic detection experiment applied the Transmission Method and Reflection Method was conducted as follows; When detecting an internal defect of the carbon brake disc, we compared the C-scan results according to whether masking was applied to the side of the object to be detected, measured at the ultrasonic transmission frequencies of 340 kHz and 400 kHz respectively, to detect internal defects of the carbon brake disc through the measured C-scan images. In this way, we confirmed that there is a significant difference in the internal defect detection performance depending on the masking and the change in the ultrasonic transmission frequency value.

Penetration impact behaviour of hot-compacted self-reinforced polyamide 6 composite laminates

Polyamide 6 based sheets are produced by a typical hot compaction procedure and characterised in terms of compaction quality and penetration low-velocity impact performances. Starting from two commercial clothes, a yellow one fully constituted by PA6 fibres and a hybrid red one containing also 1% of secondary polyurethane fibres, single polymer composite (SPC) plates involving 24, 30 and 36 layers of each fabric were prepared under controlled conditions and studied to define their potential applications. Ultrasonic C-scan results showed a quality of compaction always decreasing with the number of stacked fabric layers (thickness) and, with the same number of cloth layers, lower for red laminates compared to yellow ones, in line with previous morphological observations. Different damage propagation mechanisms occur as highlighted by typical load-deflection curves with hybrid systems showing a more pronounced ductile behaviou and an evident deviation from the penetration energy-thickness linear relationship for red samples is detected.

Characterization of disbonds in a nuclear radiation protection structure using laser ultrasonics

Characterization of disbonds in a nuclear radiation protection structure (NRPS) has been investigated by laser ultrasonics. Characteristics of laser ultrasonic waves in an NRPS have been researched by simulations and experiments. The signal-to-noise ratio of laser ultrasonic signals in the adhesive structure can be improved by wavelet threshold de-noising (WTDN) and band-pass filtering. Original laser ultrasonic signals were processed by WTDN to implement C-scans which have high contrast to noise ratio. Laser ultrasonic C-scan results can accurately characterize the disbonds, which are in reasonable agreement with the immersion ultrasonic detection result. Therefore, laser ultrasonics has potential for non-destructive evaluation of the bonding quality of NRPSs.

Kinematic constraint analysis in a twin-robot system for curved-surface nondestructive testing

Purpose – Nondestructive testing based on cooperative twin-robot technology is a significant issue for curved-surface inspection. To achieve this purpose, this paper aims to present a kinematic constraint relation method relative to two cooperative robots. Design/methodology/approach – The transformation relation of the twin-robot base frame can be determined by driving the two robots for a series of handclasp operations on three points that are noncollinear in space. The transformation relation is used to solve the cooperative motion problem of the twin-robot system. Cooperative motions are divided into coupled and combined synchronous motions on the basis of the testing tasks. The position and orientation constraints for the two motion modes are also explored. Findings – Representative experiments between two industrial robots are conducted to validate the theoretical developments in kinematic constraint analysis. Artificial defects are clearly visible in the C-scan results, thereby verifying the validity and the effectiveness of the proposed method. Originality/value – The transformation relation of the twin-robot base frame is built under a series of handclasp operations. The position and orientation constraints for the coupled and combined synchronous motions are explored. Theoretical foundations of trajectory planning method for the transmitting and receiving transducers of the cooperative twin-robot system are presented.

Experimental investigation on impact performances of GLARE laminates

An experimental investigation was carried out on the damage resistance to a concentrated quasi-static indentation force and low-velocity impact of four kinds of glass-reinforced aluminum laminates (GLARE for short). Compared with the experimental results of the CFRP (Carbon Fiber Reinforced Plastics) laminates, the performance of GLARE was determined. By means of concentrated quasi-static indentation force test, typical force–displacement response, the maximum contact force and dent depth were received. Through drop-weight low-velocity impact tests, impact force histories, indentation depths (through a new method) and dissipated energy were obtained. The test results show that the force–displacement response of GLARE 4 laminates under the concentrated quasi-static indentation force has an obvious flat roof and the failure is instantaneous, which are different from CFRP laminates. The indention will be visible once the impact happens. C-scan results find that there is no delamination besides the impact area after both the concentrated quasi-static indentation and low-velocity impact. The dissipated energy approximately equals the impact energy.

Inverse Kinematic Analysis and Evaluation of a Robot for Nondestructive Testing Application

The robot system has been utilized in the nondestructive testing field in recent years. However, only a few studies have focused on the application of ultrasonic testing for complex work pieces with the robot system. The inverse kinematics problem of the 6-DOF robot should be resolved before the ultrasonic testing task. A new effective solution for curved-surface scanning with a 6-DOF robot system is proposed in this study. A new arm-wrist separateness method is adopted to solve the inverse problem of the robot system. Eight solutions of the joint angles can be acquired with the proposed inverse kinematics method. The shortest distance rule is adopted to optimize the inverse kinematics solutions. The best joint-angle solution is identified. Furthermore, a 3D-application software is developed to simulate ultrasonic trajectory planning for complex-shape work pieces with a 6-DOF robot. Finally, the validity of the scanning method is verified based on the C-scan results of a work piece with a curved surface. The developed robot ultrasonic testing system is validated. The proposed method provides an effective solution to this problem and would greatly benefit the development of industrial nondestructive testing.

Compression-after-impact (CAI) performance of epoxycarbon fibre-reinforced nanocomposites using nanosilica and rubber particle enhancement

One of the problems in the design of automotive structures and body parts made by fibre reinforced composites is that these materials are susceptible to a small energy impact caused by for instance, accidental tool drop during maintenance or stone strike while in operation. This often lead to a barely visible impact damage which causes reduction in compressive strength of the composite part. To increase the impact tolerance of the composites, toughening agents like silica nanoparticles and rubber particles can be utilized to toughen the resin. To understand the effect of the particles enhancement, the impact tolerance was evaluated utilizing Compression After Impact (CAI) test after the impact induced by gas- gun impacting equipment. The results from CAI test after 20 J impact (high energy stone strike) shows about 30% improvement in residual compressive strength for the nanosilica enhanced composite compared to unmodified CFRP. Also C-scan results after 7 J impact shows about 50% smaller delamination area for the nano-enhanced composite.

Improving compression-after-impact performance of carbon–fiber composites by CNTs/thermoplastic hybrid film interlayer

Abstract Multi-walled carbon nanotubes (MWCNTs) have been effectively dispersed with polyetherketone cardo (PEK-C)/dichloromethane by ball milling technique. Pure PEK-C film and MWCNTs hybrid PEK-C (MWCNTs/PEK-C) films with different MWCNTs content were made employing an solvent casting technology. Transmission electron microscope (TEM) showed the MWCNTs shortened by ball milling. The neat and nano-hybrid films were used as interleaves for carbon fiber/bismaleimide (BMI) composite laminates. C-scan results showed that impact damage area decreased for PEK-C film interleaved laminates and further reduced by modification of MWCNTs. The compression after impact (CAI) strength was significantly improved for interleaved composite laminates when compared to baseline composite laminate. Maximum improvement (∼33%) of CAI value was achieved by interlayer of hybrid film with 10 wt% MWCNTs. The toughening micromechanism of enhanced resistance to low-energy impact with the incorporation of MWCNTs were investigated based on scanning electron microscopic (SEM) images of the fracture surfaces of the tested specimens. It is suggested that thermoplastics, BMI particles and MWCNTs had combined action to reduce interlaminar impact damage and thus to improve CAI strength.

Application of Phase Coded Pulse Compression Method to Air-coupled Ultrasonic Testing Signal Processing

The experiment reliability and defect quantification accuracy of air-coupled ultrasonic testing are highly affected by the bad SNR(signal to noise ratio) of the through transmitted signal because of the high damped medium property in air and giant mismatch of acoustic impedance in air-solid interface. To deal with these handicaps, the matching filter design and its implementation in air-coupled ultrasonic inspection based on the theory of phased Coded pulse compression are discussed. Influences of different sequences, bandwidth and cycles of Barker code on pulse compressed results are analyzed either. SNR of the through-transmitted signal and defect quantification accuracy of C-scan results on glass fiber reinforced polymer(GFRP) specimen are compared using single frequency excitation and pulse compression method, respectively. The results show that the smallest time width and peak amplitude of the main lobe are acquired using central frequency of the transducer as bandwidth of Barker code. Using phase coded pulse compression method, the SNR of the through-transmitted signal is increased about 12.4 dB; the quantification accuracy is increased about 12% compared to the single frequency excitation, and the edge of the defect is better highlighted. The results represent that the SNR of the through transmitted signal and defect quantification accuracy are apparently increased by using phase Coded pulse compression method in air-coupled ultrasonic testing.

Bond Interface Evaluation by Ultrasonic Testing and SEM for Radial Friction Welding Joints of Copper and Steel

The bonds of T2 copper/50steel were studied by inertial radial friction welding techniques at different conditions. Ultrasonic C-scan testing was employed to evaluate the bonding situation. In order to verify the reliability of ultrasonic C-scan testing results, the optical microscope, SEM and shear test were used to analyse the microstructure and mechanical properties. It was found that ultrasonic C-scan testing was useful to evaluate the inertial radial friction welding joint situations by analyzing the ratio of good bonded area in the bond area of ultrasonic C-scan images. And the correctness of ultrasonic C-scan testing was testified by microstructure examination. Furthermore, it was concluded that enough metalline plastic deformation occurred, oxide film of joint was destroyed and constituent elements were mutually diffused sufficiently, when two stage pressure was employed and friction pressure 78 MPa, friction speed 3200r/min were used.

Detection of Impact Damages in CFRP Composites Using Ultrasonic Burst Phase Thermography

Carbon fiber reinforced polymer (CFRP) composites have been used extensively because of their excellent performance, but they are susceptible to the impact damages, such as the impact of birds, runway stones and tools off can cause the failure of CFRP composites. Therefore, nondestructive testing of CFRP composites is necessary to promote the failure control and prevention. Ultrasonic burst phase thermography (UBP) developed a few years ago is a defect-selective and fast imaging tool for damage detection. This paper describes the principle of UBP, gives some UBP testing results of impact damages in five CFRP laminated boards. The shape and position of the damages are shown in the phase images visualized and the relationship between the damage areas and impact energy is discussed. The flash pulse thermography (PT) results and ultrasonic C-scan results are also shown as comparison. It is concluded that all of the three NDT methods have their own characteristics and the comparison verifies the feasibility of CFRP impact damage detection using ultrasonic burst phase thermography.

Pulsed eddy current imaging and frequency spectrum analysis for hidden defect nondestructive testing and evaluation

Hidden defect characterisation in some complex structures is difficult. Pulsed Eddy Current (PEC) imaging based on rectangular excitation coil is investigated in this paper and hidden defect nondestructive testing and evaluation (detection, classification, and quantification) is carried out based on the various C-scan images. Experimental results have illustrated that hidden defects can be identified effectively by particular character in C-scan imaging results and sub-surface defects can be discriminated to correct class by selecting the rising time from response in time domain. The quantification information of hidden defects is preliminarily obtained based on the contour and 3D images. In addition, PEC imaging and frequency spectrum analysis are effective to detect, classify, and evaluate the sub-surface defects under the influence of edge effect of specimen. To sum up, PEC imaging is an effective approach to characterise hidden defects and sub-surface defects.