Ultrasonic C-scan Inspection Research Articles

Experimental investigation of impacted multidirectional laminates under compressive static and cyclic loading

In this paper, the static and fatigue behaviors under compression of Multi-Directional (MD) Carbon Fiber Reinforced Polymer (CFRP) laminates with impact damage were investigated. After low-velocity impact performed on three 24-ply Quasi-Isotropic Quasi-Homogeneous (QIQH) laminates having different stacking sequences, the influence of the distribution of interfaces between two plies on the compression after impact and post-impact compression fatigue properties was evaluated. The damages on the surfaces and internal delamination were accessed via visual and optical observation and ultrasonic C-scan inspection. The thermographic approach, which has been developed and validated on numerous non-impacted composite materials, was applied for the rapid determination of the fatigue limit together with the S-N curves of post-impact fatigue specimens. Experimental results showed that with 5 J impact energy, the three stacking sequences gave similar impact results. The residual strengths measured by compression after impact tests were also similar to each other, so did the global damage modes. The determined fatigue limits of impacted specimens of different stacking sequences were nearly identical and the predicted S-N curves were also close to each other despite the exact crack propagation and orientation were not the same, under both compressive static and cyclic loads.

Effect of seawater aging on impact behavior of glass fiber/epoxy laminates with drilled holes

During their service life, composite marine structures made up of glass fiber reinforced polymers are susceptible to be affected by environmental and loading conditions such as seawater and low velocity impacts which can cause them severe structural damage. Based on this fact and concern, the present work investigates the influence of seawater aging on impact behavior of glass fiber/epoxy composite laminates with and without drilled holes. To achieve this goal, glass fiber/epoxy laminates were manufactured by resin transfer molding and then cut into specimens for low velocity impact testing. Before impact tests, specimens were immersed in artificial seawater at 70°C for 1464 h. After that time, dry (before seawater aging) and wet (after seawater aging) specimens were tested at 10 and 15 J impact energies using a drop-weight impact tower. The results of accelerated seawater aging confirmed that the presence of drilled holes in composites does not influence on their moisture absorption content and diffusion coefficient. The impact test results showed that the impact properties of wet specimens are significantly lower respect to dry specimens. It was found that the peak force of wet specimens displays a reduction of approximately 40% with respect to the dry specimens as a consequence of moisture absorption which causes plasticization of the polymeric matrix. Ultrasonic C-scan inspection showed that damage area of wet specimen increased up to 250% compared to that of dry specimen. Fractographic analyses by scanning electron microscopy and X-ray computerized tomography revealed that low velocity impact in composite laminates produced damage mechanisms associated to matrix cracking, fiber/matrix debonding, fiber breakage, and delamination, which were more prominent in laminates with drilled holes and subjected to a previous seawater exposition.

Automated immersion ultrasonic testing technology for debonding defects of the brazed joint of DOME in EAST divertor

Aiming at the problem of non-destructive evaluation of debonding defects on joint interface of DOME in EAST divertor, an automatic water immersion ultrasonic C-scan testing technology based on a six-axis robot is proposed in this research. First, the research and development ideas, principles and control logic of the testing system are explained. Second, testing technologies such as probe selection, reference sensitivity and gate setting, and scanning path planning based on robotic arm teaching technology are introduced in detail. Finally, the testing results of the test blocks and products are presented, and the distribution state and formation reasons of the debonding defects on the bonding surface are analyzed. The results show that the automatic water immersion ultrasonic C-scan inspection system based on the six-axis robot has good adaptability to the surface and high testing sensitivity, which can meet the requirements for product quality and technical specifications. In summary, the proposed technology can realize the non-destructive quantitative assessment of the debonding defects of the brazed joint of DOME.

Debonding Failure Analysis of Honeycomb Sandwich Composite Radome

The radome made of C sandwich structure failed after different service time. The failure causes were analyzed by ultrasonic C-scan inspection, fracture surface inspection, section metallographic inspection, bonding strength test, manufacturing process analysis and viscidity testing of film. The results show that: The debonding position of honeycomb composite radome is between honeycomb and outer skin. That is caused by the weak adhesive strength between honeycomb and skin. The main cause of weak adhesion is that the film loses fluidity and viscidity due to the overlong holding time. Weak adhesion is not related to the heating temperature, compressive stress and local honeycomb contamination. The holding time should be controlled to be less than 6h in later manufacture of the radome to keep fluidity and viscidity of film.

Smart Bondline Monitoring of an Efficient Industrial Thermoplastic Aircraft Window Frame

In this work, a smart thermoplastic window frame for a regional aircraft has been designed and manufactured. The aim of the work was to design a smart sensing system for monitoring of a bonded thermoplastic aircraft window frame in operation. The conductive tracks were designed and inkjet-printed onto the window frame and their disruption was used as an indication of a damage event created within the bondline. Based on the electrical resistance measurements, the method was able to detect a damage that was created in the bondline due to an impact event. To verify the proposed methodology, ultrasonic C-scan inspection was also performed.

Reduction of slamming damage in the hull of high-speed crafts manufactured from composite materials using viscoelastic layers

For high-speed crafts, the shift to new materials was initially driven by a need to obtain lighter, stronger vessels, with complex geometries and adapting the mechanical properties of the material to the needs of the design. A new experimental set-up has been designed to reproduce the slamming impacts in small samples of the material, at different velocities and impact energies. Ultrasonic C-scan inspection of each of the prepared specimens has been conducted to evaluate its quality level and the absence of porosity zones. After a certain number of cycles of impacts, each panel was removed and inspected again to assess damage evolution, and the micromechanisms of interlaminar and intralaminar damage propagation. Damage levels have been correlated to residual strengths using AITM-0010 test method for determination of compression strength after impact. It is shown how is possible to effectively mitigate the damage by slamming by inserting a viscoelastic layer into the GFRP panels. The structured combination of a polymer with high deformation capacity with domains of a second, rigid polymer, dampens the impacts and delays the damage progression, improving the dissipation of impact energy, thus protecting the structure of the hull and increasing the service life of the high-speed crafts.

Influence of low-velocity impact on residual tensile properties of nonwoven flax/epoxy composite

This paper addresses the damage resistance and post-impact damage effect on residual tensile properties of nonwoven flax fibers reinforced epoxy composites subjected to low-velocity impact. Two different impactors: hemispherical and conical, at six different impact energy levels: 4 J, 6 J, 8 J, 10 J, 12 J and 14 J were assessed. The experimental results to investigate the influence of impactor type suggest that the penetration of the impactor and induced damage are more important with a conical impactor. The post-impact damage patterns and failure mechanisms of impacted samples were characterized by ultrasonic C-scan inspection. Results suggest that damage induced by the impact included matrix cracking, and delamination, which are more important with a conical impactor. Tensile properties show a significant effect of the impact induced damage on the performance of the composite. A particular effect was identified for an impact energy from 8 J, where the tensile modulus E0 and E1 are decreased by 53.5% and 59.3%, respectively. This effect was also confirmed by the examination of the strain map, describing the deformation behavior of the material at different impact energy, where a stress concentration localized in the impacted zones was identified.

Degradation of Mode-I Fracture Toughness of CFRP Bonded Joints Due to Release Agent and Moisture Pre-Bond Contamination

An experimental investigation of the effects of pre-bond contamination on Mode-I fracture toughness of carbon fiber reinforced plastic (CFRP) bonded joints is presented in this paper. Two pre-bond contamination scenarios were considered; namely, the silicon-based release agent and moisture. The two contamination scenarios were realized in one of the composite substrates prior to bonding. The common characteristic of the two contamination scenarios is that they lead in the formation of defects in the form of weak bonds that cannot be detected by conventional non-destructive testing techniques. The contamination effects on Mode-I fracture toughness of the bonded joints were investigated by conducting mechanical tests on double-cantilever beam specimens and comparing the results with relative measurements taken from reference specimens. Prior to mechanical testing, the bonding quality of the specimens was tested using ultrasonic C-scan inspection. Both the release agent and moisture are found to significantly degrade the Mode-I fracture toughness of the joints. For the release agent, the effect was more significant for silicon concentrations over 5 at%; a complete lack of adhesion was observed for silicon concentrations over 7 at%. At low values of relative humidity, there was a small increase in Mode-I critical energy release rate while at larger values there is a decrease which reaches 26% for the higher relative humidity percentage. The results from the Non-Destructive Testing (NDT) tests verify the inability of conventional NDT to detect the defects resulting at the interface between the contaminated adherend's surface and the adhesive for both contamination scenarios.

Assessment of cumulative damage by using ultrasonic C-scan on carbon fiber/epoxy composites under thermal cycling

In recent years, structural composites manufactured by carbon fiber/epoxy laminates have been employed in large scale in aircraft industries. These structures require high strength under severe temperature changes of -56° until 80 °C. Regarding this scenario, the aim of this research was to reproduce thermal stress in the laminate plate developed by temperature changes and tracking possible cumulative damages on the laminate using ultrasonic C-scan inspection. The evaluation was based on attenuation signals and the C-scan map of the composite plate. The carbon fiber/epoxy plain weave laminate underwent temperatures of -60° to 80 °C, kept during 10 minutes and repeated for 1000, 2000, 3000 and 4000 times. After 1000 cycles, the specimens were inspected by C-scanning. A few changes in the laminate were observed using the inspection methodology only in specimens cycled 3000 times, or so. According to the found results, the used temperature range did not present enough conditions to cumulative damage in this type of laminate, which is in agreement with the macro - and micromechanical theory.

Experimental and numerical investigation of the influence of imperfect bonding on the strength of NCF double-lap shear joints

The influence of imperfect bonding, owing to partial lack of adhesive, on the strength of composite non-crimp fabric (NCF) double-lap shear (DLS) joints was experimentally and numerically investigated. Fabrics were layered and compacted using a thermoplastic veil while infiltration of the preforms was done using the vacuum assisted process. Paste adhesive bonding was carried out by implementing the novel insertion squeeze flow process. Quality of adhesive bonding was tested using X-ray imaging and ultrasonic C-scan inspection. The tensile lap shear strength of the DLS joints was determined experimentally. Digital macrographs revealed that the specimens failed due to shear failure of the adhesive (debonding) and fracture of the composite boundary layer. As a second approach, a mesomechanical model based on the FE method and the (homogenized) progressive failure analysis method was developed. In the model, the areas without adhesion, as detected by the C-scans, were included. Numerical simulations of failure initiation and progression at the NCF joint and the adhesive indicated that it is possible to predict the strength and failure mechanisms of the imperfect bonded DLS joints.

Effects of RTM Mold Temperature and Vacuum on the Mechanical Properties of Epoxy/Glass Fiber Composite Plates

The purpose of this study is to investigate the effects of mold temperature, application of vacuum at resin exit ports, and initial resin temperature on the mechanical properties of epoxy matrix woven glass fiber reinforced composite specimens produced by resin transfer molding (RTM). For this purpose, six mold temperatures (25, 40, 60, 80, 100, and 120°C), two initial resin temperatures (15 and 28°C), and vacuum (0.03 bar) and without vacuum (~1 bar) conditions are utilized. Specimens are characterized by ultrasonic C-scan inspection, mechanical tests (tensile, flexural, and impact), thermal analyses (ignition loss and TGA) and scanning electron microscopy. It is observed that mechanical properties of the specimens produced at a mold temperature of 60°C with the application of vacuum and initial resin temperature of 28°C proved to be the highest (e.g., 16, 26, and 43% higher tensile strength, flexural strength, and Charpy impact toughness, respectively, compared to the lowest values attained with mold temperatures other than 60°C while other variables are kept constant). It has been shown that application of vacuum contributes to the final mechanical properties of the produced composites by lowering the percentage of `voids'. In fact, without the application of vacuum, the deteriorations in mechanical properties are as high as 26% loss in Charpy impact toughness and 5% losses in tensile and flexural strength. Additionally, lowering the initial resin temperature is shown to alter mechanical properties (e.g., 14, 12, and 18% losses in tensile strength, flexural strength, and Charpy impact toughness, respectively, when the initial resin temperature is decreased from 28 to 15°C).

Resistance welding of thermoplastic composites skin/stringer joints

An experimental investigation of resistance welding of APC-2/AS4 PEEK/carbon fibre composite using a stainless steel mesh heating element is presented. A special specimen geometry, the skin/stringer configuration, was used to represent a typical reinforced aerospace structural joint. The specimens consisted of a flange, representing a stringer or frame, welded onto a skin laminate. The effects of the welding parameters such as the input power level and clamping distance on the weld quality and performance were investigated. The welding parameters were optimised using short beam shear tests, ultrasonic C-scan inspection and optical microscopy. The mechanical performance of the resistance-welded skin/stringer configuration was investigated using three- and four-point bending tests and the failure mechanisms were characterised by optical and scanning electron microscopy. Two methods were used to reduce the stress concentration at the flange tip. The first method was to machine a 20° taper angle at the edge of the flange and the second one was to create a resin fillet at the flange tip. No mechanical performance improvement was obtained with the resin fillet method but the taper angle method showed 25% mechanical performance improvement when the taper angles were machined after the welding operation.

A Study on the Measurement System Using an Ultrasonic Sensor in Electron Beam Welding

Electron beam welding (EBW) has an important role in industrial applications such as semi-conductor and LCD manufactures because of its high performance and accuracy. Since the operation is done in a vacuum room, it is very difficult to test the weldability as well as the correct welding area of the specimen. In this study, a measurement system of the EBW was developed based on the 3-axis LVDT controlled table. In addition, the algorithm to tracking the welding line has been developed. The weldability of the aluminum specimen was tested by a newly developed measurement system with ultrasonic C-scan inspection.

High velocity impact resistance of composite materials

Composite materials are used in applications that require protection against high velocity impacts by fragment simulating projectiles. In this work, the ballistic performance of two commercially available materials against a fragment simulating projectile (FSP) is studied. The materials used were an aramid fiber with a phenolic matrix and a polyethylene fiber with a thermoplastic film. Impact tests have been carried out, with velocities ranging from 300 m/s to 1260m/s. The projectile used is a 1.1g NATO FSP. Impact velocity and exit velocity are measured, to determine the V 50 and the energy absorbed in cases where perforation occurs. Assessment of the impact damaged area is done using ultrasonic C-scan inspection. Types of damage and damage mechanisms have been identified. Several mechanical tests have been carried out to determine the mechanical properties, at different strain rates. Future work in numerical simulation of impact will be done using commercial code Autodyn® from Century Dynamics.

Manufacture of Composite Panels with J-Shape Stiffeners

Abstract The use of composite materials in primary structure of aircraft is becoming more common. Stiffened composite panels have been designed for a variety of load carrying conditions in aircraft. In this, study, manufacturing techniques for stiffened composite panels were reviewed and discussed. Co-curing and secondary bonding manufacturing techniques were used to produce four stiffened composite panels. Experience gained with these techniques is discussed. Ultrasonic C-scan inspection was carried out to detect any defects in the panels. These panels were tested in compression to verify their structural capability in the post-buckled state. This paper presents the details of the tool design concepts and manufacturing techniques, and identifies and discusses the advantages and disadvantages of each manufacturing technique.

The Fracture Behavior of Carbon/Epoxy Laminates Containing Internal Cut Fibers

Fiber breakage in a composite laminate can be caused by impact loads. A study was conducted to investigate the effect of internal fiber breakage on the tensile strength of carbon/epoxy laminates. The specimens contained internal fiber cuts, which were considered “buried cracks” in this study. Four layups, [0, ± 45,90,02], [90, ± 45,0]s, [0, ± 68,0, ± 68]s, and [0]l2, were studied. The results showed a significant strength reduction due to the internal fiber cut. Delamination occurred under load adjacent to the internal crack prior to the final fracture, as shown by ultrasonic C-scan inspection and micro-sectioning inspection. The dimensions of the delamination zones were affected by laminate layups and the length of internal cracks. Prediction of the residual strength was attempted using a principal load carrying ply failure criterion in conjunction with a 3-dimensional anisotropic finite element analysis. Laminate failure prediction was based on the behavior of individual plies. The predicted residual strength was in general agreement with measured residual strength over all the laminates and conditions investigated, indicating the applicability of the criterion.