Solid Laminate Research Articles

Assessment of residual strength of repaired solid laminate composite materials through mechanical testing

Robust residual strength test methods are critical for assessing the validity of new repair techniques for restoring strength to damaged composite laminates. In this study, the strength of pristine, damaged and repaired solid laminate composite material has been investigated by performing four different mechanical tests. Impact damage was imparted to carbon fibre epoxy resin specimens using a drop tower instrument. Specimens were repaired using a novel resin injection technique employing a low viscosity adhesive as the repair resin. The compressive strength of specimens was determined using compression after impact (CAI) and large-scale CAI test methods. Tensile tests were performed to obtain tensile strength. Flexural strength was evaluated by four-point bend tests. The thoroughness of each test method in determining the success of the repair was investigated. This study demonstrates that the strength restoring capacity of a repair method is subject to the test method used to assess the residual strength. The four-point bend test was shown to be the best preliminary test method for screening repair techniques as it is not affected by loading anomalies. However, design criteria should always be considered when choosing a residual strength test method for assessing repair validity.

テーパー角変更によるサンドイッチパネルtapered end-closure結合部の初期損傷抑制

For the configuration of the CFRP face plates/foamed plastic core sandwich panel joint, a tapered end-closure type joint is selected and studied. In this type of joint, the sandwich panel is tapered to form the solid laminate of two face plates near the joint portion and two panels to be joined are mechanically fastened at the solid laminate portions with a splice plate. This type of joint could be suitable for the aircraft panel joint configuration, because a flat surface can be obtained at the joint portion, which is advantageous in an aerodynamic point of view. However, in a previous study, it was reported on the results of tensile strength test that the delamination crack initiating from tapered core end and propagating through the interface between two face plates was observed as an initial failure mode at much lower tensile load compared to the final failure load. In this study, the angle of tapered panel portion is focused on and the initial failure suppression effect caused by the change of taper angle is investigated through experiment and numerical analysis. And it is found that the smaller taper angle is preferred to suppress the initial failure.

Ultra-light CRH wind deflector fabricated by woven lattice sandwich composites

An alternative design method was suggested to design an ultra-light hierarchical wind deflector fabricated by woven lattice sandwich composites (WLSCs). The alternative method is based on the criterion that the alternative sandwich structure should have equal bending strength and bending rigidity to the replaced solid composite laminate. Two alternative structures for wind deflectors were suggested and analyzed based on corresponding structural models. Stress and strain predictions were well consistent with the experiments, which reveal that adopting hierarchical WLSC structure, the new designed wind deflector has greater load capacity, and meanwhile the weight is reduced at a level of more than 50%.

Nondestructive testing potential evaluation of a terahertz frequency-modulated continuous-wave imager for composite materials inspection

The sub-terahertz (THz) frequency band has proved to be a noteworthy option for nondestructive testing (NDT) of nonmetal aeronautics materials. Composite structures or laminates can be inspected for foreign objects (water or debris), delaminations, debonds, etc., using sub-THz sensors during the manufacturing process or maintenance. Given the harmless radiation to the human body of this frequency band, no special security measures are needed for operation. Moreover, the frequency-modulated continuous-wave sensor used in this study offers a very light, compact, inexpensive, and high-performing solution. An automated two-dimensional scanner carrying three sensors partially covering the 70- to 320-GHz band is operated, using two complementary measurement approaches: conventional focused imaging, where focusing lenses are used; and synthetic aperture (SA) or unfocused wide-beam imaging, for which lenses are no longer needed. Conventional focused imagery offers finer spatial resolutions but imagery is depth-limited due to the beam waist effect, whereas SA measurements allow imaging of thicker samples with depth-independent but coarser spatial resolutions. The present work is a compendium of a much larger study and describes the key technical aspects of the proposed imaging techniques and reports on results obtained from human-made samples (A-sandwich, C-sandwich, solid laminates) which include diverse defects and damages typically encountered in aeronautics multilayered structures. We conclude with a grading of the achieved results in comparison with measurements performed by other NDT techniques on the same samples.

Nondestructive Evaluation of Mechanically Loaded Advanced Marine Composite Structures

Marine composite structures subject to dynamic loading typically incorporate more than one material type, and consist of laminate sections up to hundreds of millimetres in thickness. These solid hybrid laminates exhibit different behaviour in static and fatigue loading from thin aerospace composite laminates and sandwich structures. There is therefore a need to better understand the likely damage and degradation mechanisms that will occur in these thick structures and to concurrently develop nondestructive evaluation (NDE) technology to meet the consequent inspection problems. In this paper we present details of an ongoing fatigue program on marine composite blades. The challenges for ultrasonic NDE of thick composites, and emerging inspection methods using state-of-the-art inspection systems and analysis tools will be discussed.

Compression After Impact and Fatigue of Reconsolidated Fiber-reinforced Thermoplastic Matrix Solid Composite Laminate

Carbon fiber-reinforced poly-phenylene sulfide laminate coupons were impacted at low-energy in a drop-tower machine and subsequently fatigued in a four-point bending fixture. The doubly damaged test pieces were then hot-press reconsolidated and inspected nondestructively by vibrothermography to check their structural integrity. The residual mechanical properties of the laminate in both the as-damaged and as-repaired conditions were determined by compression loading with the in-plane strain fields determined via a digital image correlation system. Cross-section views of damaged and repaired samples were analyzed by light optical microscopy and correlated to residual mechanical properties, as were the digital image correlation and nondestructive test results. Based on the values of stiffness and ultimate strength of the repaired laminates, 10J was inferred as the maximum impact energy at which it is worthwhile performing hot-press reconsolidation, in view of the applied fatigue history following impact.

Terahertz Wave Approach and Application on FRP Composites

Terahertz (THz) applications have emerged as one of the most new powerful nondestructive evaluation (NDE) techniques. A new T-ray time-domain spectroscopy system was utilized for detecting and evaluating orientation influence in carbon fiber-reinforced plastics (CFRPs) composite laminates. Investigation of terahertz time-domain spectroscopy (THz-TDS) was made, and reflection and transmission configurations were studied as a nondestructive evaluation technique. Here, the CFRP composites derived their excellent mechanical strength, stiffness, and electrical conductivity from carbon fibers. Especially, the electrical conductivity of the CFRP composites depends on the direction of unidirectional fibers since carbon fibers are electrically conducting while the epoxy matrix is not. In order to solve various material properties, the index of refraction (n) and the absorption coefficient (α) are derived in reflective and transmission configurations using the terahertz time-domain spectroscopy. Also, for a 48-ply thermoplastic polyphenylene-sulfide-(PPS-) based CFRP solid laminate and nonconducting materials, the terahertz scanning images were made at the angles ranged from0°to180°with respect to the nominal fiber axis. So, the images were mapped out based on the electrical field (E-field) direction in the CFRP solid laminates. It is found that the conductivity (σ) depends on the angles of the nominal axis in the unidirectional fiber.

T-Ray NDE Inspections on the Fiber Direction of Thermoplastic PPS-Based CFRP Composites

Terahertz ray (T-ray) imaging has been emerged as one of the most promising new powerful nondestructive evaluation (NDE) techniques, and new application systems are under processing development for the area applications. The terahertz time domain spectroscopy (THz TDS) can be considered as a useful tool using general non-conducting materials; however it is quite limited to conducting materials. In this study, a new time-domain spectroscopy system was utilized for detecting and evaluating layup effect and flaw in FRP composite laminates. In order to solve various material properties, the index of refraction (n) and the absorption coefficient (α) are derived in reflective and transmission configuration using the terahertz time domain spectroscopy. However, the T-ray is limited in order to penetrate a conducting material to some degree. So, investigation of terahertz time domain spectroscopy (THz TDS) was made and reflection and transmission configurations were studied for a 48-ply thermoplastic PPS (poly-phenylene sulfide)-based CFRP solid laminate.

Characterization of Fiber Direction Influence in CFRP Composites Using Advanced NDE Techniques

A nondestructive technique would be very useful. Advanced NDE T-ray (terahertz ray) techniques of technology and instrumentation has provided a probing field on the electromagnetic spectrum. However, the T-ray is limited in order to penetrate a conducting material to some degree. Here, the T-ray would not go through easily the CFRP composite laminates since carbon fibers are electrically conducting while the epoxy matrix is not. So, investigation of terahertz time domain spectroscopy (THz TDS) was made and reflection and transmission configurations were studied for a 48-ply thermoplastic PPS(poly-phenylene sulfide)-based CFRP solid laminate. It is found that the electrical conductivity of CFRP composites depends on the direction of unidirectional fibers.

NDE inspection of terahertz waves in wind turbine composites

An investigation of terahertz waves was made for the nondestructive evaluation of composite materials and structures in the wind turbine. The modalities of the terahertz radiation used were time domain spectroscopy (TDS) and continuous wave (CW). The composite materials and structures investigated include both non-conducting polymeric composites and conducting carbon fiber composites. Terahertz signals in the TDS mode resembles that of ultrasound; however, unlike ultrasound, a terahertz pulse was able to detect a smaller crack hidden behind a larger crack. This was demonstrated in thick GFRP (glass fiber reinforced plastics) laminates of the wind turbine using saw slots. In carbon composites the penetration of terahertz waves is quite limited and the detection of flaws is strongly affected by the angle between the electric field direction of the terahertz waves and the intervening fiber directions. The structures used in this study included both solid laminates and the parts in the wind turbine. The defects and anomalies investigated by terahertz radiation were foreign material inclusions, simulated disband, delamination and mechanical impact damage. The effectiveness and limitations of terahertz radiation are defined and noted for the NDE of composites.

Terahertz spectroscopy approach of the fiber orientation influence on CFRP composite solid laminates

Terahertz ray (T-ray) imaging applications have provided one of the most promising new powerful nondestructive evaluation techniques, and new application systems are under process development for area applications. Detecting flaws and defects in fiber reinforced plastic (FRP) composite laminates due to flaws in FRP composite laminate that affect laminate properties, including stiffness, strength, and thermal behavior, is very important. In this study, a new time-domain spectroscopy system was utilized for detecting and evaluating the flaws in FRP solid composite laminates. Extensive experimental measurements in reflection mode were made to map out T-ray images. In particular, electromagnetic properties, such as refractive index, were estimated in this characterization procedure. The estimates of properties were in good agreement with known data. Using these characteristic material properties, we successfully demonstrated the characteristics of the T-ray behavior propagating through FRP composites. Furthermore, layup effect and flaws of FRP composite laminates were observed in reflection mode, and limitations were discussed in the T-ray processing.

Methodology for exciting dynamic shear and moment failure in composite sandwich beams

The increasing use of composites in high performance structural applications subjected to dynamic loading drives the need for understanding basic material failure properties and structural response as a function of loading rate. Balsa core sandwich beams having woven carbon/vinylester composite facesheets were investigated in both shear and bending failure at quasi-static and dynamic loading speeds producing strain rates ranging from 10−5 to 100 s−1. Short sandwich beam tests show weak rate dependency, with a slight decreasing trend in the balsa core shear failure strain for increased strain rate. Wide scatter in core shear failure strain was observed due to the highly variable nature of the balsa material, as well as its heterogeneous tiled construction within the sandwich panel. Excitation of facesheet compressive failure in sandwich beams with weak core of 50.8 mm thickness required the use of 2.43 m long sandwich beams subject to high speed loading (10 m/s) in order to achieve 100 s−1 strain rate. Facesheet compressive failure strain in the long beam specimens under dynamic loading was measured to be 10% higher than for quasi-static loading. Comparison of long sandwich beam bending results to shorter solid laminate bending strength measurements show a different trend in rate dependency, and thus it is recommended that full-depth sandwich beams (and panels) be used to characterize the failure of these types of sandwich structures.

Functionalized graphene oxide for clinical glucose biosensing in urine and serum samples

A novel clinical glucose biosensor fabricated using functionalized metalloid-polymer (silver-silica coated with polyethylene glycol) hybrid nanoparticles on the surface of a graphene oxide nanosheet is reported. The cyclic voltammetric response of glucose oxidase modification on the surface of a functionalized graphene oxide electrode showed a surface-confined reaction and an effective redox potential near zero volts, with a wide linearity of 0.1–20 mM and a sensitivity of 7.66 μA mM−1 cm−2. The functionalized graphene oxide electrode showed a better electrocatalytic response toward oxidation of H2O2 and reduction of oxygen. The practical applicability of the functionalized graphene oxide electrode was demonstrated by measuring the peak current against multiple urine and serum samples from diabetic patients. This new hybrid nanoarchitecture combining a three-dimensional metalloid-polymer hybrid and two-dimensional graphene oxide provided a thin solid laminate on the electrode surface. The easy fabrication process and retention of bioactive immobilized enzymes on the functionalized graphene oxide electrode could potentially be extended to detection of other biomolecules, and have broad applications in electrochemical biosensing.

Recyclability and reutilization of carbon fiber fabric/epoxy composites

Solid carbon fiber/epoxy laminates manufactured by liquid resin infusion of twill fabric reinforcement are recycled in a bath of boiling sulfuric acid to separate the fibers from the matrix. The recycled reinforcement consists of long fibers arranged in a random, entangled mat. Using the same epoxy matrix and infusion materials and process, the recycled fibers are reutilized to manufacture solid laminates. The physical properties of the recycled laminates are evaluated by means of pulse-echo ultrasound, visual microscopy, and fiber volume content. The average fiber volume content of the recycled laminates is 33%, compared to the 62% of the twill laminates. The mechanical properties of the recycled composite include tension modulus and strength, compression modulus and strength, three-point bend flexure strength, and short beam shear strength. The properties are compared against the values of the twill reinforcement with quasi-isotropic stacking sequence. Results show that the recycled material offers promising elastic properties and strength values, similar to those of advanced carbon fiber sheet molding compounds, and therefore can be used as structural material.

Ultrasonochemically Conjugated Metalloid/Triblock Copolymer Nanocomposite and Subsequent Thin Solid Laminate Growth for Surface and Interface Studies

Polymer and metalloid nanoparticles can be conjugated in a symphonized manner using ultrasonochemical force to obtain hybrid nanocomposites. The process is demonstrated using polymer poly(ethylene glycol) (PEG), metalloid SiO(2)@Ag, and triblock copolymer ABA. The acoustic microstreaming and cavitation force from the ultrasonics are crucial parameters that determine the harmonized PEG stabilization and ABA blending of the metalloid nanocomposites that are obtained. Surface plasmon resonance in the resulting hybrid systems are examined by UV-vis absorbance spectroscopy. The resulting PEG-stabilized SiO(2)-Ag conjugated with a triblock copolymer poly(p-dioxanone-co-caprolactone)-block-poly(ethylene oxide)-block-poly(p-dioxanone-co-caprolactone) (PPDO-co-PCL-b-PEG-b-PPDO-co-PCL/ABA) (PEG-SiO(2)@Ag/ABA) shows a red shift of 20 nm (410 nm) from its initial resonance at 390 nm (PEG-SiO(2)@Ag). Nanocomposite particles were then spin-coated on a glass substrate to obtain the growth of thin solid laminates (thickness 27 microm). Structural functionality was studied by FT-IR, (1)H NMR, and FT-Raman spectroscopy. Morphological properties were ensured from FE-SEM, HRTEM, AFM, and FIB-SEM. Identity and crystallinity of the prepared nanocomposite were confirmed by XRD analysis. A very low weight percentile loss of the fabricated nanocomposites ensures its high thermal stability. Fabricated nanocomposite laminate might have a role in coating, reinforcement, and resistance and as substrate additives for a variety of surface and interface studies. Further, the ultrasonochemical approach utilized here could also be a smart system to fabricate other heteronanostructures in a single platform.

A Solid-State, Rechargeable, Long Cycle Life Lithium–Air Battery

This paper describes a totally solid-state, rechargeable, long cycle life lithium–oxygen battery cell. The cell is comprised of a Li metal anode, a highly Li-ion conductive solid electrolyte membrane laminate fabricated from glass–ceramic (GC) and polymer–ceramic materials, and a solid-state composite air cathode prepared from high surface area carbon and ionically conducting GC powder. The cell exhibited excellent thermal stability and rechargeability in the temperature range. It was subjected to 40 charge–discharge cycles at current densities ranging from 0.05 to . The reversible charge/discharge voltage profiles of the cell with low polarizations between the discharge and charge are remarkable for a displacement-type electrochemical cell reaction involving the reduction of oxygen to form lithium peroxide. The results represent a major contribution in the quest of an ultrahigh energy density electrochemical power source. We believe that the cell, when fully developed, could exceed specific energies of 1000 Wh/kg in practical configurations.

Non-destructive ultrasonic techniques on sequence error of carbon fiber/epoxy composite solid laminates

When propagating in the thickness direction of composite laminates, shear ultrasonic waves are particularly sensitive to layup sequence in the cured CF/Epoxy composite laminates manufactured from unidirectional prepregs and are interacting with the polarization direction of propagating through its thickness. The strength and stiffness in laminates often are attributable to the specific fiber orientation and layup sequence. So the layup orientation greatly influences its properties in a composite laminate. There is a good correlation between the model and the non-destructive ultrasonic technique developed to inspect the sequence error of cured symmetric layups of the laminates. The physical model is based on the decomposition and recombination of shear polarization. Non-destructive evaluation capabilities are highly desirable for reusing unidirectional prepreg sheets.

Failure Analysis of Bolted Joints in Foam-core Sandwich Composites

This study represents an effort to predict the bearing strength, failure modes, and failure load of bolted joints in foam-core sandwich composites. The studied joints have been used in a light full composite airplane. By using solid laminates, a new design for the joint zone is developed. These solid laminates include a number of glass plies with total thickness equal to core thickness. The effect of solid laminate size and interface angle of foam—solid laminate in the bonding zone on the bearing strength, failure loads and type of modes are investigated. The numerical study is performed using 3D FEM in ANSYS commercial code. Tsai—Wu failure criterion is used in the failure analysis. The results indicate that the most important parameter in the proposed joint zone design is the foam—solid laminate interface angle which plays an important role on the value of failure criterion (damage) in the bonding zone. Also, the use of squared shaped solid laminate as compared with a circular laminate will decrease the criterion value significantly. Finally, the influence of solid laminate size and interface angle on the buckling strength was discussed. As obtained through eigenvalue buckling analysis, the increase of solid laminate size or interface angle could result in considerable higher buckling strength.

ONE-SIDED ULTRASONIC CHARACTERIZATION OF CFRP COMPOSITE LAMINATES

Ultrasonic NDE of composite laminates is usually analyzed on the backwall echoes of the specimen. In the inspection of actual composite structures, the back surface of the part was curved or contained complex geometry and did not provide a usable backwall echo. In this work, a one-sided pitch-catch setup was used in the detection and evaluation of flaws and material anomalies in solid composite laminates. Two miniature potted Rayleigh wave transducers (for generating surface waves in contact mode on steel) were joined head-to-head and used in the pitch-catch mode on the surface of composites. The pitch-catch signal was found to be more sensitive than normal incidence backwall echo of longitudinal wave to fiber orientation of the CFRP composites, including low level porosity, ply waviness, and cracks.

Thermally Induced Loads of Fastened Hybrid Composite/Aluminum Structures

Large composite structures have been increasingly used in the aviation industry. New applications of composite materials include primary structures such as aircraft fuselages. These large composite parts are sometimes attached, either by the fasteners or adhesive bonding, to metallic structures. Because of the large coefficient of thermal expansion mismatch between the metallic and composite structures, the temperature change from the aircraft assembly line to the actual flight condition induces high thermal stresses during flight in both the composite fuselage and aluminum frames. An experimental program was executed to determine the interaction between the fastened Z-shaped aluminum beams and the solid composite laminate. An analytical model was also developed to simulate the thermal/mechanical behavior of the hybrid composite/metal structure. Finite element analysis was conducted to determine the parameters necessary for the analytical model. The results from the developed analytical model were found to correlate well with experimental results.