Carbon Fiber Reinforced Polymer Plates Research Articles

High-frequency vibration effects on the hole integrity in rotary ultrasonic drilling of carbon fiber-reinforced plastic composites

This investigation represented a fundamental research on the potential effects of the high-frequency vibration on the hole integrity involved in rotary ultrasonic drilling (RUD) of carbon fiber-reinforced plastic (CFRP) composites. It was found the increased thickness of the CFRP plate shrunk the flowing velocity of the coolant, which brought about the residual chippings gradually accumulated at the radial clearance between the tool and the material. Furthermore, the chipping accumulation at the clearance seriously increased the friction effects and the resultant thermal load, thus leading to the chipping adhesions on the tool surface and machined cylinder jamming at the central hole of the tool. The mutual constrain between two vertical bundles brought the delamination around the holes generated in conventional drilling (CD) process to a termination at the bundle interface. The ultrasonic superimposition reduced the thrust force of the diamond tool which provided inadequate energy for the delaminated fibers reaching the bundle interface. Moreover, hole position on the two-dimensional orthogonal fabrics significantly influenced the propagation of the delaminated fibers, which weakened the effects of the drilling parameters on the delamination dimensions. Additionally, superimposing an ultrasonic vibration prolonged the abrasive trajectories and increased their overlapping probability, and the induced smoothing effects resulted in the obvious reduction of the surface roughness. The tensile stress exerted on the margin of the machined surface was responsible for the initiation of the CD delamination. After the delaminated fibers reached the bundle interface, the further extrusion of tool brought about the margin suffered from the shear stress, thus leading to the collapse of the machined cylinder. Considering the thrust force of the diamond tool and the undrilled thickness of the machined surface, the critical conditions of the delamination initiation were developed, which revealing that the decreasing of the thrust force caused the reduction of the critical undrilled thickness.

Direct-Write Piezoelectric Transducers on Carbon-Fiber-Reinforced Polymer Structures for Exciting and Receiving Guided Ultrasonic Waves.

Advancements in the structural health monitoring (SHM) technology of composite materials are of paramount importance for early detection of critical damage. In this work, direct-write transducers (DWTs) were designed for the excitation and reception of selective ultrasonic guided waves and fabricated by spraying 25- [Formula: see text]-thick piezoelectric poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TRFE)] coating with a comb-shaped electrode on carbon fiber-reinforced polymer (CFRP) plates. The characteristics and performance of the ultrasonic DWTs were benchmarked with the state-of-the-art devices, discrete lead zirconate titanate (PZT) ceramic transducers surface-mounted on the same CFRP plates. The DWTs exhibited improved Lamb wave mode excitation (A0 or S0 mode) relative to the discrete PZT transducers. Moreover, high signal-to-noise ratio was obtained by effectively canceling other modes and enhancing the directivity with the periodic comb-shaped electrode design of the DWTs, despite the smaller signal amplitudes. The enhanced directivity overcompensates for lower amplitude attenuation, making DWT a good candidate for locally monitoring critical stress hot spot regions in the CFRP structure prone to early damage initiation. It is shown that pairing a DWT sensor with a discrete PZT actuator could further achieve balanced performance in both wave mode selection and signal amplitudes, making this combination really attractive for ultrasonic SHM.

Experimental assessment of delamination extension on carbon / epoxy drilled plates

In the last decades, composite materials have been getting predominance, due to their peculiar characteristics, making their application attractive in the most diverse areas. One of the most used machining processes is drilling. This process allows for the use of mechanical connections to join parts in non-permanent connections, unlike the adhesive joints which are permanent connections.Thus, there is a need for a broad assessment of the damage caused by the drilling to the material. This work presents a study on how the machining parameters affect the delamination extension by drilling a set of carbon fiber reinforced polymer plates. Enhanced radiography and image analysis techniques are utilized and, in the end, a correlation between these results and the mechanical characteristics of the machined plates is obtained. Different drilling parameters are used, namely two feed rates while keeping the spindle speed constant, and three different drill geometries (Step, Brad and Helical). The thrust force is monitored during drilling and after drilling, parts are non-destructively and destructively (bearing, pin-bearing) tested. The results show that, although the thrust force varies and the delamination extension is affected by machining parameters, the mechanical resistance obtained in the tests did not evidence significant variations. The results show that, although the thrust force, varying around 50% in average, and the delamination extension, with a variation of 8%, are affected by machining parameters, the mechanical resistance obtained in the tests did not evidence significant variations, with an average difference of 2%. The main outcome of this work is to demonstrate that existing bearing tests may not the most suitable ones to assess mechanical consequences of drilling damage extension.

Delamination and hole wall roughness evaluation in air-cooled drilling of carbon fiber-reinforced polymer

Drilling of carbon fiber-reinforced polymer (CFRP) is widely employed in manufacturing processes in the aeronautical, automobile, and energy industries. The evaluation of the hole region focusing on wall roughness and delamination phenomena is extremely important to predict bolted joints' performance, where at least one of the adherent is a composite material. Thus, this work performed a statistical analysis on the delamination (entrance and exit of the plate) and wall roughness of drilling holes in an 8.6 mm thick CFRP plate carried out by an uncoated carbide drill under compressed air-cooling, varying the cutting speeds and feed rates. Since air-cooling usage combines positive aspects, such as low-cost implementation and shorter process time compared to ultrasonic-assisted drilling, it turns out to be an excellent alternative for aeronautical industry. Thus, the main contribution of the present work consists on analyzing the variation of the delamination in the entrance and exit of the first and tenth holes of a CFRP plate after dry and air-cooled drilling. This variation of the delamination between the cold drill (first hole) and the heated drill due to the drilling holes' sequence (after ten holes) is investigated for 18 different combinations of parameters (runs). For instance, it is shown that if the combination of parameters values is suitable, then it is possible to reduce the mean value of adjusted delamination factor for the entrance of the tenth hole around 11% when comparing cooled-air with dry cutting.

Effect of reinforcement ratio on flexural behavior of reinforced concrete beams strengthened with CFRP plates

An experimental study was carried out to investigate the effect of tensile reinforcement ratio on the flexural capacity of reinforced concrete beams strengthened with Carbon Fiber Reinforced Polymer (CFRP) plates. This study aims to observe the effect of reinforcement ratio on debonding failure load of reinforced concrete beams externally strengthened with CFRP plates. Six reinforced concrete beams consisting of three control beams and three beams strengthened with CFRP plates were tested. The beams were simply supported and loaded with four-point bending. The test variable was tensile reinforcement ratio (1%, 1.5%, and 2.5%). CFRP plates were glued on the bottom of the beams with the purpose to increase the flexural strength of the beams. Analytical prediction using fiber element method was also carried out to obtain a full flexural response of the beams due to bending load. The test results show that beams with CFRP plates have higher flexural capacity for about 10% to 50% than the control beams. However, at a specified load level, two of beams with CFRP plates suddenly collapse due to delamination of CFRP plates. In addition, analytical calculation predicts well the test results in excellent accuracy.

Stress Intensity Factor Assessment for the Reinforcement of Cracked Steel Plates Using Prestressed or Non-Prestressed Adhesively Bonded CFRP.

The use of adhesively bonded carbon fiber reinforced polymer (CFRP) materials to reinforce cracked steel elements has gained widespread acceptance in order to extend the lifespan of metallic structures. This allows an important reduction of the stress intensity factor (SIF) at the crack tip and thus a significant increase of the fatigue life. This paper deals with the assessment of the SIF for repaired cracked steel plates, using semi-empirical analysis and finite element analysis. Metallic plates with only one crack originating from a center hole were investigated. Virtual crack closure technique (VCCT) was used to define and evaluate the stress intensity factor at crack tip. The obtained modeling results are compared with experimental investigations led by the authors for different reinforcement configurations including symmetrical and non-symmetrical reinforcement, normal modulus and ultra-high-modulus CFRP plates, and pre-stressed CFRP plates. Results show that finite element model (FEM) analysis can obviously simulate the fatigue performance of the CFRP bonded steel plates with different reinforcement configurations. Moreover, a parametric analysis of the influence of the pre-stressing level was also conducted. The results show that an increase of the pre-stressing level results in an increase of the fatigue life of the element.

Joining of thermoplastic carbon fiber reinforced plastic laminates by selective heat input during laser cutting

The drilling of carbon fiber reinforced plastic (CFRP) components is a preliminary step within the assembly of airplanes or cars. The usage of CFRP brings along different challenges and opportunities due to the heterogeneous material characteristics. Laser cutting of thermoplastic composites can lead to distinctive heat affected zones (HAZs) if the process parameters are not optimized regarding the specific boundary conditions and material characteristics. During cutting, the matrix material gets molten and solidifies again. Further heat input beyond what is needed for cutting can lead to the deterioration of the matrix material. As a result, process strategies are normally developed to reduce the HAZ to a minimum. Such strategies can be used for adapting the process and generating a selective heat input in the interaction zone of two components during the drilling process. Adapting the process can also be used to weld components together. Unlike other more common efforts to reduce the HAZ, this study’s goal was to adapt the process so that two thermoplastic CFRP plates lying upon another can be welded during the cutting process. The developed process shows potential for the preliminary fixation of components in assembly processes by finding an optimum between reachable tensile force and resulting HAZ. The investigations were performed with CFRP plates with a thickness of h = 2.4 mm and a drilled hole with a diameter of d = 5 mm. The results demonstrate the possibility of welding the laminates while reducing the HAZ width to values below b = 300 μm. Tensile tests were performed with the joined specimens to determine the resulting tensile load. Cutting the hole with parameters typically used to minimize the HAZ result either in no weld seam or in a weak weld seam, which breaks at minimum force before tensile testing. Cutting parameters resulting in a distinctive HAZ generate tensile forces of up to F = 5.82 kN within the observed process window. Furthermore, strategies were investigated allowing a defined heat input only within the transition between the two plates. With this adapted strategy, tensile forces in the range of F = 1 kN could be realized. For these samples, the major HAZ width was located in the welding area with decreasing HAZ width for the upper and lower plates.

Effect of Mechanical Pretreatments on Damage Mechanisms and Fracture Toughness in CFRP/Epoxy Joints.

Adhesive bonding of carbon-fiber-reinforced polymers (CFRPs) is a key enabling technology for the assembly of lightweight structures. Surface pretreatment is necessary to remove contaminants related to material manufacturing and ensure bond reliability. The present experimental study focuses on the effect of mechanical abrasion on the damage mechanisms and fracture toughness of CFRP/epoxy joints. The analyzed CFRP plates were provided with a thin layer of surface epoxy matrix and featured enhanced sensitivity to surface preparation. Various degrees of morphological modification and fairly controllable carbon fiber exposure were obtained using sanding with emery paper and grit-blasting with glass particles. In the sanding process, different grit sizes of SiC paper were used, while the grit blasting treatment was carried by varying the sample-to-gun distance and the number of passes. Detailed surveys of surface topography and wettability were carried out using various methods, including scanning electron microscopy (SEM), contact profilometry, and wettability measurements. Mechanical tests were performed using double cantilever beam (DCB) adhesive joints. Two surface conditions were selected for the experiments: sanded interfaces mostly made of a polymer matrix and grit-blasted interfaces featuring a significant degree of exposed carbon fibers. Despite the different topographies, the selected surfaces displayed similar wettability. Besides, the adhesive joints with sanded interfaces had a smooth fracture response (steady-state crack growth). In contrast, the exposed fibers at grit-blasted interfaces enabled large-scale bridging and a significant R-curve behavior. While it is often predicated that quality composite joints require surfaces with a high percentage of the polymer matrix, our mechanical tests show that the exposure of carbon fibers can facilitate a remarkable toughening effect. These results open up for additional interesting prospects for future works concerning toughening of composite joints in automotive and aerospace applications.

Bonding performance of CFRP plate and concrete with soft layer system

The use of CFRP (Carbon Fiber Reinforced Polymer) plate to strengthen RC (Reinforced Concrete) structure has become popular, nowadays. Meanwhile, bonding behavior between CFRP plate and concrete is an important issue in application. Many research works had been done to improve the bonding performance of this method. In prior study, it was reported that the bond strength could be increased by increasing the stiffness of CFRP or using a soft adhesive layer with a small shear stiffness. With this assumption, the bond strength will be enhanced by putting a soft layer (polyurea) between CFRP plate and usual adhesive (epoxy). To obtain the complete view of bonding behavior of CFRP plate-concrete, a double face tensile test setup was conducted in this research. Both high modulus type and high tension type of CFRP plate with soft layer and without soft layer were tested. CFRP plate used in this study had a thick of 1mm, 2mm and 4mm, respectively, and had a width of 50mm for all specimens. The results showed that soft layer system has a big influence for high tension type.

Linear creep of bonded FRP-strengthened metallic structures at warm service temperatures

Ambient cured epoxy adhesive is widely used for bonding fibre reinforced polymer (FRP) plates to metallic structures. The present paper examines a typical strengthening adhesive to investigate the effect of adhesive thermo-viscoelasticity. The response of the adhesive was determined using a series of tests using the multi-frequency scanning mode of a dynamic mechanical analyser (DMA). The thermo-mechanical properties of the adhesive were then characterised using time–temperature superposition parameters and a Prony series representation for generalised Maxwell creep. The adhesive response was in turn used within two finite element (FE) models to examine the effect of creep in the adhesive at warm temperatures (<100 °C) on the performance of a lab-scale carbon fibre-reinforced polymer (CFRP) plate strengthened steel beam and a real-scale CFRP plate strengthened cast-iron beam respectively. The study found that thermo-viscoelastic creep of the adhesive bonding layer causes an increase in the slip between the FRP and the structure, which could induce damage in the bonded joint and make the CFRP becomes less effective, potentially resulting in failure of the strengthening system during the long-term service. Differential thermal expansion effects can enhance the joint bonding stress and allow the plate to maintain its contribution to the moment capacity of the beam; however, this benefit could be lost when temperature decrease, and the additional irreversible damage caused by the increased joint stress could reduce the effectiveness of strengthening further..

Impact Behavior of Carbon Fiber/Epoxy Composites and Fiber Metal Laminates with Open Holes

This paper describes an experimental investigation performed to evaluate the low-velocity impact behavior of prepreg-based carbon fiber reinforced polymers (CFRPs) and fiber metal laminates (FMLs) with and without open holes subjected to different impact energies. The laminates were made of carbon fiber/epoxy prepregs and aluminum layers and manufactured by using autoclave. The impact responses of these laminates were experimentally obtained using a drop-weight tower at impact energies of 15, 20 and 30 J. After testing, the impact damage area of tested specimens was quantified from C-scan ultrasonic technique. In addition, an X-ray tomography analysis was performed to assess the through-thickness distribution of damage in FMLs with and without open holes. The results showed that the woven FMLs exhibit the highest impact peak force compared to that of CFRP plates due to the presence of aluminum layers, which induce a higher deformation capability to the laminates. The presence of open holes in laminates tends to augment their damage extension and decreases their impact peak force due to the local stress concentration effect. Nevertheless, it was observed by C-scan ultrasonic images that the aluminum layers reduce the extent of delamination of laminates during the impact event. Postimpact evaluation using X-ray computed tomography showed that impact causes a severe damage to the laminates around their impact point and confirmed that matrix cracking and delamination are the principal damage mechanisms induced on the through-thickness direction of the FML plates. In specific, the results confirm that the aluminum layers provide good impact properties and damage resistance when they are added to the CFRPs.

CFRP Origami Metamaterial with Tunable Buckling Loads: A Numerical Study.

Origami has played an increasingly central role in designing a broad range of novel structures due to its simple concept and its lightweight and extraordinary mechanical properties. Nonetheless, most of the research focuses on mechanical responses by using homogeneous materials and limited studies involving buckling loads. In this study, we have designed a carbon fiber reinforced plastic (CFRP) origami metamaterial based on the classical Miura sheet and composite material. The finite element (FE) modelling process’s accuracy is first proved by utilizing a CFRP plate that has an analytical solution of the buckling load. Based on the validated FE modelling process, we then thoroughly study the buckling resistance ability of the proposed CFRP origami metamaterial numerically by varying the folding angle, layer order, and material properties, finding that the buckling loads can be tuned to as large as approximately 2.5 times for mode 5 by altering the folding angle from 10° to 130°. With the identical rate of increase, the shear modulus has a more significant influence on the buckling load than Young’s modulus. Outcomes reported reveal that tunable buckling loads can be achieved in two ways, i.e., origami technique and the CFRP material with fruitful design freedoms. This study provides an easy way of merely adjusting and controlling the buckling load of lightweight structures for practical engineering.

Deep-Subwavelength-Optimized Holey-Structured Metamaterial Lens for Nonlinear Air-Coupled Ultrasonic Imaging.

Ultrasound non-destructive testing (NDT) is a common technique used for defect detection in different materials, from aluminium to carbon-fiber-reinforced polymers (CFRPs). In most cases, a liquid coupling medium/immersion of the inspected component is required to maximize impedance matching, limiting the size of the structure and materials. Air-coupled inspection methods have recently been developed for noncontact inspections to reduce contact issues in standard ultrasonic inspections. However, transmission of ultrasound in air is very inefficient because of the enormous impedance mismatch between solids and air, thus requiring a signal amplification system of high-sensitivity transducers. Hence, the captured signal amplitude may not be high enough to reveal any wave distortion due to defects or damage. This work presents a design of a holey-structured metamaterial lens with a feature size of λ/14 aiming at improvement of acousto-ultrasonic imaging using air-coupled transducers. The required effect is obtained by matching geometrical parameters of the proposed holey-structured metamaterials and the Fabry–Perot resonance modes of the structure. Transmission tests have been conducted on different fabricated metamaterial-based structures, to assess the frequency component filtering of the proposed method in both acoustic (f = 5 kHz, 20 kHz) and ultrasonic range (f = 30 kHz, 40 kHz). Results showed an improved sensitivity of damage imaging, with an increase in amplitude of the design frequencies of the lens by 11 dB. Air-coupled inspections were conducted on a stress-corrosion cracked aluminum plate and impacted CFRP plate using the holey-structured lens. Results showed an improvement in the damage-imaging resolution due to a wave-amplitude increase across the defective features, thus demonstrating its potential as an efficient and sensitive inspection tool for damage-detection improvement in geometrically complex components of different materials.

Impact monitoring of CFRP composites with acoustic emission and laser Doppler vibrometry

The use of Acoustic Emission (AE) to detect impacts is of interest within industries where vital components are prone to impact damage, in particular where Carbon Fibre Reinforced Polymers (CFRP) are used, as damage can often go un-noticed within them. For AE monitoring of impacts piezoelectric sensors are used to detect the ultrasonic wave produced by an impact. Classification is also possible of these waves enabling a distinction between damaging and non-damaging impacts. These sensors do however have resonance, so do not give an accurate picture of how the waves propagate, better knowledge would enable better selection of sensors. Laser Doppler Vibrometry is a non-contact and non-resonant method of analysing the surface displacement on a structure. In this study, a vibrometer was used to monitor CFRP plates during impact to assess its applicability for distinguishing between damaging and non-damaging impacts, compared with a surface mounted AE sensor. The vibrometer was able to detect both low frequency flexural modes due to the impact process and the higher frequency extensional modes, initiated by damage. When compared to the AE sensor the vibrometer was comparable in its results, and unlike the sensor, not susceptible to resonance or decoupling. For the tested material the vibrometer identified frequencies greater than 20 kHz to be associated with damaging impacts.

Evaluation of composite action in cross laminated timber-concrete composite beams with CFRP reinforcing bar and plate connectors using Digital Image Correlation (DIC)

This research proposes new types of shear connectors made of carbon fibre reinforced polymer (CFRP) composites for effective stress transfer between the timber and concrete sections in cross laminated timber (CLT)-concrete composite beams. New shear connectors are designed and made of bidirectional carbon fibre reinforced polymer (CFRP) composite plates and crossed CFRP reinforcing bars. The mechanical performance, bending stiffness, ductility, and interfacial slippage and strain of timber-concrete composite (TCC) beams with CFRP connectors are compared with those with steel plate and screw systems through four-point flexure testing. Local slip, interfacial slip and strain behaviour are comparatively analysed for connections with equivalent axial stiffness using a Digital Image Correlation (DIC) based technique, so that relative composite behaviour could be determined. Furthermore, a cost evaluation is undertaken to compare the feasibility of proposed shear connectors for construction of TCC systems. Results from flexural tests demonstrate that CFRP rod specimens experience higher ultimate load and bending stiffness in elastic loading stage and ductility in failure while slippage at serviceability and ultimate load is minimal. These results demonstrate that CFRP reinforcing bars can be used as an alternative to existing steel plate/screw systems. Although CFRP plate connectors show lower ultimate strength, bending stiffness and ductility, the performance of the system can be further improved by using sufficient anchorage systems at the end of CFRP plate within the concrete.

Noncontact Damage Topography Reconstruction by Wavenumber Domain Analysis Based on Air-Coupled Ultrasound and Full-Field Laser Vibrometer.

Noncontact ultrasonic detection technology is an effective method to detect damage in time. This paper proposes a noncontact damage detection system based on air-coupled ultrasound and full-field laser vibrometer, which realizes the excitation of relatively single-mode guided waves and the wavefield automatic detection. The system performance is verified through experiments, and the experimental wavenumber is consistent with the theoretical dispersion characteristics of the Lamb wave in the A0 mode. Based on this system, the topography reconstruction algorithms, including the Wavenumber Filtering Algorithm and Spatial Wavenumber Algorithm, were tested and compared with the aluminum alloy plate and the carbon fiber reinforced polymer plate. The results show that, based on the air-coupled ultrasound and full-field laser vibrometer detection system, the Spatial Wavenumber Algorithm has better imaging error and contrast, and the damage edge detection is smoother.

Comparison of strain pre-extrapolation techniques for shape and strain sensing by iFEM of a composite plate subjected to compression buckling

The inverse Finite Element Method (iFEM) is a model-based technique for structural shape sensing based on a pattern of input strain data experimentally acquired on the structure. It is independent of the external load and material properties, making it a valid approach for composite materials monitoring. In this framework, the iFEM shape sensing capability is experimentally investigated on a carbon fiber reinforced polymer plate subjected to a compressive buckling condition. As logistic constraints impede strain sensor installation in all the desired locations, Smoothing Element Analysis (SEA) and polynomial fittings are used to pre-extrapolate the whole plate's strain field. Finally, after a sensitivity analysis on the input strain pre-extrapolation, the iFEM displacement reconstruction is validated with three lasers' independent measurements and direct FEM results, investigating the local and global shape sensing capability.

Effect of the chloride environmental exposure on the flexural performance of strengthened RC beams with self-anchored prestressed CFRP plates

Carbon fibre-reinforced polymer (CFRP) plates are widely used to strengthen reinforced-concrete (RC) structures. At present, most studies focused on the short-term performance of strengthened RC beams with a prestressed CFRP plate, and very limited information is available on their durability, which is of importance in certain cases, such as high-piled wharfs and bridges in marine environments. This study investigates the effects of chloride environment exposure on the flexural performance of strengthened RC beams with self-anchored prestressed CFRP plates. In total, seven RC beams, four of which were strengthened by a self-anchored prestressed CFRP plate, were fabricated and tested under three-point bending. The testing parameters included 25% and 40% prestress level of the ultimate tensile strength of the CFRP plate, 5% and 10% tensile steel rebar corrosion ratio of mass loss and 10% and 20% anchor corrosion ratio in the chloride environment. The evolution of time-dependent prestress losses in the strengthened beam was determined, and the flexural behaviour of the strengthened beams was analysed. A theoretical model is proposed to predict the flexural strength of prestressed CFRP strengthened RC beams in a chloride environment. The experimental results indicated that the coupling of the corrosion expansion of the metal anchor and rebar in the chloride environment had a large negative effect on the flexural performance of the beams strengthened by the self-anchored prestressed CFRP plates. After exposure to the chloride ion solution, the failure mode of the RC beams strengthened by the prestressed CFRP changed from concrete crushing in the compression section to anchor pullout. The proposed model, which considered the effect of a chloride environment to predict the flexural strength of the strengthened beams, agreed well with the test results.

A hybrid support vector regression with multi-domain features for low-velocity impact localization on composite plate structure

The accurate localization of low-velocity impacts on the composite plate structure of the ship is still a great challenge. Current research mainly focuses on extracting single domain features from impact signals as the input of machine learning methods, whereas ignores multi-domain features with more comprehensive impact information. In this paper, a hybrid support vector regression with multi-domain features is proposed to increase the localization accuracy in determining the locations of low-velocity impacts on the composite plate structure. The proposed method consists of the signal preprocessing, the multi-domain feature extraction, and the impact localization. In the signal preprocessing, the trend component in the low-velocity impact signals is eliminated by adopting the empirical mode decomposition (EMD) method. Then, the multi-domain features, which include time domain features, frequency domain features, and time-frequency domain features, are extracted from the preprocessed impact signals. Finally, the optimized support vector regression based on the bat algorithm (BA-SVR) is designed to implement the localization of low-velocity impacts. The low-velocity impact localization system using four fiber Bragg grating (FBG) sensors is established on a carbon fiber reinforced plastic (CFRP) plate, and then five sets of experiments are executed. The statistical results in these experiments demonstrate the effectiveness and feasibility of BA-SVR that uses multi-domain features and four FBG sensors and the satisfactory localization performance of the proposed method in handling the low-velocity impact localization problem on the CFRP plate.

Theoretical, emulation and experimental analysis on auxetic re-entrant octagonal honeycombs and its applications on pedestrian protection of engine hood

In this article, a novel re-entrant octagonal honeycomb (ROH) with negative Poisson's ratio (NPR) configuration is proposed owing to its superior energy absorption and lightweight design performances. Regarding the effects of adjacent unit cells, a mechanics-based homogenization approach is put forward and employed to derive static mechanical properties including elastic modulus and Poisson's ratio of two-dimensional (2D) and three-dimensional (3D) ROHs under axial loading. Based on above, the predominant compression failure mode is explored. Then, the static mechanical properties and finite element model of ROH are validated by experiment and demonstrate high accuracy. Subsequently, to obtain dynamic responses of the ROH under dynamic impact loading, the relations among dynamic plateau stress and quasi-static plateau stress, initial density, dense strain and impact velocities are established under the conditions of mass conservation and momentum theorem based on one-dimensional shock wave theory and ideal rigid-perfectly plastic material model. Besides, dynamic energy absorption models of the ROH under dynamic loading are acquired in consideration of the effects of impact velocities on dense strain, the results show that theoretical models agree quite well with simulation results. On this basis, the application of ROH on automotive energy absorber is studied with the purpose of acquiring superior energy absorption and satisfying lightweight requirements simultaneously. Based on the decoupling thought of stiffness and pedestrian protection, a new design method of carbon fiber reinforced plastics (CFRP) hood with ROH is put forward in consideration of energy absorption and lightweight requirements. Of this, ROH is regarded as sandwich structure to meet the demands of energy absorption, CFRP plates are treated as outer and inner skins to satisfy the needs of stiffness. Firstly, multi-objective optimization models are set up under the constraints of reasonable structural parameters and negative Poisson’s ratio. Then, the NSGA-Ⅱalgorithm is adopted to achieve the optimal Pareto fronts with the targets for maximum unit mass energy absorption and minimum density. Afterwards, the optimization on thicknesses of CFRP layers is carried out to achieve minimum mass under the constraint conditions that the static stiffness of CFRP hood is larger than that of steel hood. In addition, the layering sequence of inner and lower skins are also optimized. Finally, improved overall stiffness, optimized HIC values and similar lightweight space of CFRP hood with ROH are achieved in comparisons with CFRP hood without ROH. Compared with steel hood, the pedestrian protection performance of CFRP hood with ROH is greatly enhanced, 1st free mode increases by 33.9%, and torsional stiffness and bending stiffness increase by 23.1% and 30.8%, respectively. While the weight decreases by 40.1%. The results indicate that the CFRP hood with ROH presents outstanding advantages of pedestrian protection and lightweight design in terms of HIC and stiffness performance.