CFRP Specimens Research Articles

Punching shear behavior for flat slabs with CFRP and openings

This study is devoted to investigate the flexural behavior of geopolymer flat slab that have transverse web openings with CFRP. The shear reinforcement is considered one of the important solutions to increase the resistance of the slabs to the failure of the punching shear and it is also possible through the use of these collars to provide service openings next to the columns. Three molds made of wood were prepared to examine 15 samples of geopolymer concrete under concentrated conditions, Dimensions of section are of total (70*70*7 cm) and Dimensions of column (15*15*15 cm). Three groups of models comprise the models for this research: the first group looks at the impact of the column's location; the second group looks at the impact of an opening next to the column or not; and the third group looks at the impact of CFRP. The findings demonstrated that the presence of transverse web openings affects the general flexural behavior of flat slab geopolymer reinforced concrete as a result of the reduction of concrete within the geopolymer concrete section. CFRP specimens demonstrated superior due to the inherent mechanical properties of the material. Among the various repairing procedures used, the full wrapping technique yielded the best results after the repairs.

Microscopic damage behavior in CFRP cross-ply laminates at cryogenic temperature

For the practical use of cryogenic propellant tanks made of CFRP laminates, experimental elucidation of the laminates’ microstructural damage propagation behavior at cryogenic temperatures is important. This paper presents a newly developed tensile test rig that enables in-situ observation of microscopic damage using an optical microscope at cryogenic temperatures using a Gifford–McMahon refrigerator. In-situ observations of microscopic damage under uniaxial tensile loading were done at room temperature (290 K, 17 °C) and at 30 K (−243 °C) on cross-ply thin-ply CFRP specimens of three kinds with different 90° layer thicknesses. The results demonstrated that the crack propagation behavior is independent of temperature, that the matrix crack density is higher, and that the onset of matrix crack initiation strain is lower at 30 K than at 290 K. Furthermore, the thermal strain within 90° layer at 30 K and at 290 K was estimated using finite element analyses (FEA). The FEA results suggest that the decrease in onset strain of matrix crack initiation at 30 K is mainly attributed to the increase in the thermal strains within the 90° layer.

Improving defect detection capability of pulse and pulse phase thermography method for CFRP plates by enhancing rear surface heat transfer coefficient

Infrared pulse thermography (PT) is a noncontact and convenient nondestructive inspection method, and its application to various objects has been reported in many papers. However, conventional PT is not suitable for detecting deep- or near-rear-surface defects. This study focuses on PT inspections of CFRP plates and improvements in their defect detection capabilities. To improve the capability of detecting near-rear-surface defects, a method for enhancing the heat transfer coefficient (HTC) of the rear surface during PT inspection was examined in this study; this would increase the temperature difference in the defective areas observed in thermal images. To investigate the effectiveness of the proposed method, experiments were conducted on CFRP specimens with artificial defects. The CFRP specimens were inspected using the PT method, during which water was brought into contact with the rear surfaces of the specimens to enhance the HTC. The experimental results showed that the capability of detecting near-rear-surface defects improved when the rear surfaces were in contact with water with forced convection. In addition, obtaining phase images by applying a Fourier transform to the experimentally obtained temperature data (that is, pulse phase thermography: PPT) was examined. Consequently, the combination of the enhancement of the rear-surface HTC and the use of phase images was significantly effective in improving the defect detection capability.

Mechanical and thermal characterization of additively manufactured novel multilayer polymer composite through experiments and finite element simulation

This study presents the investigation of mechanical and thermal properties of additively manufactured novel multilayer composites (MLC). Wood and carbon-reinforced poly-lactic acid were used as the base materials for the 3D printing of the MLC. The tensile, compressive, and flexural strengths of the developed MLC were 51.47, 70.90, and 73.3 MPa, respectively. The finite element simulation values of tensile and compressive strength of the MLC were 51.01 and 70.02 MPa depicting negligible variations of 0.90% and 1.24%. The glass transition temperatures for the WRP, CFRP, and MLC specimens were 54.85 °C, 57.29 °C, and 56.78 °C, respectively.

Effect of triphenylphosphine addition for the fast curing epoxy-anhydride CFRP system

In this study, we investigated the impact of adding Triphenylphosphine (TPP) at concentrations of 1[Formula: see text]wt.%, 3[Formula: see text]wt.%, and 5[Formula: see text]wt.% to an epoxy-anhydride system that displays, a degradation in mechanical properties at high temperatures. We conducted an analysis of cure behavior using differential scanning calorimetry and evaluated flexural, shear, and tensile strengths at room temperature. To assess heat resistance, CFRP specimens were exposed to 100∘C for 1[Formula: see text]h, and tensile strengths were subsequently measured. The addition of 3[Formula: see text]wt.% TPP resulted in a 5[Formula: see text]min reduction in the time required to reach a 95% degree of cure, but it was observed that mechanical properties at room temperature were compromised. CFRP exposed to 100∘C indicated that an increase in TPP content helped mitigate the degradation of mechanical properties.

A novel machine learning model to design historical-independent health indicators for composite structures

Developing comprehensive health indicators (HIs) for composite structures encompassing various damage types is challenging due to the stochastic nature of damage accumulation and uncertain events (like impact) during operation. This complexity is amplified when striving for HIs independent of historical data. This paper introduces an AI-driven approach, the Hilbert transform-convolutional neural network under a semi-supervised learning paradigm, to designing reliable HIs (fulfilling requirements, referred to as 'fitness'). It exclusively utilizes current guided wave data, eliminating the need for historical information. Ensemble learning techniques were also used to enhance HI quality while reducing deep learning randomness. The fitness equation is refined for dependable comparisons and practicality. The methodology is validated through investigations on T-single stiffener CFRP panels under compression-fatigue and dogbone CFRP specimens under tension-fatigue loadings, showing high performance of up to 93% and 81%, respectively, in prognostic criteria.

Prediction of creep failure life for unidirectional CFRP with heat-resistant epoxy resin as matrix exposed to high temperature under tension load

The accelerated testing methodology (ATM) developed by the authors is the methodology for predicting statistically long-term life of CFRP structures based on the viscoelasticity including the time-temperature superposition principle for matrix resin. The formulae of ATM are expressed as the product of static strength at room temperature, its dispersion, and the viscoelastic parameter of matrix resin that changes with time and temperature history. This methodology ATM was applied to the prediction of long-term creep failure life for unidirectional CFRP with heat-resistant epoxy resin as matrix under heat degradation in this study. Resin impregnated CFRP strands with heat-resistant epoxy resin were molded using a filament winding system as virgin specimens of unidirectional CFRP. Then, heat-degraded specimens were prepared by exposing the virgin specimens under acceleration conditions determined based on the time-temperature superposition principle for chemical deterioration assuming the condition of practical temperature 110°C and period 10 years. Second, the creep strengths of virgin and heat-degraded CFRP strands were statistically predicted based on ATM and the effect of heat degradation on the long-term life of CFRP strands was evaluated. As results, it was cleared that when the creep strength of CFRP strands undergoes thermal aging at the practical condition, in addition to the strength decrease to time due to the viscoelasticity of the resin, a comparable decrease in static strength and increase in this scatter occur.

In-plane shear damages consideration of hoop wound composite cylinders using acoustic emission method

The objective of this study was to examine the in-plane shear properties of filament-wound composite cylinders and assess the associated damage mechanisms using acoustic emission techniques. Two distinct composite materials, namely carbon/epoxy and glass epoxy, were manufactured for this purpose. The cylinders were subjected to torsion load conditions in accordance with ASTM D5448. Throughout the testing process, various parameters such as torque, angle of torsion, and strain were meticulously recorded. Additionally, the acoustic emission sensors were utilized to capture signals indicating the occurrence of damages. The findings of the study indicate that the in-plane shear strength and shear modulus of the carbon/epoxy specimens surpass those of the glass/epoxy counterparts. The results from acoustic emission testing indicated that in the CFRP specimens, there were no instances of fiber/matrix debonding. Instead, the main mode of damage observed was fiber breakage, accounting for approximately 71% of the total damage detected. On the other hand, in the GFRP specimens, the primary damage mechanism was found to be fiber/matrix debonding, making up approximately 50% of the overall damage recorded.

Assessing pseudo-ductile behavior of woven thermoplastic composites under tension and bending

Most fiber-reinforced composites are inherently brittle and fail suddenly at low strains without yielding and energy-absorbing capability. Still, under some conditions, they can demonstrate ductile like response known as pseudo-ductility. To investigate such a response, experimental analysis of carbon- and glass-fabric reinforced thermoplastic polymer (C/GFRP) composites was performed in on- and off-axis orientations under service loading conditions of tension and bending. Tensile tests of off-axis specimens were conducted with a full-field strain-measurement digital image correlation (DIC) technique. Cyclic bending tests of on- and off-axis C/GFRP specimens were performed to assess their ductility and damage behavior. The tests revealed that on-axis CFRP laminates failed due to fracture of brittle carbon fibers under tension, monotonic and cyclic bending. The on-axis GFRP samples demonstrated a linear-elastic brittle response under tension but a visco-elasto-plastic nonlinear behavior under monotonic and cyclic bending with hysteresis and energy absorption. This nonlinearity could be due to stress stiffening of thin laminates under large-deflection bending and mesoscopic effects in the woven fabrics. The off-axis C/GFRP specimens exhibited ductile behavior akin to metals, enduring high strains with permanent deformation before ultimate failure, and absorbing substantial amounts of energy. The pseudo-ductile response of the off-axis specimens is attributed to plasticity due to matrix cracking, fiber-matrix debonding as well as fiber trellising, whereas in the on-axis GFRP specimens, it is primarily due to visco-elasto-plastic behavior of glass fibers and the TPU matrix. It is concluded that material's response can be tailored for stiffness, strength and ductility for specific applications.

An experimental investigation on the influence of environmental ageing on strength and fatigue behavior of different single lap joints

It is essential to examine the performance of CFRP and aluminum specimens after being exposed to environmental aging and to develop a technique that can predict their mechanical function. However, there is limited research on the static and fatigue strength of adhesive, mechanical, and hybrid joints (adhesive/mechanical) after environmental ageing. The study aims to examine the effect of thermal and hygrothermal ageing on the static and fatigue performance of three-point bending joints. Three groups of samples were prepared: I) unaged, II) thermally aged, and III) hygrothermally aged. The bending performance of three different types of joints (bonded, bolted, and bonded/bolted) was evaluated by conducting bending tests using three distinct types of adherends: ATA (Aluminum-to-Aluminum), CTC (Composite-to-Composite), and ATC (Aluminum-to-Composite). Also,the strength of the joints was measured and assessed for both static and fatigue (at three different levels of loading using the new fixture) loading. Static tests showed that bonded/bolted CTC specimens had the highest strength, and hygrothermal and thermal conditions significantly affected the static performance of the three types of joints and substrates. Under fatigue load, the highest fatigue life was observed in bonded, bolted, and bonded/bolted specimens for ATA, ATC, and CTC, respectively. In a bonded/bolted single lap joint, the static and fatigue strength consists of two steps: the initial step corresponds to the strength of a bonded specimen, and the next step starts when the bonded joint fails and fits the strength of a bolted joint. Finally, both ageing and static load percentage reduced the fatigue life and shifted the failure mode of the joints.

Tension-after-impact (TAI) performance and 3D damage visualization of carbon-fiber epoxy composites with adhesively bonded patch repairs

Bonded repairs are advantageous for the aircraft industry when applying repairs to composite parts. However, their post-impact behavior under mechanical loads has to be addressed. Within this study, CFRP specimens repaired via a soft-patch repair approach as well as pristine (i.e. virgin) laminates without repairs, were impacted (energy: 5 J to 30 J). Subsequently, tensile tests were performed in order to investigate their post-impact mechanical properties. For selected impact energy levels, a detailed analysis on the extent of damage was conducted by micro-CT imaging. From these investigations, the interrelation of the impact energy, the mechanical properties obtained from the post-impact tensile tests as well as of the extent (quantitative and qualitative) and nature of damage could be studied and visualized in 2D and 3D images. Furthermore, a high-resolution image acquired on an undamaged repair-specimen was segmented, from which the individual bond constituents could be identified and visualized in 3D representations.

Automatic defect detection and three-dimensional reconstruction from pulsed thermography images based on a bidirectional long-short term memory network

Machine learning techniques have become increasingly applied to non-destructive testing based on pulsed thermography. However, existing methods need to extract characteristic data manually. The present work addresses this issue by applying a bidirectional long-short term memory (Bi-LSTM) network to identify defects, predict defect depths, and reconstruct defective materials in three dimensions automatically based on raw cooling data sequences. The network is trained and tested based on data collected for stainless-steel specimens with multiple flat-bottom holes introduced at various specimen depths. A dual-task method and a single-task method were proposed based on Bi-LSTM network. A classification model and a regression model are constructed in the dual-task method for identifying defects and predicting defect depths. Only the regression network is implemented in a single-task method to more quickly obtain the same results based on a depth threshold. Both methods are demonstrated to achieve satisfactory accuracy in the 3D reconstruction of the defects in the testing specimen. In addition, higher pulse energy and faster acquisition frequency can promote the prediction accuracy. Then the results of Bi-LSTM were compared with the results of 1D CNN and MLP. To verify the generalization of the proposed method, CFRP specimen is employed for 3D reconstruction, which also performed with good results.

Application of Improved b-Value and Clustering Analysis for Structural Integrity Assessment of CFRP Specimen under Tensile Loading

Application of Improved b-Value and Clustering Analysis for Structural Integrity Assessment of CFRP Specimen under Tensile Loading

Experimental Study on Axial Compression Performance of New-Type BFRP-Confined Square Concrete Columns

For a new type of BFRP composite based on epoxy silicate resin, the confined influence on the concrete structure was studied by model experiment and theoretical analysis. The effects of wrapping form and layers on the behavior of the square concrete column was understood. The results showed the development trend of the loading curve of specimens confined by BFRP and CFRP is basically consistent, but the confined effect of BFRP is relatively poor. Under the two layers of full coverage, the axial compressive strength of the BFRP-confined groups and the CFRP-confined groups was increased by 28.45% and 64.73%, respectively. The existing codes have good applicability to CFRP specimens, but poor applicability to BFRP specimens. The calculation model proposed here can accurately describe the axial compression bearing capacity of new BFRP reinforced square concrete columns.

Analysis of sensitivity of elastic characteristics of unidirectional carbon fiber specimens with thermoplastic matrix to loading rate at high temperature

The problem of assessing the sensitivity of unidirectional carbon fiber reinforced plastics to loading rate is considered. As a rule, when the loading rate changes, the stress-strain curves for unidirectional CFRP specimens under loading at different off-axis angles change due to the rheological properties and physical nonlinearity. An attempt is made to reveal the degree of anisotropy sensitivity of a unidirectional carbon fiber reinforced plastic with thermoplastic matrix at an elevated temperature in the range of changes in the loading rate by two orders of magnitude both in stress and in strain.

Fatigue damage prediction of CFRP specimens by means of Non-Destructive Techniques

Fatigue damage prediction of CFRP specimens by means of Non-Destructive Techniques

Experimental investigation on the influence of silanized graphene coatings on carbon fiber composites as toughening agents at low temperatures

Several applications of carbon fiber are being investigated, with the alteration of the carbon fiber surface using graphene based nanoparticles as the primary focus. But, in the present study, carbon fiber was surface modified with 3-aminopropyl trimethoxysilane functionalized graphene nanoparticles using the electrophoretic deposition (EPD) method, and the CFRP composite laminate was fabricated using the VARTM method. The matrix comprises diglycidyl ether of bisphenol A (DGEBA) and triethylenetetramine (TETA) hardener. To assess toughness behaviour, the modified CFRP samples were subjected to charpy impact tests at both room temperature (RT) and cryogenic temperature (CT). The impact strength of CFRP specimens was enhanced by 14.8% and 18.7% at RT and CT, respectively. FTIR analysis was conducted to confirm the presence of flexible silane bonding. SEM analysis was conducted to further examine the fracture surface. At CT, silane functions as a flexible coupler inside CFRP laminates, imparting toughness to CFRP laminates.

Effect of defects combined with impact damage on compressive residual strength in curved CFRP specimen

This work reports the results of a test campaign conducted on curved carbon–epoxy​ specimens subjected to impact damage combined with manufactured embedded wrinkle defects in order to study the influence of this combination on the compressive strength. A numerical model was used to model wrinkle defects and to carry out a numerical study of their influence on impact and compression after impact (CAI) tests. The model produced acceptable results for low misalignments but for higher levels the model failed. Investigations conducted in this work suggest that damage tolerance studies should consider impact, rather than wrinkle defects, as a priority.

Numerical analysis of the effects of wrinkles in the radius of curvature of L-shaped CFRP specimens on unfolding failure

Complex composite parts, and especially curved laminated composites, are more subject to defects than flat laminates. One of the most common defects is the presence of wrinkles, which can occur in the radius of curvature. Wrinkle effects must be studied to evaluate their influence on the mechanical properties of laminated composites. In this study, a numerical model was developed to take wrinkle and local delamination defects into consideration and assess their impact on the failure load of L-angle specimens under four-point bending tests. Experimental wrinkles based on a previous work were modelled using the Discrete Ply Model strategy and four-point bending simulations were run. The numerical model allowed the effects of each defect (local delamination and wrinkle) to be distinguished and their combined influence on the failure load to be studied with respect to the maximum misalignment.

Dynamic response and microstructure evolution of titanium alloy plates under low-velocity impact

This paper presents an experimental investigation into the dynamic response and microstructure evolution of titanium alloy plates subjected to low-velocity impact load. The impact experiments with different impact energies were designed and conducted. The effects of impact energy on the impact response curves and the interested impact resistance were analyzed. The macro deformation/failure mechanisms were examined by using digital microscope. The microstructure evolution induced by the variation of impact energy was further elaborated via scanning electron microscope (SEM) and electron backscatter diffraction (EBSD) technique. On the basis of same impact condition, the impact behaviors of titanium alloy specimens were compared with the ones of woven carbon fiber reinforced composite laminates (CFRP) with the same areal density. The results show that the impact response of titanium alloy specimens is mainly characterized with linear loading stage, nonlinear loading stage and unloading stage. Linear relationships of peak impact force, maximum impactor displacement and absorbed energy with impact energy are found. The increase of impact energy will increase the content of α phase due to the difference in plastic deformation resistance of microstructures. Different impact behaviors appeared for titanium alloy specimens and CFRP specimens. CFRP specimens exhibited superior impact resistance with lower peak impact force and higher energy absorption relative to titanium alloy specimens.