Terahertz Spectroscopic Characterization and Thickness Evaluation of Internal Delamination Defects in GFRP Composites

The use of terahertz time-domain spectroscopy(THz-TDS)for the nondestructive testing and evaluation(NDT&E)of materials and structural systems has attracted significant attention over the past two decades due to its superior spatial resolution and capabilities of detecting and characterizing defects and structural damage in non-conducting materials.In this study,the THz-TDS system is used to detect,localize and evaluate hidden multi-delamination defects(i.e.,a three-level multi-delamination system)in multilayered GFRP composite laminates.To obtain accurate results,a wavelet shrinkage de-noising algorithm is used to remove the noise from the measured time-of-flight(TOF)signals.The thickness and location of each delamination defect in the z-direction(i.e.,through-the-thickness direction)are calculated from the de-noised TOF signals considering the interaction between the pulsed THz waves and the different interfaces in the GFRP composite laminates.A comparison between the actual and the measured thickness values of the delamination defects before and after the wavelet shrinkage denoising process indicates that the latter provides better results with less than 3.712%relative error,while the relative error of the non-de-noised signals reaches 16.388%.Also,the power and absorbance levels of the THz waves at every interface with different refractive indices in the GFRP composite laminates are evaluated based on analytical and experimental approaches.The present study provides an adequate theoretical analysis that could help NDT&E specialists to estimate the maximum thickness of GFRP composite materials and/or structures with different interfaces that can be evaluated by the THz-TDS.Also,the accuracy of the obtained results highlights the capabilities of the THz-TDS for the NDT&E of multilayered GFRP composite laminates.

Parametric Optimization to Minimise the Surface Roughness on the Machining of GFRP Composites

The present investigation focuses on the parametric influence of machining parameters on the surface finish obtained in turning of glass fiber reinforced polymer （GFRP） composites. The experiments were conducted based on Taguchi＇s experimental design technique. Response surface methodology and analysis of variance （ANOVA） were used to evaluate the composite machining process to perform the optimization. The results revealed that the feed rate was main influencing parameter on the surface roughness. The surface roughness increased with increasing the feed rate but decreased with increasing the cutting speed. Among the other parameters, depth of cut was more insensitive. The predicted values and measured values were fairly close to each other, which indicates that the developed model can be effectively used to predict the surface roughness on the machining of GFRP composites with 95% confidence intervals. Using such model could remarkablely save the time and cost.

Optimizing the Machining Parameters for Minimum Surface Roughness in Turning of GFRP Composites Using Design of Experiments

In recent years, glass fiber reinforced plastics (GFRP) are being extensively used in variety of engineering applications in many different fields such as aerospace, oil, gas and process industries. However, the users of FRP are facing difficulties to machine it, because of fiber delamination, fiber pull out, short tool life, matrix debonding, burning and formation of powder like chips. The present investigation focuses on the optimization of machining parameters for surface roughness of glass fiber reinforced plastics (GFRP) using design of experiments (DoE). The machining parameters considered were speed, feed, depth of cut and workpiece (fiber orientation). An attempt was made to analyse the influence of factors and their interactions during machining. The results of the present study gives the optimal combination of machining parameters and this will help to improve the machining requirements of GFRP composites.

Compressive Behaviour and Failure Mechanisms of GFRP Composite at High Strain Rates

Experimental investigations into the compressive behavior of glass fiber reinforced polymer(GFRP)composite at high strain rates were carried out using a split Hopkinson pressure bar(SHPB)setup.The GFRP laminates were made from E-glass fibers and epoxy resins by vacuum assisted compression molding machine.The results of the compressive tests indicated that the mechanical behavior of the GFRP composite depends highly on the strain rate.The compressive peak stress,toughness and Young's modulus of the GFRP composite increased with the increase of strain rate,while the strain level at the initial stages of damage was shortened with the increase of strain rate.In addition,the dynamic deformation behavior and failure process of the specimens were observed directly by using a high-speed camera.Following the experiments,the fracture morphologies and damage modes were examined by scanning electron microscopy(SEM)to explore the possible failure mechanisms of the specimens.The results showed that multiple failure mechanisms appeared,such as matrix crack,fiber-matrix debonding,fiber failure and shear fracture.

Designing a Phthalonitrile/Benzoxazine Blend for the Advanced GFRP Composite Materials

High performance resin must be used in the high performance glass fiber-reinforced plastic（GFRP） composites, but it is sometimes difficult to balance the processabilities and the final properties in the design of advanced thermoset GFRP composites. In this study, a phthalonitrile/benzoxazine（PPN/BZ） blend with excellent processability has been designed and applied in the GFRP composite materials. PPN/BZ blend with good solubility, low melt viscosity, appropriate gel condition and low-temperature curing behavior could enable their GFRP composite preparation with the prepreg-laminate method under a relatively mild condition. The resulted PPN/BZ GFRP composites exhibit excellent mechanical properties with flexural strength over 700 MPa and flexural modulus more than 19 GPa. Fracture surface morphologies of the PPN/BZ GFRP composites show that the interfacial adhesion between resin and GF is improved. The temperatures at weight loss 5%（T_（5%）） and char residue at 800 °C of all PPN/BZ GFRP composites are over 435 °C and 65% respectively. PPN/BZ GFRP composites with high performance characteristics may find applications under some critical circumstances with requirements of high mechanical properties and high service temperatures.

Combination form analysis and experimental study of mechanical properties on steel sheet glass fiber reinforced polymer composite bar

The concept of steel sheet glass fiber reinforced polymer(GFRP)composite bar(SSGCB)was put forward.An optimization plan was proposed in the combined form of SSGCB.The composite principle,material selection,and SSGCB preparation technology have been described in detail.Three-dimensional finite element analysis was adopted to perform the combination form optimization of different steel core structures and different steel core contents based on the mechanical properties.Mechanical tests such as uniaxial tensile,shear,and compressive tests were carried out on SSGCB.Parametric analysis was conducted to investigate the influence of steel content on the mechanical properties of SSGCB.The results revealed that the elastic modulus of SSGCB had improvements and increased with the rise of steel content.Shear strength was also increased with the addition of steel content.Furthermore,the yield state of SSGCB was similar to the steel bar,both of which indicated a multi-stage yield phenomenon.The compressive strength of SSGCB was lower than that of GFRP bars and increased with the increase of the steel core content.Stress-strain curves of SSGCB demonstrated that the nonlinear-stage characteristics of SSGCB-8 were much more obvious than other bars.