Influence of Stitch Density and Stitch Thread Thickness on Compression After Impact Strength of Stitched Composites

This study aims to investigate the influence of stitch density and stitch thread thickness on compression after impact (CAI) strength of stitched composites. Unstitched laminated composites and specimens stitched with varying stitch density and stitch thread thickness are subjected to impact damage and then compressive loading. It is shown that stitched composites have higher CAI strength than unstitched counterpart due to smaller impact-induced delamination area, where local buckling occurs during compressive failure. However, it is revealed that the effectiveness of stitching in suppressing delamination growth and inhibiting sublaminate buckling under compressive loading is intimately related to stitch density. It is also found out that stitch thread thickness has little influence on CAI strength at low impact energy level, but has considerable effect at high impact energy level

Fatigue Performance of Glass Fiber/Epoxy Composite at Low Temperature

Due to technological advancement in the manufacturing methods of composites, these materials find a plethora of applications which include but are not limited to wind energy projects in the form of turbine blades. These blades at times are exposed to temperatures as low as -40ᵒC. Therefore, there is a need to study low-temperature effects on such materials under different loading scenarios. This study investigates the possibility of utilizing MAC/GMC as a simulation tool to match trends of mechanical properties such as fatigue performance and stiffness variation of a given Glass fiber/Epoxy composite at 23ᵒC (room temperature) and -40 ᵒC, under fully reversed (R=-1) and tensile (R=0.1) loading cases. The results show remarkable consistency with the published data

Validation of Delamination Reduction Trend for Stitched Composites using Quasi-Static Indentation Test

A novel empirical-based Delamination Reduction Trend (DRT) for stitched composites has been recently proposed. The DRT is capable of predicting the effective reduction in impact induced delamination area due to the influence of stitching. DRT simply relates two parameters: normalized delamination area and stitch fibre volume fraction, to characterize the effectiveness of stitching in impact damage suppression. This paper seeks to validate the DRT by using quasi-static indentation (QSI) test, which is considered analogous to low velocity impact test, due to similar structural response. Results from QSI test show good agreement with DRT. Furthermore, limitations in DRT have been established

Negative Effective Mass Density of Acoustic Metamaterial using Dual-Resonator Spring-Mass Model

In this paper, we propose an acoustic metamaterial with a microstructure consisting of two internal resonators. The performance of this dual-resonator metamaterial is compared to the original singleresonator metamaterial. Analytical findings show that the dual-resonator metamaterial exhibits its negative effective mass density over a larger frequency spectrum, particularly at two distinctively asymptotic regions. The wave propagation phenomenon in the metamaterial is investigated using finite element simulation. Computational results reveal that the dual-resonator metamaterial is capable of attenuating wave propagation in a larger operating frequency. Practical applications like vibration control and blast mitigation are demonstrated and discussed

Progressive Damage in Stitched Composites under Impact Loading

Damage in carbon fibre reinforced plastics (CFRP) due to impact loading is an extremely complex phenomenon that comprises of multiple failure mechanisms like intra-laminar matrix cracks, interlaminar delamination, fibre pull-out and fibre fracture. In stitched composites, impact damage behavior is further complicated by the presence of through-thickness stitching [1, 2], which not only favorably increases mode I/II interlaminar strength [3, 4], but also inevitably creates geometrical defects like weak resin-rich pockets around stitch threads and misalignment of in-plane fibres. Computational modeling has been used to simulate progressive damage effectively [5]. However, the complexity of impact damage progression in stitched composites would need to be first understood and appreciated by physical experimental observations. In this study, quasi-static indentation (QSI) test is performed for the first time on stitched composites. QSI offers a good validation and comparison with low-velocity impact (LVI) test [6], and provides good understanding on damage progression in composite structures under impact loading. Damage initiation, propagation and ultimate failure are investigated due to the effect of stitching, particularly the influence of stitch density. Nondestructive evaluation (NDE) techniques namely ultrasonic c-scan analysis, x-ray radiography and xray micro computed tomography are employed to elucidate various damage mechanisms in stitched composites

Material Selection of Z-fibre in Stitched Composites - Experimental and Analytical Comparison Approach

Strain energy release rates are measured and compared for laminated composites stitched with different fibre materials – Carbon, Kevlar and Vectran. DCB test and FE simulation are performed to evaluate the interlaminar toughness. It is proven that Vectran provides the toughest interlaminar reinforcement and is most suitable for Zfibre application

Active Infrared Sensing of Impact Damage in Carbon Fibre Reinforced Polymer

With the growing demand of carbon fibre reinforced plastic (CFRP) in aerospace, marine and automobile industries, much attention is devoted to characterizing the material strength and characteristics of failure. This paper demonstrated the feasibility to estimate the internal damage non-destructively as a result of a quantified impact applied on 16-ply fabric CFRP. On thermography images at different heating time, differences were observed between intact area and area with internal damage. The estimation of 1D extent of damage using thermographic analysis was compared with images observed with cross sectional microscopic. The results suggest that qualitative analysis using thermography shows potential to be used as a tool for measuring impact damage

Low-velocity impact damage of composites by the element- failure method and strain invariant failure theory

Master'sMASTER OF ENGINEERIN

Metacomposites Protection System against Primary Blast Injury

There is an increasing use of improvised explosive devices (IEDs) in terrorist and insurgent activities. Exposure to blast is becoming more frequent. Blast related injuries can be inflicted at various levels. Primary blast injury results from blast wave-induced changes in atmospheric pressure, causing organs and tissues to rupture due to shearing and stretching at different rates. Secondary blast injury results from objects put in motion by the blast wind impacting a person. Tertiary blast injury results from a person being blown into solid objects by the blast wind. This paper deals with the protection system against primary blast injury by mitigating blast wave.Shear and stress waves from the primary blast could potentially cause traumatic brain injury (TBI) directly. Very often no external physical injuries are detected in the affected victims, but serious damages are inflicted internally, particualarly to the brain where neurological effcts can be slow to appear. In this work, we explore the use of metacomposites to build a protection system that can mitigate dynamic loads and attenuate blast wave. Firstly, the concept of elastic metamaterials is introduced by presenting the unique property of negative effective mass density through simple analytical mass-in-mass models. Next, the impact wave annenuation and blast wave mitigation capability of the metacomposites are demonstrated using computational simulation. Finally, practical ways to design and manufacture metacomposites protection system with negative effective mass density are proposed and presented