Kwek Tze Tan

Impact Damage Resistance, Response, and Mechanisms of Laminated Composites Reinforced by Through-Thickness Stitching

In this article, the study of impact damage of laminated composites reinforced by through-thickness stitching is investigated and presented in threefold. Specimens stitched with varying stitch density and stitch thread thickness are subjected to low-velocity impact via a drop-weight machine. Impact damage resistance is first studied by examining the extent of delamination area in damaged specimens using ultrasonic C-scan analysis. It is revealed that higher stitch density is more capable of impeding delamination growth by arresting cracks at closer interval and suppressing crack propagation. The use of thicker stitch thread offers slight improvement to damage resistance by marginal reduction in delamination propagation, and is more pertinent at high impact energy levels. Impact damage response is then analyzed from the impact history response curves of impacted laminates. The impact response of load–time graphs demonstrates that the onset of delamination is not influenced by stitch density and stitch thread thickness, but the maximum residual impact force is related to the delamination size of the laminates, which is sequentially related to stitch parameters. Finally, impact damage mechanisms are elucidated by employing X-ray radiography and micro-Computed Tomography to reveal subsurface damages, primarily dominated by intralaminar matrix cracks, interlaminar delamination, and stitch fiber/matrix debonding. It is revealed that stitches act as crack initiation sites, due to the presence of weak resin-rich pockets around stitch threads, thus inadvertently resulting in densely stitched composites having more stitch-induced matrix cracks upon impact loading. Contrarily, specimens with higher stitch density and thread thickness are more capable of impeding delamination growth by effectively bridging delamination cracks and arresting crack propagation. Principal mechanisms responsible for impact resistance performance of stitching namely crack arresting and crack bridging are presented and discussed.

Optimizing the band gap of effective mass negativity in acoustic metamaterials

A dual-resonator microstructure design is proposed for acoustic metamaterials to achieve broadband effective mass negativity. We demonstrate the advantage of acoustic wave attenuation over a wider frequency spectrum as compared to the narrow band gap of a single-resonator design. We explicitly confirm the effect of negative effective mass density by analysis of wave propagation using finite element simulations. Examples of practical application like vibration isolation and blast wave mitigation are presented and discussed.

Interlaminar Fracture Toughness of Vectran-stitched Composites - Experimental and Computational Analysis

In this article, mode I interlaminar fracture toughness (GIC) of Vectran-stitched laminated composite is determined experimentally and computa- tionally. Critical strain energy release rates are measured by performing double can- tilever beam test on composites stitched with Vectran as stitch fiber, and are found to increase with increasing stitch thread thickness and stitch density. It is also revealed that the relationship between GIC and stitch density or stitch thread volume fraction appears to be linear. Interlaminar tension test is conducted to identify important fracture behavior of a single Vectran stitch fiber thread. The finite-element (FE) model of the stitched composite incorporates the novel four-step stitch fracture pro- cess, namely, interfacial debonding, slack absorption, fiber breakage, and pullout friction. The FE predictions of load-displacement curves and critical mode I strain energy release rates show good agreement with the experimental results. The differ- ences in interdependent stitch mechanisms between moderately stitched and densely stitched composites are discussed.

Numerical investigation of band gaps in 3D printed cantilever-in-mass metamaterials.

In this research, the negative effective mass behavior of elastic/mechanical metamaterials is exhibited by a cantilever-in-mass structure as a proposed design for creating frequency stopping band gaps, based on local resonance of the internal structure. The mass-in-mass unit cell model is transformed into a cantilever-in-mass model using the Bernoulli-Euler beam theory. An analytical model of the cantilever-in-mass structure is derived and the effects of geometrical dimensions and material parameters to create frequency band gaps are examined. A two-dimensional finite element model is created to validate the analytical results, and excellent agreement is achieved. The analytical model establishes an easily tunable metamaterial design to realize wave attenuation based on locally resonant frequency. To demonstrate feasibility for 3D printing, the analytical model is employed to design and fabricate 3D printable mechanical metamaterial. A three-dimensional numerical experiment is performed using COMSOL Multiphysics to validate the wave attenuation performance. Results show that the cantilever-in-mass metamaterial is capable of mitigating stress waves at the desired resonance frequency. Our study successfully presents the use of one constituent material to create a 3D printed cantilever-in-mass metamaterial with negative effective mass density for stress wave mitigation purposes.

Asymmetric wave transmission in a diatomic acoustic/elastic metamaterial

Asymmetric acoustic/elastic wave transmission has recently been realized using nonlinearity, wave diffraction, or bias effects, but always at the cost of frequency distortion, direction shift, large volumes, or external energy. Based on the self-coupling of dual resonators, we propose a linear diatomic metamaterial, consisting of several small-sized unit cells, to realize large asymmetric wave transmission in low frequency domain (below 1 kHz). The asymmetric transmission mechanism is theoretically investigated, and numerically verified by both mass-spring and continuum models. This passive system does not require any frequency conversion or external energy, and the asymmetric transmission band can be theoretically predicted and mathematically controlled, which extends the design concept of unidirectional transmission devices.

Attenuation of transverse waves by using a metamaterial beam with lateral local resonators

This study numerically and experimentally investigated the wave propagation and vibrational behavior of a metamaterial beam with lateral local resonators. A two-dimensional simplified analytical model was proposed for feasibly and accurately capturing the in-plane dispersion behavior, which can be used for the initial design. The out-of-plane wave motions, however, required advanced three-dimensional (3D) modeling. Through experimental validations, 3D finite element simulations were demonstrated to be suitable for advanced design and analysis. This study provided a basis for designing metabeams for transverse wave mitigation. The proposed concept can be further extended to 3D metamaterial plates for wave and vibrational mitigation applications.

A diatomic elastic metamaterial for tunable asymmetric wave transmission in multiple frequency bands

Unidirectional/asymmetric transmission of acoustic/elastic waves has recently been realized by linear structures. Research related to unidirectionality of wave propagation has received intense attention due to potentially transformative and unique wave control applications. However, asymmetric transmission performance in existing devices usually occurs only in a narrow frequency band, and the asymmetric frequencies are always within ultrasound range (above 20 kHz). In this work, we design and propose a linear diatomic elastic metamaterial using dual-resonator concept to obtain large asymmetric elastic wave transmission in multiple low frequency bands. All of these frequency bands can be theoretically predicted to realize one-way wave propagation along different directions of transmission. The mechanisms of multiple asymmetric transmission bands are theoretically investigated and numerically verified by both analytical lattice and continuum models. Dynamic responses of the proposed system in the broadband asymmetric transmission bands are explored and analyzed in time and frequency domains. The effect of damping on the asymmetric wave transmission is further discussed. Excellent agreements between theoretical results and numerical verification are obtained.

Dynamic impact testing of hedgehog spines using a dual-arm crash pendulum

Hedgehog spines are a potential model for impact resistant structures and material. While previous studies have examined static mechanical properties of individual spines, actual collision tests on spines analogous to those observed in the wild have not previously been investigated. In this study, samples of roughly 130 keratin spines were mounted vertically in thin substrates to mimic the natural spine layout on hedgehogs. A weighted crash pendulum was employed to induce and measure the effects of repeated collisions against samples, with the aim to evaluate the influence of various parameters including humidity effect, impact energy, and substrate hardness. Results reveal that softer samples-due to humidity conditioning and/or substrate material used-exhibit greater durability over multiple impacts, while the more rigid samples exhibit greater energy absorption performance at the expense of durability. This trend is exaggerated during high-energy collisions. Comparison of the results to baseline tests with industry standard impact absorbing foam, wherein the spines exhibit similar energy absorption, verifies the dynamic impact absorption capabilities of hedgehog spines and their candidacy as a structural model for engineered impact technology.

A novel meta-lattice sandwich structure for dynamic load mitigation

In this article, a novel meta-lattice sandwich structure is proposed and designed for impulsive wave attenuation and dynamic load mitigation. This original meta-lattice truss core sandwich structure has a similar configuration as a normal lattice sandwich structure, except that its truss bars are composed of meta-lattice truss unit cells. The design philosophy of locally resonant elastic metamaterials is integrated into the meta-lattice truss unit cell whereby a relatively heavier metal core (the resonator) is coated with a soft material layer (rubber coat), which is then connected to an outer shell. Based on this unique construction, several frequency band gaps are created by the locally resonant behavior of the specially designed resonators, in which stress waves within the stopping band gaps are not able to propagate through the material. Analytical spring-mass model is employed to predict the frequency band gaps, whereas numerical finite element simulation is utilized to model the continuum structure under impulsive loadings. The impact response, wave attenuation, and stress distribution contours between normal sandwich structure and meta-lattice sandwich structure are compared and analyzed. The mechanisms of wave mitigation and energy absorption by the internal resonators are thoroughly investigated. Results evidently show that the proposed meta-lattice sandwich structure has a more superior ability for impact mitigation and higher kinetic energy absorption capability due to the locally resonant behavior of the internal resonators.

Static flexural properties of hedgehog spines conditioned in coupled temperature and relative humidity environments

Hedgehogs are agile climbers, scaling trees and plants to heights exceeding 10m while foraging insects. Hedgehog spines (a.k.a. quills) provide fall protection by absorbing shock and could offer insights for the design of lightweight, material-efficient, impact-resistant structures. There has been some study of flexural properties of hedgehog spines, but an understanding of how this keratinous biological material is affected by various temperature and relative humidity treatments, or how spine color (multicolored vs. white) affects mechanics, is lacking. To bridge this gap in the literature, we use three-point bending to analyze the effect of temperature, humidity, spine color, and their interactions on flexural strength and modulus of hedgehog spines. We also compare specific strength and stiffness of hedgehog spines to conventional engineered materials. We find hedgehog spine flexural properties can be finely tuned by modifying environmental conditioning parameters. White spines tend to be stronger and stiffer than multicolored spines. Finally, for most temperature and humidity conditioning parameters, hedgehog spines are ounce for ounce stronger than 201 stainless steel rods of the same diameter but as pliable as styrene rods with a slightly larger diameter. This unique combination of strength and elasticity makes hedgehog spines exemplary shock absorbers, and a suitable reference model for biomimicry.