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Low-velocity impact damage is a major concern in the design of structures made of composite materials, because impact damage is hidden and cannot be detected by visual inspection. Piezoelectric sensors can be used to detect variations in structural and material properties for structural health monitoring. In this study, polyvinylidene flouride (PVDF) and lead zirconate titanate (PZT) sensors are used for monitoring impact damage initiation and propagation in composite laminates to illustrate this potential benefit. Several tests for monitoring the stress wave signals including acoustic emission due to failure modes, such as matrix cracking, delamination, and fiber breakage are performed. A series of impact tests at various impact energies by changing the impact mass and height is performed on the instrumented drop weight impact tester. The wavelet transform (WT) and short time fourier transform (STFT) are used to decompose the piezoelectric sensor signals in this study. Test results show that the particular waveform of sensor signals implying the damage initiation and propagation are detected above the damage initiation impact energy. It is found that both PZT and PVDF sensors can be used to detect the impact damage.

3D bioprinting of self-supporting stable tissue and organ structure is critically important in extrusion-based bioprinting system, especially for tissue engineering and regenerative medicine applications. However, the development of self-standing bioinks with desired crosslinking density, biocompatibility, tunable mechanical strength and other properties like self-healing, in situ gelation, drug or protein incorporation is still a challenge. In this study, we report a hydrogel bioink prepared from alginate (Alg) and hyaluronic acid (HA) crosslinked through multiple crosslinking mechanisms, i.e., acyl-hydrazone, hydrazide interactions and calcium ions. These Alg-HA gels were highly dynamic and shear-thinning with exceptional biocompatibility and tunable mechanical properties. The increased dynamic nature of the gels is mainly chemically attributed to the presence of acyl-hydrazone bonds formed between the amine groups of the acyl-hydrazide of alginate and the monoaldehyde of the hyaluronic acid. Among the different combinations of Alg-HA gel compositions prepared, the A5H5 (Alginate-acyl-hydrazide: HA-monoaldehyde, ratio 50:50) one showed a gelation time of ~60 s, viscosity of ~400 Pa.s (at zero shear rate), high stability in various pH solutions and increased degradation time (>50 days) than the other samples. The A5H5 gels showed high printability with increased post-printing stability as observed from the 3D printed structures (e.g., hollow tube (~100 layers), porous cube (~50 layers), star, heart-in, meniscus and lattice). The scanning electron microscopy analysis of the 3D constructs and hydrogels showed the interconnected pores (~181 µm) and crosslinked networks. Further, the gels showed sustained release of 5-amino salicylic acid and bovine serum albumin. Also, the mechanical properties were tuned by secondary crosslinking via different calcium concentrations. In vitro assays confirmed the cytocompatibility of these gels, where the 3D bioprinted lattice and tubular (~70 layers) constructs demonstrated high cell viability under fluorescence analysis. In in vivo studies, Alg-HA gel showed high biocompatibility (>90%) and increased angiogenesis (3 folds) and reduced macrophage infiltration (2-fold decrease), demonstrating the promising potential of these hydrogels in 3D bioprinting applications for tissue engineering and regenerative medicine with tunable properties.

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The capabilities for monitoring impact forces on an aircraft composite wing using impact response function (IRF) are examined. The IRF is derived based on the finite element method (FEM). Impact locations are identified by minimizing strain errors and force deviation errors. The reconstruction of impact forces are performed by computational inverse methods. The results of impact location identifications and impact force reconstructions are verified by numerical simulations using finite element analysis.