Fracture Mechanics of Polymers and Polymer Composites

Vitrimers have recently emerged as a class of polymers combining great processability, self-healing capability and high-temperature mechanical properties. Most of those salient features of vitrimers originate from the existence of dynamic covalent bonds in the polymer network. Although intensive research has been conducted for understanding the constitutive properties of these new materials, their fracture behaviors have been largely unexplored. Moreover, improving the creep resistance and fracture toughness of vitrimers are critical for the commercial applications, which need to be addressed. In this dissertation, we first experimentally show that, if the network with only regular covalent bonds and the network with dynamic covalent bonds are immiscible with each other, hybrid network can be synthesized with a low molar ratio of dynamic covalent bonds (less than 20 mol%), which still maintains its reprocessing and self-healing capabilities. Our discoveries will enable much greater tunability of the thermo-mechanical properties of vitrimers such as stress relaxation, creep resistance and fracture toughness, which can be important in many of their applications. Additionally, we investigate the rate-dependent fracture of vitrimers, and we, for the first time, obtain the intrinsic fracture energy and bulk dissipation of vitrimers during crack extension. Then, we find that the transient nature of a vitrimer network yields peculiar fracture characteristics that cannot be understood from existing fracture theories. Crack propagation is a non-equilibrium process whose velocity depends on the interplay between external load, bond dynamics and network damage. To explain the transient life of a crack in the vitrimer, we extend the linear elastic fracture theory to a dynamic model that predicts the time-dependent evolution of a crack during loading. Finally, we propose that a combination of crack tilting and crack bridging determines the effective fracture toughness of the fiber-reinforced composite with the plywood structure. Based on our quantitative analysis, it is found that the effective fracture toughness of the composite can be maximized for a certain pitch angle of the oscillated/twisted plywood structure, which agrees well with experiments

Research progress of ceramic matrix composite parts based on additive manufacturing technology

Ceramic matrix composites (CMCs) are materials that can be engineered for high-temperature applications in various fields including aerospace, marine, etc. It is very difficult to fabricate CMCs using traditional moulding methods due to their brittleness and high hardness. Additive manufacture (AM) technology, a digital manufacturing technology, provides multiple advantages over traditional manufacturing technologies, such as fabricating geometrically complex parts, mould-free fabrication, short development cycle, etc. In this paper, various AM technologies developed for CMCs are reviewed with emphasis on mechanisms of manufacturing, characteristics of production, and recent research progresses. With the springing up of innovative ideas and pioneering work, AM technology possesses unique forming capabilities in fabricating CMCs, demonstrating strong potentials in the application of CMCs in aerospace and other fields. However, there are still many challenges of CMCs fabricated by AM technologies, i.e. poor mechanical properties and geometric accuracies; lower reinforcement volume fraction than that of traditional manufacturing processes

Iterative Sliding Mode and Increment Feedback Attitude Control for On-Orbit Capturing Process of Spacecraft

According to the characteristics of spacecraft capturing noncooperative targets in orbit, an increment feedback controller based on nonlinear iterative sliding mode is presented. Firstly, the attitude tracking error equation is established, and then, an increment feedback control law based on bounded iterative sliding modes is proposed, which does not need to estimate the uncertain moment of inertia and external disturbances. For comparing, an adaptive sliding mode controller has been designed in the paper. Some numerical simulations have been given in the presence of spacecraft on-orbit capturing noncooperative target, and the simulation results show that the increment feedback controller has strong robustness to the unknown parametric variations and external disturbances and has a smaller control input torque in control process

Porous nanocomposites with enhanced intrinsic piezoresistive sensitivity for bioinspired multimodal tactile sensors

Abstract In this work, we propose porous fluororubber/thermoplastic urethane nanocomposites (PFTNs) and explore their intrinsic piezoresistive sensitivity to pressure. Our experiments reveal that the intrinsic sensitivity of the PFTN-based sensor to pressure up to 10 kPa increases up to 900% compared to the porous thermoplastic urethane nanocomposite (PTN) counterpart and up to 275% compared to the porous fluororubber nanocomposite (PFN) counterpart. For pressures exceeding 10 kPa, the resistance-pressure relationship of PFTN follows a logarithmic function, and the sensitivity is 221% and 125% higher than that of PTN and PFN, respectively. With the excellent intrinsic sensitivity of the thick PFTN film, a single sensing unit with integrated electrode design can imitate human skin for touch detection, pressure perception and traction sensation. The sensing range of our multimodal tactile sensor reaches ~150 Pa, and it exhibits a linear fit over 97% for both normal pressure and shear force. We also demonstrate that an electronic skin, made of an array of sensing units, is capable of accurately recognizing complex tactile interactions including pinch, spread, and tweak motions

Cavitation Induced Fracture of Intact Brain Tissue

Nonpenetrating traumatic brain injuries (TBI) are linked to cavitation. The structural organization of the brain makes it particularly susceptible to tears and fractures from these cavitation events, but limitations in existing characterization methods make it difficult to understand the relationship between fracture and cavitation in this tissue. More broadly, fracture energy is an important, yet often overlooked, mechanical property of all soft tissues. We combined needle-induced cavitation (NIC) with hydraulic fracture models to induce and quantify fracture in intact brains at precise locations. We report here the first measurements of the fracture energy of intact brain tissue that range from 1.5 to 8.9 J/m2, depending on the location in the brain and the model applied. We observed that fracture consistently occurs along interfaces between regions of brain tissue, which allow cavitation-related damage to propagate several millimeters away from the initial injury site. Quantifying the forces necessary to fracture brain and other soft tissues is critical for understanding how impact and blast waves damage tissue in vivo and has implications for the design of protective gear and tissue engineering

Accurate Cutting-Force Measurement with Smart Tool Holder in Lathe

Cutting force in lathe work is closely related to tool wear and affects the turning quality. Direct measurement of the cutting force by measuring the strain of the tool holder is challenging because the tool holder design aims to be highly rigid in order to undertake large cutting forces. Accordingly, the most popular dynamometer designs modify the standard tool holder by decreasing the structural rigidity of the holder, which reduces the machining precision and is not widely accepted. In order to solve the issue of the low stiffness of the dynamometer reducing the machining precision, in this paper, the ultra-low strain on the tool holder was successfully detected by the highly sensitive semiconductor strain gauges (SCSG) adjacent to the blade cutting insert. However, the cutting process would generate much heat, which increases the force measuring area temperature of the tool holder by about 30 °C. As a result, the readout drifted significantly with the temperature changes due to the high temperature coefficient of SCSG. To solve this problem, the temperature on the tool holder was monitored and a BP neural network was proposed to compensate for temperature drift errors. Our methods improved the sensitivity (1.14 × 10−2 mV/N) and the average relative error of the BP neural network prediction (≤1.48%) while maintaining the original stiffness of the tool holder. The smart tool holder developed possesses high natural frequency (≥6 kHz), it is very suitable for dynamic cutting-force measurement. The cutting experiment data in the lathe work show comparable performance with the traditional dynamometers and the resolution of the smart tool holder is 2 N (0.25% of total range)

3D Printing of a Biocompatible Double Network Elastomer with Digital Control of Mechanical Properties

The majority of 3D-printed biodegradable biomaterials are brittle, limiting their potential application to compliant tissues. Poly (glycerol sebacate) acrylate (PGSA) is a synthetic biodegradable and biocompatible elastomer, compatible with light-based 3D printing. In this work we employed digital-light-processing (DLP)-based 3D printing to create a complex PGSA network structure. Nature-inspired double network (DN) structures with two geometrically interconnected segments with different mechanical properties were printed from the same material in a single shot. Such capability has not been demonstrated by any other fabrication technique. The biocompatibility of PGSA after 3D printing was confirmed via cell-viability analysis. We used a finite element analysis (FEA) model to predict the failure of the DN structure under uniaxial tension. FEA confirmed the soft segments act as sacrificial elements while the hard segments retain structural integrity. The simulation demonstrated that the DN design absorbs 100% more energy before rupture than the network structure made by single exposure condition (SN), doubling the toughness of the overall structure. Using the FEA-informed design, a new DN structure was printed and the FEA predicted tensile test results agreed with tensile testing of the printed structure. This work demonstrated how geometrically-optimized material design can be easily and rapidly achieved by using DLP-based 3D printing, where well-defined patterns of different stiffnesses can be simultaneously formed using the same elastic biomaterial, and overall mechanical properties can be specifically optimized for different biomedical applications