Adam Hehr

Carbon nanotube sensor thread for distributed strain and damage monitoring on IM7/977-3 composites

Laminated composite materials are used in applications where light weight is a key requirement. However, minor delamination damage in composites can propagate and lead to the failure of components. Failure occurs because delamination reduces the local bending stiffness and increases bending stress, which leads to the propagation of damage and eventual failure. These failures may be avoided if the damage could be detected early and repaired. Although many damage detection methods have been investigated, none are in widespread use today to prevent the failure of composites. This paper describes the use of carbon nanotube sensor thread to monitor strain and damage in composite materials. Sensor thread was bonded onto an IM7-laminated composite coupon to measure surface strain in a quasi-static uniaxial tensile test. The sensor thread was calibrated against a strain gage, which was also mounted to the coupon. The sensor thread measured the average strain over the length of the sample and indicated when the strain exceeded a nominal safe level. Sensor thread was also bonded to the surface of laminated composite panels in different patterns and detected, located and partially characterized the damage caused by multiple impacts to the panel. The new findings in this paper can be summarized as; (1) carbon nanotube sensor thread was tested as a distributed sensor for the first time on IM7/977-3 composites; (2) the sensor thread was found to monitor strain and detect damage in the composites with a potential sensitivity down to the micro-crack level; (3) the sensor thread was barely visible on the composite and did not add significant mass or affect the integrity of the composite; (4) the data acquisition system developed was simple and reliable.

Micro-crack detection and assessment with embedded carbon nanotube thread in composite materials:

Carbon nanotube thread has shown strong promise to be built into or onto composite materials for strain and damage monitoring via the material’s piezoresistive property. It has been found that a distinguishing feature of sensing thread incorporated in these materials is the detection of micro-cracking. This study articulates how embedded carbon nanotube thread in unidirectional glass fiber composites can identify the onset of matrix cracking, track crack growth, and differentiate between crack breathing and closing states. This information is obtained by analyzing the resistance response of the thread with a low-speed data acquisition system and a simple Wheatstone bridge circuit. A digital optical microscope was utilized to verify that a micro-crack was indeed present in the structure at the location of the sensor thread. Additionally, to demonstrate the effectiveness of this crack detection approach compared to past crack detection approaches, a comparison against foil-type strain gauges and piezoelectric accelerometers was made. Finally, a simple crack model is presented for the sensor thread.

Effect of weld power and build compliance on ultrasonic consolidation

Purpose Ultrasonic additive manufacturing (UAM) is a fabrication technology based on ultrasonic metal welding. As a solid-state process, temperatures during UAM fabrication reach a fraction of the melting temperatures of the participating metals. UAM parts can become mechanically compliant during fabrication, which negatively influences the ability of the welder to produce consistent welds. This study aims to evaluate the effect of weld power on weld quality throughout a UAM build, and develop a new power-compensation approach to achieve homogeneous weld quality. Design/methodology/approach The study utilizes mechanical push-pin testing as a metric of delamination resistance, as well as focused ion beam and scanning electron microscopy to analyze the interface microstructure of UAM parts. Findings Weld power was found to negatively affect mechanical properties and microstructure. By keeping weld power constant, the delamination energy of UAM coupons was increased 22 per cent along with a consistent grain structure. As a result, to ensure constant properties throughout UAM component construction, maintaining weld power is preferable over the conventional strategy based on amplitude control. Research limitations/implications Further characterization could be conducted to evaluate the power control strategy on other material combinations, though this study strongly suggests that the proposed approach should work regardless of the metals being welded. Practical implications The proposed power control strategy can be implemented by monitoring and controlling the electrical power supplied to the welder. As such, no additional hardware is required, making the approach both useful and straightforward to implement. Originality/value This research paper is the first to recognize and address the negative effect of build compliance on weld power input in UAM. This is also the first paper to correlate measured weld power with the microstructure and mechanical properties of UAM parts.

Deformation Mechanisms in NiTi-Al Composites Fabricated by Ultrasonic Additive Manufacturing

Thermally active NiTi shape memory alloy (SMA) fibers can be used to tune or tailor the effective coefficient of thermal expansion (CTE) of a metallic matrix composite. In this paper, a novel NiTi-Al composite is fabricated using ultrasonic additive manufacturing (UAM). A combined experimental-simulation approach is used to develop and validate a microstructurally based finite element model of the composite. The simulations are able to closely reproduce the macroscopic strain versus temperature cyclic response, including initial transient effects in the first cycle. They also show that the composite CTE is minimized if the austenite texture in the SMA wires is 〈001〉B2, that a fiber aspect ratio >10 maximizes fiber efficiency, and that the UAM process may reduce hysteresis in embedded SMA wires.

Characterization of embedded fiber optic strain sensors into metallic structures via ultrasonic additive manufacturing

Fiber Bragg Grating (FBG) sensors measure deviation in a reflected wavelength of light to detect in-situ strain. These sensors are immune to electromagnetic interference, and the inclusion of multiple FBGs on the same fiber allows for a seamlessly integrated sensing network. FBGs are attractive for embedded sensing in aerospace applications due to their small noninvasive size and prospect of constant, real-time nondestructive evaluation. In this study, FBG sensors are embedded in aluminum 6061 via ultrasonic additive manufacturing (UAM), a rapid prototyping process that uses high power ultrasonic vibrations to weld similar and dissimilar metal foils together. UAM was chosen due to the desire to embed FBG sensors at low temperatures, a requirement that excludes other additive processes such as selective laser sintering or fusion deposition modeling. In this paper, the embedded FBGs are characterized in terms of birefringence losses, post embedding strain shifts, consolidation quality, and strain sensing performance. Sensors embedded into an ASTM test piece are compared against an exterior surface mounted foil strain gage at both room and elevated temperatures using cyclic tensile tests.

Optimal welding parameters for very high power ultrasonic additive manufacturing of smart structures with aluminum 6061 matrix

Ultrasonic additive manufacturing (UAM) is a recent solid state manufacturing process that combines ad- ditive joining of thin metal tapes with subtractive milling operations to generate near net shape metallic parts. Due to the minimal heating during the process, UAM is a proven method of embedding Ni-Ti, Fe-Ga, and PVDF to create active metal matrix composites. Recently, advances in the UAM process utilizing 9 kW very high power (VHP) welding has improved bonding properties, enabling joining of high strength materials previously unweldable with 1 kW low power UAM. Consequently, a design of experiments study was conducted to optimize welding conditions for aluminum 6061 components. This understanding is critical in the design of UAM parts containing smart materials. Build parameters, including weld force, weld speed, amplitude, and temperature were varied based on a Taguchi experimental design matrix and tested for me- chanical strength. Optimal weld parameters were identi ed with statistical methods including a generalized linear model for analysis of variance (ANOVA), mean e ects plots, and interaction e ects plots.

Dynamics of ultrasonic additive manufacturing.

Ultrasonic additive manufacturing (UAM) is a solid-state technology for joining similar and dissimilar metal foils near room temperature by scrubbing them together with ultrasonic vibrations under pressure. Structural dynamics of the welding assembly and work piece influence how energy is transferred during the process and ultimately, part quality. To understand the effect of structural dynamics during UAM, a linear time-invariant model is proposed to relate the inputs of shear force and electric current to resultant welder velocity and voltage. Measured frequency response and operating performance of the welder under no load is used to identify model parameters. Using this model and in-situ measurements, shear force and welder efficiency are estimated to be near 2000N and 80% when welding Al 6061-H18 weld foil, respectively. Shear force and welder efficiency have never been estimated before in UAM. The influence of processing conditions, i.e., welder amplitude, normal force, and weld speed, on shear force and welder efficiency are investigated. Welder velocity was found to strongly influence the shear force magnitude and efficiency while normal force and weld speed showed little to no influence. The proposed model is used to describe high frequency harmonic content in the velocity response of the welder during welding operations and coupling of the UAM build with the welder.

Embedded Carbon Nanotube Sensor Thread for Structural Health Monitoring and Strain Sensing of Composite Materials

Abstract This chapter investigates the use of carbon nanotube (CNT) sensor thread in distributed structural health monitoring (SHM) systems, specifically as embedded damage and strain sensors. The CNT sensor thread has shown potential to be integrated into/onto composite materials to provide confident damage detection, localization, and characterization in complex geometries without complicated detection algorithms and minimal sensing channels. This chapter articulates current work done with CNT thread in Nanoworld Laboratories, specifically CNT thread performance as a sensor; past, current, and future embedded sensing work; and potential SHM design architectures for aircraft, along with a description of a few potential multifunctional aspects of the material. Multifunctional here implies improving the composite material besides self-sensing of damage and strain. Some of these multifunctional characteristics include self-sensing of moisture, oxidation, and temperature; improved mechanical properties of damping, toughness, stiffness, and strength; and improved thermal and electrical transport, among many other potential areas. Besides these multifunctional characteristics, CNT thread is low in weight and small in size and the material is modest in cost. As a consequence of these strong sensor and material characteristics, the authors believe that this could be a game-changing material for high-cost composite commercial and defense vehicles. Future military and commercial composite vehicles will have “nano inside” to provide safety, reliability, durability, condition-based maintenance, and multifunctionality.

Passive damping of carbon nanotube thread

Passive damping of as-spun carbon nanotube thread was studied here as a multifunctional attribute to continuous fiber composite materials or nanocomposites. Many researchers have doped polymeric materials like epoxy with carbon nanotubes to achieve enhanced damping for composite materials; yet thread or yarn has to be rigorously explored. As a result, this study quantifies carbon nanotube thread material damping within its linear range (~1% strain) at a single frequency and analyzes this damping as a function of temperature from room temperature to roughly 100°C. From these tests, it was found that three distinct carbon nanotube threads exhibited loss factors near 0.05 at room temperature, and a single carbon nanotube thread maintained a loss factor near 0.035 from room temperature to 100°C after the first thermal cycle.

Al‐NiTi Metal Matrix Composites for Zero CTE Materials: Fabrication, Design, and Modeling

Al-NiTi composites fabricated via ultrasonic additive manufacturing (UAM) provide a light-weight solution for low thermal expansion applications. It is shown that the thermal expansion of Al 6061 can be reduced by over 50% by incorporating a 13% volume fraction of NiTi fibers. This reduction in thermal expansion occurs from the contraction of the NiTi fiber during heating, thereby offsetting the thermal expansion of the Al matrix. Al-NiTi composites are made possible by low temperature UAM process. Successful implementation of these composites requires a careful design approach that includes the processing characteristics as well as the thermo-mechanical response of the shape memory fibers and matrix. This is achieved using a NiTi microstructure based FEA model implemented that captures the underlying thermomechanical response of the NiTi fibers and calculates the complex stress state within the composite.