Traditional methods for obtaining probability of detection curves in structural health monitoring systems are often costly in terms of budget, and time. To address these limitations, we propose a novel computational framework that uses gradient- and Hessian-enhanced surrogate modeling for estimating model-assisted probability of detection curves in guided-wave structural health monitoring systems. The framework leverages the hypercomplex automatic differentiation method combined with the spectral finite element method to compute highly accurate derivatives of wave propagation phenomena with respect to model parameters. These derivatives form the basis for two derivative-enhanced surrogate modeling approaches. The first, Taylor series expansion, approximates model-assisted probability of detection curves under nominal conditions, offering a computationally efficient alternative to traditional simulations. The second, gradient-Hessian Enhanced Kriging, constructs a surrogate model capable of predicting model-assisted probability of detection curves under varying operational conditions, enabling efficient transfer of probability of detection curves across scenarios without additional simulations. Two application examples are presented: an academic example with a detailed step-by-step description, and an application study of a plate with a localized through-thickness defect detected using two piezoelectric transducers in a pitch-and-catch configuration. Our framework significantly reduces the computational cost by over 30% in obtaining model-assisted probability of detection curves while maintaining equivalent accuracy to traditional methods. Additionally, incorporating gradient and Hessian information improves surrogate model accuracy and reduces training sample requirements by a factor of two compared to derivative-free models. This approach offers an efficient solution for generating model-assisted probability of detection curves and facilitates the development of probability of detection transfer functions with negligible additional computational cost.

Interlaminar reinforcement, utilizing vertically aligned carbon nanotubes (VA-CNT), was realized for the first time with high-temperature aerospace carbon fiber reinforced plastic (CFRP) composites. Polyimide composites, widely used in high-temperature aerospace applications, have higher operating temperatures than epoxy systems (350-400 degrees C vs. 150-200 degrees C) due to their higher glass transition temperature (Tg), but suffer from lower toughness and reduced interlaminar properties. In prior work, employing VA-CNT arrays between plies has successfully toughened and strengthened the interlaminar region in aerospace epoxy CFRP, leading to broadly improved mechanical properties, including fatigue life. Here, two VA-CNT interlaminar reinforcing architectures are integrated with aerospace PI CFRP and show significant reinforcing effects. Both architectures, VA-CNTs (termed nanostitch) and a patterned and densified VA-CNT forest (termed buckled nanostitch), are integrated with PI CFRP. While maintaining the interlaminar thickness, 10 mu m thick VA-CNT films (nanostitch) show an increase in short-beam shear (SBS) strength by 22 % under static loading and 2-13 times increase in fatigue life under SBS fatigue loading. Toughness increases of 31 % and 30 % are observed in initiation and steady-state Mode I toughness, respectively, with the steady-state crack noted to bifurcate away from the reinforced inter- laminar region and into the intralaminar region. This study underscores the application of CNTs as reinforcement in relatively brittle high-temperature aerospace composite structures, such as aircraft engine components, space vehicle heat shields, and satellite structural elements.

Many modern structural health monitoring (SHM) systems use piezoelectric transducers to induce and measure guided waves propagating in structures for structural damage detection. To increase the detection capabilities of SHM systems, gradient-based optimization of sensor placement is frequently necessary. However, available numerical differentiation methods for mechanical wave propagation problems suffer from truncation and subtraction errors and are difficult to extend to high-order sensitivities. This paper addresses these issues by introducing an approach to obtain highly accurate numerical sensitivities of arbitrary order in mechanical wave propagation problems. The hypercomplex time-domain spectral finite element method (ZSFEM) couples the hypercomplex Taylor series expansion method with the time-domain spectral finite element method. We show how ZSFEM can be implemented within the commercial finite element package ABAQUS/Explicit. For verification, we compared the numerical and analytical results of the displacement and its sensitivities with respect to mechanical parameters, geometry, and boundary conditions for a rod subjected to a sudden, distributed axial load. First- and second-order sensitivities were obtained with normalized root mean square deviations below [Formula: see text]. Mesh convergence analyses revealed that [Formula: see text]-refinement offered better convergence rates than [Formula: see text]-refinement for the outputs and their sensitivities. Also, the sensitivities obtained with ZSFEM were compared with finite differences showing higher accuracy and step-size independence (e.g., no iteration is needed to determine the step size that minimizes the error). For simplicity, ZSFEM was presented only for one-dimensional truss elements, but the method is general and can be applied to other elements.

The utilization of space systems stands as a cornerstone of contemporary living. Technological advancements play a pivotal role in achieving the reusability, safety, and cost-effectiveness of space vehicles. Intelligent space systems with integrated sensors capable of structural health monitoring (SHM) are driving innovation to enhance vehicle's self-awareness, enable adaptation to operational conditions, and reduce operational expenses. SHM's primary objective is to detect, evaluate, and forecast structural behaviors, thereby supplying vital insights for mission control and retirement decisions. In this contribution, utilization of thin unobtrusive piezoelectric sensors for obtaining structural dynamic characteristics of space structures are considered. The sensors enable operational modes in which structural vibration information is acquired due to events caused by micrometeorite impacts or mission progress. The latter one may include component deployment, docking or spacecraft's operation. Alternatively, the sensors may be subject to a broadband frequency excitation signal which allows for acquisition of the local structural dynamics directly. To enable both measurement modalities, miniature SHM hardware based on the microcontroller systems is proposed and their operation with piezoelectric sensors is demonstrated. Structural vibration information acquired during impact events is analyzed and signal features characteristic of the events are proposed. Local structural vibration modes are considered for characterization of spacecraft's components and assessment of structural integrity. Recommendation on utilizing the piezoelectric sensors in orbital environment are given and a path for practical integration of the SHM into spacecraft operation is proposed.

Composite laminates utilizing autoclave-grade carbon fiber-reinforced plastic (CFRP) prepreg were manufactured using a polymer nanoporous network (NPN) interlayer that generates capillary pressure in lieu of pressure from an autoclave. The polymer nanofiber NPN film is integrated into the interlaminar region and is shown to eliminate voids in a vacuum-bag-only (VBO) curing process. After a preliminary investigation of the effect of NPN thickness on the interlaminar region and performance, an 8 μm thick polymer NPN was selected for a scaled manufacturing demonstration. Combining the polymer NPN with "out-of-oven" (OoO) electrothermal heating of a carbon nanotube (CNT)-heated tool, a 0.6 × 0.6 m void-free plate is successfully manufactured. OoO cure enables an accelerated cure cycle, which reduces the cure time by 35% compared to the manufacturer-recommended cure cycle (MRCC). X-ray microcomputed tomography (μ-CT) reveals that the laminates are void-free and of identical quality to autoclave-cured specimens. An array of mechanical tests including tension, compression, open-hole compression (OHC), tension-bearing (bolt-bearing), and compression after impact, reveal that the accelerated NPN-cured composites were broadly equivalent, with some instances of improved properties, relative to the autoclave-cured parts, e.g., OHC strength increased by 5%. With reduced capital costs, energy consumption, and increased throughput, the facile polymer NPN-enabled out-of-autoclave (OoA) fabrication method is shown to be a practical and attractive alternative to conventional autoclave fabrication.

Average satellite lifetime in orbit has been gradually increasing since the advent of the space industry. With the increase in satellite lifespan on orbit it is becoming increasingly economically advantageous to refuel and conduct in-orbit servicing rather than launching new satellites. This presents a challenging problem of automatic docking for refueling. Traditionally, docking mechanisms have used either a physical switch, a force sensor, a torque sensor, or a combination of the three. Traditional docking verification techniques using those sensors are suboptimal for use in an orbital servicing satellite as customer's satellite mass can vary in a wide range while docking velocity also varies. In addition, the refueling system produces a complex pattern of mechanical signals during docking, which is challenging to classify. For this reason, an SHM system with small unobtrusive piezoelectric sensors was proposed to identify and characterize satellite docking. It was decided that the passive monitoring of the docking is preferable over active methods not to interfere with satellite dynamics and reduce power use. The mechanical waves resulted from the satellite's docking momentum annulment, thrusters, electrical motors, and mechanical component deployment were passively monitored using an array of piezoelectric wafer sensors. Features in mechanical signals corresponding to the docking were distinguished from other mechanical events normal to satellite's operation. An algorithm was developed that utilize features specific to docking to classify the quality of the docking engagement including potential false positives and misalignment issues. This algorithm was embedded in a real-time microprocessor which was used to capture passive ultrasonic signals and run the associated data analysis algorithm. Experiments conducted on a laboratory scale docking imitator suggested the applicability of the proposed approach and verified performance on the data acquisition and classification system on exemplary signals.

A novel composite manufacturing technique, utilizing open nanoporous materials, termed Nano-Porous Network (NPN), are used to consolidate autoclave aerospace-grade epoxy prepreg carbon fiber reinforced composite (CFRP) laminates without an autoclave. The L-shape geometry parts were cured with vacuum-bag only (VBO) and shown to be void free. Previous studies have demonstrated that various NPNs, including an electrospun polymer nanofiber veil NPN as used here, can provide capillary forces that enhance resin flow at ply interfaces to enable void elimination without requiring autoclave pressure. In this study, a complex structure (i.e., L-shape) structure is manufactured with an electrospun polymer nanofiber veil between each unidirectional (UD) IM7/8552 ply and cured in a conventional oven under vacuum following the manufacturer-recommended cure cycle (MRCC) but not using autoclave pressure. The L-shape is studied extensively in the literature and is known to have a varying pressure distribution around the radius such that voids concentrate in the corner. The cured parts (baseline autoclave and NPN VBO processed) were inspected with X-ray micro-computed tomography (μCT) and shown to be void-free in both the flat regions and the curve and showed equivalent interlaminar strength to autoclave-cured parts, similar to previously reported flat panels. Exhibiting low capital cost, low energy consumption, and high manufacturing efficiency, this fabrication method has the potential to replace conventional autoclave fabrication in the aerospace industry.

Large-scale aerospace composite parts utilizing autoclave-required carbon fiber reinforced plastic (CFRP) epoxy prepreg were manufactured with a novel Nano- Porous Network (NPN) enabled and CNT-heater based out-of-oven (OoO) for a combined out-of-autoclave (OoA) approach. While prior work has established open porosity carbon nanotube films as effective NPN materials, here we utilize electrospun polymer nanofiber veils to provide capillary pressure in the inerlaminar region during curing, thus enabling void elimination without requiring an autoclave. A CNT-based surface heater was used to cure a 0.6 m  0.6 m plate. Utilizing the fast rate of the CNT heaters, an accelerated cure cycle was developed to shorten the cure cycle by 35% compared to the manufacturer-recommended cure cycle (MRCC). X-ray microcomputed tomography (μCT) inspections showed that the cured parts (autoclave baseline and combined OoA approach) were void-free. The mechanical tests (shortbeam shear, tension, and compression) revealed that the accelerated-cure combined OoA approach composites were equivalent to the autoclave cured parts. Having attributes of reducing capital cost and increasing manufacturing efficiency intrinsic to OoA, this scaled fabrication method is a facile alternative to conventional autoclave fabrication, and future work will study a broader array of mechanical testing.

View Video Presentation: https://doi.org/10.2514/6.2022-0503.vid Interest has been growing in the development and use of composite laminates based on polymer matrices with glass transition temperatures (Tg) beyond traditional epoxies, which are more suitable for high-temperature applications, such as propulsion components. However, these high-Tg composites generally present relatively poor interlaminar strength and toughness and are, therefore, prone to delamination. Here, a strategic reinforcement of the interlaminar region in high Tg (375ºC) carbon fibre reinforced polyimide (CFRP) unidirectional-ply laminates is investigated. We show for the first time that vertically aligned carbon nanotubes (VA-CNT) can be used to reinforce the interlaminar region of polyimide-based CFRP laminates to delay delamination initiation. A 20 µm VA-CNT morphology (termed nanostitch) is investigated resulting in statistically significant improvements of 7% on the static interlaminar shear strength. These results provide a strong foundation for continued research to mature and scale the technique so that it can be applied to real composite propulsion components such as stator vanes and fan ducts.

Fatigue cracks are a major concern for aging metallic structures, as they can cause unexpected catastrophic failure. They often occur on hidden surfaces, e.g., under paint and rust or on the interface between two structural layers, which makes visual detection difficult. Acoustic Emission (AE) is a passive nondestructive evaluation (NDE) method that measures the initiation and progression of defects when a structure is continuously monitored. While using this method, which has been successfully applied to monolithic and composite structures, the fatigue life is related to the cumulative AE count. However, if the method is applied after the fatigue crack is initiated, the relationship between the fatigue life and the cumulative AE count may not be applicable. In this paper, the dependence of AE behavior on the prior fatigue loading history is studied using modified compact tension specimens. The cumulative AE count, AE correlation plots, and waveform characteristics are shown to be dependent on loading conditions in addition to the coupling state. Relative behaviors between AE sensors and trends in the cumulative AE count are illustrated by two AE characteristics that are shown as being independent of prior loading history and coupling discontinuities.

This work demonstrates the curing of autoclave required prepreg in a vacuum-bag-only (VBO) environment by the integration of various nanomaterial systems with nanoscale porosity, termed nanoporous network (NPN) materials in the ply-ply interface of the composite prepreg laminates. This enables void removal by encouraging resin infusion through capillary effects. This process removes the need for applied autoclave pressure and enables VBO curing of autoclave required composites using either conductive or convective heating under vacuum. A polyamide (PA) commercially available electrospun polymer nanofiber (EPN) film as well as a bespoke-commercial polyimide (PI) aerogel are demonstrated. Void-free laminates are revealed by micro-computed tomography (µCT) in flat as well as in L-shaped geometries, and as tested in flat geometries short beam shear strength parity is achieved with autoclave cured laminates.

Several novel next-generation composite architectures are demonstrated by introducing hierarchical architectures of vertically aligned carbon nanotube (VACNT) arrays. Here, we integrate buckling and patterning of VACNT arrays with advanced aerospace-grade carbon fiber polymer-matrix composites to introduce multi-level hierarchy into the interlaminar regions of composite laminates comprised of unidirectional IM7/8552 plies. The integration of buckled and patterned VACNTs shows ~7% increase in static interlaminar shear strength and ~224% increase in fatigue life across several load levels under short beam shear tests, when compared to the baseline unreinforced system. This hierarchical nano-engineered interlaminar reinforcement alters the damage modes, leading to a significant increase in structural strength over the conventional composites, beyond the unbuckled VACNT system. This work further demonstrates the ability to utilize structural instability of the nanoscale fibers at the microscale. In addition to the improvements in mechanical performance, the hierarchical composites with these micro-buckled nanoscale arrays are expected to be extended to multifunctional purposes beyond structural applications.

Here, a nanomaterial with morphology-controlled nanoscale capillaries is utilized to overcome manufacturing challenges in layered polymeric architectures. It is demonstrated that the capillary pressure from a nanoporous film replaces the need for applied pressure to manufacture void-free layered polymeric architectures. Manufacturing of aerospace-grade advanced carbon fiber composites is performed for the first time without utilizing pressure from an autoclave. Combined with a conductive curing approach, this work allows advanced composites to be manufactured without costly oven or pressure vessel infrastructure. The nanomaterial-enabled capillary pressure is quantified as 50% greater than typical pressures used in such processing, and is anticipated to overcome the limitations imposed by the requirement of high applied pressure in many other applications such as adhesive joining of various bulk materials including metals, press forming, and closed-mold infusion processing of layered composites and polymers.

The paper presents a detection sensitivity analysis for a guided wave (GW) approach to monitoring fatigue crack growth. Piezoelectric beamforming array (PZT) sensors were used to send and receive ultrasonic waves. In this “baselined” method, changes in sensor response are recorded between the installed and test conditions. Propagating cracks create line-of-site obstacles between arrays, thus more energy is reflected (pulseecho mode) and less is transmitted (pitch-catch mode) between sensor pairs. Any change in acoustic impedance, such as reduction in stiffness or thickness would also cause partial energy reflection, proportional to the relative impedance change. Two test configuration as presented here, including fatigue in 4-point bending and more traditional tension-tension. The purpose of the present study was to evaluate the sensitivity of this GW approach to damage size using the Length at Detection (LaD) statistical approach recently developed specifically to be applied to Structural Health Monitoring (SHM).

The paper presents a detection sensitivity analysis for a novel approach to monitoring fatigue crack growth. A carbon nanotube (CNT) sensor was used based on a potential drop (PD) damage detection strategy. Resistive CNT film was laminated between impermeable membranes to create a durable crack gauge that can resist high strain levels, high temperatures and submersion in water. Any crack growth below the sensor would disrupt the CNT electrical entanglement, therefore increasing the network resistance. As opposed to traditional crack gauges with discretized output based on broken copper traces, the CNT crack gauge provides for a continuous range of output, with resistance change proportional to the square of the crack length. The purpose of the present study was to evaluate the sensitivity of this CNT sensor to damage size using statistical approaches recently developed specifically to be applied to Structural Health Monitoring (SHM), including the Length at Detection (LaD) and REpeated Measured Random Effects Model (REM2) techniques.

Future space vehicle will incorporate a broad range of technologies for increased safety, situation awareness, and autonomous operation. It is envisioned that diagnostics, prognostics and health management (PHM)will enable monitoring and decision support for a variety of spacecraft systems, components and structures. This contribution highlights PHM elements applicable to space structures. Constitutive elements of PHM are discussed in light of multifaceted management of space structures from fabrication to retirement. In addition to classical PHM elements, aspects unique to space systems are highlighted: pre-launch diagnostics as a pathway to by-pass certain qualification tests, validation of component deployment, catastrophic event monitoring and assessment, and ability to report reentry breakup event. PHM approach to space structures is discussed in light of prior development, implementation and testing of structural health monitoring systems flown in stratospheric and sub-orbital flights as well as prior laboratory work and current development of ISS experiments. Examples of propagating and standing wave damage detection approaches are presented. Physics-based model including sensor, structure and measurement methodology is reported. Impact of space environment factors on sensor and structural response is estimated. It is advocated that the PHM could be a key element of future information-centric space vehicles.

A composite manufacturing technique based on the capillary action of a nanoporous network is investigated to address the limitations of current manufacturing methods such as autoclave and out-of-autoclave prepreg processes. This manufacturing technique consists of the insertion of a nanoporous network (i.e., vertically aligned carbon nanotube arrays) into the interlaminar regions of composite laminates. Void content and mechanical tests suggest that this capillary-driven manufacturing technique enables traditional autoclave-required prepreg to be processed under vacuum-only conditions without an autoclave or any modifications to the prepreg system. Since conventional autoclave prepregs have been widely used in the aerospace industries and their material properties have been extensively researched, this manufacturing technique can enable high-quality composite structures to be achieved in a much more efficient way.

There is much interest in the potential to use Structural Health Monitoring (SHM) technology to augment traditional Nondestructive Evaluation (NDE) methods to improve safety, increase asset availability, and reduce maintenance and inspection costs. SHM has the potential to be used in many areas of application including critical components in aircraft and pipelines. Probability of detection (POD) plays a critical role in aircraft structural integrity programs. As such, there has been a high interest in developing methods that can be used to assess POD in SHM applications. In contrast to traditional NDE laboratory experiments to assess POD that involve a set of specimens with cracks, SHM sensors are fixed and SHM data are acquired over time as cracks grow or otherwise evolve. Traditional statistical methods for assessing POD (e.g., as described in MIL-HDBK 1823A 2009) no longer apply to such repeated-measures data. This purpose of this paper is to review the basic statistical concepts of probability of detection (POD) and to show how these concepts can and should be applied to SHM POD studies by modifying and extending existing methods for estimating POD. The methods presented here are applicable when there is a scalar damage index or other response that will be used to make a detect decision. The paper compares a simple model based on length at detection and a random effects model to describe repeated measures data.

Next-generation composite manufacturing processes are needed to overcome several limitations of conventional manufacturing processes (e.g., high energy consumption). Here we explore, via experiments and modeling, the characteristics of the newly developed out-of-oven (OoO) curing technique that cures a composite laminate via resistive heating of a carbon nanotube film. When compared to oven curing of an aerospace-grade out-of-autoclave (OoA) carbon fiber prepreg advanced composite laminate, the OoO curing reduces energy consumption by over two orders of magnitude (14 vs. 0.1 MJ). Thermo-physical and mechanical tests including differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), short beam shear (SBS), and ex-situ and in-situ double-edge notch tension (DENT) indicate that the physical and mechanical properties of OoO-cured laminates are equivalent to those of oven-cured (baseline) laminates. In addition to energy savings, the OoO curing process has the potential to reduce part-to-part variations through improved spatiotemporal temperature control. (C) 2018 Elsevier Ltd. All rights reserved.