Abstract Fibre-reinforced polymer (FRP) composite materials are limited in high temperature applications by the matrix glass transition temperature, T g . At and above this temperature, significant mechanical performance is lost, and degradation processes accelerated. This research explores the use of internal passages, or vascules, within the laminate to carry a coolant fluid, absorbing heat energy and cooling the material. A custom thermal chamber and four-point flexural test fixture were developed to perform in-situ thermo-mechanical testing. Vascular and non-vascular carbon/epoxy specimens were manufactured, containing arrays of four 1.1 mm diameter vascules. Specimens were exposed to temperatures from ambient to 170 °C (T g = 200 °C). Flexural modulus varied little with temperature across all tests. Non-vascular specimens at 170 °C showed a reduction in ultimate strength of 21 % compared to under ambient conditions. The presence of vascules caused a small improvement in flexural modulus and strength, due to displacement of a small number of 0° fibre tows further from the neutral axis as a result of the manufacturing process. At 15 L·min −1 coolant flow, vascular specimens showed full retention of strength compared to non-vascular specimens at ambient, demonstrating the potential mechanical performance benefits.

Abstract Shape memory alloys (SMAs) are a class of metallic alloys that possess remarkable characteristics such as superelasticity and shape memory effect. Superelastic SMAs have been considered as fiber in polymer composites due to their ability to recover their deformation upon removal of load, good energy dissipation capacity and impact resistance. Graphene nanoplatelets (GNPs) consists of small stacks of graphene sheets that are two-dimensional. Both sides of atomic lattice of GNPs contact matrix of a composite system and can generate more sites for potential chemical and physical bonding with the host material. Most importantly, graphene sheets and their derivatives can be produced at large-scale for industrial demand at low-cost. This study explores the fabrication of multi-scale reinforced epoxy matrix composites in which GNPs and SMA strands are employed as nano- and micro-scale reinforcements, respectively. First, GNPs are dispersed into a ductile and brittle epoxy matrix to produce GNP/epoxy nanocomposites. To study the effect of GNP content on the behavior of the developed nanocomposite, GNPs are added to the epoxy-hardener mixture at different weight percentages (neat, 0.1%, 0.25%, 0.5%, 1%, and 2%). Uniaxial tensile tests of the developed nanocomposites are conducted under monotonic load up to failure. The optimum GNP content for GNP-reinforced epoxy matrix is determined and used in the fabrication of SMA fiber/epoxy composite. The developed multiscale reinforced epoxy composites are tested under tensile loading and their full-field strain and temperature behavior are monitored and evaluated using a digital image correlation system and an infrared thermal camera.

This article illustrates an approach to develop innovative smart materials based on carbon fiber composites. The proposed approach relies on the use of ultra-light strain sensors that are embedded into the composite and are adopted to monitor in real-time the actual material configuration. Such sensors are composed of electrospun PVDF fibers that exploit piezoelectricity to identify strain and thanks to their extreme lightweight can easily be embedded within the composite layers without affecting the structural integrity. On the other hand, the composite is equipped with a system of internal distributed heaters that can locally and globally vary the composite temperature. Since the adopted epoxy has a considerable temperature-dependent behaviour, it is possible to control its stiffness and thus to control the structural frequencies and damping. By coupling the sensing system with the control system, the structural properties are tuned to match prescribed working conditions, thus optimizing the performance of the proposed smart system. The proposed approach is investigated experimentally by manufacturing prototypes of the smart composite and by performing multiple tests to study the material response and evaluate the obtained performance.

In many structural applications the use of composite material systems in both retrofit and new design modes has expanded greatly. The performance benefits from composites such as weight reduction with increased strength, corrosion resistance, and improved thermal and acoustic properties, are balanced by a host of failure modes whose genesis and progression are not yet well understood. As such, structural health monitoring (SHM) plays a key role for in-situ assessment for the purposes of performance/operations optimization, maintenance planning, and overall life cycle cost reduction. In this work, arrays of fiber Bragg grating optical strain sensors are attached to glass-epoxy solid laminate composite specimens that were subsequently subjected to specific levels of fully reversed cyclic loading. The fatigue loading was designed to impose strain levels in the panel that would induce damage to the laminate at varying numbers of cycles. The objectives of this test series were to assess the ability of the fiber Bragg grating sensors to detect fatigue damage (using previously developed SHM algorithms) and to establish a dataset for the development of a prognostic model to be applied to a random magnitude of fully reversed strain loading. The prognostic approach is rooted in the Failure Forecast Method, whereby the periodic feature rate-of-change was regressed against time to arrive at a failure estimate. An uncertainty model for the predictor was built so that a probability density function could be computed around the time-of-failure estimate, from which mean, median, and mode predictors were compared for robustness.

Energy harvesting from low frequency cyclic motion is possible in a variety of applications, but generating power with piezoelectric stacks at low, off-resonance frequencies is challenging. In this study, Compliant Layer Adaptive Composite Stacks (CLACS) were investigated as a toughened piezoelectric generator to increase efficiency at low frequencies and match the compliance of many commercial devices. CLACS were manufactured with PZT discs, interdigitated epoxy layers of varying thicknesses, and encapsulated in epoxy. Energy production of each CLACS type as a function of compliant layer thickness was characterized. Power amplification of CLACS was modeled assuming discs remain planar, volume of epoxy was conserved, and total epoxy deformations were small. Shear lag theory demonstrated increases in positive in-plane strains induced by external through-thickness compression. This amplified sensitivity of the entire stack to through-thickness compressions, substantially increases power generation capability. Experimental data showed that increases in compliant layer thickness resulted in increased power generation in all loading conditions. The shear lag structural mechanics model showed good correlation with theoretical predictions, assuming small deformation of the compliant layer. In addition to reducing composite stiffness, the CLACS generated 61% more power than conventional stack actuators with the same PZT volume via lateral strain amplification effects.

Conductive nanofiller-modified composites have received a lot of attention from the structural health monitoring (SHM) research community in recent years because these materials are piezoresistive (i.e. they have deformation and damage-dependent electrical conductivity) and are therefore self-sensing. To date, the vast majority of work in this area has utilized direct current (DC) interrogation to identify and/or localize damage. While this approach has been met with much success, it is also well known that nanofiller-modified composites possess frequency-dependent electrical behavior. This behavior can be roughly modeled as a parallel resistor-capacitor circuit. However, much less work has been done to explore the potential this frequency-dependent behavior for damage detection. To this end, the work herein presented covers some preliminary results which leverage high-frequency electrical interrogation for damage detection. More specifically, carbon nanofiber (CNF)/epoxy specimens are produced and connected to an external inductor in both series and parallel configurations. Because the CNF/epoxy electrically behaves like a resistor-capacitor circuit, the inclusion of an inductor enables electrical resonance to be achieved. Changes in resonant frequency are then used for rudimentary damage detection. These preliminary results indicate that the potential of SHM via the piezoresistive effect in nanofiller-modified composites can be considerably expanded by leveraging alternating current (AC) interrogation and resonant frequency principles.

Distributing a carbon nanotube sensing network throughout the binder phase of energetic composites is investigated in an effort for real time embedded sensing of localized heating in polymer bonded explosives (PBXs) through thermo-electromechanical response for in situ structural health monitoring (SHM) in energetic materials. The experimental effort herein is focused on using 70 wt% Ammonium Perchlorate (AP) (solid oxidizer used in solid rocket propellants) crystals embedded into epoxy binder having concentration of 0.1 wt% multi-walled carbon nanotubes (MWCNTs) relative to entire hybrid energetics. Electrical and dielectric properties of neat (i.e. no MWCNTs) energetics and MWCNT hybrid energetics are quantitatively and qualitatively evaluated under localized thermal loading. Electrical and dielectric properties showed variations for both neat energetics and MWCNT hybrid energetics depending on input frequency measurements. Significant thermo-electromechanical response was obtained for MWCNT AP hybrid energetics, providing a proof of concept for thermo-electromechanical sensing for realtime SHM in energetics.

To continue to meet spacecraft systems ever increasing thermal management requirements, new control methods need to be developed. While advances in metamaterials have provided the ability to generate materials with a broad range of material properties, relatively little advancement has been made in the development of adaptive metamaterials. This paper is focused on the development of a thermal management metamaterial that enables the active and passive control of a metamaterial’s thermal conductance. This variable conductivity is achieved through the application of internally or externally applied loads that induce internal contact resulting in changes in the conductive path length and the effective conductive area. This capability enables active or passive control of a metamaterial’s effective thermal conduction through the application of mechanical and thermal strain. Passively applied thermal strains can be used to design a highly nonlinear material thermal conductivity as a function of temperature. Actively, this can be used to precisely control the temperature of an interface through dynamically changing the instantaneous heat flux through the metamaterial. This work expands on the field of thermal switches by enabling a non-binary configuration where the initial air gap is slowly closed as contact sequentially introduced into the metamaterial. As internally or externally developed loading is applied, contact is introduced with an increasing contact area until full contact is achieved. This intermediate step of partial contact enables unique design capabilities that enable highly nonlinear thermal conductivity as a function of temperature as well as stability regions that allow passive thermal control. An example metamaterial was developed and evaluated to quantify the potential of this concept. The specific metamaterial configuration assessed in this paper uses offset flat and curved copper plates that are connected at the edges of the plate using a low conductivity epoxy. To evaluate the metamaterial performance, the stiffness and thermal conductivity are calculated as a function of the resulting contact area and the required applied loading. This work is focused on determining the potential of this metamaterial concept by evaluating this initial concept confirmation to establish the magnitude of the thermal conductance change, and the design of the conductivity change a function of applied loading.

This paper presents a simulation-based study to investigate the damping properties of a novel piezocomposite, consisting of piezoelectric fiber and epoxy reinforced with randomly orientated double walled carbon nanotubes (DWCNT), termed as piezoelectric fiber nano reinforced composite (PFNRC). Authors have observed that the past research dealt with the effect of aligned single walled carbon nanotubes (CNT) on active damping of piezoelectric composite in extension mode (e 13 and e 33 ). It is known from the past research that DWCNT inclusions improve the passive damping of a composite. Therefore, the authors use DWCNT inclusions to study the active-passive damping of the piezoelectric composite, in this article. The random orientation of the DWCNT is considered to replicate the physical composite as it known that aligning CNTs in a single direction is not feasible due to fabrication constraints. A multistep homogenization method involving Method of Cells (MOC) is employed to obtain effective properties of PFNRC. A modified 3D-MOC is used to obtain the effective properties of epoxy matrix with DWCNT inclusions (DWCNT-epoxy), considering the effect of nano particle agglomeration. A 2D-MOC is then implemented with long fiber PZT as the active material and DWCNT-epoxy as the matrix. This procedure is followed for computing the effective material properties of extension (e 33 ) as well as shear (e 15 ) mode of PFNRC, when DWCNT inclusions are added into the epoxy matrix at different weight percentages. The constitutive equations are derived with the help of Maple and simulated in MATLAB. These results are used to compare the active-passive damping performance of the composites using a single degree of freedom damping model, employing Newmark’s numerical integration method. The active damping performance of the composites is evaluated by varying the displacement and velocity gains in a negative feedback system. The main focus of the study is to find the most efficient operating mode of the proposed composite for damping of structural vibrations.

Active Fiber Composites (AFCs) are piezoelectric devices comprised of long cylindrical fibers, typically made of ceramic lead zirconate titanate (PZT), embedded in an epoxy polymer. AFCs use interdigitated electrodes to produce electric field lines parallel to the fibers (33-mode) rather than across the diameter, exploiting the stronger out-of-plane electromechanical coupling. Nonlinear piezoelectric and dielectric terms and non-uniform poling are often neglected in modeling AFCs due to the added complexity, however including the terms improves accuracy for strong electric fields and where the electrode geometry causes non-uniform electric fields. For that reason, a new finite element model of the AFC is developed which includes the effect of nonlinearities in piezoelectric strain constants and electric permittivity due to a non-uniform applied electric field resulting from two sets of interdigitated electrodes. The methods used to apply the nonlinear constitutive equations and poling are described. A comparison of the AFC response with linear and nonlinear material properties, with non-uniform poling, is shown for increasing applied electric fields. The difference in AFC response illustrates the necessity to include Rayleigh Law terms and non-uniform poling in the model.

The paper begins with a brief overview of historical building coverings. Thermadapt™ thermally adaptive buildings are introduced as a completely new class of shingles, siding and roofing. These elements physically change shape in response to thermal loading. In hot weather with high solar loading, the panels curl up and away from the building. As the temperature cools and the sun sets, the Thermadapt™ elements lie close to the building. In cool temperatures, the elements lie flat agains the building transferring solar energy. In extremely cold temperatures, high convexity inherently forms in the elements, forming a pocket of trapped dead air which forms a highly effective layer of insulation. Thermadapt™ elements are analytically modeled using Classical Laminated Plate Theory (CLPT). Although Thermadapt™ elements may use materials like shape memory alloys, cost concerns drive the use of coefficient of thermal expansion mismatch as the basic driving mechanism. A series of experiments were performed on a variety of Thermadapt™ elements using high CTE mismatch pairs of structural materials including graphite-epoxy and aluminum and Invar and aluminum pairings. Analytical estimates are shown to predict the performance of the Thermadapt™ panels with great accuracy with curvature levels measured and predicted in excess of 5 deg/m/°C. Analytical predictions using CLPT employed a lateral constraint, driving lateral curvature, κ y , to zero by the use of stiff lateral constraint mechanisms like edge rolls and lateral corrugations. This constraint was shown to increase deflections by roughly 33% over the unconstrained elements which were simply allowed to encounter equal curvatures in x and y directions, or “doming.”

Designing devices made from epoxy-based shape memory polymers is difficult because few material behavior parameters are available for these materials in the rubbery/shape changing region. This work examines the rubbery state, greater than 20° C above the glass transition temperature (Tg), as an elastomeric regime suited to characterization with simple tension and planar tension experiments. Differential scanning calorimetry (DSC) results show a 70° C Tg, which agrees with prior research. Simple tension experiments at 100° C exhibited nonlinear elastic behavior, and finite element analysis (FEA) agreed with the constitutive behavior exhibited in the experiments. Planar tension experiments exhibited novel results. The stress/strain response was sigmoidal with a significant plateau in stress followed by rising stress to failure. The typical 10:1 gage width/gage length ratio seemed to over constrain the material. The strain to failure is small, and suggests the material behavior is a hybrid of elastic and hyperelastic behavior.

Results are reported from an ongoing experimental investigation of the effects of thermo-oxidative aging on the mechanical behavior of an epoxy shape memory polymer (SMP). Chemo-rheological degradation due to macromolecular scission and cross-linking is one of the main factors contributing to the chemical aging of thermo-responsive SMPs. This aging may manifest as residual strain or irreversible material property changes, which can affect the performance and limit the useful life of a SMP. A relatively new epoxy SMP based on the diglycidyl ether of bisphenol A is synthesized, and specimens are tested under uni-axial tension using a dynamic mechanical analyzer. Fundamental viscoelastic behavior and thermal expansion coefficients are first characterized, showing a glass transition near 60 °C. Shape memory cycle experiments are performed at shape fixing temperatures of 80, 125, 150 and 175 °C, and the effect of fixing time at each temperature is examined upon subsequent strain recovery at 80 °C. Performance parameters such as recovery ratio, speed of recovery and residual strain are quantified as a function of shape fixing time and temperature. No effect of chemical aging was seen at a fixing temperature of 80 °C, although the recovery ratio decreases initially with increasing fixing time and stabilizes near 92 %. Only minor effects of chemical aging are seen in the mechanical responses for fixing temperatures of 125 and 150 °C, but specimens exhibit progressively more noticeable color changes that indicate oxidation. Significant effects are observed at the highest fixing temperature of 175 °C, where chemical aging at longer fixing times results in a reduction in recovery rate across the rubber-glass transition temperature, progressively larger residual strains, lack of complete strain recovery at 80 °C, and higher temperatures to achieve 90 % strain recovery.

An experimental characterization of nanocomposite strain and damage sensing in support of development of CNT-polymer nanocomposites for structural health monitoring (SHM) applications was conducted. As such, effort here is focused towards examining the piezoresistive behavior of poly(dimethyl-siloxane) (PDMS) and epoxy filled with acid treated single walled carbon nanotubes (COOH-SWNTs) under quasi-static compression and tension. Precision LCR Meter with two terminal method, in conjunction with mechanical testing and data acquisition system were used to measure instantaneous resistance values. Given the emphasis on SHM applications which correlate changes in electrical resistivity to deformation and damage, 0.1 wt% COOH-SWCNTs concentration below the nanocomposite electrical percolation threshold was considered for PDMS and epoxy. Measurements confirmed the onset of damage prior to noticeable effects in the stress-strain response. The resistance measurements were able to both detect strain for PDMS nanocomposites and damage initiation and provide continuous assessment of the damage state between damage initiation events for epoxy nanocomposites. Digital Image Correlation (DIC) System was used to observe the crack propagation on notched epoxy nanocomposite compact tension samples.

Incorporation of multifunctionality to fibre reinforced polymer composite materials delivers many benefits. One example includes improved longevity of components through increasing permissible temperatures of operation, which could be achieved via in-situ cooling. As the temperature of composite components approaches the glass transition temperature (T g ) of the matrix, thermal stress induced ageing greatly increases [1], [2], thus the incentive for integrated cooling. In order to assess the damage, which could be caused by exposure to elevated temperatures, isothermal ageing was performed at a temperature 15°C lower than the materials T g (2200 hours at 110°C). Material used in this study is a carbon/epoxy prepreg system (Gurit, SE70), with a T g of 126°C when cured at 110°C. Results have shown a significant drop in Short Beam Shear (SBS) Strength starting after exposure for 1700h and increase in fibre bridging seen in mode I Double Cantilever Beam (DCB) testing. Fracture surface analysis using SEM indicated that fibres were generally less well bonded to the matrix, with visible changes began occurring as early as 1000h exposure. These results indicate that extended exposure of a material at near T g temperatures has a detrimental effect on material properties. To mitigate against this phenomenon, a series of tests were performed on SBS and DCB specimens in a raised temperature (110°C) environment, which incorporated in-situ cooling. The specimens were placed in an oven at 110°C and were cooled down to a constant temperature of 60°C via the internal vascular cooling arrangement. Further testing is underway to assess the inhibition of ageing and maintenance of the original composite material by active cooling using embedded vascular networks.

Highly cross-linked polyurethanes have a high elastic modulus and creep resistance, but they undergo a brittle fracture below the glass transition temperature. Unfortunately, a large number of glassy polyurethanes are prone to brittle fracture without undergoing large elastic deformations; in particular, brittle failure is common under conditions such as low temperature and high strain rates. While the rigidity in polymers is required for practical applications, the lack of resistance against crack propagation is essential to avoid catastrophic failures. The toughening of polymers is a crucial aspect of improving the strength and ductility at specific temperatures and deformation rates. One method that has shown promise in recent years is the creation of local regions of reduced modulus that absorb strain energy and toughen the polymer. For instance, rubbers are typically added to epoxy which phase separate upon polymerization and create local elastic regions that significantly toughen the polymer. In this study, a variety of two-phase transparent polyurethanes in the form of single inclusions is designed to study the toughening mechanism of the local regions of reduced modulus with an embedded crack. Synthesized heterogeneous polyurethanes show a transition from brittle to ductile behavior in addition to a drastic increase in the maximum load that the polymer can withstand. Compact tension experiments demonstrate that a small reduction in the inclusion’s Young’s modulus (∼10%) leads to an increase in the toughness by factor of 7 (∼700%). Moreover, digital image correlation is performed to map the strain distribution around the crack in order to visualize possible toughening mechanism. Comparison between the induced strain field in samples with inclusion and samples without inclusion reveals an efficient toughening mechanism of the polymers.

Traditional composite materials invented to be used in structures with the purpose of high load-bearing with excellent in-plane properties. Continuous fiber reinforced composites are one of the mostly used categories of advanced composites. This class of composites has gained a lot of attention due to their light-weight and decent mechanical properties. However, additional material design is required to tune both mechanical and structural properties of these composites. Since the load transfer between reinforcement phase and polymer matrix happens at the interfacial region, a better interphase might result in a composite with higher vibration damping. In this study, a gradient interphase between carbon fiber and polymer matrix has been created by using ZnO nanowires to engineer the damping loss factor of the carbon fiber composites. For the growth of ZnO nanowires on the carbon fabric, low temperature hydrothermal reaction has been used. Then the carbon fabrics with ZnO nanowires were infiltrated with a low viscosity epoxy using vacuum assisted resin transfer molding technique. The stiffness and structural damping of the composite were examined using dynamic mechanical analysis. The results show that the damping properties of hybrid composites using ZnO nanowires are enhanced compare to the bare carbon fabric composites. Since the growth of ZnO nanowires is a tunable process, the length, diameter and aspect ratio of the nanowires and consequently the architecture of the interphase can be tailored for the desired vibration damping in the system. Thus, the hybrid composites with ZnO nanowire interphase can be used to enhance the energy dissipation in a structural system.

Utilizing conductivity changes to locate matrix damage in glass fiber reinforced polymers (GFRPs) manufactured with nanocomposite matrices is a promising avenue of composite structural health monitoring (SHM) with the potential to ensure unprecedented levels of safety. Nanocomposites depend on the formation of well-connected nanofiller networks for electrical conductivity. Therefore, matrix damage that severs the connection between nanofillers will manifest as a local change in conductivity. This research advances state of the art conductivity-based SHM by employing electrical impedance tomography (EIT) to locate damage-induced conductivity changes in a glass fiber/epoxy laminate manufactured with carbon black (CB) filler. EIT for damage detection is characterized by identifying the lower threshold of through-hole detection and demonstrating the capability of EIT to accurately resolve multiple through holes. It is found that through holes as small as 3.18 mm in diameter can be detected, and EIT can detect multiple through holes. However, sensitivity to new through holes is diminished in the presence of existing through holes unless a damaged baseline is used. These research findings demonstrate the considerable potential of conductivity-based health monitoring for GFRP laminates with conductive networks of nanoparticles in the matrix.

Carbon fiber reinforced composites have been used in a wide range of applications in aerospace, mechanical, and civil structures. Due to the nature of material, multiple types of structural damage including micro matrix cracks and delaminations can significant degrade the integrity and safety of composites. It is difficult to detect and repair such damage since they are always barely visible to the naked eye. This paper presents the development of self-sensing and self-healing functions in order to detect damage progression and conduct in-situ damage repair in composite structures. Carbon nanotubes, which are highly conductive materials, are uniformly distributed within epoxy to develop the self-sensing capability. Shape memory polymer is used in the hot spot to obtain the self-healing capability. The developed multi-functional material is applied to carbon fiber reinforced composites for the autonomic detection and heal the matrix cracking. Experimental results showed that the developed composite materials are capable of detecting and healing the matrix cracks and delaminations. The developed self-healing material has the potential to be used as a novel structural material in mechanical, civil, aerospace applications. It can be used to detect and in-situ repair matrix damage induced by low velocity impacts and fatigues.

Several ensembles of sweeping diagnostic impulsive forces were measured in a unidirectional carbon-epoxy composite beam modified locally with a soft viscoelastic patch. The spatial uniformity of the typical ensemble of diagnostic signals is addressed by a systematic spatio-temporal coherence analysis in terms of proper orthogonal decomposition (POD) modes. All samples of spanning ensembles are strongly dominated by the same POD mode characterized by a nearly uniform spatial modulation and a sharp triangular pulse time modulation. The higher POD modes have small amounts of energy. They possess an important statistics property: their spatial modulation mean value is nearly zero with standard deviation nearly identical to the nearly uniform value of the dominant POD mode. The nearly uniform spatial distribution of the dominant POD mode is a fuzzy picture of the ideal or nominal one where the impact-generated diagnostic forces should have a time waveform independent of the site of impact. Despite this energy content deficiency, the ensemble of acceleration signals acquired at a fixed point while the beam is excited by an ensemble of sweeping diagnostic forces has very robust POD modal structure. The POD modes show in a clear manner the presence of a soft viscoelastic patch simulating mass modifications. The POD-based coherence analysis of ensembles of diagnostic forces generated in this practical problem is potentially useful for a real-time verification-inspection of the integrity of networks of embedded and surface-mounted actuators and sensors.