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Spatially resolved capacitance-based stress self-sensing in unmodified concrete has been demonstrated. The spatial resolution is 45 mm in one dimension, which is in the direction of the capacitance measurement. Parallel coplanar component electrodes (aluminum, 5-mm wide), attached to the concrete using double-sided adhesive tape) separated by 45 mm are used to measure the in-plane capacitance in the direction perpendicular to the length of the electrodes. Combinations of component electrodes are electrically connected to form an electrode. The capacitance ranges from ∼200 pF to ∼750 pF. The greater is the number of component electrodes in an electrode, the higher is the capacitance. The compressive loading is applied at selected areas located between adjacent component electrodes. The stress (defined as load divided by the 300 ×300-mm2 concrete area) is up to 3000 Pa. The load decreases the capacitance monotonically and reversibly. The fractional decrease in capacitance ranges from ∼0.1 % to ∼0.5 %. More spatially concentrated loading, as for loading near the edges of the specimen, gives greater fractional decrease in capacitance. The capacitance decreases with increasing inter-electrode distance. Embedded steel rebars with a 20.0-mm concrete cover do not affect the capacitance or capacitance-based sensing. • Spatially resolved structural self-sensing of stress is reported. • It involves capacitance measurement, as shown for unmodified concrete. • Parallel coplanar component electrodes (aluminum strips) 45 mm apart are used. • Component electrodes are electrically connected to form a composite electrode. • The compressive stress reduces the capacitance reversibly and monotonically. [ABSTRACT FROM AUTHOR]

Through comparing carbons in the graphite allotrope family, namely isotropic graphite (IG), exfoliated-graphite-based flexible graphite (FG), graphite-flake thick film (not sintered), carbon fibers (PAN, pitch) and highly-oriented pyrolytic graphite (HOPG), all differing in the microstructure, this work addresses for the first time the factors (including the structure-property relationship) that govern the electric permittivity (kHz frequency) of carbons. The permittivity is more sensitive to the microstructural heterogeneities than the resistivity, which only slightly influences the permittivity. Smaller crystallite dimensions promote the permittivity. PAN-based carbon fibers exhibit higher permittivity than pitch-based carbon fibers, IG, FG or HOPG. The crystallographic preferred orientation is not influential to the permittivity, though it influences the resistivity. Interfaces (e.g., the grain boundaries in IG and the interface between the carbon layer ribbons in a fiber) promote the permittivity, being sites for the carrier-atom interaction that enables the polarization. A large number of sites is more important than a large site size for promoting the permittivity. The fiber diameter is not influential to both permittivity and resistivity. The thick film exhibits the highest relative permittivity (5 × 105), due to its low electrical connectivity. IG (grain size 12 μm) gives the lowest relative permittivity (5 × 102), due to its coarse microstructure. [Display omitted] [ABSTRACT FROM AUTHOR]

Self-sensing uses the cement-based material without sensor incorporation to sense itself. This paper reviews self-sensing cement-based materials, with coverage of the well-studied resistance-based sensing as well as the less-studied capacitance-based sensing. This review is the first that covers capacitance-based self-sensing. Capacitance-based sensing is advantageous over resistance-based sensing in that no particular admixture is required, so that it is applicable to both existing and new structures. In contrast, resistance-based sensing that is comparable to capacitance-based sensing in stress/strain sensitivity requires conductive admixtures, such as carbon fiber. Resistance-based strain sensing is based on piezoresistivity, which is associated with the resistance increasing upon tension and decreasing upon compression. Capacitance-based strain sensing is based on piezopermittivity, which is associated with the permittivity decreasing upon tension and increasing upon compression. Damage causes the resistance to increase and causes the permittivity to decrease. Increase in temperature decreases the resistance but increases the permittivity. This review also covers the methodology of the electrical measurements. [ABSTRACT FROM AUTHOR]

The dynamics of direct-current-induced electric polarization/depolarization of a conductive material (graphite) are studied. Forward-current polarization and reverse-current depolarization cause the apparent resistivity to increase by ≤ 2.2% and ≤1.0%, respectively. Polarization-related carrier-atom interaction engagement occurs more readily than depolarization-related disengagement. The number n of carriers that participate increases with the number N I of injected electrons; the rate of the n increase decreases with increasing N I. The efficiency of carrier-atom interaction (indicated by n / N I , ≤1.2 × 10−17) decreases with increasing N I (≤1.2 × 1020). For the same N I , the fraction of carriers that participate (n /(N I + N c), where N c = number of inherent carriers) increases with increasing current (rate of electron injection). At a particular current, this fraction increases and then decreases with increasing N I. The polarization/depolarization rate decreases as polarization/depolarization proceeds. At a given polarization/depolarization time, the rate and degree of polarization are higher than those of depolarization. The above fraction is higher for polarization (≤8.5 × 10−18) than depolarization (≤4.7 × 10−18). The difference between polarization and depolarization agrees with the apparent-resistivity asymmetry and true electret character indicated by the open-circuited DC electric field of 1.0 × 10−5 V/m. The above fraction is lower than the fraction of carriers that participate in the electret discharge (4.9 × 10−4) by 14 orders of magnitude. Image 1 [ABSTRACT FROM AUTHOR]

The piezopermittivity of cement mortar without functional admixture relates to capacitance-based structural self-sensing of stress. The capacitance increases with increasing water content, so the effect of the water content on the self-sensing is of concern and is thus addressed in this work. Upon compression, the transverse capacitance measured using coplanar electrodes decreases reversibly for any stress/water-content combination. The water content ˂50% influences the capacitance little. The capacitance is an ineffective stress indicator when the water content (˃20%) is not fixed. For a given water content, the stress is effectively indicated by the fractional capacitance decrease, which increases with increasing stress, regardless of the water content. The fractional change in capacitance due to stress is independent of the water content when the stress exceeds ∼30 kPa. With limitations on the stress/water-content combination, the stress sensing based on the fractional change in capacitance is effective. The capacitance decrease is mainly due to piezopermittivity, with the transverse piezopermittivity coefficient ∼+5000; the dimensional change contributes negligibly. The fractional decrease in permittivity is greater when the strain is smaller. [Display omitted] • Piezopermittivity enables capacitance-based stress self-sensing. • Upon compression, the transverse capacitance decreases for any water content. • The water content ˂50% influences the capacitance little. • The stress is effectively indicated by the fractional capacitance decrease. • The transverse piezopermittivity coefficient is high at ∼+5000. [ABSTRACT FROM AUTHOR]

Graphical abstract Reversible effects of the tensile stress on the electric field output, relative permittivity κ and electrical resistivity ρ , indicating the piezoelectric and piezoresistive behavior of aluminum. Highlights • Aluminum is the dominant lightweight metal for structures. • The first report of piezoresistivity and piezoelectricity in aluminum is provided. • These phenomena enable structural self-sensing and multifunctionality. • The piezoresistivity is strong and negative, with gage factor -1630. • The piezoelectricity is weak, with coupling coefficient d 33 -1.4 × 10−8 pC/N. Abstract This work provides the first report of piezoresistivity, piezoelectricity (piezoelectret) and dielectricity in aluminum, as observed in the in-plane direction for aluminum foil and Al 2024-O alloy bar, which exhibit similar properties, though the piezoelectricity of the latter is slightly stronger. These phenomena enable structural self-sensing and multifunctionality. The stress (≤66 MPa) and strain (≤0.096%) are in the elastic regime. The piezoresistivity is strong and negative, with gage factor -1630, and provides electrical-resistance-based stress/strain sensing. The resistance (DC) decreases monotonically and reversibly with increasing tensile stress, with the fractional decrease being up to 13%. The in-plane electric field (DC) decreases monotonically and reversibly with increasing tensile stress, with the fractional decrease being up to 29%. The capacitance (2 kHz) increases monotonically and reversibly with increasing tensile stress, with the fractional increase being up to 10%. The capacitance increase (up to 10%) relates to the increase in the relative permittivity (up to 15%). The relative permittivity is high, at 5.48 × 104 (2 kHz) in the absence of stress. The piezoelectricity is weak, with the piezoelectric coupling coefficient d 33 being -1.4 × 10−8 pC/N, the electric field output contributing -1.7 × 10−8 pC/N, the relative permittivity increase contributing +3.5 × 10-8 pC/N, and the electric field and permittivity in combination contributing -5.2 × 10-10 pC/N. In spite of its weakness, the direct piezoelectricity enables electric-field-based and capacitance-based stress/strain self-sensing. [ABSTRACT FROM AUTHOR]

Structural self-sensing refers to a structural material sensing itself without sensor incorporation. Resistance-based self-sensing has long been studied. This work addresses inductance-based self-sensing and inductance-capacitance-based self-sensing. In particular, it provides the first report of inductance-capacitance-based self-sensing. A change in the inductance causes a change in the capacitance. This is to be distinguished from the capacitance change resulting from permittivity change. Permanent mechanical deformation with shape change (such as that associated with bending or undulation, as opposed to tensile strain) increases the inductance, in accordance with Faraday's Law, thus decreasing the capacitance, as shown in this work for aluminum subjected to undulating bending. Conductivity is required for the inductance. The inductance capacitance is measured with the presence of a dielectric film (a double-sided adhesive tape) between the specimen and the aluminum foil electrode, as needed because an LCR meter is not designed for measuring the capacitance of a conductor. The specimen is subjected to undulating bending for different numbers of bends and different radii of curvature. The inductance and resistivity, as measured by direct clipping without the dielectric film, increase monotonically with the number of bends, while the inductance capacitance decreases monotonically. For radius of curvature 12.24 mm and 7.00 mm, the highest fractional increase in inductance (52% and 110%, respectively) is greater than the highest fractional increase in resistivity (2.2% and 7.3%, respectively) and the highest fractional decrease in inductance capacitance (2.04% and 2.4%, respectively). The sensing is superior for a smaller radius of curvature, whether the sensing is by measuring the inductance, resistance or capacitance. The inductance is superior to the resistivity or inductance capacitance for sensing bending. [Display omitted] • Shape-changing deformation (undulating bending) increases the resistivity (≤7.3%). • This deformation increases the inductance (≤110%) much more than the resistivity. • The inductance increase reduces the capacitance ("inductance capacitance") (≤2.4%). • The inductance capacitance change is not due to a permittivity change. • Self-sensing based on inductance capacitance has not been previously reported. [ABSTRACT FROM AUTHOR]

Highlights • Structural capacitors are multifunctional structural materials. • They provide the capacitor function for the purpose of electrical energy storage. • This paper reviews the scientific development of structural capacitors. • This paper also enunciates the design and applications of structural capacitors. • Structural capacitors will provide an untapped form of energy storage. Abstract Structural capacitors are multifunctional structural materials that provide the capacitor function for the purpose of electrical energy storage. This paper reviews the development of structural capacitors and enunciates their design and applications. A structural capacitor is commonly a polymer-matrix structural composite with a dielectric film between the electrodes, which are an electronic conductor, commonly the continuous carbon fiber laminae that serve to reinforce the composite. The dielectric film is preferably small in thickness and serves to avoid short circuiting of the two electrodes. In order to maximize the capacitance by having the structural capacitor constitute capacitors in parallel, the dielectric film is preferably positioned at every interlaminar interface of the composite, such that alternating electrodes in the stack are connected to opposite polarities of the AC electric field source. A structural supercapacitor requires the matrix to be a solid electrolyte. From the viewpoints of structural performance, safety, service life and high frequency capability, structural dielectric capacitors are closer to commercialization readiness than structural supercapacitors. Structural capacitors have not yet been commercialized, but they are expected to provide an untapped, extensive, save and distributed means of energy storage, and allow aircraft, satellites, automobile, ships, wind turbines, buildings, solar panels, display panels, outdoor lighting, computers, cell phones, etc., to store energy in their structures. [ABSTRACT FROM AUTHOR]

Abstract This work reports capacitance-based stress self-sensing in cement paste without requiring any particular admixture (whatever type). The in-plane capacitance is measured between two coplanar electrodes (aluminum foil) adhered to the cement slab by double-sided adhesive tape. The capacitance decreases with increasing normal stress, whether the stress is purely compressive or flexural compressive, due to piezoelectricity. The capacitance decrease is completely reversible in the low-stress regime (normal stress up to 19 kPa), and partially reversible above this stress. The minimum normal stress change detected is 0.2 kPa, which, in case of flexure, corresponds to a flexural stress change of 8.5 kPa. The change in capacitance per unit normal stress change is up to 0.061 and 0.101 pF/kPa for purely compressive and flexural compressive loading, respectively; the value is higher in the low-stress regime, which gives superior linearity and reversibility. Low-stress sensing is relevant to pedestrian monitoring and room occupancy monitoring. [ABSTRACT FROM AUTHOR]

This paper provides the first report of the sensing of stress in a metal (namely steel and stainless steel) by capacitance measurement. The method involves the measurement of the in-plane capacitance (2 kHz) between two coplanar electrodes that are on the same surface of the specimen. The capacitance decreases with increasing normal compressive stress. The fractional decrease in capacitance (up to 20%) is higher than the compressive strain calculated based on the elastic modulus by orders of magnitude. The capacitance decrease is essentially reversible and is attributed to the direct piezoelectric effect. Because an LCR meter is not designed to measure the capacitance of an electrical conductor, it is essential for a dielectric film to be positioned between the electrode and specimen. Two configurations are used for the electrodes. Configuration I involves aluminum foil as the electrode and a stack of 3 layers of double-sided adhesive tape as the dielectric film; no pressure is used. Configuration II involves copper sheet as the electrode and a glass fiber polymer-matrix composite film as the dielectric film, with normal pressure applied to the stack. The fractional decrease in capacitance per unit stress is much greater for configuration I than configuration II. Configuration I is better for sensing low stresses, whereas configuration II is better for sensing high stresses. Configuration I is less expensive and more practical than configuration II. [ABSTRACT FROM AUTHOR]

This paper reports unprecedentedly capacitance-based nondestructive detection of aggregate proportion variation in cement-based materials. It uses the fringing electric field of a capacitor comprising a cement-based slab and two relatively small copper electrodes. The aggregate volume fraction increases from cement paste to mortar and to concrete, thus causing the apparent permittivity (high due to the fringing field) to decrease in the same order. Each slab consists of two regions with different aggregate proportions. The through-thickness and in-plane capacitances are measured using sandwiching and coplanar electrodes, respectively. To locate the interface, the capacitance is innovatively measured using a series of electrodes. [ABSTRACT FROM AUTHOR]

This is the first review of the emerging field of capacitance-based self-sensing in structural materials. Capacitance-based self-sensing is rich in both science and applications. It is applicable to materials ranging from insulators to conductors, whereas the longstanding resistance-based self-sensing is only applicable to conductors. The attributes sensed include the stress/strain, temperature and damage. The stress/strain sensing exploits piezopermittivity, which is typically positive. The temperature sensing exploits pyropermittivity, which is typically positive. The damage sensing exploits the fact that defects reduce the permittivity. The dielectric behavior of highly conductive materials is consistent with the 1999 theoretical prediction by Jonscher concerning charge-carrier polarization and the consequent "giant polarizability". The materials for which capacitance-based self-sensing has been shown include 3D-printed polymer, asphalt (bitumen-based material), cement-based materials (without or with conductive admixture), continuous carbon fiber polymer-matrix composite, carbon-carbon composite, steel and aluminum. For the conductors, the polarization hinges on the carrier-atom interaction. For the electronic conductors (carbons and metals), the carriers are electrons. For the ionic conductors (cement-based material without conductive admixture), the carriers are ions. For the nonconductors (3D-printed polymer and asphalt), the molecules present align to cause polarization. This work is focused on the ordinary frequencies (such as kHz). For the conductors, the capacitance measurement using an LCR meter needs to be performed with a dielectric film between the specimen and an electrode. For determining the permittivity, the capacitance should be measured for three or more different values of the inter-electrode distance, so that the specimen volumetric capacitance and the interfacial capacitance of the specimen-electrode interface are decoupled. [Display omitted] • First review of the emerging field of capacitance-based structural self-sensing. • Capacitance-based self-sensing is applicable to insulators and conductors. • Longstanding resistance-based self-sensing is only applicable to conductors. • The attributes sensed include the stress/strain, temperature and damage. • Piezopermittivity and pyropermittivity are exploited for the sensing. [ABSTRACT FROM AUTHOR]

A new method of measuring the permittivity of silicon wafers is provided. It removes the previously reported dependence of the measured permittivity on the silicon resistivity and on the electrode quality. Metal-semiconductor contacts or thin-film electrodes are not required. The method innovatively uses N -segment (e.g., N = 7) sandwiching aluminum-foil electrodes adhered to both surfaces of the wafer using double-sided adhesive tape, which provides adhesion and resistance enhancement (for ensuring that the meter measures the capacitance correctly). By measuring the capacitance C for various values of the capacitor area A (as provided by different numbers of segments) and plotting 1/ C vs. 1/ A , the permittivity is obtained from the slope and the effect of the electrode-specimen interface capacitance is removed. The relative permittivity (12) equals the conventionally obtained values. By using a sufficiently high value of N , the permittivity spatial distribution can be probed, thus enabling spatially resolved permittivity-based quality sensing. • A new method of measuring the permittivity of silicon wafers is provided. • It removes the dependence of the measured permittivity on the silicon resistivity. • It removes the dependence of the measured permittivity on the electrode quality. • The effect of the electrode-specimen interface capacitance is removed. • Metal-semiconductor contacts and thin-film electrodes are not required. [ABSTRACT FROM AUTHOR]

The capacitance-based stress self-sensing in asphalt without device or functional constituent incorporation is reported for the first time, with relevance to smart pavements. Stress sensing enables weighing. Without the incorporation, the technology is applicable to existing and new pavements, and gives low cost, high durability, large sensing volume and maintained mechanical properties. Linearity of the capacitance with the normal compressive stress occurs for stress ranging from 2 Pa to 60 Pa. The ability to sense such low stress enables the sensing of even children. Nonlinearity at > 60 Pa involves the slope of the capacitance-stress curve decreasing. Monotonic and reversible capacitance increase with stress occurs for stress ≤ 600 Pa, with complete reversibility up to stress ≥ 2100 Pa. The electrodes are aluminum foil bonded to the asphalt with a thin bitumen interlayer. With coplanar electrodes (the stress not applied to the electrodes), the in-plane capacitance increases upon compression, presumably due to the compaction of the part of the asphalt near the plane of the coplanar electrodes. With sandwiching electrodes (the stress applied to the electrodes), the through-thickness capacitance increases reversibly upon compression, due to the decrease in the thickness of the bitumen interlayer. The coplanar electrodes enable self-sensing, and, compared to the sandwiching electrodes, they give much greater sensitivity and are more amenable to practical implementation. The sensing is based on the fractional change in capacitance, so the thickness of the asphalt or bitumen interlayer does not matter. The capacitance-based stress sensing is superior for asphalt than cement-based materials of prior work, due to the greater stiffness of the latter. [Display omitted] • The capacitance-based stress self-sensing in asphalt is reported for the first time. • The sensing is relevant to smart pavements. • No device or functional constituent incorporation is involved. • Without the incorporation, the sensing is applicable to existing and new pavements. • Linearity of the capacitance with the compressive stress occurs for stress 2–60 Pa. [ABSTRACT FROM AUTHOR]

• Piezopermittivity is the reversible change of the permittivity with elastic strain. • Piezopermittivity provides the basis for capacitance-based strain/stress sensing. • The direct piezoelectric effect does not require the permittivity to change. • The sensing effectiveness is the fractional change in permittivity per unit strain. • The magnitude of the fractional change in permittivity exceeds the strain magnitude. [Display omitted] Piezopermittivity refers to the reversible change of the electric permittivity (the main material property that describes the dielectric behavior) with the elastic strain. This paper addresses the emerging field of piezopermittivity, which provides the basis for capacitance-based strain/stress sensing, including self-sensing in case of structural materials. Piezopermittivity differs from piezoresistivity, which allows resistance-based strain/stress sensing. It also differs from the direct piezoelectric effect, which does not require the permittivity to change. In order to establish piezopermittivity, this paper presents the piezopermittivity theory, sensing methodology, and piezopermittivity-related materials science. Piezopermittivity stems from the effect of strain on the microstructure, which affects the permittivity. It is exhibited by electrical conductors (metals, carbons, and carbon fiber polymer-matrix and carbon-matrix composite) and nonconductors (perovskite ceramic and 3D-printed polymer), which are comparably effective for capacitance-based sensing, as shown at frequencies ≤2 kHz. All materials are unpoled. Poling of the perovskite ceramic does not alter the piezopermittivity behavior. The magnitude of the fractional change in permittivity much exceeds the strain magnitude, as expected for piezopermittivity. The sensing effectiveness, as described by the fractional change in permittivity per unit strain, is positive for positive piezopermittivity and negative for negative piezopermittivity. The majority of the materials exhibit positive piezopermittivity. The sensing effectiveness is +1.99×103 and -4.81×102 for uncoated and nickel-coated carbon fibers, respectively. The value for the uncoated carbon fiber is close to the value of +1.21×103 for a perovskite ceramic. For a 3D-printed polymer, the value is -2.87×106 in the direction perpendicular to the printed layers. Capacitance-based sensing is advantageous to the widely reported resistance-based sensing in that it does not require intimate contact of the electrodes with the specimen. The measurement of the capacitance of a conductive material using an LCR meter requires a dielectric film between the electrode and specimen. [ABSTRACT FROM AUTHOR]

• Capacitance-based self-sensing is shown for the first time for asphalt. • A pitch-based material serves as model asphalt. • No functional component is involved. • Compressive stress (5–1720 Pa) is sensed. • The capacitance increases reversibly by ≤1.2% upon unloading. Capacitance-based self-sensing is shown for the first time on a pitch-based material, which serves as model asphalt. No functional component is involved. Compressive stress (5–1720 Pa) is sensed. The capacitance increases reversibly by ≤1.2% upon unloading. It is linearly related to the stress, with the fractional change in capacitance per unit stress ≤9.3 × 10−5/Pa. The capacitance change is identically and repeatedly observed for 20 cycles at the same stress amplitude. The resistance decreases reversibly upon unloading, but it is an inferior stress indicator than the capacitance, due to the greater irreversibility tendency and less regular dependence on the stress. [ABSTRACT FROM AUTHOR]

The strong piezoresistivity and piezopermittivity inherent in cement paste without functional admixture are evaluated and unprecedentedly correlated, with relevance to resistance-based and capacitance-based structural self-sensing, respectively. Compression (low elastic regime) decreases the resistivity and increases the permittivity, indicating respectively positive piezoresistivity (unprecedentedly high gage factor ≤20400) and negative piezopermittivity (magnitude of the fractional change in capacitance per unit strain ≤4710). Both effects are strengthened by decreasing the strain (8 × 10−8–7 × 10−7), or increasing either the frequency (2–100 kHz) or electric field (0.09–0.50 V/mm). The resistivity decreases greatly with increasing frequency, while the permittivity decreases less, both because of the sluggishness of the ion movement. Electric field increase raises the capacitance/resistance. Upon unloading, the capacitance decreases to below that before loading. The electrodes sandwich the specimen, being adjacent to the stressed region. The fringing electric field reaching the stressed region facilitates the sensing, being strengthened by increasing the frequency or electric field. • Strong piezoresistivity and piezopermittivity inherent in cement paste are reported. • They relate to resistance-based and capacitance-based structural self-sensing. • Cement paste with no functional admixture is investigated in the elastic regime. • Compression decreases the resistivity and increases the permittivity. • The piezoresistivity and piezopermittivity are strengthened by reducing the strain. [ABSTRACT FROM AUTHOR]

Repeated flexure (at progressively increasing longitudinal bending stress amplitude) increases the electric field at the top surface but decreases that at the bottom surface of the steel beam. • This work reports the stress self-sensing in steel beams under elastic flexure. • The sensing is based on electric field measurement (piezoelectret). • Alternatively, the sensing is based on capacitance measurement (dielectricity). • Alternatively, the sensing is based on resistance measurement (piezoresistivity). • Coplanar electrodes on the top or bottom surface are used. This work reports for the first time the structural self-sensing of stress in steel beams under flexure (elastic regime, ≤200 MPa) through measurement of the electric field (piezoelectret), capacitance (dielectricity) and resistance (piezoresistivity). Coplanar electrodes on the top or bottom surface are used. Under flexure, the electric field and resistance increase at the top surface, but decrease at the bottom surface, while the capacitance decreases at both surfaces. Under normal compression, the electric field, capacitance and resistance decrease at both surfaces. For flexure and normal compression, the resistance and capacitance changes are essentially completely reversible, whereas the electric field change is not. Longitudinal tension (whether in flexure or normal compression) causes the electric field to decrease, whereas longitudinal compression (in flexure) causes the electric field to increase. Longitudinal tension (whether in flexure or normal compression) causes the resistance to decrease, whereas longitudinal compression (in flexure) causes the resistance to increase (negative piezoresistivity). The capacitance (2 kHz) decreases due to the stress (whether flexure or normal compression and whether at the top or bottom surface) and is attributed to the permittivity decrease, except that the capacitance decrease under longitudinal compression (in flexure) is attributed to the AC voltage used to measure the capacitance penetrating the full thickness and the effect of longitudinal tension at the bottom surface dominating over that of longitudinal compression at the top surface. In contrast, the DC voltage used to measure the resistance is much smaller and does not penetrate the full thickness, so that the resistances at the top and bottom surfaces respond to flexure differently. The magnitude of the fractional change in electric field, capacitance or resistance increases with decreasing beam width. [ABSTRACT FROM AUTHOR]

This paper provides the first report of the effect of stress on the capacitance and electric permittivity of a three-dimensionally (3D) printed polymer. This effect is relevant to capacitance-based stress monitoring. The reported capacitance, permittivity and stress are all in the out-of-plane direction. The stress is compressive up to 0.9 kPa. The printing involves bottom-up stereolithography, using an ultraviolet-curable acrylate ester resin. Each specimen has 20 layers. The layer thicknesses are 26, 33 and 46 μm. Both the capacitance and permittivity increase monotonically and reversibly with increasing stress for all three layer thicknesses, such that the increase is relatively abrupt in the low-stress regime below 0.1 kPa. This means that the capacitance can serve repeatedly as an indicator of stress, with the sensitivity being particularly high at low stresses. For all three layer thicknesses, there is a very slight degree of irreversibility in the capacitance change upon unloading, such that the capacitance is slightly higher during unloading than during the prior loading. The capacitance increase is attributed totally to the increase in permittivity. The increase in permittivity with increasing stress is attributed to the increase in the degree of in-plane molecular alignment. For any stress level, whether during loading or unloading, the permittivity decreases with increasing layer thickness. [ABSTRACT FROM AUTHOR]