Wearable strain sensors are essential for the realization of applications in the broad fields of remote healthcare monitoring, soft robots, and immersive gaming, among many others. These flexible sensors should be comfortably adhered to the skin and capable of monitoring human motions with high accuracy, as well as exhibiting excellent durability. However, it is challenging to develop electronic materials that possess the properties of skin compliant, elastic, stretchable, and self-healable. This work demonstrates a new regenerative polymer complex composed of poly(2-acrylamido-2-methyl-1-propanesulfonic acid), polyaniline, and phytic acid as a skin-like electronic material. It exhibits ultrahigh stretchability (1935%), repeatable autonomous self-healing ability (repeating healing efficiency >98%), quadratic response to strain (R 2 > 0.9998), and linear response to flexion bending (R 2 > 0.9994), outperforming current reported wearable strain sensors. The deprotonated polyelectrolyte, multivalent anion, and doped conductive polymer, under ambient conditions, synergistically construct a regenerative dynamic network of polymer complex cross-linked by hydrogen bonds and electrostatic interactions, which enables ultrahigh stretchability and repeatable self-healing. Sensitive strainresponsive geometric and piezoresistive mechanisms of the material owing to the homogeneous and viscoelastic nature provide excellent linear responses to omnidirectional tensile strain and bending deformations. Furthermore, this material is scalable and simple to process in an environmentally friendly manner, paving the way for the next-generation flexible electronics.
The analysis of notch stresses and strains is one of the key parts of fatigue life prediction of components and structures, In this paper two related approaches are introduced, covering the whole field from uniaxial to multiaxial non-proportional loading. The pseudo stress at the notch root computed by theory of elasticity is introduced as the governing variable for elastic-plastic notch analysis. Although the pseudo stress is just a specially defined nominal stress, it eases notch analysis in comparison to using arbitrarily definable nominal stresses, especially for non-proportional multiaxial loading. Additionally a pseudo strain based approach is introduced and compared to the stress approach. Both proportional and non-proportional loading are discussed, and compared with the approaches of other workers. NOMENCLATURE c = dimensional proportionality constant relating local structural response to loading, e = subscript denoting the elastic part of a quantity E = Young's modulus f("u), 3" = load-notch strain curves f o =criterion for the onset of yielding similar to K , S, ' u = fictitious local strain and local stress computed by theory of elasticity F,, Go, If,, = coefficients of Hill's yield criterion g(u) = stress-strain curve H' =work hardening modulus of an equivalent stress vs. equivalent plastic strain curve K , = (nondimensional) stress concentration factor Lo =load at the start of yielding Lp = fully plastic limit load (elastic-perfectly plastic material) m = counter for different load components Mo, No, Oo = coefficients of Hill's yield criterion n = number of different load components p = subscript denoting the plastic part of a quantity q = subscript denoting an equivalent quantity, e.g. von Mises equivalent stress s = deviatoric part of a stress tensor sij = uij -8 S = nominal stress L = load (force, moment, displacement, etc.) x, y, z = local Cartesian coordinates X, Y, Z =global Cartesian coordinates (for nominal stresses) y = shear strain E =local (notch) strain v = Poisson's ratio u = local (notch) stress a, = yield stress d = hydrostatic pressure d = $ua T = shear stress * Formerly at Technische Hochschule Darmstadt Fachgebiet Werkstoffmechanik. 98 1 982 V. B. KOTTGEN et al.
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In the current investigation, an innovative time-domain damage index is introduced for the first time which is based on local statistical features of the waveform. This damage index is called the ‘normalized correlation moment’ (NCM) and is composed of the nth moment of the cross-correlation of the baseline and comparison waves. The performance of this novel damage index is compared for some synthetic signals with that of an existing damage index based on the Pearson correlation coefficient (signal difference coefficient, SDC). The proposed damage index is shown to have significant advantages over the SDC, including sensitivity to the attenuation of the signal and lower sensitivity to the signal’s noise level. Numerical simulations using Abaqus finite element (FE) software show that this novel damage index is not only capable of detecting the delamination type of damage, but also exhibits a good ability in the assessment of this type of damage in laminated composite structures. The NCM damage index is also validated using experimental data for identification of delamination in composites.
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