Quasi-isotropic Research Articles

Twisting Characteristics and Tensile Effects of aQuasi-isotropic Strands made of Coated Conductors

With the development of second generation (2G) high-temperature superconducting (HTS) tapes, several cabling configurations, such as twisting stacked-tapes cable and conductor on round core cables, have been proposed, and some prototypes were developed. Alternating current (ac) losses arise in HTS tapes in ac operations, and those losses are an important consideration for applications since they influence the stability and operating cost. Conventionally, a twisting technique is used to effectively reduce the ac losses. This paper presents the twisting properties and tensile properties of a quasi-isotropic (QI) strand made from 2G tapes. The strand consists of 2G tapes, aluminum foil, and a metal sheath. A twisting apparatus was designed and demonstrated to research the characteristics of the strands in twisting situations. The critical currents and strain of a single CC were tested for different twist pitches in self-field and at 77 K. The results show that the critical twist pitch of the QI strand is higher than that of a single tape. The critical current of the QI strand was also measured at 77 K after the application of different tensile stresses at room temperature.

Comparison of out-of-plane tensile moduli of CFRP laminates obtained by 3-point bending and direct loading tests

The out-of-plane tensile modulus of laminated carbon fiber reinforced plastics (CFRPs) was evaluated through 3-point bending tests using laminated UD-CFRP and quasi-isotropic (QI) CFRP samples with out-of-plane span directions. Direct loading tests of QI-CFRP samples were performed for comparison. It was shown that the bending modulus measured for a UD-CFRP specimen with L/t>25 is consistent with the tensile modulus if the bending modulus is less than 15GPa. An analytical description of the out-of-plane modulus of thin QI-CFRP samples was derived by modifying the three-dimensional laminate theory and introducing conditions similar to plane-stress. Subsequently, it was demonstrated that this modified out-of-plane modulus is the lower bound of the apparent bending moduli and that the one predicted by the three-dimensional laminate theory is the upper bound. Finally, it was demonstrated that the 3-point bending test method is more practical than a direct loading test when comparing both methods.

Experimental characterisation of mixed mode traction–displacement relationships for a single carbon composite Z-pin

Abstract This paper presents an experimental characterisation of the mechanical performance and behaviour of through-thickness reinforced composite laminates. To achieve this, composite blocks with individual reinforcing pins were manufactured, quality assessed and tested. Individual specimens were inspected using X-ray Computed Tomography and only the specimens with acceptable quality pin insertions were tested experimentally under a range of mode mixities. Two stacking sequences, uni-directional (UD) and quasi-isotropic (QI) were investigated. It was found that the pins inside the UD samples experienced significantly larger pin/matrix bond strength than those in the QI laminates. The resulting experimental data indicates that a non-UD laminate type may experience pin pull-out and thus increased energy absorption for a wider range of mode mixities than a UD laminate type. Energy plots show a clear transition from a pull-out to a pin rupture region for both laminate types. Specimens that experienced pin rupture during low mode mixity tests exhibited similar failure energies to those loaded in pure mode II.

The effect of long-term exposure to high temperature atmosphere on weight change and damage progress in carbon fiber-reinforced polycyanate ester composites

Composites degradation in thermo-oxidative environment was investigated by measuring weight change and characterized using quantitative methods. Isothermal aging experiment was conducted on carbon fiber/polycyanate (T700SC/FSD-M-08178) unidirectional, angle-ply and quasi-isotropic (QI) laminate at 180 °C in air up to 1500 h. Two empirically based weight change models were used to predict weight change of unidirectional, angle-ply, and QI laminates. Microscopic observation was also conducted on exposed surfaces to investigate damage progress due to thermo-oxidative-degradation induced matrix crack.

REDUCTION OF NUMERICAL DISPERSION OF ADI-FDTD METHOD WITH QUASI ISOTROPIC SPATIAL DIFFERENCE SCHEME

In this paper, the difierence scheme of the alternating- direction-implicit flnite-difierence time-domain (ADI-FDTD) method is replaced by the quasi isotropic (QI) spatial difierence scheme to improve its numerical dispersion characteristics. The unconditional stability advantage of QI-ADI-FDTD is analytically proven and numerically verifled. The numerical dispersion of the novel method can be dramatically reduced by choosing proper weighting factor. An example is simulated to demonstrate the accuracy and e-ciency of the proposed method.

J046023 CFRP小角度差擬似等方性積層板の力学特性(〔J046-02〕高分子基複合材料の加工と評価(2))

Mechanical properties and damage behaviour are evaluated for the quasi-isotropic (QI) CFRJP laminates with small fibre orientation mismatch of 15° that consist of thin plies. The small mismatch exhibits more isotropy also in tensile strength while conventional QI laminates with 45° mismatch fail much lower stress under inclined load. The QI laminates with small orientation mismatch are found to fracture in a brittle manner due to the fibre breakage under tensile or out-of-plane loading because interlaminar delamination and matrix cracking are suppressed by the lower shear stress resulted by the small mismatch and thin plies.

FLEXURAL STRENGTH OF CFRP BOX BEAMS WITH DIFFERENT LAMINATE STRUCTURES

This paper presents an experimental study on the flexural strength of CFRP box beams with two laminate structures: a cross-ply (CP) and a quasi-isotropic (QI). Three specimens were prepared for each laminate structure. The specimens were tested under four point bending, and the material tests of each laminate structure were conducted. The bending tests showed that QI exhibited higher flexural strength than CP and that they had different failure configurations. Also, flexural strength is calculated based on beam theory with Tsai-Wu criterion or maximum stress criterion to be compared with the experimental one. Moreover, in order to discuss a buckling behavior, buckling stress is calculated. It is shown that the calculated flexural strength based on Tsai-Wu criterion agrees well with the experimental one and that the buckling stress exhibits higher value than compressive stress at the flexural strength in the bending test.

Analytical and finite element prediction of Lamb wave scattering at delaminations in quasi-isotropic composite laminates

This paper presents a theoretical and finite element (FE) investigation of the scattering characteristics of the fundamental anti-symmetric (A0) Lamb wave at delaminations in a quasi-isotropic (QI) composite laminate. Analytical models based on the Mindlin plate theory and Born approximation are presented to predict the A0 Lamb wave scattering at a delamination, which is modelled as an inhomogeneity, in an equivalent isotropic model of the QI composite laminate. The results are compared with FE predictions, in which the delamination is modelled as a volume split. The equivalent isotropic model and QI composite laminate are used to investigate the feasibility of the common theoretical approach of modelling the delamination as the inhomogeneity. A good correlation is observed between the theoretical solutions and FE results in the forward scattering amplitudes, but there exists a larger discrepancy in the backward scattering amplitudes. The FE results also show that the fibre direction of the outer laminae has a pronounced influence on the forward and backward scattering amplitudes, which is not predicted by the analytical models.

Through-the-thickness mechanical properties of smart quasi-isotropic carbon/epoxy laminates

Integrating smart materials such as fibre optic sensors or actuating shape memory alloy (SMA) wires with the host composites during manufacturing is one of the most fundamental aspects in the technology of smart structures. Sensing or actuating needs often dictate at what depth and how many sensors or wires should be embedded in the host composites. Thus manufacturing defects like resin pockets or poor interfacial bonding occur and could affect interlaminar shear (ILS) and flexural mechanical behaviour of smart composite structures. Limited information on these through-the-thickness mechanical properties is available from smart quasi-isotropic (QI) laminates of less than 8-ply thick, but no such data exists for thicker smart QI laminates. Therefore in the present research, both ILS and flexural mechanical properties of smart QI carbon/epoxy beams have been determined by using short beam shear (SBS) and Iosipescu methods as well as three-point and four-point bending methods. While SMA wires were embedded at selected interfaces of two different lay-ups in the longitudinal direction, optical fibres (OFs) were embedded at six different ply interfaces in both the longitudinal and transverse directions. It was found that neither ILS properties nor flexural modulus was affected, irrespective of the smart materials, their orientation, or through-the-thickness location. The flexural strengths did not suffer any noticeable degradation when the OFs or SMA wires were embedded either in the longitudinal direction or in the transverse direction in the tensile region. It was shown that the flexural strength degraded significantly when the OFs embedded in the transverse direction were in the compressive region.

Quasi‐shear waves in inhomogeneous weakly anisotropic media by the quasi‐isotropic approach: A model study

In inhomogeneous isotropic regions, S-waves can be modeled using the ray method for isotropic media. In inhomogeneous strongly anisotropic regions, the independently propagating qS1- and qS2-waves can similarly be modeled using the ray method for anisotropic media. The latter method does not work properly in inhomogenous weakly anisotropic regions, however, where the split qS-waves couple. The zeroth‐order approximation of the quasi‐isotropic (QI) approach was designed for just such inhomogeneous weakly anisotropic media, for which neither the ray method for isotropic nor anisotropic media applies. We test the ranges of validity of these three methods using two simple synthetic models. Our results show that the QI approach more than spans the gap between the ray methods: it can be used in isotropic regions (where it reduces to the ray method for isotropic media), in regions of weak anisotropy (where the ray method for anisotropic media does not work properly), and even in regions of moderately strong anisotropy (in which the qS-waves decouple and thus could be modeled using the ray method for anisotropic media). A modeling program that switches between these three methods as necessary should be valid for arbitrary‐strength anisotropy.

Green’s functions for inhomogeneous weakly anisotropic media

Summary Formulae for the zeroth-order principal term plus the first-order additional term of the qP- and qS-wave Green’s functions in the so-called quasi-isotropic (QI) approximation are derived for an unbounded inhomogeneous weakly anisotropic medium. The basic idea of this approximation is to seek the asymptotic solution of the elastodynamic equation as an expansion with respect to two small parameters of the same order: the small parameter used in the standard ray method and a parameter characterizing differences of tensors of elastic parameters of a weakly anisotropic medium and of a nearby ‘background’ isotropic medium. As a result, the procedure of constructing the Green’s functions is split into two steps: (1) calculation of rays, traveltimes, the geometrical spreading and polarization vectors in the background isotropic medium; (2) calculation of corrections of traveltimes, amplitudes and polarization vectors due to the deviation of the weakly anisotropic medium from the isotropic background. Application of the QI approximation to qP-wave propagation leads to useful simplified formulae found earlier by the application of the perturbation methods. Application of the QI approximation to qS-wave propagation leads to formulae of basic importance. The zeroth-order QI approximation removes the well-known problems of the standard ray method for anisotropic media and gives regular solutions for regions in which the differences between the phase velocities of qS waves in the direction of propagation are small. This is the case for weakly anisotropic media as well as for singular regions of qS waves such as in the vicinities of kiss and intersection singularities, for example. In such situations, frequency-dependent amplitudes of the qS waves in the zeroth-order QI approximation are obtained by a νmerical solution of two coupled first-order ordinary differential equations along a ray in the background isotropic medium. When a medium is strongly anisotropic and/or high frequencies are considered, approximate closed-form solutions of the two coupled differential equations have the form of the ray solutions describing two decoupled qS waves. The standard ray method for anisotropic media can substitute the QI approximation in such regions. On the other hand, in the limit of infinitely weak anisotropy, the formulae for the QI approximation smoothly converge to formulae for isotropic media. Thus the QI approximation represents a link between ray formulae for anisotropic and isotropic media. The formulae for the zeroth-order QI approximation are regular everywhere except for singular regions of the ray method for isotropic media. The accuracy of the QI approximation can be increased by considering the first-order additional terms of the QI approximation. The two coupled differential equations are equivalent to the equations of the coupling ray theory (CRT) based on a simplification of a coupling volume integral. Use of a vectorial framework along rays in the background medium that is different in the QI approximation from that used in the CRT avoids some problems of the CRT approach. The QI approximation including the first-order additional terms is expected to yield results of comparable quality to or better quality than those of the CRT. The equivalence of the zeroth-order QI approximation to the CRT promises acceptable results of the QI approximation not only in weakly anisotropic media but also in singular regions of qS waves.