Journal of Aerospace Engineering

The bond strength of reinforcing bars in concrete is a prerequisite for the evaluation of the development length in reinforced concrete structures. This study concerns these phenomena for fiber reinforced polymer (FRP) rebars in normal strength concrete (NSC). Three different types of rebars were tested using the beam specimen: Carbon, glass, and steel. This involved a total of 26 beam specimens containing 10, 16, and $19\mathrm{mm}$ rebars. The test embedment lengths were 10, 15, and 20 times the rebar diameter $\left(d_b\right)$ . For each rebar tested, the results concern load deflection curves, bond stress-slip responses, and the mode of failure. The results showed that the bond strength of a FRP rebar is, generally, lower than that of steel rebar. Based on this and previous research, proposals for the average bond strength and for the development length of straight FRP rebars under tension in NSC are made.

A damage detection technique, which does not rely on any past baseline signals, is proposed to assess damage in composite plates by using an enhanced time reversal method. A time reversal concept of modern acoustics has been adapted to guided-wave propagation to improve the detectability of local defects in composite structures. In particular, wavelet-based signal processing techniques have been developed to enhance the time reversibility of Lamb waves in thin composite laminates. In the enhanced time reversal method, an input signal at an excitation point can be reconstructed if a response signal measured at another point is reemitted to the original excitation point after being reversed in a time domain. This time reversibility is based on linear reciprocity of elastic waves, and it is violated when nonlinearity is caused by a defect along a direct wave path. Examining the deviation of the reconstructed signal from the known initial input signal allows instantaneous identification of damage without requiring the baseline signal for comparison. The validity of the proposed method has been exemplified through experimental studies on a quasi-isotropic laminate with delamination.

A full continuum aeroelastic model for bending-torsion dynamics of a slender high-aspect-ratio wing in inviscid subsonic airflow is developed avoiding finite element or Padé approximations. The structure model is the classical cantilever model of Goland. The aerodynamics is simplified to the two-dimensional typical section theory. Stability is discussed in the Laplace domain leading to the calculation of the aeroelastic modes, the stability curve, and a precise definition of flutter speed, as well as an explicit formula for divergence speed. The flutter speed is shown to be monotonic decreasing as $M$ increases for small $k$ (normalized complex frequency); if a mode flutters at $M=0$ then it flutters for every $M>0$ excepting $M=1$ . A time-domain state space model is developed requiring the language of abstract functional analysis in the form of a “convolution-evolution” equation in a Hilbert space. The time domain model for $M=0$ differs radically from $0<M⩽1$ . It helps clarify the nature of the aeroelastic modes and flutter instability. The state space model can be used for control design including self-straining actuators.

This paper deals with the problem of data validation of an instrumentation system applied in the aerospace area. The fault diagnosis method used for the validation is based on the principle of the parity space approach. Residuals are generated thanks to the analytical redundancy relations given by the model and the important number of sensors. Indeed, we propose a procedure, which permits us to compute systematically all the redundancy equations for the residual generation phase. The additional concept, called residuals structuration, is necessary to isolate the detected faults. Finally, the data validation task consists in isolating the failing data and in sending only the valid information to a control system. An application to an aerospace system illustrates the proposed algorithm.

A practical method is presented for modeling of two-dimensional arbitrary shape aggregates. An efficient algorithm is developed for mass concrete whose aggregate content reaches 60–70%. All the same kind of random triangular fundamental aggregates are generated at one time, then the two-dimensional arbitrary shape aggregates are generated by the method of random extension. The presented algorithm is superior to common methods, in which all aggregates are generated one by one. This algorithm avoids the inefficiency and low aggregate content of other algorithms. Corresponding software 2D-RAS is developed on the basis of this algorithm. And by means of this software, behaviors of the three-grading concrete specimens under uniaxial tension and bending are studied. The effect of shape of aggregates on mechanical behaviors of concrete is preliminarily investigated. The results indicate that critical load capacity of specimens with random circular aggregate is greater than that of the specimen with arbitrary polygonal aggregate. The cracking depth of the specimen with random arbitrary shape aggregate is larger than that of the specimen with circular aggregate.

This paper presents the results of an experimental investigation carried out to study the effectiveness of a passive control in the form of a cross-wire located at the nozzle exit and downstream of the nozzle exit for promoting jet mixing. The advantage of locating the cross-wire at downstream of the nozzle exit is to keep the flow at the nozzle exit undisturbed, to avoid thrust loss. Sonic jets at nozzle pressure ratios (NPR) of 3, 5, and 7 were investigated with cross-wire at the nozzle exit, at $1D_e$ and $2D_e$ from the nozzle exit. The cross-wire influenced the supersonic core, causing the significant reduction of core length and mixing enhancement. The cross-wire also results in diffusion of shocks for all the cross-wire locations and at all tested NPRs.

This paper aims to discover how the measures that improve aircraft structural safety compare with each other in terms of effectiveness. The safety measures we include here are a load safety factor of 1.5, conservative material properties, redundancy, certification tests, error reduction, and variability reduction. We consider a static point stress design with a simple redundancy model. We model individual errors in calculation (loads, stresses, failure) and in geometry and variability in loading, material properties, and geometry. We use a probabilistic model based on assumed uniform distribution for errors as we often have only upper limits on errors. For variabilities we also use some lognormal distributions. We find that error reduction is more effective than certification testing, which is more effective than using an extra load safety factor. Variability reduction is found to be a very effective way of reducing the probability of failure (more effective than error reduction), but it should be accompanied with an increased B-basis value. In addition, certification testing is found to be effective when errors are large, whereas structural redundancy is found to be more effective when errors are low. We also find that as safety measures are added and the probability of failure is reduced, the uncertainty in that probability of failure increases.

A conceptual design of an unmanned robot submarine is presented to explore one of the moons of the planet Jupiter, namely, Europa. This moon is a little smaller than our Earth’s moon but contains a lot of water; in fact the oceans on Europa are some $96\mathrm{km}$ deep and contain about 5 times the amount of water that our Earth’s oceans contain. Thus, it is likely that large sea animals may exist on Europa supported by hydrothermal vents and these animals will prove of interest to scientists. To satisfy these scientific interests, it is the writer’s belief that a robot submarine will be landing on Europa within the next $20\text{years}$ . This paper discusses some of the problems that will be encountered in the design, construction, and operation of this robot submarine. For example, even though the internal diameter of the submarine may only be $1\mathrm{m}$ , it will not be possible to build the submarine from a metal; as such a structure will have no reserve buoyancy and will sink like a stone to the bottom of the ocean. Even vessels built in glass fiber reinforced plastic and carbon fiber reinforced plastic have only fractional positive buoyancy. This paper shows that for the vessel to have adequate reserve buoyancy, it will be necessary to build the submarine hull in a metal matrix or ceramic composite. This paper discusses different methods of providing power, but favors a fuel cell for this purpose. This paper also discusses the hostile environment that the submarine is likely to encounter on this moon.

Multidisciplinary aeroservoelastic interactions are studied by the combination of knowledge acquired in two main disciplines: aeroelasticity and servocontrols. In aeroelasticity, the doublet lattice method is used to calculate the unsteady aerodynamic forces for a range of reduced frequencies and Mach numbers on a business aircraft in the subsonic flight regime by use of NASTRAN software. For aeroservoelasticity studies, there is the need to conceive methods for these unsteady aerodynamic forces conversions from frequency into Laplace domain. A new method different from classical methods is presented, in which Chebyshev polynomials theories and their orthogonality properties are applied. In this paper, a comparison between flutter results expressed in terms of flutter speeds and frequencies obtained with our method with flutter results obtained with classical Padé and least squares methods is presented for a business aircraft at one Mach number and a range of reduced frequencies. It has been found that results obtained with our method are better in terms of average error than results obtained with the two classical methods here presented.