Baseline-free damage identification of metallic sandwich panels with truss core based on vibration characteristics

A baseline-free damage identification method is proposed to identify damages in metallic sandwich panels with truss core in the article. The method is based on flexibility matrix and gapped smoothing method, with damage index defined DIm. The weight coefficient m is introduced to consider the effect of damages on both low-order modes and high-order modes. Numerical simulations and experiments are conducted to evaluate the present method. Besides, damage index DIm* is also defined by processing DIm with Teager energy operator, and comparisons between DIm and DIm* are also carried out. Results show that the proposed method is effective in detecting single damage and multiple damages of the same or different extent. The weight coefficient m plays a very important role in identification of multiple damages of different styles. When comparing with DIm*, it is found that the present index DIm is better at suppressing the singularity caused by contact nodes and detecting of multiple damages which contain small or slight damages.</p

Heat Transfer and Mode Transition for Laser Ablation Subjected to Supersonic Airflow

When laser ablation is subjected to supersonic flow, the influence mechanism of airflow on laser ablation behavior is still unclear. A coupled thermal-fluid-structure model is presented to investigate the influence of supersonic airflow on the development of a laser ablation pit. Results show that the aerodynamic convection cooling effect not only reduces the ablation velocity but also changes the symmetry morphology of the ablation pit due to the non-uniform convective heat transfer. Flow mode transition is also observed when the pit becomes deeper, and significant change in flow pattern and heat transfer behavior are found when the open mode is transformed into the closed mode

Experimental investigation of unbound nodes identification for metallic sandwich panels with truss core

Experimental investigation of metallic pyramidal sandwich panels with truss core in the cases of different extent and location of unbound nodes damages are conducted and a baseline-free damage identification method based on flexible matrix gapped smoothing method and Teager energy operator is used. The influences of sensor density and Teager energy operator on the damage identification are also discussed. Moreover a 0-1 unbound nodes identification method is also proposed to capture the shape and size of the unbound nodes damage zone. The results show that the proposed baseline-free method could identify the unbound nodes damage of metallic sandwich panels with truss core effectively. Increasing sensor density is beneficial for damage identification and there is a critical sensor density for the identification of unbound nodes damage. Teager energy operator plays a very important role in suppressing fluctuations and singularities caused by non-damage factors such as boundary condition noise and shaker. The identified damage zone obtained by the 0-1 method is in accordance with the actual damage zone. (C) 2016 Elsevier Ltd. All rights reserved

Effects of random damages on dynamic behavior of metallic sandwich panel with truss core

Random damages in the core or face/core interface often appears during fabrication or application of metallic sandwich panels with truss core and may significantly affect the dynamic behavior of the sandwich panel. Two types of evaluation parameters i.e. global parameters and local parameters are used to evaluate the effect of random damages. A finite element model in conjunction with stochastic number program that can automatically specify random damages of various extents is developed. Experiments were conducted to verify the proposed analytic model and technique. Three factors affecting dynamic properties of the damaged sandwich are considered including damage extent boundary condition and damage zone. Results show that variations of structural dynamic property are in proportional to the damage extent and the concentration degree of random damages and increases as the constraint of boundary condition is enhanced. Effects of random damages on both natural frequencies and mode shapes are usually in accordance. Damages in the zone covering structural local vibration modes or maximal vibration amplitude would have greater influences on natural frequencies and mode shapes than damages in other zones. (C) 2016 Elsevier Ltd. All rights reserved

Failure maps and optimal design of metallic sandwich panels with truss cores subjected to thermal loading

Sandwich panels with truss cores have been widely investigated due to their superior mechanical performances. When being used in the thermal protection system of a high-speed aircraft, sandwich panels are usually subjected to intense thermal loading and may fail due to various mechanisms. This paper presents a theoretical and numerical analysis on the failure mechanisms and optimal design of metallic sandwich panels with truss cores subjected to uniform thermal loading. Five failure modes are considered: global buckling, face sheet buckling, face sheet yielding, core member buckling and core member yielding. Failure maps of sandwich panels with several truss core topologies are developed based on these failure modes. Taking the five failure modes as constraint conditions, sandwich panels with truss cores are optimally designed for the minimum weight at given thermal loadings. It is found from the optimal analysis that sandwich panels with Kagome and X-type truss cores are more efficient than those with tetrahedral and pyramidal truss cores. Sandwich panels with fully-clamped boundary conditions have superior thermal loading resistance than those with simply-supported boundary conditions. (C) 2016 Elsevier Ltd. All rights reserved

Thermal post-buckling behavior of simply supported sandwich panels with truss cores

This article presents a thermal post-buckling solution for sandwich panels with truss cores under simply supported conditions, when subjected to uniform temperature rise. The Reissner assumptions are adopted and truss cores are assumed to be continuous and homogeneous. Differential governing equations are developed based on the variational principle. The perturbation technique is employed to determine the thermal post-buckling path of sandwich panels with truss cores. Based on the present method, influences of truss core configuration, relative density, aspect ratio, and initial imperfection on the thermal post buckling behavior are discussed

Modeling the Failure Behavior of CFRP Laminates Subjected to Combined Thermal and Mechanical Loadings

This paper proposes a theoretical approach to predict the failure behavior of laminated carbon fiber reinforced polymer (CFRP) under combined thermal and mechanical loadings. Two types of CFRP Laminates i.e. CCF300/BA9916 and T700/BA9916 are investigated and TGA tests in both nitrogen and oxidation environments at different heating rates are carried out to obtain the thermal decomposition kinetic parameters of polymer matrix and carbon fiber. Based on the thermal decomposition behavior and a multi-level structure model the thermal physical properties mechanical properties and thermal deformations of the laminated composites at high temperatures are obtained. Then substituting thermally degraded properties into constitutive equations of composite materials as macroscopic defects the damage mode and failure strength of the laminated composite under thermo-mechanical loadings is obtained. Predicted elastic properties and failure strength are compared with experimental results as well as previous models. Effects of heating rates and heating environments through rigorous physical model are considered in the present work. It is found that the heating rate significantly affects the thermal and mechanical properties the higher the heating rate the less degraded are the thermo-mechanical properties and failure strength at a given temperature. Young's modulus and failure strength of T700/BA9916 are higher than those of CCF300/BA9916 at high temperatures due to the higher volume fraction of carbon fibers which are less weakened in thermal environment

Coupled thermal-fluid-structure behavior of airflow over target irradiated by high-power laser

In this paper, a coupled thermal-fluid-structure numerical model is presented to investigate interactive effects of airflow, high power laser and metallic target. The numerical model is validated by experiments recently carried out by Lawrence Livermore National Laboratory. The numerical simulation also verified some experimental observations, which show that the convective heat transfer effects of airflow and the aerodynamic pressure play important roles to the damage behavior of laser irradiated target. The convective heat transfer of airflow reduces the temperature of laser irradiated area therefore delays the time reaching damage. When a thin-walled metallic panel is heated up to a high temperature below the melting point, it is softened and the strength nearly vanishes, the aerodynamic pressure becomes a dominant factor that controls the damage pattern even when it is in a low magnitude. The effects of airflow velocity and laser power on the damage behavior of irradiated metallic target are investigated with the aid of the coupled thermal-fluid-structure numerical model, where critical irradiation times to reach the yield failure yield t(yield) and melting failure t(yield) are the main concern. Results show that, when the incidence laser power increases from 500 W/cm(2) to 5000 W/cm(2), significant drop in failure times are found as the incidence laser power increases. When the Mach number of airflow increases from 1.2 to 4.0 at a given incident laser power, a critical airflow velocity is found for the irradiation time to reach the yield strength and melting point, i.e., the maximum irradiation time to reach failure is found at the Mach 1.8 similar to 2.0. The competition of aerodynamic heating before the laser is switch on and airflow cooling after the target is heated up accounts for effects

A THEORETICAL ANALYSIS ON THE THERMAL BUCKLING BEHAVIOR OF FULLY CLAMPED SANDWICH PANELS WITH TRUSS CORES

This article presents a theoretical analysis on the thermal buckling behavior of sandwich panels with truss cores under fully clamped boundary conditions, subjected to uniform temperature rise. The Reissner model is developed by ignoring the flexural rigidity of the core and considering the shear stiffness of the sandwich panel is only contributed by truss cores. By using double Fourier expansions to the virtual deformation mode, the critical temperature of sandwich panels is obtained. Theoretically predicted critical temperatures are in good agreement with those from FEM. The effect of boundary conditions and structure parameters of the sandwich panel are also discussed.</p