High-resolution synchrotron x-ray computed tomography (CT) in situ pull-out tests with stepwise increased loading were performed to investigate the force transfer between a shape memory alloy (SMA) wire and the surrounding epoxy polymer matrix. The advancing interfacial failure was observed. The stochastic surface structure of the SMA wire was utilized to determine the axial and radial strains introduced into the SMA wire during the test by performing digital volume correlation on the reconstructed surface data. The global and local strain of the embedded SMA wire volume could be correlated with the force of the first interfacial failure. Using image segmentation on the cross-sections derived from the reconstructed CT volume data, the growth of the delamination along the observed length of the embedded SMA wire for increasing load levels was measured. In addition, the advancing interfacial failure was correlated with changes in the cross-sectional area of the SMA wire due to transverse contraction. The local surface strain characteristics of an embedded SMA wire during CT of an in situ pull-out test were compared to a non-embedded SMA wire loaded in situ. It was found that the polymer matrix exerts an external stress on the SMA wire, constraining its radial strain. Thereby, the study reveals that interfacial failure is not only a shear-stress-induced failure, but shear strain and even more normal strain due to transverse contraction of the SMA wire plays an important role too.
To reduce installation space as well as weight, the use of shape memory alloys (SMA) is advantageous in the field of actuator technology. A big challenge using SMAs in for actuation is their high energy consumption due to Joule heating. To overcome this, an actuator system based on the agonist-antagonist principle, using SMA wire actuators was developed. Main advantage is the possibility of an energy-free holding of the position without the use of special locking components. A prototype made of CFRP and SMA wires was developed and built. The structure consists of a CFRP laminate on which SMA wires are anchored on the upper and lower side. This work also contents a simulation approach of the actuating structure in ABAQUS® using a simplified phenomenological model.
In this paper the suitability of active shape memory alloy hybrid composite (SMAHC) technology to realize an adaptive airfoil trailing edge is evaluated. The demonstrator component investigated highlights the potential of such concept as an extreme lightweight variant of a morphing trailing edge. The main challenges for successful implementation according to today´s typical requirements were identified. The investigation includes analysis and model based modification of an successfully flight tested concept for adaptive vortex generators,1 as well as the design and implementation of a demonstrator of a morphing trailing edge, and its experimental investigation in the laboratory.
KEYWORDS: Sensors, Signal attenuation, Acoustic emission, Structural health monitoring, Neurons, Acoustics, Signal detection, Neural networks, Wave propagation, Data acquisition
This study presents a low-cost and small-scale Structural Health Monitoring System (SHM) for thin walled carbon fiber reinforced plastics (CFRP) structures based on acoustic emission (AE) analysis. It covers the inherent geometric complexity and anisotropic properties of such structures through the implementation of an artificial neural network (ANN). The system utilizes piezoelectric sensors, a data acquisition unit and a microprocessor with a trained ANN in order to localize events that result from artificial sound sources. Besides high precision in localization the system is scalable and adaptive through adequate design and training of the ANN and system hardware. Especially for CFRP, nowadays well established for lightweight applications in the aerospace and automotive industry, such a system helps to overcome their major downside, their sensitivity towards impact loading. Impact sources like bird strikes, tool drops or stone debris can be the cause for delamination that can result in a severe drop of stiffness and early catastrophic failure. In order to guarantee structural integrity, CFRP structures therefore need to be inspected via nondestructive testing methods on a regular scheme. Due to its passive nature and in-situ capabilities AE-based SHM can reduce cost and down-time that come with regular inspections as an alternative approach that allows for a conditionbased inspection scheme.
Non-destructive testing using thermography makes it possible to detect near-surface defects in fiber-reinforced composites as part of quality assurance or maintenance. The quality of the measurement and thus also the detectability of the defects decreases continuously with increasing depth and decreasing defect size. Various post-processing methods, such as pulse-phase thermography (PPT) and higher order statistics (HOS) can be used to improve the contrast or the signal-to-noise ratio of defects, whereby it is important to choose the right parameters depending on the characteristics of the defect. This study investigates the theoretical maximum achievable depths and shows the limits of thermography. For active thermography, impulse thermography is investigated and different post-processing methods are compared. As defect types, delamination in form of air inclusions are considered and their position is varied. Furthermore the influence of the measuring equipment was investigated. The results from the simulation are discussed and compared with results from literature and from experiments.
The main advantage of high performance composite material is its exceptional light-weight capability due to individual tailoring of anisotropic fiber lay-up. Its main draw-back is a brittle and complex failure behavior under dynamic loading which requires extensive quality assurance measures and short maintenance intervals. For this reason efficient test methods are required, which not only generate good and reliable results, but are also simple in handling, allow rapid adaptation to different test situations and short measuring times. Especially the knowledge about size and position of a defect is necessary to decide about acceptance or rejection of a structure under investigation. As a promising method for contactless in-line and off-line inspection we used pulsed thermography. For the determination of the depth of the defects we used logarithmic peak second derivative, a widely accepted method. Alternatively an analytical model, describing the adiabatic heating of a solid plate by an instantaneous pulse, was fitted directly to the measurement data. For the determination of defect size four different approaches were investigated and compared with exact values. The measurements were done with continuous carbon-fiber reinforced materials.
Static vortex generators (VGs) are installed on different aircraft types. They generate vortices and interfuse the slow
boundary layer with the fast moving air above. Due to this energizing, a flow separation of the boundary layer can be
suppressed at high angles of attack. However the VGs cause a permanently increased drag over the whole flight cycle
reducing the cruise efficiency. This drawback is currently limiting the use of VGs. New active VGs, deployed only on
demand at low speed, can help to overcome this contradiction. Active hybrid structures, combining the actuation of
shape memory alloys (SMA) with fiber reinforced polymers (FRP) on the materials level, provide an actuation principle
with high lightweight potential and minimum space requirements. Being one of the first applications of active hybrid
structures from SMA and FRP, these active vortex generators help to demonstrate the advantages of this new technology.
A new design approach and experimental results of active VGs are presented based on the application of unique design
tools and advanced manufacturing approaches for these active hybrid structures. The experimental investigation of the
actuation focuses on the deflection potential and the dynamic response. Benchmark performance data such as a weight of
1.5g and a maximum thickness of only 1.8mm per vortex generator finally ensure a simple integration in the wing
structure.
For actuation purposes active hybrid structures made of fiber reinforced polymers (FRP) and shape memory alloys
(SMA) enable substantial savings concerning weight, space and cost. Such structures allow realizing new functions
which are more or less impossible with commonly used systems consisting of the structure and the actuator as separated
elements, e.g. morphing winglets in aeronautics. But there are also some challenges that still need to be addressed. For
the successful application of SMA FRP composites a precise control of temperature is essential, as this is the activating
quantity to reach the required deformation of the structure without overloading the active material. However, a direct
measurement of the temperature is difficult due to the complete integration of SMA in the hybrid structure. Also the
deformation of the structure which depends on the temperature, the stiffness of the hybrid structure and external loads is
hard to determine. An opportunity for controlling the activation is provided by the special behavior of the electrical
resistance of SMA. During the phase transformation of the SMA - also causing the actuation travel - the resistance drops
with rising temperature. This behavior can be exploited for control purposes, especially as the electrical resistance can be
easily measured during the activation done by Joule heating. As shown in this contribution, theoretical modelling and
experimental tests provide a load-independent self-sensing control-concept of SMA-FRP-hybrid-structures.
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