A versatile and broad-field technique, sonic thermography uses high intensity acoustic waves to induce frictional heating
at defect locations and the thermal signature is then detected using IR imaging. Sonic thermography has the potential to
be used as a quantitative technique for difficult inspection problems. One example is the inspection of interference fit
fasteners. In the case of poorly fitted interference fasteners, the acoustic waves induce relative motion between the
fastener and host, causing frictional heating which can then be detected. The preliminary results of an inspection of
interference fit levels in fastened metallic plates, reminiscent of the F-111C wing skin, are discussed. By improving the
repeatability of the acoustic energy transfer, the heat detected using the IR thermographic system can be correlated to the
interference fit levels of the fasteners. The results provide encouragement for the development of a quantitative
assessment capability, however one of the remaining critical issues, which has hindered the use of sonic thermography as
a quantitative technique, is the poor repeatability of acoustic excitations. This paper will also report on an experimental
study which investigates this repeatability issue, in particular the role of the interface material used between the horn tip
and the structure to enhance energy transfer.
An important property of shape memory (SMA) wires is the generation of high stresses when the strain recovery is impeded during heating. These stresses are called recovery stresses and can reach stress levels up to 800 MPa. In a first step this paper compares and discusses the recovery stress generation and mechanism in different SMA-wires based on experimental results. All experiments were performed on a specially equipped thermomechanical testing apparatus. Complex stress-, strain-, and temperature profiles can be programmed to study the thermomechanical behavior of a SMA. The knowledge of these recovery stresses was applied for composite materials. Embedding pre-strained SMA-wires in a composite result in a material with adaptive properties that are related to the reversible martensitic transformation in the SMA-wires. The behavior of the SMA-composites was studied in three ways. Starting from the experimental results on SMA-wires and the knowledge of composite materials, the behavior of the SMA- composites was predicted. A computer simulation model has been used for the same purpose. Thirdly, thermomechanical experiments were performed on the SMA-composites. The theoretically calculated and the simulated results were validated by comparison with these experimental results. In conclusion, links were established between the recovery stress behavior of a SMA-wire and the thermomechanical behavior of SMA-composites. This knowledge can be used to accurately design SMA-composites based on material data of individual SMA-wires.
It is well known that composites, although strong and lightweight, can suffer badly when impacted. This can have catastrophic consequences to a structure. By embedding superelastic shape memory alloys into a composite structure, it is possible to reduce impact damage quite significantly. Superelastic shape memory alloy (SMA) wires absorb a lot of the energy during the impact due to their 'elastic' and hysteretic behavior. The mechanism behind superelasticity is the reversible stress induced transformation from austenite to martensite. If a stress is applied to the alloy in the austenitic state, large deformation strains can be obtained and stress induced martensite is formed. Upon removal of the stress, the martensite reverts to its austenitic parent phase and recoverable strains of up to 8% can be achieved. This paper will report on the results, in which superelastic shape memory alloys were pre-strained to 1.5% and 3% and then embedded into glass fiber/epoxy composite plates. These plates were then impact tested. The effect of embedding wires at different depths of the specimen, different types of wires (martensitic NiTi and stainless steel) and also different volume fractions of wires was also investigated. The results of the impact tests were examined by ultrasonic C-scan to determine the size of the delamination area. The energy absorbed and the maximum impact force were also determined.
The peculiar thermomechanical and functional properties of adaptive composites with embedded shape memory (SMA) wires are directly related to the reversible martensitic transformation in the SMA-wires. The gradual transformation and the related strain recovery of the prestrained SMA-wires during heating is hampered by the rigid matrix. The constraining matrix thus influences the transformational behaviour of the embedded SMA-wires. The effects on the transformation heat and on the transformation temperatures of the forward and reverse transformation have been quantified and explained.
A number of techniques are available which will enhance residual static strength and fatigue life of metallic components. In the case where a crack exists a simple method is to stop drill the crack tips. More elaborate techniques consist of stop drilling the crack tips, cold expanding the resultant hole and inserting an interference-fit plug. The application of bonded composite repairs to cracked metallic structures has also become an accepted technique to enhance strength and fatigue life of cracked metallic components. This paper looks at the possibility of using smart materials such as shape memory alloys (SMA) for the life enhancement of metallic structures. An investigation into the use of SMAs was completed using finite element modeling of one particular concept known here as the annular SMA repair which was presented as one method of possibly increasing the fatigue life of holes. From the results given in this paper it was shown that the inclusion of a SMA ring, interference fitted into a machined annulus in an aluminum plate, induces a significant compressive stress around the boundary of a hole. Also, discussed are other ideas which use SMAs for repairing cracked metallic components. Although the current studies are only conceptual and qualitative they do illustrate the feasibility and potential of such techniques to enhance strength and fatigue life of metallic components.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.