Ingenieurwissenschaften und zugeordnete Tätigkeiten
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Ultrasonic echo testing is widely used in non-destructive testing to investigate concrete structures as well as to locate built-in components or inhomogeneities. Currently, Synthetic Aperture Focusing Technique algorithms (SAFT) are used for imaging. These algorithms are highly developed but have some limitations. It is not possible to image the lower boundary of tendon ducts or vertical reflectors. We adopted a geophysical imaging technique, the Reverse Time Migration (RTM), to non-destructive testing in order to improve the imaging of complicated structures in concrete. By using the entire wavefield there are fewer limitations compared to SAFT.
In a first step, simulations for polyamide and concrete structures were performed by using a 2D acoustic finite difference code. The simulations were followed by experiments at a polyamide specimen. Here we were able to determine shape and size of boreholes with a sufficient accuracy. After these successful tests we carried out experiments at a reinforced concrete foundation slab. The reconstruction of the structure of the lower boundary of the slab was improved and vertical reflectors inside the slab were imaged clearly. These tests on polyamide and concrete showed that RTM is a step forward for ultrasonic testing. However we observed migration artifacts and difficulties in imaging 3D structures.
In a second step we implemented a 2D elastic RTM code, since for our ultrasonic measurements elastic waves are emitted. The modeling code is included in the Madagascar software package. The required computing power for performing elastic RTM is significantly high. Thus we need appropriate computer hardware to obtain meaningful results within an adequate time frame. Using our hardware RTM of an ultrasonic data set takes far too long (3 months).
We applied the elastic code to ultrasonic data acquired on a concrete specimen which contains vertical reflectors. A comparison of the acoustic RTM results with those obtained by elastic RTM showed an improvement in the image quality.
Future work includes the analysis of RTM artifacts and the expansion of the algorithm to three dimensions. Another topic to be addressed is to how to account for the size of the ultrasonic transducer arrays which we are using.
Carbon fiber composites have been increasingly used in aerospace, military, sports, automotive and other fields due to their excellent properties, including high specific strength, high specific modulus, corrosion resistance, fatigue resistance, and low thermal expansion coefficient. Interlaminar fracture is a serious failure mode leading to a loss in composite stiffness and strength. Iscontinuities formed during manufacturing process degrade the fatigue life and interlaminar fracture resistance of the composite. In his study, three approaches were implemented and their results were correlated to quantify discontinuities effecting static and fatigue interlaminar fracture behavior of carbon fiber composites.
Samples were fabricated by hand layup vacuum bagging manufacturing process under three different vacuum levels, indicated High (-686 mmHg), Moderate (-330 mmHg) and Poor (0 mmHg). Discontinuity content was quantified throughthickness by destructive and nondestructive techniques. Eight different NDE methods were conducted including imaging NDE methods: X-Ray laminography, ultrasonic, high frequency eddy current, pulse thermography, pulse phase thermography and lock-in-thermography, and averaging NDE techniques: X-Ray refraction and thermal conductivity measurements. Samples were subsequently destructively serial sectioned through-thickness into several layers. Both static and fatigue interlaminar fracture behavior under Mode I were conducted. The results of several imaging NDE methods revealed the trend in percentages of discontinuity. However, the results of averaging NDE methods showed a clear correlation since they gave specific values of discontinuity through-thickness. Serial sectioning exposed the composite’s internal structure and provided a very clear idea about the type, shape, size, distribution and location of most discontinuities included. The results of mechanical testing showed that discontinuities lead to a decrease in Mode I static interlaminar fracture toughness and a decrease in Mode I cyclic strain energy release rates fatigue life. Finally, all approaches were correlated: the resulted NDE percentages and parameters were correlated with the features revealed by the destructive test of serial sectioning and static and fatigue values in order to quantify discontinuities such as delamination and voids.