Lincy Pyl

Pitfalls in the experimental recording of ultrasonic (backscatter) polar scans for material characterization

The ultrasonic polar scan (UPS), either in transmission, reflection or backscatter mode, is a promising non-destructive testing technique for the characterization of composites, providing information about the mechanical anisotropy, the viscoelastic damping, the surface roughness, and more. At present, the technique is merely being used for qualitative purposes. The limited quantitative exploration and use of the technique can be primarily ascribed to limitations of current theoretical models as well as the difficulty to perform accurate, and more importantly, reproducible UPS experiments. Over the last years, we have identified several potential pitfalls in the experimental implementation of the technique which severely deteriorate the accurateness and reproducibility of a UPS. In this paper, we make an inventory of the most important difficulties, illustrate each of them by a real experiment and present a feasible mediation, either numerical or experimental in nature. Once the experimental set-up is fine-tuned to overcome these pitfalls, it is expected that the recording of high-level UPS experiments, in combination with numerical computations, will facilitate the technique to become a fully quantitative non-destructive characterization method.

Parametric Study of an Explosive‐Driven Shock Tube as Blast Loading Tool

The need for efficient blast loading tools is increasing with the development of new protective techniques. Among these tools, one can cite the use of an explosive-driven shock tube (EDST). The purpose of the present study is to define analytical equations to predict incident and reflected pressure and impulse generated at a laboratory scale EDST end in terms of the tube length, the tube diameter, the explosive mass, and the stand-off distance. The formulae are obtained based on a dimensional analysis and a numerical parametric study. The analytical study is supported by a set of experiments, in order to validate the obtained analytical models. The EDSTs discussed in this study are open on both sides with diameters ranging from 0.15 to 0.50m and a length ranging from 0.75 to 3m. Two different tube sections, circular, and square, are examined. Explosive charges from 5 to 50 g of C4 are used. Within these conditions reflected pressures ranging from 0.15 to 11 MPa and reflected impulses ranging from 100 to 3000 Pa·s are obtained at the tube end. The obtained analytical equations are compared to several available results and formulae from the literature. In addition to that, a graphic representation is developed in order to estimate the tube geometry and the explosive mass necessary to generate a given couple of pressure–impulse at the tube end.

Calibration and correction procedure for quantitative out-of-plane shearography

Quantitative shearography applications continue to gain practical importance. However, a study of the errors inherent in shearography measurements, related to calibration of the instrument and correction of the results, is most often lacking. This paper proposes a calibration and correction procedure for the out-of-plane shearography with a Michelson interferometer. The calibration is based on the shearography measurement of known rigid-body rotations of a flat plate and accounts for the local variability of the shearing distance. The correction procedure further compensates for the variability of the sensitivity vector and separates the two out-of-plane deformation gradients when they are coupled in the measurement. The correction procedure utilizes two shearography measurements of the same experiment with distinct shearing distances. The effectiveness of the proposed calibration procedure is demonstrated in the case of a static deformation of a centrally loaded plate, where the discrepancy between experimental and finite element analysis results is minimized.

Experimental Investigation of the Deformation of Built-up Members of Cold-Formed Steel Profiles

Built-up members of cold-formed steel (CFS) profiles were tested in 4-point bending. CFS profiles (generally thin-walled) deform considerably under load, and the deformed configuration is a result of the superposition of different buckling mode shapes. Local buckling propagates through the profile walls; during distortional buckling parts of the cross-section rotate around a web-flange juncture. Alongside the buckling effects, the overall deformation of the member is considerable. To study these slender and relatively long members, a sufficient number of measuring positions on the specimens is needed. Often, this is not feasible with the conventional measuring techniques. An optical measuring device was used to record the movement of a large number of points per specimen. The obtained results are placed in a 3D coordinate system and can be exported for further data processing. The goal of the measurement campaign was to calibrate a Finite Element model that will simulate the tests. The model will be used for the analysis of composed frame members of CFS profiles, whose design is not entirely covered by the European Standard [1]. After calibration, the FEA predicts the performance of these built-up members well.

Errors in shearography measurements due to the creep of the PZT shearing actuator

Shearography is a modern optical interferometric measurement technique. It uses the interferometric properties of coherent laser light to measure deformation gradients on the µm m − 1 level. In the most common shearography setups, the ones employing a Michelson interferometer, the deformation gradients in both the x- and y-directions can be identified by setting angles on the shearing mirror. One of the mechanisms for setting the desired shearing angles in the Michelson interferometer is using the PZT actuators. This paper will reveal that the time-dependent creep behaviour of the PZT actuators is a major source of measurement errors. Measurements at long time spans suffer severely from this creep behaviour. Even for short time spans, which are typical for shearographic experiments, the creep behaviour of the PZT shear actuator induces considerable deviation in the measured response. In this paper the mechanism and the effect of PZT creep is explored and demonstrated with measurements. For long time-span measurements in shearography, noise is a limiting factor. Thus, the time-dependent evolution of noise is considered in this paper, with particular interest in the influence of external vibrations. Measurements with and without external vibration isolation are conducted and the difference between the two setups is analyzed. At the end of the paper some recommendations are given for minimizing and correcting the here-studied time-dependent effects.

Local Stiffness Identification of Beams Using Shearography and Inverse Methods

Shearography is an interferometric method that produces full-field displacement gradients of the inspected surface. In high-technology industry it is often used qualitatively to detect material defects, but quantitative applications are still rare. The reasons for that are the complicated calibration procedure as well as the denoising, unwrapping, the local sensitivity vector estimation and the local shearing angle estimation needed to get quantitative gradient-maps. To validate the technique and its calibration, results obtained from shearography are compared to results obtained from scanning laser vibrometry. Beams are acoustically excited to vibrate at their first resonant frequency and the mode shape is recorded using both shearography and scanning laser vibrometry. Outputs are compared and their properties discussed. Separate inverse method algorithms are developed to process the data for each method. They use the recorded mode shape information to identify the beam’s local stiffness distribution. The beam’s stiffness is also estimated analytically from the local geometry. The local stiffness distributions computed using these methods are compared and the results discussed.

Optical Measuring Technique for the Investigation of Built-Up Cold-Formed Steel Structural Members

The construction industry uses cold-formed steel (CFS) sheets in the form of galvanised thin-walled profiles and corrugated sheets. In the past decade, CFS profiles have been competing with their hot-rolled counterparts as primary structural members of industrial halls, office buildings and residential housing of up to 3-4 storeys. The spans and column heights achieved with CFS profiles are ever larger. Due to the large slenderness of these members, adequate strength and stability are necessary, as well as reliability in design. Thin-walled members go through buckling during all stages of their working life. Local buckling appears at loads sometimes much lower than the design load. Distortional buckling seriously reduces the member resistance. It interacts with warping and lateral-torsional buckling, being significant for these asymmetric open sections. To restrict these effects, builders employ double sections - usually two standard cold-formed shapes bolted together to form a built-up section. These sections have the advantages of symmetry, higher stability and strength. The design of built-up members involves many uncertainties - although the European standard includes guidelines on the prediction of local, distortional and global buckling, the partial integrity and interaction between the parts of the composed members is still not studied. To study the actual behaviour, built-up members are tested in bending. An optical device for 3D motion analysis measures the displacement of points of interest on the specimen. Two interacting cameras use parallax to obtain the position of an arbitrary number of reflective markers glued to the specimen. The device tracks the movement of the markers in a 3D coordinate system without any contact with the specimen. Standard displacement transducers measure vertical displacements to validate the results. The paper gives an appraisal of the applicability of the method, a summary of the difficulties faced and the outcome of the test campaign.

Estimation of the Strain Rate Hardening of Aluminium Using an Inverse Method and Blast Loading

The dynamic behaviour of structures under blast wave loading is increasingly studied due to the rising risks of intentional and accidental explosions and the development of new protective technologies and methods. To ensure the efficiency of these protective measures, the identification of the material behaviour under blast loading is of utmost importance. This paper investigates the feasibility of the identification of the strain rate hardening of aluminium using an inverse method and blast loads. The results of a series of tests on aluminium plates loaded by means of an Explosive Driven Shock Tube are reported. A numerical study is performed to develop a model that is both in concordance with the experimental measurements and suitable for loop implementation. The 3D high-speed full-field measurements and finite element simulations are used to accurately assess the dynamic response of the plates. The identification is based on the Levenberg-Marquardt formulation for damped least-squares solution. The approach is virtually validated and its sensitivity to noise is studied using virtual measurements from finite element calculations. Then, experimental data are used for the identification. The results obtained are within the range of expected values based on literature.

Structural Reliability in Design and Analysis of Tensile Structures

In order to achieve a semi-probabilistic verification format such as used for the analysis of conventional structures in the Eurocodes, research into structural reliability calculations for tensile surface structures is needed. Appropriate partial factors have to be proposed and evaluated. To gain insight, a scholastic example with three cable segments is analysed. In order to take into account the uncertainties associated to the pre-tensioned system, Latin Hypercube Sampling is applied to sample six main influencing variables. The cable system is designed according to the ultimate limit state under loading, considering a partial factor of 1.35 for the pretension. The structural reliability of both designs is evaluated. An increase of the partial factor for pretension from 1 to 1.35 results in an increase of the reliability index from 2.27 to 5.42.