Showing papers in "Experimental Mechanics in 2023"

Optimal Design, Development and Experimental Analysis of a Tension–Torsion Hopkinson Bar for the Understanding of Complex Impact Loading Scenarios

Abstract: Abstract Background Advanced testing methodologies and measurement techniques to identify complex deformation and failure at high strain rates have drawn increasing attention in recent years. Objective The objective of the current study is the development of a novel combined tension–torsion split Hopkinson bar (TTHB) conceived to generate a combination of tensile and torsional stress waves in a single loading case, and to measure material data representative of real case impact scenarios. Methods An energy store and release mechanism was employed to generate both the longitudinal and shear waves via the rapid release of a bespoke clamp assembly. A parametric study of the material and geometry of the clamp was implemented via numerical simulations to optimise critical aspects of the wave generation. Thin-walled tube specimens made of two metallic materials were utilised to examine the capability of the developed TTHB system by comparing the experimental measurements with those obtained from conventional split Hopkinson tension and torsion bars. Results The experimental results demonstrate that the synchronisation of the longitudinal and torsional waves was achieved within 15 microseconds. Different wave rise time were obtained via the controlled release of the clamp using fracture pins of various materials. The analysis indicates that the developed TTHB is capable of characterising the dynamic behaviour of materials under tension, torsion, as well as under a wide range of complex stress states. Conclusions The presented apparatus, testing and analysis methods allow for the direct population of the dynamic failure stress envelopes of engineering materials and for the accurate evaluation of existing and novel constitutive models.

Architecture-Driven Digital Volume Correlation: Application to the Analysis of In-Situ Crushing of a Polyurethane Foam

Abstract: Digital Volume correlation (DVC) consists in identifying the displacement fields that allow for the best possible registration of volume images of a sample captured at various loading stages. With cellular materials, the use of DVC faces an intrinsic limit: in the absence of an exploitable texture on (or in) the struts or cell walls, the available speckle pattern will unavoidably be formed by the material architecture itself. This leads to the inability of classical DVC techniques to measure kinematics below the cellular scale, i.e. at the sub-cellular or micro scales. Here, we extend a newly developed architecture-driven DIC technique [1] for the measurement of 3D displacement fields in real cellular materials at the scale of the architecture. The proposed solution consists in assisting DVC by a weak elastic regularization using, as support, an automatic finite-element image-based mechanical model. Complex (locally buckling) kinematics of a polyurethane foam under compression are accurately measured during an in-situ test. The method is essential to evidence the class of dominance (stretching versus bending) of the foam. The proposed method allows to confirm that the foam used is bending-dominated, which is not possible with a classical mesoscopic DVC approach. This method is a good candidate for the analysis of complex local deformation mechanisms at the architecture scale.

Determination of the Modulus of Elasticity by Bending Tests of Specimens with Nonuniform Cross Section

Abstract: Abstract Background Bending tests offer technical advantages when material testing is performed to determine the modulus of elasticity. In biomechanical studies, beam-like cortical bone specimens subjected to flexural loading are usually characterized by nonuniform cross-sectional properties along the beam axis and a comparatively large spatial variation of the local material properties. Objective A suitable evaluation method for determining the average modulus of elasticity within the volume of beam-like specimens with nonuniform cross section was to be identified. Methods A total of 138 samples of human pelvic cortical bone were extracted and tested under flexural loading. Different methods, all based on the linear-elastic flexural theory of beams, were applied to determine the average modulus of elasticity on the basis of measured deformations, and the results were compared. Some of these methods utilized the measured midspan deflection, and others used the elastic curve obtained by digital image correlation. Results The results showed that it was not appropriate to determine the average modulus of elasticity from only the measured midspan deflection. The consideration of deflections at multiple points along the beam axis is recommended. Conclusions An evaluation method based on the fitting of the analytically determined elastic curve of the beam with its nonuniform cross-sectional properties to the measured deflections is considered the most appropriate method for determining the average modulus of elasticity of the specimen.

On the Cover: Introducing Virtual DIC to Remove Interpolation Bias and Process Optimal Patterns

Abstract: Digital Image Correlation (DIC) is an image-based measurement technique routinely used in experimental mechanics, which provides displacement and strain maps of an observed surface/volume. The metrological performance of DIC has reached its limit which is directly determined by the texture of the imaged surface/volume. This paper proposes a novel DIC strategy, which relies on a virtual image. This image, noiseless and of infinite resolution, is moreover optimized for providing measurements with the best metrological performance. The so-called Virtual DIC retrieves the displacement fields by comparing this virtual image to the experimental images. No interpolation is required and processing optimal textures such as checkerboards is possible. Virtual DIC is first applied on synthetic images for comparison purposes with a usual DIC approach. Outstanding metrological performance is observed thanks to the possibility of processing checkerboard patterns. The proposed Virtual DIC is twofold: (i) thanks to the use of a closed-form expression, built-in DIC operators are elaborated without recurring to noisy and poorly defined real images. Interpolation is therefore avoided; (ii) it makes possible it to process checkerboard patterns, which offers the best metrological performance.

Introducing Virtual DIC to Remove Interpolation Bias and Process Optimal Patterns

Abstract: Digital Image Correlation (DIC) is an image-based measurement technique routinely used in experimental mechanics, which provides displacement and strain maps of an observed surface/volume. The metrological performance of DIC has reached its limit which is directly determined by the texture of the imaged surface/volume. This paper proposes a novel DIC strategy, which relies on a virtual image. This image, noiseless and of infinite resolution, is moreover optimized for providing measurements with the best metrological performance. The so-called Virtual DIC retrieves the displacement fields by comparing this virtual image to the experimental images. No interpolation is required and processing optimal textures such as checkerboards is possible. Virtual DIC is first applied on synthetic images for comparison purposes with a usual DIC approach. Outstanding metrological performance is observed thanks to the possibility of processing checkerboard patterns. The proposed Virtual DIC is twofold: (i) thanks to the use of a closed-form expression, built-in DIC operators are elaborated without recurring to noisy and poorly defined real images. Interpolation is therefore avoided; (ii) it makes possible it to process checkerboard patterns, which offers the best metrological performance.

Fatigue and Progressive Damage of Thin Woven CFRP Plates Weakened by Circular Holes

Abstract: Abstract Background In order to design thin-walled components it is necessary to consider the presence of holes and their effects. For high performance composite structures, this is still an issue, since usually only coupons are used in experimental observations and the influence of free edges and the hole affects the fatigue behavior mutually. Objective This work aims to find, through experimental trials, an empirical model that can be used to describe and predict the damage propagation, originating from a circular hole. Methods A fatigue test series is performed and the damage initiation and propagation is monitored with three-dimensional digital image correlation, with which the occurring damage can be measured. Validation of the experimentally induced damage size measured with digital image correlation is performed intermediate with an in-situ measurement with active thermography and phased array ultrasonic. The novelty of this approach is that wide specimens are used, where the influence of the free edges on the hole does not occur. Results The progression of the detected damage over the test reflects the applied loads, where higher loads cause larger damage. For all defined load levels a similar damage propagation is identified, allowing to establish an empirical model and fit it to the test data. Conclusion The proposed empirical model provides a novel approach to describe and predict damage propagation originating from a circular hole in thin-walled composite plates. In addition, it is shown that the damage propagation ceases for the selected plate configuration and thus does not lead to a complete failure.

Quantitative Full-Field Data Fusion for Evaluation of Complex Structures

Abstract: Abstract Background Validation of models using full-field experimental techniques traditionally rely on local data comparisons. At present, typically selected data fields are used such as local maxima or selected line plots. Here a new approach is proposed called full-field data fusion (FFDF) that utilises the entire image, ensuring the fidelity of the techniques are fully exploited. FFDF has the potential to provide a direct means of assessing design modifications and material choices. Objective A FFDF methodology is defined that has the ability to combine data from a variety of experimental and numerical sources to enable quantitative comparisons and validations as well as create new parameters to assess material and structural performance. A section of a wind turbine blade (WTB) substructure of complex composite construction is used as a demonstrator for the methodology. Methods The experimental data are obtained using the full-field experimental techniques of Digital Image Correlation (DIC) and Thermoelastic Stress Analysis (TSA), which are then fused with each other, and with predictions made using Finite Element Analysis (FEA). In addition, the FFDF method enables a new high-fidelity validation technique for FEA utilising a precise full-field point by point similarity assessment with the experimental data, based on the fused data sets and metrics. Results It is shown that inaccuracies introduced because of estimation of comparable locations in the data sets are eliminated, The FFDF also enables inaccuracies in the experimental data to be mutually assessed at the same scale regardless of differences in camera sensors. For example, the effect of processing parameters in DIC such as subset size and strain window can be assessed through similarity assessment with the TSA. Conclusions The FFDF methodology offers a means for comparing different design configurations and material choices for complex composite substructures, as well as quantitative validation of numerical models, which may ultimately reduce dependence on expensive and time-consuming full-scale tests.

Mechanical Characterisation and Simulation of the Tensile Behaviour of Polymeric Additively Manufactured Lattice Structures

Abstract: Abstract Background The mechanical properties of lattice structures have been primarily investigated using uniaxial compression loads. Particularly for polymers, tensile properties are rarely considered because of the difficulties of defining a suitable specimen design in which the fracture occurs within the gauge length. Objective This work proposes a novel formulation to obtain a specimen for the tensile test with a gradation of the lattice density at the interface with the bulk portion, which realises a uniform stress distribution. The aim is to combine a localisation of the fracture in the gauge length with a specimen geometry accomplishing the EN ISO 527 standard and analyse the correlation between the mechanical performance and the defects induced by the process on such thin structures. Methods The formulation is experimentally and numerically (FEM) tested by designed specimens with different cell topology, cell size, strut diameter, and number of cells in the sample thickness. Also, results from uniaxial compression tests are used to validate the tensile properties. The specimens are manufactured in different orientations in the building volume by laser powder bed fusion with Polyamide 12. The effects of the pores morphology, distribution, and inherent anisotropy are investigated using X-ray computed tomography analysis. This data is also used to tune a numerical model. Results The numerical analysis showed a uniform stress distribution; experimentally, the fracture is localised inside the gauge length in respect of the ISO standard. Remarkably, among the different strut-based architectures, the elongation at break is, in the best case, 50% of the corresponding bulk material, while the tensile strengths are comparable. Vertical printed specimens exhibited a slight decrease in tensile strength, and the elongation at break was lower than 50% compared to the counterparts built along the horizontal orientation. Modifying the numerical model according to process-related dimensional deviations between the actual and the nominal structures significantly improved the numerical results. The remaining deviation highlighted the incorrectness of modelling the lattice material from the bulk properties. Conclusion Density gradation is a reliable approach for describing the tensile behaviour of polymeric lattice structures. However, the lower amount of porosity and the different shape in the lattice led to a different material mechanical performance with respect to the corresponding bulk counterpart. Therefore, for polymeric lattice structures, the relationship between process-design-material appears crucial for correctly representing the structure behaviour.

Bilayer Stiffness Identification of Soft Tissues by Suction

Abstract: In vivo mechanical characterisation of biological soft tissue is challenging, even under moderate quasi-static loading. Clinical application of suction-based methods is hindered by usual assumptions of tissues homogeneity and/or time-consuming acquisitions/postprocessing. Provide practical and unexpensive suction-based mechanical characterisation of soft tissues considered as bilayered structures. Inverse identification of the bilayers’ Young’s moduli should be performed in almost real-time. An original suction system is proposed based on volume measurements. Cyclic partial vacuum is applied under small deformation using suction cups of aperture diameters ranging from 4 to 30 mm. An inverse methodology provides both bilayer elastic stiffnesses, and optionally the upper layer thickness, based on the interpolation of an off-line finite element database. The setup is validated on silicone bilayer phantoms, then tested in vivo on the abdomen skin of one healthy volunteer. On bilayer silicone phantoms, Young’s moduli identified by suction or uniaxial tension presented a relative difference lower than 10 % (upper layer thickness of 3 mm). Preliminary tests on in vivo abdomen tissue provided skin and underlying adipose tissue Young’s Moduli at 54 kPa and 4.8 kPa respectively. Inverse identification process was performed in less than one minute. This approach is promising to evaluate elastic moduli in vivo at small strain of bilayered tissues.