Mechanical testing of arterial tissue can provide unique insights into its behaviour. As measurement and computational techniques continue to advance, new applications can be discovered that increase the accuracy of characterising tissue behaviour. This review provides an overview of the general considerations involved in testing arterial tissue and discusses the more commonly employed *in vitro* techniques used to assess the mechanical properties of arterial tissue, as well as emerging techniques. The more common methods discussed are uniaxial, planar biaxial and inflation testing. With the increasing precision and versatility of optical measuring systems and the rising interest in the mechanical behaviour of vessels with complex geometries and material properties, we also discuss the emerging trend of full-field measurement systems. Mechanical testing can be combined with bioreactor techniques to characterise the dynamic remodelling of arterial tissue in response to its mechanical environment. The analysis and characterisation provided by advanced mechanical testing techniques can produce and inform more accurate surgical simulations, as well as aid in the risk prediction and modelling of pathological conditions.

Accurate measurements of creep strain are necessary to evaluate the condition and predict the remaining life of power plant constituent materials. Optical techniques are appropriate for this purpose as they are a non-contact method and can therefore be used to measure strain without requiring direct access to the surface. Within this class of techniques, the Auto-Reference Creep Management And Control (ARCMAC) camera system can be used to calculate the strain between two points using a series of silicon nitride (SiN) target spheres (the ARCMAC gauge). There are two iterations in system design, the Conventional ARCMAC and Digital Single-Lens Reflex (DSLR) ARCMAC.

Experiments are conducted to determine the absolute limit of accuracy of the systems in comparison to a strain gauge, and the relative accuracy across several orders of magnitude until specimen failure. In addition, tests have been performed using the ARCMAC gauge at elevated temperatures to evaluate the effect of temperature on the gauges and to investigate whether its accuracy diminishes in creep conditions.

It was found that both conventional and DSLR ARCMAC systems can be accurate to 60 *με* or less. In accelerated creep tests, the ARCMAC gauge produced similar agreement to a linear variable displacement transducer when used to measure creep strain. Strain variations (under 500 *με*) were noted on a steel plate subjected only to operational temperature and no stress. This error is very reasonable compared to a critical strain value of 93 000 *με* in a given high temperature-service material. Digital image correlation (DIC) results using the DSLR ARCMAC system show approximately 4% error in measurement for plastic strains in the specimen. The two measures of strain measurement (using ARCMAC and DIC) can serve to complement each other.

Speckle-based interferometry systems are useful tools for measuring vibration patterns at harmonically vibrating objects. The standard method for processing the speckle patterns acquired is to eliminate additive background noise and speckle phase, yielding Bessel-type fringe patterns whose values are proportional to the absolute value of the Bessel function. Fringes are covered by multiplicative speckle intensity noise on the Bessel-modulated vibration amplitude. Sine–cosine filtering is not an option because the Bessel-type fringe pattern is not a phase pattern, and thus, sine–cosine filtering would only degrade the results. Improvements can be reached by involving additional measurements acquired in the stationary state. An alternative method for processing vibration patterns using linear regression is proposed, yielding patterns where the vibration amplitude is an argument of a true Bessel function, not its absolute value. As a result, spatial frequencies of the vibration fringe pattern are only half of those obtained with standard methods, and results can be filtered and normalised conveniently. While other contributions to improve the results rely on determining indexed skeletons for high Bessel fringe densities, the proposed method aims at a very limited number of low-order fringes, allowing demodulation of the Bessel fringe pattern based on the actual Bessel values in the pattern. The method provides an effective alternative for spatial filtering. Phase differences between the stationary and vibrating states have an adverse effect on the results. Two methods, capable of handling phase jumps of 2π in the phase difference distribution, are presented to correct this.

A full automated NIR polariscope has been specially built for residual stress measurement in crystal silicon wafers for solar applications. The multiple configurations of the instrument allow measuring both the isoclinic and the isochromatic parameters on a full field. A new algorithm has also been developed to extract the maximal shear stress inside the silicon wafers without linking the isoclinic parameter to the isochromatic parameter. Hence, it is straightforward to use and the extraction errors are reduced. Coupling this improved data analysis with the comprehensive capabilities of the test rig, allowed to show that the effect of the cutting process on the residual stress inside the silicon wafers is predominant compared with the effect of the cast process, related to the thermal gradient and impurities.

The shear band development and potential hot-spot initiation of polymer-bonded explosive were investigated under low-rate punch loading and combined punch and thermal loading. The digital image correlation method was used for deformation analysis. The obtained results showed that the initiation and development of shear bands at room temperature occur in three stages and demonstrated that the material suffers a prolonged shear stress concentration in a local area. Large shear was shown to lead to the formation of slip bands in the hard-phase area, indicating that this region has the highest potential to initiate hot-spots even under low-rate punching. For high temperatures, the initiation and development of shear bands occur in two stages, with the resulting flow field being close to Hill's solution except for a small area directly underneath the punch. Large plastic flow rate was shown to be another important feature besides shear banding.

A methodology is proposed to optimize a specimen shape in a biaxial testing machine for the identification of constitutive laws based on full-field measurements. Within the framework of finite element model updating and integrated digital image correlation, the covariance matrix of the identified material parameters due to acquisition noise is computed, and its minimization is the basis of the proposed shape optimization. Two models are investigated: first, a linear elastic law, and second, an elastoplastic law with linear kinematic hardening. Two optimal fillet radii sets are assessed for the two investigated laws based on the minimization of the identification uncertainty.

Stimulated thermography is a non-destructive technique capable of detecting and quantifying defects in composite material thanks to the varying thermal behaviour they display if subjected to a thermal stimulation. Although in literature valid and consolidated thermographic techniques (lock-in and pulsed/stepped thermography) are available, the use of stimulated thermography in industry is still not widespread in the case of application of large structures, mainly because of the overall lengthy time required for testing owing to the necessary scanning approach.

In this work, the influence of the main set-up parameters of stimulated thermography is assessed, analysing simulated defects on a sample specimen made of Glass Fiber Reinforced Polymer. In particular, the attention was focused on the optimisation of testing parameters for the improvement of signal quality and to reduce testing time. In this regard, a new procedure is proposed based on a least-square fitting algorithm capable of providing various synergic thermal analyses with a modulated heat excitation and within a single test.

No abstract is available for this article.

]]>Repeating structures in the form of multiple-bladed rotors are used widely in turbomachinery. Mistuning in turbomachinery is caused by small differences in individual blade properties because of inevitable material and manufacturing variations, resulting in the splitting of vibration modes of the tuned system. Modal characteristics of the blades are quite sensitive to the level of mistuning present inside the structure. In addition, the existence of damage also results in changed dynamics of the complete system. This paper introduces a modal assurance criterion (MAC)-based approach for the investigation of small defects such as cracks in a repeating structure. In order to understand the key issues involved, initial work involved a numerical study of a simple comb-like repeating structure, followed by a detailed numerical and experimental investigation of a tuned and mistuned bladed disc. Changes to the system mode shapes and mode order arising from damage are related to the location and severity of damage. Damage, in the form of small, open cracks, is modelled using different techniques such as follows: material removal, monotonic reduction in the modulus of elasticity of selected elements at the required location and mass modification. Damage indices based on differences in the MAC that give a measure of the change in the mode shapes are introduced. MAC matrices are obtained using a reduced number of data points. The damage index is obtained from the Frobenius norm of the MAC matrix subtracted from the AutoMAC of a reference tuned model without crack. A clear correlation between the damage indices and the crack depth/location is shown. Application of this approach to the limited data obtainable from developing techniques such as blade tip timing is also explained.

Recently, a very interesting article was published in Strain where a rigid polyurethane foam specimen was submitted to longitudinal vibrational excitation in the ultrasonic range. The authors showed that it was possible to measure time-resolved strain response maps by combining digital image correlation and ultra-high-speed imaging. The objective of this discussion is to propose further analysis of the data published in that article, showing that it is possible to extract meaningful values for Young's modulus by using the acceleration field in the specimen as a load cell. The aim here is not to provide a complete solution to this problem but to alert the readers on the possibilities offered by this kind of test. This method is an interesting alternative where the energy is input repeatedly instead of in one go as in impact-based tests. Full-field vibration measurements have already been used in the past to identify stiffnesses but only in bending and at much lower strain rates. This article shows that the method can be extended to cover a much wider strain rate range. Finally, only global stiffness values were identified then, whereas here, maps of stiffnesses can be derived.

Bending of sheet materials is a common forming mode for shaping sheet components. Although many numerical models of bending, both analytical and numerical simulations based, are available in the literature, extensive experimental validations have been rather limited. A new bend test method and complementary three-dimensional finite element (FE) simulation of the experiments are employed to assess the predictions from an advanced analytical and FE model of pure bending of aluminium sheet materials. The experimental set-up developed and utilised is an open concept design that allows access to the tensile surface and through-thickness region in the vicinity of the specimen bend line to continuously record images of the deforming specimen with two cameras. The specimen images are analysed for strains using an online strain mapping system based on digital image correction method. Tangential strain distribution results from the models in terms of material thinning in the bend region are compared with those from the experiments on AA2024 aluminium sheet material by considering the responses from the specimen edges and mid-width regions at the bend line. Furthermore, the tangential and radial stress distributions on the through-thickness section of the specimen from the analytical model are compared with those from the FE model. The results from experiments, FE model and analytical model are compared and discussed in the light of the experimental data and the assumptions involved in the development of the models.

The grid method is a technique suitable for the measurement of in-plane displacement and strain components on specimens undergoing a small deformation. It relies on a regular marking of the surfaces under investigation. Various techniques are proposed in the literature to retrieve these sought quantities from images of regular markings, but recent advances show that techniques developed initially to process fringe patterns lead to the best results. The grid method features a good compromise between measurement resolution and spatial resolution, thus making it an efficient tool to characterise strain gradients. Another advantage of this technique is the ability to establish closed-form expressions between its main metrological characteristics, thus enabling to predict them within certain limits. In this context, the objective of this paper is to give the state of the art in the grid method, the information being currently spread out in the literature. We propose first to recall various techniques that were used in the past to process grid images, to focus progressively on the one that is the most used in recent examples: the windowed Fourier transform. From a practical point of view, surfaces under investigation must be marked with grids, so the techniques available to mark specimens with grids are presented. Then we gather the information available in the recent literature to synthesise the connection between three important characteristics of full-field measurement techniques: the spatial resolution, the measurement resolution and the measurement bias. Some practical information is then offered to help the readers who discover this technique to start using it. In particular, programmes used here to process the grid images are offered to the readers on a dedicated website. We finally present some recent examples available in the literature to highlight the effectiveness of the grid method for in-plane displacement and strain measurement in real situations.

Plastic strain localisation in a sheet specimen was monitored by electronic speckle pattern interferometry during uniaxial tensile tests. The experiments were carried on in the diffuse and localised necking stages until fracture. A kinematic model, which is independent of material characteristics, was used to describe the whole strain rate field with two crossing localisation bands inclined with respect to the tensile direction. Then, the physical features of localisation, such as the width of the two bands, their inclination angles and their maximum strain rates are identified by least-square from the displacements fields and their evolutions are followed from the onset of diffuse necking up to the failure. In particular, the effect of the average strain rate is considered and bandwidth evolution is analysed in detail.

It was found that:

- The band structure appears early, as soon as diffuse necking starts;
- The separation, in terms of strain rate or bandwidth, of the two bands corresponds to the transition between diffuse and localised necking. The localised necking stage can be divided into two sub-stages: in the first one, the two bands continue to evolve but at different rates, and in the second one, one of the bands stabilises. The transition between the two sub-stages is influenced by the crossbeam velocity;
- The inclination of the band leading to fracture remains quite stable, while the other rotates towards a situation perpendicular to the tensile direction;
- The band width decreases exponentially versus the maximum local strain. The two bands follow the same evolution path, but one of them progressively lags behind the other until it stops deforming.
- Although the average strain rate was only varied by a factor two, it was found that, when the strain rate increases, the two bands stay together longer and thus that the onset of localised necking is delayed.