Abstract 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.

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Abstract The effect of dynamic strain rates on failure responses of a fine‐grained granitic rock is studied experimentally and theoretically. Theoretical investigation employs a model incorporating dynamic fracture criterion with damage mechanics theory. Experimental investigation is conducted using split Hopkinson pressure bar device. In order to investigate the effects of microstructure on dynamic fracture failure under different loading rates, fragment debris of each tested specimen is collected and analyzed. It is found through the debris analysis that the granitic rock breaks down into the fragment debris in grain size scales and the effect of strain rates on the formation of fragment debris appears to be related to the microstructure of the rock. It is also found that dynamic inertia induced by the dynamic loading can reduce the effect of friction confinement generated by the contact between the cylindrical specimen and two split Hopkinson pressure bars on the dynamic responses of the specimen. Theoretical evaluations agree with the corresponding experimental observations.

Abstract The influence of carbon nanotube (CNT) concentration on the fracture toughness of poly(methyl methacrylate) (PMMA) was examined on single‐edge V‐notched‐beam (SEVNB) specimens. Six groups of SEVNB specimens containing 0.5, 1, 2, 4, 8.5 wt% of CNTs and neat PMMA as a reference were tested. First, a notch was introduced into the specimens by a specially made disk whose edge is V‐shaped with a 30∘ angle and a 30 μm tip width. As suggested by an American Society for Testing and Materials Standard for polymers, induction of a natural crack was attempted, without success. Therefore, fracture toughness values were determined with the ‘sharp’ machined notch by means of a calibration formula. These were compared to values obtained using a stress concentration factor and found to differ by less than 3%. The latter calculation takes into account the geometry of the notch. Results showed a decrease in the fracture toughness values with an increase in the CNT concentration. For specimens in which a natural crack was attempted, referred to as a razor‐cut notch, a significant increase in the apparent fracture toughness was observed, as a result of the induced damage.

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Abstract The application of the digital image correlation technique to the determination of the J‐integral at fracture initiation for carbon black‐filled natural rubber compounds is discussed. Three different compounds with varying carbon black content were tested, using two different test configurations: pure shear and biaxial tensile test. Digital image correlation was used to measure the displacement field around the crack tip in the tested specimens. From the displacement field, which was interpolated using a finite element scheme, the stresses were evaluated by using Ogden's hyperelastic model, and the J‐integral could be calculated. The results compare well with both theoretical and finite element results.

Abstract The use of prestressed near‐surface mounted fibre‐reinforced polymers (NSM‐FRP) has been long acknowledged to be a suitable approach to strengthen and retrofit existing reinforced concrete structures. The application of a certain amount of prestress to the FRP prior to its installation provides a number of benefits, mainly related to crack width and deflection requisites at serviceability limit state conditions. After transferring the prestress to a structural element, some of the existing cracks can be closed, decreasing the vulnerability of the element to corrosion, and a certain amount of deflection can be recovered due to the introduced negative curvature. However, these benefits can only be assured if the prestress is properly preserved over time. In this context, three series of reinforced concrete beams, in a total of 10 beams, were strengthened with a prestressed carbon FRP laminate (CFRP) and monitored for about 40 days. The data obtained from these tests is in this paper presented and analysed. The observed losses of strain in the CFRP laminate were found to be mainly located in the extremities of the bonded length, while in the central zone, most of the initial strain was well‐preserved over time. Additionally, the highest CFRP strain losses were observed in the first 6 to 12 days after prestress transfer, suggesting that the benefits of prestressed NSM‐FRP will not be considerably lost over time.

Abstract A novel digital image correlation (DIC) technique has been developed to track changes in textile yarn orientations during shear characterisation experiments, requiring only low‐cost digital imaging equipment. Fabric shear angles and effective yarn strains are calculated and visualised using this new DIC technique for bias extension testing of an aerospace grade, carbon‐fibre reinforcement material with a plain weave architecture. The DIC results are validated by direct measurement, and the use of a wide bias extension sample is evaluated against a more commonly used narrow sample. Wide samples exhibit a shear angle range 25% greater than narrow samples and peak loads which are 10 times higher. This is primarily due to excessive yarn slippage in the narrow samples; hence, the wide sample configuration is recommended for characterisation of shear properties which are required for accurate modelling of textile draping.

No abstract is available for this article.