Applied Mechanics and Materials Vols. 24-25

Abstract: Strain field measurement with non intrusive techniques is needed in order to characterize the behaviour of steels at high temperatures subjected to small displacements. In this work we present a technique based on laser-produced speckles coupled with a cross-correlation cross-spectrum method. This method proves more accurate than cross-correlation for small displacements. The laser wave length used (532 nm) allows to perform strain measurements, even with heat radiation.

Abstract: Optical techniques for displacement measurements have become more common in recent years. The current preferred technique is digital image correlation (DIC) which works very well but has limitations for measuring diametral contractions in cylindrical specimens using a standard 3D system. To overcome the limitations of using either a diametral clip gauge or standard 3D DIC, a method has been developed for measuring diametral contractions simultaneously in two directions using a standard 3D DIC system in conjunction with an edge detection algorithm. Results have shown the method to work well.

Abstract: We show how digital holographic interferometry in the Long Wave InfraRed spectral range (LWIR) can be used for the investigation of mechanical structures. The 10.6 μm radiation is produced by a CO2 Laser. Experimental results showing that the method can be used to locate defects in a panel are presented and advantages and disadvantages of this approach are discussed.

Abstract: Compared with the extensive research on properties of the fracture process zone (FPZ) under quasi-static loading conditions, much less information is available on its dynamic characterization, especially for high-strength concrete (HSC). This paper presents the very recent results of an experimental program aimed at disclosing the loading rate effect on the size and velocity of the (FPZ) in HSC. Eighteen three-point bending specimens were conducted under a wide range of loading rates from from 10-4 mm/s to 103 mm/s using either a servo-hydraulic machine or a self-designed drop-weight impact device. Four strain gauges mounted along the ligament of the specimen were used to measure the FPZ size. Surprisingly, the FPZ size remains almost constant (around 20 mm) when the loading rate varies seven orders of magnitude.

Abstract: The paper describes recent use of Digital Image Correlation (DIC) for in-situ measurements of deformation and cracking of large civil engineering structures like bridges and power plant. Recent work at NPL has demonstrated the potential of DIC as a novel NDT tool for measuring deformation and cracking in reinforced concrete structures. This has particular application where the area of interest is in a region where inspection is difficult or costly and where direct access may have safety implications. In this case accurate measurements from pairs of images can be very cost effective.

Abstract: Current trends in the wind turbine industry demand that manufacturers and related research facilities increase development efficiency whilst minimizing production costs. A key component developed is the blade, which needs to efficiently capture the wind energy whilst being able to survive severe weather conditions. More recently larger blades have been used, where main advantages include an increase in wind capture efficiency, a reduction in the installation and reduction in maintenance costs per MW. This paper outlines the main principles behind optical measurement techniques employed, influencing factors for using this technology, and presents the importance of the results achieved thus far and describes ongoing development activities.

Abstract: This paper is part of ongoing structural health monitoring in the National Physical Laboratory. The work is aimed at improving reliability of sensors and increasing the uptake of monitoring systems by improving the understanding of the fundamental interactions between the sensors and their environments. It is important to optimize and validate the selection of the sensors for each application in order to ensure that the results are meaningful. An example of unexpected aspects of a sensor performance exposed to temperature and solar radiation is demonstrated and the importance of temperature compensation is highlighted. More work is required fully to explain this phenomenon.

Abstract: This research deals with the sensitivity of eight types of performance-designed high-strength concrete to the loading rate. Variations in the composition of the concrete produce the desired performance, for instance having null shrinkage or being able to be pumped at elevated heights without segregation, but they also produce variations in the fracture properties that are reported in this paper. We performed tests at five loading rates spanning six orders of magnitude in the displacement rate, from 1.74  10-5 mm/s to 17.4 mm/s. Load-displacement curves show that their peak is higher as the displacement rate increases, whereas the corresponding displacement is almost constant. Fracture energy also increases, but only for loading rates higher than 0.01 mm/s. We use a formula based on a cohesive law with a viscous term to study the results. The correlation of the formula to the experimental results is good and it allows us to obtain the theoretical value for the fracture energy under strictly static conditions. In addition, both the fracture energy and the characteristic length of the concretes used in the study diminish as the compressive strength of their aggregates increases.

Abstract: The present work involves a numerical modelling of the Embedded Process Zone (EPZ) by utilizing the elastoplastic Mode I and Mode II fracture models for the simulation of plastically deforming adhesive joints. A traction-separation law was developed separately for Mode I and Mode II. For the analysis of the mixed-mode fracture processes, the cohesive zones in Mode I and Mode II fracture were assumed uncoupled. The experimental programme involved the fabrication and testing of Double Strap Joints (DSJs) and Single Lap Joints (SLJs). By fitting the numerical results to the experimental ones, the basic cohesive parameters of the problem were defined.

Abstract: In this contribution the mechanical behaviour of polymeric matrix materials is analysed for both thermoplastics (Polypropylene) and thermosets (RTM6, RIM935). The results obtained from tensile tests carried out at different velocities indicate a nonlinear, inelastic material behaviour with strainrate dependence. For the clear identification and quantification of the nonlinearities, the experimental procedure has been extended to relaxation experiments and deformation controlled loading-unloadingprocesses with intermediate relaxations. Based on the experimental observations a small-strain viscoplastic material model is derived and material parameters are identified. The stress-strain-curves computed for different load histories are compared to the experimental results.