اندازه گیری کرنش ها و جابجایی در کامپوزیت های FRP با کسری حجمی کم
Abstract
1- Introduction
2- Experimental program
3- Discussion of results of DIC on FRP materials
4- Conclusions
References
Abstract
Accurately measured strains are critical when investigating the application of Fiber Reinforced Polymer (FRP) materials, but traditional mechanical strain measurement methods have several critical drawbacks related to the installation process and the recording capabilities of the devices. FRP materials typically used in the civil engineering field feature large asymmetries and heterogeneity originated from the manual installation procedures, as opposed to the highly controlled FRP fabrication methods used in other fields that result in more homogeneous materials. The feasibility of using an optical full-field Digital Image Correlation (DIC) technique for measurement of strain fields on FRP materials used in the civil engineering industry has been investigated and the level of error in the DIC method when using more traditional methods was determined. The main advantage of using DIC over more traditional methods, which is the capacity of DIC to measure full field strains instead of strains at isolated points, has been demonstrated by providing exemplar measurements of various specimens of FRP materials. The reported strain fields are examples of what was obtained during an experimental campaign to understand the behavior of FRP anchors and other materials. The main conclusions drawn from the observation of those strain fields are discussed.
Introduction
Determining operational limits and safe working strains, as identified in international guidelines such as ACI 440.2R [1] and CNR-DT 200 [2], is paramount to the application of Fiber Reinforced Polymers (FRP) in civil engineering. One of the most common applications of FRP materials in civil engineering is as Externally Bonded Reinforcement systems (EBR-FRP) to strengthen existing structures. Accurate measurement of the FRP strains when under load in such applications provides insight and understanding of the load distribution within the FRP material. When conducting tests, a number of traditional instruments are available for the measurement of displacements and strains, including Linear Variable Differential Transformer devices (LVDT) or foil Strain Gauges (SG). These instruments have been in use for decades, but feature some significant drawbacks as the installation of the instrumentation is typically invasive, expensive, cumbersome or timeconsuming. But the most significant deficiency of these devices is that they only measure displacements or strains at specific, isolated points. Spatial strain fields allow both general strain distributions and localized strain concentrations to be observed. Spatial strain field measurements are critical when investigating EBR-FRP systems, where the FRP materials are non-homogenous and highly anisotropic as opposed to more traditional metallic or cementitious materials. This effect is exacerbated by “low-tech” in situ manufacturing methodologies typically involved in the installation of EBR-FRP systems. The manufacturing approaches typically used in EBR-FRP systems result in low fiber volume fractions, which is defined as the ratio between the volume of fibers and the total volume, and in large variations in fiber orientation both in-plane and through thickness due to the lack of molding/compression force. Modern full-field imaging techniques such as Digital Image Correlation (DIC), thermograph and particle image velocimetry enable the determination of strain fields during testing.