Measuring strain without parasitic thermal influence is vital. A temperature compensated strain sensor is fabricated in fused silica using femtosecond laser micromachining. Utilizing femtosecond laser direct writing and femtosecond irradiation followed by chemical etching, two Bragg gratings are fabricated in the bulk of a fused silica substrate. By suspending one of the Bragg gratings in a cantilever, it is mechanically isolated from the rest of the substrate. Thermal and tensile characterization showed that both Bragg gratings are sensitive to thermal changes with a sensitivity around 10.5 pm/ • C, while only the non-isolated Brag grating is sensitive to strain with a sensitivity of 1.1 pm/µϵ. Hence, it is proven that the parasitic thermal influence on the strain sensor can be compensated by taking into account the response of the isolated Bragg grating.
<p>The strength of architectural glass (soda-lime silica) is highly dependent on surface flaws generated during production, handling and service life. Fracture mechanical investigation of glass, however, is challenging due to e.g. the randomness of flaw size, flaw orientation and quality. Generation of radial and median cracks is inevitable while using a mechanical indenter with direct contact. These undesirable effects, along with uncertainty about the groove’s depth and geometry, degrade the accuracy of results and underline the need for a more reliable tool. Consequently, this contribution focuses instead on the application of ultra-short laser as a non-contact tool, which recently has proved to be a promising solution because of its precision, high speed, and repeatability. Here, artificial grooves with a well-controlled depth are realized on the surface of soda-lime silica glass to investigate the effects of loading rate, flaw size and flaw orientation on the glass strength. Four- point bending tests are performed to assess the failure loads. The method manages to capture the results with a very low standard deviation of the failure stress (approximately 1 MPa), eliminating the need for using large series of specimens.</p>
The strength of chemically pre-stressed glass depends on the depth of surface flaws and the value of the pre-stress. So far, some research has been conducted on this topic; however, there were always uncertainties regarding the flaw depth and the pre-stress profile. Consequently, this research characterises the pre-stress profile using experimental methods. The latter include measuring the depth of layer (DoL) and the surface compressive pre-stress (CS) with FSM-7000h and verifying the achieved DoL with the Na + and K + distribution through the thickness obtained from the SEM/EDS analysis. Results demonstrate that the amount of K decreases parabolically (secondorder) to a certain value and then remains constant. Based on this observation and some boundary conditions, the equation of the pre-stress profile was obtained for thin chemically pre-stressed aluminosilicate glass (Falcon®) with 8h and 24h processing durations in molten salt at 460 ˚C. A non-strengthened glass (NSG) was also used as a reference for comparison. Then, three artificial laser-induced flaws with accurately controlled depths is tested by means of a clamp bender. The results of the strength tests demonstrated very low deviations in the failure stress. It was shown that even when the depth of the flaw is higher than the DoL, which means that the flaw tip enters the zone with the pre-tensile stress, there is still considerable resistance from the surrounding intact area. Furthermore, it was confirmed that the pre-stress strain energy for 24h processing is larger than for 8h, leading to more fragmentation after failure under a similar loading condition. Finally, it was found that the fracture toughness is not constant through the pre-stressed glass thickness, and it is dependent on the pre-stress profile with the peak value at the glass surface.
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