1 INTRODUCTION

1.1 Cold Bending and Lamination

Manufacturing of curved glass via cold bending (Kassnel-Henneberg 2011, Fildhuth 2012) and lamination allows for low-curvature glass with optically exact, smooth surfaces. Stacks of fully toughened (FTG) or heat strengthened (HTG) glass panes and interlayers are deformed on a bending template, which keeps the curved shape during the following lamination in the auto-clave. After production the compound is released from the template. The coupled glass-interlayer laminate with its included bending stress is then subjected to initial, elastic springback and gradual recovery due to interlayer relaxation/creep, which reduces the initial curvature. This behaviour depends on the interlayer shear stiffness, thus e.g. SentryGlas® (SG) is often used due to its high shear modulus. The maximum curvature is limited by the acceptable tensile glass surface stress caused by cold, elastic bending. The thickness of the glass is decisive for the bending stress and the feasible bending radii of single curved glass (Fildhuth 2012).

1.2 Research Objective and Applied Methods

For the intended structural application of cold bent glass (e.g. Fildhuth 2012), knowledge about the influence of the manufacturing process, material properties and the short- and long-term be-haviour is necessary. To understand these design parameters and to apply them for calibrating a FE-simulation model, the deformation and the stress included in the bent glass panes and the in-

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Interior Stress Monitoring of Laminated Cold Bent Glass with Fibre Bragg Sensors

T. Fildhuth & J. Knippers ITKE - Institute of Building Structures and Structural Design, University of Stuttgart, Germany

ABSTRACT: Curved glass modules with shape-stabilisation via shear-compound established by lamination of cold bent glass panes are increasingly popular in research and building applica-tions. For determining the multiple parameters influencing the long-term behaviour of such lam-inates, series of cold bent glass specimen involving different interlayer material are tested at the ITKE. For monitoring the unknown glass surface stress inside the laminate, a new methodology implying fibre optical sensors with Bragg gratings has been developed. Temperature or mechan-ically induced expansion of the sensors mounted to the interior glass surfaces causes shift of light reflected at the gratings. Stress is calculated from this and compared to corresponding FE-simulation. The thin, barely visible fibre optical sensors proved withstanding laminate manufac-turing conditions providing high signal intensity in long-term service. Hence, they are suitable for continuous stress and temperature monitoring in laminated glass and could help enhancing structural safety of critical glass modules in buildings.

terlayers depending on the time have to be known. Both aspects are experimentally assessed us-ing single curved, cold bent glass laminate specimen (see 3.1).

Springback and recovery deformation is measured by photogrammetry (3.2). Monitoring of the bending stress, however, is more difficult to achieve, as it notably has to permit identifying the stress inside of the laminate: The cold bending of the glass on a shaping template first causes nearly identical tensile and compressive stress on the opposed surfaces of each glass pane. But FE-analysis has revealed a different development of stress on the interior and exterior glass sur-faces after lamination and release from the template during springback and recovery in time. Furthermore, the influence of stress irregularities from the bending template is of interest.

Polarisation filters or photoelastic scattered light polariscopes are not applicable for such lam-inates. Electrical resistance based strain gauges suffer damages and may change their electrical resistance properties in the lamination process in an autoclave with temperatures up to 140°C. The gauges and their supply lines also perturb the transparency of the laminate and are therefore not applicable for glass in service in a building or vehicle.

Coated, nearly invisible fused silica glass fibres (d=125 to 195μm) with fibre-Bragg-grating sensors (FBGS) functioning as an interference filter for light passing through the fibre core are a possible solution to meet the exigencies mentioned above. They are mounted adhesively to the glass surface prior to lamination, withstand temperatures up to 200°C and thus permit local strain measurement. Real-time stress monitoring of critical glass constructions is rendered pos-sible with these sensors and could also enhance structural glass safety.

2 FIBRE OPTICAL SENSORS

2.1 Fibre Optical Sensor Types

(Gabler 2012) describes three types of fibre-optical sensors: Spectral (e.g. FBGS), intensity-based and interference-based. In addition, the sensors can be grouped into intrinsic (sensor is part of the continuous fibre, e.g. FBG-sensors) and extrinsic system types (a single sensor placed at the fibre ending, e.g. mono-Fabry-Pérot-sensor). Fibre-Bragg-grating sensors are cho-sen for this project as they are particularly robust and multiple sensors can be allocated at any necessary position in the same fibre.

2.2 Fibre Optical Sensors with Bragg Gratings

The single mode optical fibres applied here have a fused silica core (d=90µm) with a cladding (d=125µm) and an exterior coating d=195 µm, see (fig. 1). Light modes are transmitted through the fibre core. The refractive index n of a glass fibre can be modified by optical absorption of UV radiation. The resulting fibre Bragg gratings are periodic modulations of n serving as an in-terference filter reflecting characteristic bands of light in the fibre. The reflected band can be measured with an interrogator (fig. 2, left), which identifies the peaks. Figure 2 (right) shows an example of a fibre with 6 sensors. The period of the refraction index variation is called Λ. The maximum mode-coupling occurs for the initial Bragg wavelength λ0 as per (equation 1):

, 2 ∙ , ∙ , (1)

Once the fibre is subjected to strain or temperature, both n and Λ vary and subsequently the reflected wavelength λ0 changes. Applying (equation 2) from (Gabler 2012), the behaviour of a sensor fibre subjected to mechanical load (principal strains , , ) and temperature (T) can be described as the ratio of the wavelength shift Δλ and the initial wavelength λ0:

∆ ∆ ,∆ ∆ ∆ ∆ (2)

The first term is the change of Λ due to axial strain ε1 and temperature variation ΔT. The fol-lowing terms add the modification of the refraction index n caused by the principal strain com-ponents εi (2

nd term) and temperature ΔT (3rd term). Differentiation of (equation 2) and some complements lead to the (equation 3), assuming isotropy of the fibre core and principal strain (with ε1 orientated along the fibre axis):

∆∙ 1 ∙ ∙ ∙ ∙ ∆ ∙

(3)

where p11, p12 = Pockels-coefficients (photo-elastic matrix of the fibre core); αn = thermo-elastic coeff. and αT =thermal expansion coeff. of the fibre core. For axial strain, ∙ applies. The expressions containing p11, p12 and ν can be (Trutzel 2001, Schlüter 2011) expressed as strain transmission coefficient k. If the sensor is glued to a substrate, e.g. glass, the thermal expansion of the substrate αsub is used. The axial me-chanical strain of the fibre caused by the substrate strain then reads:

, ∙∆

∙ ∙ ∆ (4)

(Equation 4) permits to directly calculate the mechanical strain of a substrate-mounted FGBS. ΔT is referring to T0 at λ0. In the case of cold bent glass, the surface strain of the glass is found by correcting the strain from the sensor by means of equation 6:

,. ∙

. ∙ (5)

where s = distance sensor axis to substrate centre axis; h = substrate thickness (see VDI 2010).

Figure 1. Functional principle of a fibre Bragg grating sensor

Figure 2. Interrogator and interpretation software (left); fibre-signal with 6 sensors as used in the project

For the typical wavelength measurement range of interrogator units of 1510 to 1590nm, a purely mechanically induced strain of 1µm/m corresponds to a wavelength shift of 1.18 to

1.24pm. A temperature change of 1K causes a wavelength shift of 10.3 to 10.9pm in a free sen-sor, composed of the expansion of the fibre and the apparent sensor “elongation” caused by the temperature sensitiveness of the refraction index n of the fibre. A fibre glued to soda lime sili-cate glass shows a temperature induced shift of 20.1pm/K. The sensitivity of the index of refrac-tion n of the fibre to temperature shifts leads to a non-negligible portion of apparent strain e.g. in the sensor signals to be excluded for strain calculation.

3 TESTING

3.1 Specimen

Three different compositions of glass laminate series measuring 2400mm*800mm each are used for testing: Type 1 consists of two glass panes laminated with PVB, type 2 is as well produced from two glass panes glued with an ionomer interlayer and type 3 is constituted of three glass panes with two SG- interlayers. See table 1 for details.

Table 1: specimen types and sensor (FBG) equipment Composition type specimen (series of 3) thickness FTG/PVB/FTG: 6/1.52/6mm

1 2esg_pvb_012 x 6 FBGs

2esg_pvb_022 x 6 FBGs

2esg_pvb_032 x 6 FBGs

13.5mm

FTG/SG/FTG: 6/1.52/6mm

2 2esg_sgp_012 x 6 FBGs

2esg_sgp_022 x 6 FBGs

2esg_sgp_032 x 6 FBGs

13.5mm

FTG/SG/FTG/SG/FTG: 6/1.52/6/1.52/6mm

3 3esg_sgp_012 x 6 FBGs 1x 12 FBGs

3esg_sgp_022 x 6 FBGs 1x 12 FBGs

3esg_sgp_032 x 6 FBGs 1x 12 FBGs

21.0mm

Figure 3. Sensor equipment layout for the interior glass surfaces of a type 1 or 2 laminate

Fibres equipped with a string of either 6 (types 1, 2) or 12 sensors (type 3) at distinct posi-

tions are glued to the interior glass surfaces as per figure 3. As sensors measure strain in axial fibre direction, their orientation has to be laid out according to the strain direction of interest. Next, the glass panes are stacked with the interlayer material and wrapped in a vacuum bag. The whole is placed on an aluminium bending template and cold bent to a radius of 5000mm by means of two transverse beams at the narrow edges. Then, specimens are laminated in an auto-clave. Afterwards, still fixed on the template (fig. 4), the compound is transported to the testing room. There, the glass laminate is released by unwinding the transverse beams. As the glass is in horizontal position, its self-weight influences the initial springback and recovery. Directly following the release, the compound is lifted from the template, turned to its curved long side and placed on PTFE sliding supports for friction reduction (fig. 5). Due to this vertical position,

self-weight influence vanishes. Deformation and interior glass surface strain are monitored dur-ing the process and the following 9 months, as pointed out in 3.2 and 3.3.

Figures 4, 5. Bent laminate (with numbered photogrammetry survey marks) fixed to the bending device (left); 3 x 3 glass laminate specimens placed on platforms in the testing room (right)

3.2 Deformation measurement

The short and long term deformation is monitored using photogrammetry survey of a grid of numbered points (figures 4, 5) fixed on both glass surfaces. Computer-aided 3D-coordinate cal-culation resulted in data with 20µm root mean square values and allowed for establishing a time-related 3D deformation model of the specimen. This part is not a subject of this paper.

3.3 Stress Monitoring

A single-optical channel BraggMETER FS2100 providing an operating range of 1500 to 1600nm is used as sensor interrogator. It applies a continuous swept laser for light transmission into the fibres and a detector for the part of the spectrum reflected at the FBGS. The 6 or 12 peak wavelength values of the reflected spectra are then identified for the local sensor locations and their orientation. The data allow calculation of strain as explained in (2.) and corresponding stress (supposing that Bernoulli / Navier hypothesis applies for the glass).

4 FINITE ELEMENT ANALYSIS

For comparison, a finite element model (ANSYS 13 Mech. APDL) of the testing specimen is used. Meshing is made with SOLID185 8-node-volume elements (6mm FTG – 1.52mm PVB or SG – 6mm FTG for type 1 and 2). The FTG material model is assumed to have linear-elastic properties with E=70,000MPa and ν=0.23. Both PVB and SG have been modelled as viscoelas-tic materials (Ferry 1980). For PVB, the shear-modulus G results from testing by (Sackmann 2008) are applied. SG ionomer is characterised by means of G provided by (Bennison 2013). In cases, the time- and temperature dependent shear modulus values are exploited for curve-fitting in the software. The values are transformed to build a Prony-Dirichlet series for parallel Max-well elements applying (equation 6) as indicated in (ANSYS 2010) for a temperature range of 17-20°C. Solution is run geometrically non-linear with time stepping.

∑ ∙ (6)

Results at locations corresponding to sensor positions 1 or 6 are shown in figure 6. From the interior stress graphs (bold curves) it can be seen, that PVB-laminated specimen show on-going stress reduction, whereas for SG-compounds (type 2) a constant stress state close to the initial bending values establishes.

Figure 6. Interior and exterior surface stress σx ≈ σ1 at sensor positions 1 or 6, calculated using FEA

5 SENSOR RESULTS

Stress calculated from the sensor signals for cold bending (phase a), transport (b), elastic springback and initial relaxation (c) of the PVB- (type 1) and SG- (type 2) laminated specimen is shown in figure 7. The SG-specimen (dotted curve) shows less stress decrease and different relaxation behaviour from 5500s – 7000s due to the high interlayer stiffness compared to the less stiff PVB-specimen. This tendency continues in long-term relaxation as per figure 8 (sensor 6 ≈ sensor 1): At a mean temperature around 17-21°C, the three PVB-laminates still show on-going, digressive stress decrease during 8-9 months testing (2.1*107s), whereas the type-2-SG-laminates have constant stress. The irregularities in cold bending and initial springback of the curves are mostly due to continuous or step-wise measurement method. Figure 9 shows such a continuous measurement: On the left, the change of the peak wavelength signal for sensor 1 due to different production sequences is visible. The initial signal pos. 1 at zero load shifts to pos. 3 due to cold bending strain. Surface-orthogonal strain ε3 caused by vacuum leads to a typical (Gabler 2012) double-peak signal (see (equation 3); pos. 2 shows vacuum prior to bending, pos. 4 the signal with vacuum after bending) not permitting correct axial stress calculation. The cor-responding continuous wavelength shift and the stress calculated from it are given at the right of figure 9 using the same numbering: Pos. 4 shows the apparent rise in σ1-stress due to the signal influenced by transverse strain. Stress calculated for pos. 4 is thus not correct due to vacuum.

Figure 7. Cold-bending, transport and release sequence compared for SG and PVB-laminates

Figure 8. Stress-time monitoring at sensor position 6 compared for PVB- (left) and SG-laminates (right)

Figure 9. Process monitoring results: wavelength peaks (left); gradual stress-wavelength tracking (right)

6 CONCLUSION

In the FE-analysis, the principal tendency of the glass stress depending on time is corresponding to the sensor results. After cold bending, a stress of 40-47 MPa is reached. It reduces by about 5 MPa due to elastic springback for PVB and by 1-2MPa for SG after release from the bending template. SG-laminated specimens show nearly constant stress. PVB-specimens suffer a stress reduction by another 5-7 MPa in the hours following release; this loss of bending stress contin-ues digressively during 9 months testing time. Whereas the FEA-compressive stress σx is close to the sensor-calculated stress σ1≈σx, the tensile stress FE-results diverge. This difference can be related to the complex contact-element calculation and the exact contact position on the bending jig influencing the local stress and needs further examination. However, it shows that cold bend-ing is influenced by many factors resulting in non-negligible, local stress differences. This is al-so shown by local deformation irregularities measured with photogrammetry, which influence the sensors. Principally, the fact that PVB above glass transition temperature shows digressive, continuing creep / relaxation behaviour underlines that it is not suited for cold bent structural glass. In contrast, SG laminates show nearly no relaxation at room temperature and nearly keep

the initial stress. As expected from sandwich theory and FEA, it can be confirmed from the sen-sors that absolute stress on the interior glass surfaces is higher than on the exterior faces.

7 ONGOING AND FUTURE RESEARCH

The sensor strain evaluation of the laminates having three glass layers is under way. It can al-ready be stated that it is showing even less springback and recovery behaviour than the type 2 laminates. Research is continuing on the enhancement of strain / stress measurement using opti-cal fibres inside of glass laminates by optimisation of the adhesive connection, the monitoring of the manufacturing process and a higher density of measurement points e.g. by all grating fi-bres to reduce the influence of local strain variations. Other effects are the enhancement of local temperature compensation, principal glass surface strain measurement (σ1, σ2) and the influence of the interlayer material embedment of parts of the glass-surface-mounted sensor: High inter-layer stiffness as for SG may lead to a so-far unknown influence of transverse, plain strain ε2 from the interlayer on the sensor, even though the adhesive connection to the glass provides higher stiffness than the interlayer. As pointed out in more detail in (Fildhuth 2013), transverse strain (ε3) influence as observed during vacuum bagging (figure 9) causes birefringence with a typical double-peak wavelength, which might occur due to exterior loading or internal strain or-thogonal to the glass surfaces. This phenomenon should be avoided in the manufacturing and application of the laminates and is therefore examined as well. A focus is laid on above-mentioned reliability perfecting of the sensors aiming at life cycle stress measurement of lami-nates in service conditions possibly allowing for more safety in structural glass application.

8 ACKNOWLEDGEMENTS

The research project is conducted with the support and participation of INGLAS Produktions GmbH Friedrichshafen/Germany, BGT Bischoff Glastechnik AG Bretten/Germany, M. Ton-dera, Y. Hu, A. Ehret and O. Hainbuch (University of Stuttgart), RFR Paris, FBGS Technolo-gies GmbH Jena and the Institute of Photogrammetry (IFP) at the University of Stuttgart.

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