This thesis concerns various aspects of the durability offibre optic sensors embedded in composite. Since repair orreplacement of embedded sensors is not generally possible, thefunctional reliability of embedded sensors is one of the mostimportant prerequisites for successful use. The main researchobjective was to investigate the interaction between the sensorand the composite, and how this is affecting the mechanical andoptical sensor response. Fibre optic sensors embedded incomposite structures induce local stress concentrations whenthe composite is subjected to mechanical loads andenvironmental changes such as temperature and moisture. Acomplex transfer of stresses through the interfaces between theembedded sensor and the composite occurs and can result inlarge local stresses in the composite and a significant changein the response of the embedded sensor. These stressconcentrations make the interfaces susceptible todebonding. The sensor performance was studied experimentally andnumerically. Some basic results were generated for the EFPI andBragg grating sensors. The phase-strain response was determinedduring static and fatigue loading. The results showed that thesensors were more reliable in compression than in tensilestatic and fatigue loading. Generally, the sensor reliabilityduring loading was significantly improved for the Bragg gratingsensors over that of the EFPI sensor, as an effect of thesensor geometry. This was also demonstrated in theinvestigations on impacts. Impacts do not necessarily result indamage in the composite, but might cause debonding or otherfailure modes in the sensor area. Large, local stressconcentrations occur at several positions in the EFPI sensor,which pointed out that this sensor type was not suitable forembedded applications. The shift in focus from the sensor concept based on the EFPIsensor to that based on the Bragg grating sensor manifesteditself in several studies. The calculated deformation fieldaround an embedded optical fibre was verified in experimentsusing a high-resolution moiré interferometric technique.Furthermore, the improvement in the coating technology wasverified. A significant higher interfacial strength wasobtained with the silane-treated glass surface. The resultsindicated that at least a twofold improvement of the shearstrength was obtained. To simultaneously measure the in-plane strain components andthe temperature change, embedded Bragg grating sensors werearranged in a rosette configuration. The relationship betweenthe optical response from each sensor and the strains in thelaminate was numerically and analytically established. Damage lead to stress redistribution in the sensor region,which may influence the output from the embedded Bragg gratingsensor. The effect was numerically evaluated for interfacialdamage, and was compared to that of a sensor with undamagedinterface. The results showed that debonding might have asignificant influence, in particular for combined thermal andmechanical loading. <b>Keywords</b>: composites, fibre optic sensor, embedded, EFPIsensor, Bragg grating sensor, durability, fatigue, impact,strain measurement, interface, stress analysis

<p>This thesis concerns various aspects of the durability offibre optic sensors embedded in composite. Since repair orreplacement of embedded sensors is not generally possible, thefunctional reliability of embedded sensors is one of the mostimportant prerequisites for successful use. The main researchobjective was to investigate the interaction between the sensorand the composite, and how this is affecting the mechanical andoptical sensor response. Fibre optic sensors embedded incomposite structures induce local stress concentrations whenthe composite is subjected to mechanical loads andenvironmental changes such as temperature and moisture. Acomplex transfer of stresses through the interfaces between theembedded sensor and the composite occurs and can result inlarge local stresses in the composite and a significant changein the response of the embedded sensor. These stressconcentrations make the interfaces susceptible todebonding.</p><p>The sensor performance was studied experimentally andnumerically. Some basic results were generated for the EFPI andBragg grating sensors. The phase-strain response was determinedduring static and fatigue loading. The results showed that thesensors were more reliable in compression than in tensilestatic and fatigue loading. Generally, the sensor reliabilityduring loading was significantly improved for the Bragg gratingsensors over that of the EFPI sensor, as an effect of thesensor geometry. This was also demonstrated in theinvestigations on impacts. Impacts do not necessarily result indamage in the composite, but might cause debonding or otherfailure modes in the sensor area. Large, local stressconcentrations occur at several positions in the EFPI sensor,which pointed out that this sensor type was not suitable forembedded applications.</p><p>The shift in focus from the sensor concept based on the EFPIsensor to that based on the Bragg grating sensor manifesteditself in several studies. The calculated deformation fieldaround an embedded optical fibre was verified in experimentsusing a high-resolution moiré interferometric technique.Furthermore, the improvement in the coating technology wasverified. A significant higher interfacial strength wasobtained with the silane-treated glass surface. The resultsindicated that at least a twofold improvement of the shearstrength was obtained.</p><p>To simultaneously measure the in-plane strain components andthe temperature change, embedded Bragg grating sensors werearranged in a rosette configuration. The relationship betweenthe optical response from each sensor and the strains in thelaminate was numerically and analytically established.</p><p>Damage lead to stress redistribution in the sensor region,which may influence the output from the embedded Bragg gratingsensor. The effect was numerically evaluated for interfacialdamage, and was compared to that of a sensor with undamagedinterface. The results showed that debonding might have asignificant influence, in particular for combined thermal andmechanical loading.</p><p><b>Keywords</b>: composites, fibre optic sensor, embedded, EFPIsensor, Bragg grating sensor, durability, fatigue, impact,strain measurement, interface, stress analysis</p>

In this thesis, fatigue life of a cantilever aluminum plate with a side notch under certain loading conditions is analyzed. Results of experimental stress analysis of the cantilever aluminum plate by using a uniaxial strain gage are presented. The strain gage is glued on a critical point at the specimen where stress concentration exists. Strain measurement is performed on the base-excited cantilever beam under random vibration test in order to examine the life profile simulation. The fatigue analysis of the test specimen is carried out in both time and frequency domains. Rainflow cycle counting in time domain is examined by taking the time history of load as an input. Number of cycles is determined from the time history. In frequency domain analysis, power spectral density function estimates of normal stress are obtained from the acquired strain data sampled at 1000 Hz. The moments of the power spectral density estimates are used to find the probability density function estimate from Dirlik&rsquo / s empirical expression. After the total number of cycles in both time and frequency domain approaches are found, Palmgren-Miner rule, cumulative damage theory, is used to estimate the fatigue life. Results of fatigue life estimation study in both domains are comparatively evaluated. Frequency domain approach is found to provide a marginally safer prediction tool in this study.

[Truncated abstract] Characterising the behaviour of calcareous sediments that possess some degree of bonding between their constituents has attracted worldwide research interest in recent years. Although many recent studies have made significant contributions in delineating the behaviour of these sediments, there is still paucity of information particularly on the cyclic behaviour of cemented calcareous soils. This thesis describes in detail the characteristic features of cemented calcareous soils and proposes methods for characterising their cyclic behaviour. Two different calcareous soils Goodwyn (GW) and Ledge Point (LP) soils representing extreme depositional environments were examined in this study. Artificially cemented sample were created using the CIPS (Calcite Insitu Precipitation Systems) technique, considering its superiority over other most commonly available cementation techniques in replicating the natural pattern of cementation, and the behaviour of natural calcarenite under monotonic loading conditions. The experimental program involved triaxial testing of both uncemented and calcite-cemented calcareous soils under different loading conditions, i.e. isotropic compression tests to high-pressure (16 MPa), monotonic shearing tests, undrained cyclic shearing tests and undrained monotonic post-cyclic shearing tests. Significant emphasis has been placed on the cyclic behaviour of these soils. Internal submersible LDVTs were used for the accurate and continuous measurement of strain down to about 10-5

The complex interface between tyre and terrain is a largely studied topic in terramechanics and vehicle dynamics research. This interface, known as the contact patch, is however hidden from view and cannot easily be measured. Several studies have focused on measuring tyre strain on the inside surface of the tyre to indirectly determine tyre parameters. The inner surface is separated from the contact patch by the tyre thickness however this difference can be considered small in comparison to the bene t gained by a safe environment for measurement systems. Static studies of tyre strain have been successful however lacks the important phenomena occurring in a rolling tyre. Tyre strain measurements in dynamic tyres have been limited to discrete points and/or once per revolution, which is an insufficient sampling rate for vehicle stability controllers such as ABS. This study performs full-fi eld and point strain measurements of the inner tyre surface of a rolling agricultural tyre at low speeds. Stereo cameras mounted on a mechanically stabilised rim will record full-fi eld measurement of the contact patch kept in constant view. Digital Image Correlation techniques are used to determine full-fi eld deformation and strain from successively captured images. Point measurements, such as strain gauges, are included in the study for a comparative measurement. An agricultural tyre hosts large lugs which include large strain concentrations within the contact patch. The complex tyre structure signi ficantly influences the strain measurements, other factors such as inflation pressure, vertical load and slip angle is also studied. Since most vehicle forces are transmitted through the tyre at the tyre-terrain interface, capabilities to measure this area will be a great benefi t for tyre research and leading towards a smart tyre. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2020. / Mechanical and Aeronautical Engineering / MEng (Mechanical Engineering) / Unrestricted

The deformation on cylindrical surfaces of holes in tensile loaded laminated composite specimens was measured using new moire interferometry techniques. These new techniques were developed and evaluated using a 7075-T6 aluminum control specimen. Grating replication techniques were developed for replicating high quality diffraction gratings onto the cylindrical surfaces of holes. Replicas of the cylindrical specimen gratings (undeformed and deformed) were fabricated onto circular steel sectors. Narrow angular regions of these sector gratings were directly evaluated in a moire interferometer. This moire interferometry approach eliminated potential sources of error associated with other moire interferometry approaches. Two composite tensile specimens, fabricated from IM7/5250-4 pre-preg with ply layups of [0₄/90₄]_3s and [+30₂/-30₂/90₄]_3s, were examined using the newly developed moire interferometry techniques. Circumferential and thickness direction displacement fringe patterns (each 3 degrees wide) were assembled into 90 degrees wide mosaics around the hole periphery for both composite specimens. Distributions of strain were calculated with high confidence on a sub-ply basis at select angular locations. Measured strain behavior was complex and displayed ply-by-ply trends. Large ply related variations in the circumferential strain were observed at certain angular locations around the periphery of the holes in both composites. Extremely large ply-by-ply variations of the shear strain were also documented in both composites. Peak values of shear strain approached 30 times the applied far-field axial strain. Post-loaded viscoelastic shearing strains were recorded that were associated with the regions of large load-induced shearing strains. Large ply-grouping related variations in the thickness direction strain were observed in the [+30₂/-30₂/90₄]_3s specimen. An important large-scale trend was observed where the thickness direction strain tended to be more tensile near the outside faces of the laminate than near the mid-ply region. The measured strains were compared with the three-dimensional analysis technique known as Spline Variational Elastic Laminate Technology (SVELT), resulting in a very close match and corroborating the usefulness of SVELT. / Ph. D.

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No description available.

<p dir="ltr">Terahertz Time Domain Spectroscopy (THz-TDS) is a powerful non-destructive, non-ionizing spectroscopic technique utilized for evaluating the optical properties of materials within the terahertz frequency range, spanning from 0.1 to 10 terahertz or wavelengths of 300 micron to 3000 micron. It effectively bridges the gap between microwave and infrared regions on the electromagnetic spectrum and its high resolution which avoiding scattering can quantify small changes in dielectric properties of media. It has high transmission through visibly opaque polymers and its ability to record both magnitude and phase information makes it a strong spectroscopic technique with applications in security, chemistry, electronics and telecommunication and non-destructive evaluation methods for solid mechanics.</p><p><br></p><p dir="ltr">This work introduces a polarization-dependent analytical model employing THz-TDS for computing strain in materials. The model establishes a correlation between volumetric strain and the change in time of arrival for a THz pulse, leveraging dielectrostrictive properties, variations in doping particle density, and changes in sample thickness due to Poisson’s effects. Validation of the analytical model is achieved through strain mapping of polydimethylsiloxane doped with highly dielectrostrictive strontium titanate (STO). Two experiments, including open-hole tensile and circular edge-notch specimens, demonstrate the efficacy of the model. Additionally, the study accounts for stress relaxation behavior to ensure measurement accuracy. Comparison of THz strain mapping results with finite element model (FEM) and surface strain measurements using digital image correlation (DIC) method highlights the technique's sensitivity to material features such as particle clumping and edge effects, while showcasing strong agreement with FEM and DIC results.</p><p><br></p><p dir="ltr">This analytical model is further expanded for experimentally mapping subsurface stress and strain in the adhesive layer of a single lap shear test. This in-situ non-destructive testing method pioneers the use of THz-TDS for stress estimation in the adhesive layer. Validation through strain mapping of STO doped Araldite 2011 epoxy adhesive with the analytical formulation is presented.</p><p dir="ltr">Finally, THz-TDS is applied for fracture front mapping in a double cantilever beam test with high-density polyethylene bonded with STO doped Araldite 2011. The phase-dependent model for mapping fracture fronts in the sub-surface adhesive layer involves analyzing convoluted waves due to interface resonances in a multi-layer structure using THz-TDS in transmission mode. The technique evaluates changes in dielectrostrictive properties and degree of separation to delineate fracture fronts. THz image enhancement algorithms facilitate crack front delineation. Error analysis on measured crack thickness is conducted to evaluate signal-to-noise ratio for THz-TDS. Additionally, an approach employing THz-TDS measured fracture propagation information for determining sub-surface stress maps in the adhesive layer and computing fracture toughness (G_Ic) is proposed. This work highlights the versatility and efficacy of THz-TDS in material characterization and stress/strain mapping in solid mechanics applications.</p>

Optical fiber sensors have already been developed from the experimental stage to practical applications in the past 20 years. There is no doubt that this technology can bring a wealth of applications, ranging from sensors in medical industry, aerospace and wind-energy industries, through to distributed sensors in oil and gas industry. Among a large amount of physical and chemical parameters which optical fiber sensors could measure, temperature and strain are the most widely studied. This thesis presents several low-cost optical fiber sensor configurations primarily for temperature and strain measurement. Several basic optical fiber components which are good candidates as optical fiber sensors are used in our experiments, such as fiber Bragg gratings (FBGs), multimode fibers (MMFs), small-core dispersion compensation fibers (SCDCFs), high-birefringence fiber loop mirrors (HBFLMs), and polarization-maintaining photonic crystal fibers (PMPCFs). Temperature and strain cross sensitivity is a crucial issue when designing high performance optical fiber sensors, since most of the sensing components are both sensitive to temperature and strain. This would introduce an error when measuring each of them independently. We developed several schemes to overcome this problem by cascading an FBG and a section of MMF, inserting an FBG into an HBFLM, and space division multiplexing two HBFLMs. By measuring the wavelength shifts of the two independent components' spectra in each scheme, simultaneous measurement of temperature and strain could be achieved. However, all the above schemes need optical spectrum analyzers to monitor the spectral information, which increases the cost of the system and limits the operation speed. In order to avoid using optical spectrum analyzers, we use an intensity-based interrogation method with MMFs and HBFLMs as edge filters. By measuring power ratio changes, instead of monitoring spectra shifts, simultaneous measurement of temperature and strain could be realized with a low cost and high speed. The resolutions of the above five configurations are between 0.26 - 1.2 ^oC in temperature and 9.21 - 29.5 με in strain, which are sufficient for certain applications. We also investigate the sensing applications with the SCDCF. Since the cutoff wavelength of this kind of fiber is around 1663 nm, which makes it naturally an MMF in the wavelength range of 1550 nm. By slightly offsetting the core of the SCDCF with respect to that of the standard single-mode fiber (SMF), a high extinction ratio could be achieved with almost 9 dB. When a lateral force (lateral strain) applied on the SCDCF, extinction ratio will decrease. The change of the extinction ratio is almost independent of temperature variation. The measured extinction ratio change has a good quadratic relationship with respect to applied lateral force. This feature could be used to measure lateral force (lateral strain). In addition, we also use this feature to realize simultaneous measurement of both the longitudinal strain and lateral strain, since the applied longitudinal strain results in the whole spectrum shift. Moreover, a miniature high temperature sensor could also be made using the SCDCF. One end of a 4-mm long SCDCF is spliced directly to SMF with the other end cleaved. By monitoring the reflection spectrum of the SCDCF, temperature information could be obtained. This sensing head is very compact and could realize high temperature measurement up to 600 ^oC. Recently, a kind of PMPCF has been found to have very small responses to temperature change. This offers an opportunity to measure other parameters without considering temperature influence. We construct a compact 7-mm long transmission-type sensor with this kind of PMPCF. The interference spectrum generated by the coupling of cladding modes and core mode is obtained by slightly offsetting the PMPCF core to SMF core. The experiment shows that the interference spectrum is almost unchanged within the temperature range of 25-60 ^oC. The presented sensor has the potential to be used to measure strain and refractive index in the normal environment without temperature discrimination for practical applications.