Residual stresses are “locked-in” within a material, and exist without any external loads. Such stresses are developed during most common manufacturing processes, for example welding, cold working and grinding. These “hidden” stresses can be quite large, and can have profound effects on engineering properties, notably fatigue life and dimensional stability. To obtain reliable and accurate residual stress measurements for uniform and non-uniform stress states, a novel and practical method using crossing-slitting and ESPI is presented here. Cross-slitting releases all three in-plane stress components and leaves nearby deformation areas intact. The ESPI (Electronic Speckle Pattern Interferometry) technique gives an attractive tool for practical use, because measurements provide a large quantity of useful data, require little initial setup and can be completed rapidly and at low per-measurement cost. A new ESPI setup consisting of shutter and double-mirror device is designed to achieve dual-axis measurements to balance the measurement sensitivities of all in-plane stress components. To evaluate data quality, a pixel quality control and correction procedure is also applied. This helps to locate bad data pixels and provides opportunities to correct them. The measurement results show that this procedure plays an important role for the success of residual stress evaluation. Based on the observed displacement data and finite element calculated calibration data, an inverse computation method is developed to recover the residual stresses in a material for both uniform and non-uniform cases. By combining cross-slitting and ESPI, more reliable results for the three in-plane residual stress components can be obtained.

Residual stresses are “locked-in” within a material, and exist without any external loads. Such stresses are developed during most common manufacturing processes, for example welding, cold working and grinding. These “hidden” stresses can be quite large, and can have profound effects on engineering properties, notably fatigue life and dimensional stability. To obtain reliable and accurate residual stress measurements for uniform and non-uniform stress states, a novel and practical method using crossing-slitting and ESPI is presented here. Cross-slitting releases all three in-plane stress components and leaves nearby deformation areas intact. The ESPI (Electronic Speckle Pattern Interferometry) technique gives an attractive tool for practical use, because measurements provide a large quantity of useful data, require little initial setup and can be completed rapidly and at low per-measurement cost. A new ESPI setup consisting of shutter and double-mirror device is designed to achieve dual-axis measurements to balance the measurement sensitivities of all in-plane stress components. To evaluate data quality, a pixel quality control and correction procedure is also applied. This helps to locate bad data pixels and provides opportunities to correct them. The measurement results show that this procedure plays an important role for the success of residual stress evaluation. Based on the observed displacement data and finite element calculated calibration data, an inverse computation method is developed to recover the residual stresses in a material for both uniform and non-uniform cases. By combining cross-slitting and ESPI, more reliable results for the three in-plane residual stress components can be obtained.

Residual stresses are “locked-in” within a material, and exist without any external loads. Such stresses are developed during most common manufacturing processes, for example welding, cold working and grinding. These “hidden” stresses can be quite large, and can have profound effects on engineering properties, notably fatigue life and dimensional stability. To obtain reliable and accurate residual stress measurements for uniform and non-uniform stress states, a novel and practical method using crossing-slitting and ESPI is presented here. Cross-slitting releases all three in-plane stress components and leaves nearby deformation areas intact. The ESPI (Electronic Speckle Pattern Interferometry) technique gives an attractive tool for practical use, because measurements provide a large quantity of useful data, require little initial setup and can be completed rapidly and at low per-measurement cost. A new ESPI setup consisting of shutter and double-mirror device is designed to achieve dual-axis measurements to balance the measurement sensitivities of all in-plane stress components. To evaluate data quality, a pixel quality control and correction procedure is also applied. This helps to locate bad data pixels and provides opportunities to correct them. The measurement results show that this procedure plays an important role for the success of residual stress evaluation. Based on the observed displacement data and finite element calculated calibration data, an inverse computation method is developed to recover the residual stresses in a material for both uniform and non-uniform cases. By combining cross-slitting and ESPI, more reliable results for the three in-plane residual stress components can be obtained. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

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The objectives of this research are to measure residual stress in both unblasted and sandblasted mild steel specimens by using three different techniques: X-ray diffraction (XRD), strain-gage hole drilling (SGHD), and electronic speckle pattern interferometry (ESPI) hole drilling, and to validate the new ESPI hole drilling method by comparing its measurement results to those produced by the SGHD method. Both the XRD and SGHD methods were selected because they are accurate and well-verified approaches for residual stress measurements. The ESPI hole drilling technique is a new technology developed based on the SGHD technique, without the use of strain gage. This technique is incorporated into a new product referred to as the PRISM system, manufactured by Hytec, Incorporated, in Los Alamos, New Mexico. Each method samples a different volume of material at different depths into the surface. XRD method is especially different compared to the other two methods, since XRD only measures stresses at a depth very close to the surface (virtually zero depth). For this reason, no direct comparisons can be made between XRD and SGHD, as well as between XRD and ESPI hole drilling. Therefore, direct comparisons can only be made between SGHD and ESPI hole drilling methods.

The residual stress profile is a major factor on the fatigue life of components that are subjected to cyclic loading. In order to measure these stresses x-ray diffraction (XRD) is commonly used. The penetration depth of x-rays is limited for this method and thus, it must be combined with material removal to determine in-depth stress profiles.At SKF Manufacturing Development Center (SKF MDC), where the work for this thesis was carried out, the current layer removal method is restricted to a depth of 0.5 mm. Consequently, an additional method of material removal is necessary to obtain information at greater depths. The purpose of this thesis was to investigate possible material removal techniques that can be implemented with XRD measurements. Two different material removal techniques were studied; electrochemical etching and milling in combination with electrochemical etching. The electrochemical etching equipment was developed at SKF MDC prior to this thesis but needed further testing and validation. The residual stress profiles of induction hardened cylinders were studied using the two different removal techniques combined with XRD measurements and the results were compared with stresses measured by Electronic Speckle Pattern Interferometry (ESPI) with hole drilling. In addition, the results were compared with simulations performed at SKF MDC India. It was concluded that both the material removal methods could be successfully combined with XRD measurements. However, for practical reasons the methods should be refined before being implemented on a regular basis. Unfortunately, poor correlation between XRD and ESPI measurements were obtained due to reasons not fully understood.

There has been widespread interest in corneal biomechanics over recent years, driven largely by the advancements in, and the popularity of refractive surgery techniques and subsequent concerns over their safety. Lately there has been interest into whether crosslinking, which is currently used for the treatment of keratoconus, could be developed as a minimally invasive technique to change the refractive power of the cornea by selectively changing the corneal biomechanics in specific regions to induce a shape change. Successful application of this technique requires a detailed understanding of corneal biomechanics and so far, little is known about the biomechanics of this complex tissue. The current lack of understanding can be mostly attributed to the absence of a suitable measurement technique capable of examining the dynamic behaviour of the cornea under physiological loading conditions. This thesis describes the development of a novel full-field, ex vivo, measurement method incorporating speckle interferometric techniques, to examine the biomechanics of the cornea before and after crosslinking in response to hydrostatic pressure fluctuations representative of those that occur in vivo during the cardiac cycle. The eventual measurement system used for the experiments detailed in this thesis incorporated; an Electronic Speckle Pattern Interferometer (ESPI), a Lateral Shearing Interferometer (LSI) and a fringe projection shape measurement system. The combination of these systems enabled the 3-dimensional components of surface displacement and the 1st derivative of surface displacement to be determined in response to small pressure fluctuations up to 1 mmHg in magnitude. The use of both ESPI and LSI together also enabled the applicability of LSI for measurement of non-flat surfaces to be assessed, and limitations and error sources to be identified throughout this work. To enable the measurement of corneal biomechanics, part of this thesis was concerned with the design of a bespoke loading rig. A chamber was designed that could accommodate tissue of both porcine and human origin. This chamber was linked to a hydraulic loading rig, whereby the cornea could be held at a baseline pressure representative of normal intraocular pressure and small pressure variations could be introduced by the automated vertical movement of the reservoir supplying the chamber. Experiments were conducted on a range of non-biological samples with both flat and curved surface topography, and both uniform and non-uniform mechanical properties, to determine if the measurement configuration was giving the expected measurement data and the loading rig was stable and repeatable. Following experiments on non-biological samples, a range of experiments were conducted on porcine corneas to develop a suitable testing methodology and address some of the challenges associated with corneal measurement, including transparency and hydration instability. During these investigations, a suitable surface coating was identified to generate an adequate return signal from the corneal surface, while not interfering with the response. Alongside this, the natural variation in the response of the cornea was investigated over the total experimental time, and a range of data was presented on corneas before and after crosslinking, which confirmed the suitability of the measurement methods for the assessment of crosslinking. Ultimately, a small sample size of six human corneas were investigated before and after crosslinking in specific topographic locations. From the experiments on human and porcine corneas, full-field maps of surface deformation have been presented, and a compliant region incorporating the peripheral and limbal areas has been identified as being fundamental to the response of the cornea to small pressure fluctuations. In addition to this, the regional effects of crosslinking in four different topographic locations on corneal biomechanics have been evaluated. From this, it has been demonstrated that crosslinking in specific regions in isolation can influence the way the cornea deforms to physiological-scale fluctuations in hydrostatic pressure and this could have implications for refractive correction.

Residual stresses are one of the major challenges in additive manufacturing of metallic materials today. They are induced during manufacturing because of the rapid local heating and cooling and may cause distortions, cracking and delamination between layers. The magnitude of the residual stresses depends on factors such as manufacturing technology, material, part geometry, scanning strategy, process parameters, preheating temperature and density of the component. In the present work, the magnitudes of residual stresses and distortions in austenitic stainless steel 316L components manufactured by Selective Laser melting have been investigated. Four specimens with rectangular cross-sections where produced with different heights and wall thicknesses. The residual stresses were measured by two different methods, x-ray diffraction and Electronic Speckle Pattern Interferometry (ESPI) incremental hole drilling in order to see how well they correspond to each other. The results showed peaks of high tensile stresses in the building direction in all specimens. The magnitudes of stresses were similar in all four specimens. The largest distortions occurred when the wall thickness was increased, and the height was reduced. It was also shown that the measurements made by x-ray diffraction and ESPI incremental hole drilling were not consistent with each other. The latter showed unrealistically high measurements near the surface.

Non-destructive testing (NDT) of glass-fibre reinforced polyester (GRP) composite materials has been becoming increasingly important due to their wide applications in engineering components and structures. Electronic Speckle Pattern Interferometry (ESPI) has promising potential in this context because it is a non-contact, whole-field and real-time measurement system. This potential has never been fully exploited and there is only limited knowledge and understanding available in this area. This reality constrains the wide popularity and acceptance of ESPI as a novel NDT technique. Therefore it is of considerable importance to develop an understanding of the capability of ESPI with respect to damage evaluation in GRP composite materials. The research described in this thesis is concerned with an investigation into the applicability of ESPI in the NDT of GRP composite materials. Firstly, a study was carried out to determine excitation techniques in terms of practicality and effectiveness in the ESPI system. Three categories of defects were artificially introduced in GRP composite materials, namely holes, cracks and delaminations each with different geometrical features. ESPI was then employed to evaluate the three kinds of defects individually. It has been found that cracks and holes on back surfaces can be defined when the technique is used in conjunction with thermal excitation. Internal Temperature Differential (ITD) induced fringe patterns were more efficient than External Thermal Source (ETS) induced fringe patterns with regard to detecting the presence of holes and cracks. In the case of delamination, ESPI was found to be capable of detecting the damage when used in combination with mechanical excitation originating from a force transducer hammer. The geometrical features and magnitudes of delaminations were also established as being quantifiable. The validation of ESPI as an NDT technique was carried out in an attempt to establish a better understanding of its suitability and have more confidence in its applications. Four damaged specimens were Subjected to ESPI examination in conjunction with visual inspection, ultrasonic C-scan and sectioning techniques. The geometrical features and magnitudes of damage evaluated using ESPI showed a good correlation with those evaluated by conventional techniques. Poor visibility and readability is an inherent problem associated with ESP! due to an overlapping between the noise and signal frequencies. An improvement of image quality is expected in an attempt to achieve a wide acceptance of ESPI as a novel NDT technique. It has also been demonstrated that this problem can be tackled using optical phase stepping techniques in which optical phase data can be extracted from the intensity fringes. A three-frame optical phase stepping technique was employed to produce the "wrapped" and "unwrapped" phase maps which are capable of indicating internal damage with high visibility and clarity. Finally ESPI was practically employed to evaluate damage in GRP composites introduced by quasi-static and dynamic mechanical loading. It was found that ESP! was capable of monitoring the progressive damage development of specimens subjected to incremental flexural loading. The initial elastic response, damage initiation, propagation and ultimate failure of specimens were clearly characterised by the abnormal fringe pattern variations. In a similar manner, ESPI was employed to evaluate the low velocity falling weight impact induced damage. A correlation was established between the magnitude of damage and the impact event parameters as well as the residual flexural properties.