Kelvin H. K. Chan

Speckle referencing: digital speckle pattern interferometry (SR- DSPI) for imaging of non-diffusive surfaces

Optical metrology has been widely employed as a key technique for modern industrial production, owing to its fast, precise and non-invasive measurement. Digital speckle pattern interferometry (DSPI) is one of these non-destructive testing methods that possess the abilities to measure surface deformation, vibration and profile. However, one of the challenges with DSPI is the incapability to address the imaging of non-diffusive surface, owing to the failure to form speckle pattern. In this paper, we demonstrate a modified DSPI system used for non-diffusive surface measurement. Experiment has been carried out to validate this modified DSPI by using metal-alloy surface as testing sample. The speckle fringe pattern generated by applying an external load was analyzed to obtain the 3-D surface deformation parameters.

Characterization and optimization of illumination vector for contouring surface form and feature using DSPI

Surface defect or damage is one of the critical factors leading to the failure of engineering materials and structures. The methodologies for the measurement of surface shape and feature or defect have been extensively explored and developed over the past few decades, including both contact and non-contact methods. Speckle pattern interferometry, as a non-contact optical method, has been demonstrated to effectively contour the surface shape through adjusting the illumination vector. However, few studies have been made to investigate the effect of the initial position of the illumination source as well as the source translation direction. In this paper, we report to carry out a study of measuring the surface form and feature using digital speckle pattern interferometry system via a slight translation of illumination source. Through theoretically analyzing the sensitivity factor along with the experimental validation, it is shown that the contouring fringe is more sensitive to the surface height with an off-axis illumination than the paraxial illumination. It is also found that translating the source along axial and lateral direction can be both used for the surface shape re-construction.

Surface topography and the impact on fatigue performance

Areal characterization was applied to plain fatigue specimens manufactured from a nickel-based superalloy, Alloy 720Li, to determine the impact of machined/finished surface topography on fatigue performance of this material. Samples were subjected to fatigue testing in the as-turned and shot peened conditions to study the interaction between residual stresses and topography in influencing the fatigue performance. The turning process was deliberately manipulated to produce three distinct finishes which were subsequently given an identical shot peening, resulting in six grades of surface topography. Surface topography was found to influence fatigue even in the presence of peened compressive residual stresses by promoting crack initiation at valley sites. Both the roughness amplitude and spatial characteristics of the surface were found to be important when correlating to fatigue performance.

Surface Roughness, Areal Topographic Measurement, and Correlation to LCF Behavior in a Nickel-Based Superalloy

Surface roughness often determines fatigue performance of advanced engineering components making definition of this parameter essential subsequent to manufacture. Traditionally, topography measurements employ an average amplitude parameter, Ra, obtained from a two-dimensional contact measurement. This parameter, however, is highly localised making it relatively unreliable. This study attempts to correlate areal (3D) topographic, measurements with the low cycle fatigue (LCF) performance defined for a nickel-based superalloy (Waspaloy). Three different surface finishes, namely longitudinal polished, circumferential ground, and longitudinal ground were applied to fatigue specimens. The height and orientation of the topographic features with respect to the loading axis were found to affect LCF performance. Results indicate a close correlation between cycles to failure and the maximum height (Sz) and ten-point height (S5z) parameters. A power fit to account for the topographic effect was generated based on the experimental data.

Experimental investigations and parametric studies of surface roughness measurements using spectrally correlated speckle images

The surface roughness parameters encoded in a speckle pattern can be effectively extracted through correlation experiments. In the case of spectrally correlated speckle images, the degree of decorrelation arises from wavelength difference in the laser light irradiated on the surface. To obtain accurate results in such methodology, a proper design of experiments is important due to more than one parameter involved in the experiment. Here, experimental investigations and parametric studies of surface roughness measurements using spectral speckle correlation methodology are presented, considering the potential variables in the system. The sources of error and factors affecting the accuracy in measurement are identified and the experimental results obtained from standard calibration plate samples are presented.

Intensity and contrast based surface roughness measurement approaches for rough and shiny surfaces

In manufacturing engineering the surface finish of a machined component is of fundamental importance in order to ensure its performance. A non-contact and non-destructive device based on optical technique, is a promising alternative to stylus based device for carrying out measurement of surface quality. In addition to this, in situ monitoring of surface roughness on a workpiece is an important requirement in modern machining process, since it would increase on-line machining rate and consequently productivity. Here, measurement approaches and system configuration for surface roughness measurement using laser speckle intensity and contrast are discussed. The technique would allow full-field measurement over sample of interest having both rough and shiny surface properties. Measurement data on standard calibration plates is presented with details on the measurement accuracy and reliability.

Fiber optic probe for surface roughness measurement of ALM samples based on laser speckle intensity

Additive layer manufacturing (ALM) is a modern 3D fabrication process for manufacturing parts with complex geometries. However, the challenge lies in the development of a suitable metrology system for the evaluation of internal surfaces of these geometrically complex ALM parts. Research presented in this manuscript discusses the design, development and demonstration of an in-house developed speckle metrology system employing specialty optical fibers to measure the surface roughness. Vibratory finishing is used to finish the surfaces of the test specimen with four distinct surface roughness (Ra) values ranging between 5μm to 20μm in steps of 5μm. A simple speckle intensity analysis method is used to evaluate surface roughness based on the speckle images acquired by the developed probe. The obtained speckle image intensity values are calibrated against the roughness values measured using a traditional stylus based metrology device (Mitutoyo SJ-301 Surface Tester) to obtain a calibration factor. This enabled the extraction of surface roughness values of unknown test samples. Since the developed probe is flexible, miniature and all fiber based, it is envisaged that this method can be suitably modified for investigation and evaluation of surface finish parameters of such 3D printed components with complex geometries which are generally challenging.

The Role of Surface Topography on Fatigue Behaviour of Nickel Based Superalloys

Advanced areal (three-dimensional) characterisation of surface topography was applied to laboratory scale fatigue test specimens manufactured from the nickel based superalloy Alloy720Li. Finishing was deliberately manipulated to offer four distinct grades of topography. Subsequent low cycle fatigue performance was then correlated to a range of parameters selected to represent the surface topography. The aim of the ongoing study is to predict fatigue performance and aid to establish correlations between topographic parameters and fatigue life.