Showing papers in "Strain in 2008"

Speckle Interferometry: A Review of the Principal Methods in Use for Experimental Mechanics Applications

Abstract: With its nearly 40 years of existence, speckle interferometry (SI) has become a complete technique, widely used in many branches of experimental mechanics. It is thus a challenging task to try to summarise in a couple of pages its principal characteristics from both theoretical and practical points of view. Admittedly, even though this goal is not met here, it appeared worth attempting to provide the photomechanics community with a discussion of the ins and outs of the technique. The necessity of a vocabulary free of ambiguity was a prerequisite, and hence the first section is a plea for a clearer definition of the discipline. Moreover, this section offers the opportunity to re-examine the basic aspects of SI. Then, the main features of the method are briefly considered following a strengths, weaknesses, opportunities and threats (SWOT) analysis. Endowed with a lot of specific advantages, compared with other whole-field methods, SI can play an increasing role in photomechanics.

On the Characterisation of Foam and Micro‐lattice Materials used in Sandwich Construction1

Abstract: The paper gives an overview of issues relating to the characterisation of the progressive collapse of core cellular materials used in sandwich construction. The specific structural application addressed is foreign object impact, and in this case the core cellular material is subject to multi-axial stresses, progressive collapse and possible rupture. The paper gives an overview of various theoretical and modelling issues, which are then related to experimental materials and structural tests for model development, calibration and validation. Most discussion concerns polymeric crushable foam, metal foam and metallic lattice structures.

Terahertz Digital Holography

Abstract: Recent years, Terahertz (THz) radiation which occupied an extremely large portion of the electromagnetic spectrum between the microwave and infrared has drawn much attention and interest, since the rapid advances in the femtosecond pulsed laser technology and the rich physical and chemical processes that occur in this region. In past decade, various THz image techniques have been proposed. The THz imaging system based on the CCD camera has also been used to study the property of the object. The THz field distribution passing through the object can be displayed on the computer screen in real-time[l]. Three dimensional (3D) T-ray imaging has also been demostrated[2]. However, all of these technologies are based on the focus plane imaging, the diffraction and dispersion of the THz in the free space have not been considered. Since the wavelenth of the THz radiation is quite longer than the visiable light, the diffraction effect is more serious. Therefore, in order to resolve the exact information about the object, the diffraction and propagation characteristics of THz radiation should be taken into account[3,4]. the and the image reconstruction is necessary.

A New Drop‐Weight Impact Machine for Studying Fracture Processes in Structural Concrete

Abstract: This paper describes the main characteristics of a new drop weight impact machine that has been specifically designed for studying the dynamic mechanical behaviour of structural concrete samples. Such type of equipment has been used to generate simple and measurable fracture processes under moderate to fast loading rates, as opposed to blast chambers, which produce complicated crack patterns that are difficult to analyze. The machine consists of two main parts, the mechanical structure and the data acquisition system. The former is just a hammer, guided by two robust columns, which can impact the specimen with energy up to 7860 J. The latter consists of piezoelectric force sensors, accelerometers and optical fibre photoelectric sensor plus oscilloscopes and signal conditioners. The paper also presents the results of some preliminary tests that show the sensitivity of the work of fracture to the loading rate.

Local Energy Approach to Steel Fatigue

Abstract: This paper presents an experimental protocol developed to locally estimate different terms of the energy balance associated with the fatigue of DP600 steel. The method involves two quantitative imaging techniques. First, digital image correlation provides displacement fields and, after derivation, strain and strain-rate fields. A variational method, associated with an energy functional, is used to simultaneously identify elastic parameter and stress fields. The deformation energy rate distribution can then be determined on the basis of the stress and strain data. Secondly, infrared thermography provides thermal images which are used to separately estimate the thermoelastic source amplitude and mean dissipation per cycle distributions. The image processing uses a local form of the heat diffusion equation and a special set of approximation functions that take the frequency spectra of the sought sources into account.

Beginnings and Early Development of Thermoelastic Stress Analysis

Abstract: Starting with Gough's first recorded observation of the thermoelastic effect in 1803, through to the 1990s, the principal steps in the evolution of the thermoelastic stress analysis (TSA) technique are identified and reviewed. The coverage includes the contributions of Weber, who first described the effect in metals, Kelvin who provided the classical theoretical treatment, Turner and Coker who saw the potential of the effect as the basis of a stress analysis technique, Belgen who demonstrated the value of infrared technology in this context and Mountain and Weber whose foresight and skills resulted in the first commercially available equipment (SPATE) for TSA. Comments on the personal qualities and attributes of those involved are also included.

Determination of Stress Intensity Factor for Cracks in Orthotropic Composite Materials using Digital Image Correlation

Abstract: This paper focuses on the application of the digital image correlation (DIC) technique to determine the stress intensity factor (SIF) for cracks in orthotropic composites. DIC is a full-field technique for measuring the surface displacements of a deforming object and can be applied to any type of material. To determine the SIF from full-field displacement data, the asymptotic expansion of the crack-tip displacement field is required. In this paper the expansion of the crack tip displacement field is derived from an existing solution for strain fields. Unidirectional fibre composite panels with an edge crack aligned along the fibre were tested under remote tensile loading and the displacements were recorded using DIC. The SIF was calculated from the experimental data by fitting the theoretical displacement field using the least squares method. The SIF thus determined was in good agreement with theoretical results and therefore demonstrates the applicability of the derived displacement field and DIC technique for studying fracture in composites.

Bone Characterization using Piezotransducers as Biomedical Sensors

Abstract: This technical note demonstrates the application of piezoelectric ceramic (PZT) transducers for evaluation of structural dynamic parameters of bones. An experimental study conducted on chicken femur is reported. The bone is instrumented with two PZT patches, one serving as an actuator and the other as a sensor. The actuator patch is excited by applying a sinusoidal sweep voltage signal, which in turn excites the bone, the vibrations of which are picked up by the sensor patch. The gain across the sensor patch as a function of frequency serves as the frequency response function (FRF). Structural dynamic parameters of the bone such as modal frequencies and corresponding damping ratios are derived from this FRF. The proposed technique detects changes in the mechanical properties of the bone through changes in the FRF. This technique can be employed in monitoring the healing of critical bones after surgery. It can also aid research related to bone injury, diagnosis of ailments such as osteoporosis and numerous other fields of biomechanics.

Measurement of Surface Shape and Position by Phase-Shifting Digital Holography

Abstract: Phase-shifting digital holography is applied to measure the shapes and positions of rough surfaces from the averaged conjugate product of the reconstructed complex amplitudes, named complex coherence factor, corresponding to dual wavelengths. The phase of the averaged product at the object plane provides the shape of the surface, whereas the peak position of the modulus with respect to reconstruction distance provides the position of the surface. The phase is almost free from speckle noise and easy to be unwrapped. The resultant resolution of surface shape amounts to a few tens of micrometres. The sensitivity of the position measurement that requires no marking on the object is several per cent of object distance from a charge-coupled device (CCD). The method corresponds to the detection of the position of maximum contrast of the fringes to be observed in conventional holographic interferometry. It makes use of fringe phase for shape mea- surement and fringe contrast for position measurement. Both the theory explaining the principle and experimental results are presented.

Calibration of a Speckle Interferometry Full‐Field Strain Measurement System

Abstract: This study describes the calibration of a full-field speckle interferometry strain measurement system using the calibration specimen and protocol defined in the Standardisation Project for Optical Techniques of Strain measurement (SPOTS) standard. The specimen was based on the monolithic embodiment of a four-point bending test and was manufactured from aluminium following the SPOTS design. Strain-gauge rosettes attached to the upper and lower faces of the beam were used to derive two correction factors of an analytical expression that predicted the strains generated in the gauge section of the beam. Following the SPOTS protocol, the comparison of measured and predicted strains yielded two fit parameters and their associated uncertainties for each of three displacement-load steps which indicated the closeness of the data sets. An acceptable calibration was achieved for the single normal component of in-plane strain considered in this study, for each load step employed. For the highest load range, which generated a maximum strain of approximately 810 μstrain in the gauge section, the overall calibration uncertainty was found to be 35.3 μstrain, which in relative terms can be expressed as 2.2% of the strain measurement range for which the instrument was calibrated.

Effect of Strain Rate, Adiabatic Heating and Phase Transformation Phenomena on the Mechanical Behaviour of Stainless Steel

Abstract: The influence of strain rate on the stress–strain behaviour of an AISI 304 austenitic stainless steel sample was investigated. For this purpose, uniaxial tensile tests were performed at room temperature for different strain rates. Microstructural measurements of transformed martensitic phase as a function of plastic strain, and thermal analyses of the specimens were carried out as well. It was found that increasing the strain rate from 10−4 to 10−1 s−1 leads to a 25% improvement in uniform elongation. Moreover, a ‘curve-crossing’ phenomenon was observed for the hardening behaviour measured at different strain rates. These results were rationalized in terms of martensitic phase transformation suppressed by a temperature increase in the specimens deformed with high strain rates.

Friction Coefficient Definition in Compression-fit Couplings Applying the DOE Method

Abstract: The design of the fork-pin compression-fit couplings of front motorbike suspensions is uncertain mainly because of the poor knowledge of the mean coupling pressure p, due to the not symmetric geometry of the fork, and of the first friction coefficient μ11. The axial releasing force F11=μ11p.A, which is the fundamental design parameter, indeed depends on the mentioned two factors, usually unknown. In this paper is presented a generalized methodology which is useful to calculate the μ11 parameter concerning the fork-pin couplings of the front motorbike suspensions. The present production is differentiated by the different material of the two elements in contact, the fork and the pin. The possible combinations are: the fork and the pin in steel, the fork in aluminium and the pin in steel and the fork and the pin in aluminium. In previous works two mathematical models have been defined: the first (Croccolo et al [1]) is useful to calculate the mean contact pressure p in every fork-pin coupling by introducing an overall mathematical function, which is able to correct the theoretical formulas valid only for axial-symmetric elements [2]; the second (Croccolo and Reggiani [3]) is useful to calculate the first friction coefficient μ11, as a function of the production and assembly specifications, in couplings with both the fork and the pin in steel. The fundamental goal of this work is to define a mathematical model useful to calculate the first friction coefficients μ11 for the other two combinations of couplings, aluminium-steel and aluminium-aluminium. The second goal is to update an innovative software (Fork Design©), realized by the authors in Visual Basic® programming language, which is useful to perform the design and the verification of the all fork-pin couplings. The mathematical model for μ11 has been defined through FEM analyses, performed with Ansys 9.0® and applying the Design Of Experiment (DOE) method.

Monitoring of Impacted Aramid‐Reinforced Composites by Embedded PVDF Acoustic Emission Sensors

Abstract: An embedded piezoelectric [poly(vinylidene fluoride) (PVDF)] thin film sensors system for acoustic emission (AE) was realized to investigate the possibility of monitoring, in real time, the post-impact damage in aramid woven fabric-reinforced epoxy. The same sensors have been used in a previous work on similar specimens tested in flexure but not previously impacted, with the aim of verifying the suitability of these sensors to be embedded and their ability to detect AE signals under loading. This work is a continuation of the previous one aiming at evaluating the ability of these embedded PVDF sensors to point out the presence of impact damage, issue widely studied in literature. Aramid fibre/epoxy composite specimens with embedded PVDFs, previously impacted at different energies, namely 5, 10 and 15 J, were tested using three-point bending tests. It appeared from mechanical tests that the flexural strength decreased passing from non-impacted specimens to those impacted with the highest energy and that the embedment of PVDFs in the laminates did not markedly affect the structural integrity of the impacted composites. The degree of impact damage, represented by the decrease in mechanical properties, has been correlated with the AE activity by means of a parametric analysis of the AE signals detected during post-impact mechanical tests.

Analysis of Fatigue Damage under Block Loading in a Low Carbon Steel

Abstract: The fatigue damage accumulation behaviour of the P355NL1 steel is characterised using block loading fatigue tests. First, the constant amplitude low-cycle fatigue behaviour of the P355NL1 steel is evaluated through strain-controlled fatigue tests of smooth specimens. Both fatigue and cyclic elastoplastic behaviours are analysed. Then, block loading is applied to identify the key features of the fatigue damage accumulation phenomena for the P355NL1 steel. The block loading is composed of two distinct low-cycle constant amplitude strain-controlled blocks. The first block is applied for a predefined number of loading cycles, being followed by a second block which is applied until failure. The block loading illustrates that fatigue damage evolves nonlinearly with the number of load cycles as a function of the strain amplitude. These observations suggest a nonlinear damage accumulation rule with load sequence effects. The linear Palmgren–Miner's rule used extensively in design is not verified for the P355NL1 steel. Finally, using the generated experimental data, the cyclic elastoplastic behaviour of the P355NL1 steel is modelled using a continuum plasticity model with nonlinear kinematic hardening, available in the commercial finite element code ansys®.

Application of Digital Image Correlation Method to Study Dental Composite Shrinkage

Abstract: Although resin-based composites are widely used in dental restoration, these materials shrink during polymerisation. Polymerisation shrinkage results in distortion of the restoration and bonded tooth and also generates internal stress at the resin–tooth interface. Digital image correlation (DIC) is used to determine the in-plane displacement field by matching different zones of two characterised pictures. The objective of this study was to examine the applicability of DIC in measuring the deformation of the composite restoration and the surrounding tooth. A preliminary experiment examined the shrinkage of composites in a simulated cavity using the DIC method. The measured shrinkage pattern was consistent with a corresponding finite element model. Subsequently the deformation of composite restorations on human molars was examined using this validated DIC method. The greatest deformation was found on the free occlusal surfaces, and the least on the gingival wall. The increased deformation on the post-cured images indicated that the shrinkage continued even after termination of light activation. DIC method facilitates a full-field measurement of shrinkage profile. These experimental results did not only demonstrate the spatial and temporal relationship of displacement in a dental restoration, but also provide validation of computational models to examine the polymerisation consequence.

Mechanical Characterization of KMPR by Nano‐Indentation for MEMS Applications

Abstract: The ability to create high-aspect ratio structures (HARS) are extremely important for many MEMS sensors and actuators applications such as gyros, accelerometers, and pumps. Traditionally, HARS are fabricated using LIGA [1] and ICP techniques. However, the expensive and the consuming LIGA process is usually unaffordable and the silicon ICP process usually suffers from poor sidewall quality. As a result, the UV based LIGA-Like process becomes a competitive fabrication process for creating HARS in polymeric materials. During the past decade, with the negative photosensitive resin, SU-8, UV LIGA-Like process has successfully demonstrated its capability in shaping HARS. However, SU-8 needs stringent temperature and time control in baking cycle to avoid cracking and potentially, the reliability of the fabricated structure become a concern. Recently, a new negative photosensitive resin, called KMPR™ [2], has been reported to have better characteristics in fabrication and it does not require tight baking control. It has the potential to replace SU-8 in the future. However, to our best knowledge, its mechanical properties are not yet reported and this represents a gap for related MEMS design. The purpose of this paper is therefore to provide specific material properties, namely hardness and elastic modulus of KMPR after different thermal treatments via nano-indentation [3] for MEMS design.

Damage localisation in a stiffened composite panel

Abstract: This work was conducted as part of the Aircraft Reliability Through Intelligent Materials Application (ARTIMA) European Union project. It presents a case study of damage detection in a curved carbon-fibre reinforced panel with two omega stiffeners which was investigated using ultrasonic lamb waves. The statistical technique of outlier analysis was used here as a way of pre-processing experimental data prior to damage classification. Multilayer perceptron neural networks were used here for both classification and regression problems of damage detection. It was then investigated whether using wavelet analysis to perform prior wavelet decompositions of experimental data could facilitate damage classification.

Acoustical and Poromechanical Characterisation of Titanium Scaffolds for Biomedical Applications

Abstract: Biocompatible materials are designed so as to mimic biological materials such as bone as closely as possible. As regards the mechanical aspect of bone replacement materials, a certain stiffness and strength are mandatory to effectively carry the loads imposed on the skeleton. In this paper, porous titanium with different porosities, produced on the basis of metal powder and space holder components, is investigated as bone replacement material. For the determination of mechanical properties, i.e. strength of dense and porous titanium samples, two kinds of experiments were performed – uniaxial and triaxial tests. The triaxial tests were of poromechanical nature, i.e. oil was employed to induce the same pressure both at the lateral surfaces of the cylindrical samples and inside the pores. The stiffness properties were revealed by acoustic (ultrasonic) tests. Different frequencies give access to different stiffness components (stiffness tensor components related to high-frequency-induced bulk waves versus Young's moduli related to low-frequency-induced bar waves), at different observation scales; namely, the observation scale the dense titanium with around 100 μm characteristic length (characterised through the high frequencies) versus that of the porous material with a few millimetres of characteristic length (characterised through the low frequencies). Finally, the experimental results were used to develop and validate a poro-micromechanical model for porous titanium, which quantifies material stiffness and strength from its porosity and (in the case of the aforementioned triaxial tests) its pore pressurisation state.

In-Field Measurement of Forces and Deformations at the Rear End of a Motorcycle and Structural Optimisation: Experimental–Numerical Approach Aimed at Structural Optimisation

Abstract: The present study concerns the analysis of the asymmetric displacement behaviour of the rear part of a motorcycle. Stylistic reasons led to the design of a vehicle with only one suspension located on the left-hand side. Experimental tests performed on a circuit with seven obstacles along a straight line confirmed that the bending displacement is higher on the right-hand side than on the left. This study aimed to perform a structural optimisation of the components involved at the rear end of the motorcycle, to find a solution to the problem. A hybrid approach is applied: the force acting on the suspension and bending displacements at the rear end were simultaneously measured in working conditions; a finite-element method model was then set up, validated and applied for design optimisation purposes. Both methodological aspects and applicative results are presented and discussed. Finally, a solution in accordance with design specifics is proposed.

Test Conditions Effect on the Fracture Toughness of Hollow Glass Micro‐sphere Filled Composites

Abstract: Low-density sheet moulding compounds based in hollow glass micro-spheres are being increasing used namely in automotive industry where can present advantages compared with traditional metal, such as: lower weight, less expensive for low volume production in consequence of lower tool costs, no corrosion effects, a more design freedom, etc. These materials are usually classified as syntactic foams if the filler content is relatively high. Syntactic foams are potential good materials for applications where impact loads occurs once they are able to reduce impact force [1,2]. The addition of hollow micro-spheres trends to reduce the Young modulus and ultimate strength [2,3] and even the specific values are only increased in terms of impact force and marginally in flexural modulus for high volume fractions of micro-spheres [2].

An Approach to Analyse Errors Introduced in the Random Grid Strain Measurement Method

Abstract: The present work refers to the errors imposed by the recently introduced random-grid mesh-free full field strain measurement method. Excluding systematic errors of the digital camera, the method itself is not an error-free procedure. A possible cause of errors could be the misplacement of the spot-centre (centroid) with regard of the spot boundaries. Another cause of errors is the limited order of approximation in the field function. A third one, emerges from the so-called ‘sub-pixel effect’. This kind of error is difficult to trace, so direct comparison between the results of the method and exact solutions is required. In the present work, proper analytical or numerical derivations of those errors are presented and reasonable upper limits are estimated. Finally, numerical and experimental examples are presented to demonstrate the accuracy of the method.

Evaluation of Residual Stresses During Fatigue Test in an FSW Joint

Abstract: At present, friction stir welding (FSW) represents one of the most interesting techniques in the field of welding. The process is has been implemented in industrial practice for joining aluminium alloys, while the welding of the titanium alloy and the steels is still primarily in a developmental stage.

Energy Balance of a Semicrystalline Polymer During Local Plastic Deformation

Abstract: This paper, in a first part, presents the characteristics of a new experimental set-up using digital image correlation (DIC) and infrared thermography (IRT). In a second part, the kinematical data, obtained by DIC, are used to track the material surface element temperature variations deduced from thermal images. They are then combined to construct the local energy balance. To illustrate the interest of such an approach, the paper then describes the calorimetric effects accompanying the propagation of necking in a plasticised PolyAmide 11. A thermodynamic analysis of cyclic loading finally aims to show the existence of an entropic elastic effect generally associated with rubber-like materials.

Fabrication of Cu Nanowires at Predetermined Positions by Utilising Stress Migration

Abstract: Stress migration (SM) is a phenomenon whereby atoms diffuse from a region of lower stress towards a region of higher stress because of the hydrostatic stress gradient. In this study, the fabrication of Cu nanowires at predetermined positions by controlling the direction of the atomic flux caused by SM is analysed. Cu nanowires are fabricated by rearranging accumulated atoms into wire-like crystals. First, the sites at which Cu atoms accumulate are found from finite element (FE) analysis and from the results obtained from experiments. Next, a technique is proposed for accumulating atoms by SM at predetermined sites. Finally, the successful fabrication of Cu nanowires at these sites is demonstrated.

Full-Field Vibration Measurement by Time-Average Speckle Interferometry and by Doppler Vibrometry - a Comparison

Abstract: Identification of dynamic material properties, non-destructive testing and study of vibroacoustic behaviour of different structures impose complex, pointwise and full-field measurements. Among other techniques, optical non-contact techniques are representing today the favourite choice since they do not add any mass to the structure under test. When the range of vibration amplitudes is small, most of these techniques are based on interferometric principles.

Strain Measurement of Nickel Thin Film by a Digital Image Correlation Method

Abstract: Digital image correlation (DIC) is a whole-field and non-contact deformation measuring method. It could provide deformation information of a specimen by processing two digital images that are captured before and after the deformation. In 1982 Peters and Ranson [1] first employed digital image correlation for displacement and strain measurement under the assumption that there is a one-to-one correspondence on the intensity pattern of surface images before and after deformation. Chu et al. [2] utilized a cross-correlation coefficient as an objective function and measured the rigid body translation, rigid body rotation, and uniaxial uniform strain. Initially, the searching method was a coarse-fine iterative technique, in which the searching range was progressively reduced until the stop criterion was satisfied. Newton-Raphson method was later applied to search the six deformation components [3].