Showing papers in "Strain in 2012"

Artificial Accelerated Ageing of GFRP Pultruded Profiles Made of Polyester and Vinylester Resins: Characterisation of Physical–Chemical and Mechanical Damage

Abstract: This article presents results of experimental investigations of the durability of glass-fibre-reinforced polymer (GFRP) pul- truded profiles exposed to typical environments of civil engineering applications. Specimens obtained from commercial GFRP profiles made of unsaturated polyester and vinylester resins were subjected to immersion in (i) demineralised water and (ii) salt water at 20 � C, 40 � C and 60 � C for up to 18 months, (iii) continuous condensation at 40 � C for up to 9 months and (iv) accelerated ageing in a QUV chamber for up to 3000 h. The effects of such exposure conditions on both types of profiles were analysed in what concerns their (i) mass changes, (ii) viscoelastic response, evaluated by means of dynamic mechanical analysis (DMA), (iii) mechanical response in tension, bending and interlaminar shear, and (iv) chemical changes, assessed through Fourier transformed infrared spectroscopy (FTIR). Hygrothermal ageing had significant influence on the material performance, namely on the mechanical response - demineralised water immersion caused a higher level of degradation, compared with salt water immersion, and results show a clear competition between moisture-induced plasticisation and residual post-curing of the composite matrix. Following QUV exposure, although considerable aesthetical changes were observed, the viscoelastic response and mechanical performance of both profiles were not remarkably affected, confirming that UV radiation affects essentially the outermost layers of GFRP profiles. In general, the GFRP profile made of vinylester resin exhibited better durability perfor- mance, when compared with its polyester counterpart.

High-Resolution Strain Mapping Through Time-of-Flight Neutron Transmission Diffraction with a Microchannel Plate Neutron Counting Detector

Abstract: Conventional neutron radiography can be strongly enhanced by obtaining Bragg-edge information spatially correlated with the attenuation coefficient. This can now be achieved through time-of-flight techniques at pulsed neutron sources, utilising a neutron counting detector with high-spatial and high-temporal resolution. In these measurements, the positions of Bragg edges can in principle be obtained for each 55 × 55 μm2 pixel of the radiographic image. The combination of both Bragg-edge and attenuation information enables high spatial resolution studies to be carried out on material composition, phase transitions, texture variations, as well as residual strain mapping. In this article, we present the results of high-resolution strain maps of a ferritic steel cantilever sample measured at different loads by both transmission and conventional diffraction modes, as well as strains in an austenitic steel compact-tension (CT) crack sample. The proof of principle experiments performed on the ENGIN-X beamline on a bent cantilever arrangement resulting in a uni-axial stress field verified that the strain values measured in diffraction and transmission mode are in good agreement. The characteristics of the transmission mode detector as well as the measured strain maps and future possibilities of this technology are discussed.

Ultra-High-Speed Full-Field Deformation Measurements on Concrete Spalling Specimens and Stiffness Identification with the Virtual Fields Method

Abstract: For one decade, spalling techniques based on the use of a metallic Hopkinson bar in contact with a concrete sample have been widely employed to characterise the dynamic tensile strength of concrete at strain rates ranging from a few tens to hundreds of s-1. However, the processing method based on the use of the velocity profile measured on the rear free surface of the sample (Novikov formula) remains quite basic. In particular, the identification of the whole softening behaviour of the concrete material is currently out of reach. In the present paper, a new processing technique is proposed based on the use of the virtual fields method (VFM). First, a digital ultra-high-speed camera is used to record the pictures of a grid bonded onto the specimen. Then, images of the grid recorded by the camera are processed to obtain full-field axial displacement maps at the surface of the specimen. Finally, a specific virtual field has been defined in the VFM equation to use the acceleration map as an alternative ‘load cell’. This method applied to three spalling tests with different impact parameters allowed the identification of Young’s modulus during the test. It was shown that this modulus is constant during the initial compressive part of the test and decreases in the tensile part when microdamage exists. It was also shown that in such a simple inertial test, it was possible to reconstruct average axial stress profiles using only the acceleration data. It was then possible to construct local stress–strain curves and derive a tensile strength value.

Investigation of the Uncertainty of DIC Under Heterogeneous Strain States with Numerical Tests

Abstract: In this research, numerical 2D digital image correlation (DIC) tests are carried out to assess the uncertainty of DIC under heterogeneous strain states. DIC is implemented to measure the deformation of the numerically deformed images with respect to the undeformed counterparts, which are taken from the real tensile specimens. The tensile specimens are made of three materials, i.e. steel DC06, steel DX54D+Z, and aluminium alloy Al6016, and cut into three different geometries, namely one standard design and two complex designs. The specimens are all painted manually with random speckle patterns. The original images are deformed by imposed displacement fields, which are obtained by simulating uni-axial tensile tests of the specimens with finite element analysis (FEA). In this way, the errors sourcing from the hardware of the image system are excluded. According to the geometries of the specimens, homogeneous and heterogeneous strain states are achieved by FEA. The optimum mesh sizes of the models are identified in order to minimize theirs influence on the imposed fields. The impacts of sub set sizes, step sizes and strain window sizes are studied for an optimum correlation. Finally, the influence of the strain state is in vestigated. It is found that the DIC accuracy and precision decrease under highly heterogeneous strain states.

Compressive Viscoplastic Response of 6082‐T6 and 7075‐T6 Aluminium Alloys Under Wide Range of Strain Rate at Room Temperature: Experiments and Modelling

Abstract: In this work, the viscoplastic behaviour of 6082-T6 and 7075-T6 aluminium alloys is examined over a wide range of strain rates. Three different testing techniques were applied to this investigation: low-rate experiments were performed using a regular servo-hydraulic testing machine, high-rate tests were conducted using a split Hopkinson bar apparatus and very-high-rate experiments were carried out using a miniaturised direct impact test arrangement. The latter testing set-up allowed for the characterisation of material flow at strain rates up to . These experimental results showed a sharp increase in the rate sensitivity of the materials once a threshold loading rate of is exceeded. This behaviour may be attributed to the presence of viscous drag on high-velocity dislocation motion. In addition, the thermo-viscoplastic behaviour of the 6082-T6 and 7075-T6 aluminium alloys was analytically described using the extended Rusinek–Klepaczko model of viscous drag effects. Satisfactory correlation was observed between the experiments and the constitutive model results over the entire range of strain rates studied,

Strain Measurement by Digital Image Correlation: Influence of Two Types of Speckle Patterns Made from Rigid or Deformable Marks

Abstract: For both 2D and 3D measurements by correlation techniques, speckle patterns can be composed of rigid particles (RP). In this paper, a study of the influence of these RP on digital image correlation (DIC) strain error assessment is performed experimentally and numerically. Firstly, it is shown that prior to the influence of pattern, it is necessary to have an appropriate calculation strategy of initial values for DIC iterative process. Then for simulated images, the RP pattern presents a similar behaviour as the one of a deformable pattern to which a noise is added. However, for experimental images, this noise is less predominant on error assessment than the one because of deteriorations of patterns for large strains.

Shear Modulus Determination of Cuboid Metallic Open-Lattice Cellular Structures by Analytical, Numerical and Homogenisation Methods

Abstract: The shear response of irregular open-lattice cellular cores made of interconnected metallic struts is analysed, and the core shear modulus in the three material principal directions is derived. The analytical approach is based on the technical beam theory, which is used for the determination of the unit-cell response under complex loading. The influence of the strut geometrical parameters and unit-cell shape and size on the cellular core shear stiffness is determined. The analytical determination of the unit-cell properties is successfully validated by a reference numerical model of the unit cell, which is developed for comparison purposes. Furthermore, the homogenisation principles are applied to the prediction of the shear response of a core block structure comprising a high number of unit cells, for which experimental results were available; the comparison revealed that the experimental results coincide well with the results obtained from the homogenised model.

Fatigue Damage of the Gesso Layer in Panel Paintings Subjected to Changing Climate Conditions

Abstract: Numerical modelling was used to follow the moisture movement and strain in a composite system – an unrestrained, single wood panel coated with a layer of gesso, in response to cyclic sinusoidal variations in relative humidity (RH). The allowable magnitude of the variations, below which physical damage of the gesso layer on the wood does not occur over a selected time of exposure, was derived as a function of cycle duration, panel thickness and moisture diffusion configuration. The dimensional response of wood substrate becomes subject to restraint by the applied layer of gesso. The panels do not respond significantly to diurnal fluctuations or shorter irrespectively of the panel thickness. The panels respond more and more significantly when the duration of the fluctuations increases until a certain critical period at which the panel fully responds to each cycle. The analysis of the data obtained indicates that moderate RH variations within the approximate range 50 ± 15% are safe. This safe range was derived using the extremes of conservative criteria of the gesso’s fatigue fracture and assumption of worst-case wooden substrate response. The reduction of allowable amplitude of RH cycles because of decrease in the gesso’s modulus of elasticity and thickness is discussed.

Fracture Behaviour Investigation into a Polymer‐Bonded Explosive

Abstract: Polymer-bonded explosive (PBX) is used widely in weapon systems. Failure of PBX caused by mechanical damage is one of the sources of accidental ignitions. A brittle crack of PBX produces local heating, creating a ‘hot spot’ finally. Investigation into the tensile fracture behaviour of PBX is one of the main works to determine the failure mechanism. Although many researchers have carried out the quasi-static Brazilian test to understand the damage evolution of PBX, the fracture feature of PBX under dynamic impact is rarely reported. In this article, dynamic Brazilian tests were conducted. A single-pulse loading apparatus was used to ensure that specimen was loaded only once during a dynamic Brazilian test. High-speed camera, digital image correlation and micro-observation techniques were adopted for strain measurement and microfracture observation. All the dynamic tensile crack exhibits transgranular fracture, which indicates more heat would released by the propagation of crack and more friction between fractured crystal surfaces. On the basis of the theories of interface debonding and transgranular fracture, larger crystals are more prone to crack, whereas smaller crystals simply debond with neighbouring binders. Discrete element method simulation results show that specimen with interface debonding microcracks was able to sustain additional load until transgranular fracture begins.

Study on Fracture Behaviour of a Polymer‐Bonded Explosive Simulant Subjected to Uniaxial Compression Using Digital Image Correlation Method

Abstract: Quasi-static uniaxial compression experiments were conducted on a polymer-bonded explosive (PBX) simulant. At macroscale, the deformation and fracture process of samples were recorded using a charge-coupled-device camera. Microscopic examination was conducted to in situ observe the deformation and fracture processes of samples using SEM equipped with a loading stage. Microscopic damage modes, including interfacial debonding and particle fracture, were observed. The digital image correlation (DIC) technique was used to calculate the recorded images, and the macroand micro-scale displacement and strain fields were determined. Crack initiation, crack propagation, fracture behaviour and failure mechanism of samples were studied. The effects of aspect ratios on fracture behaviour and failure mechanism of PBX simulant were analysed.

Bond Analysis of Timber Structures Strengthened with FRP Systems

Abstract: This study focuses on the adhesion analysis of glued joints between Norway Spruce and Carbon fibre reinforced polymer (CFRP) regarding timber reinforcement. An experimental programme was carried out on two different test series to evaluate the behaviour of timber-CFRP bonded joints. ‘Pull-Off’ tests were achieved to evaluate the strength of the adhesion connection between materials. Two strengthening techniques [externally bonded reinforcement (EBR), near-surface mounted (NSM)] and several composite systems (laminate, sheets and textile) were analysed. Through ‘Four Point Bending’ tests, the adhesion behaviour of timber-CFRP joints along the fibres direction was evaluated. Three trials were performed on models strengthened with different criteria (one EBR and two NSM series), each with three different bonding lengths. The maximum anchor strength of the composite, the effective bonding length between the materials, the medium shear strength developed along the interface and the maximum composite strain for the EBR and NSM reinforcements were obtained. The experimental results were compared with equivalent values from concrete-CFRP joints found in literature. Furthermore, the theoretical model proposed in Fib Bulletin 14 was calibrated to predict the maximum anchor strength of the composite according to its bonding length. Finally, some recommendations are proposed for the design of timber structures reinforced with CFRP systems.

Response of Wood Supports in Panel Paintings Subjected to Changing Climate Conditions

Abstract: The finite element method was used to model the moisture movement and strain in the wood supports of panel paintings, in response to changing climate conditions – temperature and relative humidity (RH). The material properties of lime wood (Tilia sp.), determined experimentally, were used in the modelling. Critical amplitudes of cyclic sinusoidal RH fluctuations generating strain of 0.002 in the most responsive tangential direction of the unrestrained, single wood panel, which the pictorial layer was assumed to endure without damage, were derived for the mid-RH region as a function of cycle duration, panel thickness and diffusion configuration. Panels do not respond significantly to diurnal fluctuations or shorter. The panels respond more and more significantly when the duration of the fluctuations increases until the panel fully responds to each cycle. These fluctuation periods are 14 and 90 days at 20 °C for a panel thickness of 10 and 40 mm, respectively, with two faces of a panel diffusively opened. Sinusoidal RH variations bringing about wood’s full response have the critical amplitude of ±6% RH, that is strain of 0.002 endangering the pictorial layer is produced at such amplitude in the tangential direction of the unrestrained panel.

Health Monitoring of Aerospace Structures Using Fibre Bragg Gratings Combined with Advanced Signal Processing and Pattern Recognition Techniques

Abstract: The main purpose of this work is to develop an innovative system for structural health monitoring of aerospace composite structures based on real-time dynamic strain measurements. The dynamic response of a composite panel, which represents a section of a typical aeronautical structure, is measured using fibre Bragg grating (FBG) dynamic sensors. Damage is simulated by slightly varying locally the mass of the panel at different zones of the structure. A finite element model of the structure has been developed to simulate the dynamic behaviour based on the modal superposition principle. The numerical model was calibrated against experimental results, and it was used for the placement of the FBG sensors. The proposed damage detection algorithm utilises the collected dynamic response data, and through various levels of data processing, an artificial neural network identifies the damage size and location. Feature extraction is the first step of the algorithm. Novel digital signal processing techniques, such as the wavelet transform, are used for feature extraction. The extracted features are effective indices of damage location and its extension. The classification step comprises a feed-forward back propagation network, whose output determines the simulated damage location. Finally, dedicated training and validation activities are carried out by means of numerical simulations and experimental procedures.

Performance Analysis of the Mesh-Free Random Grid Method for Full-Field Synthetic Strain Measurements

Abstract: In responding to the needs of the material characterization community, the recently developed mesh-free random grid method (MFRGM) has been exhibiting very promising characteristics of accuracy, adaptability, implementation flexibility and efficiency. To address the design specification of the method according to an intended application, we are presenting a sensitivity analysis that aids into determining the effects of the experimental and computational parameters characterizing the MFRGM in terms of its performance. The performance characteristics of the MFRGM are mainly its accuracy, sensitivity, smoothing properties and efficiency. In this paper, we are presenting a classification of a set of parameters associated with the characteristics of the experimental set-up and the random grid applied on the specimen under measurement. The applied sensitivity analysis is based on synthetic images produced from analytic solutions of specific isotropic and orthotropic elasticity boundary value problems. This analysis establishes the trends in the performance characteristics of the MFRGM that will enable the selection of the user controlled variables for a desired performance specification.

A Study of Bistable Laminates of Generic Lay‐Up for Adaptive Structures

Abstract: Asymmetric laminates are known to exhibit two stable cylindrical states and one unstable saddle state. Such bistability has attracted attention in aerospace applications because of the potential low energy requirement to achieve a large deflection or change in shape. This paper presents experimental observations of a generic asymmetric [−30/60] laminate with and without piezoelectric actuation and compares against both energy-based analytical and finite element (FE) models. It is observed that the analytical model offers a qualitative understanding of bistable behaviour, degree of curvature and overall shape but is unable to model the distinctive curvature changes near the boundaries which can be captured by the FE model. The investigation also presents the use of piezoelectric actuation to achieve snap-through in both analytical and FE models, which is compared and validated with experimental characterisation.

A Review of the Challenges and Limitations of Full-Field Measurements Applied to Large Heterogeneous Deformations of Rubbers

Abstract: This paper presents an overview of the use of full-field measurement techniques, more precisely digital image correlation (DIC) and coupled DIC and infrared thermography, for the material and structure characterisation of rubber reported in the literature. Even though such techniques have increasingly been applied for approximately 30 years for moderate deformations in metal and composite materials, they are still under-employed in the measurement of full kinematic and thermal fields in the case of large deformations undergone by rubber materials. To date, the applications addressed are crack propagation at both macroscopic and microscopic scales, model validation and constitutive parameter identification.

Mechanical Characterisation of Metals by Indentation Tests: An Experimental Verification Study for On‐site Applications

Abstract: Mechanical characterisation techniques have been recently proposed, which use as main information source the geometry of the residual imprint left on metal surfaces by hardness or instrumented indentation tests. Relevant identification procedures have been developed but the problem has been investigated, so far, mainly from a methodological point of view, exploiting pseudo-experimental data. This contribution presents the results of a verification study based on the real deformation measurements, collected from tests performed at scales consistent with those of structural applications. It is shown that the recovered mechanical properties compare satisfactorily well with those resulting from traditional tensile tests.

Zero-Point Correction Method for Nanoindentation Tests to Accurately Quantify Hardness and Indentation Size Effect

Abstract: An original treatment method is proposed to accurately determine by nanoindentation, the macrohardness and the indentation size effect (ISE). This method is applied to stainless steel specimens having different rough surfaces. It uses load versus indentation depth curves and is based on two main original features. The first one concerns the correction of the zero point (i.e. depth equals to 0) to minimise the scattering between experimental curves. The latter are all described by usual hardness equations and are shifted by minimising the distance from a leading curve chosen in a random way among the experimental curves. The second feature is the simultaneous treatment of all the nanoindentation curves to compute the macrohardness and evaluate the ISE. The standard deviation for the estimated macro- hardness is small, which indicates the robustness of the approach. It is shown that using a single nanoindentation curve can alter macrohardness estimation because of a bad consideration of the ISE. To prevent this misinterpretation, the curves should be treated simultaneously instead of averaging results of separately treated curves. A correlation is identified between the standard deviations of both surface roughness and correction of zero point, which highlights the effect of surface roughness on the scattering of the indentation curves. searching for the initiation of contact, they use a point that is consistent with the Hertz's theory. They perform a regression analysis on the initial elastic part of the loading curve and fit it with a relationship established using Hertz's theory. However, this new definition can only be applied with spherical indenters. The previous methods are designed to correct the zero-point position through the calculation or the extrapolation of experimental data. Most of them focus their research on the first few nanometres or on a position near zero force. But, the experimental noise has a particularly important effect at the nanometre scale. In this work, we propose to correct the zero-point errors using the evolution predicted by macroscopic behaviour laws. An original approach for the data analysis of nanoin- dentation curves is proposed. The latter is based on the def- initionofadeviationbetweentheexperimentaldataandthe evolution depicted by usual hardness equations. The effec- tiveness of our method is also reinforced by the consider- ationoftheindentationsizeeffect(ISE)duringthetreatment of the curves. Both features aim at accurately determining the material macrohardness and quantifying the size effects. Finally, we ensure the result accuracy by having a good sta- tistical representativeness of the specimen properties thanks to the simultaneous use of several loading curves. In this study, one hundred loading curves are studied as a whole. This method is applied to four specimens of 316L stainless steel having mirror-like surface (paper grit 4000) to rough surface (paper grit 80) to determine the evolution of the macrohardness and the ISE with roughness. The different magnitudes of abrasive surface are then used to quantify the effect of roughness on the first- contact detection. The second part of this study describes the experimen- tation and the pre-treatment realised on the experimental

Damage Operator-Based Lifetime Calculation Under Thermomechanical Fatigue and Creep for Application on Uginox F12T EN 1.4512 Exhaust Downpipes

Abstract: The article focuses on the application of a recently developed damage operator-based lifetime calculation to a thermomechanically loaded exhaust downpipe. The damage operator approach enabling online continuous damage calculations for isothermal and non-isothermal loading with mean stress corrections is reviewed. The article also highlights an extension of the strain-life approach to take into account viscoplastic effects and creep. The transient results from thermal and structural analyses using finite element analyses have been applied to the exhaust downpipe in LMS Virtual.Lab and the damage predicted. Tested exhaust downpipes were then subjected to the same loading conditions as in the calculation, and load cycles were repeated up to the point of failure. Simulated and test results are comparable.

High Strain Rate Compressive Tests on Wood

Abstract: The penetrating split Hopkinson pressure bar was used to study the response of dry maple wood under high strain rate impact load. Using longer bar and shorter specimens utilised the assumption of one-dimensional stress waves travelling along the bars and specimen because the experiment fulfilled the ratio of diameter to length of bars condition in Kolsky bar experiments. The stress–strain relationships and behaviour of the fibre structure materials’ failure were investigated during the compressive dynamic tests at strain rates between 9501 and 2000 s−1. The mechanics of dynamic failure was studied and it was confirmed that deformation of specimen is a linear function of energy absorption by specimens.

Experimental behaviour of RC beams shear strengthened with NSM CFRP laminates

Abstract: The authors wish to acknowledge the support provided by the 'Empreiteiros Casais', Degussa, S&P (R) and Secil (Unibetao, Braga). The study reported in this paper forms a part of the research program supported by FCT, PTDC/ECM/73099/2006.

A Simple Method for Estimation of Residual Stresses by Depth-Sensing Indentation

Abstract: The evaluation of residual stresses is an important aspect in many engineering applications, such as surfaces produced by mechanical or thermal treatment processes or even thin films deposited on substrates. Currently, there are several techniques for residual stress measurement. However, its application is limited by problems associated with the precision and simplicity of measurement, as well as their applicability to a wide variety of materials and situations. Indentation tests are widely used in determining the mechanical properties of materials, so it is very important to assess their sensitivity to the presence of residual stresses. In this context, recourse to numerical simulation of indentation testing proves to be an important tool to study the effect of residual stresses in the determination of hardness and modulus as well as the actual determination of residual stresses. This paper investigates the influence of the presence of equibiaxial residual stresses in the indentation test results. It proposes a methodology for reverse analysis to determine the sign and value of the equibiaxial residual stresses present in the surface of materials, from depth-sensing indentation results. For applying this methodology, the elastic and plastic behaviour of the material must be previously determined.

Application of Elastic Image Registration and Refraction Correction for Non-Contact Underwater Strain Measurement

Abstract: The refraction-induced image distortion introduces large errors in the deformation measurement of fluid submerged specimens using digital image correlation (DIC). This study provides a review of the nature of the refraction-induced image distortion, assesses experimental conditions that interact with refraction and proposes an elastic image registration technique to correct the refraction distortion of underwater images. In the elastic image registration technique, control points are selected on reference and refracted images of a template object and locally sensitive transformation functions that overlay the two images are obtained. The transformation functions so obtained are then used to reconstruct undistorted images from underwater images and the former are used as input to a DIC system. The proposed approach has shown to improve the refraction error in the order of 5–8% for typical material test samples undergoing deformation inside a water-filled glass chamber.

On the Variation of Clamping Force in Bolted Double Lap Joints Subjected to Longitudinal Loading: A Numerical and Experimental Investigation

Abstract: This study investigates the variation of clamping force and its concomitant effects on the performance of bolted double lap joints subjected to longitudinal loading. Two different amounts of clamping force were applied to bolted double lap joints made of Aluminium 2024-T3, and variations of clamping force were measured under the application of longitudinal loading. Finite element modelling was also performed to compare with experiments. The results unanimously revealed a gradual initial reduction of clamping force followed by a significant increase as the longitudinal load was increased. Also affected, was the load transfer mechanism in the joint resulting in variation of friction force between the plates, but in a different trend compared to clamping force. Finally, the key parameters have been discussed and highlighted pertaining to the performance of the joint.

3D Residual Stress Field in Arteries: Novel Inverse Method Based on Optical Full-field Measurements

Abstract: Arterial tissue consists of multiple structurally important constituents that have individual material properties and associated stress-free configurations that evolve over time. This gives rise to residual stresses contributing to the homoeostatic state of stress in vivo as well as adaptations to perturbed loads, disease or injury. The existence of residual stresses in an intact but load-free excised arterial segment suggests compressive and tensile stresses, respectively, in the inner and outer walls. Accordingly, an artery ring springs open into a sector after a radial cut. The measurement of the opening angle is commonly used to deduce the residual stresses, which are the stresses required to close back the ring. The opening angle method provides an average estimate of circumferential residual stresses but it gives no information on local distributions through the thickness and along the axial direction. To address this lack, a new method is proposed in this article to derive maps of residual stresses using an approach based on the contour method. A piece of freshly excised tissue is carefully cut into the specimen, and the local distribution of residual strains and stresses is determined from whole-body digital image correlation measurements using an inverse approach based on a finite element model.

Fatigue Behaviour of SiC Particulate-Reinforced A359 Aluminium Matrix Composites

Abstract: In this work, we describe the fatigue behaviour of silicon carbide (SiCP)-reinforced A359 aluminium alloy matrix composite considering its microstructure and thermo-mechanical properties. A variety of heat treatments have been performed for the 20 vol. % SiCp composite, which resulted in different strength and elongation behaviour of the material. The fatigue behaviour was monitored, and the corresponding S–N curves were experimentally derived for all heat treatments. The fatigue strength was found to depend strongly on the heat treatment. In addition, the fatigue behaviour was monitored non-destructively via the use of lock-in thermography. The heat wave, generated by the thermo-mechanical coupling and the intrinsic dissipated energy during mechanical loading of the sample, is detected by a thermal camera.

In Situ Evaluation of Tensile Properties of Heat-Affected Zones from Welded Steel Pipes

Abstract: Currently, measuring the local tensile strength in inhomogeneous materials is not standardised, nor accepted techniques are available despite such technique would be beneficial in a variety of technological applications. Thus, this work introduces an innovative method for assessing stress–strain properties at a sub-millimeter scale and illustrates the potential of the technique by evaluating the strength of a sub-region in the HAZ from welded steels pipes. The method employs a fully instrumented stage inside a scanning electron microscope that stretches small tensile specimens (2.0 mm × 0.5 mm cross-section, 12.5 mm gage length) while registering detailed images of the deformed region. The specimens, cut from full-scale welds, include in their gage length weld metal, base metal and HAZ and have an 85 μm period grid of evaporated lead on their surface to visualise the deformation. Upon straining, local strain is determined by correlating sequential images of the specimen surface with an open source code for particle image velocimetry. The calculated local strain within the HAZ and the load values recorded during testing are converted into a local stress–strain response. The results for two different heat inputs agree with usual, but indirect and less accurate assessments procedures, including local hardness measurements and notched bar testing.

Multiple‐view Shape and Deformation Measurement by Combining Fringe Projection and Digital Image Correlation

Abstract: We present a new method that combines the fringe projection and the digital image correlation (DIC) techniques on a single hardware platform to simultaneously measure both shape and deformation fields of three-dimensional (3-D) surfaces with complex geometries. The method in its basic form requires only a single camera and single projector, but this can be easily extended to a multi-camera multi-projector system to obtain complete 360o measurements. Multiple views of the surface profile and displacement field are automatically co-registered in a unified global coordinate system, thereby avoiding the significant errors that can arise through the use of statistical point cloud stitching techniques. Experimental results from a two-camera two-projector sensor are presented and compared to results from both a standard stereo-DIC approach and a finite element model.

Characterisation of the Local Deformation Behaviour of Engineering Plastics Rolls

Abstract: Previous research on the ‘Characterization of the global deformation behaviour of engineering plastics rolls’ investigated the deformation behaviour of loaded Polyoxymethylene (POM) and Polyetheretherketone (PEEK) rolls regarding global considerations only. This study concentrates on the analysis of the local strain distribution in these rolls. For this purpose, the former component rolling tests were repeated and images of the front side of the rolls were taken. A full-field strain analysis of the images was conducted for the static stage and the rolling process. The different behaviour of POM and PEEK under the investigated loading conditions, which has been discovered previously, could be confirmed in the local scale. Moreover, the results permit conclusions concerning the material behaviour under these loading conditions: Whilst POM shows typical viscoelastic properties, the behaviour of PEEK can be interpreted as elastic-plastic combined with time dependence of the irreversible deformation part.

Microstructure–Stiffness Relationships of Common Yew and Norway Spruce

Abstract: Yew (Taxus baccata L.) exhibits among conifers a unique macroscopic elastic behaviour. For example, it shows a comparatively low longitudinal elastic modulus related to its comparatively high density. We herein explore the microstructural origin of these peculiarities, aiming at the derivation of microstructure–stiffness relationships. We measure stiffness properties of yew at different hierarchical levels and compare them to corresponding stiffnesses of Norway spruce (Picea abies [L.] Karsten). Cell wall stiffness is investigated experimentally by means of nanoindentation in combination with microscopy and thermogravimetric analysis. On the macroscopic level, we perform uniaxial tension and ultrasonic tests. Having at hand, together with previously reported stiffnesses, a consistent data set of mechanical, chemical and physical properties across hierarchical levels of wood, we discuss influences of microstructural characteristics at different scales of observation. Moreover, a micromechanical model is applied to predict trends of effects of the microstructure on the investigated stiffness properties. On the cell wall level, particularly, the amount of cellulose and its orientation – which was earlier reported to be distinctly different for yew and spruce – result in differences between the two considered species. On the macroscopic scale, model predicted effects of the annual ring structure on transverse stiffness and shear stiffness are found to be smaller than effects of the microfibril angle and mass density.