Constant Load Control Research Articles

Susceptibility of a Fe-Mn-Al-Ni-Cr shape memory alloy to environment-assisted corrosion cracking

In recent years, iron-based shape memory alloys (SMAs) have become increasingly interesting for application in civil engineering structures. Promising candidates in the field are iron-based Fe-Mn-Al-Ni-X (X = Ti, Cr) SMAs. These alloys have demonstrated the potential to be used in various types of applications. While the fully recoverable martensitic phase transformation can be used for structures where high damping capacities are required, i.e. for high rise buildings in earthquake prone areas, it has been successfully demonstrated that the material can also be used as prestressing element in concrete structures. However, for the application in civil engineering structures the problem of stress corrosion cracking and environment-assisted cracking has to be considered. The simultaneous occurrence of tensile stresses and unfavorable environmental conditions, such as carbonation of concrete or the penetration of salt, can lead to spontaneous failure of the building structure. In the present study, the susceptibility of Fe-Mn-Al-Ni-Cr to stress corrosion cracking was studied in neutral chloride containing solutions. For that purpose, tensile tests under constant load control at different stress levels have been conducted. The pitting corrosion that occurred during immersion in the electrolytic solution has a decisive influence on the martensitic phase transformation and the crack evolution. The high density of dislocations at the martensite-to-austenite interfaces promotes the formation of transgranular cracks as well as stress-induced corrosion cracks originating from corrosion pits. Assessment of the fracture surfaces revealed dominating intergranular crack growth, most likely induced by hydrogen embrittlement. The detrimental mechanisms lead to failure of the specimens well before the targeted test time of 1000 h.

Load-control vs. displacement-control strategy in fatigue threshold analysis of adhesives: Effects of temperature

For the design of adhesive joints for service in fatigue, parameters such as the rate of fatigue crack growth and the fatigue threshold energy release rate are of extreme importance. These parameters can be obtained through fatigue fracture tests using different strategies, namely Constant load control and constant displacement control. Despite the extensive studies conducted on the fatigue performance of bonded joints, the role of testing methodology on the fatigue fracture behaviour of adhesives is less considered, especially in conjunction with the effect of temperature in these tests. Accordingly, in this study, a structural polyurethane adhesive suitable for automotive applications is characterized in terms of its fatigue behaviour in pure mode I using DCB (double cantilever beam) specimens. Fatigue tests were performed at different temperatures under both constant load control and constant displacement control conditions. A compliance-based method was used to measure the crack size throughout tests. The threshold energy release rate was calculated for different test temperatures and the two testing strategies were compared. It was seen that a constant displacement control methodology is superior to a constant load control methodology when testing for the threshold energy release rate, due to its shorter duration and low sensitivity to initial testing conditions. Differences observed between fatigue testing methodologies were exacerbated by the effect of temperature on the adhesive.

Experimental characterization of the thermo-electro-mechanical properties of a shape memory composite during electric activation

This works investigates in detail the electro-thermo-mechanical properties of a shape memory composite (SMC) during shape memory cycles in which the heating is a result of resistive heating. The SMC is a covalently cross-linked poly(ϵ-caprolactone) network filled with 3 wt% of multiwall carbon nanotubes. The characterization is performed with the help of a custom-made tensile test bench that is able to couple the mechanical characterization with the thermal and electrical ones. A proportional integral controller using the lambda tuning method is used in order to control the temperature achieved by resistive heating of the SMC. The electrical resistivity of the SMC shows a non-linear and non-monotonic dependence on temperature and strain. The resistivity is also found to vary among successive shape memory cycles, suggesting that a (first) training cycle is necessary not only to stabilize the mechanical but also the electrical properties of the SMC. A fuzzy logic controller for constant load control is also used to investigate the strain variation with temperature related to the two-way shape memory effect of the SMC. The results give evidence of the strong interplay between the electrical and (thermo-)mechanical characteristics of electroactive SMCs.

Effect of Fatigue Life of Dissimilar Aluminium Alloy (AA 5083 - AA 6062) Joining by Friction Stir Welding

The effect of processing parameters on the mechanical and microstructural properties of dissimilarAA5083–AA6062 joints produced by friction stir welding was analysed in this study. Different samples were produced by varying the advancing speeds of the tool as 5 mm/min and by varying the alloy positioned on the advancing side of the tool. In all the experiments the rotating speed is fixed at1200 RPM. All the welds were produced perpendicularly to the rolling direction for both the alloys. Microhardness (HV) and tensile tests performed at room temperature were used to evaluate the mechanical properties of the joints. For the mechanical fatigue tests, a resonant electromechanical testing machine was employed under constant loading control up to 10 Hz sine wave loading. The fatigue tests were conducted in the axial total stress–amplitude control mode, with R = rmin/rmax = -1. In order to analyse the microstructural evolution of the material, the welds’ cross-sections a SEM observation was made of the fracture surfaces.

Creep-fatigue crack growth behavior of G115 steel under different hold time conditions

The creep-fatigue crack growth behavior and fracture mechanisms of G115 steel at 650 °C were studied using compact tension specimens in a constant load control mode. The crack growth length and load-line displacement analyses reveal that G115 steel is a creep-ductile material and the crack growth life and initiation time increase with increasing dwell time. Meanwhile, the parameter ΔK was used to characterize the crack growth behavior and the effect of dwell time on da/dN depends on ΔK. These data were also compared with those getting from P91 steel under 625 °C and P92 steel at 630 °C. When the crack growth rate, da/dtavg, is represented by Ctavg, all the data seem to collapse within a single narrow scatter band. Further, the approximate Nikbin-Smith-Webster (NSW) crack growth model was utilized to predict the creep-fatigue crack propagation behavior of G115 steel. And the microstructures of the cross-sections along the crack growth direction were studied through scanning electron microscopy to reveal the crack growth mechanisms of G115 steel.

Effect of boundary conditions on the evolution of lattice strains in a polycrystalline austenitic stainless steel

The effect of boundary conditions (constant load, constant strain and elastic follow-up) on lattice strain evolution during creep in a polycrystalline austenitic stainless steel was studied using in situ neutron diffraction at 550 °C. The lattice strains were found to remain constant under constant load control. However, under constant strain and elastic follow-up control, the lattice strains relaxed the most in the elastically softest lattice plane {200} and the least in the elastically stiffest lattice plane {111}. The intergranular stresses created between different grain families were constant during creep tests irrespective of the boundary conditions with the initial applied stresses of 250 MPa.

3D multiscale simulation of a silicon slider on a PE substrate

A multiscale approach based on Pseudo Amorphous Cell (PAC) is employed to study nanoscale sliding of a silicon ball against a polyethylene (PE) substrate. In this multiscale approach, the domain is firstly constructed as a tessellation of identical Amorphous Cells (ACs). For small deformation regions, the number of degrees of freedoms (DOF) is then reduced by computing the displacements of only vertices of the ACs instead of the atoms within. The reduction is achieved by determining an accurate mapping relationship between the atomistic displacements and the displacements of the vertices of the PACs. During sliding, the polymer chains generally line up in the sliding direction. The coefficient of friction is comparable to some experiment values. Under constant load control, it is found that the penetration depth increases and then decreases to a steady value during the sliding.

Effect of welding parameters on mechanical and microstructural properties of dissimilar AA6082–AA2024 joints produced by friction stir welding

The effect of processing parameters on the mechanical and microstructural properties of dissimilar AA6082–AA2024 joints produced by friction stir welding was analysed in this study. Different samples were produced by varying the advancing speeds of the tool as 80 and 115 mm/min and by varying the alloy positioned on the advancing side of the tool. In all the experiments the rotating speed is fixed at 1600 RPM. All the welds were produced perpendicularly to the rolling direction for both the alloys. Microhardness (HV) and tensile tests performed at room temperature were used to evaluate the mechanical properties of the joints. The mechanical tests were performed on the joints previously subjected to annealing at 250 °C for 1 h. For the fatigue tests, a resonant electromechanical testing machine was employed under constant loading control up to 250 Hz sine wave loading. The fatigue tests were conducted in the axial total stress–amplitude control mode, with R = σ min/ σ max = 0.1. In order to analyse the microstructural evolution of the material, the welds’ cross-sections were observed optically and SEM observations were made of the fracture surfaces.

Creep Model Based on Theta Projection and Omega Method for Predicting the Creep Behavior of Alloy

Creep testing of Alloy 718 has been carried out at various loads in the temperature range near 650°C in constant load control mode in order to understand how to predict the creep behavior including tertiary creep. The test results have been used for evaluating the existed models, such as Theta projection and Omega method that have been widely used for predicting long term creep strain and rupture time. After determining variables and material parameters of each method with the test results, estimated creep data from each model have been compared with the each measured creep data from the creep tests. The root cause of the discrepancy between estimated and measured data has been analyzed in order to improve the existed methods. The reliability of the improved model has been evaluated in relation to creep data.

Effect of welding parameters on mechanical and microstructural properties of AA6056 joints produced by Friction Stir Welding

The effect of processing parameters on mechanical and microstructural properties of AA6056 joints produced by Friction Stir Welding was analysed in the present study. Different samples obtained by employing rotating speeds of 500, 800 and 1000 rpm and welding speeds of 40, 56 and 80 mm/min were produced. The mechanical properties of the joints were evaluated by means of microhardness (HV) and tensile tests at room temperature. Fatigue tests on the welds were carried out by using a resonant electro-mechanical testing machine under constant loading control up to 250 Hz sine wave loading. The low cycle (LCF) and high cycle (HCF) fatigue tests were conducted in the axial total stress-amplitude control mode with R = σ min/ σ max = 0.1, for all the welding and rotating speeds used in the present study. It was observed that the specimens welded at 56 mm/min showed the best behaviour in the low cycle regime. The microstructural evolution of the material was analysed by optical observations of the welds cross sections.

Effect of Welding Parameters on Mechanical and Microstructural Properties of Dissimilar AA6082-AA2024 Joints Produced by Friction Stir Welding

The effect of processing parameters on mechanical and microstructural properties of dissimilar AA6082-AA2024 joints produced by Friction Stir Welding was analysed in the present study. Different samples were produced by employing a fixed rotating speeds of 1600 RPM and by using the advancing speeds of the tool of 80 and 115 mm/min. All the welds were produced in direction perpendicular to the rolling one for both the alloys and by changing, for all the processing conditions, the alloy positioned on the advancing side of the tool. The mechanical properties of the joints were evaluated by room temperature tensile tests. Fatigue tests on the welds were carried out by using a resonant electro-mechanical testing machine under constant loading control up to 250 Hz sine wave loading. The fatigue tests were conducted in the axial total stress-amplitude control mode with R=smin/smax=0.1. The microstructural evolution of the material was analysed by optical observations of the welds cross sections and SEM observations of the fracture surfaces.

Mechanical and Microstructural Behaviour of 6061/AL

The aim of the present paper was the study of mechanical and microstructural behaviour of 6061/Al2O3/20p metal matrix composite sheets joined by Friction Stir Welding. The material was welded into the form of sheets of 7 mm thickness after T6 treatment and was tested in tension and fatigue at room temperature. The mechanical behaviour of the material was found to depend strongly on the particles fracture across the weld. The tensile properties in longitudinal direction resulted higher respect to the transversal ones. The fatigue endurance (S‐N) curve of the welds was obtained by using a resonant electro‐mechanical testing machine under constant loading control up to 250 Hz sine wave loading. The cyclic fatigue tests were conducted in the axial total stress‐amplitude control mode with min max R = σ / σ The microstructure resulting from the FSW process was studied by employing optical and scanning electron microscopy.

Effect of friction stir processing on the fatigue properties of a Zr-modified 2014 aluminium alloy

The fatigue properties of a Zr-modified 2014 aluminium alloy resulting from friction stir processing (FSP) were analysed in the present study. The sheets were processed parallel to the extrusion direction; the tensile mechanical properties were evaluated at room temperature in the longitudinal direction with respect to the processing one in order to observe the differences from the parent material as a function of the strong grain refinement due to the friction stir process. The fatigue endurance ( S– N) curve of the FSP material was obtained by using a resonant electro-mechanical testing machine under constant loading control up to 250 Hz with sine wave loading. The cyclic fatigue tests were conducted in the axial total stress–amplitude control mode with R = σ min/ σ max = 0.1. The microstructure resulting from the FSW process was studied by employing optical and scanning electron microscopy.

화학적 초음파가공을 이용한 유리의 미세가공

An ultrasonic machining process has been known as efficient and economical means for precision machining of glass or ceramic materials. However, because of its complexity, the mechanism of the machining process is still not well understood. Therefore, it is hard to optimize the process parameters effectively. The conventional ultrasonic machining which uses the abrasive slurry only, furthermore, is time-consuming and gives the relatively rough surface. In order to increase the material removal rate and improve the integrity of the machined surface, we have introduced the novel ultrasonic machining technique, Chemical-assisted UltraSonic Machining(CUSM). The desktop-style micro ultrasonic machine has been developed and the z-axis feed is controlled by the constant load control algorithm. To obtain the chemical effects, the low concentration HF(hydroj1l1oric acid) solution, which erodes glass, added to alumina slurry. Through various experiments and comparison with conventional results, the superiority of CUSM is verified. MRR increases over 200%, the surface roughness is improved and the machining load decreases dramatically.br/

초음파에 의한 고 세장비 유리가공 특성

An ultrasonic machining process is efficient and economical means for precision machining of glass and ceramic materials. However, the mechanism of the process with respect to the crack initiation and propagation and the stress development in the ceramic workpiece subsurface arc still not well understood. In this research, we have investigated the basic mechanism of ultrasonic machining of ultrasonic machining of glass by the experimental approach. For this purpose, we designed and fabricated the desktop micro ultrasonic machine. The feed is controlled precisely by using the constant load control system. During machining experiments, the effects of abrasive characteristics and machining conditions on the surface roughness and the material removal rate are measured and compared.

“EFFECT OF LOADING MODE ON THE CYCLIC RESPONSE AND THE ASSOCIATED SUBSTRUCTURE OF POLYCRYSTALLINE COPPER IN THE HIGH‐CYCLE REGIME”

AbstractStudies on the influence of loading mode on the cyclic response of small‐grained polycrystalline copper and the associated dislocation structures have been carried out in the high‐cycle regime. It is found that the saturation behaviour under constant load control, for two sets of specimens, with and without initial ramp‐loading, exhibits strong differences in the “intermediate” range of stress amplitudes, i.e., between 70 and 98 MPa. Within this range the ramp‐loading mode promotes a gradual substructure evolution which leads to localization of slip in primary systems and formation of persistent slip bands (PSBs), whereas conventional loading leads to the formation of elongated cells and multiple sets of wall structures (e.g., labyrinth structure), both intimately associated with multiple slip conditions. At low stress amplitudes observed differences in the plastic strain amplitudes obtained at saturation, as an effect of different loading modes, are relatively small and related to equally small differences in the uniformity and homogeneity, from grain to grain, of the dislocation structures associated with that stage. At high stress amplitudes equiaxed cell structures are promoted under both loading modes, the deformation is homogenized, and the cyclic response as a function of loading mode shows no differences.

Characteristics of the carotid baroreflex in man during normal and flow-restricted exercise.

Eight subjects were studied in the supine position at rest, during normal dynamic leg exercise (control exercise) and with blood-flow restriction in the working legs (flow-restricted exercise). Graded muscle blood-flow restriction was accomplished by applying a supra-atmospheric pressure of 50 mmHg to the working legs. During incremental-load exercise, flow restriction reduced exercise performance and peak heart rate by 36% and 13%, respectively. The function of the cardiac branch of the carotid baroreflex was studied over its full operational range, at rest and during constant-load control and flow-restricted exercise, by measuring R-R intervals during application of pulse-synchronous graded pressures (40 to -65 mmHg) in a neck-chamber device. Heart rate and arterial pressure were higher during flow-restricted than control exercise, indicating that the flow restriction activated the muscle chemoreflex. Raising the carotid transmural pressure (systolic arterial pressure minus neck-chamber pressure) was accompanied by increasing R-R intervals in all conditions. The set point (point of baseline carotid transmural pressure and R-R interval) coincided with the midportion of the pressure-response curve at rest and with the threshold point of the curve during exercise. The maximal rate of change in relative R-R intervals and the corresponding carotid transmural pressure range were higher during control exercise than at rest and highest during flow-restricted exercise, indicating that exercise and especially flow-restricted exercise increased carotid baroflex sensitivity, and shifted the carotid baroreflex optimal buffering range to higher pressures. The results suggest that the carotid baroflex attenuates exercise heart rate increases mediated by the muscle chemoreflex and/or by central command.

２・１／４Ｃｒ‐１Ｍｏ鋼のクリープき裂成長挙動の評価

Creep crack growth behavior of 21/4 Cr-1 Mo steel at 550°C was investigated by energy rate line integral, C* under conventional constant load control and deflection rate control. The same relationship of da/dt versus C*-integral was obtained by both of the control methods. The increasing deflection rate control test proposed in this paper was found to be a very efficient method because a wide range of creep crack growth data can be obtained stably on a single specimen. Various formulae to experimentally determine C*-integral of CT specimens were compared. Equation (3) considering the axial force as well as the bending effect agreed with the analytical results by the finite element method in the range of a/W≥0.5.The result of C*-integral estimation using the fully plastic solution, agreed with the experimental results in steady state creep conditions, when the plane strain solution was applied. Although the C*-integral approach with the fully plastic solution seems prospective for practical creep crack growth prediction, more investigations are needed under various conditions.