Modeling of the dynamic characteristics of two‐bolt‐joints

Abstract

In this paper, a method to identify the damping and stiffness properties of bolted joints from a substructure synthesis scheme is presented. The easily measured frequency response functions of substructures and the assembled structure are the only data needed in this method. A synthesis formula used to predict the frequency response functions of two‐bolt‐joint structures is proposed. The formula makes use of the extracted properties of a single‐bolt‐joint. Some experiments with two free‐free steel beams jointed with one or two bolts are made to check this method. The close correlation between predicted and measured results demonstrates that this method is acceptable.     

Stress–strain behaviour and abrupt loss of stiffness of geopolymer at elevated temperatures

This paper reports stress versus strain curves of geopolymer tested while the specimens were kept at elevated temperatures, with the aim to study the fire resistance of geopolymer. Tests were performed at temperatures from 23 to 680 °C and after cooling. Hot strengths of geopolymer increased when the temperature increased from 290 to 520 °C, reaching the highest strength at 520 °C, which is almost double that of its initial strength at room temperature. However, glass transition behaviour was observed to occur between 520 and 575 °C, which was characterised by abrupt loss of stiffness and significant viscoelastic behaviour. The glass transition temperature is determined to be 560 °C. Further, the strength reductions occurred during cooling to room temperature. This is attributed to the damage due to brittle nature of the material making it difficult to accommodate thermal strain differentials during cooling phase.     

Cyclic stress–strain characteristics of two microalloyed steels

Abstract

The cyclic stress–strain behaviour of two microalloyed steels with different microstructures has been characterised at room temperature under strain controlled low cycle fatigue. The cyclic stress–strain curve in the double logarithmic plot shows a linear relation for both steels. A transition of the cyclic stress–strain curve from softening to hardening with increasing strain amplitude has been observed with respect to the corresponding tensile curve. The strain amplitude for the onset of cyclic softening to hardening transition has been found to be dependent on grain size. The strain lifetime behaviour, estimated from modified universal slopes equation, shows similar trends as Nb or V bearing microalloyed steels. The cyclic characteristics of the two microalloyed steels have been compared with corresponding predeformed state carried out under stress controlled conditions. While, cyclic saturation was observed in case where the extent of predeformation was within the Lüders strain, cyclic softening occurred when it exceeded the Lüders strain. It has been attempted to provide a mechanistic understanding of the differences in the cyclic behaviour of the two steels owing to the microstructure and predeformation.     

Measurements of the “Magnex DC” characteristics at microwave frequencies

This paper deals with the measurements of the equivalent impedance, insertion loss (IL), and return loss (RL) of a conductive composite material called Magnex DC, performed at microwave frequencies. It is found that the equivalent impedance of this composite material decreases as the frequency increases in the X-band (8–12.4 GHZ) and the IL of a 1.4 mm thick specimen is greater than 9 dB over the whole band. The utilization of this material in electromagnetic shielding is considered. Furthermore, results of the measurements performed on a tapered specimen are also reported.     

The influence of stress-controlled tensile fatigue loading on the stress–strain characteristics of AISI 1045 steel

This study analyses the influence of fatigue loading on the residual tensile properties of AISI 1045 steel. The fatigue tests were carried out under stress-controlled tensile loadings at a stress ratio equal to 0. The maximum applied stresses were within the range from 550 MPa to 790 MPa. An analysis of ratcheting strain and plastic strain amplitude evolution due to fatigue loading was performed on the experimental data. In the next stage of this study, the initial fatigue loadings were introduced. Two maximum stresses, 550 MPa and 750 MPa, and three cycle lengths, 25%, 50% and 75% of the total number of cycles required to fracture the material at a given stress, were used. The pre-fatigued specimens were subjected to tensile testing at strain rates from 10⁻⁴ to 10⁰ s⁻¹. A large number of fatigue cycles, equal to 75% of the fatigue life, induces material softening as well as a drop in elongation and a reduction of area. Pre-fatigue at maximum stress equal to 550 MPa results in the increase of the elastic limit and offset yield point as well. Both parameters reach almost constant value after number of cycles equal to 25 % of the fatigue life. The further increase in the number of cycles does not affect elastic limit and offset yield point in a clearly visible way. The increase of maximum stress of the initial fatigue loadings up to 750 MPa induces similar but stronger effect i.e. increase and stabilization of elastic limit and offset yield point values, however decrease of both parameters value is observed at large number of pre-fatigue cycles corresponding to 75% of the fatigue life.     

Stress–strain relationships of polycarbonate over a wide range of strain rate,including a shock wave regime

Stress–strain relationships of polycarbonate (PC) are determined over a very wide range of strain rates, including a shock wave regime. Plate impact tests, drop-weight tests, and quasi-static tests using universal and Instron testing machines are used for the high strain rate (10⁷ s⁻¹), medium strain rate (10² s⁻¹) and low strain rate (10⁻⁴ s⁻¹) tests, respectively. A newly modified unsteady wave sensing system (NM-UWSS) for plate impact tests is developed to determine the stress–strain relationships of PC. The system consists of a powder gun for plate impact tests, three embedded polyvenyliden fluoride (PVDF) gauges, and NM-UWSS. As originally proposed, UWSS is aimed at obtaining experimental inputs for the Lagrangian analysis used in determining the dynamic behavior of materials. We revise this standard system (UWSS) twice to gain a higher time resolution. In the past, the conventional charge mode (Q₂ method) was used. The first modified system (M-UWSS) has been used to study two classes of materials: (1) metallic materials and (2) polymeric materials, where the Q₁ method coupled with a transient differential equation for the equivalent circuit of the measurement circuit for the PVDF stress gauge was used. The latest method (Q_t method) for gaining the highest time resolution of shock wavefront structure by considering the effects of a piezofilm's thickness is proposed for PC at particle velocities of up to 1 km/s. Here we show from basic equations of piezoelectricity that the charge density q, i.e., the charge release per unit area, of the active electrode is proportional to the ratio of the thickness of the shocked region to the total thickness of the piezofilm. It is demonstrated that the rise time of shock charge density q in the piezofilm induced by such shock in the Q₂, Q₁ and Q_t methods, in this order, is becoming much shorter. The latest Q_t method has the highest accuracy among these three methods. Power law relations between stress and strain rate are observed again with PC under conditions of uniaxial strain over a very wide range of strain rates, i.e., 10⁻⁴–10⁷ s⁻¹ including a shock wave regime. For the PC, the effects of strain rate on the stress–strain relationships are estimated using empirical formula.     

Effect of strain on the I–V characteristics of discontinuous silver films and determination of their gauge factor

Three films (A, B and C) of discontinuous silver films whose mass thicknesses (d _m ) are 6, 12 and 18 Å, respectively were deposited onto Corning 7059 glass substrates at ambient temperature using a thermal evaporation technique. The increase in dc resistance (R _dc ), in air, with time (time-ageing) was monitored till short-term stability was achieved. The effect of strain on the I–V characteristics of discontinuous silver films and determination of their gauge factor (v) was studied and it was found that; (1) a deviation from linearity is observed at higher voltages (>60 V) and at particular voltage, the electric current increases as d _m increases (2) R _dc increases as the tensional strain (?) increases and the dependence of fractional change of resistance on ε is linear with no hysteresis (3) v decreases as d _m increases and the high values of ν for these films candidates them to be miniature strain sensors. The data of this work are discussed on the ground that the thermally activated tunneling is the mechanism responsible for the electrical conduction in discontinuous silver films.     

Dynamic characteristics of wide tapered‐land slider bearings

Abstract

Taking into account squeezing‐action effects, the present study is concerned with the dynamic characteristics of a tapered‐land slider bearing with infinite width. According to the results obtained, the bearing characteristics are significantly affected by the shoulder parameter δ and the geometric parameter α of tapered‐land bearings. For a fixed α, there exist some δ such that the bearing possesses a maximum load, a minimum friction parameter and a maximum stiffness. Compared to the plane‐bearing case, the tapered‐land bearing produces higher damping coefficients, and provides higher load capacity and dynamic stiffness at large α. These results provide useful information for engineers in selecting and designing bearing systems.     

The charges,currents and near‐field characteristics of the transparent source for the FDTD method

Abstract

The electromagnetic characteristics of a transparent source for the finite‐difference time‐domain (FDTD) method are investigated. The approximate formulations for the corresponding electric and magnetic fields at the source are first reported, while the associated charges and current are also demonstrated. The approximate E field and H field are very closely related to the FDTD simulated results. It is shown that the magnetic field is proportional to the temporal derivative of the electric field at the source node, while the electric field is proportional to the temporal integration of the driving function. Static charges may be built up to produce static E fields after the driving function has passed, which are responsible for the dc offset problem in the pulsed FDTD algorithm. The dc offset is verified for the case of Gaussian pulse excitation and is avoided via the use of a bipolar pulse.     

Comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials

This paper presents the results of a comprehensive study of the effects of rolling resistance on the stress–strain and strain localization behavior of granular materials using the discrete element method. The study used the Particle Flow Code (PFC) to simulate biaxial compression tests in granular materials. To study the effects of rolling resistance, a user-defined rolling resistance model was implemented in PFC. A series of parametric studies was performed to investigate the effects of different levels of rolling resistance on the stress–strain response and the emergence and development of shear bands in granular materials. The PFC models were also tested under a range of macro-mechanical parameters and boundary conditions. It is shown that rolling resistance affects the elastic, shear strength and dilation response of granular materials, and new relationships between rolling resistance and macroscopic elasticity, shear strength and dilation parameters are presented. It is also concluded that the rolling resistance has significant effects on the orientation, thickness and the timing of the occurrence of shear bands. The results reinforce prior conclusions by Oda et al. (Mech Mater 1:269–283, 1982) on the importance of rolling resistance in promoting shear band formation in granular materials. It is shown that increased rolling resistance results in the development of columns of particles in granular materials during strain hardening process. The buckling of these columns of particles in narrow zones then leads to the development of shear bands. High gradients of particle rotation and large voids are produced within the shear band as a result of the buckling of the columns.     

Effects of strain rate and temperature on the stress–strain response of solder alloys

To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between–10 and 75°C and strain rates between 10^–1 and 10^–3 s^–1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized.     

Strength and stress–strain characteristics of traditional adobe block and masonry

Traditional unstabilized adobe low-rise buildings are common in many Chinese small towns and villages. This paper presents a study on the uniaxial compressive strength and stress–strain behavior of traditional unstabilized adobe blocks and masonry prisms with various compositions. The adobe blocks were manually produced by Chinese traditional technique in various proportions of natural soil and sand. The influence of various proportions on unconfined compressive strength, dry density and initial tangent modulus are discussed. Following this, soil mortars in three different proportions were used to construct adobe masonry prisms, with the purposes of understanding the influence of mortar strength to block strength ratio on compressive strength and stress–strain characteristics. The result shows that the compressive strength, initial tangent modulus and Poisson’s ratio of prism are influenced by the ratio of mortar strength to block strength. In addition, tangent modulus and Poisson’s ratio increase with the ratio of stress to peak strength. It was also found that although coefficients of variation of experimental results are reduced by load–unload cycles, peak strains are largely increased.     

Measurement of strain distribution by the moiré fringe multiplication method at a tip of propagating fatigue crack

The moiré fringe multiplication method which used interference of the +1st and -1st order diffracted beams from a 1000 lines/mm phase-type grating on the specimen surface was applied to the measurement of strain distribution at the tip of a propagating fatigue crack in a steel plate having a central crack. The sensitivity of the measurement was equivalent to the sensitivity obtained from a 2000 lines/mm grating on the specimen when the conventional moiré fringe method is used. The gage length was of the order of 10m. The result of the measurement of strain distribution was applied to the estimation of the fatigue crack propagation rate.

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Résumé La méthode de multiplication des franges de Moiré qui utilise les interférences d'ordre +1 et -1 de rayons lumineux diffractés par une grille de 1000 lignes/millimètre, du type à phase sur la surface de l'éprouvette, a été appliqué à la mesure de la distribution des déformations à l'extrémité d'une fissure de fatigue en cours de propagation dans une tôle d'acier présentant une fissure centrale.La sensibilité de la mesure était équivalente à la sensibilité obtenue par une grille de 2000 lignes/millimètre appliquée sur l'éprouvette et utilisant la méthode de franges de Moiré conventionnelle. La longueur de référence était de l'ordre de 10 microns. Le résultat des mesures de distribution de déformation a été appliqué à l'estimation de la vitesse de propagation d'une fissure de fatigue.

     

Morphology characteristics of α precipitates related to the crystal defects and the strain accommodation of variant selection in a metastable β titanium alloy

Profound and comprehensive investigations on the morphology characteristics of α precipitates are essential for the microstructural control of metastable β titanium alloys.At the very beginning of aging treatment,intragranular α precipitates with a dot-like morphology begin to generate nearby the dislo-cations,then those dot-like α precipitates with the same crystallographic orientation tend to connect with each other to develop a lath-like morphology.With the progress of aging treatment,the orientated lath-like α precipitates gradually combine with each other to form the V-shaped clusters or the triangular ones.The dislocations of{1(-1)0}β < 11 (-1)>β edge type are evidenced within the β grains,and it is found that variant selection ofα precipitates induced by the transformation strain and the interplay between α vari-ants and the dislocations are confirmed as the key factors for the formation of the V-shaped or triangular clusters.The results of this work could provide underlying knowledge on the morphology characteristics of intraguranular α precipitates related to the crystal defects and the strain accommodation of α variants in metastable β titanium alloys.     

Dependence of dynamic strain ageing on strain amplitudes during the low-cycle fatigue of TP347H austenitic stainless steel at 550 °C

Low-cycle fatigue (LCF) tests are carried out on TP347H stainless steel at a strain rate of 8 × 10⁻³ s⁻¹ with total strain amplitudes (Δε_t/2) of ±0.4% and ±1.0%, at room temperature (RT) and 550 °C. It is found that the stress responses and dislocation structures under cyclic loading strongly depend on the value of strain amplitude at 550 °C. Compared with those at the same strain amplitude at RT, the material shows a rapid strain softening, and finally attains a stabilized state at Δε_t/2 = ±0.4% and 550 °C, but the one presents an anomalous behavior, i.e., first a rapid hardening to the maximum stress, followed by a reducing softening at Δε_t/2 = ±1.0% and 550 °C. More cells resulting from dislocation cross-slip and planar structures due to dynamic strain ageing (DSA) restricting cross-slip develop at low strain amplitude of ±0.4% at the first cycle. However, there are more complicated dislocation structures, such as cells, elongated cells, walls/channels and planar structures at Δε_t/2 = ±1.0%. The observations of scanning electron microscopy (SEM) and transmission electron microscopy (TEM) exclude the effects of martensitic transformation, creep, oxidation, and precipitations on these stress responses and microstructure evolutions, which result from DSA appearing at 550 °C.     

The θ projection method applied to small strain creep of commercial aluminium alloy

Abstract

The paper describes the early uniaxial creep behaviour of an airframe aluminium alloy (2419-T851). Tests were conducted over the temperature range 373 to 463 K and for rupture lives up to 7000 ks. The shapes of creep curves were fitted using either the conventional 4-Θ equation or the recently proposed 6-θ equation. Coefficients allowing the interpolation of times to small strains were produced and the accuracy of such interpolations was checked against experimental values. At small strains, the 4-Θ equation produced systematic errors but the 6-θ function gave expected values of errors which could not be distinguished from zero. At larger strains, both methods yielded zero mean errors. The procedures are discussed in terms of the likely deformation mechanisms which might lead to the observed forms of the projection equations. It seems likely that very early creep contains some strain due to anelastic grain boundary relaxation.     

Influence of shear bond strength on compressive strength and stress–strain characteristics of masonry

The paper is focused on shear bond strength–masonry compressive strength relationships and the influence of bond strength on stress–strain characteristics of masonry using soil–cement blocks and cement–lime mortar. Methods of enhancing shear bond strength of masonry couplets without altering the strength and modulus of masonry unit and the mortar are discussed in detail. Application of surface coatings and manipulation of surface texture of the masonry unit resulted in 3–4 times increase in shear bond strength. After adopting various bond enhancing techniques masonry prism strength and stress–strain relations were obtained for the three cases of masonry unit modulus to mortar modulus ratio of one, less than one and greater than one. Major conclusions of this extensive experimental study are: (1) when the masonry unit modulus is less than that of the mortar, masonry compressive strength increases as the bond strength increases and the relationship between masonry compressive strength and the bond strength is linear and (2) shear bond strength influences modulus of masonry depending upon relative stiffness of the masonry unit and mortar.