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To analyze the stress wave propagation associated with the vortex-induced vibration (VIV) of a marine riser, this paper employed a multi-signal complex exponential method. This method is an extension of the classical Prony’s method which decomposes a complicated signal into a number of complex exponential components. Because the proposed method processes multiple signals simultaneously, it can estimate the “global” dominating frequencies (poles) shared by those signals. The complex amplitude (residues) corresponding to the estimated frequencies for those signals is also obtained in the process. As the signals were collected at different locations along the axial direction of a marine riser, the phenomena of the stress wave propagation could be analyzed through the obtained residues of those signals. The Norwegian Deepwater Program (NDP) high mode test data were utilized in the numerical studies, including data sets in both the in-line (IL) and cross-flow (CF) directions. It was found that the most dominant component in the IL direction has its stress wave propagation along the riser being dominated by a standing wave, while that in the CF direction dominated by a traveling wave.

The solution treatment (ST) and the the second phase morphology changing duing the aging precipitation process of 0Cr21Ni6Mn9N austenitic stainless steel were investigated using optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) with EDS and transmission electron microscope (TEM). The results showed that the precipitation phase was Cr2N which initially nucleated along austenitic grain boundaries and then grew towards into the inner grains in strip morphology. Also, with the longer aging time the proportion of Cr2N increased. The mechanical properties of alloys with and without the presence of the precipitation Cr2N were also studied. It was observed that due to the exiting of the precipitation Cr2N, the strength of 2169N stainless steel reduced during a certain range of aging time, and then improved when the aging time reached to 48h, while the elongation decreased thoroughly.

According to the behavior of shape memory alloys, a structure of removable shape memory alloy pipe joint was designed. The contact manner of two parts of this structure is cone-globe. Cone-globe sealing is the key point of this structure’s sealing. Therefore the finite element analysis software ANSYS was used to simulate and calculate the sealing. The result shows that the width of the sealing surface and the contact stress altered with the change of structural parameters, which affected the sealing most. According to the value of the sealing surface’s width and the contact stress, the structural parameters including cone angle can be definitely settled.

All time-domain methods for experimental modal analysis (EMA) begin with a mathematical model. Based on either a high-order matrix polynomial model or a first-order state-space model, this paper emphasizes the comparison of numerical conditioning and stability, as well as the modal parameter estimation, among EMA methods. Numerical conditioning pertains to the perturbation behavior of a mathematical problem (model) itself and stability pertains to the perturbation behavior of an algorithm used to solve that problem on a computer. As various EMA methods are modeled differently with distinct solution algorithms, implementing these methods would have different conditioning and stability. In this paper, both deterministic and stochastic EMA methods are covered. Three different scenarios for the response signal are considered: (1) clean response from impulse loading, (2) noisy response from impulse loading, and (3) noisy response from ambient noise excitation. Comparing the numerical conditioning of various EMA methods, this paper theoretically illustrates that methods based on first-order state-space models are more likely to be well-conditioned (with a smaller conditioning number) than those based on high-order polynomial models. Furthermore, the numerical observation of a case study for a 6 degree-of-freedom system also suggests that first-order state-space model methods are more robust and accurate for the estimation of modal frequency and damping.

The effects of annealing on the phase transformation behavior and superelasticity of cold-rolled Ti50Ni48Fe2 shape memory alloy were extensively investigated. Curves of temperature dependence of electrical resistivity reveal that both the cold-rolled and annealed specimens exhibit a B2 → R→B19′ two-stage martensitic transformation upon cooling and a B19′ → B2 one-stage transformation upon heating, although the austenitic transformation temperature decreases with the increase of the annealing temperature. Tensile stress–strain curves show the critical stress for stress-induced martensite (σ SIM) of Ti50Ni48Fe2 alloys decreases with the increase of annealing temperature due to the decrement of dislocation density caused by the recrystallization. As a result, the σ SIM decreases. Upon a cold-rolling and annealing at 623 K for 30 min, the Ti50Ni48Fe2 alloy exhibits excellent superelasticity with the maximum recoverable strain of 5.8 % at a loading strain of 7 %. In such a case, a complete superelasticity of 5 % can be obtained in the Ti50Ni48Fe2 alloy after deformation increasing to 15 cycles.

Effects of mechanical treatment, pre-strain and pre-deformation temperature on shape recovery stress of Ti50Ni47Fe3 shape memory alloys were investigated. Mechanical treatment, consisting of hot-rolling and hot-forging, improved σrmax σrmax (maximum recovery stress), dσr/dT (rate of recovery stress with temperature) and Ts (the temperature at which recovery stress rises sharply). Martensitic transformation and reverse transformation temperatures also increased with increasing the hot-rolling deformation. σrmax and dσr/dT increased with increasing pre-strain at first and then decreased with further increasing of pre-strain, and the maximum values of σrmaxand dσr/dT were obtained at a pre-strain of 10%. On the other hand, σrmaxand dσr/dT decreased with increasing pre-deformation temperature. Upon a hot-rolling of 88% area reduction and pre-deformation of 10% at-150°C, the Ti50Ni47Fe3 alloy exhibited excellent recovery stress property with the σrmax of 391MPa in combination with dσr/dT of 5.09.

The microstructures and crystallization behavior of Ti–47 at% Ni–3 at% Fe shape memory alloy wire under the condition of severe cold drawing at room temperature and different post-deformation annealing processes were intensively investigated using transmission electron microscope (TEM) and differential scanning calorimetry (DSC). It is indicated that the amorphous phase is dominant in the Ti50Ni47Fe3 wire after the cold drawing of 78 % areal reduction. The critical temperature for recrystalization is determined at about 300 °C. The average grain size grows from 7 up to 125 nm when annealing temperature rises from 300 to 500 °C. Post-deformation annealing process exerts significant influence on the crystallization temperature which climbs up with the increase of annealing temperature.

A new damage assessment method is proposed for aging offshore platforms based on dynamic tests. The stiff nesses of the measured model corresponding to the two measurement moments are estimated by a series of stiffness-correction factors, respectively. Based on these stiffness-correction factors a new damage indicator is defined aiming at reducing influences of existed damages accumulated before the first measurement on damage assessment occurred between the two adjacent measurements. One theoretical improvement is that the requirement using the stiffness matrix of FEM to replace the one of the measured model can be ignored in the calculation of MSE of the measured model. The other development is influences of damages accumulated before the first measurement on damage detection occurs between the two measurements could be reduced greatly, which is very important for aging platforms because these platforms have not been tested aiming at damage detection during their previous service life, especially for most platforms in China. A jacket platform is chosen for numerical studies, and numerical results show that the proposed method could identify damages occurred between the two measurements properly, even including damage severity estimation.

In this work, transformation behaviors and mechanical properties of cold-rolled shape memory alloy Ti50Ni49Fe1 by severe plastic deformation (SPD) were intensively investigated. The phase transformation behaviors, phase analysis, and microstructures were characterized by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and transmission electron microscopy (TEM), respectively. Tensile testing was performed to analyze the effect of SPD on the mechanical properties and shape memory of Ti50Ni49Fe1 alloy. When the thickness reduction is beyond 30 %, the martensitic transformation is suppressed. After cold-rolling, the alloy is mainly composed of B2 parent phases with some stress-induced martensitic B19′ phases, and high density of dislocations are generated and the grains are obviously refined. The yield stress σ b significantly raises from 618 MPa of 0 % cold rolling to 1,338 MPa of 50 % SPD. Shape-memory effect increases from 6.5 % without cold rolling to 8.5 % after 30 % SPD, ascribed to the induced defects in cold rolling. Those results indicate that Ti50Ni49Fe1 alloy has improved mechanical properties and potential commercial applications after SPD.

The microstructures and mechanical properties of Ti–47 at% Ni–3 at% Fe shape memory alloy wire under the condition of severe cold-drawing at room temperature and different postdeformation annealing processes were intensively investigated using transmission electron microscope (TEM), X-ray diffraction (XRD), Vickers microhardness tester and electron tensile tester. It is indicated that the structure of the alloy evolves into a predominant amorphous structure with a trace of nanocrystalline B2 phase after the cold-drawing of 76 % areal reduction. Postdeformation annealing process exerted significant influence on the microstructure and mechanical properties. Crystallization occurs when the cold-drawn wire was annealed at 300 °C for 30 min. The ultimate tensile strength and ductility as well as the superelasticity of the wire are improved significantly by cold-drawing plus postdeformation annealing.

The effects of heat treatments on the phase transformation behavior of Ti49Ni49.5Fe1V0.5 and Ti48Ni48.5Fe1V2.5 alloys were investigated. The results indicate that the alloys subjected to different heat treatments have B2 structure at room temperature. All the specimens exhibit a two-stage B2 → R → B19′ martensitic transformation on cooling, but a B19′ → B2 one-stage reverse martensitic transformation on heating except aged A1 alloy, which undergoes an abnormal two-stage transformation upon heating. The phase transformation temperatures are affected by heat treatments and V content, which can be attributed to the variation of the second-phase particles content in the matrix.

This paper extends the recently developed cross modal strain energy (CMSE) method for damage localization and severity estimation, using limited modal data, with iterative process. For investigating the performance and verifying the effectiveness of the present CMSE method, the numerical study of this article chooses a damaged beam that was investigated in a published article. The numerical result indicates that the CMSE method is capable of detecting the damage locations and estimating their severities properly, even when only few spatially incomplete, noisy modes are utilized.

The direct mode shape expansion method is an iterative technique, then one can conclude that the convergence performance maybe challenged when applied to three-dimensional structures. In addition, mode shape values at different DOFs (degrees-of-freedom) sometimes are not in a same order of magnitude, which will produce much error for the estimation of small values of unmeasured mode components. Thus, this paper proposed a non-iterative mode shape expansion method based on coordinate decomposition and neglecting modelling errors between the finite element model and the experimental structure. The advantage of coordinate decomposition is that the unmeasured components of mode shape values could be estimated with different weighting coefficients, even in a physical meaningful interval. Numerical studies in this paper are conducted for a 30-DOF (degree-of-freedom) cantilever beam with multiple damaged elements, as the measured modes are synthesized from finite element models. The results show that the approach can estimate unmeasured mode shape values at translational and rotational DOFs in x, y and z directions with different weighting coefficients, respectively; and better mode shape expansion results can be obtained when proper constraints are employed.

The aim of the study was to investigate the epidemiology and risk factors of acute urinary retention (AUR) during pregnancy. We included all cases of pregnancies with AUR reported in Taiwan's Longitudinal Health Insurance Database from January 1, 1998, to December 31, 2011. Cases of AUR onset 1 day before delivery were excluded. The Cochrane-Armitage trend test and logistic regression analysis were used to evaluate the age distribution and types of deliveries of pregnant women. Chi-square tests and Fisher's exact test were performed to examine the association among all covariates. The odds ratios (OR) and 95% confidence intervals (CI) were estimated. We identified 308 cases of AUR in 65,490 pregnancies. The risk of AUR during pregnancy was 0.47%. The peak incidence occurred between the 9th and 16th gestational weeks. Patients who experienced preterm delivery exhibited the highest risk for AUR (2.18%). Those with post-term delivery had the second highest risk (0.46%), and patients with a normal delivery exhibited the lowest risk (0.33%). Compared with normal delivery, preterm delivery carried a higher risk of AUR (OR: 6.33, 95% CI: 4.94–8.11). The AUR risk was higher for patients with advanced maternal age (>35 years old) than it was for those in the younger group (< 20 years old) (OR: 2.62, 95% CI: 1.18–5.81). Within the normal delivery group, higher incidences of urogenital infection, gestational diabetes mellitus, previous abortion, abnormal pelvis, disproportion, and endometriosis were noted in women with AUR than in those without AUR (all P values

The influence of aging treatment on transformation behavior and shape memory of the Ti50−xNi48Fe2Nbx (x=0, 0.6, 0.8, 1.0, and 1.2) alloys was investigated using differential scanning calorimeter (DSC), mechanical drawing machine, and microhardness tester in this paper. It is indicated that the aging treatment has a significant effect on the phase transformation temperatures (Ms, Mf, Mp, As, Af, and Ap) and microhardness of the samples. The phase transformation temperatures are found to decrease initially with the increasing aging temperature from 300 to 500 °C and increase with further increase of the aging temperature. The aging treatment at intermediate temperature between 400 and 500 °C results in an improved shape memory effect. In addition, the highest microhardness value is also obtained.

The effect of annealing treatments and thermomechanical cycling on the transformation behaviors and shape memory effect of Ti48.5Ni48Fe2Nb1.5 shape memory alloys were investigated using electrical resistivity measurement and tensile testing. It is found that the transformation behaviors are influenced considerably by the annealing treatments. Both Ms and As increase with increasing annealing temperature and cooling rate. Martensite stabilization occurs during thermomechanical cycles, thus resulting in lower Ms and recovery ratio. Moreover, thermomechanical cycling leads to pseudo-elasticity. The yield point of stress-induced martensite (σSIM) increases with increasing cycles and pre-formation.

Jacket-type platforms may be the most common type of offshore structures, and damage localization and severity estimation is important for these structures. This paper employs a multi-hierarchical damage identification method based on BP neural network to detect damages in jacket platforms. Firstly, the damaged storey of the jacket is detected, and the numbers of the elements among the detected storey are then detected. According to this method, the learning samples can be more targeted and the number can be reduced largely. In the end, a jacket model is used to investigate the performance of this method, and the results indicate that this approach is more effective and has higher accuracy than direct diagnosis method.

The influence of Fe addition on the phase transformation and the microstructure of Cu67Al27-XMn6FeXshape memory alloys are investigated by means of electrical resistivity, X-ray diffraction, scanning electron microscopy and microhardness test. It was shown that the Ms (Martensitic start transformation) temperature of the 850°C heat-treated alloy exhibit a sharp increase as Fe content increases, by comparison with the alloy without heat-treatment. For example, after 850°C heat-treatment, the Ms temperature of the alloy increases from 52K to 135K when Fe is added from 0 at.% to 1.5 at.%. The microstructure of as-homogenized Cu67Al27-XMn6FeXalloys consists of Cu3Al matrix, γ2(Cu9Al4) and α (Cu) phases. Fe element was distributed in precipitates and matrix.

The influence of Nb addition on phase transformation, especially the intermediate R phase transformation and microstructure of TiNiFe shape memory alloys were investigated by means of electrical resistivity, optical microscope, energy dispersive spectroscopy and X-ray diffraction. The results showed that the R phase transformation was inhibited with the increase of Nb content. The martensite start transformation temperature decreased from 227K to 158K when the Nb content increased from 0.5 at.% to 2 at.%. The microstructure of the alloys were mainly characterized by TiNi matrix phase in which Fe and Nb resolved.

For the health monitoring of civil structures, the study of finite element model updating or damage detection mainly focuses on undamped systems. Thus, their objective has been exclusively restricted to the correction of the mass and stiffness matrices. In contrast, this paper performs the model updating and damage detection for damped structures. A theoretical contribution of this paper is to extend the cross-model cross-mode (CMCM) method to simultaneously update the mass, damping and stiffness matrices of a finite element model when only few spatially incomplete, complex-valued modes are available. Numerical studies are conducted for a five-story frame structure with multiple damaged elements. The numerical results reveal that applying the CMCM method, together with an iterative Guyan reduction scheme, can yield good damage detection in general.

Nowadays, damping is still the important dynamic characteristic that is the least understood and the most difficult to quantify. The advantage of the proposed damping matrix identification method is that several low order complex modes achieved by Experimental Modal Analysis (EMA) are capable of calculation precision. In order to identify the damping matrix, this paper also proposes a general damping matrix decompositon technique, which can be used in the situation that different parts of the structure have different damping. Then a five-story frame structure is applied to demonstrate the proposed method ,and the results obtained show the suitability of this approach for damping matrix identification in frame structures.

The phase transformation characteristics, the dynamic elastic modulus and the static tensile elastic modulus of Ti50Ni47.5Fe2.5 alloy were investigated. It is found that, the two mutations in the dynamic elastic modulus is caused by reverse martensite phase transformation and austenite phase transformation respectively; Static tensile test can not reflect the intrinsic elastic modulus when the test temperature is close to martensite transformation temperature(Ms). The static elastic modulus and the dynamic elastic modulus have the same trend when the test temperature is enough higher than Ms.

Microstructures evolution, mechanical properties, transformation behaviors and shape memory effects of low niobium content (5at.%) NiTiNb shape memory alloys were investigated. It was found that the amount of Nb resolved in the NiTi matrix is closely related to the Ni/Ti ratio and has significant influence on the mechanical properties, especially the yield stress of the alloys. Ti 45.2 Ni 49.8 Nb 5 and Ti 45 Ni 50 Nb 5 alloys exhibits as excellent shape memory effect as the equiatomic binary NiTi alloys.

The influences of Co-addition on phase transformation behavior and mechanical properties of TiNi alloys were investigated. Results indicate that, as a substitute for Ni, Co added to TiNi alloys can dramatically decrease the martensite transformation temperature, and R phase transformation and martensite transformation are accordingly separated. When Co-content reach 10 at.%, the martensite transformation temperature is lower than that of liquid nitrogen. During deformation at room temperature, Ti 50 Ni 48 Co 2 alloy exhibits good ductility with a lower stress plateau caused by stress-induced martensite and martensite reorientation.

A damage detection method reflecting initial damage of the elastic structure is presented. It is based on the idea that the damage will decrease the structure stiffness. From eigenvalue issue, regard Poison ratio and elastic modulus as a constant and a random variable, respectively, and in terms of known data and a combination of the FEM, then equations solving random modulus of the undamaged and the damaged structure are obtained, respectively. Based on the element damage indicator, initial damage of the structure can be detected. At last, a five-storey shearing structure is simulated, and the results indicate that initial damage situation can be accurately calculated, and initial damage locations can be estimated based on the proposed method, which is simple and effective, and contributes to the application in Engineering.

Two layer thermal barrier coatings (TBCs) were prepared by EB-PVD (Electron Beam-Physical Vapor Deposition) at different substrate temperatures in the range of 823 to 1123K, and their microstructure was investigated with SEM and AC impedance as a function of substrate temperature and thermal cycling time. YSZ layer of all TBCs samples is in column structure, but the grain size and growth orientation are different with substrate. In this research, impedance spectra (IS) was measured as a function of thermal cycling between 1323K and 298K for these thermal barrier coatings. Grain boundary and bulk can be distinguished from analysis of AC impedance spectroa to provide information about the relation between microstructure and electric properties.The change in IS until failure was found to be related with the thickness, microcracks and macrocracks of TGO and the change in the interfacial of TGO/YSZ.

Static and dynamic mechanical behavior of Ti50Ni47Fe3 shape memory alloy was investigated by mechanical tensile test and instrumention impact test under a wide temperature range from -196°C to 20°C. The instrumention impact test was carried out under different temperatures at which the alloys are with martensite, austenite and R phase, respectively. It is found that the alloy with austenite performed the best ductility, the samples with martensite and R phase exhibit superior and inferior ductility, respectively.

In this article, the influence of Co addition on phase transformation behavior and mechanical properties of TiNiFe shape memory alloy was investigated extensively. Differential scanning calorimetry (DSC) measurements shows that martensitic start transformation temperatures (M s) decrease drastically with increasing Co content, while the R phase transformation start temperatures (R s) vary slightly. Nevertheless, the substitution of Ni with Co does not exert substantial influence on the two-stage transformation behavior of the TiNiFe alloy. The results from stress–strain curves indicate that higher critical stress for stress-induced martensitic transformation (σ SIM) has been obtained because of Co addition. In such cases, the Ti50Ni48Fe1Co1.0 alloy maintains a good shape memory effect, and a maximum recoverable strain of 7.5 % can be obtained.