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Creep age forming (CAF) is an advanced forming technology that combines creep deformation and age hardening processes. When compared with the conventional forming technologies including roll bending and shot-peen forming, CAF has many advantages of low residual stress, excellent dimensional stability, good service performance and short production cycle. It is an optimal technique for precise manufacturing for shape and properties of large-scale complicated thin-walled components of light-weight and high strength aluminum alloys in the aviation and aerospace industries. Nevertheless, CAF has an inevitable disadvantage that a large amount of springback occurs after unloading, which brings a challenge on the accurate shape forming and property tailoring of components. Therefore, how to achieve accurate prediction and control of springback has always been a bottleneck hindering the development of CAF to more industrial applications. After the factors of affecting springback and measures of reducing springback are summarized from the internal and external aspects, constitutive models for predicting springback and springback compensation methods for CAF of aluminum alloy panel components are reviewed. Then, a review of research progresses on tool design for CAF is presented. Finally, in view of the key issue that it is difficult to predict and control the shape and properties of components during CAF, the technical challenges are discussed and future development trends of CAF are prospected.

Natural gas hydrates, as a new energy source, are attracting more and more attention. Safe and efficient exploitation has become one of the hot issues in current studies. In order to investigate the creep behaviour of sediments containing methane hydrates, a series of triaxial compression tests and creep tests was conducted under various conditions. First, stress–strain curves were drawn, and then the failure strength of the sediments was obtained. Next, on the basis of creep tests, the creep curves were recorded. The results showed that the attenuation- and steady-creep stages always existed during creep, but the acceleration-creep stage just appeared under higher-temperature and higher-stress conditions. The creep test results indicated that the confining pressure, temperature and stress level affected the creep characteristics of sediments, such as the initial strain ε 0, the rheological initial time t m, the rheological initial strain ε m, the failure time t f and the failure strain ε f. The findings suggested that, during the creep tests, more hydrate dissociation accelerated the damage to the sediments’ strength under higher stress level, temperature or confining pressure.

The wheel tracking test (WTT) has been extensively used for laboratory characterization of permanent deformation of hot mix asphalt (HMA). However, the fully-confined setup of the device is unable to capture the tertiary zone where shear deformation takes place. This makes the permanent deformation resistance characteristics of HMA difficult to analyze. Hence, a modified wheel tracker (MWT), with unconfined lateral sides along the wheel tracking direction was utilized in this study, which can capture the tertiary flow (referred to as the flow number (FN)). The dynamic creep test, which yields FN, is a common laboratory test which has the ability to characterize permanent deformation considering the shear behavior. The study investigated the relationship of the MWT and the dynamic creep test utilizing the permanent deformation and permanent microstrain data from the respective tests. A novel parameter, FN-Index was explored. The permanent deformation parameters derived from the MWT were well correlated with the dynamic creep test. The MWT showed promising repeatability for the FNs.

The short-term vibration response of uncracked prestressed concrete (PC) members under long-age conditions was investigated. No study has examined long-term phenomena of concrete, such as its tendon relaxation, creep, curing, and shrinkage, under the aforementioned conditions. Therefore, a research sub-program was initiated at the National Center for Research on Earthquake Engineering on the basis of an experimental campaign on uncracked PC bridge members initiated in 2015. In particular, a simply supported PC beam composed of a parabolic-bonded tendon and high-strength concrete made in Taiwan was allowed to short and, consequently, long-term prestressing losses measured for approximately 9.5 months. During short-term prestressing losses and after long-term prestressing losses, free short-term transverse vibrations were induced in the PC beam characterized by a second-order initial curvature and an axial end constraint. The time-dependent Young’s modulus of the beam was determined by conducting compression tests on cylinders during the short-term prestressing losses. Moreover, compression tests were conducted on cores drilled along the PC beam’s span after free vibration tests. The theoretical time-dependent Young’s modulus of the beam was evaluated according to the compression test results. The experimental data were compared with a reference Euler–Bernoulli solution and finite-element (FE) model that describe the dynamics of PC girders by considering the effective prestressing force. The fundamental frequencies of the PC beam were mainly sensitive to the variation in its initial tangent Young’s modulus due to the consolidation or hardening of concrete. These frequencies were unaffected by the second-order initial curvature. In conclusion, the reference solution and FE model can accurately predict the short-term dynamics of uncracked PC members exposed for long times.

This study presents a prediction of creep behavior concrete matrix based on waste glass powder during 2000 h of loading and describe a procedure for modeling the primary creep law and we have presented the intergranular contact simulation bodies using ANSYS software. The behavior of this type of concrete is in the reaction of pozzolan reactivity of waste glass powder, which acts as a substitute for cement powder. The modeling procedure of creep behavior consist to applied the finite element method (FEM) based on a model called (strain hardening model) using the computer code ansys14.0. This paper illustrates a new approach of study by the FEM of the intergranular concrete matrix on the microscopic level and to note closely the phenomena of creep behaviors. This item presents ANSYS ® modeling interface Glass Powder – granular in a concrete Matrix structure. Contact problems fall into two general classes: rigid-to-flexible and flexible-to-flexible. In general, any time a soft material comes in contact with a hard material, the problem may be assumed to be rigid-to-flexible. To model a contact problem, you first need to identify the parts to be analyzed for their possible interaction. If one of the interactions is at a point, the corresponding component of your model is a node. If one of the interactions is at a surface, the corresponding component of your model is an element. The finite element model recognizes possible contact pairs by the presence of specific contact elements. These contact elements are overlaid on the parts of the model that are being analyzed for interaction. ANSYS ® 14.0 software has been used to perform the numerical calculation in this paper.

In this work, we applied a multilevel modeling technique to estimate the diametral creep in the pressure tubes of Korean Canada Deuterium Uranium (CANDU) units. Data accumulated from in-service inspections were used to develop the model. To confirm the strength of the multilevel models, a 2-level multilevel model considering the relationship between channels for a CANDU unit was compared with existing linear models. The multilevel model exhibited a very robust prediction accuracy compared to the linear models with different data pooling methods. A 3-level multilevel model, which considered individual bundles, channels, and units, was also implemented. The influence of the channel installation direction was incorporated into the three-stage multilevel model. For channels that were previously measured, the developed 3-level multilevel model exhibited a very good predictive power, and the prediction interval was very narrow. However, for channels that had never been measured before, the prediction interval widened considerably. This model can be sufficiently improved by the accumulation of more data and can be applied to other CANDU units.

The thermostructural behavior of a regeneratively cooled thrust chamber for aerospace applications is strongly non-linear as plastic and creep phenomena come into play. Indeed, the fuel combustion ...

Variable creep load is one of the most important failure modes for hot-component of aero-engine. To accurately predict creep damage and remaining life, a novel nonlinear creep damage accumulation model is proposed based on the Wilshire function and isodamage line, which takes the influence of load history into account and uses ultimate tension strength to compensate the temperature effect. Experimental result of tests on four kinds of material were utilized to verify the accuracy of the proposed model and to compare it with existing models. It was determined that the novel model was better at predicting damage accumulation than all others model. Furthermore, the proposed model elucidates the evolutionary process of creep damage, and four cases of damage evolution process are discussed.

There are limited studies in the development of testing apparatus on unsaturated soils due to the complex structure, tedious calibration process, and large demands on test time and skills. In this study, a novel multifunctional apparatus (MFA) has been designed and manufactured for testing the unsaturated soils. This state-of-the-art testing apparatus is based on modular design and can be configured to conduct a variety of tests such as pressure plate test, oedometer test, direct shear test, and triaxial creep test under suction-controlled conditions. This apparatus has a high-grade stainless-steel chamber which is used to measure the total volume changes of an unsaturated soil specimen in triaxial creep tests. Based on the calibration results, the creep coefficient for MFA is about half of the coefficient for a typical double-cell system. Additionally, the MFA is designed in a manner to enable testing specimens with different dimensions and to control soil suction based on different techniques. Typical calibration results, test procedures, and experimental results obtained from this apparatus are presented and discussed in detail. Finally, the performance of this apparatus is well verified by the experimental results. It is found that the MFA is capable of and reliable for performing above-mentioned tests and accurately investigating engineering behaviour of both saturated and unsaturated geomaterials.

Background Web creep is the most common long-term complication requiring revision after syndactyly surgery; however, few methods have been reported. The aim of this study was to introduce a newly designed asymmetric dorsal gull wing flap to reconstruct web for the postoperative web creep. Methods A retrospective analysis was performed for 20 patients from January 2016 to May 2019. Sex, age, original malformations, original surgical procedure, complications, time between the 2 operations, operation records, preoperative and postoperative photographs, and Withey score were reviewed. Results Eleven boys and 9 girls with average age of 60.65 ± 44.76 months underwent revision. Twenty-nine web spaces were affected (web creep, 12 cases; web creep and scar contracture, 17 cases). The original surgical procedure consisted of syndactyly separation in 15 cases, syndactyly separation with a full-thickness skin graft in 5 cases. There was 1 case of postoperative infection. All patients received an asymmetric dorsal gull wing flap and a zigzag incision, 15 patients received an additional full-thickness skin graft. The average time interval between the 2 operations was 34.60 ± 35.94 months. The follow-up time was 34.30 ± 20.73 months. No complications were noted, none of the patients redeveloped web creep. The median values for web creep, flexion-extension deformity, total Withey scores in the postoperative period were significantly lower than the preoperative values. The appearance and function of all digits were good. Conclusions The asymmetric dorsal gull wing flap is a good choice for web reconstruction when web creep is caused by syndactyly surgery.

WRe/TZM gradient material was successfully fabricated simultaneously by spark plasma sintering technique with the gradient die to achieve the one-step bonding. The gradient sintering behavior and the effect of the sintering parameters on microstructure and mechanical properties of WRe/TZM joint were investigated. The results showed that the powders of WRe and TZM alloys could be densified and bonded simultaneously using gradient die to construct different sintering temperatures, and the relative densities of gradient material with WRe and TZM alloys were over 98%. The optimal joint with no pores and cracks was obtained at T = 1600 °C, t = 8 min and P = 30 MPa, under which the joint has the highest shear strength of 279.92 MPa. The Berkovich hardness of interface was mainly affected by the grain size and the solid solution of W and Mo. The resistance to creep at interface was between WRe alloy and TZM alloy. The joint exhibited excellent thermal diffusivity of 44.86 mm2/s. The WRe/TZM gradient material with excellent properties indicates this process is a promising technique route for fabricate the anode target for X-ray tube of CT scanner.

Tunnels are generally designed for a sustained usage of 80 to 100 years, during which the safety of tunnel structures must be guaranteed. A common supporting form utilized in contemporary tunnel engineering is composite lining. To derive applicable parameters of the supporting form and therefore ensure the long-term safety of the tunnel structure, it is imperative to determine the extra acting force exerted onto the composite lining by the creep of the rock surrounding the tunnel and to calculate the stress-strain characteristics of composite lining. In the current study, this paper proposes an approach termed surrounding reinforcement, which is based on the homogenization method. Specifically, this paper defined the bolt force as the internal force of the surrounding rock, analyzed their viscoelastic-plastic properties using the unified strength theory, and derived an equation for calculating the stress-strain relationship of the composite lining. To further validate the method in tunnel structures, this paper applied the derived equation to a representative instance. The results of this paper show that the initial support force has also increased during the creep process of the surrounding rock, indicating that engineers should pay close attention to the coordination between the strength of initial support and the secondary lining and thus ensure an optimal distribution of the pressure from the surrounding rock when designing composite lining tunnel within weak strata. This paper proposes that the initial support not only would guarantee the tunnel safety during the construction stage but also could cooperate with the secondary lining to brace the stress caused by the creep, ensuring that the supporting structure stays stable across the whole period of tunnel operation. This paper provides an alternative to previous methods that is more comprehensive, with simpler calculations, and more applicable to the composite lining supporting design within weak strata.

Under long-term load, the creep deformation of concrete materials has a serious impact on the structural safety of hydraulic structures, especially under the action of ultra-high stress levels, the concrete materials will undergo nonlinear creep, which is extremely easy to cause structural damage. In this study, the uniaxial nonlinear creep test of concrete specimens was used to establish the damage index based on the wave velocity value of ultrasonic flaw detection, and the creep and damage degree curve of the concrete specimen were obtained. The ideal elastic element, the Kelvin body, and the nonlinear viscoplastic element are connected in series, and a new viscoelastic–plastic model considering the creep characteristics of concrete is proposed. Based on the principle of least squares, the Levenberg–Marquardt (LM) algorithm is used to inverse the parameters of the nonlinear creep test. In addition, the model is verified by the measured data of linear creep. At the same time, the sensitivity of each model parameter is analyzed. The research shows that the LM algorithm can give the fitting parameters of the model better and faster, and the fitting values of the model are similar to the experimental results. The sensitivity analysis of the parameters shows that the proposed model has good stability and good adaptability. The model has a more accurate description of the various stages of creep, and may be conveniently applied to concrete creep calculations in actual projects.

Coal mines in Pingdingshan mining area, Henan province, China, are commonly mined at a great depth up to 1000 m, and inevitably reside in high temperature, high pressure, high permeability, strong mining disturbance, and strong time effect environment. Support structures of the mines in the area are vulnerable to failure and destruction while roadway stability has been a long-standing issue in the region. Based on the specimens collected from the roof of the return-air rise at Pingdingshan No.1 coal mine, here, we investigated the creep characteristics of sandy mudstone under triaxial conditions and developed a constitutive relationship to predict the creep behavior. Parameter identification and sensitivity analysis were performed to determine the influential factors of creep deformation. We showed that the creep deformation and steady-state creep rate at Pingdingshan coal region grow as axial stress is elevated while descend with increased confining pressure. However, the maximum creep deformation of sandy mudstone increases with a higher confining pressure. Thereby, stress difference, elastic modulus and coefficient of viscosity are the major parameters controlling creep deformation. Hence, it is important to enhance the support strength and mechanical properties of the surrounding rock, to better the roadway stability. In addition, numerical simulation was also conducted for simple excavation geometries to analyze different support structures. On this basis, the “cable bolts steel arch, deep and shallow reinjection, floor pressure relief and local reinforcement” scheme was proposed and field practice showed promising results. The findings in this study can be used to guide the support design for the deep soft rock roadway (DSRR).

International audience; The use of structural adhesive bonding is under development for offshore applications and more specifically for fastener connection to existing steel structures. For this application, creep appears to be one of the main long-term phenomena that needs to be considered during the design approach. To investigate this issue, experimental creep investigations were realised at real scale for two bonded fastener geometries at different stress levels and under different load situations (tension and shear). Local measurements were carried out during these investigations to better describe the long-term behaviour of the bonded fasteners. Despite the non-uniform adhesive thickness in the assembly, numerical investigations have highlighted that the stress field mainly depends on the fastener geometry and the applied load level. This article is dedicated to the description of a methodology that relies on a precise inverse identification of creep parameters and on simplifying hypotheses on the stress field to be able to use simple analytical tools. The methodology enables to identify creep parameters much quicker than a method coupling an optimisation algorithm and a finite element analysis and could easily be used for more complex and precise models.

Purpose Meniscal surgery is one of the most common orthopaedic surgical interventions. Total meniscus replacements have been proposed as a solution for patients with irreparable meniscal injuries. Reliable fixation is crucial for the success and functionality of such implants. The aim of this study was to characterise an interference screw fixation system developed for a novel fibre-matrix-reinforced synthetic total meniscus replacement in an ovine cadaveric model. Methods Textile straps were tested in tension to failure (n = 15) and in cyclic tension (70–220 N) for 1000 cycles (n = 5). The textile strap-interference screw fixation system was tested in 4.5 mm-diameter single anterior and double posterior tunnels in North of England Mule ovine tibias aged > 2 years using titanium alloy (Ti6Al4Va) and polyether-ether-ketone (PEEK) screws (n ≥ 5). Straps were preconditioned, dynamically loaded for 1000 cycles in tension (70–220 N), the fixation slippage under cyclic loading was measured, and then pulled to failure. Results Strap stiffness was at least 12 times that recorded for human meniscal roots. Strap creep strain at the maximum load (220 N) was 0.005 following 1000 cycles. For all tunnels, pull-out failure resulted from textile strap slippage or bone fracture rather than strap rupture, which demonstrated that the textile strap was comparatively stronger than the interference screw fixation system. Pull-out load (anterior 544 ± 119 N; posterior 889 ± 157 N) was comparable to human meniscal root strength. Fixation slippage was within the acceptable range for anterior cruciate ligament graft reconstruction (anterior 1.9 ± 0.7 mm; posterior 1.9 ± 0.5 mm). Conclusion These findings show that the textile attachment-interference screw fixation system provides reliable fixation for a novel ovine meniscus implant, supporting progression to in vivo testing. This research provides a baseline for future development of novel human meniscus replacements, in relation to attachment design and fixation methods. The data suggest that surgical techniques familiar from ligament reconstruction may be used for the fixation of clinical meniscal prostheses.

To quantify the intensity of automobile brake creep groan, both experimental and analytical studies are innovatively conducted on the friction-induced vibration and noise of the disc brake in this paper. Experimentally, three factors, brake disc initial temperature, terrain, and gear position, are comprehensively contemplated to design six different test conditions, based on which a creep groan vehicle road test is conducted. Depending on the subjective evaluation and statistical analysis of the annoyance degree caused by vibration and noise in the starting and braking process under different test conditions, the influence of each factor on creep groan intensity is obtained. By processing the brake caliper and lower suspension arm acceleration and the interior sound pressure signals acquired in the test, the occurrence conditions and transient dynamic characteristics of creep groan are explored. One of the creep groan vibration modes is triggered by the rapid removal of the force exerted on the brake pedal when the vehicle starts at an extremely low speed, which presents intermittent impact and chaotic characteristics. The other vibration mode shows periodic harmonic characteristics, whose energy is concentrated on specific frequencies in the form of frequency doubling. This vibration mode occurs when continuously maintaining a low brake oil pressure or slowly releasing the force exerted on the brake pedal during the vehicle low-speed starting process, the velocity-displacement phase-plane portrait of which shows a stable limit cycle of stick-slip motion. Analytically, four vibration indexes and four noise indexes are established to evaluate the intensity of creep groan, the validity of which is verified by linear regression analysis. Finally, by combining all effective indexes, multiple linear regression analysis is performed, based on which the mapping relationship between the subjective evaluation of creep groan and effective indexes is obtained. The results demonstrate that the combination of the logarithmic vibration dose value of maximum brake caliper acceleration pulse and the loudness of interior noise can accurately describe the annoying degree caused by creep groan. The regression model can quantify and predict creep groan intensity of the test vehicle under different test conditions, which has guiding significance for engineering applications.

Inconel-625 is used for the fabrication of bellows in high-temperature sodium valves operating at 843 K in fast breeder reactors. Creep deformation coupled with the damage is one of the major failure modes of the bellow. The creep data required for the damage tolerant design analysis are not available in the published literature for Inconel-625 at 843 K. In view of the above, the present study focuses on the generation of creep data and analyzing the creep deformation and damage behaviour of Inconel-625 at 843 K over a stress range of 650–710 MPa. It was observed that the variations in steady-state creep rate and rupture life as a function of applied stress followed the power-law relationship. For the complete description of creep strain–time data of Inconel-625 at 843 K, two different theta-projection methodologies (four-theta projection and five-theta projection) were employed. The relative applicability of both relationships has been verified based on their fitting capability of creep strain vs. time as well as creep rate vs. time data. It has been observed that five-theta projection model provides a better description of experimental data compared to the four-theta model across different stress levels, except at 710 MPa. Due to the occurrence of minimum creep rate, instead of steady-state creep rate, four-theta projection provides a better prediction than five-theta projection at 710 MPa. This methodology also helps in understanding the evolution of strain contribution from different stages of creep life for a given time using the optimized parameters.

Rehabilitating the deteriorated deck of post-tensioned concrete box-girder bridges by ultra-high-performance concrete (UHPC) overlay is studied in this paper. Three post-tensioned concrete box-girder bridges in California were selected for the deck overlay analysis. Rehabilitation construction procedures for each bridge were described, including the practical lane closure and traffic re-routing. Detailed finite element models were established incorporating the staged construction modeling of the rehabilitation process, the time-dependent properties for existing concrete and prestressing tendon, and a “double-layer” modeling technique for the simulation of the deck. The time-dependent curves and equations of a specified UHPC were fitted based on experimental data, including creep, shrinkage, and elastic modulus, and this particular mix has been assumed in the FE analysis. How the thickness of the deck layer to be replaced affected the stress distributions in the UHPC overlay and the existing box girder was parametrically studied. Results indicated that stress variations in the existing box-girder were acceptable during the deck rehabilitation construction, but significant tensile stresses were developed in the newly placed UHPC deck overlay. Shrinkage-mitigation measures, such as adding the shrinkage-reducing admixture (SRA), should be considered during the placement of the UHPC overlay to control the shrinkage-induced tensile stress. Steel reinforcement may be adopted in the negative bending moment region to resist the flexural tensile stress by the live loads. The rehabilitation thickness of 3.8–5.1 cm (1.5–2.0 in.) is deemed appropriate for the bridges selected in this study.

Reinforced concrete bridges constitute the major part of existing bridges. The purpose of this paper is to present a serviceability evaluation methodology for RC bridges under probabilistic conditi...

Infallible creep rupture life prediction of high temperature steel needs long hours of robust testing over a domain of stress and temperature. A substantial amount of effort has been made to develop alternative methods to reduce the time and cost of testing. This study presents a finite element analysis coupled with a ductility based damage model to predict creep rupture time under the influence of multiaxial stress state of ex-service and as-received Grade 91 steel at 873 K. Three notched bar samples with different acuity ratios of 2.28, 3.0 and 4.56 are modelled in commercial Finite Element (FE) software, ABAQUS v6.14 in order to induce different stress state levels at notch throat area and investigate its effect on rupture time. The strain-based ductility exhaustion damage approach is employed to quantify the damage state. The multiaxial ductility of the material that is required to determine the damage state is estimated using triaxiality-ductility Cock and Ashby relation. Further reduction of the ductility due to the different creep mechanisms over a short and long time is also accounted for in the prediction. To simulate the different material conditions: ex-service and as-received material, different creep coefficients (A) have been assigned in the numerical modelling. In the case of ex-service material, using mean best fit data of minimum creep strain rate gives a good life prediction, while for new material, the lower bound creep coefficient should be employed to yield a comparable result with experimental data. It is also notable that ex-service material deforms faster than as-received material at the same stress level. Moreover, higher acuity provokes damage to concentrate on the small area around the notch, which initiates higher rupture life expectancy. It also found out that, the stress triaxiality and the equivalent creep strain influence the location of damage initiation around the notch area.

Three-dimensional finite element (FE) thermal and stress models were created to compute the temperature evolution, thermal stress and potential of cracking in a segmental concrete box girder segment during construction. User-defined subroutines were developed in the ANSYS program to activate the degree of hydration-dependent heat rate and material properties, and creep behavior in the thermal and stress calculations. The developed FE model was verified with experimental measurements of a concrete cube. Adiabatic temperature rise, together with compressive strength and splitting tensile strength for a high-strength concrete mix typically used in construction of box girders were tested and incorporated in the subroutines. The effect of casting time and placement season (summer and winter), initial concrete temperature change, and insulation on the risk of cracking in a cast-in-situ box girder segment at early ages was investigated using the proposed model. The results indicate that the temperature difference between the segment’s middle and the gate corner is very large leading to a high cracking risk. Use of an insulation material such as blankets along with casting concrete at the nighttime would significantly lessen the thermal tensile stress and thus could reduce cracking risk in the segment.

Due to the demand, especially by the power generation and the aerospace industries, for materials withstanding prolonged high temperatures – and relatively high stresses – it has become imperative ...

The life of turbine blades often determines the life of the aero-engine, and creep rupture is one of the main fracture forms of turbine blades in high temperature environments. In order to accurate...

Proverbially ‘To travel hopefully is better than to arrive.’ Many roads have been travelled in search of a short-term mechanical test that would reliably predict long-term component creep life. The...

The basic principle of a noncontact method of measurement of conductivity of liquids, presented earlier, is analysed to evolve design rules to have minimal error in the measurement. In this method, the current is sent through the liquid whose conductivity is to be measured by induction and the voltage across the liquid column is measured using capacitive coupled electrodes and the conductivity computed therefrom. Hence, problems that arise due to contamination or corrosion of contacting electrodes are eliminated. The signal conditioning circuit is developed such that the measurement of the voltage-drop across the column of the liquid and hence the determination of the conductivity of the liquids unaffected by the coupling capacitances of the potential electrodes. Thus, small deviations that can creep in during manufacturing of the probe (manufacturing tolerances) do not affect the performance. An error analysis of the proposed system was carried out and the practicality of the scheme verified by developing and testing a prototype. The test results showed a worst-case error of 0.82% in conductivity measurements. The facts that the proposed method is unaffected by the coupling capacitors and the presence of materials kept outside of the probe were also verified.

In 3D IC integration, a critical demand of interfacial joints in high-end devices is ultra-fine pitch, below 2 μm. The pitch cannot be fabricated by using solder joints because of its low melting point. Microelectronic industry prefers a solid state process, but the processing temperature cannot be too high, preferable near 250 °C. Thus, Cu-to-Cu direct bonding has attracted much attention. We have bonded the highly (111)-oriented Cu microbumps to highly (111)-oriented Cu films as well as to randomly-oriented Cu films for comparison, at temperatures from 200 to 350 °C. We examined the microstructures of bonded interfaces and conducted shear tests to evaluate the mechanical strength of the Cu joints. The bonded interfaces for the (111) Cu microbumps to (111) Cu films had fewer voids than those bonded to randomly oriented Cu films. In addition, the bonding strength of the latter is much less than the former. We propose that the fast surface diffusivity on the (111) surfaces play a critical role on surface creep, which increases the bonding strength of the Cu joint. A surface creep model and its creep rate are presented, and a comparison to the experimental data has been made; the agreement is reasonable.

In order to explore the influence of creep pre-damage on bursting liability of coal and rock, the creep pre-damage treatment of coal with different stress levels (0%, 30%, 45%, and 60%) was carried out, and the bursting liability index test was carried out. The variation rules of uniaxial compressive strength, impact energy index, elastic energy index, and dynamic failure time with different degrees of creep pre-damage was analyzed. The bursting liability of coal is evaluated comprehensively. The results show that with the increase of creep pre-damage degree, the uniaxial compressive strength, elastic energy index, and impact energy index of coal sample first increase and then decrease, and the dynamic failure time first decreases and then increases. The comprehensive evaluation result of coal sample bursting liability changes from weak impact to strong impact, then to weak impact, and finally to no impact. This paper provides a reference for the study of rock burst of coal under the influence of creep.

Laminated Veneer Lumber (LVL) Sengon is classified as one of the engineering products having a significant increase of both physical and mechanical properties compared with Sengon solid wood. Considering its short planting years and sustainable production, Sengon wood is very potential to be used as construction materials of low-rise houses to support the housing needs in Indonesia. Creep behaviour of LVL Sengon material is one of the mechanical properties that needs to be considered. This article evaluated value of creep factor of the open web truss joist (OWTJ) LVL Sengon test and compared this experimental creep factor with the numerical results developed by FE model taking into account the viscoelastic parameters of authors' previous study. The viscoelastic parameters were based on a 217-day creep test of compression and tension parallel to the grain of LVL Sengon at 20 % of stress level that were further modeled using Prony series creep model having n equals to 3. The reduction in the modulus of elasticity over time resulted in creep deflection and creep factor values at 217 days of testing results and FE numerical analysis of the OWTJ LVL Sengon ranging from 1.50–1.54; while the predicted creep factor at 25 years of service life is 1.57 or greater than the creep factor value provided in SNI 7973: 2013 of 1.5.

This work summarizes the prediction models for creep strain that taken place in concrete under compression as well as under tension and shear. In literature, many researchers have carried out experimental studies on creep, RILEM has gathered these experimental results within a computerized data bank. This work briefly reviews the most widely used models and can predict observations up to 5000 days assessed by ACI209R-1992 Model, B3 Model, CEB C99 Model, GL 2000 Model, BS 8110 (1985), and AS3600 (1988). Since creep is one of the most important time-dependent properties of concrete as it increases cracking and harmfully distresses the function, durability, and structural appearances. The objective of this work is to understand the behavior of concrete when creep strain takes place at different ages by incorporating spreadsheets which simplify the calculations for engineers when estimating the creep strain for the design purpose. Based on the RILEM data bank, most of the studies confirm that the GL2000 Model is the closest to the experimental results.

Many analytical and numerical analysis and design methods for geosynthetic-reinforced soil structures require a single-value (constant) estimate of reinforcement stiffness. However, geosynthetic reinforcement products are rate-dependent polymeric materials meaning that they exhibit time and strain-dependent behaviour under load. Hence, the appropriate selection of a constant (elastic) stiffness value requires careful consideration. A simple hyperbolic stiffness model is shown to be a useful approximation to the constant-load isochronous creep-strain behaviour of these materials at low load levels applicable to operational (serviceability) conditions of geosynthetic-reinforced soil structures. A large database of 606 creep tests on 89 different geosynthetic reinforcement products falling within seven different product categories was collected. From these data, isochronous stiffness values were determined for different combinations of duration of loading and strain level. Data from products falling within the same category were collected together to provide approximations linking the isochronous load-strain (creep) stiffness to the ultimate tensile strength of the material. These approximations are useful for analytical and numerical modelling particularly when parametric studies are undertaken to identify the sensitivity of model outcomes to reinforcement stiffness. Finally, three different geosynthetic-reinforced soil application examples are provided to demonstrate the important role of tensile stiffness on analysis and design outcomes.

In deep coal and rock mass engineering, extensive initial damage problems caused by excavation and unloading are critical to the long-term deformation and stability of underground projects. An innovative initial damage coal sample test method was proposed, and coal samples with different initial damage were prepared by the MTS pressure tester's circular displacement cyclic loading and unloading control. The creep characteristics of coal samples with different initial damages were tested by the staged loading creep test method. With the increase of initial damage, the coal samples’ creep time became shorter, and the rate of accelerated creep damage increased sharply. The relationship between fractional viscosity coefficient and stress was introduced to describe the non-linear relationship of creep strain increase at each stage. The creep model in the acceleration phase was improved by establishing the relationship between the damage variable and the initial damage. Based on the fractional calculus theory, a viscoelastic-plastic model considering the initial damage of coal samples was established, and the fractional order model was verified. The coefficient of correlation between the experimental data and the creep model was higher than 0.95, indicating that the fractional creep model can better describe the creep characteristics of coal samples with initial damage.

The variability of the virtual time of stepped isothermal method (SIM) was observed during the creep master curves construction of a new high-performance geosynthetics material, which led to the significant errors in predicting the long-term creep. After analyzing the process of construction, the difference in selecting the beginning and end segments is the reason causing the errors. Therefore, a set of computer-based routine was programmed to eliminate the errors. Based on the amount of data of SIM and conventional creep tests, the virtual time of each corresponding step, the curves of scaled creep data, and the comparison between these two creep data were obtained in the present study by the routine. They were used to study the influence of different beginning segments and end segments on the master creep curve. Overall, The 5% of elevated temperature step and 50% of the previous step is suggested to regard as the beginning segment and end segment respectively to obtain the more well-founded virtual time and more accurate creep curve. This study not only provides a reference for research focusing on accelerated creep tests but proposed a fast and accurate methodology to construct the creep master of SIM tests.

Coal mining activity disturbs the natural equilibrium of in situ stress state such that the induced stresses deform the surrounding rock mass. Although the installation of roof supports typically prevents rock deformation, rock mass failure may still occur in bolted roof. The erratic failure of rock mass after few days to months and years of excavation is the result of time-dependent deformation of roof rock under constant surrounding stresses. In the current research, the triaxial laboratory creep experiments proved that Marcellus shale exhibits time-dependent deformation under constant stress that was smaller than the failure strength. However, unlike conventional experimental results, the time-dependent deformation did not depend on the level of constant stress. The often-neglected initial state of specimens and intrinsic heterogeneity highly influenced the time-dependent failure of Marcellus shale.

Reliability assessment of solder joints under thermomechanical cycling is one of the long lasting concerns in the field of electronic materials. To pay attention to this issue, an artificial neural...

Through the adiabatic temperature rise experiment, the adiabatic temperature rise of concrete with hydration time was recorded. Based on the maturity degree theory, the relationship between the hydration degree of the concrete and the equivalent age was determined. Then, the hydration degree prediction model of the concrete's early elastic modulus and tensile strength was established. The local temperature and humidity of the concrete were measured by the shrinkage experiment, and based on the capillary water tension theory, a temperature-humidity prediction model for the early shrinkage of the concrete was designed. According to the ratio of the creep deformation and elastic deformation of concrete which were obtained through the restraint ring experiment, a model for predicting the early creep coefficient of concrete was proposed. Based on the coupling effect of “hydration-temperature-humidity,” a prediction model of early cracking risk coefficient of concrete under multifield coupling was proposed. Finally, several groups of slab cracking frame experiments were carried out, and the cracking risk prediction results of concrete were consistent with the actual situation, which indicated the correctness of the early cracking risk prediction model of concrete.

This paper presents a methodology for the development of two creep models for the welds and the steel base material that are capable of predicting the thermal creep behavior of transverse welded lap joints. This methodology is based on the experimental program of the thermal creep effect on transverse welded lap joints conducted previously as a part of this research. The experimental results are in the form of creep curves and they are used to develop a Norton–Bailey power law creep equation for the welded lap joints. Then, two creep models for the welds and steel base material are proposed by introducing temperature-dependent scaling factors to the Norton–Bailey power law creep equation of the welded lap joints. Finite element (FE) simulations are then developed in ABAQUS to predict the thermal creep behavior of the welded lap joint using the two proposed creep models for the welds and the steel base material. The two creep models are calibrated by changing the power law creep constants in order to predict the experimental creep curves with reasonable accuracy. Another series of FE simulations is conducted using Fields and Fields creep model for the steel base material instead of the proposed one for comparison purposes. The weld and structural steel creep models are intended to support the development of modeling the creep behavior of large-scale welded connections for structural-fire engineering application.

This study presents a new mechanistic procedure for determining the critical cracking temperature of asphalt concrete (AC) using data from bending beam rheometer (BBR) test of asphalt binder and indirect tension (IDT) test of AC. This new procedure uses BBR creep data to generate the mixture relaxation modulus mastercurve by utilizing the Hirsch model, time-temperature superposition principle, and Prony series-based interconversion method. The Hirsch model parameters are calibrated by comparing creep data from BBR and IDT creep tests performed at the same temperature. Boltzmann hereditary integral and second-order heat equation are then used to calculate thermal stress from the developed relaxation modulus mastercurve. IDT strength data is transferred from test strain rate to thermal strain rate using the viscoelastic continuum damage model. Since a strain gauge is not attached for traditional laboratory IDT strength testing, this study derived an analytical equation based on the Hondros solution to compute the horizontal strain rate from the applied vertical displacement rate. Finally, the critical cracking temperature is determined by coupling the thermal stress and strength profiles. Using the procedure presented in this paper, the critical cracking temperatures of four AC mixtures were predicted from BBR and IDT data. Their actual critical cracking temperatures were measured using thermal stress restrained specimen test performed in the laboratory to validate the method. The predicted critical cracking temperatures are found to be very close to the laboratory measured values. The developed procedure has substantial practical and technical importance in predicting the critical cracking temperature of AC because it utilizes widely available BBR and IDT tests.

Next-generation concentrating solar power (CSP) tower technologies target operating temperatures exceeding 700 °C to increase the thermal-to-electric conversion efficiency. Molten chloride salts are one possible alternative to current commercial molten nitrate salts to enable the higher operating temperature. This paper analyzes the predicted optical, thermal-fluids, and structural performance of traditional external tubular solar receiver configurations applied with a chloride salt heat transfer fluid (HTF) and inlet/outlet temperatures of 500 °C/735 °C, and considers sensitivity analysis and optimization relative to receiver sizing, tube sizing, number of panels, flow circuit configurations, and solar flux concentration under constraints on internal velocity, pressure drop, wall thickness, and required creep-fatigue lifetime. The high temperature conditions increase the significance of inelastic deformation mechanisms such as creep relative to that expected in commercial 565 °C nitrate salt designs. High-temperature creep and creep-fatigue damage in the metal alloy tubes are the key factors that limit allowable solar flux concentration and achievable receiver thermal efficiency at the near-800 °C wall temperature conditions. For a traditional external cylindrical receiver configuration, the design parameters and conditions capable of satisfying all constraints produced, at best, a design point receiver efficiency of 78.2%, or 80.5% when excluding receiver intercept efficiency. Variation in the optimal receiver performance relative to uncertainty in the binding maximum velocity, minimum wall thickness, and minimum lifetime constraints is presented.

Following the increase in the number and quantity of waste materials and their negative impact on the environment, this research has sought to examine the benefits of recycling these waste materials and check their impact on the creep and fatigue of asphalt concrete mixture when they are added as fillers. Four waste materials were used in the tests conducted: waste toner, waste medical ash, Waste Electrical Arc Furnace Dust (EAFD), and waste tire rubber. Limestone was added as a control material. Adding these waste materials as fillers was made by the Marshall Test method. Then, the Universal Testing Machine (UTM) was used to simulate creep (dynamic and static) and fatigue tests. Two different temperatures were used (25, 40 °C), and for the dynamic creep, three different loading frequencies were utilized (1, 4 and 8 Hz) to simulate vehicle speeds of 20, 40 and 60 miles/hr. respectively. Based on the laboratory test results, testing temperature and loading frequency for each type of filler had a significant influence on the accumulated strain, creep stiffness, and resilient modulus. Consequently, the increase in loading frequency from 1 to 8 Hz meant speed increase, and thus decreased-time of contact between the stress of the load and asphalt concrete mixture. In this case, rutting decreased with the increase of Hz. Thus, axial micro-strain increased with time at loading frequency and temperature, showing that waste toner was the best filler that could be added to the asphalt concrete mixture. Similarly, in static creep test, accumulated strain increased with time at different levels of temperature for all types of fillers. Conversely, creep stiffness decreased with time at different levels of temperature for all types of fillers. Experimental results on fatigue indicated that for the best filler investigation, a linear relationship was obtained between logarithm strain and logarithm number of cycles (Nf). As a result, the waste toner as filler was the best filler as it had the highest fatigue life for the tested mix design.

Purpose The purpose of this study was to determine whether aging imparts a clinically significant effect on the (1) mechanism of graft failure and (2) structural, material, and viscoelastic properties of patellar tendon allografts by evaluating these properties in younger donors (≤30 years of age) and older donors (>50 years of age). Methods A total of 34 younger (≤30 years of age) and 34 older (>50 years of age) nonirradiated, whole bone-tendon-bone allografts were prepared for testing by isolating the central third of the patellar tendon using a double-bladed 10-mm width scalpel under a 10-N load to ensure uniformity of harvest. Bone blocks were potted in polymethylmethacrylate within custom molds. Tendon length and cross-sectional area were measured using an area micrometer. A mechanical loading system was used to precondition the grafts for 100 cycles with a load between 50 N and 250 N (1 Hz). A creep load (500 N) was then applied at a rate of 100 mm/min (10 minutes). Grafts were allowed to recover at 1 N (10 minutes), followed by pull-to-failure at a rate of 100% strain per second. Mechanisms of failure (midsubstance vs avulsion) were noted and the structural, material, and viscoelastic properties calculated and compared between groups. Results There were 33 (97%) midsubstance tears in the younger group and 28 (82%) in the older group (P = .034). Younger grafts showed greater ultimate load to failure (1,782 N [1,533, 2,032] vs 1,319 N [1,103, 1,533]) (P = .006) and ultimate tensile stress (37.4 MPa [32.4, 42.4] vs 27.5 MPa [22.9, 32.0]) (P = .006). There were no significant differences in displacement (P = .595), stiffness (P = .950), strain (P = .783), elastic modulus (P = .114), creep displacement (P = .881), and creep strain (P = .614). Conclusions This in vitro study suggests that aging weakens the bone-tendon junction and decreases the ultimate tensile strength of patellar tendon allografts. However, aging did not affect the displacement, strain, stiffness, elastic modulus, creep displacement, or creep strain of patellar tendon allografts. Clinical Relevance Surgeons should be aware that patellar tendon allografts from donors >50 years of age have a lower ultimate tensile stress than donors ≤30 years of age.

Traditional alloy design which requires deep understanding of the Process–Structure–Property relationship, involves numerous tests and iterations. It is not cost-effective and very time-consuming. Current study explored the idea of using machine learning to build quantitative models to predict creep life of Cr–Mo steel with the help of convention creep study wisdom. Three time–temperature parameters, namely Larson-Miller parameter (LMP), Manson-Haferd parameter (MHP) and Manson-Succop parameter (MSP), were used as the target feature instead of creep life. Moreover, Pearson correlation coefficient (PCC) and Spearman correlation coefficient (SCC) were used to discuss the feature importance of creep life. The results demonstrated that the prediction accuracy was effectively improved by transforming creep life into LMP, MHP and MSP. In the case of using MSP as target feature, random forest (RF) had the best accuracy and robustness in all regression models. Feature importance ranking provided new insights for domain experts to analyze the main influencing features to creep property of Cr–Mo steel. This study has successfully proved the feasibility of combining machine learning and time–temperature parameters in predicting creep life and provided a basic idea and workflow for researchers to predict creep life of steel.

This paper surveys advances in the understanding and modelling of wheel–rail creep forces. The main focus is placed on tribological aspects, for which significant progress was made in the past two ...

The aim of this paper is to estimate the damage of railway carriage wheels caused by lock-up braking. First, we investigate the typical characteristics of wheel-rail contact including the parameters of contact patch, longitudinal creep and coefficient of adhesion. Then we show the current requirements concerning the wheel-slide protection systems and determine the points which should be reviewed. To predict the effect of the sliding phenomenon, material properties are needed, as well. Afterwards, a simplified thermal simulation is built to estimate where martensite formation may occur.

The paper presents and compares the results on the reliability and remaining life assessment of a reactor (coxing box) from a petrochemical plant. The reactor shell is made of 16Mo5 (W1.5423) steel, with a thickness of 25 mm, plated with 3 mm thick X6CrAl13 (W1.4002) stainless steel. The assessment was made in two steps. For preliminary remnant life assessment, specifications of section VII of the ASME code was used followed by iRiS‑Thermo expert system. Further, experimental creep and metallographic replica analysis were performed. Results comparison of the two methods applied revealed a reduction of the preliminary estimated remaining live obtained using metallographic replica analysis. Based on the results obtained, the possibility to extend the service duration of the coxing box in the safety condition, using current process parameters, with of 20.000 hours was highlighted.

There are a large number of concrete bonding interfaces in assembled concrete structures. Moreover, the problems of deformation and coordination at the interfaces lead to weak bonding under shrinkage and creep. The theoretical methods and analytical solutions for the long-term performance of composite structures in recent literatures are relatively complicated for the review or optimization in the design stage. Hence, a method for calculating the mid-span deflection of and the cross-sectional stress on composite beams under shrinkage and creep was developed based on the average curvature method. The calculation method was then verified by the experiments and the numerical simulations. Further, factors affecting the deflection of and the stress on the assembled concrete composite beams, including the loading age, the reinforcement ratio, the section dimensions, and the surface area in contact with the atmosphere were analyzed by using the developed method. The results revealed that the adjustment of the section dimensions and the loading time is the effective optimization approach. A framework for review or optimization of composite concrete beams based on the sensitivity analysis is proposed and a case study is carried out. The results of this work can helpfully be used to control the risk of additional deflections or cracking during the long-term operation of buildings.