Institute of Fundamental Technological Research

The measurement and control of the heat transfer of sheet alloys in successive non-isothermal forming cycles is crucial to achieve the desired post-form properties and microstructure, which could not as of yet be realized by using traditional test facilities. In the present research, a novel heat transfer measurement facility was designed to generate and subsequently measure the in-situ heat transfer from a sheet alloy to multi-mediums such as forming tools, air, lubricant and coating. More importantly, the facility was able to use a single sheet alloy sample to perform successive non-isothermal forming cycles, and subsequently obtain high throughput experimental results including the temperature evolution, cooling rate, mechanical properties and microstructures of the alloy. The high throughput in-situ heat transfer measurement facility identified that the cooling rate of AA7075 was 152 °C/s and the mechanical strength was over 530 MPa in the 1st forming cycle. However, it decreased to less than the critical value of 100 °C/s in the successive 10th forming cycle, leading to a low mechanical strength of only 487 MPa. The identified variations that occur in the successive non-isothermal forming cycles would improve the consistency and accuracy of part performance in large-scale manufacturing.

High throughput in-situ measurement,Heat transfer,Successive non-isothermal forming,Sheet alloys,Microstructure

The aim of this research was to investigate an effect of low temperature on the mechanical properties and mi-crostructure of 6061-T6 aluminium alloy (AA6061-T6) subjected to dynamic loading. The specimens were subjected to dynamic compression at a low temperature of −80°C in a range of strain rates from 1.25 × 10 3 1/s to 3.4 × 10 3 1/s to compare their mechanical responses. The deformation mechanisms were analysed through EBSD observations during which dynamic recovery, was found as the dominant one. Furthermore, microstruc-tural analysis indicated that deformation under high strain rate conditions and temperature of-80°C enables to keep the constant initial grain size of the material after the loading applied. The material behaviour was modelled using mechanism-based viscoplastic constitutive equations. Furthermore, an accuracy of the developed model was validated by comparing it to experimental data. The set of constitutive equations proposed has been successful in modelling the stress-strain behaviour of the material for the range of strain rates and temperatures encountered in aluminium-forming processes under low-temperature conditions.

Split Hopkinson pressure bar (SHPB),Low temperature,AA6061-T6,Microstructure

In this paper, three different directions of 0°, 45° and 90° were used to manufacture the Haynes 282 alloy bars by using the Direct Metal Laser Sintering (DMLS) method. The additively manufactured specimens as well as these of the wrought Haynes 282 were subsequently subjected to comparative fatigue tests in the range of stress amplitude from ±400 MPa to ±800 MPa. The AM process enhanced the fatigue response of the nickel-based alloy in question by 200 MPa. Furthermore, it was found, that the printing direction does not affect the fatigue response of additively manufactured specimens significantly as minor differences in service life were observed for the entire stress amplitude range adopted. Finally, fatigue damage measure φ and fatigue damage parameter D approaches were used to reveal the dynamics of damage development and to monitor damage development due to fatigue.

Haynes 282, nickel alloys, fatigue, additive manufacturing, Direct Metal Laser Sintering (DMLS)

In the present work, the compatibility relationship on the failure criteria between aluminium and polymer was established, and a mechanics-based model for a three-layered sandwich panel was developed based on the M-K model to predict its Forming Limit Diagram (FLD). A case study for a sandwich panel consisting of face layers from AA5754 aluminium alloy and a core layer from polyvinylidene difluoride (PVDF) was subsequently conducted, suggesting that the loading path of aluminium was linear and independent of the punch radius, while the risk for failure of PVDF increased with a decreasing radius and an increasing strain ratio. Therefore, the developed formability model would be conducive to the safety evaluation on the plastic forming and critical failure of composite sandwich panels.

formability, M-K model, failure criteria, composite sandwich panel

Adhesive and stretchable nanofibrous hydrogels have attracted extensive attraction in wound dressings, especially for joint wound treatment. However, adhesive hydrogels tend to display poor stretchable behavior. It is still a significant challenge to integrate excellent adhesiveness and stretchability in a nanofibrous hydrogel. Herein, a highly adhesive, stretchable, and breathable nanofibrous hydrogel was developed via an in situ hybrid cross-linking strategy of electrospun nanofibers comprising dopamine (DA) and gelatin methacryloyl (GelMA). Benefiting from the balance of cohesion and adhesion based on photocross-linking of methacryloyl (MA) groups in GelMA and the chemical/physical reaction between GelMA and DA, the nanofibrous hydrogels exhibited tunable adhesive and mechanical properties through varying MA substitution degrees of GelMA. The optimized GelMA60-DA exhibited 2.0 times larger tensile strength (2.4 MPa) with an elongation of about 200%, 2.3 times greater adhesive strength (9.1 kPa) on porcine skin, and 3.1 times higher water vapor transmission rate (10.9 kg m–2 d–1) compared with gelatin nanofibrous hydrogels. In parallel, the GelMA60-DA nanofibrous hydrogels could facilitate cell growth and accelerate wound healing. This work presented a type of breathable nanofibrous hydrogels with excellent adhesive and stretchable capacities, showing great promise as wound dressings.

nanofibrous hydrogels, hybrid cross-linking, adhesivity, stretchability, breathable capability

The worldwide energy crisis and environmental deterioration are probably humanity’s greatest challenges. Thermoelectricity, which allows for the mutual conversion between thermal and electrical energy, has become a promising technology to alleviate this challenge. Increasingly more research focuses on how to fabricate and apply thermoelectric materials for harvesting energy and regulating the indoor thermal environment. However, only a few studies have focused on cementitious materials with thermoelectric potential. Thermoelectric cement is a composite material in which particular additives can enhance the thermoelectric performance of ordinary cement. By potentially replacing traditional construction materials with thermoelectric cement in building ap-plications, electricity could be generated from waste heat, reducing the use of fossil fuels, and supplementing other renewable energy sources like solar and wind. This article presents a review of fundamentals, fabrication, characterization, composition, and performance, as well as modeling methods and opportunities for thermo-electric cement composites. The literature reviewed covers the period from 1998 to 2020 related to thermo-electric cement. It also presents the challenges and problems to overcome for further development and provide future research directions of thermoelectric cement.

thermoelectric cement composites, additives, thermoelectric generator, thermoelectric cooler, seebeck coefficient, figure of merit

The interfacial heat transfer coefficient (IHTC) for titanium alloys is an important parameter in non-isothermal hot stamping processes to determine the temperature field as well as temperature-dependent material behaviours that consequently affect the post-form properties of the formed components. However, the IHTC for titanium alloys in hot stamping processes has seldom been studied before. In the present research, the effects of contact pressure, lubricant, surface roughness, tooling material and initial blank temperature on the IHTC for the titanium alloy Ti-6Al-4V were studied and modelled to characterise the IHTC values under various hot stamping conditions as well as identify the functional mechanisms affecting the IHTC. Furthermore, the results of hot stamping of Ti-6Al4V wing stiffener components were used to verify the simulation results of the temperature field of the formed component with an error of less than 5%.

interfacial heat transfer coefficient (IHTC), Ti-6Al-4V, hot stamping, experimental validation

Assessing the effect of defect induced stresses on magnetic flux leakage (MFL) signals is a complicated task due to nonlinear magnetomechanical coupling. To facilitate the analysis, a multi-physics finite elemental simulation model is proposed based on magnetomechanical theory. The model works by quasi-statically computing the stress distribution in the specimen, which is then inherited to solve the nonlinear magnetic problem dynamically. The converged solution allows identification and extraction of the MFL signal induced by the defect along the sensor scanning line. Experiments are conducted on an AISI 1045 steel specimen, i.e. a dog-bone shaped rod with a cylindrical square-notch defect. The experiments confirm the validity of the proposed model that predicted a linear dependency of the peak-to-peak amplitude of the normalized MFL signal on applied stress. Besides identifying the effect of stress on the induced MFL signal, the proposed model is also suitable for solving the inverse problem of sizing the defects when stress is involved.

magnetic flux leakage, magnetomechanics, Jiles–Atherton model, non-destructive testing, finite element method, multiphysics numerical simulation

Because of the thermoelectric (TE) effect (or Seebeck effect), a difference of potential is generated as a consequence of a temperature gradient across a sample. The TE effect has been mostly studied and engineered in semiconducting materials and it already finds several commercial applications. Only recently the TE effect in cement-based materials has been demonstrated and there is a growing interest in its potential. For instance, a temperature gradient across the external walls of a building can be used to generate electricity. By the inverse of the TE effect (or Peltier effect), one can also seek to control the indoor temperature of a building by biasing TE elements embedded in its external walls. In designing possible applications, the TE properties of cement-based materials must be determined as a function of their chemical composition. For instance, the TE properties of cement paste can be enhanced by the addition of metal oxide (e.g., Fe2O3) powder. In this paper, a single thermoelectric leg is studied using the finite element method. Metal oxide additives in the cement paste are modelled as spherical inhomogeneities. The thermoelectric properties of the single components are based on experimental data, while the overall thermoelectric properties of the composites are obtained from the numerical model. The results of this numerical study are interpreted according to the effective medium theory (EMT) and its generalisation (GEMT). KEY WORDS: Cement composites; Thermoelectrics; Seebeck C

Cement composites; Thermoelectrics; Seebeck Coefficient; Electrical Conductivity; Thermal Conductivity

hot stamping, titanium alloys, FAST