Thermomechanical Tests Research Articles

A 3D anisotropic visco-hyperelastic constitutive model for unidirectional continuous fiber reinforced shape memory composites

A general 3D anisotropic visco-hyperelastic constitutive model for unidirectional continuous fiber reinforced shape memory composites (SMPCs) is proposed based on energy decomposition. The total energy of SMPCs is decomposed into isotropic part from matrix and anisotropic part from fiber reinforcements, respectively. The matrix part is further decomposed into hyperelastic part and visco-hyperelastic part. Meanwhile, a temperature-dependent energy contribution weight factor is introduced to define the variation of energy contribution of matrix and fibers with phase transition in the thermodynamic process and shape memory cycle. The proposed model is demonstrated on a carbon fiber reinforced SPMC. Thermomechanical and shape memory tests of SMP and SMPCs with different fiber inclination angles are carried out and simulated with the proposed model. Comparisons between simulation results and experimental data validate the proposed model.

The Use of Nanoscale Montmorillonite (MMT) as Reinforcement for Polylactide Acid (PLA) Prepared by Fused Deposition Modeling (FDM)-Comparative Study with Biocarbon and Talc Fillers.

The subject of the presented research focuses on a comparative assessment of three types of polymer fillers used to modify highly crystalline poly(lactic acid) PLA intended for the FDM technique. The aim of the presented work was to determine the performance of the developed materials. The key aspect of the work was the use of polymer fillers of three different types. Nano-sized montmorillonite (MMT), biobased biocarbon (BC) and mineral talc. The several types of composites were prepared using extrusion technique. The maximum content for BC and talc filler was limited to 20 wt%, while for MMT it was 5 wt%. Prepared samples were subjected to detailed material analysis including mechanical tests (tensile, flexural, Charpy), thermal analysis (DSC, DMTA), HDT/Vicat tests and structure analysis. The results of the test confirmed that even relatively small amount of nano-type filler can be more efficient than micrometric particles. The used type of matrix was highly crystalline PLA, which resulted in a significant nucleation effect of the crystalline structure. However, thermomechanical tests revealed no improvement in thermal resistance. Microscopic survey confirmed that for MMT and talc filler the structure anisotropy was leading to more favorable properties, especially when compared to structures based on spherical BC particles.

Modelling of Strain-Controlled Thermomechanical Fatigue Testing of Cast AlSi7Cu3.5Mg0.15 (Mn, Zr, V) Alloy for Different Aging Conditions

Thermomechanical fatigue loadings (TMF) applied on components in a certain temperature range with a variable state of stress (tensile and/or compression) produce a localized concentration of plastic strains that results in crack initiation and propagation. The time evolution of plastic strains must be known a priori to predict the lifetime of a part submitted to TMF loadings, which requires an extensive campaign of mechanical characterization conducted at different temperatures and aging conditions. Such a campaign was proposed for the aluminum alloy AlSi7Cu3.5Mg0.15 (Mn, Zr, V), which is recognized as being creep resistant. Combined isothermal low-cycle fatigue and isothermal creep tests were performed on this alloy to determine the constitutive parameters based on the Lemaître and Chaboche (LM&C) viscoplastic model. These laws were implemented within the finite element simulation software (Z-set) to model the response of the alloy to a thermomechanical fatigue test. The results of TMF Z-Set simulations, using the LM&C model adapted for two aging conditions, were then compared with results obtained from “Out of Phase” thermomechanical fatigue testings (OP-TMF) performed on a Gleeble 3800 machine. The modelling of the OP-TMF test revealed the complexity of the mechanical behavior of the material induced by the temperature gradient prevailing along with the cylindrical specimen. It was found that a better prediction of the evolution of plastic strains requires taking into account a larger range of plastic strain rates conditions for the determination of the constitutive law and eventually includes the role of the microstructure in the evolution of the material behavior, starting first with the yield stress.

Interfacial Delamination at Multilayer Thin Films in Semiconductor Devices.

With the evolution of semiconducting industries, thermomechanical failure induced in a multilayered structure with a high aspect ratio during manufacturing and operation has become one of the critical reliability issues. In this work, the effect of thermomechanical stress on the failure of multilayered thin films on Si substrates was studied using analytical calculations and various thermomechanical tests. The residual stress induced during material processing was calculated based on plate bending theory. The calculations enabled the prediction of the weakest region of failure in the thin films. To verify our prediction, additional thermomechanical stress was applied to induce cracking and interfacial delamination by various tests. We assumed that, when accumulated thermomechanical-residual and externally applied mechanical stress becomes larger than a critical value the thin-film cracking or interfacial delamination will occur. The test results agreed well with the prediction based on the analytical calculation in that the film with maximum tensile residual stress is the most vulnerable to failure. These results will provide useful analytical and experimental prediction tools for the failure of multilayered thin films in the device design stage.

Dynamic strain aging and microstructural damage mechanism of austenitic stainless steel under thermomechanical fatigue in the temperature range of 250–400 °C

Thermomechanical fatigue tests were performed on austenitic stainless steel in the temperature range of 250–400 °C. Results show that the increase in strain amplitude reduces fatigue life and enhances the impact of phase angle. DSA is only observed in the tensile direction under out-of-phase while in the compression direction under other phase angles at a low strain amplitude. The slip bands with a high dislocation density enhance fatigue resistance, and dislocation tends to form cell structures at high strain amplitudes. Moreover, oxidation damage contributes a lot to the reduction of fatigue life at high strain amplitudes.

Thermomechanical and isothermal fatigue crack propagation testing method for 316LN austenitic stainless steel

AbstractInvestigating fatigue crack growth behavior of materials is crucial to the life design and safety evaluation of engineering structures. During the practical service condition, the temperature keeps changing, which cannot be studied by the traditional fatigue crack growth test method at constant temperature. In this study, we focus on constructing a new thermomechanical fatigue (TMF) crack propagation testing method, developing a thermal deformation compensation method, and proposing an equivalent compliance method suitable for variable temperature conditions based on ASTM E647 standard to measure the crack length. Furthermore, the load‐ and strain‐controlled TMF tests under different phase angles are carried out to prove the validity of the new method.

Characterization and modeling of the hardening and softening behaviors for 7XXX aluminum alloy subjected to welding thermal cycle

The evolution of thermo-elasto-plastic stress-strain during welding process plays a key role in determining the welding residual stresses and mechanical properties of welded joints for age-hardening 7XXX aluminum alloy, which is influenced by the softening and hardening behaviors. In this study, a Gleeble 3800 thermo-mechanical testing machine was employed to simulate the welding thermal cycle process of 7XXX aluminum alloy, and dynamic yield strengths under different temperatures were measured to elucidate the softening behavior. An isothermal multi-level cyclic loading tests at various temperatures were conducted to characterize the cyclic hardening feature. The calculation formulas for the proportion of kinematic and isotropic hardening model were deduced, and the cyclic hardening behavior was quantitatively described. A softening model and a nonlinear mixed hardening model were explored to simulate the evolution of dynamic yield strength and the cyclic hardening behavior, and excellent agreements were obtained between predicted and measured dynamic yield strengths and cyclic stress-strain curves.

Microscopic testing of carbon fiber laminates with shape memory epoxy interlayer

For the first time, microscopic testing has been performed on shape memory polymer composites (SMPCs) which were manufactured by commercial materials already used in aerospace. Results from micro-tests have been compared with those from conventional memory-recovery cycling on macro-scale. Two shape memory polymer composite (SMPC) laminates were fabricated with different shape memory (SM) interlayer: one in the form of an uncured epoxy powder and the other in the form of a thin epoxy foam. The latter, in particular has been studied to evaluate lightweight and stiff sandwich structures with SM properties. The assessment of the manufacturing process by a hot press moulding technique was assessed through micro scale analysis using SEM and MicroCT analysis. DMA analyses were carried out to understand the interaction mechanisms between raw constituents. A Vickers micro-indentation examination before and after heating was able to assess the shape recovery behaviour at the micro-scale level. A nano-instrumental indentation was used to characterise the shape memory response under different loads at elevated temperatures. Whilst an instrumented thermo-mechanical test allowed to investigate the shape memory behaviour at macro-scale level. Results allow identifying the recovery mechanisms at the micro-scale which are responsible for the shape memory performances at the macro-scale. The higher recovery ability of the SM foam is confirmed in comparison with bulk interlayers.

Carbonated 3D-Printable Polymer Composite for Thermo-Mechanically Stable Applications

In this report, we investigate the infusion of carbon dioxide into a 3D-printable photosensitive polymer. The result is a carbonated polymer composite material. In use, polymer composite materials expect to succeed where ordinary polymers and metals fail. This is due to the tailorability of composite materials for specific applications. Usually, micro/nano-particulates are embedded as fillers within a polymer matrix, enhancing the overall material properties. Here, carbon dioxide (CO2) microbubbles serve as the filler within a nylon-like polymer matrix. Additive manufacturing by stereolithography (SLA) of the carbonated polymer composite proved possible using the digital light projection (DLP) 3D printing technique. Post-heat treatment using thermogravimetric analysis of the samples at elevated temperatures resulted in a 33% mass reduction, indicative of nearly complete solvent removal and curing. An initial increase in polymer carbonation duration showed a 16% increase in porosity, more stable thermal profiles, and a 40% decrease in specific heat capacity. Thermo-mechanical compressive tests on an optimal carbonated sample revealed a 70% increase in compressive strength over its neat counterpart and a peak modulus at 50 °C of 60 MPa. Such 3D-printable carbonated polymer composites may find use in applications requiring high strength-to-weight ratio thermally stable polymers and applications requiring a versatile and convenient storage medium for on-demand CO2 deposition or supercritical fluid phase transformation.

Modelling of the transition from mode I to crystallographic crack growth in a single crystal gas turbine blade alloy under service-like loading conditions

In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.

Temperature and ice form effects on mechanical behaviors of ice-richmoraine soil of Tianmo valley nearby the Sichuan-Tibet Railway

Moraine soils are widely distributed in the Qinghai-Tibet Plateau. With increased global warming, moraine soil-related geohazards have become increasingly common, posing a serious threat to the infrastructure (e.g., the Sichuan–Tibet Railway) and inhabitants of this region. This paper aims to investigate the mechanical behavior of ice-rich moraine soil in the Tianmo Valley by conducting a series of triaxial constant strain rate (CSR) and coupled thermomechanical (CTM) tests on artificially moraine soil containing different ice forms (crushed ice and block ice). The results show that in general, compared to moraine soil containing crushed ice, moraine soil with block ice has a higher peak strength, a similar internal frictional angle and a considerably larger cohesive strength. The stress–strain curve of the soil containing crushed ice shows a strain-hardening form, while that of the soil containing block ice is represented by a strain-softening model similar to that of dense soil. In the CTM tests, it is revealed that the rising temperature could cause a sharp increase in strain and lead to sample failure, even when the axial load is far below the material's peak strength. A comparison between samples with different ice forms reveal that the soil containing crushed ice is more sensitive to temperature change. The tests demonstrate that the ice form has a significant influence on the mechanical behavior of moraine soil, and the temperature rise can result in a dramatic decrease in soil strength. Therefore, efforts should be made to detect the occurrence form of buried ice and the changes in the environmental temperature as well as the stress state of moraine soil slopes in situ.

Time-gated active imaging digital image correlation for deformation measurement at high temperatures

We proposed a novel time-gated active imaging digital image correlation (DIC) method based on bandpass filtering and gated single-photon imaging techniques for high-temperature deformation measurement. The proposed method uses a powerful pulsed laser for illumination and then records the reflected pulsed laser light using a gated single-photon camera with an ultrashort exposure time. As a result, the received radiation light from the heated object and the ambient light can be significantly reduced to a negligible level compared with the illumination light intensity from the pulsed laser. Experiments including the varying ambient light test and thermal deformation measurement of a Ni-based alloy at around 700 °C verified the performance of the proposed method. It is observed that the established time-gated active imaging DIC system is dramatically superior to existing blue-light DIC and ultraviolet-DIC (UV-DIC) systems in preserving the image quality at high temperatures, thus showing promising application prospects in extremely challenging scenarios, such as ultra-high-temperature material testing, or thermomechanical testing of hypersonic materials and structures subjected to transient aerodynamic heating simulated by quartz lamps or an arc-heated wind tunnel.

Under-extrusion challenges for elastic filaments: the influence of moisture on additive manufacturing

The applicability of Additive Manufacturing for operational parts expands with the availability of new materials with specific properties. For elastomeric components produced with Fused Filament Fabrication, challenges associated with the printing process due to the nature of the material are faced. This paper investigates the effect of under-extrusion in this process regarding the feeding system and, predominantly, the moisture for thermoplastic polyurethanes with 3D printing experiments and thermomechanical testing. In particular, the filament flow control with a Bowden extruder provides a challenge. A microscopic analysis reveals the signs of under-extrusion, along with the influence of material drying to reduce the moisture content. The drying may depend not only on time and temperature, but also on mass and surface effects. Water uptake measurements exhibit absorptions up to 1.89% in weight, most of which take place during the first 24 h of the experiments. Tensile tests performed on samples with different moisture contents show their influence in the ultimate stresses. The moisture in the material causes under-extrusion induced failures. Those failures are less likely to happen at lower moisture levels, resulting in occasional higher tensile strengths. Overall, the importance of proper storage of the material throughout printing is verified, even under moderate humidity conditions due to its hygroscopic nature.

Influence of thermomechanical exposure on the structure of hydrides in irradiated E110 alloy cladding pipes under the conditions of long-term dry storage of spent nuclear fuel

The penetration of atomic hydrogen into the cladding material of fuel elements of WWER-1000 reactors due to interaction with the coolant during operation can subsequently significantly reduce their plasticity characteristics with a decrease in temperature during long-term dry storage of spent nuclear fuel (SNF) due to the formation of brittle hydrides. The morphology of hydrides influenced by the hydrogen content, storage temperature and circumferential stresses plays a decisive role in the embrittlement of the fuel cladding material. Associated radial hydrides are of particular danger; they constitute the most favorable path for crack propagation.In the present work, thermomechanical tests of irradiated fuel claddings samples made of the E110 alloy were carried out, simulating normal and emergency conditions of long-term dry storage. It was shown that under the conditions considered, the formation of a significant amount of radially oriented hydrides was observed, leading to degradation of mechanical properties (embrittlement) fuel cladding.

Investigation into the Performance of NiTi Shape Memory Alloy Wire Reinforced Sn-Bi Self-Healing Metal Matrix Composite.

Self-healing materials have the potential to create a paradigm shift in the life cycle design of engineered structures, by changing the relation between material damage and structural failure, affecting structures’ lifetime, safety, and reliability. However, the knowledge of self-healing capabilities in metallic materials is still in its infancy compared to other material systems because of challenges in the synthesis of organized and complex structures. This paper presents a study of a metal matrix composite system that was synthesized with an off-eutectic Tin (Sn)-Bismuth (Bi) alloy matrix, reinforced with Nickel–Titanium (NiTi) shape memory alloy (SMA) wires. The ability to close cracks, recover bulk geometry, and regenerate strength upon the application of heat was investigated. NiTi wires were etched and coated in flux before being incorporated into the matrix to prevent disbonding with the matrix. Samples were subjected to large deformations in a three-point bending setup. Subsequent thermo-mechanical testing of the composites confirmed the materials’ ability to restore their geometry and recover strength, without using any consumable components. Self-healing was accomplished through a combination of activation of the shape memory effect in the NiTi to recover the samples’ original macroscopic geometry, closing cracks, and melting of the eutectic material in the matrix alloy, which resealed the cracks. Subsequent testing indicated a 92% strength recovery.

Rate and temperature dependent compaction-creep-recovery and void analysis of compression molded prepregs

This study investigates the compaction-creep response of a glass/epoxy prepreg system at different temperatures, pressures, and compaction speeds, using a compaction fixture with heated platens, housed in-situ in an X-ray computed tomography (XCT) machine. It has been shown that by increasing the compaction speed from 0.1 mm/min to 1 mm/min, the total strain, the percentage of compaction creep and the resulting permanent deformation increased for all of the temperature and pressure conditions considered in this study. It was noted that for all compaction speeds and pressures, the percentage of compaction-creep increases from 25 °C up to 55 °C and then decreases with further increases in temperature. A decrease in creep deformation with post creep-thickness recovery, due to the resin squeezing out from the prepreg, was observed at higher temperatures. The void content in the post-creep, fully cured laminates was determined using an analytical model, the results of which were compared with XCT-developed geometrical models. Good agreement between both approaches was observed, with differences in void content results of less than 12% being recorded. The optimum process parameters (55 °C, 0.3 MPa and 0.5 mm/min) were identified in compression molded prepreg samples to choose the first isothermal dwell period . The combination of these process parameters resulted in moderate levels of compaction-creep and permanent deformation as well as a low void volume fraction in the test samples. • Rate dependent thermo-mechanical compaction-creep tests were performed on uncured glass/epoxy prepreg samples. • Percentage of compaction-creep and permanent deformation are found to be increasing with the increase of compaction speed. • Void volume fraction was determined in cured samples using analytical and XCT-aided geometrical models and compared. • Normal distribution statistical analysis was employed to study the size and shape of individual voids.

Variable-order fractional dynamic behavior of viscoelastic damping material

Abstract Viscoelastic damping material has been widely used in engineering machinery to absorb vibration and noise. Dynamic behavior of the viscoelastic damping material is mainly affected by temperature and frequency. Classical dynamic behavior equations of the viscoelastic damping material have complex structures with multiple and ambiguous parameters. So a novel variable-order fractional constitutive model (VOFC) is established based on the variable-order fractional operator. Then the viscoelastic dynamic equations are derived by Laplace transform of the VOFC model. The dynamic thermomechanical analysis tests by the three-point bending mode are carried out at variable temperatures and frequencies, and the frequency spectrum of the dynamic behavior, i.e. the loss modulus, storage modulus and loss factor are obtained. Against the test data, the VOFC model is compared with classical models such as the integer-order Maxwell model, constant fractional-order Kelvin-Voigt model, constant fractional-order Maxwell model and constant fractional-order standard linear solid model. The results show that the VOFC model can describe dynamic behavior of the viscoelastic damping material at different temperatures and frequencies more accurately. And the VOFC model has simpler structure and only two parameters with clearly physical meaning. Finally, a calculation software is designed and developed, which simplifies the process of predicting viscoelastic dynamic behavior in a wide frequency range.

Memory behaviour of polyester knitted fabric integrated with temperature-responsive shape memory polymer filament

The present article highlights a novel approach towards investigating the memory behaviour of shape memory filaments integrated into a knitted textile structure. This study is the first step to investigate the interaction behaviour of such smart materials within textile structures. This integration can further open vast possibilities in designing smart textiles with unique functionalities such as sensing and actuating. A shape memory fabric (SMF) was successfully knitted using shape memory filament and polyester yarn. A systematic investigation is carried out to quantify the memory behaviour of SMF by thermo-mechanical tensile test. The experimental results reveal excellent shape recovery ( R r) (>90%) and good shape fixity ( R f) (∼80%) at strains of 20% and 60%, and temperatures of 30°C and 50°C. Memory filament behaves differently in a fabric structure compared to its pristine state at different temperatures and strains, as confirmed by experimental results. Under various thermo-mechanical conditions, the cyclic test of SMF revealed almost complete R r with an improvement in R f.

Static recovery of A5083 aluminum alloy after a small deformation through various measuring approaches

Static recovery was confirmed to be the dominant softening mechanism during annealing for the studied A5083 aluminum alloy. The kinetics of static recovery, described based on the activation parameters (activation energy and volume of static recovery) and the static softening fraction, were mainly studied through two thermomechanical tests, namely, double-pass compression and stress relaxation tests. A new approach was proposed to measure the static softening fraction in the stress relaxation test. In general, a higher temperature or strain rate accelerates static recovery, while interestingly, the effect of pre-strain on static recovery is opposite in cases with and without external stress, owing to the enhanced static recovery by external stress during annealing. In addition, microhardness tests and electron backscatter diffraction (EBSD) characterization were also conducted to verify the accuracy of the results. The grain average misorientation approach based on EBSD characterization was confirmed to be effective in distinguishing and quantifying static recovery. It is noteworthy that the special stress hardening phenomenon occurring at 400 °C and 0.1/s is caused by strain aging, showing the complexity of the material behaviors after deformation of the studied aluminum alloy.

Understanding softening of amorphous materials for FFF applications

Abstract The work explores the fundamentals of the softening process for amorphous materials expanding on the work presented by Stammers and Beek (Stammers, E. and Beek, W.J. (1969). The melting of a polymer on a hot surface. Polym. Eng. Sci. 9). The pressure induced melt removal process was studied in detail. The process, although fundamental, allows for an understanding of the phase transition of materials during the fused filament fabrication (FFF) process. Experimental results were analyzed using state-of-the-art equipment. A total of four materials were tested. PVC and PC are amorphous materials, and PA6 and PP are semi-crystalline materials. A setup was built to allow the application of a constant force. The resulting melt film was analyzed using computed tomography technology. The experimental results were compared with numerical simulation results originating from COMSOL Multiphysics and the analytical solution derived by Stammers and Beek. Rheological and thermo-mechanical tests were used to find the softening temperature of amorphous materials. Results showed a straightforward transition for the melting temperature of semi-crystalline materials and softening temperature of amorphous materials. It is suggested that for amorphous materials, the glass transition temperature can be used as the softening temperature (T s = T g ). When comparing all three results (experimental, analytical, and numerical), values are within the same range and magnitude. A good approximation was achieved for PA6 and PVC, which have semi-crystalline and amorphous structures, respectively. Results provide insights into the phase transition process and phase change mechanisms during the extrusion of materials in the fused filament fabrication process.