Tensile Tests Research Articles

Additive manufacturing of polyamide 12 bulk structures using directed energy deposition with a thulium laser: Effect of process conditions on morphology and mechanical performance

Laser-based directed energy deposition of polymers (DED-LB/P) is a highly flexible additive manufacturing process capable of fabricating three-dimensional structures with a high degree of customization on free-form surfaces. In this article, the manufacturability of polyamide 12 bulk structures using DED-LB/P in combination with a thulium laser operating at a wavelength of 1.94 μm is investigated for the first time. The typical absorption bands at this wavelength eliminate the need for additives such as carbon-based nanoparticles, which would limit the range of applications. The generated structures were analyzed regarding porosity, thermal behavior, and mechanical properties to evaluate the potential of DED-LB/P with a thulium laser. As a consequence of the evaporation of absorbed moisture or ethanol residues resulting from powder preparation, a thermal pretreatment of the polymer powder reduced the porosity of the DED-LB/P structures to 0.9%. Depending on the processing parameters, the crystallinity of the produced structures ranged from 25.6% to 29.9%. For the tensile tests, the required geometries were milled from the DED-LB/P bulk structures. The results show that an ultimate tensile strength of up to 52 MPa, an elongation at break of up to 58%, and Young's modulus of up to 1590 MPa can be achieved. These remarkable mechanical properties are primarily attributed to the complete melting of the powder particles in DED-LB/P and the improved surface quality resulting from the milling process.

Development of Polymer Composites Using Surface-Modified Olive Pit Powder for Fused Granular Fabrication.

In this study, olive pit agro-waste from the olive oil industry is valorized by incorporating it as an additive in a polyethylene terephthalate glycol (PETG) matrix to develop bio-based composite materials for large format additive manufacturing (LFAM). The olive pits were first ground into olive pit powder (OPP) and then functionalized by polymerizing poly(butylene adipate-co-terephthalate) PBAT on their surface, resulting in a hydrophobic, modified olive pit powder (MOPP) with enhanced compatibility with the PETG matrix. OPP and MOPP composites were compounded and 3D-printed via Fused Granular Fabrication (FGF) using 5, 10, and 15 wt.% concentrations. The PBAT coating increased the degradation temperature and specific heat capacity of the material, contributing to a lower melt viscosity during printing, as confirmed by MFR, MDSC, and TGA analyses. Tensile testing revealed that MOPP composites generally exhibited superior mechanical properties compared to OPP composites, likely due to the improved compatibility between PBAT on the MOPP surface and the PETG matrix. SEM analysis further validated these findings, showing a highly irregular and porous fracture surface in OPP composites, while MOPP composites displayed a smooth surface with well-integrated MOPP in the PETG matrix.

Development and characterization of a clay-based composite reinforced with doum fibers

This paper investigates the effects of Doum fiber as a local materials, on the mechanical strength properties of cylindrical earth samples with a diameter of 3 cm and a height of 4.5 cm. Untreated and Alkali treated Doum fiber with a NaOH concentration of 1%, 4%, 6% are introduced in local clay with a ratio of 0.5%, 1% and 1.5% by weight of the earth samples. A tensile test of the Doum fiber and a compressive strength test of the elaborated specimens is conducted. The study findings outline an improvement in the mechanical properties of the earth samples and adherence between surface fiber and soil when the fiber is treated. And a dosage of 0.5% by weight of Alkali treated Doum fiber with a NaOH concentration of 1% is sufficient to achieve higher mechanical performances. Modifying the fiber surface through NaOH treatment enhances fiber adhesion to clay and improves the mechanical behavior of the biocomposite.

Dual‐Modal Sensing Skin Adaptive to Daylight, Darkness, and Ultraviolet Light for Simultaneous Full‐Field Deformation Measurement and Mechanoluminescence Responses

AbstractMechanoluminescence (ML) and digital image correlation (DIC) have emerged as promising optical methods to visualize and measure deformation fields. In this study, a dual‐modal sensing skin, called the ML‐DIC skin is introduced, that is capable of emitting ML and facilitating DIC measurements under various lighting conditions, including daylight, night or darkness, and UV irradiation. Four ML‐DIC skins are fabricated with or without carbon nanotubes (CNTs) using a composite powder consisting of SrAl2O4: Eu,Dy (SAO), and acrylic resin, with CNT milling times of 48, 72, and 96 h for three of four skins, respectively. DIC measurements are performed under multiple lighting conditions for measuring photoluminescence, persistence luminescence, and reflection. Uniaxial tension tests demonstrate the superior performance of ML‐DIC skins with CNTs compared with pristine SAO skins, with the skin subjected to 48 h of CNT dispersion exhibiting optimal performance. Further investigations focus on ML emission and DIC measurements near the crack‐tip vicinity of static and propagating cracks as well as on surfaces above subsurface cracks. The integration of ML and DIC techniques offers a versatile approach for comprehensive deformation analysis applicable to diverse environments, with implications for materials science, engineering, and structural health monitoring.

Evaluation of Tensile Strength of Dissimilar Metals SS 316-pure Zn Joining by Friction Welding Method

Welding of dissimilar metals has always been a challenge in industrial applications. In this study, the joining of SS 316 and 99.9% pure zinc by continuous drive friction welding has been carried out. The welding process is carried out with parameters such as rotation speed, friction pressure, forging pressure, and burn length being kept constant for all weld joint samples. This friction time is selected through the process parameter approach carried out on aluminium alloy material. The experimental results were analysed by tensile test and microstructure using a scanning electron microscope (SEM) to characterise the microstructure. Tensile test results from friction welding with variations in friction time from 30 to 80 seconds obtained a higher tensile strength of the joint in the 30 to 50 seconds friction time range. In this experiment, the maximum joint tensile strength is obtained at 53.93 MPa, and the maximum modulus elasticity of 128.60 GPa at 35 seconds of friction. Solid-state welding makes it possible to join SS 316 and pure zinc, which is difficult to join in conventional fusion welding methods.

Effect of high pressure-low plasticity burnishing on the mechanical and microstructural behaviour of copper foil interlayered FSW aluminium alloys

Abstract This study delves into the impact of high pressure-low plasticity burnishing (HP-LPB) on the mechanical and microstructural behaviour of friction stir welding (FSW) joints, specifically involving AA5052 and AA6082 aluminium alloys. Notably, copper foil (CF) is introduced as a novel element in the HP-LPB process to enhance the weld strength. The experiments were conducted with the tool rotational speeds (TRS) of 1000, 1100, and 1200 RPM, each paired with the welding speeds (WS) of 20, 25, and 30 mm min−1, and tool tilt angle (TTA) of 0°, 1°, and 2°. Mechanical behaviour is assessed through tensile testing along with microhardness measurements, revealing the advantages of HP-LPB with CF in enhancing joint strength and toughness. The microstructural analysis reveals the dissolution of precipitates, highlighting the influential role of copper foil in the improvement of joint efficiency. From the weld without a CF interlayer, a maximum tensile strength of 188 MPa was achieved at a TRS of 1200 RPM, WS of 25 mm min−1 and TTA of 2°. The post-processed FSW sample interlayered with copper foil, exhibited an improvement in joint efficiency by 87% at the optimum process parameter. This research demonstrates that the use of copper foil interlayers combined with HP-LPB treatment can substantially enhance the mechanical properties and structural integrity of FSW aluminum alloys, offering a valuable solution for advanced industrial applications.

Preparation and study of attapulgite reinforced PVA fiber with cross‐linking of boric acid

AbstractPolyvinyl alcohol (PVA) fibers were manufactured by gel spinning with reinforcing of attapulgite (ATT) and cross‐linking of boric acid (PVA/B/ATT fibers), in order to improve the mechanical properties. Furthermore, the final fibers were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), differential scanning calorimetry (DSC), thermogravimetric analyzer (TGA), powder x‐ray diffraction (XRD) and tensile strength tester. In addition, when ATT amount was no more than 2.0 wt%, FTIR analyses suggested that the degrees of cooperation combining of ATT and boric acid with PVA macromolecules were enhanced. SEM images indicated that PVA/B/ATT fibers had smooth surfaces. DSC, TGA and XRD results illustrated that the melting temperatures and maximum thermal decomposition temperatures first increased and then dropped as the content of ATT increased. Moreover, mechanical properties results indicated that the optimal ATT amount was 2.0 wt%, and the tensile strength of PVA/B/ATT‐2 fiber reached the highest of 14.2 ± 0.3 cN/dtex. It was largely because that the cooperation combining of boric acid and ATT with PVA molecules improved the mechanical properties of PVA/B/ATT fibers.

Electrospun Cellulose Acetate/Poly(Vinyl Alcohol) Nanofibers Loaded with Methyl Gallate and Gallic Acid for Anti-Staphylococcus aureus Applications.

Methyl gallate (MG) and gallic acid (GA) are natural compounds with potent activity against methicillin-resistant Staphylococcus aureus (MRSA), a significant global health concern. In this study, MG and GA were incorporated into cellulose acetate (CA) blended with poly(vinyl alcohol) (PVA) to create electrospun nanofibers aimed at combating both methicillin-susceptible S. aureus (MSSA) and MRSA. Key electrospinning parameters-DC voltage, injection flow rate, and syringe tip-collector distance-were optimized, with the best conditions being a 1.5 mL/h flow rate, 30 cm distance, and 20 kV voltage. The resulting nanofiber mats were characterized by SEM, FTIR, DSC, tensile strength testing, contact angle measurement, swelling behavior, and release profiling. Antibacterial properties were assessed using the agar diffusion test. The obtained nanofibers had diameters ranging from 879.33 to 906.13 nm. Among the samples, MG-GA-CA/PVA exhibited the highest tensile strength, good flexibility, and improved stiffness, which was related to enhanced thermal stability and chemical interactions as shown by DSC and FTIR analyses. This formulation also displayed excellent hydrophilicity, swelling properties, and a consistent release profile over 8 to 24 h. Furthermore, MG-GA-CA/PVA showed superior antibacterial activity against both MSSA and MRSA, suggesting its potential as a strong, flexible, and effective anti-S. aureus material.

Stress relaxation and improved fracture toughness of metal bonding using flexible monolith sheets and an epoxy adhesive

AbstractWhile epoxy resins exhibit excellent mechanical and insulating properties as well as excellent stability against heat and chemicals, epoxy adhesives also have drawbacks such as brittleness and stress concentration. Rubber-based materials are often added to epoxy adhesives to increase toughness, but they are sensitive to heat and moisture, limiting their effectiveness in harsh environments. In this study, we propose a new sheet-type adhesive consisting of a conventional liquid epoxy adhesive and an epoxy monolith sheet with internal continuous pores, using the advantageous properties of the flexibility and toughness of the epoxy monolith to avoid stress concentration. We evaluated the adhesion strength for metal bonding using the sheet-type epoxy adhesives via a lap-shear tensile adhesion test at various temperatures. The total destruction energy was also estimated via a tapered double cantilever beam test. Furthermore, a heat cycle adhesion test was conducted using two types of metallic materials with different coefficients of thermal expansion to elucidate the effect of the monolith sheet on the improvement of interfacial failure induced by stress concentration.

Predictions of sheet molding compound composite structures using probabilistic finite element simulations with bimodal Weibull strength distributions

Due to the large scatters in Sheet Molding Compound (SMC) mechanical properties, the uncertainty in the prediction of mechanical responses of SMC composite structures is significantly high as compared to that for continuous fiber composite structures. Another challenge is the size effect resulting from the scatter in mechanical strengths, i.e., the measured material flexural strength is much higher than its tensile strength. As a result, classical approaches that rely on the mean tensile strength can substantially underpredict the flexural responses in Finite Element Analysis (FEA). This work investigates the use of a material model with bimodal Weibull distributions for its strength properties in Probabilistic FE (PFE) simulations. A Random Search-based optimization procedure was developed to obtain model parameters which are difficult to determine. The material model was verified by PFE simulations of standard experiments, and then examined in simulations of open-hole tension and disk bending tests.

Revealing grain refinement and hydrogen trapping mechanism for anti-hydrogen susceptibility of Nb-alloyed 34MnB5 press hardened steel

Hydrogen embrittlement (HE) extant a substantial concern to press-hardened steel (PHS) owing to superior strength. The high strength to light-weight automobile structures necessitates the advancement of superior HE resistance PHS. This study investigated the HE susceptibility of Nb-microalloyed PHS by slow strain rate tensile testing, u-shaped constant bending load test, and thermal desorption spectroscopy. Nb enhances microstructure and HE resistance by introducing retained austenite, refining prior austenite grains (21.14–13.73 μm), forming low-angle grain boundaries, and nano-scale precipitates. Nb-alloyed steel exhibits no-cracking over 300 h under high pre-bending stress and decreases elongation loss up to 48% in hydrogen environment as compared to Nb-free steel. Diffusible H-content in 0.12 wt% Nb-steel reduces to 14.9% of that in Nb-free steel owing to enhanced hydrogen traps, the Fcc/Bcc matrix, and carbide precipitation. The multi-phase microstructure with nano-scale NbC precipitation impeded the localized H-dispersion, enhancing the HE resistance in PHS despite its high strength.

A Preisach Model Defining Correlation between Monotonic and Cyclic Response of Structural Mild Steel

This article delivers a new Preisach model representing the correlation between the elastoplastic behavior of structural mild steel under axial monotonic and cyclic loading with damage. The newly formed model is based on the experimentally defined correlation between axial monotonic and cyclic behavior of structural mild steel. To examine the monotonic and cyclic behavior of structural mild steel and find fitting material properties for the model, monotonic and cyclic axial tensile tests are performed. Tests are executed on coupons of the commonly used European structural steel S275. The model represents a mathematical description of modified single-crystal material behavior under monotonic loading. Two different approaches were used to describe damage in the multilinear mechanical model. The excellent agreement with experimental results is achieved by infinitely linking many single-crystal elements in parallel, forming the polycrystalline model. This model provides a good solution for everyday engineering practice due to its geometric representation in the form of the Preisach triangle and the lower costs of monotonic tests used to define material properties compared to cyclic tests.

Mathematical Modelling of Tensile Mechanical Behavior of a Bio-Composite Based on Polybutylene-Succinate and Brewer Spent Grains.

A model based on the fitting of stress-strain data by tensile tests of bio-composites made of a bioplastic (polybutylene succinate (PBS)) and brewer spent grain filler (BSGF) is developed. Experimental tests were performed for various concentrations of BSGF in the range from 2% to 30%. The model is suitable for describing the elastic-plastic behavior of these materials in terms of two mechanical parameters, tensile stress and tensile stiffness (or Young's modulus), depending on the filler concentration. The mechanical characteristics, derived from the fit parameters, show good agreement with the experimental data. The mathematical model used here could be an important aid for the experimentation and manufacturing process as it allows the prediction of the mechanical tensile parameters of a mixture with different filler concentrations, avoiding the long and complex preparation cycle of bio-composites, as well as the specific mechanical tests. The physical properties required by the objects created with the PBS-BSGF bio-composite by the partners/stakeholders of the research project co-financing this research can be quite different; therefore, a mathematical model that predicts some of the mechanical properties in terms of the mixture composition may be useful to speed up the selection of the required amount of BSGF in the mixture.

The Mechanical Properties of Orthodontic Aligners of Clear Aligner After Intraoral Use in Different Time Periods.

Although the technique of orthodontic aligners has risen in popularity, their mechanical properties have not been thoroughly investigated. The aim of this study was to evaluate the mechanical properties of the orthodontic aligners Clear Aligner after intraoral use for 7, 10 and 14 days, and to compare them with as-received aligners (0 days). It was also sought to examine the properties of the unprocessed raw material (polyethylene glycol terephthalate) used to manufacture these aligners. Thirty-two aligners by four patients were evaluated and studied at 0, 7, 10, 14 days of use. Each aligner was divided into three segments (two posterior and one anterior), which resulted in 96 samples. Also, 16 samples of unprocessed material were studied. For all samples, elastic modulus, ultimate tensile stress (UTS) and yield stress were calculated by conducting tensile testing. Additionally, material hardness was tested. The two-tailed Mann-Whitney test was performed, having set the level of significance at p = 0.05. Analysis of the measurements indicated a statistically significant decrease in elastic modulus between days 0 and 14 of use, of UTS between days 0 and 7, 7 and 10, and of yield stress between days 0 and 7. For hardness, in every period, posterior segments demonstrated significantly higher values than anterior segments. All properties of the unprocessed material were statistically significantly higher than the processed samples. The unprocessed material presented significant differences in every property tested in comparison to the processed aligners. The processed material showed further deterioration over time during use. The present study provides evidence that thermoforming and ageing affect the mechanical properties of the aligners.

Production and Characterization of Hybrid Al6061 Nanocomposites

Aluminum-based hybrid nanocomposites, namely the Al6061 alloy, have gained prominence in the scientific community due to their unique properties, such as high strength, low density, and good corrosion resistance. The production of these nanocomposites involves incorporating reinforcing nanoparticles into the matrix to improve its mechanical and thermal properties. The Al6061 hybrid nanocomposites were manufactured by conventional powder metallurgy (cold pressing and sintering). Ceramic silicon carbide (SiC) nanoparticles and carbon nanotubes (CNTs) were used as reinforcements. The nanocomposites were produced using different reinforcement amounts (0.50, 0.75, 1.00, and 1.50 wt.%) and sintered from 540 to 620 °C for 120 min. The characterization of the Al6061 hybrid nanocomposites involved the analysis of their mechanical properties, such as hardness and tensile strength, as well as their micro- and nanometric structures. Techniques such as optical microscopy (OM) and scanning electron microscopy (SEM) with electron backscatter diffraction (EBSD) were used to study the distribution of nanoparticles, the grain size of the microstructure, and the presence of defects in the matrix. The microstructural evaluation revealed significant grain refinement and greater homogeneity in the hybrid nanocomposites reinforced with 0.75 wt.% of SiC and CNTs, resulting in better mechanical performance. Tensile tests showed that the Al6061/CNT/SiC hybrid composite had the highest tensile strength of 104 MPa, compared to 63 MPa for the unreinforced Al6061 matrix. The results showed that adding 0.75% SiC nanoparticles and CNTs can significantly improve the properties of Al6061 (65% in the tensile strength). However, some nanoparticle agglomeration remains one of the challenges in manufacturing these nanocomposites; therefore, the expected increase in mechanical properties is not observed.

The addition of Bi2WO6 nanoparticles to polyvinyl alcohol film: Improvements in optical, mechanical, and electrical properties

This study deals with the influence of adding Bi2WO6 (BWO) nanoparticles on the optical, structural, dielectric, and mechanical properties of polyvinyl alcohol (PVA). BWO nanoparticles synthesized by solvothermal method were characterized by X-Ray diffraction (XRD) and scanning electron microscope (SEM-EDS). The thick film nanocomposites were analyzed by Fourier Transformed Infrared (FTIR) spectrometer, XRD device, SEM, atomic force microscope (AFM), ultraviolet visible (UV–Vis) spectrophotometer, impedance analyzer, and tensile test machine. While FTIR analyses confirmed the formation of composite structure and the presence of BWO in the composites, AFM views indicated the reduced roughness for composites. Additionally, the UV–Vis light absorption spectra revealed a narrowed band gap in the composites. Moreover, with increasing BWO contribution, gradual increases were recorded in Young's modulus, toughness and % breaking strain parameters. The frequency dependent dielectric properties was also shown that the energy storage ability of the PVA matrix enhances 25 % and 50 % due to 2 % and 3 % BWO additives, respectively. However, the composite with higher ε′ and lower ε″ values than the pure PVA in the entire frequency range was the 2 % BWO-doped nanocomposite. Based on the serious and simultaneously improvements in electrical, optical and mechanical properties, it can be suggested that 2 % BWO additive provides the opportunity to expand the application area of PVA synergistically.

Non-Equal Contributions of Different Elements and Atomic Bonds to the Strength and Deformability of a Multicomponent Metallic Glass Zr47Cu46Al7.

Multicomponent metallic glasses (MGs) are a fascinating class of advanced alloys known for their exceptional properties such as limit-approaching strength, high hardness and corrosion resistance, and near-net-shape castability. One important question regarding these materials that remains unanswered is how the different elements and atomic bonds within them control their strength and deformability. Here, we present a detailed visual and statistical analysis of the behaviors of various elements and atomic bonds in the Zr47Cu46Al7 (at%) MG during a uniaxial tensile test (in the z-direction) simulated using molecular dynamics. Specifically, we investigate the identities of atoms undergoing significant shear strain, and the averaged bond lengths, projected z-lengths, and z-angles (angles with respect to the z-direction) of all the atomic bonds as functions of increasing strain. We show that, prior to yielding, the Zr element and the intermediate (Zr-Zr, Cu-Al) and stronger (Zr-Al, Zr-Cu) bonds dominate the elastic deformation and strength, while the Cu and Al elements and the weaker Al-Al and Cu-Cu bonds contribute more to the highly localized shear transformation. The significant reconstruction, as signified by the cessation of bond-length increment and bond-angle decrement, of the intermediate and the stronger bonds triggers yielding of the material. After yielding, all the elements and bonds participate in the plastic deformation while the stronger bonds contribute more to the residual strength and the ultimate (fracture) strain. The results provide new insights into the atomic mechanisms underlying the mechanical behavior of multicomponent MGs, and may assist in the future design of MG compositions towards better combination of strength and deformability.

The Influence of the Degree of Tension and Compression of Copper on the Indentation Size Effect (ISE)

The present work deals with the relationship between the degree of cold plastic deformation (up to 55.3% in the neck area in the tensile test and up to 66.6% in the compression test) and the parameters of the Indentation Size Effect (ISE). The tested material consists of 97% wrought copper. The Hanemann tester (Carl Zeiss, Jena, Germany) was used to measure micro-hardness. The loads applied during the micro-hardness test were between 0.09807 N and 0.9807 N. The influence of the load on the degree of the micro-hardness and simultaneously on the ISE, expressed by the Meyer’s index n, was significant. The influence of load on the ISE parameters was also evaluated using the Meyer’s index n, the PSR method, and the Hays–Kendall approach. For the undeformed sample, the Meyer’s index was close to 2, with the increase in the degree of tensile and compressive deformation increasing its “reverse” character (n > 2).

Anisotropy in tensile properties of a high strength metastable β titanium alloy

High strength metastable β titanium alloys are widely employed in the aircraft industry due to their outstanding strength-to-weight ratio. While components can endure complex in-service mechanical loading, the anisotropy in tensile properties has been the subject of limited attention. In this study, its origin was investigated focusing on the role played by millimeter scale β grains as they were recently identified as a source of heterogeneous deformation. Tensile properties of Ti-10V-2Fe-3Al processed via different thermomechanical routes were assessed using multiple sampling directions. In particular, elongation values were observed to vary significantly depending on the testing direction. A combination of SEM, EBSD, µ-CT and in-situ DIC during tensile tests was employed to clarify the underlying causes of this behavior. Substantial differences in strain heterogeneity and localization were found related to features of β grains, including their crystallographic and morphologic orientations. Furthermore, multiple fracture mechanisms were observed to derive from the differences in deformation behavior, and eventually compete to trigger specimen failure. Elongation values are then determined by both the degree of strain heterogeneity and the operating fracture mechanisms. These findings provide a new understanding of the role of the microstructure in the tensile behavior of high strength metastable β titanium alloys.

Physico-mechanical Properties of Nano Bio-films For Application in Food Packaging

Petroleum-based packaging has long dominated the packaging industry, raising environmental concerns. As a result, the packaging industry has started using nano bio-plastics. Incorporating nanoparticles, nano bio-plastic packaging helps in prolonging product shelf life. Additionally, packaging manufactured of nano bio-plastic is made of biodegradable polymers. Nano bio-films made from polylactic acid (PLA) and nanoclay were produced using a solvent casting process. 1.0%, 3.0% and 5.0% (wt.) of nanoclay were added to the PLA solution. The mechanical, chemical, morphological and transparency properties of nano bio-films were investigated. To improve their suitability for food packaging applications, tensile test, Fourier-transform Infrared Spectroscopy (FTIR), scanning electron microscopy (SEM) and UV-barrier test were performed. With tensile strength (TS) of 30.36 MPa and elongation at break (EAB) of 7.05%, respectively, nano bio-films with 3.0 wt. % nanoclay demonstrated higher mechanical properties than pure PLA film with about 35% of increment. In aspects of FTIR analysis, no significant differences were found with both nano bio-films and pure PLA film. The SEM morphology shows that adding nanoclay at 3.0 wt. % improves the adhesion between the PLA matrix and the nanoclay. The addition of nanoclay had a minimal impact on the PLA film's transparency, and it was interesting to see that nanoclay concentration of 1.0 wt. % and 3.0 wt. % had roughly the same light transmission values of 84.86% and 83.95%, respectively. According to the findings of this study, the PLA nano bio-film with 3.0 wt% nanoclay exhibits the optimum physico-mechanical properties.