Material Stiffness Research Articles

Mechanical, thermal, and microstructure analysis of 3D printed polyolefin elastomer-acrylonitrile butadiene styrene blends for tunable mechanical properties

Fused Deposition Modeling (FDM) has emerged as a cost-effective and user-friendly technique that has garnered significant attention in recent years. However, challenges persist when printing soft materials using FDM technology. This study proposes a solution by blending a soft polyolefin elastomer (POE) with a stiffer material, acrylonitrile butadiene styrene (ABS). A comprehensive experimental investigation utilizing a direct pellet printing method was conducted to 3D print POE-ABS blends in various ratios (10%, 30%, and 50% ABS content). The study encompassed material preparation, 3D printing, SEM (Scanning Electron Microscopy) analysis, Dynamic Mechanical Thermal Analysis (DMTA), uniaxial tensile testing, and compression testing to assess the mechanical properties and energy absorption capabilities of the 3D printed blends. The results of the DMTA showed that the blends with a higher amount of ABS have a larger area under the Tan δ curve, indicating their ability to absorb more energy. The SEM images revealed that as the proportion of ABS increases, the gaps between layers and beads become smaller. The mechanical properties of the pure printed POE, such as its elongation at break and tensile strength, were 2138% and 2.83 MPa, respectively. However, as the ABS content increased, the samples exhibited more rigid behavior, with a final transition to 50/50 ratio resulting in 9.4 MPa tensile strength and 160% elongation at break. The energy absorption test demonstrated that the sample with 30% ABS content has the highest energy absorption capability among all the tested samples. This research underscores the potential of blending soft and stiff materials to tailor properties for additive manufacturing applications, emphasizing the significance of material composition in achieving desired mechanical performance, printability, and functionality in printed objects.

Experimental Investigation of Low-Cost Bamboo Composite (LCBC) Slender Structural Columns in Compression

This paper experimentally investigates the behavior of innovative sustainable Low-Cost Bamboo Composite (LCBC) structural columns under compressive loading. The LCBC columns are manufactured from bamboo culms in combination with bio-based resins to form composite structural columns. Different LCBC cross-sectional configurations are investigated in this study, including the Russian doll (RD), Big Russian doll (BRD), Hawser (HAW), and Scrimber (SCR). Two bio-based resins, including one bio-epoxies and one furan-based resin, in addition to a soft bio-based filler and a synthetic epoxy resin, are applied. The bamboo species used as the cast-in-place giant bamboo for all configurations include Moso, Guadua, and Tali. Slender LCBC columns showed maximum stress equal to 60 MPa at failure. The study found that the samples with bio-epoxy resin (BE2) exhibited enhanced material stiffness when compared to synthetic epoxy (EPX) and furan-based resin (PF1), while PF1 specimens demonstrated increased ductility. Among the specimens with Moso bamboo and BE2 resin, those with SCR and HAW configurations achieved the highest compressive strengths.

Tensile and Bending Strength of Birch and Beech Lamellas Finger Jointed with Conventional and Newly Developed Finger-Joint Profiles.

In this study, the tensile and bending strength of birch and beech lamellas finger jointed with conventional (Standard) and newly developed finger-joint profiles (New) are presented. Polyurethane (PUR), Melamine-Urea-Formaldehyde (MUF) and Phenol-Resorcinol-Formaldehyde (PRF) adhesive systems were used to bond the finger joints. The objective of the New profiles was to reduce the stress concentrations within the finger joint by cutting the cross-grooved fingers perpendicular to the main orientation of the finger-joint profile. In the first trials of the development, larger cross-grooved fingers were cut with the aim to improve the stress distribution and to reinforce the finger joint by filling gaps in the finger joint with adhesive. As the study progressed, initial optimisations of the New profile were made. Smaller cross-grooved fingers were cut as it was assumed that they are beneficial for the manufacturing and integrity of the New profile. In combination with the MUF adhesive system, the New profile achieved the highest increase in the bending and tensile strengths compared to the Standard profile. In addition to the increased strength, other advantages such as reduced cracking in the finger joint were observed when using the New profile. The high strength and stiffness of hardwoods or other high-performance materials used in timber construction can probably be better exploited in combination with the New profile. Further tests will be carried out by considering different configurations of the New profile and different materials.

A novel approach for improving material stiffness using a direct method in below-knee prosthetic sockets

The conventional techniques for producing a socket are time-consuming disproportionate to the significant population afflicted by limb amputations. Although the new manufacturing direct method, the modular socket system (MSS) method, involves reduced labor time, the technique produces sockets with high stiffness that cause discomfort for those with lower limb amputations during walking. This study investigated the tensile characteristics of numerous materials in below-knee prosthetic sockets. Initially, a vacuum molding approach was used to produce the sockets, which involved various polymers and composite materials to improve the prosthesis socket properties. An F-socket device was also employed to ensure efficient production and optimized pressure distribution at the interface between the socket and the residual limb. A SOLIDWORKS® software was then applied to determine the numerical analysis (stress distribution and the maximum internal pressure). The samples from Group E involved utilizing a novel mixture compared to the direct and traditional methods of various materials. This study presents a novel prosthetic limb socket made from a mixture of four carbon fiber layers, utilizing 20% polyurethane resin and 80% acrylic as the matrix. The resulting material demonstrated acceptable stiffness, extended socket life, and reduced curing time. During the patient's gait cycle, peak pressure of 300 KPa was recorded using the F-socket, while SOLIDWORKS® software indicated an internal pressure of 343 KPa, aligning closely with F-socket measurements. The new direct-fit socket design prioritizes comfort and flexibility using materials with reduced stiffness.

Shear-Wave-Velocity Profiling of a Test Embankment for a High-Speed Rail Project using the Spectral-Analysis-of-Surface-Waves (SASW) Method

Nondestructive seismic testing was conducted at 24 locations on and around the embankment to evaluate the compaction effort and lateral variabilities at the site. This embankment was being constructed as a test embankment for a high-speed railroad section in the U.S.A. Different compaction techniques were used in different embankment areas, and nuclear-gauge density measurements were performed to evaluate the compaction quality in all areas. Additionally, Spectral-Analysis-of-Surface-Waves (SASW) testing was performed to determine the Vs profiles at numerous locations on and around the embankment. The Vs values determined from the SASW testing were used to compare and evaluate areas where different compaction methods were used, as well as to help understand the overall site. The embankment was divided into three zones. In each zone, five shorter and one longer SASW arrays on top of the embankment were used to determine the Vs profiles. Also, in each zone, Vs profiles in the natural soil were determined using two additional long SASW arrays on the natural ground, one array on each longitudinal side of the embankment. These 24 Vs profiles on and around the embankment were used to evaluate lateral variability in the shear stiffnesses of the geotechnical materials at the site.

Failure resistance of single-implant crowns assembled from polyetheretherketone and lithium disilicate abutments and different crown materials after artificial aging.

The objective of the present study was to investigate the effect of using different materials for the fabrication of implant abutments and crowns on the mechanical behavior of implant-supported single crowns after artificial aging. The materials were tested in different combinations to reveal whether using stiff or resilient materials as an abutment or a crown material might influence the fracture strength of the whole structure. A total of 40 implants (blueSKY) were restored with identical custom-made CAD/CAM abutments milled out of lithium disilicate or ceramic-reinforced polyetheretherketone (PEEK) and divided into five test groups (n = 8). Forty crowns made of three different materials (zirconia, lithium disilicate, and ceramic-reinforced PEEK) were used to restore the abutments. Specimens were subjected to mechanical loading up to 1,200,000 cycles in a chewing simulator (Kausimulator) with additional thermal cycling. The surviving specimens were subjected to quasi-static loading using a universal testing machine (Z010). The PEEK abutments with zirconia crowns showed the highest median failure load (3890.5 N) while the PEEK abutments with lithium disilicate crowns exhibited the lowest (1920 N). Fracture and deformation occurred in both the crowns and abutments. The failure load of the restorations was influenced by the material of the abutment and crown. Restoring PEEK abutments with zirconia crowns showed a high failure load and no screw loosening.

Cellular solids and prestressed affine networks as models of the elastic behavior of soft biological structures.

We reviewed two microstructural models, cellular solid models and prestressed affine network models, that have been used previously in studies of elastic behavior of soft biological materials. These models provide simple and mathematically transparent equations that can be used to interpret experimental data and to obtain quantitative predictions of the elastic properties of biological structures. In both models, volumetric density and elastic properties of the microstructure are key determinants of the macroscopic elastic properties. In the prestressed network model, geometrical rearrangement of the microstructure (kinematic stiffness) is also important. As examples of application of these models, we considered the shear behavior of the cytoskeleton of adherent cells, of the collagen network of articular cartilage, and of the lung parenchymal network since their ability to resist shear is important for their normal biological and physiological functions. All three networks carry a pre-existing stress (prestress). We predicted their shear moduli using the microstructural models and compared those predictions with existing experimental data. Prestressed network models of the cytoskeleton and of the lung parenchyma provided a better correspondence to experimental data than cellular solid models. Both cellular solid and prestressed network models of the cartilage collagen network provided reasonable agreements with experimental values. These findings suggested that the kinematic stiffness and material stiffness of microstructural elements were both important determinants of the shear modulus of the cytoskeleton and of the lung parenchyma, whereas elasticity of collagen fibrils had a predominant role in the cartilage shear behavior.

The Interior Contact-Aided Rolling Element (I-CORE)

Abstract This work introduces an interior contact-aided rolling element (I-CORE) compliant mechanism that draws upon the concepts used for the contact-aided rolling element, cross axis flexural pivot, and pre-curved flexible beam. The I-CORE incorporates a bilinear compressive stiffness response with an initial tailorable stiffness governed by the flexural geometry, followed by a stiffness curve governed by the material stiffness at the contact point. The I-CORE mechanism can achieve 1 or 2 degrees of rotational freedom as well as 1 degree of translational freedom. The purpose of the present work was to introduce the I-CORE mechanism, as well as a pseudo-rigid-body replacement model (PRBM) of the I-CORE mechanism which was subsequently validated using both finite element analysis and bench top mechanical testing. A pseudo rigid body model was created for the I-CORE to simplify rapid adaptation of this mechanism to different design applications. This model was validated using both finite element analysis and benchtop mechanical testing under both compression and rotation loading conditions. Additionally, multiple configurations of the device were created and evaluated in order to test its sensitivity to certain design features including the flexure width, flexure thickness, and the radius of the rounded contact surfaces. It was found that the model is sensitive to the thickness of the flexures and that despite some limitations, the pseudo rigid body model is sufficiently accurate for initial design work. Some possible applications of the mechanism are proposed.

Ultra-Tough, yet Rigid and Healable Supramolecular Polymers with Variable Stiffness for Multimodal Actuators.

Variable stiffness materials have shown considerable application in soft robotics. However, previously reported materials often struggle to reconcile high stiffness, stretchability, toughness, and self-healing ability, because of the inherently conflicting requisite of these properties in molecular design. Herein, we propose a novel strategy that involves incorporating acid-base ionic pairs capable of from strong crosslinking sites into a dense and robust hydrogen-bonding network to construct rigid self-healing polymers with tunable stiffness and excellent toughness. To demonstrate these distinct features, the polymer was employed to serve as the strain-regulation layers within a fiber-reinforced pneumatic actuator (FPA). The exceptional synergy between the configuration versatility of FPA and the dynamic molecular behavior of the supramolecular polymers equips the actuator with simultaneous improvement in motion dexterity, multimodality, loading capacity, robustness, and durability. Additionally, the concept of integrating high dexterity at both macro- and micro-scale is prospective to inspire the design of intelligent yet robust devices across various domains.

Programmable Truncated Cuboctahedral Origami Metastructures Actuated by Shape Memory Polymer Hinges

AbstractOver the past few decades, origami‐inspired structures have attracted great attention across various engineering fields due to their unique geometric and mechanical characteristics. Additionally, combining origami structures with active materials is employed to achieve programmable mechanical properties and self‐reconfigurability under external stimuli. In this work, a novel family of truncated cuboctahedral origami metastructures is proposed. These designs integrate shape memory polymers (SMPs) to actively achieve programmable mechanical properties and shape memory behavior. By utilizing SMPs for the creases and stiff materials for the panels, this approach enables deformation along the creases while enhancing the overall structural robustness. The mechanical properties and shape memory processes of these structures are investigated in detail. The proposed origami metastructures exhibit a negative Poisson's ratio and demonstrate excellent energy storage capabilities. Notably, their mechanical properties can be programmed by controlling both temperature and geometric parameters. More particularly, their Poisson's ratio can be tuned within a range of zero to −1. As a result, these truncated cuboctahedral origami metastructures hold significant potential for applications across various engineering domains, particularly in composite structures and active metamaterials.

An alternative finite element formulation to predict ductile fracture in highly deformable materials

Abstract An alternative finite element formulation to predict ductile damage and fracture in highly deformable materials is presented. For this purpose, a finite-strain elastoplastic model based on the Gurson-Tvergaard-Needleman (GTN) formulation is employed, in which the level of damage is described by the void volume fraction (or porosity). The model accounts for large strains, associative plasticity and isotropic hardening, as well as void nucleation, coalescence and material failure. To avoid severe damage localization, a nonlocal enrichment is adopted, resulting in a mixed finite element whose degrees of freedom are the current positions and nonlocal porosity at the nodes. In this work, 2D triangular elements of linear-order and plane-stress conditions are used. Two systems of equations have to be solved: the global variables system, involving the degrees of freedom; and the internal variables system, including the damage and plastic variables. To this end, a new numerical strategy has been developed, in which the change in material stiffness due to the evolution of internal variables is embedded in the consistent tangent operator regarding the global system. The performance of the proposed formulation is assessed by three numerical examples involving large elastoplastic strains and ductile fracture. Results confirm that the present formulation is capable of reproducing fracture initiation and evolution, as well as necking instability. Convergence analysis is also performed in order to evaluate the effect of mesh refinement on the mechanical response. In addition, it is demonstrated that the nonlocal parameter alleviates damage localization, providing smoother porosity fields.

From paper to innovation: revolutionising technology with the ancient art of origami and kirigami

From paper to innovation: revolutionising technology with the ancient art of origami and kirigami The S-FOAM project aims to enhance metamaterial design by integrating origami/kirigami techniques into cellular structures, allowing for self-foldability and shape-morphing in response to stimuli. This research seeks to optimise material stiffness, strength and resilience. Potential applications include soft robotics, wearable devices, adaptive medical devices and flexible solar panels that adjust their shape according to sunlight.

Research on the development and performance of flexible protective fabrics with knitted structure

Personal protective textiles (PPTs) protect humans from sharp objects. However, PPTs are designed primarily for protective performance, and their stiff material and single function make them unsuitable for long-term wear. In this study, we developed flexible protective textiles using advanced flat knitting technology. These textiles offer excellent cut resistance, good abrasion resistance, high flexibility, and warmth-retention properties. A flexible protective fabric (FPF) with knitted structure was designed with an outer layer that provides cut and abrasion resistance, an intermediate layer, and an inner layer for heat storage. The study examined the cut resistance, abrasion resistance, flexibility, thermal management, breathability and stability of the developed fabrics. FPF has excellent abrasion and cut resistance. The cut resistance of the fabric was tested in three directions. The warp cutting load was 2145 gf, while the weft cutting load was 2347 gf. The diagonal cutting load was the largest at 2364 gf, resulting in an A5 cutting grade for high cutting hazards. Compared with other protective fabrics available on the market, FPF stands out due to its softness and the smallest bending stiffness in both the warp and weft directions. Additionally, it has excellent thermal management performance and breathability, providing 53% insulation and 146 mm/s breathability. Stability tests have shown that FPF has excellent color fastness to perspiration, good cleaning stability and aging stability for cut resistance, flexibility, warmth-retention properties and breathability.

Elastic Modulus Prediction of Ultra-High-Performance Concrete with Different Machine Learning Models

Elastic modulus, crucial for assessing material stiffness and structural deformation, has recently gained popularity in predictions using data-driven methods. However, research systematically comparing different machine learning models under the same conditions, especially for ultra-high-performance concrete (UHPC), remains limited. In this study, 10 different machine learning models were evaluated for their capacity to predict the elastic modulus of UHPC. The results showed that XGBoost demonstrated the highest accuracy in predictions with large training datasets, followed by KNNs. For smaller training datasets, Decision Tree exhibited the greatest accuracy, while XGBoost was the second-best performing model. Linear regression displayed the lowest accuracy. XGBoost demonstrated the most potential for accurately predicting the elastic modulus of UHPC, particularly when a comprehensive dataset is available for model training. The optimized XGBoost exhibited better predictive performance than fitting equations for different UHPC formulations. The findings of this study provide valuable insights for researchers and engineers working on the data-driven design and characterization of UHPC.

Utilizing porous dielectric material to enhance the performance of capacitive MEMS accelerometers for shaft monitoring

This research investigates the optimization of the performance of a capacitive-based microelectromechanical systems (MEMS) shaft monitoring accelerometer using a porous soft dielectric material with high polarization capability and low Young’s modulus. The nonlinear governing equations of a microbeam with a proof mass coupled with the squeeze motion equation of the polymeric dielectric material excited by shaft vibrations are extracted and solved in both the spatial and temporal domains. The studied model considers the stiffness, dissipative, and inertial effects of the polymeric material and incorporates porosity as a displacement-dependent variable, which introduces an additional nonlinearity. After discretizing the coupled nonlinear differential equations using the Galerkin method, the response in the frequency domain is obtained by applying an energy balance method, where the coefficients of the Fourier expansion of the response are found by arc-length continuation through a physical gradient descent learning-based approach method. The occurrence of secondary and primary resonances in different harmonic responses is studied for different harmonic acceleration inputs. The equivalent rigidity of the design is investigated for different shaft frequency magnitudes and different proof mass geometries. The effect of the initial porosity volume fraction of the elastomeric dielectric material on the dynamic response of the accelerometer is also examined. The sensor’s sensitivity is dependent on its geometry and properties and can vary based on the structure’s material parameters. Consequently, the selection of different geometries and materials may result in either an improvement or a deterioration of the sensor’s performance.

Short arm splints for wrist stabilization: A mechanical material test and cadaveric radiography study.

Documentation of the wrist stabilizing effect and mechanical properties of common splinting materials is warranted to support evidence-based condition-specific recommendations for wrist immobilization. The objectives of this study were to assess the wrist stabilizing properties of volar and dorsal short-arm splints made of four different materials and evaluate the mechanical properties of the splinting materials. Dorsal and volar short arm splints made of plaster of Paris (PoP) (eight layers), Woodcast (2 mm, rigid vented), X-lite (classic, two layers) or a 3D-printed material (polypropylene) were sequentially mounted on 10 cadaveric arm specimens and fixed in a radiolucent fixture. This enabled the evaluation of maximum wrist flexion and extension relative under an orthogonal load of 42 N via radiographic images. In addition, a three-point bending test was performed on ten sheet duplicates of each of the four splinting materials. When applied as a volar splint, PoP had the highest capability to resist wrist flexion and extension. However, when applied as a dorsal splint, Woodcast exhibited a lower wrist flexion and a similar wrist extension. The 3D-printed splints-both volar and dorsal-showed the highest mean wrist flexion and extension. The mechanical properties of the Woodcast, X-lite and 3D-printed splinting materials were very similar. PoP exhibited distinct properties with a stiffness of 146 (95% confidence interval [CI]: 120-173) N/mm and a deflection at F max of 0.6 (95% CI: 0.5-0.7) mm compared to ≤7.7 (95% CI: 7.4-7.9) N/mm and ≥20 (95% CI: 18-22) mm for the other materials. PoP displayed better wrist stabilizing properties and material stiffness than Woodcast, X-lite and 3D-printed polypropylene. When considering wrist stabilizing properties, PoP may still prove to be the preferred choice for wrist immobilization. Not applicable.

Age- and sex-specific biomechanics and extracellular matrix remodeling of the ascending aorta in a mouse model of severe Marfan syndrome.

Thoracic aortic aneurysm (TAA) is associated with Marfan syndrome (MFS), a connective tissue disorder caused by mutations in fibrillin-1. Sexual dimorphism has been recorded for TAA outcomes in MFS, but detailed studies on the differences in TAA progression in males and females and their relationships to outcomes have not been performed. The aims of this study were to determine sex differences in the diameter dilatation, mechanical properties, and extracellular matrix (ECM) remodeling over time in a severe mouse model (Fbn1mgR/mgR = MU) of MFS-associated TAA that has a shortened life span. Male and female MU and wildtype (WT) mice were used at 1-4 mo of age. Blood pressure and in vivo diameters of the ascending thoracic aorta were recorded using a tail-cuff system and ultrasound imaging, respectively. Ex vivo mechanics and ECM remodeling of the aorta were characterized using a biaxial test system and multiphoton imaging, respectively. We showed that mechanical properties, such as structural and material stiffness, and ECM remodeling, such as elastic and collagen fiber content, correlated with diameter dilatation during TAA progression. Male MU mice had accelerated rates of diameter dilatation, stiffening, and ECM remodeling compared with female MU mice which may have contributed to their decreased life span. The correlation of mechanical properties and ECM remodeling with diameter dilatation suggests that they may be useful biomarkers for TAA progression. The differences in diameter dilatation and life spans in male and female MU mice indicate that sex is an important consideration for managing thoracic aortic aneurysm in MFS. NEW & NOTEWORTHY Using a mouse model (Fbn1mgR/mgR = MU) of severe thoracic aortic aneurysm in Marfan syndrome (MFS), we found that male MU aorta had an accelerated time line and increased amounts of dilatation, stiffening, and extracellular matrix (ECM) remodeling compared with female MU aorta that may have contributed to an increased risk of fatigue failure with cyclic loading over time and a reduced life span. We suggest that aortic stiffness may provide useful information for clinical management of aneurysms in MFS.

Running shoe cushioning properties at the rearfoot and forefoot and their relationship to injury: study protocol for a randomised controlled trial on leisure-time runners

Previous work has demonstrated the protective effect of shoe cushioning on injury risk in leisure-time runners, but most models currently available on the market have greater cushioning than those investigated...

Variation of Secant Young’s Modulus in an Unsaturated Gap-Graded Granular Fill

It is of great significance to understand the variation of stiffness in unsaturated granular materials for predicting performance of earth structures. Variation of secant Young’s modulus in an unsaturated gap-graded granular fill was investigated using unsaturated triaxial tests with local strain measurements in this work. Unlike the insignificant suction impact on shear strength in coarse-grained granular material, it seems that suction has greater effect on stiffness of the tested soil at small strain. Young’s modulus enhances as suction and net confining pressure rise since average skeleton stress increases and soil skeleton gets more stable by meniscus water. In addition, specimen at larger suction possesses greater degradation speed than that at lower ones. One equation was adopted to estimate Young’s modulus of soil. It shows that this equation may capture the variations of secant Young’s modulus of soil under different suctions and net confining pressures.

Toward Superior Stiffness in Carbon: The Interplay between Bond Strength and Anisotropic Response.

Carbon's ability to form diverse structures with varying hybridizations motivates the search for novel materials surpassing diamond's exceptional mechanical rigidity. We analyze previously predicted superhard phases and find that some exhibit average bond stiffness and density higher than those of diamond, hinting at potentially superior stiffness. However, these structures show a lower bulk modulus. We delve into this contradiction and demonstrate that it stems from the structures' anisotropic response to isotropic deformation. The concept of strain anisotropy is introduced to quantify this phenomenon. Our findings reveal a key connection between bond stiffness and bulk modulus in carbon materials, highlighting the importance of considering anisotropic behavior for a comprehensive understanding of the mechanical properties.