Search Results (15)

This study examines the microstructural evolution, mechanical properties, and wear behavior of medium-carbon dual-phase steel (AISI 1040) processed via Multi-Axis Compression (MAC). The DP steel was produced through inter-critical annealing at 745 °C, followed by MAC at 500 °C, resulting in a refined grain microstructure. Optical micrographs confirmed the presence of ferrite and martensite phases after annealing, with significant grain refinement observed following MAC. The average grain size decreased from 66 ± 4 μm to 18 ± 1 μm after nine MAC passes. Mechanical testing revealed substantial improvements in hardness (from 145 ± 9 HV to 298 ± 18 HV) and ultimate tensile strength (from 557 ± 33 MPa to 738 ± 44 MPa), attributed to strain hardening and the Hall–Petch effect. Fractographic analysis revealed a ductile failure mode in the annealed sample, while DP0 and DP9 exhibited a mixed fracture mode. Both DP0 and DP9 samples demonstrated superior wear resistance compared to the annealed sample. However, the DP9 sample exhibited slightly lower wear resistance than DP0, likely due to the fragmentation of martensite induced by high accumulated strain, which could act as crack initiation sites during sliding wear. Furthermore, wear resistance was significantly enhanced due to the combined effects of the DP structure and Severe Plastic Deformation (SPD). These findings highlight the potential of MAC processing for developing high-performance steels suitable for lightweight automotive applications. Full article

To accurately characterize the damage of rock-like materials under simultaneous or alternating tensile and compressive loading, a dynamic damage constitutive model for rock-like materials based on elastic tensile strain is developed by integrating the classical compressive plastic damage model and the tensile elastic damage model. The model is based on the Holmquist–Johnson–Cook (HJC) and Kuszmaul (KUS) models, categorizing the element stress state into tensile and compressive states through positive and negative elastic volumetric strain. It utilizes elastic tensile strain to enhance the calculation method for tensile cracks, determining the tensile strength of the principal direction based on the contribution rate of tensile principal stress for uniaxial/multiaxial loading. Additionally, it establishes a maximum elastic tensile strain rate function to rectify the model’s effect on the tensile strain rate. Through the LS-DYNA subroutine development, the model proficiently delineates the distribution of ring-shaped cracks on the frontal side and strip-shaped cracks on the rear side of the reinforced concrete slab subjected to impact loading. Numerical simulations demonstrate that the model provides more accurate damage prediction results for stress conditions involving simultaneous or alternating compression and tension, offering valuable insights for damage analysis in engineering blasting or impact penetration. Full article

In this study, we optimized the parameters of diffusion bonding on multi-layered stainless steel 316L and 430 stacks. The preparation process for diffusion bonding is crucial, as the bonding surfaces need to be polished and meticulously cleaned to ensure a smooth bonding process. We fabricated twelve-layer plates consisting of 55 mm × 55 mm × 3 mm and 100 mm × 50 mm × 3 mm dimensions, and the bonding response was investigated by evaluating the tensile strength of the bonding zone under varying bonding conditions, with a bonding temperature ranging from 1000 to 1048 °C, a bond time ranging from 15 to 60 min, pressure ranging from 10 to 25.3 MPa, and under a vacuum environment. SS430 exhibits a significantly higher compression creep rate than SS316L. The compressibility of diffusion welding materials does not impact the diffusion bonding strength. Multi-axial tensile strength tests confirmed strong bonding joint strength in various axes. The tensile strengths of monolithic and Diffusion bonding (DB) specimens tested in parallel are essentially identical. The optimized diffusion bonding parameters (Condition G2C: 1048 °C/25.3 MPa/15 min) are ideal for producing SS316L stainless steel cores in compact heat exchangers, offering a superior bonding quality and reduced costs. These findings have practical implications for the production of stainless steel cores in compact heat exchangers, demonstrating the relevance and applicability of our research. Full article

Fine-grained high-strength concrete has already been tested extensively regarding its uniaxial strength. However, there is a lack of research on the multiaxial performance. In this contribution, some biaxial tests are investigated in order to compare the multiaxial load-bearing behaviour of fine-grained concretes with that of high-strength concretes with normal aggregate from the literature. The comparison pertains to the general biaxial load-bearing behaviour of concrete, the applicability of already existing fracture criteria and the extrapolation for the numerical investigation. This provides an insight into the applicability of existing data for the material characterisation of this fine-grained concrete and, in particular, to compensate for the lack of investigations on fine-grained concretes in general. It is shown, that the calibration of material models for fine-grained concretes based on literature results or normal-grained concrete with similar strength capacity is possible, as long as the uniaxial strength values and the modulus of elasticity are known. For the numerical simulation, a Microplane Drucker–Prager cap plasticity model is introduced and fitted in the first step to the biaxial compression tests. The model parameters are set into relation with the macroscopic quantities, gained from the observable behaviour of the concrete under uniaxial and biaxial compressive loading. It is shown that the model is able to capture the yielding and hardening effects of fine-grained high-strength concrete in different directions. Full article

Aeronautical composite primary structures must evidence sufficient residual strength in the presence of damage for compliance with damage tolerance requirements. The study of stiffener debonding on panels subjected to compression after impact is performed in that scope. Compression leads to the buckling of the skin between the stiffeners, and thus a complex loading of the bonding between the skin and the stiffener. This paper describes the development of a stiffened specimen for the VERTEX multiaxial test rig as a first step towards the study of the damage tolerance evaluation of stiffened structures, under combined loadings and at the intermediate scale of the test pyramid. By using virtual testing, the specimen was designed to produce the phenomenology of interest as the first damage, i.e., the debonding of the stiffener from the centre. Three samples were manufactured and subjected to low velocity impacts at various locations and energies. Then the three samples were subjected to compression after impact, up to the stiffener debonding, under a post-buckling regime of the skin. Test loading evolution is described with force fluxes and global strains, obtained from in situ stereo-correlation. The different impacts were found to give different types of damage but similar residual strength to compression after impact. Full article

This paper presents the results of a stress analysis and fatigue life calculation of an HPDC mould core. The calculations were performed using Abaqus and fe-safe software. The numerical model of a core cooled by a conformal channel was based on an existing and working counterpart made of additively manufactured high-strength 1.2709 maraging steel. This study shows that the conformal channel results in a lower average core temperature as compared to the temperature of the same core shape cooled by the conventional method. The course of the stress changes during the mould cycle was also determined. It was found that stresses on the core surface caused the cyclic compression and tension of the material. The necessary strength tests of 1.2709 steel produced by selective laser melting (SLM) within a temperature range of 25 to 550 °C, which were necessary to define the fatigue coefficients by the Seeger approximation method, were also carried out in this study, along with metallographic tests of the fractures of the specimens. Based on the multiaxial fatigue criterion and using the maximum principal deformation hypothesis, the fatigue life of the core and channel surfaces was determined. Based on the calculations, it was shown that crack initiation on the channel surface can occur earlier than on the outer surface of the core. Full article

The multiaxial Mises-Schleicher and Drucker-Prager unified (MSDP_u) criterion has been shown to exhibit several specific features compared to other yield and failure criteria, including a nonlinear mean stress dependency, influence of the Lode angle, use of independent uniaxial compressive and tensile strength values and absence of an apex (singularity) on the envelope surface in the negative stress quadrant. However, MSDP_u has been seldom used in practice to solve geotechnical and geomechanical engineering problems mainly because it had not yet been fully implemented into three-dimensional (3D) numerical codes. To fill this gap, a 3D elastoplastic MSDP_u formulation is developed and implemented into FLAC3D. The proposed MSDP_u elastic-perfectly plastic (EPP) constitutive model is then validated against existing analytical solutions developed for calculating the stress and displacement distributions around cylindrical openings. The FLAC3D MSDP_u-EPP model is then applied to evaluate the vertical and horizontal stress distributions in a three-dimensional vertical backfilled stope. The numerical results obtained with the MSDP_u-EPP model are compared with those obtained with the Mohr-Coulomb EPP model, to highlight key features of the new formulation. Full article

The building industry accounts for half of the global resource consumption and roughly one third of global CO₂ emissions. Global population growth and increasing resource scarcities require engineers and architects to build for more people with less material and emissions. One promising solution are adaptive load-bearing structures. Here, the load-bearing structure is equipped with actuators, sensors, and a control unit which allows the structure to adapt to different load cases, resulting in substantial material savings. While the first prototypes use industry standard actuators to manipulate deformations and stress states, it is essential to develop actuator concepts which fit the specific requirements of civil engineering structures. This paper introduces new concepts for linear actuators, developed within the Collaborative Research Centre (SFB) 1244 Adaptive Skins and Structures for the Built Environment of Tomorrow, which can be used as adaptive concrete columns. The concept of an actuator which actuates a concrete column by external compression through hydraulic pressure is discussed in further detail. This concept allows for controlled axial extension while also increasing the compressive strength of the concrete column. Full article

Biocompatible β-type Ti-45Nb alloy with a low elastic modulus is promising in alleviating the stress shielding effect of Ti-based hard-tissue replacement implants. In this work, the ultra-fine-grained (UFG) microstructures with different grain sizes were prepared by multi-axial compression (MAC) processing of Ti-45Nb alloys, and the mechanical properties and the fretting wear properties of Ti-45Nb alloys in different grain sizes were investigated. The results show that the yield strength and ultimate tensile strength of the sample processed by 27 passes MAC increase by 76% and 91%, respectively, with an elongation of more than 9%. After MAC processing, the friction coefficient and volume wear rate gradually decrease. In addition, before MAC processing, the Ti-45Nb sample shows a wear mechanism of severe adhesive wear, oxidative wear and fatigue delamination; while after MAC processing, the wear mechanism switches to abrasive wear and slight adhesive wear with slight oxidative wear, indicating that grain refinement helps to improve the anti-fretting properties of Ti-45Nb alloys. Full article

The mechanical properties and fracture behaviour of concretes under different triaxial stress states were investigated based on a 3D mesoscale model. The quasistatic triaxial loadings, namely, compression–compression–compression (C–C–C), compression–tension–tension (C–T–T) and compression–compression–tension (C–C–T), were simulated using an implicit solver. The mesoscopic modelling with good robustness gave reliable and detailed damage evolution processes under different triaxial stress states. The lateral tensile stress significantly influenced the multiaxial mechanical behaviour of the concretes, accelerating the concrete failure. With low lateral pressures or tensile stress, axial cleavage was the main failure mode of the specimens. Furthermore, the concretes presented shear failures under medium lateral pressures. The concretes experienced a transition from brittle fracture to plastic failure under high lateral pressures. The Ottosen parameters were modified by the gradient descent method and then the failure criterion of the concretes in the principal stress space was given. The failure criterion could describe the strength characteristics of concrete materials well by being fitted with experimental data under different triaxial stress states. Full article

The strain energy released plays a crucial role in generating macroscopic failure in unidirectional (UD) composites. This paper proposes two new strain energy-based failure criteria, regarding fiber-dominated and matrix-dominated failure mode as independent and interactive, respectively. The failure expression is formulated based on rigorous mathematical deducing, accompanied by physical interpretation. Based on the lack of experimentally feasible multi-axial strengths, a predefined assumption of infinite strength under bi-axial and tri-axial compressive stress provides the possibility for determining all coefficients only by using conventional uniaxial strengths. The failure envelopes predicted by the proposed criteria have been validated against experimental results under biaxial, off-axis and tri-axial loading cases. A better agreement with physical reality was achieved by the failure mode-interactive criterion, suggesting a wide range of applicability. Full article

Ultra-high molecular weight polyethylene (UHMWPE) Dyneema^® SK-76 fibers are widely used in personnel protection systems. Transverse ballistic impact onto these fibers results in complex multiaxial deformation modes such as axial tension, axial compression, transverse compression, and transverse shear. Previous experimental studies on single fibers have shown a degradation of tensile failure strain due to the presence of such multi-axial deformation modes. In this work, we study the presence and effects of such multi-axial stress-states on Dyneema^® SK-76 yarns via transverse loading experiments. Quasi-static transverse loading experiments are conducted on Dyneema^® SK-76 single yarn at different starting angles (5°, 10°, 15°, and 25°) and via four different indenter geometries: round (radius of curvature (ROC) = 3.8 mm), 200-micron, 20-micron, and razor blade (ROC $~$2 micron). Additionally, transverse loading experiments were also conducted for a 0.30 cal. fragment simulating projectile (FSP) and compared to other indenters. Experimental results show that for the round, 200-micron indenter, and FSP geometry the yarn fails in tension with no degradation in axial failure strain compared to the uniaxial tensile failure strain of SK-76 yarn (2.58%). Whereas for the 20-micron indenter and razor blade, fibers fail progressively in transverse shear followed by progressive strength degradation of the yarn. Strength degradation of yarn occurs at relatively low strains of 0.6–0.7% with eventual failure of the yarn at approximately ~1.8% and ~1.5% strain for the 20-micron indenter and razor blade, respectively. Breaking angles (range of 10°–30°) are observed to have little effect on the failure strain for all indenter geometries. Full article

The paper provides an overview of ways to increase the strength of polymer products obtained by fused filament fabrication (FFF) technology. An algorithm for calculating the spiral toolpaths for the material deposition using multi-axis printing is proposed. The design of the five-axis device for spiral-shaped deposition of the material is shown. The description of the proposed printing method is given. The results of comparative three-point bend and compression tests are presented. The standard samples obtained in the usual way by FFF technology, as well as samples with 2, 4, 6, 8 and 10 reinforcing layers obtained by spiral deposition of the material were investigated. The description of the tests is given, the dependences of the strength of the products on the number of reinforcing layers are obtained. Conclusions about the influence of the layer deposition method on the strength of the products are formulated. Full article

Polyvinyl alcohol (PVA)-steel hybrid fiber reinforced engineered cementitious composites (ECC) characterized by optimal combination of high strength and high ductility were developed recently. These composites exhibit even tighter crack width than normal ECC, showing great potential for lower permeability in cracked state, and consequently improving the durability of ECC structures. In addition, the wide variety of promising applications in underground or hydraulic structures calls for knowledge on the mechanical behavior and corresponding permeability properties of strained ECC under multiaxial stress, as they are essential for structural analysis and durability design. Experimental investigations into the compressive properties and the in-situ gas permeability of PVA-steel hybrid fiber ECC were performed in this study, with special focus on the impact of additional steel fiber content and confining pressure. The test results show that the presence of a low confinement level allows ECC to attain a substantial improvement on compressive behavior but impairs the enhancement efficiency of additional steel fiber. The permeability evolution of strained ECC corresponds to the variation of radial strains, both of which experience little change below the threshold stress but a rapid increase beyond the peak axial strain. Apart from exhibiting low permeability at relatively small strains in the pre-peak stage, ECC can also exhibit low permeability at higher levels of compressive strain up to 2.0%. However, unlike the case in tensile loading, impermeability of cracked ECC in compression would be weakened by additional steel fibers, especially in the post-peak stage. The present research is expected to provide insight into performance-based durability design of structures made of or strengthened with ECC. Full article

Ballistic impact induces multiaxial loading on Kevlar^® and polyethylene fibers used in protective armor systems. The influence of multiaxial loading on fiber failure is not well understood. Experiments show reduction in the tensile strength of these fibers after axial and transverse compression. In this paper, we use molecular dynamics (MD) simulations to explain and develop a fundamental understanding of this experimental observation since the property reduction mechanism evolves from the atomistic level. An all-atom MD method is used where bonded and non-bonded atomic interactions are described through a state-of-the-art reactive force field. Monotonic tension simulations in three principal directions of the models are conducted to determine the anisotropic elastic and strength properties. Then the models are subjected to multi-axial loads—axial compression, followed by axial tension and transverse compression, followed by axial tension. MD simulation results indicate that pre-compression distorts the crystal structure, inducing preloading of the covalent bonds and resulting in lower tensile properties. Full article