Compressive Fatigue Properties Research Articles

Effects of PVA fibers and multi-walled carbon nanotubes reinforcement on uniaxial compression fatigue properties of Engineered geopolymer composites

As a new type of green and environmentally friendly cementing material, engineered geopolymer composites (EGC) have broad application prospects. In this study, uniaxial compression fatigue tests were conducted to investigate the fatigue properties of geopolymer material. A total of 137 samples were tested to investigate the effects of carbon nanotubes content, polyvinyl alcohol (PVA) fibers volume content, and maximum stress level on the fatigue life and deformation of geopolymers. The results showed that the fatigue life of geopolymers obeys Weibull distribution, and PVA fibers and multi-walled carbon nanotubes (MWCNTs) achieved multiscale enhancement of the fatigue life by up to 16.34 times. The total strain accumulation, residual strain, and fatigue stiffness of the geopolymers under fatigue loading exhibited a three-stage evolution rule, with stability within the secondary creep branch. In the process of EGC production, the innovative adoption of optimized MWCNTs surface modification and dispersion strategies improved the uniformity of MWCNTs distribution in the matrix. Scanning electron microscopy (SEM) confirmed the hybridization of MWCNTs and PVA fibers at the microscopic level, which can achieve multi-scale enhancement of geopolymers. This study successfully optimized the content of MWCNTs and PVA fibers, further expanding the application range of composite materials.

Influence of free water on compressive fatigue properties of ECC

Engineered cementitious composites (ECC) with strain hardening characteristics have potential applications in hydraulic structures subjected to repeated or fatigue loads. In order to clarify the effect of free water on the compressive fatigue performance of ECC, the compressive fatigue tests of dry, saturated and water-submerged ECC under different stress levels were carried out. The failure mode and S-N curve of ECC were determined, and the fatigue life of ECC decreased in turn under dry, saturated, and water-submerged conditions. The strain rate of the second stage of ECC compression fatigue increases with the increase of water content and stress level. The failure strain of ECC fatigue increases with the decrease of stress level, and the fatigue failure strain of saturated and immersed specimens is larger than that of dry specimens at the same stress level. In addition, the stress level-fatigue failure strain (P-S-ε) equation considering the failure probability is established. To improve the credibility of this equation, the confidence γ is considered, and the γ-P-S-ε curves of all samples are obtained, which can predict the fatigue failure strain of ECC at a specific stress level.

Compression fatigue properties of bioinspired nacre-like composites compared with natural nacre: Effects of architectures and orientations

Bioinspired architectures mimicking natural nacre demonstrate a remarkable efficiency in toughening materials, yet their fatigue properties remain to be explored, specifically regarding the effects of architectural types and orientations. Here the compression-compression fatigue properties of bioinspired ceramic-polymer composites with nacre-like lamellar and brick-and-mortar architectures along with natural nacre were investigated by cyclically loading along different orientations. The fatigue performance and damage mechanisms of these materials were revealed to be closely related to their architectural types and orientations. The nacre-like architectures played a role in hindering the propagation of fatigue cracks by inducing crack deflection, promoting the contact and mutual friction between cracking faces, and via the plastic deformation of polymer phase. This work may provide experimental basis for the application of nacre-like composites and give insights for designing new bioinspired fatigue-resistant materials.

Experimental investigation of impacted multidirectional laminates under compressive static and cyclic loading

In this paper, the static and fatigue behaviors under compression of Multi-Directional (MD) Carbon Fiber Reinforced Polymer (CFRP) laminates with impact damage were investigated. After low-velocity impact performed on three 24-ply Quasi-Isotropic Quasi-Homogeneous (QIQH) laminates having different stacking sequences, the influence of the distribution of interfaces between two plies on the compression after impact and post-impact compression fatigue properties was evaluated. The damages on the surfaces and internal delamination were accessed via visual and optical observation and ultrasonic C-scan inspection. The thermographic approach, which has been developed and validated on numerous non-impacted composite materials, was applied for the rapid determination of the fatigue limit together with the S-N curves of post-impact fatigue specimens. Experimental results showed that with 5 J impact energy, the three stacking sequences gave similar impact results. The residual strengths measured by compression after impact tests were also similar to each other, so did the global damage modes. The determined fatigue limits of impacted specimens of different stacking sequences were nearly identical and the predicted S-N curves were also close to each other despite the exact crack propagation and orientation were not the same, under both compressive static and cyclic loads.

Fatigue performance and fatigue equation of crumb rubber concrete under freeze–thaw cycles

To prepare crumb rubber concrete (CRC) by grinding rubber particles is one of the most efficient ways to recycle waste tires. In cold regions, the concrete is simultaneously subjected to fatigue loading and freeze–thaw cycles. In order to reveal the effect of freeze–thaw cycles on fatigue strain and fatigue life of CRC, CRC was made by substituting sand with equivalent volumes of rubber granules of 30 mesh, and after freeze–thaw cycles, the experimental study and theoretical analysis of the uniaxial compression and constant amplitude fatigue properties of the specimens were carried out. The results show that the fatigue strain of CRC presents a three-stage variation and is basically unaffected by freeze–thaw cycles. The fatigue life of CRC after freeze–thaw cycles accords with the Weibull distribution, The fatigue life of CRC mainly depends on the actual stress level. The test results of the fatigue life of CRC were analyzed for reliability by probabilistic statistical methods to obtain fatigue equations for different survival rates. Based on the principle of equivalent damage, the fatigue equation considering freeze–thaw cycles was established which can be used to predict fatigue life of CRC in engineering structures in the seasonal frozen regions.

Investigation on effect of pillar size on the comprehensive properties of Ti6Al4V lattice fabricated by selective laser melting

To investigate the effect of pillar size on the mechanical properties of Ti6Al4V lattice fabricated by selective laser melting (SLM), seven groups of Ti6Al4V lattice structures with different pillar sizes were fabricated by SLM technology. The compression process of the lattice structure of body-centered cubic element was studied by numerical simulation method, and then the numerical simulation results were verified by experiments. The porosity of the lattice structure was characterized by dry weight method, and the compressive and compressive fatigue properties of the lattice structure were tested. The results showed that the actual porosity of all lattice structures with different pillar sizes was less than the designed porosity due to the thermal adsorption of the surrounding powder on the pillar. The numerical simulation was in good agreement with the experimental results, which can be used to predict the displacement changes in the elastic region of the lattice structure. With the increase of the pillar size, the difference between the actual porosity and the designed porosity of the lattice structure increased first and then decreased. The compression resistance of the unit cell topology was better. Meanwhile, the compression and yield strength of the lattice structure were increased. The larger the pillar size of lattice structure, the better the plasticity. It can withstand higher cyclic load and have better fatigue performance.

The role of pore structures on the fatigue properties of additively manufactured porous tantalum scaffolds produced by electron beam powder bed fusion

In this work, porous tantalum scaffolds with structures of cubic, G7 and rhombic dodecahedron with different strut angles were fabricated by electron beam powder bed fusion (EB-PBF). Uniaxial compressive testing and compression–compression fatigue testing were conducted on the EB-PBF built tantalum scaffolds. The role of pore structures on the deformation behavior and fatigue properties of the tantalum scaffolds were analyzed. The effect of buckling component in the rhombic dodecahedron structure on fatigue performance of the porous tantalum scaffolds was investigated. It was demonstrated that the G7 and cubic porous tantalum scaffolds were dominated by buckling deformation under the axial force. Due to the presence of vertical struts, the G7 scaffold showed higher fatigue strength and fatigue ratio than the dodecahedron which deformed mainly via bending, the cubic scaffold showed the highest fatigue ratio. As the strut angel increased from 35.3° to 43°, the fatigue strength and fatigue ratio of the rhombic dodecahedron tantalum scaffolds improved by 59% and 20% at 2 × 106 cycles, respectively. The ratcheting strain and fatigue damage strain during the fatigue test were both decreased with the increase of buckling component, leading to the boosting of compressive fatigue properties of tantalum scaffolds accordingly.

Longitudinal compression fatigue properties of 3D five-directional braided composites at different temperatures

3D braided composites are widely used in many areas due to their unique properties of high designability, excellent damage tolerance and integrality. Longitudinal properties of 3D five-directional braided composites (3D5dBC) deserve more extensive attention owning to addition of the fifth yarns for potential applications in marine at low temperature and aerospace at elevated temperature. In this work, longitudinal compression fatigue properties and failure mechanisms of 3D5dBC are investigated, where parameters of temperature, braiding angle and stress level are simultaneously considered. The results show that the composite at low temperature has better fatigue resistance ability, and fatigue resistance of small braiding angle composite is superior to that of large braiding angle composite. The strain change could be divided into three stages; the creep rate at stage II are similar respectively at −50 °C and 100 °C, which shows that strain change is temperature-independent. Meanwhile, the hysteresis loop at cycle of 3000 reveals that energy dissipation increases with the increase of stress level for large braiding angle composite. The failure modes of composite under longitudinal fatigue are shear expansion failure. At low temperature, brittle shear feature is more obvious. In addition, fiber/matrix interfacial debonding is more distinct at elevated temperature.

Fatigue and dynamic biodegradation behavior of additively manufactured Mg scaffolds

Additive manufacturing (AM) has enabled the fabrication of biodegradable porous metals to satisfy the desired characteristics for orthopedic applications. The geometrical design on AM biodegradable metallic scaffolds has been found to offer a favorable opportunity to regulate their mechanical and degradation performance in previous studies, however mostly confined to static responses. In this study, we presented the effect of the geometrical design on the dynamic responses of AM Mg scaffolds for the first time. Three different types of porous structures, based on various unit cells (i.e., biomimetic, diamond, and sheet-based gyroid), were established and then subjected to selective laser melting (SLM) process using group-developed Mg-Nd-Zn-Zr alloy (JDBM) powders. The topology after dynamic electropolishing, dynamic compressive properties, and dynamic biodegradation behavior of the AM Mg scaffolds were comprehensively evaluated. It was found that dynamic electropolishing effectively removed the excessive adhered powders on the surfaces and resulted in similar geometrical deviations amongst the AM Mg scaffolds, independent of their porous structures. The geometrical design significantly affected the compressive fatigue properties of the AM Mg scaffolds, of which the sheeted-based gyroid structure demonstrated a superior fatigue endurance limit of 0.85 at 106 cycles. Furthermore, in vitro dynamic immersion behaviors of the AM Mg scaffolds revealed a decent dependence on local architectures, where the sheeted-based gyroid scaffold experienced the lowest structural loss with a relatively uniform degradation mode. The obtained results indicate that the geometrical design could provide a promising strategy to develop desirable bone substitutes for the treatment of critical-size load-bearing defects. Statement of significanceAdditive manufacturing (AM) has provided unprecedented opportunities to fabricate geometrically complex biodegradable scaffolds where the topological design becomes a key determinant on comprehensive performance. In this paper, we fabricate 3 AM biodegradable Mg scaffolds (i.e., biomimetic, diamond, and sheet-based gyroid) and report the effect of the geometrical design on the dynamic responses of AM Mg scaffolds for the first time. The results revealed that the sheeted-based gyroid scaffold exhibited the best combination of superior compressive fatigue properties and relatively uniform dynamic biodegradation mode, suggesting that the regulation of the porous structures could be an effective approach for the optimization of AM Mg scaffolds as to satisfy clinical requirements in orthopedic applications

Ultrasonic fatigue testing of concrete

Cyclic compression fatigue properties of concrete are studied with the ultrasonic fatigue testing method with cycling frequency 19 kHz and are compared to servo-hydraulic tests performed at 60 Hz. Ultrasonic testing was found applicable for rapid generation of very high cycle fatigue (VHCF) data of concrete. Fatigue cracks can be initiated, however specimens do not rupture, since cyclic stresses decrease with increase of compliance in displacement controlled ultrasonic tests. Observation of resonance frequency, analysis of higher order harmonics of vibration, and computed tomography of specimens are successful methods to analyse fatigue damage. Calorimetric evaluations can be used to calculate the cyclic irreversible strain, which is about 1% of the elastic strain in the ultrasonic VHCF test.

Rigid and flexible polyimide aerogels with less fatigue for use in harsh conditions

Polyimide aerogels possess various applications in harsh environment but are often limited by poor mechanical property. The ice template method to construct a layered aerogel has structural advantages, but there are still problems of insufficient compression recovery and anisotropy. We investigated the method of forming a spring-like bridge in layered structure, and verified the effect of bridges on the compression deformation, compressive strength, energy dissipation, and fatigue properties. Based on structural observations, a corresponding force mechanism was proposed. After 10,000-cyclic compression at 50% strain, PI aerogels had only 5.8% plastic deformation, 82% stress retention and stable energy dissipation coefficient below 0.3. Interestingly, the compressive performance was isotropic in horizontal direction. Combined with low thermal conductivity, PI aerogels had high structural stability when repeatedly used at 300 °C. This research may provide an available method for design of high-strength and high-toughness polymer aerogels suitable for harsh environment.

Mechanical properties of TiAl fabricated by electron beam melting — A review

As a typical intermetallic material, TiAl is inevitably difficult to process by conventional methods. Additive manufacturing (AM) has recently become a new option for making net-shape TiAl components. Among all AM methods, electron beam melting (EBM) shows the potential to make TiAl components with good mechanical properties and is used for low pressure turbine blades. The mechanical properties, including tensile and compression properties, fracture toughness, fatigue and creep properties of EBM TiAl are reviewed and compared to the conventionally fabricated alloys. Results show that the tensile strength of EBM alloys is higher than cast alloys, and other properties are comparable to the cast/forged alloys. The sensitivity of mechanical properties and microstructure to EBM processing parameters is presented. Issues including layered microstructure, anisotropy in mechanical properties, and fatigue failure from defects are also reviewed. Finally, some opportunities and challenges of EBM TiAl are identified.

Compression fatigue properties and damage mechanisms of a bioinspired nacre-like ceramic-polymer composite

Fatigue resistance is invariably critical for structural materials, but is rarely considered in the development of new bioinspired materials. Here the fatigue behavior and damage mechanisms of a nacre-like ceramic (yttria-stabilized zirconia) - polymer (polymethyl methacrylate) composite, which resembles human tooth enamel in its stiffness and hardness, were investigated under cyclic compression to simulate potential service conditions. The composite has a brick-and-mortar structure which exhibits a staircase-like fracture behavior; it displays a transition in cracking mode from the fracture of the ceramic bricks to separation along the inter-brick polymer phase with increasing stress amplitude. The nacre-like structure functions to induce crack deflection, increase the roughness of the crack surfaces, and promote the mutual sliding between bricks during fracture; this results in high fatigue resistance, which enhances the potential of this composite for dental applications.

Design and Compressive Fatigue Properties of Irregular Porous Scaffolds for Orthopedics Fabricated Using Selective Laser Melting.

An irregular porous structure plays a major role in bone tissue engineering, and it is more suitable for bone tissue growth than a regular porous structure. The response surface method was used to establish a relationship between the average pore size and the design parameters. The technology of selective laser melting was utilized to fabricate the porous Ti-6Al-4V scaffolds with an irregularity of (0.4) and porosities of (70, 80, and 90%) designed using the Voronoi-tessellation method. Compression tests of porous scaffolds showed an elastic modulus range of 0.84-1.97 GPa and an ultimate strength ranging within 21.0-99.1 MPa. The elastic modulus was mainly influenced by the porosity and heat-treatment process. Furthermore, the fatigue test results suggested that the number of cycles (9 × 104 to 1.8 × 106) was greatly influenced by the porosity and heat-treatment process. The heat treatment of annealing greatly improved the fatigue performance of porous scaffolds. The irregular porous scaffolds with lower porosity and after full annealing exhibited the best fatigue behavior.

Compressive fatigue properties of additive-manufactured Ti-6Al-4V cellular material with different porosities

Selective laser melting (SLM) is a novel additive manufacturing (AM) technique for producing cellular metallic materials with designed pore structures. SLM Ti-6Al-4V cellular alloys have been extensively studied for biomedical and other applications. The objective of this study was to investigate the effects of porosity (33 vol%, 50 vol%, and 84 vol%) on the compressive fatigue performance and fracture mechanism of an SLM Ti-6Al-4V cellular material with a new cuboctahedron COH-Z unit cell. Furthermore, the influences of hot isostatic pressing (HIP) at 1000 °C/150 MPa on the fatigue properties and fracture were also examined in this study.The results showed that the fatigue strain slowly accumulated when the stress cycle was below the 70% fatigue life. After the 70% fatigue life, the ratcheting strain rate (dε/dN) gradually increased. A shear band along a direction inclined at ~45° to the stress axis could be definitely identified at about 95% fatigue life, as demonstrated by the digital image correlation technique. With further increases in the stress cycle, the shear band was intensified and finally led to rapid fatigue failure. The HIP process did not alter the fracture behaviors of the SLM cellular alloy, although the fatigue life was much improved by several times. On the other hand, the fatigue endurance ratio of the SLM cellular alloy with 33 vol% was 0.5. Raising the porosity from 33 vol% to 84 vol% reduced the endurance ratio from 0.5 to 0.15 due mainly to the high notch sensitivity of α′-martensite in the SLM cellular alloy. The HIP treatment changed the microstructure from α′-martensite to lamellar α+β phases and thus significantly improved the fatigue endurance performance. With the combination of the COH-Z unit cell and HIP treatment, the endurance ratios at 106 cycles of the Ti-6Al-4V cellular alloy with porosities of 33 vol% to 84 vol% were as high as 0.5. This superior endurance ratio is advantageous to the long-term application of SLM Ti-6Al-4V cellular alloy.

Fatigue and fracture mechanical properties of selected concrete for subtle precast structural elements

Precast concrete elements used as a civil structure are usually made of a cement-based matrix and natural aggregates (such as sand, gravel, crushed stone, etc.). These structures are usually exposed not only to a static load but also to a cyclic load if they load the bearing part of a bridge (traffic etc.). The knowledge of fatigue and fracture mechanical characteristics is important in designing and modelling new structures. This paper introduces and compares fracture mechanical properties obtained from static and fatigue tests for three kinds of concrete. The focus was set on the bulk density, flexural and compressive cube strength, fracture toughness and fatigue properties (S−N − Wöhler curve). All of these tests are important for a practical application in the design of precast concrete structures. The experimental results were statistically analysed and they showed that the fatigue and mechanical fracture properties improved with improved mechanical parameters of concrete.

Experimental and analytical study on uniaxial compressive fatigue behavior of self-compacting rubber lightweight aggregate concrete

In view of the excellent deformation capacity and energy absorption capacity, rubber particles had potential possibility to improve the fatigue properties of lightweight aggregate concrete (LC). However, the fatigue properties of LC containing rubber particles were not adequately understood. This paper focused on the effect of rubber particles on the uniaxial compressive fatigue properties of self-compacting rubber lightweight aggregate concrete (SCRLC). The results of uniaxial compressive fatigue tests indicated that fatigue life and fatigue strain of SCRLC added generally with increase of rubber particles substitution percentage, fatigue strain of SCRLC also raised with number of cycles increased. Analyses results indicated that the fatigue life of SCRLC conformed to two-parameter Weibull distribution. Based on the experimental results, fatigue equation of SCRLC was established by double logarithmic fatigue equation. From the curves of lgS-lgNf of SCRLC, it could be discovered that increasing of rubber particles substitution percentage in SCRLC resulted in a decrease of fatigue limit strength, but the stress level at fatigue life of 2 × 106 increased firstly and then decreased. The highest stress level of SCRLC would be obtained as rubber particles substitution percentage was 30%. By comprehensive consideration, under the same strength level, the fatigue properties of SCRLC was better than that of LC.

Mechanical Properties of Double Network Poly (Acrylic Acid) Based Hydrogels for Potential Use as a Biomaterial.

Load-bearing applications of hydrogels include soft robots, tissue engineering, and stretchable electronics. This paper presents an extensive study of double network poly (acrylic acid) based hydrogel on stress relaxation, compression fatigue, shear stress, and shock absorption properties as a potential load-bearing soft tissue replacement biomaterial. Double network poly (acrylic acid) hydrogel was selected due to simple processing and availability. The optimized formulation of poly (acrylic acid) hydrogel was used for samples preparation. The compression modulus varied with hydrogel formulation, crosshead speed and swelled amount of the hydrogel. Stress relaxation and shock absorption properties of hydrogel were compared with polyurethane gel used in soft insoles (Shore 5A). Developed hydrogel displayed good fatigue properties up to 10,000 loading cycle at maximum stress of 390±30 kPa and at 84±4% strain. Further, maximum average shear stress and shear modulus of 80 kPa and 140 kPa respectively were observed at 84% strain before fracture.

Change of Fatigue and Mechanical Fracture Properties of a Cement Composite due to Partial Replacement of Aggregate by Red Ceramic Waste

Fine-grained cement-based composites used in civil engineering and construction industry are usually made of cement-based matrix and natural aggregate (such as sand, gravel, crushed stone, etc.). Red ceramic waste aggregate is considered as a perspective replacement of a part of natural aggregate in modern environmentally oriented building materials. Fine-grained cement composite with natural aggregate partially replaced by ceramic waste aggregate usually show different mechanical fracture characteristics from ordinary fine-grained concrete. The specimens were tested at six different ages. This was the reason for conducting the research programme. Altogether, 6 fine-grained cement mixtures with various proportions of natural and red ceramic waste aggregate were prepared. The aim of this paper is to present and compare mechanical fracture properties obtained from static and fatigue tests. Bulk density, flexural and compressive cube strength, fracture toughness and fatigue properties (S−N − Wöhler curve) were of special interest. All of these tests are important for a practical application of concrete with ceramic aggregate for structures. All the results were statistically analysed and they showed that the fatigue and mechanical fracture properties were improved or at least kept up with the increasing levels of red ceramic waste aggregate. Environmental impact of application in construction industry of composites in question is discussed.

Study on the fatigue properties of as-extruded α + β dual phase Mg-Li-Al alloy

The as-cast dual phase Mg-9Li-3Al (LA93) alloy was designed and prepared by vacuum resistance furnace under the protection of argon atmosphere, then processed by hot extrusion. Tension and compression fatigue properties of as-extruded dual phase LA93 alloy was studied. The as-extruded LA93 alloy had a certain hardening phenomenon during axial tension and compression fatigue. The fatigue limit of the as-extruded LA93 alloy is estimated to be σ−1 = 78.15 MPa, which is 0.37 times the tensile strength. The measured fatigue life values of the samples are in line with lognormal distribution and strong linear correlation. The S-N curve can be obtained by straight-line fitting and it is lgN = −13.72lgσ + 32.63. The P-S-N equation of confidence 90% is σ = −14.35lgN + 171.65. The crack source of the alloy sprouted on the surface of the sample and radiated diffused in the shape of the fan. There was obvious fatigue stripe on the fracture surface of the fracture surface in the crack propagation area.