Compression-compression Fatigue Research Articles

A novel normalized fatigue progressive damage model for complete stress levels based on artificial neural network

The fatigue life model and fatigue stiffness degradation model are most crucial components in establishment of fatigue progressive damage model (FPDM). Based on numerous static and fatigue experiments of glass fiber reinforced polymer-matrix composites (GFRP) 3232A/Glass, a novel normalized FPDM for complete stress level based on artificial neural network (ANN) has been built: (1) By analyzing fatigue experimental data, the construction method of fatigue life model is deeply investigated. Furthermore, a normalized fatigue life model (NFLM) predicting fatigue life considering simultaneously tension–tension (T-T) fatigue and compression-compression (C–C) fatigue is developed, which has been validated by experimental data. It is expected to reduce experimental time and costs by nearly half; (2) To fully consider the influence of stress level and remove the constraint of the shape of stiffness degradation function, the traditional models are no longer suitable. Thus, a fatigue stiffness degradation model based on ANN (FSDM-ANN) has been constructed, which overcomes drawbacks of the traditional models. The results predicted by FSDM-ANN are consistent with fatigue data in different stress levels; (3) Due to distortion of the fatigue life prediction of extreme high-cycle and low-cycle, an empirical data extrapolation method is proposed here to construct a modified FSDM-ANN, which greatly improves prediction accuracy of stiffness degradation for complete stress levels.

Low-cycle compression-compression fatigue behavior of MEX-printed PLA parts

Due to cyclic loads, additively manufactured (AM) parts can collapse at lower stress values than the strength corresponding to static loads. To date, there is limited research on the fatigue life of AM polymers. The present work investigates the fatigue behavior of material extrusion (MEX) printed Polylactic acid (PLA) parts. In this regard, the low-cycle fatigue (LCF) domain was studied and a compression-compression fatigue test setup was adopted. Initially, before the experimental tests, the MEX-printed samples were subjected to physical assessments. The printed fatigue samples presented a high accuracy with dimensional relative errors below 0.04%, while 6.66% was noted as their mass relative error. Subsequently, quasi-static and cyclic tests were performed in order to determine the mechanical behavior of the AM parts, with special focus on the low-cycle domain of the S-N (or Wöhler) curve. The quasi-static compression behavior of the PLA samples is typical of cellular materials, the samples highlighting a shear band as a failure mechanism. Five load levels were used to plot the low-cycle domain of the S-N (or Wöhler) curve with its 95% level confidence bands. The tests were performed in force-controlled mode with the stress asymmetry ratio R = 0.1 and at the test frequency of f = 10 Hz. An exponential relationship was also found between the strain at break and the number of cycles until failure. The failure of the samples in fatigue load occurred suddenly.

Compression-compression fatigue performances of hybrid unidirectional/woven carbon-fiber reinforced composite laminates after LVI

Compression-compression fatigue behavior of hybrid unidirectional/woven carbon-fiber reinforced composite laminates after low-velocity impact (LVI) was studied in this paper. The paper contains two parts. Firstly, different levels of impact energy were introduced on the specimens. Impact damage modes were obtained and characterized as fiber breakages, matrix cracks, indentation and delamination. The relationships between indentation depth/damage area and impact energy were established. Secondly, compression-compression fatigue tests were conducted on the impact damaged specimens. Fatigue life degradation trends were analyzed and fatigue limit for different types of specimens was determined. Delamination areas by depth mode and amplitude mode of C-scan showed an increasing trend with the fatigue life increasing. Stiffness decreased sharply in the early and ending stages of fatigue life. However, stiffness exhibited stable or slightly decreasing rates in the mid-stage of fatigue life. A line-shaped failure cracks going through the impact point were observed in the fatigue failure specimens.

Fiber fracture behavior of high-pressure resin transfer molded (HP-RTM) carbon/epoxy multidirectional laminates under compression-compression fatigue loading

The influence of different stress levels on the fatigue life and fracture behavior of laminated composites have been evaluated under compression-compression (C–C) loading conditions. Carbon/epoxy multidirectional (MD) laminate specimen was designed with eight layers [0°, ± 45°, 0°]s of unidirectional plies. The preforms were molded by a high-pressure resin transfer molding (HP-RTM) process. As part of the study, a static compression test was performed. The C–C fatigue loads were applied at −260 MPa, −280 MPa, −300 MPa, and −320 MP compression stress until failure or two million cycle loadings. The fatigue limit has been obtained at −265 MPa stress by stress-cycle number (S-N) curve analysis, which is 51.4 % of static compression strength. The damage behavior was identified after C–C fatigue tests. By using optical and scanning electron microscopy, fiber fracture, matrix cracking, and fiber/matrix interface debonding were observed. Under the C–C fatigue loading conditions, fiber fracture modes were characterized by rough cross-sections, shear, and line-shaped edges.

Research on compression-compression fatigue properties of carbon nanotubes reinforced closed-cell aluminum matrix composite foams by reinforcement content design

As the requirement of compressive fatigue loading on the mechanical properties of aluminium foams increase rapidly, the limited fatigue performance of them significantly restricts their engineering applications. In this study, the compression-compression fatigue properties of carbon nanotubes (CNTs) reinforced closed-cell Al-Si composite foams (C-ASCFs) were investigated. The fatigue strength of C-ASCFs with 0.50 wt% CNTs increased by 54% than that of Al-Si foams, where the CNTs contributed to decrease the crack expansion rate, transfer the fatigue load and hence change the fatigue failure mode, causing a combination of plastic and brittle deformation in them. Meanwhile, the appropriate incorporation of CNTs decreased pore nucleation threshold, optimized the pore morphology, and promoted the formation and refinement of eutectic Si. Such inhibited the probability of fatigue cracks initiation, resulting in the enhancement of fatigue life. This work provides theoretical and practical basis of design strategy of Al-Si composite foams and broadens their application prospect.

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.

The effect of microstructure on the fatigue behavior of titanium alloy graded porous structures fabricated by selective laser melting

The bionic-inspired Ti–15Mo gradient porous structures were fabricated by selective laser melting (SLM) for the first time in this work. The compression-compression fatigue behavior of functional gradient Ti–15Mo porous structures is investigated, with emphasis on understanding the relationship between fatigue performance and microstructure. The microstructure of the samples was analyzed both before and after fatigue, and correlated with the fatigue performance. During cyclic deformation, the pillars exhibited a non-uniform local stress distribution, primarily attributed to the disparity of gradient density distribution. Fatigue cracks typically initiate at the place with the defects and subsequently propagate until structural damage occurs. Fatigue ratcheting effect occurs throughout the entire fatigue process while the fatigue damage typically initiates at a later stage in the fatigue process. The results indicated that the fatigue performance of the lateral gradient (LG) structure was superior to that of the homogeneous structure, while the vertical gradient (VG) structure exhibited the poorest fatigue performance.

Fatigue behaviour of closed-cell polyurethane rigid foams

This paper investigates the compression-compression fatigue behaviour of polyurethane rigid (PUR) foams of three relative densities (RDs): 0.0854, 0.1239 and 0.2564 respectively. Preliminary tensile and compression tests were performed in order to obtain the stress–strain characteristics of the investigated materials and to determine the yield stresses in compression, required for the establishment of the relative stress levels used in the fatigue tests. It was observed that the failure mechanism during fatigue consisted in the collapse of a region of the specimens, which was characterized by a sudden increase in strain. In consequence, stress–strain cycles were recorded at various cycle intervals in order to narrow the degradation cycle interval. The resulting compression-compression fatigue curves show a similar behaviour between the RDs of 0.0854 and 0.1239 (with the latter exhibiting the best fatigue strength of the three investigated RDs), that can be described by a single trend line for each, while the foam at 0.2564 RD showed the lowest fatigue strength, the curve being characterized by two regions.

A data driven methodology for upscaling remaining useful life predictions: From single- to multi-stiffened composite panels

In this paper we execute a complex test campaign to develop a novel methodology for the Remaining Useful Life (RUL) estimation of complex multi-stiffened composite aeronautical panels utilizing Machine Learning models trained with Structural Health Monitoring (SHM) data from hierarchically simpler elements, i.e., single-stiffened panels. Distributed Fiber Optical sensors (DFOS) are employed to monitor the panels’ behavior undergoing variable amplitude compression-compression fatigue after multiple impacts. A data processing methodology is first applied to the DFOS data, to both alleviate the effect of the variable loading conditions on the monitored strain and ease the computational burden. In this upscaling endeavor, an advanced strain-based Health Indicator (HI) based on Genetic algorithms, created and validated on the single-stiffened panel data, is utilized as the prognostic feature for the RUL estimations of the multi-stiffened panels. The HI displays favorable characteristics in terms of monotonicity and prognosability which are highly desirable for more accurate RUL estimations. For the prognostic task, standard machine learning models are trained using the historical degradation data of the single-stiffened panels and a similarity analysis is performed to enhance the accuracy when predicting the RUL of the multi-stiffened panels. Despite the increased structural complexity of the multi-stiffened panels, we demonstrate that the RUL is able to be predicted with reasonable accuracy. The present work paves the road for upscaling and applying prognostic methodologies to more complex structures beyond simple coupons or generic elements.

Compressive fatigue behavior of graded tantalum scaffolds produced by electron beam powder bed fusion

In this study, graded tantalum scaffolds were manufactured by electron beam powder bed fusion (EB-PBF). Compression-compression fatigue testing was conducted on the EB-PBF built graded tantalum scaffolds. The compressive fatigue behavior and compressive fatigue strength of graded tantalum scaffolds with vertical gradient (VG) and parallel gradient (PG) states were investigated. In the fatigue testing, the failure of PG graded scaffold was concentrated on the lowest strength constituent, resulting in higher cyclic ratcheting strain and fatigue damage strain. In terms of the VG graded scaffold, due to the simultaneous uniform deformation of each structure, the strain accumulation in a single cycle was small, resulting in the low cyclic ratcheting strain. In addition, the propagation of fatigue cracks in pore structure with higher strength can be retarded by the stress redistribution, resulting in the low fatigue damage strain. Therefore, the design idea of graded pore structure under identical strain condition is beneficial to obtain porous tantalum scaffold with high compressive fatigue strength.

Corrosion fatigue behavior and anti-fatigue mechanisms of an additively manufactured biodegradable zinc-magnesium gyroid scaffold

Additively manufactured biodegradable zinc (Zn) alloy scaffolds constitute an important branch in orthopedic implants because of their moderate degradation behavior and bone-mimicking mechanical properties. This work investigated the corrosion fatigue response of a zinc-magnesium (Zn-Mg) alloy gyroid scaffold fabricated via laser-powder-bed-fusion additive manufacturing at the first time. The high-cycle compression-compression fatigue testing of the printed Zn-Mg scaffold was conducted in simulated body fluid, showing its favorable fatigue strength, structural reliability, and anti-fatigue capability. The printed Zn-Mg scaffold obtained a 227% higher fatigue strength than that of the printed Zn scaffold but 17% lower strain accumulation at 106 cycles. The accumulative strain of the Zn-Mg scaffold at its fatigue strength was dominant by fatigue ratcheting, since the fatigue damage strain of the scaffold was approximately zero. The corrosion products (ZnO and Zn(OH)2) were conducive to the inhibition of fatigue ratcheting and fatigue damage. Dislocation pile-up and solid solution phases at the grain boundaries of the Zn-Mg scaffold could retard the spreading of the crack tip and impede excessive grain coarsening, improving its fatigue endurance limit. Notably, the printed Zn-Mg scaffold could dissipate the fatigue energy through moderate grain boundary migration, thus reducing its plastic deformation. These findings illuminated the anti-fatigue mechanisms related to microstructural features and corrosive environments and highlighted the promising prospects of additively manufactured Zn-Mg scaffolds in orthopedic applications. Statement of significanceAdditive manufacturing (AM) of biodegradable metals shows unprecedented prospects for bone tissue regeneration medicine. The corrosion fatigue property is one of the key determinants in the performance of AM biodegradable scaffolds. In this study, a Zn-Mg gyroid scaffold was additively manufactured with admirable fatigue endurance limit and anti-fatigue capability. We reported that the corrosion fatigue performance was highly relevant to the microstructural features, validating that the grain boundary engineering strategy improved fatigue strength and inhibited crack penetration. Notably, moderate grain boundary migration could dissipate fatigue energy and reduce plastic deformation. Furthermore, corrosion products were conducive to impeding fatigue ratcheting and fatigue damage, indicating the promising potential of AM Zn-Mg scaffolds in treating load-bearing bone defects.

Multi-level damage diagnosis on stiffened composite panels based on a damage-uninformative digital twin

In this study, a multi-level Structural Health Monitoring methodology for stiffened composite panels is introduced. A digital twin (DT), that is, a three-dimensional finite element (FE) model, representing the pristine state baseline of the test article, is developed and verified for compressive loading in the post-buckling regime. The detailed FE model is utilized to train a surrogate model with respect to exogenous input, that is, axial load magnitude. The surrogate assists the DT concept that would allow prediction of the load acting on the structure based on an influx of strain data, acquired from fiber Bragg grating sensors permanently attached along the stringer feet. For this purpose, we leverage on the observation that remote from the damage, the strain field remains virtually unaltered with regard to the pristine state. The load is estimated by a sensor placed far from the damage whilst the diagnostic actions are performed by exploiting measurements from the remaining sensing locations. A health indicator, which compares the experimentally received strains with those from the surrogate representing the pristine state, is utilized to (1) detect, (2) localize, and (3) characterize the damage. As damage, we consider either skin-to-stringer disbond or initial impact damage propagation as well as overall stiffness degradation during thousands or millions of fatigue cycles. The sensors that have detected a disbond are dedicated to evaluating the potential propagation of it, while the remaining sensors evaluate the overall stiffness degradation. The proposed methodology is tested for one artificially disbonded and two impacted single-stringer panels subjected to block loading compression-compression fatigue.

Assessing stiffness degradation of stiffened composite panels in post-buckling compression-compression fatigue using guided waves

The application of structural health monitoring (SHM) in composite airframe structural elements under long-term realistic fatigue loading needs to consider the structural behavior on the global level, which is an intricate task. The overall structural stiffness is a key design parameter for composite structures and the stiffness degradation under fatigue loading is closely related to the damage accumulation and failure mechanism which can be used as an indicator for the structural degradation. Therefore, this paper investigates the use of guided waves in axial stiffness degradation estimation for stiffened carbon fiber reinforced polymer (CFRP) composite panels under post-buckling compression-compression (C-C) fatigue loads. Impacted or artificially debonded stiffened composite panels are tested under fatigue until failure and guided waves are acquired using a network of piezoelectric (PZT) sensors at fixed cycle intervals. The guided wave phase velocity along the loading direction is extracted to estimate the axial stiffness degradation with the consideration of mode conversion and failure of PZT sensors. The estimated stiffness of five stiffened composite panels matches well with the stiffness calculated from the load–displacement curves. The estimated stiffness is also assessed using prognostic performance metrics and shows good potential for being used as a health indicator for prognostic purposes.

Fatigue characteristics of sandwich composite joints in ships

The aim of this study is to investigate the fatigue characteristics of sandwich composite joints in ships based on the experimental and numerical methods. Sandwich composite L-joints are subjected to tension-compression, tension-tension and compression-compression fatigue loading at several load levels. Residual stiffness, strain, and surface temperature were measured during fatigue tests. The experimental results show that the sandwich composite L-joints have lower fatigue lives than expected and the fatigue load amplitude has a perceptible effect on the fatigue lives of the joints. In order to illustrate the influence of stress concentration on fatigue life of the joints, a quasi-static numerical analysis was conducted. Moreover, the fatigue damage mechanism and damage growth were revealed by experimental observation, stiffness curves and numerical stress distribution. Finally, a damage model based on the maximum displacements is proposed for the fatigue life prediction of the sandwich composite joints, and the results show that the predicted fatigue lives are in good agreement with the experimental ones.

Damage evolution in cross-ply laminates under tension–compression and compression-compression cyclic loads

Tension-Compression and Compression-Compression fatigue tests were carried out on glass/epoxy cross-ply laminates, with different values of the minimum to maximum load ratio R (−1, −3, −5, −7, −10 and 20). During the tests, the damage evolution in the 90° ply was monitored at the macro-and micro-scales. Three damage scenarios were observed, depending on the load ratio: i) initiation and propagation of transverse cracks for R ≥ −3, ii) initiation of multiple inclined short cracks for R = 20, iii) a mixed damage type with the presence of both transverse cracks and inclined short cracks for −10 ≤ R ≤ −5. Finite Element analyses of micro-scale Representative Volume Elements, combined with a recently developed crack initiation criterion, revealed the role of the Local Hydrostatic Stress in the matrix in controlling the different damage scenarios. Eventually, quantitative analyses showed a highly detrimental effect of the compressive part of the cycle on the transverse crack initiation and propagation.

Mechanisms of fatigue crack initiation and propagation in auxetic meta-biomaterials

The fatigue performance of additively manufactured auxetic meta-biomaterials made from commercially pure titanium has been studied only recently. While certain assumptions have been made regarding the mechanisms underlying their fatigue failure, the exact mechanisms are not researched yet. Here, we studied the mechanisms of crack formation and propagation in cyclically loaded auxetic meta-biomaterials. Twelve different designs were subjected to compression-compression fatigue testing while performing full-field strain measurement using digital image correlation (DIC). The fatigue tests were stopped at different points before complete specimen failure to study the evolution of damage in the micro-architecture of the specimens using micro-computed tomography (micro-CT). Furthermore, finite element models were made to study the presence of stress concentrations. Structural weak spots were found in the inverted nodes and the vertical struts located along the outer rim of the specimens, matching the maximum principal strain concentrations and fracture sites in the DIC and micro-CT data. Cracks were often found to originate from internal void spaces or from sites susceptible to mode-I cracking. Many specimens maintained their structural integrity and exhibited no signs of rapid strain accumulation despite the presence of substantial crack growth. This observation underlines the importance of such microscale studies to identify accumulated damage that otherwise goes unnoticed. The potential release of powder particles from damaged lattices could elicit a foreign body response, adversely affecting the implant success. Finding the right failure criterion, therefore, requires more data than only those pertaining to macroscopic measurements and should always include damage assessment at the microscale. Statement of significanceThe negative Poisson's ratio of auxetic meta-biomaterials makes them expand laterally in response to axial tension. This extraordinary property has great potential in the field of orthopedics, where it could enhance bone-implant contact. The fatigue performance of additively manufactured auxetic meta-biomaterials has only recently been studied and was found to be superior to many other bending- and stretch-dominated micro-architectures. In this study, we go beyond these macroscopic measurements and focus on the crack initiation and propagation. Full-field strain measurements and 3D imaging are used to paint a detailed picture of the mechanisms underlying fatigue. Using these data, specific aspects of the design and/or printing process can be targeted to improve the performance of auxetic meta-biomaterials in load-bearing applications.

Fatigue properties of Ti-6Al-4V Gyroid graded lattice structures fabricated by laser powder bed fusion with lateral loading

Graded lattice structures (GLSs) have drawn much attention in engineering and biological areas due to the enhanced mechanical properties and energy absorption capacity that benefit from the graded design. This study aims to investigate the effect of gradient on fatigue behaviors of lattice structures under the cyclic loading lateral to the gradient direction. The Gyroid-type triply periodic minimal surface was utilized to design GLS with continuously varied volume fraction (VF) from 10% to 40%. Uniaxial compressive testing and compression-compression fatigue testing were conducted on the Ti-6Al-4V Gyroid lattice structures with both uniform and graded VFs fabricated by laser powder bed fusion. Three typical fracture modes and mixed fracture modes were all observed on the fracture surfaces of struts after fatigue testing. The fatigue life of Gyroid GLSs is 1.21–1.67 times that of uniform counterparts with an identical overall VF. The enhancement mechanism on fatigue properties coming from the gradient design is elaborated through the finite element analysis and experimental characterization. The lower level of tensile stress, bigger macro-area and micro-plastic zone on struts of the main loading-bearing constituent of Gyroid GLS relieve the stress concentration around the crack tip, thus, lower the crack propagation rate and provide stronger fatigue crack resistance. Furthermore, the struts in the main loading-bearing constituent are more prone to stretching, therefore, provide long-term carrying capacity even after the occurrence of penetrating cracks. Experiment results show that the carrying capacity can be still provided by these struts even with penetrating cracks. • Gyroid graded lattice structures were designed and made via laser powder bed fusion. • Three basic fracture modes and mixed modes were observed from the fracture surfaces. • Thick struts in the graded lattice structure has a lower tensile stress. • Few penetrating cracks do not lower the carrying capacity of the graded lattice. • The gradient design dramatically improves the fatigue properties.

Progressive damage modelling and fatigue life prediction of Plain-weave composite laminates with Low-velocity impact damage

This paper developed a fatigue-driven residual strength model considering the effects of low-velocity impact (LVI) damage and stress ratio. New fatigue failure criteria based on fatigue-driven residual strength concept and fatigue progressive damage model were developed to simulate fatigue damage growth and predict fatigue life for plain-weave composite laminates with LVI damage. To validate the proposed model, LVI tests of plain-weave glass fibre reinforced polymer 3238A/EW250F laminates were conducted, followed by post-impact constant amplitude tension–tension, compression-compression fatigue tests and multi-step fatigue tests. Experimental results indicate that the LVI damage degrades fatigue strength of plain-weave glass fibre composite laminate drastically. The load history also plays an important role on the fatigue accumulation damage of post-impact laminates. The new fatigue progressive damage model achieves a good agreement with fatigue life of post-impact laminates and is able to capture the load sequence effect, opening a new avenue to predict fatigue failure of composite laminates.

Fatigue performance of auxetic meta-biomaterials

Meta-biomaterials offer a promising route towards the development of life-lasting implants. The concept aims to achieve solutions that are ordinarily impossible, by offering a unique combination of mechanical, mass transport, and biological properties through the optimization of their small-scale geometrical and topological designs. In this study, we primarily focus on auxetic meta-biomaterials that have the extraordinary ability to expand in response to axial tension. This could potentially improve the longstanding problem of implant loosening, if their performance can be guaranteed in cyclically loaded conditions. The high-cycle fatigue performance of additively manufactured (AM) auxetic meta-biomaterials made from commercially pure titanium (CP-Ti) was therefore studied. Small variations in the geometry of the re-entrant hexagonal honeycomb unit cell and its relative density resulted in twelve different designs (relative density: ~5–45%, re-entrant angle = 10–25°, Poisson's ratio = -0.076 to -0.504). Micro-computed tomography, scanning electron microscopy and mechanical testing were used to respectively measure the morphological and quasi-static properties of the specimens before proceeding with compression-compression fatigue testing. These auxetic meta-biomaterials exhibited morphological and mechanical properties that are deemed appropriate for bone implant applications (elastic modulus = 66.3–5648 MPa, yield strength = 1.4–46.7 MPa, pore size = 1.3–2.7 mm). With an average maximum stress level of 0.47 σy at 106 cycles (range: 0.35 σyσy- 0.82 σyσy), the auxetic structures characterized here are superior to many other non-auxetic meta-biomaterials made from the same material. The optimization of the printing process and the potential application of post-processing treatments could improve their performance in cyclically loaded settings even further. Statement of SignificanceAuxetic meta-biomaterials have a negative Poisson's ratio and, therefore, expand laterally in response to axial tension. Recently, they have been found to restore bone-implant contact along the lateral side of a hip stem. As a result, the bone will be compressed along both of the implant's contact lines, thereby actively reducing the risk of implant failure. In this case the material will be subjected to cyclic loading, for which no experimental data has been reported yet. Here, we present the first ever study of the fatigue performance of additively manufactured auxetic meta-biomaterials based on the re-entrant hexagonal honeycomb. These results will advance the adoption of auxetic meta-biomaterials in load-bearing applications, such as the hip stem, to potentially improve implant longevity.

Fatigue Properties of Ti-6al-4v Gyroid Graded Lattice Structures in Comparison with Uniform Counterparts Fabricated by Selective Laser Melting

Graded lattice structures (GLSs) have drawn much attention in engineering and biological areas due to the enhanced mechanical properties and energy absorption capacity that benefited from the graded porosity design. The aim of this study is to investigate the effect of gradient on fatigue behaviours of lattice structures. A GLS consisted of the Gyroid unit cell, one of the triply periodic minimal surface structures, was designed with continuously varied porosity from 60% to 90%. Uniaxial compressive testing and compression-compression fatigue testing were conducted on the Ti-6Al-4V Gyroid lattice structures with both uniform and graded porosity fabricated by selective laser melting. All three typical fracture modes and mixed fracture modes were all observed on the fracture surfaces of struts after fatigue testing. The fatigue life of Gyroid GLSs is 1.21~1.67 times that of uniform counterparts with an identical overall porosity. The enhancement mechanism on fatigue properties coming from gradient design is elaborated through finite element analysis and experimental characterization. The lower level of tensile stress, bigger macro-area and micro-plastic zone on the strut surface of the main loading-bearing constituent of Gyroid GLS relieve the stress concentration around the crack tip, thus, lower the crack propagation rate and provide stronger fatigue crack resistance. Furthermore, the struts in the main loading-bearing constituent are more prone to stretching, therefore, provide long-term carrying capacity even after the occurrence of penetrating cracks. Experiment results show that the carrying capacity can be still provided by these struts even with penetrating cracks.