Influence of screw length and diameter on tibial strain energy density distribution after anterior cruciate ligament reconstruction

Postoperative tunnel enlargement has been frequently reported after anterior cruciate ligament（ACL）reconstruction.Interference screw,as a surgical implant in ACL reconstruction,may influence natural loading transmission and contribute to tunnel enlargement.The aims of this study are（1）to quantify the alteration of strain energy density（SED）distribution after the anatomic single-bundle ACL reconstruction;and（2）to characterize the influence of screw length and diameter on the degree of the SED alteration.A validated finite element model of human knee joint was used.The screw length ranging from 20 to 30 mm with screw diameter ranging from 7 to 9 mm were investigated.In the post-operative knee,the SED increased steeply at the extra-articular tunnel aperture under compressive and complex loadings,whereas the SED decreased beneath the screw shaft and nearby the intra-articular tunnel aperture.Increasing the screw length could lower the SED deprivation in the proximal part of the bone tunnel;whereas increasing either screw length or diameter could aggravate the SED deprivation in the distal part of the bone tunnel.Decreasing the elastic modulus of the screw could lower the bone SED deprivation around the screw.In consideration of both graft stability and SED alteration,a biodegradable interference screw with a long length is recommended,which could provide a beneficial mechanical environment at the distal part of the tunnel,and meanwhile decrease the bone-graft motion and synovial fluid propagation at the proximal part of the tunnel.These findings together with the clinical and histological factors could help to improve surgical outcome,and serve as a preliminary knowledge for the following study of biodegradable interference screw.

Structural Damage Identification via Pseudo Strain Energy Density and Wavelet Packet Transform

Based on strain signals, a new time-domain methodology for detecting the beam local damage has been developed. The pseudo strain energy density （PSED） is defined and used to build two major damage indexes, the average pseudo strain energy density （APSED） and the average pseudo strain energy density rate （APSEDR）. Probability and mathematical statistics are utilized to derive a standardized damage index. Furthermore, by applying the analytic relation between the strain energy release rate and the stress intensity factor, an analytic solution of crack depth is derived. For the dynamic strain signals, the wavelet packet transform is used to pre-process measured data. Finally, a numerical simulation indicates that this method can effectively identify the damage location and its absolute severity.

Local Strain Energy Density Applied to Bainitic Functionally Graded Steels Plates Under Mixed-Mode(Ⅰ+Ⅱ) Loading

In this paper, the averaged value of the strain energy density （SED） over a control volume is used to predict the critical load of V-notched specimens made of functionally graded steels （FGSs） under mixed-mode loading. The studied FGSs contain ferritic and austenite phases in addition to bainitic layer produced by electroslag remelting. The mechanism- based strain gradient plasticity theory is used to determine the flow stress （yield stress or ultimate stress） of each layer. The Young＇s modulus and the Poisson＇s ratio have been assumed to be constant, while other mechanical properties vary exponentially along the specimen width. The control volume is centered in relation to the maximum principal stress present on the notch edge and assumes a crescent shape. The points belonging to the volume perimeter are obtained numerically. In the present contribution, the effects of notch radius and notch depth on the SED and the critical load are studied. The notch radius varies from 0.2 to 2.0 mm, and the notch depth varies from 5 to 7 ram. By using the SED approach and finite element simulations, the critical load is determined, and the obtained results show a sound agreement with the experimental results.

Multiscale Study on Low Temperature Crack Resistance Mechanism of Steel Slag Asphalt Mixture

The objective of this paper was to study low temperature crack resistance mechanism of steel slag asphalt mixture(SAM).Thermal stress restrained specimen test(TSRST)and three-point bending test were carried out to evaluate the low-temperature crack resistance of SAM and basalt asphalt mixture(BAM).Based on the digital image correlation technique(DIC),the strain field distribution and crack propagation of SAM were analyzed from the microscopic point of view,and a new index,crack length factor(C),was proposed to evaluate the crack resistance of the asphalt mixture.The crystal phase composition and microstructure of steel slag aggregate(SA)and basalt aggregate(BA)were studied by X-ray diffraction(XRD)and scanning electron microscopy(SEM)to explore the low-temperature crack resistance mechanism of SAM.Results show that the low-temperature crack resistance of SAM is better than that of BAM;SAM has good integrity and persistent elastic deformation,and its bending failure mode is a hysteretic quasi-brittle failure;The SA surface is evenly distributed with pores and has surface roughness.SA has the composition phase of alkaline aggregate-calcite(CaCO3),so it has good adhesion to asphalt,which reveals the mechanism of excellent low-temperature crack resistance of SAM.

Moving line crack accompanied with damage zone subject to remote tensile loading

In the 1920s, a closed-form solution of the moving Criffith crack was first obtained by Yoffe. Based on Yoffe＇s solution, the Dugdale model for the moving crack case gives a good result. However, the Dugddle model fails when the crack speed is closed to the Rayleigh wave speed because of the discontinuity occurred in the crack opening displacement （COD）. The problem is solved in this paper by introducing a restraining stress zone ahead of the crack tip and two velocity functions. The restraining stresses are linearly distributed and related to the velocity of the moving crack. An analytical solution of the problem is obtained by use of the superposition principle and a complex function method. The final result of the COD is continuous while the crack moves at a Rayleigh wave speed. The characteristics of the strain energy density （SED） and numerical results are discussed, and conclusions are given.

FATIGUE CRACK PROPAGATION FOR BIAXIAL STRESS CYCLING

Based on the experimental study of complex biaxial mode Ⅰ fatigue crack growth and the discussion on Von Mises'theory,a new approach is proposed for correlating crack propaga- tion rate under both in-phase and out-of-phase biaxial stress cycling.The results emphasize the contribution of plasticity to fatigue crack growth.

An energy-based low-cycle fatigue life evaluation method considering anisotropy of single crystal superalloys

The crystal orientation significantly affects the low-cycle fatigue (LCF) propertiesof single crystal (SC) superalloys. However, the orientation-dependent LCF life model withprecise mechanisms and strong applicability is still lacking. This investigation aims at establishing an energy-based LCF life evaluation method that could consider the orientation effect. First,the influencing factors of anisotropy were identified through the literature review. Secondly, themultiaxial formula of the Ramberg-Osgood (ReO) equation was established to describe theanisotropic cyclic deformation characteristics. Furthermore, the strain energy density of SC superalloys was determined based on this equation, and the effective strain energy density wasintroduced to account for the effect of orientation. Finally, the energy-based method was validated by its application to several SC superalloys. Results showed that the crystallographicorientation with a lower Young’s modulus usually exhibits better LCF resistance. This phenomenon could be attributed to the different values of strain energy density dissipated in one cycle.The multiaxial ReO relationship could capture the anisotropic cyclic deformation response ofDD6. Compared with the classical methods, the energy-based model is favored by its precisemechanism and strong applicability. And it also exhibited better prediction accuracy. Most datapoints of different crystallographic orientations lay within the
3 error band.

Tensile ratcheting behaviors of bronze powder filled polytetrafluoroethylene

A series of tensile and ratcheting experiments for compacted polytetrafluoroethylene （PTFE） and bronze filled PTFE （PTFE/bronze） were conducted on dynamic mechanical analyzer （DMA-Q800）. The effects of mean stress, stress amplitude and temperature on the ratcheting behaviors of PTFE and PTFE/bronze were investigated. It is found that the stress-strain response of PTFE/bronze is nonlinear and its elastic modulus is higher than that of pure PTFE. For uniaxial ratcheting test, the dissipation strain energy density （DSED） decreases rapidly in the first l0 cycles and approaches a constant after 20 cycles. The ratcheting strain and the DSED corresponding to 100 cycles increase with increasing mean stress, stress amplitude and temperature. Additionally, the DSED and ratcheting strain of PTFE/bronze are much lower than those of pure PTFE under the same experimental conditions. It is also found that both pure PTFE and PTFE/bronze present cyclic hardening characteristics. Above all, the addition of bronze can improve both the uniaxial tensile property and the cyclic property of PTFE.

A review on material models for isotropic hyperelasticity

Dozens of hyperelastic models have been formulated and have been extremely handy in understanding the complex mechanical behavior of materials that exhibit hyperelastic behavior(characterized by large nonlinear elastic deformations that are completely recoverable)such as elastomers,polymers,and even biological tis-sues.These models are indispensable in the design of complex engineering com-ponents such as engine mounts and structural bearings in the automotive and aerospace industries and vibration isolators and shock absorbers in mechanical systems.Particularly,the problem of vibration control in mechanical system dy-namics is extremely important and,therefore,knowledge of accurate hyperelastic models facilitates optimum designs and the development of three‐dimensional finite element system dynamics for studying the large and nonlinear deformation beha-vior.This review work intends to enhance the knowledge of 15 of the most com-monly used hyperelastic models and consequently help design engineers and scientists make informed decisions on the right ones to use.For each of the models,expressions for the strain‐energy function and the Cauchy stress for both arbitrary loading assuming compressibility and each of the three loading modes(uniaxial tension,equibiaxial tension,and pure shear)assuming incompressibility are pro-vided.Furthermore,the stress–strain or stress–stretch plots of the model's pre-dictions in each of the loading modes are compared with that of the classical experimental data of Treloar and the coefficient of determination is utilized as a measure of the model's predictive ability.Lastly,a ranking scheme is proposed based on the model's ability to predict each of the loading modes with minimum deviations and the overall coefficient of determination.

LIMITATION OF AVERAGE ESHELBY TENSOR AND ITS APPLICATION IN ANALYSIS OF ELLIPSE APPROXIMATION

By the aid of irreducible decomposition, the average Eshelby tensor can be expressed by two complex coefficients in 2D Eshelby problem. This paper proved the limitation of complex coefficients based on the span of elastic strain energy density. More discussions yielded the constraints on the sampling of module and phase difference of complex coefficients. Using this information, we obtained that the maximum relative error is 65.78% after an ellipse approximation. These results, as a supplement to our previous paper, further implied that Eshelby＇s solution for an ellipsoidal inclusion could not be applied to non-ellipsoidal inclusions without taking care.