Modeling and analysis of passive dynamic bipedal walking with segmented feet and compliant joints

Passive dynamic walking has been developed as a possible explanation for the efficiency of the human gait. This paper presents a passive dynamic walking model with segmented feet, which makes the bipedal walking gait more close to natural human-like gait. The proposed model extends the simplest walking model with the addition of fiat feet and torsional spring based compliance on ankle joints and toe joints, to achieve stable walking on a slope driven by gravity. The push-off phase includes foot rotations around the toe joint and around the toe tip, which shows a great resemblance to human normal walking. This paper investigates the effects of the segmented foot structure on bipedal walking in simulations. The model achieves satisfactory walking results on even or uneven slopes.

Piezoelectric sensing method for segmental joint contact stress during shield tunnel construction

The emergence of curved shield tunnels poses a significant construction challenge.If the quality of the segment assembly is not guaranteed,many segment cracks and damage will result from the stress concentration.Sensing the contact stresses between segmental joints is necessary to improve the quality of segments assembled for shield tunnel construction.Polyvinylidene difluoride(PVDF)piezoelectric material was chosen for the sensor because it can convert contact stresses into electrical signals,allowing the state of the segmental joints to be effectively sensed.It matches the working environment between the segmental joints of the shield tunnel,where flexible structures such as rubber gaskets and force transfer pads are present.This study proposes a piezoelectric sensing method for segmental joints in shield tunnels and conducts laboratory tests,numerical analyses,and field tests to validate the feasibility of the method.The results indicate that the PVDF film sensor can effectively sense the entire compression process of the gasket with different amounts of compression.The piezoelectric cable sensor can effectively sense the joint offset direction of the gasket.For differently shaped sections,the variation in the force sensed by the piezoelectric cable sensors was different,as verified by numerical simulation.Through the field test,it was found that the average contact stress between the segmental joints was in the range of 1.2–1.8 MPa during construction of the curved shield tunnels.The location of the segmental joints and the type of segment affect the contact stress value.The field monitoring results show that piezoelectric sensing technology can be successfully applied during assembly of the segments for effective sensing of the contact stress.

Bearing capacity of foundations on rock slopes intersected by non-persistent discontinuity

Rocks encountered in foundations of heavy structures are invariably intersected by discontinuities(joints).In the past,several studies have been performed by researchers to incorporate the effect of fully persistent joints in the assessment of the load-carrying capacity of rocks.However,in the field,the joints are non-persistent,and an assumption of full persistency will underestimate the capacity.Recently,Shaunik&Singh have studied the influence of non-persistency,number of joint segments and discontinuity orientation on the strength behaviour of rock specimens(Shaunik and Singh,2019).Bell’s approach can be used to obtain the bearing capacity of shallow foundations placed in jointed rocks.In the present study,results of the experimental work(Shaunik and Singh,2019)conducted by Shaunik&Singh have been used to suggest expressions by extending Bell’s approach for computing bearing capacity of the foundation placed near the crown of a rock slope.Easy to use design charts are also presented for field application.Finally,a real-life problem from Indian Garhwal Himalayas is considered,and the approach suggested in this study is utilised to obtain the bearing capacity of a bridge foundation as a function of uniaxial compressive strength(UCS)of intact rock,joint friction,spacing and orientation of joint,nonpersistency and number of joint segments.

Experimental study on effect of ductile-iron panel stiffness on mechanical properties of segmental joints of shield tunnels

In circular shield tunnels bearing high inner pressure or rectangular shaped shield tunnels,as the axial force at the segmental lining decreases,the bolt load at the segmental joint increases.It is essential to adopt high-stiffness segmental joints to improve the bearing capacity and control the deformation at the joint position.When designing high-stiffness segmental joints,the selection of ductileiron joint panel is crucial.In this study,two types of segmental joints with different joint panels were fabricated,and the effects of joint panel stiffness on the mechanical properties of segmental joints were analyzed through full-scale sagging and hogging bending tests.The results showed that the failure mode of high-stiffness segmental joint was similar to that of large eccentric compression section.According to the difference of panel stiffness,the failure modes can be specified into two types.If the stiffness is sufficient,the joint failure occurs due to the yielding of bolts;otherwise,it occurs due to the large deformation of ductile-iron joint panels.As for the design requirement of segmental joint,the stiffness of joint panel should be sufficient,i.e.,the opening and failure of the joints are finally induced by the bolt deformation.Otherwise,before the plastic deformation of the bolts,the large deformation of the joint panels will occur under a bendingmoment-dominant load,and the bearing capacity of the joints will greatly decrease.

Development of sandwich joining piece to fabricate segmented half-Heusler/skutterudite thermoelectric joints

Sn/Ni-Sn/Sn sandwich joining piece was developed and applied to fabricate segmented half-Heusler/skutterudite thermoelectric joints,and high-temperature service behavior of the joints was studied.The microstructure and contact resistance of the joint before and after aging were investigated.The joints are well bonded and no cracks appear at the interfaces of the joint before and after aging,which can be attributed to the formation of high-melting point intermetallic compounds.The electrical resistance crosses the joining layer smoothly and the contact resistance is low.These results show that the sandwich joining piece is effective and flexible,and promising for the preparation of segmented thermoelectric devices.

Preface

Micromechanics aims mainly at establishing the quantitative relation between the macroscopic mechanical behavior and the microstructure of heterogeneous materials.

Spectral Density Estimation of Continuous Time Series

This paper studies spectral density estimation of a strictly stationary r-vector valued continuous time series including missing observations. The finite Fourier transform is constructed in L-joint segments of observations. The modified periodogram is defined and smoothed to estimate the spectral density matrix. We explore the properties of the proposed estimator. Asymptotic distribution is discussed.

Experimental investigation on waterproofing performance of segmental joint with double gaskets for shield tunnel

The waterproofing capacity of segmental joints is an essential indicator for the long-term service performance of shield tunnels.The segmental joints with double gaskets have been adopted to improve the waterproof performance for certain tunnels with high water pressure,especially water-conveyance tunnels.This paper presents a series of waterproofing tests on joints with different settings of double gaskets via an improved test apparatus.Four cases are investigated,including(i)double gaskets with equal high/low waterproof capacity,(ii)the outer gasket with higher waterproof capacity,and(iii)the outer gasket with lower waterproof capacity.Different joint openings are also considered.Moreover,the water pressures at two cavities are continuously monitored to highlight the waterproofing mechanism and failure pattern of double gaskets.The results demonstrate that the gasket with the higher waterproofing capacity dominates the overall waterproofing capacity of joint with double gaskets;only a little enhancement of the waterproofing capacity is found for the joint with the same double gaskets.The waterproofing capacity of joints with double gaskets decreases with the increase of the joint opening.The failure pattern depends on the arrangement of double gaskets,and four stages can be identified during the whole failure process.The advantages of double gaskets in case of joint rotation are also discussed.

Analytical algorithms of compressive bending capacity of bolted circumferential joint in metro shield tunnels

The integrity and bearing capacity of segment joints in shield tunnels are associated closely with the mechanical properties of the joints.This study focuses on the mechanical characteristics and mechanism of a bolted circumferential joint during the entire bearing process.Simplified analytical algorithms for four stress stages are established to describe the bearing behaviors of the joint under a compressive bending load.A height adjustment coefficient,α,for the outer concrete compression zone is introduced into a simplified analytical model.Factors affectingαare determined,and the degree of influence of these factors is investigated via orthogonal numerical simulations.The numerical results show thatαcan be specified as approximately 0.2 for most metro shield tunnels in China.Subsequently,a case study is performed to verify the rationality of the simplified theoretical analysis for the segment joint via numerical simulations and experiments.Using the proposed simplified analytical algorithms,a parametric investigation is conducted to discuss the factors affecting the ultimate compressive bending capacity of the joint.The method for optimizing the joint flexural stiffness is clarified.The results of this study can provide a theoretical basis for optimizing the design and prediciting the damage of bolted segment joints in shield tunnels.