Experimental investigation and numerical modeling for elastoplastic notch-root stress/strain analysis under monotonic loadings

Experimental investigation and numerical modeling on elasto-plastic notch-root stress/strain distributions under monotonic loadings of both the Ni-based directionally solidified(DS)superalloy and Titanium alloy were carried out simultaneously.For measuring inhomogeneous deformation fields at notch roots,an optical-numerical full-field surface deformation measurement system was developed based on the digital image correlation(DIC)method.The obtained strain distributions were then verified with reasonable accuracy by finite element simulation,where an anisotropic elastic-viscoplastic constitutive model was developed for DS superalloy and a simple isotropic stress-strain relationship was adopted for Titanium alloy.Meanwhile,factors affecting elasto-plastic notch-root stress/strain distributions were systematically investigated numerically,where the emphasis was placed on temperature,loading stress rate,sample shape,anisotropy and notch features.The results show that stress/strain behavior at notch root is significantly affected by the mentioned factors,which are concretely embodied in the distribution of tensile stress/strain,equivalent stress and accumulative equivalent plastic strain.

Response of Skirted Suction Caissons to Monotonic Lateral Loading in Saturated Medium Sand

Monotonic lateral load model tests were carried out on steel skirted suction caissons embedded in the saturated medium sand to study the bearing capacity. A three-dimensional continuum finite element model was developed with Z_SOIL software. The numerical model was calibrated against experimental results. Soil deformation and earth pressures on skirted caissons were investigated by using the finite element model to extend the model tests. It shows that the ＂skirted＂ structure can significantly increase the lateral capacity and limit the deflection, especially suitable for offshore wind turbines, compared with regular suction caissons without the ＂skirted＂ at the same load level. In addition, appropriate determination of rotation centers plays a crucial role in calculating the lateral capacity by using the analytical method. It was also found that the rotation center is related to dimensions of skirted suction caissons and loading process, i.e. the rotation center moves upwards with the increase of the ＂skirted＂ width and length; moreover, the rotation center moves downwards with the increase of loading and keeps constant when all the sand along the caisson＇s wall yields. It is so complex that we cannot simply determine its position like the regular suction caisson commonly with a specified position to the length ratio of the caisson.

Constitutive model for monotonic and cyclic responses of loosely cemented sand formations

This paper presents a model to simulate the monotonic and cyclic behaviours of weakly cemented sands.An elastoplastic constitutive model within the framework of bounding surface plasticity theory is adopted to predict the mechanical behaviour of soft sandstone under monotonic and cyclic loadings. In this model, the loading surface always passes through the current stress state regardless of the type of loading. Destruction of the cementation bonds by plastic deformation in the model is considered as the primary mechanism responsible for the mechanical degradation of loosely cemented sands/weak rock.To model cyclic response, the unloading plastic and elastic moduli are formulated based on the loading/reloading plastic and elastic moduli. The proposed model was implemented in FLAC2D and evaluated against laboratory triaxial tests under monotonic and cyclic loadings, and the model results agreed well with the experimental observations. For cyclic tests, hysteresis loops are captured with reasonable accuracy.

Influence of the Loading Protocol and Loading Rate on the Characteristics of Timber Nail Joints

Nail joints are one of the key components that control the lateral performance of light wood frame shear walls.In previous experimental studies,researchers have used different loading rates,which failed specimens from less than a minute to more than an hour,to study the characteristics of nail joints.Moreover,there have been different loading protocols used for testing of timber nail joints or shear walls.Although some efforts have been made to address this subject,it is still unclear how the loading protocol and loading rate may influence the performance of nail joints.In this study,a total of 96 nail joints tests were carried out under monotonic and reversed-cyclic loads at three different loading rates,and under three different loading protocols(ISO,CUREE and SPD).The results of the test revealed that the loading rate did not have a noticeable effect on the mechanical properties of nail joints,while the loading protocol did influence those properties.Especially,protocols with larger cumulative damage demands lead to lower ultimate displacement and ductility of nail joints.

Dilatancy and liquefaction behaviour of clean sand at wide range of confining stresses

The state of clean sand was mainly dependent on its void ratio(density)and confining stress that greatly influenced the mechanical behavior(compression,dilatancy and liquefaction)of clean sand.Confirming whether the confining stress was a state variable of sand required precise element tests at different confining stress,especially the tests under very low confining stress whose test data were very limited.In this study,static-dynamic characteristics of clean sand was comprehensively investigated by a unified test program under low and normal confining stress ranging from 5 to 98 kPa,under monotonic/cyclic and drained/undrained conditions,together with the literature available data under confining stress of 1.0 to 3.0 MPa.For monotonic loading tests,the contraction/dilation phase transition was observed for loose sand at low confining stress,and dilatancy angles were stress-dependent.In addition,the liquefaction resistance was observed to increase with reducing of confining stress,and the axial strain varied from compressive to dilative when confining stress increased.Special attention was also paid to the enhancement effect of membrane,and it was observed that its influence on the test results was limited.In addition,the experimental results were proved reliable by reproducibility.