Resilient Properties of Soil-Rock Mixture Materials: Preliminary Investigation of the Effect of Composition and Structure

The physical composition and stress state of soil-rock mixture (SRM) materials have a crucial influence on their mechanical properties, and play a vital role in improving the performance of subgrade. To reveal the resilient behavior and mesostructure evolution of SRM materials, triaxial tests and discrete element method (DEM) numerical analysis have been carried out. In the triaxial test section, the mechanical response of SRM materials was investigated by preparing samples under different stress states and physical states and conducting triaxial tests on samples. Simultaneously, a new irregular particle modeling method was developed and applied to the discrete element modeling process to analyze the mesostructure evolution of SRM materials under cycling loading. First, a cyclic triaxial test of SRM material is performed on the SRM material, and the effects of bulk stress, octahedral shear stress and rock content on the resilient modulus of the SRM material are analyzed. It is revealed that the resilient modulus increases with increasing bulk stress and rock content, and decreases with increasing octahedral shear stress. Based on a new resilient modulus prediction model, the relationships among the rock content, stress state and resilient modulus are established. Then, based on an improved DEM modeling method, a discrete element model of the SRM is established, and the influence of rock content on coordination number and mesostructure evolution of the SRM is analyzed. The results show that in SRM materials, the increase of crushed rock changes the mesostructure of the SRM material. With the increase of rock content, the internal contact force changes from “between soil and rock” to “between rocks”, and the skeleton formed in the rocks gradually develops overall stiffness. Under the condition of low stress, the anisotropy of the SRM material is mainly caused by the shape and grade distribution of crushed rock. The induced anisotropy caused by the change of stress state has little effect on its mechanical behavior, which may lead to the greater dispersion of multiple SRM test results.

Multiaxial fatigue criteria applied to motor crankshaft in thermoelectric power plants

Fatigue failures of motor crankshafts operating in thermoelectric power plants have recently been reported. Stress fields provided by finite element calculations at critical points of a crankshaft that failed in service are used to test the structural integrity of the component. Taking into account the fact that the stresses acting at a given point are most likely out of phase, multiaxial fatigue criteria based on the von Mises stress are considered to be most suitable for predicting the fatigue behavior of the crankshaft. Using the von Mises stress, it was also possible to apply octahedral shear stress-based criteria and the results obtained have indicated that the crankshaft made of DIN 34CrNiMo6 steel should not suffer fatigue failure under the action of the stress fields in question. However, such failures have been occurring and this apparent discrepancy is presented and briefly discussed in the present study.

Multiaxial Behavior and Viscoelastic Constitutive Modeling of Epoxy Polymers

A nonlinear viscoelastic constitutive model previously proposed has been modified to account for hydrostatic stress influence. The modification is achieved by incorporating the hydrostatic stress in the equivalent stress definition. The present model predicts a shift of stress envelopes along equibiaxial compressive stress direction. It also predicts different octahedral shear stress versus octahedral shear strain curves for various strain paths. These trends indicate improved agreement between the model prediction and multiaxial experiment data.

Analysis of Resilient Modulus of Dense- and Open-Graded Aggregates

The results of analyses of laboratory resilient modulus testing conducted on dense-graded and open-graded aggregates are presented. The testing program included three different aggregate materials (crushed limestone, natural stone, and slag), five different gradation specifications, and three different moisture conditions (dry, moist, and saturated). In addition to the five aggregate specifications, test specimens were prepared so that they would satisfy the lower, central, and upper bounds for the gradations. Resilient modulus tests were conducted as closely as possible according to Strategic Highway Research Program Protocol P-46 (AASHTO T 294-92 I). The test results were analyzed using log-linear regression analysis with two-parameter (bulk stress) and three-parameter (bulk stress and octahedral shear stress) expressions for resilient modulus. The results of the testing indicate that the resilient modulus of aggregates and regression constants vary significantly depending on the type of material and vary less significantly depending on material gradation and moisture condition. The natural stone aggregates had the higher moduli and the slag aggregate had the lowest moduli. The resilient modulus does not vary significantly as the moisture content increases unless the aggregate becomes saturated. The R2 values are consistently higher using a three-parameter expression than a two-parameter expression.

An Optimum Connecting Rod Design Study—A Lubrication Viewpoint

The effect of support structure on the performance of a connecting rod bearing is analyzed by elastohydrodynamic lubrication theory. The support structure is modified iteratively until an optimum design is obtained. With the connecting rod of a typical gasoline engine as an example, it was found that a 150 percent improvement in film thickness and a 34 percent reduction in peak pressure can be obtained by changing the structural design. This design, however, increases the peak octahedral shear stress in the rod. Structural design changes considered were found to have surprisingly little effect on the power loss of the bearing.

The Effects of Surface Irregularities on the Elastohydrodynamic Lubrication of Sliding Line Contacts. Part II—Wavy Surfaces

The micro-EHD effects caused by wavy surfaces have been analytically investigated. The investigation includes the effects of phase, wavelength, and wave amplitude on film thickness, pressure distribution and subsurface octahedral shear stress field. The presence of a wavy surface with a given wavelength produces pressure oscillations of the same wavelength. With increasing wave amplitude and decreasing wavelength, the micro-EHD action increases. This results in a maximum value of the octahedral shear stress which is greater in magnitude and closer to the surface than the corresponding smooth surface case. The slope of the wavy surface in the inlet region determines whether the average film thickness is smaller or larger than the smooth surface value.

The Effects of Surface Irregularities on the Elastohydrodynamic Lubrication of Sliding Line Contacts. Part I—Single Irregularities

A full line contact solution, under isothermal conditions, is obtained in which the effects of single stationary surface irregularities on the EHD lubrication process are studied under pure sliding conditions. The irregularities studied are furrows, furrows with built-up edges, and asperities. The effects of these irregularities on film thickness, pressure, and subsurface octahedral shear stress are presented. The pressure and film thickness resulting from such surface irregularities are significantly changed from their smooth surface values. These changes alter the state of stress in the subsurface region by increasing the maximum value of octahedral shear stress and bringing the location of this maximum stress closer to the surface. The film thickness in the contact is significantly changed from the smooth surface value only when the irregularities are located in the inlet region while the maximum value of the octahedral shear stress increases to the greatest extent when the irregularities are located in the outlet half of the contact.

The Stress Field Generated in an Elastic Layer by Normally Loaded, Periodically Spaced Circular Contacts

The stress field generated in an elastic layer by normally loaded, periodically spaced circular contacts is analyzed. It is intended that this model represent a pair of rough, plated contact surfaces whose plating is much more compliant than the substrate material. The effect of plating thickness and spacing of load-bearing asperities upon the state of stress within the layer is studied. It is shown that there exist values of layer thickness and contact-spot spacing such that the octahedral shear stress within the layer is maximized. Further, for very thin layers, the maximum octahedral shear occurs at the surface of the layer as in tangentially loaded contacts.