Size effect on contact behavior in DEM simple shear tests

The 2–2.5 times the simulated sand diameter is widely accepted in giving reasonable DEM simulation results for geotechnical testing. However, it neglects the effect of a specimen height to maximum particle diameter ratio in a specific laboratory test, which may lead to a strong stress concentration and flawed simulations. This study compared laboratory simple shear tests with corresponding DEM simulations with different particle sizes. The DEM model used clump rings to simulate physical rings in the test, and decreased the additional stress applied by the widely used wall-type rings. Results showed that (1) DEM models with tested particle size and twofold sand particle size (1D and 2D tests) can better capture the tested stress–strain behavior, volumetric changes, and noncoaxiality, the 4D model has an asymmetrical distribution of contact force and contact number, indicating the specimen is inhomogeneous and has a strong stress concentration. (2) a specimen height to maximum particle diameter ratio smaller than 10 (it is greater than 10 in the ASTM D6528) could provide reasonable macro-meso mechanical behaviors. Similar studies should be carried out after trial tests on determining a reasonable specimen height to maximum particle diameter ratio under the guidance of ASTM D6528.

Numerical investigation of the plastic deformation behaviour of graded crushed stone

A numerical model of the dynamic triaxial test of graded crushed stone was established based on DEM (Discrete Element Method) to study its dynamic characteristics. The influence of test conditions on simulation results was analysed through numerical simulation. A method for determining the test conditions was proposed, and the reliability of the simulation was verified. We studied the accumulation rule and failure standard of permanent deformation. We then determined the critical failure stress level. The prediction model of permanent deformation cumulative failure was then established. When the calculation time step is greater than 1E−4 s per step, the stability of the dynamic triaxial numerical test of graded crushed stone is good. The plastic deformation of the simulated specimen tends to be stable under 10,000 dynamic loading cycles. When the specimen height and diameter are greater than 20 and 10 cm, respectively, the specimen size has little influence on the simulated axial strain. The recommended specimen size is 10 cm (Ф) × 20 cm (h). The action time curve results are consistent with indoor measurement results, which proves the simulation’s reliability. The critical failure stress is approximately linearly correlated with the confining pressure, and the cumulative failure equation of plastic deformation is established.

Load bearing capacity of thin-walled rectangular and I-shaped steel sections of short both empty and concrete-filled columns

In this experimental work, strength results obtained on short columns subjected to concentric loads are presented. The specimens used in the tests have made of cold-rolled, thin-walled steel. Twenty short columns of the same cross-section area and wall thickness have been tested as follows: 8 empty and 12 filled with ordinary concrete. In the aim to determine the column section geometry with the highest resistance, three different types of cross-sections have been compared: rectangular, I-shaped unreinforced and, reinforced with 100 mm spaced transversal links. The parameters studied are the specimen height and the cross-sectional steel geometry. The registered experimental results have been compared to the ultimate loads intended by Eurocode 3 for empty columns and by Eurocode 4 for compound columns. These results showed that a concrete-filled composite column had improved strength compared to the empty case. Among the three cross-section types, it has been found that I-section reinforced is the most resistant than the other two sections. Moreover, the load capacity and mode of failure have been influenced by the height of the column. Also, it had noted that the experimental strengths of the tested columns don’t agree well with the EC3 and EC4 results.

Electro-welded lattice reinforcement in reinforced concrete ribbed slabs

Shear strength of precast concrete ribbed slabs reinforced with electro-welded lattice girders of variable height is experimentally verified. Technical justification is given for the use of high latticework in the reinforcement of precast lightweight double-tee ribbed slabs and precast reinforced concrete slabs 30 cm in thickness. It was established that the contribution of concrete to shear strength was always higher than the expected values according to Spanish standards. Furthermore, as low lattice ribbed slabs exhibited lower ultimate shear strength values than the ones expected based on the standards, the need arose to provide an adequate height of lattices in ribbed slabs to ensure anchorage of the compressed area in order to develop the strut-and-tie mechanisms. Consequently, lattice girders with approximately 80% of specimen height are highly recommended in reinforced concrete ribbed slabs, when the length of lattice chords is 20 cm.

Investigation on Dynamical Mechanics, Energy Dissipation, and Microstructural Characteristics of Cemented Tailings Backfill under SHPB Tests

As mining depth increases, the backfill mining method is more and more widely used in underground mines. The dynamic load generated by the blasting can affect the stability of the cemented tailings backfill (CTB). The CTB samples were prepared to conduct a test of the split Hopkinson pressure bar (SHPB) to investigate the dynamic disturbance of CTB. The present paper discusses dynamical mechanics, energy dissipation, and microstructure analysis of CTB. Micro-computer tomography (micro-CT) scanning of CTB samples after the SHPB test was performed to analyze the evolution of internal cracks. The experimental results showed that when the average strain rate (ASR) increased from 30 to 98 s−1, the dynamic uniaxial compression strength (DUCS) of the CTB showed a trend of first increasing and decreasing with the increase in ASR. The dynamic stress–strain pre-peak curve of CTB directly enters the linear elastic stage. As ASR increases, the absorbed energy of the CTB shows a trend of first increasing and then decreasing. Moreover, according to the micro-CT scanning results, the crack area of CTB accounts for about 16% of the sample near the incident bar and about 1% near the transmitted bar. The crack area ratio is exponentially related to the specimen height. These findings can provide reasonable dynamical CTB strength data selection for underground pillar mining.

Estimation of the perpendicular-to-the-grain tensile strength of Scots pine glued laminated timber via three-point bending tests

Three-point bending tests were performed on specimens of glued laminated timber with different specimen heights to failure to determine the relationship between specimen height and bending strength under tension perpendicular to the grain. For the three-point bending tests, two types of glued laminated timber composed of homogeneous grade timber, as specified in the Japanese Agricultural Standard, were used. The laminae used for the glued laminated timber were L80 grade Scots pine and L110 grade Scots pine. The specimens used in the three-point bending tests had dimensions of 105 mm (width) and 10–300 mm (height). The experimental results showed that the bending strength decreased as the specimen height increased, but the rate of decrease in the bending strength decreased with increasing specimen height when the specimen height exceeded 100 mm. From the relationship between the bending strength and specimen height, parameters that fit Bažant’s size-effect law were derived, and for a specimen height of approximately 100 mm, the bending strength was equal to the perpendicular-to-the-grain tensile strength.

Influence of Duty Ratio and Current Mode on Robot 316L Stainless Steel Arc Additive Manufacturing

Wire and arc additive manufacturing (WAAM) is usually for fabricating components due to its low equipment cost, high material utilization rate and cladding efficiency. However, its applications are limited by the large heat input decided by process parameters. Here, four 50-layer stainless steel parts with double-pulse and single-pulse metal inert gas (MIG) welding modes were deposited, and the effect of different duty ratios and current modes on morphology, microstructure, and performance was analyzed. The results demonstrate that the low frequency of the double-pulse had the effect of stirring the molten pool; therefore, the double-pulse mode parts presented a bigger width and smaller height, finer microstructure and better properties than the single-pulse mode. Furthermore, increasing the duty ratio from 35% to 65% enlarged the heat input, which then decreased the specimen height, increased the width, and decreased the hardness and the tensile strength.

Brittle material strength and fracture toughness estimation from four-point bending test

The failure stress under four-point bending cannot be considered as an intrinsic material property because of the well-known size effect of increasing maximum flexural stress with decreasing specimen size. In this work, four-point bending tests are analyzed with the coupled criterion for different sample sizes. The maximum flexural stress only tends towards the material tensile strength provided the specimen height is large enough as compared to the material characteristic length. In that case, failure is mainly driven by a stress criterion. Failure of smaller specimens is driven both by energy and stress conditions, thus depending on the material tensile strength and fracture toughness. Regardless of the material mechanical properties, we show that the variation of the ratio of maximum flexural stress to strength as a function of the ratio of specimen height to material characteristic length follows a master curve, for which we propose an analytical expression. Based on this relation, we propose a procedure for the post-processing of four-point bending tests that allows determining both the material tensile strength and fracture toughness. The procedure is illustrated based on four-point bending experiments on three gypsum at different porosity fractions.

Experimental Investigation of an Influence of Coupled Compressive Loading on Porcine Spine Specimens

The human spine shortens approximately by 1% of its height during the daily activities and returns to its primary height during the night rest. Cyclic loading is important in order to ensure diffusion and convention of the nutrients and metabolites within the intervertebral discs. On the other hand, cyclic loading could lead to the damage of the intervertebral discs and the vertebra bodies if the magnitude and frequency of the loads applied to the spine exceed the allowable limits. As most of the in vitro studies that investigate the influence of cyclic loading deal with functional spinal units consisting of single intervertebral disc the purpose of this study is to investigate an influence of cyclic compression and flexion on the structural integrity and geometrical parameters of the spinal specimens consisting of more than one intervertebral disc. Two specimens consisting of four adjacent vertebrae and three intervertebral discs were scanned by using computed tomography then loaded with combined cyclic compression and flexion and then scanned for the second time in order to capture the current condition of the specimens. Obtained images were used to evaluate the changes of structural integrity and geometrical parameters of the discs. A significant decrease of the specimen height was observed during loading, mainly due to the loss of the fluid content within intervertebral discs. In total, the difference of the height of the two specimens immediately after the loading was 1.577 mm. No obvious damage to the specimens was observed when comparing images before and after the loading. A statistically significant differences between the height of the intervertebral discs before and after loading in both the first specimen (p = 0.0224) and the second specimen (p = 0.0155) were calculated with the lowest disc of both specimens decreasing the most and obviously losing the highest water content. The cross-sectional area of the lowest disc in both specimens also decreased the most. This once again confirms that lower part of the spine such as intervertebral discs L4-L5 and L5-S1 are the least prone to the injuries and degeneration due to disturbed nutrition and loss of water content.

A wear rate model incorporating inflationary cost of agro-waste filled composites for brake pad applications to lower composite cost

Wear rate appraisals are currently indispensable on agro-waste filled composites for brake pads as they predict the expected lifespan of the materials. However, existing wear rate models are inaccurate as predictions omit the inflationary cost of the materials. In this paper, the idea is to account for the inflationary cost of the materials and adjust that into a pseudo wear rate model. The wear rate of agro-waste fillers in an organic matrix to create brake pads under dry sliding wear experiments was considered. Five composite specimens were fabricated in cylindrical specimen height of 14.5 mm and varying diameters of 8, 10, 12 and 15.5 mm and the material wear loss was measured. The 8, 10 and 12 mm diameter specimens revealed that the composite with the best and worst wear resistance were the wear rates of 0.6, 1.4, 1.73 mm3/Nm, and 3.07, 3.54, 4.19 mm3/Nm, respectively. The 15.5 mm diameter specimen showed lower wear rates of 2.13 and 2.14 and 1.56 mm3/Nm than commercial brake pad’s 2.58 mm3/Nm. The pseudo wear rate model predicts the impact of the independent variable i.e. inflationary cost, opportunity cost, time, and sample size. The utility of this effort is to assist the composite manufacturers to take cost-effective decisions and design optimisation can be accomplished to lower the cost of composite products.