Initial Shear Strength Research Articles

Elastic modulus deterioration index to identify the loosened zone around underground openings

In order to effectively identify the loosened zone around underground openings by using continuum modeling, this paper proposes an index called elastic modulus deterioration index based on the following two steps. First, the response of surrounding rock mass in underground openings as obtained from ultrasonic tests before and after excavation is investigated. Based on the attenuation of the P-wave velocities of the rock mass after excavation compared to the P-wave velocities before excavation obtained from ultrasonic tests, an index (χ) is defined to assess the excavation-induced rock mass damage. Second, by introducing an empirical relationship between the static and dynamic elastic moduli, the index χ is modified to represent the static elastic modulus evolution, volume change, and stress redistribution of the rock mass based on an elasto-plastic numerical model. Furthermore, this study verifies the performance of the elastic modulus deterioration index to identify the loosened zone around underground openings by comparing its predictions with the results from ultrasonic testing of the loosened zones around two tunnels in the Jinping II hydropower station. The comparisons show that the loosened zones as identified by the elastic modulus deterioration index agree well to those from ultrasonic testing. Analysis of the effects of key input parameters and initial shear strength parameters on the elastic modulus deterioration index in identifying the loosened zone demonstrate that: (1) the sensitivity of the index to the key input parameters except the residual static elastic modulus is not significant; and (2) the index has better fault tolerance performance compared to the indices based on plastic zone in terms of the selection of initial shear strength parameters.

Printed wire interconnection using Ag sinter paste for wide band gap power semiconductors

This study reports a novel high-temperature and high-current wire interconnection by printing Ag sinter paste for WBG semiconductor power devices. The Ag sinter paste wire interconnection (APW) was successfully fabricated connecting two Cu electrodes coated by Ag. Both the high-temperature reliability and the high-current reliability of the APW were evaluated by aging test at 250 °C for 1000 h and by electro-migration (EM) test at 2.2 × 104 A/cm2 for 2000 h. In the aging, a resistivity of about 4.1 × 10−6 Ω cm was achieved, which further decreased to 3.4 × 10−6 Ω cm after aging for 1000 h. This reduction can be attributed to further sintering of the sintered Ag network. The initial shear strength was 17 MPa, which, while high enough, was maintained even after an aging test of 1000 h. In the EM test, the resistance of APW was not changed significantly up to 1000 h and then only slightly increased at 2000 h. This only slight change in resistance demonstrates greater stability than that of conventional wire interconnection material such as Al and Cu under high temperature and high current density. The influence of EM on the interface between APW and a Cu substrate was observed by scanning electron microscopy with EDX. The change of resistance of the APW was induced by the formation of Cu oxide at a cathode side, which leads to a crack extension along the interface between APW and a Cu substrate. Thus, the results obtained in the present work show the APW as a promising new alternative to wire bonding technology for WBG power electronic devices.

In-Plane Strengthening of Clay Brick and Block Masonry Walls Using Textile-Reinforced Mortar

This study evaluates the use of a composite material composed of multiaxial hybrid glass-fibers and polypropylene fabric coated in a hydraulic lime-based mortar (referred to as textile-reinforced mortar) to improve the in-plane shear strength and displacement capacity of solid clay-brick and hollow clay-block masonry walls with the aim of defining a comprehensive test protocol. Design parameters of the adopted strengthening system are investigated, and both single-sided and double-sided configurations for application on wall surfaces are considered. The influence of the presence of an external thermal insulation layer on walls’ in-plane performance is also considered. The first stage characterizes the materials, their bonding properties, and the initial shear strength of the masonry bed-joints. The second stage investigates the in-plane behavior of strengthened masonry wall panels using diagonal compression testing. No premature debonding is observed, even with no mechanical anchorage between the masonry substrate and the strengthening layer. Out-of-plane deformation due to structural asymmetry is observed in the single-sided strengthening configuration.

Determination of shear strength properties of coarse‐grained materials for stone column

AbstractThe stone columns created using a vibro – replacement technique are often used technology of ground improvement. Suitable geological conditions for stone columns include loose coarse – grained soils and soft to firm fine – grained soils. The stone columns cause increasing of strength and deformation properties of the original subsoil. The design of stone columns must be based on properly determined properties of original subsoil as well as properties of the material of stone columns. The article presents and analysis results of direct shear tests of three different types of coarse – grained materials which are used for stone columns. The shear strength of coarse – grained material was determined using large – dimensional direct shear apparatus, which is required for this type of soil. Shear tests were executed for different values of the unit weight. High values of peak and residual angle of shear strength were determined for all three tested materials also in the loosest state. The initial shear strength was measured in the case of coarse – grained material consisting of the quarry stone.

Influence of Initial Shear Strength on Time-to-Failure of Copper (Cu) Wire Bonds in Thermal Aging Condition

Abstract Copper (Cu) wire bond is used in a majority of microelectronic devices but has not been fully accepted by all industries due to potential reliability issues. Good quality bond is believed to provide high reliability. Shear strength and intermetallic (IMC) coverage are being used as indicators, however there is no developed model. High shear strength is a result of large IMC coverage laterally under the ball, but the reliability of the bond is also related to IMC longitudinal growth (thickness). This work involves studying effect of shear strength on reliability of a variety of experimental Cu ball bonds by performing temperature aging experiments on test devices, in QFN packages. The ball bonds are made by altering ultrasonic power and time to obtain different IMC coverage. All test packages are monitored for resistance change at specific intervals by performing four point resistance measurement. Resistance increase is analyzed with initial shear strength to determine whether “good quality” bonds always lead to high reliability.

Numerical simulation and parametric analysis of multi-anchor walls using the finite element method

Abstract The paper describes the first reported attempt to numerically simulate the performance of full-scale steel multi-anchor reinforced soil walls (MAWs) during construction using a commercially available two-dimensional finite element method (FEM) model. The numerical simulation results are compared to the measurements taken from three 6 m-high MAW structures that were constructed under carefully controlled in-door laboratory conditions. The walls were nominally identical but constructed with three different sandy soils varying with respect to soil stiffness and shear strength. Earth pressures and anchor loads were monitored during construction and in-situ anchor pullout tests were also performed. Numerical predictions are judged to be in reasonable agreement with most physical measurements. A novel technique was the use of in-situ anchor plate pullout test results to back-calculate initial elastic modulus values for the hyperbolic constitutive soil model used in the study. The physical and numerical results are extended by considering a wider range of wall parameters (initial soil stiffness, shear strength, anchor length and wall height) using the same FEM model. The results of this parametric analysis provide further insight into the influence of the magnitude of different input parameters including wall height on wall performance. The main objective of this study was to generate a satisfactory definition of end-of-construction conditions. The paper also reports the results of a series of case studies that can be used by design engineers to benchmark-test FEM models proposed for MAW projects that are too complicated for analysis and design using conventional simple strength-based internal stability methods and where performance-based design requires estimates of wall deformations.

Experimental investigations on the initial shear strength of masonry with earth mortars

In this paper, a comparative study on the initial shear strength of masonry with earth mortars is presented. Triplet tests were carried out to characterise the shear bond strength of five different types of earth mortar, three purely mineral and two with vegetable additives (wood and straw chaff), using calcium silicate blocks. In spite of their lower bulk densities, mortars with chaffs reached a value of compressive strength comparable to the values shown by the purely mineral mortars. The characteristic initial shear strengths of all the tested earth mortars were between two and five times higher than the minimum values for initial shear strengths required by standards. To assess the influence of blocks pre-wetting, a comparison between calcium silicate blocks and earth blocks was performed to evaluate the results obtained from the standard test procedure compared to the more common practice of using earth mortars in combination with earthen blocks.

Experimental investigations on the initial shear strength of masonry with earth mortars

In this paper, a comparative study on the initial shear strength of masonry with earth mortars is presented. Triplet tests were carried out to characterise the shear bond strength of five different types of earth mortar, three purely mineral and two with vegetable additives (wood and straw chaff), using calcium silicate blocks. In spite of their lower bulk densities, mortars with chaffs reached a value of compressive strength comparable to the values shown by the purely mineral mortars. The characteristic initial shear strengths of all the tested earth mortars were between two and five times higher than the minimum values for initial shear strengths required by standards. To assess the influence of blocks pre-wetting, a comparison between calcium silicate blocks and earth blocks was performed to evaluate the results obtained from the standard test procedure compared to the more common practice of using earth mortars in combination with earthen blocks.

Reliability of Die Attach on DBC Substrates With Different Ni Surface Finishes Using BiAgX Solder

Ni plating is a commonly used barrier layer for direct bond copper substrates used in high-temperature and high-power applications. The Ni plating method can be electrolytic or electroless and the composition options include pure Ni, Ni:P, Ni–Co, or Ni:B. BiAgX solder paste is a promising lead-free solder for applications below 200 °C. The reactivity of these different Ni layers with the BiAgX solder paste has been compared. Specially the reaction to form NiBi3 during 200 °C storage and −55 °C to + 195 °C thermal cycling test. The initial shear strength of all these Ni plating is ~4.8 kg/mm2. A degradation in shear strength was observed in further aging. Among them, the electrolytic Ni:P plated samples had the lowest degradation. The reason of degradation is determined to be the formation of NiBi3 intermetallic, since it had been found in all the other Ni plating samples beside the electrolytic Ni:P. Similar results were observed in thermal cycling test.

Thermal Stability of Silver Paste Sintering on Coated Copper and Aluminum Substrates

The thermal stability of silver (Ag) paste sintering on coated copper (Cu) and aluminum (Al) substrates has been investigated. Instead of conventional zincating or nickel plating, magnetron sputtering was used to achieve coating with titanium (Ti) and Ag. Silicon (Si) chips were bonded to coated Cu and Al substrates using a mixture of submicron Ag flakes and particles under 250°C and 0.4 MPa for 30 min. The joints were then subject to aging testing at 250°C for duration of 200 h, 500 h, and 1000 h. Two types of joints exhibited satisfactory initial shear strength above 45 MPa. However, the shear strength of the joints on Al substrate decreased to 28 MPa after 1000 h of aging, while no shear strength decline was detected for the joints on Cu substrate. Fracture surface analysis revealed that the vulnerable points of the two types of joints were (1) the Ag layer and (2) the interface between the Ti layer and Cu substrate. Based on the results of scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and simulations, cracks in the Ag layer were identified as the cause of the shear strength degradation in the joints on Al substrate. The interface evolution of the joints on Cu substrate was ascribed to Cu migration and discontinuity points that initialized in the Ti layer. This study reveals that Al exhibited superior thermal stability with sintered Ag paste.

Reliability of AuGe Die Attach on DBC Substrates With Different Ni Surface Finishes

Plated Ni is a commonly used barrier layer for direct bond copper (DBC) substrates used in high-temperature and high-power applications. The Ni plating method can be electrolytic or electroless, and the composition may be pure Ni, Ni:P, or Ni:B. With a melting point of 356 °C, eutectic Au88Ge12 (wt.%) solder preforms can be used for die attach in high-temperature applications up to 325 °C. The reactivity of these different Ni layers with the Ge in the AuGe solder preform has been compared. With Ni and Ni:B, the reaction was rapid during the reflow process, and initial shear strengths were low. With electroless and electrolytic Ni:P, the initial shear strengths were high and decreased ~30% after 3000 h aging at 300 °C and 325 °C. The formation of Ni–Ge intermetallic compounds (IMCs) during high-temperature storage was studied, and the growth kinetics of the Ni–Ge IMCs were calculated. A −55 °C to +300 °C thermal cycling test was conducted, in which the Cu-to-alumina DBC substrate interface failed before the die attach.

Influence of ENIG defects on shear strength of pressureless Ag nanoparticle sintered joint under isothermal aging

An Ag nanoparticle sintering is a promising die attach method for use in high-temperature electronics and could potentially be substituted for conventional high Pb-containing solders. An electroless Ni/immersion Au (ENIG) is a common surface finish for solder, wire bonding, and Ag nanoparticle sintering. However, there is no report on the influence of ENIG defects on the shear strength and bonding quality of Ag nanoparticle sintered joint. In this study, the relationship between the presence of ENIG defects and shear strength of Ag nanoparticle sintered joint after bonding and isothermal aging test using two types of ENIG surface finished Cu substrates (with ENIG defects and without defects) was investigated. The initial shear strength of Ag sintered joint showed similar values of approximately 36 MPa despite the presence of ENIG defects. However, the ENIG defects could affect the shear strength degradation of Ag sintered joint after thermal aging at 250 °C. This degradation came from the Ni oxidation by the presence of ENIG defects and subsequent brittle fracture of joint alone the Ni oxidation layer.

Shear resistance verification of late 19th century masonry according to EC 6 – State of the art

AbstractA large part of buildings in Central European cities like Vienna was built in the Gründerzeit period between about 1840 and 1918 [1]. These buildings were constructed according to traditional rules. Current urban development requires historic buildings to be structurally adapted, which requires retroactive analysis of the masonry walls; in Austria according to ÖNORM EN 1998‐3 [2] and ÖNORM EN 1996‐3 (EC 6) [3]. Here, special focus is on the load transfer of horizontal earthquake loads, i. e. the shear strength of masonry walls.This paper describes the verification of historic masonry in detail and discusses individual components. Initial shear strength, load‐influenced friction and the length of the compressed part of the wall are first determined using results from experimental testing and relevant literature and then compared to the approaches in EC 6. Based on this analysis, recommendations are provided to make theoretical approaches more realistic.

Evaluation of the Performance of Composite Wood Decking Bonded with Phenol Resorcinol Formaldehyde and Polyurethane Adhesives after Accelerated Aging Tests

Abstract The feasibility of applying an innovative process to manufacture composite decking boards with quartersawn or starsawn southern pine lumber bonded with phenol resorcinol formaldehyde (PRF) and polyurethane (PU) adhesives was studied. It was feasible to make quatersawn-based composite decking panels with possible improved surface qualities. Accelerated aging tests showed that composite quartersawn decking samples were more resistant to aging than flatsawn decking samples in terms of shear strength, especially after exposure to 12 wet–dry aging cycles. Compared with the PRF adhesive, the PU adhesive resulted in greater initial dry shear strength but had lower shear strength after the accelerated aging test. Furthermore, the amount of wood failure increased for the PRF-bonded samples and decreased for the PU-bonded samples after accelerated aging tests. Compared with solid quartersawn decking boards, the quartersawn composite decking boards had the same modulus of rupture and modulus of elasticity.

Experimental seismic performance of steel- and composite steel-panel wall strengthened steel frames

This paper presents an experimental study to investigate the seismic performance of steel- and composite steel-panel wall strengthened steel frames (SPWF and CPWF). A detailed experimental investigation of five 1/3-scaled specimens with one-bay and single-story was conducted. The failure modes, load-carrying capacity, hysteretic behavior, ductility, energy dissipation capacity were presented and analyzed. The effects of the length-to-height ratio, stiffeners, and the type of walls on the seismic behavior were also investigated. The experimental results show that the specimens tolerate 4% to approximately 5% story drift, the steel- and composite steel-panel wall improved the seismic performance of the steel frame. The length-to-height ratio has had large effect on load-carrying capacity, initial stiffness, ductility and response modification factor. In addition, finite element (FE) models of SPWFs and CPWFs were established to simulate their nonlinear behavior, and the results were verified by the experimental results. The failure mode obtained from the numerical simulation was in accordance with the experimental phenomenon. Furthermore, formulas were developed to estimate initial lateral stiffness and shear strength of the test specimens, and such theoretical predictions were verified by the experimental results.

A Methodology for Enhancing the Shear Response of Sandwich Composite Panel: Sandwich Composite Panel with Stair Keys

Objectives: In composite sandwich panel, delamination between the foam core and face sheet is due to inability to withstand shear load. Hence a methodology of introducing shear keys for enhancing the shear response to resist delamination is analysed in the present study. Methods/Statistical Analysis: The conventional sandwich panel under analysis is composed of Glass Fibre Reinforced Polymer (GFRP) skin with PVC (Polyvinyl chloride) foam core impregnated with epoxy resin. The present study is to introduce stair keys in between GFRP and PVC which are made by overlapping GFRP face sheets, with the length and depth to get the required step size. Parametric Finite Element (FE) investigation using ANSYS software has been performed to evaluate the foam core performance by grooving the foam material as stepped shapes. The effect of different length of the stepped grooves in foam material are analysed and its results are compared with conventional sandwich panel. The stair key response for number of steps ranging from n = 1, 3, 5 and 7 are investigated. Findings: The FE results showed an improvement in the shear stress response of the sandwich panel with stair grooved foam core with stair keys compared to conventional model. Also the stair inserts between the GFRP skin and the foam core increases the initial shear stiffness and ultimate shear strength of the proposed model of the sandwich panel. Application/Improvements: The present methodology acts as a new model of peel stopper mechanism, where the shear failure due to debonding of face sheet and foam core is rerouted to foam core. Also it is free from initial damage as there is no loss of solidarity of the bulk material at the foam core, cost effective and easy to design and manufacture compared to other pinning and stitching process.

Seismic strengthening of infilled reinforced concrete frames by CFRP

Abstract The objective of the presented research was to investigate seismic strengthening of non-structural brittle masonry infill walls of reinforced concrete (RC) frames by Carbon Fiber Reinforced Polymers (CFRP). The contribution of CFRP strengthening to the overall behaviour in terms of lateral strength and stiffness, effects of different application techniques and connection details were investigated experimentally and analytically. Experimental work consisted of two parts. The first part included diagonal tension tests of 26 masonry wall panel specimens. These tests were conducted to observe the effects of different CFRP types and applications over the initial stiffness, shear strength and failure modes. In the second part, five one story-one bay RC frames were tested under cyclic in-plane lateral loads. Lateral load-top displacement hysteresis curves and failure modes were obtained. Initial stiffness, peak-to-peak stiffness, equivalent displacement ductility, strength reduction factors, energy dissipation and hysteretic damping values of specimens were evaluated comparatively. A damage table was built for strengthened specimens using experimental observations. In the analytical part, structural models of the CFRP strengthened one story-one bay RC frames were established in order to achieve the generalization of a number of test results. Experimental results and observations were also utilized in this process. Analytical results complied well with the experimental results in terms of lateral load capacity and initial stiffness.

Alternative Technology of Constructing Masonry Structures Designed for Areas with Increased Seismic Activity

The Czech Republic is a country which generally sees low seismic activity; however, this activity needs to be taken into account in ca 50% of the country's area with 10 districts being considered as having seismic activity higher than 0.08g. In terms of masonry structures, this issue concerns mainly the execution of the head joints of the masonry and its reinforcement (both in the direction of the bed joint and in the perpendicular direction). All these technologies are rather difficult and expensive to implement. The research focuses on assessing the possibilities of constructing dry masonry and binding it with polyurethane foam. This method allows for significantly higher shear strength of the masonry and appears to be an interesting alternative in the area of constructing buildings in seismically active areas. The structure was stiffened by filling hollow masonry units with large cavities with polyurethane foam. The PUR foam was sprayed into the units during construction. The foam thus applied hardens perpendicularly to the bed joint of the masonry and, having expanded throughout the clay units, it functions as a binder and a stiffening component to the masonry as a whole. The initial shear and flexural strength of the masonry segments were determined. The newly developed method of filling the cavities of masonry units with PUR foam was compared with the conventional method of constructing masonry by means of bonding the blocks with mortar or PUR foam in the bed joints (with no cavities filled). The filling of the masonry unit cavities brought a significant increase in the shear and flexural strength of the masonry. This indicates the stiffness of the structure increased as well.

Mechanical Properties of Masonry Walls Made of Calcium Silicate Materials Made in Poland. Part 2. Shear and Flexural Strength

In the article are presented the results of the shear and flexural strength of the masonry specimens with the thin joints made of 15 types of the calcium-silicate masonry units produced by 12 Polish manufacturers. The tests of initial shear strength of the masonry fvk0 were carried out according to the PN-EN 1052-3:2004 [12] standard (117 specimens). The dependence of the shear strength on the vertical hole's percentage were considered as well. The article describes also the results of flexural strength tests (48 specimens) at failure in the plane parallel and perpendicular to the bed joints. The shear and flexural strength is compared with characteristic values proposed in the Polish NA to the PN-EN 1996-1-1:2013 [10] code and national annexes of other European countries.

SHEAR STRENGTH DEGRADATION OF SEMARANG BAWEN CLAY SHALE DUE TO WEATHERING PROCESS

The effect of weathering processes in decreasing the shear strength of clay shale had been done in this study. The drying process of clay shale with sunlight in the laboratory up to 80 days had been conducted to create the conditions of weathered sample. The peak and residual shear strength parameters of unsaturated and saturated clay shale were obtained from triaxial laboratory test, and all samples were tested on each 8 days of weathering process. Decrease of shear strength in peak and residual condition was obtained during 80 days of the drying process. The residual shear strength parameters were distinguished between residual shear strength without stress release and with stress release of confining pressure. The results up to 80 days of unsaturated clay shale showed that the cohesion at peak stress conditions reduced to 30 % based on initial shear strength before the occurrence weathering, while the internal angle friction reduced to 64 %. Residual cohesion without and with stress release reduced to 4 % and 1 %, respectively while residual internal angle friction without and with stress release reduced to 15 % and 5 %. Similar situation also occurs for the saturated clay shale samples.