Enabling and Understanding Failure of Engineering Structures Using the Technique of Cohesive Elements

In this paper, we describe a cohesive zone model for the prediction of failure of engineering solids and/or structures. A damage evolution law is incorporated into a three-dimensional, exponential cohesive law to account for material degradation under the influence of cyclic loading. This cohesive zone model is implemented in the finite element software ABAQUS through a user defined subroutine. The irreversibility of the cohesive zone model is first verified and subsequently applied for studying cyclic crack growth in specimens experiencing different modes of fracture and/or failure. The crack growth behavior to include both crack initiation and crack propagation becomes a natural outcome of the numerical simulation. Numerical examples suggest that the irreversible cohesive zone model can serve as an efficient tool to predict fatigue crack growth. Key issues such as crack path deviation, convergence and mesh dependency are also discussed

Numerical Modeling of the Constraint Effects on Cleavage Fracture Toughness

Cleavage fracture has been an important subject for engineers primarily because of its catastrophic nature and consequences. Experimental studies of cleavage fracture did reveal a considerable amount of scatter and provided evidence of noticeable constraint effects. This did provide the motivation for the development of statistical-based and micromechanics-based methods in order to both study and analyze the problem. The Weibull stress model, which is based on the weakest link statistics, uses two parameters (m and σ u) to effectively describe the inherent distribution of the micro-scale cracks once plastic deformation has occurred and to concurrently define the relationship between the macro-scale and micro-scale driving forces for cleavage fracture. In this paper, we present the results of a recent study at evaluating the constraint effects on cleavage fracture toughness. This was done numerically using a constraint function (g(M)) derived from the Weibull stress model. The non-dimensional function (g(M)) describes the evolution of constraint loss effects on fracture toughness relative to the reference plane-strain, small scale yielding (SSY) condition (T-stress = 0). We performed detailed finite element analyses of single-edge notched bending specimens and computed the non-dimensional g(M) functions for them. The g(M) function varies with (i) the Weibull modulus, (ii) material flow properties, and (iii) specimen geometry, but not with absolute size of the test specimen. Knowing the g-function, the fracture driving force curve can be constructed for each absolute size of interest

Mechanical Behavior of a Magnesium Alloy Nanocomposite under Conditions of Static Tension and Dynamic Fatigue

In this paper, the intrinsic influence of nano-alumina particulate (Al2O3p) reinforcements on microstructure, microhardness, tensile properties, tensile fracture, cyclic stress-controlled fatigue, and final fracture behavior of a magnesium alloy is presented and discussed. The unreinforced magnesium alloy (AZ31) and the reinforced composite counterpart (AZ31/1.5 vol.% Al2O3) were manufactured by solidification processing followed by hot extrusion. The elastic modulus, yield strength, and tensile strength of the nanoparticle-reinforced magnesium alloy were noticeably higher than the unreinforced counterpart. The ductility, quantified by elongation-to-failure, of the composite was observably lower than the unreinforced monolithic counterpart (AZ31). The nanoparticle-reinforced composite revealed improved cyclic fatigue resistance over the entire range of maximum stress at both the tested load ratios. Under conditions of fully reversed loading (R = −1) both materials showed observable degradation in behavior quantified in terms of cyclic fatigue life. The conjoint influence of reinforcement, processing, intrinsic microstructural features and loading condition on final fracture behavior is presented and discussed

A Study Aimed at Characterizing the Interfacial Structure in a Tin-Silver Solder on Nickel-Coated Copper Plate during Aging

This paper highlights the interfacial structure of tin-silver (Sn-3·5Ag) solder on nickel-coated copper pads during aging performance studies at a temperature of 150°C for up to 96 h. Experimental results revealed the as-solidified solder bump made from using the lead-free solder (Sn-3·5Ag) exhibited or showed a thin layer of the tin-nickel-copper intermetallic compound (IMC) at the solder/substrate interface. This includes a sub-layer having a planar structure immediately adjacent to the Ni-coating and a blocky structure on the inside of the solder. Aging performance studies revealed the thickness of both the IMC layer and the sub-layer, having a planar structure, to increase with an increase in aging time. The observed increase was essentially non-linear. Fine microscopic cracks were observed to occur at the interfaces of the planar sub-layer and the block sub-layer

Influence of material and process parameters on reduction-swelling characteristics of sintered iron pellets

This paper investigates the use of shop-floor ferrous scrap that contains iron ore as a raw material for the purpose of making steel products through an in situ carbothermic reduction. The technique of powder metallurgy (PM) was used for the purpose of studying reduction followed by densification during sintering. Two sources of iron oxide—ferrous grinding-sludge powder and iron ore—and three sources of the carbonaceous material—graphite, charcoal, and carbon black—were considered. The carbonaceous material was added to the iron oxide after calculating the stoichiometric carbon requirement for facilitating both direct reduction and direct–indirect reduction. This involves a simultaneous change in weight and volume. During sintering, an in situ reduction of the iron oxide takes place that often results in severe volumetric changes. The test results revealed the degree of reduction (DOR) and degree of densification (DOD) of the grinding sludge (GS) to be 15% and 45% higher, respectively, than that of iron ore (IO). This is essentially due to the presence of distinct iron-oxide phases coupled with a greater amenability to the occurrence of carbothermic reduction. Indirect reduction also took place and contributed to improving the degree of reduction (DOR) and degree of densification (DOD) of the final products. Overall, the shape stability of the sintered grinding-sludge (GS) powder was found to be optimized when parameter settings of graphite (from 25% in excess to 50% in excess) were added, a compaction pressure of 1050 MPa was applied, and a sintering temperature of 1200 °C was employed. Hence, ferrous scrap can be chosen as direct reduced iron for the manufacture of steel and can also be used for cost-efficient and eco-friendly structural components with a marginal compromise on both the purity and strength of the ferrous products

Understanding the Influence of Pressure and Radial Loads on Stress and Displacement Response of a Rotating Body: The Automobile Wheel

This paper highlights the use of the finite element technique for analyzing stress and displacement distributions in wheels of automotive vehicles when subject to the conjoint influence of inflation pressure and radial load. The most commonly used considerations in the design of the rotating body are elucidated. A potentially viable technique for finite element modeling of radial wheel, subjected to loading, is highlighted. The extrinsic influence of inflation pressure on performance of the rotating body, that is, the wheel, is rationalized

Synthesis of an aluminum alloy metal matrix composite using powder metallurgy : role of sintering parameters

Powder metallurgy-based metal matrix composites (MMCs) are widely chosen and used for the development of components in the fields spanning aerospace, automotive and even electronic components. Engineered MMCs are known to offer a high strength-to-weight (σ/ρ) ratio. In this research study, we synthesized cylindrical sintered samples of a ceramic particle-reinforced aluminum metal matrix using the technique of powder metallurgy. The samples for the purpose of testing, examination and analysis were made by mixing aluminum powder with powders of silicon carbide and aluminum oxide or alumina. Four varieties of aluminum composite were synthesized for a different volume percent of the ceramic particle reinforcement. The hybrid composite contained 2 vol.% and 7 vol.% of silicon carbide and 3 vol.% and 8 vol.% of alumina with aluminum as the chosen metal matrix. Homogeneous mixtures of the chosen powders were prepared using conventional ball milling. The homogeneous powder mixture was then cold compacted and subsequently sintered in a tubular furnace in an atmosphere of argon gas. Five different sintering conditions (combinations of temperature and sintering time) were chosen for the purpose of this study. The density and hardness of each sintered specimen were carefully evaluated. Cold compression tests were carried out for the purpose of determining the compressive strength of the engineered MMC. The sintered density and hardness of the aluminum MMCs varied with the addition of ceramic particle reinforcements. An increase in the volume fraction of the alumina particles to the Al/SiC mixture reduced the density, hardness and compressive strength. The sintering condition was optimized for the aluminum MMCs based on the hardness, densification parameter and cold compressive strength. The proposed powder metallurgy-based route for the fabrication of the aluminum matrix composite revealed a noticeable improvement in the physical and mechanical properties when compared one-on-one with commercially pure aluminum

HT2003-47477 AN INVESTIGATION OF COPPER DISSOLUTION AND THE FORMATION OF INTERMETALLIC COMPOUNDS IN MOLTEN TIN AND Tin-Silver SOLDERS

ABSTRACT This paper presents an experimental study of copper dissolution in molten tin and tin-silver (Sn-Ag) solders and the formation and presence of the Cu-Sn intermetallic compound at solder/copper interfaces. During the experiments, copper (99.9% pure) samples, coated with a RMA flux, were dipped vertically in a molten solder for different time periods ranging from 5 seconds to 10 minutes. The molten solder was maintained at temperatures of 232 o C, 250 o C and 300 o C for pure tin and 221 o C, 250 o C, and 300 o C for Sn-3.5%Ag respectively. The samples were then cut, cleaned and cold mounted in epoxy at ambient temperature. Mechanical grinding, finish polishing, etching, and optical metallographic procedures were utilized for examining the microstructures of the polished and etched samples. The average thickness of the intermetallic compound and the amount of copper dissolved was determined. Experimental results indicate the temperature of molten solder to control the rate of dissolution of copper and the formation and presence of intermetallic compounds at the interfaces. At a given temperature of the solder temperature, the rate of dissolution of copper in the solder revealed a rising trend with an increase in dwell time of copper in the solder. For short contact time periods, the dissolution rate is low and the thickness of the intermetallic compound is small. With an increase in dwell time, the dissolution rate of copper rapidly increases and eventually reaches a plateau. Initiation of dissolution of copper causes a layer of the Sn-Cu intermetallic compound to form around the copper substrate. This in turn prevents direct contact of the copper substrate with the molten solder. The rate of formation of the layer of intermetallic compound reveals a similar trend. Based on experimental results, the kinetic parameters involved in governing the growth of the intermetallic were determined for the two solders. The parameters can be used to estimate the kinetics of copper dissolution and intermetallic compound formation during soldering

Detailed deletion mapping of chromosome band 14q32 in human neuroblastoma defines a 1.1-Mb region of common allelic loss

Neuroblastoma (NB) is a well-known malignant disease in infants, but its molecular mechanisms have not yet been fully elucidated. To investigate the genetic contribution of abnormalities on the long arm of chromosome 14 (14q) in NB, we analysed loss of heterozygosity (LOH) in 54 primary NB samples using 12 microsatellite markers on 14q32. Seventeen (31%) of 54 tumours showed LOH at one or more of the markers analysed, and the smallest common region of allelic loss was identified between D14S62 and D14S987. This region was estimated to be 1-cM long from the linkage map. Fluorescence in situ hybridization also confirmed the loss. There was no statistical correlation between LOH and any clinicopathologic features, including age, stage, amplification of MYCN and ploidy. We further constructed a contig spanning the lost region using bacterial artificial chromosome and estimated this region to be approximately 1.1-Mb by pulsed-field gel electrophoresis. Our results will contribute to cloning and characterizing the putative tumour-associated gene(s) in 14q32 in NB. © 2000 Cancer Research Campaig