Variation of monotonic strain in copper thin films during fatigue testing

The plastic deformation behavior is important to evaluate the fatigue damage of copper thin films. In spite of the importance of the deformation behavior of thin films with load cycling, there is no trial to measure the strain directly on the surface during fatigue testing because of difficulties in measuring the strain.In this study, the monotonic strain of copper films of 12 μm thickness during fatigue testing was measured by using the DIC method. The DIC method provides full field deformation of the specimen with high precision, and can directly measure the strain of the gauge section without any assumption. With increasing number of cycles, the monotonic strain increases similarly to the conventional creep curve, and the fluctuations of the strain on the specimen surface seriously increases and the increase in the fluctuation range is very large compared to that in the mean strain due to the roughness of the specimen.     

A neural-network approach to describe the scatter of cyclic stress–strain curves

In order to predict a product’s durability in the early phases of development it is necessary to know the stress–strain behaviour of the material, its resistance to fatigue and the loading states in the material. These parameters, however, tend to exhibit a considerable degree of uncertainty. Due to a lack of knowledge of the actual circumstances in which the product is used, during the early development phase, simulations based on statistical methods are used. The results of the experiments show that the cyclic stress–strain curves demonstrate not only a large amount of scatter, but also a dependence on the temperature, the size of the cross-section, the content of alloying elements, the loading rate, etc.This article presents a method for modelling cyclic stress–strain curve scatter using a hybrid neural network for an arbitrary selection of the influencing factors. In an example of the measured data for a high pressure die-cast aluminium alloy it is clear that the suggested method is suitable for describing cyclic stress–strain curves. The main advantage of a hybrid neural network in comparison with a conventional method is the neural network’s ability to precisely describe the influence of various factors, and their combinations, based on the form and scatter of the cyclic stress–strain curve families. Defining the model parameters, i.e., training the neural network, is a procedure that does not require any additional user interventions; however, it enables us to gather knowledge that would otherwise require a lot of research. Thus, the trained neural network is a robust tool that can be used to predict cyclic stress–strain curves for random values of influencing factors. The capabilities of the presented method are only limited by the quantity of the measured data used for the neural-network training.     

Tension behavior of unidirectional glass/epoxy composites under different strain rates

By considering wide applications of composite materials, having a proper knowledge of them under dynamic loading is necessary. In order to study the effects of strain rates on the behavior of the materials, special testing machines are needed. Most of the research in this field is focused on applying real loading and gripping boundary conditions on the testing specimens. In this study, behavior of unidirectional glass fiber reinforced polymeric composites under uni-axial loading is determined at quasi-static and intermediate strain rates of 0.001–100 s⁻¹. The tests were performed using a servo-hydraulic testing apparatus equipped with a strain rate increase mechanism. For performing the tests, a jig and a fixture are designed and manufactured. The performance of the test jig was evaluated and found to be adequate for testing of composites. Dynamic tests results are compared with the results of static tensile tests carried out on specimens with identical geometry. Experimental results show a significant increase of the tensile strength by increasing the strain rate. The tensile modulus and strain to failure are also observed to increase slightly by increasing the strain rate.     

Modeling of cyclic stress–strain behavior and damage mechanisms under thermomechanical fatigue conditions

A multi-component model was applied to predict the cyclic stress–strain response of different alloys under thermomechanical fatigue conditions based upon isothermal hysteresis loops. A ductile AISI 304 L-type stainless steel and two high strength alloys, the near-α titanium alloy IMI 834 and the nickel-base superalloy IN 100, were chosen as test materials. These represent alloys with rather different dislocation slip modes, stress–strain characteristics and damage mechanisms. Model predictions are compared with experiments and the differences in cyclic stress–strain response and damage mechanisms under isothermal and thermomechanical fatigue conditions, respectively, are discussed based upon microstructural observations.     

Compressive response of glass–fiber reinforced polymeric composites to increasing compressive strain rates

The utilization of composite materials instead of traditional materials in structural high-speed applications has induced the need for a proper knowledge of dynamic behavior as well as static behavior of them. The material and structural response vary significantly under dynamic loading as compared to static loading conditions. In order to investigate the dynamic responses of composite materials under dynamic loading at various strain rates, special testing machines are needed. Most of the researches in this field are focused on applying real loading and gripping boundary conditions on the testing specimens.The present study is carried out in order to characterize the compressive properties of unidirectional glass–fiber reinforced polymeric composites using a servo-hydraulic testing apparatus at varying strain rates, ranging from 0.001 to 100 s⁻¹. For performing practical tests, a jig and a fixture are designed and manufactured, which could insure the alignment of axial loads on the specimens. During of tests, the performance of the test jig is evaluated. It is found that the designed jig and the fixture perform very well during the test process. The results of the dynamic tests are compared with the results of the static tests carried out on specimens with identical geometry. Based on the experimental results obtained from the tests, empirical functions for the mechanical properties are proposed in terms of strain rates. The results of the study indicate that strain rate has a significant effect on the material response. It is found that the compressive strength and modulus both increased with increasing the strain rate. Also, the results show that the compressive strain to failure is generally insensitive to strain rate.     

On the hysteretic nature of variable-amplitude fatigue crack growth

Local Stress–Strain (LSS) approach as applied in notch fatigue analysis is extended to the fatigue crack in order to simulate the effect of variable-amplitude loading on near-tip stress and strain. Simulated near-tip stress–strain response appears to explain load sequence sensitivity of near-threshold fatigue crack growth. This observation is supported by fractographic evidence of near-threshold variable-amplitude fatigue crack in an Al–Cu alloy under closure-free conditions.     

Stress in particulate reinforcements and overall stress response on aluminum alloy matrix composites during straining by analytical and numerical modeling

SiC and Al₂O₃ (10–20v%) particle-reinforced Al-2618 matrix composites subjected to tensile loading were selected to simulate stress–strain curves and average stress in particles, and to examine mechanical properties experimentally in comparison. A particle-compounded mechanical model was established based on Eshelby equivalent inclusion approach to simulate stress–strain curves by introducing secant modulus and tangent modulus techniques, and to calculate stress in particles and in matrices. The same modeling work was carried out by FEM analysis based on the unit cell model using a commercial ANSYS code. The modeling and experiment were also applied to compare the mechanical behaviors between hard matrix and soft matrix, which were produced under different heat treatments. Through the comparison of the results between simulations and experiment, it is shown that Eshelby particle-compounded mechanical model can predict the stress–strain curve of the composites with both hard matrix and soft matrix, while the FEM model can match the experimental data with only hard matrix. The modeling was also carried out to study the influence of different volume fractions and aspect ratios on elastic modulus and yield strength of the composites with different reinforcing particles to get a better understanding of strengthening mechanisms of the composites.     

Effects of alloying elements on elastic properties of Ni by first-principles calculations

The effects of alloying elements (Al, Co, Cr, Cu, Fe, Hf, Mo, Nb, Pt, Re, Ta, Ti, W, Y and Zr) on the elastic constants (c_ij’s) of Ni have been investigated using first-principles calculations within the generalized gradient approximation. The results are compared with the available experimental data and analyzed based on the volume changes and electron density. It is found that the shear modulus decreases with increasing volume caused by alloying addition and the bulk modulus (B) is related to the total molar volume (V_m) and electron density (n) with the relationship, . The melting temperatures of Ni–X dilute solutions calculated from the available thermodynamic databases have been compared to those obtained from the empirical relationship with the elastic constant c₁₁. The calculated elastic constants show good relationships with the mechanical properties such as 0.2% flow stress and give us a guideline to understand and develop Ni-based superalloys.     

Plastic deformation characteristics of cross-equal channel angular pressing

For the first time, the plastic deformation characteristics of cross-equal channel pressing (cross-ECAP), a modified equal channel angular pressing, using a cross-shaped channel instead of a conventional l-shaped channel, was analyzed by using finite element analysis. The deformation in the cross-ECAP is more complicated and the strain induced is much more severe than that in the conventional ECAP. However, the plastic strain is localized in the central linear region of the workpiece and very small strain is developed in the edge regions, which is in good agreement with experimental results in the literature showing nonuniformity in microstructure and hardness distribution. The load requirements of cross-ECAP are much higher in comparison to conventional ECAP and T-ECAP processes.     

Stress–strain curves of metallic materials and post‐necking strain hardening characterization: A review

For metallic materials, standard uniaxial tensile tests with round bar specimens or flat specimens only provide accurate equivalent stress–strain curve before diffuse necking. However, for numerical modelling of problems where very large strains occur, such as plastic forming and ductile damage and fracture, understanding the post‐necking strain hardening behaviour is necessary. Also, welding is a highly complex metallurgical process, and therefore, weldments are susceptible to material discontinuities, flaws, and residual stresses. It becomes even more important to characterize the equivalent stress–strain curve in large strains of each material zone in weldments properly for structural integrity assessment. The aim of this paper is to provide a state‐of‐the‐art review on quasi‐static standard tensile test for stress–strain curves measurement of metallic materials. Meanwhile, methods available in literature for characterization of the equivalent stress–strain curve in the post‐necking regime are introduced. Novel methods with axisymmetric notched round bar specimens for accurately capturing the equivalent stress–strain curve of each material zone in weldment are presented as well. Advantages and limitations of these methods are briefly discussed.     

Mechanical and swelling properties of thermoplastic elastomer blends

The mechanical behavior, microhardness and abrasion resistance of acrylonitrile butadiene rubber (NBR) vulcanizate loaded with 40 phr fast extrusion furnace (FEF) carbon black nanopowder and different concentrations of suspension polymerization polyvinyl chloride (PVC) were studied. The measured parameters (i.e., the Young’s modulus, tensile strength, and elongation at break) varied with the concentration of PVC. Both the elastic modulus and the tensile strength increased with increasing PVC loadings while the elongation at brake recorded a linear decrease. The hardness degree and the abraded mass increased as the concentration of PVC increased. The classical theory of rubber elasticity was used to calculate the rubbery modulus, the number of effective chains per unit volume and the average molecular weight. Swelling measurements were done on the mentioned samples. Addition of PVC was found to decrease the maximum degree of swelling, the penetration rate and the average diffusion coefficient. Swelling was found to slightly affect the degree of hardness and elastic modulus.     

Sensitivity of plastic strain localization zones to boundary and initial conditions

 The plastic strain localization phenomena are studied with the use of the theory of viscoplasticity and finite element method. The aim of the study is to check if the strain localization is sensitive to variation of initial and boundary conditions. Total and local plastic deformation energy as candidate response functionals for sensitivity analysis are discussed by selected numerical examples of wave propagation problems. The paper considers sensitivity of the form and intensity of plastic strain localization to the variation of the above parameters. Different response functionals for structures demonstrating plastic strain localization were proposed and studied. Received: 4 February 2002 / Accepted: 10 October 2002     

An experimental study on energy absorbing structures made of fabric composites

Egg-box shaped energy absorbing structures made of fabric composites were fabricated to find out the compressive characteristics and energy absorption capacity. Various stacking sequences and boundary conditions (unconstrained and bonded) were examined to investigate the stress–strain curves during compression. Failure modes of composite egg-box panels were observed and investigated correlating each step of meaningful collapsing behaviour. In order to check out the possibility as an ideal energy absorbers foam filled composite egg-box panels were fabricated and tested. From the test results it was found that the foam filled composite egg-box panels had good energy absorption capacity with smooth stress–strain curves which resembles the ideal energy absorber. The energy absorption per unit mass of composite egg-box panels made of different types of material and stacking sequences was calculated and compared with.     

Effect of deformation and oxygen content on mechanical properties of different copper wires

Investigations were done at dip-forming, contirod and SCR copper wire rods. These continuously cast and hot rolled copper wire rods (8 mm in diameter) were cold drawn in two drawing programmes (soft and hard drawn) to various diameters without any intermediate annealing. The results of influence of deformation and oxygen content on the flow stress, tensile elongation and number of twist to failure are given. Strength and strain hardening rates of hard-drawn wires always were higher than the soft-drawn wires. Oxygen effect on the ductility of copper wires measured with the tensile elongation and the number of twist to failure is not clear at large drawing prestrains.     

Biaxial deformation behavior and mechanical properties of a polypropylene/clay nanocomposite

Polymer nanocomposites offer the potential of enhanced properties such as increased modulus and barrier properties to the end user. Much work has been carried out on the effects of extrusion conditions on melt processed nanocomposites but very little research has been conducted on the use of polymer nanocomposites in semi-solid forming processes such as thermoforming and injection blow molding. These processes are used to make much of today’s packaging, and any improvements in performance such as possible lightweighting due to increased modulus would bring significant benefits both economically and environmentally. The work described here looks at the biaxial deformation of polypropylene–clay nanocomposites under industrial forming conditions in order to determine if the presence of clay affects processability, structure and mechanical properties of the stretched material. Melt compounded polypropylene/clay composites in sheet form were biaxially stretched at a variety of processing conditions to examine the effect of high temperature, high strain and high strain rate processing on sheet structure and properties.     

Effect of friction stir processing conditions on fatigue behavior and texture development in A356-T6 cast aluminum alloy

Friction stir processing (FSP) was applied to A356-T6 cast aluminum alloy to modify the microstructure and to eliminate casting defects under two different tool rotational speeds. Plane bending fatigue tests had been conducted, revealing that FSP could enhance the fatigue strength where the lower rotational speed condition gave better results. The enhancement of fatigue strength was attributed to the elimination of casting defects. Crystallographic analysis by EBSD revealed that the texture induced by FSP had detrimental effect on growth resistance. The lower rotational speed condition resulted in the weaker texture, and consequently, further increase of fatigue strength was achieved compared with the higher rotational speed condition.     

Effect of strain rate on low cycle fatigue of 316LN stainless steel with varying nitrogen content: Part-I cyclic deformation behavior

The effect of strain rate and nitrogen content on cyclic deformation and substructural changes in 316LN stainless steel is investigated at temperatures 773, 823 and 873 K. Dynamic strain aging (DSA) and/or thermal-recovery processes are observed to control cyclic deformation, and the regimes of their predominance are mapped. An increase in nitrogen content and DSA enhanced cyclic stress and are found to offset thermal-recovery induced cyclic strength reduction. In addition, strain localization in the form of slip-bands impinging on grain boundary is observed. The predominance of thermal-recovery over DSA manifested as dislocation-poor channels, dislocation cells within and in-between planar slip-bands.     

Laboratory investigation on strength and deformation characteristics of ice-saturated frozen sandy soil

To investigate the properties of ice-saturated frozen sandy soil, a series of triaxial compression tests on frozen sandy soil with a volumetric ice content of about 50% were carried out at a temperature of − 2.0 °C. The effect of confining pressure on strength and deformation features is analyzed according to the experimental results. The results show that the strength changes with increasing confining pressure in three distinct phases. According to the effective stress principle, the mechanism of strength is explained. A strength criterion is proposed to describe the strength characteristic. The equivalent stress versus axial strain curve shows strain-softening under each confining pressure, and the extent of strain-softening decreases with the increase in confining pressure, until it behaves the so-called perfect elasto-plastic feature when the confining pressure is large enough. The improved Duncan–Chang hyperbolic model is taken to simulate the stress–strain behaviors. The simulation shows that the model can well describe the strain-softening. The dependency of the volumetric deformation on the confining pressure is also discussed.     

Effect of texture on high temperature deformation behavior at high strain rates in a Mg–3Al–1Zn alloy

A microstructure optimization design method of the forging process is proposed. The optimization goal is a small grain size and a homogeneous grain distribution of the forgings. The optimization object is the preforming die shape. The microstructure optimization code is developed using the micro-genetic algorithm and the finite element method. The two forming steps including the preforming process and the final forging process of H-shape forgings are analyzed using the self-developed code. The optimization results show that small grain size and homogeneous grain distribution can be achieved by controlling the shape of the preforming die. Samples of the same size as in the optimization are preformed and then forged to the desired H-shape forgings under the same deformation conditions as in the optimization. Micrographs in the symmetry section of samples show that the grain sizes of the forgings almost coincide with the optimization results.