The purpose of this paper is to characterize the dynamics and direction of self-folding of pre-strained polystyrene (PSPS) and non-pre-strained styrene (NPS), which results from local shrinkage using a new process of directed self-folding of polymer sheets based on a resistively heated ribbon that is in contact with the sheets. A temperature gradient across the thickness of this shape memory polymer (SMP) sheet induces folding along the line of contact with the heating ribbon. Varying the electric current changes the degree of folding and the extent of local material flow. This method can be used to create practical three-dimensional (3D) structures. Sheets of PSPS and NPS were cut to 10 × 20 mm samples, and their folding angles were plotted with respect to time, as obtained from in situ videography. In addition, the use of polyimide tape (Kapton) was investigated for controlling the direction of self-folding. Results show that folding happens on the opposite side of the sample with respect to the tape, regardless of which side the heating ribbon is on, or whether gravity is opposing the folding direction. The results are quantitatively explained using a viscoelastic finite element model capable of describing bidirectional folds arising from the interplay between viscoelastic relaxation and strain mismatch between polystyrene and polyimide. Given the tunability of fold times and the extent of local material flow, resistive-heat-assisted folding is a promising approach for manufacturing complex 3D lightweight structures by origami engineering.

A novel non-bonded interface technique (NBIT) is used to analyze internal residual strain by combining a pre-split sample of AISI 4340 steel with the circular grid residual strain analysis technique. NBIT is compared with an implicit non-linear finite element (FE) model using LS-DYNA. A split FE model was compared with a quarter FE model to determine the split interface that causes an average difference of 9.0% on the residual von Mises strain field from a 588.6 N indentation. The homogeneous FE quarter model was then compared with the experimental split model using 588.6, 981.0, and 1471.5 N indentation forces. An average displacement difference of 3.92 µm was found when comparing the experimental split and FE homogeneous samples from a 588.6 N indentation. The internal residual major and minor principal strains from the split experimental sample and homogeneous FE model were compared for each indentation force. The minor principal strain results show the 588.6, 981.0, and 1471.5 N indentation forces resulted in a difference between the experimental split and homogeneous FE model of 28.5%, 34.8%, and 26.0%, respectively. The difference between the comparisons was explained by the inability of the FE model to simulate local non-homogeneous material properties such as grain composition and orientation whereas NBIT does. NBIT can be used for micro- or macro-scale residual strain analysis as the spatial resolution is highly adjustable.

This work documents the development of a tool to perform automated parameter fitting of constitutive material models. Specific to this work is the fitting of a Swift hardening rule and isotropic linear plasticity model to aluminum 2024-T351, C36000 brass, and C10100 copper. Material characterization was conducted through the use of compressive, cold upsetting tests. A noncontact, optical displacement measurement system was applied to measure the axial and radial deformation of the test specimens. Nonlinear optimization techniques were then applied to tune a finite element model to match experimental results through the optimization of material model parameters as well as frictional coefficient. The result is a system, which can determine constitutive model parameters rapidly and without user interaction. While this tool provided material parameters for each material and model tested, the quality of the fit varied depending on how appropriate the constitutive model was to the material's actual plastic behavior. Aluminum's behavior proved to be an excellent match to the Swift hardening rule while the behavior of brass and copper was described better by the linear plasticity model.

Shot peening is a cold working process, which is used to enhance the properties of materials, especially the fatigue life as it induces large compressive residual stresses in the subsurface of materials. In this paper, the effect of the shot peening process on the topography of the shot peened surface and the distribution of the residual stresses in the subsurface of the material was systematically investigated. A technique to estimate the shot peening coverage was employed using a finite element model which was further developed using experimental results for increased accuracy. The comparison between the numerical and experimental studies gives a good agreement of the distribution of the residual stresses in the subsurface of the shot peened material. The shot peening pressure and media size are two main factors affecting on the presence of compressive residual stresses in the subsurface of the material.

Characterization of materials undergoing severe plastic deformation requires the careful measurement of instantaneous sample dimensions throughout testing. For compressive testing, it is insufficient to simply estimate sample diameter from an easily measured height and volume. Not all materials exhibit incompressibility, and friction during testing can lead to a barreled sample with diameter that varies with height. Video extensometry has the potential to greatly improve testing by capturing the full profile of a sample, allowing researchers to account for such effects. Common two-dimensional (2D) video extensometry algorithms require thin, planar samples, as they are unable to account for out-of-plane deformation. They are, therefore, inappropriate for standard compressive tests which use cylindrical samples that exhibit large degrees of out-of-plane deformation. In this paper, a new approach to 2D video extensometry is proposed. By using background subtraction, the profile of a cylindrical sample can be isolated and measured. Calibration experiments show that the proposed system has a 3.1% error on calculating true yield stress—similar to ASTM standard methods for compressive testing. The system is tested against Aluminum 2024-T351 in a series of cold upsetting tests. The results of these tests match very closely with similar tests from the literature. A preliminary finite element model constructed using data from these tests successfully reproduced experimental results. Diameter data from the finite element model undershot, but otherwise closely matched experimental data.

Warm hydromechanical deep drawing (WHDD) has increasingly been implemented by automotive industry due to its various benefits including mass reduction opportunities in auto body-in-white components and improved formability for lightweight alloys. In the first part of the current study, WHDD of AA 5754-O was studied. In order to obtain the highest formability, an optimization study was performed for AA 5754-O WHDD process parameters (tool temperature, hydraulic pressure (HP), and blank holder force (BHF) loading profiles) through finite element analysis (FEA) + experimentation approach. Results showed that the optimal temperature for punch is 25 °C and 300 °C for die and blank holder. In addition, HP was found to be more effective on formability when compared to BHF. Both fast increasing HP and blank holder loading profiles contributes to higher formability.

Cold expansion (CX) is a material processing technique that has been widely used in the aircraft industry to enhance fatigue life of structural components containing holes. CX introduces compressive hoop residual stresses that slow crack growth near the hole edge. The objective of this paper is to predict residual stresses arising from cold expansion using two different finite element (FE) approaches, and compare the results to measurement data obtained by the contour method. The paper considers single-hole, double-hole, and triple-hole configurations with three different edge margins. The first FE approach considers process modeling, and includes elastic–plastic behavior, while the second approach is based on the eigenstrain method, and includes only elastic behavior. The results obtained from the FE models are in good agreement with one another, and with measurement data, especially close to the holes, and with respect to the effect of edge margin on the residual stress distributions. The distribution of the residual stress and equivalent plastic strain around the holes is also explored, and the results are discussed in detail. The eigenstrain method was found to be very useful, providing generally accurate predictions of residual stress.

Design of high-performance power lines with advanced materials is indispensable to effectively eliminate losses in electrical power transmission and distribution (T&D) lines. In this study, aluminum conductor composite core with carbon nanostructure (ACCC–CNS) coating in a multilayered architecture is considered as a novel design alternative to conventional aluminum conductor steel-reinforced (ACSR) transmission line. In the multiphysics approach presented herein, first, electrothermal finite element analysis (FEA) of the ACSR line is performed to obtain its steady-state temperature for a given current. Subsequently, the sag of the ACSR line due to self-weight and thermal expansion is determined by performing thermostructural analysis employing an analytical model. The results are then verified with those obtained from the FEA of the ACSR line. The electrothermal finite element (FE) model and the thermostructural analytical model are then extended to the ACCC–CNS line. The results indicate that the ACCC–CNS line has higher current-carrying capacity (CCC) and lower sag compared to those of the ACSR line. Motivated by the improved performance of the ACCC–CNS line, a systematic parametric study is conducted in order to determine the optimum ampacity, core diameter, and span length. The findings of this study would provide insights into the optimal design of high-performance overhead power lines.

The manufacturing parameters such as curing process cause residual stresses in polymeric laminated composites. Therefore, an accurate method of measurement of residual stresses is essential for the design and analysis of composites structures. The slitting method is recently used for measurement of the residual stresses in laminated composites. However, this method has some drawbacks such as high sensitivity to noise of measurements and high scattering in the final results, which necessitate using of normalization techniques. Moreover, the form of polynomials, used in the conventional slitting method for calculation of the stiffness matrix, has a significant effect on final results. In this paper, it is shown that the major reason of the drawbacks of the slitting method in calculating the residual stresses is a direct use of the elastic released strains recorded by strain gages. In the present study, instead of direct calculation of residual stresses from the elastic released strains, eigen strain distribution as a constant and invariant field has been calculated from the recorded elastic strains. Then, by using the calculated eigen strain field in a finite-element model, the residual stress filed was obtained. Also, instead of using polynomials to calculate the compliance, a superposition method was used. The results show that the new method decreases the sensitivity of the final results to noise and scattering of the experimental data. It means that the normalization methods are not needed any more.

Many mathematical models based on the advanced theory of bending to predict bending characteristics for monolithic sheet materials are available in the literature. In this work, a similar approach is utilized to develop bending models for a bilayer laminated sheet material. The principal stresses and strains through the thickness and change in relative thickness, at specified bend curvatures, are obtained as a function of increasing curvature during bending. Additionally, three-dimensional (3D) finite element (FE) based models for bilayer laminate bending are developed to overcome simplifications of the analytical models. In order to experimentally validate the two models, a new experimental bend test-jig is developed and experiments are performed on bilayer steel–aluminum laminate for different clad to matrix thickness ratios. These experiments have enabled continuous measurements of strain along the width at the bend line and through the laminate thickness at one of the specimen edges using an online strain mapping system based on digital image correlation (DIC) method. Analytical model results indicate how the through-thickness strain distribution and relative thickness of the specimen in bending are influenced by the location and thickness of the soft clad material. The FE model and experimental results exhibit similar trends in the relative thickness change for different geometric arrangements of steel–aluminum layers. The tangential and radial stresses decrease in magnitude with increasing aluminum clad thickness ratios. The 3D FE model of laminate bending provided strain predictions across the specimen width at the bend line on the tension and compression sides that increased with increasing clad thickness ratios. Also, relative thickness data from the 3D FE model showed uniaxial and plane strain stress states at the edge and midwidth sections of the test specimen. The results from analytical and FE models and from DIC and microscopic thickness measurements indicate that thickness at the bend line increases with increasing clad thickness for the case of clad layer on the compressive side of the laminate (i.e., C-C case) and vice versa for clad layer on the tensile side (C-T).

This paper is intended to quantify the relationship between the peen forming effectiveness and various involved parameters through a realistic numerical study. For this purpose, a new finite element (FE) model is proposed with full geometry representation, random shots generation, and rate-dependent material law of kinematic strain-hardening. The mesh sensitivity and effects of boundary conditions are carefully examined. The FE model is validated by comparing the results with the experimental measurements. The proposed model is then used to investigate the effects of the peening intensity (represented as the shot velocity) and the strip thickness on the peen-formed deflection and the residual stress distribution for strips made of Ti-6Al-4V. Our results indicate the existence of a maximum convex deflection for different strip thicknesses. In addition, a reversed deflection (i.e., concaved curvature) is observed for severe peening conditions (i.e., thin strip under high peening intensity). Our simulations verify the previous proposition that a concaved curvature can be generated only when the whole cross section is plastically deformed.

In this study, a crystal plasticity finite element model (CPFEM) has been revisited to study the microstructure effects on macroscopic mechanical behavior of ultrafine-grained (UFG) nickels processed by severe plastic deformation (SPD). The microstructure characteristics such as grain size and dislocation density show a strong influence on the mechanical behavior of SPD-processed materials. We used a modified Hall–Petch relationship at grain level to study both grain size and dislocation density dependences of mechanical behavior of SPD-processed nickel materials. Within the framework of small strain hypothesis, it is quite well shown that the CPFEM predicts the mechanical behavior of unimodal nickels processed by SPD methods. Moreover, a comparison between the proposed model and the self-consistent approach will be shown and discussed.

It is the objective of this study to conduct realistic simulations of the arc-height development in shot-peened Almen strips using the finite element (FE) method. Unlike our earlier work which is devoted to relaxation of shot peening induced residual stress, in this paper, the focus is on peen forming as a result of repeated spherical impingement. Specifically, a 3D FE model with 1500 randomly distributed shots bombarding an Almen strip was developed. Strain rate dependent plasticity was considered and an artificial material damping was applied to control the undesired high-frequency oscillations. The solution further adopts both explicit dynamic and implicit quasi-static analyses to simulate the entire arc-height development in the Almen strips. Quantitative relationships between the resulting equivalent plastic strain and the associated residual stress distribution for a given shot velocity and shot numbers are established and discussed. The work also considers the effect of repeated impacts upon the induced residual stress field using a large number of random shots. Attention was further devoted to the effect of the strip constraint upon the outcome of the impingement. Our results indicate that the proposed FE model is a powerful tool in investigating the underlying mechanisms of the peening treatment.

Forming limit diagrams (FLD) have been widely used as a powerful tool for predicting sheet metal forming failure in the industry. The common assumption for forming limits is that the deformation is limited to in-plane loading and through-thickness bending effects are negligible. In practical sheet metal applications, however, a sheet metal blank normally undergoes a combination of stretching, bending, and unbending, so the deformation is invariably three-dimensional. To understand the localized necking phenomenon under this condition, a new extended Marciniak–Kuczynski (M–K) model is proposed in this paper, which combines the FLD theoretical model with finite element analysis to predict the forming limits after a sheet metal undergoes under continuous-bending-under-tension (CBT) loading. In this hybrid approach, a finite element model is constructed to simulate the CBT process. The deformation variables after the sheet metal reaches steady state are then extracted from the simulation. They are carried over as the initial condition of the extended M–K analysis for forming limit predictions. The obtained results from proposed model are compared with experimental data from Yoshida et al. (2005, “Fracture Limits of Sheet Metals Under Stretch Bending,” Int. J. Mech. Sci., 47(12), pp. 1885–1986) under plane strain deformation mode and the Hutchinson and Neale's (1978(a), “Sheet Necking—II: Time-Independent Behavior,” Mech. Sheet Metal Forming, pp. 127–150) M–K model under in-plane deformation assumption. Several cases are studied, and the results under the CBT loading condition show that the forming limits of post-die-entry material largely depends on the strain, stress, and hardening distributions through the thickness direction. Reduced forming limits are observed for small die radius case. Furthermore, the proposed M–K analysis provides a new understanding of the FLD after this complex bending-unbending-stretching loading condition, which also can be used to evaluate the real process design of sheet metal stamping, especially when the ratio of die entry radii to the metal thickness becomes small.

The flexural response of a three-dimensional (3D) layer-to-layer orthogonal interlocked textile composite has been investigated under quasi-static three-point bending. Fiber tow kinking on the compressive side of the flexed specimens has been found to be a strength limiting mechanism for both warp and weft panels. The digital image correlation (DIC) technique has been utilized to map the deformation and identify the matrix microcracking on the tensile side prior to the peak load in the warp direction loaded panels. It has been shown that the geometrical characteristics of textile reinforcement play a key role in the mechanical response of this class of material. A 3D local–global finite element (FE) model that reflects the textile architectures has been proposed to successfully capture the surface strain localizations in the predamage region. To analyze the kink banding event, the fiber tow is modeled as an inelastic degrading homogenized orthotropic solid in a state of plane stress based on Schapery Theory (ST). The predicted peak stress is in agreement with the tow kinking stress obtained from the 3D FE model.

The use of externally bonded carbon fiber-reinforced polymers (CFRPs) as strengthening systems to improve the condition and overall capacity of existing reinforced concrete structural members is found to be a promising scheme. This field has drawn the attention of many researchers in the past two decades through the implementation of much theoretical work as well as experimental studies. It was evident through many experimental investigations conducted by a number of researchers that the bond action between fiber-reinforced polymers and reinforced concrete members is considered one of the main factors affecting the performance and reliability of external strengthening systems and warrants further investigation. Debonding failure is a brittle mode of failure that may occur prematurely before strengthened members develop their full composite and expected capacities. This paper aims to investigate the bond behavior between the CFRP-concrete interface via experimental work and finite element (FE) simulations. The experimental study consisted in testing nine concrete prisms with different bonded lengths of the CFRP plates that vary between 25% (60 mm), 50% (120 mm), and 75% (180 mm) of the total length of the concrete prisms and instrumented with strain gauges. A FE simulation model was created and validated using the experimental results of the tested specimens, and a parametric study was carried out to investigate the effect of several parameters on the bond behavior between CFRP and concrete surfaces. The trend of the FE simulation results shows a good agreement with the experimental results and those available in the literature. It was observed that the optimum length of the CFRP plate is in the range between 150 mm and 160 mm when bonded to concrete surfaces. Other conclusions and observations were drawn based on the experimental and numerical results.

As a method for mechanical surface treatment, shot peening has been widely used to improve the fatigue strength of materials. However, the influence of residual stresses introduced by shot peening depends on their stability. The stability of residual stresses during fatigue may be studied in two stages: the first cycle and successive cyclic loading. In this study the stability and development of the residual stresses during the first cycle of strain controlled fatigue of normalized steel was investigated. The influence of total strain amplitude and the loading direction was studied. The residual stresses were obtained using the x-ray diffraction technique. It was shown that the stability and relaxation of the residual stresses depend both on the amount and the direction of the loading stresses. Finite element modeling (FEM) was used to rationalize the experimental data. Very good agreement between the experimental and FEM results were observed.

A finite element method is employed to numerically evaluate the stiffness and energy absorption properties of an architecturally hybrid composite material consisting of unidirectional and random glass fiber layers. An ls-dyna finite element model of a composite hollow square tube is developed in which the position of the random fiber layers varies through the thickness. The assessment of the stiffness and energy absorption is performed via three-point impact and longitudinal crash tests at two speeds, 15.6 m/s (35 mph) and 29.0 m/s (65 mph), and five strain rates, ɛ · = 0.1 s −1 , 1 s −1 , 10 s −1 , 20 s −1 , and 40 s −1 . It is suggested that strategic positioning of the random fiber microstructural architecture into the hybrid composite increases its specific absorption energy and, therefore, enhances its crashworthiness. The simulation data indicate that the composite structure with outer layers of unidirectional lamina followed by random fiber layers is the stiffest due to the considerable superior specific energy absorption of the random fiber micro-architecture. Moreover, it is illustrated that the specific energy absorption increases with the increased ratio of impact contact area over cross-section area. Of all the parameters tested the thickness of the unidirectional laminate on the specific energy absorption does not appear to have a significant effect at the studied thickness ratios.

Nanoindentation is widely used to characterize the mechanical and interfacial properties of thin film systems. However, the effects of substrate compliance on the indentation response of compliant substrate systems are not well understood. This paper investigates the effects of the large compliance mismatch between the film and the substrate and of the film thickness for model systems using nanoindentation tests, finite element simulations, and an analytical model based on a classical plate-bending solution. The results showed that for displacements less than the film thickness and for ratio of the substrate to film modulus less than 100. The indentation force-displacement response exhibits a linear relationship that can be predicted accurately by the linear plate-bending model. The effective stiffness depends linearly on the film thickness and also on the substrate and film moduli. For larger displacements, the indentation response exhibits the scaling relationship of the nonlinear plate-bending model.

When measuring residual stresses using the hole-drilling strain-gauge method, plasticity effects arise if the residual stress level exceeds about 60% of the material yield strength. In this case the classical methods, which are based on the linear elastic material behavior, do not work properly and residual stresses are overestimated. This paper presents a numerical study of the influence of plasticity on residual stress measurement by using the hole-drilling strain-gauge method in those cases in which stress does not vary with depth. The study investigates the effects of the most important loading, measuring, geometry, and material variables. An iterative method, which can be applied to obviate these errors, is then presented. The method was implemented in ANSYS using the APDL macrolanguage ( ANSYS Parametric Design Language Guide, Documentation for ansys 11.0 ) to automatically execute the procedure steps. A finite element model of the hole, which allows for plasticity, is requested. Employing the readings of a standard three elements strain-gauge rosette, the method makes it possible to extend the measurement limit in comparison to that of the ASTM E837 standard ( ASTM E837-08, “Standard Test Method for Determining Residual Stresses by the Hole-Drilling Strain-Gauge Method” ).