Dog-bone Specimens Research Articles

Characterization of Yield Surface Evolution of AZ31 from Shear to Equibiaxial Tension

This research characterized the strain hardening behavior of AZ31 under different stress states from shear to balanced biaxial tension with a newly proposed yield function. Experiments are conducted for AZ31 magnesium alloy by in-plane shear specimens, dogbone specimens, notched specimens and bulging specimens to characterize the flow behavior under different stress states. The flow behaviors are characterized by a newly proposed yield function in a form of the three stress invariants. The proposed yield function is implemented into ABAQUS/Explicit to predict the plastic response of the alloy under different stress states. It is shown that the proposed yield function can precisely predict the distinct flow behaviors and reaction forces from shear to equibiaxial tension from the initial yielding to fracture.

Identification of Swift Law Parameters Using FEMU by a Synthetic Image DIC-Based Approach

Computer-aided engineering systems rely on constitutive models and their parameters to describe the material behaviour. The calibration of more elaborated material models with a larger number of parameters becomes very time and cost consuming. The development of image-based technology has enhanced the interest in inverse identification methods, which, when coupled with full-field measurements, have the potential to reduce the number of experimental tests required to accurately identify material properties. This work aims to identify the Swift hardening law parameters of a dual-phase steel using a tensile test on a heterogeneous dogbone specimen under uniaxial and quasi-static loading conditions using the finite element model updating (FEMU) technique. The numerical results were used to generate synthetic images, which were then processed by digital image correlation (DIC) and used as the reference in the identification procedure. Two different approaches were tested: (i) directly comparing the numerical results to the reference; (ii) using DIC-levelled numerical data by iteratively generating synthetic images and using the DIC filter with the same settings as were used on the reference (virtual experiment). The identification results obtained from both approaches are compared and discussed.

Finite element modeling of fatigue properties of polymer nanocomposites

This paper presents a 3-dimensional finite element modeling of fatigue properties of polymer nanocomposites with a Representative Volume Element (RVE) approach using ANSYS. Epoxy was used as a matrix, and alumina nanoparticles of spherical shape (diameter 33 nm) were used as reinforcement. Convergence analysis was conducted to select the proper size of RVE. A geometric model of a flat dog bone specimen was created to model the fatigue properties. MATLAB code was used to generate randomly distributed nanoparticles. The sample was loaded in uniaxial tension-tension fatigue loading with a stress ratio of 0.1. S-N data for epoxy and alumina were given as input to determine the fatigue life of polymer nanocomposites. The results obtained from modeling were compared with experimental data. The fatigue life was determined by taking the average life at five different locations in the gauge region for a sample. The average of five different samples was calculated for each wt.% (0.5, 1.0, and 1.5). Reinforcement of nanoparticles up to 1 wt.% caused an increment in the fatigue life of nanocomposites, and fatigue life decreased at 1.5 wt.% of nanoparticles. The finite element modeling was able to predict the trend of improvement provided by the alumina nanoparticles (similar to experimental data).

The Influence of Temperature on the Tensile Mechanical Properties of PPA

In the automotive industry and not only, more and more attempts are being made to replace parts made of metals with those made of plastics, due to their low weight and low manufacturing cost. Thus, a detailed knowledge of the mechanical properties of polymeric materials is a must. This paper presents a study of the influence of temperature on the tensile mechanical properties of semicrystalline polyphthalamide (PPA). Dog bone specimens were obtained by injection molding and, in order to eliminate the possible moisture absorbed in the place of storage of the specimens, they were subjected to a drying treatment in an airtight oven at a temperature of 80 degrees for 12 hours. The specimens were tested at 5 different temperatures (25, 40, 80, 120, and 160°C) with a loading speed of 5 mm/min. For each test temperature 3 specimens were used, according to the ISO 527-2 standard. With the data extracted from the experimental tests, the mechanical properties were calculated. Also, the force – displacement and stress – strain curves were plotted. After the experimental tests, it was concluded that the dry specimens are more resistant, and with increasing temperature, the PPA material changes from a brittle to a ductile behavior. Moreover, the main tensile properties decrease significantly with increasing temperature (the modulus of elasticity decreases by 66.8% from temperature of 25 °C to temperature of 160 °C).

The challenges of quasi-static and fatigue experiments of structural adhesives

This paper presents the results of an experimental campaign designed specifically to investigate unexpected and unwanted failures observed during the quasi-static and fatigue tensile loading of structural adhesives. Simple finite element models were also developed to support the arguments drawn from the experimental investigations. Particular parameters causing tab and asymmetrical failures in the bulk of dog-bone adhesive specimens were investigated. Artificial defects at the tab-adhesive interface, causing stress concentrations, intentionally misaligned tabs creating bending moments, different tab manufacturing, different fatigue loading frequency, and different grip pressure, were all considered in this analysis. In addition to the traditional instrumentation, a high-speed camera was also implemented to assist the observations. Experimental results showed that the tab failure occurrence is linked to the grip pressure, which shall be minimized. Other parameters such as grip alignment, tab taper, and loading frequency proved to also have a sizable effect on tab failure.

EVALUACIÓN DE LAS PROPIEDADES MECÁNICAS Y MICROESTRUCTURA DE PROBETAS DE LA ALEACIÓN AISI 316L IMPRESAS POR DEPOSICIÓN DE FILAMENTO FUNDIDO

The additive manufacturing process by extrusion can provide an economical alternative to the manufacture of metal parts compared to powder bed fusion technologies, which are more expensive and with much lower production rates. The alternative’s viability is based on knowing how to print by extrusion metallic parts with adequate mechanical properties. This work explains how to print traction dog-bone specimens by fused fabrication of a filament loaded with stainless steel 316L powder. Subsequent, postprocesses of debinding and sintering for increasing specimen’s density are also described. The specimens were tested to assess the mechanical properties of traction achieved: elastic limit, youth modulus, maximum stress, rotational stress and Poisson's modulus. Finally, the results were compared with those obtained in an AISI 316L rolled steel specimen, and the obtained micro-structured was studied. Keywords: FFF, 316L, 3D printing, Sintering, Debinding.

Size and Shape Effects on Fatigue Behavior of G20Mn5QT Steel from Axle Box Bodies in High-Speed Trains

In this paper, the axial loading fatigue tests are at first conducted on specimens ofG20Mn5QT steel from axle box bodies in high-speed trains. Then, the size and shape effects on fatigue behavior are investigated. It is shown that the specimen size and shape have an influence on the fatigue performance of G20Mn5QT steel. The fatigue strength of the hourglass specimen is higher than that of the dogbone specimen due to its relatively smaller highly stressed region. Scanning electron microscope observation of the fracture surface and energy dispersive X-ray spectroscopy indicate that the specimen size and shape have no influence on the fatigue crack initiation mechanism. Fatigue cracks initiate from the surface or subsurface of the specimen, and some fracture surfaces present the characteristic of multi-site crack initiation. Most of the fatigue cracks initiate from the pore defects and alumina inclusions in the casting process, in which the pore defects are the main crack origins. The results also indicate that the probabilistic control volume method could be used for correlating the effects of specimen size and shape o the fatigue performance of G20Mn5QT steel for axle box bodies in high-speed trains.

Machine learning-based modeling of the coupling effect of strain rate and temperature on strain hardening for 5182-O aluminum alloy

This research characterizes the dynamic hardening behavior of an aluminum alloy sheet of 5182-O for the coupling effect of strain rate and temperature. Tests are carried out for dogbone specimens at different loading conditions to experimentally characterize the strain rate hardening and thermal softening effect for the alloy. The behaviours are then modeled by the Johnson-Cook, Zerilli-Armstrong and Lim-Huh models. In addition, the FEA-friendly polynomial model and artificial neural network (ANN) model are used to describe the highly non-linearity and coupling of strain hardening. Factors affecting ANN predicting accuracy and numerical computing efficiency are comprehensively studied including network structure, parameter settings and optimization algorithms. All the analytical and ANN models are also implemented into ABAQUS/Explicit to numerically compute the reaction force of tensile tests of dogbone specimens. The strain hardening curves are predicted by the analytical and ANN models for the comparison with experimental measurements to evaluate their performance. The experimental results show that the strain rate is slightly negative at room temperature, while the strain rate effect turns to be positive as temperature rises. The comparison of the flow curves between prediction and experiments reveals that the coupling effect is reasonably illustrated by the proposed polynomial model and the ANN model illustrates the flow curves with the dramatically much better accuracy than all the other models. The numerically predicted reaction forces prove that the ANN model accurately illustrates the load capability with the best agreement among the models studied in this research. The numerical computation also shows that the numerical computation efficiency of the ANN model is slightly reduced compared with analytical models, but the reduction is not much and worthwhile compared with the high accuracy of ANN.

Development of Starch-Based Bioplastic from Jackfruit Seed

In this article, jackfruit seed starch plasticized with common plasticizers was developed and characterized. At the first step, the research papers that dealt with the fabrication and characterization of starch-based bioplastics were synthesized and analyzed. Next, jackfruit seeds were selected as a source for starch because of their large availability, low price or even free, and high starch capacity. Afterward, a starch-based bioplastic fabrication procedure was proposed. From preliminary tests, plasticizers were sufficiently selected, including water, glycerol, natri bicarbonate, and acid citric. Using different combinations of these plasticizers, four types of bioplastics were then fabricated to study the effect of the plasticizers as well as to characterize the properties of the corresponding bioplastics. A cutting tool for ASTM D412 type A standard tensile testing specimen was then designed and fabricated. Using these dog-bone specimens, tensile results showed that the hardness of the fabricated bioplastic was positively proportional to the ratio of the starch. Furthermore, from SEM characterization, the bioplastic specimens were fully plasticized. Although the fabricated bioplastic has lower mechanical properties than petroleum-based plastics, its environmental friendliness and high potential added value promise to be a material of the future.

Effect of printing speed on tensile and fracture behavior of ABS specimens produced by fused deposition modeling

The current paper deals with the influence of printing speed on the tensile and fracture strength of Acrylonitrile Butadiene Styrene (ABS) specimens made by Fused Deposition Modeling (FDM) technique. Four different printing speeds of 10, 30, 50, and 70 mm/s are used to fabricate dog-bone and Semi-Circular Bending (SCB) specimens for examining the mechanical and fracture performance of FDM-ABS parts, respectively. Due to the plastic deformation in the crack tip zone of SCB specimens prior to fracture initiation, the critical value of J-integral is chosen as the fracture characterizing parameter. Therefore, elastic–plastic finite element analyses are performed to calculate the critical values of J-integral (Jc). According to the experimental results, the fabricated specimens with a printing speed of 70 mm/s shows the best performance with the maximum elongation and fracture resistance compared to the other printed specimens with different nozzle speeds. For exploring the failure mechanisms in the tensile specimens Scanning Electron Microscopy (SEM) is utilized and various failure mechanisms have been presented and discussed. These observations are then linked to the tensile and fracture properties of the studied specimens.

Experimental Study on Direct Tensile Properties of Cemented Paste Backfill

Cemented paste backfill (CPB) has been increasingly utilized in mines for efficient mineral obtaining and regional ground support. To guarantee the work performance, the mechanical properties of CPB have long been a topic of study among researchers. But the research progress on the tensile strength of CPB is limited, mainly because of the lack of an appropriate test method due to the low tensile strength of CPB. Therefore, instead of the conventional splitting indirect tensile strength test method, a new direct tension test method, which utilizes the specifically designed compression to tension load converter (CTLC) and dog-bone-shaped specimen, has been applied to study the direct tensile properties of CPB. In this study, the direct tensile strength (DTS) of 47 CPB mix designs were measured using CTLC, and the unconfined compressive strength (UCS) of the corresponding mix design was also tested. The experimental results showed that the increase in the binder content, solid mass content, and curing period led to higher CPB direct tensile strength, and the DTS of CPB was most sensitive to the binder content. Furthermore, the influence of the slurry mass solid content on the tensile strength of CPB was not linear. The influence of the binder content became increasingly notable with the increase in the solid content, especially if the binder content exceeded 75%. The effect of the curing period was found to be rather marginal due to the decreasing amount of un-hydrated cementitious materials left with the increase of the curing period. Overall, the DTS generated using dog-bone specimens and the CTLC apparatus are valid for better mine backfill designs. Finally, a linear correlative between UCS and DTS with a formula in the form of σDT (DTS) = 0.171 σc (UCS) was obtained, and the correlation was sufficient for further calculation of DTS using measured UCS.

Experimental characterization and finite element validation of orthotropic 3D-printed polymeric parts

Components fabricated via Fused Filament Fabrication (FFF) have an anisotropic response, which is further complicated by an intra-part and a part-to-part variation of their mechanical properties. In addition, the mechanical characterization and analysis process has not been standardized yet, making it difficult to assess the structural behavior and verify the compliance of a part with the performance criteria in service. This paper intends to fill this gap for specific printing process parameters. First, it speculates that a linear infill with a 100% infill could help to reduce the anisotropy of the parts to a mild orthotropy. Thin components have provided a quick and preliminary confirmation of the approach. After an initial test setup design, which was required to standardize the method, the in-plane behavior was studied. Classical dog-bone specimens returned unsatisfactory results when coupled with the internal structures. As a result, the paper takes inspiration from the test methods for Uni-Directional Composites (UDCs). It uses sets of Design of Experiments (DoEs) to determine the optimal shape of the tabs. This method managed to quantify the factors of the 3 × 3 reduced elastic coefficients matrix. Finally, the paper presents a set of three-point bending, simple bending, and bending–torsion tests on samples featuring different laminations. 2D FE models tuned with the experimental properties simulated them, following the Classical Lamination Theory (CLT) approach. For consistency, a shear penalization was introduced for the out-of-plane shear. The FE models delivered an excellent mechanical response prediction; this result appears to validate the approach and the method and endorses 2D FEM and CLT as reliable tools to analyze linear infill FFF parts.

Anisotropic yield surfaces of additively manufactured metals simulated with crystal plasticity

The mechanical anisotropy created by additive manufacturing (AM) is not yet fully understood and can depend on many factors, such as powder material, manufacturing technology and printing parameters. In this work, the anisotropic mechanical properties of as-built, laser powder bed fusion (LPBF) austenitic stainless steel 316L and titanium alloy Ti-6Al-4V are investigated through crystal plasticity simulations. Periodic representative volume elements (RVEs) are used that are specific to each material. The RVE for austenitic stainless steel consists of FCC crystals with a crystallographic texture measured by X-ray diffraction. The α′ martensite microstructure of Ti-6Al-4V is captured with a multi-scale RVE, including internal lamellar structures, using HCP crystals and a synthetically generated texture. For both materials, the crystal plasticity parameters are calibrated against tensile tests carried out on dog-bone specimens printed in different orientations. The RVEs, calibrated to experiments, are applied in virtual material testing and subjected to multiple load cases to generate the Hill-48 and Yld2004-18p yield surfaces of the materials.

Determination of the fatigue behavior of mechanical components through infrared thermography

The determination of the fatigue behavior at a component level usually requires dedicated test rigs and an expressive amount of time. The hours spent on such machinery are expensive; therefore, solutions to reduce experimentation time are most welcomed. In this context, this investigation aims at developing a procedure for rapid determination of the fatigue strength of crankshafts by means of a thermographic methodology. The use of infrared cameras for fatigue strength analysis was first assessed in standard dog-bone specimens. Crankshafts were then tested in an in-house fatigue test rig using the conventional staircase method and the thermographic method. Sample batches with different manufacturing parameters were produced and tested to assess the robustness of the proposed alternative technique. Results of the dog-bone test campaign revealed a good correlation between fatigue strength estimates obtained with the conventional Wöhler curve and the thermographic methodology. Finally, the thermographic technique also delivered results in close agreement with the staircase method for all crankshaft batches. The proposed procedure was found to be a viable, rapid alternative to conventional fatigue test programs, with potential application for complex structural components such as crankshafts, among others.

Tensile and compressive strength of polyethylene engineered cementitious composite (PE-ECC) at elevated temperature

The fire performance of engineered cementitious composites (ECC) made with polyethylene (PE) fibers has not been addressed extensively in the literature. Therefore, this study’s main objective is to investigate the residual compressive and tensile properties of PE-ECC specimens after high-temperature exposures. The compression cube samples were heated up to 400 °C, and the tensile specimens were exposed to sub-elevated temperatures up to 120 °C. After the samples were heated, they were allowed to cool down naturally to room temperature. During the heating process, no explosive spalling was observed. Cube specimens exposed to the temperature of 200 °C, maintained almost all the compressive strength of unheated specimens. The compressive strength of the samples heated to the temperature of 400 °C, was almost 60% of that of the control ones. All the tensile dogbone specimens at different temperature exposures showed strain-hardening and multiple cracking behaviors. At 80 °C, the tensile strength of dogbone specimens was increased by 13% and then decreased by 10% at 120 °C compared to the control unheated specimens.

Multi-Fractal Scaling Law applied to VHCF with an emphasis on statistical fluctuations

In recent decades, ultrasonic testing machines have allowed to investigate the very-high cycle fatigue (VHCF) range, featuring feasible gigacycle fatigue testing in a very short time, although the influence of frequency is still partly controversial. The fatigue behaviour depends on the random distribution of initial defects, whatever the stress range amplitude is. As a consequence, the structural size comes into play and the so called ”size-effects” are usually observed. On the other hand, the scaling is not uniform when sufficiently large dimensional ranges are analysed. To this aim, the multi-fractal formalism was adopted to explain the observed specimen-size effect in the VHCF regime when a wide dimensional range is investigated. In addition, the statistical dispersion of experimental results was accounted by adopting different probabilistic functions. Among them, the Generalized Extreme Value Distribution Type-I was selected. In this way, it was possible to derive the analytical relationship for probabilistic-scale dependent P-S-N-b curves.Subsequently, the proposed model was used to analyse the experimental data obtained in an experimental campaign carried out at Politecnico di Torino by the present authors. More in detail, ultrasonic fully reversed tension–compression fatigue tests in the very-high cycle fatigue (VHCF) range were conducted on a set of hourglass and dog-bone specimens made of EN AW-6082 aluminium alloy, with a diameter in the middle cross-section ranging from 3 mm up to 30 mm.The comparison between the theoretichal model and the experimental data allowed to demonstrate the ability of Multi-Fractal Scaling Law (MFSL) to provide objective values of the fatigue strength of full-size components subjected to VHCF and to guarantee safe structural design.

Effect of heat treatment on crystallinity and mechanical properties of flexible structures 3D printed with fused deposition modeling

Fused Deposition Modeling (FDM) is a widely used 3D printing technique, which works based on the principle of melted polymer extrusion through nozzle(s) and depositing them on a build plate layer by layer. However, products manufactured with this method lack proper mechanical strength. In this work, 2/1 twill weave fabric structures are 3D printed using poly (lactic) acid (PLA). The ultimate tensile strength in the warp and weft directions and the modulus (stiffnesses) are measured for non-heat-treated (NHT) samples. The printed samples were heat-treated (HT) to improve the strength and stiffness. The variation in ultimate tensile strength is statistically insignificant in warp direction at all temperatures; however, the tensile strength in weft direction decreased after heat treatment. The modulus in warp direction increased by 31% after heat treatment while in the weft direction it decreased after heat treatment. Differential scanning calorimetry (DSC) tests showed the highest crystallinity at 125°C. The properties of the twill fabrics were compared with a standard dog-bone (DB) specimen using uniaxial tensile tests, Differential scanning calorimetry tests, and optical microscope (OM). For dog-bone specimens, the maximum values of crystallinity, ultimate tensile strength, and modulus were found to be at 125°C. The maximum crystallinity percentages are higher than that of the NHT samples. The ultimate tensile strength of NHT DB specimen 3D printed in horizontal orientation improved after heat treatment. The ultimate tensile strength of DB samples in vertical directions increased after heat treatment as well. The stiffness increased in both directions for DB samples.

Engineered Cementitious Composites using Chinese local ingredients: Material preparation and numerical investigation

Characterized with remarkable tensile ductility, toughness and multi-cracking behavior under tension, Engineered Cementitious Composites (ECC) with Chinese local ingredients was developed in this paper, aiming to reduce the cost and to match a tensile strain capacity of 4%. Compression, tension and bending tests were carried out to determine the mechanical performance of this composite material. In addition, a numerical study to characterize the tensile and flexural behaviors of ECC specimens was conducted based on Lattice Discrete Particle Model (LDPM), which was recently developed to simulate quasi-brittle materials through a three dimensional assemblage of spherical particles. The effect of fiber reinforcement was investigated using the so-called LDPM-F which accounts for dispersed fibers at the meso-level. Plain mortar specimens without fibers were tested to calibrate the relevant LDPM parameters, while the LDPM-F parameters related to the fiber-matrix interaction were identified by matching three-point bending test of ECC specimens. Results show that the validated ECC numerical model realistically predicts the tension response of dogbone specimens. In addition, typical multi-cracking phenomenon and strain-hardening behavior of ECC are successfully captured. Finally, the effect of fiber length on ECC tensile response was further studied, and the results suggest that the best tensile performance of ECC is obtained for 18-mm-long polyvinyl alcohol (PVA) fiber reinforcement.

3d printing of stainless steel 316L and its weldability for corrosive environments

Critical to its successful deployment, metal 3D printed parts must demonstrate not only corrosion resistance to operate in harsh environments but also good weldability to existing assets for installation and long-term durability. To address this issue, this paper evaluates the corrosion resistance of additively manufactured Stainless Steel (SS) 316 L via Laser powder bed fusion (L-PBF) and its weldability to its wrought counterpart under corrosion distress. First, 3D printed pellets are separately characterized by accelerated corrosion testing (i.e. chronoamperometry, cyclic potentiodynamic polarization and electrochemical impedance spectroscopy) following ASTM standards, and its improved corrosion resistance is demonstrated and compared to its wrought counterpart. Next, to evaluate the weldability, 3D printed SS 316 L plates are welded to wrought counterparts, dissimilar welded joints dog-bone specimens are machined from it and submitted for uniaxial mechanical tensile testing. When compared against similar welded joints wrought dog-bone specimens (i.e. wrought on both halves), dissimilar welded joints dog-bone specimens exhibit 36.02% less ducility and 12.59% lower yield strength, and those values reduce to 74.65% and 27.29%, respectively, when the former undergoes accelerated corrosion. These results evaluate 3D printed SS 316 L parts against wrought for applications as replacement parts in harsh environments that require welding for installation.

Probabilistic analysis of additively manufactured polymer lattice structures

Filigree lattice structures are sensible to geometrical imperfections and the scatter of material parameters which all depend on the stability of the manufacturing process. The aim of this study is to analyze these effects for polymer lattice structures and incorporate them in a finite element model for robust design. Micrographs of lattice structure slices show a smaller diameter for vertical struts. Basic mechanical tests on bulk material exhibit a tension–compression asymmetry which is captured with a Drucker-Prager material model in simulations. Digital image correlation measurements allow to determine true material properties. Plateau stress and failure strain are a result of the biggest flaw in the specimen. Hence, a new model to determine their probability distribution is proposed. This model outperforms standard approaches deriving the probability distribution from the central moments. A spatial correlation of geometric deviations and scatter of the material is investigated with variography subsequently allowing to model the varying properties with random fields. Simulations of dog-bone specimens show that the probability distributions of material properties are captured well. Also simulations of lattice structures are able to represent the probability distributions of their homogenized mechanical properties. The whole stress–strain response and the failure progression agree well with experimental results.