Dog-bone Specimens Research Articles

Fabrication and Mechanical Testing of Mesoscale Specimens

The mechanical response of metallic materials results from a complex hierarchy of deformation mechanisms across length scales. The need to understand these mechanisms independently has driven the miniaturization of testing samples, including small scale samples and single crystal micropillars. However, difficulties in machining and testing small samples have hampered the evaluation of the mechanical response of mesoscale samples with dimensions between tens to hundreds of microns. This paper innovates with a simple approach for the manufacturing and test of dog-bone specimens with a minimum gauge width up to 50 μm. The results demonstrate a pronounced sample size effects on the mechanical response for the dimensions analysed and highlights the need to advance the characterization of mesoscale samples. We also demonstrated the capability of testing the tensile response of single crystals from engineering alloys.

Manufacture and Characterization of Polylactic Acid Filaments Recycled from Real Waste for 3D Printing.

This paper studies the thermal, morphological, and mechanical properties of 3D-printed polylactic acid (PLA) blends of virgin and recycled material in the following proportions: 100/0, 25/75, 50/50, and 75/25, respectively. Real waste, used as recycled content, was shredded and sorted by size without a washing step. Regular dog-bone specimens were 3D printed from filaments, manufactured in a single screw extruder. Thermogravimetric analysis indicated that adding PLA debris to raw material did not significantly impact the thermal stability of the 3D-printed samples and showed that virgin and recycled PLA degraded at almost the same temperature. Differential scanning calorimetry revealed a significant reduction in crystallinity with increasing recycled content. Scanning electron microscopy showed a more homogenous structure for specimens from 100% pure PLA, as well as a more heterogeneous one for PLA blends. The tensile strength of the PLA blends increased by adding more recycled material, from 44.20 ± 2.18 MPa for primary PLA to 52.61 ± 2.28 MPa for the blend with the highest secondary PLA content. However, this study suggests that the mechanical properties of the reprocessed parts and their basic association are unique compared with those made up of virgin material.

Towards optimization of polymer filament tensile test for material extrusion additive manufacturing process

Material extrusion (MEX) is a popular additive manufacturing (AM) method that can process a wide range of feedstock materials, most commonly in filament form. Currently, there is no standardized testing method for filament tensile properties, and researchers resort to 3D-printed dog-bone specimens, which necessarily include the effects of the printing process. In this study, the impact of the strain measurement device, knife-edge type, gage length, testing speed, and oven treatment on filament tensile properties was explored using an off-the-shelf fixture. It was observed that an extensometer with blunt knife edges, a filament gage length of 165 mm, and a 6.35 mm/min (0.25 in./min) testing speed could accurately evaluate the tensile properties of acrylonitrile butadiene styrene (ABS) filaments. In addition, an optimized raster path, 3D printing design, and process parameters were used to manufacture dog bone tensile specimens according to ASTM D638-22 from the same ABS filament spool. The tensile properties of the filaments were validated using the results of 3D-printed dog-bone specimens. Young's modulus, stress at yield, and stress at break for the optimum filament test set (2.20 GPa, 43.9 MPa, and 39.1 MPa) were very similar to those of the 3D-printed specimens (2.26 GPa, 44.9 MPa, and 37.3 MPa). The optimum filament tensile testing parameters determined in this study for ABS can be used for the initial test setup for other filament materials to provide baseline values that can serve as the foundation for AM process development.

Tuning mechanical properties of 3D printed composites with PLA:TPU programmable filaments

Polylactic acid (PLA) filament is widely used for desktop 3D printing purposes due to its exceptional mechanical properties such as high strength; however, its brittleness restricts its use for producing flexible objects. Thermoplastic polyurethane (TPU) filament which is also widely used for desktop 3D printing, on the other hand, is flexible and commonly used in printing compliant objects with relatively low load-bearing performance. This study investigates the ability to tune the mechanical properties of specimens that are printed using programmable filaments composed of PLA and TPU filaments with different volume ratios of PLA and TPU. Two types of PLA and TPU filament arrangements, i.e., series and parallel, are considered. The PLA:TPU programmable filaments are used to print dogbone specimens for tensile testing. In printing the dogbone specimens, the raster angle is varied, i.e., 0, 45, and 90° with respect to the transverse direction of the specimen. To examine their mechanical behaviors based on different PLA and TPU filament arrangements, compositions, and raster angles, tensile tests are conducted on both programmable filaments and dogbone specimens. This study demonstrates the ability to tune the mechanical properties of printed objects by designing programmable filaments and varying raster angles during printing.

Cyclic plasticity and ULCF behavior of steel butt-joints considering different welding methods

In order to reveal the influence of welding method on the cyclic plasticity and ultra-low cycle fatigue (ULCF) behavior of low-alloy butt-joints, three different welding methods, namely, shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and submerged arc welding (SAW), were considered. Dog-bone flat-plate specimens with the welds were cut off from the plates for cyclic tests. The load-carrying capacity, energy dissipation property, and ULCF performance of the specimens were discussed. According to the experimental results, GMAW and SAW lead to the highest and lowest peak loads, respectively. The various welding methods have a slight influence on the equivalent viscos damping (EVD) coefficient of the specimens. Compared with those of the base metal (BM) specimen, ductility of the welded specimens is degraded by 34.62–61.54%, yield strength is deteriorated by 2.86–13.20%, ULCF life is decreased by 71.33%-92.31%. Metallurgical and scanning electron microscope (SEM) analysis were further conducted to reveal the mechanism behind the significantly compromised ULCF behavior of the welded specimens. In-depth numerical analysis was also conducted to reproduce the experimental processes. In the finite element (FE) analysis, inverse calibration technique was applied to determine the Voce-Chaboche (VC) combined hardening constitutive model parameters, and then cyclic void growth model (CVGM) was adopted for fracture simulation.

Green electrochemical polishing of EBM Ti6Al4V samples with preliminary fatigue results

Additive manufacturing technologies have several well-known advantages; however, the surface quality of the printed parts is poor, compromising the corrosion and mechanical properties. Thus, surface finishing treatments are necessary. The most post-processing treatments proposed in the literature are mechanical or chemical. The formers are contact techniques and cannot be suitable for complex geometry, while the latter use strong and dangerous reagents. This paper is aimed to investigate the effect of the electropolishing treatment, a contactless technique, using an eco-friendly electrolytic solution in reducing the roughness and improving the fatigue properties of Ti6Al4V specimens produced by electron beam melting technology. Several parameters affecting the effectiveness of the electropolishing treatment have been investigated, as interelectrode distance, stirring rate and duration time. An optimal combination of them allowed a uniform surface treatment and a significant roughness reduction (64% of Sa reduction for cylindrical specimens). Under sinusoidal loading conditions with a maximum stress of 378 MPa and a stress ratio equal to R = 0.1, the mean fatigue life of electropolished dog-bone specimens was 250% longer than that of as-built specimens.

Uniaxial tensile ductility behavior of ultrahigh-performance concrete based on the mixture design – Partial dependence approach

This study examined how UHPC mixture proportions influence its ductility parameters using random forest models. These parameters were energy absorption capacity (g) and strain at peak stress (εpc) under uniaxial tension. This purpose was achieved using importance analyses of input variables and partial dependence plots (PDPs). The numerical models were developed using the R statistical language and 34 input variables, 600 observation databases, including 50 experimental results. Direct tensile tests were conducted on 46 dogbone specimens to verify the prediction capacity of both the model and the results obtained from the PDP analysis. The tests were carried out in accordance with the JSCE recommendations for eighteen UHPC series resulting from the combination of two cementitious matrices with compressive strengths of 124 and 156 MPa and three-fiber monofiber reinforced systems at three volumetric dosage levels (i.e., 1%,2%, and 3%). The findings indicated that it is more likely to reach exceptional values of the considered responses (εpc ≥0.3% and g ≥50 kJ/m3) when using deformed high-strength steel macro fibers than with another type. Additionally, g rose with the compressive strength value. Nevertheless, the εpc value decreased with the increase of the cementitious matrix compressive strength up to the value of 190 MPa, after which the trend is reversed, and εpc presents exponential growth due to the improvement in the adherence between the matrix and the fiber caused by the enhancement of the matrix microstructure.

Size-dependence of AM Ti–6Al–4V: Experimental characterization and applications in thin-walled structures simulations

Previous studies show that the properties of parts manufactured via additive manufacturing, such as selective laser melting, depend on local feature sizes like lattice wall thickness and strut diameter. Although size dependence has been studied extensively, it was not included in constitutive models for numerical simulations. In this work, flat dog-bone tensile specimens of different thicknesses were manufactured and tested under quasi-static conditions to characterize the size-dependent properties experimentally. It was observed that key mechanical properties decrease with specimen thickness. Through curve-fitting to experimental data, this work provides approximate analytical expressions for the material properties values as a function of specimen thickness, furnishing a phenomenological size-dependent constitutive model. The interpolating capability of the model is cross-validated with existing experimental data. Two numerical examples demonstrate the application of the size-dependent material model. The axial crushing of thin-walled lattices at varying wall thicknesses was simulated by the size-dependent material model and one that ignores size effects. Results show that ignoring size effects leads to overestimated peak crushing force and specific energy absorption. The two material models were also compared in the topology optimization of thin-walled structures. Results show that the size-dependent model leads to a more robust optimized design: having higher energy absorption and sustaining less material fracture.

Biocomposite panels with unidirectional core stiffeners − 3-point bending properties and considerations on 3D printing and extrusion as a manufacturing method

Sandwich panels with unidirectional core stiffeners are known for their relatively high bending stiffness at low weight, stability under compressive and shear loads and energy absorption capability. In this study, 3D printing was used to screen biocomposite sandwich panels easily and preliminarily with different unidirectional core stiffener designs. Thermomechanical pulp (TMP) fibre-reinforced poly(lactic acid) (PLA) was used in this study.A corrugated, trapezoid and arched cell structure were tested experimentally and numerically using a bimodular material model, accounting for different behaviour in tension and compression. The trapezoid structure showed the best flexural properties of the three 3D-printed sandwich beams. It was chosen to be explored further, manufacturing it by extrusion. Extrusion is a production process likely to be used in industry on a larger scale.Basic material properties of the biocomposites were obtained from injection moulded dogbone specimens. The flexural properties of the extruded panels were measured experimentally and simulated using finite element analysis. Simulations were done with a hyperelastic material model. Predictions and experiments were in adequate agreement, allowing such kind of simulation to be used for extruded biocomposite sandwich panels.

Loading rate effects on mixed-mode I/II fracture envelope of epoxy resins with nonlinear behavior

The tangential stress contour (TSC) method in combination with the equivalent material concept (EMC) was used to predict the fracture load, fracture initiation angle, and fracture resistance of an epoxy resin with nonlinear and ductile behavior. In this analysis, both the loading rate and mode mixing effects were taken into account to obtain the fracture envelopes of the tested resin. Based on the considered approach, only a tensile test performed on dogbone specimens and a pure Mode I fracture toughness test using semi-circular bend (SCB) samples are required to predict the ultimate load and crack initiation angle of the resin at different mode mixities. According to this technique, the non-linear ductile behavior of the tested epoxy is simulated by a virtual brittle linear response, which greatly simplifies the problem. The predicted fracture envelope was validated by experimental results. A very good agreement between the prediction and the experimental data was obtained.

A Strain-Gauge-Based Method for the Compensation of Out-of-Plane Motions in 2D Digital Image Correlation

When performing a digital image correlation (DIC) measurement, multi-camera stereo-DIC is generally preferred over single-camera 2D-DIC. Unlike 2D-DIC, stereo-DIC is able to minimise the in-plane strain error that results from out-of-plane motion. This makes 2D-DIC a less viable alternative for strain measurements than stereo-DIC, despite being less financially and computationally expensive. This work, therefore, proposes a strain-gauge-based method for the compensation of errors from out-of-plane motion in 2D-DIC strain measurements on planar specimens. The method was first developed using equations for the theoretical strain error from out-of-plane motions in 2D-DIC and was then applied experimentally in tensile tests to two different dog-bone specimen geometries. The compensation method resulted in a clear reduction in the strain error in 2D-DIC. The strain-gauge-based method thus improves the accuracy of a 2D-DIC measurement, making it a more viable option for performing full-field strain measurements and providing a possible alternative in cases where stereo-DIC is not practical or is unavailable.

Investigating the effect of raster orientation on fracture behavior of 3D-printed ABS specimens under tension-tear loading

Most of the engineering components are servicing under complex loading conditions or have complex geometries that lead to a multiaxial stress state in the component. Thus, the fracture behavior of parts under mixed-mode loading conditions should be investigated. The Fused Deposition Modeling (FDM) is a subcategory of the Additive Manufacturing (AM) techniques that has some important manufacturing parameters. The raster orientation (or raster angle) is one of fabricating parameters that significantly affect the mechanical behavior of fabricated parts. Therefore, this study aims to investigate the fracture behavior of FDM specimens made of Acrylonitrile Butadiene Styrene (ABS) under mixed-mode I/III loading conditions. To this goal, four different raster configurations of ψR = 0/90°, 15/-75°, 30/-60°, and 45/-45° were selected for FDM fabrication of edge cracked rectangular specimens, and five loading angles of β = 0° (mode I), 40°, 65°, 72°, and 90° (mode III) were used for performing mixed-mode I/III fracture experiments on the pre-cracked specimens. Besides, some tensile experiments were performed on dog-bone specimens fabricated with the mentioned raster configurations. In order to theoretically predict the failure loads in the tested cracked specimens, a theoretical model named Equivalent Material Concept (EMC) was utilized for equating the ductile behavior of FDM-ABS material with a brittle one. The mentioned concept was coupled with J-integral (EMC-J) and Maximum Tangential Stress (EMC-MTS) criteria to predict the failure loads under mixed-mode loading conditions. The results showed that both EMC-J and EMC-MTS criteria could predict the experimental failure loads well. Additionally, Scanning Electron Microscopy (SEM) was performed on the fracture surfaces to analyze the failure mechanisms of the tested specimens. SEM analysis confirmed the presence of three failure features namely air gaps, filament pullouts, and hackle regions that were discussed comprehensively.

The role of loading mode on the property degradation of adhesives at high temperatures

AbstractDespite existing studies on the effects of temperature on the behavior of bonded joints, the role of loading mode and test type (strength, fatigue, and fracture tests) on the rate of degradation of adhesive/joint properties at high temperatures has not been adequately investigated. The aim of the current study is to determine if adhesives experience the same rate of degradation at high temperature when they are subjected to different mode mixities. To do so, an epoxy adhesive was subjected to strength, fracture, and S–N fatigue tests, for modes I and II, at different temperatures. The results show that there is a significant difference between the rate of degradation in mode I/tensile and mode II/shear loading conditions. It was also found that the rate of degradation in adhesive properties is a function of test type. Based on the results, by increasing the temperature, a less reduction in tensile strength was found in the Arcan joints (almost 50% reduction) compared with the dogbone specimen (where the strength was reduced by less than 40%). According to the results, for all the considered tests, temperature sensitivity was more noticeable in tensile loading conditions compared with the shear mode. It is shown that despite the reduction in tensile fracture energy with temperature (by around 23%), the shear fracture energy was improved (by over 20%) at 50°C. Based on these results, an S–N model was also developed to create a master S–N curve, plotted as a function of temperature.

Wolf and Dog: What Differences Exist?

A morphological study of the skeletal specimen of Canis lupus L. from an archeological dig of Agnano (Pisa) (Fauna Laboratory, Department of Archaeological Sciences, University of Pisa, Italy) that is chronologically placed in the Wurm period (last glaciation) was done to perform an anatomical comparison between this wild ancestor and osteological specimens of Canis familiaris L. present in the Veterinary Anatomy Museum (University of Pisa). Marked morphological differences in the splanchnocranium (nasal bone, zygomatic arch and orbital angle), neurocranium (sagittal crest) and temporomandibular joint (due to different developments of the masticatory muscles) are highlighted on the wolf specimen compared to those in the domestic dog specimens present in Museum. The appendicular skeletal bones of the wolf show anatomical features similar to those of dog bone specimens, confirming their belonging to the same family (Canidae). This result confirms that domestication has almost exclusively affected the anatomical features of the skull that have changed due to the difference in dietary approach between wolves and dogs.

Characterization of glass fiber reinforced polymer via digital volume correlation: Quantification of strain activity and damage growth

Strain and damage distributions within a dogbone specimen containing a rectangular notch were assessed in-situ via X-ray computed tomography coupled with digital volume correlation. The specimen cut from a continuous glass fiber mat reinforced polyester resin composite plate was subjected to cyclic tensile loading. The strain–damage interplay was evaluated by analyzing major eigen strain and correlation residual fields. The horizontal strained band emanating from the notch root was present from the beginning of loading. The final failure proceeded along this strained band. The first cracks did not initiate in the notched region, despite high strain gradients. Strain activity and damage growth were quantified by analyzing the cumulative probabilities of major eigen strains. The comparison was made between the notched region and the remaining part of the inspected specimen. It is shown that, although the mean levels were higher in the notched region, the standard deviations of the major eigen strain distributions were higher in the remaining part of the specimen.

Studying the compressive, tensile and flexural properties of binary and ternary fiber-reinforced UHPC using experimental, numerical and multi-target digital image correlation methods

Compressive, tensile, and flexural properties of ultra-high-performance concrete (UHPFRC) specimens were studied in this research. Binary and ternary combinations of micro steel (MS), round crimped (RC), crimped (C), hooked-end (H), and polypropylene (PP) fibers were used in overall ratios of 2 % by volume of concrete. For this purpose, 100×200 mm cylindrical specimens, dog-bone specimens (length: 330 mm, width: 80 mm, thickness: 40 mm), and prismatic beams with a dimension of 100×100×500 mm (clear span: 450 mm) were cast and tested under compressive, tensile, and four-point bending tests (4PBT). A digital image correlation (DIC)-based method namely, multi-target digital image correlation (MT-DIC) was used to record the displacement and deflection values in tension and flexure tests. Furthermore, experimental findings were used in numerical simulations and additional analyses were carried out as complementary studies to provide a better understanding of the governing parameters; length, width, depth, and overall size of the beams. Results revealed that a hybrid combination of micro and macro steel fibers performs better than other specimens in all the investigated parameters and the MT-DIC method proved to be a very useful tool in capturing the displacement and deflection values. Furthermore, the inverse analysis approach for the numerical simulation of beams and nonlinear regression-based models captured the direct tension and flexural results with coefficient of determination (R2) values above 0.90.

Correlation Analysis of Established Creep Failure Models through Computational Modelling for SS-304 Material

To maintain safety and reliability in power plants, creep-life prediction models have received much attention over the years. This article was designed to focus on the conditions when a material structure is exposed to extremely high temperatures and pressures with the help of finite element analysis. A direct comparison of the feasibility of different models’ fitness and suitability in predicting creep damage was presented in this article by simulating the damage evolution of a uniaxial SS-304 specimen under a pre-defined load, using established constitutive creep models. Comparative assessments of minimum creep strain rate, creep deformation, and stress rupture were demonstrated using the Norton–Bailey (NB), Kachanov–Rabotnov (KR), Theta projection (TP), and sine-hyperbolic (SH) models while standardizing them with the Omega model. The FE results of a dog-bone specimen, while implementing the models, were compared with the actual creep experiment results to check for the models’ reliability and validation. Subsequently, sensitivity studies of the established creep models were conducted using the statistical tools RSM and ANOVA, with an analysis of how the parameters for operation, design, and material dependency came into effect. Thus, quantitative and qualitative correlation analyses of the FE creep response for these five established models were conducted together, resulting in finalizing the selection of the most suitable model, the sine-hyperbolic model, for the SS-304 material under the defined boundary conditions. The 0.84 R2 value of the sine-hyperbolic model proved the model’s selection for predicting the creep response of stainless steel 304. The method can be applied to select a suitable creep damage model as per the feasibility of the operating conditions.

Characterization of glass fiber reinforced polymer via Digital Volume Correlation: Investigation of notch sensitivity

The results of in-situ cyclic tensile experiments performed on two dogbone specimens made of glass fiber mat reinforced polyester resin are presented (the second specimen contains a machined rectangular notch). The experimental data were obtained by using X-ray Computed Tomography. The reconstructed volumes were analyzed via Digital Volume Correlation. The investigated material was notch-insensitive since both specimens failed at equal stress levels. To further confirm this hypothesis, and to study strain–damage interactions, the major eigen strain and correlation residual maps of both specimens were analyzed. Even in the first loading cycle, an inner strained band extending through the whole ligament area developed within both specimens, already indicating the path to final fracture. It is shown that the final failure of the studied material was primarily driven by the yarn mesostructure, i.e., the influence of the underlying heterogeneities prevailed over the effects due to specimen machining and/or geometric singularity.

Elasto-Plastic Mechanical Modeling of Fused Deposition 3D Printing Materials

Fused deposition modeling (FDM) is one of the most common 3D printing technologies and is becoming a well-established production method. Short fiber-reinforced polymers represent a new class of printing materials that enhance the mechanical properties of final components, thus informing an interesting subject of analysis for this study. FDM-printed parts are characterized by a strong anisotropy, so their behavior should be analyzed accordingly. The authors proposed a modeling strategy based on a transversely isotropic behavior hypothesis, representing material behaviors associated with an elasticity matrix in relation to the elastic field and a combination of Hill's yield criterion and Voce's isotropic hardening law for the plastic field. Material properties of materials were experimentally identified through tensile tests on dog bone specimens printed with different orientations in space. The numerical model was then calibrated using the finite element software LS-DYNA and the optimization software LS-OPT. An agreement between numerical and experimental results showed the robustness of the modeling strategy proposed to describe the stress behaviors of printed materials until a maximum load is reached, while strain behaviors have yet to be correctly defined due to the difficulties associated with evaluating an equivalent deformation.

The effect of process parameters on mechanical characteristics of specimens obtained via DLP additive manufacturing technology

Digital Light Process (DLP) is one of the Additive Manufacturing (AM) methods to produce three-dimensional (3D) polymeric components with high dimensional stability. The technique is based on light-induced polymerization, consisting in spreading the light of a suitable wavelength in a spatially controlled area according to the component’s digital model. Starting from a liquid monomer solution, a 3D solid polymeric object is created. The mechanical and fracture properties of such components are highly influenced by the process parameters which must be carefully considered when load bearing parts have to be produced by AM. This paper investigates the effect of the DLP process parameters on tensile properties using dog bone specimens and fracture toughness determined by using Single Edge Notch Specimens loaded in three-point bending. Five different process parameters were considered by testing six specimens; for each of them, the influence of the printing orientation on the fracture toughness of UV-sensitive resin “translucent green” (curing UV light wavelength 405 nm) is considered. Among the various parameters, the study considers also the post-processing effects on the fracture toughness.