Tensile Specimen Geometry Research Articles

Optimization of a Tapered Specimen Geometry for Short-Term Dynamic Tensile Testing of Continuous Fiber Reinforced Thermoplastics

Continuous fiber reinforced thermoplastics (cFRTP) are one of the most promising lightweight materials. For their use in structural components, reproducible and comparable material values have to be evaluated, especially at high strain rates. Due to their high stiffness and outstanding strength properties, the evaluation of the material behavior at high strain rates is complex. In the presented work, a new tensile specimen geometry for strain rate testing is virtually optimized using a metamodel approach with an artificial neural network. The final specimen design is experimentally validated and compared with rectangular specimen results for a carbon fiber reinforced polycarbonate (CF-PC). The optimized specimen geometry leads to 100% valid test results in experimental validation of cross-ply laminates and reaches 9% higher tensile strength values than the rectangle geometry with applied end tabs at a strain rate of 40 s−1. Through the optimization, comparable material parameters can be efficiently generated for a successful cFRTP strain rate characterization.

Displacement based Simulation and Material Calibration based on Digital Image Correlation Part II - Application

An alternative approach to calibrate material models is proposed. The entire displacement field of a specimen measured with a DIC system was converted into boundary conditions by the Envyo® mapping software. These boundary conditions were imposed on all nodes in the corresponding LS-DYNA® simulation model. In this way a digital twin of the specimen geometry and the experimental kinematics was obtained. Considering the local stress equilibrium over the entire specimen and the equilibrium of global force integrated over many hypothetical cross sections, LS-OPT® was used to iteratively determine the isotropic model parameters of the tested material. A new tensile specimen geometry is introduced to get the most out of a single test. The findings suggest that, at present, convergence in iterative parameter optimization cannot be attained without prior filtering and smoothing of the DIC input data in time and space, despite the utilization of the chosen material model.

Identification of micro‐failure processes of HDPE‐henequen fiber composite material by using acoustic emission monitoring: Effect of fiber surface modification

AbstractIt is well known that fiber‐matrix interface region is crucial to ensure an efficient stress transfer between matrix and reinforcement and that it controls the composite behavior when subjected to external loads. In this experimental study, micro‐damage mechanisms of HDPE/surface modified henequen fiber reinforced composites are investigated in tensile quasi‐static mode. Two essential parameters of the materials composition were taken as main indicators: fiber length and quality of the fiber matrix adhesion. Essential Work of Fracture theory (EWF) was applied on double edge notch tensile (DENT) specimen geometry. Acoustic Emission (AE) technique was coupled to composite samples to obtain the characteristics of stress waves signals to monitor in real time damage evolution and failure mechanisms detection. Results demonstrated the substantial effect of chemical bonds interaction in the interface area on the composite mechanical properties. The total fracture work (Wf) exhibited an increasing value in both components, We and Wp, in chemically treated fiber composites. Likewise, mechanical properties are reported to be higher by 37% in maximum load in such materials. Elastic waves detected by AE were identified and correlated to micromechanical events during composite damage sequence. Most of the signals corresponded to microcracks (signals of low amplitude, short duration), propagation of microcracks (signals of medium amplitude), fracture of fibers and matrix (signals of long duration and high amplitude). AE signals detected exhibiting higher energy were associated to an enhanced fiber/matrix interface.

On Ausferrite Produced in Thin Sections: Stability Assessment through Round and Flat Tensile Specimen Testing

Ductile irons were produced into different casting wall sections, that is, 25 mm, 5 mm and 3 mm. The alloys were then austenitized with the same conditions at 875 °C for 2 h and austempered for three different combinations of temperatures and times: 250 °C for 6 h, 310 °C for 3 h and 380 °C for 1 h. The aim of the investigation was to study the ausferrite stability of austempered ductile irons with three different nominal contents of nickel produced in thin sections through tensile testing. So, strain hardening analysis of tensile flow curves was carried out since it has been found to be a reliable support to ductility analysis in assessing the optimal austempering conditions. Because of different wall sections, round and flat tensile specimens with geometries complying with ASTM E8/E8M-11 were tested. Austempered ductile irons from 5 and 3 mm wall sections were tested through flat geometry specimens only, while 25 mm wall sections were tested through both round and flat geometries. Though the ausferrite was affected by Ni content and the graphite morphology was improved with reduced thin sections, the ausferrite stability and the tensile mechanical behavior were insensitive to Ni content and section thickness below 25 mm. Furthermore, it resulted that the tensile plastic behavior was sensitive to the specimen geometry in a consistent way, increasing the instability of ausferrite and indicating that a proper analysis and comparison of tensile properties of austempered ductile irons must take into account the tensile specimen geometry.

Influence of stress triaxiality on hydrogen assisted ductile damage in an X70 pipeline steel

The presence of hydrogen in steel components affects their structural integrity through a phenomenon called hydrogen embrittlement. While it is known that hydrogen affects the mechanical damage development upon loading, the specific mechanisms are still unclear. An experimental study is presented that investigates the plastic anisotropy and ductile fracture behavior of API 5L X70 pipeline steel with and without hydrogen charging at multiple scales. Three different tensile test specimen geometries (smooth and notched axisymmetric) are employed to investigate the influence of stress triaxiality thereon. The macromechanical responses during tensile tests are analyzed, along with micromechanical features of the resulting damage obtained using High-Resolution X-ray Computed Tomography. For all stress triaxialities tested, the presence of hydrogen does not affect the macroscopic plastic anisotropy, but accelerates ductile damage development and fracture, which is in agreement with the plasticity dominated hydrogen embrittlement mechanisms HELP and HESIV. Increasing stress triaxiality leads to a larger susceptibility to hydrogen embrittlement. Hydrogen accelerated void nucleation and enhanced lateral void growth are observed and quantified. The presented results can aid the development of numerical models describing hydrogen embrittlement in high-strength low-alloy steel.

Derivation and Validation of Linear Elastic Orthotropic Material Properties for Short Fibre Reinforced FLM Parts

Additively manufactured parts play an increasingly important role in structural applications. Fused Layer Modeling (FLM) has gained popularity due to its cost-efficiency and broad choice of materials, among them, short fibre reinforced filaments with high specific stiffness and strength. To design functional FLM parts, adequate material models for simulations are crucial, as these allow for reliable simulation within virtual product development. In this contribution, a new approach to derive FLM material models for short fibre reinforced parts is presented; it is based on simultaneous fitting of the nine orthotropic constants of a linear elastic material model using six specifically conceived tensile specimen geometries with varying build direction and different extrusion path patterns. The approach is applied to a 15 wt.% short carbon-fibre reinforced PETG filament with own experiments, conducted on a Zwick HTM 5020 servo-hydraulic high-speed testing machine. For validation, the displacement behavior of a geometrically more intricate demonstrator part, printed upright, under bending is predicted using simulation and compared to experimental data. The workflow proves stable and functional in calibration and validation. Open research questions are outlined.

Effect of design and tensile testing specimen geometry on final tensile properties of powder bed fusion plastic

In Laser Powder Bed Fusion there are certain considerations that need to be accounted for when designing for thin-walled or complex shapes. Much is known of how parameters such as build orientation and scanning strategy can affect the resultant tensile properties. The tensile property results are also influenced by factors such as the shape of the specimen. The specimens’ cross-sectional geometry and length ratio are carefully considered to obtain accurate and reliable tensile properties. Eighteen different tensile geometries were manufactured using an EOS P110 and PA12 powder. These different geometries were chosen to evaluate different influencing factors such as width, gauge length, specimen geometry and scanning strategy. This knowledge is used in conventional standards when determining specimen geometries. This work aims to combine conventional tensile specimen shape and L-PBF factors to best represent the actual tensile properties of different polymer geometries.

Optimization of Manufacturing Parameters and Tensile Specimen Geometry for Fused Deposition Modeling (FDM) 3D-Printed PETG.

Additive manufacturing provides high design flexibility, but its use is restricted by limited mechanical properties compared to conventional production methods. As technology is still emerging, several approaches exist in the literature for quantifying and improving mechanical properties. In this study, we investigate characterizing materials’ response of additive manufactured structures, specifically by fused deposition modeling (FDM). A comparative analysis is achieved for four different tensile test specimens for polymers based on ASTM D3039 and ISO 527-2 standards. Comparison of specimen geometries is studied with the aid of computations based on the Finite Element Method (FEM). Uniaxial tensile tests are carried out, after a careful examination of different slicing approaches for 3D printing. We emphasize the effects of the chosen slicer parameters on the position of failures in the specimens and propose a simple formalism for measuring effective mechanical properties of 3D-printed structures.

Development of an innovative specimen geometry for tensile split Hopkinson tests

Specimen manufacturing is one of the critical issues to solve during a dynamic material characterization, especially by mean of the Hopkinson bar technique. In particular, concerning tensile testing, the sample is generally axisymmetric with two threaded ends to directly connect it to the Hopkinson bars. In this context, this work presents the development of an innovative sample geometry and related fixtures that can be widely adopted when conventional manufacturing procedures are problematic or when there are numerous specimens. The Tensile Hopkinson Extruded Design Sample (THEDS) geometry requires properly shaped bar ends that can be obtained by wire electrical-discharge machining. The THEDS geometry has been efficiently applied to a test campaign on electron-beam welded copper and niobium specimens. The performances of the developed geometry have first been assessed using digital image correlation (DIC) to compare the results obtained at quasi-static and intermediate velocities with a standard tensile specimen geometry. Secondly, dynamic tests at up to about 1000 s-1 have been performed with a modified Hopkinson bar system at the European Commission (JRC) using the THEDS geometry.

Method for Evaluating Irradiation Effects on Flow Stress in Fe-9%Cr ODS Using TEM In Situ Cantilevers

Transmission electron microscopic (TEM) in situ mechanical testing has become a widely utilized tool for simultaneously measuring mechanical properties and understanding fundamental deformation mechanisms in irradiated and nuclear materials. Although tensile and compression specimen geometries are among the most common, opportunities remain for investigating alternative geometries that could provide unique insights into the plasticity of irradiated materials. This work demonstrates a new TEM in situ cantilever beam configuration. Cantilevers are produced from as-received and proton-irradiated (1 dpa, 500°C) Fe-9%Cr oxide dispersion-strengthened steel. Flow stress is measured using a TEM in situ depth-sensing mechanical testing holder. A 200-MPa increase in flow stress is measured due to irradiation. Size effects arise when the intrinsic (i.e., microstructural) size approaches the extrinsic (i.e., external dimensions) size and can be described using a power law relationship as a function of the material microstructure and cantilever dimensions.

Impact damage of hemispherical shells formed by direct metal laser sintering

Abstract Cellular structures have excellent potential to become alternatives to composites with the advent of 3D printing due their environment robustness, ease of manufacturing, light weight and higher energy absorption capabilities. The behavior of the cellular structures at high impact energies remains largely unexplored. The current work consists of carrying out impact test on hemispherical shells. The hemispherical shells having hexagonal grid patterns are modeled and printed using Direct Metal Laser Sintering (DMLS) with IN718 material. The printed shell structures are then subjected to impact of steel ball using a gas gun for a range of impact velocities and locations on the shells. Tensile test specimen geometries are developed and printed using In718 material to study the material properties. The test results are presented and future scope of work is discussed

Development and Validation of a Miniature Tensile Specimen for Determination of Mechanical Properties

Abstract In this work, a miniature tensile specimen with nominal dimensions of 3.0-mm gage length (GL), 1.5-mm gage width, and 0.5-mm thickness that was carved out of a 10.0-mm-diameter disk has been developed and standardized using analytical and experimental methods. The geometry of the miniature specimen, called ultra sub-size (USS), was optimized using finite element analysis to determine the fillet radius and appropriate geometrical tolerances for specimen dimensions like gage width and thickness. The methods of specimen preparation, gripping, and tensile testing with the use of digital image correlation for strain measurement were standardized. The 0.2 % offset yield strength, ultimate tensile strength, and uniform strain of different materials obtained using this specimen geometry were analyzed and compared with the results of ASTM sub-size (GL: 25 mm) and further sub-size geometry (GL: 12.5 mm). The results of this study show that USS tensile specimen geometry developed in this work can be reliably employed for mechanical property evaluation in situations where tensile testing using standard-size specimens is practically not possible.

Introduction of New Tensile Specimen Geometry for Property Evaluation in Stamped Parts

Abstract Reliable tensile data are crucial in the automotive design process, particularly for the implementation of new Advanced High Strength Steel grades that can be used in new intricate shapes. The property evaluation of specific subsections of a complex part, such as a metal stamping, requires deviation from standard-sized specimens because of material quantity limitations which force the use of smaller specimens. A new tensile geometry is introduced for testing regions within parts that were previously difficult to characterize. However, the specimen size effect is not consistent between different steels, as demonstrated for this new geometry herein. Tensile property measurements taken with noncontact extensometers from specimens of three different sizes from three different sheet steels showed varying material responses to changes in specimen geometry.

On the onset of deformation twinning in the CrFeMnCoNi high-entropy alloy using a novel tensile specimen geometry

Deformation-induced nanoscale twinning is one of the mechanisms responsible for the excellent combination of strength and fracture toughness of the single-phase, face-centered cubic CrMnFeCoNi (Cantor) alloy, especially at cryogenic temperatures. Here, we use a novel, modified dogbone geometry that permits the sampling of varying stress and strain regions within a single tensile specimen to characterize the onset of twinning in CrMnFeCoNi at 293 K, 198 K and 77 K. Electron backscatter diffraction (EBSD) and backscattered electron (BSE) imaging revealed the presence of deformation nano-twins in regions of the samples that had experienced plastic strains of ∼25% at 293 K, ∼16% at 198 K, and ∼8% at 77 K, which are similar to the threshold strains described by Laplanche et al. (Acta Mater. 118, 2016, 152–163). From these strains we estimate that the critical tensile stress for the onset of twinning in this alloy is on the order of 750 MPa.

Application of micro-tensile test for material characterization of mild steel DC01

During recent years, material characterization based on miniature tensile specimens has been investigated extensively. The Small Punch Test (SPT) is often used for determination of tensile properties but alternative miniature tensile specimen geometries have been suggested due to the limitations of the SPT specimen geometry. However, compared to SPT, the Micro-Tensile Test (M-TT) has a significant advantage since it does not need previous established correlations and enables direct results conversion into standard terms. In this paper, the applicability of the M-TT is investigated for material characterization including Lankford coefficients, hardening laws and stress-strain curves. For this purpose, mild steel DC01 M-TT samples were extracted from sheet in Rolling, Transverse and Diagonal Directions. Moreover, M-TT samples were machined to achieve different thicknesses for indicating thickness effects on strain path.

Analysis of thermal history effects on mechanical anisotropy of 3D-printed polymer matrix composites via in situ X-ray tomography

The tensile behavior of an additively manufactured (AM) polymer matrix composite (PMC) is studied with in situ X-ray computed microtomography (CT) and digital volume correlation (DVC). In this experiment, the effects of recycled material content and print direction on the selective laser-sintered (SLS) material’s mechanical response are explored. The PMC samples are printed in a tensile specimen geometry with gage lengths parallel to all three orthogonal, primary sintering directions. In situ tensile-CT experiments are conducted at Argonne National Laboratory’s Advanced Photon Source 2-BM beamline. Analysis of the AM PMC’s tensile response, failure, and strain evolution is analyzed both from a conventional standpoint, using the load–displacement data recorded by the loading fixture, and from a microstructural standpoint by applying DVC analysis to the reconstructed volumes. Significant variations on both strength and ductility are observed from both vantages with respect to print direction and the recycled material content in the printed parts. It is found that the addition of recycled source material with a thermal history reduces the tensile strength of the SLS composite for all directions, but the effect is drastic on the strength in the layering direction.

Benchmark study of failure criteria for ship collision modeling using purpose-designed tensile specimen geometries

Finite element analysis is extensively employed to predict the structural response of marine structures when subjected to operational loads. The quality of the obtained results is to a high degree dependent on the models used to describe the mechanical behavior of the materials involved. The present study reports on an extensive experimental program setup to evaluate constitutive models for shipbuilding materials. Models used for the elasto-plastic behavior, strain rate sensitivity and material failure are presented for a carbon steel sheet. Four failure criteria developed particularly for ship collision modeling are critically assessed: the Equivalent Plastic Strain criterion (EPS), the Germanischer Lloyd criterion (GL), the Rice-Tracey-Crockroft-Latham criterion (RTCL) and the Bressan-Williams-Hill criterion (BWH). To evaluate these failure criteria performance over a wide range of triaxialities, tensile experiments are carried out on six purpose-designed coupon geometries. The plastic strains, strain rates and triaxialities are evaluated using experimental full-field photogrammetric techniques and numerical finite element simulations. Comparison of the experimental and numerical results shows a good performance of the RTCL, BWH and EPS criteria.

Coupon specimen–based approach for the simulation of crossbore stress and strain state

Direct investigation of the stress and strain state of structures with crossbore geometries is typically complex to achieve. Crossbores are often located at the center of reciprocating machinery with few options for access. Thus, the number of studies that have directly characterized these structures is limited and continues to be an experimental challenge. In order to provide an alternative approach for the investigation of material performance in these structures, a coupon specimen is presented as an alternative to direct measurement. A notched tensile specimen geometry is adopted in this study and is designed to match the stress gradient in a representative crossbore intersection. An approach for specimen design is presented and the design is validated using digital image correlation to measure the full-field stress gradient. The proposed design is shown to match the stress gradient to within 10% for the first 50% of the specimen. In addition to the validation of the gradient, an initial set of fatigue experiments is also conducted in order to compare to the modified Smith–Watson–Topper fatigue life approach. Finally, a simulated autofrettage cycle is completed and the resulting residual stress is analyzed by digital image correlation.

Design of an Experimental Program to Assess the Dynamic Fracture Properties of a Dual Phase Automotive Steel

The dynamic behavior of materials must be considered when determining the crashworthiness of a vehicle and the safe design of the vehicle components. Series of mechanical tests at wide ranges of stress and strain rates are essential to identify the material's damage and fracture behavior identical to realistic conditions. In the present contribution, an extensive experimental program has been developed to assess the dynamic fracture properties of dual phase steel (DP-K 1000). Various tensile specimen geometries covering wide range of stress states are employed for testing at quasi static conditions. The limitations imposed on the sample geometries by clamping technique and requirements of high strain rate test based on split Hopkinson bar test principle restrict the use of static geometries for the dynamic range. An optimization approach based on finite element simulations has been adopted to determine the most suitable dimensions for various tensile specimen geometries quantified by stress triaxiality. The possibility of introducing a standard for dynamic sample geometries has also been investigated. Moreover, the influence of transition zone on the deformation of the specimen has been analyzed and incorporated into the optimization strategy. The optimized specimen geometries are finally adopted for dynamic material testing so as to derive enough inputs for constitutive material modeling and to facilitate fundamental material research.

Failure Analysis and Anisotropy Evaluation of 3D-Printed Tensile Test Specimens of Different Geometries and Print Raster Patterns

Anisotropic mechanical properties related to build orientation is a characteristic of parts fabricated with 3D printing technologies. In the development of new materials for 3D printing processes, understanding the effects of 3D printer build orientation and raster pattern on physical property and failure mode differences is extremely important. While there is currently no standard for the evaluation of build orientation-based mechanical performance, such analysis has typically been achieved through the fabrication and scrutiny of tensile and other test coupons which were printed in different build orientations. In some cases, printing specimens in the ZXY (or vertical) build orientation can be difficult due to the capability of a given 3D printer platform. There are also multiple tensile test specimen geometries specified in the ASTM D638 standard for the tensile testing of polymer materials and understanding which specimen geometry works best for 3D printing is not currently well understood. The work presented here explores the effect of tensile test specimen geometry on the anisotropy of mechanical properties related to the build orientation of tensile test specimens. The test coupons were fabricated using a material extrusion 3D printing platform based on fused deposition modeling technology using a grade of acrylonitrile butadiene styrene not typically used in 3D printing in order to simulate the testing of a new material. The effects of raster pattern and the geometric dependence of mechanical property anisotropy were explored, and validation of the use of “faux vertical” specimens in lieu of ZXY-printed specimens was demonstrated. Finally, scanning electron microscopy was used to perform fractography on the various versions of the printed tensile test specimens in order to determine the effect of raster pattern on failure mode.