Standard Tensile Tests Research Articles

Recipe Development and Mechanical Characterization of Carbon Fibre Reinforced Recycled Polypropylene 3D Printing Filament

Recycled polypropylene filaments for fused filament fabrication were investigated with and without 14 wt% short fibre carbon reinforcements. The microstructure and mechanical properties of the filaments and 3D printed specimens were characterized using scanning electron microscopy and standard tensile testing. It was observed that recycled polypropylene filaments with 14 wt% short carbon fibre reinforcement contained pores that were dispersed throughout the microstructure of the filament. A two-stage filament extrusion process was observed to improve the spatial distribution of carbon fibre reinforcement but did not reduce the pores. Recycled polypropylene filaments without reinforcement extruded at high screw speeds above 20 rpm contained a centreline cavity but no spatially distributed pores. However, this cavity is eliminated when extrusion is carried out at screw speeds below 20 rpm. For 3D printed specimens, interlayer cavities were observed larger for specimens printed from 14 wt% carbon fibre reinforced recycled polypropylene than those printed from unreinforced filaments. The values of tensile strength for the filaments were 21.82 MPa and 24.22 MPa, which reduced to 19.72 MPa and 22.70 MPa, respectively, for 3D printed samples using the filaments. Likewise, the young’s modulus of the filaments was 1208.6 MPa and 1412.7 MPa, which reduced to 961.5 MPa and 1352.3 MPa, respectively, for the 3D printed samples. The percentage elongation at failure for the recycled polypropylene filament was 9.83% but reduced to 3.84% for the samples printed with 14 wt% carbon fiber reinforced polypropylene filaments whose elongation to failure was 6.58%. The SEM observations on the fractured tensile test samples showed interlayer gaps between the printed and the adjacent raster layers. These gaps accounted for the reduction in the mechanical properties of the printed parts.

Potential of Date-Seed/Snail Shells as a Carburizer for Enhanced Mechanical Properties of Mild-Steel

The suitability of date-seed/snail shells as a carburizer for enhanced mechanical properties of mild-steel using the packed carburization technique was investigated in this work. Standard tensile, impact and hardness test samples prepared from mild-steel were subjected to pack-carburization process using mixtures of date-seed and snail shell in the ratio 60:40 respectively at 800, 900, and 1 000 °C for 3 hours. The carburized samples were quenched in water at room temperature and further tempered at 300 °C for 30 minutes for residual stress relief of the quenching effect. The mechanical properties and optical microstructure of carburized specimen were performed. Results indicated an enhanced mechanical property of the carburized mild-steel using date-steel/snail shell as a carburizer compared to un-carburized same steel material. The tensile strength and hardness increased with increasing carburizing temperature, though with an associated decrease in ductility. The peak hardness (32.82 HRB) and tensile strength (521 MPa) with equivalent 31.28 and 51.45 percentage increments respectively were obtained at carburizing temperature of 1 000 °C. Hence, using date-seed/snail shell powder as a carburizer can enhance the mechanical properties of mild-steel.

Study on the effects of fiber orientation on the mechanical properties of natural fiber reinforced epoxy composite by finite element method

Natural Fibers being environment friendly and sustainable materials are gaining the interest of scientists, researchers and industrialists all over the world. Natural Fibers composite being lighter and cheaper than the synthetic fiber composites are seen as a scope for advancement in the ever evolving field of material science. Moreover, natural fibers unlike synthetic fibers are biodegradable in nature which draws the eyes of environmentalists towards them in these high times. In the present study, the optimization of most commonly discussed natural fiber reinforced composites such as Banana fiber, Jute fiber, Kenaf fiber, Hemp fiber and Pineapple Leaf fiber (PALF) on the basis of fiber orientation has been discussed. Finite element models of each fiber reinforced epoxy composite was prepared at constant fiber volume percent of 30% with fiber orientation angles of 0°, +22.5°/−22.5°, 45°/−45°, +67.5°/−67.5° and 90° measured from horizontal axis. The models were loaded for different standard loading conditions, i.e., tensile, flexural, and hardness test. From the results it was observed that for tensile and flexural test all the fibers showed minimum stress at the orientation of 0° and 90°. In Brinell harness test, composites with 45o angle of orientation shows the best results.

New procedure for determining the strain hardening behavior of sheet metals at large strains using the curve fitting method

The stress–strain relationship of a sheet metal is conventionally evaluated from the experimental data obtained from the uniaxial tensile test before the onset of localized necking. The identification of the hardening behavior beyond the neck or post-necking behavior is essential for studying material fracture. This study presents a new procedure of identifying the flow stresses of sheet metals at large strains. In this procedure, the experimental data obtained from the uniaxial tensile test and the measured forming limit at a plane–strain tension mode FLC0 are adopted in the curve fitting procedure for calibrating the parameters of different hardening laws. The measured FLC0 is introduced in the cost function of the fitting to provide an additional constraint for the hardening parameters in large strain ranges. Using the uniaxial tensile test data ensures the accuracy of the pre-necking description of the hardening. The proposed method decreases the variation of the predicted flow stresses beyond the uniform elongation for different hardening laws. For validation purposes, the developed approach is applied to calibrate the parameters of the Kim–Tuan and the linear combined Swift/Voce hardening models for three sheet metals tested in Numisheet (2014) benchmarks. The hardening laws with newly identified parameters are implemented into the finite element software ABAQUS/Explicit to simulate the standard uniaxial tensile and reverse drawing tests. The comparative results show that the simulated tensile force in the uniaxial tensile tests and the simulated punch force in the reverse drawing tests agree well with the experiments, thereby validating the accuracy and the efficiency of the proposed identification procedure of large strain hardening in sheet metals.

CAPTURING OF UNEVEN DEFORMATIONS OF LIGHTENED 3D PRINTED PARTS

This paper deals with the problematics of tensile testing of maraging steel lightweight cylindrical samples with internal structures, intended for automotive application. The samples were made with DMLS technology and tensile tested under the static strain rate of 0.002 s-1. During the loading, uneven deformations and multiple necking occurred on the samples. The mentioned effect significantly affects the state of stress and deformation at the final fracture zone, as well as the total deformation into the fracture, which means that it actually distorts the standard tensile test record. Therefore, a methodology for capturing the deformation response using the ARAMIS optical system was proposed. The methodology is presented and verified by a set of experiments for BCC internal structure.

Effects of the corrosion pitting parameters on the mechanical properties of corroded steel

To study the effects of corrosion on the mechanical property of corroded steel, the standard monotonic tensile tests and corresponding finite element (FE) simulations with a modified fracture criterion to analyze the ductility degradation and fracture behaviour are systematically conducted on corroded steel specimens. The results showed that the degeneration of the mechanical property of corroded steel is mainly affected by the presence of single or double critical pits, the pit sizes (a or c), as well as the spatial distance (S) between the adjacent critical pits; the spatial parameter must be ignored during the interaction of the adjacent critical pits. The strength and deformation of the corroded steel rapidly degenerated because the critical pit led to the prematurely initiated ductile cracks, which were enhanced by the presence of double critical pits, as their interactions facilitated a faster accumulation of local plastic strains on the specimen; The initiation and propagation of local ductile cracks were closely associated with the critical pits because of the loss of sectional stiffness near the critical pits. By increasing the pit size and reducing the spatial parameter, the effect of the critical pit on local ductile crack evolution gradually increased, and the stress triaxiality was continuously elevated; thus, the equivalent deformation capacity defined by the equivalent plastic fracture strain (εplf) was reduced. Finally, an all-encomplete concept, including fracture initiation and propagation, was employed with the FE analysis to distinguish between the contributions of different pitting parameters on the ductility degradation of corroded steel.

Structural failure criterion for spatially-reinforced carbon-carbon composite materials

This paper defines the structural strength criterion for 4DL-reinforced carbon-carbon materials. For this scheme, fiber reinforcement consists of four groups of reinforcing elements, three of them are located in parallel planes with the angles of 120° between them and the fourth one is normal to them. The paper addresses the first failure of the material corresponding to its yield stress, in this point, one of the material components deviates from linear elastic behavior. A composite material is considered to be non-uniform structurally and consists of a matrix and reinforcing elements, rods. Those rods, in their turn, represent a unidirectional composite. To analyze the stress-strain state of individual components of the material, a three-level elastic model is built that uses the analytic approach at the micro level, while at higher levels it uses the finite element method. For numerical calculations, a structural cell of the material is taken. The boundary conditions provide small to negligible influence of the edge effects, thus simulating the behavior of the infinite volume of the material. For the material components, local strength criteria are introduced, where the fields of the criterion quantities are averaged over the volume of the structural cell. The strength surface of the material that corresponds to its first failure is obtained, and the conclusion is made that the suggested criterion provides a reasonable agreement with the available data on the typical carbon-carbon composite characteristics. Based on the calculated dependencies of the material’s yield stress on the load direction, a procedure is suggested to identify the model parameters based on the material failure behavior analysis using standard tensile and compressive tests. Estimated discrepancies between the results calculated using the suggested criterion and those obtained using the limiting stress criterion for biaxial stress states are given. It is shown that the discrepancy may reach tens of percent and in some cases the material strength increases in comparison with that in the uniaxial stress state. The results are subject to verification tests in order to verify the model for advanced spatially reinforced carbon-carbon composite materials.

Prediction of Failure in Ceramic Matrix Composites Using Damage-Based Failure Criterion

This paper presents a damage-based failure criterion and its implementation in order to predict failure in ceramic matrix composites (CMC) manufactured via filament winding. The material behavior of CMCs is anisotropic and strongly depends on the angle between fiber orientation and loading direction. The inelastic behavior of laminates with different fiber orientations under tension and shear is modeled with the help of continuum damage mechanics. The parameters required for the damage model are obtained from a standard tensile and shear test. An isotropic damage law determines the evolution of damage in thermodynamic space and considers the interaction of damage parameters in different principal material directions. A quadratic damage-based failure criterion inspired by the Tsai-Wu failure criterion is proposed. Failure stress and strain can be predicted with higher accuracy compared to the Tsai-Wu failure criterion in stress- or strain-space. The use of the proposed damage limits allows designing a CMC component based on the microstructural phenomenon of stiffness loss. With the help of results obtained from modeling and experiments, fracture mechanics during the Iosipescu-shear test of CMCs and its capability to determine the shear strength of the material is discussed.

Single hair fiber assessment techniques to discriminate between mineral oil and coconut oil effect on hair physical properties.

To utilize a matrix of single-fiber hair testing methodologies to mechanistically understand the impact of common oiling treatments-coconut oil and mineral oil-on hair strands. Further, the effect of hair twisting-experienced in everyday grooming practices-on hair strength was investigated under different scenarios. Study involved multiple surfactant wash cycles of hair swatches with and without overnight hair oil treatments. Instrumental testing was done on strands from hair swatches-Tensile Extension, Torsional Stretching, and Tensile Extension of twisted hair fibers. Differentiation was observed in tensile and torsional testing parameters with 20 wash cycles, while no statistical significance was observed in single wash. However, when we combine the two stresses together by extending the twisted hair strands, a clear differentiation was seen even in single cycle for coconut oil in comparison with mineral oil and surfactant wash. The differentiation in tensile parameters for twisted fibers becomes much more prominent with multiple cycles. Penetration of coconut oil in hair strands makes the fiber core more flexible and thus helps negotiate the torsional stress at the time of extension. Product benefit discrimination in single-strand testing can be amplified by combining multiple stresses in one testing methodology. Observing the consumer habits and incorporating the torsion component in standard tensile testing of hair helps differentiate the two commonly used hair oiling treatments. Coconut oil was found to significantly increase the tensile strength of twisted fibers owing to its penetration inside hair core.

The Failure of Carbon Fiber Reinforced Composite Analyzed by Acoustic Emission

This paper focus on evaluation of behavior, damage and failure processes occurring during the loading cycles in the carbon fiber reinforced composite by acoustic emission method. Since acoustic emission is physical phenomenon that detects the released energy in form of waves spreading through the material due to stimulation of material by external or internal stress, it is possible to evaluate complex damage and failure processes. For that purpose, the standard and open holes tensile testing specimens with different number of plies were manufactured. Selected acoustic emission parameters were correlated with data obtained from mechanical test. Linear localization method together with signal analysis using Fast Fourier transform algorithm were used as another tool for detection and evaluation of spreading damage processes inside the composite during the load. Basic damage types inside the composite material were identified by metallographic analysis using light microscopy. More complex damage processes were observed on the fracture surface using stereomicroscopy and scanning electron microscopy.

Mechanical properties and modelling of superior high-performance steel at elevated temperatures

Steel structures have been increasingly applied in contemporary infrastructures, which always require high loading capacities and strong resistance to disasters such as earthquake actions, fire and environmental corrosion, etc. Recently a superior high-performance (SHP) steel is developed accordingly, possessing combined beneficial features of high-strength, high-ductility, corrosion-resistance and fire-resistance, which may well satisfy the structural requirements. The present study aims to investigate mechanical properties of the SHP steel at elevated temperatures and to propose its constitutive models for further usage in fire design and analyses of structural members. A comprehensive experimental programme including a series of standard tensile coupon tests at both ambient and elevated temperatures is described herein, and key mechanical properties including modulus of elasticity, yield strength, ultimate tensile stress as well as stress-strain curves of the SHP steels are obtained based on the test results. Reduction effects of elevated temperatures on their mechanical properties are quantitatively clarified, and further comparisons with prediction results in accordance with national standards as well as with independent test results reported in literature are made. It is indicated that the mechanical properties of the SHP steels differ significantly from that of conventional mild steel and fire-resistant steel. Specific prediction equations are proposed for evaluating the temperature-dependent mechanical properties of the SHP steel; furthermore, an adapted Ramberg-Osgood model is developed for describing its full range stress-strain relation. The research outcomes of the present research may serve as an essential basis for better understanding mechanical performance of the SHP steel at elevated temperatures as well as for further investigations on its structural members' behaviour; and it is helpful to promote its application in practice.

Mechanical behaviour of double side high performance PSA adhesive applied to painted naval structures

The use of adhesives constitutes a well-established technology in the aeronautical and automotive industries. A rising interest in the use of these materials has appeared in naval industry, where using adhesives in non-structural areas implies the reduction of welding to fix low weight components, making the joining process cheaper and faster. Among the different families of available adhesives, double-sided Pressure Sensitive Adhesives (PSAs) are considered of great industrial interest. A high performance PSA has been employed to join specimens of carbon steel coated with an epoxy painting scheme approved by the naval sector. The present paper reports for the first time the influence of some experimental application variables of this PSA-coated naval steel system on its mechanical behaviour. Standard shear and tensile tests have shown that the curing conditions, surface preparation and paint roughness have considerable effects on the resistance of these adhesive joints.

Thermoset powder coating wastes as filler in LDPE – Characterization of mechanical, thermal and morphological properties

In this study, three different electrostatic powder coating wastes of thermoset structure were used as fillers for low-density polyethylene (LDPE). The mechanical, thermal, and morphological properties of these composites materials are reported. The thermoset powder coating wastes were hydrolyzed using water and alcohol, and then 10%, 20%, and 30% by weight of the deactivated wastes were mixed with LDPE, first in a mechanical dry mixer, and subsequently homogenized by melt compounding in an extruder at about 160 °C. Standard tensile test bars and plate-shaped samples were produced from these homogeneous compounds by injection molding. The effects of adding hydrolyzed powder coating wastes on the properties of LDPE were determined by mechanical, thermal, and morphological tests. Tensile strength and perforation threshold of the sample filled with powder coating wastes decreased to some extent compared to un-filled sample. However, an increase in izod impact and bending strengths has been observed. The results of this investigation demonstrate that powder coating wastes with thermoset structure can be used as fillers in LDPE.

Effect of surface roughness on tensile ductility of artificially corroded steel plates

Physically, corrosion damage results in a reduction in thickness and a change in surface geometry due to differential corrosion rates on the surface of a steel member. The effects of thickness reduction on the mechanical characteristics of structural steel have been investigated intensively, but the same cannot be said for the surface configuration. This study investigated the variation of corrosion thickness and surface roughness characteristics with increasing corrosion degradation and their contribution to a reduction in the tensile ductility of corroded steel. Twenty-two SS400 type steel plates were corroded by an underwater accelerated exposure (bubbling) test. Specimens were exposed for a varying number of days after which measurement and evaluation of thickness reduction and surface roughness characteristics were conducted. Then, the remaining load and elongation capacity were determined experimentally from the standard uniaxial tensile test. The distribution of the surface strains was studied employing digital image correlation techniques. Findings from this work revealed that surface unevenness due to corrosion causes stress concentration and uneven yielding, and strain localization after yielding during the tensile test. This translated to an early attainment of maximum load, resulting in reduced ductility in corroded specimens. A general trend is such that ductility decreases with increasing surface roughness. A plastic ductility loss of up to 80% is experienced when maximum surface height ratio, ζsz ≥ 0.9 and average surface height ratio, ζsa ≥ 0.15. Finally, empirical equations for estimating the remaining tensile ductility of corroded steel based on surface roughness characteristics were proposed.

Phenomenon of Spall Fracture during Tension

The phenomenon of spall fracture of high-strength steel specimens with the formation of two fracture surfaces during standard tensile tests is discussed. The fracture along two planes is shown to occur not simultaneously: first, fracture induced by a tensile load is observed, and then a second fracture plane forms, presumably, according to the spallation mechanism. The sequence proposed for the two fractures is confirmed by the results of fractographic analysis. The spallation during tension of standard specimens is found to differ from the classical spallation under an intense external action in the scheme of deformation wave formation and the source of spallation energy.

A new approach to evaluate the elastic modulus of metallic foils

The accurate determination of the elastic properties is non-trivial for metallic foils. The measured elastic modulus is often described in literature as significantly smaller than the respective modulus of the bulk counterparts. This paper describes a straight forward way to minimize the influence of the measurement on the calculated values using a standard tensile testing device combined with a laser speckle extensometer and advanced data evaluation. The elastic modulus obtained with a monotonic tensile procedure is compared to values obtained from multiple loading–unloading cycles in the elastic regime. The latter were found to lead to more reproducible and reasonable values with significantly smaller standard deviations. To enable interpretation of the modulus values electron backscatter diffraction and X-ray diffraction were used to emphasize the effects of texture and microstructure. Two different electrodeposited copper foils were characterized since copper is widely used in many industrial applications.

A virtual fields method for identifying anisotropic elastic constants of fiber reinforced composites using a single tension test: Theory and validation

A novel inverse identification method using only a single tensile specimen for obtaining four anisotropic elastic constants of fiber reinforced composites sheet is proposed in this study. The method is based on virtual fields method (VFM) for solving weak form equilibrium equation with an optimized tension-type non-standard specimen specially designed with an elliptic hole at the center of the specimen (though not limited to this special shape). Full-field strain measured by a digital image correlation (DIC) technique is applied as experimental strain fields which are used as inputs of the VFM. For optimizing the tension-type VFM specimen, finite element (FE) analysis is conducted as a virtual experiment to generate ideal strain fields. The determined tension-type VFM specimen has an elliptic hole with 45° inclined to the loading direction, which is optimized from the error analysis between the FE input and VFM calculated material constants. For the validation of the proposed approach, experiments are performed with unidirectional carbon fiber reinforced plastic (UD-CFRP) sheets. The comparison of VFM identified anisotropic constants to those determined from the standard tensile tests with three different orientated samples shows satisfactory accuracy ranging between −5.94 and 0.71%, errors which confirm the validity of the new inverse method for the characterization of anisotropic elasticity of composites sheets.

Determination of a full range constitutive model for high strength S690 steels

In order to enable accurate analysis and prediction on structural behaviour of high strength S690 steel member at large deformations, an integrated experimental, theoretical and numerical investigation to formulate a full range constitutive model is reported in this paper. It is well known that standard tensile tests are effective means to obtain key mechanical properties of coupons for deformations up to on-set of necking. However, owing to non-uniform cross-sectional distributions in both stresses and strains within the necking regions of the coupons, it is important to allow for these effects when analysing measured forces and extensions to give true stresses and strains of the coupons.The proposed instantaneous area method has been developed and calibrated against test results of a total of 30 standard tensile tests on S690 cylindrical coupons, and it is formulated with key mechanical properties of engineering stress-strain curves readily obtained from these tests. High resolution digital images of the deformed coupons are captured and analysed to obtain instantaneous dimensions of the necking areas. Through theoretical formulation, true stress-strain curves of the coupons after on-set of necking are derived, and modified with numerical correction factors through successive approximations. After normalization on all the corrected true stress-strain curves, a set of formulae to describe the full-range constitutive model for the high strength S690 steels is proposed. Consequently, the proposed model is readily applicable to both analytical and numerical analyses of the high strength S690 steels and their structural members undergoing both small and large deformations up to fracture.

Single sample method for assessing the Baushinger effect

The Bauschinger effect is one of the fundamental properties of most metal alloys exposed to plastic deformation under non-monotonic loading. Development of the methods for quantifying this effect is one the important issues of the theory of plasticity. Calculation of the parameter characterizing the aforementioned effect is required for determination of the stress state in plastically deformable blanks upon pressure metal treatment. The value of the parameter (determined in standard tensile tests followed by subsequent compression of samples) is defined by the ratio of the conditional yield strength of the sample under compression to the value of the preliminary tensile stress. A series of cylindrical samples (~10 pcs.) is usually taken for tensile-compression tests. According to the traditional procedure, long-size standard specimens are pre-stretched to various degrees of plastic deformation. After that short specimens are cut out from those specimens for compression tests to determine the conditional compressive yield strength with a tolerance of 0.2% for plastic deformation. Such a procedure is rather time consuming and expensive. We propose and develop a new single-model method for estimating the Bauschinger effect which consists in testing of a single long-size specimen for tension followed by compression of the specimen in a special device providing deformation of a previously stretched specimen without flexure under conditions of a linear stress state. The device was designed, manufactured and underwent the appropriate tests. The device contains supporting elements in the form of conical-shaped sectors that prevent flexure of a long cylindrical specimen upon compression, a ratio of the working part length to diameter ranges from 5 to 10. The results of experimental determination of the parameter β characterizing the indicated effect are presented. The results of comparing the values of the parameter β determined by the developed and traditional methods revealed the possibility of determining the parameter β using the proposed method. To reduce the complexity of performing tests related to determination of the parameter β we approximated it in the form of an exponent as a function of the magnitude of plastic deformation and determine the only one value of β0 under plastic deformations exceeding 0.05. In this regard, β0 can be considered a new characteristic of the material. The calculated data are in good agreement with the experimental results. The values of β0 are determined for a number of studied steel grades.

Impact of atmospheric corrosion on the mechanical properties of B235 steel rods

It is well known that atmospheric corrosion greatly affects the stress-strain properties of reinforcing steel. To be able to assess and forecast the remaining service life of constructions which have been subjected to atmospheric corrosion, it is necessary to have good knowledge of this effect. Research is often complicated to do due to the lack of accurate information on the initial dimensions of the steel parts and the properties of the material of which those parts are made. In this paper an attempt is made to overcome this obstacle. Reinforcing steel rods are tested. Those rods have been subjected to natural atmospheric conditions in a temperate climate zone for 25 years, in vertical orientation. To determine the initial stress-percentage extension curve of those rods test pieces have been made by machining on a lathe – to remove the corrosion layer and to set a standard diameter. The chemical composition and the density of the material are determined by appropriate experiments. The yield strength, the ultimate strength, the percentage elongation after fracture are determined by standard tensile testing. To determine the final stress-percentage extension curve, tensile tests are carried out with the corroded rods without any treatment. A comparison of both curves and analysis of the atmospheric corrosion effect on the stress-strain behavior is made.