Gauge Section Of Specimens Research Articles

Grain refinement and superplastic flow in friction stir processed Ti–15V–3Cr–3Sn–3Al alloy

The rolled Ti–15V–3Cr–3Sn–3Al (Ti-15-3) alloy (metastable β titanium alloy) sheet with an average grain size of 44.0 μm was subjected to friction stir processing (FSP) at a tool rotation speed of 250 rpm and a tool traverse speed of 100 mm/min (250–100). Thereafter, a fine-grained (∼6.6 μm) and relatively equiaxed microstructure with a high angle grain boundary (HAGB) ratio of 74.5% was observed in the stir zone (SZ). Superplastic tensile tests were then conducted on this microstructure at the temperatures ranging from 600 °C to 800 °C and strain rates range of 1 × 10−4-1 × 10−2 s−1, and an excellent low-temperature superplasticity (LTSP) with the elongation of 463% was obtained at 650 °C and 1 × 10−4 s−1. In addition, the microstructure in the gauge section of the tensile specimens interrupted at different engineering strains of 20%, 50%, 200%, and 463% (tensile fractured) at the optimal superplastic tensile condition of 650 °C and 1 × 10−4 s−1 was studied. It was found that the precipitated α phase increased with the increasing strain, which contributed to the achievement of an enhanced LTSP by inhibiting the grain growth. Moreover, the α grains with a finer grain size of 4.4 μm was observed in the gauge section of the tensile fractured specimen and this was attributed to the occurrence of continuous dynamic recrystallization (CDRX). Therefore, the superplastic deformation mechanism of the Ti-15-3 alloy is recognized as grain boundaries sliding (GBS) accompanied with dislocation movement and CDRX at 650 °C and 1 × 10−4 s−1.

Correction of specimen strain measurement in Kolsky tension bar experiments on work-hardening materials

Cylindrical dog-bone (or dumbbell) shaped samples have become a common design for dynamic tensile tests of ductile materials with a Kolsky tension bar. When a direct measurement of displacement between the bar ends is used to calculate the specimen strain, the actual strain in the specimen gage section is overestimated due to strain in the specimen shoulder and needs to be corrected. The currently available correction method works well for elastic-perfectly plastic materials but may not be applicable to materials that exhibit significant work-hardening behavior. In this study, we developed a new specimen strain correction method for materials possessing an elastic-plastic with linear work-hardening stress–strain response. A Kolsky tension bar test of a Fe-49Co-2V alloy (known by trade names Hiperco and Permendur) was used to demonstrate the new specimen strain correction method. This new correction method was also used to correct specimen strains in Kolsky tension bar experiments on two other materials: 4140 alloy, and 304L-VAR stainless steel, which had different work-hardening behavior.

Interrupted fatigue testing with periodic tomography to monitor porosity defects in wire + arc additive manufactured Ti-6Al-4V

Porosity defects remain a challenge to the structural integrity of additive manufactured materials, particularly for parts under fatigue loading applications. Although the wire + arc additive manufactured Ti-6Al-4 V builds are typically fully dense, occurrences of isolated pores may not be completely avoided due to feedstock contamination. This study used contaminated wires to build the gauge section of fatigue specimens to purposely introduce spherical gas pores in the size range of 120–250 micrometres. Changes in the defect morphology were monitored via interrupted fatigue testing with periodic X-ray computed tomography (CT) scanning. Prior to specimen failure, the near surface pores grew by approximately a factor of 2 and tortuous fatigue cracks were initiated and propagated towards the nearest free surface. Elastic-plastic finite element analysis showed cyclic plastic deformation at the pore root as a result of stress concentration; consequently for an applied tension-tension cyclic stress (stress ratio 0.1), the local stress at the pore root became a tension-compression nature (local stress ratio −1.0). Fatigue life was predicted using the notch fatigue approach and validated with experimental test results.

Criticality of porosity defects on the fatigue performance of wire + arc additive manufactured titanium alloy

This study was aimed at investigating the effect of internal porosity on the fatigue strength of wire + arc additive manufactured titanium alloy (WAAM Ti-6Al-4V). Unlike similar titanium alloys built by the powder bed fusion processes, WAAM Ti-6Al-4V seldom contains gas pores. However, feedstock may get contaminated that may cause pores of considerable size in the built materials. Two types of specimens were tested: (1) control group without porosity referred to as reference specimens; (2) designed porosity group using contaminated wires to build the specimen gauge section, referred to as porosity specimens. Test results have shown that static strength of the two groups was comparable, but the elongation in porosity group was reduced by 60% and its fatigue strength was 33% lower than the control group. The stress intensity factor range of the crack initiating pore calculated by Murakami’s approach has provided good correlation with the fatigue life. The kink point on the data fitting curve corresponds well with the threshold value of the stress intensity factor range found in the literature. For predicting the fatigue limit, a modified Kitagawa-Takahashi diagram was proposed consisting of three regions depending on porosity size. Critical pore diameter was found to be about 100 µm.

Cu addition effects on TRIP to TWIP transition and tensile property improvement of ultra-high-strength austenitic high-Mn steels

Austenitic high-Mn steels have been nominated as desirable ultra-high-strength cold-rolled steels whose mechanical properties are greatly improved by powerful deformation mechanisms of transformation- and twinning-induced plasticity (TRIP and TWIP). In this study, an austenitic high-Mn TRIP steel was suggested to achieve a good strength-ductility balance, and 1–2 wt.% Cu was added as an element for increasing stacking fault energy (SFE) as well as an austenite stabilizer to exploit a transition from TRIP to TWIP. The non-Cu-added steel showed the highest yield and tensile strengths (502 MPa and 1137 MPa, respectively) and the lowest elongation (34.6%) with a serrated flow. Yield and tensile strengths decreased with increasing Cu content, while the elongation was the highest in the 1%-Cu-added steel. TRIP and TWIP mechanisms showed good agreements with calculated SFEs in consideration of (Mn,Cu)-segregated bands. In the non-Cu-added steel, the TRIP occurred step by step as localized deformation bands passed through the specimen gage section to activate the serrated flow, which were reduced (or improved) by the transition from TRIP to TWIP with increasing Cu content. In the 1%-Cu-added steel, overall tensile properties were improved (yield strength; 461 MPa, tensile strength; 1093 MPa, elongation; 65.1%) as both TRIP and TWIP were well homogenized to produce synergic effects.

Effects of solute segregation on tensile properties and serration behavior in ultra-high-strength high-Mn TRIP steels

Austenitic high-Mn TWinning- and Transformation-Induced Plasticity (TWIP and TRIP) steels are strong candidates for GPa-grade cold-rolled steel sheets. The reduction in C or Mn content from high-Mn TWIP steels help generate a TRIP mechanism and prevent serration. However, these high-Mn TRIP steels show low yield strength because of the inherent characteristics of austenite, and often contain a band-shaped segregation of solutes, making the steels acts as hetero-structural materials. Therefore, in this study, we investigate the effects of compositionally-segregated microstructures on tensile properties and serration behavior in precipitation-hardened high-Mn TRIP steels. The present TRIP steels showed high yield strength (778–824 MPa) and an excellent strength-ductility balance, along with serration in their stress-strain curves which could not be explained by existing theories of dynamic strain aging. A considerable amount of martensite was formed step by step as localized deformation bands passed through the specimen gage section, which implied that the serration occurred only when the transformation rate increased substantially. In microstructural aspects, the martensitic transformation occurred sequentially along Mn-segregated bands due to differences in austenite stability and Mn content between high- and low-Mn bands, thereby leading to discontinuous transformation and consequently the serrated flow.

Identification of the role of Al-Fe-Mn-Si large casting dispersoids in age-hardenable aluminum alloys using small angle X-ray scattering

Aluminum flat-stock incorporates large dispersoids to facilitate grain-size refinement upon casting and to prevent galling in downstream operation. Industrial-practice has reduced the deleterious effects of stringers and the focus for cost-effective Al-Mg-Si alloys has been the characterization of nano-precipitates to attain optimum strength conditions. Such studies record the presence of large intermetallic particles but their role during work-hardening is ignored. Constitutive relation analyses (CRA) which can replicate the stress-strain diagram have revealed that parabolic hardening beyond yield point elongation is due to these particles. At maximum matrix strengthening by artificial ageing, the enforced strains around these non-shearable µm-sized particles are accommodated by rotated structures; that is, dislocation arrays leading to increased strain hardening. The spreading of lattice orientations due to these events enhance the occurrence of double Bragg scattering (DBS) detected by small angle X-ray scattering (SAXS). Separating the anisotropic SAXS intensity from the total intensity, the role of layered precipitates can be differentiated from that of DBS by comparing data from the undeformed grip to that from the deformed gauge section of tensile specimens aged for various times. Although the presence of layered precipitates will be similar for grip and gauge sections, the lattice rotation inherent during deformation may alter the SAXS response. Moreover the variation of anisotropic SAXS data (streaking) with artificial ageing times is attributed to the evolution of epitaxial nano-precipitation on the dispersoids interfaces.

Strain evaluation using a non-contact deformation measurement system in tensile tests of irradiated F82H and 9cr ODS steels

We developed a non-contact deformation measurement system to accurately evaluate strain for post irradiation tensile testing, since conventional strain gages cannot be used for small size specimens. The strain calculated from cross-head displacement generally includes deformation from specimen shoulders, fixtures, and the test frame in addition to the deformation from the specimen gauge section. In our system, the distance between painted marks within the specimen gauge section was measured using a high resolution video camera to evaluate the specimen deformation during room temperature tensile testing. The test materials were F82H and 9Cr ODS steels irradiated up to ≈71 displacements per atom (dpa) at about 573 K in High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL). Our system yielded accurate stress strain curves without deformations other than the specimen gage section, and the elongation was less than that calculated from cross-head displacement. This system can contribute to expanding the technically reliable database for the design activity of fusion reactor blanket, including the effects of irradiation on tensile properties.

Hoop tensile strength of tubular carbon fiber reinforced silicon carbide matrix composites

C/SiC (carbon fiber-reinforced silicon carbide) composites are widely used in the aerospace industry and for the fabrication of high-temperature parts owing to their excellent heat and chemical resistances. In this study, the tensile properties of C/SiC composite tubes manufactured by a filament winding method were examined via hoop tensile testing, and the optimal testing conditions were determined by comparing them with the properties of a composite plate specimen. It was found that during tensile testing, bending stress was generated and the strain inside and outside of the gauge section of the composite specimen is shown as tensile and compressive, respectively; as a result, premature damage occurred, which was not observed for the plate specimen. In order to minimize this effect, tensile testing was performed under the improved conditions, at which the side of fixture was close to the side of the test specimen, and the distance between the upper and lower fixtures was small. However, the relatively low strain to failure (which is a unique characteristic of C/SiC composites) suppressed the induction of both strain inside and outside the specimen in the tensile direction, which led to lower tensile strengths of the composite tubes as compared to that of the plate specimen. Because the tensile modulus calculated in this study is very close to that of the plate specimen, the hoop tensile testing method is suitable for determining the modulus values of C/SiC composites.

Effects of {10–12} Twins on Dynamic Torsional Properties of Extruded AZ31 Magnesium Alloy

Effects of initial twins on dynamic torsional properties of extruded AZ31 alloy were investigated by introducing {10–12} twins into it through precompression to 3 and 6% strains along the extrusion direction and performing torsional testing at a strain rate of 1.4 × 103 s−1 using a torsional Kolsky bar system. The as-extruded sample without twins showed higher dynamic torsional properties than the precompressed samples with many initial twins; the maximum shear strength and fracture shear strain decreased with increasing amount of initial twins. In the as-extruded sample, twinning occurred vigorously throughout the gage section of the tubular specimen during high-strain-rate torsional tests, resulting in heavily deformed morphology, many macrocracks, and rough fractured surfaces. The increased amount of initial twins suppressed the twinning behavior and localized the applied torsional deformation; this resulted in an almost unchanged sample shape, no secondary cracks, and a flat fracture plane, thereby deteriorating the dynamic torsional properties of the extruded alloy.

In situ analysis of damage evolution in an Al/$$\hbox {Al}_{2}\hbox {O}_{3}$$Al2O3 MMC under tensile load by synchrotron X-ray refraction imaging

The in situ analysis of the damage evolution in a metal matrix composite (MMC) using synchrotron X-ray refraction radiography (SXRR) is presented. The investigated material is an Al alloy (6061)/10 vol $$\%$$ $$\hbox {Al}_{2}\hbox {O}_{3}$$ MMC after T6 heat treatment. In an interrupted tensile test the gauge section of dog bone-shaped specimens is imaged in different states of tensile loading. On the basis of the SXRR images, the relative change of the specific surface (proportional to the amount of damage) in the course of tensile loading was analyzed. It could be shown that the damage can be detected by SXRR already at a stage of tensile loading, in which no observation of damage is possible with radiographic absorption-based imaging methods. Moreover, the quantitative analysis of the SXRR images reveals that the amount of damage increases homogeneously by an average of 25% with respect to the initial state. To corroborate the experimental findings, the damage distribution was imaged in 3D after the final tensile loading by synchrotron X-ray refraction computed tomography (SXRCT) and absorption-based synchrotron X-ray computed tomography (SXCT). It could be evidenced that defects and damages cause pronounced indications in the SXRCT images.

Experimental investigation on the mechanical behaviour of 3D carbon/carbon composites under biaxial compression

The effects of complex state of stress on the compressive behaviour of 3D carbon/carbon composites are investigated by application of uniaxial and biaxial loadings using a specially developed Zwick cruciform testing facility. The shape of the biaxially loaded cruciform specimen is optimised to avoid premature fracture outside the gauge section. A semi-analytical method is proposed to determine the stress components in the gauge section of the biaxial specimen. The experimentally obtained failure stress relation, which traces an elliptical path in the principal stress space, can be well represented by the Tsai criterion with a stress interaction parameter of F12 = −0.85. Macro-fracture morphology and SEM micrographs are examined and the results show that the failure mechanisms of the composites vary with the loading ratio. The results also suggest that the biaxial stress interaction effect is represented by a domain in the biaxial specimen, which is characterised by torsion and bending fractures in the dislocated fibres between two adjacent Z yarns.

Test Method Development and Determination of Three-Dimensional Strength and Failure Modes of Polyvinyl Chloride Structural Foams

A comprehensive study has been conducted to develop proper test methods for accurate determination of failure strengths along different material directions of closed-cell polymer-based structural foams under different loading modes. The test methods developed are used to evaluate strengths and failure modes of commonly used H80 polyvinyl chloride (PVC) foam. The foam's out-of-plane anisotropic and in-plane isotropic cell microstructures are considered in the test methodology development. The effect of test specimen geometry on compressive deformation and failure properties is addressed, especially the aspect ratio of the specimen gauge section. Foam nonlinear constitutive relationships, strength and failure modes along both in-plane and out-of-plane (rise) directions are obtained in different loading modes. Experimental results reveal strong transversely isotropic characteristics of foam microstructure and strength properties. Compressive damage initiation and progression prior to failure are investigated in an incremental loading–unloading experiment. To evaluate foam in-plane and out-of-plane shear strengths, a scaled shear test method is also developed. Shear loading and unloading experiments are carried out to identify the causes of observed large shear damage and failure modes. The complex damage and failure modes in H80 PVC foam under different loading modes are examined, both macroscopically and microscopically.

Mechanical behavior and failure mechanisms of Li-ion battery separators

Anisotropic mechanical properties were experimentally determined and compared for three types of commercially available Li-ion battery separators: Celgard 2325, Celgard PP2075 dry-processed polymer separators, and DreamWeaver Gold 40 non-woven separator. Significant amount of anisotropy of properties was determined, with the Young's modulus being different by up to a factor of 5 and ultimate strength being different by a factor of 10 between orthogonal directions within a polymer separator layer. Strain rate sensitivity was investigated by applying strain rates ranging from 1 ⋅ 10 − 4 s −1 to 0.1 s −1 . Significant strengthening was observed and the strain rate strengthening coefficients were determined for both elastic modulus and yield stress in case of polymer separators. Digital image correlation technique was used to measure and map the strains over the specimen's gage section. Significant strain concentration in bands running perpendicular to the tensile axis was observed in polymer separator samples oriented in transverse direction. Such localized necking allows for extremely high strains close to 300% to develop in the material. The failure mode was remarkably different for all three types of separators which adds additional variable in safe design of Li-ion batteries for prevention of internal short circuits. • Mechanical behavior of three types of Li-ion battery separators investigated. • Strain rate sensitivity studied in two orthogonal directions. • Inhomogeneity of strain distribution determined by digital image correlation. • Separators show very different failure modes and strength.

Modeling slip system strength evolution in Ti-7Al informed by in-situ grain stress measurements

Far-field high-energy X-ray diffraction microscopy is used to asses the evolution of slip system strengths in hexagonal close-packed (HCP) Ti-7Al during tensile deformation in-situ. The following HCP slip system families are considered: basal 〈a〉, prismatic 〈a〉, pyramidal 〈a〉, and first-order pyramidal 〈c+a〉. A 1 mm length of the specimen's gauge section, marked with fiducials and comprised of an aggregate of over 500 grains, is tracked during continuous deformation. The response of each slip system family is quantified using ‘slip system strength curves’ that are calculated from the average stress tensors of each grain over the applied deformation history. These curves, which plot the average resolved shear stress for each slip system family versus macroscopic strain, represent a mesoscopic characterization of the aggregate response. A short time-scale transient softening is observed in the basal 〈a〉, prismatic 〈a〉, and pyramidal 〈a〉 slip systems, while a long time-scale transient hardening is observed in the pyramidal 〈c+a〉 slip systems. These results are used to develop a slip system strength model as part of an elasto-viscoplastic constitutive model for the single crystal behavior. A suite of finite element simulations is performed on a virtual polycrystal to demonstrate the relative effects of the different parameters in the slip system strength model. The model is shown to accurately capture the macroscopic stress-strain response using parameters that are chosen to capture the mesoscopic slip system responses.

Forward models for extending the mechanical damage evaluation capability of resonant ultrasound spectroscopy

Finite element (FE) modeling has been coupled with resonant ultrasound spectroscopy (RUS) for nondestructive evaluation (NDE) of high temperature damage induced by mechanical loading. Forward FE models predict mode-specific changes in resonance frequencies (ΔfR), inform RUS measurements of mode-type, and identify diagnostic resonance modes sensitive to individual or multiple concurrent damage mechanisms. The magnitude of modeled ΔfR correlate very well with the magnitude of measured ΔfR from RUS, affording quantitative assessments of damage. This approach was employed to study creep damage in a polycrystalline Ni-based superalloy (Mar-M247) at 950°C. After iterative applications of creep strains up to 8.8%, RUS measurements recorded ΔfR that correspond to the accumulation of plastic deformation and cracks in the gauge section of a cylindrical dog-bone specimen. Of the first 50 resonance modes that occur, ranging from 3 to 220kHz, modes classified as longitudinal bending were most sensitive to creep damage while transverse bending modes were found to be largely unaffected. Measure to model comparisons of ΔfR show that the deformation experienced by the specimen during creep, specifically uniform elongation of the gauge section, is responsible for a majority of the measured ΔfR until at least 6.1% creep strain. After 8.8% strain considerable surface cracking along the gauge section of the dog-bone was observed, for which FE models indicate low-frequency longitudinal bending modes are significantly affected. Key differences between historical implementations of RUS for NDE and the FE model-based framework developed herein are discussed, with attention to general implementation of a FE model-based framework for NDE of damage.

A New Technique for Conducting Split Hopkinson Tensile Bar Test at Elevated Temperatures

In this paper, a new experimental technique is developed for conducting split Hopkinson tensile bar (SHTB) test at elevated temperatures, which includes a Quick Hook Joint to join the specimen and the loading bars and a system to heat, transmit and mount the specimen. The effectiveness of the Quick Hook Joint is demonstrated through numerical simulation. The cold contact time (CCT) experienced by the specimen is measured by laser sensors and the two cold contact processes are numerically simulated. The simulation results suggest no evident temperature decrease in the specimen gage section and no significant temperature increase in the loading bars due to thermal diffusion. Finally SHTB tests of a 45# steel are conducted at various temperatures to provide validations and to demonstrate the feasibility of the developed experimental apparatus and testing method.

Optimisation of thickness of miniature tensile specimens for evaluation of mechanical properties

Experiments were carried out to determine the minimum material volume in the gauge section of miniature tensile test specimens required to get mechanical properties data comparable to those from standard test specimens. The experimental results were verified and aided by numerical and metallurgical analysis simultaneously. In order to generalize the results, three different materials, having large variation in their mechanical properties, were selected for the study.

Dynamic tensile properties of a single crystal Nickel-base superalloy at high temperatures measured with an improved SHTB technique

In this study, to characterize the high-rate tensile properties of a Nickel-base single crystal superalloy at high temperatures, an improved technique for conducting split Hopkinson tensile bar (SHTB) tests at high temperatures (up to 1200°C) is developed. A Quick Hook Joint method to rapidly mount the specimen to the SHTB system is adopted in the technique. Firstly, the specimen is heated to target temperature while keeping the bars at the room temperature. Then the Quick Hook Joint method is used to quickly assemble the specimen to the SHTB by a synchronically dual-actuator assembled system, immediately before loading is imposed to the specimen. The cold contact time (CCT) experienced by the specimen is measured by laser sensors and the cold contact process is numerically simulated. The results show that no evident temperature decrease in the specimen gauge section and no significant temperature increase in the loading bars. Finally, tensile experiments of a single-crystal Nickel-base superalloy over a temperature range of 298–1473K and over a strain-rate range of 0.001–3000s−1 are conducted. Anomalous high temperature peak of flow stress is noticed in the experimental results, and the peak of the flow stress shifts to higher temperature as the strain rate increases. Anomalous temperature valley of the fracture strain is formed over the selected temperature range, and the valley of the fracture strain also shifts to higher temperature as the strain rate increases. A model is applied to quantitatively describe the strain-rate effect on the anomalous peak of the flow stress over a wide range of strain rates.

An ultra-high temperature testing instrument under oxidation environment up to 1800 °C.

A new testing instrument was developed to measure the high-temperature constitutive relation and strength of materials under an oxidative environment up to 1800 °C. A high temperature electric resistance furnace was designed to provide a uniform temperature environment for the mechanical testing, and the temperature could vary from room temperature (RT) to 1800 °C. A set of semi-connected grips was designed to reduce the stress. The deformation of the specimen gauge section was measured by a high temperature extensometer. The measured results were acceptable compared with the results from the strain gauge method. Meanwhile, tensile testing of alumina was carried out at RT and 800 °C, and the specimens showed brittle fracture as expected. The obtained Young's modulus was in agreement with the reported value. In addition, tensile experiment of ZrB2-20%SiC ceramic was conducted at 1700 °C and the high-temperature tensile stress-strain curve was first obtained. Large plastic deformation up to 0.46% and the necking phenomenon were observed before the fracture of specimen. This instrument will provide a powerful research tool to study the high temperature mechanical property of materials under oxidation and is benefit for the engineering application of materials in aerospace field.