Miniature Specimens Research Articles

Microstructure and fatigue performace of additively manufactured AlSi10Mg.

Laser powder bed fusion technology (L-PBF) can readily fabricate near-net shape metal parts. Therefore, the automotive and aerospace industries have been investigating the L-PBF production of AlSi10Mg parts because of low specific density, good hardenability, and low powder costs. Further, local melting of the atomized powder and subsequent rapid solidification generates fine structures having mechanical properties that are competitive with conventionally produced Al alloys.If the products remain in the as-built state (i.e., no post fabrication heat treatment), residual stresses are expected in the part and are superimposed on the operating stress with often unpredictable effects on its fatigue life. As-built part surfaces are rough compared to machined surfaces with a negative influence on the fatigue strength of L-PBF AlSi10Mg parts. On the other hand, surface machining is not only expensive but often impossible for L-PBF parts due to their geometric complexity.This study investigates the fatigue behavior of L-PBF AlSi10Mg under the combined effect of untreated condition and as-built (i.e., rough) surface quality. Three sets of miniature specimens, each with a different orientation (A, B, C) with respect to the build direction were printed in an SLM 280 HL system operating with a layer thickness of 50 μm. Each set consisted of approx. 15 samples. The as-built samples were tested in cyclic plane bending at a load ratio R = 0 at a frequency of 25 Hz and a significant directional influence on the fatigue behavior quantified. To investigate the origin of this behavior, samples for each orientation were examined using metallographic techniques to determine the structure and quality of surfaces. Surface features depending on printing strategy and printing parameters of the different specimens qualitatively explain the observed directional fatigue behavior.

Fracture properties and crack tip constraint quantification of 321/690 dissimilar metal girth welded joints by using miniature SENB specimens

By using miniature SENB specimens, the fracture properties of the materials in the region of welded metal, 321 stainless steel heat affected zone, 690 alloy heat affected zone of 321/690 dissimilar metal girth welded joints were tested. Both the J-resistance curves and critical fracture toughness of the three different materials are affected by the crack size because of the effect of crack tip constraint. Groups of constraint corrected J-resistance curves of the three materials are obtained according to J-Q-M approach. The welded metals exhibit the best fracture resistance but the worst fracture resistance is observed in the material of 690 alloy heat affected zone.

Pore-affected fatigue life scattering and prediction of additively manufactured Inconel 718: An investigation based on miniature specimen testing and machine learning approach

Fatigue life scattering and prediction of Inconel 718 fabricated by selective laser melting were investigated using miniature specimen tests combined with statistical method and machine learning algorithms. The relationship between pore features and fatigue life of the selective laser melting-fabricated specimens was analyzed statistically. The results show that the increase in the size and/or the number of the pores in the specimens, and/or the decrease in the distance from a pore center to the specimen surface degraded the fatigue life. The machine learning and statistical analysis results reveal that the fatigue life are most closely related to the location of the pores compared with the size and the number of pores in the specimens. The finding may provide a potential way to get high-throughput statistical data helping in evaluating defect-dominated scattering and prediction of fatigue life of additive manufactured metallic parts using miniature specimen testing assisted by the machine learning approach.

Stir zone anisotropic work hardening behavior in friction stir processed EN8 medium carbon steel

The SZ anisotropic work hardening behavior in duplex ferrite-martensite phase (DFM) microstructures has not been investigated yet. It is an important consideration for the selection of the stretch formability direction of the friction stir processed blank used for sheet metal forming applications. The anisotropic work hardening behavior is characterized by using miniature tensile specimens prepared along longitudinal (LD) and transverse (TD) directions. The tribological parameters such as coefficient of friction (COF) and wear rate at LE value of 6 are 0.57 and 0.5 × 10 − 10 m 3 / Nm . The effect of surface texture on the work hardening response is observed to be negligible. The plastic anisotropy in terms of three and multi-stage work hardening behaviors is investigated at a linear energy ( LE) value of 6. The global true stress-strain responses and fracture surface morphologies suggested that the deformation occurs in different stages. In the initial stage, the deformation is mainly localized in the soft ferrite phase followed by the distinct work hardening stages in the plastic flow curves. In the elastic and elastoplastic region of the flow curve, the soft ferrite phase plastically deformed, followed by stress transfer to the martensite/retained austenite phases in the fully plastic region. The (110) and (111) pole figure suggested that the active in-habit-plane slip system is (110) and [ 1 1 ‾ 1 ] . The strain localizes more in the V19 + V22 block having the critical resolved shear stress (CRSS) value of 445.5 MPa. The retained austenite (RA) is having ( − 11 − 1 ) A / / ( 0 − 11 ) F and [ 011 ] A / / [ 111 ] F type Kurdjumov-Sachs (K–S) orientation relationship with the ferrite phase. A BCC {112} <111> -type twinned laths is found to be present within the martensite laths. The recommended stretch formability direction (SFD) should coincide with the longitudinal or processing direction of the friction stir processed blank.

Evaluation of tensile properties of various regions of creep exposed dissimilar weld joint using miniature specimen testing

Strength variations in various regions of a dissimilar weld joint made between 316LN stainless steel (SS) and P91 steel after creep exposure at 550 °C and stress of 200 MPa was estimated using automated ball indentation (ABI) test. The ABI results were used to derive equivalent uniaxial tensile flow behaviour using established correlation factors. A comparison of the results obtained in the creep tested and post weld heat treated (PWHT) samples showed loss of strength in the P91 steel side. It was observed that, creep exposure under this testing condition strengthened the 316LN SS side of the weld joint. ABI derived results showed that, the inter critical-heat affected zone (IC-HAZ) of P91 steel is the weakest region across the weld after creep exposure. The narrow IC-HAZ created a strain constraint which caused the specimen to fail in the P91 steel base metal region.

Construction of True Stress-Strain Curve of Metallic Material by Artificial Neural Network and Small Punch Test

Small punch test (SPT) is used to evaluate mechanical properties of metallic materials by a miniature specimen. A method combining SPT and artificial backpropagation neural network for determining the true stress-strain curve of metallic materials is proposed. The load-displacement curves of different hypothetical materials were obtained by the finite element model of SPT with considering Gurson-Tvergaard-Needleman (GTN) damage parameters and used to train a backpropagation neural network. The relationship between the load-displacement curve of SPT and the true stress-strain curve of the conventional uniaxial tensile test was established based on the trained neural network, which is validated by the experimental results of X80 pipeline steel. The results demonstrate that the established relationship can be used to predict the true stress-strain curve of the metallic materials and then to determine their elastoplastic properties by SPT.

The effect of wash treatment on the mechanical properties and energy absorption potential of a 3D printed polymethyl methacrylate (PMMA)

Stereolithographic 3D-printing of a Polymethyl Methacrylate (PMMA) resin was used to manufacture miniature dog-bone specimens that were subject to a variety of wash treatments consisting of Isopropyl Alcohol (IP), Hydrogen Peroxide (HP) and a Detergent (D) solution. The washed dog-bone specimens were then tested in uniaxial tension and the mechanical properties were quantified to assess the potential use of the resultant materials for the design of energy absorbing lattice structures. Compared to an As-Printed material, a detergent wash treatment resulted in almost no loss in peak stress, but a 61 % increase in toughness due to enhanced elongation. Moderate decreases in peak stress and high elongation were observed for HP washes. A 5 % (concentration) HP wash with detergent (HP5 + D) resulted in a 90 % greater toughness than the As-Printed material. Isopropyl washed specimens showed low toughness due to low peak stresses. A finite element study was conducted to simulate compression tests of an octet-truss lattice structure for which the tensile test properties were used as model inputs. The simulations revealed some relationships between the predicted energy absorption-tensile properties and showed that the HP5 + D (moderate peak force) and As-Printed + D (high peak force) washed conditions have potential application as energy absorbing octet-truss lattice structures.

Strength assessment of austenitic steel grain boundaries by impact bending tests for miniature specimens

The paper proposes methods for assessing the strength of grain boundaries according to the results of testing miniature specimens by impact bending. Results of bending at low temperature are given to assess the strength of grain boundaries in austenitic chromium-nickel steels. The test temperature was determined when the proportion of brittle intergranular fracture of embrittled chromium-nickel steel 10Kh18N9 is at least 90%. Three types of miniature specimens of different geometric shapes have been developed, providing approximately the same absorbed energy when tested for impact bending. It is shown when it is necessary to use such miniature specimens.

Evaluation of resistance to corrosion cracking of irradiated austenitic chromium-nickel steels by impact bending tests on miniature specimens

In this work, we present results of a bending test of miniature specimens made of chromium-nickel stainless austenitic steels of grades 321 (08Kh18N10T), 316 (06Kh16N11M3) and 304 (02Kh18N9) irradiated to various damage doses from 4 to 125 dpa in different energy neutron spectra. Effects of the damage dose and the neutron energy spectrum on the intergranular fracture energy, which determines the strength of grain boundaries, are studied. Two neutron spectra are considered: one characteristic of the active zone of PWR and WWER reactors, and the other is typical for the active zone of fast core reactors. The relationship between the resistance to corrosion cracking of irradiated chromium-nickel steels 321, 316 and 304 and the strength of grain boundaries is considered.

Toughening mechanisms in cost-effective carbon-epoxy laminates with thermoplastic veils: Mode-I and in-situ SEM fracture characterisation

This study investigates the influence of thermoplastic non-woven veils on the interlaminar fracture energy and toughening mechanisms under mode-I dominant loading conditions in out-of-autoclave resin-infused carbon fibre-epoxy laminates. Two different non-woven micro-fibre veils, Polyetherimide (PEI) and Polyphenylene Sulfide (PPS), of low areal weight (∼10 g/m2) are introduced between carbon fibre non-crimp fabric (NCF) laminae prior to vacuum assisted resin infusion (with a low viscous two-part epoxy resin, i.e. Araldite 564/Aradur 2954). Double Cantilever Beam (DCB) specimens are tested for characterising mode-I fracture energies, and a novel miniature specimen geometry with an embedded fibre-discontinuity to trigger delamination under mode-I dominant loading is employed for in-situ SEM characterisation of micro-damage evolution and toughening mechanisms. The results obtained from DCB specimens showed that the micro-fibre thermoplastic veils considerably improved the mode-I fracture energy when compared with the untoughened fracture energy (i.e. ∼13% increase with PEI and ∼60% with the PPS veils, at the onset of crack propagation). Optical and scanning electron microscopy revealed that the veil parameters such as fibre dispersion, fibre diameter and specific surface area play an important role in enhancing fracture energy. Fracture surface micrographs indicated that progressive fibre-matrix debonding followed by fibre pull-out and fibre bridging resulted in higher fracture energy within the PPS veils. Interleaving a relatively low-cost NCF composite with thermoplastic veils could be as resistant to delamination as aerospace-grade carbon fibre-epoxy systems.

Effect of initial microstructure on the deformation heterogeneities of 316L stainless steels fabricated by selective laser melting processing

The selective laser melting (SLM) is a popular additive manufacturing (AM) technique used for the fabrication of metal parts. In the present study, two 316L stainless steel specimens (SLM-I and SLM-II) with different microstructures were fabricated with different levels of energy density by changing the laser power and scanning speed, which are the main SLM process conditions. The deformation and fracture behavior of miniature SLM specimens under uniaxial tension were experimentally measured via optical microscopy (OM), field emission scanning microscopy (FE-SEM), and an electron back-scattered diffraction (EBSD) technique. In order to analyze the deformation heterogeneities under uniaxial tension, the inverse pole figure (IPF) map, kernel average misorientation (KAM) map, Taylor factor (TF) map, grain boundaries (GBs), Σ3 twin boundaries (TBs), and melt pool boundaries (MPBs) developed in deformed SLM specimens were analyzed at different strain levels. The effect of microstructural factors on the deformation heterogeneities of SLM specimens was explained by the evolution of KAM, GBs, MPBs, and Σ3 TBs. The initial microstructures of the SLM specimens significantly influenced the generation and propagation of cracks under uniaxial tension.

Creep Strength Evaluation of a Long-Term Used Boiler Weldment Part on 2.25Cr-1Mo Steel by Using Miniature Specimens

Creep Strength Evaluation of a Long-Term Used Boiler Weldment Part on 2.25Cr-1Mo Steel by Using Miniature Specimens

INFLUENCE OF ROUGH AS-BUILT SURFACES ON SMOOTH AND NOTCHED FATIGUE BEHAVIOR OF L-PBF AlSi10Mg

Near-net shape metal parts of great geometrical complexity are fabricated by the Laser Powder Bed Fusion (L-PBF) technology directly from a CAD model. Therefore, parts can be lightweight, less expensive in terms of material use and with shapes that may be impossible to produce by conventional technology. The fatigue behavior of L-PBF part in as-built condition is negatively affected by poor surface quality. Surface finishing after fabrication may be either unacceptably costly or impossible because the surface is inaccessible. Fatigue performance can be further reduced by the notch effect due to local geometrical variations. Among the Al-alloys, AlSi10Mg is readily processed with L-PBF and it is of interest for different industrial sectors.In this contribution two aspects, that is: i) the directional smooth fatigue behavior of as-built AlSi10Mg, and ii) the notch fatigue behavior with as-built surfaces are investigated. Eight sets of un-notched and notched miniature specimens of AlSi10Mg were produced as a single batch by L-PBF and tested in the as-build state under cyclic plane bending loading. The smooth fatigue behavior was determined as very sensitive to applied stress direction with respect to the build direction. The directional nature of the fatigue behavior was confirmed by notch fatigue data. Therefore, four notch fatigue factors that depend on the PBF technology were introduced and determined. The fatigue behavior of L-PBF AlSi10Mg obtained here was compared satisfactorily against recent data obtained with standard specimen geometries and test methods. The present methodology using mini specimens under cyclic bending efficiently determines the fatigue response of L-PBF metals.

Development of miniature cruciform specimen and testing machine for multiaxial creep investigation

High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to extreme temperature conditions and their complex shapes. A multiaxial creep investigation is required for the safety of high temperature components in the design phase of the manufacture of such a component. However, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. A miniature cruciform specimen having a plane stress condition gauge section that is 5 mm square was designed using finite element (FE) analysis. Further, a biaxial testing machine capable of tensile loading was designed to conduct multiaxial creep tests. The testing machine had a 2 kN loading capacity and a 1 kW furnace. We also developed a non-contact displacement measuring method for the miniature specimen, which had an excessively small gauge section to permit the attachment of a mechanical extensometer. The proposed method uses a conventional optical camera to capture some images of the surface. We obtained the displacement value of the specimen, by tracking the trace of the target marks painted on the surface of the specimen. The measured strain value that was obtained from the non-contact displacement measuring method corresponds to the strain obtained by the mechanical method at room and high temperatures. By using the developed multiaxial creep testing machine and the non-contact observation system, we can investigate not only the deformation of the testing specimen, but also the surface conditions of materials during the creep test.

Non-contact strain evaluation for miniature tensile specimens of neutron-irradiated F82H by digital image correlation

• Non-contact strain evaluation by DIC using speckle patterns of surface flaws was applied and reliability of the technique was demonstrated. • Room temperature tensile behavior of Japanese RAFM, F82H, irradiated to ∼70 dpa at 573 K was preliminarily evaluated. • No possible irradiation-induced change of the elastic properties in both axial and transverse directions for irradiated F82H was demonstrated. The small specimen tensile test is commonly used as the basic test to evaluate the irradiation effect on the strength of fusion structural materials after irradiation, and a lot of attempts were made to qualify the data obtained by small size specimen, but the specific attention on the evaluation of strain were not satisfactory given. This study therefore aims to propose a novel non-contact strain measurement technique, based on digital image correlation (DIC) method, for reduced-activation ferritic/martensitic (RAFM) steel, e.g., F82H, as the leading candidate structural material of fusion in-vessel components in Japan. A flat rectangular miniature specimen, e.g., SS-J3 type, was adopted to evaluate non-irradiated and neutron-irradiated tensile properties. In the proposed DIC, very tiny surface machining flaws were recognized by the DIC computation program and utilized to set several pairs of the gauge end points with a fixed gauge length. In case of the room-temperature test, it was clearly demonstrated that this procedure could provide more precise and reliable data compared with the previous approach of the single-scan measurement under floated gauge length. Superior linearity was achieved and recognized DIC points of concern were traceable during the entire period of the test. By applying this technique, superior high-dose stability of elastic properties of F82H was first demonstrated.

Low cycle fatigue testing using miniature specimens

This study discussed a low cycle fatigue testing method using miniature specimens of SUS304 stainless steel and S35C carbon steel. A stable low cycle fatigue testing method was developed for miniature specimens with a 1 mm diameter that gives similar low cycle fatigue lives of bulk specimens with a 6.5 mm diameter. The testing method assessed the effects of surface roughness and residual stress. The results showed that low cycle fatigue damage could be evaluated using miniature specimens taken from the cracked parts of bulk specimens.

High Cycle Fatigue Performance of LPBF 304L Stainless Steel at Nominal and Optimized Parameters.

In additive manufacturing, the variation of the fabrication process parameters influences the mechanical properties of a material such as tensile strength, impact toughness, hardness, fatigue strength, and so forth, but fatigue testing of metals fabricated with all different sets of process parameters is a very expensive and time-consuming process. Therefore, the nominal process parameters by means of minimum energy input were first identified for a dense part and then the optimized process parameters were determined based on the tensile and impact toughness test results obtained for 304L stainless steel deposited in laser powder bed fusion (LPBF) process. Later, the high cycle fatigue performance was investigated for the material built with these two sets of parameters at horizontal, vertical, and inclined orientation. In this paper, displacement controlled fully reversed (R = −1) bending type fatigue tests at different levels of displacement amplitude were performed on Krouse type miniature specimens. The test results were compared and analyzed by applying the control signal monitoring (CSM) method. The analysis shows that specimen built-in horizontal direction for optimized parameters demonstrates the highest fatigue strength while the vertical specimen built with nominal parameters exhibits the lowest strength.

On the mechanical integrity of AA6082 3D structures deposited by hybrid metal extrusion &amp; bonding additive manufacturing

Hybrid Metal Extrusion and Bonding Additive Manufacturing (HYB-AM) is a new solid-state process for the production of 3D metal structures. In HYB-AM, the wire feedstock is continuously processed through an extruder and deposited in a stringer-by-stringer manner to form layers and eventually a near net-shape component. In this work, the layer bonding of AA6082 samples produced by this process has been investigated by means of tensile testing, hardness measurements and microscope analyses. Furthermore, a novel method for the fabrication of miniature tensile specimens for assessing the bond strength across the layers is presented and applied. The test results reveal that the ultimate tensile strength is approaching that of the substrate material of the same alloy, yet with a somewhat lower elongation prior to fracture. Microscope analyses show that the bonded interfaces are fully dense; however, the fracture surfaces reveal regions of kissing-bonds and lack of bonding. Still, these preliminary investigations indicate that the HYB-AM process, upon further optimization, has the potential of processing high quality aluminum alloy components.

Microstructural investigation and mechanical properties evaluation using miniature specimen testing of various constituents of dissimilar weld joint

Microstructural evolution in P91 (modified 9Cr–1Mo) steel and 316LN Stainless Steel (SS) bi-metallic weld joints fabricated by arc welding technique using Inconel-182 electrode has been studied in as-welded and post weld heat treated (PWHT) conditions. The PWHT brought about significant changes in the microstructure which altered the strength gradient especially in the ferritic steel side. The differences in weld thermal cycling which occurred in various regions of the P91 steel side caused M23C6 precipitates to evolve in varying sizes. The size variation in these precipitates was the paramount cause for the variation in strength in this region which could be probed by miniature specimen testing. Using this testing it could be established that PWHT resulted in offsetting the strength variation in the HAZ of the ferritic steel side. It could also be understood that PWHT generates a weak region in the P91 side making it most susceptible to damage.

An adaptable flexural test fixture for miniaturised polymer specimens

An adaptable flexural test fixture is proposed to characterise the mechanical properties of miniature beam specimens (≤10 mg) at ambient conditions or in the presence of fluids at elevated temperatures. The fixture is validated using representative amorphous and semi-crystalline polymers. The response of miniature specimens is compared against that of medium-sized specimens (≤1 g) on the same fixture and on conventional test equipment. Good agreement is found between the specimen sizes for all materials, but the comparisons highlight small differences attributed to factors such as specimen dimensional accuracy, crystallinity and span-to-thickness ratios. Flexural tests in water at 37 °C using both specimen sizes were performed to investigate the evolution of mechanical behaviour of hydrolytically degraded polylactides. Here, specimen size influences the diffusion timescale of acidic by-products which can reduce or enhance autocatalysis.