Multilayering of metal/ceramic combinations can help to achieve better strength and toughness than the individual material constituents. The effect of elastic-plastic mismatch in multilayers on the crack driving force and eventually on fracture resistance has been analyzed in this work. The enhancement in fracture toughness by decreasing layer spacing has been predicted from finite element calculations and verified by micro-cantilever fracture tests. Further, calculations have been carried out for a more stable clamped beam bend geometry to determine R-curve behavior in such multilayers.
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Themes: Small Scale Specimen Testing
OPTIMIZATION AND USE OF HIGH-THROUGHPUT MICROMECHANICAL TESTING DESIGN FOR 3D-PRINTED POLYMERS
Modern materials behave differently on a micro-scale level than in bulk applications. Therefore, with ever present miniaturization, the materials’ testing on a micron-level is gaining importance. 3D printing with a sub micron precision, such as direct laser writing by two-photon lithography, allows for relatively fast manufacturing of miniaturized specimens for micromechanical testing. In combination with precise loading by a nanoindenter tip, high throughput micromechanical testing is enabled. Presented research shows design process of miniaturized cantilever and push to pull device specimens for fracture mechanics testing, aided finite element modelling, together with high throughput testing of polymeric materials with varied printing parameters and loading conditions. Such in situ and ex situ experimental setup allows for systematic fracture mechanics testing on the small scale for common materials used in small-scale 3D printing.
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MICRO-BENDING FOR MULTI-SCALE FRACTURE CHARACTERIZATION OF CEMENT-BASED MATERIALS AND CERAMICS
The last few years have seen the development of several testing techniques at the micro-scale to characterize the mechanical properties of multi-scale materials. One such novel methods are micro-cantilever bending tests to assess mechanical properties of materials. Micro-cantilever tests allow for a variety of test configurations, including scaled chevron-notch geometry allowing controlled crack-growth prior to critical failure load. Chevron notches are convenient at such length scales since they allow for ex-situ measurements of crack length – avoiding the need to directly measure crack length, which could be a challenge at such scale. The test approach provides access to R-curve behavior for quasi-brittle materials on the micro-scale and opens the door to investigate more sophisticated topics, i.e., creep crack growth. Moreover, the technique complemented with visualization and scanning tools, such as Scanning Electron Microscopy and Microtomography, allows for a proper and extensive analysis of the crack growth phenomena.
The motivation of this work revolves around expanding currently available knowledge on multiscale characterization of cement-based materials with the focus on fracture testing at the micro- and mesoscale through the novel micro-bending technique. This test opens the door to understanding fracture, toughening mechanisms, and evolution of these properties regarding processing variables.
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HIGH-CYCLE FATIGUE IN THE TEM: NANOCRYSTALLINE METALS [Keynote]
In-situ TEM high-cycle fatigue experiments on electron transparent thin films of nanocrystalline Pt and Cu have revealed not only microstructural-sensitive crack propagation, but also unexpected microstructural-scale crack healing. Based on the experimental observations, atomistic modeling, and continuum-scale microstructural modeling, the mechanism appears to be crack flank cold welding facilitated by local compressive microstructural stresses and/or grain boundary migration. While these observations are specific to pure nanocrystalline metal thin films under a high-vacuum environment, there are potentially much broader ramifications. The existing observations can be used to help rationalize suppressed fatigue crack propagation rates in vacuum, subsurface, or under contact-inducing mixed-mode stresses; and even the precipitous decline in propagation rates near the fatigue threshold.
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UNDERSTANDING OF TOUGHENING IN CEMENTED CARBIDES BY MEANS OF SMALL-SCALE MECHANICAL TESTING AND CHARACTERIZATION
Small-scale mechanical testing (massive nanoindentation, compression of micropillars and fracture of notched microcantilevers) and characterization (cross-section FIB-tomography and FESEM inspection) are proposed and validated as effective tools for studying fracture mechanics and toughening mechanisms governing stable crack growth in cemented carbides. Crack growth resistance behavior of cemented carbides and corresponding microstructural effects are sucessfully described and understood on the basis of ductile ligament reinforcement behind the crack tip as the key toughening mechanism for these materials.
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RELATING NANOSCALE STRUCTURE AND PROPERTIES TO MACROSCALE FRACTURE TOUGHNESS FOR BULK METALLIC GLASSES [Keynote]
Bulk metallic glasses (BMGs) can range from exceptionally tough to brittle depending on their structural state; however, quantifying their structure-property relationships has been an unresolved challenge. Our findings revealed that local hardness variations within the BMG microstructure strongly affect the fracture behavior. Moreover, the hardness heterogeneities are controlled by the size and volume fraction of FCC-like medium-range order (MRO) clusters. We have proposed a model of ductile phase softening whereby relatively soft FCC-like MRO clusters sit in a matrix of harder icosahedral dominated ordering, while micropillar compression testing has revealed how the activation of these clusters into shear transformation zones can be negatively affected by oxygen impurities which in turn lower the fracture toughness.
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CHARACTERIZATION METHODOLOGY OF PIPELINE STEELS USING MINIATURE SPECIMENS
A possible solution to check the fitness-to-service of existing pipeline steels for hydrogen transport is to extract small coupons without interrupting supply operations. From these coupons, it is possible to machine sub-size specimens to characterize ductility and fracture toughness of the base and weld (weld metal and heat affected zone) materials in both air and pressurized hydrogen environments. Using sub-size requires specific facilities. This paper describes a new setup and the associated methodology developed to test sub-size specimens. The method is applied to tests under pressurized hydrogen gas.
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IN SITU SEM HIGH-THROUGHPUT CYCLIC TESTING OF FREESTANDING THIN FILMS
This work presents a small-scale high-throughput technique to characterize the cyclic behavior of freestanding thin films. The technique consists of the microfabrication of a Si carrier composed of an array of grips and freestanding dogbone thin films and of the automated in situ Scanning Electron Microscope (SEM) fatigue testing of the microfabricated carrier. The Si carrier functions as a nanomechanical testing device in which multiple dogbones can be simultaneously and independently tested under the same applied mechanical conditions. As a proof-of-concept, the fatigue behavior of nanocrystalline Al thin films was investigated. The technique allows for the simultaneous evaluation of crack nucleation and propagation across the fatigue life of several dogbones, facilitating the understanding of deformation mechanisms in nanocrystalline metals and providing statistically significant data. This technique reduces total testing time by orders of magnitude and allows for the investigation of the stochastic variability in fatigue failure. The current technique can be further expanded to account for different materials, new geometries and different loadings modes.
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EFFECT OF ELECTRIC CURRENT ON PRE-CRACKED THIN METALLIC SHEETS: FROM CRACK PROPAGATION TO CRACK HEALING [Keynote]
Both scientifically and technologically, it is important to study the effects of electric current pulses on the structural integrity of metallic components. As an electric current reverses its direction across a crack, massive current crowding occurs at the crack tip, thereby generating a non-uniform temperature field sourcing away from the crack tip, and considerable electromagnetic forces are generated on the crack faces that open the crack in Mode I. Recent studies have shown that due to the synergistic effects of the above two stimuli, a pre-existing flaw may grow as well as heal upon application of an electric current pulse of high density. While one is a bane for structural integrity, the other one is a boon to in-service components. Here, we will discuss the reasons behind crack propagation upon application of an electric current and then explore the attributes responsible for a transition from flaw propagation to flaw healing upon passage of an electric current pulse. Furthermore, the synergetic role of mechanical load and magnetic field in the propagation of a pre-existing flaw will be discussed. We will establish the complementary roles of electric current, magnetic field and mechanical load in the failure of pre-cracked metallic sheets.
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UNDERSTANDING FATIGUE DAMAGE PROGRESSION IN A STRUCTURAL STAINLESS STEEL THROUGH CYCLIC BALL INDENTATION TESTING
Understanding fatigue damage progression and estimating remaining life of dynamically loaded components has been a major challenge for several safety critical in-service components. Towards this, a few small specimen fatigue test methods are available, such as, cyclic ball indentation, cyclic small punch test and cyclic bulge test etc. Cyclic ball indentation has the potential to be deployed in-situ during plant maintenance to record fatigue response of localized spots. The method uses a spherical indenter of 1/16” (~1.58 mm) diameter which applies cyclic compression-compression loading on the material at selected location and monitors the load-displacement response continuously to identify failure event due to fatigue.
To capture a complete picture of this, controlled experiments using carefully prepared dog bone fatigue specimens of SS 304 have been conducted. The dog bone specimen is fatigue cycled under tension-tension uni-axial loading till failure, with Acoustic Emission (AE) signature capture during fatigue cycling. the fatigue cycling is interrupted periodically and cyclic ball indentation tests are carried out again at some locations of gage length to identify failure life cycle data of fatigue cycled specimen through displacement sensing and hysteresis area. Data obtained from cyclic ball indentation is then correlated with loss of stiffness.
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