Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method (i.e., neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45 deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension.

An Experimental Investigation into Additive Manufacturing-Induced Residual Stresses in 316L Stainless Steel

Structural ceramic nanocomposites are reviewed with emphasis on the Al 2 O 3 SiC and Si 3 N 4 SiC systems. The incorporation of a nanosized second phase, such as SiC, into a ceramic matrix can lead to an improvement in mechanical properties. It is still unclear, however, whether those improvements can directly be related to an intrinsic ‘nanocompossite effect’ or to other factors. This review is divided into three parts. First, basic processing routes for nanocomposites, namely conventional powder processing, sol-gel processing and polymer pyrolysis are presented in detail. Second, the mechanical properties of different nanocomposites are compared. Finally, models which attempt to explain the improvements in these properties are explored. It will be shown that the strength increase can best be related to a reduction in processing defect size. For applications the most interesting properties of nanocomposites are their wear, creep and high temperature performance.

Structural ceramic nanocomposites

There has been growing interest in incorporating single-wall carbon nanotubes (SWNTs) as toughening agents in brittle ceramics. Here we have prepared dense Al2O3/SWNT composites using the spark-plasma sintering (SPS) method. Vickers (sharp) and Hertzian (blunt) indentation tests reveal that these composites are highly contact-damage resistant, as shown by the lack of crack formation. However, direct toughness measurements, using the single-edge V-notch beam method, show that these composites are as brittle as dense Al2O3 (having a toughness of 3.22 MPa m0.5). This type of unusual mechanical behaviour was also observed in SPS-processed, dense Al2O3/graphite composites. We argue that the highly shear-deformable SWNTs or graphite heterogeneities in the composites help redistribute the stress field under indentation, imparting the composites with contact-damage resistance. These composites may find use in engineering and biomedical applications where contact loading is important.

Contact-damage-resistant ceramic/single-wall carbon nanotubes and ceramic/graphite composites

Bending and free vibration response of layered functionally graded beams: A theoretical model and its experimental validation

Indentation techniques have been extensively used for mechanical characterisation of materials. Development of instrumented indentation techniques at low and ultra low load levels has further improved their utility for understanding the mechanical responses of solids at micro and nano scales. The variation of hardness with the load/depth of indentation, known as indentation size effect, has led to difficulties in using hardness as a fundamental or characteristic mechanical property of materials. Detailed discussions are focused on the issue of indentation size effect in brittle and ductile solids. Various theoretical models accounting for the indentation mechanics are highlighted. The results obtained from these techniques with quasicrystals, bulk metallic glasses and nanomaterials are reviewed. The issues related to phase transformation during indentation tests are briefly discussed. The industrial use of the indentation technique has been pointed out. Some of the current issues and directions fo...

Micro- and nanoindentation techniques for mechanical characterisation of materials

The dependence of K IC on the notch-root radius has been examined for a notch radius as small as a few micrometers in a dense, fine-grained, polycrystalline alumina ceramic. The notch radius can be systematically varied by using a semimanual procedure in a special jig which polishes out rather than cuts the specimen. K IC is independent of the notch sharpness for notch-root radii <10 μm. The results are critically compared with those obtained by other standard techniques and discussed in terms of residual compressive stresses introduced during the notching procedure

Effect of Notch‐Root Radius on the Fracture Toughness of a Fine‐Grained Alumina

Grain size influence on residual stresses in alumina/zirconia composites

Fracture energy and R-curve behavior of Al2O3/Mo composites

Bridging stresses that result both from elastic tractions and frictional interlocking in the wake of an advancing crack have been evaluated quantitatively via in situ Raman microprobe spectroscopy in a toughened Si3N4 polycrystal. Crack opening displacement (COD) profiles of bridged cracks also have been measured quantitatively via scanning electron microscopy to substantiate the piezospectroscopic determination of microscopic stresses via Raman spectroscopy. The highest spatial resolution of the stress measurement in the Raman apparatus was 1 µm, as dictated by the optical lens that was used to focus the laser on the sample. Measurements of the bridging stresses were performed both at fixed sites (as a function of the applied load) and along the profile behind the crack tip (under a constant load). Rather high stress values (i.e., 0.4-1.1 GPa) were measured that corresponded with unbroken ligaments that bridged the crack faces in elastic fashion, whereas frictional sites were typically under a lower tensile stress (0.1-0.5 GPa). Mapping the near-tip COD profile and the bridging stresses at the (normal) critical load for catastrophic fracture enabled us to calculate the crack-tip toughness and to explain the rising R-curve behavior of the material. From a comparison with conventional fracture-mechanics data, a self-consistent view of the mechanics that govern the toughening behavior of the Si3N4 polycrystal could be obtained. In particular, crack bridging is proven to be, by far, the most important mechanism that contributes to the toughening of polycrystalline Si3N4 materials.

In situ measurement of bridging stresses in toughened silicon nitride using Raman microprobe spectroscopy

Fracture of a large-grained alumina polycrystal has been examined in situ by optical microscopy. Concurrently, local bridging stresses, as generated by friction or tension of unbroken ligaments in the wake of the crack path, were measured by piezospectroscopy. Stress measurements were performed both at fixed sites as a function of the external load and at a fixed external load along the crack profile. Frictional stresses were similar/congruent50 MPa, while unbroken ligaments between the crack faces were found to support tensile stresses up to similar/congruent100 MPa. The maximum bridging stress was dictated by the weak (intrinsic) interface bonding of the polycrystal. Average bridging stresses, either theoretically calculated from R-curve data or experimentally measured by piezospectroscopy on frictional/bridging sites, were similar. Such a circumstance enables us to explain the fracture behavior of polycrystalline alumina by considering crack-wake shielding as the main micromechanism contributing to toughening.

Toshihiko Nishida

Papers

Effect of Notch‐Root Radius on the Fracture Toughness of a Fine‐Grained Alumina

Grain size influence on residual stresses in alumina/zirconia composites

Fracture energy and R-curve behavior of Al2O3/Mo composites

In situ measurement of bridging stresses in toughened silicon nitride using Raman microprobe spectroscopy

In Situ Measurements of Frictional Bridging Stresses in Alumina Using Fluorescence Spectroscopy