Increase In Apparent Fracture Toughness Research Articles

Grain size effect on the mechanical properties of nanocrystalline magnesium aluminate spinel

To develop transparent materials with superior mechanical properties, nanocrystalline magnesium aluminate (MgAl2O4) spinel with grain sizes ranging from 3.7 to 80 nm has been synthesized by environmentally controlled pressure assisted sintering. In this study, we investigated the microstructure and grain size dependence of the mechanical properties of nanocrystalline MgAl2O4 by performing transmission electron microscopy, nanoindentation, uniaxial micropillar compression, and micro-cantilever bending. Electron microscopy confirmed that the environmentally controlled pressure assisted sintering technique produces a nearly fully dense grain structure with a porosity of less than 1% in larger grain-sized ceramics and observably pore-free grain structures in the smaller grain-sized ceramics. Mechanical characterization revealed that nanoindentation hardness, compressive fracture strength, and fracture toughness each exhibit distinct grain size dependence. Our experimental results and numerical analyses point to a change in dominant strain accommodating mechanisms from dislocation-based plasticity to shear banding as the grain size is reduced, as previously suggested by the literature. Practical implications of the change in strain accommodation mechanisms manifest as the emergence of indentation size effect, weak grain size dependence of hardness and strength, and a ∼2-fold increase in apparent fracture toughness for the smaller grain-sized ceramics.

Mechanical properties and tempering resistance of an ultrafine grained Tool Steel-PSZ composite fabricated by high energy mechanical milling and spark plasma sintering

The mechanical properties and tempering resistance of a powder metallurgical (PM) tool steel (AISI H13) reinforced with partially stabilized zirconia (PSZ) were investigated. The choice of particles reinforcement was made in view to the well-known phase transformation toughening effect in PSZ. Composite powders were produced by high energy mechanical milling and subsequently consolidated by spark plasma sintering (SPS). An ultrafine grain microstructure was achieved showing a homogenous distribution of refined PSZ particles within the severely deformed matrix. Thanks to the fast SPS consolidation, a small amount (less than 2 vol%) of reaction products were formed. The hardness of the composites was significantly higher than the unreinforced counterpart, due to the microstructural refinement, increased dislocation density, and dispersion hardening. Tempering resistance at 550 °C and 650 °C was also significantly improved. The hot compressive yield strength of the composites measured 650 °C and 450 °C was increased up to 1.8 times compared to unreinforced steel. PSZ phase transformation during hot compression contributed to increased work hardening. Fracture toughness was significantly decreased compared to the base material. However, compared to a TiC reinforced H13 with the same hardness, a marginal increase in apparent fracture toughness was observed.

Stress Delocalization in Crack Tolerant Electrospun Nanofiber Networks

The fracture toughness of a noncontinuum fibrous network produced by electrospinning polyamide 6 nanofibers is investigated. The mechanical properties of the nanofiber network is observed to be independent of various incorporated macroscopic crack lengths, resulting in an apparent increase in fracture toughness with increasing crack length as evaluated using conventional fracture mechanics. Strain mapping of the nanofiber network indicates stress delocalization mechanisms operating around these macroscopic cracks in the network. The deformation behavior of the nanofiber network will therefore depend on the volume of fibers being loaded in the network and not the number of fibers in the cross-sectional width defining continuum sample mechanics. These results indicate a propensity for both the synthetic electrospun nanofibrous network in this work and potentially other nanofibrous networks to resist failure from macroscopic cracks incorporated within the material.

Hydrogen effect on fracture toughness of thin film/substrate interfaces

The effect of hydrogen on the interface fracture toughness of two nano-film/substrate structures, Ni/Si and Cu/Si, were evaluated using four-point bend specimens with and without hydrogen charging. Hydrogen typically decreases the fracture toughness of materials. However, we found in this study that the interfacial toughness between the Ni film and the Si substrate increased due to the presence of hydrogen, while that of Cu/Si decreased. Nanoindentation experiments for the Ni and Cu films revealed that local plasticity in the Ni and Cu films is promoted by the charged hydrogen. The critical stress intensity at the Ni/Si interface crack considering the plasticity of Ni, namely the true fracture toughness, is scarcely influenced by the existence of hydrogen. The apparent increase in fracture toughness of the Ni/Si interface is due to the large stress relaxation near the crack tip caused by softening due to the presence of hydrogen. Although the promotion of plastic deformation of Cu relaxes the stress intensity at the Cu/Si interface crack, the apparent interfacial toughness still decreases because of the significant decrease in the true toughness due to the presence of hydrogen.

Analysis of the Mechanical Properties in Short Carbon Fiber-toughened ZrB2—SiC Ultra-high Temperature Ceramic

ZrB2—20 vol.% SiC ceramics with additional 20 vol.% short carbon fibers were developed to promote graceful failure using hot-pressing technique. The influence of short carbon fiber on the strength, and modulus of composite was analyzed and estimated using a modified rule of mixtures. The calculation results showed that a little decrease in flexural strength from 502 to 445 MPa and elastic modulus from 414 to 322 GPa was due to the lower elastic modulus of short carbon fiber. An apparent increase in fracture toughness from 2.0 to 6.56 MPa m1/2 was obtained with the addition of short carbon fiber. And the fracture toughness was analyzed and calculated based on the toughening mechanisms of crack deflection, fiber pullout, and crack bridging. Calculated results were in good agreement with experimental results.

Design of Si3N4-based ceramic laminates by the residual stresses

Ceramic laminates with strong interfaces between layers are considered a very promising material for different engineering applications because of the potential for increasing fracture toughness by designing high residual compressive and low residual tensile stresses in separate layers. In this work, Si3N4/Si3N4-TiN ceramic laminates with strong interfaces were manufactured by rolling and hot pressing techniques. The investigation of their mechanical properties has shown that the increase in apparent fracture toughness can be achieved for the Si3N4/Si3N4-20 wt.%TiN composite, while further increase of TiN content in the layers with residual tensile stresses lead to a formation of multiple cracks, and as a result, a significant decrease in the mechanical performance of the composites. Micro-Raman spectroscopy was used to measure the frequency shift across the Si3N4/Si3N4-20 wt.%TiN laminate. These preliminary Raman results can be useful for further analysis of residual stress distribution in the laminate.

Mechanical characterization of Al 2O 3/ZrO 2 hybrid laminates

Significant residual stresses due to the thermal expansion mismatch were found in the Al 2O 3/Ce–ZrO 2 laminated composites investigated in this study. The residual stresses were measured by evaluating the difference in indentation crack length, with and without the residual stresses. The obtained residual stress values are in good agreement with the calculated residual stresses from the measured coefficients of thermal expansion and elastic constants. The laminated structure was found to give rise to an increase in indentation strength compared to monolithic alumina. This apparent increase in fracture toughness can be explained as the effect of the compressive residual stress in the outer Al 2O 3 layer and can be controlled by changing the relative layer thickness. For the size scale studied here, the zirconia-based layers did not appear to play a major role in the final failure process.

Effect of fracture mode on warm prestressing

Warm prestressing (wps) is the phenomenon by which the fracture resistance of a material containing a crack is apparently enhanced if the material is (warm) prestressed at a temperature above that at which it is eventually fractured. The wps effect is manifested by materials, such as ferritic steels, which exhibit a fracture toughness transition temperature. Above this temperature a cracked specimen may be loaded, without fracturing, to a crack-tip stress intensity level, K(wps), which significantly exceeds the fracture toughness of the material below the transition temperature, K(ic). If this load is held constant and the specimen is cooled below the transition temperature, K(wps) is unaltered but the specimen does not fracture, even though K(wps) > K(ic). Additional load is required to fracture the specimen. If the crack-tip stress intensity factor calculated from the fracture load at the lower temperature is K(f), then K(f)-K(ic) is the apparent increase in fracture toughness produced by the wps. Tests of this type, which involve loading, cooling under load and then fracture at the lower temperature, are termed LCF tests. A similar wps effect is observed if, following the warm prestress, the specimen is unloaded, cooled to the lower temperature and then fractured. The apparentmore » stress intensity level at fracture, K(f), is again found to be in excess of K(ic) in the absence of prestressing. Tests which involve an unloading step between wps and low temperature fracture are known as LUCF tests. There is a clear wps effect in steels in which the fracture mode is either mixed intergranular/cleavage or wholly intergranular. The magnitude of the effect appears to be similar to that when purely cleavage fracture is involved, though, in the case of LUCF tests, this needs to be confirmed. It would appear that the beneficial effects of wps can be relied upon in circumstances where metallurgical changes alter the fracture mode from cleavage to intergranular fracture.« less

AN INVESTIGATION OF THE WARM PRESTRESSING (WPS) EFFECT IN A533B WELD METAL

Abstract— The WPS effect has been shown to be beneficial in increasing the lower shelf fracture toughness of A533B weld metal for both the load‐unload‐cool‐fracture (LUCF) and the load‐cool‐fracture (LCF) cycles. The LCF cycle gives a greater apparent increase in fracture toughness than the LUCF cycle but only a small increment of load in the lower shelf region is needed to propagate failure compared with a much increased fracture load which is required in the LUCF case. Models predicting the increase in apparent fracture toughness by superposition of the stress and strain distributions from each step in the prestress cycle are shown to be non‐conservative for failures occurring in the lower shelf brittle regime. There is evidence for the operation of a “microcrack blunting” mechanism, but the extent to which it contributes to the WPS effect is small. Stress‐relief heat‐treatments after prestressing remove nearly all the beneficial effects of WPS. Residual stresses clearly play an important role in the WPS phenomenon, and the models successfully predict experimentally observed trends. Prestressing “deactivates” failure initiation from the larger inclusions in the weld metal. This however does not affect the intrinsic fracture toughness of the material. Fracture in this weld metal, where initiation sites (inclusions) abound, appears to be a co‐operative process, and fracture is controlled by inclusions of the mean (and hence most plentiful) diameter rather than the maximum diameter.

Some comments on the effect of biaxial stress on fatigue crack growth and fracture

The influence of biaxial loading on plastic zone size and crack opening displacement has been examined. Loading parallel to the crack plane was accounted for in satisfying the yield criteria, proposed by Von Mises, along the crack plane. Comparing plastic zone sizes for biaxial and uniaxial loading, when the applied stress normal to the crack plane is the same in both cases, shows a reduction of up to 65 per cent in plastic zone size for the biaxial loading case. The crack opening displacement is also reduced up to 33 per cent for biaxial loading. Based on critical crack opening displacement, fatigue crack propagation rates would be lower than for uniaxial loading, and an apparent increase in fracture toughness would also occur. The theoretical predictions are supported by a limited amount of experimental data.