Precracked Bend Bars Research Articles

On the influence of specimen size and geometry on the fracture toughness of tungsten heavy metal alloys

We previously showed that commercial tungsten (W)-based heavy metal alloy (WHA) composites containing 90 to 97 wt% W, reinforced with 3 to 10 wt% of a NiFe ductile phase (DP), have room temperature (RT) maximum load elastic-plastic fracture toughness (KJm) values from 69 to 110 MPa√m. This is far higher than for monolithic RT W toughness (≈ 8 ± 4 MPa√m). In all cases, fracture took place by stable crack growth. However, these results were based on very small (16/3.3/1.65 mm) pre-cracked bend bar tests, which naturally raises the question of size effects on the measured KJm. While they are best thought of as being composites, here we experimentally assess size and geometry effects on the KJm of WHA, in the broad context of validity criteria that have been developed for both cleavage and ductile tearing, R-curve fracture in steels. Specifically, we examine the relation between KJm and a dimensionless elastic-plastic zone small scale yielding deformation limit criterion, M ≥ σobo/Jm. Here σo is the flow stress, Jm is the measured J at maximum load, and bo is the initial unbroken ligament dimension. Tests on specimens from 3x to 8x larger than the original small (1x) bend bars show only weak or no size effect up to 95W with KJm ≈ 82 ± 5 MPa√m. However, 3x 97W alloy fractured elastically at KIm ≈ 38 ± 4 MPa√m. The corresponding average for the 1x specimens was KJm 100 ± 15 MPa√m for the 90–95W and 69 ± 12 MPa√m for the 97W alloys. The approximately constant KJm for 3–8x specimen sizes suggests that the M required for constraint-related size independence is ≈ 200. However, DP fraction also plays a role, and there are additional effects of compact tension versus bend bar specimen geometries.

The effect of hot rolling on the strength and fracture toughness of 90W–7Ni3Fe tungsten heavy metal alloys

Due to their balance of strength, ductility and toughness, liquid-phase sintered tungsten heavy metal alloys (WHAs) are considered to be an attractive alternative to monolithic polycrystalline tungsten (W) for plasma facing components. Here we explore the effect of hot rolling (HR) on the room temperature (RT) mechanical properties of a 90W–7Ni3Fe (by wt%) WHA, at different thickness reductions of 0, 62, 74 and 87%. High elastic-plastic KJm maximum load toughness for LS-oriented pre-cracked bend bars is observed in the 0 and 87% HR WHA, along with stable crack growth. In contrast, the 62 and 74% HR WHA experience elastic fracture, at much lower KIc. The opposite trend is observed in the microhardness, as well as the tensile yield and ultimate stresses, which are lowest in the 0 and 87% HR condition and highest in the 62 and 74% HR WHA. The reason for the difference in strength at 87 versus 74% HR reductions is not fully understood. However, the strength differences partially rationalize the toughness trends. Unfortunately, HR results in highly anisotropic brick-and-mortar microstructures, leading to much lower elastic KIc in the LT-orientation.

On the Remarkable Fracture Toughness of 90 to 97W-NiFe Alloys Revealing Powerful New Ductile Phase Toughening Mechanisms

Tungsten is generally too brittle to serve a robust structural function. Here, we explore the fracture toughness of 90 to 97%W (by wt.) liquid phase sintered tungsten heavy alloys (WHAs). The room temperature (RT) maximum load fracture toughness (KIc or KJm ≈ 38 to 107 MPa√m) of WHA, containing only 3 to 10 wt.% of a NiFe-based ductile phase (DP), is ≈ 5 to 13 times higher than KIc typical of monolithic W (≈ 8 MPa√m). RT tests for a range of precracked bend bar sizes generally show extensive stable ductile tearing (DT), except in the case of the 97 wt.%W alloy, where elastic fracture occurs in all but the smallest specimen tests. Nevertheless, even in this case the KIc = 38 ± 4 MPa√m, which is still almost 5 times higher than that for monolithic W. Tests with the smallest specimens down to -196°C, to partially emulate irradiation hardening, show decreasing toughness and a transition to elastic fracture at a temperature of -150°C for 90W to -25oC for 97W. However, even at -196°C, the 97W KIc is ≈ 3 times that of monolithic W at RT. In contrast to classical ductile phase toughening by macrocrack bridging, WHA toughening mainly involves new mechanisms associated with arrest, blunting and bridging of numerous dilatationally shielding process zone microcracks.

AE analysis of damage processes in cast iron and high-strength steel at different temperatures and loading rates

Acoustic emissions originating from damage processes in nodular cast iron and high-strength steel were analyzed. Three point bending tests of notched and precracked bend bars were performed to investigate the onset of crack growth at quasi-static and dynamic loading rates and temperatures of 20 and −40 °C. Additionally, the materials were subjected to uniaxial tensile loading. It was found that the steel emits most of the transient acoustic signals during globally elastic behavior. In contrast, for the cast iron, most of the transient acoustic emissions were detected at small plastic strain. Since the acoustic emission were correlated with the volume of the plastically deformed material in the steel, the size of the plastic zone during fracture toughness testing was estimated. Furthermore, the onset of stable crack initiation was derived from the acoustic signal.

On the fracture toughness of irradiated F82H: Effects of loss of constraint and strain hardening capacity

Constraint loss adjustments, based on finite element (FE) stress analysis and local critical stress, σ*, critical stressed volume, V*, cleavage criteria were applied to fracture toughness data from small pre-cracked bend bars of the IEA heat of F82H irradiated to 6.6dpa at 300°C in the High Flux Isotope Reactor. The 100MPa√m master curve method reference temperature shifts were evaluated based on both the measured toughness (ΔTm) and after size-adjusting the toughness to small-scale yielding conditions at reference specimen size (ΔTo) with values of ≈142 and 205°C, respectively. The model based prediction ΔTo=CoΔ〈σfl〉, where Co≈0.68 and Δ〈σfl〉 is the change in the average flow stress over 0–10% strain is in excellent agreement with ΔTo. The FE analyses also demonstrates an upper-bound KJc that can be measured with these small bend bars for irradiated alloys that suffer severe loss of strain hardening.

Assessment of irradiation embrittlement of the Eurofer97 steel after 590 MeV proton irradiation

The irradiation hardening of the Eurofer97 steel following 590 MeV proton irradiations was determined at three different irradiation temperatures, 50 °C, 250 °C and 350 °C, and various doses up to about 1.3 displacement per atom. The dose and temperature dependence of the irradiation hardening was characterized by a linear relationship between the irradiation hardening and the square root of the dose as: Δ σ 0.2 = k( T) dpa 1/2. Mini pre-cracked bend bar (1 × 1 × 16 mm 3) were also tested in the lower ductile to brittle transition region before and after irradiation at 300 °C and 0.5 dpa. The effective fracture toughness-temperature curves, K e( T), were indexed on an absolute temperature scale at T 0 for K e = 100 MPa m 1/2 for both the unirradiated and irradiated condition. The irradiation-induced temperature shift Δ T 0 of the K e( T) curves yielded a coefficient C 0, defined as C 0 = Δ T 0/Δ σ 0.2, of about 0.53. For these low doses, helium effects could not be identified on the fracture properties.

Deformation and fracture toughness of a bulk amorphous Zr–Ti–Ni–Cu–Be alloy

The flow behavior and fracture toughness of two different plate thicknesses (i.e. 4 and 7 mm) of a bulk amorphous Zr–Ti–Ni–Cu–Be alloy was investigated. It is shown that the flow/fracture stress was independent of superimposed hydrostatic pressure over the range 50–575 MPa, suggesting that the flow behavior follows the von Mises criterion. However, the macroscopic orientation of the fracture plane relative to the stress axis was strongly affected by changes in stress state, suggesting some normal stress dependence to the flow/fracture behavior. The fracture behavior was also studied on both notched and precracked bend bars for both plate thicknesses. The average fracture toughness obtained from seven fatigue precracked specimens taken for both plate thicknesses was 17.9±1.8 MPa√m, while the notched toughness obtained on specimens with notch root radii ranging from 65 to 250 μm taken from both plate thicknesses were in the range of 9l–131 MPa√m.

Fracture Toughness Anisotropy and Toughening Mechanisms of a Hot‐pressed Alumina Reinforced with Silicon Carbide Whiskers

The fracture toughness of a hot‐pressed alumina and that of a hot‐pressed alumina/SiC‐whisker composite containing 33 vol% SiC whiskers were measured by four‐point bending on single‐edge precracked bend bars having sharp precracks created by “bridge indentation.” Two batches of the composite were examined, one exhibiting a greater degree of whisker clustering than the other. The fracture toughness of the alumina was around 4 MN·m‐3/2 whereas that of the composite varied between 5 and 8 MN·m‐3/2 depending on microstructural uniformity and crack‐propagation direction. Crack deflection in combination with a change in fracture from intergranular to transgranular fracture is proposed as an explanation of the superior fracture toughness of SiC‐whisker‐reinforced alumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack‐propagation direction in identical crack planes. This effect could with good accuracy be described in terms of crack deflection for the composite with uniform whisker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack‐growth direction was greater and could not entirely be explained by crack deflection.

Crack resistance curves of ceramics determined on notched and precracked bend bars

Crack resistance curves of ceramics determined on notched and precracked bend bars

Fracture Toughness of Ceramic Precracked Bend Bars

Fracture toughness was measured for four ceramic materials using precracked bend bar specimens. The effect of the precracking parameters, used for the bridge indentation method on fracture toughness values, was determined. Excellent agreement was obtained between fracture toughness values measured by this method and values obtained by other techniques.

The fracture behaviour of PMMA in mixed modes I and II

The brittle fracture behaviour of PMMA under mixed mode I/II loading has been investigated using pre-cracked bend bar specimens loaded in antisymmetric and symmetric four point loading. The accuracy of the antisymmetric configuration was verified by a photoelastic investigation, and by comparison with results obtained using other mixed mode load arrangements. The results are used to test and discuss three mixed mode fracture criteria. It is concluded that mixed mode I/II brittle fracture of PMMA at room temperature is described best by a maximum tangential tensile stress criterion based on the linear elastic stress field. This is found to be consistent with the present understanding of the micro-mechanisms involved in fracture of PMMA.