Fracture toughness

About: Fracture toughness is a research topic. Over the lifetime, 39642 publications have been published within this topic receiving 854338 citations.

The fracture and fatigue behaviour of nano-modified epoxy polymers

Abstract: The introduction of nano-silica particles into an epoxy polymer has increased both the initial toughness, as measured by the fracture toughness, KIc, and also significantly improved the cyclic-fatigue behaviour of the epoxy polymer. Thus, the significant increases recorded in the values of the range of applied stress-intensity factor at threshold, ΔKth, from the cyclic-fatigue tests for the nano-silica modified materials are very noteworthy, since these increases are accompanied by significant improvements being recorded in the initial toughness.

Fracture toughness of an epoxy system

Abstract: The fracture toughness of epoxy used in the bulk and adhesive form was measured by a previously developed technique. The uniform double cantilever-beam specimen, which was described earlier, was modified to a tapered beam, which simplified the experimental procedure and calculations for obtaining toughness measurements. by varying the ratio of hardener to resin and post-cure temperature on a single epoxy system (DER 332-TEPA), it was found that the toughness of the epoxy used in either bulk or bond form varied by a factor of approximately five. A particular combination of composition and post-curing temperature generally yielded higher toughness in the bulk than in the bond form. This was not always the case, however. At high post-cure temperatures, where the bonds were very tough, their toughness exceeded that of the bulk material. Hence, it does not appear possible to predict joint toughness from bulk toughness measurements. The toughness of joints was found to be a single-valued function of tensile modulus. For the bulk material, on the other hand, the toughness obtained on the epoxy having a specific modulus depended on the combination of composition and post-cure temperature. Joint toughness for any combination of composition and post-cure temperature depended only on the cracking rate. If the epoxy was the type that caused cracks to jump rapidly, the epoxy was tough and vice versa. For a particular epoxy system, toughness was increased by driving the crack at an increasing rate.

Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering

Abstract: Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB 2 - and HfB 2 -based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB 2 with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB 2 with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La 2 O 3 addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.

How does “phase transformation toughening” work in semicrystalline polymers?

Abstract: The possibility of phase transformation toughening is demonstrated by the example of the β-modification of isotactic polypropylene (β-iPP), which undergoes βα-transformation (i.e., from hexagonal to monoclinic) during mechanical loading. The resulting α-iPP exhibits a higher crystalline density than the initial β-modification. That, along with the exothermic character of the βα-recrystallization, is responsible for the improvement in toughness that occurs. The occurrence of this βα-transformation is evidenced by differential scanning calorimetry (DSC). Toughness of the α- and β-iPP is studied and compared with the “essential work of fracture” concept by using static-loaded deeply double-edge-notched tensile (DDEN-T) specimens. The main effect of the βα-transformation is a large increase in the specific plastic work consumed in the necked zone. Light microscopic (LM) and infrared thermographic (IT) pictures reveal that the plastic zone becomes larger and its shape more circular when βα-transformation takes place. It is suggested that the principle of mechanical stress-induced phase transformation from a less toward a more dense crystalline state may be a universal tool for toughness upgrading in semicrystalline polymers.