Primary Fracture Mode Research Articles

Thermal shock and thermal fatigue study of Sr- and Mg-doped lanthanum gallate

Thermal shock and thermal fatigue resistance of La 0.85Sr 0.15Ga 0.85Mg 0.15O 2.85 (LSGM) were investigated using a water-quench technique. Thermal shock resistance test shows that the residual strength decreases gradually as the thermal shock temperature increases, and the critical temperature difference, Δ T c, is about 75 °C. The SEM observations show that the primary fracture mode does not change and intergranular fracture is the primary fracture mode before and after thermal shock test. Thermal fatigue measurements of LSGM and 2 wt% Al 2O 3 strengthened LSGM (named as LSGMA) show that the addition of Al 2O 3 did not enhance the crack extension resistance.

Investigation on surface treatments of CFRP and aluminum to improve fracture toughness of adhesively-bonded CFRP–aluminum joints

In this study, the role of surface treatments of CFRP (graphite/epoxy composite) and aluminum (7075-T6) on the adhesively-bonded CFRP-aluminum joints has been investigated. The CFRP was surface-treated by Ar+ ion irradiation in an oxygen environment and the aluminum was surface-treated using a DC plasma. Ar+ ion irradiation treatment was carried out at Ar+ ion dose of 1016 ions/cm2. Plasma treatment was carried out at a volume ratio of acetylene gas to nitrogen gas of 5:5 and the treatment time was 30 s. The effect of surface treatments on the fracture behavior CFRP-aluminum joints was determined from fracture tests using three different CLS (cracked lap shear) specimens: (1) untreated CFRP/untreated aluminum, (2) ion-irradiated CFRP/untreated aluminum and (3) untreated CFRP/plasma-treated aluminum. Fracture behaviors (fracture load, fracture toughness, fracture surfaces) of these three different specimens were compared. The results showed that both fracture load and fracture toughness of CFRP-aluminum joints were in the following order: ion-irradiated CFRP/untreated aluminum specimen > untreated CFRP/plasmatreated aluminum specimen > untreated CFRP/untreated aluminum specimen. SEM examination of fracture surfaces showed that fracture occurred as an interfacial failure for untreated specimens. On the other hand, a cohesive failure in the adhesive was the primary fracture mode for specimens surface-treated by ion irradiation or plasma.

Hydrogen embrittlement of a Ti-strengthened 250 grade maraging steel

Constant extension-rate tensile tests are performed to investigate the effects of strain rate and environmental hydrogen concentration on the tensile properties of various aged T-250 specimens. The 426 °C (800 °F) underaged specimens are very sensitive to strain rate; the 482 °C (900 °F) peak-aged specimens exhibit a reduced ductility under low strain rates; and the 593 °C (1100 °F) overaged specimens are insensitive to strain rate when tested in air. The excellent resistance to embrittlement of the overaged specimens in gaseous hydrogen could be associated with the extensive formation of reverted austenite and the incoherent Ni3Ti precipitates. The tensile-fractured surfaces of such specimens reveal a ductile dimple fracture. However, the peak-aged specimens are susceptible to gaseous hydrogen embrittlement, and the embrittled region shows a primary fracture mode of quasi-cleavage. The least resistant to hydrogen embrittlement of the underaged specimens is characterized by a more brittle fracture appearance, that is, intergranular fracture, under a low strain rate or in the gaseous hydrogen environment.

Fracture behavior in medium-carbon martensitic Si- and Ni-steels

A study of fracture behavior in medium-carbon martensitic Ni- and Si-steels has been made. Charpy impact testing was conducted in order to investigate fracture mode as a function of test temperature. Whereas the transgranular cleavage fracture becomes the primary brittle fracture mode with decreasing test temperature in the Si-steel, intergranular fracture is the only brittle fracture mode observed at low temperatures in the Ni-steel. The different fracture behavior between these steels appears to be due to variation in intrinsic matrix toughness. Since Si may impair the intrinsic matrix toughness, the occurrence of transgranular cleavage fracture becomes relatively easy with decreasing test temperature. On the other hand, since Ni may improve considerably the intrinsic matrix toughness, the transgranular cleavage fracture is not able to occur although the test temperature decreases. Thus, the intrinsic matrix toughness can play a significant role in controlling the fracture behavior.

The nature of quasicleavage fracture in tempered 5.5Ni steel after hydrogen charging

Quenched and tempered 5.5Ni steel was embrittled by hydrogen charging and broken in air at room temperature. The primary fracture mode was transgranular quasicleavage. The quasicleavage facets were studied by scanning electron fractography and by transmission electron microscopy of profile fractographic specimens. The latter were prepared by plating the fracture surface with nickel and thinning so that the fracture surface was contained within the region of the specimen that was transparent to the electron beam. The fracture surface generally followed martensite lath boundaries. In addition, interlath microcracks were frequently found in the material immediately beneath the fracture surface. These results suggest that transgranular hydrogen embrittlement in this steel is primarily an interlath cracking phenomenon. Since the lath boundary planes tend to lie in {110}, the results also explain the prevalence of {110} quasicleavage in the embrittled specimens, which contrasts with the {100} cleavage found in uncharged specimens broken below the ductile-to-brittle transition temperature.

The influence of the M23 (C,N)6 compound on the mechanical properties of type 422 stainless steel

Ductility and toughness of Type 422 stainless steel was shown to be strongly influenced by either the presence or absence of large grain boundary precipitates of the M23(C,N)6 compound. A lengthy, isothermal solution heat treat cycle combined with a longer, lower temperature precipitation heat treatment was used to exaggerate the M23(C, N)6 compound along the boundaries of large austenite grains. After a subsequent conventional heat treatment, the presence of this compound along the prior austenite grain boundaries substantially reduced the ductility and the Charpy V-notch impact energy. The primary fracture mode was intergranular failure along these prior austenite grain boundaries. This is a reversible process and this compound can be completely dissolved. The M23 (C, N)6 compound was removed from these prior austenite grain boundaries by another lengthy, isothermal resolution heat treatment. After a subsequent conventional heat treatment, the ductility returned to values which are near those found in as-received and heat treated material; the Charpy V-notch impact energy increased to values above that found in commercially processed Type 422 stainless steel. The fracture appearance returned to the more normal transgranular ductile dimple mode.