Change In Fracture Surface Morphology Research Articles

The combined effects of hydrogen and aging condition on the deformation and fracture behavior of a precipitation-hardened nickel-base superalloy

The effect of hydrogen (H) on the deformation behavior of Monel K-500 in various isothermal heat treatment conditions (non-aged, under-aged, peak-aged, and over-aged) was assessed via uniaxial mechanical testing. H-charged and non-charged specimens were strained to failure to facilitate a comparison of ductility, fracture surface morphology, strength, and work hardening behavior. For all examined heat treatment conditions, H charging leads to a significant reduction in ductility, which is accompanied by a consistent change in fracture surface morphology from ductile microvoid coalescence to brittle intergranular fracture. While H charging led to a systematic enhancement in the yield strength of all heat treatments, the three age-hardened conditions exhibited a more than 2-fold increase relative to the non-aged heat treatment. This suggests that H modifies the dislocation–precipitate interactions, which also manifest themselves through changes in work hardening metrics related to the dislocation storage and recovery rates. In particular, the H-charged peak-aged specimen exhibited a significant increase in initial hardening (dislocation storage) rate relative to the H-charged under-aged specimen. Transmission electron microscopy of these samples revealed the onset of widespread dislocation looping in the H-charged peak-aged sample, in addition to the planar slip bands characteristic of the non-charged condition. This result suggests that hydrogen induces the particle shearing-to-looping transition at smaller particle sizes. Possible mechanistic explanations for this observed behavior are presented.

Mechanical properties and morphological characteristics of ARALL reinforced with TRGO doped epoxy resin

ABSTRACT Mechanical properties in tension, bending, fatigue and lap-shear in two different proportions (0.5%wt and 1%wt) of TRGO (Thermally Reduced Graphite Oxide) doped ARALL (Aramid Aluminium Laminate) were examined. The materials and their failure modes were characterized morphologically by examination through SEM (Scanning Electron Microscopy). Some mechanical properties of ARALL were improved when doped with 0.5% of TRGO, showing a significant increase of fatigue properties, as well as a change in fracture surface morphology. Tension andbending properties showed variable results and further studies should be carried out to arrive to definitive conclusions, while lap-shear testing showed lower shear values. The results were statistically validated through mono-factorial variance analysis. Comparing the present results with previous work on CNT (Carbon Nanotubes) doped ARALL, it can be stated that: (a) TRGO doped ARALL showed improved fatigue properties when compared with non-doped ARALL, but in a less effective way than doping with CNT, (b) TRGO doped ARALL tension properties showed no significant variation as compared with ARALL alone, showing no deleterious influence as in the CNT doping case, (c) TRGO doped ARALL bending properties resulted better than non-doped ARALL, but similar than those obtained when doping with CNT and (d) TRGO decreased the adherence between aramid fiber impregnated L20 epoxy resin and aluminium. These last results are sustained based on observed improvements as a percentage value, without a statistical variance analysis made on CNT doped ARALL.

Mechanical properties and morphological characteristics of ARALL reinforced with TRGO doped epoxy resin

ABSTRACT Mechanical properties in tension, bending, fatigue and lap-shear in two different proportions (0.5%wt and 1%wt) of TRGO (Thermally Reduced Graphite Oxide) doped ARALL (Aramid Aluminium Laminate) were examined. The materials and their failure modes were characterized morphologically by examination through SEM (Scanning Electron Microscopy). Some mechanical properties of ARALL were improved when doped with 0.5% of TRGO, showing a significant increase of fatigue properties, as well as a change in fracture surface morphology. Tension andbending properties showed variable results and further studies should be carried out to arrive to definitive conclusions, while lap-shear testing showed lower shear values. The results were statistically validated through mono-factorial variance analysis. Comparing the present results with previous work on CNT (Carbon Nanotubes) doped ARALL, it can be stated that: (a) TRGO doped ARALL showed improved fatigue properties when compared with non-doped ARALL, but in a less effective way than doping with CNT, (b) TRGO doped ARALL tension properties showed no significant variation as compared with ARALL alone, showing no deleterious influence as in the CNT doping case, (c) TRGO doped ARALL bending properties resulted better than non-doped ARALL, but similar than those obtained when doping with CNT and (d) TRGO decreased the adherence between aramid fiber impregnated L20 epoxy resin and aluminium. These last results are sustained based on observed improvements as a percentage value, without a statistical variance analysis made on CNT doped ARALL.

Effects of sample temperature on spall fracture in laser shock-loaded metals between 30 K and 1000 K

For many years, spall fracture of shock-loaded materials has been one of the most widely studied phenomena in shock physics, for both fundamental and technological motivations. Laser driven shocks provide a means to investigate this process over ranges of extremely high strain rates and short durations, and they allow recovering spalled samples more easily than plate impact or explosive loading techniques. In this paper, we present laser shock experiments on gold and aluminium in cryogenic conditions (relevant in the context of inertial confinement fusion), and on iron at high temperatures up to about 1000 K. Time-resolved measurements of the free surface velocity are used to determine the evolution of the spall strength with sample temperature. They are complemented by post-test observations of the recovered targets, which reveal clear changes in fracture surface morphology in the spall craters. In the case of iron, possible influences of pressure-induced phase transformations prior to tensile loading are discussed on the basis of hydrodynamic simulations.

Effect of nanoclay addition on mechanical and thermal behavior of vinyl ester based nanocomposites obtained by casting

This paper presents the experimental study of mechanical and thermal properties of organically modified montmorillonite clay (Nanoclay) (0, 1, 2, 3, 4 and 5 wt.%) in the vinyl ester matrix by ultrasonic stirrer. The changes in mechanical properties are investigated by using tensile and impact testing machine. It was found that the addition of nanoclay particles significantly improved tensile properties of pure vinyl ester, but impact properties of pure vinyl ester were affected negatively with the nanoclay content in the nanocomposite. It was found that the absorbed energy and impact resistance of the nanocomposites decreased with increasing the nanoclay content. DSC curves showed the glass transition temperature change in the nanoclay reinforced vinyl ester nanocomposites as compared to the pure vinyl ester. XRD analysis was performed to identify the structure of nanocomposites. SEM results showed the change in fracture surface morphology of nanoclay reinforced vinyl ester nanocomposite. Also, homogeneous distribution of nanoclays in the matrix was showed by SEM micrographs. This observation helped in identifying the morphology of the nanocaly in the vinyl ester matrix.

Hydrogen-Assisted Fatigue Lifetime Characteristic of AF1410 Steel

Results of hydrogen-assisted fatigue lifetime testing indicated a substantial but gradual increment in crack growth rate as a function of increasing hydrogen content. Hydrogen was introduced into both sides of the specimen simultaneously via galvanostatic charging. Extensive scanning electron microscope fractographic analyses revealed a clear shift in the modes of failure and changes in fracture surface morphology as a function of increasing hydrogen content. A methodology is outlined that can be used to predict fracture surface features as a function of applied stress intensity factor. A convenient deterministic model is proposed that seems to reasonably accurately capture the crack growth rate behavior under strain controlled testing conditions. In addition, successful application of acoustic emission technique to classify various cracking stages in full spectrum testing was performed on hydrogen charged samples.

Effects of theα−εphase transition on wave propagation and spallation in laser shock-loaded iron

Despite extensive research work on the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\epsilon}$ phase transition occurring in shock-loaded iron, the kinetics of this transformation remain largely unknown. Here, we present time-resolved free surface velocity measurements in iron foils of thicknesses ranging from 150 to $520\text{ }\ensuremath{\mu}\text{m}$ subjected to laser shocks of peak pressure of about 130 GPa and duration of about 3 ns. The records show an elastic precursor followed by a plastic front, but the double wave structure that is usually associated with the phase change does not appear over such short propagation distances. The measured profiles are compared to the predictions of one-dimensional simulations involving rate-dependent descriptions of both twinning and phase transitions. Such comparisons provide an estimate of a time constant governing the transformation kinetics, which is found to strongly condition the evolution and attenuation of the pressure pulse during its propagation. The spall stress evaluated from the records is shown to be significantly higher after the $\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\epsilon}\text{\ensuremath{-}}\ensuremath{\alpha}$ cycle. Metallurgical observations of the recovered samples confirm both the phase transition and the spall damage inferred from the velocity profiles. Finally, they show the very clear change in fracture surface morphology to the so-called smooth spall expected above the phase transformation.

The Influence of Loading Rate on the Ultimate Properties and Fracture Mechanism of Styrenic Thermoplastic Elastomers

The influence of loading rate on the tensile fracture of polystyrene-polyisoprenepolystyrene (SIS) and polystyrene-poly(ethylene-co-butylene)-polystyrene (SEBS) has been investigated. The tensile strength of SIS initially increased with increasing strain rate, eventually reaching a plateau at elevated strain rates. In contrast, the tensile strength of SEBS was relatively unaffected by strain rate. The fracture surfaces of the tensile test specimens were examined by scanning electron microscopy. The fracture surface morphologies indicated that fracture initiated via cavitation, followed subsequently by void coalescence and catastrophic fracture. For both materials there was no qualitatively obvious change in fracture surface morphology with increasing strain rate. The results indicate that the ultimate strength of styrenic thermoplastic elastomers is governed by the nature of the dominant failure mechanism at the molecular scale; when chain scission dominates, the tensile strength is independent of the strain rate, but when chain pull-out dominates, the tensile strength increases with increasing strain rate.

Dynamical Fracture Instabilities Due to Local Hyperelasticity at Crack Tips

ABSTRACTWhen materials break and cracks propagate, bonds between atoms are broken generating two new material surfaces. Most existing theories of fracture assume a linear elastic stress-strain law. However, the relation between stress and strain in real solids is strongly nonlinear due to large deformation near a moving crack tip, a phenomenon referred to as hyperelasticity or nonlinear elasticity. Cracks moving at low speeds create atomically flat mirror-like surfaces, whereas cracks at higher speeds leave misty and hackly fracture surfaces. This change in fracture surface morphology is a universal phenomenon found in a wide range of different brittle materials, but the underlying physical reason has been debated over an extensive period. Using massively parallel large-scale atomistic simulations employing a new, simple atomistic material model allowing a systematic transition from linear elastic to strongly nonlinear material behaviors, we show that hyperelasticity can play a governing role in dynamical crack tip instabilities in fracture of brittle materials. We report a generalized model that treats the instability problem as a competition between different mechanisms controlled by local stress field and local energy flow near the crack tip. Our results indicate that the fracture instabilities do not only appear in defected materials, but instead are an intrinsic phenomenon of dynamical fracture. Our findings help to explain controversial experimental and computational results, including experimental observation of crack propagation at speeds beyond the shear wave speed in rubber-like materials.

Elevated temperature fatigue crack propagation behavior of an Al-Li-Cu-Mg-Ag-Zr alloy

Preliminary indications concerning the applicability of X2095-type alloys for warm temperature applications (125--200 C) are encouraging. The strength capability is clearly demonstrated. Before commitments are made to this alloy system for elevated temperature applications, data need to be gathered on microstructures after long term exposure to the service temperature. Fracture toughness, impact resistance, and creep data are also required to make a complete assessment. The fatigue crack propagation data at 10Hz is also encouraging. More detailed experiments are required to determine the cause for a change in fracture surface morphology compared to room temperature. The fact that these fracture surfaces are so dissimilar, yet the crack propagation rates are nearly identical (in the near threshold region) is perplexing. Additional data are required at lower frequencies and tensile hold times to assess the influence of creep-environment-fatigue interactions, and their impact on component life at these service temperatures.

Effects of stress ratio on fatigue crack propagation in Q1N (HY80) pressure vessel steel

The effects of the stress ratio on fatigue crack propagation under plane strain conditions are evaluated by load-shedding tests and closure measurements for stress ratios ( R = K min K max ) of 0.2, 0.35, 0.5 and 0.7. The changes in fracture surface morphology associated with crack closure and mechanisms are explained, and the trends in the crack closure data with increasing stress ratio and stress intensity are explained by considering the effects of the increase in crack opening displacement with increasing stress ratio and stress intensity.

Changes in fracture surface composition and morphology with time in vacuum in YBa2Cu3O7−x

Changes in fracture surface composition and morphology with time after fracture in a high vacuum environment are observed in the high transition temperature oxide superconductor YBa2 Cu3 O7−x . Morphological changes involving several grains and extending over distances of tens of microns are seen. Changes in fracture surface morphology are, at least in some cases, preceded by changes in surface composition as determined from Auger electron spectroscopy, and can occur at room temperature in the absence of electron beam excitation.

Comparison of techniques for the measurement of plastic work of fatigue crack growth in low carbon steel

The cyclic plastic work to produce a unit area of fatigue crack, U, was calorimetrically measured in a low carbon steel. The results of the calorimetric U measurements are roughly an order of magnitude higher than U measurements obtained using other techniques. A drop off in U observed at low crack growth rates was observed to be associated with a change in fracture surface morphology. Several improvements in data analysis and acquisition are also reported.

Toughening of polyester resins by rubber modification

The microstructures of two polyester resin systems, with reactive liquid rubber additions incorporated, were investigated using electron microscopy. Fracture surface morphologies of failed fracture toughness specimens were examined using scanning electron microscopy. The fine structures of unfractured toughened resins were examined by imaging ultra thin sections in the transmission electron microscope. Three of the four rubber additives investigated produced, upon curing, a dispersion of second-phase rubber-rich particles in the polyester resin matrix. The fourth additive, which was more compatible with polyester than the other three, did not produce any detectable particle dispersion upon curing, even at relatively high concentrations. The fine structures of the particle distribution were found to be highly dependent upon both rubber and resin formulations. Rubber additions modified the mode of fracture observed in double torsion tests of polyester resins, from continuous crack propagation to “slip-stick”, and distinctive changes in fracture surface morphology were observed. In zones of crack arrest and slow stable crack growth, crack blunting occurred and highly deformed structures were seen on the fracture surface. In one system, this zone was split into two distinct regions, due to crack blunting and the initiation of new, sharp cracks. In zones of rapid crack growth, there was no evidence of crack blunting. The amount of crack blunting was highly dependent upon speed of testing.

Microstructure and mechanical properties of rapidly quenched L11 alloys in Ni-Al-X systems

Ductile Ll2-type y′ compounds with rather high strengths and large elongations have been found in rapidly quenched Ni-Al-X (X = Cr, Mn, Fe, Co, or Si) ternary systems. The y′ compounds consist of a metastable phase which contains numerous APD with a size as small as about 50 to 75 nm. Further, the ductile y′ wires with circular cross section have been manufactured directly from the liquid state by an in-rotating-water spinning method. The wire diameter is in the range of 85 to 190 µm and the average grain size is about 2 to 5 µam. The Hv, σy, σf, and eρ of the y′ wires are about 240 to 400 DPN, 390 to 590 MPa, 580 to 910 MPa, and 4 to 10 pct, respectively, for Ni-Al- (Cr, Fe, Co, or Si) systems and about 220 DPN, 260 MPa, 440 MPa, and 27 pct, respectively, for the Ni-Al-Mn system. A cold drawing causes a significant increase in σy and σf and the attained values are about 2450 MPa and 2480 MPa, respectively, for Ni-20Al-10Cr wire drawn to about 90 pct reduction in area. Around the temperatures where the APB disappear on annealing, the Hv, σy, σf, and egr of the y′ wires decrease significantly accompanied with a drastic change in fracture surface morphology from a transgranular type to an intergranular type. It has been therefore inferred that the high strengths and good ductility of the melt-quenched y′ compounds are due to the structural changes to a low degree of ordered state containing a high density of APB and the suppression of grain boundary segregation.

Failure modes in low density aluminous cement bonded composites

Acoustic emission analysis showed that the failure of some low density aluminous cement bonded perlite composites cannot be explained adequately using the classical Griffith type approach in which it is assumed that failure is caused by the catastrophic propagation of the most severe flaw. It was found that the general concept of cumulative weakening could be applied to the failure of these composites. A change in failure mode from that of cumulative weakening to Griffith type fracture occurred when the bulk density of the composite was greater than about 1.6 lb/bd ft and/or the cement content of the material was greater than approximately 30%. In addition, microscopic examination showed a change in fracture surface morphology from primarily interparticle failure at low density or low cement contents to primarily transparticle failure at high density or high cement content.

Effect of elevated temperature exposure on the structure, stability, and mechanical behavior of aluminum-stainless steel composites

The effect of elevated temperature exposure on subsequent ambient temperature tensile behavior of aluminum-stainless steel composities (V f= 6.5 pct) has been studied. In particular, ambient temperature tensile yielding, flow, and fracture were correlated with the associated interface microstructures, matrix substructure, and fracture morphology in the as-pressed condition and following elevated-temperature exposure at 550°C (823 K) or 625°C (898 K) for 24 h (86.4 ks). Compared to the as-pressed condition, exposure at either temperature results in a small increase (≲4 pet) in initial modulus, and a decrease in the level of residual stress (tensile) in the matrix; tensile stress-strain behavior in stage II (matrix plastic, reinforcement elastic) is essentially unaffected. Lower strength levels in stage III (matrix and reinforcement plastic) after exposure are due to premature cracking in the interface reaction zone, primarily a ternary (Fe, Cr) Al intermetallic, with associated notch effects on the wire reinforcement. Changes in fracture surface morphology of the composites confirm the degradation. Wires extracted from composites after hot pressing or following exposure at 550°C (823 K) possess a unique strength. Exposure at 625°C (898 K) leads to a bimodal distribution in the strength of extracted wires. In each condition, a matrix dislocation cell structure develops in stage III; the invariant form and size of the cell structure withV f and distance from the matrix wire interface confirm isostrain conditions.