Elastic-plastic Fracture Mechanics Methods Research Articles

Fracture toughness of the stomatopod dactyl club is enhanced by plastic dissipation: A fracture micromechanics study

The dactyl club of stomatopods is a biological hammer used to strike on hard-shell preys. To serve its function, the club must be imparted with a high tolerance against both contact stresses and fracture. While the contact mechanics of the club has been established, fracture toughness characterization has so far remained more elusive and semi-quantitative using nanoindentation fracture methods. Here, we used microcantilever fracture specimens with a chevron-notched crack geometry to quantitatively evaluate the fracture response of the impact region of dactyl clubs. The chevron-notched geometry was selected as it minimizes surface-related artefacts due to ion milling, and further allows to carry out fracture tests on samples free of pre-cracks with stable crack propagation even for brittle materials. Both linear elastic as well as elastic-plastic fracture mechanics methods, together with finite element modelling, were employed to analyse the fracture data. We find that crack-tip plastic dissipation is the main mechanism contributing to the fracture properties of the dactyl club material. Our study also suggests that the chevron-notched crack geometry is a suitable method to quantitatively assess the fracture toughness of hard biological materials. Statement of significanceCharacterizing the fracture resistance of biomineralized structures is essential to draw their structure-properties relationships. Yet measuring the fracture properties of such materials is often hampered by their small size and irregular shape. Indentation fracture is used to circumvent these issues but does not discriminate between the elastic and elastic-plastic contributions to the fracture resistance. The dactyl club “hammer” of mantis shrimps is a biological material whose fracture properties are central to its function. A microfracture study was conducted using microcantilever specimens with chevron-notched crack geometry to assess the fracture toughness. Adopting linear elastic and elastic-plastic fracture mechanics protocols, we find that plastic dissipation is the major contribution to the fracture response of the hypermineralized impact region of the dactyl club.

Fracture Toughness of the Stomatopod Dactyl Club is Enhanced by Plastic Dissipation: A Fracture Micromechanics Study

The dactyl club of stomatopods is a biological hammer used to strike on hard-shell preys. To serve its function, the club must be imparted with a high tolerance against both contact stresses and fracture. While the contact mechanics of the club has been established, fracture toughness characterization has so far remained more elusive and semi-quantitative using nanoindentation fracture methods. Here, we used microcantilever fracture specimens with a chevron-notched crack geometry to quantitatively evaluate the fracture response of the impact region of dactyl clubs. The chevron-notched geometry was selected as it minimizes surface-related artefacts due to ion milling, and further allows to carry out fracture tests on samples free of pre-cracks with stable crack propagation even for brittle materials. Both linear elastic as well as elastic-plastic fracture mechanics methods, together with finite element modelling, were employed to analyse the fracture data. We find that crack-tip plastic dissipation is the main mechanism contributing to the fracture properties of the dactyl club material. Our study also suggests that the chevron-notched crack geometry is a suitable method to quantitatively assess the fracture toughness of hard biological materials.

Fracture of Silicon: Influence of rate, positioning accuracy, FIB machining, and elevated temperatures on toughness measured by pillar indentation splitting

The pillar indentation splitting test is a novel technique for assessing the fracture behavior of materials using micro-scale pillar samples. One typical limitation of this technique is the necessity of fabricating samples using focused ion beam (FIB) machining, which both creates damage to the samples and limits the number of samples which can be manufactured in a set timeframe. An alternative fabrication technique, lithography, is used here to fabricate a large number of (100)-oriented, Silicon micro-pillar samples. This allowed parametric studies of pillar splitting to be performed to study the influence of testing rate and positioning accuracy. Further, it allows the comparison of samples produced using different methods (lithography, Gallium FIB, and Xenon FIB) as a function of size. FIB damage was found to significantly increase the apparent toughness at smaller pillar sizes, but the influence diminishes to negligibility at pillar diameters > 10 μm. Lastly, the fracture behavior of Silicon was investigated as a function of temperatures up to 300 °C. Apparent toughness values began increasing at 175 °C due to crack blunting due to partial dislocation-mediated plasticity. At temperatures > 250 °C, the plasticity was sufficient to prevent splitting – requiring elastic-plastic fracture mechanics methods for further analysis.

Does a nucleated crack propagate?

Conditions for propagation or non-propagation of an incipient crack from stress concentrations such as notches, holes, etc., are evaluated using elastic-plastic fracture mechanics methods and the Kitagawa-Takahashi diagram, hence linking smooth specimen behavior to fracture mechanics. The analysis differs from all other previous models which are either empirically based or assume that short crack growth behavior differs from that of long cracks due to crack closure or lack of it. It is shown that the stress intensity factor of an incipient crack decreases and then increases as it moves away from the internal stress field of a notch. Crack propagation is ensured only when the minimum is equal to or greater than the unique fatigue threshold for crack growth. Otherwise non-propagation conditions prevail. Equations are developed to establish the conditions for the minimum stress required for continuous propagation of the originated crack. It is shown that by analyzing published results on notch-fatigue that specimen failure occurs only if the applied stresses exceed the minimum stress required for crack propagation. Otherwise crack arrest occurs resulting in non-propagating cracks. The analysis can be readily used to determine the conditions for failure of a notched specimen if the elastic stress concentration factor, kt, the notch-tip radius, ρ, and fatigue crack threshold, Kth, for the material are known. The analysis, in principle, is applicable to all subcritical crack growth processes.

Fracture mechanics assessment of thermal aged nuclear piping based on the Leak-Before-Break concept

The Leak-Before-Break (LBB) concept has been accepted to design the primary piping system of the pressurized water reactor (PWR). Due to thermal aging of long term operation, the cast stainless steels (CSSs) which are used for the primary piping of PWR, suffer a significant loss of fracture toughness, and as a consequence the safety margin of the thermal aged pipe decreases. Therefore, the aged piping should be analyzed and validated by the LBB concept. In this paper, elastic–plastic fracture mechanics (EPFM) assessments of the thermal aged piping are presented according to the LBB concept. The critical break size of crack unstable tearing is calculated by the EPFM method. The crack driving force diagram (J–a diagram), the stability assessment diagram (J–T diagram) and a numerical method are applied to calculate the critical crack size of crack break. The effects of thermal aging on the plastic limit load, J–T diagram, critical crack size of the EPFM and the critical failure mode are studied. The results show that the thermal aging effect decreases the maximum allowed J-integral at a certain ductile tearing modulus by more than 50% and it increases the flow stress and plastic limit load by 11.78%. The results based on the J–T diagram are about 40% conservative than those based on the direct numerical method for the high loading case. For the thermal aged piping, it is important to consider the competition failure modes between plastic collapse and unstable ductile tearing.

OS0817 参照応力法に基づく周方向複数欠陥を有する管の弾塑性破壊力学評価法

The elastic plastic fracture mechanics methods defined in the Fitness-for-Service Codes of Japan Society of Mechanical Engineers is applicable limited to the single defect. However, multiple stress corrosion cracks were observed in the same weld joint of recirculation system piping of boiling water reactors. The J integral evaluation methods based a reference stress method was developed for a pipe having multiple circumferential cracks on the same plane, and Z-factor used in the elastic plastic fracture mechanics methods was proposed.

Effect of low temperature on hydrogen-assisted crack propagation in 304L/308L austenitic stainless steel fusion welds

• Measured crack growth resistance of welds at 223 K with 140 wppm H (gas charged). • H reduced fracture initiation toughness by >59% and altered fracture mode. • 223 K altered fracture mode but had no effect on J IC of precharged welds. • At 293 K, microcracks initiate at δ-ferrite, and ferrite governed crack path. • At 223 K, microvoids form at γ deformation band intersections near phase boundaries. Effects of low temperature on hydrogen-assisted cracking in 304L/308L austenitic stainless steel welds were investigated using elastic–plastic fracture mechanics methods. Thermally precharged hydrogen (140 wppm) decreased fracture toughness and altered fracture mechanisms at 293 and 223 K relative to hydrogen-free welds. At 293 K, hydrogen increased planar deformation in austenite, and microcracking of δ-ferrite governed crack paths. At 223 K, low temperature enabled hydrogen to exacerbate localized deformation, and microvoid formation, at austenite deformation band intersections near phase boundaries, dominated damage initiation; microcracking of ferrite did not contribute to crack growth.

Hydrogen-assisted crack propagation in 304L/308L and 21Cr–6Ni–9Mn/308L austenitic stainless steel fusion welds

Elastic–plastic fracture mechanics methods were used to characterize hydrogen-assisted crack propagation in two austenitic stainless steel gas tungsten arc (GTA) welds. Thermally precharged hydrogen (140wppm) degraded fracture initiation toughness and crack growth toughness and altered fracture mechanisms. Fracture initiation toughness in hydrogen-precharged welds represented a reduction of >67% from the estimated toughness of non-charged welds. In hydrogen-precharged welds, microcracks initiated at ferrite, and dendritic microstructure promoted crack propagation along ferrite. Deformation twinning in austenite interacts with ferrite, facilitating microcrack formation. While hydrogen altered fracture mechanisms similarly for both welds, the amount of ferrite governed the severity of hydrogen-assisted crack propagation.

Thermal Fatigue and Fracture Mechanics Analysis of Aluminium Alloy

The in-phase and out-of-phase thermal fatigue of aluminum alloy were experimentally studied. The fatigue life was evaluated analytically by using the elastic-plastic fracture mechanics method (mainly J integral). The results of experiments and calculations showed that the life of out-of-phase fatigue was longer than that of in-phase fatigue within the same strain range. This is the same as the results of other materials such as medium and low carbon steel. On the other hand, the predicted life was consistent with experimental results. This suggests that J integral as a mechanics parameter for characterizing the thermal fatigue strength of aluminum alloy and the calculation method developed here is efficient. A parameter ΔW was proposed from energy aspect to characterize the capacity of crack propagation. The in-phase thermal fatigue life was the same as the out-of-phase thermal fatigue life for identical ΔW values.

Experimental Investigation Into the Fracture Toughness of Polyethylene Pipe Material

The mechanical behavior of the recently produced gas pipes material PE100 is investigated and compared to the commonly used material PE80 to determine their relative advantages. The two materials show plastic behavior at room temperature. The fracture toughness of the two materials is experimentally determined using the two common elastic plastic fracture mechanics methods: the American Society for Testing and Materials (ASTM) multiple specimen test method for determining the J-R curve of the materials, and the crack opening displacement method. The investigation of the fracture behavior of the two materials includes the effect of the specimen thickness as well as specimen configuration. The experimental tests were carried on the compact tension specimens and the single edge notch bending (SENB) specimens. At −70°C, the materials show elastic behavior, the ASTM test method for determining fracture toughness is applied to SENB specimens to determine KIC of both materials. PE80 shows greater resistance to fracture than PE100.

Liquid-Metal Embrittlement of Type 316L Stainless Steel by Gallium as Measured by Elastic-Plastic Fracture Mechanics

Abstract In order to bound failure of austenitic stainless steel storage containers housing Ga-containing compounds, the liquid-metal embrittlement of Type 316L (UNS S31603) stainless steel (SS) by Ga was investigated. Type 316L SS compact tension specimens were exposed to liquid Ga using a depassivation technique to wet the specimen. Linear elastic and elastic-plastic fracture mechanics methods were used to compare the fatigue and fracture behavior. Mild liquid-metal embrittlement was observed, as indicated by increased fatigue crack growth rate, decreased number of fatigue cycles to failure, decreased crack initiation resistance, and increased crack growth rate. Stable cracking was observed for all test conditions. A small amount of intergranular cracking was observed following Ga exposure. No effect of test temperature on embrittlement was observed over the small temperature range examined (35°C to 75°C). Decreasing crosshead displacement rate promoted Ga embrittlement. Based on fractography, profilometry, and mechanics, it appears that both adsorption-induced decohesion and adsorption-enhanced plasticity mechanisms are operative in the Type 316L SS-Ga system.

Thin Layer Splitting Along the Elastic-Plastic Solid Surface

Thin layer splitting along the elastic-plastic solid surface is studied based on the elastic-plastic fracture mechanics method. In the splitting process, since the split arm does not undergo the reversed plastic bending, comparing with the conventional peel test method, the split test has remarkable advantages in measuring the material fracture behavior and is recommended as a new test method. Moreover, besides the driving force parameter, the split test method provides an additional measurable parameter, a residual curvature (or curvature radius) of the split arm. Comparing with the peeling force, the split force also has the connection with the total energy release rate, which is related with the crack tip separation energy (or material fracture toughness), separation strength, and the plastic dissipation work. Through measuring the driving force and the residual curvature, the fracture toughness and separation strength can be obtained. The primary objective of the present research is to develop a series of relations of the split force, the residual curvature, as well as the crack tip slope angle, respectively with the split layer thickness and material parameters, when crack tip advances steadily. Frictionless (or smooth) contact between splitter head and split arm surface is assumed. Another objective of the present research is to explore a connection between the split test solutions and the peel test solutions. Finally, the split test analysis is applied to a wedge-loaded double-cantilever beam experiment for Al-alloy material, a considerably similar test method with the split test, conducted by Thouless and his collaborators, and the fracture parameters from both test systems are correlated.

Load-carrying capacity and crack resistance of a cladding by the Sigma-oscillating wire technique

If cracks are postulated in the ferritic base material beneath the austenitic cladding, their initiation and propagation under hypothetical loading cases is influenced by the load carrying capacity of the cladding. The toughness of the KKS-RPV cladding was assessed by means of elastic-plastic fracture mechanics methods. Sub-sized tensile and bend specimens were fabricated by reconstitution technique from broken halves of standard ISO-V Charpy specimens, representing crack extension in radial and circumferential direction. They were tested and evaluated and further analyzed with the Gurson model. For temperatures relevant to the loss of coolant and upset conditions analyzed, a sufficient toughness of the cladding in terms of J and CTOD resistance curves could be shown.

Analysing Effect Of Residual Stresses Based On Crack Closure Model For Part Through Fatigue Crack Growth Problems

The strip yield crack closure model has been extended for the fatigue crack growth in the residual stress field for the part through cracks. The analysis of residual stress includes not only the initial stress, but also its relaxation due to the crack growth and the fatigue load The investigation shows that it is possible that the fatigue crack growth in the residual stress field can be reasonably analysed based on the elastic-plastic fracture mechanics method

Linear elastic vs elastic-plastic fracture mechanics methods in nuclear vessel integrity assessments

This paper compares analytical failure predictions for a flawed PWR vessel from linear elastic and elastic-plastic fracture mechanics, as currently permissible by ASME code. The significant conservatism under upper-shelf conditions provided by the first approach has been quantified in terms of internal pressure and wall-thickness strain gradient. Monotonic crack growth withstood by the assessed component has been evaluated through several elastic-plastic criteria, and leak-before-break and related crack-arrest events have been inferred to be likely the deeper the postulated pre-crack is. Research results indicate that logarithmic J-R curve data fitting is more appropriate and conservative than the conventionally used power law in regard to extensive crack propagation. Conservative linear elastic and elastic-plastic predictions have also been confirmed by testing small-scale and/or deeply sidegrooved testpieces.

Creep brack growth behavior of Two Al-Li alloys

Creep crack growth behavior of two Al-Li-Cu-Mg-Zr alloys with and without the addition of Ge was determined under static load at 150 °C and the results were analyzed using the linear elastic and elastic-plastic fracture mechanics methods. The alloys were produced through conventional ingot metallurgy processing and extruded into a plate form. Texture is more pronounced in the alloy that does not contain Ge. Because of the texture, crack growth rates in the two alloys differ significantly for the L-T and T-L orientations. The growth occurs by the nucleation of microcracks ahead of the main crack which grow and coalesce with the main crack. While this leads to rapid crack growth in the T-L orientation, it causes crack tip bifurcation and even complete crack arrest in the L-T orientation.