Brittle-Ductile Transition in Heterogeneous Metallic Materials [PowerPoint Submission]

2006 ◽  
Vol 978 ◽  
Author(s):  
Silvester John Noronha ◽  
Nasr M Ghoniem
Keyword(s):  
Fracture Toughness ◽  
Tensile Stress ◽  
Size Distribution ◽  
Crack Tip ◽  
Transition Curve ◽  
Crack Plane ◽  
Metallic Materials ◽  
Brittle Ductile Transition ◽  
Discrete Dislocation ◽  
The Matrix

AbstractWe present a model for the brittle - ductile transition in heterogenous metallic materials based on two dimensional discrete dislocation simulations of crack-tip plasticity. The sum of elastic fields of the crack and the emitted dislocations defines an elasto-plastic crack field. Effects of crack-tip blunting of the macrocrack are included in the simulations. The plastic zone characteristics are found to be in agreement with continuum models, with the added advantage that the hardening behavior comes out naturally in our model. The present model is composed of a macrocrack with microcracks ahead of its tip. These microcracks represent potential fracture sites at internal inhomogenities, such as brittle precipitates. Dislocations that are emitted from the crack-tip account for plasticity. When the tensile stress at the microcrack situated along the crack plane attains a critical value over a distance fracture is assumed to take place. The brittle-ductile transition curve is obtained by determining the fracture toughness at various temperatures. Factors that contribute to the sharp upturn in fracture toughness with temperature are found to be: the decrease in tensile stress ahead of the crack tip due to increase in blunting, and the increase in dislocation mobility. The inherent scatter in fracture toughness measurements are studied by using a size distribution for microcracks, distributed on the crack plane of the macrocrack. The scatter in fracture toughness measurements is found to be an effect of the size distribution of microcracks rather than their spatial distribution on the matrix ahead of the crack plane. When compared, the obtained results are in agreement with the existing experimental data.

A Dislocation Simulation Approach to Physical Basis of Master Curve

2008 ◽  
Author(s):  
Silvester J. Noronha ◽  
Heshan P. Gunawardane
Keyword(s):  
Fracture Toughness ◽  
Tensile Stress ◽  
Fracture Stress ◽  
Master Curve ◽  
Transition Curve ◽  
Crack Plane ◽  
Brittle Ductile Transition ◽  
Discrete Dislocation ◽  
Increasing Temperature ◽  
Two Stages

Discrete dislocation simulations of crack-tip plasticity are used to study the sharp increase in fracture toughness around ductile-brittle transition temperature of ferritic steels. The model used composed of a macrocrack with a microcrack ahead of it in its crack plane. The microcrack represents potential fracture sites at internal inhomogenities, such as brittle precipitates. The simulation has two stages: at first the fracture stress of microcrack, σF is calculated from dislocation simulation of microcrack-tip plasticity. In the next stage the fracture toughness is estimated by the macrocrack tip plasticity simulation; the fracture toughness is applied stress intensity at the macrorack when the tensile stress at the microcrack position attains σF. The brittle-ductile transition curve is obtained by determining the fracture toughness at various temperatures. Factors that contribute to the sharp upturn in fracture toughness with increasing temperature are found to be the increase in dislocation mobility, the decrease in tensile stress ahead of the macrocrack tip due to blunting and increase in mircocrack fracture stress due to increase in plasticity at the microcrack tips. The shape of the curve obtained is similar to the Master Curve.

Quantification of Residual Stresses Induced by Prior Loading at High Temperatures

2011 ◽  
Author(s):  
Ali N. Mehmanparast ◽  
Catrin M. Davies ◽  
Robert C. Wimpory ◽  
Kamran M. Nikbin
Keyword(s):  
Fracture Toughness ◽  
Residual Stress ◽  
Residual Stresses ◽  
Crack Growth ◽  
Creep Crack Growth ◽  
Crack Tip ◽  
Crack Plane ◽  
Residual Stress Field ◽  
Compression Direction ◽  
Fracture Specimen

High temperature components generally undergo cyclic loading conditions. Prior tensile/compressive loading of a fracture specimen can induce compressive/tensile residual stress fields at the crack tip. These residual stresses will influence the subsequent fracture behaviour of the cracked body. This work forms part of a project to examine the influence of creep induced damage at a crack tip on subsequent fatigue crack growth and fracture toughness properties of austenitic type 316H stainless steel. Creep damage is introduced local to the crack tip of a fracture specimen by interrupting a creep crack growth test, performed at 550 °C. Prior to testing, the material was pre-compressed in order to strain harden the material. The compact tension, C(T), specimen geometry has been considered in this work. Since residual stresses are known to influence fatigue and fracture toughness properties of a cracked body, it is important that the residual stress levels at the crack tip are quantified. Neutron diffraction (ND) measurements have therefore been performed to quantify the extent of residual stress in these samples after initial loading, and compared to finite element model predictions. Two specimens have been considered with the crack plane orientated in parallel and perpendicular to the pre-compression direction. Compressive residual stresses of around 100 MPa have been measured directly ahead of the crack tip. Reasonable predictions of the principal residual stress distributions have been obtained by the simplified FE analysis. Though the tensile properties differ significantly in for specimens orientated parallel and perpendicular to the pre-compression direction, no significant differences in the residual stress field are predicted in the C(T) specimens orientated in both directions.

scholarly journals Effects of HfC Particles on Crack Initiation and Propagation Behavior of WC/Co Composites

2019 ◽  
Vol 37 (3) ◽  
pp. 628-635
Author(s):  
Xinyu Yan ◽  
Shouren Wang ◽  
Daosheng Wen ◽  
Gaoqi Wang ◽  
Wentao Liu
Keyword(s):  
Fracture Toughness ◽  
Crack Propagation ◽  
Bending Strength ◽  
Crack Tip ◽  
Crack Deflection ◽  
Simulation Software ◽  
Homogeneous Distribution ◽  
The Matrix ◽  
Crack Pinning ◽  
Crack Bifurcation

Tungsten carbide composites were prepared by cold-pressing and hot-pressing sintering; fracture toughness and bending strength of the specimens were tested. The microstructures of HfC/WC/Co composites were observed with the SEM. The mathematical models were established to investigate the relationship between stress intensity factors of crack straight-through, crack deflection, and crack bifurcation with crack length, based on the crack propagation energy release rate. The simulation software ABAQUS was used to verify the four crack propagation methods of crack straight-through, crack deflection, crack bifurcation and crack pinning. The simulation results show that adding appropriate amount of HfC can effectively improve the fracture toughness and bending strength of the composites. The homogeneous distribution of HfC and Co in the matrix has a significant effect on the improvement of the strength and toughness of the composites, and the improvement mechanism is to disperse or transfer the stress at the crack tip to HfC by crack deflection, crack bifurcation, crack pinning, transcrystalline fracture, etc. As a result, the stress concentration at the crack tip in the matrix is reduced, and the toughness of the composites is improved.

A New Rationale for Scatter in Fracture Toughness Measurements

2008 ◽  
Author(s):  
Silvester J. Noronha
Keyword(s):  
Fracture Toughness ◽  
Size Distribution Function ◽  
Ferritic Steels ◽  
Crack Plane ◽  
Weibull Analysis ◽  
Weakest Link ◽  
Brittle Ductile Transition ◽  
Link Theory ◽  
Detailed Statistical Analysis ◽  
Good Agreement

The observed scatter in fracture toughness is investigated based on a dislocation simulation model that has been proposed to predict the brittle ductile transition in ferritic steels. We carried out a series of Monte-Carlo simulations using uniform distribution of microcracks on the crack plane of macrocrack. Detailed statistical analysis of the simulation results showed that the fracture is always initiated at one of the largest microcracks, whose size correspond to the tail of the size distribution function, and the inherent scatter arises from the distribution in the size of the critical microcrack that initiates the fracture and not from the variation of the location of the critical microcrack. Utilizing the weakest-link theory, Weibull analysis shows good agreement with the Weibull modulus values obtained from fracture toughness measurements.

Annealing Effects on Toughened Intra-Type Nanocomposites

2006 ◽  
Vol 45 ◽  
pp. 1632-1639 ◽  
Author(s):  
Hideo Awaji ◽  
Seong Min Choi
Keyword(s):  
Fracture Toughness ◽  
Elevated Temperatures ◽  
Volume Fraction ◽  
Process Zone ◽  
Nitrate Solution ◽  
Crack Tip ◽  
Hydrogen Atmosphere ◽  
Second Phase ◽  
High Porosity ◽  
The Matrix

Intra-type nanocomposites, in which nanosized second-phase particles are embedded within matrix grains, generate dislocations around the dispersed nanoparticles. The intra-type nanostructure induces a thermal expansion mismatch between the matrix and the dispersed particles, which will yield nanoscale stress distribution around the particles and generate lattice defects, such as dislocations. The dislocations of ceramics can be generated at elevated temperatures, become sessile dislocations at room temperature, and serve as nanocrack nuclei in highly stresses fields, e.g. at a main crack tip. The frontal process zone size ahead of a crack tip is expanded due to creation of nanocracks and hence the fracture toughness is improved. Annealing after sintered nanocomposites is important in controlling the dislocation activities. Appropriate annealing will disperse dislocations into the matrix grains. However, dislocations are sensitive to temperature, and higher temperature or longer annealing time result in dislocation disappearance and cause the reduction of the strength and fracture toughness of nanocomposites. In this study, commercially available γ-alumina agglomerated powder with high porosity was used to create the intra-type nanostructure. Nickel nitrate solution was infiltrated into nanopores of the γ-alumina agglomerates in vacuum. The alumina/nickel composite powder following reduction in hydrogen atmosphere was sintered using a pulse electric current sintering method. The volume fraction of nickel was about 3 vol %. After appropriate annealing, the highest fracture toughness was obtained to be 7.6 MPam1/2, which is two times higher than that of monolithic alumina.

scholarly journals An Assessment of ASTM E1922 for Measuring the Translaminar Fracture Toughness of Laminated Polymer Matrix Composite Materials

Polymers ◽  
2021 ◽  
Vol 13 (18) ◽  
pp. 3129
Author(s):  
Islam El-Sagheer ◽  
Amr A. Abd-Elhady ◽  
Hossam El-Din M. Sallam ◽  
Soheir A. R. Naga
Keyword(s):  
Fracture Toughness ◽  
Glass Fibers ◽  
Crack Tip ◽  
Laminated Composite ◽  
Fiber Reinforced Polymer ◽  
Fiber Direction ◽  
Reinforced Polymer ◽  
The Matrix ◽  
American Society ◽  
Cracked Specimens

The main objective of this work is to predict the exact value of the fracture toughness (KQ) of fiber-reinforced polymer (FRP). The drawback of the American Society for Testing Materials (ASTM) E1922 specimen is the lack of intact fibers behind the crack-tip as in the real case, i.e., through-thickness cracked (TTC) specimen. The novelty of this research is to overcome this deficiency by suggesting unprecedented cracked specimens, i.e., matrix cracked (MC) specimens. This MC exists in the matrix (epoxy) without cutting the glass fibers behind the crack-tip in the unidirectional laminated composite. Two different cracked specimen geometries according to ASTM E1922 and ASTM D3039 were tested. 3-D FEA was adopted to predict the damage failure and geometry correction factor of cracked specimens. The results of the TTC ASTM E1922 specimen showed that the crack initiated perpendicular to the fiber direction up to 1 mm. Failure then occurred due to crack propagation parallel to the fiber direction, i.e., notch insensitivity. As expected, the KQ of the MC ASTM D3039 specimen is higher than that of the TTC ASTM D3039 specimen. The KQ of the MC specimen with two layers is about 1.3 times that of the MC specimen with one layer.

Fracture and R-curves in high volume fraction Al2O3/Al composites

2000 ◽  
Vol 15 (5) ◽  
pp. 1131-1144 ◽  
Author(s):  
N. Nagendra ◽  
V. Jayaram
Keyword(s):  
Fracture Toughness ◽  
Volume Fraction ◽  
Porous Alumina ◽  
Crack Tip ◽  
High Volume ◽  
Fracture Mechanisms ◽  
Constraint Factor ◽  
Ductile Phase ◽  
Crack Bridging ◽  
The Matrix

Fracture toughness and fracture mechanisms in Al2O3/Al composites are described. The unique flexibility offered by pressureless infiltration of molten Al alloys into porous alumina preforms was utilized to investigate the effect of microstructural scale and matrix properties on the fracture toughness and the shape of the crack resistance curves (R-curves). The results indicate that the observed increment in toughness is due to crack bridging by intact matrix ligaments behind the crack tip. The deformation behavior of the matrix, which is shown to be dependent on the microstructural constraints, is the key parameter that influences both the steady-state toughness and the shape of the R-curves. Previously proposed models based on crack bridging by intact ductile particles in a ceramic matrix have been modified by the inclusion of an experimentally determined plastic constraint factor (P) that determines the deformation of the ductile phase and are shown to be adequate in predicting the toughness increment in the composites. Micromechanical models to predict the crack tip profile and the bridge lengths (L) correlate well with the observed behavior and indicate that the composites can be classified as (i) short-range toughened and (ii) long-range toughened on the basis of their microstructural characteristics.

Failure Analysis on Rubber-Modified Epoxy Resin under Various Loading Speed Conditions

2005 ◽  
Vol 297-300 ◽  
pp. 1907-1912 ◽  
Author(s):  
Deok Bo Lee ◽  
Joo Hyung Kim
Keyword(s):  
Fracture Toughness ◽  
Composite Material ◽  
Epoxy Resin ◽  
Structural Reliability ◽  
Fiber Composite ◽  
Damage Zone ◽  
Crack Tip ◽  
Rubber Content ◽  
The Matrix ◽  
Modified Epoxy

A rubber-modified epoxy resin is widely used as adhesive and matrix materials for fiber composite material. The structural reliability of composite material depends on the fracture toughness of the matrix resin. In this study, the fracture toughness and the damage zone around a crack tip in rubber-modified epoxy resin were investigated. The volume fractures of rubber (CTBN1300×8) in the rubber-modified epoxy resin were 0%, 5% and 15% under several loading speeds. The fracture toughness(KIC) and the fracture energy(GIC) were measured by using 3-point bending specimens. The 4-point bending specimens were also used to observe damage zones at the vicinity of a crack tip in modified resins. The results show that the values of the fracture toughness and the sizes of damage zones at 5% and 15% rubber content decrease with increase in loading speed.

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