Closure Effect ofⅠ+ⅡMixed-mode Crack for EA4T Axle Steel

The crack-closure effect is a crucial factor that affects the crack growth rate and should be considered in simulation analysis and testing.A mixed-mode I+II loading fatigue crack growth test was performed using EA4T axle steel specimens.The variation of the plastic-induced crack closure(PICC)effect and the roughness-induced crack closure(RICC)effect during crack deflection in the mixed-mode is examined in this study.The results show that the load perpendicular to the crack propagation direction hinders the slip effect caused by the load parallel to the crack propagation direction under mixed-mode loading,and the crack deflection is an intuitive manifestation of the interaction between the PICC and RICC.The proportion of the RA value change on the crack side caused by contact friction was reduced by the interaction between PICC and RICC.The roughness of the crack surface before and after the crack deflection is different,and the spatial torsion crack surface is formed during the crack propagation process.With the increase of the crack length,the roughness of the fracture surface increases.During the crack deflection process,the PICC value fluctuates around 0.2,and the RICC value is increased to 0.15.

Investigation on behavior of crack penetration/deflection at interfaces in intelligent coating system

Based on the three-phase model, the propagation behavior of a matrix crack in an intelligent coating system is investigated by an energy criterion. The effect of the elastic mismatch parameters and the thickness of the interface layer on the ratio of the energy release rate for infinitesimal deflected and penetrated crack is evaluated with the finite element method. The results show that the ratio of the energy release rates strongly depends on the elastic mismatch al between the substrate and the driving layer. It also strongly depends on the elastic mismatch a2 between the driving layer and the sensing layer for a thinner driving layer when a primary crack reaches an interface between the substrate and the driving layer. Moreover, with the increase in the thickness of the driving layer, the dependence on a2 gradually decreases. The experimental observation on aluminum alloys monitored with intelligent coating shows that the established model can better explain the behavior of matrix crack penetration and can be used in optimization design of intelligent coating.

Crack deflection occurs by constrained microcracking in nacre

For decades, nacre has inspired researchers because of its sophisticated hierarchical structure and remarkable mechanical properties, especially its extreme fracture toughness compared with that of its predominant constituent,CaCO3, in the form of aragonite. Crack deflection has been extensively reported and regarded as the principal toughening mechanism for nacre. In this paper, our attention is focused on crack evolution in nacre under a quasi-static state. We use the notched three-point bending test of dehydrated nacre in situ in a scanning electron microscope(SEM) to monitor the evolution of damage mechanisms ahead of the crack tip. The observations show that the crack deflection actually occurs by constrained microcracking. On the basis of our findings, a crack propagation model is proposed, which will contribute to uncovering the underlying mechanisms of nacre’s fracture toughness and its damage evolution. These investigations would be of great value to the design and synthesis of novel biomimetic materials.

MICROSTRUCTURE AND MECHANICAL PROPERTIES OF TiB_2-B_4C CERAMIC COMPOSITES

The microstructure and mechanical properties of TiB_2 /B_4C composites have been investi- gated.It was found that both the strength and hardness for TiB_2 greatly increase with the ad- dition of 20 to 30 wt-% B_4C,and the fracture toughness K_(IC) value remaines on the original high level.The flexure strength,Vicker's hardness and fracture toughness are 782 MPa,26.2 GPa and 7.2 MPam^(1/2),respectively,for the TiB_2-30 wt-% B_4C composite,compared to 450 MPa,21 GPa and 7.0 MPam^(1/2) for monolithic TiB_2.The toughening and strengthening mechanisms,have also been discussed.

INVESTIGATION OF MULTIPHASE-REINFORCED Si_3N_4 COMPOSITE MATERIAL

By means of whisker reinforce and paricle dispersion, the routes of property improvement on Si3N4 ceramic material have been studied. The medhaniacl properties of Siw/Si3N4 and Siw/Si3N4TiC material was compared, which proved that multiphase reinforce had overlap effect. Microstructure of the rnaterial was investigated by means of SEM and the mechanisms of SiC. and TiCP reinforces had been disussed.

A Novel Maximum Resolved Shear Stress Gradient Criterion and Its Application for Crack Deflection Behavior in Polycrystalline Silicon Materials

In this paper,a development of new resolved shear stress gradient criteria is performed for the study of crack propagation behavior in polycrystalline materials with an emphasis on the effect of slip plane and slip direction.The prediction of crack deflection behavior by the maximum resolved shear stress gradient criterion shows good agreement with the experimental results.Comparison study for the prediction of crack propagation behavior in poly crystalline materials by other criteria demonstrates that the maximum resolved shear stress gradient criterion is superior to other fracture criteria.This suggests that the new criterion could be further applied for predicting the crack deflection behavior in other polycrystalline materials.

Dual precipitate simultaneous enhancement of tensile and fatigue strength in(FeCoNi)_(86)Al_(7)Ti_(7) high-entropy alloy fabricated using selective laser melting

Recent studies have indicated that precipitation-strengthened high-entropy alloys(HEAs)show superior mechanical performance and have been successfully fabricated by additive manufacturing.However,the lack of fatigue and fracture research has limited the engineering applications of additive manufacturing HEAs.This work explored a dual precipitation-strengthened(FeCoNi)_(86)Al_(7)Ti_(7) HEA with excellent tensile and fatigue strength,prepared through selective laser melting and heat treatment.Compared with the as-built samples,the tensile properties and fatigue endurance limit improved through aging by 48.7%and 30%,respectively.The strengthening mechanism and enhanced fatigue performance were clarified in detail.The improvement in fatigue strength was attributed to the improved resistance of the L1_(2) and L2_(1) precipitates.During deformation,the dislocation shear coherent L1_(2) precipitates reduced slip band energy and inhibited slip band expansion,while the L2_(1) particles acted as obstructions for further slip band propagation,severely limiting the rapid formation and propagation of crack growth.In-situ TEM cyclic tensile-tensile testing also clarified the fatigue crack growth behavior,demonstrating that crack deflection due to L2_(1) precipitate obstruction slowed down the crack growth rate and efficiently promoted the closure of the microcrack tips.This work offers im plications for a new strategy to develop additive manufacturing HEAs.

Role ofδ-ferrite in fatigue crack growth of AISI 316 austenitic stainless steel

FF sample(nearly free ofδ-ferrite)and CF sample(containing∼4%δ-ferrite)were prepared from the AISI 316 austenitic stainless steel plate to elucidate the role ofδ-ferrite in the fatigue crack growth under the solution treated and accelerated aged conditions.It is found that the fatigue crack growth resistance of the CF sample is higher than the FF sample under the solution treated condition.However,a significant deterioration of the fatigue crack growth resistance is observed in the CF sample while little variation is found in the FF sample after accelerated aging treatment at 750°C for 10 h.In the solution treated condition,deflected crack growth path is present when the main crack encounters theδ-ferrite in the CF sample due to the differences in the fatigue responses between austenite andδ-ferrite.The measured growth rate of the deflected crack is significantly slower than that of the flat crack of the same length.After the accelerated aging treatment,microcracks are produced at the M_(23)C_(6)/δinterface due to the strain incompatibility between M_(23)C_(6) and retainedδ-ferrite when the decomposedδ-ferrite is subject to plastic deformation in the crack tip plastic zone.The preexisting microcracks in the front of crack tip provide a viable path for crack propagation,resulting in the relatively flat crack path.

Effect of Carbon Migration on Interface Fatigue Crack Growth Behavior in 9Cr/CrMoV Dissimilar Welded Joint

Fatigue crack growth(FCG)behavior of 9 Cr/CrMoV dissimilar welded joint at elevated temperature and different stress ratios was investigated.Attention was paid to the region near the fusion line of 9 Cr where carbon-enriched zone(CEZ)and carbon-depleted zone(CDZ)formed due to carbon migration during the welding process.Hard and brittle tempered martensite dominated the stress ratio-insensitive FCG behavior in the coarse grain zone(CGZ)of 9 Cr-HAZ.For crack near the CGZ-CEZ interface,crack deflection through the CEZ and into the CDZ was observed,accompanied by an accelerating FCG rate.Compared with the severe plastic deformation near the secondary crack in 9 Cr-CGZ,the electron back-scattered diffraction analysis showed less deformation and lower resistance in the direction toward the brittle CEZ,which resulted in the transverse deflection.In spite of the plastic feature in CDZ revealed by fracture morphology,the less carbides due to carbon migration led to lower strength and weaker FCG resistance property in this region.In conclusion,the plasticity deterioration in CEZ and strength loss in CDZ accounted for the FCG path deflection and FCG rate acceleration,respectively,which aggravated the worst FCG resistance property of 9 Cr-HAZ in the dissimilar welded joint.