Mixed-mode impact behaviour of a low carbon steel

Abstract

As fracture mechanics has developed as a discipline, many parameters have been developed to characterize the instability condition. However, a majority of this work has been confined to the investigation of mode I fracture. Thus, we have standardized methods for experimentally determining K,o, J~o and J-resistance curves for mode I crack propagation. However, cracks in real materials can be subjected n o t just to tensile sU'esses but to complex stress states so that the development of suitable parameters to characterize mixed-mode crack initiation and propagation is important in the evolution of suitable design criteria. Further, observations indicate that initially fiat cracks in some tough materials tend to reorient themselves to oblique planes during growth. For these materials, crack propagation can be said to occur under combined mode conditions.A considerable amount of work on mixed-mode I/III fracture toughness of materials [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] is available. Of this work, a portion involves non-proportional loading, while a consistent body of work has focused on proportional loading using modified compact tension specimens [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19]. The superposition of mode III loading results in drastic reduction in fracture toughness in some materials whereas in other materials it has little effect or even results in an increase in the fracture toughness. Fracture mechanism maps delineating regions of susceptibility to tensile and shear loads have been proposed [27,28].In the mixed-mode fracture toughness tests outlined above, the use of the modified compact tension specimen has enabled the testing of materials under a variety of combinations of mode I and mode III loadings. By using appropriately defined n-fixed-mode versions of the stress intensity factor K and the J integral, the susceptibility of these materials to mixed-mode fracture can be quantified. In addition to compact tension specimens, three point bend specimens with an inclined crack can also be used to determine the mixed-mode fracture behaviour of materials [29].The aim of the present study was to study the feasibility of extending the mixed-mode fracture concept to impact testing using a Charpy test specimen. The proposed modified Charpy test specimen is shown in Fig. 1. The crack inclination angle can be varied to provide a range of mode I and mode III load combinations. Four angles, 90 ° , 75 ° , 60 ° and 45 ° were used in the present study. This modified Charpy test specimen provides for a simple method to determine the mixed-mode impact resistance of materials. Apart fi'om providing information on dynalnic fracture under mixed-mode conditions this method

The Influence of Texture on the Ductile-to-Brittle Transition Behavior in Fe20Cr4.5Al Oxide Dispersion Strengthened Alloy

“…The influence of texture on the variation with temperature of notch toughness (ductile to brittle transition temperature) was considered for cleavage (lower shelf region), mixed ductile-cleavage (transition region) and ductile (upper shelf region) fracture modes. The LSE depends on the orientation to maximum principal stress direction and density of (100) planes, since these planes are the only crystallographic planes in which cleavage can occur [7][8][9][10][11]. The above results of notch toughness at the LS region have shown the LSE values are the same disregarding of condition (microstructure and texture) of material.…”

“…Cleavage in the lower shelf region. In this region, the effect of texture on the notch toughness is determined by the density and relative orientation regarding to the maximum principal stress of the (100) crystallographic planes since this plane is the cleavage plane [7][8][9][10][11].…”

“…Ductile in the upper shelf region. In this region, the effect of texture on notch toughness is determined by the volume fraction and relative orientation regarding to the maximum principal stress of the (110), (123) and (112) planes because these planes are responsible of the plastic deformation of the material [8,11].…”