Determination of fracture toughness using the area of micro-crack tracks left in brittle materials by Vickers indentation test

Abstract

Abstract:In this article, a new method has been presented for the estimation of fracture toughness in brittle materials, which enjoys improved accuracy and reduced costs associated with fracture toughness testing procedure compared to similar previous methods, because a vast range of specimens with irregular cracks can be accommodated for testing. Micron-sized alumina powders containing 0.05 wt% magnesium oxide (MgO) nanoparticles were mixed and also together with 2.5 vol%, 5 vol%, 7.5 vol%, 10 vol%, and 15 vol% of silicon carbide (SiC) nanopowders separately. By making and testing various types of ceramics with different mechanical properties, and considering the irregular cracks around the indented area caused by Vickers diamond indenter, a semi-empirical fracture toughness equation has been obtained.

A correlation between fracture toughness and cohesion strength of molybdenum thermal sprayed coatings

“…It was observed that the microcraks generated by Vickers indentation occur after a critical load has been reached and depending on the type of coating and indentation direction they usually propagate on the directions of the indent diagonals. This mechanism of propagation of microcracks appears usually in homogeneous materials and in particular, in brittle materials [3], [8], [9]. Generally, after the Vickers indentation two types of microcracks can be generated, described as Half-penny cracks (median/radial cracks) or Palmqvist cracks (radial cracks) (Fig.2b), where l is the average length of the cracks, a is the half average length of the diagonal and c is the sum of l and a.…”

“…To distinguish between these two types of cracks, it can be applied a criterion based on the ratio of cracks length/diagonal indentation [9]. A general rule admits that if c/a ≥ 2, the model is the type Half-penny and if c/a < 2, the model is Palmqvist [3]. Other authors assert that the cracks are Palmqvist type only if 0.25 ≤ l/a ≥ 2.5 [10] and they are Half-penny only if c/a ≥ 2.5 [11].…”

“…Firstly, the microcracks in these inhomogeneous thermal coatings are propagating in an unpredictable way and finally will lead to a delamination of the coating. The morphology and propagation behavior of microcraks in thermal sprayed coatings, which can occur when mechanical stresses are applied or as a result of residual stresses generated by the cooling and solidifications of hot Mo splats, define a mechanical property of coatings, called critical stress intensity factor or fracture toughness that is generally used for predicting of the mechanical behavior of brittle materials [3]. The fracture toughness is the ability of the coating to withstand at the highest level of stress intensity without fracturing and can be evaluated by using several methods such as scratch testing, double cantilever beams loaded in tension, Chevron notch bar and single-edge notched beams tested in three or four point bending [3][4].…”

“…The morphology and propagation behavior of microcraks in thermal sprayed coatings, which can occur when mechanical stresses are applied or as a result of residual stresses generated by the cooling and solidifications of hot Mo splats, define a mechanical property of coatings, called critical stress intensity factor or fracture toughness that is generally used for predicting of the mechanical behavior of brittle materials [3]. The fracture toughness is the ability of the coating to withstand at the highest level of stress intensity without fracturing and can be evaluated by using several methods such as scratch testing, double cantilever beams loaded in tension, Chevron notch bar and single-edge notched beams tested in three or four point bending [3][4]. One of the most used methods in recent years, providing good results for the evaluating of fracture toughness of thermal sprayed coatings, is the indentation method by Vickers indenter [3][4][5][6][7][8][9][10][11][12].…”

“…The fracture toughness is the ability of the coating to withstand at the highest level of stress intensity without fracturing and can be evaluated by using several methods such as scratch testing, double cantilever beams loaded in tension, Chevron notch bar and single-edge notched beams tested in three or four point bending [3][4]. One of the most used methods in recent years, providing good results for the evaluating of fracture toughness of thermal sprayed coatings, is the indentation method by Vickers indenter [3][4][5][6][7][8][9][10][11][12]. Beside the accessibility of this method, the major advantage is that the fracture toughness of coatings can be tested locally, on several levels such as, at the coating-substrate interface, also called interfacial fracture toughness, on the perpendicular and parallel direction to the coatingsubstrate interface, between layers or individual splats.…”