Introducing platform surface interior angle (PSIA) and its role in flake formation, size and shape

Four ways archaeologists have tried to gain insights into how flintknapping creates lithic variability are fracture mechanics, controlled experimentation, replication and attribute studies of lithic assemblages. Fracture mechanics has the advantage of drawing more directly on first principles derived from physics and material sciences, but its relevance to controlled experimentation, replication and lithic studies more generally has been limited. Controlled experiments have the advantage of being able to isolate and quantify the contribution of individual variables to knapping outcomes, and the results of these experiments have provided models of flake formation that when applied to the archaeological record of flintknapping have provided insights into past behavior. Here we develop a linkage between fracture mechanics and the results of previous controlled experiments to increase their combined explanatory and predictive power. We do this by documenting the influence of Herztian cone formation, a constant in fracture mechanics, on flake platforms. We find that the platform width is a function of the Hertzian cone constant angle and the geometry of the platform edge. This finding strengthens the foundation of one of the more influential models emerging from the controlled experiments. With additional work, this should make it possible to merge more of the experimental results into a more comprehensive model of flake formation.

Efeito de Diferentes Tempos de Condicionamento Ácido na Carga de Fratura de Duas Vitrocerâmicas

O estudo avaliou a carga de fratura de uma vitrocerâmica de silicato de lítio reforçada por zircônia (ZLS) e uma vitrocerâmica de dissilicato de lítio (LD) condicionadas com ácido fluorídrico 10% por diferentes tempos. Amostras cerâmicas (12 mm x 14 mm x 1,2 mm de espessura) foram obtidas pelo corte de blocos para CAD/CAM. Após a cristalização e o acabamento, as amostras de cada material foram divididas aleatoriamente em 3 grupos (n10) para o condicionamento ácido por 20s (G20), 40s (G40) e 60s (G60), seguido de lavagem sônica e secagem. Bases cilíndricas de resina epóxica reforçada com fibras (NEMA G10) foram confeccionadas. Silano e adesivo foram aplicados sobre a superfície da cerâmica e do G10. Cimento resinoso fotopolimerizavel foi aplicado no centro da cerâmica que foi posicionada sobre a base de G10, e foi fotoativado. Em máquina de ensaios universal e em água destilada a 37ºC, uma força compressiva (0,1 mm/s) foi aplicada por um pistão com ponta plana de 3 mm de diâmetro no centro da cerâmica até o primeiro sinal acústico do início da fratura. O modo de falha foi analisado com transiluminação. Os resultados foram avaliados por Kruskal-Wallis (0,05). Não houve diferença na carga de fratura (N) entre os grupos (p0,1). O modo predominante de falha foi trinca do tipo radial. Somente um corpo-de-prova do grupo G40 de ZLS, sofreu fratura catastrófica com origem de trinca na superfície cerâmica (Hertzian/cone). Portanto, o tempo de condicionamento ácido (20s, 40s e 60s) não influencia a carga de fratura imediata das vitrocerâmicas cimentadas sobre um análogo da dentina.Palavras-chave: Cerâmica. Programa Auxiliado por Computador. Cimentação.

Dynamic fracture of inorganic glasses by hard spherical and conical projectiles

In this article, high-speed photographic investigations of the dynamic crack initiation and propagation in several inorganic glasses by the impact of small spherical and conical projectiles are described. These were carried out at speeds of up to approximately 2×10 6 frames s −1 . The glasses were fused silica, ‘Pyrex’ (a borosilicate glass), soda lime and B 2 O 3 . The projectiles were 0.8–2 mm diameter spheres of steel, glass, sapphire and tungsten carbide, and their velocities were up to 340 m s −1 . In fused silica and Pyrex, spherical projectiles' impact produced Hertzian cone cracks travelling at terminal crack velocities, whereas in soda-lime glass fast splinter cracks were generated. No crack bifurcation was observed, which has been explained by the nature of the stress intensity factor of the particle-impact-generated cracks, which leads to a stable crack growth. Crack bifurcation was, however, observed in thermally tempered glass; this bifurcation has been explained by the tensile residual stress and the associated unstable crack growth. A new explanation has been proposed for the decrease of the included angle of the Hertzian cone cracks with increasing impact velocity. B 2 O 3 glass showed dynamic compaction and plasticity owing to impact with steel spheres. Other observations, such as total contact time, crack lengths and response to oblique impacts, have also been explained.

Bi-Axial Flexure Strength, Weibull Modulus and Fracture Mode of Alumina Glass-Infiltrated Core/Veneer Ceramic Composites

The purpose of this study is to determine the bi-axial flexural strength, weibull modulus and fracture mode of bilayered alumina glass-infiltrated core and the veneering porcelain. Forty disk specimens were fabricated from alumina glass-infiltrated core (HSDC-A) and veneer porcelain (Vintage AL). The specimens were equally divided into four groups as: MV, monolithic specimens of veneer material; MC, monolithic specimens of core material; BV, bilayered specimens with the veneer in tension; BC, bilayered specimens with core material in tension. Mean flexure strength, standard deviation and associated Weibull modulus were determined using bi-axial flexure (ball-on-ring) for each group. Both optical and scanning electron microscopy were employed for identification of the fracture mode and origin. The surface loaded in tension influenced the bi-axial flexural strength and reliability of the composites. The frequency of specimen delamination, Hertzian cone formation and sub-critical radial cracking in the bilayered discs are also dependent on the surface loaded in tension.