Search

• Pop-in or serrations cannot be detected at large loading rates. • Indentation hardness decreases with a constant elastic modulus under small loads. • Fracture toughness can be obtained from the total indentation energy. • Area of shear band scales with contact and residual areas. • Scaling relationships among indentation variables of Zr-based BMG were quantified. Berkovich nanoindentation experiments were conducted on Zr 55 Cu 30 Al 10 Ni 5 bulk metallic glass under a constant loading rate. Indentation hardness decreased linearly with contact depth, which defied the direct application of conventional indentation size effect model for crystalline material. Elastic modulus remained invariant under small loads, but decreased linearly under large loads. The scaling relationships between indentation variables such as contact depth, permanent depth, the maximum indentation displacement, plastic energy, the total work of indentation and their ratios were investigated. The area of shear band circle, the projected contact area at the maximum load, and the residual projected area were all found to be proportional to one another. A new methodology to determine fracture toughness was proposed based on the total work of indentation, provided that the critical indentation displacement at the onset of fracture corresponds to zero value of elastic modulus extrapolated from curve fitting. [ABSTRACT FROM AUTHOR]

Rockwell C diamond indenter with a spherical tip of radius of 108 μm was used to slide on bulk polycarbonate under linearly increasing normal load from 5 mN to 20 N in order to investigate deformation and damage of ductile polymers during scratching. The peak value of the ratio of residual depth over penetration depth corresponds to a critical normal load, which can be used as the piecewise point for the subsection functions describing variation of tangential load, penetration depth and residual depth with normal load. The true adhesion interfacial friction coefficient was found to be 0.3 for contact between PC and diamond by three-dimensional model. Residual groove widths were found to be proportional to the square root of normal load, and provided useful information to characterize the radius of spherical tip. Values of fracture toughness of PC obtained by scratch-based methodologies under sufficiently large normal loads associated with brittle fracture and crack plane, are in excellent agreement with literature values. The material is under triaxial compressive stress state, making the flow and shear stresses much larger than the ones for uniaxial tensile test. • True adhesion friction coefficient can be obtained by three-dimensional model rather than two-dimensional model. • Fracture toughness of polycarbonate can be measured by scratch-based methodologies. • Scratch-based approaches of assessing fracture toughness of PC are only applicable under large loads. • The radius of spherical tip can be characterized by residual scratch groove widths. • Flow and shear stresses during scratching are larger than the ones for uniaxial tension. [ABSTRACT FROM AUTHOR]

Fracture toughness can be measured by microscratch test from objective force and depth measurement, which is much convenient than indentation approach and conventional means, since initial notch or pre-crack is not required. Micro-scratch test was conducted on copper under a spherical probe of radius 105 μm and constant applied normal load with the aim of characterizing fracture toughness of copper by sliding technique with a sphere. Scratch variables such as normal load, lateral load, friction coefficient, and penetration depth were analyzed, and different methodologies for calculating fracture toughness were compared. Linear elastic fracture mechanics was found to be inapplicable to scratch of elastic-plastic material with sufficient plasticity under a spherical probe, while energetic size effect law can be successfully applied to characterize fracture toughness by microscratch test with a sphere abrading metallic material. • Fracture toughness of copper was obtained from scratch test with a spherical indenter. • Correlations among scratch variables for a sphere sliding on copper were examined. • Spherical indentation and scratch test with a spherical indenter were compared. • Different methodologies for computing fracture toughness by scratch test were compared. • The energize size effect law is applicable to characterize fracture toughness of metallic material by scratch test. [ABSTRACT FROM AUTHOR]

Micro-scratch tests have been carried out on borosilicate K9 glass using a Rockwell C diamond indenter with a spherical tip radius 105 μm in order to study deformation/damage regimes and microcracking patterns of K9 glass with increasing normal load. The abrupt changes of test data together with optical microscopy observation can be used to determine three transition loads corresponding to initiation of sub-surface median crack, onset of surface ring crack, and lateral cracks reaching the surface, respectively. Based on linear elastic fracture mechanics, fracture toughness obtained from scratch test under a low driving speed and a suitable fitting range of nominal size is found to be 0.85 MPa•m1/2 for K9 glass, which is close to 0.82 MPa•m1/2 obtained from Vickers hardness technique, making micro-scratch a powerful means to assess fracture toughness of material, and opening new application of scratch test as a quantitative method. [ABSTRACT FROM AUTHOR]