Fracture analysis is complicated for polymers since, besides of temperature and time dependence, there are involved effects from plastification, chain orientation and adiabatic temperature rise. It is the aim of this section to describe several fracture processes which are specific of polymers. Fracture behavior is determined experimentally by the mode and time profile of loading. Stress- and strain controlled loading yields different behavior as well as loading in tension, compression or shear (torsion). Compressive strength is higher than tensile strength; shear strength is the lowest one.

Fracture Behavior of Polymers

The successful use of plasma-sprayed hydroxyapatite (HA) coatings on Ti-alloy implants for implant-to-bone fixation requires strong adherence of the ceramic coating to the underlying metal substrate. In this study, the metal-ceramic interface was evaluated using mechanical, chemical, and structural characterization methods. Evaluations of an HA-coated Ti-6A1-4V implant system using a modified short bar technique for interfacial fracture toughness determination revealed relatively low fracture toughness values. Additionally, conventional tensile bond strength testing indicated much lower values than previously reported. Using high resolution electron spectroscopic imaging, evidence of chemical bonding was revealed at the plasmasprayed HA/Ti-6AI-4V interface, though bonding was primarily due to mechanical interlock at the interface. This study illustrates the benefits of, and the need for, a multilevel approach to evaluate and improve these plasma-sprayed ceramicme tai subst rate interfaces.

Student research award in the undergraduate, Master candidate category, or health science degree candidate category, 17th annual meeting of the society for biomaterials, scottsdale, AZ may 1–5,1991. Characterization of the interface in the plasma‐sprayed HA coating/Ti‐6Al‐4V implant system

A comparative evaluation of rock strength measures

Fracture toughness of a sintered A12O3 was determined with four-point-bend specimens having either straight-through or chevron notches. For the straight-through notched specimens, measured KIc decreased with decreasing notch width. For the smallest notch width (66 μm) KIc= 3.42±0.13 MN m−¾. For specimens with chevron notches, a crack initiates and extends from the tip of the notch under increasing load. KIc is calculated from the maximum load without measuring crack length, under the assumption that the derivative of the compliance is the same as that for a specimen with a straight-through crack. A refined calculation accounts for the truncated chevron crack shape at maximum load using Bluhm's slice model. For the chevronnotch configuration, a value of KIc= 3.49±0.11 MN m−¾ was measured, which appears to be independent of the initial notch length a0 (distance from the crack mouth to the tip of the triangular notch). An effect of a1 (length of the chevron notch at the surface) on KIc was observed, independent of whether the calculation of KIc was based on the straight-through crack assumption or on the slice model.

Fracture Toughness Determination of A12O3 Using Four‐Point‐Bend Specimens with Straight‐Through and Chevron Notches

Research within the past two decades has achieved a dramatic upsurge of improvements in the mechanical properties of engineering ceramics. These improvements have often been made through increased toughness by novel toughening mechanisms such as the stress induced phase transformation, microcracking, fibre/whisker crack bridging, etc. These may occur not only in the frontal process zone ahead of a sharp crack, but also in the following crack wake region. The consequences of these microfracture processes and mechanisms in the wake and the crack bridging regions are significant, for they result in very complex fracture processes and they create many critical issues and difficulties in the experimental determination of the fracture resistance of brittle materials. The lack of a physical basis for a fracture criterion in the present fracture mechanics framework adds further confusion to fracture mechanics studies. This paper is a state of the art review of the application of fracture mechanics to brit...

Fracture toughness testing of brittle materials

For the determination of fracture toughness especially with brittle materials, a short bar specimen with rectangular cross section and chevron notch can be used. As the crack propagates from the tip of the triangular notch, the load increases to a maximum then decreases. To obtain the relation between the fracture toughness Kic and maximum load Pmax, calculations of Srawley and Gross for specimens with a straight-through crack were applied to the specimens with chevron notches. For the specimens with a straight-through crack, an analytical expression was obtained. This expression was used for the calculation of the Kic−Pmax relation under the assumption that the change of the compliance with crack length for the specimen with a chevron notch is the same as for a specimen with a straight-through crack.

Compliance and stress intensity coefficients for short bar specimens with chevron notches

The short rod chevron-notch specimen has the advantages of (1) crack development at the chevron tip during the early stage of test loading and (2) convenient calculation of KIc from the maximum test load and a calibration factor which depends only on the specimen geometry and manner of loading. For generalized application, calibration of the specimen over a range of specimen proportions and chevron-notch configurations is necessary. Such was the objective of this investigation, wherein calibration of the short rod specimen was made by means of experimental compliance measurements converted into dimensionless stress intensity factor coefficients.

D. Munz

Papers

Compliance and stress intensity coefficients for short bar specimens with chevron notches

Compliance calibration of the short rod chevron-notch specimen for fracture toughness testing of brittle materials

Extended range stress intensity factor expressions for chevron-notched short bar and short rod fracture toughness specimens