TWI Industrial Member Report Summary 891/2007
By D Zhou
Fracture toughness KIC and critical J-integral Jc have been used extensively in fracture mechanics as material parameters. There are various standards for measurement of KIC and Jc under static loading. These standard test procedures generally include either a three-point single edge notched bend (SENB) test or a compact tension test under static loading. In general, these procedures cannot be directly used for determining the dynamic fracture toughness of materials. Under the dynamic loading, dynamic inertia might become dominant and a stress wave could be introduced at early time to complicate the fracture event. For ductile materials, the constitutive relationship becomes complex due to the strain rate effect. So far, there is no generally accepted measurement standard for determining the dynamic fracture toughness of materials in the entire range of loading rates.
Although standard tests do not exist for dynamic fracture toughness, a number of measurement techniques have been proposed for specific applications. The accuracy of these techniques needs to be evaluated because some of them do not consider the inertia effect. In order to quantify the effect of dynamic inertia, a characteristic transition time is introduced. During the transition time, the inertia effects dominate fracture behaviour and beyond that, the inertia effect can be ignored. Thus the transition time can be used to identify whether the fracture behaviour is a dynamic or quasi-static event.
Traditionally, most measurement techniques require special trained personnel or time-consuming calibration of strain gauges. Alternatively, the fracture toughness can be determined using a hybrid numerical/experiment approach, ie, numerically calculating dynamic stress intensity factor (DSIF) or J-integral and experimentally measuring the time-to-fracture (the time-to-fracture is defined as the duration from the beginning of the impact event to the initiation of the fracture). This approach provides a straightforward way to calculate dynamic fracture toughness provided that the time-to-fracture is known; this fracture time can be readily obtained by performing a simple un-calibrated strain gauge test.
The hybrid numerical/experiment approach requires a calculation of the DSIF or dynamic J-integral. Although static J-integral can be calculated readily from most commercial finite element softwares, such as ABAQUS/standard, these softwares are incapable of calculating dynamic J-integral. Therefore, the focus of this report is to develop an algorithm to calculate dynamic J-integral of a specimen with incorporation of inertia effects. With the dynamic J-integral available, the hybrid numerical/experiment approach can be used to investigate the validity of conventional static tests in a dynamic event. Also, the conditions for the validity of the J-integral as a transferable material characterising parameter can be further discussed.
- Develop a numerical procedure for determining fracture toughness by calculating dynamic J integral using a domain integral method at the time when fracture occurs in a dynamic test.
- Review different testing procedures and provide guidelines on application of these testing procedures depending upon the time-to-fracture in comparison with the characteristic transition time.