The ductile failure in metals has long been associated with void nucleation, growth and coalescence. Many micromechanics-based damage models were developed to study the effects of the voids sizes, shape and orientation to the nucleation, growth and coalescence of voids. However, the experimental methods to quantitatively validate these models were lacking. This paper is aimed to experimentally investigate at the microscale and nanoscale the effects of the shapes, sizes, orientation and density to the nucleation, growth and coalescence of voids and their relation to the ductility of the metal. In this work, notched tensile specimens with various radii were designed along different orientations. These specimens were tensile loaded up to different percentage of ultimate failure strain. The deformed specimens were then sectioned both along and perpendicular to the loading direction to microscopically study the voids size, shape and density. On the other hand, microtensile specimens were made out of these already deformed specimens. Using the advanced imaging capabilities of AFM and SEM combined with in-situ loading, the growth and coalescence of voids were in-situ studied at the microscale and nanoscale.
Nowadays composite materials have been extensively utilized in many military and industrial applications. For example, the newest Boeing 787 uses 50% composite (mostly carbon fiber reinforced plastic) in production. However, the weak delamination strength of fiber reinforced composites, when subjected to external impact such as ballistic impact, has been always potential serious threats to the safety of passengers. Dynamic fracture toughness is a critical indicator of the performance from delamination in such impact events. Quasi-static experimental techniques for fracture toughness have been well developed. For example, end notched flexure (ENF) technique, which is illustrated in Fig. 1, has become a typical method to determined mode-II fracture toughness for composites under quasi-static loading conditions. However, dynamic fracture characterization of composites has been challenging. This has resulted in conflictive and confusing conclusions in regard to strain rate effects on fracture toughness of composites.
Quasi-static experimental techniques for fracture toughness have been well developed and end notched flexure (ENF) technique has become a typical method to determined mode-II fracture toughness. ENF technique also has been implemented to high-rate testing using SHPB (Split Hopkinson Pressure Bar) technique for dynamic fracture characterization of composites. In general, the loading condition in dynamic characterization needs to be carefully verified that forces are balanced if same equations are used to calculate the fracture toughness. In this study, we employed highly sensitive polyvinylidene fluoride (PVDF) force transducers to measure the forces on the front wedge and back spans of the three-point bending setup. High rate digital image correlation (DIC) was also conducted to investigate the stress wave propagation during the dynamic loading. After careful calibration, the PVDF film transducer was made into small square pieces that are embedded on the front loading wedge and back supporting spans. Outputs from the three PVDF transducers as well as the strain gage on the transmission bar are recorded. The DIC result shows the transverse wave front propagates from the wedge towards the supports. If the crack starts to propagate before reaching force balance, numerical simulation, such as finite element analysis, should be implemented together with the dynamic experimental data to determine the mode-II fracture toughness.
It is essential to characterize the nonlinearity in scanning probe microscopes (SPMs) in order to acquire spatial measurements with high levels of accuracy. In this paper, a new characterization method is presented that combines a high-resolution image processing technique used by the experimental mechanics community known as Digital Image Correlation (DIC) with digital images from a standard type of SPM known as an atomic force microscope (AFM). The characterization results using this new method match those from the conventional method using micromachined calibration gratings. However, the new method uses the texture of a specimen surface and not a precisely micromachined calibration grating. As a consequence, the new characterization technique is a more direct method for measuring scanning errors that can be conducted in situ when imaging a specimen surface at any scale within the scanning range of the SPM. It also has the advantage of reconstructing the position error curve more continuously with less noise than the conventional method.
