Richard W. Klopp

The strength behavior of granulated silicon carbide at high strain rates and confining pressure

The dynamic Mohr–Coulomb behavior of silicon carbide (SiC) was inferred from symmetric pressure/shear plate‐impact experiments which entail planar impact of two SiC plates inclined at 15° to the impact direction. The transverse velocity of the free rear surface of the target plate was recorded using a laser Doppler velocimeter system, and the experiments were simulated using a postulated viscoplastic constitutive model that accounts for comminution and dilatancy. Model parameters were varied until the computed and measured velocity histories agreed. The results indicate that comminution occurred soon after loading, and thus the experiment measures the behavior of granulated material at shear strain rates of ≊105 s−1 and mean stress ranging from 1 to 9 GPa. A friction coefficient of 0.23 was obtained, which is about half the value for quasistatic compression of precomminuted ceramic reported in the literature. The simulation results were strongly affected by the values chosen for the friction coefficient a...

Assessment of fatigue load spectrum from fracture surface topography

Abstract The possibility of deducing load spectrum parameters from fatigue failure surfaces is explored by applying innovative, three-dimensional topographic characterization and analysis techniques to failure surfaces in aluminum sheet. Precise, high-resolution elevation maps of fracture surfaces were obtained using confocal optics scanning laser microscopy. Elevation power spectral density curves resulting from a fast Fourier transform of the elevation data appear sensitive to stress intensity range and environment. A conjugate fracture surface matching procedure, FRASTA, can detect and may provide a way to quantify overloads.

Analysis of tilt in the high‐strain‐rate pressure‐shear plate impact experiment

An analysis is presented of the effect of tilt in the high‐strain‐rate pressure‐shear plate impact experiment. Normal and shear tractions and the velocity difference across the specimen are obtained from free‐surface particle velocity, tilt angle, closure direction, and impact velocity data. The effect of tilt in high‐strain‐rate pressure‐shear plate impact is shown to be small unless the velocity difference across the specimen is itself small.

Fracture toughness measurements using small cracked round bars

This study investigated using small precracked round bars (CRBs) for measuring fracture toughness values representative of material fracture resistance in large, thick components (so-called plane strain fracture toughness). The three alloys tested with CRBs (titanium-10V-2Fe-3Al, HY-130 steel, and A508 class 2A steel) cover a broad range of fracture behaviors. K or J values at crack initiation were calculated using the experimental load and displacement records. The fracture tests were complemented with scanning electron microscope observations and finite element analyses of round bars with different ratios of crack depth to bar diameter. The CRB data were compared with independently measured toughness data obtained with larger bend specimens. In two cases, toughness values measured with precracked round bars for a given material and mode of fracture were in good agreement with those measured with larger standard fracture mechanics specimens; the exception was A508 steel at 204°C, and the lower values in this case may have occurred because the microstructural features influencing fracture were large in relation to the uncracked ligament size of the CRB. Finite element simulations of the CRB indicate that in the fully plastic regime the constraint factor Q is low (-0.75 to -0.85) for shallow crack specimens but increases with deformation (from -0.6 to -0.1) for deeply cracked bars. Experimentally, this difference in constraint was evidenced by a transition from ductile to cleavage fracture mode in (embrittled) HY-130 steel specimens with shallow and deep cracks, respectively.

Using Small Cracked Round Bars to Measure the Fracture Toughness of a Pressure Vessel Steel Weldment: A Feasibility Study

The research objectives were to demonstrate the feasibility of using small fatigue pre-cracked round bars to measure the initiation fracture toughness of ductile nuclear pressure vessel steels and weldments and to refine and validate experiment and analysis procedures. Initiation fracture toughness values were measured for a duplicate of HSSI Weld 72W, unirradiated, in the temperature range -150° to 50°C, using small cracked round bar (CRB) specimens. The results were compared with the values obtained with 1T-CT specimens. The good agreement between the toughness values measured with CRB and 1T-CT specimens indicates that using small CRB specimens (possibly cut from Charpy bars) to measure fracture toughness is feasible. A relationship between J and the displacement due to the crack δ c r , where δ c r is obtained from extensometer measurements, was established experimentally. Fracture initiation in CRBs of the size investigated here occurred at or near maximum load, with the crack growth prior to maximum load being less than 200 μm. This observation, together with the unique relationship between J and δ c r , open the possibility of greatly simplified testing and data reduction procedures for fracture experiments with CRB.

Microdamage Observations in Dynamically Fractured Ti-10V-2Fe-3Al Microstructures and Preliminary Modeling Attempts

This paper presents the results of a study of titanium Ti-10V-2Fe-3Al in two heat treatments for which we determined the static and dynamic flow and fracture properties. Complementary fractographic observations revealed the dominant microstructural failure mechanisms. We then used these results to guide preliminary finite element modeling of the two-phase microstructures aimed at rationalizing how the observed micromechanisms affect their global mechanical behavior.

THREE-DIMENSIONAL ANALYSES OF PLATE IMPACT EXPERIMENTS WITH CIRCULAR AND STAR GEOMETRIES

Flier impact experiments are used in the study of dynamic material response and failure. In general, experiments study the material response to the shock processes within the one-dimensional response region of the target before lateral waves from the plate edges arrive. Prior work has indicated that star-shaped flier and target plates reduce the magnitude of the edge effects or extend the one-dimensional response time within the central region of the target plate. To further quantify this effect, we performed three-dimensional finite element simulations of plate impact experiments with circular and two different star geometries for the flier and target plates. Calculations showed that star-shaped plates indeed reduce the magnitude of the edge effects compared to circular plates. However, the star geometries can reduce the one-dimensional response time at the center of the target plate and are more difficult to fabricate than circular geometries.

Effect of Microstructure and Loading Rate on the Fracture Behavior of Titanium–10V–2Fe–3Al

ABSTRACT Initiation and propagation toughness of Ti-10V-2Fe-3Al in three microstructural conditions were measured under static and dynamic loads using bend tests. Materials of equivalent strengths, but having 40%, 12% and 0% primary alpha phase, exhibited similar static and dynamic initiation toughnesses but markedly different propagation toughnesses. Fractographic evidence suggests that primary alpha enhances propagation toughness by a resistance curve effect rather than a crack velocity effect.