Jacques H. Giovanola

Adiabatic shear banding under pure shear loading Part I: direct observation of strain localization and energy dissipation measurements

Adiabatic shear banding under pure shear loading was studied in VAR 4340 steel (HRC 40) using a high-speed photography technique. The development of a shear band was directly observed and the strain and strain rate during localization were measured. The stress-strain curve for the shear banded material and estimates of the energy dissipated and the temperature in the shear band were also obtained. It was found that shear localization occurs in two sequential stages over widths of 60 μm and 20 μm, respectively. Significant strength reduction occurs only during the second stage of localization. Strains of 17 and strain rates approaching 1.4×106s−1 were measured in the band. The total energy dissipated per unit area of shear band was of the order of 0.18 MJ/m2; the corresponding temperature elevation was in excess of 1000°C. The information obtained both in this investigation and in the metallographic and fractographic investigation presented in a companion paper provide insight into the mechanisms leading to the development and fracture of a shear band. Such information can be used to develop micromechanical models of adiabatic shear banding.

Adiabatic shear banding under pure shear loading Part II: fractographic and metallographic observations

Abstract A metallographic and fractographic investigation was performed of dynamically loaded torsion specimens of VAR 4340 steel (HRC 40). Macroscopically, it was found that the shear bands propagated on several distinct planes around the circumference of the specimen. Microscopically, scanning electron microscope observations of the fracture surfaces reveal that microvoid nucleation and growth is the mechanism leading to shear band fracture. Patches of highly smeared and knobbly material, produced by sliding of the fracture surfaces against each other, were also found at several locations and suggest very high local temperatures. Metallographic cross sections revealed white-etching material, characteristic of transformed bands, only at limited locations, corresponding to regions of the fracture surface exhibiting a highly smeared or knobbly texture. These results indicate that microvoid nucleation and growth play a significant role in adiabatic shear banding and that one of the mechanisms for the formation of transformed bands is the extensive sliding and rubbing of the shear fracture surfaces against each other. Thus transformed bands may be, at least in some cases, a consequence rather than a cause of the final shear band fracture.

Investigation and Application of the One-Point-Bend Impact Test

This paper investigates a dynamic fracture test that produces smoothly varying stress intensity histories of controlled amplitude and duration. A test configuration has been adopted in which a simple edge-cracked specimenis loaded in bending by impacting it at the midsection without supporting it at the outer edges. The stress intensity history is measured by a strain gage near the crack tip and has approximately a sinusoidal time dependence. The dependence of stress intensity amplitude and duration on the impact velocity and on the specimen material and dimensions has been determined both analytically and experimentally. The limit on the specimen aspect ratio to obtain a smooth stress intensity history controlled only by the first mode of vibration of the specimen has also been established. Stress intensity pulses of durations varying between 90 and 500 its can be readily achieved, and the maximum amplitude can be independently adjusted over an order of magnitude (20 to 200 MPa m 1 / 2 ). The new experimental procedure has been used to measure the dynamic fracture toughness of 4340 steel (HRC 50) at three loading rates. Comparison of the dynamic results with the value of the static toughness indicates little strain-rate sensitivity of the fracture toughness of 4340 steel in the range of loading rates investigated (quasi-static to 3 × 10 6 MPa m 1 / 2 s - 1 ).

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.

FRACTURE OF GEOMETRICALLY SCALED, NOTCHED THREE-POINT-BEND BARS OF HIGH STRENGTH STEEL

Abstract The purpose of the experimental work reported in this paper was to provide data that may serve for the development of scaling rules for ductile fracture initiation at blunt notches. Fracture experiments were performed with three sizes of geometrically scaled notched bend specimens of high strength low hardening HY-130 steel using carefully scaled fixturing. Fracture initiation, defined as the appearance of the first sub-millimeter crack with a tensile opening, was reliably detected, using acoustic emission and direct visual inspection with a microscope. Comparison of the normalized load versus load-point displacement curves revealed a significant scale effect on the condition for fracture initiation, with large specimens fracturing at smaller normalized displacement than smaller specimens. The normalized displacement at fracture does not decrease in direct proportion to specimen size. Metallographic cross-sections of the specimens and fractographic observations revealed that at the microscale, fracture initiates by shear localization followed by nucleation and growth of voids under shear and tension in the localization zone. This mechanism profoundly modifies the stress and strain gradients at the notch root so that microstructural length scales become fracture controlling parameters. Both changes are probably responsible for the observed scaling behavior.

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.

Corrosion-Fatigue Crack Nucleation in Alclad 2024-T3 Commercial Aircraft Skin

The processes that influence the transition from corrosion pit to crack formation in Alclad 2024-T3 under cyclic loads in aqueous environments were examined in laboratory experiments in salt water. Results suggest that the nucleation of corrosion-fatigue cracks involves two competing mechanisms: hydrogen effects in the cladding and electrochemical dissolution at constituent particles in the core alloy. Cracks in the clad did not necessarily nucleate at the largest corrosion pit, suggesting that a contributing factor to crack nucleation from a pit may be the creation of a local region of weakness. This paper presents results from experiments to assess the effects of pH and cladding on the nucleation and propagation of corrosion-fatigue cracks.

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.

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.