David P. Kihl

Thermomechanical response of AL-6XN stainless steel over a wide range of strain rates and temperatures

Abstract To understand and model the thermomechanical response of AL-6XN stainless steel, uniaxial compression tests are performed on cylindrical samples, using an Instron servohydraulic testing machine and UCSDs enhanced Hopkinson technique. True strains exceeding 40% are achieved in these tests, over the range of strain rates from 0.001/s to about 8000/s, and at initial temperatures from 77 to 1000 K. In an effort to understand the underlying deformation mechanisms, some interrupted tests involving temperature and low- and high-strain rates, are also performed. The microstructure of the undeformed and deformed samples is observed by optical microscopy. The experimental results show: (1) AL-6XN stainless steel displays good ductility (strain >40%) at low temperatures and high-strain rates, with its ductility increasing with temperature; (2) at high-strain rates and 77 K initial temperature, adiabatic shearbands develop at strains exceeding about 40%, and the sample breaks, while at low-strain rates and 77 K, axial microcracks develop at strains close to 50% or greater; (3) dynamic strain aging occurs at temperatures between 500 and 1000 K and at a strain rate of 0.001/s, with the peak value of the stress occurring at about 800 K, and becoming more pronounced with increasing strain and less pronounced with increasing strain rate; and (4) the microstructure of this material evolves with temperature, but is not very sensitive to the changes in the strain rate. Finally, based on the mechanism of dislocation motion, paralleled with a systematic experimental investigation, a physically based model is developed for the deformation behavior of this material, including the effect of viscous drag on the motion of dislocations, but excluding the dynamic strain aging effects. The model predictions are compared with the results of the experiments. Good agreement between the theoretical predictions and experimental results is obtained. In order to verify the model independently of the experiments used in the modeling, additional compression tests at a strain rate of 8000/s and various initial temperatures, are performed, and the results are compared with the model predictions. Good correlation is observed.

Stochastic fatigue damage accumulation under broadband loadings

Abstract Fatigue tests were conducted on 72 high-strength welded steel cruciform-shaped specimens subjected to stochastic loadings. Results of these tests are used to investigate experimentally the effects of loading non-normality and frequency bandwidth and truncation on the rate of fatigue damage accumulation. Test results are compared with predictions made using Rayleigh approximation and rainflow analysis in terms of cycles and time to failure. Results indicate that non-normality can significantly increase the rate of fatigue damage accumulation, and can result in non-conservative fatigue life estimates if its effect is not accounted for, regardless of frequency content. Likewise, frequency content was also found to influence the rate of fatigue damage accumulation, but to a lesser extent than non-normality. When higher-frequency components were included, shorter fatigue lives were observed. Fatigue life predictions using rainflow analysis produced good agreement with experimental results; predictions made using Rayleigh approximation produced non-conservative fatigue life estimates.

Mean stress effects in fatigue of welded steel joints

Mean stress effects in steel weldments were examined under both constant and random narrowband amplitude fatigue loadings. The purpose of these tests was to provide experimental data with which to substantiate the use of analytical expressions to account for mean stress effects. Fatigue tests were performed under both tensile and compressive mean stress levels. Test results indicate agreement with the modified Goodman equation to be favorable in accounting for the effect of tensile mean stresses on fatigue life. However, test results from high fatigue loadings (maximum stresses nominally above half ultimate) were found to possess better agreement with the Gerber formulation than with the modified Goodman one. Behavior under compressive mean stresses indicated a linear correction relationship was required, which was less conservative than any of the relationships considered. Test results obtained under random amplitude fatigue loadings exhibited trends similar to those observed under constant amplitude loadings. This finding, along with supporting analysis, indicates that the same correction relationship can be used in the same manner for both constant amplitude and random (narrowband) amplitude loadings.

Fatigue of welded joints under narrowband non-Gaussian loadings

Abstract Analytical and experimental results are presented to determine the effect of non-normality of the loading process on the rate of fatigue damage accumulation in welded steel joints. All stochastic loadings used are narrowband and the parameter kurtosis is used to measure the degree of non-normality of the loading processes. Non-normal loadings are generated by passing Gaussian loadings through non-linear transformation functions. p] Results of the analytical study as well as results of fatigue experiments on welded cruciform specimens indicate that non-normality can significantly influence the rate of fatigue damage accumulation. Errors in fatigue life prediction can vary from slightly conservative to very unconservative by assuming a normal response when it is not. Results of random load experiments indicate that an endurance limit observed under constant amplitude tests does not convey to random load response.

Fatigue of welded steel joints under wideband loadings

Abstract Fatigue tests were conducted on high-strength welded steel cruciform-shaped specimens subjected to random loadings to investigate the effects of loading intensity, nonnormality and frequency bandwidth on the rate of fatigue damage accumulation. The test result are compared with predictions made using the Rayleigh approximation and rainflow analysis in terms of cycles and times to failure. Results indicate that nonnormality can significantly increase the rate of fatigue damage accumulation and result in nonconservative fatigue life estimates if it is effect is not accounted for properly. Likewise, frequency content was also found to influence the rate of fatigue damage accumulation, but to a lesser extent than nonnormality.

Stochastic fatigue damage accumulation in a T-welded joint accounting for the residual stress fields

Abstract The residual stresses that occur as a result of nonhomogeneous heating and cooling during welding may have a significant effect on the accumulation of fatigue damage in a welded joint. The problem is complicated not because of the complex spatial distribution of the residual stress fields, but because those fields typically change under an applied load. The present study considers the effect of residual stresses on fatigue damage accumulation in a welded joint subjected to stochastic loading. The influence of residual stresses on stochastic fatigue damage accumulation is accounted for by a simple approach based on an elastic–perfectly-plastic material model and the Gerber correction factor. The model assumes that the residual stress remaining at the critical location depends on the largest nominal stress ever endured by a welded joint. The model predicts that the residual stresses during stochastic loading randomly decay to zero. The effect of material yielding is additionally investigated by considering an elastic–plastic material model with linear kinematic hardening. The residual stresses in this case are computed through Monte Carlo simulations. It is demonstrated that the effect of material hardening is to reduce the rate of residual stress decay and thus to accelerate the rate of fatigue damage accumulation.

Reduction of Overwelding and Distortion for Naval Surface Combatants. Part 2: Weld Sizing Effects on Shear and Fatigue Performance

As high-strength thinner-steel implementation in ship designs increase, dimensional management becomes critical to control construction costs and schedule in ship production. In the U.S. shipbuilding industry, improvements to shipbuilding facilities and processing technology have not kept pace with the rate of change in ship design. Additionally, new designs using thinner steels are subject to legacy weld sizing criteria possibly leading to inappropriately sized welds on lightweight materials. These two factors result in widespread overwelding, causing severe plate deformation in naval vessels during construction and nonvalue added labor to correct as needed for fit-up tolerances. Historically, shear and fatigue strength data has been focused on the larger welds and thicker steel plates typical of the state of the practice when these legacy weld sizing criteria were developed. In order to optimize weld design and production in modern, lightweight naval surface vessels, there is a need to develop more accurate data about the performance of precision fillet welds for thin steels.