Robert R. Stephens

The effect of temperature on the behaviour of short fatigue cracks in Waspaloy using an in situ SEM fatigue apparatus

Abstract Short fatigue crack growth behaviour was studied in Waspaloy at 25°C, 500°C and 700°C under cyclic loading conditions at a stress ratio of 0.1. Specimens were tested within an elevated temperature fatigue apparatus coupled to a scanning electron microscope which allowed in situ observations to be made of the fatigue process. Crack formation was dominated by slip band cracking at 25°C and 500°C, while at 700°C cracks formed along slip bands and twin boundaries. Crack propagation proceeded by means of slip band cracking at 25°C and 500°C, while at 700°C crack growth was primarily stage II-mode I type growth. Short fatigue crack growth rates were faster at 500°C than at 25°C for an equivalent stress intensity range, ΔK1, owing to a change in slip character. Short crack growth rates tended to be lower at 700°C than at 500°C, owing to changes in the material leading to precipitate coarsening. As the temperature was increased, microstructural contributions to crack growth decreased, resulting in a reduced microstructural short crack effect.

Fatigue crack growth of Ti-62222 titanium alloy under constant amplitude and miniTWIST flight spectra at 25°C and 175°C

Abstract Fatigue crack growth behavior was investigated for Ti-62222 titanium alloy under constant amplitude ( R = 0.1 and 0.5) and miniTWIST flight spectra at 25 and 175°C. Crack closure was observed at threshold and near threshold for both temperatures with R = 0.1 but not for R = 0.5. Based upon applied or effective Δ K , fatigue crack growth behavior for a given R ratio showed no significant temperature effect. MiniTWIST flight spectra fatigue crack growth life at 175°C, however, was 2.5–3.5 times greater than at 25°C. Fatigue crack growth life calculations using NASA/FLAGRO were conservative. Fatigue crack growth was transgranular with SEM fracture surface morphology dependent upon both Δ K and temperature with significant increase in secondary cracking at 175°C.

Thermal Stresses Due to Laser Welding in Bridge-Wire Initiators

In ongoing research at the University of Idaho, potential failure mechanisms of airbag initiators are being investigated. Cracking of the cylindrical glass-to-metal seal (GTMS) present in these devices has been observed. These cracks could be a path for moist gas to diffuse into the initiator, potentially leading to bridge-wire degradation and late-fire or no-fire initiator failure. Previous research has shown that cracking may be caused by thermal stresses induced by the GTMS formation process. The goal of this research was to determine if welding of the output-can onto the initiator header could produce stresses in the glass great enough to cause cracking. A finite element analysis solution was chosen to model the transient heat transfer and temperature distribution in the initiator assembly during the welding process. The thermal stresses were calculated with a mechanical analysis once the temperature distribution was determined. Compressive stresses induced by pressing the header assembly into the output-can as part of the manufacturing process were also investigated with a closed-form mechanics of materials solution. The welding thermal stress model initially predicted radial stresses greater than the tangential stresses. This conflicts with observed radial cracks, which would be induced by tangential stresses. Subsequent investigations with an interface region stiffness model showed that when the stiffness of the bond at the pin-glass and glass-header interfaces is decreased, the maximum radial stress is greatly reduced and that the maximum tangential stress stays relatively constant. These predicted stresses were still in excess of the range of glass strengths reported in literature. However, superposition of the compressive stresses due to the press-fit and residual stresses created when the GTMS is formed with these thermal stresses results in the total radial and tangential stresses being on the same order as the reported strengths. It was determined that when initiators are overheated during welding, radial stresses due to thermal expansion cause the bond to fail and separation to occur over a portion of the pin-glass interface. Tangential stresses developed for the same reason are sufficient enough to cause radial cracking, where the bond is still intact.

Thermal Induced Stresses in Bridge-Wire Initiator Glass-to-Metal Seals

Cracks have been observed in the insulating glass of bridge-wire initiators that may allow moisture to penetrate the assembly, potentially leading to the corrosion and degradation of the bridge wire and the pyrotechnic material. Degradation of the pyrotechnic or the bridge wire may result in initiator failure or diminished performance. The goal of this research is to determine if the manufacturing processes could produce thermal stresses great enough to crack the glass. A parametric plane stress closed-form solution was used to determine the effects of changing material properties and dimensions of the initiator, and to determine potential stresses within the initiator from two different manufacturing scenarios. To verify and expand the plane stress closed-form solution, a two-dimensional axisymmetric finite element analysis was performed. To reproduce the two manufacturing scenarios, lumped models and models that included the effects of cooling the initiator were used. Both models showed that if the manufacturing process involves pouring molten glass into the initiator, the potential for cracking exists. Furthermore, if the surface of the initiator cools faster than the center, cracking is more likely.

Relative stiffness of 3 bandage/splint constructs for stabilization of equine midmetacarpal fractures

OBJECTIVE Determine the relative stiffness of 3 bandage/splint constructs intended for emergency fracture stabilization. DESIGN Experimental model. A single plane free end deflection model was developed to simulate the forces placed on a bandage/splint construct during stabilization of a complete mid-metacarpal bone fracture. The total deflection of the model in one plane was measured following application of 3 different bandage/splint combinations including a classic, 3 layered Robert Jones Bandage (RJB) with a splint placed on the outside of the bandage (RJB-3), an RJB with splint placed after the first of 3 bandage layers (RJB-1), and a single layer full limb bandage with external splint (SS). Comparisons were made between the deflections of the model with each bandage/splint combinations in an effort to determine the most effective method for field fracture stabilization. SETTING Laboratory. ANIMALS No animals were utilized in data collection for this study. Two live horses were utilized during the pilot study. INTERVENTIONS Application of bandage and splint to a model intended to simulate the bending force on a lower forelimb fracture in a horse MEASUREMENTS AND MAIN RESULTS Deflection was determined by the difference between the height of the models supported free end before application of a 4.5 kg weight and at the conclusion of the deflection test. There was no significant difference in the amount of deflection between bandage/splint combinations (78 ± 32 mm (RJB-1), 94 ± 44 mm (RJB-3), and 93 ± 33 mm (SS)) CONCLUSIONS: The one-layer bandage with splint was equivalent to either RJB configuration in the mean amount of deflection in the simple model of a fracture.