ADVANCED MATERIALS & PROCESSES | APRIL 2025 15 for examination. The hydraulic actuator is an assembly that converts hydraulic pressure into linear movement of a piston, which then rotates a spindle, extending the landing gear leg until it reaches a locked position. If fluid pressure is lost in the cylinder, actuation cannot occur. As shown in Fig. 2, these actuators were 14.5 in. long with a diameter of 2.25 in. The cracked barrel was made of 7075-T6 aluminum. Each failed actuator exhibited near identical cracking patterns with two segments through the barrel: a circumferential crack (130° to 300° long) and a longitudinal crack that extended partway along the housing (typically 2.5 to 4 in.). LABORATORY EXAMINATION After initial photography, the barrel of the actuator was disassembled by removing the set screws and any safety wire present. Subsequently, the flange opposite the endcap typi- cally separated past the clip, either being held by a small filament or falling off entirely from the housing. These conditions were consistent with nearly or complete circumferential fracturing. In most cases, the retaining or snap ring also fell out—this ring was made of cadmium-coated alloy steel. To examine the longitudinal crack on the side of the barrel, an angled cut was made with a handheld cutoff wheel and then intentionally overstressed just outside of the crack tip. This process created a small exemplar area of overstress features to compare with pre-existing cracks, and the region was visually distinguishable, consistent with lack of prolonged exposure to outdoor service environments. Optical and scanning electron microscopy (SEM) examinations revealed distinct regions in the circumferential crack. As shown in Fig. 3, the fracture surface area nearer to the housing groove that held the retaining ring exhibited a smoother, more reflective visual appearance, whereas the outer regions were darker and dull, with a rough, fibrous texture. The inner region, annotated in Fig. 3b, exhibited striations, as typified in Fig. 4. Outside the fatigue crack, the fracture surface exhibited equiaxed dimpled rupture, consistent with overstress fracture. The longitudinal crack also showed dimpled rupture, with more directional quality as this crack was parallel to the part grain direction. A closer examination of the fatigue region found the cracks had originated at multiple initiation Fig. 3 — View of (a) the circumferential portion of the crack on the hydraulic actuator, after disassembly, with (b) the areas of fatigue cracking and overstress annotated. (b) (a) Fig. 4 — Secondary electron (SE) micrograph of a typical view of fatigue striations in the propagation region of the circumferential crack.
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