Análisis de fallas

Shear failure occurs when the thrust load exceeds the material's shear strength, effectively 'cleaving' the ring along the line of the groove edge. This is a brittle failure mode and is more common in rings with very high hardness (HRC > 52). While higher hardness increases the shear strength $S_s$, it reduces the material's fracture toughness. If the ring is subjected to impact loads, a high-hardness ring may snap rather than deform. For most applications, a hardness range of HRC 44-48 for carbon steel or HRC 40-45 for 17-7PH provides the optimal balance between shear strength and impact resistance.

Rotation in the groove occurs when the retained component rotates relative to the ring, or when the ring lacks sufficient radial tension to 'grip' the groove bottom. This causes abrasive wear on the groove sides and the ring's faces. Over time, the groove becomes wider and the ring becomes thinner, leading to a loss of thrust capacity. Detection during inspection is evidenced by 'polishing' or circular scoring marks on the ring and groove surfaces. In severe cases, the ring may show signs of 'bluing' due to frictional heat. Prevention involves ensuring proper ring tension or using a 'self-locking' ring that prevents rotation via a tab-in-slot design.

Dishing (or coning) occurs when the thrust load $P$ exceeds the moment resistance of the ring's cross-section. As the ring deforms into a cone shape, it expands radially and eventually slips out of the groove. This is usually caused by either a groove that is too shallow or a groove material with low yield strength. If the groove wall deforms, it provides a 'ramp' for the ring to climb. The solution is to use a harder groove material, increase the groove depth $d$, or use a heavier cross-section ring. Multi-turn rings are also more resistant to dishing than single-turn rings because the individual turns provide mutual support against the bending moment.

Lateral buckling occurs when a Crest-to-Crest wave spring's free height $L_0$ is significantly larger than its mean diameter $D_m$ (typically $L_0/D_m > 4$). Under load, the spring behaves like a slender column and bows outward. This leads to non-axial loading, uneven stress distribution, and potential contact with the housing walls, which increases friction and wear. The solution involves using an internal guide rod or an external sleeve to provide lateral support. Mathematically, the critical buckling load $P_{cr}$ can be estimated using a modified Euler's formula $P_{cr} = \frac{\pi^2 \cdot E \cdot I_{eff}}{(K \cdot L)^2}$, where $I_{eff}$ is the effective moment of inertia of the wave structure.

The primary sign of hydrogen induced delayed fracture is a 'clean' brittle fracture that occurs hours or days after the spring has been installed and placed under load, despite no immediate failure during assembly. The fracture surface usually shows intergranular cracking under a Scanning Electron Microscope (SEM). This is caused by hydrogen atoms migrating to areas of high tensile stress (the wave peaks). To prevent this, the spring must be baked at $190^{\circ}C \pm 10^{\circ}C$ within 1-4 hours of plating to drive out the diffused hydrogen. Failure to bake promptly allows the hydrogen to trap at grain boundaries, leading to embrittlement.

Edge tearing occurs during the coiling or stamping process if the tooling is dull or the material ductility is insufficient. These micro-cracks on the radial edge of the spring act as severe stress risers. Under cyclic loading, the stress intensity factor $K$ at the crack tip is defined by $K = Y \cdot \sigma \cdot \sqrt{\pi \cdot a}$, where $a$ is the crack depth. If $K$ exceeds the fracture toughness $K_{Ic}$ of the material (e.g., 17-7PH), the crack will propagate rapidly. Visual inspection at 10x magnification or fluorescent penetrant inspection (FPI) is required for high-criticality aerospace parts to ensure no edge defects are present.

Fretting corrosion occurs at the contact points between the wave peaks and the mating surface when micro-oscillations (typically 1-100 μm) are present. In aerospace connectors, vibration causes the spring to rub against the housing. This generates fine metallic debris that oxidizes and acts as an abrasive, leading to material loss and a decrease in preload. Mitigation strategies include applying a dry-film lubricant (e.g., MoS2 per MIL-L-46010) to reduce the coefficient of friction, or using a material with higher surface hardness. Additionally, increasing the spring's preload can sometimes 'lock' the surfaces together, preventing the relative micro-motion.

Permanent set, or 'taking a set,' occurs when the actual stress exceeds the material's yield strength. If theoretical calculations suggest the spring is safe, the failure often stems from 'stress relaxation' or 'creep' due to operating temperatures higher than specified. Another cause is 'dynamic surging,' where high-frequency vibrations cause localized deflections beyond the design work height. This can be analyzed using the natural frequency formula $f_n = \frac{1}{2 \cdot \pi} \cdot \sqrt{\frac{k}{m}}$. If the operating frequency matches $f_n$, the resulting resonance can cause the spring to compress to its solid height, exceeding the yield point and causing permanent deformation.

Hydrogen embrittlement occurs when atomic hydrogen penetrates the carbon steel crystal lattice (typically high-carbon spring steels such as SAE 1070-1090) during electroplating, acid pickling, or in highly corrosive operating environments.

Mechanism:
Hydrogen atoms diffuse and accumulate in areas of high tensile stress, micro-voids, or dislocations. When the spring is loaded under stress, these atomic pockets recombine into hydrogen gas molecules ($H_2$), generating high internal gas pressure that initiates micro-cracking and leads to immediate, brittle, catastrophic structural failure under low design loads.

Mitigation and Preventive Protocol:
1. Avoid Acid Cleaning: Use mechanical descaling or alkaline cleaning instead of hydrochloric or sulfuric acid baths.
2. Relief Baking (Mandatory): Within 1 hour (maximum 4 hours) of electroplating (e.g., zinc or cadmium), the wave springs must undergo stress-relief baking at $375^\circ F \text{ to } 400^\circ F$ ($190^\circ C \text{ to } 205^\circ C$) for a minimum of 4 to 24 hours (refer to ASTM F1940 / ASTM B850).
3. Baking Window: Delayed baking after electroplating allows the hydrogen to permanently localize, rendering baking ineffective.