Mechanical springs are used in mechanical components to exert force, provide flexibility, and absorb or store energy. This article provides an overview of the operating conditions of mechanical springs. Common failure mechanisms and processes involved in the examination of spring failures are also discussed. In addition, the article discusses common causes of failures and presents examples of specific spring failures, describes fatigue failures that resulted from these types of material defects, and demonstrates how improper fabrication can result in premature fatigue failure. It also covers failures of shape memory alloy springs and failures caused by corrosion and operating conditions.

A flat spring for the main landing gear of a light aircraft failed after safe execution of a hard landing. The spring material was identified by chemical analysis to be 6150 steel. The fracture was found to have occurred near the end of the spring that was inserted through a support member about 25 mm thick and attached to the fuselage by a single bolt. Brinelling (plastic flow and indentation due to excessive localized contact pressure) was observed on the upper surface of the spring where the forward and rear edges of the spring contacted the support member. It was indicated by chevron marks that brittle fracture had started beneath the brinelled area at the forward edge of the upper surface of the spring. The origin of the brittle fracture was found to be a small fatigue crack that had been present for a considerable period of time before final fracture occurred. Fracture of the landing-gear spring was concluded to have been caused by a fatigue crack that resulted from excessive brinelling at the support point. Regular visual examinations to detect evidence of brinelling and wear at the support in aircraft with this configuration of landing-gear spring were recommended.

Several of the springs, made of 1.1 mm diam Inconel X-750 wire and used for tightening the interstage packing ring in a high-pressure turbine, were found broken after approximately seven years of operation. Intergranular cracks about 1.3 mm in depth and oriented at an angle of 45 deg to the axis of the wire were revealed by metallographic examination. A light-gray phase, which had the appearance of liquid-metal corrosion, was observed to have penetrated the grains on the fracture surfaces. The spring wires were found to fracture in a brittle manner characteristic of fracture from torsional loading (along a plane 45 deg to the wire axis). Liquid-metal embrittlement was expected to have been caused by metals (Sn, Zn, Pb) which melt much below maximum service temperature of the turbine. The springs were concluded to have fractured by intergranular stress-corrosion cracking promoted by the action of liquid zinc and tin in combination with static and torsional stresses on the spring wire. As a corrective measure, Na, Sn, and Zn which were present in pigmented oil used as a lubricant during spring winding was cleaned thoroughly by the spring manufacturer before shipment to remove all contaminants.

The safety valve on a steam turbogenerator was set to open when the steam pressure reaches 2400 kPa (348 psi). The pressure had not exceeded 1790 kPa (260 psi) when the safety-valve spring shattered into 12 pieces. The steam temperature in the line varied from about 330 to 400 deg C (625 to 750 deg F). Because the spring was enclosed and mounted above the valve, its temperature was probably slightly lower. The 195 mm (7 in.) OD x 305 mm (12 in.) long spring was made from a 35 mm (1 in.) diam rod of H21 hot-work tool steel. It had been in service for about four years and had been subjected to mildly fluctuating stresses. Analysis (visual inspection, 0.3x photographs, 0.7x light fractographs, and metallographic examination) supported the conclusions that the spring failed by corrosion fatigue that resulted from application of a fluctuating load in the presence of a moisture-laden atmosphere. Recommendations included replacing all safety valves in the system with new open-top valves that had shot-peened and galvanized steel springs. Alternatively, the valve springs could be made from a corrosion-resistant metal-for example, a 300 series austenitic stainless steel or a nickel-base alloy, such as Hastelloy B or C.

The power-type counterbalance spring, formed from hardened-and-tempered carbon steel strip and subsequently subjected to phosphating treatment, fractured at the two locations during fatigue testing. A rust colored dark band at the inside edge of the fracture surface was disclosed during investigation. Etch pits were revealed by the cleaned surface which were never observed on properly phosphated coating. It was interpreted that the spring had been subjected to an abnormal acid attack in pickling or phosphating which had resulted in considerable absorption of hydrogen by the metal and hence embrittlement. The part was concluded to have cracked during phosphating or excessive acid pickling before phosphating.

The engine of an automobile lost power and compression and emitted an uneven exhaust sound after several thousand miles of operation. When the engine was dismantled, it was found that the outer spring on one of the exhaust valves was too short to function properly. The short steel spring and an outer spring (both of patented and drawn high-carbon steel wire) taken from another cylinder in the same engine were examined in the laboratory to determine why one had distorted and the other had not. Investigation (visual inspection, microstructure examination, and hardness testing) supported the conclusion that the engine malfunctioned because one of the exhaust-valve springs had taken a 25% set in service. Relaxation in the spring material occurred because of the combined effect of improper microstructure (proeutectoid ferrite) plus a relatively high operating temperature. Recommendations included using quenched-and-tempered steel instead of patented and cold-drawn steel or using a more expensive chromium-vanadium alloy steel instead of plain carbon steel; the chromium-vanadium steel would also need to be quenched and tempered.

A cadmium-plated music-wire return spring that operated in a pneumatic cylinder designed for infinite life at a maximum stress level of 620 MPa failed after 240,000 cycles. An extremely hard and small kernel, which looked like a weld deposit, was observed at the center of the fractured surface. The kernel was assumed to have resulted from extreme localized overheating. These springs were reported to have been barrel electroplated after fabrication. The intermittent contact with the dangler (suspended cathode contact) as the barrel rotated allowed high local currents when the last contact was broken was revealed to have resulted in an arc that caused local melting of the metal being plated. The molten metal was interpreted to have been quenched instantly by the plating solution and by the mass of the cold metal of the spring. The hard spot caused by arcing during plating was concluded to be the reason of the fatigue failure. Rack plating or barrels with fixed button contacts at many points instead of dangler-type contacts were recommended to avoid hard spots.

Grease-wiper springs for cams formed from stampings of 0.25-mm thick carbon spring steel (0.65 to 0.80% C) fractured at the 0.025 mm radius on the stamped 135 deg corner at a 90 deg bend after 5,000,000 cycles. Tool marks 2 to 2.3 mm from the center of the stamped bend were disclosed by visual examination. Fatigue striations originating from cracks at the 0.025 mm radius inside corner at the bend were revealed by SEM of the fractured surface. The maximum stress at the bend, in stock of maximum thickness and as a function of the radius of the 135 deg corner, was indicated by stress calculations to be very close to the maximum allowable fluctuating stress for the material. The wiper springs were concluded to be fractured in fatigue and the cyclic loading resulted from cam rotation. The maximum applied stress approached the allowable limit due to high stress-concentration factor in the spring (caused by the very small inside radius). The corner radius was increased to 0.76 mm and the tools were re-polished to avoid tool marks.

A medium-carbon helical spring was installed in a machine assembly that was welded into its final location. Weld spatter was not prevented from landing on the wire surface by any shield. An elongated drop and two tiny droplets of metal were observed a short distance from the fracture. No droplets were revealed at the origin of the fracture, but it was assumed that a drop of molten metal landed at the origin. Adherence of the spatter drop was expected to have been affected by the opening and closing of the fatigue crack. Weld spatter bead was concluded to have caused the fatigue fracture.

A 6150 flat spring was found to be failed. The face of the spring was revealed to be under tensile stress. The failure was concluded to have begun at the dark spot on the edge where roughness resulted from shearing during the blanking operation.

Two outer valve springs made from air-melted 6150 pretempered steel wire broke during production engine testing. The springs were 50 mm in OD and 64 mm in free length, had five coils and squared-and-ground ends, and were made of 5.5 mm diam wire. It was revealed that fracture was nucleated by an apparent longitudinal subsurface defect. The defect was revealed by microscopic examination to be a large pocket of nonmetallic inclusions (alumina and silicate particles) at the origin of the fracture. Partial decarburization of the steel was observed at the periphery of the pocket of inclusions. Torsional fracture was indicated by the presence of beach marks at a 45 deg angle to the wire axis. It was established that the spring fractured by fatigue nucleated at the subsurface defect.

Presence of transverse marks which were remnant of grinding was indicated in a failed valve spring made from ground rod. The shot-peening pattern was light at this location. A transverse crack was found to grow from one such mark under the influence of local stress fields until it was reoriented to the plane normal to the major tensile axis by sufficient loading. The shot-peening procedure was altered to create adequate surface compression at all stressed points on the springs.

A copper alloy C51000 (phosphor bronze, 5%A) failed prematurely during life testing of several such springs. The wire used for the springs was 0.46 mm (0.018 in.) in diam and was in the spring-temper condition. The springs were revealed to be subjected to cyclic loading, in the horizontal and vertical planes during the testing. The fracture was revealed to have occurred in bend 2. An indentation, presumably caused by the bending tool during forming, at the inner surface of the bend where fracture occurred was revealed by microscopic examination. Spiral marks produced on springs during rotary straightening were observed. A crack that had originated at the surface at the inside bend and had propagated toward the outside of the bend was revealed by microscopy of a longitudinal section taken through bend 2. The small bend radius was interpreted to contribute to spring fatigue as a result of result in straining at the bend zone. The spring was concluded to have failed in fatigue. It was recommended that the springs should be made of wire free from straightener marks and the bending tool should be redesigned so as not to indent the wire.

The springs formed from 3.8 mm diam cold-drawn carbon steel wire failed to comply with load-test requirements. A split wire in the spring was revealed by investigation. A smooth heat-tinted longitudinal zone was observed in the fracture. It was concluded that the spring failed in the load test due to the split wire. The reason for the condition was interpreted to be overdrawing which resulted in intense internal strains, high circumferential surface tension, and decreased ductility.

The conical helical spring sealed, within each switch enclosure, fractured to lead to the failure of several electrical toggle switches. The spring was fabricated from 0.43 mm diam AISI type 302 stainless steel wires. Appreciable amount of scale was observed on the fracture surface and tool marks were revealed on the inner surface of the broken spring. A typical fatigue fracture that originated at a tool mark on the wire surface was revealed by inspection of a fracture surface of the broken springs. Regions which displayed beach marks around the fracture origin and parallel striations within the beach-mark regions were revealed by scanning electron microscopy. As a corrective measure, the spring-winding operation was altered to eliminate the tool marks.

Two large tension springs fractured during installation. The springs were manufactured from a grade 9254 chromium-silicon steel spring wire. The associated material specification allows wire in the cold-drawn or oil-tempered (quenched-and-tempered) condition. The specified wire tensile strength range was 1689 to 1793 MPa (245 to 260 ksi). The finished springs were to be shot peened for greater fatigue resistance. Investigation (visual inspection, 3x images, 2% nital etched 148x SEM images, chemical analysis, hardness testing, and EDS analysis) supported the conclusion that the springs failed during installation due to the presence of preexisting defects. Crack surfaces were found to be corroded and phosphate coated, indicating that the cracks occurred during manufacture. Installation, which presumably entailed some axial extension, resulted in ductile overload failure at the crack sites. Recommendations included evaluating the manufacturing steps to identify the process(es) wherein the cracking was likely occurring. It was further recommended that a suitable nondestructive method such as magnetic particle inspection be implemented.

Majority of the water feeders in a new chicken house had stopped working. The water feeders were found to be operated on the principle that when the chickens pecked a plastic bowl, a compressed spring released a squirt of water. The small compression springs were made from type 302 stainless steel, and the operating stresses were safely within the design limits given by the Goodman diagram. The springs were revealed by scanning electron microscopy to contain numerous cracks on their inside surface, and these cracks were all at 45 deg to the wire axis. The solution was recommended as to select a grade of spring steel that would be more corrosion resistant than 302 stainless.

A helical compression spring with ten turns made of 1.8 mm thick wire which was under high pressure during tension applied to a rocker arm broke on the test stand in the third turn. The fracture was a torsion fracture that initiated in the highly loaded inner fiber and showed in its origin the characteristics of a fatigue fracture. A longitudinal fold was located at the fracture crack breakthrough which could still be observed at the fourth and fifth turns, where a further incipient crack originated. A metallographic section was made directly next to the fracture path and the fold was cut. It showed decarburized edges in the outer slanted part and this most likely occurred during rolling. The inner radially proceeding part, however, was probably a fatigue fracture originating in the fold. The fracture of this highly stressed spring was therefore accelerated by a rolling defect. In order to decrease the stress, the construction has meantime been modified.

A valve spring made of 4.1 mm diam wire, designed to withstand 10,000,000 stress cycles, fractured after only 2,000,000 cycles. The surface displayed impressions which indicated it had been treated by shot blasting. The spring has broken in two places. Fracture 1 was a torsional fatigue fracture which has started from a lobe-like surface defect and not, as is usual, from a point on the most highly stressed inner surface. Fracture 2, on the other hand, was a bending fatigue fracture with a starting point on both the inner and the outer surface of the spiral. The objective of the shot blasting, to put the surface into a state of even compressive internal stress, which must first be overcome during subsequent bending and torsional loading before the boundary zone comes under tensile stress, was therefore not realized in this case. On the contrary, the shot blasting led to a state of internal stress which favored fracture of the spring.

Spring failures were investigated in this study. A seam that extended more than 0.05 mm below the wire surface was revealed and the fatigue-fracture front progressed downward from several origins. A crack that is triangular in outline was produced by each of the fronts. This was reported to have occurred when the fracture plane changed to an angle with the wire axis in response to the torsional strain. The spring failure was concluded to have originated at the seam.