Chapter 5

Failure analysis is the science of determining the probable cause of a failure to a particular part or structure. Metallurgists often view failure analysis as an examination of microscopic metallic structures. Engineers often perform failure analysis by looking at macrostructures or gross deformations of a part. Figure 1 is a view of a failed truck

Figure 1

Figure 2

engine crank shaft. In this case, microscopic failure analysis indicates that improper heat treating of the shaft surface resulted in a premature, costly fatigue failure. Figure 2 is a view of a failed side wall to a grain bin. Macroscopic failure analysis indicates that sidewall buckling resulted from unusual wind loads at the time of the accident. In each case, the failure analysis influenced subrogation action taken by companies insuring the loss.

Failure analysis can uncover two situations of interest to the adjuster, i.e., the design or manufacturing defect. Let’s look at the design defect first. A design defect is a product deficiency that occurs in the engineering design process. A bolt too small for the load, a sharp edge causing stress concentration, poor material selection, are all examples of a design defect. Recovering the part that broke is helpful but obtaining information on the design of the part is critical since this is the essence of the investigation. Availability of engineering drawings or a similar part significantly helps the analysis.

A manufacturing defect is a deficiency that occurs on the assembly line during installation. Improperly torquing bolts, improper wire routing or substituting an unspecified part are examples of a production defect. Design information is helpful, but obtaining the unique part that failed is critical. The adjuster is often the first person at the failure scene who has an interest in the failure and is in the best position to influence the outcome of the failure analysis.

Here are some recommendations:

1. Call in the failure analyst immediately to remove the part and view the environmental influences on the failure.

2. If this is impractical, photograph the failed object as soon as possible. Make sure photographs contain environmental information such as the proximity of the failed part to the total structure.

3. Move the failed part to storage for future analysis. Be careful not to damage the part or fracture surface which will yield clues to the probable cause of the failure.

4. Protect the part or fracture surfaces from corrosion by applying with oil, grease or similar protective coatings.

5. Document transfer of the part from one party to another to maintain the continuity of the chain of custody.

Metal fatigue is a metallurgical term describing the failure of a metal part resulting from repetitive forces. As the metal is subjected to this alternating stress, a small crack develops, eventually growing until the part fails. Many of us have broken a coat hanger by repeatedly bending the wire until it breaks. This is essentially using metal fatigue to break the wire. Metal fatigue does not imply that the metal is "old and tired." Metal fatigue develops as a result of the number of repetitive stress cycles, not aging. Unlike the human body that gets tired and rebuilds after rest, metal fatigue damage is progressive with the mechanical part eventually failing. The insurance adjuster may encounter fatigue failures as the cause of loss of the insured's property. Since the insurance adjuster is often the first individual to report on the probable cause of loss, a basic familiarity with fatigue failures can help in the failure analysis. Why the interest in fatigue as a cause of failure? Metal fatigue can be a result of:

1. Improper mechanical design

2. Improper factory assembly

3. Improper assembly by the insured

Let's look at each reason separately. The manufacturer's engineering department should design the equipment such that it lasts its intended life. A fatigue failure during that expected life implies that the manufacturer did not take fatigue strengths of the material into account when designing the mechanical device. This is a design defect and the insurer has a valid subrogation interest against the manufacturer.

In the second case, the device could have been designed properly but manufactured improperly. For example, the coupling on a tractor engine may be adequately designed, yet the assembly department failed to torque bolts to specification and the bolts failed as a result of metal fatigue. Again subrogation interest is justified against the manufacturer because of a manufacturing defect.

The third reason for the fatigue failure may rest with the insured who may have improperly serviced or operated the mechanical device.

Figure 1

The following are recommendations that may help in establishing the existence of a fatigue failure:

1. Figure 1 shows a typical fracture surface of a crankshaft that failed in fatigue. A fatigue fracture surface usually is composed of several concentric lines called striations which emanate from the origin of the crack. If the striations are visible, the chances are it is a fatigue crack. If the striations are not visible, it could still be a fatigue crack.

2. Next, protect the fracture surface and save the fractured part for analysis by an independent engineer. Oil the fracture surface to prevent corrosion and protect the surface with tape to prevent surface abrasion and deterioration.

3. Take photographs of the mechanical apparatus from which the part was removed. This helps the failure analyst verify the cause of failure.

4. Determine how long the mechanical apparatus operated before the failure occurred. This helps the failure analyst determine if the time to failure was reasonable or unreasonable.

5. Finally consult your failure analyst to obtain an independent opinion on the probable cause of failure.

Fatigue failures are a result of cyclic stresses in mechanical components and can be a result of improper design or manufacture. Recognition of a fatigue failure, preservation of the failed parts and consultation with a failure analyst may result in a recovery for the company.

Billions of dollars are lost annually to corrosion related failures. There are many types of corrosion such as pitting corrosion, stress corrosion cracking (sometimes called environmentally assisted cracking), galvanic corrosion, concentration cell corrosion, fretting corrosion and crevice corrosion. For the purpose of this article, metallic corrosion is defined as the deterioration of a metal or alloy as a result of interaction with an adverse environment. Several corrosion failure case studies are presented along with a brief discussion on corrosion characteristics causing the loss.

Figure 1

Figure 1 is a view of a hole in the high pressure line feeding the gas meter at a residential building. Natural gas had leaked from the hole resulting in an explosion in the home and total loss of the structure. The hole is a result of pitting corrosion caused by a soil environment rich in chlorides. Newer natural gas piping of this type is typically coated to reduce the chance of such deterioration. Figure 2 is a photograph of a typical flexible brass connector used to service

Figure 2

gas appliances in a home. The arrow points to a fracture in the brass tubing. Figure 3 is a view of a metallographic cross-section of the tube in the vicinity

Figure 3

of the fracture surface. The dark areas indicated by the arrow show intergranular stress corrosion cracking as the cause of the failure. The brass chosen for this tube was susceptible to stress corrosion cracking (environmentally assisted cracking) which is aggravated by household cleaning chemicals such as ammonia. The crack allowed natural gas to leak into the home causing an explosion and two fatalities.

Figure 4

Figure 4 is a view of a dump trailer that sustained a frame failure when driving over a bump. Figure 5 is a view of the failure origin showing severe areas of

Figure 5

surface corrosion. The corrosive environment is the typical salt/water mixture resulting from the deicing of roads during the winter. Periodic inspection of trucking equipment is recommended, but often overlooked. A visual inspection can often warn of such a failure allowing repair work to be performed.

Figure 6 shows a tanker trailer that leaked chemical waste while parked on a small hill. The chemical waste flowed down the hill and polluted a sewage lagoon in a treating plant, resulting in a substantial economic loss. Figure 7 is a view of one of the holes that was discovered in the bottom of the tanker.

Figure 6

Figure 7

The corrosion of the aluminum shell came from within the tanker as a result of an improper cleaning with sodium hydroxide. Globules of sodium hydroxide remained in the tanker causing chemical corrosive attack to the aluminum and penetration of the shell.

Figure 8 is a view of a straight truck that

Figure 8

Figure 9

failed to stop at a stop sign when the brake pedal was pressed, causing an accident. Figure 9 is a view of a brake line that leaked severely in the vicinity of a metal clip. The line was originally under a clip which partially covered the brake line forming a crevice. The resulting crevice corrosion caused penetration of the brake line and loss of hydraulic pressure. Crevice corrosion is a complex form of loss of passivity in a metal in a crevice.

Figure 10

Figure 10 is a view of natural gas piping that was cast into a concrete sidewalk. Over a period of several years, severe corrosive attack occurred to the pipe resulting in several holes. The concrete sidewalk limited natural gas escape since it encapsulated the pipe. When the sidewalk was being excavated to replace the gas line, severe leakage resulted from the areas where the concrete had fractured, causing an explosion. The use of chloride salt for ice removal on the concrete contributed to the corrosion. Another form of corrosion, termed concentration cell corrosion, also contributed to the leakage.

Figure 11

Figure 11 is a view of a corroded water pipe that had been buried in acidic soil. Severe localized corrosion had occurred in the water pipe as indicated by the arrow. The acid attack of the surface is a significant ingredient in the corrosion of the pipe. The corrosion also appeared to be aggravated by ground electrical currents in the water pipe. A coated pipe with better control over electrical grounding should correct this problem.

As can be seen from several examples, environmental influences are a significant player in corrosion failures. Some failures are design related; others can be attributed to poor maintenance and misuse. In each case there was significant property loss or personal injury.

A bolt is a threaded fastener utilized throughout industry to secure two or more mechanical parts together. There are a wide variety of threaded fasteners available including hex head bolts, carriage bolts, machine screws, studs, tapping screws, socket head bolts and plow bolts. A representative hex head bolt, shown in Figure 1, is characterized

Figure 1

Figure 2

by a threaded portion, an unthreaded shank and a hexagonally shaped cap or head. Figure 2 shows the cap stamped with the name of the manufacturer and a symbol indicating it to be a grade 5 bolt with a minimum tensile strength of 120000 psi. Bolts may be subjected to thousands of pounds of tensile force as well as alternating forces at a range of frequencies. When a threaded fastener cannot sustain the expected loading and becomes detached, a significant loss may occur. Although rare, bolt failure has caused wheel detachment on vehicles, structural failure in buildings, and crashes of aircraft.

This article presents several examples of bolt failures that have been involved in serious losses. The claims professional is usually involved early in the analysis of the accident and is influential in the early decisions as to the failure analysis of the threaded fastener involved. Through a review of these case studies, the claims professional can gain insight into how to handle future assignments involving bolt induced losses.

FAILURE AS A RESULT OF AN OVERLOAD

Many accidents can be characterized as an impact with a non-compliant object such as a truck impacting a concrete bridge support. In these cases, bolt failure due to overload can occur. Figure 3 is a view of a bolt that fractured in the threaded area. The 45 degree full-slant

Figure 3

fracture surface indicates high tensile loads. The fine, gray appearance of the fracture surface is consistent with a sudden overload failure. In this case, other bolts on the mechanical part had failed, transferring the load to the remaining bolt shown in Figure 3, resulting in an overload.

Figure 4

Figure 4 is a view of the fracture surface of a steering gear output shaft of a large truck. The truck was involved in an accident and a question arose as to the role of the steering unit as a possible cause. Microscopic examination of the fracture shows a full-slant fracture surface (about 45 degrees) and evidence of a shear-face tensile fracture, characteristic of an overload. It was concluded that the fracture of the output shaft was most likely a result of the accident and not a cause.

Figure 5

Figure 5 is a view of a similar fracture surface at the threaded end of a wheel spindle with its characteristic 45 degree fracture surface and fine gray appearance. This is a typical overload during a vehicle rollover accident.

Figure 6

Figure 6 shows a typical treaded tie rod end on a vehicle steering system. The severe distortion of the bolt prior to failure suggests that an external force from the impact deformed the tie rod end, causing a failure.

FAILURE FROM LACK OF LOCKING MECHANISM

In order to prevent bolts from loosening over time, various locking mechanisms are employed. They include lock washers, locking nuts, jam nuts, mechanical deformations, wire wrap, cotter pins, metal locks, expansion anchors, helical coils and polymer locking compounds. Machinery that is subject to vibratory environments usually is equipped with

Figure 7

some sort of locking mechanism. If the locking mechanism is not applied to the machinery during manufacture, a catastrophic event may result. Figure 7 is a view of a hoist transmission used in a

Figure 8

large crane. The bolt shown in Figure 8 was found out of position after the crane transmission “jumped out of gear” dropping a heavy load. In Figure 7, the arrow points to the location of the bolts. Specifications called for a polymer locking compound to be applied to the bolt threads to prevent backing out. No compound was found on the bolt threads or in the threaded hole. Consequently, over a period of time, the bolts loosened, resulting in the loss of control to the shift fork in the transmission.

METAL FATIGUE

Metal fatigue is the phenomenon characterized by progressive crack growth during cyclic loading. A crack is often initiated at a flaw or stress riser (sharp notch) in a part. Cyclic forces such as vibrations or repeated impact cause the crack to increase in size until the part can no longer sustain the load, and a final fracture occurs. Figure 9 is a view of a classical reverse bending fatigue fracture of a bolt. The arrows point to the initiation sites of the fatigue crack. The small lines or striations on the metal surface show the advance of the crack from the exterior to the inside of the bolt. The rutted gray area in the middle of the bolt is the area of final fracture where the bolt cross-section was reduced and the bolt could

Figure 9

not carry the load. Metal fatigue can be a result of a design deficiency as well as improper assembly of the part.

FAILURE FROM IMPROPER TORQUE

When threaded fasteners are utilized, the amount of tightening or bolt torque is often important. Motor vehicle wheel studs require torques ranging from about 100 ft-lbs for smaller vehicles to over 400 ft-lbs for large trucks. The appropriate torque is required in order to prevent relative flexing of the two parts being fastened and to assure an acceptable mechanical connection. Bolt failures as a

Figure 10

result of improper torque have occurred in automobile applications. Figure 10 shows a view of a failed wheel stud compared to a new one. This bolt failed as a result of insufficient torque. Figure 11 shows a part of the stud that was bearing on the wheel rim causing severe wear of the thread, another indicator of insufficient bolt torque.

Figure 11

FAILURE FROM IMPROPER DESIGN

Figure 12 is a view of the front ski suspension system for a snowmobile. The operator of the snowmobile was badly

Figure 12

injured when the sled suddenly veered to one side, throwing him into a tree. As shown in Figure 12, the bolt failed in the threaded section at a shear point in the bracket. It is generally considered poor design to allow significant alternating shear or bending forces in the vicinity of the threaded section of the bolt since the threads form a stress riser and tend to initiate fatigue cracks, as happened in this case. A better design would be to utilize a bolt with a shorter threaded section so that the unthreaded shank material is at the shear area of the bracket. This eliminates the stress riser from the threaded section and increases the effective bolt diameter.

Figure 13

Figure 13 is a view of a rock truck that sustained a left front wheel mount failure. Figure 14 shows the initiation of the

Figure 14

failure mode where an over-stressed bolt failed and fell out of position, thereby transferring higher loads to the remaining bolts. Eventually the remaining bolts failed, causing detachment of the wheel mount and an accident. Figure 15 shows wear on the bolt threads a result of bolt movement due to insufficient clamping force between flanges.

Figure 15

FAILURE FROM IMPROPER MANUFACTURE

Figure 16 is a view of a failed tie rod end bolt, a critical steering system component in an automobile. The vehicle

Figure 16

suddenly pulled to the right after traveling over a bump in the road. The fracture surface revealed an area of progressive fracture that had been occurring over time. This was initiated by a heat treating related defect in the outer surface of the tapered shank. The crack grew by metal fatigue and finally failed when traveling over a modest road surface perturbation.

Figure 17

Figure 17 is a view of the right rear control arm of a midsize automobile that rolled over while traveling on an interstate highway. The driver suddenly experienced extreme difficulty in steering the vehicle and lost control. In Figure 17, it is apparent that the control arm bolt is out of position and, in fact, fractured near the threaded end. With little evidence of an extreme force application at the right rear suspension, it appeared unusual that a bolt would fracture from an overload in such a manner. The bolt was removed and tested. The exterior surface hardness was found to vary considerably along the bolt length, resulting in a stress discontinuity at the fracture surface. The non uniformity of hardness occurred from improper heat treatment of the bolt during manufacture.

CORROSION FAILURE

Corrosion of metals can be disastrous to threaded fasteners. Surface and pitting corrosion attacks threaded fasteners as a result of contact with moisture or other corroding media. Since bolts often carry high loads, stress corrosion cracking (also called environmentally assisted cracking) is another corrosion related failure mode. Corrosion, coupled with forces in a bolt, tends to accelerate cracking. Figure 18 shows a damaged

Figure 18

dump trailer after a rollover accident. Figure 19 is a view of a suspension related clamp on the dump trailer. The clamp failed, causing the axle to part from the vehicle and an accident. Views of the fracture surfaces show progressive environmentally assisted cracking as a cause of the bolt failure. Such failures

Figure 19

are normally discovered during periodic inspections, a typical maintenance procedure on large vehicles.

EVIDENCE HANDLING

As can be seen by the previous examples, the fracture surface plays a significant role in the analysis of threaded fastener failures. Consequently, the appropriate handling of the failed bolt as evidence encompasses the protection and preservation of the fracture surface. A light oil coating on the fracture surface helps reduce corrosion, provided the surface films on the fracture surface are not significant. If surface films must be preserved, the sealing in a dry, air tight

Figure 20

container is helpful. Removal of the bolt from the vehicle or piece of machinery requires care. Figure 20 shows the result of improper removal. The person removing this bolt used a hammer and screw driver which damaged the fracture surface. In Figure 21, a torch cut has badly damaged the fracture surface. In some cases, the bolt cannot be dislodged. Then the whole part should be removed and possibly be cut away at a later date.

CLOSING

When threaded fastener failure appears to be a cause of a loss, a few fundamental investigative measures are in order. First, thoroughly photograph the parts

Figure 21

involved, preferably in their undisturbed state. Save the mechanical system, i.e. the automobile, piece of machinery or device for possible future analysis. If parts must be removed, avoid damage to the fracture surfaces of failed parts. Avoid hammering or torching the parts as depicted in Figures 20 and 21. Save the parts in an environment that intends to inhibit the onset of corrosion and reduces the chances of additional deformation from handling. Obtain as much history as possible on the usage and maintenance of the mechanical system. Finally, place interested parties on notice as to testing and disassembly to avoid the pitfalls of spoliating the evidence.

Wood structures fail for a variety of reasons causing losses that are dealt with by the claims administrator. Recognizing the failure mode or cause of the failure is necessary for efficient claims handling. Shown below are examples of typical wood related failures and probable causes of the failure. Figure 1 is a view of a typical bending related failure. The

Figure 1

wood is in good condition. Splintering indicates the wood was probably overloaded causing the collapse. Rotten wood fails in a manner not characterized by splintering. This is often called a brash fracture or brash wood. Check for biological or insect damage as a cause of the failure. Figure 2 is a typical view of a

Figure 2

brash failure in wood. Biological decay has caused significant reduction in the wood strength of the bow truss connection resulting in the collapse of the roof it supported.

Figure 3

Figure 3 shows a supermarket roof that collapsed during a rain storm. Figure 4 shows a severe compression failure of the bow truss connection due to moisture

Figure 4

related deterioration. The attic was poorly ventilated allowing for the excess moisture accumulation. Figure 5 shows a loss on a relatively new roof structure. This was a result of poor design where the roof collapsed onto a small manufacturing facility. Figure 6 shows the metal connectors at the truss joints severely deformed from the overload.

Figure 5

Figure 6

Figure 7

Figure 7 shows a failure of floor joists in a home causing the floor to sag significantly. Flooding of the crawl space plus lack of proper ventilation caused deterioration of the joists and columns.

Figure 8

Figure 8 is a view of biological decay that has rendered a balcony a safety hazard.

Defects in wood can cause failures. Splits, checks and unusual grain orientations can cause failure in otherwise normal woods. Mechanical testing can yield information on the strength of a particular piece of wood. Figure 9 shows a typical bending test on a specimen taken from a failed beam. Microscopic analysis also helps determine wood type as well as wood condition and existence of biological activity.

Figure 9

Failed water pipes can cause substantial property loss as well as personal injury. The following case studies show what to look for when reconstructing the water pipe failure scenario.

Figure 1 is a view of a failed solder joint on a 3 inch pipe. The water drained into the basement of a large office complex

Figure 1

causing significant damage to a telephone switching system and computer complex. The failed joint was a result of insufficient heat during the solder process causing poor solder penetration and a weak joint. Figure 2 shows a similar cold solder joint that caused a substantial

Figure 2

Figure 3

water loss in an apartment complex.

Figure 3 is a view of an elbow taken from a 4 inch line. The pipe suddenly fractured causing water to drain downward through several floors in an office building. Microscopic analysis revealed a casting defect as shown by the arrow. This tended to raise the stress in the pipe wall resulting in final failure. The fracture surfaces were well preserved by field personnel facilitating the analysis. Figure 4 shows a typical sprinkler head

Figure 4

that was found to have an unusually high failure rate. Several heads had activated for no apparent reason watering down offices and industrial facilities. In Figure 4, the arrows point to cracks in the bronze support frame. This was probably a result of shrinkage during the casting operation, a manufacturing defect.

Figure 5

Figure 5 shows over stress related cracks as a result of excessive tightening of a pipe fitting. This eventually resulted in leakage and significant property loss to a residential building. Figure 6 shows a

Figure 6

leaking grooved coupling that failed due to severe strain on a pipe from poor installation. This was a 5 inch water line that flooded an office building basement nearly drowning the janitorial staff. Figure 7 shows the classic "fish mouth" fracture associated with excessive localized hoop stress consistent with freezing

Figure 7

in the water pipe. Figure 8 shows fittings that vibrated loose on a water pump motor causing substantial water damage to a residential building. The pump couplings were not secured properly to withstand the pump vibration.

Figure 8

Anti-friction bearings which include ball, roller and needle bearings are used in many industrial and consumer product applications. Automobiles, industrial machinery, household appliances and aircraft are examples of mechanical apparatus that rely on anti-friction bearings for proper function. Figure 1 is a view of a typical ball bearing. The arrow to the upper right points to the outer race, essentially an outer track that directs the rolling action of the balls. The arrow to

Figure 1

the left of Figure 1 points to the inner race that is the inner track of the assembly and also typically connects to a shaft. The center arrow points to the retainer or cage which provides equal spacing for the bearing balls. Figure 2 is an example of a loss involving an anti-friction

Figure 2

bearing, in this case a roller bearing, in the axle of a motor home. The insured was driving the vehicle at highway speeds when one of the axle bearings failed, causing excessive heating and ignition of the motor home structure, resulting in a total loss of the vehicle and

Figure 3

contents. Figure 3 shows the axle shaft supporting the bearing that failed. Figure 4 shows the wheel drum and the remains of the roller bearing retainer or cage as indicated by the arrow. When the

Figure 4

bearing failed, the wheel drum began dragging on the brake shoes, resulting in excessive heating and fracture of the wheel drum. Red hot drum fragments were thrown into the vehicle structure causing the fire. Improper preload of the bearing (tightening of the axle nut) is the suspected cause of the failure.

Typical failure symptoms of anti-friction bearings include corrosion, seizing, race flaking, retainer failure, electrical erosion, brinelling and fretting.

Corrosion is typically a result of improper storage, poor maintenance or insufficient rust inhibitor.

Seizing is a result of overloading, improper preload, inadequate lubrication or improper lubricant.

Race flaking (rapid metal fatigue of bearing surfaces) is a result of improper installation, misalignment, vibration, contamination of lubricant and overloading.

Retainer failure (cage failure) is often a result of contamination, improper lubrication or excessive speed.

Electrical erosion happens when electrical currents travel though bearings, causing arcing as the bearing surfaces bounce against each other.

Brinelling is a result of excessive forces placed on a bearing in a static mode such as when a piece of machinery has been dropped. Small dents in bearing surfaces are a result.

Fretting is the rubbing of bearing surfaces causing oxidation of the surfaces in a manner similar to brinelling.

Figure 5 is a view of a bearing used to support a drive gear shaft on a high speed printing press. Infrared thermography (Figure 6) shows significant heating of the bearing, a leading indicator of a failure.

Many of the previously mentioned failure modes manifest themselves in overheating. Figure 7 is an example of

Figure 5

Figure 6

Figure 7

retainer (cage) failure from contaminated lubricant. Figure 8 shows a pitted inner bearing surface resulting from excessive vibration. Figure 9 is an example of seizing as a result of improper lubrication.

Figure 8

Bearing failures are typically a result of poor maintenance, improper manufacture or improper design. Vandalism resulting in a bearing failure is also a possible cause. Information from the insured regarding the particular maintenance performed on the machinery in question helps distinguish between manufacturing and maintenance deficiencies. Oil samples or grease samples should be analyzed for contaminants and metallic content. Preserve the bearing for failure analysis, especially when improper design or manufacture is suspected. Finally, inquire as to any modifications or unusual operating procedures that may violate warranties and manufacturer's instructions.

Figure 9

Steel cable or wire rope is used throughout industry, providing a variety of tasks such as structural support, lifting and control functions. Figure 1 is a view of a typical wire rope showing many steel wire strands woven in a manner similar

Figure 1

Figure 2

to regular hemp rope. Wire rope is very strong for its size and weight and performs many functions well. However, it is susceptible to corrosion, wear and metal fatigue, which can cause failure, economic loss and personal injury. Figure 2 shows a wire rope loop, complete with compression sleeve and thimble. The purpose of the thimble is to provide a surface that withstands wear better than the bare wires. Without the thimble, the wire strands would eventually abrade or fatigue causing a failure. The compression sleeve keeps the wire from sliding around the thimble. Sometimes mechanical clamps are used, especially for large wire ropes. Figure 3 is a view of a large grain auger that collapsed during a windstorm. Peak wind speeds were

Figure 3

Figure 4

approximately 50 MPH, a speed easily resisted by the structure. Figure 4 is a view of the upper portion of the tower. Inspection of the cable indicated that a cable failure had occurred in the upwind cable. Figure 5 shows the cable loop that had failed at a pier in the ground. Several wire strands had been broken. The cable clamp was adequate, but no thimble had been provided. None of the other cables had a thimble at the loop end.

Figure 5

Figure 6

Figure 6 is a close-up of failed strands at the end of the loop. Failure analysis of the strands suggested that corrosion fatigue was the likely cause of failure. This was brought on by the lack of a thimble that would have protected the cable end. It was concluded that this failure was a result of improper construction of the tower cable system.

Overloading of cable systems is a common cause of failure. Serious injury can occur if a cable fails near personnel. For instance, if a wire rope cable fails when lifting or winching a load, the cable

Figure 7

often whips violently and can cause serious injury. Figure 7 is a view of a radio tower that collapsed during an ice storm. The cable failure origin showed evidence of severe overloading in an otherwise properly designed and maintained cable system.

When inspecting for the cause of a cable failure, it is desirable to save the failed cable, especially the failure origin, for future analysis. Inquire as to the loading on the cable, i.e., wind, lifting, high-speed operation, etc. If abrasion or wear is a cause of the failure, analysis of the design or manufacturing method is in order. Other design considerations may also be a cause of failure, especially in those cases where excessive rope flexing occurs from using cable sheaves (pulleys) that are too small for the wire rope.

Compressed gas cylinders are vessels that contain high pressure gases such as propane and natural gas. The ubiquitous propane cylinder can be found throughout the country in applications from temporary heating to industrial vehicle fuel reservoirs. As the propane cylinders are used and refilled, inspections are periodically performed to assess pressure vessel integrity as required by the U.S. Department of Transportation regulations. Among the several inspection requirements, is the visual inspection of the outside surfaces of the cylinder. Impact marks, scrapes, corrosion, erosion and wear are noted during these inspections. The underlying premise of these requirements is to remove from service any cylinder that exhibits deficiencies on the outside surface that could cause a failure. Inspection of the extent of corrosion of the exterior metal is an important examination that is performed in order to requalify a propane cylinder for service. Inspection is visually made and areas of suspicious corrosion damage are noted. In the instance of severe surface corrosion, a small tool is used to measure the depth of the corrosion. Pitting and crevice corrosion cause thinning of the pressure vessel wall, resulting in higher stresses and stress concentration. When excessive corrosion depth is found, the propane tank is deemed to be unacceptable and is retired.

When these inspections are not performed and the propane cylinder suffers from corrosion damage, the condition could prove disastrous. Figure 1 shows a building that sustained considerable

Figure 1

explosion damage after the failure of a propane cylinder. This building was undergoing a renovation with a 100 lb. propane cylinder on the first floor, connected to a temporary construction heater. One evening when the building was unoccupied, an explosion occurred on the first floor. The 100 lb. propane cylinder rocketed upward through the first 3 floors and became lodged in the fourth floor. Figure 2 shows the top of the propane cylinder with resulting

Figure 2

impact damage and part of a wood beam that was in the path of the trajectory.

Figure 3

Figure 3 is an overall view of the 100 lb. propane cylinder. The bottom of the cylinder separated and was found on the

Figure 4

first floor (Figure 4). As would be expected, as the propane at approximately 100 psi escaped from the bottom of the cylinder, over 7 tons of force pushed the cylinder upward. The accident investigation focused on the cylinder end cap and why it had fractured. The arrow in Figure 5 points to the fractured end of the cylinder at the foot ring. Severe crevice

Figure 5

corrosion was evident. Crevice corrosion occurs at a corner between two pieces of steel where trapped debris, dirt and moisture accumulate in the crevice. Figure 6 depicts the corroded area in the crevice between the tank envelope and the foot ring, with the arrow pointing to

Figure 6

Figure 7

severe crevice corrosion. Figure 7 is a cross-sectional cut through the foot ring showing the failure origin. The left arrow points to the original cylinder wall. The right arrow points to the foot ring support which does not carry pressure but merely the weight of the cylinder. The center arrow points to the base of the bottom cap on the cylinder. The Y like crevice traps moisture and debris, resulting in the corrosion that significantly reduced the end cap steel thickness (center arrow). In this case, approximately 25% of the original wall thickness remained at time of failure. This condition was obviously hazardous and lead to a large property loss. Apparently, this propane cylinder had not been properly inspected.

When a loss occurs that mandates a failure analysis of a propane cylinder, the following recommendations are offered. Preserve the fracture surfaces from corrosion by applying a protective film such as mineral oil. Attempt to recover as many parts of the cylinder as possible. This may require extensive excavation at the scene to find lost parts such as the one described above. Perform a failure analysis as soon as possible since the process of corrosion is ongoing and can lead to loss of evidence.