Introduction

Leaf Springs are widely used in the automobile and railway industries for suspension applications. The simplest variation is the single beam spring. The more normal application is the laminated ﴾multiple ﴿ leaf spring which provides a more efficient stress distribution.

Leaf Springs have the following characteristics.

They are suitable for low and medium load forces They have reasonably linear working characteristics They have relatively low spring constant They are long items with relatively low cross section They are relatively low cost items

Design of Leaf Spring

Failures in Leaf Springs

Leaf Springs and common causes of failure.

Ever have a leaf spring crack, become fatigued or break and wonder what the cause is? We will take a look at a few common reasons why leaf springs fail.

Loose U-Bolts

The image below shows center bolt failure due to a truck spring not having tight enough u-bolts. U-bolts, especially on newly installed truck springs should be checked periodically to verify they are tight. Even if you have a professional installer install springs on your truck, you should stop by after 500 miles and have your u-bolts inspected to make sure they didn't come loose.

 

Corrosion and Fatigue

Corrosion and fatigue are typically caused by a combination of time and the elements. Numerous variables will weigh on how long your leaf spring will last before it suffers from corrosion or fatigue. How much weight you haul, what part of the country you live in, etc. will play a role in the life of your springs. Making sure salt and other corrosive materials are washed off and not overloading your truck will help prevent corrosion and early spring fatigue. 

Overloading Your Truck Springs

Everyone says they never do it , but we see it all the time. Overloading is pretty self-explanatory. You have more weight in the back of your truck or you are towing a load heavier than what your truck is rated at. We all love to do it, but the negative effect on your truck springs is unavoidable. On the left image below you can see the radial shear marks from an overloaded spring. The image on the right shows distortion of the spring eye from overloading.

How do you prevent overloading your truck springs? Obviously you could carry less weight or purchase a truck with a higher weight rating. If buying a new truck or hauling less weight is not an option, you could add an overload kit like a Firestone Air Spring kit, Hellwig Helper springs or a Air Lift Air Spring kit. Overload kits are designed to take some of the weight off the leaf springs and place it onto the overload kit.

Another option that is more popular with commercial vehicles it to add another piece of steel to the spring pack. This may cause the ride to be stiffer, but if you are always hauling heavier loads in a commercial setting this may be your best bet.

We do not recommend adding a new piece of steel to leaf springs that already suffer from fatigue. You are better off replacing both of your truck springs with new ones with a higher weight rating. It's very common that we will have someone only want to replace one busted leaf in a spring pack and a few months later a different leaf breaks. If one has cracked, the others are probably in the same condition and will not last much longer.

Weld Splatter

Weld splatter is more common in commercial trucks and vans that come from the factory without a bed or on motorhome leaf springs . This is caused by welding a body or accessories to your truck in the same area as your leaf springs. Just a small amount of weld spatter can be disastrous to the life of your spring. About the only way to prevent this is to inspect your springs after and make sure no one was careless with the welder.

Above are a few common reasons why leaf springs fail. There are other manufacturing reasons such as quench cracks, tight eyes, notches, incorrect temperatures, etc. that can cause failure. With today's modern, high-tech manufacturing processes and quality control, they are not nearly as commons as they were in the past.

Leaf Spring Failure Analysis

Factors influencing fatigue life:

Overloading

The higher the loads or deflections seen by a spring, the lower its fatigue life.

Shock Absorbers

A properly functioning shock absorber will tend to reduce the spring deflection as the vehicle hits a bump. Lower spring deflections mean lower operating stresses on the spring which in turn gives longer fatigue life. This is especially true for full taper springs which do not have the high interleaf friction to help dampen spring deflections. Worn or missing shock absorbers must be replaced to maximize spring life.

Brake Adjustments

Improperly adjusted brakes can also reduce spring life. Under braking, springs are expected to absorb some of the braking forces. If the brakes on an axle are unevenly

adjusted one spring will have to absorb more than its share of braking force which can reduce its fatigue life.

Protective Coatings

Corrosion is one of the major factors in reducing spring life. Proper paints and care during handling and installation can help to slow the spread of spring corrosion. On full taper springs the only acceptable coating is the individual painting of each leaf with zinc-rich paint. This paint may be recognized by its characteristic gray color.

Surface Condition

The condition of the spring surface also has an effect on fatigue life. Generally, a fatigue crack will start at some sort of surface defect on the spring leaf. Therefore, care needs to be used when manufacturing and installing springs to reduce these defects to a minimum.

Shot Peening

Extensive testing indicates that shot peening can increase the life of springs by a factor of three or more. It is not enough, however, to simply shot peen the first one or two leaves in an assembly-all leaves must be shot peened. All major vehicle manufacturers specify that their OEM springs have each leaf shot peened.

Decarburization and Steel Quality

Improper manufacturing methods can also reduce fatigue life. For example, poorly controlled heat-treat furnaces can excessively decarburize the leaf surface. Decarburization is the loss of carbon from the steel surface which will result in a soft leaf surface once heat-treating is complete. This soft layer will not be able to handle the spring stresses and will lead to early failure. Poor steel quality can also influence spring life. If the steel has excessive impurities in it, the fatigue life will be reduced.

Maintenance

Finally, improper maintenance will affect spring life. Spring eyes and other suspension components should be regularly greased to prevent

binding. U-bolts should never be reused. Axle seats, top plates and other components should be periodically inspected and

replaced as required.

Spring failures may be categorized into three types :

Early Life Failures

These type of failures occur generally due to a spring defect, installation problem or overload. This may be due to the material used, the manufacturing processes or improper

installation techniques. This type of failure may also be caused by a short-term overload condition.

Midlife Failures

Once the spring has passed the time in service which would expose early life failures, a very low failure rate should be observed, assuming the spring is subjected to normal service.

Late Life Failures

At this point, the frequency of spring failures will tend to increase rapidly as the useful life of the spring has been reached. By this time the spring steel has been fatigued and corroded to a point where its useful life is over.

Failures occurring in early and midlife of the spring are usually most economically handled by repairing the broken leaf rather than replacing the spring. Failures in older springs occur at a point when all leaves have reached their fatigue life the spring should now be replaced. The difficulty, of course, is determining what type of failure the spring has experienced. Basically, the condition of the spring, as well as its service history, will indicate if the spring should be repaired or replaced.

When To Repair

If the spring has not been repaired or repaired only once. Stamping a 1 in the clip for the first repair and a 2 for a second repair will help identify the number of previous repairs.

If the spring mileage is less than half of normal life. If the repair cost is less than 1/2 the cost of a new spring. If no more than two or three leaves are broken. If the failure is not of a fatigue type. For example, a leaf broken through the center hole is

caused by improper spring clamping brought on by loose U-bolts or worn axle seats, not fatigue. This spring should be repaired, if possible, and the cause of failure corrected.

Even when it appears to make sense to repair, the following should be kept in mind :

Repair leaves are usually not shot peened and must often be heavily hand-fit to match the old spring. Therefore, the repair leaf will not be as durable as a leaf in a new spring would be.

Since the remaining leaves have lost some of their strength, the replaced leaves will be carrying more of the load than they were originally designed for.

When the leaves first broke the remaining leaves in the spring had to carry more load and were probably overstressed.

Replacing the broken leaves does nothing to restore the fatigue life of the reused leaves. These leaves will continue to fail since their fatigue life is essentially over.

When To Replace

The spring has already been repaired once or, at most, twice. The spring service mileage has exceeded 1/2 its normal life. The repair cost exceeds 1/2 the cost of a new spring. More than two or three leaves are broken. If small fatigue cracks can be seen running across the leaf width near the U-bolts on the

unbroken leaves. If the leaf tips have separated away from the leaf above. Never attempt to repair a full taper spring.

Mechanical Failure of Materials

The usual causes of mechanical failure in the component or system are:• Misuse or abuse• Assembly errors• Manufacturing defects• Improper or inadequate maintenance• Design errors or design deficiencies• Improper material or poor selection of materials• Improper heat treatments• Unforeseen operating conditions• Inadequate quality assurance• Inadequate environmental protection/control• Casting discontinuities.

The general types of mechanical failure include:• Failure by fracture due to static overload, the fracture being either brittle or ductile.• Buckling in columns due to compressive overloading.• Yield under static loading which then leads to misalignment or overloading.• Failure due to impact loading or thermal shock.• Failure by fatigue fracture.• Creep failure due to low strain rate at high temperature.• Failure due to the combined effects of stress and corrosion.• Failure due to excessive wear.

Failure Due to FractureFracture is described in various ways depending on the behavior of material under stress upon the mechanism of fracture or even its appearance.

Ductile FractureDuctile fracture is characterized by tearing of metal and significant plastic deformation. The ductile fracture may have a gray, fibrous appearance. Ductile fractures are associated with overload of the structure or large discontinuities. This type of fracture occurs due to error in design, incorrect selection of materials, improper manufacturing technique and/or handling.

Brittle FractureBrittle fracture is characterized by rapid crack propagation with low energy release and without significant plastic deformation. Brittle metals experience little or no plastic deformation prior to fracture. The fracture may have a bright granular appearance. The fractures are generally of the flat type and chevron patterns may be present. Materials imperfection, sharp corner or notches in the component, fatigue crack etc.

Ductile-to-Brittle TransitionThe temperature at which the component works is one of the most important factors that influence the nature of the fracture.

Factors Affecting the Fracture of a MaterialThe main factors those affect the fracture of a material are:• Stress concentration• Speed of loading• Temperature• Thermal shock.

Stress ConcentrationIn order to break a small piece of material, one way is to make a small notch in the surface of the material and then apply a force. The presence of a notch, or any sudden change in section of a piece of material, can vary significantly change the stress at which fracture occurs. The notch or sudden change in section produces what are called stress concentrations.

A crack in a brittle material will have quite a pointed tip and hence a small radius. Such a crack thus produces a large increase in stress at its tip. One way of arresting the progress of such a crack is to drill a hole at the end of the crack to increase its radius and so reduce the stress concentration.

A crack in a ductile material is less likely to lead to failure than in a brittle material because a high stress concentration at the end of a notch leads to plastic flow and so an increase in the radius of the tip of the notch. The result is then a decrease in the stress concentration.

Speed of LoadingAnother factor which can affect the fracture of a material is the speed of loading. A sudden blow to the material may lead to fracture where the same stress applied more slowly would not. With a

very high rate of application of stress there may be insufficient time for plastic deformation of a material to occur under normal conditions, a ductile material will behave in a brittle manner.

TemperatureThe temperature of a material can affect its behavior when subject to stress. Many metals which are ductile at high temperatures are brittle at low temperatures. For example, steel may behave as a ductile material above, say, 0 _C but below that temperature it becomes brittle. The ductile–brittle transition temperature is thus of importance in determining how a material will behave in service.

Failure Due to FatigueMetal fatigue is caused by repeated cycling of the load. It is a progressive localized damage due to fluctuating stresses and strains on the material. Metal fatigue cracks initiate and propagate in regions where the strain is most severe. S–N curve for the fatigue strength of a metal.

The process of fatigue consists of three stages:• Initial crack formation• Progressive crack growth across the part• Final but sudden fracture of the remaining cross section.

Prevention of Fatigue FailureThe most effective method of improving fatigue performance is improvements in design. The following design guideline is effective in controlling or preventing fatigue failure• Eliminate or reduce stress raisers by streamlining the part or component.• Avoid sharp surface tears resulting from punching, stamping, shearing, or other processes.• Prevent the development of surface discontinuities during processing.• Reduce or eliminate tensile residual stresses caused by manufacturing.• Improve the details of fabrication and fastening procedures.

Failure Due to CorrosionCorrosion of metallic materials occurs in a number of forms which differ in appearance. Failure due to corrosion is a major safety and economic concern.

Several types of corrosion are General corrosion, galvanic corrosion, crevice corrosion, pitting, intergranular, stress corrosion etc. This can be controlled using galvanic protection, corrosion inhibitors, materials selection, protective coating and observing some design rules.

Corrosion is chemically induced damage to a material that results in deterioration of the material and its properties. This may result in failure of the component.

Several factors should be considered during a failure analysis to determine the effect of corrosion in a failure. Examples are listed below:• Type of corrosion• Corrosion rate• The extent of the corrosion• Interaction between corrosion and other failure mechanisms.

Material of Leaf Spring