Operational Information

Fatigue

 
 

 

A metal subjected to a repetitive or fluctuating stress will fail at a stress much lower than that required to cause fracture on a single application of load. Failures occurring under conditions of dynamic loading are called fatigue failures, presumably because it is generally observed that these failures occur only after a considerable period of service. Fatigue accounts for at least 90 percent of all service failures due to mechanical causes.

 

Fatigue occurs when a material is subject to alternating or cyclic stresses, over a long period of time. Examples of where fatigue may occur in a marine diesel engine are: crankshafts, valve springs, turbocharger blades, piston crowns, bottom end bolts, piston skirts at the gudgeon pin boss and tie bolts.

 

Stresses can be applied in three ways, torsionally, axially and by bending.

 

The symbol for stress is the Greek letter sigma s and the units are force/ unit area i.e N/m2 or psi (imperial)

 

TORSIONAL

 

This is where the material is twisted and untwisted along its axis. Any rotating shaft driving a load will be subject to torsional vibration at the natural frequency of the shaft. However torsional vibration is most easily visualised in an engine crankshaft where the compression and firing forces are applied to the crankpin through the piston and con rod. These forces vary according to angle of thrust applied by the conrod and the cylinder firing pressure but are greatest at about 10 either side of TDC.

 

The crank also has to absorb the inertia loading due to the conrods and pistons, which easily amounts to several tons on each cylinder.

 

 

AXIAL

 

This is where the material is subject to tension or compression along its axis. An example of this are the bottom end bolts on a four stroke engine.

 

The bolts and their nuts are subject to tensile stress when tightened and additional varying tensile stress during operation. The total stress level is high and varies with time, giving rise to the risk of fatigue. The connecting rod is in compression during the compression and power strokes, but due to the inertia forces in the running gear when the piston changes direction between the exhaust and inlet strokes, the connecting rod is put into tension. This increases the tension in the bottom end bolts, leading to cyclic stressing.

 

BENDING

 

When material is bent, the inside of the bend will be in compression and the outside of the bend will be in tension. This type of stress can be easily visualised in a piston crown under the gas load and is compounded by the stresses induced by the difference in temperature on the top surface and the underside of the crown (thermal stressing).

It also occurs in crankshafts where the gas load on the piston is bending the crankshaft. If the main bearings are of different heights (i.e out of vertical alignment), then the bending is increased.

 

STRESS CYCLES

 

Reversed

Repeated

Random

 

 

There are three stress cycles with which loads may be applied to the component under consideration. The simplest being the reversed stress cycle . This is merely a sine wave where the maximum stress and minimum stress differ by a negative sign. An example of this type of stress cycle would be in an axle, where every half turn or half period as in the case of the sine wave, the stress on a point would be reversed. The most common type of cycle found in engineering applications is where the maximum stress (smax)and minimum stress (smin) are asymmetric (the curve is a sine wave) not equal and opposite. This type of stress cycle is called repeated stress cycle. A final type of cycle mode is where stress and frequency vary randomly. An example of this would be hull shocks, where the frequency magnitude of the waves will produce varying minimum and maximum stresses.

 

THE S-N CURVE

 

The S-N curve is just a graph plotted of stress, S against the number of cycles N.

N is a logarithmic scale i.e 105 cycles, 106 cycles 107 cycles etc.

 

The line plotted for the particular material will indicate how many stress reversals it can go through before it fails.

 

If the material is loaded below the fatigue limit, which in the example shown is 14103 psi  (95103  kN/m2) then it will not fail regardless of the number of stress cycles.

 

Material such as aluminum, copper and magnesium do not show a fatigue limit, therefore they will fail at any stress and number of cycles. Other important terms are fatigue strength and fatigue life. The stress at which failure occurs for a given number of cycles is the fatigue strength. The number of cycles required for a material to fail at a certain stress is the fatigue life.
 

 

CRACK INITIATION, PROPAGATION AND FAILURE

 

Failure of a material due to fatigue may be viewed on a microscopic level in three steps:

  1. Crack Initiation: The initial crack occurs in this stage. The crack may be caused by surface scratches caused by handling, or tooling of the material; threads ( as in a screw or bolt), flaws in the material, slip bands or dislocations intersecting the surface as a result of previous cyclic loading or work hardening.

  2. Crack Propagation: The crack continues to grow during this stage as a result of continuously applied stresses

  3. Failure: Failure occurs when the material that has not been affected by the crack cannot withstand the applied stress. This stage happens very quickly.

Fatigue failure can be identified by examining the fracture. A fatigue fracture will have two distinct regions; One is smooth or burnished as a result of the rubbing of the bottom and top of the crack as it is growing. The second is granular, due to the rapid failure of the material.

 

Other features of a fatigue fracture are Beachmarks and Striations. Beachmarks, or clamshell marks, may be seen in fatigue failures of materials that are used for a period of time, allowed to rest for an equivalent time period and the loaded again as in factory usage. Striations which can be seen through a microscope, are thought to be steps in crack propagation, were the distance depends on the stress range. Beachmarks may contain thousands of striations

 

Visible beachmarks on a tiebolt failure

Magnification of fatigue failure showing striations

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