Understanding Physics of Failure Science is vital in the Plant and Equipment Wellness Way EAM Methodology
World Class Enterprise Asset Management, Reliability and Maintenance Success Is Guaranteed When You Eliminate and Prevent Physics of Failure Causes
Physics of Failure is the first foundation topic presented in the Plant and Equipment Wellness Way EAM training course. The science of Physics of Failure explains how parts and components break and consequently lead to your plant, machines and equipment failing. The video clips below explain Physics of Failure and its use in the Plant Wellness Way EAM methodology.
3-day training course: DAY 1 INTRODUCTION
PEW/PWW EAM 3-day Course Content
Why do Machines and Equipment Continue to Fail in Companies?
PEW/PWW EAM Course Day 1 – Foundations Session 1 – Physics of Failure
Understand How Machines are Designed
The Unforgiving Nature of Machine Design
As soon as a machine part deforms outside of its tolerance limits it is on the way to premature failure. Plant, machinery and equipment can only be expected to be reliable if kept within the design stresses and the internal and external environmental conditions it is designed to handle. Once the stresses or environment conditions are beyond its capability, it is on the way to an unwanted breakdown at sometime in future.
Stress from Distortion
Here are common situations where soft-foot occurs. If the items are bolted down without fixing their soft-foot problem, the equipment is distorted out-of-shape, or the mounting feet do not fully contact the base and properly support the forces created when the equipment is used. Soft-foot is a Physics of Failure cause that must always be eliminated when machines and equipment are mounted.
Another common Physics of Failure cause is shaft misalignment that distorts and bends shafts, which in turn combines with running loads and can overload the shaft bearings when the machine is operating with normal duty loads.
The Physics of Parts Failure
Theoretically, if the strength of materials are well above the loads they carry, they should last indefinitely. In reality the load-bearing capacity of a material is probabilistic – i.e. there will be a range of stress-carrying capabilities. We can show the probabilistic nature of parts by drawing a curve from the stress level at which the first part failed to the stress required to make the last part fail. If a part with a low stress capability is used in a situation where the probability of experiencing high loads is greater, then there is a chance that a load will arise that is above the capacity of the part and it will fail.
Physics of Failure science explains how component microstructures are destroyed. Many materials degrade with time, or from the service conditions, or from accumulated fluctuating stresses. The parts fatigue and they are no longer able to carry the original loads and stresses. In such situations the probability that the item will see stresses above its remaining capacity to sustain them increases. Eventually the part will fail.
Fatigue Limit of Parts Material of Construction
Because of the probabilistic statistical nature of failure several specimens have to be tested at each stress level. Some materials, notably low-carbon steels, exhibit a flattening off at a particular stress level as at (A) in the figure which is referred to as the fatigue limit. As a rough guide, the fatigue limit is usually about 40% to 50% of the tensile strength. In principle, components designed so that the applied stresses do not exceed this level should not fail in service. The difficulty is a localised stress concentration may be present or introduced during service which leads to initiation, despite the design stress being normally below the ‘safe’ limit. Most materials, however, exhibit a continually falling curve as at (B) and the usual indicator of fatigue strength is to quote the stress below which failure will not be expected in less than a given number of cycles which is referred to as the endurance limit.
Although fatigue data may be determined for different materials it is the shape of a component and the level of applied stress which dictate whether a fatigue failure is to be expected under particular service conditions. Surface finish condition is also important to prevent crack initiation. Often complete components or assemblies, e.g. railway bogie frames or aircraft fuselage, will be tested by subjecting them to an accelerated loading spectrum reproducing what they are likely to experience over their entire service lifetime.
Stress at Atomic and Microstructure Levels Destroy
This graph is called a stress-life cycle curve. A great deal of fatigue load testing, where the load cycles in one direction and is then reversed, has been done with a wide range of metals. From these tests graphs of tensile strength verses number of cycles to failure have been developed. An example of one for wrought (worked) steel is shown in the Figure. It helps us to understand how much load a material can repeatable take and still survive.
Have you ever bent a metal wire back and forth until it breaks from being worked? If you have then you were performing a stress life-cycle test. The wire does not last long when bent to 90 degrees one way and then back 90 degrees the other way. Each bend was an overstress, and eventually the overstress accumulated and the wire fatigued and failed.
The vertical scale on this log-log plot shows the applied stresses as a proportion of the steel’s ultimate tensile stress ‘Su’ while the horizontal scale is the number of stress cycles to failure. The left hand sloping line tells us is that a steel part put under high cyclic loads producing stresses in high proportion to its ultimate tensile stress will fail after a given number of cycles. Whereas the right hand side of the curve indicates that if cyclic stresses are maintained below a definable limit the part will have infinite life. The curve also tells us that a steel part made of this metal will fail if it has just one load cycle with a stress greater than its ultimate tensile strength. (Like when a small bolt snaps-off if over-tightened) It will also fail in less than two thousand cycles if the imposed stresses are 90% or more of the tensile strength. But if the stresses are kept below half of the tensile strength it will never fatigue.
Note that not all metals have a fatigue limit.
Activity – Stressing Steel Parts
In this activity we see the effect of over-stressing parts. We also see a distribution of failure events once the number of cycles to failure are plotted for each of the Participants. Following the plotting of Participant’s results we plot the distribution from the Instructor’s procedure over that of the Participants and show how narrow the Instructor’s distribution is compared to the spread of the combined Participants.
The Instructor then shows and trains the Group to mimic the Instructor’s process to bend clips and gets the Participants to break another clip as trained. The number of cycles to failure is plotted and a second distribution results. There will still be a spread that is much wider than the Instructor’s distribution.
The second distribution means either that the training was not delivered well, the Participants did not learn the instructions properly, or the Participants do not have the physical attributes to do the task as required even after being train.
There is much that can be extracted from the Activity to show the importance of understanding the effect of process and the importance of using consistent procedures on the achievement of specific outcomes.
One other activity to include is to change to a second, different paper clip manufacturer (even use three separate manufacturers) and have the Instructor use the same clip breaking procedure on the new make of clip. Since the same procedure is consistently used any variation must be due to the material of construction properties.
Impacts of Out-of-Roundness
Impacts of Out-of-Roundness – The Need for Precision
Failing Roller Bearing Degradation Curve and the Worsening Defect Severity
Stage 1. Earliest detectable indication of bearing failure using vibration analysis. Signals appear in the ultrasonic frequency bands around 250 KHz to 350 KHz. At this point, there is approximately 10 to 20 percent remaining bearing life.
Stage 2. Bearing failure begins to “ring” at its natural frequency, (500 to 2,000 Hz) signal appears at the first harmonic bearing frequency. Five to 10 percent remaining bearing life.
Stage 3. Bearing failure harmonics of the fundamental frequency are now apparent. Defects in the inner and outer race are now apparent and visible on vibration analysis of the noise signal. Temperature increase is now apparent. One to five percent of remaining bearing life.
Stage 4. Bearing failure is indicated by high vibration. The fundamental and harmonics begin to actually decrease, random ultrasonic noise greatly increases, temperatures increase quickly. Remaining life one hour to one percent.
Operating Stresses Cause Failure
The diagram shows how 3 different operating methods stress a dragline boom. The way a machine is used affects its rate of failure. Table 1 is a measure of the operating impact of each practice. Method B causes a lot of damage – the loads are higher and the fatigue from stress cycling accumulates faster. Method C is more gentle. ‘B’ has an expected 5 failures a year and ‘C’ only 2 a year. Which operating practice is best for the business?
To make the necessary assessment, we need to know the DAFT Costs for each option. Then we can see if the extra throughput from ‘B’ actually produces a lower unit cost product. If it does, then ‘B’ should become the standard way to operate. But if it does not, then Method B must be abandoned.
Recall that … Unit Cost = Total Cost of Production ÷ Total Throughput. If the DAFT Costs of the extra 3 failures from using Method B, cost more than the extra 22 million units produced from Method C, the company will be loosing money.
The Operating Overload Cycle
We know that parts also fail from being overstressed. This overstress is imposed on the part. Each overstress takes away a portion of the part’s strength. When enough overstress accumulates, or there is one large overload incident, the part suddenly fails.
Cause of Aging Failures
Wear-out failures are any failure mechanism that result from parts weakening with age and usage. Included are processes involving material fatigue, wearing between surfaces/substances in contact, corrosion, degrading insulation, and wear-out in light bulbs and fluorescent tubes. Initially the strength is adequate for the applied load, but over time the strength deceases. In every case the average strength value falls and the spread of strength distribution widens. This makes it very difficult to provide accurate predictions of operating life for such items.
Building for the Physics of Failure
The mechanisms of failure caused by stressing components has become known as the Physics of Failure (PoF).
The chart shows the best-practice process now adopted in designing equipment following recognition of the influences and effects of PoF. The parts are modelled by Finite Element Analysis (or other test regimes) and their behaviours analysed under varying operating conditions. The probabilistic life cycle performance is identified under the various situations and the results indicate the design limitations and operating constraints to achieve a desired failure-free life.
PEW SOLUTION: Identify Physics of Failure Causes of Atomic and Microstructure Stress
Operating stresses work on the atomic structure of a material. The loads and forces of operation are absorbed by the atoms and crystals of the material of construction. If the stresses in the atomic bonds are too great they break the bond. Where operating stresses are beyond the capacity of the material structure the structure fails. Once enough stress failures accumulate the item fails and a part breaks.
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