Why good engineering design must consider more than simply whether a structure can carry the load
When most people think about structural failure, they imagine a component being overloaded until it bends, buckles or breaks. While this can certainly happen, some of the most dangerous failures in engineering occur under loads that are well below the material’s ultimate strength.
This is fatigue failure.
Fatigue can cause a component that has performed reliably for thousands—or even millions—of operating cycles to suddenly fail. The danger is that fatigue damage can often develop internally or in areas that are difficult to inspect, with little obvious warning before significant damage has occurred.
What is fatigue failure?
Fatigue is the progressive development of cracks in a material as a result of repeated or fluctuating loading.
Importantly, a component does not necessarily need to experience loads approaching its maximum design capacity for fatigue to occur. A relatively modest load, when repeatedly applied in the same location, can eventually initiate a crack.
Once a crack has formed, each subsequent loading cycle can cause the crack to grow. Eventually, the remaining section of material may no longer be capable of carrying the applied load, resulting in sudden fracture or collapse.
This is what makes fatigue particularly concerning:
The component may appear adequately strong when assessed for a single static load, yet still be unsuitable for its intended service life.
Why static strength analysis isn’t always enough
A conventional strength calculation might ask:
“Can this component withstand the maximum applied load without yielding or breaking?”
For fatigue-sensitive equipment, that is only part of the question.
The engineer also needs to consider:
- How frequently is the load applied?
- How many cycles will the component experience?
- Does the load fluctuate between tension and compression?
- Where are the stress concentrations?
- Are there welds, holes, sharp corners or changes in section?
- How does the geometry influence local stresses?
- What material and manufacturing processes are being used?
- What level of inspection and maintenance will be possible?
- What is the intended design life?
A component can therefore have an apparently adequate static factor of safety while still having an unacceptable fatigue life.
Stress concentrations: where fatigue often begins
Fatigue cracks commonly initiate at locations where stress is concentrated.
These locations can include:
- Weld toes and weld terminations
- Sharp internal corners
- Sudden changes in section
- Bolt holes
- Keyways
- Cut-outs
- Notches
- Threads
- Attachments and brackets
- Poorly designed transitions between structural members
A component may experience a relatively uniform overall load, but its geometry can cause the local stress to be significantly higher than the nominal calculated stress.
This is why simply calculating the average stress in a component is not always sufficient.
A well-designed structure attempts to manage these stress concentrations through appropriate geometry, smooth transitions, adequate radii, suitable weld details and careful load-path design.
Small cracks can have serious consequences
One of the most concerning aspects of fatigue failure is that the initial damage can be extremely small.
A fatigue crack may begin at a weld or geometric discontinuity and initially have little effect on the overall appearance or operation of the equipment.
However, every subsequent load cycle can potentially extend the crack.
Over time, the crack can become large enough to significantly reduce the effective cross-sectional area of the component. In many cases, the final fracture occurs very quickly compared with the time taken for the crack to develop.
This means that the absence of visible deformation does not necessarily mean that a component is safe.
Why operating conditions matter
Engineering design cannot be separated from how equipment is actually used.
The loads experienced during real operation may be significantly different from the loads assumed during the original design.
For example, equipment may experience:
- Repeated impacts
- Vibration
- Start/stop cycles
- Shock loading
- Reversing loads
- Unbalanced loads
- Wind loading
- Seismic loading
- Changes in operating speed
- Increased payloads
- Modified attachments
- Changes in operating procedures
Even relatively small changes to operating conditions can alter the fatigue behaviour of a component.
This is particularly important for older machinery where the original design assumptions may no longer reflect the way the equipment is currently being operated.
Design life matters
Fatigue design is fundamentally related to number of cycles.
A structure designed for 10,000 cycles is not necessarily suitable for 1,000,000 cycles.
Similarly, a component that has successfully operated for many years is not automatically safe to continue operating indefinitely.
The cumulative loading history matters.
For equipment with a known operating cycle, engineers can estimate the number of cycles expected over the intended service life and assess the component accordingly.
For more complex equipment, different load cases and operating conditions may need to be considered, with the cumulative effect of those cycles assessed as part of the fatigue analysis.
Fatigue is particularly important in dynamic machinery
Fatigue considerations become especially important where equipment is repeatedly moving, rotating, lifting, vibrating or accelerating.
Examples include:
Amusement devices
Ride structures and components can experience thousands or millions of repeated operating cycles, often with significant dynamic loading.
Lifting equipment
Cranes, lifting frames, attachments and lifting points can experience repeated loading throughout their service life.
Mobile plant
Excavators, drilling equipment, earthmoving machinery and other mobile plant can experience significant cyclic and shock loading.
Transport structures
Vehicle chassis, trailers and transport frames are subjected to continually changing loads while in service.
Industrial machinery
Rotating equipment, conveyors, presses and automated machinery can experience large numbers of operating cycles.
In each case, the question is not simply whether the structure can carry the load today, but whether it can continue to safely carry that load throughout its intended service life.
The importance of good engineering design
Fatigue resistance starts with good design.
An engineer should consider the complete load path through the structure and identify where stresses are likely to concentrate.
Good fatigue-resistant design may involve:
- Eliminating unnecessary stress concentrations
- Improving transitions between sections
- Increasing local section capacity where required
- Selecting appropriate materials
- Optimising weld details
- Improving weld geometry and termination
- Providing appropriate radii
- Avoiding abrupt changes in stiffness
- Designing connections appropriately
- Considering the actual operating environment
- Establishing a realistic design life
- Allowing for inspection and maintenance
Importantly, making a component stronger is not always the answer.
Simply increasing the thickness of a plate or increasing the size of a structural member may not resolve a fatigue problem if the fundamental issue is poor load transfer or a local stress concentration.
Sometimes the most effective solution is to change the geometry or improve the way loads flow through the structure.
Engineering analysis can reveal what isn’t obvious
Modern engineering analysis, including finite element analysis (FEA), can be particularly useful in identifying areas of elevated stress.
FEA allows engineers to examine how loads are distributed through a structure and identify potential areas of concern.
However, analysis is only as good as the assumptions behind it.
Accurate fatigue assessment requires appropriate consideration of:
- Material properties
- Geometry
- Boundary conditions
- Applied loads
- Load combinations
- Dynamic effects
- Stress concentrations
- Weld details
- Operating cycles
- Manufacturing conditions
A visually impressive computer model does not automatically constitute a valid engineering analysis.
The results need to be interpreted by an appropriately qualified engineer who understands both the limitations of the modelling and the physical behaviour of the equipment.
Don’t wait for the crack to appear
One of the most important lessons from fatigue failures is that inspection and engineering design need to work together.
Where fatigue is a potential failure mechanism, inspection programs should focus on known fatigue-critical locations.
This may involve targeted visual inspection, non-destructive testing or more detailed engineering assessment depending on the equipment and risk involved.
If cracking is identified, simply repairing the visible crack may not address the underlying problem.
The engineer should determine:
Why did the crack occur in the first place?
Was it caused by:
- Excessive cyclic loading?
- Poor geometry?
- Inadequate weld detailing?
- Unexpected operating loads?
- Excessive vibration?
- A change in operating conditions?
- An inadequate original design?
Repairing the crack without addressing the cause may simply result in the crack returning.
Retrospective engineering assessment
For existing equipment, particularly older or modified machinery, it may not be possible to obtain complete original design information.
In these circumstances, a retrospective engineering assessment can help determine whether the equipment remains suitable for its current application.
This may include:
- Inspecting the equipment and identifying fatigue-critical areas.
- Reviewing available drawings, calculations and operating information.
- Establishing realistic design and operating loads.
- Assessing structural load paths.
- Identifying stress concentrations and potential failure locations.
- Undertaking structural and/or fatigue analysis where appropriate.
- Identifying required repairs, modifications or improvements.
- Establishing appropriate inspection and maintenance requirements.
This type of assessment can be particularly valuable where equipment has been modified, its operating conditions have changed, or failures have already been observed.
Engineering for the life of the equipment—not just the day it is built
Good engineering design considers the entire lifecycle of a piece of equipment.
The objective should not simply be to produce a structure that passes a single load case. The design should be appropriate for the loads, environment, operating cycles and expected service life of the equipment.
Fatigue failure is often a progressive process, but the final failure can be sudden and catastrophic.
That is why identifying fatigue risks early is so important.
Good engineering asks more than “Will it carry the load?”
It asks:
“Will it continue to safely carry the load, repeatedly, throughout its intended service life?”
That distinction can be the difference between equipment that merely works and equipment that has been properly engineered.
Need an engineering assessment?
If you have machinery, structures, lifting equipment or other plant that is experiencing cracking, unexpected deformation, repeated component failures or changes in operating conditions, an engineering assessment can help identify the underlying cause and determine an appropriate path forward.
At MAD Eng, we provide mechanical engineering, structural assessment, design verification and inspection services for plant and equipment, with a focus on practical engineering solutions and safety-critical applications.