One day you are sitting in your office chair at work and suddenly the leg breaks and you fall over. Has anyone else had this experience? The service life of a product is not always apparent and sometimes comes to an abrupt end in unexpected ways. The phenomenon of ‘fatigue fracture’ is often hidden behind this situation.
This article delves into the mechanisms of fatigue fracture and preventive measures through the case of office chairs.
What is fatigue failure?
Fatigue Failure refers to the phenomenon whereby a material is subjected to repeated loading, eventually leading to failure. It is important to note that fatigue failure is not caused by a single large force, but by repeated small forces.
In the case of office chairs, the loads on the legs and frame are repeated every time the user sits or stands up. Invisible small damages accumulate and one day lead to fatal destruction.
S-N curves of materials and the mechanism of fatigue fracture
All structures and products, not just office chairs, are at risk of fatigue failure.
The graph below shows the ‘S-N curve’ (stress-repetition curve) in metallic materials. This is a basic indicator of fatigue fracture behaviour.
How to read the graph
Horizontal axis (N): indicates the number of cyclic loads. It is expressed on a logarithmic scale, the further to the right the higher the number.
Vertical axis (σ): Stress Amplitude. The higher the number of cycles, the higher the stress.
Characteristics of the curve
At low repetitive loads (e.g. less than 1000 cycles), relatively high stresses are unlikely to lead to failure.
For high cyclic loads (e.g. more than 100 000 cycles), the likelihood of fracture increases at low stresses.
Endurance Limit Some metallic materials (e.g. steel) have an area where failure does not occur if the stress is kept below a certain value. This value is called the ‘fatigue limit’. On the graph, this corresponds to the area where the stress levels off at the right-hand end.
Mechanisms of fatigue fracture.
1. generation of micro-cracks
When legs and frames are subjected to repetitive loading, micro-cracks occur in the areas where stress is concentrated. At this stage, the cracks are not visible to the naked eye. 2.
2. crack propagation
Repeated loading causes the crack to grow gradually. The rate of growth depends on the material properties and stress conditions, but the strength of the structure gradually decreases.
3. critical failure.
Finally, when the crack reaches a critical point, the residual strength is not sufficient to support the load and failure occurs. Only at this stage is it recognised as a ‘sudden break’.
Causes of fatigue fracture in office chairs
Factors that contribute to fatigue failure include material properties, design and operating environment. In the case of office chairs, the following points are particularly influential
1. choice of materials
Legs and frames are made of metal or plastic, and cracks are more likely to occur if materials with low fatigue strength are chosen. 2.
2. stress concentrations
The risk of fatigue failure is higher at connections and welds, where stresses are concentrated.
3, conditions of use
The risk of failure increases if the magnitude and frequency of loading exceeds the design assumptions due to weight, sitting and frequent use.
How can fatigue fracture be prevented?
To reduce the risk of fatigue fracture, a consistent approach is needed from the manufacturing stage through to the use stage.
1. design considerations
Stress distribution: incorporate curvilinear shapes and reinforcements to prevent stress concentration.
Ensuring a safety factor: design with a margin of safety appropriate to the operating environment.
2. material selection
Select materials with high fatigue strength and, where necessary, apply treatments (e.g. hardening or surface treatment) to improve durability.
3. maintenance and inspection
It is important to carry out regular inspections to detect cracks and abnormalities at an early stage. Particular attention should be paid to products that have been in use for a long time.
4. proper use
Observe the conditions of use described in the operating instructions and take care not to apply excessive loads.
Summary: Fatigue fracture is an invisible crisis.
Sudden breakage of office chairs is not uncommon, but behind it lies an unseen crisis: fatigue fracture. Appropriate action at the design and material selection stage and regular inspections during use are key to significantly reducing this risk.
Next time you buy an office chair, why not pay attention to ‘safety’ and ‘durability’ as well? Paying attention to what you can't see is the secret to choosing a product you can continue to use with peace of mind.
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