1. Packaging Dynamics
Lecture 5: Fatigue, Test Acceleration and Product Fragility presented by David Shires
Editor-in-Chief, Packaging Technology & Science
Chief Consultant, Pira International

2. Product Fragility Packaged products can fail in many ways:
Excessive damage to the pack itself
Surface scuffing
Clips or connectors coming loose
Leakage
Compressive collapse
Of major concern is component breakage

3. Damage Boundaries Drop a hen’s egg onto a hard surface
What happens depends on the drop height
Below a certain height the egg will be OK – the impact velocity is below a damage boundary.

4. Damage Boundaries
Now drop the egg from higher, but onto some cotton wool
What happens depends on the thickness and softness of the cotton wool
If the impact is sufficiently cushioned the egg will be OK – the impact acceleration is below a damage boundary

5. Damage Boundaries
We can characterise any product’s fragility by its damage boundaries
Damage Zone
Critical acceleration
The boundaries will only be straight lines for ideal pulses
Critical velocity

6. Product Fragility
Component will break if we apply a single acceleration > damage threshold 3x
Lower pulses can cause damage if repeated 6x

7. Fatigue Damage Boundary
Damage Zone X1 X3
X10

8. Fatigue
Fatigue is the effect of repeated cycles of stress (cyclic loading)
In shock impacts the stress levels may be high and the number of cycles low
In vibration the stress levels are relatively low but the number of cycles high
At 20Hz we have stress cycles per hour.

9. Fatigue
Intuitively we might expect a stress threshold below which materials do not fatigue

10. Miner’s Rule Based on earlier work by Basquin and Palmgren
Applies to damage accumulated from varying stress cycles

11. Miner’s Rule Comparing 2 broadband vibration histories
Same PDS profile but different intensity
Same time
Formula for time compression of a vibration test
Da = accumulated damage, n = number cycles, σ = stress level, ά = constant, G = acceleration, t = time

12. Vibration Test Time Compression
We apply Miner’s rule, and typically use a value of 2 for the constant
To reproduce a 5 hour journey in 1 hour’s testing:

13. Vibration Test Time Compression
Thoughts:
ά is not a universal constant – it varies with material and typically is between 4 and 8
Most packaging tests are based on 2
Many packaging responses are not fatigue responses
In the main it seems to work
Some reports of poor results
For non-fatigue failures there may be other damage thresholds or changes in response
Pallet bounce on the test table is one example

14. Shock & Fragility Testing
Why are we interested in the damage boundaries for a product?
Damage Zone
The critical velocity of the product tells us if we need protective packaging
Drop Height (m)
Impact Velocity (m/s)
0.2
2.0
0.5
3.1
1.0
4.4

15. If the critical velocity < expected drop height then the critical acceleration is the design target for our cushion.
Damage Zone

16. “Can the product be re-designed to be more robust?”
Before we start designing cushioned packs there is an important question
“Can the product be re-designed to be more robust?”
Make the product more reliable
Save packaging materials and transport volume over and over and over again.
Damage Zone

17. Shock Testing Machine
Free-fall Machine

18. Shock Testing Machines
Types of Programmer
Elastomer
Half sine pulse
Characteristic of cushioned impacts
Cast Cone
Asymetric saw-tooth pulse
Flat shock response spectrum (SRS)
Single use – makes test very expensive
Pneumatic Piston
Constant deceleration (trapeziodal pulse)
Reasonable flat SRS
Cost effective alternative to cone system

19. Shock Testing Machines
Peak Acceleration
Pulse Duration
Half Sine
Free-fall Height
Programmer Stiffness
Terminal saw-tooth
Cone material and angle
Trapezoid
Programmer Pressure

20. Shock Testing Machines
Synthesized shock – vibration table
Vibration controller plays single pulse to table
Works best with EM shakers with higher velocities and thrust
Limited by stroke and thrust capability
Easily reproduce all classic pulses
Synthesise complex pulse to match recorded SRS

21. Measuring Damage Boundary
Critical Velocity
Damage Zone
Short duration <3ms half-sine pulses ensure critical acceleration is exceeded
Drop height is increased until damage occurs

22. Measuring Damage Boundary
Damage Zone
Trapezoidal pulses are applied with a drop height set to give impact velocity exceeding critical velocity
Pneumatic programmer pressure is increased until damage occurs

23. Fragility & Shock Testing
Practicalities
A data capture system is used to record shocks
An accelerometer on the test table records the applied shock
Accelerometers on key parts of the test item record transmitted shock
Transmitted shocks can be much higher than the applied shock
Transmitted shocks can be very complex or noisy
they can be filtered (with care) or interpreted by calculating the SRS
The test item must be fastened rigidly to the test table

24. Fragility & Shock Testing
Practicalities
The test item has to be tested in 6 directions
Requires a minimum of 12 products – all destroyed

25. Specified Fragility
Many items are produced against a shock specification
eg 50G, 11ms half sine pulse
This simplifies testing considerably and reduces the number of test samples required
Still need to test in 6 directions

26. Shock v Drop Testing
Shock testing assesses the responses of a product to prescribed shocks.
It allows us to understand the fragility of the product and the protection it may require
Drop testing allows to us to check whether packaged products are durable to handling hazards
The shock resulting from a drop impact may be complex but the drops are simple to define and test.
We can use accelerometers to make sure the product does not experience a high shock

27. Drop Testing
The product will experience a cushioned shock – longer duration, lower G and approximately half sine in shape
The pack will experience a short complex shock with high G levels.
It is difficult to predict this shock, but easy to reproduce it.

28. Why don’t we test the pack on the shock machine ?
We would apply a very controlled, defined reproducible shock pulse to the pack
We could set the table drop height to the test drop height
The applied shock would be different to a drop
The shock transmitted to the product may be different to a drop
If fshock < 0.3fpack-cushion then transmitted shock unaffected

29. Applied and transmitted shocks
Packaging drop test
Packaging shock tests

30. Shock testing is usually for products
Drop testing is usually for packaged products
And sometimes products