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

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

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.

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

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

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

Fatigue Damage Boundary Damage Zone X1 X3 X10

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 72000 stress cycles per hour.

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

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

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

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:

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

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

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

“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

Shock Testing Machine Free-fall Machine

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

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

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

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

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

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

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

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

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

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.

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

Applied and transmitted shocks Packaging drop test Packaging shock tests

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