# Product Fragility: The G Ceiling and the Drop Height Beside It _The G number is half the answer; the missing half is usually the drop height it was paired with._ **The short answer** **A fragility figure is two numbers, not one.** A product has a speed change it can absorb, measured in inches per second, and a deceleration it can take, measured in G. **The G number is only usable with the drop height beside it.** It becomes a line drawn across a cushion curve, and that curve has to come from the same drop height your package will see. Somebody asks how many G your product can take. The honest first answer is that the question is half of one. A shock has a size and a severity, and a product can fail from either. Drop it far enough and the sheer speed change breaks something, no matter how softly it lands. Land it hard enough and the deceleration breaks something, even from a short fall. So a fragility result is a pair, and the pair is what makes a cushion specifiable. One number on its own will not turn into foam. This reference covers where the pair comes from and the order the tests run in. It also covers how the pulse gets its shape, and the step where a correct number still produces the wrong cushion. - **in/s and G** - Both are needed to size foam - **A then B** - Speed change first, deceleration second - **Same drop height** - Or the cushion curve does not apply - **Destructive** - Run it last, on scarce units ## Where the G Ceiling Comes From The figure comes from a shock machine and a published method. [ASTM D3332](https://standards.nimonik.com/products/astm/d3332/) covers determination of the shock fragility of products, and says the information serves the design of shipping containers that transport them. That phrasing sets the purpose. Fragility is not a quality score for a product; it is an input to a container design, and it is worth nothing on its own. The standard is also careful about what counts as the product. A unit or consumer package carried inside an outer container counts as the product. So a retail box with its own insert goes on the machine as the thing being protected. Which matters for who owns the number. If your item ships inside a branded carton that then goes into a shipper, the fragility that matters belongs to the branded carton and its contents together. ## Two Numbers, in a Fixed Order The method runs two tests, and not in an arbitrary sequence. Method A comes first and finds the critical velocity change, then Method B finds the critical acceleration. The units are worth stating because they get dropped. Critical velocity change is a speed, in [metres or inches](https://www.vibrationdata.com/tutorials/product_fragility.pdf) per second, while peak pulse acceleration is in G. The order is doing real work: finding the speed change first tells the lab how hard it can afford to hit the product while hunting the deceleration limit. That saves units from pulses that were never going to be informative. Together the two values bound a region. Below a certain speed change nothing breaks whatever the deceleration, and above it the deceleration ceiling takes over, which is the shape the pair describes. > Figure: Only the deceleration ceiling crosses to the right-hand plot. The speed change limits how far the package may fall at all. ## The Pulse Is Shaped on Purpose A shock machine does not simply drop things. It produces a pulse of a chosen shape, and the shape changes the answer. For the speed change test a half sine waveform is the usual pick. It programs easily on most machines, and the response in that region does not depend much on shape. For the deceleration test the choice is deliberate in a different way. A square wave is the normal choice because the spring-mass response is a maximum, which makes the resulting figure a conservative one. Then physics intervenes: shock machines always produce pulses with some finite rise and decay time. So the real pulse is closer to a trapezoid, and the figure comes out slightly less conservative than a true square wave would give. None of that invalidates the number. It does mean a fragility figure carries the machine and the programmer that made it, which is a fair thing to ask about when two labs disagree. ## Turning the Ceiling Into a Cushion Here is the step the number exists for: the critical acceleration becomes a line drawn across a set of cushion curves. The [six-step method](https://www.lansmont.com/resources/six-step-method) is explicit about the mechanics. Locate the critical acceleration level on each of the cushion curves, then draw a horizontal line across the plots through that point. Any portion of the curve falling below the line indicates the usable static loading range. Static loading is simply how much product weight sits on each square inch of foam. The curve is high at both ends, because too little loading leaves the foam too stiff and too much bottoms it out. Which material carries that loading well is a separate question, and packaging foam density is where it gets settled. Where the curve dips below your ceiling the loading works, and that band is the answer the fragility figure was for. Choosing within that band is an economic call. Designing at the highest possible static loading uses less material, though the lowest can make sense where compressive creep matters over a long storage life. One step in that sequence is easy to skip, and it changes the economics. Between defining fragility and evaluating cushions, the method puts product improvement feedback, and the order is the point. If the measured ceiling comes in too low to protect affordably, the cheaper answer is sometimes a change to the product rather than more foam around it. A bracket, a screw or a different internal mount can lift a ceiling permanently, while every unit shipped pays for foam again. So a fragility result is feedback to two teams at once: the packaging engineer reads it as a constraint, and the product engineer can read it as a target. | What has to come with a fragility figure | What goes wrong without it | | --- | --- | | The critical velocity change, in in/s | No limit on how far the package may fall at all | | The drop height the figure is paired with | The cushion curve you read may describe a different fall | | The pulse shape used | Two labs can report different ceilings for one product | | The orientation tested | A corner-down ceiling does not cover a flat face | | How many units it came from | A single specimen is a sample of one | ## The Drop Height Has to Match This is where a correct number still produces the wrong foam, because a cushion curve is not a property of a material alone. One curve exists for each combination of material type, material thickness and drop height. Change any of the three and it is a different curve. So the check is specific. Confirm that the cushion curves came from the same drop height you chose for the package. Read a 30-inch curve while designing for a 36-inch fall and the foam will look adequate on paper. The product meets a harder landing than the curve ever described. Design drop height is itself a decision about weight and handling, not a constant. Getting it settled before reading any curve is what keeps the two halves of the calculation talking about the same event. ## The Ceiling Assumes a Brittle Failure The whole ceiling-line method rests on an assumption about how things break. It is usually a safe one, and it is worth knowing when it is not. Traditional damage boundary work [assumes brittle failure](https://www.labequipment.com/wp-content/uploads/2020/11/FDB_20Bkgnd_20Status.pdf), and many real products do fail in that mode. For a brittle product a single shock above the critical level breaks it, and repeated shocks below that level cause essentially no degradation. That is why one line works: if nothing below the ceiling accumulates, the number of impacts does not enter the calculation at all. Ductile products behave differently. Low-level shocks may not cause damage at first, yet they degrade the material, and multiple applications of those low shocks can eventually cause failure. For those the single ceiling is optimistic, since a route with many small impacts can retire a product that would pass a one-drop test comfortably. A fuller approach exists and adds a third parameter, the number of impacts. It would find that dependence, where the traditional method cannot. The reason it stays rare is cost, and the cost is blunt: one complete published procedure could require 372 shock tests and 36 broken products. So the practical question is about failure mode. Ask whether the product fails by cracking or by gradual degradation, because the second kind needs more caution than a single ceiling offers. ## How Few Units This Rests On One more thing belongs on any fragility figure, and it is the sample: testing to a damage boundary means breaking things on purpose. Specimens are usually in short supply, and the availability of testable units is normally very restricted. That shortage shapes the whole program. It is why the order of testing matters beyond the two methods. Work that rarely destroys a unit should run before the potentially destructive boundary test, so scarce samples are not spent early. The cushion side is more generous and more explicit about counts. Five drops are performed at a given static loading, with a minimum of five loadings tested to plot one curve, and a new sample used at each loading. That asymmetry is worth carrying into any conversation about confidence. The foam data behind a design usually rests on more units than the product ceiling it is being matched against. ## Common Questions ### Is one G number enough to specify a cushion? No. A fragility result is a pair: the critical velocity change bounds how far the package may fall, and the critical acceleration becomes the ceiling line drawn across a cushion curve. Without the drop height the figure was paired with, the curve you read may describe a different fall. ### What does static loading mean on a cushion curve? It is the product weight carried per square inch of foam. The curve runs high at both ends, because too little loading leaves the foam stiff and too much bottoms it out. The usable band is where the curve dips below your ceiling. ### Why do two labs report different fragility figures? Usually the pulse. A square wave gives a conservative deceleration figure because the spring-mass response peaks, while a real machine produces a trapezoid with finite rise and decay that reads slightly less conservative. Orientation and sample count also move the result. ### Do repeated small drops matter if each one is under the ceiling? It depends how the product fails. For a brittle product, repeated shocks below the critical level cause essentially no degradation. For a ductile one, low shocks that do no damage at first still degrade the material, and enough of them can eventually cause failure. ### Does fragility testing destroy the product? Finding a damage boundary means driving the product to failure, so yes, units are consumed. Testable specimens are usually scarce, which is why non-destructive work should run first and the boundary test last. **How to weigh this page.** It gathers what the published method and the engineering literature state, so it can tell you which numbers to ask for and what a complete answer looks like. It cannot give you a fragility figure for your own product. That comes from testing it on a shock machine, in the orientations it will actually meet. Where a design protects something expensive, fragile or regulated, the figures belong to a lab and the sign-off belongs to whoever carries the risk. **Key takeaways** - A fragility result is two values: a critical velocity change in inches per second, and a critical acceleration in G. - ASTM D3332 runs them in order, Method A for the speed change and Method B for the deceleration, and treats a retail package inside an outer container as the product. - The G value is used by drawing it as a horizontal line across cushion curves; the portion below the line gives the usable static loading range. - One cushion curve exists per material, thickness and drop height, so the curve must come from the drop height the package is designed for. - A square wave gives a conservative ceiling, while a real machine's trapezoid reads slightly less conservative, so the pulse shape belongs with the figure. - Testable units are scarce and the boundary test consumes them, so non-destructive work runs first.