A drop test simulation is a transient dynamic finite element analysis of a product striking a surface at the speed it reaches after falling from a given height. It follows the impact through the first few milliseconds and the rebound after it, and predicts the contact force and deceleration, the peak stress and strain in every part, where material yields and stays deformed, and which component cracks or comes loose first. It also shows what happens inside: a battery shifting in its pocket, a circuit board flexing on its bosses, a display panel bending against its frame.
Its job is to shape the design before prototypes exist and to make each physical test count. The physical drop test a product must pass under the code or standard you specify still happens; the simulation means the prototype that goes to it has already had its weak points found and fixed, and if a test does fail, it shows why.
What a simulated drop shows
The starting point is the impact velocity. Ignoring air resistance, a product dropped from height h reaches
v = √(2gh)
which gives about 3.1 m/s from 0.5 m, 4.4 m/s from 1 m and 5.4 m/s from 1.5 m. The simulation begins at the instant of contact with the product already travelling at that speed, so no time is spent modelling the fall itself.
What happens next depends on how far the product can travel while it stops. If the deceleration were constant over a stopping distance d, its value would be
a = v² ÷ 2d = gh ÷ d
so a 1 m drop onto a hard floor, with 2 mm of local crush and flex, averages about 500 g. Real impacts are not uniform and the peak is higher than the average, but the relationship explains why stiff housings on hard floors see such severe loads, and why a few millimetres of soft bumper changes the result so much.
From the full transient solution, a drop study reports:
- Force and acceleration histories at the point of contact and at internal components such as a battery, sensor or connector.
- Peak stress and strain in every part, and the moment each peak occurs, which is often not the moment of first contact.
- Plastic deformation: dents, bent brackets and stretched clips that remain after the product comes to rest.
- Likely failures: parts where strain exceeds what the material can tolerate, such as a cracked screw boss, a sheared snap fit or a broken glass panel.
- Internal motion: gaps that close, secondary impacts between components and the housing, and board flexure near sensitive parts such as ball grid array packages.
- Energy: how the kinetic energy at impact is absorbed by elastic and plastic deformation and by friction, and how much returns as rebound.
Choosing heights and orientations
Drop height usually comes from the code or standard you specify, or from the realistic use case: the height of a hand, a pocket, a desk or a vehicle tailgate for portable products, and handling heights for packaged goods, where heavier packages are commonly dropped from lower heights. Simulation can also go beyond the required height to show how much margin a design has.
Orientation matters as much as height. A rectangular product has 6 faces, 12 edges and 8 corners, giving 26 distinct impact orientations, and each one loads the product differently:
| Orientation | What it tends to load | Typical concerns |
|---|---|---|
| Flat face | A large contact area and a very short stopping distance | Often the highest overall deceleration; inertia of heavy internal parts; display and panel bending |
| Edge | Bending of the housing walls along the edge | Seams opening, clips releasing, walls cracking |
| Corner | A small contact area and high local crush | Cracking of brittle housings at the corner; local permanent deformation |
| Angled, with rotation | A first impact, then rotation onto a second edge or face | A second impact that can be harder than the first |
The last case catches out many test plans. A long product, such as a handheld tool or instrument, that lands on one end at an angle rotates about the first contact and slaps down on the other end, and that second impact can be the more severe one for components at the far end. Simulation shows which of the 26 orientations, and which angled cases, drive the design, so physical test time goes on the ones that matter.
The target surface matters too. Concrete and steel plate are close to rigid. Softer surfaces, such as plywood over concrete or flooring materials, absorb some of the energy. The model should use the surface that the standard or the use case defines.
Material behaviour at high strain rates
In a standard tensile test, a specimen is stretched slowly, at a strain rate of around 10⁻³ per second. In a drop, local strain rates can be several orders of magnitude higher, and many materials respond differently:
- Metals. Yield strength generally rises with strain rate. The effect is pronounced in mild and low-carbon steels and much smaller in most aluminium alloys.
- Thermoplastics. Stiffness and strength usually rise with rate, but ductility can fall, so a material that stretches in a slow test can crack at impact speed, particularly at low temperature or next to a sharp notch.
- Elastomers and foams. Their response depends on both rate and temperature. Foams absorb energy by crushing at a nearly constant stress over a large strain, which is why packaging inserts work.
- Glass and ceramics. Brittle, with strength governed by surface flaws and edge finish, so they are assessed against a fracture stress rather than a yield point.
Material data is often the limiting input. Rate-dependent properties are published for some grades and not others, and polymer properties vary with filler, moisture and moulding conditions. Where only slow-test data exists, it generally errs on the cautious side for the strength of ductile metals but not for the ductility of plastics, and the material model should be chosen with that in mind.
How simulation complements physical drop testing
Physical and simulated drops answer different questions, and the strongest development programmes use both.
| Physical drop test | Drop test simulation | |
|---|---|---|
| What it provides | Evidence that real parts, as made and assembled, survive | An understanding of why a design survives or fails, and by how much |
| Visibility | The outside, and the outcome after the event | Every part, at every instant of the impact |
| Orientation control | Approximate; the real impact angle varies from drop to drop | Exact and repeatable |
| Design changes | Each one needs a new prototype | Wall thickness, ribs, materials and mounts changed in the model |
| Timing | Needs hardware, so comes later in development | Possible from the first CAD, before tooling |
| Captures | Manufacturing variation, assembly and the real material | The design as specified |
Simulation earns its place at three points: early, to choose wall thicknesses, rib layouts, materials and internal mounting before tooling is committed; before testing, to identify which orientations and heights are most severe; and after a failed test, to understand the failure and assess a fix before the next prototype is built. The physical test brings in what a model can only represent as well as its inputs allow: weld lines in mouldings, porosity in castings and variation in screw tightening.
Design changes a drop study commonly leads to
- Larger corner radii and thicker corner sections where corner drops concentrate the load.
- Ribs and gussets on screw bosses, which often crack at their base.
- Bumpers or overmoulded elastomer at corners and edges, adding stopping distance and lowering the peak deceleration.
- Better retention of heavy internal parts such as batteries and transformers, so they do not hammer the housing.
- Relocated circuit board fixings, to reduce board flexure near sensitive packages.
- A tougher or impact-modified material grade, or a change of manufacturing process.
What to have ready for a drop study
A useful study starts with CAD of the full assembly, the mass of each significant component, the material and grade of each part, the heights, orientations and impact surface from the code or standard you specify, and a clear definition of failure: a crack in the housing, a component coming loose, a display that stops working, or permanent deformation beyond a stated limit. Products for consumer markets often add a cold-temperature case, since many plastics lose toughness in the cold.
When to bring in a specialist
Drop studies combine fast transient dynamics, changing contact between many parts and non-linear material behaviour, which makes them among the more demanding analyses to set up well. SixGate™’s drop test and impact simulation tells you which orientation is most severe, which part fails first and what change would fix it. The same team can then carry that change through the detailed design and drawings, into the prototypes that go to the physical test and on to production. For the material and contact side, see non-linear analysis, and for the background, our guide to linear vs non-linear FEA.