Drop, Slam & Component Impact Analysis

Drop and Slam Events as Impact Problems

A drop, slam or component strike is an impact problem: a body with mass and velocity contacts a target, and the structure must absorb the kinetic energy and transmit the load through its internal load paths. The physics is the same as any impact — mass, velocity, momentum, energy, contact, material response, structural deformation — but the specific considerations differ from crash or ballistic impact. The initial condition is usually a drop height or a prescribed velocity rather than a continuous motion; the target may be a floor, a fixture or another component; the orientation may be arbitrary; and the internal contents of the dropped assembly have their own inertia and their own load paths to the casing. Each of these must be represented faithfully or the local structural load will be wrong.

DROP HEIGHT DEFINES POTENTIAL ENERGY — IT DOES NOT BY ITSELF DEFINE THE LOCAL STRUCTURAL LOAD. The local load depends on the contact stiffness, the structural compliance, the orientation and the internal inertia distribution.

Initial Velocity from Drop Height

For a free-fall drop from a known height, the impact velocity is derived from conservation of energy: the potential energy at the drop height equals the kinetic energy at impact. This gives the familiar relation v = √(2gh). This relation is exact for a frictionless free fall in a uniform gravitational field. In practice, drag, initial rotation, release mechanism dynamics, guidance friction and orientation changes during the fall can modify the actual impact velocity. For low drop heights and dense objects, the ideal relation is usually adequate. For high drop heights, light objects or controlled drop rigs with guidance, the actual velocity should be measured or estimated with these effects included. The initial velocity is a load definition — getting it wrong makes the entire analysis wrong.

v  =  √( 2 · g · h )

where:
v  =  impact velocity  [m/s]
g  =  gravitational acceleration  [m/s²]
h  =  drop height  [m]

Caveats:
• Exact for frictionless free fall in uniform gravity.
• Drag may reduce actual velocity for light objects or high drop heights.
• Release mechanism, guidance friction and rotation can modify velocity.
• Where velocity is critical, measure it — do not assume the ideal value.

Impact Orientation and Rotational Velocity

The orientation at impact determines which part of the structure contacts first, which load paths are engaged, and how the impact energy is distributed. A drop that lands on a corner engages a very different load path from one that lands flat. For many drop events the orientation at impact is not deterministic — the body may rotate during the fall — and the worst-case orientation must be considered. Rotational velocity at impact adds a rotational kinetic energy component and creates a different deceleration history across the structure: the leading edge decelerates first and fastest, while the trailing edge may still be moving. A purely translational initial condition with no rotation is a simplification that must be justified against the actual drop dynamics.

Contact Stiffness and the Target Representation

The contact stiffness between the dropped body and the target determines how abruptly the velocity is arrested and therefore how high the peak contact force is. A stiff contact (rigid floor, hard target) produces a short, high-force deceleration; a compliant contact (deformable target, cushioning) produces a longer, lower-force deceleration. The same drop height and the same mass can produce very different local loads depending on the contact stiffness. The target must be represented appropriately: a rigid floor is a valid idealisation only if the floor is effectively rigid relative to the dropped structure — if the floor deforms, absorbs energy or redistributes load, it must be modelled as deformable. The contact definition itself — penalty stiffness, friction, damping — must be calibrated so the contact behaviour is physical and the contact energy is a small fraction of the total.

Internal Component Inertia and Load Paths

A dropped assembly is rarely a single solid body. It has an outer casing, internal components, mounts, boards, payloads and fasteners. When the casing contacts the floor, the internal components continue moving due to their own inertia until they are arrested by their mounts or by contact with the casing. The deceleration history of an internal component is not the same as the deceleration history of the casing — it depends on the mount stiffness, the internal clearance, the component mass and whether the component bottoms out against the casing. A rigid outer casing that survives the impact without visible damage does not guarantee that internal components survive; the internal load path must be explicitly modelled and the internal component responses extracted.

The internal components of a dropped assembly have their own inertia. A rigid outer casing does not guarantee that internal components experience the same deceleration history.

Rigid Floor vs Deformable Target

The decision to model the target as rigid or deformable depends on the relative stiffness and the purpose of the analysis. If the target is a concrete floor or a steel test plate that is effectively rigid compared to the dropped structure, and the purpose is to assess the dropped structure, a rigid target is a reasonable idealisation that saves computational cost. If the target is another structure, a cushion, a mount or a floor that may deform, the target must be modelled as deformable so the energy shared between the dropped body and the target is represented. The test is the same as for any contact pair: does the target absorb significant energy or redistribute significant load? If yes, model it; if no, rigid is acceptable — but justify the decision.

Advanced Assembly During Controlled Impact

The diagram below shows an advanced electronic or aerospace assembly during a controlled impact. The outer casing contacts the floor; the internal components — mounted on frames or isolators — have their own inertia and their own load paths to the casing. The load transfer chain runs from the floor contact through the casing wall, through the mounts, to the internal components. Each interface in this chain has a stiffness and a possible failure mode, and each must be represented if the internal component loads are to be predicted.

CONTROLLED IMPACT — ADVANCED ASSEMBLY WITH INTERNAL LOAD PATHS

         ┌─────────────────────┐
         │   OUTER CASING      │
         │  ┌───┐    ┌───┐    │
         │  │PCB│    │PLD│    │  ← internal components
         │  └─┬─┘    └─┬─┘    │     (own inertia)
         │    │ mount  │ mount│
         │    ▼        ▼      │
         │  ═════════════════ │  ← internal frame
         └────────╤════════───┘
                  │ contact
  ════════════════╧════════════════  ← floor / target
         ↑
    Load transfers: floor → casing wall → mounts → components
    Each component decelerates through its own mount stiffness.
    Casing deceleration ≠ component deceleration.

Drop and Impact Event Types

Different drop and impact events have different initial conditions, key physics and modelling considerations. The table below characterises the common event types in this category.

Verification for Drop and Slam Models

A drop or slam model should be verified against the energy balance — initial kinetic energy should transfer into internal energy with small artificial components — and against the physical plausibility of the deceleration history. Where test data is available, the force-time or acceleration-time history should be compared, not just the final deformation. The internal component responses should be checked for physical plausibility: do the mounts bottom out, do the components contact the casing, are the internal decelerations consistent with the mount stiffness and clearance?

• Initial velocity consistent with drop height and release conditions — Account for drag, rotation and guidance where relevant
• Impact orientation represents the intended / worst case
• Contact stiffness produces physical deceleration duration and peak force
• Target representation (rigid vs deformable) justified
• Internal component inertia and load paths modelled explicitly
• Energy balance checked — KE transfers to IE, artificial energies small
• Internal component deceleration histories extracted and checked for plausibility

Key Takeaways

• Drop height defines potential energy, not the local structural load — contact stiffness, compliance, orientation and inertia determine the load.
• The initial velocity from v = √(2gh) is exact only for ideal free fall; account for drag, rotation and release dynamics where relevant.
• Internal components have their own inertia — a surviving casing does not guarantee surviving internals.
• The target must be deformable if it shares energy or redistributes load; rigid is justified only by relative stiffness.
• Orientation and rotational velocity at impact can change the load path and must be considered.