Drop Test Studies

Drop test studies evaluate the effect of the impact of a part or an assembly with a rigid or flexible planar surface. Dropping an object on the floor is a typical application and hence the name. The program calculates impact and gravity loads automatically. No other loads or restraints are allowed.

Setup

The Drop Test Setup PropertyManager allows you the following options to setup the drop test study:

• You define the drop height (h), the acceleration of gravity (g), and the orientation of the impact plane. The program calculates the velocity (v) at impact from: v = (2gh)1/2. The body moves in the direction of gravity as a rigid body until it hits the rigid plane.

• You define the velocity at impact (v),  the acceleration of gravity (g), and the orientation of the impact plane. The program determines the region of impact based on the direction of the velocity at impact.

No rotations are considered until the initial impact occurs.

Computations

The program solves a dynamic problem as a function of time. The general equations of motion are:

FI(t) + FD(t) + FE(t) = R(t)

where FI(t) are the inertia forces, FD(t) are the damping forces, and FE(t) are the elastic forces. All of these forces are time-dependent.

In static analysis this equation reduces to: FE(t) = R(t) since the inertia and damping forces are neglected due to small velocities and accelerations.

Damping is not currently considered. The external forces R(t) include the gravitational and impact forces.

There are two basic classes of methods to directly integrate this equation in the time domain; implicit methods and explicit methods. Explicit methods do not require assembling or decomposing the stiffness matrix; an appealing feature that saves computer time and resources. However, they require the time step to be smaller than a critical value for the solution to converge. The critical time step is typically very small.

Implicit integration schemes give acceptable solutions with time steps usually one or two orders of magnitude larger than the critical time step required by explicit methods. However, they require intensive calculations at each time step.

The software uses an explicit time integration method to solve drop test studies. It automatically estimates the critical time step based on the smallest element size and uses a smaller value to prevent divergence. You can suppress very small features, when appropriate, or use mesh control to try to prevent the generation of very small elements.  The program internally adjusts the time step as the solution progresses.

For further reading on explicit methods, refer to: An Explicit Finite Element Primer by Paul Jacob & Lee Goulding, 2002 NAFEMS Ltd.

Convergence

Good transition in the mesh helps convergence. Fast mesh transition can lead to divergence. The solver checks for this condition by monitoring the energy balance. It gives a message and stops when the energy balance indicates divergence.  

Will the Model Break?

The study does not answer this question automatically.  It also does not predict the separation of bonded components due to impact. You can use the results to assess the possibility of such events to occur. For example, you can use maximum stresses to predict material failure and contact forces to predict separation of components.

Related Topics

Drop Test - Contact

Drop Test - Materials

Drop Test - Result Options

Drop Test - Setup

Drop Test - Solution Time After Impact

Drop Test - Viewing Results

Performing Drop Test Analysis