TPT September 2018

AR T I C L E

Advanced Machine & Engineering/AMSAW

that is generally true from machine tools: the stiffer the better. However, this is not necessarily true when dealing with shock loads.

This confirms that a quasi-static calculation with an ampli- fication factor (impulse factor) is possible, and conducting dynamic simulations as a first approach is not necessary. This formula can also be applied to a static displacement calcula- tion of a beam with the use of the Euler-Bernoulli beam theory. For example, let us assume a 100 kg mass is hitting a support beam and the contact force and resulting stress needs to be known: E (Young’s modulus) is 210 GPa; h is 25mm; l is 1m; and the beam cross section is a 50mm x 25mm rectangle.

Advanced Machine & Engineering/AMSAW

Verification of durability The transient nonlinear dynamic analysis in FEA software verifies the product in an early stage and conducts damage analysis of it. This provides insight on the contact, reaction forces, and the stress as a function of time. The impact takes place in a matter of milliseconds, making the adequate discretisation of space and time crucial. The boundary condition represents the interface to the floor. The initial condition is the impact velocity – with g being the gravity constant and h the starting height from where a steel billet is dropped.

Figure 2: A visual representation calculating the contact force and resulting stress of a dropped support beam

Moment of inertia for a rectangular profile is:

Figures 3 & 4: FEA software analysis of heavy load drop contact, reaction forces and stress as a function of time

The static displacement is:

Improved ways to design material load tables

Bending stress due to static load – M is the maximum bending moment; W is the moment of resistance:

With today’s technology there are improved ways to design material load tables so they are not overloaded with costs. Generally, designers have two choices: over-engineer the structure to withstand any heavy shock load, increasing the cost of the load table for the customer; or, through the use of FEA software, design the structure to allow elastic flexing while avoiding the effects of plastic deformation. We have all dropped our smartphones. To reduce the risk of cracking or shattering the screen, we have placed the phone within a shock-absorbing case. This simple approach reduces the stiffness and impact forces that correspond with the stresses of the phone when dropped. The same is true for dropping material onto a material load table.

The impact factor is:

This demonstrates that the impact force from a 25mm height will be seven times higher while the static load of the mass is 981 N. The bending stress will rise in the same fashion. The formula for the impact factor shows that an increase in stiffness reduces the static displacement. This increases the impact factor and impact force. The result shows the paradigm

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SEPTEMBER 2018