Drop Test Analysis with Bolt Pre-Stresses

This article introduces a time-saving and a smart approach for drop test analysis with pre-stresses using LS-Dyna. 

Many products that are subject to handling during transport, installation, or repair are at risk of being dropped. Granted, handlers generally try to avoid these types of mishaps. When equipment is out of your hands, its safe transportation is out of your control. One way to ensure that your product survives its journey from the factory to the point of installation is to perform drop test analysis and verify that it survives without damage. That way, your company isn’t answering warranty claims from customers who received damaged goods that left your warehouse in mint condition.

Although the methodology for drop test is fairly standard, it is challenging to capture the finer details that happen in reality. This article introduces a time-saving and a smart approach for drop test analysis with pre-stresses using LS-Dyna.

Motivation

While drop test problems involving huge appliances, the effects of bolt-load or pre-stresses are generally ignored. However, in some cases, it is desirable to have a pre-stress loading of a structure before performing a transient dynamic analysis or, simply, drop test analysis. This is because nowadays the product safety has increased the demand for accurate simulation models.

In this article, I used LS-DYNA. It is a highly advanced, general-purpose, nonlinear, finite element program that is capable of simulating complex real world problems.

Firstly engineers need to perform a pre-stress analysis for the bolts before conducting the drop test analysis. Then you will need to integrate the stresses and strains obtained from the pre-stress analysis into the drop test analysis setup.

Drop Test with Included Pre-Stresses (two-step method)

In LS-DYNA, I define bolt pre-load (non-iterative loading type) using *INITIAL_AXIAL_FORCE_BEAM (Type 9 beams only) and *INITIAL_STRESS_SECTION (solid elements only). These keywords work with *MAT_SPOTWELD. The failure models apply to both beam (Type 9) and solid elements (Type 1).

*INITIAL_AXIAL_FORCE_BEAM will pre-load beam elements to a prescribed axial force.

Screenshot of keyword in drop test analysis

In the above screenshot of the keyword, BSID is Beam Set ID. I define the preload curve (axial force vs. time) with *DEFINE_CURVE. LCID is the Load Curve ID.

The below video show the pretension in the beams.

*INITIAL_STRESS_SECTION will pre-load a cross-section of solid elements to a prescribed stress value. Pre-load stress (normal to the cross-section) is defined via *DEFINE_CURVE.

Screenshot of keyword in drop test analysis

In this screenshot placed above, ISSID is section stress initialization ID,  CSID Cross-Section ID, LCID Load Curve ID (pre-load stress versus time), PSID Part Set ID, VID Vector ID (direction normal to the cross section). You can define the vector if *DATABASE_CROSS_SECTION_SET is
used to define the cross section.

In the video, you can see the pre-stresses in solid elements when I used *INITIAL_STRESS_SECTION.

Video Courtesy: LSTC

*INTERFACE_SPRINGBACK_LSDYNA allows LS-DYNA to create a DYNAIN file at the end of the simulation containing deformed geometries, residual stresses, and strains. This file sets me up for the next phase of analysis where I use it with the *INCLUDE keyword. However, the DYNAIN file neither includes contact forces nor contains nodal velocities. These quantities from the pre-stress analysis do not automatically carry over to the drop test.

Drop Test with Included Pre-Stresses – Both in One Step!

In the previous method, there is always manual intervention which can lead to unknown errors. Drop test of an appliance by considering pre-stresses in one step can be specified by using *DEFINE_TRANSFORM.

*DEFINE_TRANSFORM allows to scale, rotate and translate the appliance and you must define before you use the *INCLUDE_TRANSFORM command.

Screenshot of keyword in drop test analysis

Screenshot of keyword in drop test analysis

In the above screenshots, TRANSID refers to Transformation ID that is available in *DEFINE_TRANSFORM and the part which is specified in the file name will include the TRANSID.

The video below shows the drop test analysis of an appliance where *DEFINE_TRANSFORM allows the appliance to pre-stress and then the actual drop happens. The von-Mises stress contours show that the stresses get developed in the parts due to pre-stressing of the beams before the actual impact.

Saves time!

Using this approach, I can save about 20% of the time required to setup a pre-stress analysis and drop test analysis together. In addition, we can eliminate manual intervention.

Thanks to this, I get to submit my simulation jobs to the solver before I head into the weekend. I return in the following week to view and post-process the final results.

If you want to learn more about this, please talk to us.

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