s t r e s s e n g i n e e r i n g s e r v i c e s

These methods allow transportation and warehouse stacking

performance of unitized loads to be studied earlier in the

package development process, before physical samples are

available, with commensurate reductions in risk, cost, and

speed to market.

More Information Means Better DecisionsThe development of a successful packaging system requires

information about the fragility of the product being shipped

and knowledge about the hazards of the distribution environ-

ment. This information must then be used to make informed

decisions regarding cushioning materials, primary boxes or

packaging, and shipping boxes or cases. For high-volume

products that are typically transported in larger quantities,

unit load design adds an extra layer of packaging that must

be considered.

Each new package design project starts with considerable

assistance in the form of an extensive body of packaging

industry knowledge and experience to draw upon. Many sup-

pliers are available who have developed a range of excellent

packaging materials and are happy to offer their expertise.

In situations where multiple design choices are available, it is

often not difficult or expensive to prepare samples and run

tests to measure the actual performance in, for example, a box

compression or drop test.

Package design decisions associated with unit load perform-

ance have not always been so easy to assess as those for indi-

vidual cartons or boxes. Unit load tests require larger

quantities of product and packaging to conduct and can be

more difficult to perform. Products may be packaged in unit

loads for extended periods of time and travel through most of

the distribution system in that form. The International Safe

Transit Association (ISTA), ASTM, ISO and other organizations

have developed tests for evaluating various attributes of unit

load performance but it is still necessary to have one or more

unit loads available to conduct these tests. The same is true of

the “stack-and-ship” tests that are sometimes used to judge

how a product will perform in the distribution environment.

ADVANCED F INITE ELEMENT ANALYSIS (FEA)

TECHNIQUES ARE MAKING IT POSSIBLE TO PERFORM

“VIRTUAL” COMPUTER SIMULATIONS OF STANDARD

INDUSTRY UNIT LOAD TESTS.

Virtual Simulation of

ISTA Unit Load Tests

In our packaging work at Stress Engineering Services we find that the

cost and time required to evaluate changes to unit load design pres-

ent an obstacle to making smart changes. Everyone wants to reduce

material usage, or use different materials, to save costs. The desire to

make more sustainable packaging choices has added a further moti-

vation to continuously reevaluate packaging systems and implement

changes. When considering a potential unit load design change,

however, the cost and time to conduct a thorough evaluation can be

daunting. All too often the result is:

1. Change is not made at all

2. Change is made without enough understanding

The first outcome obviously results in never realizing the potential

benefits. The second runs the risk of being detrimental to the organi-

zation if the new design proves to be unsuitable. Damage claims can

increase and packaging costs may actually go up to implement a fix

if the design doesn’t work as intended.

Physics-Based ApproachTo try to be more predictive about the effects of changes in unit load

design, and mitigate the associated risks, we have found value in

applying a physics-based approach to understanding packaging per-

formance. This has involved the use of tools ranging from hand cal-

culations based on the theories of classical mechanics to

computer-based simulation techniques such as finite element analy-

sis. This paper discusses recent work with finite element analysis to

simulate ISTA unit load tests.

Most transport packaging hazards fall in the general categories of

• Compression

• Vibration

• Impact

• Shock

Common industry test standards for packages reflect these hazards

in the nature of the tests they prescribe. Impact, in the sense of

being struck by another sliding or falling object, is not so common a

hazard for unit loads as it is for small parcels or other packages that

are individually handled or sorted. The typical unit load environment

is more complex than might be suggested by the simple descriptions

of “compression, shock, and vibration” and this is reflected in the

ISTA tests for unit loads.

CompressionIn the most general sense, compression testing involves determining

the force required to crush a package or unit load. Alternately, unit

load tests like those in ISTA Test Procedure 3E, Unitized Loads of

Same Product, serve to verify that a unit load can support the com-

pressive loads it is expected to experience in service.

Compressive loads can occur while a unit load is being transported if

products are stacked atop one another in a trailer, rail car or inter-

modal container. When this is true there can be a combination of

compression and vibratory loads present.

Figure 1 shows the results of a finite element model simulating a

stack of two unit loads consisting of corrugated boxes. The left image

shows buckling in the box sidewalls. The color code in the right

image shows the stress in the corrugated panels with blue being low

stress and red indicating high stress.

An advantage of techniques like finite element analysis is the ability

to expose inner features of the packaging system which are not usu-

ally visible and examine their behavior. In Figure 2 the contact pres-

sure between product layers is shown which highlights the more

highly-stressed load path through the case sidewalls and corners.

A further consideration for unit load compression is the time-

dependence of material properties that may be significant during

warehouse storage. This type of creep behavior can be an issue with

corrugated fiberboard boxes as well as plastic bottles and other

types of packaging. Over time, the progressive crushing or collapse

of packages can result in warehouse unit load stacks shifting and

leaning. In some situations this can reach a point where the center of

gravity (CG) of the stack has shifted so far that it becomes unstable

and risks collapsing.

figure 2: Detail of stress in corrugated boxes from stacking of unit loads

figure 1: Finite element model of stacked unit loads ofcorrugated cases

Figure 3 shows measurements of unit load CG shift for 35 days and

extrapolates those results out to one year. With information about

the product geometry and stack height it is possible to estimate

whether such movement will lead to a unit load stack that is danger-

ously unstable.

ShockShock can be experienced from being roughly handled with a fork

truck, sharp jolts in the back of a truck, rail car coupling or other

events. Clearly shock can occur in both the vertical and horizontal

directions and both are addressed in ISTA 3E. FEA results from a

simulation of a horizontal shock event are shown in Figure 4.

ISTA 3E addresses shock with a rotational edge drop test. A vertical

drop or rotational flat drop might also be appropriate in some situa-

tions. Figure 5 shows results from a rotational edge drop of a unit

load. For comparison, a vertical drop is shown in Figure 6. Pure verti-

cal drops of unit loads can sometimes be difficult to perform and this

simulation illustrates an advantage of analytical methods in that they

can allow difficult tests to be safely investigated.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0 30 60 90 120 150 180 210 240 270 300 330 360

TIME (days)

Measured CG

Extrapolation

CG

CH

AN

GE

(in

)

figure 4: Stress on face of boxes at 42 in/sec horizontal impact

figure 5: Stress in boxes at impact during unit load rotational edge drop test

figure 6: Stress in boxes at impact during unit load 8” vertical drop test

figure 3: Stacked unit load CG shift with time

w w w . s t r e s s . c o m

#432

on the web at www.stress.com Cincinnati • Houston • N e w O r l e a n s • Baton Rouge

© 2010 Stress Engineering Services, Inc.

To Talk With A Unit Load Virtual Simulation ExpertCall SES today at 513-336-6701

VibrationVibration is, like shock, a multidimensional phenomenon; though the

packaging industry has focused on vertical vibration for many years.

Studies of vibration in the transportation environment have shown

the most significant vibration to be vertically oriented and this has

been the most severe loading direction for the ubiquitous corrugat-

ed fiberboard box. As packaging is reduced to save costs and use

less material, some of the stability provided by full corrugated cases

is being lost and unit loads are being encountered which have con-

siderably reduced horizontal stability. Horizontal vibration can then

play a more important role in assessing overall unit load perform-

ance in a vibratory environment. Finite element modeling can be

used to simulate vibration but it is not easy to predict damage and

much of the success of the method depends on the details of the

specific package design. Single axis or multiple axis vibration can

be simulated.

Simpler Means More Cost-EffectiveFEA models can be constructed with varying levels of detail depend-

ing on the nature and size of the model, the desired outcome, the

information available as inputs, and the computational resources

available to run the model. Highly detailed models can be time-con-

suming to construct and take a long time to run.

A key factor in obtaining a satisfactory solution—which is also cost-

effective—is to identify valid simplifications to the model which

reduce the complexity but preserve the overall behavior. An example

might be reducing the level of detail on the inner product or even

representing it as a simple shape with an appropriate mass. If the

goal of a particular analysis is to understand the gross behavior of

the unit load, then the solution will be little changed by leaving out

small details. All of the ISTA unit load tests are good candidates for

careful simplification.

Proceed With CautionIt is very important to point out that these models have gaps in

knowledge and capability that keep them from being as accurate

and true-to-life as we might want them to be. All such models

depend heavily on having knowledge about the properties of the

materials that they are simulating. It is not possible to model every

aspect of a unit load in exquisite detail. Some of the properties and

behaviors they exhibit are simply difficult to measure and determine

with certainty. The images and animations that can be produced are

terrific, but don’t let yourself be misled by the pretty pictures.

Some key limitations to current modeling technology as it applies to

packaging and unit loads are in the areas of manufacturing variabili-

ty and damage accumulation. It is easy to make perfect products

and packages in a computer. The greater challenge is often to make

the computer model realistically imperfect. After all, not every box

and package is identical.

Corrugated fiberboard is such a common and widely used material;

it may come as a surprise to learn that it can be difficult to predict

how and when it will fail. Many packaging tests are, of course, not a

single test to failure but repetitive drops, shakes and squeezes that

collectively represent the distribution environment and progressive-

ly damage the package. Excellent research has been done to under-

stand how damage forms and develops, but the technology to fully

predict failure is still evolving.

Modeling is Still ModelingFinite element modeling of transport packaging is not in any way a

substitute for testing and experimentation. It is not magic, it is not

perfect, it is just a tool. Like all tools, it must be used carefully and

knowledgeably to get the most benefit while staying out of trouble.

Analysis and testing complement each other very nicely and togeth-

er can achieve more than either can alone.