Post on 02-Jan-2022
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.