How and Why Cases Fail Under Compressive Loading


Case compression strength is a function of board caliper and board rigidity. Factors which affect these change the case failure load (C.F.L.) of a particular pack.

How cases fail under compression loading

{Concentration of load on a case}

Anyone who has ever observed a corrugated box under compression, or a stack of filled boxes standing in a warehouse, is familiar with the stress pattern which loaded containers develop. These are bulge failure lines around the edges of the case.

These stress lines indicate that the load concentrates near the edges. It is this concentration of load near the edges which makes it possible to cut hand-holes etc. in the middle of a panel without drastically affecting C.F.L.

Experimentation has shown that compression load is distributed 64% of the total load on the edges and the remaining 36% on the panels. This is why a 5 panel liner will be stronger than a 4 panel liner.

The limitations of laboratory compression tests

First, it is necessary to appreciate the difference in conditions between a long term stacking load, where packs are subjected to a constant sustained weight, and a laboratory compression test carried out using a machine with driven plates. The latter test takes place in a very short period of time (a few minutes) under an increasing load at a constant rate of case deformation. Furthermore, the plates are rigid planes arranged so that the force is applied evenly around the perimeter of the case and generally the point of failure is sharply defined.

In a field stack test the weight continues for an appreciable period of time (days or weeks) under a constant load with uncontrolled rate of case deformation. Corrugated fiberboard is a visco-elastic material and therefore exhibits "creep" under these conditions, i.e. there is a continuing deflection of the material when subjected to constant force.

A laboratory test also takes place in a controlled atmosphere. This of course, is not the case with a field test.

When estimating the compression strength of a case, a compression test of a minimum, of five samples should be carried out as a means of cross-checking results, but what these results represent should always be borne in mind. The results obtained from laboratory testing will vary with the speed that the plates apply the load. For this reason the procedures laid down in ASTM D642-90 Standard Test Method for Determining Compressive Resistance of Shipping Containers, Components, and Unit Loads should always be followed.

When cases are in their distribution cycle they experience compressive loading during simple static loads, as in a warehouse and in more difficult situations, such as during transportation on trucks, railroads and ships. All of these shipping conditions magnify the compressive loads felt by the cases via the vibration that causes movement in the stack. This is sometimes termed dynamic loading, as opposed to the static loading of a warehouse.

The effect of asymmetrical board grades and heavy weight flutings

If we compress a corrugated box, the panels deflect under load. In practice we deal with filled boxes, so the bulge is outward. The outside liner is stressed in tension, while the inside liner is in compression.

As long as we have a balanced combination, the load does not affect the "inside" or "outside" differentially. If the board weights are asymmetrical, however, then the heavier (stiffer) facing inside the box will accept a higher compression load than if the lighter, less stiff facing had been in that position. The stiffer liner in that case being outside, it is stressed in tension, so it's greater stiffness does not come into play.

Increasing the weight, and hence stiffness, of the fluting medium can be the most economic way to increase the compression strength of a pack. That the use of these materials alters the distribution of fiber between liners and medium is obvious. However, if the disproportion between medium and liners is excessive, the stiffness potential of either one cannot be fully utilized. A 80K/21/80K (400K/105/400K) board grade could be compared to a tank that has got the engine of a compact car.

Effects of converting processes

Certain converting processes can have a major effect on the final compression strength of a case. High print coverage is, of course, an obvious example, but deep slotting, bundle strapping around case corners and even badly set pull rollers will have a detrimental effect.

Effects of the Environment

The relative humidity conditions that a pack is experiencing and the percentage of moisture in the corrugated board are factors that are strongly linked and have a major effect on the performance of a pack.

The laboratory conditions under which cases are normally compression tested are 23 degrees Celsius and a relative humidity of 50%. Conditions in the field are often much harsher than this. Below is a small calculator program that will enable the user to find the correlation between the compression figures obtained in the laboratory and those that can be expected at other given relative humidity values.


The length of time during which cases are under load is another major factor in how well or badly a given pack will perform. There is a logarithmical  relationship between the time a pack is under load and the reduction in compressive strength. Below we have provided another calculator program that will demonstrate the likely reduction in a pack's laboratory compression strength during a given time period. Note how the values fall off quite sharply during the initial few weeks and then start to level off. To aid you in evaluating this two charts are also shown, the first has a standard linear scale and the one below it has a logarithmical scale.

Problems with Unitization and Distribution Damage

Cases are rarely stacked on the floor of a warehouse, instead to enable easy shipment they are stacked onto some form of unitize, most frequently this is a pallet. There are many types of pallet construction and many of them are not fully boarded at the top, full decking at the bottom of the pallet is even rarer. Please CLICK HERE for further details on unitization.

Product Support

As with the need for liners and flutings to be matched, to maximize the synergistic effect, so it is also important that, where possible the case's product and / or internal fittings act in concert with the main pack to ensure that the pack's maximum strength is achieved. The fact that a pack is failing to live up to its full potential can be most easily seen on the graph produced by a compression test machine. When rather than producing a smooth curve, a lumpy curve showing fail and rally is seen this indicates that not all parts are acting in unison. Quite minor discrepancies in size or pack bending allowances can produce surprising reductions in the compression strength. Examples of this is the gap in the inner flaps of an RSC, which means that any product situated there can not act fully in unison with the pack, or inner flaps of a die-cut design that do not fully reach the bottom of the pack when the lid is closed.

Poor fit of the product in the case, causing bulge will also reduce the packs final compression strength due to the forced deformation of the pack prior to load even being applied. Liquid and granular products can be particularly challenging to pack due to this phenomenon.

Safety Factors

No matter how carefully the environmental and mechanical hazards that a pack must face are calculated and even after empirical data has been gathered, some form of safety factor, an increase over and above the calculated requirement, should be applied. There are no hard and fast rules that can be applied, each pack should be treated individually if over-packing is to be avoided. However, as long as a good compression estimation program, such as BoxComp has been used and the information that has been supplied to it has been verified, then a safety factor of 2 to 1 should be sufficient. If the pack is doing through a particularly arduous distribution chain that is likely to present more than the usual opportunities for damage, then a safety factor of 3 to 1 may be more appropriate. Safety factors in excess of 5 to 1 are rarely necessary and should you find yourself contemplating the use of such high figures then it normally means that pack's distribution chain should be re-examined.