The basic principle of the compressive strength tester
A compressive strength tester is a precision instrument used to evaluate the mechanical properties of a material or structure under pressure. Its core working principle is to apply a vertical compressive force to the specimen through a precisely controlled loading system and simultaneously measure its deformation until the specimen fails or reaches a predetermined deformation. The instrument is usually composed of a frame, a force sensor, a displacement sensor, a pressure plate and a control system. During the test, the force value and displacement data are recorded in real time and a force-displacement curve is generated, so as to calculate the maximum compressive strength, elastic modulus, yield point and other key parameters of the sample. For packaging materials such as cartons and paper tubes, their compressive performance is directly related to the protection ability in stacking, transportation and other links, so this test is of great practical significance.
Carton compressive strength
The compressive strength test of a carton mainly evaluates its ability as a packaging container to resist crushing when stacked vertically. Tests usually follow standards such as GB/T 4857.4, ISO 12048, ASTM D642, etc. During the test, the prepared empty carton is placed in the center of the lower pressure plate of the tester, and the upper pressure plate moves down at a constant rate and applies pressure to the top of the carton. The test records the entire process from the start of loading to when the carton collapses or reaches a predetermined maximum force.
The compressive strength (BCT) of a carton can be theoretically estimated using the following formula, which combines material and structural factors:
BCT = k × ECT × √(P × D)
Among them, BCT is the compressive strength of the carton (N), ECT is the edge compressive strength of the containerboard (N/m), P is the circumference of the carton (m), D is the depth of the carton (m), and k is the comprehensive coefficient related to the structure and manufacturing process of the carton. In actual testing, ambient temperature and humidity have a significant impact on the results, so standards usually stipulate strict pretreatment conditions.
Compressive strength of paper tubes
Paper tubes (also known as paper cores) are widely used in winding mandrels in textile, film, paper, and other industries, and their radial compressive strength is a key indicator to ensure that the coil does not deform during storage and transportation. The test mostly refers to ISO 11093-4, GB/T 22906 and other standards. During the test, a piece of paper tube specimen of the specified length is placed vertically between the pressure plates, and its radial bearing capacity is tested by compression. The test should focus on the characteristics of its maximum crushing force and load-deflection curve.
For cylindrical structures such as paper tubes, the radial compressive strength is closely related to the ring compressive strength (RCT) and geometric dimensions of the material. The stability analysis can be based on the mechanical model of thin-walled cylinders. The critical buckling load (P_cr) can be approximated as:
P_cr ∝ (E * t³) / R
E is the elastic modulus (MPa) of the material, t is the thickness of the tube wall (mm), and R is the average radius of the paper tube (mm). This relationship reveals the effect of improving material stiffness and increasing wall thickness on enhancing the anti-instability ability of paper tubes.
Influencing factors
In order to ensure the accuracy and comparability of test results, a number of parameters need to be strictly controlled during operation. The main influencing factors included the temperature and humidity pretreatment state of the specimen, the parallelism and size of the loading platen, the loading speed, and the neutrality of the specimen placement. Any deviation can lead to uneven stress distribution, deviating from the true level of the test value.
| Test objects | Core test parameters |
| Carton | Compressive strength, deformation amount, force-displacement curve |
| Paper tubes | Radial crushing force, radial stiffness, buckling mode |
| Share the influencing factors | temperature and humidity, loading rate, specimen centering, and platen flatness |
Testing process
Standardized testing processes are the basis for reliable data. First, the specimen needs to be pre-treated to equilibrium in a standard temperature and humidity environment. Subsequently, the geometric dimensions of the specimen are precisely measured and recorded. During the test, the specimen is correctly placed in the center of the compression tester's platen, the specified loading speed is set and the test is initiated. The instrument automatically records compressive force and displacement data and stops when the specimen fails or a preset termination condition is reached.
Data analysis not only focuses on the maximum compressive force value, but also needs to be combined with the entire force-displacement curve. The slope of the initial linear segment of the curve can reflect the overall stiffness of the specimen; The shape of the curve can reveal failure patterns of the specimen, such as sudden crush, progressive buckling, or material yield. This information is valuable for material improvement, structural optimization, and quality control.
Epilogue
The compressive strength tester provides a quantitative scientific means for quality control and performance evaluation of cartons and tubes. By following standardized testing methods and deeply understanding the relationship between material properties, structural design and test conditions, relevant industries can more effectively evaluate the reliability of packaging products and optimize design solutions, thereby ensuring the safety of products in the logistics chain and reducing losses. With the advancement of technology, the automation and intelligence of testing instruments continue to improve, which will further enhance the efficiency of testing and the in-depth application of data.
References
GB/T 4857.4-2023, Packaging - Basic tests for transportation packaging parts - Part 4: Compressive and stacking test methods using pressure testing machines
ISO 12048:2022， Packaging — Complete, filled transport packages — Compression and stacking tests using a compression tester
ISO 11093-4:2020， Paper and board — Testing of cores — Part 4: Determination of compressive strength
ASTM D642-20， Standard Test Method for Determining Compressive Resistance of Shipping Containers, Components, and Unit Loads
GB/T 22906.4-2022, Determination of paper cores - Part 4: Determination of radial compressive strength