Application Note | Wood Mechanical Properties
Wood Hardness & Elastic Modulus Using Microindentation
Cherry, Maple and Walnut Compared by Instrumented Vickers Indentation
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
Wood has a long history of being used as an important construction material for building houses, furniture, shelters, boats and many others, thanks to its structural strength, beautiful natural patterns, relatively low cost and ease of construction. As a naturally grown material, the individual cellular and fiber structure of the wood makes it a heterogeneous and anisotropic material of infinite variation of details. The same type of wood may possess different mechanical properties when the trees are subjected to altered environmental influences such as growing areas, climate, soil conditions, and growing space1.
Importance of Microindentation for Wood
Hardness and Young’s modulus are two of the most important mechanical properties of wood. A precise and reliable measurement of these wood properties is critical in determining their level of durability and suitability for different uses. The high-precision load-displacement curves from the microindentation measurement directly provide the physico-mechanical properties including hardness, Young’s modulus, and creep.
Compared to conventional hardness measurements, the microindentation tests by the NANOVEA Mechanical Tester avoid user errors during observing the imprint and determining its edge and size under the microscope. In addition, the elastoplastic properties such as Young’s modulus and creep are measured at the same time, which are essential in selecting the wood of the best mechanical properties for targeted applications.
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Measurement Objective
In this application, the NANOVEA Mechanical Tester, in microindentation mode, is used to compare the mechanical properties of three different types of wood. We would like to showcase the capacity of the NANOVEA Mechanical Tester in performing hardness and Young’s modulus measurements on wood samples with high precision and reproducibility.
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Mechanical Tester
Test Conditions
The Micro Module of the NANOVEA Mechanical Tester was applied to perform microindentation on three wood samples (Cherry, Maple and Walnut) using a Vickers indenter. All indentations were performed to a maximum load of 5 N. The test conditions are summarized in Table 1. Six tests were carried out on each sample to ensure result reproducibility.
As an orthotropic material, the mechanical properties of wood are independent in three directions, namely, longitudinal, tangential and radial. Moreover, irregular grains such as knots, burls, and “crotch” wood or porosity and cracks may also cause variation in the measured values of hardness and Young’s modulus. In this study, we located the areas for measurements on a smooth wood surface with regular wood patterns. This is followed by measuring the side hardness and Young’s modulus on the plank surface perpendicular to the wood grain, in order to achieve better repeatability and tighter standard deviation.
| Parameter | Value |
|---|---|
| Maximum force (N) | 5 |
| Loading rate (mN/min) | 10 |
| Unloading rate (mN/min) | 10 |
| Creep (s) | 5 |
| Indenter type | Vickers |
Table 1: Test conditions of the microindentation test.
Results and Discussion
The representative load-displacement curves of the indentations on the three wood samples are shown in Fig. 2 below. The Maple sample shows a penetration depth of ~90 µm, compared to that below 60 µm for Cherry and Walnut, indicating its relatively soft characteristics. The values of hardness and Young’s modulus calculated using the Oliver and Pharr method2 are summarized and compared in Fig. 3. The Cherry sample possesses a high hardness of 0.095 GPa, compared to 0.031 and 0.078 GPa, respectively, for Maple and Walnut. The Young’s modulus values of the Cherry, Maple and Walnut samples are 1.63, 0.88 and 2.28 GPa, respectively.
Conclusion
In this study, we showcased the capacity of the NANOVEA Mechanical Tester in performing micro hardness tests on wood samples. It is an ideal tool for measuring the mechanical properties such as hardness and Young’s modulus of different types of wood. The Cherry sample shows the highest hardness of 0.095 GPa, compared to 0.031 and 0.078 GPa, respectively, for Maple and Walnut. The Young’s modulus values of the Cherry, Maple and Walnut samples are 1.63, 0.88 and 2.28 GPa, respectively.
The Nano, Micro or Macro modules of the NANOVEA Mechanical Tester all include ISO and ASTM compliant indentation, scratch and wear tester modes, providing the widest and most user friendly range of testing available in a single system. In addition, optional 3D non-contact profiler and AFM Module are available for high resolution 3D imaging of indentation, scratch and wear track in addition to other surface measurements such as roughness.
References
1. Forest Products Laboratory, Wood Handbook, Chapter 4: Mechanical Properties of Wood (USDA Forest Service, FPL-GTR-113).
2. Oliver, W. C.; Pharr, G. M., Journal of Materials Research, Volume 7, Issue 6, June 1992, pp. 1564–1583.
Frequently Asked Questions About Wood Hardness Testing
Why measure wood hardness by microindentation?
The load-displacement curve gives hardness, Young’s modulus and creep directly, without the user error of reading an imprint’s edge and size under a microscope, so different woods can be compared precisely for durability and suitability.
Which wood was hardest?
Cherry, at 0.095 GPa, against 0.078 GPa for Walnut and 0.031 GPa for Maple.
Which wood was stiffest?
Walnut, with a Young’s modulus of 2.28 GPa, against 1.63 GPa for Cherry and 0.88 GPa for Maple.
How deep did the indenter go?
About 90 µm in Maple, compared with below 60 µm in Cherry and Walnut at the same 5 N maximum load, reflecting Maple’s softer character.
How is wood’s variability handled?
Wood is orthotropic and contains knots, burls, pores and cracks, so the indents were placed on smooth areas with regular grain, on the plank surface perpendicular to the grain, with six tests per sample.
How are hardness and modulus calculated?
With the Oliver and Pharr method from the loading and unloading curve of each indentation.
