This paper is a review of the past research of mechanical testing methods for natural fibre honeycomb sandwich structure as well as failure modes analysis at a microscopic level by using Scanning Electron Microscope (SEM). As the world is garnering attention towards renewable resources for environmental purposes, studies of natural fibre have been increasing as well as the application of natural fibre throughout various industries such as aerospace, automobiles, and construction sectors. This paper is started with brief information regarding the honeycomb sandwich structure, introduction to natural fibre, its applications as well as the factors affecting the performances of the structure. Next, the mechanical testing methods are listed out as well as the expected outcomes obtained from the respective testing. The mechanical properties are also identified by conducting lab tests according to the ASTM standard for sandwich and core structures. The microstructure of the deformed samples is then examined under Scanning Electron Microscope (SEM) by using different magnifications to study the failure mechanisms of the samples. The images obtained from the SEM test are analyzed by using fractography which will show the failure modes of the samples. This article is based on past research conducted by professional on the related topic.
A sandwich structure composite consists of a thin, dense, and strong face sheet layered with a lightweight core. It is broadly applied in industries requires lightweight design such as automotive and aerospace sectors [
Honeycomb structure is recently being considered as the core as it offers good mechanical and crushing as well as acoustic properties, fluid control, low dielectric properties, low thermal conductivity coefficients, small cross-sectional area, and a large exposed area within the cells [
Natural fibres are fibres that can be easily gained and extracted from plants, vegetables, and animals’ sources. Natural fibres are advantageous and sustainable than synthetic fibres which led the industrial sectors to utilize natural fibres in various field such as automotive, aerospace and construction [
Other than that, the behaviours of natural fibre also depend on the weight percent of the fibre, its orientation, length, texture, and its hydrophilic nature. Nonetheless, hydrophilic fibre and hydrophobic matrix have poor congruent which will give an impact on the structure, yet it can be improved by using compatibilizers to develop better adhesion between the fibre and the matrix [
Natural fibre honeycomb sandwich structure offers various advantages in industries due to the mechanical properties and high-performance. Nevertheless, the number of researches on factography analysis that have been done on natural fibre honeycomb sandwich structure is low. Hence, as this paper will provide a review on the testing methods and standards to study the mechanical behaviours of natural fibre honeycomb sandwich structure, as well as fractography analysis to identify the failure mechanism, this paper will be very beneficial as it can be referred for future studies of natural fibre honeycomb sandwich structure to broaden the implementation of this structure especially in construction industry.
As the performance and properties of the face sheets, core and the adhesive varies depending on the materials, design parameters and the manufacturing process, an adequate testing is required to prevent failures. Therefore, in order to select the most suitable materials, a correct development process, help in design process as well as ensuring the quality control, standardized or specific tests are vital for these purposes.
The testing for natural fiber-based sandwich composites is vital to assess the quality of the materials used for manufacturing purposes. It will help in evaluating and optimizing the usage of materials in the structures. To study the effects of equipment and tool design, the sandwich panels also need to be tested according to the suitable testing methods. From the test, the quality of product can be determined. Hence, it will bring huge opportunities to manufacturer to overcome any problems faced in the structure whether the materials are unsuitable or other mechanical defects of the structure that impacted the performances such as crack and fractures.
In attempt to study the mechanical properties and the structural integrity of the face sheets and the honeycomb core, there are various test methods that can be used according to ASTM Standards Test Methods and the Military Standard 401B, Sandwich Constructions and Core Materials: General Test Methods.
| Name of the Test | Type of Test | Outcome/Observation |
|---|---|---|
| Wear and Tear | Ø Investigate performance properties for long-term usage of the material. | |
| Tensile Strength & Modulus Test | Shear | Ø Max. load required to shear. |
| Flatwise Tensile Test | Ø Measure the adhesive bond strength. | |
| Strength Test | Flexural Strength Test & Modulus Test | Ø Determine the flexibility of the specimen under load to prevent deformation. Ø Measure the stress experienced by the materials at the failure moment. |
| Material Construction Tests | Ø Analyze construction issues (adhesive bond strength, resistance to deformation under load, exposure to moisture). | |
| Edge Bearing Test | Ø Determine the bearing strength of fasteners in a laminate under tension loading. | |
| Impact (Puncture) Test | Ø Measure the ability of materials to resist puncture, withstand shock loading and fractures during shock loading. |
Before the test is conducted, the samples involved need to be made sure to be properly conditioned as the temperature and humidity of the surrounding will affect the sample. Thus, ASTM D618-61 provides the outline for specimen conditioning requirements before actual testing as shown in
The honeycomb core is another important component of a lightweight sandwich structure. The different geometric parameters of the honeycomb core have a great influence on the mechanical properties of the sandwich structure. Thus, to further investigate the bending response brought from honeycomb core, sample testing should be done according to the most suited testing standard. The ASTM standard that has been used for the past research is shown in
Scanning Electron Microscope (SEM) is a tool that has the function to magnify small features or objects that cannot be observed by human sight. Different from typical microscope which using optical light to observe, SEM use electron beam with high energy on the sample surface to form images. In the previous study, the mechanical properties of samples are tested by some physical testing. After
| No. | Parameter | Requirement |
|---|---|---|
| 1. | Relative Humidity | 23˚C ± 2˚C |
| 2. | Temperature | 50 ± 5% |
| 3. | Duration (min) relative to test specimen dimension | 40 hrs for specimen ≤ 6.33 mm thick 88 hrs for specimen ≥ 6.35 mm thick |
| No. | ASTM Specification | Findings |
|---|---|---|
| Cores | ||
| 1. | C273/C273M | Core Shear Properties |
| 2. | C365/C365M | Core Flatwise Compressive Properties |
| 3. | C393/C393M | Core Shear Properties by Beam Flexure |
| 4. | C394 | Core Shear Fatigue |
| 5. | D7336/D7336M | Core Static Energy Absorption |
| Sandwich Panels | ||
| 1. | C297/C297M | Sandwich Flatwise Tensile Strength |
| 2. | C364/C364M | Sandwich Edgewise Compressive Strength |
| 3. | C480 | Sandwich Flexural Creep |
| 4. | D6416/D6416M | Sandwich 2D Plate Flexural Properties |
| 5. | D7249/D7249M | Sandwich Facing Properties by Beam Flexure |
| 6. | D7250/D7250M | Sandwich Beam Flexural and Shear Stiffness |
that, the deformed samples will be examined under Scanning electron Microscope (SEM) by using several magnifications to study the microstructure defects of the sample. The failure analysis of the sample obtained from the images of SEM will be analyzed by using fractography.
Typically, the honeycomb sandwich structure constructed from various types of materials is tested according to the standard. When the sample is observed under SEM, there are many failure modes that can be found on the sample surface and core. From the previous studies, the images obtained from the SEM micrographs were analysed by using fractography which shown failure mechanisms of the structure such as matrix breakage [
More failure modes can be studied from SEM and fractography analysis can be seen from the research that has been done by Cecilia Daimin et al. on the micro failure of eco board. The magnifications of SEM that has been used are different to identify the failure modes of the samples ranging in between 20X to 85X of magnifications. The SEM images of the specimens are shown in the
| Specimen | Coated | Uncoated |
|---|---|---|
| OPEFB | ||
| OPEFB:AO |
| OPEFB: RH | ||
The micrographs obtained from the SEM imaging showed dimple, void, fibre pull out and matrix cracking as the main reasons of the breakage of the specimens. Hence, to study the microstructure of the sample under Scanning Electron Microscope, fractography analysis is required to identify the exact failure outcomes. Despite of the utilization of SEM in observing the samples, there are several challenges that needed to be taken into consideration while conducting SEM test. An experience personnel is needed to prepare the samples and conduct SEM test as there is a risk of radiation exposure resulted by the scattered electron from the sample surface if it is not properly handled. Other than that, only a small portion of sample can be fitted into the vacuum chamber which can handle the vacuum pressure. Hence, the surface area observed under the SEM will be limited.
In summary, there are various advantages of natural fibre in the construction industry. Their mechanical properties have contributed to the implementation of the honeycomb sandwich panel in construction industries. However, there are not many researches that have been done on natural fibre honeycomb sandwich structure in order to improve the performances. Thus, this paper will help to review the testing methods and the standard that can be referred to study the mechanical behaviours of natural fibre honeycomb sandwich structure. The microstructure defects that have been found in previous research under Scanning Electron Microscope examination also will help to tackle the failure mechanisms of the structure. Further research of natural fibre honeycomb sandwich structure is highly required as it will be very beneficial in the academic world to find ways of improvisation as an attempt to tackle the problems faced for future development of natural fibre honeycomb sandwich structure.
The author would like to thank the whole research team who were involved in this research. On behalf of the Advance Composite Group, the author would like to thank Universiti Malaysia Sabah for providing the opportunity, funding and platform for this research to take place.
The authors declare no conflicts of interest regarding the publication of this paper.
Al-Azad, N., Dedifitrianto, N.B. and Shah, M.K.M. (2021) A Mini Review on Testing Methods for Mechanical Properties of Natural Fibre Honeycomb Sandwich Structure and Fractography Analysis. Journal of Materials Science and Chemical Engineering, 9, 29-38. https://doi.org/10.4236/msce.2021.95003