Title: On the compressive strength and stiffness behaviour of common structures made from MDF
Contributors: Derek Covill….
University of Brighton, started December 2019
Abstract: Background: Understanding how designs behave structurally is an important consideration for any product designer. Description: In this report, four different structure types were tested for their strength and stiffness properties in compression using samples made from 6 mm MDF. Results/discussion: There was a clear variation in the strength results (with mean values ranging from 118 – 294 N), and in order from weakest to strongest the structures were: cantilever, triangle, rectangle, triangulated rectangle. Future work: Future work will include characterising the material behaviour of MDF and developing Finite Element (FE) models to parametrically explore the behaviour of various structural designs.
Introduction
This report is the first in a series of technical reports to help students understand how to present experimental or technical work in a clear, concise, objective and rigorous way. It has been written in the standard format for writing up a scientific report (Monash University, 2019). The Harvard method has also been used for the referencing (Imperial College, 2017) to show how this is done.
The aim of this report was to experimentally determine the strength and stiffness of a range of common structures. These structures were laser cut out of MDF sheets (thickness = 9 mm). The results of this report are important to help understand how the shape of the structures influence their physical behaviour. In particular, the strength and stiffness under a compressive load were chosen since these are fundamental quantities which are relevant for designing anything that is structural (e.g. furniture, buildings, vehicles etc). It is hoped that this will be a useful learning exercise for design and engineering students.
In this report, strength (Fmax) has been defined as the maximum compressive force (F) that the structure withstands before failure. Furthermore, stiffness (k) was defined as the resistance to deformation (d) under an applied compressive force (F), expressed as k = F / d (in the region between F0 and Fmax). This is a standard approach for determining the static structural behaviour of objects (Covill et al, 2016), although typically this is limited to elastic deformation only.
Materials and Methods
Four different structures were tested in compression using a Lloyds LR10K Universal Testing System with a 10 kN load cell (calibration to BS EN ISO 7500: 2018) at a deformation rate of 60 mm / minute until the sample failed or until 30 mm of deformation had been recorded. The four structures included a cantilever, a triangulated structure, a rectangular structure and a triangulated rectangular structure and these are shown below in Figure 1. For each structure type, two samples were produced to provide some statistical confidence (n = 2).
Figure 1. The four structures tested in this report included: (a) a cantilever, (b) a triangulated structure, (c) a rectangular structure, and (d) a triangulated rectangular structure.
Figure 2 below shows the experimental setup with the samples clamped in the non-slip wedge-type grips and the load applied using a horizontally mounted mild steel rod (diameter = Φ10 mm). The samples were clamped with 34 mm of the stem held within the grips.
Figure 2. The experimental setup. Samples were clamped in the non-slip wedge-type grips and the load applied using a horizontally mounted mild steel rod (left) and the position of the clamp for each sample (right).
Results and Discussion
The latest version of the data can be found here: DP402-19-20-MDF-structures-compression
Table 1 below shows the strength and stiffness results for all structures. There was a clear variation in the strength results with the cantilever structures providing the lowest strength (mean = 64 N). The triangle provided the second lowest strength and was almost double that of the cantilever (mean = 166 N). The rectangle was only marginally stronger than the triangle (mean = 208 N), while the triangulated rectangle structures were the strongest (mean = 250 N).
Table 1. Strength and stiffness results for all structures, with data shown as mean ± standard deviation (n = 2).
Cantilever |
Triangle |
Rectangle |
Tri-rect |
|
| Mass, m (g) | 11.6 | 15.8 | 16.4 | 20.4 |
| Strength, Fmax (N) | 64 ± 2 | 166 ± 15* | 208 ± 3 | 250 ± 4 |
| Stiffness, k (N/mm) | 9.7 ± 0.6 | 34.3 ± 3.0 | 59.1 ± 1.8 | 69.7 ± 3.8 |
Table 2 below shows the strength to mass ratio and stiffness to mass ratio results for all structures. Interestingly the rectangular structure demonstrated the highest strength to mass ratio out of all the structures.
Table 2. Strength to mass ratio and stiffness to mass ratio results for all structures.
| Cantilever |
Triangle |
Rectangle |
Tri-rect |
|
| Strength to mass ratio F/m (N/g) | 5.5 | 10.5 | 12.7 | 12.3 |
| Stiffness, k/d.m (N/mm.g) |
Figure 3 below shows the failure locations for each of the four types of structure, with some failures occurring in the
Figure 3. The failure locations for each of the four types of structures (a) cantilever, (b) triangulated structure, (c) rectangular structure, and (d) triangulated rectangular structure.
A one-way Analysis of Variance (ANOVA) was carried out to determine if there was a statistically significant difference between these structure types. For the strength data there was a highly significant difference between the structure types (p < 0.001) and for the stiffness data TBA………. However, it should be noted that with only two samples (n = 2) for each structure type which limits the confidence in these statistical findings.
Assumptions…need to tighten this up…
- material was consistent
- setup was consistent (note that we had many different people doing the setup and this probably influenced our results somewhat)
- pieces were consistent
- temperature was constant
- other?
Future work that would be valuable to complete this study would be to generate the stiffness data (eek!). Furthermore, it would be interesting to also compare the strength to mass ratios and the stiffness to mass ratios of the various structures to see how this might compare to the original strength and stiffness data respectively. It would also be of interest to complement this study with an explanation of the types of loading that occurs internally with each structure in this load case (i.e. bending, tension and compression).
Future work that would be valuable to complement this report would be to carry out experiments to characterise the material behaviour of MDF (similar to Engineering Toolbox 2011, or Granta 2019) in tension, compression and bending (i.e. strength and stiffness). Furthermore, by developing Finite Element (FE) models and validating these against the experimental results it is possible to parametrically explore the behaviour of such structures by varying the shapes and materials used in the designs.
Conclusions
The results showed clear differences in the abilities of the four structures to withstand compressive forces. While it is clear that triangulation of a structure will provide additional strength, without statistical analysis it is not known at this stage if these differences are significant (although it is thought that they will be!).
References
BS EN ISO 7500: 2018. Metallic materials. Calibration and verification of static uniaxial testing machines. Tension/compression testing machines. Calibration and verification of the force-measuring system.
Granta, 2019. CES edupack: material database software.
Covill, D., Allard, P., Drouet, J-M., Emerson, N. 2016. An Assessment of Bicycle Frame Behaviour under Various Load Conditions Using Numerical Simulations. Procedia Engineering, Vol 147, pp 665-670, https://doi.org/10.1016/j.proeng.2016.06.269
Engineering ToolBox, 2011. Wood, Panel and Structural Timber Products – Mechanical Properties. [online] Available at: https://www.engineeringtoolbox.com/timber-mechanical-properties-d_1789.html (accessed 4/12/2019)
Imperial College London, 2017. Harvard referencing method. https://www.imperial.ac.uk/media/imperial-college/administration-and-support-services/library/public/harvard.pdf (accessed 4/12/2019).
Monash University, 2019. Writing scientific and lab reports. https://www.monash.edu/rlo/assignment-samples/science/science-writing-a-lab-report (accessed 4/12/2019).
