J. FAJRIN ET AL.

Copyright © 2013 SciRes. OJCE

summarize statistical comparisons by declarations of

statistical significance or non -significance [2]. In a scien-

tific research, a hypothesis is proposed prior to data col-

lection and analyses. The statistical analysis of the data

will produce a number that is statistically significant if it

falls below a certain percentage called the confidence

level or level of significance. Further, Reference [2] ex-

plains that statistical significance is used to reject or ac-

cept what is called the null hypothesis, which usually

states that there is no relationsh ip between two variables.

In a simple expression, statistical sign ificance means that

there is a good re lat ionshi p exis ti ng bet ween two variables.

This paper focuses on the significance analysis of the

experimental results on the flexural behaviour of the sand-

wich pane l s develope d by the authors.

2. Significance Analysis

Although significance or statistical analysis is rarely

found as a primary approach in composite sandwich pan-

el research, it has actually been extensively used in the

field of composite material research. A number of re-

searches reported in the literature provide the idea of how

it was applied to composite material research. A study on

the significance effect of microwave curing on tensile

strength of carbon fibre composites was reported by [3].

The statistical analysis employed was two-way analysis

of variance (ANOVA) using statistical software SPSS

14.0. The results showed that the curing time and mi-

crowave process had significant effect on the tensile

strength of the carbon fibre composites. Reference [4]

reported their work on the optimization of processing

variables in wood-rubber composite panels manufactur-

ing process. The results of experiments were statistically

analysed using response surface method (RSM). The

Design-Expert statistical software was employed to de-

termine the significant factors that affected the properties

of the composite panels. It was concluded that the den-

sity and the interaction of different variables were the

significant factors affecting the final properties of the

boards.

Reference [5] used design of experiments (DoE) to

study the significance of low energy impact on modal

parameters for composite beams. The experiment was

designed as a 5 × 2 full factorial design. The results

showed that damping ratio is more sensitive parameter

for the damage detection than the natural frequency.

Reference [6] reported their work on the mechanical and

absorption properties of woven jute/banana hybrid com-

posites. Statistical analysis using one-way ANOVA was

employed to analyze the results of tensile, flexural and

impact tests of various composite configurations. The

results suggested that th e layering pattern had significant

effect on the mechanical properties of the composites.

A response surface methodology (RSM), which is a

statistical design of experiment method, was employed

by [7] to analyze the factors influencing deflection in

sandwich panels subjected to low-velocity impact. The

results revealed that the deflection increased with the

increasing of height of fall the mass of impactor. The

deflection was only slightly increased with the incr easing

in the core thickness.

It is worth to note that when a statistical test res ulted in

a significant outcome it means that a finding has a

chance of being true due to the relation between variables,

not just really a chance occurrence. Statistical signific-

ance does not always mean that the finding is important

or that it has any decision-making utility, so that the re-

searcher must always examine both the statistical and the

practical significance of any research finding.

3. Experimental Program

Before the significance analysis is performed, the flex-

ural testing program will be briefly reviewed in the fol-

lowing sections.

3.1. Testing Specimens

The sandwich samples were cut and shaped into a span

length of 450 mm and the size of 550 × 50 × 22 mm for

length, width and thickness, respectively. An aluminium

5005 H34 sheet with the thickness of 0.5 mm was used

as the skins for all samples. An expanded polystyrene

(EPS) is used for the core of this hybrid sandwich panel.

The thickness of EPS core for control level was 21 mm

and 15 mm for the other two levels to maintain a constant

overall thickness of 22 mm. Several types of natural fiber

composites have been investigated and the two best per-

formed natural fibers were employed as the intermediate

layer; jute and hemp fiber composites with a thickness of

3 mm. Each level was replicated 5 times; hence the total

of samples tested was 15 samples. The arrangements of

the flexural test specimens are shown in Table 1.

Sandwich panel specimens were manually prepared

using a pressing system. All constituent parts were cut

into the same length and width and glued together using

structural grade adhesive. The NFCs intermediate layers

were sanded-up using sanding machine to obtain uniform

thickness while alu minium sheet were roughed manually

using sandpaper. The EPS core was sliced using hot

knife foam cutter to obtain the required thickness. When

all constituents ready, they were glued and placed in the

pressure system. The process of sample preparation is

shown in Figure 1.

3.2. Testing Results

The static flexural test was conducted in accordance with