Fracture of natural biomaterials
TA Charlie Florek
In basement of fiber optics building
Department of Biomedical Engineering
Rutgers, The State University of New Jersey
Safety:
1. Safety glasses must be worn during the impact fracture tests.
2. Use gloves when working with the impact apparatus.
3. Stand well clear of the impact apparatus during operation
4. NEVER stand in front or behind the machine during testing (the front is the direction in
which the pendulum swings).
[1] Purpose
There are two goals to this lab:
1. to understand how materials fracture and how this is related to their microstructure.
2. to relate the concepts of materials fracture to the properties and structure of natural
biomaterials.
You will examine the surfaces of materials which have undergone brittle and ductile fracture to
see the typical morphological features of the fracture surface. You will perform impact fracture
tests on a ceramic, wood (across the grain and with the grain) and bone.
After completing the experiments you should be able to:
 Explain how natural biomaterials have evolved to reduce the risk of fracture.
 Explain why pure ceramics are poor synthetic biomaterials.
 Estimate the fracture surface energy for a brittle material.
[2] Introduction
Natural biomaterials such as bone, teeth and wood have evolved to have a complex
microstructure which enables them to withstand large mechanical stresses and impacts without
fracturing. They are also relatively light and have a high strength to mass ratio. Synthetic
biomaterials based on pure polymers, ceramics or metals cannot match the natural materials they
replace in terms of strength, fracture resistance and mass. However, composite artificial
biomaterials which contain more than one material and have a carefully engineered
microstructure can have similar properties to the natural biomaterials they replace.
This lab will use a series of impact fracture tests to examine the relative fracture resistance of
natural and synthetic materials. The test is loosely based on the standard Charpy impact test for
examining brittle and ductile fracture in bulk samples.
[3] Apparatus
The apparatus is a purpose built impact tester based upon a swinging pendulum (see Fig. 1). The
pendulum impacts the test sample at the bottom of its downward swing. The impact breaks the
sample and the pendulum continues on its upward swing. Some of the pendulum’s kinetic energy
is lost in fracturing the sample and this reduces the height of the pendulum’s upward swing. The
length of the upwards swing is found by examining the maximum angle of the pendulum during
its upswing. By evaluating the lost energy it is possible to estimate the energy needed to create
the fracture surface.
pendulum
sample
Fig. 1: The impact testing apparatus.
[4] Experiments: Part A – Fracture Surfaces
You have been provided with several samples that have already been fractured. These are:
(a) glass microscope slide
(b) wood broken across the grain
(c) wood broken with the grain
(d) ceramic tile
(e) rib bone (cooked)
You have also been given a short piece of copper tubing which you can fracture by bending it
(you may need to bend it several times before it breaks in two).
For each of the samples visually examine the fracture surface, then describe and sketch it on
pages 1 & 2 of the results sheet (at the end of this document).
[5] Experiments: Part B – Impact Tests
You have been provided with 8 samples for fracture testing. These are:
(a) 3 pieces of ceramic tile
(b) 2 pieces of wood across the grain
(c) 2 pieces of wood with the grain
(d) 1 bone
Before you begin you must calibrate the impact tester. This requires you to find the maximum
angle of the pendulum on its upward swing when it does not strike a sample. To find this, one of
you must lift the pendulum until it is vertical (Fig. 2a). The pendulum is then released and
swings downwards and then upwards (Fig. 2b). One of you should be position at the side of the
pendulum to visually see how high the pendulum swings.
(a)
(b)
Fig. 2: (a) pendulum at the top of its swing and (b) pendulum at the end of its swing.
You should repeat this procedure several times until you are confident that you can assess the
correct angle for the end of the pendulum’s swing.
Now you can being testing the samples. Test the ceramic tile first, then the wood samples and
lastly the bone. It is important that you test the bone last as you only have one sample and you
need to be certain that you test it correctly.
For each sample place it in the vise with the scratch or groove just above the vise’s jaws and
parallel with them. DO NOT over-tighten the jaws as this may fracture the ceramic or bone
samples. The correct mounting is shown in Fig. 3.
Fig. 3: Correct sample mounting.
Now repeat the procedure you used for calibrating the impact testing apparatus. This time the
sample will break when the pendulum impacts it. The upward swing of the pendulum should be
reduced. Record the angle of the upward swing on page 3 of the results sheet and then complete
the other sections of the sheet.
The energy lost in fracturing the sample is the maximum potential energy for the upward swing
from the calibration minus the maximum potential energy for the upward swing from the sample.
[6] After the Experiments
Collect all the broken fracture samples and place them in the box provided by the TA. Leave the
area tidy for the next group. Make sure you have completed all 3 pages of the results sheets.
[7] References
Fracture of Brittle Solids (2nd edition) B.R.Lawn (Cambridge University Press)
http://www2.umist.ac.uk/material/research/intmic/features/charpy/notes.htm
PART A: Results - For each sample sketch and describe the fracture surface.
Glass
Description:____________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Wood (across grain)
Description:____________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Wood (with grain)
Description:____________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Ceramic
Description:____________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Bone
Description:____________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
Copper
Description:____________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
______________________________________________________________________________
PART B: Results - For each sample enter the maximum angle of the pendulum’s upward swing.
Sample
Maximum angle (°)
Example
156°
Maximum Height
Energy Lost
0
Calibration 1
0
Calibration 2
0
Calibration 3
0
Calibration 4
0
Calibration 5
0
Example
144°
Ceramic 1
Ceramic 2
Ceramic 3
Example
154°
Wood with grain 1
Wood with grain 2
Example
138°
Wood across grain 1
Wood across grain 2
Example
140°
Bone 1
To calculate the maximum height you need the pendulum’s length (76cm). To calculate the
energy lost in the impact you need the pendulum’s mass (1 kg) and then you should use the
standard method to find gravitational potential energy (PE=mgh). Note that there is slight offset
in the pendulum arm of -2 at the bottom of the swing.
Example:
Maximum angle = 156;
Angle at bottom of swing = -2;
Total swing length = 158;
Maximum height (h) = 0.76[1+sin(158-90)]=1.465 meters;
P.E. = 1 x 9.8 x 1.465 = 14.357 joules