Journa I of Dentistry, 4,
1 l-l 4
A sphere compression test for measuring the mechanical properties of dental composite materials R. J. Verrall Dental Science Advancement
Foundation,
London
18) compressive, tensile (bending), hardness and impact tests to determine dynamic fracture properties. These were perfectly satisfactory as long as the tests were carried out on singlephase materials. However, with the advent of composite materials certain difficulties have been found which can be summarized as cavity and also is more suitable for glass-reinforced materials because they are subject to ‘brittle’ follows : 1. Mechanical tests are normally performed fracture. The theoretical aspects and examination on specially machined or formed specimens, of the way the spherical specimens break, together with the empirical relationships, have heen but dental composite materials cannot be analysed. Finally, results of tests performed on formed satisfactorily in this way. The reason commercially available restorative materials are is that when preparing composite materials for presented. testing, setting begins as soon as the catalyst is added to the resin. The setting time, of which the initial part is also the mixing time, is constant, no matter how large the volume INTRODUCTION of the specimen being prepared. It is therefore DURING the past decade a generation of much easier to mix small rather than the large materials has become available to dentists for samples required for traditional tests to obtain restorative work. These materials are mostly a homogeneous structure. Since all dental composites consisting of a hard glass filler in composite materials suffer from microstruca relatively soft matrix. Because these poly- tural defects or voids (Ribbons et al., 1972), phase materials behave differently in terms of it is also easier to attain a more homogeneous handling and structural properties it has been structure, with fewer voids, by using a smaller found necessary to develop appropriate mech- volume of material. anical tests compatible with these properties. 2. A smaller volume represents the situation The general objective of this paper is, therefore, in the tooth cavity more closely. A sphere to describe an example of such a test and compression test to determine the relative stiffspecifically to measure the stiffness and mech- ness of a composite restorative material is of anical strength of a variety of dental composite particular interest to dentists because it very nearly represents the type of loading and stress materials. Traditionally, the tests performed on matericonsequences involved in teeth and fillings in the mouth. als to determine their mechanical properties, For example, an occlusal filling which that is, their resistance to deformation and expanded too much under loading would very fracture, have been standard (BS specification
ABSTRACT Traditional methods of determining the mechanical strength properties of composite restorative materials have for various reasons been found to be unsatisfactory. A new method of testing is described which follows more closely the situation in the tooth
Journal of Dentistry, Vol. ~/NO.
12
1
P
Tooth
////////L//,/,/I I
1 \
: Possible break
filling material
Fig. I.-Diagram
showing possible break of the by an occlusal filling which expanded too much under loading.
tooth
CUSP
\
I I
I I \ I \ I \ / 1 / / / ’ ’ / / ’ ’ ’ ’ / ’ 1 ‘1 Fig. 2.-Cylinder of material used for the standard compression test. P, Compressive load.
P
Fig. I.-Fractured segments.
specimen showing lunar
Fig. 3.-Distribution
of transverse tensile stresses within a sphere under axial compressive loading (after Wilshaw, 1972). P, Compressive force; h, diameter of specimen; Ah, displacement of specimen (compressive); u,, tensile stress.
described here also serves as a simple, cheap and effective means of quality control, which has hitherto been difficult.
soon break away the tooth cusps, as shown in Fig. 1. 3. Because these materials are very brittle
The standard compression test is carried out using a cylinder of material as shown in Fig. 2. The specimen has to be manufactured with flat and parallel ends. When the load is applied and deformation takes place, friction due to sliding occurs between the platen surfaces and the material. The new composite materials may be classed as ‘brittle’, that is, permanent irreversible deformation of the material before fracture is very small. When a force is applied to a sphere of composite materials as shown in Fig. 3 the brittle fracture takes place in two stages. First, Hertzian contact fractures may occur at the top and bottom contact points,
SPHERE COMPRESSION
they suffer considerably from chipping at the edges when, for example, they are cut or cast into beams for bend tests, thereby increasing the amount of experimental scatter. 4. Composite materials are very expensive. The cost of testing with small samples is many times cheaper than when preparing for traditional tests. In addition, this relative cheapness allows for a greater number of tests, thus giving a better indication of variability in properties, which is very important in these The sphere compression test evaluations.
TEST
Verrall : Sphere Compression Test for Composite Materials
followed, as the load increases, by the fracture of the bulk material due to the tensile stresses acting normal to the diametral sections, as shown in Fig. 3. The cracks extend longitudinally, forming a number of lunar segments (Fig. 4), and the load drops instantaneously. The magnitude of the compressive load at fracture is used to calculate the tensile strength. Also, the intense compressive region in the contact zone may alter the structure of a porous material. The magnitude of the tensile stress ot is related to the compressive force P on the diameter h (diameter of sphere) :
13
-r---A
D
Fig. 5.-Diagram
TENSILE STRENGTH Hiramatsu and Oka (1966) have analysed the relationship between the compressive force and resultant tensile stresses of an elastic sphere when tested as described above and have shown theoretically and empirically that the tensile strength (i.e. maximum stress a,) is related to the maximum compressive force P and the diameter h by: 0*9P St = 7
of the apparatus used. A, Uniformly driven loading beam; B, specimen jig; C, load cell ; D, specimen ; E, load applicator; F, dial gauge.
specimen and Ah = displacement of specimen (compressive). Thus, the relative stiffness,
(2) (1)
where S, = tensile strength.
RELATIVE STIFFNESS The stiffness of an elastic solid is defined as the applied force divided by the resultant displacement within the elastic limit of that material. In the case of a simple cylinder the stiffness may be expressed in terms of a ratio of stress/ strain. When considering a sphere, the normal force/displacement is considered, but because the transverse diameter of the sphere is the area over which the force is applied, the results can only be strictly used to compare the stiffness of like bodies, i.e. spheres. However, for a set of spheres whose radii do not vary by more than a few per cent, the formula for calculating the relative stiffness of spheres E(S) may be derived empirically from Young’s modulus, whereby P = applied force, h = diameter of
EXPERIMENTAL
DETAILS
The apparatus consists basically of a tensile testing machine used in its compressive mode, with a jig set up as shown in Fig. 5. The specimen jig is specially designed with two hardened steel platens of which the lower is fixed and the upper is guided by three aligned guides. It is important that there is no play in the guides, because lateral movement could result in rolling or tipping and hence invalidate the results. The load is applied by means of a surface ground ball bearing fitted into the end of the applicator, which is free to rotate. This allows forany misalignment between the jig applicator, load cell or machine. The dial gauge measures the height of the specimen and its deformation. The specimens are prepared as stated in the manufacturer’s instructions, hand-rolled into
Journal of Dentistry, Vol. ~/NO.
14
1
Bulk fracture or yield point
l.S4 6
0.1
Fig. 6.-Results
0.3 0.2 Displacement Ah (IO-‘ m)
0.4
of a typical test.
spheres of 6(f0*5) x 10 -3 m diameter and then left for at least 24 hours before testing. The sphericity of the specimens is not critical within reasonable limits. Each sphere is then placed centrally in the specimen jig, and a slow loading rate of 4 x 10 -6 ms -l applied compressively until the sphere fractures. The spheres appear to break up into many (4 or 5 mainly) lune-shaped segments (Fig. 4). As shown in equations 1 and 2 both the relative stiffness and tensile strength may be calculated from the applied load and deformation coordinates. The results of a typical test are shown in Fig. 6. The small curve at the start of the straight line is due to Hertzian fractures taking place. These are ignored because it is the secondary bulk fracture that is of primary importance in this experiment. The results for various commercially available composite restorative materials are shown in Fig. 7.
Composites
A
6
C
Composites
Fig. 7.-Tensile
strength and relative stiffness of some commercially available composite materials. Twenty specimens of each type were tested. Shaded area represents 1 standard deviation. REFERENCES
Acknowledgements The author wishes to thank Dr T. R. Wilshaw of the Materials Science Division within the School of Applied Sciences, University of Sussex, for his help and encouragement throughout the preparation of this paper, and the University of Sussex for providing the facilities for carrying out the tests.
Y. and OKA Y. (1966) Determination of the tensile strength of rock by a compressive test of an irregular test piece. Znt. J. Mech. 3,
HIRAMATSU
89-99.
RIBBONS J. W., PEARSON G.
J. and VERRALL R. J. (1972) Effects of various packing techniques on cavity adaptation using composite filling materials. Znt. Assoc. Dent. Res. Paper 113. WILSHAW T. R. (1972) Personal communication.
1 l-l 4
A sphere compression test for measuring the mechanical properties of dental composite materials R. J. Verrall Dental Science Advancement
Foundation,
London
18) compressive, tensile (bending), hardness and impact tests to determine dynamic fracture properties. These were perfectly satisfactory as long as the tests were carried out on singlephase materials. However, with the advent of composite materials certain difficulties have been found which can be summarized as cavity and also is more suitable for glass-reinforced materials because they are subject to ‘brittle’ follows : 1. Mechanical tests are normally performed fracture. The theoretical aspects and examination on specially machined or formed specimens, of the way the spherical specimens break, together with the empirical relationships, have heen but dental composite materials cannot be analysed. Finally, results of tests performed on formed satisfactorily in this way. The reason commercially available restorative materials are is that when preparing composite materials for presented. testing, setting begins as soon as the catalyst is added to the resin. The setting time, of which the initial part is also the mixing time, is constant, no matter how large the volume INTRODUCTION of the specimen being prepared. It is therefore DURING the past decade a generation of much easier to mix small rather than the large materials has become available to dentists for samples required for traditional tests to obtain restorative work. These materials are mostly a homogeneous structure. Since all dental composites consisting of a hard glass filler in composite materials suffer from microstruca relatively soft matrix. Because these poly- tural defects or voids (Ribbons et al., 1972), phase materials behave differently in terms of it is also easier to attain a more homogeneous handling and structural properties it has been structure, with fewer voids, by using a smaller found necessary to develop appropriate mech- volume of material. anical tests compatible with these properties. 2. A smaller volume represents the situation The general objective of this paper is, therefore, in the tooth cavity more closely. A sphere to describe an example of such a test and compression test to determine the relative stiffspecifically to measure the stiffness and mech- ness of a composite restorative material is of anical strength of a variety of dental composite particular interest to dentists because it very nearly represents the type of loading and stress materials. Traditionally, the tests performed on matericonsequences involved in teeth and fillings in the mouth. als to determine their mechanical properties, For example, an occlusal filling which that is, their resistance to deformation and expanded too much under loading would very fracture, have been standard (BS specification
ABSTRACT Traditional methods of determining the mechanical strength properties of composite restorative materials have for various reasons been found to be unsatisfactory. A new method of testing is described which follows more closely the situation in the tooth
Journal of Dentistry, Vol. ~/NO.
12
1
P
Tooth
////////L//,/,/I I
1 \
: Possible break
filling material
Fig. I.-Diagram
showing possible break of the by an occlusal filling which expanded too much under loading.
tooth
CUSP
\
I I
I I \ I \ I \ / 1 / / / ’ ’ / / ’ ’ ’ ’ / ’ 1 ‘1 Fig. 2.-Cylinder of material used for the standard compression test. P, Compressive load.
P
Fig. I.-Fractured segments.
specimen showing lunar
Fig. 3.-Distribution
of transverse tensile stresses within a sphere under axial compressive loading (after Wilshaw, 1972). P, Compressive force; h, diameter of specimen; Ah, displacement of specimen (compressive); u,, tensile stress.
described here also serves as a simple, cheap and effective means of quality control, which has hitherto been difficult.
soon break away the tooth cusps, as shown in Fig. 1. 3. Because these materials are very brittle
The standard compression test is carried out using a cylinder of material as shown in Fig. 2. The specimen has to be manufactured with flat and parallel ends. When the load is applied and deformation takes place, friction due to sliding occurs between the platen surfaces and the material. The new composite materials may be classed as ‘brittle’, that is, permanent irreversible deformation of the material before fracture is very small. When a force is applied to a sphere of composite materials as shown in Fig. 3 the brittle fracture takes place in two stages. First, Hertzian contact fractures may occur at the top and bottom contact points,
SPHERE COMPRESSION
they suffer considerably from chipping at the edges when, for example, they are cut or cast into beams for bend tests, thereby increasing the amount of experimental scatter. 4. Composite materials are very expensive. The cost of testing with small samples is many times cheaper than when preparing for traditional tests. In addition, this relative cheapness allows for a greater number of tests, thus giving a better indication of variability in properties, which is very important in these The sphere compression test evaluations.
TEST
Verrall : Sphere Compression Test for Composite Materials
followed, as the load increases, by the fracture of the bulk material due to the tensile stresses acting normal to the diametral sections, as shown in Fig. 3. The cracks extend longitudinally, forming a number of lunar segments (Fig. 4), and the load drops instantaneously. The magnitude of the compressive load at fracture is used to calculate the tensile strength. Also, the intense compressive region in the contact zone may alter the structure of a porous material. The magnitude of the tensile stress ot is related to the compressive force P on the diameter h (diameter of sphere) :
13
-r---A
D
Fig. 5.-Diagram
TENSILE STRENGTH Hiramatsu and Oka (1966) have analysed the relationship between the compressive force and resultant tensile stresses of an elastic sphere when tested as described above and have shown theoretically and empirically that the tensile strength (i.e. maximum stress a,) is related to the maximum compressive force P and the diameter h by: 0*9P St = 7
of the apparatus used. A, Uniformly driven loading beam; B, specimen jig; C, load cell ; D, specimen ; E, load applicator; F, dial gauge.
specimen and Ah = displacement of specimen (compressive). Thus, the relative stiffness,
(2) (1)
where S, = tensile strength.
RELATIVE STIFFNESS The stiffness of an elastic solid is defined as the applied force divided by the resultant displacement within the elastic limit of that material. In the case of a simple cylinder the stiffness may be expressed in terms of a ratio of stress/ strain. When considering a sphere, the normal force/displacement is considered, but because the transverse diameter of the sphere is the area over which the force is applied, the results can only be strictly used to compare the stiffness of like bodies, i.e. spheres. However, for a set of spheres whose radii do not vary by more than a few per cent, the formula for calculating the relative stiffness of spheres E(S) may be derived empirically from Young’s modulus, whereby P = applied force, h = diameter of
EXPERIMENTAL
DETAILS
The apparatus consists basically of a tensile testing machine used in its compressive mode, with a jig set up as shown in Fig. 5. The specimen jig is specially designed with two hardened steel platens of which the lower is fixed and the upper is guided by three aligned guides. It is important that there is no play in the guides, because lateral movement could result in rolling or tipping and hence invalidate the results. The load is applied by means of a surface ground ball bearing fitted into the end of the applicator, which is free to rotate. This allows forany misalignment between the jig applicator, load cell or machine. The dial gauge measures the height of the specimen and its deformation. The specimens are prepared as stated in the manufacturer’s instructions, hand-rolled into
Journal of Dentistry, Vol. ~/NO.
14
1
Bulk fracture or yield point
l.S4 6
0.1
Fig. 6.-Results
0.3 0.2 Displacement Ah (IO-‘ m)
0.4
of a typical test.
spheres of 6(f0*5) x 10 -3 m diameter and then left for at least 24 hours before testing. The sphericity of the specimens is not critical within reasonable limits. Each sphere is then placed centrally in the specimen jig, and a slow loading rate of 4 x 10 -6 ms -l applied compressively until the sphere fractures. The spheres appear to break up into many (4 or 5 mainly) lune-shaped segments (Fig. 4). As shown in equations 1 and 2 both the relative stiffness and tensile strength may be calculated from the applied load and deformation coordinates. The results of a typical test are shown in Fig. 6. The small curve at the start of the straight line is due to Hertzian fractures taking place. These are ignored because it is the secondary bulk fracture that is of primary importance in this experiment. The results for various commercially available composite restorative materials are shown in Fig. 7.
Composites
A
6
C
Composites
Fig. 7.-Tensile
strength and relative stiffness of some commercially available composite materials. Twenty specimens of each type were tested. Shaded area represents 1 standard deviation. REFERENCES
Acknowledgements The author wishes to thank Dr T. R. Wilshaw of the Materials Science Division within the School of Applied Sciences, University of Sussex, for his help and encouragement throughout the preparation of this paper, and the University of Sussex for providing the facilities for carrying out the tests.
Y. and OKA Y. (1966) Determination of the tensile strength of rock by a compressive test of an irregular test piece. Znt. J. Mech. 3,
HIRAMATSU
89-99.
RIBBONS J. W., PEARSON G.
J. and VERRALL R. J. (1972) Effects of various packing techniques on cavity adaptation using composite filling materials. Znt. Assoc. Dent. Res. Paper 113. WILSHAW T. R. (1972) Personal communication.
