- Email: [email protected]
Effect of impact velocity and specimen stiffness on contact forces in a weight-controlled chewing simulator
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Available online at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of impact velocity and specimen stiffness on contact forces in a weight-controlled chewing simulator Stefan Rues a,∗ , Gerhard Huber b , Peter Rammelsberg a , Thomas Stober a a b
Department of Prosthodontics, Heidelberg University Hospital, Heidelberg, Germany Department for Soil Mechanics, Karlsruhe Institute of Technology, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Chewing simulators are used for preclinical evaluation of newly developed den-
Received 17 November 2010
tal restorative materials. To guarantee the independence of test conditions, contact forces
Received in revised form
during chewing simulation should be independent of the specimen. Because of its mode of
20 April 2011
operation, i.e., impact of an antagonist, this requirement is not met for a widely used chew-
Accepted 15 September 2011
ing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany). This study was therefore intended to clarify the extent to which specimen stiffness affects maximum contact force at different impact velocities. Possible differences between the forces in the eight
Keywords:
test chambers were also of interest.
Chewing simulation
Methods. From each of five dental materials differing in Young’s modulus, eight cylindri-
Simulation device
cal disks were manufactured and embedded in specimen holders. Alumina spheres were
In vitro simulation
used as antagonists. During chewing simulations with different impact velocities and den-
Wear simulation
tal materials, vertical acceleration was recorded and contact forces were estimated on the
Dental materials
basis of these measurements.
Specimen stiffness
Results. Specimen stiffness and impact velocity had a substantial effect on maximum contact
Contact force
force. The force overshoot relative to the static load ranged from 4% for small specimen stiffness and low impact velocity to values greater than 200% for high specimen stiffness and high impact velocity. Large differences between the chambers were also detected. Significance. Weight-controlled chewing simulations should be performed either with a low impact velocity or with a spring-damper system (placed between mass and specimen) which efficiently reduces the effects of contact force variation. Influence of specimen stiffness on contact forces must be considered at data interpretation. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Chewing simulators are used for preclinical wear [1–3] and fatigue [4,5] testing or to simulate aging before an ultimate load test [6]. According to FDA guidelines on good laboratory practice, most commonly used wear simulators have
shortcomings in terms of control and regulation of force development [7–10]. This could be one reason for the poor reproducibility and great variation of wear-testing results [8]. For standardized testing and to achieve comparable results it is essential that the magnitudes of the forces generated by chewing simulators are exclusively defined by the test conditions, and that these are the same for every specimen.
∗ Corresponding author at: Department of Prosthodontics, Heidelberg University Hospital, Im Neuenheimer Feld 400, D-69120 Heidelberg, Germany. Tel.: +49 6221 566069; fax: +49 6221 565371. E-mail address: [email protected] (S. Rues). 0109-5641/$ – see front matter © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2011.09.007
1268
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Fig. 1 – The Willytec chewing simulator. Fig. 2 – Cylindrical disk (2 mm height, 8 mm diameter) embedded in the specimen holder with Technovit. The Willytec chewing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany) is a widely used weightcontrolled chewing-simulation device [2,8]. Although it has been reported that the forces generated by this chewing simulator are not well defined by the applied weights and are affected by the speed of descent of the countersample, this device seems to be a good compromise with regard to practicability, cost, and reliability [8,9]. Another factor which may possibly effect contact forces in this and other chewingsimulation devices, has so far been neglected. Because of the dynamic mode of operation of this chewing simulator, we hypothesize that the contact force during the impact phase is not solely given by the test conditions (mass and velocity of the countersample), but also depends on the stiffness of the specimen. The objective of this study was, therefore, to investigate the effect of the specimen’s stiffness on the maximum contact forces for different test conditions (mass and velocity). Because a series of eight specimens can be tested simultaneously in the Willytec chewing simulator, possible differences between the test chambers of the chewing simulator were also of interest.
2.
Materials and methods
The mode of operation of the Willytec chewing simulator (Fig. 1) seems to be simple, and has been described in detail elsewhere [8,9]. A rod loaded with extra weights (total mass m) and an antagonist at the lower end is moved up and down by a crossbar. In our tests, the downward movement consisted of an acceleration on a distance of 2 mm, 4 mm way with a defined velocity v, and a deceleration on a distance of 2 mm. The impact of the antagonist on the specimen occurred at about half distance, i.e. during the phase with constant velocity. After the impact, rod and extra mass rest on the specimen as the crossbar moves further down. One cycle is completed when the antagonist is lifted from the specimen during the upward movement of the crossbar. Human enamel as well as four commonly used restorative dental materials differing in Young’s modulus were used to perform chewing simulation (Table 1). For each restorative material, eight cylindrical disks (diameter 8 mm, height 2 mm, surface roughness Ra < 1 m) were manufactured. For human enamel, almost planar sections of the buccal side of molars
were used and trimmed approximately (deviations for height and diameter ±0.3 mm) to the same dimensions as the disks made of restorative dental materials. The enamel surface was not changed, i.e., grinding took place on all sides except the former tooth surface. All specimens were embedded in the specimen holders (Fig. 2) with methacrylate (Technovit; Heraeus Kulzer, Hanau, Germany). Alumina spheres with a diameter of 4.5 mm and surface roughness Ra = 0.2 m (Degussit; Friatec, Mannheim, Germany) were used as antagonists. These alumina spheres are also used in in vitro wear tests at our department. Force measurement with a very stiff sensor, placed between mass and specimen, would be a direct and accurate approach to measure the contact forces. However, if only one force sensor is available, the eight chambers have to be measured separately and due to the height of the force sensor only the investigated chamber could be loaded. To enable observation of the chewing simulator in its original state, i.e., no force sensor between mass and specimen, no readjustment when moving the sensor to the next chamber, and loading of all eight chambers simultaneously, the contact forces were not determined directly. Instead acceleration in the vertical direction (positive values in the direction of gravity) was measured at the upper ends of the rods. Preliminary tests showed that (a) periodic behavior occurred after 5–10 starting cycles, i.e., within each chamber a reproducible acceleration signal (<1% deviation of the signal magnitudes, identical shape) was measured for each impact, and (b) the recorded signal could be accurately reproduced after removing the sensor and placing it on the rod once more. With two accelerometers (B&K type 4383; Brüel & Kjaer GmbH, Bremen, Germany), the acceleration caused by impacts of a mass m = 5 kg moving with velocities v = 8, 15, 30, and 60 mm/s was measured for (1) all test chambers with the stiffest specimens, i.e., disks made of alumina, and for (2) the two test chambers showing the greatest deviation in (1) with the other specimens. In all tests, 100 cycles were performed and accelerations were recorded after 20 starting cycles to guarantee measurement of reproducible signals. The eigenfrequency of the accelerometers fixed to the rods was 20 kHz, thus stating the upper limit for accurate signal recording. Therefore, the output signal was recorded with
1269
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Table 1 – Young’s moduli of the five disk materials. Material Composite resin veneering material Human enamel Gold alloy Zirconia Alumina a b
Brand
Manufacturer
Sinfony – Degulor M Cercon base In-Ceram AL
3M-ESPE, Seefeld, Germany – DeguDent, Hanau, Germany DeguDent, Hanau, Germany VITA, Bad Säckingen, Germany
Young’s modulus [GPa] 3.1a ∼80b 100a 210a 380a
Values provided by the manufacturer. According to [12].
a resolution of 5 s (corresponds to a sampling frequency of 200 kHz). In (1) the first accelerometer stayed on the rod of test chamber 1 (reference) and the second accelerometer was placed on the rods of test chamber 2 to test chamber 8, one after another. Therefore, all signals could be arranged on the same timescale with the acceleration signal of test chamber 1 as reference. Estimation of the contact forces on the basis of the measured acceleration is explained in the following. Rigid body movement of the complete rod can be assumed as long as the length of the steel rod is small relative to the wavelength associated with the actual vibration frequency f. With a rod of length L = 0.4 m and a longitudinal wave speed c = 5400 m/s this assumption is valid for wavelengths = c/f > 10L, i.e., frequencies f < c/(10L) = 1350 Hz. The contact force between antagonist and specimen, which is the only constraining force, is then given by F = −ma for rigid body movement. The measured acceleration signals were, therefore, low-pass-filtered at a frequency of 1000 Hz (<1350 Hz). The maximum contact force was therefore estimated as Fmax = −malf,max , where alf,max is the magnitude of the low-pass-filtered acceleration signal. This estimate is valid as long as the lower frequencies are dominant, i.e., higher frequencies can be neglected without causing a great error in contact force estimation. For this reason, the frequency spectrum of each acceleration signal was analyzed. With increasing high frequency part (which is
neglected), an increasing error will be implemented in our estimation. Since the influence of specimen stiffness and impact velocity were of interest, linear regression (SPSS 18.0, SPSS Inc., USA) was performed to test the dependence of relative force overshoot on Young’s modulus and impact velocity.
3.
Results
In general, increasing specimen stiffness and impact velocity caused an increase in estimated contact forces, test chamber differences, and acceleration magnitudes associated with high frequencies. With the alumina specimens, results from test chambers 1 (leftmost) and 4 were the most different. Therefore, acceleration in combination with the other materials was only recorded for these two test chambers. High impact velocities, i.e., v = 60 mm/s, caused bouncing (multiple impacts in one cycle) even on the most resilient specimens. For the stiffest specimens (alumina), this bouncing of the antagonist occurred even for low impact velocities in some test chambers. Fig. 3 shows the synchronized acceleration signals for all test chambers when v = 30 mm/s was used. The impact sequence for each material did not change with varying impact velocity, but the time intervals were proportional to
Fig. 3 – Synchronized vertical acceleration signals (unfiltered, positive in the direction of gravity) for all eight test chambers.
1270
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Table 2 – Calculated relative force overshoot based on measured and low-pass-filtered (1000 Hz) acceleration. Since signals were reproducible (coefficient of variation cv < 1%), the exact deviations are not listed. Values in parentheses show tendencies of the estimated forces, only, because frequencies >1000 Hz are dominant. v [mm/s]
Chamber
Relative force overshoot (Fmax – mg)/mg [%] Sinfony
8 15 30
1 4 1 4 1 4
4 16 17 39 35 114
Enamel
Degulor M
25 22 46 43 77 113
1/v. It is, therefore, most likely that adjustment inaccuracies of approximately ±0.3 mm caused this effect. Acceleration varied substantially, i.e., magnitude ratios >2 both for different test chambers and for different materials (Fig. 4) could be observed. Furthermore, the acceleration signals consisted of a ‘carrier wave’ (low frequency part) and a superposed high frequency signal. When analysing the frequency spectrum, the most prominent high-frequency peak was always associated with a frequency f = 6640 Hz. The relative force overshoot, i.e., the difference between maximum contact force and static force relative to the static force, is given in Table 2 for test chambers 1 and 4, different impact velocities, and different materials. For the alumina specimens, the estimated force was subject to substantial error because of bouncing and pronounced high frequencies in the acceleration signal. These values must therefore be regarded with caution. Linear regression showed a fair dependence of the relative force overshoot on the Young’s modulus of the specimen (Fig. 5a, 0.361 < R2 < 0.760) and a clear correlation between relative force overshoot and impact velocity (Fig. 5b, 0.517 < R2 < 0.970). Since there exists no force overshoot for static loading (impact velocity v = 0 mm/s), lines through the origin were used for linear regression in this case.
Cercon base
19 23 42 32 51 136
32 30 59 67 149 130
In-Ceram AL (16) (37) (26) (57) (63) (156)
Finally, a positive result was obtained: All signals were reproducible after 10 starting cycles, indicating a (quasi-) periodic state during material testing with the Willytec chewing simulator.
4.
Discussion
This study revealed a substantial effect of specimen stiffness on contact forces which confirms the results of a study by Conserva et al. [10]. But also the impact velocity correlates with the relative force overshoot, thus confirming the hypothesis we stated before the tests. Furthermore, large differences between the eight test chambers were found. Besides specimen stiffness, the density of the specimen may also affect the acceleration, especially at high frequencies. As can be seen in Fig. 4, the unfiltered acceleration signals for gold alloy show higher magnitudes for high frequencies than those for enamel. This could explain the slight decrease in the estimated force overshoot from enamel to gold alloy (Fig. 5a and b) due to a higher error in force estimation. In addition, inaccuracies (less than 0.3 mm) during adjustment of the antagonists to the height of the specimen caused a sequence of impacts in the eight test chambers within a time span >10 ms clearly
Fig. 4 – Measured vertical acceleration (unfiltered, positive in the direction of gravity) for chambers 1 and 4 directly after impact at a velocity of v = 15 mm/s on disks made of five different materials. Each interval on the horizontal time axes is 6 ms.
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Fig. 5 – Effect of Young’s modulus (a) and impact velocity (b) on the force overshoot relative to the static load.
exceeding the impact times (<5 ms). These differences will not be the same for different test series (with eight specimens each) because the distribution of adjustment inaccuracies and, therefore, the impact sequence, cannot be reproduced but will differ from one test series to another. The effect of this on contact force differences between the chambers cannot be resolved in this study, however. Other studies have also reported the problems of differences between the test chambers and a force overshoot depending on the weight of the mass chosen [9]. The impact velocity was not explicitly measured in our test setup. However, the manufacturer states
1271
mean deviations <2% and maximum deviations <5% of the defined velocity. With regards to the high differences between the chosen impact velocities, these possible inaccuracies state no problem for our findings. Direct force measurement would usually be the best method for determining the contact forces and will be performed in further studies to confirm the results gained from this investigation. Measurement of acceleration at the upper end of the rod and subsequent estimation of the contact forces was, however, deliberately chosen because a force sensor would have to be placed directly behind the antagonist, resulting in a change of the set-up and possibly in a change of its dynamic reaction. It would also have been necessary to readjust the antagonists each time the force sensor was switched from one test chamber to another. The low-frequency part (f < 1000 Hz) used in estimation of this force could have been extracted from measurements at any point of the rod (quasirigid body movement). Because of this, it is no problem that acceleration was measured at the upper ends of the rods. The prominent high frequency of 6640 Hz was found in all measurements and corresponds to a resonance frequency: half of the wavelength (approximately 0.8 m) equals the length of the steel rod. The vertical movement investigated in this study is associated with fatigue testing. During wear testing vertical and horizontal forces acting on the specimen during the additional horizontal movement are also of interest. Motor vibrations may also lead to forced oscillations during the horizontal movement. On the basis of this consideration, we assume that vertical and horizontal contact forces during wear testing will also depend on the specimen stiffness, and thus influence the result of wear tests. However, further studies have to be performed to clarify this issue. Validation of dental material testing has been discussed in recent years, e.g., by Heintze [7,8], because results of numerous test methods resulted in large coefficients of variation. When the Willytec chewing simulator is used, part of this variation is probably caused by fluctuations in contact force over the eight test chambers. The comparability of wear or fatigue tests of specimens differing in stiffness, e.g. disks made of composite or ceramics, is also questionable with this arrangement. A limitation of this study is that the observed relative force overshoots are only valid for the weight-controlled chewing simulator used, and are, in general, not transferable to chewing simulator devices using other force-generating principles [10,11]. As a consequence, an additional device for the Willytec chewing simulator is proposed which reduces dynamic effects, and which enables prediction of maximum contact forces on the basis of the test conditions.
5.
Conclusion
In the Willytec chewing simulator, both impact velocity and specimen stiffness have large effects on the force overshoot relative to the static load, i.e., the stiffer the specimen and the higher the impact velocity, the higher the force overshoot in the experiments. High impact velocities (v = 60 mm/s) always
1272
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
led to several impacts (“bouncing”) in one chewing cycle. Influence of specimen stiffness on contact forces must be considered at data interpretation. Low impact velocities should be favored, especially when stiff specimens are loaded. High impact velocities can be recommended only when dynamic effects are drastically reduced by an additional device, e.g., a spring-damper system. In the actual configuration, very different measured acceleration signals were obtained from the different test chambers of the chewing simulator. Because of adjustment inaccuracies of less than ±0.3 mm, there is an impact sequence in the eight test chambers which may contribute to chamber differences.
Acknowledgments We thank Ian Davies, copy editor, for English-language revision and Clemens Schmitt for assistance with the measurements.
references
[1] Ghazal M, Yang B, Ludwig K, Kern M. Two-body wear of resin and ceramic denture teeth in comparison to human enamel. Dent Mater 2008;24:502–7. [2] Lambrechts P, Debels E, Van Landuyt K, Peumans M, Van Meerbeek B. How to simulate wear? Overview of existing methods. Dent Mater 2006;22:693–701.
[3] Stober T, Lutz T, Gilde H, Rammelsberg P. Wear of resin denture teeth by two-body contact. Dent Mater 2006;22:243–9. [4] Blatz MB, Oppes S, Chiche G, Holst S, Sadan A. Influence of cementation technique on fracture strength and leakage of alumina all-ceramic crowns after cyclic loading. Quintessence Int 2008;39:23–32. [5] Naumann M, Metzdorf G, Fokkinga W, Watzke R, Sterzenbach G, Bayne S, et al. Influence of test parameters on in vitro fracture resistance of post-endodontic restorations: a structured review. J Oral Rehabil 2009;36:299–312. [6] Ohlmann B, Marienburg K, Gabbert O, Hassel A, Gilde H, Rammelsberg P. Fracture-load values of all-ceramic cantilevered FPDs with different framework designs. Int J Prosthodont 2009;22:49–52. [7] Heintze SD, Cavalleri A, Forjanic M, Zellweger G, Rousson V. Wear of ceramic and antagonist – a systematic evaluation of influencing factors in vitro. Dent Mater 2008;24:433–49. [8] Heintze SD. How to qualify and validate wear simulation devices and methods. Dent Mater 2006;22:712–34. [9] Steiner M, Mitsias ME, Ludwig K, Kern M. In vitro evaluation of a mechanical testing chewing simulator. Dent Mater 2009;25:494–9. [10] Conserva E, Menini M, Tealdo T, Bevilacqua M, Pera F, Ravera G, et al. Robotic chewing simulator for dental materials testing on a sensor-equipped implant setup. Int J Prosthodont 2008;21:501–8. [11] Rosentritt M, Siavikis G, Behr M, Kolbeck C, Handel G. Approach for valuating the significance of laboratory simulation. J Dent 2008;36:1048–53. [12] Huang TTY, He LH, Darendeliler MA, Swain MV. Nano-indentation characterisation of natural carious white spot lesions. Caries Res 2010;44:101–7.
Available online at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Effect of impact velocity and specimen stiffness on contact forces in a weight-controlled chewing simulator Stefan Rues a,∗ , Gerhard Huber b , Peter Rammelsberg a , Thomas Stober a a b
Department of Prosthodontics, Heidelberg University Hospital, Heidelberg, Germany Department for Soil Mechanics, Karlsruhe Institute of Technology, Germany
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Chewing simulators are used for preclinical evaluation of newly developed den-
Received 17 November 2010
tal restorative materials. To guarantee the independence of test conditions, contact forces
Received in revised form
during chewing simulation should be independent of the specimen. Because of its mode of
20 April 2011
operation, i.e., impact of an antagonist, this requirement is not met for a widely used chew-
Accepted 15 September 2011
ing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany). This study was therefore intended to clarify the extent to which specimen stiffness affects maximum contact force at different impact velocities. Possible differences between the forces in the eight
Keywords:
test chambers were also of interest.
Chewing simulation
Methods. From each of five dental materials differing in Young’s modulus, eight cylindri-
Simulation device
cal disks were manufactured and embedded in specimen holders. Alumina spheres were
In vitro simulation
used as antagonists. During chewing simulations with different impact velocities and den-
Wear simulation
tal materials, vertical acceleration was recorded and contact forces were estimated on the
Dental materials
basis of these measurements.
Specimen stiffness
Results. Specimen stiffness and impact velocity had a substantial effect on maximum contact
Contact force
force. The force overshoot relative to the static load ranged from 4% for small specimen stiffness and low impact velocity to values greater than 200% for high specimen stiffness and high impact velocity. Large differences between the chambers were also detected. Significance. Weight-controlled chewing simulations should be performed either with a low impact velocity or with a spring-damper system (placed between mass and specimen) which efficiently reduces the effects of contact force variation. Influence of specimen stiffness on contact forces must be considered at data interpretation. © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Chewing simulators are used for preclinical wear [1–3] and fatigue [4,5] testing or to simulate aging before an ultimate load test [6]. According to FDA guidelines on good laboratory practice, most commonly used wear simulators have
shortcomings in terms of control and regulation of force development [7–10]. This could be one reason for the poor reproducibility and great variation of wear-testing results [8]. For standardized testing and to achieve comparable results it is essential that the magnitudes of the forces generated by chewing simulators are exclusively defined by the test conditions, and that these are the same for every specimen.
∗ Corresponding author at: Department of Prosthodontics, Heidelberg University Hospital, Im Neuenheimer Feld 400, D-69120 Heidelberg, Germany. Tel.: +49 6221 566069; fax: +49 6221 565371. E-mail address: [email protected] (S. Rues). 0109-5641/$ – see front matter © 2011 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2011.09.007
1268
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Fig. 1 – The Willytec chewing simulator. Fig. 2 – Cylindrical disk (2 mm height, 8 mm diameter) embedded in the specimen holder with Technovit. The Willytec chewing simulator (Willytec/SD Mechatronik, Feldkirchen-Westerham, Germany) is a widely used weightcontrolled chewing-simulation device [2,8]. Although it has been reported that the forces generated by this chewing simulator are not well defined by the applied weights and are affected by the speed of descent of the countersample, this device seems to be a good compromise with regard to practicability, cost, and reliability [8,9]. Another factor which may possibly effect contact forces in this and other chewingsimulation devices, has so far been neglected. Because of the dynamic mode of operation of this chewing simulator, we hypothesize that the contact force during the impact phase is not solely given by the test conditions (mass and velocity of the countersample), but also depends on the stiffness of the specimen. The objective of this study was, therefore, to investigate the effect of the specimen’s stiffness on the maximum contact forces for different test conditions (mass and velocity). Because a series of eight specimens can be tested simultaneously in the Willytec chewing simulator, possible differences between the test chambers of the chewing simulator were also of interest.
2.
Materials and methods
The mode of operation of the Willytec chewing simulator (Fig. 1) seems to be simple, and has been described in detail elsewhere [8,9]. A rod loaded with extra weights (total mass m) and an antagonist at the lower end is moved up and down by a crossbar. In our tests, the downward movement consisted of an acceleration on a distance of 2 mm, 4 mm way with a defined velocity v, and a deceleration on a distance of 2 mm. The impact of the antagonist on the specimen occurred at about half distance, i.e. during the phase with constant velocity. After the impact, rod and extra mass rest on the specimen as the crossbar moves further down. One cycle is completed when the antagonist is lifted from the specimen during the upward movement of the crossbar. Human enamel as well as four commonly used restorative dental materials differing in Young’s modulus were used to perform chewing simulation (Table 1). For each restorative material, eight cylindrical disks (diameter 8 mm, height 2 mm, surface roughness Ra < 1 m) were manufactured. For human enamel, almost planar sections of the buccal side of molars
were used and trimmed approximately (deviations for height and diameter ±0.3 mm) to the same dimensions as the disks made of restorative dental materials. The enamel surface was not changed, i.e., grinding took place on all sides except the former tooth surface. All specimens were embedded in the specimen holders (Fig. 2) with methacrylate (Technovit; Heraeus Kulzer, Hanau, Germany). Alumina spheres with a diameter of 4.5 mm and surface roughness Ra = 0.2 m (Degussit; Friatec, Mannheim, Germany) were used as antagonists. These alumina spheres are also used in in vitro wear tests at our department. Force measurement with a very stiff sensor, placed between mass and specimen, would be a direct and accurate approach to measure the contact forces. However, if only one force sensor is available, the eight chambers have to be measured separately and due to the height of the force sensor only the investigated chamber could be loaded. To enable observation of the chewing simulator in its original state, i.e., no force sensor between mass and specimen, no readjustment when moving the sensor to the next chamber, and loading of all eight chambers simultaneously, the contact forces were not determined directly. Instead acceleration in the vertical direction (positive values in the direction of gravity) was measured at the upper ends of the rods. Preliminary tests showed that (a) periodic behavior occurred after 5–10 starting cycles, i.e., within each chamber a reproducible acceleration signal (<1% deviation of the signal magnitudes, identical shape) was measured for each impact, and (b) the recorded signal could be accurately reproduced after removing the sensor and placing it on the rod once more. With two accelerometers (B&K type 4383; Brüel & Kjaer GmbH, Bremen, Germany), the acceleration caused by impacts of a mass m = 5 kg moving with velocities v = 8, 15, 30, and 60 mm/s was measured for (1) all test chambers with the stiffest specimens, i.e., disks made of alumina, and for (2) the two test chambers showing the greatest deviation in (1) with the other specimens. In all tests, 100 cycles were performed and accelerations were recorded after 20 starting cycles to guarantee measurement of reproducible signals. The eigenfrequency of the accelerometers fixed to the rods was 20 kHz, thus stating the upper limit for accurate signal recording. Therefore, the output signal was recorded with
1269
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Table 1 – Young’s moduli of the five disk materials. Material Composite resin veneering material Human enamel Gold alloy Zirconia Alumina a b
Brand
Manufacturer
Sinfony – Degulor M Cercon base In-Ceram AL
3M-ESPE, Seefeld, Germany – DeguDent, Hanau, Germany DeguDent, Hanau, Germany VITA, Bad Säckingen, Germany
Young’s modulus [GPa] 3.1a ∼80b 100a 210a 380a
Values provided by the manufacturer. According to [12].
a resolution of 5 s (corresponds to a sampling frequency of 200 kHz). In (1) the first accelerometer stayed on the rod of test chamber 1 (reference) and the second accelerometer was placed on the rods of test chamber 2 to test chamber 8, one after another. Therefore, all signals could be arranged on the same timescale with the acceleration signal of test chamber 1 as reference. Estimation of the contact forces on the basis of the measured acceleration is explained in the following. Rigid body movement of the complete rod can be assumed as long as the length of the steel rod is small relative to the wavelength associated with the actual vibration frequency f. With a rod of length L = 0.4 m and a longitudinal wave speed c = 5400 m/s this assumption is valid for wavelengths = c/f > 10L, i.e., frequencies f < c/(10L) = 1350 Hz. The contact force between antagonist and specimen, which is the only constraining force, is then given by F = −ma for rigid body movement. The measured acceleration signals were, therefore, low-pass-filtered at a frequency of 1000 Hz (<1350 Hz). The maximum contact force was therefore estimated as Fmax = −malf,max , where alf,max is the magnitude of the low-pass-filtered acceleration signal. This estimate is valid as long as the lower frequencies are dominant, i.e., higher frequencies can be neglected without causing a great error in contact force estimation. For this reason, the frequency spectrum of each acceleration signal was analyzed. With increasing high frequency part (which is
neglected), an increasing error will be implemented in our estimation. Since the influence of specimen stiffness and impact velocity were of interest, linear regression (SPSS 18.0, SPSS Inc., USA) was performed to test the dependence of relative force overshoot on Young’s modulus and impact velocity.
3.
Results
In general, increasing specimen stiffness and impact velocity caused an increase in estimated contact forces, test chamber differences, and acceleration magnitudes associated with high frequencies. With the alumina specimens, results from test chambers 1 (leftmost) and 4 were the most different. Therefore, acceleration in combination with the other materials was only recorded for these two test chambers. High impact velocities, i.e., v = 60 mm/s, caused bouncing (multiple impacts in one cycle) even on the most resilient specimens. For the stiffest specimens (alumina), this bouncing of the antagonist occurred even for low impact velocities in some test chambers. Fig. 3 shows the synchronized acceleration signals for all test chambers when v = 30 mm/s was used. The impact sequence for each material did not change with varying impact velocity, but the time intervals were proportional to
Fig. 3 – Synchronized vertical acceleration signals (unfiltered, positive in the direction of gravity) for all eight test chambers.
1270
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Table 2 – Calculated relative force overshoot based on measured and low-pass-filtered (1000 Hz) acceleration. Since signals were reproducible (coefficient of variation cv < 1%), the exact deviations are not listed. Values in parentheses show tendencies of the estimated forces, only, because frequencies >1000 Hz are dominant. v [mm/s]
Chamber
Relative force overshoot (Fmax – mg)/mg [%] Sinfony
8 15 30
1 4 1 4 1 4
4 16 17 39 35 114
Enamel
Degulor M
25 22 46 43 77 113
1/v. It is, therefore, most likely that adjustment inaccuracies of approximately ±0.3 mm caused this effect. Acceleration varied substantially, i.e., magnitude ratios >2 both for different test chambers and for different materials (Fig. 4) could be observed. Furthermore, the acceleration signals consisted of a ‘carrier wave’ (low frequency part) and a superposed high frequency signal. When analysing the frequency spectrum, the most prominent high-frequency peak was always associated with a frequency f = 6640 Hz. The relative force overshoot, i.e., the difference between maximum contact force and static force relative to the static force, is given in Table 2 for test chambers 1 and 4, different impact velocities, and different materials. For the alumina specimens, the estimated force was subject to substantial error because of bouncing and pronounced high frequencies in the acceleration signal. These values must therefore be regarded with caution. Linear regression showed a fair dependence of the relative force overshoot on the Young’s modulus of the specimen (Fig. 5a, 0.361 < R2 < 0.760) and a clear correlation between relative force overshoot and impact velocity (Fig. 5b, 0.517 < R2 < 0.970). Since there exists no force overshoot for static loading (impact velocity v = 0 mm/s), lines through the origin were used for linear regression in this case.
Cercon base
19 23 42 32 51 136
32 30 59 67 149 130
In-Ceram AL (16) (37) (26) (57) (63) (156)
Finally, a positive result was obtained: All signals were reproducible after 10 starting cycles, indicating a (quasi-) periodic state during material testing with the Willytec chewing simulator.
4.
Discussion
This study revealed a substantial effect of specimen stiffness on contact forces which confirms the results of a study by Conserva et al. [10]. But also the impact velocity correlates with the relative force overshoot, thus confirming the hypothesis we stated before the tests. Furthermore, large differences between the eight test chambers were found. Besides specimen stiffness, the density of the specimen may also affect the acceleration, especially at high frequencies. As can be seen in Fig. 4, the unfiltered acceleration signals for gold alloy show higher magnitudes for high frequencies than those for enamel. This could explain the slight decrease in the estimated force overshoot from enamel to gold alloy (Fig. 5a and b) due to a higher error in force estimation. In addition, inaccuracies (less than 0.3 mm) during adjustment of the antagonists to the height of the specimen caused a sequence of impacts in the eight test chambers within a time span >10 ms clearly
Fig. 4 – Measured vertical acceleration (unfiltered, positive in the direction of gravity) for chambers 1 and 4 directly after impact at a velocity of v = 15 mm/s on disks made of five different materials. Each interval on the horizontal time axes is 6 ms.
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
Fig. 5 – Effect of Young’s modulus (a) and impact velocity (b) on the force overshoot relative to the static load.
exceeding the impact times (<5 ms). These differences will not be the same for different test series (with eight specimens each) because the distribution of adjustment inaccuracies and, therefore, the impact sequence, cannot be reproduced but will differ from one test series to another. The effect of this on contact force differences between the chambers cannot be resolved in this study, however. Other studies have also reported the problems of differences between the test chambers and a force overshoot depending on the weight of the mass chosen [9]. The impact velocity was not explicitly measured in our test setup. However, the manufacturer states
1271
mean deviations <2% and maximum deviations <5% of the defined velocity. With regards to the high differences between the chosen impact velocities, these possible inaccuracies state no problem for our findings. Direct force measurement would usually be the best method for determining the contact forces and will be performed in further studies to confirm the results gained from this investigation. Measurement of acceleration at the upper end of the rod and subsequent estimation of the contact forces was, however, deliberately chosen because a force sensor would have to be placed directly behind the antagonist, resulting in a change of the set-up and possibly in a change of its dynamic reaction. It would also have been necessary to readjust the antagonists each time the force sensor was switched from one test chamber to another. The low-frequency part (f < 1000 Hz) used in estimation of this force could have been extracted from measurements at any point of the rod (quasirigid body movement). Because of this, it is no problem that acceleration was measured at the upper ends of the rods. The prominent high frequency of 6640 Hz was found in all measurements and corresponds to a resonance frequency: half of the wavelength (approximately 0.8 m) equals the length of the steel rod. The vertical movement investigated in this study is associated with fatigue testing. During wear testing vertical and horizontal forces acting on the specimen during the additional horizontal movement are also of interest. Motor vibrations may also lead to forced oscillations during the horizontal movement. On the basis of this consideration, we assume that vertical and horizontal contact forces during wear testing will also depend on the specimen stiffness, and thus influence the result of wear tests. However, further studies have to be performed to clarify this issue. Validation of dental material testing has been discussed in recent years, e.g., by Heintze [7,8], because results of numerous test methods resulted in large coefficients of variation. When the Willytec chewing simulator is used, part of this variation is probably caused by fluctuations in contact force over the eight test chambers. The comparability of wear or fatigue tests of specimens differing in stiffness, e.g. disks made of composite or ceramics, is also questionable with this arrangement. A limitation of this study is that the observed relative force overshoots are only valid for the weight-controlled chewing simulator used, and are, in general, not transferable to chewing simulator devices using other force-generating principles [10,11]. As a consequence, an additional device for the Willytec chewing simulator is proposed which reduces dynamic effects, and which enables prediction of maximum contact forces on the basis of the test conditions.
5.
Conclusion
In the Willytec chewing simulator, both impact velocity and specimen stiffness have large effects on the force overshoot relative to the static load, i.e., the stiffer the specimen and the higher the impact velocity, the higher the force overshoot in the experiments. High impact velocities (v = 60 mm/s) always
1272
d e n t a l m a t e r i a l s 2 7 ( 2 0 1 1 ) 1267–1272
led to several impacts (“bouncing”) in one chewing cycle. Influence of specimen stiffness on contact forces must be considered at data interpretation. Low impact velocities should be favored, especially when stiff specimens are loaded. High impact velocities can be recommended only when dynamic effects are drastically reduced by an additional device, e.g., a spring-damper system. In the actual configuration, very different measured acceleration signals were obtained from the different test chambers of the chewing simulator. Because of adjustment inaccuracies of less than ±0.3 mm, there is an impact sequence in the eight test chambers which may contribute to chamber differences.
Acknowledgments We thank Ian Davies, copy editor, for English-language revision and Clemens Schmitt for assistance with the measurements.
references
[1] Ghazal M, Yang B, Ludwig K, Kern M. Two-body wear of resin and ceramic denture teeth in comparison to human enamel. Dent Mater 2008;24:502–7. [2] Lambrechts P, Debels E, Van Landuyt K, Peumans M, Van Meerbeek B. How to simulate wear? Overview of existing methods. Dent Mater 2006;22:693–701.
[3] Stober T, Lutz T, Gilde H, Rammelsberg P. Wear of resin denture teeth by two-body contact. Dent Mater 2006;22:243–9. [4] Blatz MB, Oppes S, Chiche G, Holst S, Sadan A. Influence of cementation technique on fracture strength and leakage of alumina all-ceramic crowns after cyclic loading. Quintessence Int 2008;39:23–32. [5] Naumann M, Metzdorf G, Fokkinga W, Watzke R, Sterzenbach G, Bayne S, et al. Influence of test parameters on in vitro fracture resistance of post-endodontic restorations: a structured review. J Oral Rehabil 2009;36:299–312. [6] Ohlmann B, Marienburg K, Gabbert O, Hassel A, Gilde H, Rammelsberg P. Fracture-load values of all-ceramic cantilevered FPDs with different framework designs. Int J Prosthodont 2009;22:49–52. [7] Heintze SD, Cavalleri A, Forjanic M, Zellweger G, Rousson V. Wear of ceramic and antagonist – a systematic evaluation of influencing factors in vitro. Dent Mater 2008;24:433–49. [8] Heintze SD. How to qualify and validate wear simulation devices and methods. Dent Mater 2006;22:712–34. [9] Steiner M, Mitsias ME, Ludwig K, Kern M. In vitro evaluation of a mechanical testing chewing simulator. Dent Mater 2009;25:494–9. [10] Conserva E, Menini M, Tealdo T, Bevilacqua M, Pera F, Ravera G, et al. Robotic chewing simulator for dental materials testing on a sensor-equipped implant setup. Int J Prosthodont 2008;21:501–8. [11] Rosentritt M, Siavikis G, Behr M, Kolbeck C, Handel G. Approach for valuating the significance of laboratory simulation. J Dent 2008;36:1048–53. [12] Huang TTY, He LH, Darendeliler MA, Swain MV. Nano-indentation characterisation of natural carious white spot lesions. Caries Res 2010;44:101–7.