Procedia Engineering 101 ( 2015 ) 413 – 420

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1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Czech Society for Mechanicsdoi: 10.1016/j.proeng.2015.02.050

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3rd International Conference on Material and Component Performance under Variable Amplitude Loading, VAL2015

Study of the fatigue behavior of the polychloroprene rubber with stress variation tests

Berton, G., Cruanes, C., Lacroix, F., Méo, S., and Ranganathan, N. * Laboratoire de Mécanique et Rhéologie, Université François Rabelais de Tours, Polytech Tours, 7 avenue Marcel Dassault 37200 Tours, France

Abstract

In this study we study the fatigue behavior of a polychloroprene rubber using designed specific variable amplitude tests to gather insight into such material behavior. Firstly, increasing force amplitude block load tests were carried out that permits us to determine the stress amplitude at which fatigue damage is significant. In a second series of tests block programmed tests were carried out. During these tests the hysteresis energy and stiffness were also measured. These measurements bring out possibly a competition between two mechanisms the crystallization effect and the effect of crack propagation. The first mechanism tends to increase the stiffness while the second decreases the stiffness.

© 2015 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Czech Society for Mechanics.

Keywords: Rubber, Fatigue Behavior, Strain-Induced Crystallization,block load tests, stengtehening mechanism

1. Introduction

Rubber and rubbery materials are widely used in industry because of their ability to undergo large deformation and damp energy. Therefore, knowledge of the mechanical characteristics and, in particular, the fatigue behaviour is a very active topic of research.

* Corresponding author. Tel.: +33-24-736-1336; fax: .: +33-24-736-1311.

E-mail address:ranganathan@univ-tours.fr

© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the Czech Society for Mechanics

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For the past sixty years researchers have looked for fatigue criteria in order to shorten experimental fatigue campaigns. Lately, energetic criteria have been investigated coupling several techniques (Le Saux et al. 2010 [1], Ayoub et al. 2012 [2]) or focusing on the study of a single parameter (Mars 2001 [3], 2002 [4] with, for example, Cracking Energy Density). Lacroix 2004 [5] and then Poisson et al. 2011 [6] have been working with the hysteresis energy and their crack initiation approach provided good results regarding fatigue life predictions of a polychloroprene rubber. These authors [6] developed a Haigh diagram for a polychloroprene rubber (fig 1) and observed that below a force ratio of R=0.2, fatigue lives decrease with increase of R-ratio, whereas above this threshold value, the fatigue lives increase clearly: fatigue lives at R=0.5 are more than 10 times greater than those at R=0.2. The suggested mechanism is possible strain-induced crystallization of the polychloroprene rubber that influences the fatigue life above R=0.2 [7]. Polychloroprene rubber is known for being subjected to strain-induced crystallization (SiC) and it was shown by (Le Cam 2008 [8]), for example, that when cracks occur in such an elastomer there is competition between crack propagation and the transformation due to strain-induced crystallization (SIC).

Fig. 1. Haigh Diagram after Poisson et al. [6]

The aim of this study was to investigate hysteresis energy and stiffness evolution during carefully programmed block force tests in order to gain insight into the damage evolution in an elastomer. The outline of the present paper is as follows: Section 2 presents the details of the experimental procedure; Section 3 describes and discusses the block program tests. In Section 4, a short discussion is presented and Section 5 focuses on conclusion and perspectives.

2: Experimental protocol, material and specimen

2.1.Specimen, material and fatigue tests

The material studied during this research was a vulcanized polychloroprene rubber (CR) filled with N990 carbon black (Table 1). The specimen used were dumbbell-shaped made of a rubber part 30 mm long, bounded to two metal grip parts at each extremity that were subsequently attached to the fatigue machine with screws (Fig 2). Those specimens were molded at 175°C by an injection press for 4 minutes. The fatigue tests were conducted with a servo-hydraulic fatigue-testing machine at room temperature. The fatigue campaign was focused on uniaxial force-controlled tests with a sinusoidal signal at a frequency of 5Hz. All tests were conducted at a force ratio of R=0.1 with R defined by (1):

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max

min

FFR (1)

where; Fmin and Fmax are minimum and maximum force in a cycle respectively. The stress amplitude σa is also defined by (2):

aFmax2A

1 R (2)

where; A is the area of the section of the median part of the sample.

Table 1. Details about the formulation of the CR. Elastomer CR type G Filler Thermal carbon black

(N990) Curative system S-ZnO-MgO

Fig. 2. Dumbbell-type specimen – lengths in mm

2.2 Block program tests

In this part, the fatigue tests were set up in blocks of a limited number of cycles (Fig 3). Each block consisted of Nb=5,000 cycles performed at constant frequency of f=5Hz and a force ratio of R=0.1. The stress amplitude evolved during the test as follows:

- An increasing phase during which the stress amplitude would increase from the N-1 block to the N’th block, until reaching the maximum value set at the beginning of the test.

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- Then, a decreasing phase during which the maximum force would decrease from the N-1 block to the N’th block, taking the same values in the reverse order as during the increasing phase.

This range of values was selected to describe both low and high fatigue life tests [5][6][10]. In the Table 2 are presented the maximal force and the stress amplitude tested for the blocks during the fatigue campaign.

Table 2. Range of forces and stress amplitudes tested during the fatigue campaign. Maximum force Fmax (N) 85 100 115 125 140 150 160 175 Stress amplitude a (MPa) 0.33 0.38 0.44 0.48 0.54 0.57 0.61 0.67

Fig.3. Loading sequence consisting of increasing and decreasing blocks

Two tests were set up in order to compare the influence of the damage accumulated during the increasing phase on the behaviour of the sample during the decreasing phase. The two maximum values of the stress amplitude investigated are 0.67 and 0.57MPa. Our objective was then to observe the impact of the damage made during the block N-1 on the mechanical behaviour of the block N during both the increasing (Fblock N-1 < Fblock N) and the decreasing (Fblock N-1 > Fblock N) phase.

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3 Experimental results

3.1. Loading level test (increasing – decreasing phases) up to Fmax=175N

Fig. 4. Hysteresis energy evolution during a loading level test (increasing – decreasing) up to Fmax = 175N

First observation: the evolutions of the hysteresis energy and stiffness differ according to the increasing and the decreasing phase. Concerning the evolution of the hysteresis energy, we note first an increase in energy during each block during the increasing phase whereas it appears to reach steady-state values depending on the stress amplitude during the decreasing phase (Fig’s 4, 5 and 6).

Fig. 5. Hysteresis evolution by block during the increasing phase up to Fmax= 175N

Fig. 6. Hysteresis evolution by block during the phase decreasing from Fmax= 175N

In the case of stiffness, a steady decrease is observed per block during the increasing phase. During the decreasing

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phase, we distinguish two behaviours depending on the loading level’s block: the stiffness is constant for any block such as Fblock > 115N and is increasing at lower force levels Fblock < 115N (Fig’s 7, 8 and 9).

Fig. 7. Stiffness evolution during a loading level test (increasing – decreasing) up to Fmax = 175N

3.2 Analysis of mechanical parameters evolution Increasing phase

As shown in Fig. 5, the evolution of the hysteresis energy along the blocks is compared with the results of a constant force amplitude (CA) tests (reference results). The reference results are represented in dashed lines [9]. For blocks of loading level less than 115N, the evolution of the hysteresis energy is similar to that observed under CA tests. For blocks of loading level greater than 115N, the differences are consequent and the hysteresis energy for a given force amplitude in the BL tests is clearly greater than that observed for CA tests [10]. Fblock=115N can be considered as a threshold. Considering this change in hysteresis energy as a damage indicator, we propose that damage appears for force levels greater than this threshold value.

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Fig. 8. Stiffness evolution by block during the increasing phase at Fmax= 175N

Fig. 9. Stiffness evolution by block during the decreasing phase at Fmax = 175N

Decreasing phase

As also shown in Figs. 4 and 7, the evolution of both the hysteresis energy and the stiffness constant during the first blocks of the decreasing phase. After the block at 115N, the evolution of the two parameters is the opposite of what can be observed during a classical fatigue test in force-control: the hysteresis energy decreases and the stiffness increases. We can note significant differences of the values reached during the decreasing phase in comparison with the values reached during the increasing phase. These variations tend to diminish for lower loading levels. For the evolution of the stiffness, the behaviour is drastically different during the increasing phase as compared with the decreasing phase: In the first case, decrease in stiffness per block is observed systematically, whereas in the decreasing phase the stiffness seems to be constant per block during the decreasing phase. For a loading level this constant value is clearly lower during the decreasing phase than during the increasing phase. For similar tests conducted at a max force of 150N, similar evolutions in hysteresis and stiffness are observed as described above for test at 175N. 4. Discussion The evolution of the stiffness and the hysteresis area during a steady loading level test shows two distinct During the increasing phase, if there is no damage induced by the current loading, the evolution of the two parameters are very close to that observed under CA tests However, if the sample is damaged during the current block, it will impact the stiffness and the hysteresis energy during the following block, depending on whether it is during the increasing or decreasing phase. During the increasing phase, the only change in behaviour of the two parameters is a shift of the average value when compared with CA tests. In this case, there is “damage accumulation”. During the decreasing phase, the shift induced by the increasing phase is still there but an inverse evolution of the hysteresis energy and the stiffness is observed. At this point, the sample has undergone significant damage and we propose the cracks have most likely propagated, particularly during the block at 175N. Therefore, there are a significant number of “sites” where the strain-induced crystallization could create crystallized areas. The consequences are that those crystallized areas reinforce the sample and this mechanism could explain the evolution of the two parameters. The strain-induced crystallization being known to be temperature sensitive [11], the self-heating of the sample could

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prevent the crystallized area to grow or even to appear. That is not the case here because there is a 5 second mechanical transition phase between two blocks and the cooling curves of the sample after a fatigue test at 175N shows that in the first 5 seconds, the temperature decreases by almost 10°C. If the sample reaches the temperature at which the SiC transformation is very difficult to achieve, it would be only during the more damaging blocks (175N and possibly 160N as well). However the very rapid decrease between two blocks would imply that if at a given block the temperature was too high to cause an SiC impact on the behaviour of the sample, it is not necessarily the case for the following block, especially if it is during the decreasing phase. Hence, if there are more crystallized areas, meaning a reinforced sample, the evolution of the hysteresis energy and the stiffness during the decreasing phase is logical.

5. Conclusions and perspectives

Block force tests with increasing force steps followed by decreasing force steps have been conducted for a polychloroprene rubber. Comparing hysteresis energy and stiffness evolutions during these tests with those observed under constant amplitude tests permits us to understand the possible effects of strain-induced crystallization and crack propagation.

References

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