Local strain and damage measurements on a composite with digital image correlation and acoustic emission

TL;DR: In this paper, the authors used both digital image correlation and acoustic emission measurements to correlate strain measurements and damage location on a complex composite material during a monotonic tensile test, which is a continuous fibre-reinforced friction material used in car clutches.

Abstract: Non-linear stress–strain behaviour under monotonic loading can be caused by continuous damage. In order to understand this non-linearity, simultaneous strain and damage measurements were taken on composite specimens using digital image correlation and acoustic emission as these techniques give valuable local information. The latter is essential in the case of inhomogeneous or anisotropic materials, such as continuous fibre composites. On the one hand, digital image correlation gives access to full field strain and on the other hand, acoustic emission recording can be used for damage monitoring and location if, at least two sensors are placed on the specimen under loading. In this work, these two techniques were combined to correlate strain measurements and damage location on a complex composite material during a monotonic tensile test. The composite is a continuous fibre-reinforced friction material used in car clutches. These measures were used to understand the non-linearity of the stress–strain curve of the as-received material as well as detect volume damage after thermal cycling. In depth study of the strain field and acoustic emission events location revealed a correlation between non-homogeneous damage kinetics throughout the specimen and the evolution of the strain distribution.

Summary

Introduction

• Government authorities use resources on information campaigns in order to inform citizens about relevant policy changes.
• Individuals are rarely fully informed about proposed and existing policies, and a growing body of evidence suggests that individuals are not fully informed about the public policies relevant for their choices (Chetty, Looney, & Kroft, 2009; Chetty & Saez, 2013).
• The aim of the present paper is to investigate if the short-term effects, as documented in a previous paper (Finseras & Jakobsson, 2013) of public information materials on knowledge among the general public persists in the longer run.

The Experiment

• To study the effect of public information on knowledge the authors sent a survey questionnaire to 3,000 individuals between 40 and 67 years of age.
• 1,500 of these individuals were randomly allocated to a treatment group and 1,500 to a control group.
• The treatment consisted of receiving a standard information brochure from the Norwegian Labour and Welfare Service (NAV) on the recently implemented changes in the pension system.
• The brochure was emailed to the respondents in the treatment group as a link to a web page with a pdf (Portable Document Format) of the brochure six days before they received the first survey.
• The respondents were recruited from Yougov’s panel of individuals who have volunteered to participate in surveys.

Data and Descriptive Statistics

• A total of 662 individuals from the treatment group answered both rounds of the survey (44%), while 736 individuals from the control group answered (49%).
• Respondents in the treatment group both received the brochure and were asked to answer the questionnaire, while those in the control group were only asked to answer the questionnaire.
• Since the focus is the difference between the short- and medium-term effects of the treatment, the unbalanced sample is not of the same concern as in a standard experiment.
• With regards to generalizability to the general population, their sample includes more individuals with university education than does the population between 40 and 67 years of age (52.7% compared to 31.2%).
• In the present paper the authors report results including individuals that participated in both waves so their findings will not depend on changes of composition between the waves, however the generalizability of the results are weaker as compared to a sample that is more representative of the total population.

Results

• In this section the authors show the Intention-to-treat effects (ITT).
• The concern in their situation regards the possibility to extrapolate out of sample.
• Before moving on to the regression results the authors report the raw means for the treatment and control groups in the first and second wave (Table 2).
• The treated group is also 10 percentage points more likely to correctly identify that the new pension system will imply a higher pension if you decide to retire later and 7 percentage points more likely to correctly state that an increasing unemployment in your own cohort will not reduce your pension.
• Finally, the authors have also (for each of the four outcome variables) tested if the change in the knowledge between wave 1 and wave 2 between the treated and control group is statistically significant within each of the outcome variables.

Conclusion

• The authors find that respondents who were allocated to a group that receives an information brochure about the pension system are more likely to answer basic questions on the pension system correctly a week later.
• The monthly pension payment will be lower Life expectancy=1 if the respondent correctly states that her pension will be lower if the life expectancy of her cohort increases, 0 otherwise.

Figures

Figure 2. Schematic view of the process of DISC.

Figure 6. Type A damage kinetics (a) cumulated number of events versus time for different Y positions. (b) strain distribution, between AE sensor #1 and #2, at t1, t2 and t3 with the circled corresponding Y position of type A damage kinetics.

Figure 5. Position of the two AE sensors and the region of interest (ROI).

Figure 10. Thermal cycles

Figure 9. Cumulative number of acoustic events and strain field prior to failure.

Figure 3. Outline of the experimental set up (a) and photo (b).

Figure 7. Type B damage kinetics (a) cumulated number of events versus time for different Y positions. (b) strain distribution at t1, t2 and t3, between AE sensor #1 and #2, with the circled corresponding Y position of type B damage kinetics.

Figure 1. Preform and view of the rectangular specimen cut into the disc.

Figure 12. Acoustic emission rate versus stress for the as received and thermally cycled material.

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Local strain and damage measurements on a composite
with digital image correlation and acoustic emission
Camille Flament, Michelle Salvia, Bruno Berthel, Gérard Crosland
To cite this version:
Camille Flament, Michelle Salvia, Bruno Berthel, Gérard Crosland. Local strain and damage measure-
ments on a composite with digital image correlation and acoustic emission. Journal of Composite Ma-
terials, SAGE Publications, 2016, 50 (14), pp.1989-1996. �10.1177/0021998315597993�. �hal-01522942�

C. Flament, M. Salvia, B. Berthel, G. Crosland, Journal of Composite Materials, 2016, 50, 1989.
Local strain and damage measurements on a composite with digital image
correlation and acoustic emission
Camille Flament
1
,
2
, Michelle Salvia
1
, Bruno Berthel
1
and Gérard Crosland
2
Abstract
Non linear stress-strain behaviour under monotonic loading can be caused by continuous damage. In
order to understand this non linearity, simultaneous strain and damage measurements were taken on
composite specimens using Digital Image Correlation (DIC) and Acoustic Emission (AE) as these
techniques give valuable local information. The latter is essential in the case of inhomogeneous or
anisotropic materials, such as continuous fibre composites. On the one hand, DIC gives access to full field
strain and on the other hand, AE recording can be used for damage monitoring and location if, at least two
sensors are placed on the specimen under loading. In this work these two techniques were combined to
correlate strain measurements and damage location on a complex composite material during a monotonic
tensile test. The composite is a continuous fibre reinforced friction material used in car clutches. These
1
Laboratoire de Tribologie et Dynamique des Systèmes (LTDS), Ecole Centrale de Lyon, France
2
VALEO Matériaux de friction, Rue Thimonnier, 87020 Limoges, France
Corresponding author :
Michelle Salvia, Ecole Centrale de Lyon, LTDS, UMR CNRS 5513, 69134 Ecully Cedex, France
Email : Michelle.Salvia@ec-lyon.fr

C. Flament, M. Salvia, B. Berthel, G. Crosland, Journal of Composite Materials, 2016, 50, 1989.
2
measures were used to understand the non linearity of the stress-strain curve of the as received material as
well as detect volume damage after thermal cycling. In depth study of the strain field and AE events
location revealed a correlation between non homogeneous damage kinetics throughout the specimen and
the evolution of the strain distribution.
Keywords
Composite material, digital image correlation, acoustic emission rate, damage location.
Context
To predict the behaviour of a structure, a good knowledge of the constitutive relations of each part is
essential. To obtain the stress/strain relations strain measurements are conducted on simple mechanical
tests. For certain materials, the stress-strain curve may be non linear. Damage is one of the causes of non
linearity in the stress/strain curve. To identify such a behaviour strain and damage must be measured
simultaneously. To measure strain many techniques can be used (extensometers, strain gauges, ..),
however, when working with inhomogeneous or anisotropic materials, such as continuous fibre
composites, accurate strain measurements call for full field data. Amongst the different full field
measurement techniques, Digital Image Correlation (DIC) is one of the most popular
1–6
. It is an optic
contactless experimental technique to measure full field displacements and strain with sub-pixel accuracy.
Failure mechanisms can be identified with different non destructive techniques such as Acoustic Emission
(AE) recording which can be used for damage monitoring
7–12
. In fact elastic waves are emitted as a
consequence of crack initiation and propagation. Therefore analysing the acoustic activity gives
information on the type of damage that occurs in the material
13,14
. Furthermore, if at least two sensors are

C. Flament, M. Salvia, B. Berthel, G. Crosland, Journal of Composite Materials, 2016, 50, 1989.
3
placed on the specimen under loading, and the wave propagation velocity is known, location of each
acoustic event is possible
15
.
The aim of this work is to combine these two techniques to correlate strain measurements and damage
location during a monotonic tensile test and determine if the non-linearity of the stress-strain curve is due
to volume damage. Damage location will also be compared with strain distribution. In addition, as car
clutches are submitted to thermal cycling, volume damage resulting in reduced strength was characterised
on thermally cycled specimens using AE signals emitted during a monotonic tensile test.
Material and loading
The material studied is the organic clutch facing that transmits the rotary motion between the engine and
the wheels. It is an annular shaped continuous fibre composite constituted by a fibre glass yarn fitted with
copper strips. The studied clutch facing has external and internal diameters of 240 and 160 mm
respectively. The composite matrix is mainly composed of a phenolic thermosetting resin. The steps of
the process of fabrication are described by Bezzazi and Khamlichi
16
. During the preforming operation, a
machine guides the impregnated fibres coupling a uniform rotation with a radial translation. The two
movements have different frequencies resulting in a fibre organisation such as presented Figure 1. The
number of sin wave per 2π phase angle (N) is an important parameter as it defines the fibre orientation
17
.
The preform is put into a heated mould pressed and cured at 250°C. The fibre organisation confers an
orthotropic behaviour to the material. The orthotropic axes are the radial and the tangential axes
18
. In this
paper only mechanical properties in the tangential direction were studied. Rectangular specimens
(120x20x1.8) were cut into the disc to perform the tensile tests as shown Figure 1.

C. Flament, M. Salvia, B. Berthel, G. Crosland, Journal of Composite Materials, 2016, 50, 1989.
4
Figure 1. Preform and view of the rectangular specimen cut into the disc.
Experimental Techniques
Digital Image Correlation
The digital image correlation (DIC) technique provides displacements and strain maps on deformed
surfaces. The correlation is possible only if the surface has a random texture, such as a black and white
speckle. The surface is often painted with black and white spray paint. A region of interest (ROI) is
defined on the specimen surface. The ROI is divided into subsets which are tracked by the algorithm

Frequently asked questions

Q1. What have the authors contributed in "Local strain and damage measurements on a composite with digital image correlation and acoustic emission" ?

In this work these two techniques were combined to correlate strain measurements and damage location on a complex composite material during a monotonic tensile test. 

Q2. What is the effect of the engagement manoeuvre on the material temperature?

During the engagement manoeuvre, sliding contact occurs between the clutch facing and the counter faces (flywheel or pressure plate) which increases the material temperature. 

Q3. What is the way to identify failure mechanisms?

Failure mechanisms can be identified with different non destructive techniques such as Acoustic Emission (AE) recording which can be used for damage monitoring7–12. 

Q4. What was the aim of this work?

The aim of this work was to have a better understanding of the non-linearity of the stress-strain curve of the composite used in car clutch facings. 

Q5. How was the AE system used in the laboratory?

To test the accuracy of the AE system when locating events, 100 dB events were generated on a marked specimen using pencil lead break. 

Q6. What is the kinetics of the damage kinetics in the specimen?

The damage kinetics is non homogenously distributed in the specimen and the distribution is related to the local strain evolution during the tensile test. 

Q7. What was the effect of the AE signals on the strain field?

Simultaneous measures of the strain field, using DIC, and damage location, using AE, were done on a rectangular specimen under monotonic tensile loading. 

Q8. What is the AE rate in phase 1?

over a load of 60% of the maximum load the AE rate is constant and around 100 events per second, the flaw growth is controlled15. 

Q9. How many times were the AE events counted?

Considering the location accuracy and the repositioning of each coordinate system, the AE events were counted on 5 mm strips, every five millimetres (no overlap). 

Q10. What is the purpose of this work?

The aim of this work is to combine these two techniques to correlate strain measurements and damage location during a monotonic tensile test and determine if the non-linearity of the stress-strain curve is due to volume damage. 

Q11. What was the AE signal on the surface of the 300°C cycled specimens?

Macroscopic cracks were found on the surface of the 300°C cycled specimens, however nothing was visible, at that scale, on the 200°C cycled. 

Q12. How many thermal cycles did the specimens undergo?

To identify and quantify this degradation mechanism, specimens were submitted to 6 thermal cycles, described in Figure 10, with a maximum temperature of 200°C or 300°C.