analysis of woven fiber glass

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ANALYSIS OF WOVEN GLASS FIBER REINFORCED THERMOPLASTIC COMPOSITES UNDER VARYING STRAIN RATES

Germán Reyes1, 2 and Uday Sharma3

1Department of Mechanical Engineering 2Center for Lightweighting Automotive Materials and Processing

3Department of Automotive Systems Engineering University of Michigan - Dearborn

4901 Evergreen Road, Dearborn, Michigan 48128

ABSTRACT

Polymer matrix composites are increasingly being used in aerospace, marine, civil and automotive industries as a result of their high specific stiffness, strength, superior corrosion resistance and low coefficient of thermal expansion. Further increases depend in having a deeper understanding of their mechanical response under varying strain rates. In this study the mechanical behavior of thermoplastic woven composites was investigated under varying strain rates between 5.0 x 10-5 s-1 and 5.0 x 102 s-1 using a screw driven universal testing machine and an impact testing and imaging apparatus. Tensile testing was performed on samples with 50/50 fiber volume fractions in the warp and fill directions. Furthermore, fiber angles of 0° and 90° were used to have a deeper understanding of the effect of fiber orientation and architecture in their mechanical response under high loading rates. A high resolution state of the art non-contact strain measurement system was used to monitor real time-specimen deformation, strain evolution and distribution at varying strain rates. Results yielded stress vs. strain curves over the full range of loading rates highlighting the strain rate sensitivity exhibited by the thermoplastic composites. In addition, the non-contact strain measurement system revealed the effect of woven architecture on the mechanical behavior of thermoplastic woven composites. A formation of strain concentrations was identified and global and local strain evolutions within these woven composites were identified.

1. INTRODUCTION

Polymer matrix composites are used in a number of applications requiring service under varying loading conditions. Properties such as high specific stiffness and strength, fatigue properties and corrosion resistance make these materials especially appealing to the aerospace, civil, marine and automotive industries [1]. Limited information is available for effect of strain rate on mechanical properties of fiber reinforced composites compared to other conventional materials such as metals.

Composite materials with a woven configuration are made of fibers oriented in two perpendicular directions: the warp and the fill. Here, fibers are woven together and the fill yarns pass over and under the warp yarns following a fixed pattern. Physical parameters of the fabric, such as weight, thickness and tensile strength are directly proportional to the types and numbers of yarns used to wave it. In general, woven composites show a brittle linear elastic behavior when subjected to tensile loading conditions in the fiber directions [2]. Woven composites with same fiber orientation layers also show highly anisotropic mechanical behavior

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due to their orthotropic fiber reinforcements [3]. Although studies have been conducted to investigate the failure mechanisms and damage mechanics of woven composites, very little has been done to experimentally investigate their local and global strain distributions when subjected to tensile loading under varying loading rates. This information is very important to understand the basis for the development and validation of more accurate FEA models and to predict the mechanical behavior of composite structures [4]. A number of studies have revealed that failure of composites under compression loading conditions is progressive in nature and is a function of strain rate and applied stress. Matrix cracking, crushing of fibers and lateral flow of fiber bundles have been found to be the main damage mechanisms under through thickness dynamic loading [5-7].

In this paper an attempt is made to investigate the tensile properties of a woven glass fiber reinforced polymer composite at varying strain rates. A state of the art non-contact strain measurement system (ARAMIS 3D) is used to present a better understanding of various failure modes as well as the local and global strain distributions.

2. EXPERIMENTATION

2.1 Composite laminate manufacture

Composite panels tested in this study were based on a woven glass fiber polypropylene with fiber content of approximately 60 % by weight. Twintex® PP-60, developed and patented by Vetrotex, is a prepreg made by the commingling of continuous glass filaments and PP filaments. Here, the commingling occurs during the glass fiberizing process, which guarantees a homogeneous distribution of two types of filament at an industrial price. Panels with dimensions of 240 x 200 mm were manufactured by stacking two layers of the woven prepeg and placed in a picture frame mold. The mold was then positioned in an air circulating oven, heated to 180 ºC and then removed for stamping in a cold press under a pressure of 0.715 N/mm² (Figure 1). This rapid cooling was used to ensure a lower degree of crystallinity of the polypropylene matrix [8]. Once the mold had cooled below 55ºC, the panels were removed from the mold and visually inspected for defects.

Upper mold

Lower mold

Composite

prepreg

Figure 1. Schematic illustration of the stacking arrangement for the manufacture of composite panels.

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2.2 Tensile testing

Composite panels were cut and specimens were prepared for tensile testing. Coupons were subjected to strain rates between 5.0 x 10-5 s-1 to 5.0 x 10-2 s-1 by varying the crosshead displacement rate of the Instron universal testing machine. Table I shows the strain rates

achieved with the screw driven universal testing machine. The test events were recorded using high resolution state of the art non-contact strain measurement system (ARAMIS 3D from Trilion). This system was used to monitor real time-specimen deformation, strain evolution and distribution under the aforementioned strain rates. In order to determine the shape and size of the specimens for high rate testing care was taken to ensure that the working section was short enough to reach stress equilibrium as soon as possible and wide enough to guarantee representative mechanical properties of the material [9]. In this study dog-bone specimens with gauge length of 10 mm and width of 8 mm were tested in both 0° and 90° directions (Figure 2). After testing, low magnification optical micrographs were then taken in order to elucidate the tensile failure modes.

For the strain rates between 230 s-1 and 440 s-1 an in house developed-pneumatic driven impact testing and imaging apparatus was used. This equipment is instrumented with load, velocity and displacement sensors. Extensive testing was performed in order to calibrate and validate the high strain rate data ensuring quality and consistency of the results (additional details can be requested to the authors once the technology transfer office has cleared the patent rights). Dynamic strain rates achieved are listed in Table II. In order to get a better understanding of the failure modes under dynamic loading conditions, a high velocity digital camera was used.

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Specimen group Strain Rate Crosshead Speed

)( 1s (mm/min)

1 15105 s 0.03

2 14105 s 0.3

3 13105 s 3

4 12105 s 30

Table I: Strain rates generated by varying the crosshead speed.

Specimen group Strain Rate Pressure

)( 1s (psi)

5 440 90

6 300 75

7 230 60

Table2: Strain rates generated by varying pressure for high strain rates.

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(a) (b) (c)

Figure 2: Schematic illustration of the tensile testing geometry and composite samples (a) In 0° direction, (b) Side view and (c) 90° direction.

3. RESULTS AND DISCUSSION

The mechanical behavior of thermoplastic woven composites was investigated. Tensile testing regime was performed onto glass fiber reinforced samples with 50/50 fiber volume fractions in the warp and fill directions respectively. Four specimens, each with two different principal axis directions 0o and 90o were tested. Tests were conducted at strain rates of 5.0 x 10-

5 s-1to 5.0 x 10-2 s-1 by varying the crosshead speed of an Instron 4469 universal testing machine and at 230 s-1 to 440 s-1 using an in house developed-pneumatic driven impact testing and imaging apparatus. During testing, the load and displacement data was recorded. A non-contact strain measurement system was used to monitor specimen deformation and strain distributions. Here, the samples were prepared by applying a regular and random high contrast dot pattern to the surface, typically with spray paint. Processing of the acquired data yielded in-plane strains.

Figure 3 shows typical engineering stress vs. strain curves for specimens 1 and 2 of the thermoplastic woven composite under tension in the principal axis directions (0o) and (90o) at a strain rate of 5.0 x 10-5 s-1. It is worth noting that this composite has the same fiber volume fraction in the warp and fill directions, however the properties in the 90o are different. From Figure 3, it is clear that the woven thermoplastic composite exhibited a typical brittle behavior with sudden failure at stress levels around 246.45 MPa and strain values of 0.3 with principal axis directions (0o) whereas for principal axis directions (90o) failure occurred at stress levels around 188.94 MPa and strain values of 0.32.

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Figure 3: Typical stress vs. strain curves for the thermoplastic woven composites in the principal axis directions (0o) and (90o) under a strain rate 5.0 x 10-5 s-1.

Figure 4 shows a typical output snapshot from the non-contact strain measurement

system of the sample tested at a strain rate of 5.0 x 10-5 s-1 for the principal axis direction of 0o. A deeper insight in to the global and local strain distributions within these woven composites can be achieved by selecting specific sections in any direction. Local strains can be measured by selecting a specific point. Here two points were selected to show the variation of epsilon y

( y ) during loading. The plot shows the buildup of stress in the gauge area with time, until the

specimen fails at 6007.704 seconds from the start of the test. The initiation of failure in the specimen is graphically shown in figure by the area with high values of stain for a particular surface. Stage points 0, 1 and 2 are selected on the surface representing points of maximum strain.

Figure 4: Strain distribution at maximum load in the thermoplastic woven composite with principal axis directions (0o) under a strain rate 5.0 x 10-5 s-1.

Section 0

Stage point 1

Stage point 0

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The output from the system and the plot of strain vs. time, both conclude that the failure occurs at 6007.704 seconds from the start of the test, establishing a clear relationship between the data obtained from the test and the non-contact strain measurement system. The location of specimen failure i.e. the location of maximum strain build up given by the non-contact strain measurement system is supported by the micrograph of the specimen taken after failure as shown in Figure 5.

Front view: Rear view: Before After Before After

Figure 5: Micrographs of failed specimen 1 (0o) showing failure location under a strain rate 5.0 x 10-5 s-1.

Figure 6 shows a typical output snapshot from the non-contact strain measurement system for the sample tested at a strain rate of 5.0 x 10-5 s-1 with a principal axis direction 90o. Here, it is clear that strains are localized mainly in three points due to twill weave of the woven

fabric. As a result, in stage point 1 maximum values of y of approximately 10.9 % were

reached. A closer examination of the evolution of y on stage point 1 shows a constant increase until failure occurs. The location of maximum strain build up for specimen 2 (90o) resulting in failure is supported by the micrograph of the specimen taken after failure as shown in Figure 7.

Figure 6: Strain distribution for specimen 2 with principal axis directions 90o at 5.0 x 10-5 s-1.

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Stage point 0

Stage point 1

Section 0

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Figure 7: Micrographs of 90o failed specimens after testing at strain rate 5.0 x 10-5 s-1.

Figure 8 shows a typical engineering stress vs. strain curves for the thermoplastic woven

composite loaded under tension in the principal axis directions of 0o and 90o at strain rate of 5.0 x 10-4 s-1. Here, failure took place at stress levels of around 298.27 MPa and strain values of 0.32 with principal axis directions of 0o, whereas for principal axis directions of 90o failure occurred at stress levels of around 229.92 MPa and strain values of 0.31.

Figure 9 shows typical strain distributions for 0o tensile tested specimens under a strain rate of 5.0 x 10-4 s-1. From the figure it can be seen that areas labeled as stage point 1 and 2 exhibit higher values of strain localization.

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Figure 9: Strain distribution for 0o specimens tested at a strain rate of 5.0 x 10-4 s-1.

A closer examination of the evolution of y on stage point 0 shows a constant increase until failure occurs. The location of maximum strain build up for these 0o specimens resulted in the failure location as shown by the low magnification optical micrographs exhibited in Figure 10.



Figure 11 shows typical stress and strain vs. time plots for 90o tested specimens at a

strain rate 5.0 x 10-4 s-1. The stages 155 and 165 shown in Figure 11 represent the stages when the specimens reached its maximum tensile strength and failed under tensile loading. A closer

examination of the evolution of y on stage point 0 shows a constant increase until failure occurs. Here, the location of maximum strain build up resulted in failure as shown by the micrographs of the specimen taken after failure (Figure 12). A similar mechanical behavior and failure mode was apparent under a strain rate of 5.0 x 10-3 s-1.

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Section 0

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Figure 11: Typical stress and strain vs. time plots for 90o tested specimens at 5.0 x 10-4 s-1.



Figure 13 shows a typical engineering stress vs. strain curves for the thermoplastic

woven composite loaded under tension in the principal axis directions of 0o and 90o at strain rate of 5.0 x 10-2 s-1. Here, failure took place at stress levels of around 310 MPa and strain values of 0.35 with principal axis directions of 0o, whereas for principal axis directions of 90o failure occurred at stress levels of around 280 MPa and strain values of 0.34.

Figure 14 shows typical strain distributions for 0o tensile tested specimens under a strain rate of 5.0 x 10-2 s-1. From the figure it can be seen that areas labeled as stage point 2 and 3 exhibit higher values of strain localization up the maximum load. A closer examination of the

evolution of y on stage point 2 shows a constant increase until failure occurs as shown in Figure 15. A similar mechanical behavior and failure mode was apparent in the 90o tested specimens (Figures 16 and 17).

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Front view: Rear view: After After


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Front view: Rear view: After After


Figure 18 shows typical engineering stress vs. strain curves for the thermoplastic woven composite under tension at strain rates between 5.0 x 10-5 s-1 to 5.0 x 10-2. It is clear that as the stiffness and strength of the composite material increases with strain rate. This behavior could be associated with the viscoelastic nature of the polymeric matrix and the time-dependent nature of accumulating damage. It is suggested that at slower rates damage accumulates more gradually, but at higher rates, however, damage does not have enough time to develop and thus the damage accumulation process has a diminishing effect on the stress–strain curve as the strain rate increases. This is confirmed by the in-situ data yielded by the non-contact strain measurement system. Such data will set the basis for the development and validation of more accurate FEA models to predict the mechanical behavior of composite structures under different strain rates. The unfiltered stress vs. strain curves yielded after tensile testing under dynamic strain rates are shown in Figure 19. Here strain rates range between 230 s-1 and 440 s-1. As expected, an increase in stiffness is apparent between the strain rates of 230 s-1 and 380 s-1 [1, 3-4, 6-7]. However, as the strain rate is increased to 440 s-1, a decrease in stiffness is observed.

Stage point 2

Section 0

Stage point 0

Stage point 3

Section 1

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Figure 18: Typical stress vs. strain curves for woven composite at varying strain rates between 5.0 x 10-5 s-1

to 5.0 x 10-2 s-1.

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ss (

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440380

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5.0 x 10-4 5.0 x 10-5

Figure 19: Typical stress vs. strain curves for woven composite under dynamic loading conditions.

Stage 5

Stage 4

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A closer examination of the specimens shown in Figure 20 revealed that a couple of bundles in the gage region are oriented at an angle of approximately 7o of the loading axis, thus affecting the results. This highlights the importance of fiber misalignment on the mechanical behavior of woven composite materials and more importantly, on the design and manufacture of advance dcomposite structures.

380s-1 440s-

Figure 20: Micrographs of stage1 for strain rates 380s-1and 440s-1.

In order to get a better understanding of the failure modes under dynamic loading conditions, a high velocity digital camera was used. Figure 21 shows images of the woven thermoplastic composite tested at a strain rate of 230 s-1. Each of the five sages corresponds to 1/5 of the loading time. A description of the failure mode is as follows, initially the tensile load causes failure in the bundles oriented 90o of the loading axis (stage 2), since in this direction, the fibers are carrying very little or no load at all. Following this, the bundles of fibers oriented parallel to the loading axis continue to carry the load. It is worth noting that at this point the curved nature of the bundle starts to straighten as the tensile load is increased (stage 3). This results in inter-bundle debonding acting as stress concentration points were crack initiates (stage 4). Once the tensile strength of the composite is reached, a catastrophic failure follows (stage 5). A similar failure behavior was observed on the samples tested at a strain rate of 440 s-1 as shown in Figure 22.

Finally, the stress vs. strain curves of the thermoplastic woven composite material tested in this study under tensile loading conditions at strain rates between 5.0 x 10-5 s-1 and 5.0 x 102 s-1 are shown in Figure 23. Here, it is evident that the composite system exhibits clear strain rate sensitivity highlighting the importance of determining the mechanical behavior of composites under high strain rates to successfully design of structures where dynamic loading may occur.

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Stage 1 Stage 2

Stage 3 Stage 4

Stage 5

Figure 21: Micrographs of stages for high strain rate testing at 230s-1.

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Stage 1 Stage 2

Stage 3 Stage 4

Stage 5

Figure 22: Micrographs of stages for high strain rate testing at 440s-1.

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Figure 23: Typical stress vs. strain curves for woven composite at varying strain rates between 5.0 x 10-5 s-1 to 5.0 x 102 s-1.

4. CONCLUSIONS

The mechanical behavior of thermoplastic woven composites has been investigated under varying strain rates ranging from 5.0 x 10-5 s-1 to 5.0 x 102 s-1. Tensile testing was performed on samples using fiber angles of 0° and 90°. Experimental results provided stress vs. strain curves over the full range of loading rates. The strain rate sensitivity exhibited by the thermoplastic composites resulted in an increase of stiffness and strength coupled with a reduction of strain to failure. The effect of fiber angle and woven architecture on the mechanical behavior of thermoplastic woven composites under quasi-static loading conditions has been revealed by the non-contact strain measurement system. A clear formation of strain concentrations was identified and a deeper insight in to the global and local strain evolution and distributions within these woven composites was achieved. Higher strains were present in places were the fibers were oriented perpendicular to the loading direction. These locations acted as stress concentration points were failure initiated. In addition, the failure modes occurring under dynamic loading conditions were identified and resulted to be very similar to those apparent under quasi-static rates if loading. Finally, it has been shown that this woven thermoplastic composite material retains its properties under high strain rates and can be used in structures that may be subjected to dynamic loading conditions.

5. AKNOWLEDGMENTS

The authors are grateful to the Office of the Vice President for Research (UM-AA), the College of Engineering and Computer Science (UM-D) and the Society of Plastics Engineering (SPE Automotive Composites Conference & Exhibition Scholarship 2008) for supporting this work.

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8. Davies, P., Cantwell, W. J. “Fracture of glass /polypropylene laminates; influence of cooling rate after moulding”. Composites 25 (1994): 869-877.

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analysis of woven fiber glass

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