Compressive behavior of notched and unnotched carbon woven-ply PPS thermoplastic laminates at different temperatures

Accepted Manuscript Compressive behavior of notched and unnotched carbon woven-ply PPS thermoplastic laminates at different temperatures Yue Wang, Jipeng Zhang, Jiazhen Zhang, Zhengong Zhou, Guodong Fang, Shiyu Wang PII:

S1359-8368(16)32724-X

DOI:

10.1016/j.compositesb.2017.09.027

Reference:

JCOMB 5274

To appear in:

Composites Part B

Received Date: 16 November 2016 Revised Date:

31 July 2017

Accepted Date: 13 September 2017

Please cite this article as: Wang Y, Zhang J, Zhang J, Zhou Z, Fang G, Wang S, Compressive behavior of notched and unnotched carbon woven-ply PPS thermoplastic laminates at different temperatures, Composites Part B (2017), doi: 10.1016/j.compositesb.2017.09.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Compressive behavior of notched and unnotched carbon woven-ply PPS thermoplastic laminates at different temperatures

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Yue Wang, Jipeng Zhang, Jiazhen Zhang, Zhengong Zhou, Guodong Fang∗, Shiyu Wang Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 15001, China

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Abstract

The temperature dependence of compressive behavior for notched and unnotched

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specimens is essential for structural design and applications of woven carbon fabric/polyphenylene sulfide (CF/PPS) laminates. Thermal properties of CF/PPS composite were studied by DMA and DSC analysis. Compressive experiments of notched and unnotched specimens at different temperatures (25 oC, 95 oC, and 125 oC) were conducted. Compressive tests for notched specimens at 200 oC were also taken

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into account. Failure modes were examined by scanning electron microscope (SEM) and stereomicroscope. It was found for unnotched specimens that with the increase of temperature the compressive strength exhibited an obvious decline due to the softening

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of matrix, while the modulus decreased initially and then increased because of the cold crystallization. For the notched specimens, the strength decreased slightly compared

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with that of unnotched specimens, which could be attributed to the stress concertation accommodation mechanism. There was a transition failure mode from brooming to kind band for the unnotched specimens with the increase of temperature, which were closely associated with the matrix state and weave architecture characteristics. But the notched specimens appeared more complex failure modes, especially at 95 oC. Two different ∗

Corresponding Author.

Tel.: +86 451 86402396; fax: +86 451 86402386 E-mail: [email protected] (Guodong Fang)

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ACCEPTED MANUSCRIPT failure modes were detected, which could be recognized as a critical transition state. Keywords: A. Thermoplastic composite, Notch; B. Temperature effect, Compressive behavior

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1. Introduction High-performance thermoplastic composites have been widely used in the aeronautic industrial fields due to their prominent advantages of recyclability and improved processing technologies [1-10] over conventional thermoset composites. In

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order to take full advantages of thermoplastic composites, it is important to clarify the mechanical behaviors of thermoplastic composites under different loading conditions

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[11-25]. Except the investigations at room temperature, considerable attentions have been paid upon the mechanical properties of thermoplastic composites at elevated temperatures [26-36]. However, the complex compressive failure behaviors of fiber-reinforced thermoplastic laminates at high temperature were seldom studied. The main compressive failure modes of the fiber reinforced polymer (FRP)

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composites include delamination, brooming, through-thickness shear-fault, kink-band and splitting, as well as their combination failures [37-43], which depend on the fiber manufacturing process, reinforcement architecture, surrounding interface and matrix

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properties [38, 39, 44].

Among them, the ductility of matrix plays a significant role on the failure modes of

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FRP composites. For example, unidirectional CF/epoxy composite laminate failed in the form of kink-band initiated from a damage zone including local crushed and broken fibers [45, 46]. While the kind-band in the unidirectional CF/PEEK was originated from the free edge [47, 48]. In addition, for the woven CF/epoxy [23] and woven CF/polyimide [49] composites, the dominant compressive failure modes were the through-thickness shear-fault accompanying with inter-ply delamination and kink-band, respectively. Thus, the main compressive failure modes were delamination for the composites with brittle resins, but kink-band and shear failure for the composites with tougher resins. 2

ACCEPTED MANUSCRIPT Besides, initial imperfection induced by the undulation of reinforcement in textile composite also dominates the failure modes. As mentioned in Refs. [23, 45, 46], the failure modes in unidirectional composite and 2D woven composite were kink-band and shear-fault, respectively. While the failure modes of 2D braided textile composites and

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the compressive responses were studied in Refs. [50-52], the dominant damage mechanism of axial tow kinking was captured by a PUC size 3D model whilst the compressive strength under uniaxial compression loading was predicted. In order to

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understand the compressive instabilities of axial fiber tows, a 3D FE model of an axial tow was established and the mode of deformation was accurately obtained using a combination of geometric nonlinearity and material nonlinearity. The numerical results

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suggested 2D triaxial braided composite failed through a combination of matrix micro-cracking and tow buckling, which further formed kink-band. The failure modes of 3D woven textile composite was investigated with the aid of experimental and numerical methods [53-55]. The results indicated 3D woven composites presented delamination between layers in the center and kink-band on the outer layers. The

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composites had higher strength in SHPB test in all three directions, accompanied by a transition in failure mechanism. Moreover, the computational models that included initial geometric perturbation to the “perfect” textile architecture were able to accurately

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predict the failure in the in-plane compression response. Compared with the studies above, the main concern in this paper is the effect of temperature on the thermoplastic

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matrix and the special regions induced by the undulation of reinforcement under compressive loading.

The localized buckling of fiber and matrix shear failure as the main compressive

mechanisms depend greatly on temperature. It will be exacerbated when the temperature is around or higher than the glass transition temperature ( Tg ) of the thermoplastic composites. The compressive mechanical responses and failure mechanisms for the thermoplastic composites will change with the increase of temperature. The glass transition temperature is a critical temperature, which can make the

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ACCEPTED MANUSCRIPT amorphous phase transfer from glass state to rubbery state. The effect of

Tg

on the

mechanical properties of FRP composites is always characterized by non-linearity of stress-strain curves, especially for off-axial plies [56]. The nonlinear response of the

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composites, resulted from the shear deformation of the polymer matrix, will be enhanced at elevated temperatures due to the viscoplastic nature of the resin [57]. And the dependence of failure modes on Tg is more prominent. Vieille et al [58] showed that woven CF/PPS specimens presented a through-thickness shear-fault at RT and a

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kink band at 120 oC because of the reduction of PPS ultimate strength. The failure modes of woven CF/polyimide laminate were also greatly affected by temperature. The

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failure mode was overall shear-fault combining delamination at room temperature (RT). With the increase of temperature, the delamination length and shear-fault angle were reduced, and the kind band was formed when the temperature reached Tg [59]. Thus the elevated temperature has a detrimental influence on the compressive properties of thermoplastic composites.

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It is inevitable to drill holes on the thermoplastic composites, such as angles and clips for connection, for complex structures, which will induce more complex failure modes for the composites. The high stress gradient will lead to local failures in the

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vicinity of the hole, and then the stress will be redistributed [60]. For notched unidirectional composites at RT, the damage usually initiated from the hole edge. The

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failure modes were mainly fiber microbuckling and delamination [61, 62]. Since fabric laminates are more resistant to splitting and delamination [63], the failure of notched plain weave composite was initially dominated by matrix cracking and fiber microbuckling along the plane of fracture. And a localized failure in the vicinity of the hole was observed prior to catastrophic failure [64]. Some present investigations were focused on their tensile properties of notched specimens. The damage propagation behaviors were greatly depended upon the temperature [30]. However, to the author’s knowledge, few studies have been published about the notched compressive behavior of

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ACCEPTED MANUSCRIPT CF/PPS composites at elevated temperatures. The purpose of the present study is to evaluate the influence of temperature on the compressive behavior of CF/PPS. Compressive behaviors and failure modes for the notched and unnotched CF/PPS specimens are investigated. The failure mechanisms are

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studied by using experimental observations. The relationship of compressive behavior between the matrix properties at different temperature is discussed. The different failure modes for notched and unnotched CF/PPS specimens are analyzed. And the transition of

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failure modes with the increase of temperature is emphatically investigated. 2. Materials and methods 2.1 Materials

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The thermoplastic 5-harness satin weave composite laminates were supplied by TenCate Advanced Composites Company, in which the reinforcement and matrix materials were T300JB carbon fabric and Fortron 0214 PPS, respectively, as shown in Fig. 1 where the locations marked as a, b and c denote respectively crossover point, resin-rich region and warp fiber. The fiber volume fraction was 50% in CF/PPS The

composite

laminates

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laminate.

with

quasi-isotropic

lay-up

sequence:

[(±45)/(0,90)]3s were manufactured using hot pressing technique and panel thickness

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was 3.72 mm.

Fig. 1 Weave structure on the surface and lateral observations. 2.2 Experimental methods The compressive tests were conducted on Instron 3382 universal mechanical testing machine, which was equipped with a 100kN capacity load cell and an 5

ACCEPTED MANUSCRIPT environmental chamber. Tests on unnotched and notched specimens were carried out based on ASTM D6641 and ASTM D6484, respectively, and the loads were applied with displacement controls at a same crosshead speed of 0.5 mm/min. For the strain measurements, laser extensometer and strain gauge were used in notched and unnotched

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compressive tests, respectively. The geometries of the specimens and experimental

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fixture are shown in Fig. 2.

Fig. 2 Geometries of compressive specimens and experimental fixture. Prior to the mechanical testing, a combination of DSC and DMA were used to

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establish a relationship between the thermal property and mechanical property. A differential scanning calorimetry performed by Pyris Diamond DSC was conducted on the CF/PPS composite. Both the heating and cooling rate used were 10 oC /min.

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Three-point bending dynamic mechanical analysis was carried out by Perklin Elmer Pyris Diamond DMA. Temperature scans from room temperature to 250 oC with a

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heating rate of 5 oC and a frequency of 1Hz were performed in Nitrogen atmosphere. 3. Results and discussion 3.1 Thermal analysis of CF/PPS composite In a composite, the matrix state plays a dominant role in mechanical properties,

especially for the semi-crystal polymer, which may experience recrystallization, melting and decomposition with the increase of temperature. The thermal properties of CF/PPS composite obtained by DMA and DSC analysis are shown respectively in Fig. 3 and Fig. c 4, where Tg , Tc and Tm denote respectively glass transition temperature of

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T1

and

T2

means

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respectively the temperature of the initiation and end of the peak.

Fig. 3 DMA curve of CF/PPS composite.

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c In Fig. 3, Tg means the glass transition temperature of CF/PPS composites and

equals to 105 oC, which was defined as loss modulus peak temperature, which was r

higher than the glass transition temperature of neat resin ( Tg = 90 oC) supplied by TenCate Company and nearly the same with that presented in Ref. [24]. It can be

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attributed to the increase of PPS crystallinity after the addition fibers and the negligible deviation was probably resulted from the different stacking sequence. It should be noted that lots of resin-rich regions (Fig. 1) exist in the CF/PPS composite due to the

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undulation nature of woven fabric. Thus, these resin regions will keep the same glass transition temperature as that of the neat PPS [29], which will induce some detrimental

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effect on the mechanical properties of the composites. Furthermore, the locations of crystallization and melting peaks in DSC curves were

presented in Fig. 4(a), (b) and (c), which were also different from that of neat rein provided in Ref. [65] due to the addition fibers. Based on the thermal properties of CF/PPS composites as seen in Fig. 3 and Fig. 4, the compressive behavior of notched and unnotched specimens were studied at the potential service temperatures, 95 oC and 125 oC. In addition, compressive behavior for the notched specimen at 200 oC was also investigated for the application in the peripheral engine parts.

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second heating cycle.

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Fig. 4 DSC curves of CF/PPS composite: (a) first heating cycle, (b) cooling cycle and (c)

3.2 Effect of temperature on mechanical response of specimens

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The stress-strain curves of the unnotched CF/PPS specimens at 25 oC, 95 oC, and 125 oC are shown in Fig. 5. And the strength and moduli are also listed in Tab. 1. As

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seen in Fig. 5, the relationship between stress and strain is almost linear at the initial part of the curve at RT, but a slightly nonlinear at the end of curve. It is similar with that of epoxy [66] and other thermoplastic resins [23] based composites.

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Fig. 5 Compressive stress-strain curves of unnotched specimens at different temperatures.

Table 1 Compressive behavior of notched and unnotched specimens at different temperatures.

Temperature (oC)

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Properties

95

125

200

σ n (MPa)

257.08

218.26

149.61

100.088

σ u (MPa)

505.34

337.71

236.18

Ch

0.51

0.65

0.63

8.96

6.87

8.88

37.06

31.45

38.80

ε n (%)

2.48

2.26

1.22

ε u (%)

1.73

1.13

0.74

(GPa)

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En

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RT

Eu

(GPa)

7.50

0.71

σn: notched compressive strength, σu: unnotched compressive strength, Ch : hole factor, En: notched compressive modulus, Eu: unnotched compressive modulus, εn: notched compressive strain, εun: unnotched compressive strain. The slight nonlinearity of the unnotched specimen at RT may be associated with

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ACCEPTED MANUSCRIPT the damage of matrix and interface. Stress-strain curve of unnotched CF/PPS at 95 oC r c ( Tg < T < Tg ) is also linear, but the ultimate compressive strength has a significant

reduction. As discussed in Ref. [33], the flexural strength had a pronounced decrease

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r when the temperature reached to Tg . As discussed in Section 3.1, the strength reduction

of the CF/PPS laminates at 95 oC is mainly due to the softening of matrix in resin-rich c region. When temperature increases to 125 oC ( T > Tg ), the nonlinearity of the

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stress-strain curve becomes apparent, which can be attributed to the onset of plastic deformation of the whole matrix. During the compressive process, the fibers will lose the support due to the soften matrix, and the compressive bearing capacity of the

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specimens will have greatly reduction.

From the microscope point of view, the crystal structure of PPS matrix is always stable enough and can hardly be affected at temperatures below the melting point (283 o

C for present CF/PPS in Fig. 4). The molecule chains and segments of the amorphous

parts in semi-crystalline polymers are immovable at lower temperatures. With the

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increase of temperature, the segments become more active, and more energy will be got by them. As the temperature reaches Tg , the energy of segments is enough to overcome the friction between them. These segments can move freely to reflect the plastic

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deformation of matrix in the macroscopic [66]. The compressive stress-strain curves of notched specimens at different

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temperatures are shown in Fig. 6 and the compressive strength, moduli and failure strain are listed in Tab. 1. The strength of notched specimens are calculated by dividing the ultimate applied load by the gross width of the specimen.

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Fig. 6 Compressive stress-strain curves of notched specimens at different temperatures.

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It can be seen that the stress-strain curve of notched specimen at RT has a similar linear trend as that of the unnotched one. Nonlinearity also exists for notched specimens at elevated temperatures, which also can be associated with the soften matrix. It is interesting to note that the notched specimens at 125 oC and 200 oC can retain a certain

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bearing capacity after the specimens fail, which can be related with their special failure modes. For the notched specimens, a hole factor

Ch

is defined to evaluate the hole

sensitivity of ultimate strength, as shown in Table 1. The hole factor can be calculated as

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Ch =σunotched σuunnotched , where σ unotched and σ uunnotched are notched and unnotched ultimate strength, respectively. As listed in Tab. 1, the hole factor of specimens at 95 oC

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and 125 oC are almost the same, but higher than that at RT. It is attributed to the plastic deformation of matrix at elevated temperatures, as later proved in Fig. 8, which is benefit for the stress concentration accommodation, as discussed in Ref. [30]. The compressive strength and moduli of unnotched and notched specimens as

functions of temperature are shown in Fig. 7. It can be found in Fig. 7(a) that the strength of both kinds of specimens decrease with the increase of temperature. But, the strength retention rate of notched specimen is higher than that of unnotched one at the same temperature, which can also be attributed to the stress concentration

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ACCEPTED MANUSCRIPT accommodation mechanism at elevated temperatures. It is also worth noting that the strength retention of notched specimen at 200 oC is nearly 40%, which is higher than that given in Ref. [59] for CF/PEI composites whose Tg and testing temperatures are

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be attributed to the high crystallinity rate of PPS matrix.

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305oC and 375oC, respectively. The high compressive strength retention of CF/PPS can

Fig. 7 Compressive strength and moduli of unnotched and notched specimens as functions of temperature: (a) strength; (b) moduli

The compressive moduli of the notched and unnotched specimens decrease

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significantly at 95 oC, but at 125 oC it becomes higher, which may be caused by the cold crystallization of the amorphous part, as expected in Fig. 4(a) that the cold crystallization appears at a temperature range of 109 oC to 159 oC. As studied in Refs.

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[33, 67], the strain-induced crystallization mechanism was proposed to describe the cold crystallization phenomenon. Namely, the chains were relaxed and re-orientated under

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the couple effect of external force and high temperature. The modulus of notched specimen at 200oC is a little lower than that at 125oC. At 200oC, the reconstructed crystals remain in an unstable state, but part of them will be destroyed by a relative higher temperature.

3.3 Effect of temperature on matrix state Matrix will play a dominant role in the compressive behavior and failure modes of FRP composites at different temperatures. Matrix state also closely relates with the bonding performance between fiber and matrix. The fracture morphologies as shown in

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ACCEPTED MANUSCRIPT Fig. 8 can indicate the matrix state at different temperatures to capturing the failure

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mechanism of CF/PPS at different temperatures.

Fig. 8 SEM micrographs of matrix morphologies at different temperatures: (a) RT; (b)

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95 oC; (c) 125 oC.

As shown in Fig. 8, large size matrix hackles along the fibers are observed at RT,

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and all the fibers are covered by this thick matrix layer. It is indicated that the matrix at RT is almost brittle, as well as the fiber and matrix is well bonded. At 95 oC, the size of hackles decreases, and the covered matrix layer gets thinner. While at 125 oC, less matrix adheres to fibers, and plastic hackles are exhibited. It is illustrated that the matrix flows at a large degree and has a weak interface bonding.

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It is just this matrix degradation that results in the reduction of strength and modulus, as above discussed in Section 3.2. It should be noted that the matrix degradation at 95 oC is not so much serious, but significant reduction of strength and

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modulus were obtained (Fig. 7), which suggests that some other influence factors may exist. As above discussed in section 3.1, some matrix in the crossover regions still keeps

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the same glass transition temperature as that of the neat resin, which can be softened as the temperature reaches 95 oC. Consequently it can be concluded that the strength reduction at 95 oC may also be attributed to the softening of matrix in the widespread resin-rich regions. The softening behavior of matrix in the resin-rich region is difficult to examine by conventional methods, which may be obtained by a well-designed examination technique in the future. 3.4 Effect of temperature on failure modes In the macroscopic level, the compressive behavior and failure modes greatly

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ACCEPTED MANUSCRIPT depend upon the experimental temperature. The fracture morphologies shown in Fig. 9 were examined by means of the Olympus stereo microscope for the failed unnotched

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specimens at different temperatures.

Fig. 9 Failure modes of unnotched specimens at different temperatures: (a) RT; (b) 95 C; (c) 125 oC.

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The failure of unnotched specimen exhibits a brittle fracture manner at RT and there are lots of matrix cracks in the resin-rich regions as well as interface cracks, which

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are mainly formed in fiber bundles/matrix and fiber bundles/fiber bundles interfaces. The connection of the seriously propagate cracks result in the delamination, especially

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near the surface of the laminate where usually exists a higher interlaminar stress [68]. Some of these outer delamination have propagated to the grip region and separated laminas are formed. In addition, fiber bundles in the cross-section are all broken. It should be noted that the failure mode of woven CF/PPS at RT is different from those of woven CF/Epoxy [49] and woven CF/PEI [23, 69], which have kink band and brooming without fracture, respectively. At 95 oC, a wedge shear failure with an angle of approximately 30o with respect of the loading direction is detected. A few fibers kinked due to the softening of matrix in resin-rich regions are observed in the vicinity of the wedge whilst significant 14

ACCEPTED MANUSCRIPT delamination close to the surface of specimen and matrix cracks in resin-rich regions are exhibited. Compared with RT failure mode, the delamination length is relative shorter due to the increase of the mode-I interlaminar fracture toughness, which also can prevent the interlaminar cracks from extending [23, 70]. At 125oC, matrix becomes

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soften seriously and the kink-band with an angle of 41o referring to the loading direction is formed. It should be noted that the width of the kink-band is almost equal to the diameter of a fiber bundle and the formation of kink-band is closely related to the stress

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concentration in the crimp regions. The phenomenon is similar with that of 8H satin CF/PEI tested at Tg [59].

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The failure morphologies in lateral views of the notched specimens at different temperatures are shown in Fig. 10. The matrix and interface cracks can also be observed in notched specimens at RT and 95 oC, which are similar with that of unnotched specimens. There exists a transition of failure modes from wedge shear failure and obvious delamination at RT to kind bands or even microbuckling at elevated

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temperatures. The strength retention of specimens at 125 oC and 200 oC as shown in Fig. 6 can be attributed to these microbuckling. Thus, these specimens do not fail completely, and the end parts of their stress-strain curves correspond to the plastic flow of the

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matrix.

Fig. 10 Compressive failure morphologies for notched specimens at four temperatures, (a) RT; (b) 95 oC; (c) 125 oC; (d) 200 oC.

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ACCEPTED MANUSCRIPT It should be noted that there exist two different failure modes in lateral views of the r c specimens at 95 oC, as shown in Fig. 10(b). Thus, 95 oC between Tg and Tg is a

transition temperature for notched specimens. It also can be found that the angle β

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between failure plane and loading direction increases with the increase of the temperature for the notched specimens. Compared with the failure morphologies of unnotched specimens, the existence of hole in the notched CF/PPS specimen can intensify the influence of temperature on failure modes. The similar failure modes with

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that of unnotched specimens can be obtained by notched specimens at relative lower temperatures. It may be attributed to that the matrix in notched specimens can be

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softened earlier by the combination of temperature and stress concentration effects. 3.5 Effect of temperature on local damage of unnotched specimens Besides the matrix softening behavior which can result in the variation of failure modes, the local damage behavior in microscopic level may be another key factor. A typical local damage in the crossover region between 0o and 90o fiber bundles in a

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single layer within the CF/PPS laminates was shown in Fig. 11.

Fig. 11 SEM micrographs of 90° fiber bundle in cross-ply at different temperatures: (a) RT; (b) 95 oC; (c) 125 oC. 16

ACCEPTED MANUSCRIPT It can be seen that cracks in the crossover regions between 0o and 90o fiber bundles (solid arrow) as well as in the 90o fiber bundles (dash arrow) are detected, regardless of the testing temperature. It is expected that the former cracks initiate earlier than the later ones due to the stress concentration at the crimps. The cracks in 90o fiber bundles

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exhibit the different characteristics with increasing the temperature. At RT, crack in 90o fiber bundle passes throughout the cross-section. The normal of crack plane is parallel to the fiber plies. At elevated temperatures, the propagations of the cracks are prevented,

the plastic deformation of matrix [35].

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which can be attributed to that the energy required for propagation is partly absorbed by

In addition, cracks in 90o fiber bundle are also tilted with the increase of

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temperature, where the angles between the crack paths and loading directions are 33.41o and 42.17o for specimens at 95 oC and 125 oC, respectively. They are similar to the values of β corresponding to the same conditions provided in Fig. 9. It is illustrated that the local damage of woven CF/PPS composite plays a dominant role in the variation of failure modes with temperature. Thus, it can be concluded that the failure mechanism of

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unnotched woven CF/PPS composite at elevated temperatures are dominated by the matrix state and local damage behavior in the characteristic regions (crossover between

the next study.

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0o and 90o fiber bundles), which will be helpful for the progressive damage analysis in

3.6 Effect of temperature on local damage at the hole edge of notched specimens

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For notched specimens, the damage usually initiated in the vicinity of the hole due

to the stress concentration. The post-failure notched specimens at different temperatures was cut along the center line in loading direction to examine the local damage in the vicinities of the holes as shown in Fig. 12.

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Fig. 12 Local damage at the edge of the hole of notched specimens at different temperatures.

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Up the hole regions of notched specimen, the delamination cracks was propagated from the hole regions for the specimens at RT and 95 oC. Two different failure modes

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appear in the vicinity of hole of notched specimen at different temperatures. It is noted that there are no apparent difference about quality for specimens. At RT, the notched specimen shows a wedge shear failure. An analogous kink-band is formed at 125 oC. And some fiber bundles are shear failure. At 95 oC, the failure modes of notched specimens exhibit a transition state, which includes the shear failure and fiber kinking.

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It should also be noted that the kind band observed at 125 oC is not the same with that of the unnotched case. Less fiber bundles are broken but appear slight microbuckling. At 200 oC, the microbuckling of fiber bundle becomes more obvious, which can be attributed to the serious softening of matrix.

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4. Conclusions

The effects of temperature on the strength and failure modes of notched and

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unnotched woven CF/PPS composite under compression were investigated in this paper. The results can be summarized as follows: (1) The nonlinear stress-strain curves, reduction of strength and modulus for woven

CF/PPS laminates were attributed to the softening of matrix at elevated temperatures. But the special cold crystallization mechanism can increase the moduli at relative higher temperature. The strength reduction of notched specimens at elevated temperatures was less than that of unnotched specimens because the stress concentration was accommodated by the plastic deformation of matrix. Additionally, the hole factor

Ch

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interface bonding weakened, which resulted in a transition of failure modes from brooming to wedge shear and kink-band for the unnotched specimens, and another transition from wedge shear to kink-band until microbuckling for the notched specimens.

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The presence of hole in the notched specimens can intensify the influence of temperature. The failure mechanism for the transition of failure modes can be

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considered as the matrix state and local damage at the crossover regions. Acknowledgements

The present work was supported by the National Science Foundation of China under Grant Nos. 51271067, 11272105 and 11572101. References

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