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A modified V-notched beam test method for interlaminar shear behavior of 3D woven composites
Composite Structures 181 (2017) 46–57
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
A modified V-notched beam test method for interlaminar shear behavior of 3D woven composites Gang Liu, Li Zhang, Licheng Guo ⇑, Qimei Wang, Feng Liao Department of Astronautic Science and Mechanics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 15 May 2017 Revised 27 July 2017 Accepted 10 August 2017 Available online 24 August 2017 Keywords: V-notched beam test fixture Interlaminar shear 3D woven composites Experimental study
a b s t r a c t The V-notched beam test (VNB) is recognized as a standard test method to determine the shear properties of composite materials. But it is not easy to make sure that the specimen placed in a predetermined position; the force conditions of the specimen become very complex when the large deformation of the specimen is generated, the friction between the internal components of fixture cannot be ignored when bending moment is generated as well when using traditional V-notched beam test fixtures. A modified V-notched beam test fixture has been proposed and designed based on standardized apparatus. After improvement, specimen with different thicknesses can be positioned easily. Meanwhile, effects of friction and bending moment are greatly reduced, and it has been validated by finite element simulation. Furthermore, some typical of experiments have been carried out by using the proposed fixture to determine the interlaminar shear modulus and shear strength of a type of 3D woven composites. Some new experimental data are presented in this paper, which can provide guidance and help for future study and industrial design of the 3D woven composites. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction As a new type of composite material, three-Dimensional (3D) woven composite is widely used in the advanced engineering structure of the plane, launcher helicopter, civil architecture and medical devices, owing to their multifarious advantages such as excellent mechanical properties in the thickness direction, good impact damage resistance and good integrity. It is well known that composite material has complicated damage evolution and failure modes since its inhomogeneity, anisotropy, and internal cracks, holes and interfacial defects caused by the production process. Therefore, many scholars have studied the stiffness, strength, residual strength and stability of the composite materials in recent years. It is essential to obtain real, vital and reliable material properties, but sometimes it is not that easy. Up to now, there is no specific test standards for the performance testing of 3D woven composites. The advanced design of multidirectional fiber-reinforced polymers shows an increased need for reliable intra- and interlaminar shear parameters [1,2]. However, the reliability and accuracy of data measurement of material properties depend on the quality
⇑ Corresponding author. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.compstruct.2017.08.056 0263-8223/Ó 2017 Elsevier Ltd. All rights reserved.
of test equipment and experimental procedures. Considerable shear test methods have been put forward to obtain shear moduli and shear strengths of composites by former researchers. For example, Iosipescu V-notched beam test method was developed from Iosipescu [3] by Walrath D. E. et al. [4] and Adams D. F et al. [5], who applied the Iosipescu shear test method to the field of composites [6–8] and suggested that the Iosipescu shear test techniques employing a double edge-notched flat specimen with two counteracting moments would satisfy the conditions of providing a region of pure, uniform shear stress [9]. This method was then developed eventually by Swanson et al. [10], Gipple et al. [11], Morton et al. [12], Melin et al. [13], and some other researchers [14–15]. Shear test methods like V-notched rail shear test developed by Whitney et al. [16] and Adams et al. [17], standardized by [18], and modified by Gude et al. [2] and Totry et al. [19]. Methods of three rail shear test [20,21], ±45° tensile test [22] and Arcan test [23,24] were also widely used. Extensive research on the Iosipescu shear test method has been conducted and developed by a large number of investigators [2,25,26]. Appropriate shear test method should be chosen and developed according to the type of materials and the purposes of measurement by users [27–29]. Problems still exist and tend to be solved in future though the Iosipescu shear test method are developed and applied by many researchers [30–32].
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For some textile reinforced composites, the effect of the large deformation of the specimens and the bending moment generated on the guide column must be taken into consideration. When using traditional V-notched beam test fixtures, first of all, it is difficult to make sure that the specimen placed in a predetermined position. Secondly, the force conditions of the specimen become very complex when the specimen in the condition of large deformation. Thirdly, the friction between the internal components of fixture cannot be ignored when bending moment is generated. In order to solve the above problems effectively and sufficiently, this paper proposed a modified V-notched beam test fixture on the basis of standardized apparatus suitable for various thicknesses. The newly modified fixture can better ensure the pure shear state of the specimen even under the condition of relatively larger deformation. Meanwhile, the specimen can be easily positioned, the effects of friction and bending moment are greatly reduced, which means the area contacted by the fixture components in the specimen will not be easily crushed caused by stress concentration. The second part of this paper will give a brief introduction and analysis of the test fixture in ASTM D5379. Then some improvements, validations will be made on the fixture, which are well designed and manufactured as presented in the third part. Finally, some typical experiments will be carried out by using the proposed modified fixture to determine the interlaminar shear modulus and shear strength of a type of 3D woven composites. 2. The analysis of test device according to ASTM D5379-05 The standard V-notched beam (VNB) method was demonstrated in ASTM D5379/D 5379M-05 [14] at length, which was adopted to obtained the shear properties of composites by some researchers [33–37]. The standard test V-notched device (Fig. 1) in this method is composed of two halves (an upper grip and a lower grip). The lower grip is fixed on a holder and the upper grip can slide through by a linear bearing post. The grip holder and the bearing post are fixed on the baseplate resting on the testing machine. A specimen can be inserted into the test device, adjusted
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and fastened by using two adjustable jaws tightened by thumbscrews and a specimen alignment pin. The two halves of the clamping apparatus are compressed by a testing machine while monitoring load. Then an approximately uniform shear stress distribution on the specimen between the two notches. For fiber reinforced composites, especially for threedimensional woven/braided composites, good in-plane and inter layer shear behavior and complex shear damage process make it difficult to obtain accurate and reliable shear properties of these materials unless robust test devices and procedures are used. If the standardized VNB test fixture is adopted to obtain the shear properties of 3D woven/braided composites, the following issues should be taken into consideration. Firstly, improvements ought to be made to position the specimen in the predetermined place since the specimen is easy to generate skewing in the vertical direction (See in Fig. 2). Secondly, high requirement is needed on the strength of the adjustable jaws (wedges) when thicker specimens are tested, because the adjustable jaws (wedges) might be crushed and deformed under the condition of large load. Simultaneously, stress concentration will occur inevitable in the area the specimen pressed by the wedges, resulting in the crush at edge of the specimen. Thirdly, unwanted bending will appear ineluctable due to the friction between the upper grip and the bearing post when the load becomes large (Fig. 3). Furthermore, the approximately uniform shear stress distribution between the two notches will not exist when large deformation occurs on the specimen. The specimen will subject to both shear and tensile loading simultaneously. With the increasing of the load, the specimen will slide, the assumed approximately uniform shear stress state between the notches turns into the state of a combination of shear and tension, which may lead to the increase of the friction between the upper grip and the bearing post. Eventually the experimental results may deviate from what we expected. If the fixture is kept on loading, premature failure in the specimen becomes imminent due to excessive bending of the bearing post and the sliding of the specimen. An modified V-notched beam test fixture based on ASTM D5379/D 5379M-05 are designed and manufactured to take the
Fig. 1. V-Notched Beam Test Fixture Schematic [14].
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Fig. 2. Specimen Place in the Fixture (left) [14].
Fig. 3. Specimen Place in the Fixture (right) [14].
above issues into consideration, the feasibility and applicability of which is verified by numerical simulation. This will be addressed in Section 3. 3. Modification of VNB test fixture 3.1. Proposed modifications To eliminate the locating difficulty and misalignment of the specimen, to tackle the handling difficulties of the fixture during the installation process, to reduce the stress concentration at the end of the specimen and the deformation of the adjustable jaws
(wedge) and to weaken the unwanted bending caused by friction between the upper grip and the bearing, were aimed at. The thickness of the specimen is not defined in the ASTM D5379/D 5379M-05, it is suggested to be designed according to the practical needs. For different composite materials, laminates or textile composites, the requirements of the thickness of the specimen are not the same. To ensure specimens with different thicknesses can be tested using the VNB test fixture with precision positioning and no skewing in the vertical direction, three square notches on the surface contacted by the specimen in the fixture are designed. Users can design three gaskets each with a groove according to the thickness of specimens to ensure the specimen is just into the grooves, and then put the gaskets into the notches and fasten them with bolts. Thus the various thicknesses of specimens can be tested by replacing the gaskets and the specimens can be positioned accurately. To prevent the deformation of the adjustable jaws (wedges) and eliminate the stress concentration (to some extent) in the area of the specimen pressed by the wedges, a simple method is adopted to avoid using the wedges. In doing so, the stress situation of the specimen will be changed and the approximately pure shear zone on the specimen will decrease, and this will not affect the determination of shear properties. An effective way to reduce the impact of unwanted bending is to diminish the friction between the upper grip and the bearing post. The sliding bearing is changed to the ball bearing and the number of the bearing posts is increased to two. Furtherer, the bearing posts are designed closer to the loading axis, which will greatly decrease the unwanted bending. 3.2. The newly designed V-notched beam test fixture The modifications described in previous section have been put into effect. A modified V-notched beam test fixture has been designed and manufactured. Fig. 4(a) is the three-view sketch of the new V-notched beam test fixture with specimen. Fig. 4(b) provides the exploded view of the modified fixture with specimen and different specimens within the corresponding gaskets. Meanwhile, a photo of practical manufactured test device is revealed in Fig. 5(a).
G. Liu et al. / Composite Structures 181 (2017) 46–57
Fig. 4. Diagram of Modified VNB Test Fixture: (a) Three-view Sketch, (b) Exploded View.
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Fig. 5. Modified V-Notched Beam Test Fixture: (a) Assembled Fixtures with Specimen, (b) Gaskets, (c) Ball Bearings.
It is shown that the overall size of the fixture is increased so that the stiffness of the test fixture is enhanced. Obviously, it is a considerable change on the modified fixture compared with the standard one since the new fixture is more flexible. In the first place, various thickness of specimens could be tested by replacing the gaskets. Three kinds of gaskets are designed to be suitable for specimens of three types of thicknesses, 5 mm, 8 mm, 10 mm, respectively (Fig. 5b). When testing, we fasten the selected gaskets in the corresponding notches on the grips (Grip A, Grip B, Grip C in Fig. 4b) with bolts, then insert the specimen into the grips fastening with bolts, thus the specimens could be positioned accurately without skewing. We can also redesign the adaptor (Fig. 4b) according to the indenter of the testing machine. Inevitably, unwanted bending may appear due to the friction between the upper grip and the bearing post in the conventional shear test. To decrease the effect of the bending, two symmetrical bearing posts are adopted and designed closer to the loading axis, and the ball bearings (Fig. 5c) are used instead of a sliding bearing as well. 3.3. Finite element evaluation of the modified fixture A Finite element model was built to evaluate the suitability of the modified VNB test fixture. Simulations were performed by using finite element software ABAQUS. The model is shown in Fig. 6. In this model, parameters of steel and aluminum are used for material parameters of fixture components and specimen, respectively. The bottom of the baseplate is constrained in x, y and z directions. Uniform displacement loads of 1 mm, 5 mm and 10 mm were applied in negative y direction on the adapter. Considering the case of small deformation, a relatively larger friction factor was given to the contact surfaces of specimen and fixture. For ball bearings, a friction factor of 0.005 was given to the contact surfaces of bearing posts and the ball bearings. The concerning mesh areas were refined to improve the accuracy of calculation. Mises stress distribution of fixture and shear stress distribution of specimen under three kinds of displacement loads (1 mm, 5 mm, 10 mm) were obtained and shown in Fig. 7, Fig. 8, and Fig. 9. Obviously, the structure of the modified fixture is relatively stable and reliable to test V-notched specimens with large deformation and high modulus.
Fig. 6. Finite element model of modified VNB test fixture.
The Von Mises stress distribution of the entire fixture under different displacement loads show that the design of the modified fixture can meet the requirements of the original stress state of the specimen. At least, the specimen will not distort during the testing process since the gaskets each with a groove position the specimen well. There is almost no serious area of stress concentration in the fixture. The bending moment generated on the bearing posts is relatively small, which guarantees and maintains the pure shear stress state of the specimen. This may due to the great reduction of the friction by replacing the sliding bearing by two ball bearings. The Von Mises stress distribution in Fig. 9(c) and shear stress distribution in Fig. 9(d) indicate that the edge areas of the specimen pressed by the fixture components will not be crushed even if it subjects to a relatively larger load. In a word, the modified fixture performs well and shows its superiorities when testing specimens with different thickness and large deformation.
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Fig. 7. Stress distribution (1 mm of displacement) (a) Von Mises stress of entire fixture (b) Von Mises stress of Lower Grip with specimen (c) Von Mises stress of specimen (d) Shear stress of specimen.
Fig. 8. Stress distribution (5 mm of displacement) (a) Von Mises stress of entire fixture (b) Von Mises stress of Lower Grip with specimen (c) Von Mises stress of specimen (d) Shear stress of specimen.
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Fig. 9. Stress distribution (10 mm of displacement) (a) Von Mises stress of entire fixture (b) Von Mises stress of Lower Grip with specimen (c) Von Mises stress of specimen (d) Shear stress of specimen.
4. Experimental investigation of interlaminar shear properties of a type of 3D woven composites by using the modified test fixture Warp enhanced 2.5D (shallow bend-joint) carbon/epoxy braided composites, as a particular type (layer-interlock) of 3D woven composites, has superior performance to interlaminar properties [38,39]. In this part, the modified fixture is used to characterize the shear properties of this kind of composite material. 4.1. Materials and specimens The warp enhanced 2.5D (shallow bend-joint) carbon/epoxy braided composite plates (provided by Center of Composite and Structure of Tianjin University of Technology) are fabricated with 12K T700 carbon fibers in the warp and weft yarns, 3K T300 carbon fibers in the binder yarns and TDE-86 resin in the matrix through the RTM process [40,41]. The specimens are obtained from the composite plates by waterjet cutting technology and manual grinding processing. The structure of the warp enhanced 2.5D braided composites and the tested specimen shape are shown in Fig. 10(a), the appearance of the fabric and the composite place are shown in Fig. 10(b) and Fig. 10(c), respectively. In this part, interlaminar shear test is conducted, so only specimen shapes of plane 2–3 and plane 1–3 are exhibited in Fig. 10a. By studying the microstructure of this kind of materials, it is found that the dimensions of the minimal repeating unit of structure (also called representative volume element, Fig. 10(a)) in three directions are 6.6 mm, 6.6 mm and 0.6 mm, respectively. Thus it is required to guarantee that at least one RVE be included in each direction, which means the thickness and height of the specimen must be greater than 6.6 mm.Considering the relatively
high stiffness and strength of this kind of composite, we enlarged the size of the V notches of the specimen in order to make it easier to observe the failure modes. The schematic diagram of the newly designed specimen is shown in Fig. 11(a). The tested specimens attached with bonded resistance strain gages (+45 and 45° to the loading axis at each side, respectively) are shown in Fig. 11 (b). The test device is shown in Fig. 11(c). Two types of specimens (plane 1–3, plane 2–3) are tested and the specimens subject to monotonic loading with a speed of 0.5 mm per minute. Sampling frequency of force sensor and strain meter are set as 10 Hz per second. The geometric parameters of tested specimens are listed in Table 1. Since waterjet cutting technology cannot ensure symmetrical in the thickness direction, we grinding the side and surface to make it symmetry results in slight differences on the thicknesses as well as on minimum widths. 4.2. Results and discussions The interlaminar shear experiments are conducted by the modified V-notches beam test fixture. The stress-time curves of shear on plane 1–3 (ost-8) are shown in Fig. 12(a), the strain-time curves are exhibited in Fig. 12(b). The corresponding curves of shear on plane 2–3 (osp-8) are shown in Fig. 13(a) and (b). Interlaminar shear moduli and strengths are calculated according to the formulas in standard ADTM D5379/D [14]. Therefore, strain of abscissa in Fig. 12(b) and Fig. 13(b) (stress-strain curves) are defined as |e+45°| + |e-45°| to facilitate linear fitting. The results of the interlaminar shear tests are given in Table 2. It is noticed that the shear deformation of the specimen is relatively large in the experiment process due to the particularity of the composite structure. Actually, in most cases, the strain gauges may fail before the specimens reach the limit stresses [14]. Therefore, only the stress-strain curves before failure are obtained,
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Fig. 10. The appearance of warp enhanced 2.5D braided composites: (a) structure and specimen shape, (b) fabric surface, (c) composite plate surface.
Fig. 11. Specimen details: (a) schematic diagram, (b) tested specimens, (c) test device.
Table 1 Geometric parameters of specimens (mm). Labels
length
Width
Minimum Width
Thickness
Cross-sectional area
ost-8-1 ost-8-2 ost-8-3 osp-8-1 osp-8-2 osp-8-3
76 76 76 76 76 76
20 20 20 20 20 20
8.98 8.94 9.14 8.98 8.86 8.90
8.06 8.26 8.74 7.00 8.62 9.10
72.3788 73.8444 79.8836 62.8600 76.3732 80.9900
seeing Fig. 12(b) and Fig. 13(b). Obviously, the relationship of shear stress-strain exhibits slight nonlinearity. Fig. 12(a) and Fig. 13(a) show that the shear stress-time curves on both plane 1–3 and plane 2–3 are relatively stable and basically consistent, which reflects the reliability of the modified fixture in a certain extent.
The average shear modulus (on plane 1–3 and plane 2–3) are determined by the slopes of the shear stress-strain curve (strain range: 1500–4000 micro-strains), which are 4.1958 Gpa and 4.3213 GPa, separately. While the specimens (plane 1–3 and on plane 2–3) failed with maximum shear stresses of 63.6416 MPa
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Fig. 12. Shear on plane 2–3 (osp-8): (a) stress-time curves, (b) stress–strain curves.
and 72.5055 MPa, respectively. Theoretically, for orthotropic materials, the modulus and the shear strengths on plane 1–3 and plane 2–3 are no difference. But in the case of this type of structure, the binder yarn plays a role in a certain extent. It is discussed in Dhiman et al. [42].
The interlaminar 1–3 and 2–3 specimens all revealed valid failure modes illuminated in Ref. [14]. The experimental results show that almost all specimen cracks initiated near the notch root and then propagated into the specimen. Examples of failure modes are shown in Fig. 14.
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Fig. 13. Shear on plane 1–3 (osp-8): (a) stress-time curves, (b) stress–strain curves.
5. Conclusion A modified V-notched beam (VNB) shear test fixture is proposed to ensure that: 1) the specimen can be positioned easily. 2) specimens with various thicknesses can be tested. 3) the specimen is in a pure shear state even under large deformation.
4) the effects of friction and bending moment in the bearing posts are be reduced. A thought of modification of a V-notched beam test fixture suitable for various thicknesses is proposed and manufactured by design three gaskets each with a groove according to the thickness of specimens to ensure the specimen is just into the grooves. So that various thicknesses of specimens could be tested by replacing the gaskets and the specimens can be positioned accurately.
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Table 2 Results of the interlaminar shear tests. Labels
Modulus GPa
ost-8-1 ost-8-2 ost-8-3 osp-8-1 osp-8-2 osp-8-3
4.1611 4.2146 4.2117 4.3614 4.3439 4.2605
Average(M) GPa
4.1958
4.3213
Failure Stress MPa 63.9125 64.7651 62.2471 72.0677 72.8359 72.6129
Average (S) MPa
63.6416
72.5055
Fig. 14. Examples of failure modes. (a) specimens in plane 1–3, (b) TEM result.
Meanwhile, the wedges are removed to avoid the premature failure and earlier crush on the specimen. The sliding bearing is changed to the ball bearing and the bearing posts are increased into two to greatly decrease the unwanted bending. Validations of the modified fixture are simulated by finite element method successfully. The simulation results shown that the modified fixture performs well and shows its superiorities when testing specimens with different thickness and large deformation experiments were carried out by using the proposed modified fixture to test the interlaminar shear modulus and shear strengths of a type of 3D woven composite. The experimental method and results will provide guidance and help for future research and industrial design. Acknowledgement This work is sponsored by National Natural Science Foundation of China (NSFC) (Nos. 11322217, 11432005). References [1] Hufenbach W, Schirner R, Andrich M. Intra- and interlaminar shear behaviour of textile composites tested with V-notched rail shear test method. In: Car Z, Kudlacek J, Pepelnjak T, editors. Proceedings of the International Conference on Innovative Technologies: IN-TECH2012. Rijeka: Faculty of Engineering, University of Rijeka; 2012. p. 73–7. [2] Gude M, Hufenbach W, Andrich M, Mertel A, Schirner R. Modified V-notched rail shear test fixture for shear characterization of textile-reinforced composite materials. Polym Testing 2015;43:147–53. [3] Iosipescu N. New accurate procedure for single shear testing of metals. J Mater 1967;2:537. [4] Walrath DE, Adams DF. The Iosipescu shear test as applied to composite mechanics. Exp Mech 1983;23(1):105–10.
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Composite Structures journal homepage: www.elsevier.com/locate/compstruct
A modified V-notched beam test method for interlaminar shear behavior of 3D woven composites Gang Liu, Li Zhang, Licheng Guo ⇑, Qimei Wang, Feng Liao Department of Astronautic Science and Mechanics, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 15 May 2017 Revised 27 July 2017 Accepted 10 August 2017 Available online 24 August 2017 Keywords: V-notched beam test fixture Interlaminar shear 3D woven composites Experimental study
a b s t r a c t The V-notched beam test (VNB) is recognized as a standard test method to determine the shear properties of composite materials. But it is not easy to make sure that the specimen placed in a predetermined position; the force conditions of the specimen become very complex when the large deformation of the specimen is generated, the friction between the internal components of fixture cannot be ignored when bending moment is generated as well when using traditional V-notched beam test fixtures. A modified V-notched beam test fixture has been proposed and designed based on standardized apparatus. After improvement, specimen with different thicknesses can be positioned easily. Meanwhile, effects of friction and bending moment are greatly reduced, and it has been validated by finite element simulation. Furthermore, some typical of experiments have been carried out by using the proposed fixture to determine the interlaminar shear modulus and shear strength of a type of 3D woven composites. Some new experimental data are presented in this paper, which can provide guidance and help for future study and industrial design of the 3D woven composites. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction As a new type of composite material, three-Dimensional (3D) woven composite is widely used in the advanced engineering structure of the plane, launcher helicopter, civil architecture and medical devices, owing to their multifarious advantages such as excellent mechanical properties in the thickness direction, good impact damage resistance and good integrity. It is well known that composite material has complicated damage evolution and failure modes since its inhomogeneity, anisotropy, and internal cracks, holes and interfacial defects caused by the production process. Therefore, many scholars have studied the stiffness, strength, residual strength and stability of the composite materials in recent years. It is essential to obtain real, vital and reliable material properties, but sometimes it is not that easy. Up to now, there is no specific test standards for the performance testing of 3D woven composites. The advanced design of multidirectional fiber-reinforced polymers shows an increased need for reliable intra- and interlaminar shear parameters [1,2]. However, the reliability and accuracy of data measurement of material properties depend on the quality
⇑ Corresponding author. E-mail address: [email protected] (L. Guo). http://dx.doi.org/10.1016/j.compstruct.2017.08.056 0263-8223/Ó 2017 Elsevier Ltd. All rights reserved.
of test equipment and experimental procedures. Considerable shear test methods have been put forward to obtain shear moduli and shear strengths of composites by former researchers. For example, Iosipescu V-notched beam test method was developed from Iosipescu [3] by Walrath D. E. et al. [4] and Adams D. F et al. [5], who applied the Iosipescu shear test method to the field of composites [6–8] and suggested that the Iosipescu shear test techniques employing a double edge-notched flat specimen with two counteracting moments would satisfy the conditions of providing a region of pure, uniform shear stress [9]. This method was then developed eventually by Swanson et al. [10], Gipple et al. [11], Morton et al. [12], Melin et al. [13], and some other researchers [14–15]. Shear test methods like V-notched rail shear test developed by Whitney et al. [16] and Adams et al. [17], standardized by [18], and modified by Gude et al. [2] and Totry et al. [19]. Methods of three rail shear test [20,21], ±45° tensile test [22] and Arcan test [23,24] were also widely used. Extensive research on the Iosipescu shear test method has been conducted and developed by a large number of investigators [2,25,26]. Appropriate shear test method should be chosen and developed according to the type of materials and the purposes of measurement by users [27–29]. Problems still exist and tend to be solved in future though the Iosipescu shear test method are developed and applied by many researchers [30–32].
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For some textile reinforced composites, the effect of the large deformation of the specimens and the bending moment generated on the guide column must be taken into consideration. When using traditional V-notched beam test fixtures, first of all, it is difficult to make sure that the specimen placed in a predetermined position. Secondly, the force conditions of the specimen become very complex when the specimen in the condition of large deformation. Thirdly, the friction between the internal components of fixture cannot be ignored when bending moment is generated. In order to solve the above problems effectively and sufficiently, this paper proposed a modified V-notched beam test fixture on the basis of standardized apparatus suitable for various thicknesses. The newly modified fixture can better ensure the pure shear state of the specimen even under the condition of relatively larger deformation. Meanwhile, the specimen can be easily positioned, the effects of friction and bending moment are greatly reduced, which means the area contacted by the fixture components in the specimen will not be easily crushed caused by stress concentration. The second part of this paper will give a brief introduction and analysis of the test fixture in ASTM D5379. Then some improvements, validations will be made on the fixture, which are well designed and manufactured as presented in the third part. Finally, some typical experiments will be carried out by using the proposed modified fixture to determine the interlaminar shear modulus and shear strength of a type of 3D woven composites. 2. The analysis of test device according to ASTM D5379-05 The standard V-notched beam (VNB) method was demonstrated in ASTM D5379/D 5379M-05 [14] at length, which was adopted to obtained the shear properties of composites by some researchers [33–37]. The standard test V-notched device (Fig. 1) in this method is composed of two halves (an upper grip and a lower grip). The lower grip is fixed on a holder and the upper grip can slide through by a linear bearing post. The grip holder and the bearing post are fixed on the baseplate resting on the testing machine. A specimen can be inserted into the test device, adjusted
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and fastened by using two adjustable jaws tightened by thumbscrews and a specimen alignment pin. The two halves of the clamping apparatus are compressed by a testing machine while monitoring load. Then an approximately uniform shear stress distribution on the specimen between the two notches. For fiber reinforced composites, especially for threedimensional woven/braided composites, good in-plane and inter layer shear behavior and complex shear damage process make it difficult to obtain accurate and reliable shear properties of these materials unless robust test devices and procedures are used. If the standardized VNB test fixture is adopted to obtain the shear properties of 3D woven/braided composites, the following issues should be taken into consideration. Firstly, improvements ought to be made to position the specimen in the predetermined place since the specimen is easy to generate skewing in the vertical direction (See in Fig. 2). Secondly, high requirement is needed on the strength of the adjustable jaws (wedges) when thicker specimens are tested, because the adjustable jaws (wedges) might be crushed and deformed under the condition of large load. Simultaneously, stress concentration will occur inevitable in the area the specimen pressed by the wedges, resulting in the crush at edge of the specimen. Thirdly, unwanted bending will appear ineluctable due to the friction between the upper grip and the bearing post when the load becomes large (Fig. 3). Furthermore, the approximately uniform shear stress distribution between the two notches will not exist when large deformation occurs on the specimen. The specimen will subject to both shear and tensile loading simultaneously. With the increasing of the load, the specimen will slide, the assumed approximately uniform shear stress state between the notches turns into the state of a combination of shear and tension, which may lead to the increase of the friction between the upper grip and the bearing post. Eventually the experimental results may deviate from what we expected. If the fixture is kept on loading, premature failure in the specimen becomes imminent due to excessive bending of the bearing post and the sliding of the specimen. An modified V-notched beam test fixture based on ASTM D5379/D 5379M-05 are designed and manufactured to take the
Fig. 1. V-Notched Beam Test Fixture Schematic [14].
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Fig. 2. Specimen Place in the Fixture (left) [14].
Fig. 3. Specimen Place in the Fixture (right) [14].
above issues into consideration, the feasibility and applicability of which is verified by numerical simulation. This will be addressed in Section 3. 3. Modification of VNB test fixture 3.1. Proposed modifications To eliminate the locating difficulty and misalignment of the specimen, to tackle the handling difficulties of the fixture during the installation process, to reduce the stress concentration at the end of the specimen and the deformation of the adjustable jaws
(wedge) and to weaken the unwanted bending caused by friction between the upper grip and the bearing, were aimed at. The thickness of the specimen is not defined in the ASTM D5379/D 5379M-05, it is suggested to be designed according to the practical needs. For different composite materials, laminates or textile composites, the requirements of the thickness of the specimen are not the same. To ensure specimens with different thicknesses can be tested using the VNB test fixture with precision positioning and no skewing in the vertical direction, three square notches on the surface contacted by the specimen in the fixture are designed. Users can design three gaskets each with a groove according to the thickness of specimens to ensure the specimen is just into the grooves, and then put the gaskets into the notches and fasten them with bolts. Thus the various thicknesses of specimens can be tested by replacing the gaskets and the specimens can be positioned accurately. To prevent the deformation of the adjustable jaws (wedges) and eliminate the stress concentration (to some extent) in the area of the specimen pressed by the wedges, a simple method is adopted to avoid using the wedges. In doing so, the stress situation of the specimen will be changed and the approximately pure shear zone on the specimen will decrease, and this will not affect the determination of shear properties. An effective way to reduce the impact of unwanted bending is to diminish the friction between the upper grip and the bearing post. The sliding bearing is changed to the ball bearing and the number of the bearing posts is increased to two. Furtherer, the bearing posts are designed closer to the loading axis, which will greatly decrease the unwanted bending. 3.2. The newly designed V-notched beam test fixture The modifications described in previous section have been put into effect. A modified V-notched beam test fixture has been designed and manufactured. Fig. 4(a) is the three-view sketch of the new V-notched beam test fixture with specimen. Fig. 4(b) provides the exploded view of the modified fixture with specimen and different specimens within the corresponding gaskets. Meanwhile, a photo of practical manufactured test device is revealed in Fig. 5(a).
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Fig. 4. Diagram of Modified VNB Test Fixture: (a) Three-view Sketch, (b) Exploded View.
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Fig. 5. Modified V-Notched Beam Test Fixture: (a) Assembled Fixtures with Specimen, (b) Gaskets, (c) Ball Bearings.
It is shown that the overall size of the fixture is increased so that the stiffness of the test fixture is enhanced. Obviously, it is a considerable change on the modified fixture compared with the standard one since the new fixture is more flexible. In the first place, various thickness of specimens could be tested by replacing the gaskets. Three kinds of gaskets are designed to be suitable for specimens of three types of thicknesses, 5 mm, 8 mm, 10 mm, respectively (Fig. 5b). When testing, we fasten the selected gaskets in the corresponding notches on the grips (Grip A, Grip B, Grip C in Fig. 4b) with bolts, then insert the specimen into the grips fastening with bolts, thus the specimens could be positioned accurately without skewing. We can also redesign the adaptor (Fig. 4b) according to the indenter of the testing machine. Inevitably, unwanted bending may appear due to the friction between the upper grip and the bearing post in the conventional shear test. To decrease the effect of the bending, two symmetrical bearing posts are adopted and designed closer to the loading axis, and the ball bearings (Fig. 5c) are used instead of a sliding bearing as well. 3.3. Finite element evaluation of the modified fixture A Finite element model was built to evaluate the suitability of the modified VNB test fixture. Simulations were performed by using finite element software ABAQUS. The model is shown in Fig. 6. In this model, parameters of steel and aluminum are used for material parameters of fixture components and specimen, respectively. The bottom of the baseplate is constrained in x, y and z directions. Uniform displacement loads of 1 mm, 5 mm and 10 mm were applied in negative y direction on the adapter. Considering the case of small deformation, a relatively larger friction factor was given to the contact surfaces of specimen and fixture. For ball bearings, a friction factor of 0.005 was given to the contact surfaces of bearing posts and the ball bearings. The concerning mesh areas were refined to improve the accuracy of calculation. Mises stress distribution of fixture and shear stress distribution of specimen under three kinds of displacement loads (1 mm, 5 mm, 10 mm) were obtained and shown in Fig. 7, Fig. 8, and Fig. 9. Obviously, the structure of the modified fixture is relatively stable and reliable to test V-notched specimens with large deformation and high modulus.
Fig. 6. Finite element model of modified VNB test fixture.
The Von Mises stress distribution of the entire fixture under different displacement loads show that the design of the modified fixture can meet the requirements of the original stress state of the specimen. At least, the specimen will not distort during the testing process since the gaskets each with a groove position the specimen well. There is almost no serious area of stress concentration in the fixture. The bending moment generated on the bearing posts is relatively small, which guarantees and maintains the pure shear stress state of the specimen. This may due to the great reduction of the friction by replacing the sliding bearing by two ball bearings. The Von Mises stress distribution in Fig. 9(c) and shear stress distribution in Fig. 9(d) indicate that the edge areas of the specimen pressed by the fixture components will not be crushed even if it subjects to a relatively larger load. In a word, the modified fixture performs well and shows its superiorities when testing specimens with different thickness and large deformation.
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Fig. 7. Stress distribution (1 mm of displacement) (a) Von Mises stress of entire fixture (b) Von Mises stress of Lower Grip with specimen (c) Von Mises stress of specimen (d) Shear stress of specimen.
Fig. 8. Stress distribution (5 mm of displacement) (a) Von Mises stress of entire fixture (b) Von Mises stress of Lower Grip with specimen (c) Von Mises stress of specimen (d) Shear stress of specimen.
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Fig. 9. Stress distribution (10 mm of displacement) (a) Von Mises stress of entire fixture (b) Von Mises stress of Lower Grip with specimen (c) Von Mises stress of specimen (d) Shear stress of specimen.
4. Experimental investigation of interlaminar shear properties of a type of 3D woven composites by using the modified test fixture Warp enhanced 2.5D (shallow bend-joint) carbon/epoxy braided composites, as a particular type (layer-interlock) of 3D woven composites, has superior performance to interlaminar properties [38,39]. In this part, the modified fixture is used to characterize the shear properties of this kind of composite material. 4.1. Materials and specimens The warp enhanced 2.5D (shallow bend-joint) carbon/epoxy braided composite plates (provided by Center of Composite and Structure of Tianjin University of Technology) are fabricated with 12K T700 carbon fibers in the warp and weft yarns, 3K T300 carbon fibers in the binder yarns and TDE-86 resin in the matrix through the RTM process [40,41]. The specimens are obtained from the composite plates by waterjet cutting technology and manual grinding processing. The structure of the warp enhanced 2.5D braided composites and the tested specimen shape are shown in Fig. 10(a), the appearance of the fabric and the composite place are shown in Fig. 10(b) and Fig. 10(c), respectively. In this part, interlaminar shear test is conducted, so only specimen shapes of plane 2–3 and plane 1–3 are exhibited in Fig. 10a. By studying the microstructure of this kind of materials, it is found that the dimensions of the minimal repeating unit of structure (also called representative volume element, Fig. 10(a)) in three directions are 6.6 mm, 6.6 mm and 0.6 mm, respectively. Thus it is required to guarantee that at least one RVE be included in each direction, which means the thickness and height of the specimen must be greater than 6.6 mm.Considering the relatively
high stiffness and strength of this kind of composite, we enlarged the size of the V notches of the specimen in order to make it easier to observe the failure modes. The schematic diagram of the newly designed specimen is shown in Fig. 11(a). The tested specimens attached with bonded resistance strain gages (+45 and 45° to the loading axis at each side, respectively) are shown in Fig. 11 (b). The test device is shown in Fig. 11(c). Two types of specimens (plane 1–3, plane 2–3) are tested and the specimens subject to monotonic loading with a speed of 0.5 mm per minute. Sampling frequency of force sensor and strain meter are set as 10 Hz per second. The geometric parameters of tested specimens are listed in Table 1. Since waterjet cutting technology cannot ensure symmetrical in the thickness direction, we grinding the side and surface to make it symmetry results in slight differences on the thicknesses as well as on minimum widths. 4.2. Results and discussions The interlaminar shear experiments are conducted by the modified V-notches beam test fixture. The stress-time curves of shear on plane 1–3 (ost-8) are shown in Fig. 12(a), the strain-time curves are exhibited in Fig. 12(b). The corresponding curves of shear on plane 2–3 (osp-8) are shown in Fig. 13(a) and (b). Interlaminar shear moduli and strengths are calculated according to the formulas in standard ADTM D5379/D [14]. Therefore, strain of abscissa in Fig. 12(b) and Fig. 13(b) (stress-strain curves) are defined as |e+45°| + |e-45°| to facilitate linear fitting. The results of the interlaminar shear tests are given in Table 2. It is noticed that the shear deformation of the specimen is relatively large in the experiment process due to the particularity of the composite structure. Actually, in most cases, the strain gauges may fail before the specimens reach the limit stresses [14]. Therefore, only the stress-strain curves before failure are obtained,
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Fig. 10. The appearance of warp enhanced 2.5D braided composites: (a) structure and specimen shape, (b) fabric surface, (c) composite plate surface.
Fig. 11. Specimen details: (a) schematic diagram, (b) tested specimens, (c) test device.
Table 1 Geometric parameters of specimens (mm). Labels
length
Width
Minimum Width
Thickness
Cross-sectional area
ost-8-1 ost-8-2 ost-8-3 osp-8-1 osp-8-2 osp-8-3
76 76 76 76 76 76
20 20 20 20 20 20
8.98 8.94 9.14 8.98 8.86 8.90
8.06 8.26 8.74 7.00 8.62 9.10
72.3788 73.8444 79.8836 62.8600 76.3732 80.9900
seeing Fig. 12(b) and Fig. 13(b). Obviously, the relationship of shear stress-strain exhibits slight nonlinearity. Fig. 12(a) and Fig. 13(a) show that the shear stress-time curves on both plane 1–3 and plane 2–3 are relatively stable and basically consistent, which reflects the reliability of the modified fixture in a certain extent.
The average shear modulus (on plane 1–3 and plane 2–3) are determined by the slopes of the shear stress-strain curve (strain range: 1500–4000 micro-strains), which are 4.1958 Gpa and 4.3213 GPa, separately. While the specimens (plane 1–3 and on plane 2–3) failed with maximum shear stresses of 63.6416 MPa
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Fig. 12. Shear on plane 2–3 (osp-8): (a) stress-time curves, (b) stress–strain curves.
and 72.5055 MPa, respectively. Theoretically, for orthotropic materials, the modulus and the shear strengths on plane 1–3 and plane 2–3 are no difference. But in the case of this type of structure, the binder yarn plays a role in a certain extent. It is discussed in Dhiman et al. [42].
The interlaminar 1–3 and 2–3 specimens all revealed valid failure modes illuminated in Ref. [14]. The experimental results show that almost all specimen cracks initiated near the notch root and then propagated into the specimen. Examples of failure modes are shown in Fig. 14.
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Fig. 13. Shear on plane 1–3 (osp-8): (a) stress-time curves, (b) stress–strain curves.
5. Conclusion A modified V-notched beam (VNB) shear test fixture is proposed to ensure that: 1) the specimen can be positioned easily. 2) specimens with various thicknesses can be tested. 3) the specimen is in a pure shear state even under large deformation.
4) the effects of friction and bending moment in the bearing posts are be reduced. A thought of modification of a V-notched beam test fixture suitable for various thicknesses is proposed and manufactured by design three gaskets each with a groove according to the thickness of specimens to ensure the specimen is just into the grooves. So that various thicknesses of specimens could be tested by replacing the gaskets and the specimens can be positioned accurately.
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Table 2 Results of the interlaminar shear tests. Labels
Modulus GPa
ost-8-1 ost-8-2 ost-8-3 osp-8-1 osp-8-2 osp-8-3
4.1611 4.2146 4.2117 4.3614 4.3439 4.2605
Average(M) GPa
4.1958
4.3213
Failure Stress MPa 63.9125 64.7651 62.2471 72.0677 72.8359 72.6129
Average (S) MPa
63.6416
72.5055
Fig. 14. Examples of failure modes. (a) specimens in plane 1–3, (b) TEM result.
Meanwhile, the wedges are removed to avoid the premature failure and earlier crush on the specimen. The sliding bearing is changed to the ball bearing and the bearing posts are increased into two to greatly decrease the unwanted bending. Validations of the modified fixture are simulated by finite element method successfully. The simulation results shown that the modified fixture performs well and shows its superiorities when testing specimens with different thickness and large deformation experiments were carried out by using the proposed modified fixture to test the interlaminar shear modulus and shear strengths of a type of 3D woven composite. The experimental method and results will provide guidance and help for future research and industrial design. Acknowledgement This work is sponsored by National Natural Science Foundation of China (NSFC) (Nos. 11322217, 11432005). References [1] Hufenbach W, Schirner R, Andrich M. Intra- and interlaminar shear behaviour of textile composites tested with V-notched rail shear test method. In: Car Z, Kudlacek J, Pepelnjak T, editors. Proceedings of the International Conference on Innovative Technologies: IN-TECH2012. Rijeka: Faculty of Engineering, University of Rijeka; 2012. p. 73–7. [2] Gude M, Hufenbach W, Andrich M, Mertel A, Schirner R. Modified V-notched rail shear test fixture for shear characterization of textile-reinforced composite materials. Polym Testing 2015;43:147–53. [3] Iosipescu N. New accurate procedure for single shear testing of metals. J Mater 1967;2:537. [4] Walrath DE, Adams DF. The Iosipescu shear test as applied to composite mechanics. Exp Mech 1983;23(1):105–10.
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