Industrial Applications of 3D Textiles for Composites

1. Industrial Applications of 3D Textiles for Composites Christopher M. Pastore Philadelphia University Philadelphia, Pennsylvania, USA

2. Textile Reinforced Composites • Fiber reinforced composites whose repeating volume element (RVE) is characterized by more than one fiber orientation. • Formed with hierarchical textile processes that manipulate individual fibers or yarn bundles to create an integral structure. • It is possible to join various sub-assemblies together to form even more complex structures.

3. History of Textile Composites • The primary work in structural textile composites was initiated in the 1960’s and 70’s • The motivation was primarily elimination of delamination • From impact • From ablation • Many 3D textiles were developed in this activity around the world.

4. Advantages of 3D Textiles • The use of textiles in composites revealed two sets of benefits: • Delamination resistance • Primarily derived from through thickness orientation of yarns • Potential for reduced cost • Pre-assembled layers of fibers reduce touch labor • Part consolidation can be realized with near-net-shape manufacturing

5. Disadvantages to Textiles • Crimp • Need for new machinery • Development cost • Difficulties in structural characterization

6. Evaluation of Textiles • The true effectiveness of textiles for applications is very specific, depending on • Fabric type • Size of part • Mechanical performance requirements • Availability of processing equipment

7. Textiles are considered to have significant cost savings compared to tape lay-up. Individual layer of fabric is much thicker than tape. Fewer lay-up steps are necessary to create the final structure. Formed from dry fiber and infiltrated with resin in a secondary operation. Handling and storage requirements of the material are reduced compared to prepreg. A single product is suitable for a variety of matrix materials, reducing inventory and manufacturing costs. Perceived Benefits

8. XYZ Orthogonal Nonwoven A variation on non-wovens is the XYZ system which has no interlacings, but uses fibers or yarns to create the structure.

9. Jersey Knits • The simplest weft knit structure is the jersey. • Inherently bulky due to curvature of the yarn. • The “natural” thickness of a jersey knit fabric is roughly three times the thickness of the yarns, resulting in maximum yarn packing factors of 20-25%, and thus Vf around 15%. • High extensibility (up to 100% strain to failure) which allows complex shape formation capabilities.

10. Conformable Rib Knit

11. Warp Knits • In the WIWK, the load bearing yarns are locked into the structure through the knitting process

12. Types of 2D Braids

13. 3D Braiding Machine

14. Basic weave structures

15. 3D Weaves Through thickness Layer-to-layer XYZ

16. Crimp in Textiles • The crimp is defined as one less than the ratio of the yarn's actual length to the length of fabric it traverses. • Crimp levels influence fiber volume fraction, thickness of fabric, and mechanical performance of fabric. • High crimp leads to • Reduced tensile and compressive properties • Increased shear modulus in the dry fabric and the resulting composite • Fewer regions for localized delamination between individual yarns.

17. New Machinery/Processes • Very complex shaped objects can be produced with textile processes • Sometimes new processes or machinery are required. • Particular emphasis is on placement of bias yarns in woven fabrics.

18. Doubly Stiffened Woven Panel

19. Variations in Weave Design • Consider the formation of a tapered fabric • Weaves can have gradients in a single or double axis by changing yarn size in the width or length • Complex shapes can be achieved through “floating” and cutting yarns to reduce total number of yarns in some section of the part

20. Gradations through yarn size

21. Shape through floats

22. Issues with shaping woven fabrics • Tailoring the cross-section of a weave results in • a change in weave angle, • a change in the distribution of longitudinal, weaver, and fill, and • a change in fiber volume fraction in consequence to the change in thickness. • Some fiber volume fraction effects can be controlled by tooling. The tailoring occurs in a discrete manner, using individual yarns, whereas most tooling will be approximately continuous.

23. Mechanical Property Predictions • to model the structural response it is necessary to describe the mechanical properties of the material. • The simplest form is to treat as homogenous medium with anisotropic properties. • This is termed homogenization of the material. • If the volume of material to be homogenized is small compared to the structural component, this approach seems reasonable. • In the case of textile reinforced materials, the RVE is typically quite large, on the order of cm in some cases. It may not be reasonable to consider the RVE as representing the response of the material • Special analytical tools need to be developed to understand the local response within the RVE.

24. Homogenization of Properties • Analytical techniques have been developed to predict the elastic properties of textile composite RVE's. • averaging mechanical properties of the constituent materials, • Bolotin (1966), Nosarev (1967), Tarnopol'skii et al. (1967), and Sendeckyj (1970), Roze and Zhigun (1970), Kregers and Melbardis (1978), Kregers and Teters (1979), Chou et al. (1986), Ishikawa and Chou (1982), Jortner (1984), Whyte (1986), Ko et al. (1987), Ko and Pastore (1989) , Howarth (1991) , Jaranson et al. (1993), Singletary (1994), Pochiraju et al. (1993) • property predictions based upon detailed geometric descriptions of the reinforcement, and • Foye (1991), Gowayed (1992), Bogdanovich et al. (1993), Carter et al. (1995). • finite element methods treating matrix and fiber as discrete components. • Kabelka (1984), Woo and Whitcomb (1993), Sankar and Marrey (1993), Yoshino and Ohtsuka (1982), Whitcomb (1989), Dasgupta et al. (1992), Naik and Ganesh (1992), Lene and Paumelle (1992), Blacketter et al. (1993) and Glaesgen et al. (1996), Hill et al. (1994), Naik (1994)

25. Non-RVE Considerations • The size of the RVE is relatively large compared to test specimens and some actual structures. • The application of RVE based analysis may not be appropriate • Even experimental data can be effected by this assumption • The strain gage used in tensile testing usually covers only a few RVEs of the textile, and sometimes even less than 1.

26. If the measurement system does not contain a large number of RVEs, then the measurements do not reflect a true average value. The location of the gage will affect the measured values. Some of the perceived high variation in tensile modulus may be due to the relationship between strain gage and RVE size. Measurements of Elastic Properties

27. Moiré Interferometry Field on Axially Loaded Braided Composite

28. Elastic Modulus vs. Gage Area for Braided and 3D Woven Composites

29. Location of Test Cell with Respect toUnit Cells in a Triaxial Braid

30. Predicted Tensile Moduli for 60° Triaxial Braid AS-4/ Epoxy Test Cell with y1 = b and x1 = 4.1a

31. Predicted and Experimental Tensile Modulus of a Triaxially Braided AS-4/ Epoxy Composite with 45° Braid Angle and 12% Longitudinal Yarns

32. Predicted and Experimental Tensile Modulus of a Triaxially Braided AS-4/Epoxy Composite with 45° Braid Angle and 46% Longitudinal Yarns

33. Predicted and Experimental Tensile Modulus of a Triaxially Braided AS-4/ Epoxy Composite with 70° Braid Angle and 46% Longitudinal Yarns

34. Outlook • Tremendous variety of textile reinforcements available for composites applications. • Range from very traditional processes such as weaving to novel techniques such as three-dimensional fabrics. • The most obvious advantage of these materials is labor savings.

35. Physical Limitations • Current cost of production. • modifications to machines are needed for shaping capabilities, • capital cost is applied to a few prototypes, the unit cost is tremendous (no economy of scale) • Processing difficulties. • infiltration at high pressure, and thermal effects during curing. • frequently results in internal yarn geometry distortions. • elastic and strength properties have high variation. • thermal effects can result in local disbonds from yarns. • One approach that seems promising is the use of cold cure systems such as e-beam curing to reduce the temperature of cure and thus reduce the effect of different coefficients of thermal expansion between the fiber and resin.

36. Analytical Shortcomings • Analytical techniques are still not adequate to satisfy structural analysts planning to apply these materials to load bearing structures. • Some “variation” in elastic performance is expected due to a non-integer number of RVE's. • the design allowables for the materials are greatly reduced, frequently making them appear unsuitable for structural application due to the perception of high weight penalty. • It is possible to account for this behavior even with simple tools such as stiffness averaging if the non-RVE element is modeled.

37. Failure Analysis • Understanding of failure initiation and growth is still required. • Greater resolution of the internal stress state is needed than that for establishing homogenized elastic constants. • Failure modes are poorly understood. • These modes are associated with local curvature and distortion of the yarns at crossover points, and cracking between yarn bundles (inter-bundle cracking). • Transverse cracking and fiber failure within the yarns (intra-bundle cracking) are also a function of the complex stress state inherent in a textile. • An important issue is how curvature and inter-bundle cracking affect compression by reducing the stability of the yarn.

38. Conclusions • Textile composites have been intriguing composites researchers since the 1960s. However they have not gained true acceptance in the industry as yet. • Textile composites will remain only promises until: • cost of production are greatly reduced, • material forms are refined to meet arbitrary mechanical property requirements, and • analytical techniques are developed fully.

39. Conclusions • It seems that military demands will not drive the necessary technology to turn these dreams into reality. • What is needed is aggressive development on the part of the textile manufacturer to find appropriate industrial placement. • The most likely market forces driving future development will be the biomedical, automotive, and civil infrastructure industries.