Measurement and Prediction of Fabric and Garment Drape
Rahamat Ullah Joy
B.Sc. in Textile Engineering (Daffodil International University)
M.Sc. (University of Oulu, Finland)
A critically important, in fact essential, property of a textile fabric and one which distinguishes it from other materials, such as paper or steel, is its ability to undergo large, recoverable draping deformation by buckling gracefully into rounded folds of single and double curvature. It is this characteristic that plays a critical role in the fit, body conformation and wear comfort of garments and when translating three-dimensional (3D) body shapes into two-dimensional (2D) patterns and vice versa. According to the Textile Terms and Definitions of the Textile Institute, “Drape is defined as ‘the ability of a fabric to hang limply in graceful folds, e.g. the sinusoidal-type folds of a curtain or skirt’. It refers to the fabric shape as it hangs under its own weight; Cusick defined the drape of a fabric as ‘a deformation of the fabric produced by gravity when only part of the fabric is directly supported’. Drape, together with the effect of seams, determines the way in which a garment moulds itself to the shape of the body, this being a critical factor in comfort and aesthetic-related aspects of a garment and its fit. Ayada and Niwa4 showed that the visual beauty and total quality of gathered skirts are closely related to the fabric mechanical properties of bending, shear and fabric weight and can be described by the parameters of formability, elastic potential and drape.
Drape, in which the fabric shearing properties play a dominant role, is also a critically important parameter in the application of body scanning, mass customization, computer-aided design and computer-aided manufacturing (CAD-CAM) and automatic pattern making to clothing design and manufacturing. The most significant developments in recent years have been the empirical prediction and modelling of drape as well as the move towards 3D design, simulation and virtual modelling (3D virtual prototyping) which enables the designer to ‘drape and validate’ their design onto a computer generated manikin or one built off a body scan of a fit model, taking into account technical information, fabric type, colour, drape and stretch as well as the effect of seams. Transforming 2D patterns into a 3D configuration that follows a body surface (and vice versa), of necessity, involves modelling the fabric physical properties such as drape.
Measurement of Drape
Fabric drape characteristics and behaviour are manifested in the appearance and fit of the garment and are usually assessed subjectively. Nevertheless, considerable research and development has been directed to the routine objective measurement and characterization of drape and to relate drape, so measured, to objectively measured fabric mechanical properties, notably bending stiffness and shear stiffness. Chung presented a detailed review of studies on drape, both static and dynamic, on both unseamed and seamed fabrics, and investigated the effect of seam allowance, type and position on woven fabric drape. She found that bending length increased with the insertion of a vertical seam, while drape coefficient increased with the addition of radial seams; increasing the seam allowance had little effect. The highest drape coefficient occurred with the circular seam located just out of the pedestal. Schenk developed a new method to measure the effect of seam stiffness on the stiffness of adjacent fabrics.
Pioneering work was carried out by Chuetal who developed a method of measuring drape by means of the F.R.L. Drapemeter, quantifying drape as a dimensionless drape coefficient (DC%). Cusick subsequently developed what has become known as Cusick’s drapemeter (Fig.) and which is still the standard and most common method of measuring drape. It has a parallel light source that causes the shape of the draped fabric to be projected onto a circular paper disc. The drape of a fabric is popularly defined as the area of the annular ring covered by the vertical projection of the draped fabric expressed as a percentage of the area of the flat annular ring of fabric, this being termed the ‘drape coefficient’. In practice, the contour of the shadow is often traced onto the paper and cut out for weighing. Cusick defined the drape coefficient (DC%) as the weight of the paper of the drape shadow (W2) expressed as a percentage of the paper weight (W1) of the area of the full annular ring DC% = W2/W1 × 100
Bhatia and Phadke10 stated that since the draped sample will form pleats it will not remain in one plane and that the traced image is not necessarily the true projected one. They stated that understanding the drape mechanism requires a study of the following factors.
A. The drape geometry, i.e. the configuration of the draped sample, the drape measurement being employed to study the effects of fabric geometry.
B. The drape diagrams, i.e. the projected 2D simplification of the 3D draped sample, which contains three significant items:
- The area, which is the basis of the drape coefficient;
- The number of nodes – formed as a result of material buckling, the phenomenon of buckling, the type of load applications and the boundary conditions;
- The shape of the nodes – when the nodes are uniform, the drape diagram is a cyclic function in polar co-ordinates. Converting these polar coordinates into rectangular co-ordinates simplifies the analysis between the shape factor and the drape coefficient.
The drape geometry is predictable from the drape coefficient, the number of nodes decreasing as the drape coefficient increases (inverse relationship). Behera and Mishra found a negative correlation between the number of nodes and fabric bending rigidity.
Typical examples of ‘drapemeters’ include those of Cusick, F.R.L. and I.T.F., and the M.I.T. Drape-O-Meter. Other principles of measuring drape include the force to pull a circular fabric sample at a constant speed through a ring, the force being termed the ‘drape resistance’ of the fabric. Collier developed a digital drapemeter. Matsudaira et al. used an image analysis system to measure static and dynamic drape. Vangheluwe and Kiekens also used image analysis (video digital camera and computer-based image processing system) to measure the drape coefficient, while Stylios et al. developed the next generation of drapemeters, enabling 3D static and dynamic drape to be measured by means of a charge-coupled device (CCD) camera as a vision sensor. Image analysis enables many measurements to be made in a relatively short time. The following are some of the standard test methods used to measure fabric drape:
- BS 8357;
- BS 5058/EN 9073;
- UNI 8279;
- AFNOR G07-109;
- ERT 90-1.
Another factor: Empirical prediction of drape
A number of experimental studies have been undertaken to identify those fabric properties that affect drape and to quantify such effects empirically, by means of regression equations and other analytical techniques. Peirce carried out one of the earliest studies on fabric drape, early studies demonstrating the dominant role of fabric stiffness on drape, fabric weight playing a lesser role. Chu et al. showed that drape depended upon three basic fabric properties, namely Young’s modulus (Y), cross-sectional moment of inertia (I) and fabric weight (W) [drape coefficient = f(B/W), where B = YI].
Later studies demonstrated the effect of fabric shear and also shear hysteresis on drape for both woven and knitted fabrics, ‘shearing’ being the deformation that results in a flat fabric when opposing forces act parallel to each other (shear stiffness being the shear angle at which a fabric begins to buckle). Xu and Wang derived the following prediction equations for the shearing rigidities of worsted fabrics with short floats (e.g. plain, 2/1 twill, 1/2 twill and 2/2 twill)
Initially, work on drape concentrated on its accurate measurement and on the empirical prediction of drape from the fabric mechanical properties, notably bending and shear rigidity and hysteresis. More recently, however, attention has increasingly focused on modelling garment drape, this being important for developing 3D garment CAD systems. Ideal drape models should not only be able to display the static drape of the garment realistically with 3D renderings of design features, colours and surface textures, but simulate the animated dynamic drape. It should have the capability to convert 3D shapes into 2D patterns or vice versa. Although most apparel CAD systems or drape models on the Internet are claimed to present realistic draping effects, the real performance needs to be evaluated by the end user.
Significant improvements in the drape models have occurred over the past two decades; however, further development in this area is still needed. As Wentzel pointed out, ‘the imagery of the virtual 3D sample is still flat; the stand and garment look somewhat sterile. Although fabric coefficients can be entered, the representation of the fabric drape still leaves some room for improvement.’ When 3D animation is to be achieved, the challenge is greater. The resolution of the 3D virtual garment is still low in real-time presentation. Owing to the complexity and high polygon calculation, it takes a long time to achieve accurate performance of 3D animation. When the virtual garment is presented in a dynamic way or in 360° rotation, the figure tends to show a lot of shading and poor texture effects.
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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.