Getting Fit Faster (Clothes, That Is)
When you think about underwear, pantyhose, sportswear, and other tight-fitting clothing, you probably don’t think of finite element analysis (FEA). But the people at Osaka-based Toyobo Co., Ltd., do. At least now they do.
“In fields such as clothing, healthcare, and sports, a simple and accurate understanding of the clothing pressure and clothing pressure distribution during body movements is essential to the design of clothing and clothing materials,” says Chisato Nonomura, Ph.D., and manager of the Computational Research Group at Toyobo (toyobo.co.jp/e). As part of their R&D in the areas of fabrics and textiles, they’ve developed a simulation system that measures the pressure of clothing against the skin. Says Nonomura: “The adoption of realistic simulation is indispensable to efficiently design garments that create optimal clothing pressure.”
To perform their pressure analyses, Toyobo chose Abaqus FEA software from SIMULIA (simulia.com), a Dassault Systèmes brand. It estimates that by using simulation, it reduced the time and cost of the product design process by 80 to 90%.
To create the virtual human body model for the pressure simulation in Abaqus, Toyobo engineers used a dummy supplied by Nanasai, Co., Ltd., that represents a 20-year-old Japanese woman. They performed 3D measurements of the model, then used the results to create a virtual body in Abaqus as a rigid model. As for clothing, they selected a short-sleeved, tight-fitting knitted undershirt made of a polyester and polyolefin blend, and a pair of pants made of a nylon and polyurethane blend. Paper sewing patterns were produced of the two.
With the CAD models for the body and clothing prepared, the engineers performed meshing in I-DEAS, then created Abaqus input files, adding analysis inputs—the rigid element R3D3 for the body and the shell element S4R for the clothing models. In total there were some 18,400 elements for the body, 4,300 for the shirt, and 3,400 for the pants.
Essentially, the engineers took the digital body model and laid over it the digital models of the clothing, then analyzed the contact and pressure that resulted when the models touched. The simulation was performed on a 3.6-GHz HP workstation and took about six hours.
Validation of the simulation results was performed with an air-pressure measuring device, which involves measuring the clothing pressure by calculating the difference between atmospheric pressure and the pressure pneumatically transmitted from air packs attached to parts of the body where the clothing contacts the skin.
What’s important to understand is that fabrics don’t behave like homogeneous materials, like steel, which respond identically to a load applied in any direction. Knitted fabric demonstrates hysteresis (think shape memory) in uniaxial extension and unloading; its behavior differs depending on the tensile direction. Woven fabric behaves differently than knitted fabric, and it demonstrates different characteristics depending on which direction it is stretched. All of which is to say that creating models for these materials—without making them exceedingly complex—is difficult.
For the knitted fabric the engineers ignored hysteresis, extracted only extension data, and assumed nonlinear elastic behavior. They also ignored the effect of material constriction and used a neo-hookean hyperelastic body (this doesn’t exhibit a linear relationship to applied stress and strain) as the matrix to account for anisotropy. Additionally, they assumed the material was of uniform thickness and used non-compression conditions for the model.
For the woven fabric, which exhibits different characteristics, depending on whether it is stretched north-south (the warp), east-west (the weft) or on the bias, at 45°. So the engineers used a feature in the Abaqus software called “rebar layers,” which reinforces the material uniaxially, like rebar in structural concrete, so that the model of the woven fabric is a hyperelastic matrix shell. The model allows for simple expression of orthotropy and nonlinearity.
“In an industry that is old and quite conservative,” says Nonomura, “it can be difficult to introduce new technology.” But in this case, the benefits of simulation compared with the conventional approach of garment development (design, fabric, sewing pattern, sewing, wearing, measuring) are notable.
Japan’s Ministry of Economy, Trade and Industry supported both the simulation system and subsequent studies on material modeling that were conducted by Hirohisa Noguchi and Masata Tanaka at Keio University, and Takaya Kobayashi and Shuya Oi at Mechanical Design & Analysis Corp. Clearly, this is a rather extensive undertaking.
