PSI - Issue 75

Aijia Li et al. / Procedia Structural Integrity 75 (2025) 318–333

321

4

Aijia Li, Christian Garnier,, Marie-Laetitia Pastor, Xiaojing Gong, Clément Keller/ Structural Integrity Procedia (2025)

Diameter of the plane in the specimen’s reduced area

dp

2. Material and methods 2.1. Material properties and layups of the specimen

In order to validate the specimen design, two types of MD composite specimens, named cross-ply-[(0/90) 6 ] s and quasi-isotropic [(0/45/90/-45) 3 ] s specimen, were manufactured from the same unidirectional CFRP (Carbon Fiber Reinforced Polymer) composite prepreg “SE84LV/HEC/150/400/37+/ - 3%popa”, whose physical properties for finite element simulation are concluded in Table 1. The reason for selecting the two layups is that they are the most commonly used configurations, not only in scientific research but also in engineering applications (Skinner et al. (2019) and Skinner et al. (2020)). The geometry of the specimen considered in this work is cruciform with four arms whose shape fits well with the testing machine “ Instron 8800 ” . To optimize the geometry of the specimen, the key point is the competition between the stresses around the gauge region and those around the transition region, as shown in Fig. 1. Hence, the shape and the dimension of these two areas need to be determined through the numerical simulation, described in the 3 rd section.

Table 1. physical properties of the unidirectional CFRP composite

Property

Symbol

Value

Unit MPa MPa MPa MPa MPa MPa MPa MPa MPa MPa -

Tensile strength in the fiber direction Tensile strength in the transverse direction

2000

X t Y t

50

In-plane shear strength

100

S

Compression strength in the fiber direction Compression strength in the transverse direction Elastic modulus in the fiber direction Elastic modulus in the transverse direction

1000

X c Y c E 1 E 2

50

138000

9500 0.28 5200 5200 1450

Poisson ’ s ratio

Nu 12

Shear modulus in the XY direction Shear modulus in the XZ direction Shear modulus in the YZ direction

G 12 G 13 G 23

2.2. Criteria for specimen design To ascertain the most suitable specimen design, four design criteria are proposed by considering the failure location, stress field, thermography measurement, and manufacturing. The first criterion is to ensure final damage in the gauge region based on the Tsai-Hill failure criterion, so a parameter B , defined as the ratio of the maximum Tsai-Hill factor in the gauge region and the transition one , is proposed to recognize the location of the damage in the gauge region, as detailed in equation (1). Specifically, the occurrence of final damage in the gauge region is predicted when the parameter B exceeds 1, and a higher value of the parameter B means a larger probability of a successful test. For the second criterion, the stress field should be uniform and continuous in terms of the distribution of different stress components. The third criterion is to provide an adequate area for the infrared thermography, so the area of the observation region (gauge region) should exceed 100 mm 2 to ensure at least 100 valid pixels, considering that most infrared and digital cameras have a resolution of around or over 640*512 pixels, an observation distance of 0.7 m, and 50 mm focal length lens (Jia et al. 2023). The last criterion is to minimize difficulties in manufacturing by taking into account the common fabrication setup and equipment.