## 1 Introduction  
Polymethyl methacrylate (PMMA) is extensively applied in different industrial areas, such as aircraft, automotive industries, and biomedical materials, for its excellent properties [1; 2; 3; 4; 5; 6]. For these widely used materials, mechanical strength is the first and foremost characteristic to ensure safety [7; 8; 9; 10; 11; 12]. Therefore, many studies focused on the common mechanical properties, i.e., compression [2; 13], tension [14], and shear [2], of PMMA. Besides, some studies raise concerns on the mechanical responses of PMMA under complex stress states, such as combined loading conditions [13; 14; 15; 16]. The mechanical behaviors of PMMA under combined stress states always differ to that under uniaxial loading because of the introduction of additional shear stress. The yielding and failure of PMMA are sensitive to shear stress, i.e., the introduced shear stress decreases its normal stress strength. Many studies focus on the instrumented indentation behaviors of PMMA due to its increasing applications in microelectromechanical systems, i.e., the improved capabilities of recent instruments. Thus, tests should be performed on a small volume of materials [17; 18]. However, the phenomenon in which the mechanical properties of materials with a small volume are always enhanced compared with those with large volume is called the indentation size effects (ISE). The indentation hardness of tested materials, caused by the strain gradients, usually depends on the indentation depth and increases with decreasing depth [19; 20; 21]. For metallic materials, ISE can be characterized by the statistically storeddislocations and the geometrically necessary dislocations, which grow with the plastic strain, and are produced by the heterogeneous plastic flow attributable to the gradients of plastic strain [22; 23; 24]. These concepts are widely accepted to describe the ISE in crystalline materials. However, the PMMA as glass polymer (GPs) is a typical amorphous material that has a significantly different deformation mechanism compared with other crystalline materials. Therefore, the dislocation-mediated plasticity may not be suitable for describing the ISE in GPs. By contrast, the cooperative-localized rearrangement of atomic or molecular clusters in small distinct regions, defined as shear transformation-mediated plasticity, is a widely accepted mechanism for plastic deformation in GPs [1]. As displayed by [PERSON] et al. [17; 23; 25], shear transformation-mediated plasticity has been successfully applied in the modeling of ISE in PMMA and PC (College of automobile). The mechanical behaviors of such materials are always sensitive to the loading conditions, such as strain rate, temperature, light, and humidity [5; 26; 27; 28; 29]. The yield strength always increases with the strain rate, and the opposite is true for temperature [30]. This finding means that the correlation between the mechanical behaviors of material with environmental conditions is an important issue. This is because the mechanical behavior of PMMA is sensitive to heat. Moreover, the heat can aggravate the molecular motion of PMMA, resulting in more moisture intruding into the free volume of glassy state PMMA through the surface micro-voids. Then, the intruded water molecules will be combined with the hydrophilic groups, such as methyl groups in PMMA through hydrogen bonds. Therefore, the mechanical properties of PMMA under the condition of hygothermal aging are quite different from that without hygothermal treatment. Furthermore, the evolution of the mechanical behaviors of materials during aging is also an important parameter to judge or evaluate the useful life of materials in practical applications. However, as mentioned above, ISE, which cannot be avoided in materials during indentation test, has been clarified in normal test conditions for PMMA. This paper aimed to study the indentation behavior of aged PMMA. Then, the sensitivity of mechanical behaviors to hygrothermal aging, such as ISE, elastic modulus, and hardness can be analyzed.