## 1 Introduction  
When defects such as U-shaped notches accumulate damage at their tip, leading to the appearance of short cracks, a common repairing method consists of drilling a hole with a radius (generally) equal to the crack length. Obviously, this repairing alternative will result in the creation of a keyhole notch if the crack length is greater than the original notch tip radius [1]. In addition, materials with brittle or quasi-brittle behavior like most polymers require extra attention, because fracture in these materials usually occurs abruptly. In what follows, some studies on brittle fracture of notched components are briefly described. Regarding brittle fracture in notched components under pure mode I, pure mode II, or mixed-mode I/II loading conditions, a number of papers have been published employing various failure criteria (e.g., [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16]), generally aiming to obtain the corresponding load-carrying capacity (LCC). At the same time, polymers have shown a wide range of applications due to their broad spectrum of properties [17], and since polymeric engineering components usually contain notches with different shapes and sizes, understanding their fracture behavior by means of notch fracture mechanics (NFM) is crucial. In [12, 13, 18, 19], the fracture behaviors of poly(methyl methacrylate) (PMMA) and general-purpose polystyrene (GPPS) have been studied in the presence of different notches and under various loading conditions. The -fracture behavior of other notched or cracked brittle polymeric components has also been investigated in [20; 21; 22]. The earliest research works on the brittle fracture of notched components under compressive stresses were undertaken before the year 2000 (e.g., [23; 24; 25; 26; 27; 28; 29]). Numerous fracture experiments were performed in [30] on rectangular graphite specimens containing V-notches with end holes (VO-notches), with the goal of recording the fracture loads of the specimens. In [30], also, the results of the experiments were successfully estimated by means of the strain energy density (SED) criterion. In 2014, by employing the point stress (PS) and mean stress (MS) criteria, [PERSON] and [PERSON] [31] successfully estimated the fracture loads reported in [30]. [PERSON] et al. [32] suggested two new test specimens made of PMMA for fracture testing under compression, and they used stress-based fracture criteria in order to predict the fracture loads. Other investigations regarding the brittle fracture of specimens made of PMMA and GPPS polymers weakened by round-tip V-notches, keyhole notches, and VO-notches under mixed-mode I/II loading with negative mode I contributions have been reported in [33; 34], respectively. Recently, [PERSON] et al. [35] conducted purely compressive fracture tests on PMMA specimens weakened by notches with different shapes and sizes, and using a high-speed camera, they analyzed the strain evolution during the fracture processes. Their valuable work has provided an effective tool for understanding the mechanisms of failure, and the types of crack nucleation and propagation from the notch edge caused by the local compressive stresses. In 2012, with the aim of simplifying the prediction of fractures in notched/cracked ductile members with nonlinear behavior, a novel concept, called the equivalent material concept (EMC), was proposed by [PERSON] [36]. This concept has been successfully used together with traditional linear elastic fracture mechanics (LEFM) criteria for the estimation of the nonlinear failure of components containing different kinds of stress concentrations [36; 37; 38; 39; 40; 41; 42]. For the first time, the experimental and theoretical fracture assessments of keyhole notched specimens made of the general-purpose polystyrene (GPPS) and poly(methyl methacrylate) (PMMA) under negative mode I loading are carried out in the present research.