## 1 Introduction
[31] returned to the research of this topic and performed numerous fracture experiments on rectangular graphite specimens containing double V-notches with end holes (VO-notches) under pure compression, and recorded the fracture loads of the specimens. By means of the strain energy density (SED) criterion, they could successfully estimate the observed experimental fracture loads. [PERSON] and [PERSON] [32] used point stress (PS) and mean stress (MS) criteria to estimate the fracture loads reported in [31]. [PERSON] et al. published a paper [33] on the compressive fracture of PMMA weakened by round-tip V-notches. They proposed two new test specimens for fracture testing under compression, and could predict the fracture loads using stress-based fracture criteria. A few papers have also been published recently analyzing the brittle fracture of round-tip V-notched [34], key-hole notched [35], and VO-notched [36] specimens made of PMMA and GPPS polymers under mixed mode I/II loading with negative mode I contributions. These references report that two distinct areas form on the notch blunt border under tensile and compressive stresses, with the polymeric specimens breaking from the tensile side, given that the compressive critical stresses for these materials are significantly greater than the tensile ones. [PERSON] et al. [37] have recently performed compressive fracture tests on PMMA plane specimens containing sharp V-notches, blunt V-notches, and U-notches, and recording the deformations and fracture processes during loading/unloading process using a high-speed camera. Various fracture initiation points were indicated and discussed in this reference. It should be emphasized that the work reported in [37] is purely experimental, and no theoretical/numerical fracture prediction was performed. The mechanical properties of polymers, especially their fracture toughness, depend significantly on whether or not they are able to develop ductile (i.e., non-linear) processes. Recently, a novel concept, the so-called the equivalent material concept (EMC), has been proposed by [PERSON] [38] and employed in several works as a simple method to predict the failure of notched/cracked ductile materials with nonlinear behavior, using linear-elastic approaches. This concept has been successfully used in conjunction with traditional linear elastic fracture mechanics (LEFM) criteria to estimate the non-linear fracture of components containing different kinds of defects (e.g., [39; 40; 41; 42; 43; 44]). In the present work, and for the first time, the fracture of U-notched polymeric components under negative mode I loading was investigated, both experimentally and theoretically. The critical loads of U-notched specimens made of general-purpose polystyrene (GPPS) and polymethyl-methacrylate (PMMA) were experimentally obtained under compressive loading. In the theoretical section, two brittle fracture criteria, namely the maximum tangential stress (MTS) and mean stress (MS) criteria, are described as engineering methodologies to predict the load-carrying capacity (LCC) of the polymeric specimens being tested. These approaches require previous time-consuming experimental calibration, but a comparison of the experimental results with the theoretical predictions derived from the two criteria showed that they could both accurately predict the LCC of the notched specimens. With the aim of avoiding any complex calibration processes, yet providing good LCC estimations, and due to the non-linear behavior of the polymers being analyzed, the equivalent material concept (EMC) was used for the first time under compressive loading to compute the critical stress of the two polymers and to predict the LCC of the U-notched specimens. It was found that the combination of the EMC with MTS and MS criteria also provides excellent predictions of the experimental critical loads.