Dataset on open/blind hole-hole interaction in barely visible impact damaged composite laminates

This dataset contains the mechanical properties and structural characteristics with images of the carbon fibre reinforced epoxy composite (CFRP) laminates with open/blind holes. The mechanical dataset are the fracture strength, strain at fracture, strain energy density for resilience, strain energy density to fracture and stiffness of the CFRP laminates for different setups (namely 1 hole, 2 holes parallel to applied load, and 2 holes normal to applied load) from pristine and barely visible impact damage (BVID) specimens, determine from in-plane compression test. The structural-related dataset include thermographs, images of BVID specimens, drilling-induced damage BVID specimens and video clips of crack propagation during in-plane compression testing.


Specifications
Engineering, Composites, Mechanics of materials, Mechanical Engineering Specific subject area Hole application for composite repair, strength of materials, hole-hole interaction and orientation Type of data

Value of the Data
The data is valuable because it contains important information concerning how hole type, hole-hole interaction and orientation of holes within the damage area affects the mechanical properties of the composite laminate.
The data could be valuable for informing materials engineers about (1) the damaged material structure integrity following the drilling of holes, (2) effects of the hole-hole interaction and the orientation of holes on the mechanical behaviour of the BVID specimen, and (3) performance of different type of holes, namely blind and open holes.
The data could direct future research relating to the optimization of the design of resininjection method for repairing damage structures [1,2] , as well as new approaches relating to the response of drilling holes in composite materials.

Data Description
The data consists of the mechanical properties and structural information of the carbon fibre epoxy composite (CFRP) laminate containing different hole types (blind holes and open holes) and hole setups (1 hole, 2 holes parallel to the load, and 2 holes normal to the load) in pristine and barely visible impact damaged (BVID) conditions.
The mechanical data comprises of the fracture strength ( σ U ), strain at fracture ( ε U ), strain energy density for resilience ( u E ), strain energy density to fracture ( u F ) and stiffness (E) of the CFRP laminates derived from the stress-strain curves of the specimens subjected to in-plane compression testing to fracture. ( Table 1 ) shows the derived data of the mechanical properties of CFRP laminates with the presence of blind holes. Table 2 shows the derived data of the mechanical properties of CFRP laminates with the presence of open holes. The complete mechan- Table 1 Mechanical properties of CFRP laminates with different blind hole orientations in pristine and damaged state.   ical data can be found in the "Mech_Properties" spreadsheet in the supplementary excel file (Data_Mech_Struct). Of note, the data related to the CFRP laminates without any holes can be retrieved from another data article [3] . The structural data comprises images of BVID laminates and drilling-induced damage around holes in BVID laminates taken using infrared thermography, and the following quantitative data: • diameter of the BVID, • diameter of the holes, • drilling-induced damage diameter around the holes, were derived from the images.

Experimental design
The data presented in Section 2 were derived from a series of experimental activities. Fig. 3 presents the diagram of the experimental workflow. The detailed information of (a) specifications of the CFRP laminate, (b) creating BVID using QSI, (c) process of drilling holes and hole setups, (d) inspection of BVID and hole damage using infrared thermography, (e) in-plane com-  Pristine specimen  3  3  3  3  3  3  Damaged specimen  3  3  3  3  3  3  Sub-total  12  12  12  Total  36 pression testing, and (f) determining the mechanical properties of the CFRP laminates were described in section 3.2 and 3.3. Fig. 3 (a) illustrates the schematics of the 24-ply CFRP laminates used to derive for the mechanical test data. More information regards to the specification of the 24-ply CFRP laminates could be found in another data article [3] . Table 3 shows the sample distribution for different treatments to the CFRP laminates used to derive for the mechanical test data. A total of 36 pieces of CFRP specimens were mechanically tested to rupture using in-plane compression test method. The specimens were apportioned for three treatment groups (12 specimens per group). The three-treatment group were corresponded to 1 hole, 2 holes parallel to applied load, and 2 holes normal to applied load. In each of these treatment group, the CFRP specimens were divided to 6 specimens for each type of holes namely open hole and blind hole. Of note, three out of the six specimens were subjected to BVID (BVID specimens), while the other three specimens remained as pristine (pristine specimens).

Mechanical test data
With regard to the BVID specimens, the CFRP laminates were subjected to impact damage using quasi-static indentation method (QSI), following the ASTM methods [4] . Fig. 3 (b) shows the experimental setup of the QSI method. Of note, an impact energy of about 12 J was exerted into the CFRP samples during QSI; the imparted energy was deemed as the range of energies associated to BVID as reported in a literature [5] . Fig. 3 (c) illustrates the schematics of the different hole types (blind hole and open hole) and hole setups (1H, 2Hp, and 2Hn). The holes were created by mechanical drilling using a 2-mm diameter drill bit. The different treatments to the CFRP laminates namely hole types and hole setups were related to resin-injection method used to repair BVID CFRP laminates. Several literatures reported that the blind holes [6 , 7] and open holes [8] were used as a preparatory steps for their repair method; but the structural effects of the CFRP laminates when such holes were introduced into the damaged site and the arrangement of the holes for the purpose of repair were not reported. With regard to the hole setup (1H, 2Hp, and 2Hn), the process of mechanical drilling of a single circular holes into the CFRP laminates removed fibres and matrix resulted in weak interfacial properties [9] . When a load was applied, the crack propagation occurs longitudinally along the weak interface; whereas at the strong interface, the cracks propagate across the fibres until the specimens failed completely [9] . Dependant on the parameters (namely the loading axis, and the specimen geometry and specification), the cracks growth may spread parallel or normal to the loading and fibre directions [9] ; therefore, the two-holes setup was arranged in parallel (2Hp) or normal (2Hn) to the axis of the applied load were employed to derive for the structural and mechanical data. Fig. 3 (e) shows the experimental setup of the in-plane compression test used to derive the dataset of the mechanical properties of the CFRP samples. The in-plane compression test follows a ASTM protocol [10] . The data recorded from the mechanical test consists of the load relative to the change in displacement ( L). Fig. 3 (f) shows a typical stress-strain ( σ −ε) curve of the CFRP specimens with hole. The load and displacement data obtained was used to derive for the stress ( σ ) and strain ( ε). The σ versus ε plots was further analysed to derive for the compressive strength ( σ U ), compressive strain at rupture ( ε U ), modulus of resilience (u E ), fracture toughness (u F ), and stiffness (E).
The information related to (1) QSI method used to create BVID in CFRP laminates, (2) inplane compression test method, and (3) mechanical properties derived from the stress-strain curve can be found in another data article [3] .