J O U R N A L O F M A T E R I A L S S C I E N C E 3 9 (2 0 0 4 ) 2543 – 2546 Investigation about the effect of previous impacts on the impact behavior of high impact polystyrene (HIPS) T . S¸AH ˙ I N , T . SINMAZ C¸ EL ˙ I K ∗ , A . A R I C I Mechanical Engineering Department and Plastic Materials Research Centre, Veziro ˘ glu Campus, Kocaeli University, 41040 ˙ Izmit, Turkey E-mail: tamersc@yahoo.com A process of adding rubbers to rigid plastics in order to increase their fracture resistance was first used commer- cially in 1948, with polystyrene being the matrix. The early success of high impact polystyrene (HIPS) led to the development of similar blends based on other rigid polymers, giving rise to the rubber-toughened grades, which are now available for most commercial plastics and thermosets of any significance [1–4]. Rubber toughening causes extensive plastic deforma- tion at crack tips and leads to a considerable increase in impact strength. The initiation and propagation of cracks in glassy thermoplastics is essentially a com- petition between crazing and shear yielding. In brittle polymers, such as PS, crazing is the dominant mode whereas in more ductile materials, such as PC or PES, plane-stress shear yielding plays a more important role and, indeed, is responsible for the ductile fracture. The rubber particles not only initiate multiple crazing at low applied stresses, but also extend and deform with the crazed matrix, providing stability against premature fracture. Rubbers are unique in their ability to perform both functions, and therefore can toughen brittle PS. Other types of particles, including glass beads, can ac- celerate crazing sufficiently to cause yielding, but only well-bonded rubber particles enable essentially brittle polymers to reach large strains [5–9]. It is known that micro voids form in the polymer during deformation and they can grow and coalesce to form larger cavities and crazes, which can be observed by the naked eye. Eventually fracture will occur, at least in a glassy polymer, by the breakdown of crazes into a crack. In these materials, the impact energy is most effectively dissipated by the formation of large craze envelopes at the crack tip. The dispersed impact mod- ifier must arrest these crazes and must be sufficiently large so as not to be engulfed by the approaching craze. The rubber particle size must exceed the craze thickness and the interfacial adhesion must be sufficient to permit the effective transfer of stress to the rubber inclusion to blunt the craze. The required size is of the order of mi- crons. The interparticle distance must be sufficiently small to prevent the occurrence of a catastrophic crack [5–7]. A number of quite different mechanisms for tough- ening have been proposed but they all rely on the disper- sion of rubber particles within a glassy matrix. These ∗ Author to whom all correspondence should be addressed. include energy absorption by rubber particles, debond- ing at the rubber-matrix interface, matrix crazing, shear yielding or a combination of shear yielding and craz- ing,fracture of rubber particle, trans-particle failure, crack deflections by particles, plastic zone at crack tip, stretching and tearing of rubber particles. More energy is being absorbed than for an equivalent volume of the polystyrene matrix. The amount of energy absorbed in impact is attributed to the sum of the energy to frac- ture the glassy matrix and the work to break the rubber particles [5–7]. HIPS polymer is used in mechanical engineering ap- plications where machine parts are subjected to impact loading. In fact,mostof the machine parts are sub- jected to impact loading repeatedly during their service life. This study is aimed to investigate the repeated im- pact behavior and the crack initiation and propagation mechanism of the HIPS material. The testmaterial HIPS waskindly supplied by PETKIM (The TurkishPetrochemical Company). “A-825 E” is the commercial name of the styrene— butadiene blend. Instrumented Charpy impact tests wereperformed on aCeastpendulum type tester (Resil 25). A charpy hammer having a strike range of 1.08 kN was used. Hammer length and mass were 0.327 m and 1.254 kg, respectively. Sampling time was 8 µs.At a falling angle of 35 ◦ ,impactveloc- ity was 0.93 m/s, and maximum available energy was 0.54 J. Impact test samples were prepared according to ASTM D 256 standard. Notched samples with dimen- sions of 3.2 × 12.7 × 138 mm were used. The span was 63.5 mm. For each parameter, 10 experiments were per- formed and the average is reported. Preliminary exper- iments were performed in order to find the appropriate falling angle, which was chosen to be 35 ◦ in order to re- duce the inertial oscillations in the contact load between striker and sample. Before discussing these results, it is important to understand the approach used in the analy- sis of force-time curves, which is critical in determining the impact characteristics of materials. Upon impact of the pendulum the force rises sharply to a maximum value (F max ) and then gradually falls to zero due to catastrophic failure. The total area under a force-time curve gives the impact energy for the system ( E max ). This curve can be divided into two regions. These re- gions give the energies of crack initiation ( E i ) and crack 0022–2461C 2004 Kluwer Academic Publishers 2543