40TH ANNIVERSARY thermal shock behaviour of CMCs of other configurations thin glassy layer over the material surfaces, probably a has been limited [14, 15 by-product of oxidation processes, which obscured crack This paper presents comprehensive experimental data observation. To overcome this problem, specimens were on the damage in cross-ply CMCs resulting from a thermal held at the highest temperatures for shorter periods of shock treatment Four different configurations were inves- time. i.e. 7-10 min. tigated for shocks of increasing severity. The results pre- Microscopic examination of the thermally-shocked sented include the determination of the onset of cracking, specimens was carried out mainly using reflected light the identification of the modes of fracture, and damage microscopy. Each surface under investigation was pho- quantification at each shock. The effect of the thickness tographed section by section and the stored images we s of the material on its behaviour under thermal shock is then assembled using suitable image assembling softwar also highlighted to produce an image of the whole surface. The cracking pattern was imposed manually on the resulting image after careful observation of the real surface using microscopy More detailed observation of crack patterns was also per 2. Materials and experimental techniques formed using a scanning electron microscope Two plates of cross-ply CMC comprising Nicalon fibres in calcium aluminosilicate(CAS)matrix were supplied by Rolls-Royce plc. The first was made by stacking together four plies of unidirectional material to create a composite 3. Results with thickness 0.7 mm with a(0%90%)s configuration. 3.1. Simple cross-ply Nicalon/CAs laminates The second plate consisted of twelve plies of unidirec- 3. 1. 1. The(0/90)s laminate tional Nicalon/CAS with a total thickness of 2.2 mm and The description of thermal shock damage on this laminate a(0/90%)3s configuration. In both plates the fibre volume is given with reference to the nomenclature of Fig. 1 fraction was 0.34 As can be seen, the central, thick transverse(90)ply Both plates were cut using a water-cooled diamond saw is designated as Tl (Transverse 1) while the adjacent into specimens with dimensions 6 mm x 6 mm x 0.7 mm longitudinal (0)plies are designated as LI (Longitudinal (0°909))and6mmx6mm×2.2mm(0°/90°)3s).1) Longitudinal faces (6 mm x 0. 7 mm for the(0/90%)s No damage was observed on the surfaces of material and 6 mm x 2.2 mm for the(0/90%)3s)were ground samples after quenching through temperature differen- using silicon carbide paper with grain size 320-4000 grit tials lower than 450C, i.e. for AT< 450oC. Some of and were subsequently polished using diamond paste to a the samples quenched through AT=450C, the major I um finish By preparing the longitudinal faces adjacent ity of the samples quenched through AT=480C and to these initial faces, the effect of thermal shock treatment almost all of the samples tested through AT=500C could be assessed on four different configurations: simple showed evidence of cracking in the form of shallow, (0/90)s and(90/0%s from the samples cut from the first hair-like cracks. Thus, it was decided that the critical plate and multi-layer(0%/90%)3s and(90/0%)3s from the quenching temperature differential for this laminate lies samples obtained from the second. in the range450-500°C,i.e.△Tc=450-500°C. The ac then e water-quench test was employed to produce the tual value of ATe seems to vary depending on exper- mal shock condition. Each specimen, after being imental details, such as the angle of impact with the heated for a short period of time in an electric muffle quenching medium and the extent of pre-existing damage furnace at a pre-determined temperature, was dropped on the surfaces of the material. Generally, surfaces that into a container with a large quantity(>10 D)of room- exhibit at least some open porosity crack at 450oC or at temperature (20oC)water. It was then removed from temperature differentials close to this value the water bath and allowed to dry before microscopic The fracture mode identified on the surfaces of mate- examination rial samples shocked through△T≥40° C was matrix The quenching temperature difference, AT, is defined cracking If the direction of matrix cracks relative to the as the difference between the temperature at which the horizontal (i.e. the x-axis) is taken into account, matrix material was held in the furnace and the temperature of cracks can be further divided into those that run paralle the water bath. The critical quenching temperature dif- and those that run perpendicular to the horizontal. These ference, ATe, is the temperature differential that results two types of cracking phenomena are termed 'Horizon- in the onset of cracking. Temperature differentials in the tal Matrix Cracks'(HMCs)and 'Perpendicular Matrix range 100 to 800oC were investigated, with 2 or 3 speci- Cracks(PMCs), respectively. It should be noted that mens used at each AT. All specimens were initially held fibre breaks/failures could be observed even at the highes at high temperature for 15-20 min before quenching. It temperature differential investigated (AT=700-8000C) was found, however, that at the highest ATs investigated HMCs were the first form of damage seen after quench- (AT=700-800oC)this resulted in the formation of a ing through AT=450-500oC(Fig 2). They were located40TH ANNIVERSARY thermal shock behaviour of CMCs of other configurations has been limited [14, 15]. This paper presents comprehensive experimental data on the damage in cross-ply CMCs resulting from a thermal shock treatment. Four different configurations were inves￾tigated for shocks of increasing severity. The results pre￾sented include the determination of the onset of cracking, the identification of the modes of fracture, and damage quantification at each shock. The effect of the thickness of the material on its behaviour under thermal shock is also highlighted. 2. Materials and experimental techniques Two plates of cross-ply CMC comprising Nicalon fibres in a calcium aluminosilicate (CAS) matrix were supplied by Rolls-Royce plc. The first was made by stacking together four plies of unidirectional material to create a composite with thickness ∼0.7 mm with a (0◦/90◦)s configuration. The second plate consisted of twelve plies of unidirec￾tional Nicalon/CAS with a total thickness of 2.2 mm and a (0◦/90◦)3s configuration. In both plates the fibre volume fraction was 0.34. Both plates were cut using a water-cooled diamond saw into specimens with dimensions 6 mm × 6 mm × 0.7 mm ((0◦/90◦)s) and 6 mm × 6 mm × 2.2 mm ((0◦/90◦)3s). Longitudinal faces (6 mm × 0.7 mm for the (0◦/90◦)s and 6 mm × 2.2 mm for the (0◦/90◦)3s) were ground using silicon carbide paper with grain size 320–4000 grit and were subsequently polished using diamond paste to a 1 µm finish. By preparing the longitudinal faces adjacent to these initial faces, the effect of thermal shock treatment could be assessed on four different configurations: simple (0◦/90◦)s and (90◦/0◦)s from the samples cut from the first plate and multi-layer (0◦/90◦)3s and (90◦/0◦)3s from the samples obtained from the second. The water-quench test was employed to produce the thermal shock condition. Each specimen, after being heated for a short period of time in an electric muffle furnace at a pre-determined temperature, was dropped into a container with a large quantity (>10 l) of room￾temperature (∼20◦C) water. It was then removed from the water bath and allowed to dry before microscopic examination. The quenching temperature difference,
T, is defined as the difference between the temperature at which the material was held in the furnace and the temperature of the water bath. The critical quenching temperature dif￾ference,
Tc, is the temperature differential that results in the onset of cracking. Temperature differentials in the range 100 to 800◦C were investigated, with 2 or 3 speci￾mens used at each
T. All specimens were initially held at high temperature for 15–20 min before quenching. It was found, however, that at the highest
Ts investigated (
T = 700–800◦C) this resulted in the formation of a thin glassy layer over the material surfaces, probably a by-product of oxidation processes, which obscured crack observation. To overcome this problem, specimens were held at the highest temperatures for shorter periods of time, i.e. 7–10 min. Microscopic examination of the thermally-shocked specimens was carried out mainly using reflected light microscopy. Each surface under investigation was pho￾tographed section by section and the stored images were then assembled using suitable image assembling software to produce an image of the whole surface. The cracking pattern was imposed manually on the resulting image after careful observation of the real surface using microscopy. More detailed observation of crack patterns was also per￾formed using a scanning electron microscope. 3. Results 3.1. Simple cross-ply Nicalon/CAS laminates 3.1.1. The (0◦/90◦)s laminate The description of thermal shock damage on this laminate is given with reference to the nomenclature of Fig. 1. As can be seen, the central, thick transverse (90◦) ply is designated as T1 (Transverse 1) while the adjacent longitudinal (0◦) plies are designated as L1 (Longitudinal 1). No damage was observed on the surfaces of material samples after quenching through temperature differen￾tials lower than 450◦C, i.e. for
T< 450◦C. Some of the samples quenched through
T=450◦C, the major￾ity of the samples quenched through
T=480◦C and almost all of the samples tested through
T=500◦C showed evidence of cracking in the form of shallow, hair-like cracks. Thus, it was decided that the critical quenching temperature differential for this laminate lies in the range 450–500◦C, i.e.
Tc=450–500◦C. The ac￾tual value of
Tc seems to vary depending on exper￾imental details, such as the angle of impact with the quenching medium and the extent of pre-existing damage on the surfaces of the material. Generally, surfaces that exhibit at least some open porosity crack at 450◦C or at temperature differentials close to this value. The fracture mode identified on the surfaces of mate￾rial samples shocked through
T ≥ 450◦C was matrix cracking. If the direction of matrix cracks relative to the horizontal (i.e. the x-axis) is taken into account, matrix cracks can be further divided into those that run parallel and those that run perpendicular to the horizontal. These two types of cracking phenomena are termed ‘Horizon￾tal Matrix Cracks’ (HMCs) and ‘Perpendicular Matrix Cracks’ (PMCs), respectively. It should be noted that no fibre breaks/failures could be observed even at the highest temperature differential investigated (
T = 700–800◦C). HMCs were the first form of damage seen after quench￾ing through
T = 450–500◦C (Fig. 2). They were located 952